Pneumatics
BasicLevelTP .101 Textbook
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Pneumatics BasicLevelTP 101 Textbook
FestoDidacticKG, D-7300Esslingen1
FESTCI
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Orderno.: Description: Designation: 4th Edition: ComputerLayout: Author:
093131 PNEUM.GSLEHRB D.LB.TP1O1-GB 01t92 D. Bonner S. Broadbent, P. Croser
@Copyright by FestoDidactic KG,D-7300Esslingen 1, 1989 All rightsreserved, including rights. No partof this publication translation may be reproducedor transmittedin any form or by any means,electronic, photocopying, mechanical, or otheruise, withoutthe priorwrittenpermission of FestoDidactic KG. ISBN:3-8127-3131-2
trces on the layoutof the book
Section A:
4
Gourse
Ctr+ter 1 Characterlstlcsand apflications of pneumatics in review Pneumatics ' 2 Pneumatics and controlsystemdeVblopmeht. . . . . :
7 11 14
Chfler 2 Componentsof a pneuthaticsystem
15 16 18 21 26 27 28
Z2 23 Zt 25 Z6
and distribution Air generation Valves : valvesand logicelements Processors controlvalves Actuators: workingelementsand directional Systems:controlcircuits
Cfider 3 Symbolsand standardsIn pneumatics : ' of components Symbolsand descriptions for pneumaticsystems =2 Safetyrequirements
33 34 44
Ctrapter4 Systematlcapproachto pneumaticsolutlons. . . t.1 Designof the circuitdiagram t2 Circuitlayout 4 3 Designation elements of individual pneumatic system The life of a cycle '.4
47 48 49 50 51
..... 55 Chapters Developmentofslngleactuatorclrcuits ........ 56 apneumaticcylinder 5 . 1 Di r ectcon tr olof . ...... 57 of asingleactingcylinder 1 : Directcontrol 3.2 Example . . . . . . . 58 of a doubleactingcylinder 5 3 Exercise1 : Directcontrol ...... 60 apneumaticcylinder 5 . 4 In d ir e ctcon trolof ...... 60 single acting cylinder Indirectcontrol of a 2 : 5.5 Example . . . .. . 62 of adoubleactingcylinder 2 : lndirectcontrol 5.5 Exercise ...64 5 . 7 Lo g icfun cti ons:A ND,OR.. p re s s u re v a lv.e. . . . . . 6 4 function;the t wo 3 : ThelogicA ND 5 . 8 E xam p le v a lv e . . . . . . . 6 6 function; thetwop re s s u re 3 : ThelogicA ND 5 . 9 Exe r cise ....... 68 v a lv e shut t le 4 :The logicOR function;the 5 . 1 0 Exa m p le . . . . . . . 70 the shuttlevalve. 4 : The logicOR function; 5.11 Exercise andspeedcontrolof acylinder.. . . .... 72 5.12 Example 5 : Memorycircuit 5.13 Exercise 5 : Memorycircuitandspeedcontrolof a cylinder . . . ' ' . ' 74 ........76 5 . 1 4 Exer cise 6 :T hequickexhaustvalve plastic .. ... 78 of embossing T : Pressure dependentcontrol; 5.15 Example plastic. . . . . . 80 of embossing control; 7 Pressure dependent 5.16 Exercise .....82 Thetimedelayvalve 5.17 Example 8 .....84 Thetimedelayvalve 5.18 Exercise I
Contents
. . . . . . . . 87 Chapter6 Developmentof multipleactuatorcircuits ........88 6.1 Controlofm u lt ip le a c t u a t o rs . . . . 89 6.2 Example 9: Co-ordinated motion . . . 92 6.3 Example10:Signaloverlap . .lv . . .e s . . . . . . 94 6.4 S ignalelimin a t io n b y re v e rs in g v. a 6.5 E xample 11:S ig n aol v e rla pre ; v e rs invga lv e s o lu t io.n. . . . . . . . . . . 94 valves . . . . . . 96 6.6 Example12:Transfer station;usingreversing ChapterTTrouble-shooting of pneum?ticsystems 7.1 Document a t io . .n 7.2 Thecausesandeffectsof malfunctions .. 7.3 Maintenan c e . . . .
.... 99 . . . 1 0 '1 . . 101 ...104
SectionB:
Theory
pneumatics C h a p ter l Fu n d amentalsof 1 . 1 Ph ysica lp ropertiesof air. 1 . 2 Ch a r a cteristicsofair... 1 . 3 C on tr oltheory
....;.
....... 107 ,...108 .....110 .......111
C h ap ter 2 Airge nerationanddistribution 2 . 1 Air co m p r essor 2 . 2 Air r e ce iver 2 . 3 Air d r ye r s j..... 2 . 4 Air se r viceequipment 2 . 5 Air d istr i bution...
.....117 ......120 ...121 .....123 .......126 ....134
C h ap ter 3 Dir e ctional controlvalves 3 . 1 Co n fi gu r ationandconstruction 3 . 2 2 l? wa yva lve, 3 . 3 3 l2 wa yva lve. 3 . 4 4 l2 wa yva lve. 3 . 5 4lSw ayvalve. 3 . 6 Sl? wa yva lve, 3.7 Reliable operation of valves
....137 ......138 .......138 .......139 ....... 148 .......150 .......151 . . 153
C h a p ter 4Va lves 4 . 1 No n - r e turnvalves 4.2 Flowcontrolvalves 4 . 3 Pr e ssu r e valves 4 . 4 Co m b in a tionalvalves
...155 ....156 . . . 162 ......165 .......167
C h ap ter 5Actuatorsandoutputdevices 5 . 1 Sin g le a ctingcylinder 5 . 2 D ou b le a ctingcylinder 5 . 3 R od le sscylinder .. t ic s 5 . 4 C yl i nd e r performancecharacteris ...:.. 5 . 5 M otor s 5 . 6 R otar ya ctuators 5 . 7 l nd ica tor s
.......171 .......172 .......175 .....183 .....184 .......185 .....186 .....187
.... . . C h apter 6 systems 6 . 1 Se le cti on andcomparison of mediums 6 . 2 C on tr olsystemdevelopment..... 6 . 3 Fie ld systems(actoric) 6 . 4 Sp e ciaun l itsandassemblies
.......189 ...... 190 ...190 ......194 ....,.. 195
SectionC:
Solutions
So luti on s L i s to f stan d a r dsand references.. Index
....199 ......217 .......221
Notesregarding the conceptbehindthe textbook Thistextbookformspartof F,estoDidactic'sLearningSystemfor Automation. lt has beendesignedfor trainihgcoursesand is also suitablefor the purposeof self-tuition. Thebookis dividedintothefollowing sections: PartA: Course PartB: Theory PartC: Solutionsto the exercises PartA: Course The aim of the courseis to providestudentswith the information whichthey will requirein orderto gaina thoroughunderstanding of the subjectconcerned. This is achievedusingbothexamplesand exercises.Thesegraduallyincrease in complexityand the studentther:efore is advisedto work throughthem in sequence.Subjectswhichare dealtwith in greaterdepthin the Theorysection are markedin the text. PartB: Theory This sectioncontainsthe basictheoryrelatingto the subjectarea in question. Topicsare set out in a logicalmanner.The studentcan eitherwork through this sectionchapterby chapteror use it for referencepurposes. PartC: Solutions Thispartcontainsthe solutionsto the exercisesin PartA. A comprehensive indexis providedat the end of the textbook. Thistextbookmaybe incorporated intoan existingtrainingprogramme.
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SectionA Course
Chapter1 Characteristics and applicationsof pneumatics
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Pneumaticshas for some considerable time been used for carryingout the simplestmechanical tasks, but in morerecenttimeshas playeda moreimportant rolein the development for automation. of pneumatic technology ln the majorityof applications compressedair is usedfor one or more of the following functions: . The useof sensorsto determinestatusof processes . Informationprocessing . Switchingof actuatorsby meansof finalcontrolelements . Carryingout work Beforethe 1950s,pneumatics was mostcommonlyusedas a workingmedium in the form of storedenergy. Duringthe 1950sthe sensingand processing roles developedin parallelwith working requirements. This development enabledworkingoperationsto be controlledusing sensorsfor the measurementof machinestatesand conditions.The development of sensors,processorsand actuatorshas led to the introduction systems. of pneumatic In parallelwith the introduction of total systems,the individualelementshave furtherdevelopedwith changesin material,manufacturing and designprocesses.
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The pneumaticcylinderhas a significantrole as a lineardrive unit,due to its relativelylow cost, ease of installation, simple and robustconstructionand readyavailability in varioussizesand strokelengths. The pneumaticcylinderhasthe followinggeneralcharacteristics: . Diameters . Strokelengths . Availableforces . Pistonspeed
6 to 320 mm 1 to 2000mm 2 to 50000N 0.02to 1 m/s
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Pneumatic components canperformthefoilowing typesof motion: . Linear . Swivel . Rotary \ D
Someindustrial applications employing pneumatics are listedberow:
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. Generalmethodsof material handling: . Clamping . Shifting . Positioning . Orienting
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. Generalapplications: . Packaging . Feeding . Metering . Dooror chutecontrol . Transferof materials . Turningand invertingof parts . Sortingof parts . Stacking of components . Stamping andembossing of components
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Pneumatics is usedin carryingoutmachining andworkingoperations. Forexample: . Drilling . Turning . Milling . Sawing . Finishing . Forming . Qualitycontrol
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t Advantages and distinguishing characteristics of compressed air;
1.1. PneumatlcsIn revlew
Availability:
Air is availablepractically everywhere in unlimitedquantities.
Transport;
Ajr can be easilytransported in pipelines,evenoverlarge distances.
Storage:
A compressor neednot be in continuous operation.Compressedair can be storedin a reservoirand removedas required.ln addition,the reservoircan be transportable.
Temperature :
Compressed air is relativelyinsensitive to temperature fluctuations.This ensuresreliableoperation, evenunder extremeconditions.
Explosionproof: Compressed air offersminimalriskof explosionor fire, henceno expensiveprotection againstexprosion is required. Cleanliness :
Unlubricated exhaustair is clean.Any unlubricated air which escapesthroughleakingpipesor components doesnot causecontamination. This is an importantpointwhen considering the food,woodandtextileindustries.
Components:
The operatingcomponents are of simpleconstruction and arethereforerelativelyinexpensive.
Speed:
Compressed air is a veryfastworkingmedium.Thisenables highworkingspeedsto be attained.
Adiustable:
With compressedair components,speedsand forcesare infinitelyvariable.
Orcrload safe :
Pneumatic toolsand operatingcomponents can be loaded to the pointof stoppingand are thereforeoverloadsafe.
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it ls aisc In orderto accuratelydefinethe areasof applicationof pneumatics, withthe negativecharacteristics: necessary to be acquainted Preparation : Compressedair requiresgood preparation.Dirt and condens@ shouldnot be present. Compressible: It is not alwayspossibleto achieveuniformand constantpiston speedswithcompressed air. Forcerequirement: Compressedair is economicalonly up to a certainforce requirement. Underthe normalworkingpressureof 6-7 bar (600to 700 kPa) and dependenton the travel and speed,the outputlimit is between20 000 and 30 000 Newtons. Noiselevel: The exhaustair is loud. This problemhas now, however,been largelysolveddue to the development material of soundabsorption and silencers. Costs: Compressedair is a relativelyexpensivemeans of conveying power. The high energycostsare partiallycompensated by inexpensivecomponents and higherperformance. A comparisonwith otherformsof energyis an essentialpart of the selection processwhen consideringpneumaticsas a controlor workingmedium.This evaluationembracesthe total systemfrom the inputsignal(sensors)through All factorsmust the controlpart (processor) to the outputdevices(actuators). be considered suchas: . Workor outputrequirements . Preferred controlmethods . Resources and expertiseavailableto supportthe project . Systemscurrentlyinstalledwhichare to be integrated withthe newproject
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Choiceof workingmedia:
Criteriafor a workingmedium
. Electrics ' Hydraulics . Pneumatics . A combination of the above Sebctioncriteriafor the workingsection: . . . . . . . . .
For@ Sfoke Type of motion(linear,rotary) Speed Size Servicelile Sensitivrty Sdety and reliability Energycosts - conrollability . l-landling . $rage Cfdce of controlmedia: . . . . . '
llechanical Electical Elecfonic Pneumatic, normalpressure Pneumatic,low pressure l-lydraulic
S*ctbn criteriafor the controlsection: . . . . . . . . .
Reliabilityof components influences Sensitivityto environmental Faeeof maintenanceand repair $ribhing time of components Signal speed Spacerequirements Senricelife Trainingrequirements of operatorsand maintainers f/bdificationof the controlsystem
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1.2. Pneumatics and control systemdevelopment
in a numberof in pneumatics The productdevelopment can be considered areas: . Actuators . Sensorsandinputdevices . Processors . Controlsystems . Accessories in the development of pneumatic Eachof theseproductgroupsare important reliability butwith: Thedemandsarefor system/component solutions. . . . . . . . . . .
or Accessibility for repairand/ormaintenance, Lowcostof replacement Easeof mounting andconnection requirements Lowplannedmaintenance Interchangeability andflexibility Compact design withthe above Costscommensurate Readilyavailable Documentationsupport to supportthe product Minimum trainingrequired
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Chapter2
Components of a pneumatic system
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A pneumaticsystemcan be brokendowninto a numberof levelsrepnesentirq hardwareand signalflow. The variouslevelsforma control path for signal flow from the signal (input)side to the work (output) side.'
Signalflow ACTUATING DEVICES Outputs Fi nol control el ement
PROCESSING ELEMENTS Processorsignols
INPUTELEMENTS Input signols
2 .1. Pneumaticsystem structureand signal flow
systemare: Theprimarylevelsin a pneumatic . Energysupply . Inputelements (sensors) . Processingelements(processors) . Actuatingdevices(actuators) The elementsin the systemare representedby symbolswhich indicatethe functionof the element.The symbolscan be combinedto representa solution for a particularcontroltask usingthe circuitdiagram.The circuitis drawnwith the samestructureas the signalflow diagramabove.At the actuatorlevelthe additionof the controlelementcompletesthe structure.The controlelement controlsthe actionof the actuatorafterreceivingsignalssentby the processor elements.
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Pneumatic cylinders Rotaryactuators Indicators
) FINALCONTROL ELEMENT Control signals
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ELEMENTS CONTROL Directional controlvalves
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PROCESSORS Directional controlvalves Logicelements Pressurecontrolvalves
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SENSORS Directional controlvalves Limitswitches Pushbuttons Proximity sensors
t ENERGY SUPPLY Compressor Receiver Pressureregulator Air serviceequipment
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rhe directional controlvalve(DCV)may havea sensing,a processing or an actuating controlfunction.lf the DCVis usedto controla cylindermotion,then : is a controlelementfor the actuatorgroup.lf it is used in the functionof rocessingsignals,then it is definedas a processor element.lf it is usedto sensemotions,then it is definedas a sensor.The distinguishing featurebe:,veeneachof theserolesis normallythe methodof operating the valveand *'nerethe valveis situatedin the circuitdiagram. Circuitdiagramand pneumatic elements
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rr 2.2. Air generationand distribution
The air supplyfor a particularpnelmaticapplication shouldbe sutficientand of adequatequality. Air is compressed to approximately 1/7thof its volumeby the air compressor and deliveredto an air distribution systemin the factory.To ensurethe quality of the air is acceptable,air serviceequipmentis utilisedto preparethe air beforebeingappliedto the controlsystem. reducedin the systemif the compressed Malfunctions can be considerably air is correctlyprepared.A numberof aspectsmustbe consideredin the preparationof the serviceair: . . . . . . . . . . . .
Quantityof air requiredto meetthe demandsof the system Typeof compressor to be usedto producethe quantityrequired Storagerequired for air cleanliness Requirements humiditylevelsto reducecorrosionand stickyoperation Acceptable if necegsary Lubrication requirements, Lowtemperature of the air and effectson the system Pressure requirements Linesizesandvalvesizesto meetdemand Materialselectionand systemrequirements for the environment Drainagepointsand exhaustoutletsin the distribution system Layoutof the distribution systemto meetdemand.
As a rule pneumaticcomponents are designedfor a maximumoperatingpresto operate sureof 8 - 10 bar (800-1000kPa)but in practiceit is recommended at between5 and 6 bar (500-600kPa)for economicuse. Due to the pressure lossesin the distributionsystemthe compressorshoulddeliverbetween6.5 and 7 bar (650-700kPa)to attainthesefigures.
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An air receiver should be fitted to reduce pressure fluctuations.In normal operationthe compressorfills the receiverwhen requiredand the receiveris availableas a reserveat all times. This reducesthe switchingcycles of the comDressor.
Air supplysystem
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Reservoir within a pneumatic system
Compressor
Serviceunit -;;il;; Intermediate reservoir for severalconsuming devices Condensate collector tank
lf oil is requiredfor the pneumaticsystemthen this shouldbe separately meteredusing air serviceequipment. In a normalsituation,components shouldbe selected for the controlsystemthatdo notrequirelubrication.
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Air serviceunit
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Dueto the highdemandat certainstagesof the air distribution system,a ring mainwith cross-feed connections is recommended. In this way the fluctuations are reduced.The ring main shouldbe laid out with a1-zo/ogradientto allow drainagepointsfor condensatefrom the compressor.lf there is a relatively highcondensate level,then air dryingequipmentshouldbe fittedspecifically to dry the air to the requiredquality.condensateis a commoncauseof failurein pneumatic controls. The air serviceunitis a combination of the following: . Compressed air filter . Compressed air regulator . Compressed air lubricator. The correctcombination, size and type of these elementsare determinedby the application and the controlsystemdemand.An air serviceunit is fittedat each controlsystemin the networkto ensurethe qualityof air for each in, dividualtask.
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Compressed air filter
The compressedair filter has the job of removingall contaminants from the compressedair flowingthroughit as well as water which has alregdycondensed. The compressed air entersthe filterbowlthroughguideslcits.Liquid particlesand largerparticlesof dirt are separatedcentrifugally collectingin the lowerpart of the filter bowl. The collectedcondensatemust be drainedbefore the levelexceedsthe maximumcondensatemark,as it will otherwisebe reentrainedin the air stream.
Compressed air regulator
The purposeof the regulatoris to keep the operatingpressure(secondary pressure)virtuallyconstantregardlessof fluctuationsin the line pressure (primarypressure)andthe air.consumption.
Compressed air lubricator
The purposeof the lubricatoris to delivera meteredquantityof oil mist into a leg of the air distributionsystemwhen necessaryfor the use by pneumatic controlandworkingcomponents.
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to theirfunctionin 2.3 Valves Valvescan be dividedinto a numberof groupsaccording relationto signaltype, actuationmethodand construction.The primaryfunction of the valveis to alter,generateor cancelsignalsfor the purposeof sensAdditionally the valveis usedas a powervalve ing,processingand controlling. for the supplyof workingair to the actuator.Thereforethe followingcategories are relevant:
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Directionalcontrolvalves - Signalling elements elements - Processing - Powerelements . Non-return valves . Flowcontrolvalves . Pressurecontrolvalves . Combinationalvalves
The directionalcontrolvalvecontrolsthe passageof air signalsby generating, Directional controlvalves or redirecting signals. cancelling
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of the valve is of In the field of controltechnology,the size and construction less importancethan the signalgenerationand the actuationmethod.Directionalcontrolvalvescan be of the poppetor slidetype,with the poppetutilised for smallflow ratesand for the generationof inputand processsignals.The slidevalveis ableto carrylargerflow ratesand hencelendsitselfto the power and actuatorcontrolrole. The way valveis describedby : . . . . .
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(ways): Numberof portsor openings Numberof positions: Methodsof actuationof the valve: Methodsof returnactuation: Specialfeaturesof operation:
2 wa y , 3 wa y ,4 wa y , 5wa y ,e t c . 2 positions, 3 positions, etc. Manual,air pilot,solenoid, etc. Springreturn,air return,etc. Manualoverrides, etc.
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As a signalelementthe directionalcontrolvalve is operatedby for example,a roller lever to detect the piston rod positionof a cylinder.The signal element can be small in size and createa small air pulse.A signalpulsecreatedwill be at full operatingpressurebut have a smallflow rate.
312wayrollerlevervalve(without andwithidlereturn)
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As a powerelementthe directional controlvalve must deliverthe required quantityof air to matchthe actuatorrequirements and hencethereis a need icr largervolumeflow ratesand thereforelargersizes.This may resultin a argersupplyportor manifold beingusedto deliverthe airto the actuator. 5 2 wayvalvefor cylindercontrol:doublepilotvalve
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-^e non-return valveallowsa signalto flowthroughthe devicein onedirection Non-return valves ard in the otherdirectionblocksthe flow.Thereare manyvariations in cons:':ctionand sizederivedfromthe basicnon-return valve.other derivedvalvalveby the incorporationzof non-return "es utilisefeaturesof the non-return s ements.The non-return valvecan be foundas an elementof the one way 'clv controlvalve,quick exhaustvalve,shuttlevalve and the two-pressure r A
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Flowcontrolvalves
The flow controlvalverestrictsor throttlesthe air in a particular directionto reducethe flow rate of the air and hencecontrolthe signalflow. lf the flow controlvalveis leftwideopenthenthe flow shouldbe almostthe sameas if the restrictoris not fitted.In some casesit is possibleto infinitelyvary the restrictor fromfullyopento completely closed.lf the flowcontrolvalveis fitted with a non-return valvethen the functionof flow-control is unidirectional with full free flow in one direction.A two way restrictorrestrictsthe air in both valve.The flow control directions of flow and is not fittedwith the non-return valveshouldbe fittedas closeto the workingelementas is possibleand must be adjusted to matchthe requirements of the application. Flowcontrolvalve
Pressure controlvalves
Pressurecontrolvalvesare utilisedin pneumaticsystems.Thereare three maingroups: . . .
Pressure valves regulating Pressure limitingvalves Pressure valves sequence
pressurein a controlcirThe pressureregulating valvecontrolsthe operating cuit and keepsthe pressureconstantirrespective of any pressurefluctuations in the system. The pressurelimitingvalvesare utilisedon the up-stream sideof the compressor to ensurethe receiverpressureis limited,for safety,and that the supply pressureto the systemis set to the correctpressure.
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The sequencevalvesensesthe pressureof any externalline and compares the pressureof the line againsta presetadjustable value,creatinga signal whenthe presetlimitis reached. valve sequence r Pressure
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-.e combined functions of variouselementscan producea newfunction. The Combinational valves ^ew component can be constructed by the combination of individual elernents :r manufactured in a combined configuration to reducesizeandcomplexity. An exampleis the timerwhichis the combination of a one way flowcontrolvalve, a reservoir anda3l2 way directional controlvalve. Timedelayvalve
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valvesincludethe oneway flowcontroland the two hand Othercombinational startvalve.
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2.4 Processors:valvesand logicelements
To supportthe directional controlvalvesat the processing level,there are variouselementswhichcondition the controlsignalsfor a task.The elements are: . Twopressure valve(ANDfunction) . Shuttlevalve(ORfunction) Theseelementsderivetheirconstruction fromthe non-return valveand have logicbasedroles.A specialfeatureis thattheyare fittedat a junctionof three lines,therefore theyhavethreeconnections, two in andoneout. Twopressure valve(ANDfunction)
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J The furtherdevelopment of processing elementsin pneumatics has brought aboutthe modularsystems,whichincorporate directional controlvalvefunctionsand logicelements to performa combinedprocessing task.This reduces size,costandcomplexity of the system. processing Modular unit
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-.e actuatorgroupincludesvarioustypes of linearand rotaryactuatorsof 2.5 Actuators: working . a'vingsize and construction. The actuatorsare complemented by the final elementsand directional ::^trol element,whichtransfersthe requiredquantityof air to drivethe accontrolvalves :-a:cr.Normally thisvalvewillbe directlyconnected to the mainair supplyand lossesdueto resistance. ':ed closeto the actuator to minimise
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- ^earactuators - Singleactingcylinder - Dcubleactingcylinder . ictary actuators - \laneWpe - A r motors r:--r:ators,linearand rotary
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2.6 Systems: controlcircuits
Control circuitforthesinole actingcylinder
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Thecontrolof thecylinder is an important conslderation in the development of control solutions. Thepneumatic energyis to be transferred to thecylinder via a finalcontrolelement or directional controlvalve.Thedirection of motionof thecylinder is controlled by thevalvepushbutton. A circuitis developeo.
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The Problem Thepistonrod of a singleactingcylinderis to advance whenair is applied. when the air is removed, the pistonis to automaticallv returnto the initial position.
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Solution A valveis to createa signalwhena pushbutton is pressed andto cancelthis signalwhenthepushbutton is released. The3i2waydirectional control valveis a signalgenerating valve.lt is suitablefor the controlof a singleacting cylinder. Thecircuittherefore includes primaryfeatures: thefollowing . Singleactingcylinder withonesupplyconnection andoneexhaust or vent portandthespringfor returnforce . 312way directional controlvalve:3 portsand 2 positions, pushbutton for operation andspringfor returnforce . Supplyairsourceconnected to the312wayvalve . Airconnection between valveandcylinder Controlof a singleactingcylinder
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The 3/2 way controlvalvehas3 ports.The supplyport,the exhaustportand the outletport.Therelationship betweentheseportsis determined by the passagesthroughthe valve.The numberof variations possible is determined by thenumber of positions drawn.In thiscasetwopositions arepossible.
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circuit)is definedas the 'rest' Initialposition:The initialposition(left-hand position made andthereis no manualinterAll are of the system. connections is off and the cylinderpistonrod The air supply shut ventionby the operator. port (by is to the exhaustport The connected return). cylinder retracted spring by the via the body of the valve.The supplypressureis shut off internally valve. Pushbuttonoperation: Pressingthe pushbuttonmovesthe 3/2 way valve circuit)showsthe againstthe valve returnspring.The diagram(right-hand valvein the actuatedor workingposition.The air supplyis now connectedvia the valvepassageto the singleactingcylinderport.The build-upof pressure causesthe pistonrod of the cylinderto extendagainstthe forceof the cylinder 'eturnspring.As soonas the pistonrod arrivesat the forwardend position,the in the cylinderbodyreachesa maximumlevel. airpressure hrshbutton release:As soon as the pushbuttonis released,the valvereturn soringreturnsthe valveto its initialpositionand the cylinderpistonrod retracts. f the pushbuttbnis operatedand then releasedbeforethe cylinderfully exretracts.Thereforethere is a direct :ends,the cylinderpistonrod immediately operationand cylinderpistonrod posibetweenpushbutton oontrolrelationship tron.lt is possible thatthe cylindermaynotfullyextendin suchcases. Note:The speedof extensionand the speedof retractionare different.As the rod movesforwardit is undercontrolof the air supply,Duringretraction Di:ston by the sizeof the cylinderreturnspring.Therefie speedis mainlycontrolled {oreforwardmotionis fasterthanreturnmotion.
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Control circuitforthedouble actingcylinder
The Problem is The pistonrod of a doubleactingcylinderis to advancewhena pushbutton is released. operatedand to returnto the initialpositionwhenthe pushbutton of motion,due Thedoubleactingcylindercancarryout workin bothdirections for extension beingavailable and retraction. to thefullair supplypressure
-a \
\-a I
Solution is A valveis to createone signaland cancelanothersignalwhena pushbutton pressedand to changeoverthe signalswhenthe pushbutton The is released. valvewith two signal 412way directional controlvalveis a signalgenerating The circuit outputports.lt is suitablefor the controlof a doubleactingcylinder. includes features: therefore thefollowing
-
. Doubleactingcylinderwithtwo supplyconnections . 4/2 way directional pushbutton for conlrolvalve:4 portsand two positions, operation andspringfor returnforce . Supplyair sourceconnectedlo lhe 412way valve . Twoairconnections betweenvalveandcylinder
=
Controlof a doubleactingcylinder !r E :-
!r .iE
t
t
I
I -l :{
3r -
_J
30
I
I
lnitial position: In the initialposition(left-handcircuit)all the connections are madeand thereis no manualintervention by the operator.In this unactuated position,air is appliedto the cylinderpistonrod side,whilethe pressureon the plbtonsideof the cylinderis exhausted. Rrshbutton operation: Pressingthe pushbuttonoperatesthe 4/2 way valve againstthe valve returnspring. The diagram(right-handcircuit)showsthe pressure valvein the operatedor actuatedposition.In this position,the_q!,rppty s connectedto the pistonside of the cylinder,while the pistonrod side is the pistonrod. Once Eihausle-d.Thd pressureon the pistonside advHnces full extensionis reached,the air pressureon the pistonside reachesa maximum. Pushbuttonrelease:Oncethe pushbuttonis released,the valvereturnspring rushes the valve into the initialposition.]-he- supplypressureis now connectedto the pistonrod side,while the pistonside is exhaustedvia the exruust port of the valve. The pistonrod retracts. llf the pushbuttonis releasedbeforethe cylinderis fully extended,then the cylinderpistonrod immediately returns.Therefore, thereis a directrelationship Hrreen pushbutton operationand cylinderpistonrod position.
31
Chapterg Symbolsand standards in pneumatics
33
.I I
3.1 Symbolsand descriptionsof components
The development of pneumatic systemsis assistedby a uniformapproachto the representation of the elementsand the circuits.The symbolsusedfor the individual mustdisplaythe following characteristics: elements . Function . Actuationand returnactuationmethods . Numberof connections (alllabelled for identification) . Numberof switching positions . Generaloperating principle . Simplified representation of theflowpath A symboldoesnot represent the followingcharacteristics: . . . . .
Sizeor dimensions of the component Particular methodsof construction manufacturer, or costs Orientation of the ports Anyphysical detailsof the element Anyunionsor connections otherthanjunctions
The symbolsusedin pneumatics are detailedin the standardDIN ISO 1219, "Circuitsymbolsfor fluidicequipment A summarylistingof the and systems". relevantsymbolsare shownand a more completelistingis detailedin the TP102book. The relevantstandardsfor the construction, testingand designof pneumatic controlsystemsare listedin the referencesectionof this book.
34
The symbolsfor the energysupplysystemcan be representedas individual Air supplyand generation elementsor as combinedelements.The choicebetweenusing simplifiedor detailedsymbolsis dependentupon the purposeof the circuitand its complexity.In generalwhere specifictechnicaldetailsare to be given such as for non-lubricated requirements air or micro-filtering, thenthe completedetailed symbolshouldbe used. lf a standardand commonair supplyis used for all thenthe simplifiedsymbolscan be used.Fortrouble-shooting components, the detailedsymbolsare moresuitable.But the detailshouldnot add to the complexityof the circuitfor reading.
andpreparation Symbolsusedin energyconversion Supply Compressor
,Al
With fixed copocity
(E
Y
n
Air receiver ond 'T' junction
I
Service equipment Filter
Seporotion ond filtrotion of porticles
Woter seporotor
Portiol woter removol
Lubricotor
Metered quontities of oil possed to the oir streom
Pressure regulotor
Relievingtype vent hole for excess upstreom pressure - odjustoble
$
!3
+ + -^
Y
il
!
Combined symbols Air service unit
Filter, Regulotor, Gouge, Lubricotor I
b
I
Simplifiedoir service unit
Pressure source
o35
\,/alvesymboldescription
In generalthe symbolsare similarfor pneumatics but each and hydraulics controlmediumhasspecificcharacteristics thatare unique. Directional controlvalves:symboldevelopment
Vnlvc switr-hinn nositions ore representeo os squores
The number of squores shows how mony switching nnsitinns ihc vnlve hos
Lines indicote flow poths, orrows show the direction of flow
Sh rrt o ff n o s i ti ons
ore
identified in t h e b o x e s b y lines drown of riq h t o n g le s
The connections (inlet ond
-. - s n o wn Dy lines on the o u t s id e o f t h e oox ond ore drown in t h e in it io l nnqi l i nn nr r ilel
JO
nnr tq)
nr c
The directional controlvalveis represented by the numberof controlled con- Directional controlvalves: nectionsand the numberof positions. Eachpositionis shownas a separate way valves square.The designation of the portsis important wheninterpreting the circuit symbolsand the valve as fittedto the physicalsystem.To ensurethat the :crrectlines,connections and valvesare physically in place,theremustbe a reiationship betweenthecircuitandthecomponents used. -hereforeall symbolson the circuitmustbe designated and the components withthe correctsvmbolanddesionations. -sed shouldbe labelled Directional (ways) controlvalves:portsandpositions
Number of p orts Number of p ositions
Woy directionol control volve
Woy directionol control volve Normolly closed
-7 /a
Woy directionol control volve Normolly open
L/2
'/
g '| g -
-
z- -
Woy directionol control volve
Woy directionol control volve
Woy directionol control volve Mid nosition r:losed
I
controlvalvesand is in directional A numbering systemis usedto designate systemwas utilised with ISO5599(Draft).Priorto this a lettering accordance are presentedhere: and bothsystemsof designation Port or Connection
tso 5599
LetteringSystem
port Pressure Exhaustport Exhaustports Signaloutputs Pilotlineopensflow1 to 2 Pilotlineopensflow1 to 2 Pilotlineopensflow1 to 4 Pilotlineflowclosed pilotair Auxiliary
1 3 5,3 2,4 12 12 14 10 81,91
P R (3/2way valve) R,S(5/2wayvalve) B,A Z (singlepilot3/2way) Y (512way valve) Z (512way valve) Z,Y Pz
Examples of designations
,I 'I
38
I ra
ra
e
a
rl
The methodsof actuation of pneumatic directional controlvalvesis dependent Methodsof actuation vary,i.e.mechanical, of thetask.Thetypesof actuation uponthe requirements pneumatic, The symbolsfor the methodsof and combinedactuation. electrical in lso 1219. actuation aredetailed
t
t
, t
J
mustbe givento the controlvalve,consideration Whenappliedto a directional methodof initialactuation of the valveand alsothe methodof returnactuation. Theyare bothshownon the symbol Normally thesearetwo separatemethods. eitherside of the positionboxes.Theremay also be additionalmethodsof indicated. whichareseparately actuation suchas manualoverrides, Methodsof actuation
t
Mechonicol t
Generol monuol operotion t
(t' rr-l_
Pushbutton \
of-
Lever operoted
+
Detent lever operoted
Ff r-f.-
Foot pedol
:[-
\ t
Ff L
-l-r__r-
+--hnnit
Spring return
lr
Rolle r
:
oner ot ed
-rJ-|
ta)
a
ldle return, roller Pneumotic Direct pneumotic octuotion Indirect pneumotic octuotion (piloted)
-
I
r-l V/Vl- -j\i\ /\
Spring centred :
'
: Pressure releose ra
r
ffi_ --*fL -r-> --J-I --=f L
E lec t r ic ol S inc le s o l e n o i d 'Y'-"
n n p r n t ia n
Double solenoid
n n o r n iin n
VI ----t_
,fla
Combined Double solenoid n n d n iln t operotion with monuol override
-JtH\ I i/tt\m
?_
--l--- -t a-
___J4
39 t
t
Non-return valvesandderivatives
The non-return valve(checkvalve)is the basisfor the development of many combinedcomponents. Thereare two mainconfigurations for non-return valves,withandwithoutthe springreturn. Non-return valvesandderivatives
-+-
Check volve Spring looded check volve Shuttle volve: ,^-, UK
T UN C t I O N
Tw o p re s s u re v o lv e 'AND' f u n c t io n
Quic k e x h o u s t v o lv e
Flowcontrol
Mostflow controlvalvesare adjustable.The one way flow controlvalvepermitsflowadjustment'in onedirection onlywiththe non-rqurnfitted.The airow showsthat the componentis adjustablebut does not reierto the directionof flow;it is diagrammatic only. Flowcontrolvalves
Flow control volve odjustoble One-woy flow con trol volve
I 'h,
40
---T--_t
Pressureregulating valvesare generallyadjustable againstspringcompres- Pressure regulating valves types: sion.Thesymbolsaredistinguished according to thefollowing . Pressure sensing: downstream, upstream or external . Relieving pressure or non-relieving andfluctuating . Adjustable or fixedsettings Thesymbolsrepresent valveas a singleposition valvewitha flow the pressure paththatis eitheropenor closedinitially. In the caseof the pressureregulator the flow is alwaysopen,whereasthe pressuresequencevalveis closeduntil the pressure reachesthe limitvalueas set on the adjustable spring" Pressure valves
Ad ju stoblepressure reguloting volve , n on-relieving type ArJ ir retnhlo
nrpeqr
tr e
r ca t r ln lin n
volve, relieving type (overloods ore vented) Qa n rra n n a
r r nlr r o
externot source
Qcn rro n n a
r r nlr r a
r-line
Qonrranna
fiw f^[/,7T/T-
#lu/^, -T-
il,f' l-
rrnlrra
: onrb ino tion
I t
I
41
Linearactuators
w w
and The linearactuatorsor cylindersare describedby theirtypeof construction methodof operation. The singleactingcylinderand the doubleactingcylinderform the basisfor The use of cushioning designvariations. to reduceloadson the end capsand mountingsduringdeceleration of the piston is importantfor long-lifeand smoothoperation.The cushioningcan be either fixed or adjustable. The cushioningpistonis shownon the exhaustair side of the piston.The arrow indicatesadjustable and notthe directionof cushionedmotion. cushioning Linearactuators
S ingle octin g c y lin d e r
Double oct in g c y lin d e r Double octin g c y lin d e r with double e n d e d p is t o n ro d
r-TF -FIrJ-
Double octin g c y lin d e r with non-od ju s t o b le c u s h io n in g in one dire c t io n
mr---------l-
Double octin g c y lin d e r with single o d ju s t o b le c u s h io n in g
li/+
Double octin g c y lin d e r with odjust o b lec u s h io n in g of both end s
42
[Frrr,rr---lr
tu- ;li--;r-
l|{l]F-----7--------T
r-
+ \
Rotaryactuatorsare dividedinto continuousmotionand limitedangleof rota- Rotaryactuators tion.The air motoris normallya highspeeddevicewith eitherfixedor adjustablespeedcontrol. Unitswith limitedangleof rotationare fixedor adjustablein angulardisplacement.. The rotaryactuatormay be cushioneddependingupon the load and speedof operation
t
w W
Rotarymotion , h\ t
Ai r m otor, rototion in one dire c t io n fi xed copocity
\ni
, \ )' I
b.-
Air m otor, rototion in one dir e c t io n vor io blecopocity Ai r m otor, rototion in both di re c t io n s vo r io b le copocity R otor y octuotor l i m i te d trovel r ototi on in both directions
/9\ (E
Y
,u
,1F rd,
W --t\_
-+)-
Thereare a numberof important symbolsfor accessories whichare utilisedin conjunction with pneumatics. Theseincludethe exhaustair symbols,visual indicators andthe methodsof connection of components.
F:
Auxiliarysymbols Exho ust port
-F:
Exho ust port with threoded connection hr
Si l encer
i-
Lin e connection (fixed) I L-
C r ossinglines ( no t connected)
I
I
hh
P ro q q t
r
tr a
n^t
t^^
i
:r Vi suo l indicotor
V
+V -{D _l_
+ (a Y
-a
h-
-F:
!-t\
aa
43
3.2. Safetyrequirementsfor pneumatlcsystems
Thereare a limitednumberof standardsand regulations for pneumatic systems. Thereforefor all factorsaffectingsafety,ieferencemust be made to existinggeneralguidelinesand regulations from a numberof differenten_ gineering fields.
Safetyrequirements for pneumatic clamping devices
consideration shouldbe given to the followingguidelineswhen operating pneumatic cylinders underclamping conditions. The controlsystemfor the pneumatic clampingdevicesshouldbe designedor arrangedin sucha way as to avoidaccidentaloperation.Thiscan be achieved by meansof: ' .
Manually-operated switchingdeviceswithprotectivecovers,or Controlinterlocks
. .
Pressure transducers, or Pressuresequencevalves
A failureof the air supplt'-must not causethe clampingdeviceto openduring the machiningof a clampedworkpiece. Thiscan be ach-ieved by meansof: . Pressurereservoirs . Controlinterlocks(memoryvalves) Environmental pollution
Twoformsof environmental pollution mayoccurin pneumatic systems: . '
44
Noise:causedby the escapeof compressed air oil mist:causedby lubricantswhichhavebeen introducedat the compressor or via a serviceunit and which are dischargedinto the atmosphere duringthe exhaustcycles
l )
Measuresmust be taken against excessiveexhaust noise. This can be achieved by meansof:
t
.
)
I I t t
Exhaustnoise
Exhaust silencers
Silencersare used to reducethe noiseat the exhaustportsof valves.They operateon the principleof exhaustair flow controlby creatinga greateror lesserflow resistance. Normalsilencershave only limitedinfluenceon the speedof the pistonrod. In the case of throttlesilencers,however,the flow resistanceis adjustable. Thesesilencersare usedto controlthe speedof the cylinderpistonrod andthe valveresponsetimes. Anothermethodof noisereductionis to fit manifoldswith connections to the exhaustportsof the powervalvesand thus to dischargethe air via a large sornmonsilenceror to returnit to a reservoir. -he exhaustair of mechanically driven tools or mechanically controlled Oilmist atomised which remains in for contains oil often a room some con-achines sderabletimein the formof a vapour,whichcan be breathedin. pCliutionof the environment is particularly acutein caseswherea largenumaer of air motorsor large-diameter cylindersare fittedin an installation. trSectivecountermeasures mustbe takento reducethe amountof oil mistthat :vf€s to the atmosphere. rVrenmaintaining or workingwith pneumaticsystems,care mustbe takenin Operational safety :e removaland the reconnection of air lines.The energystoredin the tube or soe will be expelledin a very shorttime with enoughforceto causesevere rnudashof the line whichcan endangerpersonnel. Wherepossiblethe air srould be isolatedat two differentpoints,removingthe air pressurebeforethe dsconnectionis made.An additionaldangeris in the disturbance of particles are:o the air blastwhichcauseseye hazards. 3l- rost controlsystemssafetydevicesand guardswill be fittedfor the protecrr cr personnel. Theseshouldneverbe manipulated as the safetyof person. ret Tay be at risk.
45
t-At l4i
Chapter4 Systematic approach to pneumatic solutions
47
gt-uarErE
qFwl
no rlt]llEEsllElli
rtcslrE
The solutionto a controlproblemis workedout accordrng to a S;sre- r'r'1playingan important documentation rolein communicating the f n3 rg5r,1-rg circuitdiagramshouldbe drawnusingstandardsymbolsand labellinEC:-prehensive is requiredincluding mostof the followrng: documentation . Displacement-step or motiondiagram . Flowchartof the process . Circuitdiagram . Partslistof all components in the system . Description of the operationof the system . Maintenance information andfault-finding . Sparepartslist . Technical dataon the components 4.'l
Designof the circuit diagram
The layoutof the circuitdiagramshouldcorrespond to the controlchainflowchart,i.e.thereshouldbe a signalflowfromthe bottomof the circuitto thetop. The energysupplyis important and mustbe includedin the circuit.The elefor the energysupplyshouldbe drawnat the bottom.Simplified mentsrequired or full component symbolscan be utilised.In largercircuits,the energysupply connections section(serviceunit,shut-offvalve,variousdistribution etc.)can be drawnseparately.
J
J
Controlchainflowchart
A CT UA T I NG DE V I CE S
J
O ut p ut s F I NA LCO NT RO L E L E ME NT S Co n t ro l s ig n o ls = J LrJ
PROCESSINGELEMENTS
z-
Processor signols
a o
zI NP UTE L E ME NT S E. ul
l nnr r i
c i nnnl c
J J J J J J J
z
L!
E NE RG YS UP P L Y S o urc e
48
J
J J J J J
) )a
t t
I
This layoutmeansthat the circuitdiagrammust be drawnwithoutconsidering the actualphysicallocationsof each of the elements.lt is recommended that all cylindersand directionalcontrol valves be drawn horizontallywith the cylindersoperatingfrom left to right,so that the circuitcan be more easily understood. The Problem 4.2 Circultlayout pneumaticcylindertravelsout if eithera The piston rod of a double-acting manualpushbuttonor a foot pedalis operated.The cylinderreturnsto its startThe pistonrod will returnprovidedthe manual ing positionafterfullyextending. actuatorshavebeenreleased. Circuitdiagram
1(P)
2(A)
1.4
1.3
5 3
Solution The valve 1.3 is mountedat the full extensionpositionof the cylinder.The circuildiagramshowsthis elementsituatedat the signalinput leveland does not direcllyreflectthe orientationof the valve.The mark on the circuitat the extendedcylinderpositionindicatesthe physicalpositionof the valve 1.3 for orruil operation.
5 b
t' the controlis complexand containsseveralworkingelements,the control shouldbe brokendown into separatecontrolchains. A chaincan be formed 'or eachfunctionalgroup. Whereverpossible,these chainsshouldbe drawn next to each other in the sameorderas the operatingsequence.This is not alwayspossible.
49
<
4.3 Designation of individual All elementsshould be Actuatedinitialposition elements shown in the circuit diagram in the initial position:here the valve is initiallyactuated.This must be indicated (for example,by an arrow,or in the case of a limit switch by drawing a cam). The valve is shown in the oDerated state with the left hand positionin line with the supplyport. The signal at 2(A) is initiallyactive due to operationof the rollerelement. The numbering systemof the individual elementsin the circuitdiagrambelow relatesto theworkinggroupnumberandthefollowing criteria: 0. 1, 2, 3 etc. 1.0,2.0etc . .1 .01,.02etc. .2, .4 etc. .3, .5 etc.
Energysupplyunit Numbering of individualworkinggroupsor controlchains Workingelement Controlelement Elements between the controlelementand workingelement Elements whichinfluence the advancestrokeof the cylinder Elements whichinfluence the returnstrokeof the cylinder
Circuitdiagram
50
. Signalflowfrombottomof circuitto the top. Summary . Energysourcecanbe shownin simplified form. . Physicalarrangement of the elementsis ignored. . Drawthe cylindersand directional controlvalveshorizontally whereverpossible,cylindersextendingfromleftto right. . Designateall elementsin the completedinstallation the same as in the circuitdiagram. . ldentifypositionof the input signalsby a mark (limitvalve).lf signalsare issuedin one directiononly, showan arrowon the mark. . Showelementsin the initialpositionof the control.ldentifyactuatedelementsby a cam or arrow. . Drawpipelinesstraightwithoutcross-overwhereverpossible.Junctionsare indicatedby a dot. Pneumatic systemdevelopment
C O N T R OLP R OB L EM
(,' A N A L YS IS ll e fi n e th e o b i e c ti v e of th e p n e u mo ti c s y s te m
The development of solutionsfor pneumatic control systems is dependentuponmethodical planning. The various phases involvedin the life cycleof such systemsare shown here.
(, D ES IG N De v e l o p o s o l u ti o n fo r t he p n e u mo ti c s y s te m
Jt \/ s.
3
IMP L EME N T A T IO N Cons tru c t, i n s to l l , te s t o n d c om m is s i o n th e p n e u m o ti c s y s te m J tE VA L U A T IO N Chec k th e p e rfo rmo n c e o f th e pneu m o ti c s y s te m to m e e t t h e o n o l y s l s o b j e c ti v e \7
M AIN T EN AN C E S er v ic e on d m o i n to i n th e p n e u mo ti c s y s t em t o me e t th e p e rfo rm o n c e o f t he o n o l y s i s ,re q u i re m e n ts It U P G R AD IN G M odif y o r u p g ro d e th e s y s te m t o me e t n e w re q u i re m e n ts
51
The life cycle of a pneumaticsystem
Analysisof a pneumatic controlproblem
The firststepis the definitionof objectivesfor the projectwith a cleardefinition Designor development of the problem(s). of the solutionis not involvedat the analysisstage.A flowchartof the totalprojectplancan be developedto define processes. the step-orientated
Design
Therearetwo stagesof designdevelopment. The first is the overallsystemdesignwhere generalsystemshardwareand controlmediumdecisionsare made. At this stagealternative solutionsmay be for consideration. addressed Thenextstageof thedesignprocessinvolves thefollowing: . Development of hardwaresystems . Documentation development documentation - preliminary . Definition requirements further of . Timeschedules for projectimplementation . Productlistsand specifications . Costingdata
lmplementation
Evaluation
The projectis implementedusing the designspecifications.The hardware componentsare orderedand then constructed to form a system. A delivery datecan be estimated for the systemcompletion and a schedulefor commissioningdrawnup. Priorto the systeminstallation, the system'sfunctionsmust be fullytested. This is important to ensurethat on-sitework is not delayed. processinvolvesthe mountingof controls,actuators,sensors The installation prior and connection of serviceunits.The installation mustbe fullycompleted any installation is to attemptto operatethe controlsystem. Once completed, the commissioning stage is reached. When the functionaltest of all componentsis completed,the systemas a whole can be functionallychecked. Finallyto ensurethe sequenceoperatesunderall conditions, the machine must be cycledunderall of the expectedand specifiedoperatingconditions, e.g.productfailure,emergency conditions, manualcycle,autocycle,blockages etc.The machineis not considered commissioned untila quantityof products havebeenpassedthroughthe machineunderproduction conditions. qnd processthe finalresultis evaluated Uponcompletion of the commissioning The benefitsof improvedproduction and comparedto the originalspecification. reducedcostswillbe apparent if the machineor systemis properlymaintained.
J J J J
J J
J J J -J J -J
-J -J
-r -J __
=J
52
-.t -
Maintenance is essentialtominimise the systemdowntimes.
Maintenance
Regularand carefulmaintenance helpsto increasethe reliabilityof a system andto reducethe operatingcosts. After a certainnumberof cycles,some componentsmay show signsof early whichmightbe due to incorrectproductselectionor a changein deterioration Basicpreventivemaintenance operatingconditions. carriedout at regularintervals helpsto diagnosefailuresof this kindandthusavoidssystemdowntimes. Afterthe systemhas beenin servicefor sometime,the reliability of the system can be improvedthroughcomponentreplacement or upgrade.
s
13 qI
53
ChapterS
55
Thereare two primarymethodsfor constructing circuitdiagrams: . The so-calledintuitivemethods,alsofrequentlytermedconventional or trialand-errormethods . The methodicaldesignof a circuitdiagramin accordancewith prescribed rulesand instructions Whereasmuchexperience and intuitionis requiredin the firstoaseand above all, a greatdeal of time wherecomplicated circuitsare concerned;designing circuitdiagramsof the secondcategoryrequiresmethodicalworkingand a certainamountof basictheoretical knowledge. Regardless of whichmethodis usedin developing the circuitdiagram,the aim is to end up with a properlyfunctioningand reliable operatingcontrol. Whereaspreviouslyemphasiswas placedon the least expensivehardware solution,moreimportance is now attachedto operational reliabilityand easeof maintenance by a clearlayoutand documentation. This inevitablyleadsto increasedusageof methodicaldesignprocesses.In such cases,the controlis alwaysconstructedin accordancewith the given procedureand is less dependentupon personalinfluencesfrom the designer. In manycases,however,morecomponents will be requiredfor the methodical solutionthanin a circuitdevisedby the intuitivemethod. This additionalmaterialrequirement will usuallybe rapidlycompensated for by time-savingat the project stage and also later in terms of maintenance. Generally,it must be ensuredthat the time spent in projectdesignand particularlyin simplifyingthe circuit,is in reasonableproportionto the overalleffort. Regardless of whichmethodand whichtechniqueis usedto producea circuit diagram,the basic requirements are sound fundamentalknowledgeof the devicesconcernedand knowledgeof the switchingcharacteristics of the componentsused. 5 . 1 . Directcontrol of a pneumaticcylinder
The simplestlevel of confrolfor the singleor doubleactingcylinderinvolves directcontrolsignals.Directcontrolis used.wherethe flow rate requiredto operatethe cylinderis relativelysmall,and the size of the controlvalveis also smallwith low actuatingforces.lf the valve is too large,the operatingforces requiredmaybe too greatfor directmanualoperation.
56
-l
The Problem A singleactingcylinderof 25mm diameteris to clamp a componentwhen a pushbutton is pressed.As longas the pushbutton is activated, the cylinderis to remainin the clampedposition.lf the pushbuttonis released,the clampis to retract.
5.2 Dlrect control of a slngle acting cylinder
Clamping device
Solution The controlvalve used for the singleactingcylinderis the 3/2 way valve.ln this case,sincethe cylinderis of smallcapacity,the air consumption is low and the operationcan be directlycontrolledby a pushbutton3/2 way directional controlvalvewith springreturn. Circuitdiagram
h
On operatingthe pushbutton the air passes through the valve from 1(P)to the 2(A)portand extends the piston rod againstthe force of the cylinderreturnspring.On releaseof the button,the valve spring returnsthe 3/2 way valveto its initial positionand the cylinder retracts.The air returns fromthe cylindervia the exhaust3(R)port. Sincethe cylinderis the only workingelementor actuatorin the circuit,it is designated1.0. The final controlelementthat extends the cylinder is designated 1.1.
57
w P
5.3 ExercisE1: Directcontrol of a doubleacting cylinder
The Problem A,double.acting cyrinderis to extendwhen a pushbutton is operated.Upon releaseof the pushbutton the cylinderis to retract.The cylinderis of smallbore (25mmdiameter)requiringa smailfrowrateto operateaithe correctspeed.
Exercise Drawthe circuitdiagramfor theproblem. Designatethe valves and indicatethe numbering system for the connections (ports). Describethe operationof thecircuit.
P W W Notes:
58
Question
Circuitdiagram
What happens to the cylinder,if the pushbutton is pressed for a very short period,and is then immediatelyreleased?
HJ@-F
Notes:
59
5.4 Indirectcontrol of a pneumaticcylinder
Example2: Indirectcontrol of a singleactingcylinder
For controlling cylindersat high speedor of largediameter,the air flow requireddetermines whethera largersize controlvalveshouldbe used.The operatingforce to actuatethe valve may be relativelylargeand in this case indirectcontrolis preferable. A similarsituationexistswhena cylinderoperates at highspeed,and requiresa largevalvethat cannotbe directlyoperated.The controlelementwill havea largeorificesize and flow rateand be operatedby pilotair to assistopeningagainstthe switchingforce.This is indirectcontrol. The supplyline can also be short,sincethe controlvalvecan be mounted Theotheradvantage closeto the cylinder. is thatthe signalelement(i.e.pushbutton3/2 valve)can be smallin size,sinceit onlyprovidesa signalto operate the controlvalveand is not required Thissignal to operatethe cylinderdirectly. elementwillbe of smallersizeandhavea shorterswitching time. The Problem A largediametersingleactingcylinderis to extenduponoperation of a pushbuttonvalve. The pushbutton valveis situatedat a remoteposition.Thereforeindirectcontrol shouldbe used to operatethe cylinder.The cylinderis to retractonce the remotepushbutton is released. Clampingdevice
J J I r-
J
t
J J
_l
J -l J J -J J
60
J J J
t
I \ a
it
Solution In the initialposition,the singleactingcylinderis retractedand the control position, due to the springreturn.The pushbutvalve1.1 is in the unactuated ton valveis in the springreturnpositionwith the connectionat 2(A) exhausted Therefore the onlyactivelinesare the 1(P)linesof the two 3/2 to atmosphere. way valves.
\t
Circuitdiagram
\ t hr\
L\ t
I
hr.r
lt I
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The 312way pushbutton valve (1.2) opens the 1(P) air supply to the 2(A) port and generates a signalat the 12(Z)pilot port of the controlvalve. The controlvalve 1.1 is actuated against spring forceand the 1(P)lineis openedto the 2(A) port, causingthe singleacting cylinderto extend.The signalat the 12(Z) line remainsas long as the pushbutton is helddown, therefore the and cylinder remains extendeduntilthe pushbutton is released.This is an indirect pushbutton controlof the cylinder.
is released, lf the pushbutton the springreturnclosesthe 1(P)portof the 3/2 This removesthe actuatway valveand exhauststhe 2(A) lineto atmosphere, ing pilotsignalat the controlvalve.The controlvalveis returnedto the initial positionby the returnspringand the singleactingcylinderlineis exhausted to The springin the cylinderretractsthe cylinderto the initialposiatmosphere. tion. The controlvalve can be fitted closeto the cylinderand be large in size to The pushbutton can be smallin sizeand fitted controlthe largeborecylinder. remotely.
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5.6 Exercise2: lndirectcontrolof a doubleacting cylinder
The Problem A doubleactingcylinderis to extendwhen a pushbutton is operated.Upon releaseof the pushbutton the cylinderis to retract. Thecylinderis 250mmdiameter andconsumes a largevolumeof air.
Exercise
Positional sketch
Drawthe circuitdiagramtor theproblem.
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Designate thevalvesand indicate the numberingsystemfor the connections.
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What happens to the cylinder,if the pushbutton is pressed for a very short periodand is then immediatelyreleased?
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5.7 Logicfunctions:
AND,OR
5.8 Example3:
The logic ANDfunction; the two pressurevalve
The pneumatic shuttlevalveand the two pressurevalvehavelogicfunctions. The shuttlevalvehas the characteristic of an OR function,wherebyat least eitherof two inputsX or Y are requiredto generatean outputat portA of the valve.In the caseof the two pressurevalve,the characteristic is that of the ANDfunction, wherebybothinputsX andY are required to initiatean outputA. Thetwo pressure valveandthe shuttlevalvenormally controlthe inputsignals' passageto ensurespecialconditions are met in a circuit. For example,interlocks,safetymeasures andoperating whichare requiredpriorto the conditions actuation of a cylinder.The logicelementshaveprocessor rolesin a circuit, wherebysignalsareprocessed to meetthe specialconditions. The Problem The pistonrod of a doubleactingcylinderis to extendwhen two 312way pushbutton valvesare actuated. lf eitherof the pushbuttons then are released, the cylinderis to returnto the initialposition. Circuitdiagram
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The controlvalvecan be a 412way or a 512way valveand can be of a size whichsuits the flow rate requiredfor the cylinderspeed.lf eitherof the two via the pushbutton signalsproduced valvesis removed, thenthe two pressure valvewill relievethe 14(Z)signalbackthroughthe exhaustport of the nonoperated3/2 way valve.The springin the controlvalveswitchesthe 5/2 way valveto the initialposition.The outlet2(B) is activewith the outlet4(A) exhaustedto atmosphere andthe cylinderretracts. I
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An alternativesolutionto usingthe two pressurevalve is to use two 3/2 way valvesin series.Herethe signalis passedfromvalve1.2to valve1.4andthen on to the 14(Z)portof the 5l2way controlvalvebut only if bothpushbuttons lf eitherpushbuttons are released, thenthe signalat 14(Z)of the are operated. pushbutton valve. controlvalveis exhaustedat the non-operated Circuitdiagram
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5.9 Exercise3: The logicANDfunction; the two pressurevalve
The Problem
Exercise Drawthe circuitdiagramfor theproblem. Designate thevalvesand indicatethe numberingsystem for the connections (ports).
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5.10Example 4: Thelogic OR function: the shutile valve
The Problem cyrinder is to extendif one or bothof two pushbuttons l9oybrg Sgting are operated' lf bothpushbuttons arethenreleased, thecylinder is to retract.
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Solution The shuttrevarveis connectedto the junction betweenthe two 3/2 way push_ buttonvarves.Uponoperation of on"ditn" pushbuttons, a signaris generated at the X or y side of the shuttrevatve. inis signatpassesthioughthe shuttre valve and is emittedat port A. This *u"r.", the controrvarvevia pirotport 14(z), andthe cyrinderextends.Tne conirotvarvecan oe a-itz wayor a 5/2 way valve and can be sized to suit the.frow ,,"1.i"qrirJo-for the cyrinder speed'lf bothof the signalsproducedui" tn" pushbutt-o-|-u"i",are removed, then the shurilevarvewirrreieasetni tilzl pir"i .igrJ o"it"tnrough rhe ex_ haustport of one ot the *uy u"iu"r. ihe returnspringin the controrvarve .3rz yuv-y?r,ve to the init[r ptsition.The ouilet2(B) is .rrit:T:,tlg. ?/2 activewith tne outlet4(A)exhausted to atmosphere,'and the cyrinde'ar"E.. Conclusion lf the cylinderis to retracton.reaching its fuily extendedposition,roilerrimit valvesshourdbe usedto confirmtn"t il-.,i.positionhas beenreached.In addi_ tion,a memoryvarveshourdue titteJiol'i'n" .ontrorof the cyrinder.
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The rollerlimit switchgeneratesa signalwhichreversesthe controlvalve 1.1 when the cylinderis fully extended.The rollerlimitswitch1.3 is a 312way valvewith springreturn.This is definedas a stroke-dependent controlwhere the positionof the cylinderis confirmedby limitswitch.In this casethe pushbuttoncan be operatedfor a shortdurationand the cylinderwill fully extend. The 5/2 way memoryvalveretainsthe 14(Z)switchedpositionuntilthe reversing signalis receivedfrom rollerlimitswitch1.3. A furtherrefinement to the op€rationof this cycleis to ensurethat the cylinderis fully retractedbeforethe pushbutton Thisrequiresan additional limitswitch1.6. canextendthecylinder.
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The additionof the two pressurevalve 1.10 and the rollerlimit switch1.6 beforethe cylindercan be extended. ensuresthatthe cylinderis fullyretracted requiredto initiateextensionare eitherof the pushbuttons in The conditions additionto the cylinderin the retractedposition(1.6). Whenthe cylinderis are still extendedat position1.3,the cylinderretractseven if the pushbuttons ooerated sincethe limit1.6is inactive.
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5.11Exercise4: The logic OR function; the shutflevalve
The Problem a magazine. lf eithera pushbutton or a r is to extend.Oncethe cylinderis fully sition.A 312wayrollerlevervalveis to re cylinder.
Exercise Drawthe circuitdiagramfor tneproblem. Designate thevalvesand in_ dicate the numberingsys_ tem for the connections (ports).
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5.12 Erample5: The Problem Memorycircuit and speed The pistonrod of a doubleactingcylinderis to extendwhena 3/2 way pushcontrol of a cylinder buttonvalveis actuated. The cylinderis to remainextendeduntila secondpushbuttonis actuatedand onlyif the firstpushbutton has beenreleased. The cylinderis to thenreturnto the initialposition. The cylinderis to remainin the initialpositionuntil.a newstartsignalis given.The sfeed of the cylinderis to be adjustablein bothdirections.
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Solution 4/.2way or 512way doublepilotvalvespossessthe requiredmemoryfunction. The valveretainsits lastswitchedpositionuntilan opposingsignalis received. r9r !nr"sreasonsignatscreatedby the pushbuttonsignallin!ddvicescan be of shortduration. The flow controlvalvescontrolthe cylinderspeedin both directionsand are independently adjustable.
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Uponoperation of pushbutton 1.2,a signalis generated at the 2(A) port and ihe pilotpott14(z)of valve1.1.The 5/2way memoryvalveswitchesand the siEnalfromport4(A)fullyextendsthe cylinder1.0.lf the pushbutton valve1.2 s released, the signalal14(z) is exhausted at the 3(R)portof the pushbutton ';alve'1.2.The valve1.1 remainsin the switchedpositionuntilthe pushbutton ,'aive1.3 is operated.lf the pushbutton valve 1.2 is releasedand therefore iere is no signalat 14(Z)then the signalgeneratedby 1.3 will returnthe "nemoryvalve to its initialpositionand the cylinderretracts.The cylinder 'ernainsretracted untila newsignalis generated al14(Z)by the valve1.2.The piston rod will extendand retractif thereare no obstructions, but there rylinder s no confirmation that the cylinderis in its fullyextendedposition.lf boththe '4rZ) signaland the 12(Y)signalare activedue to both pushbuttons being rcerated,thenthe memoryvalvewillremainin the lastpositionattained. -'re flow controlvalveshave been fittedto throttlethe exhaustingair in both :'ections of pistonmotion.The supplyair is transferred throughthe by-pass :ieck valve of the llow controlvalves,giving unrestricted supply to the rl'linder.
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5.13 Exercise5: Memorycircuitand speedcontrol of a cylinder
The Problem
Exercise Drawthe circuitdiagramfor theproblem. Designate thevalvesand indicate the numberingsystem for the connections (ports).
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1. What is the switching statusof the memoryvalve when first fitted to the system and therefore what positionwill the cylinderbe in?
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3. lf the pushbutton is held operatedeven after full extensionis reached,whateffect will this have on the cylinderretraction?
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4. Whatis the effecton the operation of fittingthe roller levervalveat the mid-stroke positionof the cylinder,i.e. not at the full extension position?
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5.14Exercise6: Thequlckexhaust valve
The Problem The operationof two identicalpushbuttonvalvesadvancesa formingtool on an edge foldingdevice.For rapid forwardtravel,the circuitutilisesa quick exhaustvalve. The forwardmovementfoldsthe edgeof a flat sheet.lf eitherof the two pushbuttonsare released,the doubleactingcylinderis to returnslowlyto the initial position.
Exercise Positional sketch Drawthe circuitdiagramfor theproblem. Designate the valvesand indicate the numberingsystem for the connections (ports).
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5.15Example7: Pressuredependent control;embossingof plasticcomponents
The Problem A plasticcomponentis embossedusing a die drivenby a doubleacting is cylinder.The die is to advanceand embossthe plasticwhena pushbutton operated.The returnof the die is to be effectedwhen a presetpressureis pressure reached. Theembossing is to be adjustable. Embossing of plasticcomponents
Solution Ihe 5/2 way memoryvalveas fittedmay not be in the correctposition.lf the cylinderextendswhenthe air supplyis turnedon, it is incorrect to reversethe connections on the 512way valve.Instead, the circuitshouldbe placedin the correctinitialpositionby operating the manualoverrides on the 5l2wayvalve. The 5/2 way directional controlvalve is switchedat the 14(Z)port by the valve 1.2 and the cylinderextends.The plastic operationof the pushbutton component is embossedby the die underpressure, untilthe presetpressure set on the sequencevalveis reached.The pressureon the inletside of the cylinderis connectedto the sequencevalve pilot line 12(Z) and this acts againstthe presetcompression of the adjustable spring.lf the presetvalueis reached,thenthe sequencevalveopensfrom 1(P)to 2(A) and sendsa pilot signalto port 12(Y)of the controlvalve.lf thereis no signalall4(Z) thenthe memoryvalveswitchesand air is suppliedfrom the 2(B) port to retractthe cylinder.At the sametime the air in the 4(A) port is exhausted and the pilot valveis relieved. signalat the sequence Therefore valvecancels the sequence the signalat 2(A)andthe pilotsignaltothe controlvalveat 12(Y).Thecylinder The pilotsignalsat 14(Z)and 12(Y)needonlyto retracts to the initialposition. be veryshortpulsesto effectthe positionof the 5/2wayvalve.
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lf the pressuresensing. line from the junctionof the pressuregaugeto the sequencevalve is too lo.ng,it is possiblethat the sequencevilve-may not switchcorrectly. lf the cylinderencounters any resistance to motionourini tne forwardtravel,the sequencevalvemay be prematurely triggered.Thereforeit is advisable to includea rollerlimitswitchai ttreextension-iosition to confirm the full travel.The rollervalveshouldbe placedin series'withthe pressure sensingsignalto preventearlytriggering of ihe sequence vatve.The nextexer_ ciseincorporates thisadditional condition.
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5.16Exercise7: Pressuredependent control;embossing of plasticcomponents
Exercise
The Problem A plasticcomponentis embossedusing a die poweredby a doubleacting cylinder.The die is to advanceand embossthe plasticwhena pushbutton is The returnof the die is to be effected operated. whenthe cylinderrod hasfully extendedto the embossingpositionand the presetpressureis reached.A rollerlimitvalveis to be usedto confirmfull extension. The cylinderis thento gauge. retract.Thepressure in the pistonchamberis indicated on a pressurb Positional sketch
Drawthe circuitdiagramfor theoroblem. Designate the valvesand indicate the numberingsystem for the connections (ports).
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Circuitdiagram
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5.17 Example8: The time delay valve
The Problem A doubleactingcylinderis usedto presstogethergluedcomponents. Upon operation of a pushbutton, theclamping cylinderextendsandtripsa rollerlever valve. positionis reached, Oncethe fullyextended the cylinderis to remainfor a time retractto the initialposition. of T= 6 secondsandthenimmediately A newstart cycleis onlypossibleafterthe cylinderhasfullyretracted. The cylinderextensionis to be slowandthe retraction adjustable but relativelyfast. process Cementing
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Solution lnitiallythe cylindershouldbe at the restpositionbut this is dependent on the positionof the 5/2 way valve 1.1. This memoryvalve must be positioned manually to ensurethatthe cylinderwillbe retracted initially.
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5.18Exercise8: Thetime delayvalve
Exercise
The Problem A doubleactingcylinderis usedto presstogethergluedcomponents. Upon operation of a pushbutton, the clamping cylinderextendsandtripsa rollerlever valve. positionis reached, Oncethe fullyextended the cylinderis to remainfor a time of T= 6 secondsand then immediatelyretractto the initialposition. A new start cycle is only possibleafter the cylinderhas fully retractedand after a delayof 5 seconds.Duringthis delaythe finishedpart is manuallyremoved and replacedwith new partsfor gluing.The cylinderextensionis to be slow andthe retraction adjustable, but relativelyfast. Positional sketch
Drawthe circuitdiagramfor theproblem. Designate thevalvesandindicate the numberingsystem for the connections (ports).
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of multiple Development actuatorcircuits
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In the case of multiplecylindercircuits,a clear definitionof the problemis important.The representationof the desired motion of all actuatorsis describedusingthe displacement-step diagram.The specialconditionsfor the startof the sequencemustalsobe defined. lf the-motion diagramand auxiliaryconditionshavebeenclearlydefined,drawing.of the circuitdiagramcan commence.Usingthe standardrepresentation and drawinglayout,the circuitis now developedin accordancewith the requirements. The designprocessand the basicdevelopment of the circuitdiagramdepend uponthe type of signalprocessingselected.where simplertype! of controls are concerned,the less favouredmethodof signalcut-outusing idle return rollersis possible. In most cases,signalcut-outby meansof reversingvalvesshouldbe incorporated. Anothermethodical approachtowardsproducinga circuitdiagramis alsocalled the "Cascade Method". This methodfor constructing a circuitdiagramis certainlythe easiestto learn for controlswheresignalcut-outis effectedby meansof reversingvalves. The samebasicthinkingis alsobehindthe designof a steppercircuit. Anotherpointto be observedis the inclusionof auxiliaryconditionsin a control. It is expedientto considerand includetheseconditions only afterthe basic circuitfunctionshavebeencompleted.Theseauxiliaryconditionsshouldthen be incorporated in a step-wisemanner,i.e.,the circuiidiagramshouldbe expandedstepby step. This is a way to ensurethat the circuitretainsits overall clarity,evenwhereelaboratecontrolsare concerned. Examples followwhichshouldensurean understanding of the methodsused.
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The Problem Two cylinders are usedto transferpartsfroma magazine ontoa chute.when a pushbuttonis pressed,cylinder1.0 extends,pushingthe part from the magazineand positionsit in preparation for transferby cylinder2.0 ontothe outfeedchute.Oncethe partis transferred, the firstcylinderretracts, followed by the second. The speedof bothcylinders are to be adjustable. Confirmation of all extended positions andretracted are required. Transferof parts
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Example9: Co-ordinated motion
Circuitdiagram
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Solution taskis dependent on rolleroperatedlimitvalves. The circuitfor thissequential for the controlare that cylinder1.0 is retractedand the The startconditions start button is operated.The processcan be brokendown into steps and of diagram.Whenlookingat the placement shownon the displacement-step These the rollervalves,the currentactionandthenreactionmustbe analysed. The sequenceof the actuatorscan be will determinethe valvepositioning. by: described . . . .
Valves1.2and1.4 Yalve2.2 Valve1.3 Valve2.3
1.0extend + Cylinder 2.0extend = Cylinder =+Cylinder1.0retract 2.0 retract + Cylinder
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When cylinder1.0 extendsa limit switch is operated. This must have the effectof advancingcylinder2.0, thereforeit is designatedas valve2.2 (even number2 for forwardmotionon cylinder2) and is connectedto the 14(Z)port of the 512way valve 2.1. Next,cylinder2.0 extends,and the reactionis the operationof the forwardlimit valve which causescylinder1.0 to retractand hencethe valveis designated1.3 (oddnumber3 for returnmotionon cylinder 1) and connectsto the 12(Y)port of valve1.1.The next reactionis that the limit switch at the retractedpositionof cylinder1.0 operatesand this must returncylinder2.0. Thereforethis valveis designated2.3 and is connectedto the 12(Y)portof the 512way valve2.1. The cylinder2.0 retractsand operates the rear limit switchon cylinder2.0. The end of the cycle is confirmed.To obtaina new start,valve 1.1 must receivea signalat the 14(Z)port from a logicalcombination of the startbuttonand the limitvalvedesignated1.4.
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6.3 Example10: Signaloverlap
The Problem In co-ordinated motioncontrol,the 5/2way memoryvalvecan onlyswitchpositionswhena singlepilotsignalis present.lf two signalsare operatedsimultaneously, i.e.bothpilotsignalson the 512wayvalveare active,a signaloverlapproblemoccurs. Thereare variousmethodsof solvingthe problem,but firstlythe overlapmust be identified. Displacement-step diagram
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The firstcontrolvalve1.1 has an overlapproblemin the firststep,sincethe valve1.2 and 1.3 can be activeat the sametime.Therefore the firstof these signalsmustbe cut shortso that it is not activepastthe usagein step4. The valve 1.3 shouldbe an idle rollerlevervalvethat only operateswhen the retraction of the cylinderoccurs.Whenthe startof the cycleis initiated, the valve1.3is deactivated sincethe idlerolleris overrun. The secondoverlapproblemis with valve2.1, wherethe signals2.2 and2.3 coincide.The two signalsare activein step 3, whenthe cylinder2.0 is fully Therefore extended. the firstof the two signalsis requiredto be cut shortto allowthe secondto operatealone.The valve2.2 mustbe an idle rollervalve whichis only activein step 2 lor a shortdurationwhen the cylinder1.0 is movingforwardto the extendedposition.Thereforethe circuitutilisingidle rollervalves1.3and2.2 is correct,if thismethodof signalcut-outis utilised. Circuitdiagram
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6.4 Signaleliminationby reversingvalves
Signalelimination by meansof a reversing valveis a frequently usedsolution. Usingthis method,savingscan be madein cuttingotf severalsignalsfrom individualreversingvalves.The methodis relativelyreliablein operation.This basicidea is to allowthe signalto be activeonly for as long as is requiredto switchthe memoryvalves.This is achievedby blockingthe signaldownstream of the signalelementby meansof a reversing valve,i.e.by supplying energy to the signalelementonly when the signalis required.An impulsevalveis usedto effectthe reversal.The maindifficultylies in the selectionof the correctsignalsfor the reversingvalve.
6.5 Example11: The Problem Signaloverlap;reversing lf idle returnrollersare not to be usedto providethe signalcut-out,then an valvesolution additionalreversingvalve is introduced.The circuitin section6.3 showsthe idlereturnvalvesto eliminate solutionutilising the overlapof signals.The problemareasfor the two memoryvalves1.1and 2.1 arethe signalsgenerated by the valves1.3 and 2.2, whichin this casehavebeeneliminated in one direction of motion.Anotherway to shortenthe timethe signals,is to removethe air supplyto thesetwo valvesexceptat the step required.The signalgenerated by 1.3 is only requiredduringthe start of the sequenceand the signal generatedby the valve2.2 is only requiredduringsteps2 and 3 and can be very shortin durationif thereare no opposingsignalsat the memoryvalve1.1 and2.1. Solution Referringto the circuitopposite,the reversingvalve0.3 activatesline 51 and 52 consecutively and createsa situationwherethe signalscan neveroverlap at the memoryvalves1.1and2.1. Initially Rollervalve1.3is the valve1.4is activesincecylind",t.O is retracted. operatedand initiallyfed with supplyfrom the line (S2),which is still active from the end of the cycle.Thereforebeforethe manualvalve 1.2 is pressed, the cylinder1.0 is heldretracted due to the signalfromline 52 on the 12(Y) portof thevalve2.1. Afteroperationof the valve1.2,the reversingvalve0.3 is switchedat the 14(Z) port:the line 52 is exhausted of air via port3(S)and port4(A)feedsline 31 withsignalair.The line51 is directlyconnected to the valve1.1port14(Z)and thb cylinder1.0 extendstowardsthe limitswitch2.2. Yalve2.2 is fed from line 51 and appliesa signalto the pilotport 2(141ol valve2.1 whichthen extends cylinder2.0. When cylinder2.0 extendsto limit switch2.3, a signal is generatedat the 12(Y)port of the reversingvalve 0.3. The outputsignalswitchesthe supply linesfromline 51 to line 52. Therefore, sinceline 51 is no longeractive,the limitswitch2.2 andthe cylinderadvancing signalsof the memoryvalves1.1 and 2.1 havebeeneliminated.The energising of line52 causesthe immediate reversalof valve2.1 and the retraction of cylinder2.0 to the limitvalve1.3. The limitvalve 1.3 is activeand reversesthe valve 1.1 whichretractscylinder 1.0to the initialposition. Thisis confirmed of limitswitch1.4, by the operation but this valveis not yet active,sincethereis no air supplyuntilthe startbutton 1.2 is operated.Finally,the circuitis set in the initialstatuswith line 52 active andwaitingfor a newstartsignal.
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Example12: Transferstation; using reversing valves
The Problem Usinga transferstation,partsare to be transferred froma verticalmagazine to a processing station.The partsare pushedby cylinder1.0andthentrinsferred ontoa chuteby cylinder2.0 for transferto the-processing station.The piston rodof cylinder2'0 is onlyto retractafterconfirmation that"cyfinOer 1.0hasfully retracted. The cycleis to startwhena startbuttonis pressed.Limitswitches are usedto confirmcylinderpositions.
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Developmentof multipleactuatorcircuits
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The sequencehas three steps and overlapis potentiallya problemat two positions.The firststepis the extensionof cylinder1.0,followedby step2, the immediateretraction of cylinder1.0.Thereforeit is possiblethat overlapoccurs on the valve1.1at ports14(Z)and12(Y),unlessthe reversing valvetechnique is used.The valve1.4 is heldoperatedinitiallyby cylinder2.0, but the start buttonis a momentarypushbutton,and in most cases,it would not be expectedthat the start buttonbe operatedfor an extendedperiod.Thereforeit may be satisfactory that the valve1.2 be usedas the meansof cancellingthe firstoverlapsignal. The secondoverlapproblemoccurswith cylinder2.0 and valve2.1, Herethe problemoccursin step 3, whenthe cylinderis to retractimmediately afterthe extensionis completed.The first of the two signalson the controlvalve 2.1 mustbe shortin duration. To removethe overlapthe reversingvalvecircuitis developedwith threelines feedingthreesteps.The lines51 to 53 representthe individualsteps1 to 3. In step1 thecylinder1.0is extended, therefore the pilotsignal1.1at port1a@)is fed from step 1 to line 51. Two movementsoccur in step 2: cylinder1.0 retractsandcylinder2.0 extends. Therefore line2 feedsthe pilotsignals1.1at port12(Y)and2.1at port14(Z).The thirdstepis cylinder2.0 to retractdue to the pilotsignal2.1 at 12(Y)whichis fed from line 53. The inputside of the circuitshowsthatthe valves1.2and 1.4 haveto be operatedto startthe cycle. The valve 1.3 next operatesand is fed to port 12(Y)of reversingvalve 0.3. This switchesto activateline 52 and exhaustsline 51. The next movements are cylinders 1.0backand 2.0 out,confirmed by valves2.3 and 2.5 whichare seriesconnectedto the 12(Y)port of the reversingvalve 0.4. The line 52 is exhaustedand line 53 activated.The cycleis noWpreparedfor a startcondition,oncevalve1.4is initiated by cylinder2.0.
98
!
Chapter7 Trouble-shooting of pneumaticsysterns
99
Faultdiagnosis
Fromthe time that energyis suppliedto the controlsystem,faultdiagnosis time and becomesan importantpart of the processto reducecommissioning downtimein the operationof a system. prior to Faultdiagnosisinvolvesa numberof distinctstagesof identification solvingthe problem.Faultsgenerallyoccureither: . .
Dueto externalfailureof the machinecomponents or due to stoppages failureswithinthe controlsystem. Internal
Experiencehas shownthat the occurrenceof controlsystemfailureis rare comparedwithexternalsensoror machinefailure. Trouble-shooting : whatcan we expectto achieve?
In the case of systemfailure,actionmust be taken by the end-userof the system.The actionto be takenis dependenton the complexityof the required repairwork. lf the problemis relativelysimple,the work will be carriedout immediately. Wheremorecomplexrepairsare required, will servicepersonnel be calledin. Externalfaults and most internalfaults,such as externalsensoror machine failures,can be identifiedand often solvedby the experienceduser or the maintenance engineer.lf the problemcannotbe quicklysolved,the characteristicsof the machineat the point of failureand the statusof the control systemshouldbe recorded.Usingtl"iisinformation, the maintenance engineer can makea decisionas to the kindof failurethat has occurredand the necessaryactionsto be taken. Faultdiagnosis shouldbe carriedout at the timeof failure,withoutdelay. The processesof recordingresultsand potentiallyidentifyingthe simplerfaults shouldtakeonlyminutes.Therefore, machinedowntime will be minimal,if the trainedpersonneland diagnostic toolsare available.
100
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7.1 Documentatlon
F The documentation of a pneumaticsystemcomprisesthe following:
\ U
. . . . . . . .
Clearsystemlayoutdiagramwith labelledvalvesand lines Circuitdiagram Listof components datasheets Component Displacement-stepdiagram Operatinginstructions manuals lnstallation and maintenance Listof sparepartsfor criticalitems
The documentation shouldbe suppliedwhenthe pneumaticsystemis installed and the machine or delivered.Whenthe systemis modified,the documentation historyshouldbe updatedto reflectany changes.This is to ensurethat the potentialusersand maintenance engineershaveavailablethe currentstatusof the machine.
\ ! I
ln general,malfunctions of a systemfall intothe followingcategories: .
7.2 The causes and effects of malfunctlons
Wear and tear on componentsand lines which can be acceleratedby influences: environmental . Qualityof compressed air . Relativemotionof components . Incorrectloadingof components . In@rreclmaintenance . Incorrectmountingand connection(i.e.signallinesare too long)
*
t r{
Tttese environmentalinfluencescan lead to the followingmalfunctionsor fail.res of the system: . Seizureof units . Breakages . Leakages . Pressuredrop . lrpnect switching
1n'
Preventive maintenance
Eventhe most intensivemaintenance is of little use when the systemincorporatesa designor planningerror or is incorrectlyinstalled.This leads to in the courseof time by prematurewear and damagewhichis characterised failureof components.Carefulplanning,evenif this costsa littlemorein some cases,can considerably reducethe frequencyof malfunctions and thus failure and downtime.lmportantpreventive measuresinclude: .
Selectionof the appropriatecomponentsand signal generators.They shouldbe adjustedto suit the environmental and operational conditionsof the system(e.9.switchingfrequency, heavyloads) . Protection of components againstcontamination . Mechanical absorptionof the actuatingforcesthroughadditionalshockabsorbers . Shortlinelengths,fittedwith amplifiers wherenecessary Faultfindingof pneumatic systems
As a rule,a newly-designed and installedpneumaticsystemwill run troublefree for sometime afterinitialadjustments havebeencarriedout. Any instances of prematurewear may not becomenoticeableuntilweeksor monthslater. Normalwear may not becomenoticeablefor years. Eventhen,faultsor the effectsof wear frequentlydo not showdirectlyapparent,with the resultthat it is not easyto identifythe defectivecomponent.lt is obviouslynot possibleto cover all the faults which may occur.The malfunctions describedhere are thereforethose which frequentlyoccur and which are difficultto localisein pneumaticsystems. Evenmorecomplexcontrolscan be dividedinto smaller unitsand checked.ln manycasesthe operatorcan eliminatethe faultimmediately,or at leastidentifythe cause.
102
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It frequentlyoccursthat sectionsof pneumaticsystemsare extendedwithout Malfunctions causedby enlarging the necessary air supply.Depending on the sequence and designof under-sized air supply the plantsection,malfunctions then occurnot continuously but sporadically, withthe resultthatfault-finding is madeincreasingly ditficult.
,
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Possibleeffects:
t
The pistonrod speedis not alwayscorrect,sincethe actuationof additional components can causesuddenpressuredrops. The force at the powercylinderdropsfor a short time duringa pressure drop.
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The samesymptomsmay occuras the resultof changesin orificecross-sections causedby contamination or by leaksat connectorswhich have worked loose(a reduction in diameter ot20y" meansa doubling of the pressure drop). The necessityto ensurethat the compressed air fed intothe networkis free of condensate is emphasised.Whateffectscan occurin practicaltermswhenthe proportion of condensate in the compressed air is too high?
Malfunctions causedby condensate
Apartfromthe corrosivedamagecausedto surfacesby the condensate which is. in manycasesextremelyaggressive, thereis the considerable dangerof seizureof valve componentsif they need to be reset by springforcJ after beingheldin one switchingpositionfor a considerable time. tubricantswithout additives havea tendency to emulsifyand createresinor gumming.All close_ toleranceslidingfits in valvesare particularly susceptible io these resistances to movement.
ia
Malfunctions caused by contamination
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' the caseof systemswhichhavebeenin servicefor sometime,an excessive orcportion of condensate in the compressed air may producerustparticlesin :a'eeswherelinesare fittedwithoutcorrosionprotection.Thiscontamination of :€ nesmayproducethefollowing effects: . . .
Shckingor seizureof slide-valve seats Leaksin poppetvalves Biockageof flowcontrolvalvenozzles.
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133
7.3 Maintenance
routinesare recomFor pneumaticsystemsthe followingregularmaintenance mended: Checkthe filter and serviceunits - drain water regularlyfrom traps and whereused replenishand adjustlubricators Discusswith the operatorsof the systemany noteddifferencesin performanceor unusualevents Checkfor air leaks,crimpedair linesor physicaldamage for wearor dirt Checksignalgenerators Checkcylinderbearingsand mountings
routines Plannedmaintenance
P
Daily: from the filtersif the air has a highwatercontentand if no Draincondensate automaticcondensatedrainagehas been provided. With large reservoirs,a water separatorwith automaticdrain shouldbe fitted as a generalprinciple. lubricatorand checkthe settingof the Checkthe oil levelin the compressed-air oil metering. Weekly: for possibledepositsof dirt or swarf. Checkthe presChecksignalgenerators sure gaugeof the pressureregulators.Checkthat the lubricatoris fundioning correctly, Every3 months: for leaks. lf necessary,re-tightenthe conCheckthe sealsof the connectors nectors.Replacelinesconnectedto movingparts. Checkthe exhaustportsof the valvesfor leaks. Cleanfilterca(ridgeswith soapywater(do not use solvents)and blowthemout with compressed air in the reverseof the normalflow direction. Every6 months: Checkthe rod bearingsin the cylindersfor wear and replaceif necessary. Alsoreplacethe scraperand sealingrings.
104
Section Theory
Chapter1 Fundamentals of pneumatics
1.1
Physicalpropertiesof alr The surfaceof the globeis entirelycoveredby a mantleof air. lt is an abundantgas mixturewiththe followingcomposition : . Nitrogenapprox.78 vol."/" . Oxygen approx.21 vol.Y" It alsocontainstracesof.carbondioxide,argon,hydrogen,neon,helium,krypton andxenon. To assistin the understanding of the naturallawsas well as the behaviourof air,the physicaldimensions whichare employedand theirclassification in the systemsof unitsare documented below. The followingtermsand unitsarerequiredfor definitions in pneumatics: BaseQuantities Unit
Symbol
Length L Mass m Time t Temperature T
Units and unit symbols TechnicalSystem SystemSl Metrg(m) Kp.s'lm Second(s) DegreesCelsius(oC)
Metre(m) Kilogram(kg) Second(s) Kelvin(K)
DerivedQuantities Unit
Symbol
Derivedunits and unit symbols TechnicalSystem ' SystemSl
Force Area Volume Flowrate Pressure
F A V O p
Kilopond(kp) ^ Squaremetre(p') Cubicmetre(mo) (mo/s)
(at) Atmosphere
NeMon(N) = 1 kg.m/s2 Squaremetre(qr') Cubicmetre(m") 1mo/s)
iT;:'li,?,
1 bar = 1o5Pa
The international andtechnicalsystemsof unitsare linkedby : NewtonsLaw:
Force= mass x acceleration F = tTl'€l
wherea is replacedby the acceleration $ue to gravity g = 9.81m/s'r Pressure : 1 Pascalis equalto the constantpressureon a surfacearea of 1m2with the verticalforceof 1 N (NeMon).100kPais equalto14.5psi (poundsper square inch).
108
l-
rSinceeverythingon earth is subjectedto the absoluteatmospheric pressure (p"t), this pressurecannot be felt. The prevailingatmosphericpressureis thereforeregardedas the baseand any deviationis termed: GaugePressure
= pg
or Vacuum
=Pv
Thisis illustrated bythefollowing diagram :
IJ
Air pressur:e. relationship
lj kPa (bar)
Pat pl bar
pressuredoes not have a constantvalue. lt varieswith the The atmospheric geographical locationand the weather. The rangefromthe absolutezero line to the variableatmosphericpressureline is called the vacuum range and abovethis,the pressure range.
r--
The absolutepressurepaois composedof pressurepatand pressurgpg. ln practice,gaugesare usgdwhichshowonly the excesspressurepg. Pressure pabis approximately onebar (100kPa)greaterthanthe Psvalue.
f-l,-. 109
of pneumatics
1.2 Characteristics of air
Festo Didaetic
shape. lts shapechangeswith In commonwithall gases,air has no particular Air can it assumesthe shapeof its surroundings. resistance,'i.e. the slightest to expand. andit endeavours be compressed law:pressure andvolumerelationship Boyle- Mariotte's
V3 p3
V1 p1
Law; "At constant is given in Boyle-Mariottes The applicablerelationship proportional to the the volumeof a givenmassof gas is inversely temperature, i.e. the productof absolutepressureand volumeis conabsolulepressure", stantfor a givenmassof gas. p r . V r = P 2 ' Y z = P s ' V s = Co n s t a n t the aboveprinciples. Thefollowing exampleillustrates Example calculation
pressureis compressed to 1i7ththe by an air compressor Air at atmospheric volume.Whatis the gaugepressureof the air assuminga constanttemperatureprocess:
pr .Vr =pz.Vz P2= P1
V1
Note: Yz lYt = 1ft
v2 p1 = pat= 1 bar = 100 kPa
p2 = 1' 7 =7 bar= 700 kPa absolute Therefore:
= pab- pat = (7-1) bar = 6 bar = 600 kPa. Pgauge
= 600 kPa musthavea compression A compressor that producesair at pgauge pressure (This is 1 bar or 100kPa). thatthe atmospheric assumes ratioof 1:7.
110
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5 The allocationof a controlsystemto one of the threecontroltypesis depend- 1.3 Gontroltheory efit on the task involved.ln the caseof the programcontrolsystem,the planfor programcontrol. ner hasthe choicebetweenthe threesub-groups ControlsystemprocessingtYPes
Plot control system betweenthe commandor referencevalue Tlse b alwaysa clearrelationship provided disturbancevariablesdo not cause any deviand firg ouFut value tirls (DlN 19226). Pilotcontrolsdo not havea memoryfunction. Enory control system Whenthe commandor referencevalue is removedor cancelled,in particular trr completionof the input signal,the output value achievedis retained (nrennrised). A ditferentcommandvalue or an opposinginput signalis reqi€d to returnthe outputvalueto an initialvalue(DlN 19226),. U ,
E3
Tlrr: (schedule)control hr a time (schedule)controlsystem,the commandvaluesare suppliedby a of a timing tinedependentprogramgenerator(DlN 19226\.Characteristics cofthd systemare,thus,the existenceof a programgeneratorand a time-de' pendentprogramsequence.Programgeneratorsmay be : . Cam shafts . Gatns . hrncfied cards . Punchedtape . Programsin an electronicmemory Goord:natedmotion control system motioncontrolsystem,the referencevaluesare suppliedby a h a coordinated s{fal generatorwhose output values are dependenton the path coveredor tre pcitibn of a movablepartwithinthe systembeingcontrolled(DlN 19226).
Ft F3 EI 111
Sequencecontrol system The sequenceprogramis storedin a programgeneratorwhichrunsthrough the programstep-by-step accordingto the statusattainedby the systembeing controlled. Thisprogrammayeitherbe permanently installed or elsereadfrom punchedcards,magnetic tapesor othersuitablememories . icationin accordance Classif withthe signaltype
Analoguecontrol system A controlsystemwhichoperatespredominantly with analoguesignalswithin the signalprocessing section(DlN19237). Digitalcontrol system A controlsystemwhichoperateschieflyusingnumerical digitalsignalswithin the signalprocessing section(DlN19237). Binarycontrol system A controlsystemwhichoperatespredominantly with binarysignalswithinthe signalprocessing sectionand wherethe signalsare not part of numerically represented data(DlN 19237). Controlsystemsignals
----?
Digitalsignal
112
Tims
Binarysignal
The classificationof control systemsaccordingto the type of data repre- Classification accordingto the sentationis of a moretheoreticalnatureand is independent of the methodof typeof signalprocessing solution.Moresuitablefor practiceis the classification accordingto the typeof signalprocessingsinceit givesinformation aboutthe methodof solutionto be chosen.
Controlsystemtypes
Syncfironous control system A wffol systemwheresignalprocessingis synchronous to a clockpulse(DlN -%Jn. Asvnchronous control system A controlsystemoperatingwithoutclockpulseswheresignalmodifications are *{ rQgeredby a changein the inputsignals(DlN 19237). Logic control system A controlsystemwhere specificsignalstatusfor the outputsignalsare assqred to the signalstatus of the input signalsby means of Booleanlogic :cFnections(DlN 19237). S:queFce control system A csrrol systemwith compulsorysteppedoperationwhereswitchingon from tr€ sEp to the nextin the programis dependentuponcertainconditions'being sdisfied(DlN 19237). frrdependent sequencecontrol system A seqrencecontrolwhose switchingconditionsare dependentonly on time DD{ 19237). ne.Cependent A sequencecontrolsystemwhoseswitchingconditionsare dependentonly on $qnab fromthe systembeingcontrolled(DlN 19237).
113
Signalflowandthecontrol chain
The controllercan be Controlchain representedas a selfcontainedblock, which can be brokendown further. A controlcan alA CT UA T I NG DE V I CE S ways be broken down into the blocksto show F in o l c o n t ro l e le me n t s the arrangementof the individual components. At the same time, this showsthe signalflow. P RO CE S S I NG E L E ME NT S
I NP UTE L T ME NT S S I G NA LF L O W
The controlchainis thus characterised by a signal flowfromsignalinputvia signalprocessing to signal outputand execution of instructions. In hardwareterms, this meansthat inputdevices processingdevices and outputdevicesmustexist for thesesignals.
Signalprocessing
S I G NA LO UT P UT
S I G NA LP RO CE S S I NG
S I G NA LI NP UT HA RDWA RE B RE A K DO W N
114
' 'Fd$t8'oliactic
,.iFitf iidHffi:€iiidffitfur-rduiiiAti es'
tEr l1i-1
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Thefollowing diagramshowssomeexamples of the assignment of devicesand signalflow:
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Pneum otics/H ydroulics Cylinders Motors lnd icotors
Actuoting devices
l ir ec t ionol cont r ol v olv es
Finol control elements
Dir ec t ion ol c ont r ol v olv es \o n- r et ur n volves l ' es s ur e v olv e s
Processing elements (control elements)
S',vit ches r.; shbutton s -'rrit switches )-aaram w9r
ur rr
aanaralnrc Ysr r9r u Lvr
Electrics/Electronics
o
='cximity signcllers Sersors
Input elements
Electric motor Solenoids Lineor motors Power contoctors Power tronsistors Semiconductors Contoctors Reloys Electronic components Switches Pushbutton s Limit switches Progrom generotors Proximity signollers Indicotors Generotor
'
' ' :
115
Example
Circuitdiagram
Actuator Finalcontrol element Signalprocessor Signalinput (sensors) Energy.supply (source)
Thecircuitandthe rayoutof the controrchaincanbe
identified.
The powersectionor worksectionconsistsof the actuator and the finalcontrol element. The controlelementreceivescontrolsignals from the processor. The signalprocessor processes information sentfromthe signarinputdevices or sensors.Thesignalflowis fromthe energysource to the [ower section.
116
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Chapter2 Air generation and distribution
tu
performance For the continuing of controlsystemsand workingelementsit is necessary to guarantee that the air supplyis clean,dry and at the required pressure.lf these conditionsare not fulfilled,then short to mediumterm degeneration of the systemwill be accelerated. The effectis downtimeon the machinery in addition to increased costsfor repairor replacement of parts.The preparation of the air startsfrom the point of generation and can be conpointsin the systemrightup to the pointof use. taminated by manypotential preparing goodqualityair and thenallowingincorrect Thereis no use in componentselection to reducethe quality.The_ sqglpment to be Qo,nSi-d-ered in lhe generation andpreparation of air include: . . . . . . . .
Air comoressor Air receiver Air filter Air dryer Air lubricator Pressure regulator points Drainage Oilseparators J
The locationand typeof compressor influences to a greateror lesserdegree the amountof particles, oil andwaterthat is takenin or suppliedto a system. The compressed air shouldthereforebe properlypreparedto preventmalfunctioning of the consuming deviceswhichareconnected to it. Poorly-prepared compressed air will inevitably lead to malfunctions and may manifest itselfin the systemas follows: . Rapidwearof'sealsandmovingpartsin thecylinders andvalves . Oiled-up valves . Contaminated silencers Preparation is achievedby meansof pre-filters on the compressorintake, series-connected dryers,filtersand separators for oil and condensate. Their mustmatchthetaskrequirements. selection Pressure level
As a rule, pneumaticconsumingdevicessuch as cylindersand valvesare pressureof 8-10bar. Practical for a maximumoperating designed experience has shown,however,that approximately 6 bar shouldbe usedfor economic operation.The purchasepriceof the compressor system,the efficiency and wearof the cylinders and valvesand the installation costsfor the pipesystem in this range. Pressure are at theirmostfavourable lossesof between0.1 and 0.5 bar mustbe expecteddue to the restrictions, bends,leaksand pipe-runs, depending on the size of the pipingsystemand the methodof layout. The compressor's systemshouldprovideat least6.5 to 7 bar for a desiredoperatingpressure levelof 6 bar.
118
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of sudden andfluctuating consumption, a coffiressed air l-!Ltn"pis-thedanger
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receivercan be installedto stabilisethe pressurein the compressed air network' In normaloperation, this receiveris filledby the compressor, with the resultthat a reserveis availableat all times. This also makesit possibleto frequency reducethe switching of the compressor.
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valuesof consumption for large-size Characteristic shouldbe determined factor com- Utilisation pressorsaccording to normal,mediumand peak loads. Practicehas shown that with a varyingair consumption severalindividualcompressors can be put to use moreeffectivelythan one largecompressor.An approximate valueof 75% shouldbe takenas the utilisation factorto be aimedfor with mediumload operation.ln orderto makethe correctselection,it is vitalto havea list of all the consumingdevicesconnectedto the compressed air networktogetherwith their averageand maximumair consumption, duty cycle and frequencyof ooeration.
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The atmospheric air takenin by the compressor alwayscontainsa proportion Whydrycompressed air? of moisturein the form of watervapour. The higherthe air temperature, the greaterthe quantityof watervapourwhich it can take up, expressedin % of relativehumidity. lf the saturationpoint of 100% is reached,the water is precipitatedin the form of droplets. The effectsof this processcan be explainedby meansol an example: i0% humidity)compresses Beforecompression, the ;aturatedcontentfor 20oC r takesup 85 g/h of water. At the compressor outletthe air is saturatedand contains51 g/m" of water. Aftercompression, risesto 4OoC. thetemperature For 1.43m3/hof compressed airthe condensate willthenamountto:
-
1.+3m3/h. 51 g/m3= 72.93gth. -
The precipitated condensate aftercompression thusamountsto:
-
85 g/h - 72.93g/h = 12.07glh
-=
lf this moistureis allowedto enterthe pneumatic system,the consequences areas follows:
= =
Corrosionin pipes,cylindersand othercomponents.This increases wear costs. and maintenance in the cylinders Thebasiclubrication is washedaway. functionof valvesis impaired, i.e. moremalfunctions The switching during sequence. the operating Contamination and damageat pointswherethe compressed air comes (paintshops,foodindustry). materials directlyintocontactwithsensitive
-r= =
'3
It thereforefollowsthat the watermustin all casesbe removedfromthe compressedair beforeit cancausedamage;the air mustbe adequately dried.
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119
2.1 Air compressor
The selectionfrom the varioustypesof compressors availableis dependent upon quantityof air, pressure,qualityand cleanliness and how dry the air shouldbe. Thereare varyinglevelsof thesecriteriadepending on the typeof compressor. Compressor types
Reciprocating piston compressor
Reciprocating compressorsare very commonand providea wide range of pressures and deliveryrates. For higherpressures multistage compression is usedwith intercooling betweeneachstageof compression. The optimum range of pressuresfor reciprocatingcompressorsare approximately: up to 400kPa (4barlS8psi) up to 1500kPa(15barl217.5psi) over1500kPa(15bar/217.5psi)
Singlestage Doublestage Trebleor multistage
Also,it is possiblebut not necessarily economicto operatein the following ranges: up to 1200kPa(12barl174psi) up to 3000kPa(30bar/435psi) (220barl3190psi) up to 22000kPa
120
Singlestage Doublestage Trebleor multistage
The diaphragmcompressoris used whereoil is to be excludedfrom the air Diaphragm compressor supply,for examplein the food,pharmaceutical and chemicalindustries. Here thereis no needfor lubrication in the compression area. The rotarygroupof compressors use rotatingmembersto compressand in- Rotarypistoncompressor creasethe pressureof the air. They are smoothin operationbut the compressionis not as highas with multistagereciprocating compressors. Flow compressorsproducelarge volumesof air at small increasesin stage Flow(turbine)compressors pressure. The air is accelerated by the bladesof the compressor'but thereis only a small increasein pressureof about1.2 timesthe inlet pressureper . stage. Receiversprovideconstantair pressurein a pneumaticsystem,regardlessof 2.2 Air receiver varyingor fluctuating consumption.This enablesbriefly-occurring consumption peaksto be balanced out,whichcannotbe madeup by the compressor. A furtherfunctionof receiversis the emergencysupplyto the systemin cases of powerfailure. The reservoircan*Fejtittedeitnerdownstreamof the compressor,to act as an air chamber,or selectively at pointswhereconsumption is high. Air receiver
Temperature Pressure gauge gauge\
Pressurerelief valve zlHF. ('ltlh) Air receiver Waterdrain
In addition,the largesurfaceareaof the receivercoolsthe air. Thus,a portion of the moisturein the air is separateddirectlyfrom the receiveras water.lt is thereforeimportantto drainthe condensate regularly. The sizeof a compressed air receiverdependson the: . Deliveryvolumeof the compressor . Air consumption for the applications . Networksize . Typeof compressor cycleregulation . Permissible pressure dropin the supplynetwork 121
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Air receiversize
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Air receiversizechart
\ Switchingcycles/h
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PressuredifferenceA p 102kpa (bar/14.5psi)
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Example Delivery volume Switching cyclesper hour Pressuredrop
V = 20 m3/min z'=20 A P = 1x lo s p a
Result: Receiversize
V=15m3(referto the chart)
122
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Waterproducesa hardeningof seals,corrosionand the washing-out of the 2.3 Air dryers originallubrication of cylinders. Oil andwatermaycausesealsanddiaphragms plants,waterand dust causecontamination, poor to swell.In paint-spraying paintadhesionand the formation of blisters.In the food,pharmaceutical and chemicalindustries, oil, dirt,bacteriaand germsdestroythe storageproperties of oroducts.
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The servicelife of pneumaticsystemsis considerably reducedif exeessive moistureis carriedthroughthe air systemto the elements. Thereforeit is important1o1l!lhe necessary air dryingequipment to reducethe moisture content to a levelwhichsuitsthe application and the elementsused.Thereare three methodsof reducing auxiliary the moisture contentin air:
''i t,*i',1 r""" t $
t
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. Lowtemperature drying . Adsorption drying . Absorption drying
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lihe additional costof installing air dryingequipment can be amortised overa pay-back perioddue to the reductionin maintenance short costs,_red!'ced reliability oowntime andincreased oiTneiystem.
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-ne mostcommontypeof dryertodayis the relrigeration dryer. Theseunits Lowtemperature drying rcerateeconomically and fe,liabfy and the maintenance costsare low. With 'e'rigerated drying,the compressed air is passedthrougha heat-exchanger s!stemthroughwhicha refrigerant flows.The aimis to reducethe temperature :' the air to a dewpointwhichensuresthatthe waterin the air condenses and :'ops out in the quantityrequired.
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DewPoint: The dew pointtemperature is the temperature to whicha gas mustbe cooled to condense watervapourcontained in the gas. The lowerthe temperature the morethe waterwill condenseand reducethe amountentrappedin the air. Usingrefrigeration methods,it is possibleto achievedewpointsof between2oCand SoC. Beforethe compressed air is outputintothe network, the air is heatedto bring the air backto ambientconditions. The outlettemperatures are approximately l OoCin winterand approx.3OoCin summer. Adsorption dryers
The lowestequivalentdew points(downto -gOoC)can be achievedby means of adsorptiondrying. In this process,the compressed air is passedthrougha gel and the wateris depositedon the surface,i.e., it is adsorbed.(Adsorb: wateris deposited on the surfaceof solids.) The dryingagentis a granular materialof sharp-edged shapeor in beadform. This dryingagentconsists almostentirelyof silicondioxide. Adsorption drying
Moist oir I
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Hre-ltlter (orl ttlter,) Shut-off volve (open)
Adsorber 1
Shut-off volve (closed)
Adsorber 2
Ho t o ir ' 1 Shut-off volve (open)
Shut-off volve (closed)
Heotel
After filter
Fon
Dry o ir
In practice,two tanksare used. Whenthe gel in one tank is saturated,the air flow is switchedto the dry, secondtank and the first tank is regenerated by hot-airdrying,
124
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Absorption dryingis a purelychemicalprocess. The moisturein the com_ Absorption dryers pressedair formsa compound withthe dryingagentin the tank. Thiscauses the dryingagentto breakdown;it is then dischirgedin the formof a fluidat the baseof the tank.Absorption dryingis not of majorsignificance in present_ day practice, sincethe operating costsare too highandihe efficiency too low for mostapplications.
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c llgqogr and.oil particlesare also separatedin the absorption dryer. Larger :uantitiesof oil havean effecton the efficiency of the dryer,howeuer.Forthis 'easonit is advisable to includea finefilterin frontof the dryer.
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Tre featuresof the absorption processare :
-
. Simpleinstallation of the equipment wearbecausethereare no movingpartsin thedryer ' Lowmechanical . No external energyrequirements.
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125
2.4 Air serviceequipment
As a rulethe compressed air whichis generated shouldbe dry, i.e..freeof oil. For somecomponents lubricated air is damaging, for others,it is undesir,able, but for powercomponents it may in certaincasesbe necessary.Lubrication of the compressed air shouldthereforealwaysbe limitedto the plantsections which requirelubrication.For this purpose,mist lubricatorsare fittedto feed the compressed air withspecially selectedoils. Oilswhichare introduced into 'the air fromthe compressor are not suitablefor the lubrication of controlsysiemcomponents. Theproblems thatoccurluithexcessive lubrication include: . Malfunctions dueto excessively lubricated components . Oil mistpollution of the environment . Gumming-up of paftsoccursafterlengthyplantstandstills . Difficulties in adjusting the lubricator correctly. Despitetheseproblems,lubrication of the compressed air by meansof mist lubricators maybe necessary in certaincases: . Whereextremely rapidoscillating motionsare required . Withcylindersof laigediametei,from approximately 125mmupwards.As far as possiblelubricators shouldbe installed onlydirectlyupstreamof the consuming cylinders. ; of the correctsizeof lubricator is determined by the air consumpI Theselection tion of the cylinders, sinceall lubricators requirea minimumvolumetric flow beforethey beginto deliveroil (checkthe responsethreshold).This means that if the lubricator is sizedtoo large,it may undercertaincircumstances be iwhilejf it is too small,leikageaiiwhichmaybe presentcan cause 11gffective, the lubricator to dry out duringidle periodF.As a generalprinciplecylinders with heat-resistant sealsmustnot be suppliedwith lubricated compresiedair, sincethe specialgreasewhichformsthe originallubrication wouldbe washed out.
126
Air generationand dislribution
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: :c^ipressedair passingthroughthe lubricatorcauses a pressuredrop -=':.2.^ the orl reservoirand the upper pari of the lubricator.The pressure : -:':^oe is sufficient to forcethe oil upwardsthrougha viaductwhere it then -' -: .io a nozzlewhichcan be seen throughan inspectionglass. Here the : s a:Jnrrsed and takenup by the air streamto a greateror lesserextent. : = :cssibleto checkthe oil dosaqeas follows:
Checkingthe oil dosage
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20 - : .ce of white cardboardshouldbe held at a distanceof approximately -- ''3nr the exhaustport of the powervalve. lf the systemis then allowedto ::='aie for some time rt shouldbe possibleto see only a pale yellowcolour In this case : ^ :'e cardboard.Drippingoil is a clearsign of over-lubrication '-= readjusted. should be -oricator
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127
Removingoil
Up to a few yearsago, the generalview was that the oil discharged by the compressor couldbe usedas a lubricant for the powercomponents.Now it has beenrecognised that this is not the case. As the levelof heatproducedin is very high, the oil is carbonisedand the oil vapourexthe compressor hausted.Thisleadsto an abrasiveactionon cylindersand valves,and service is considerably reduced. Moreover, the oil is deposited on the innerwallsof the pipesand is eventually absorbedin an uncontrolled way intothe air flow. This fact alonemakescontrolledand effectivedistribution impossible.A pipe whichhas becomeconin thisway can no longerbe cleanedwithoutdismantling. taminated A further disadvantage is gumming,whichmeansthat aftera systemhas been at a for sometime (afterweekendsand publicholidays), standstill lubricated componentsdo not at firstfunctioncorrectly.A basicrequirement thereforeis that the oil discharged by the compressor be removedor the compressed air must be generated in oil-freeform. pointsshouldbe observed Thefollowing practice: in everyday . As far as possiblecompressor oils shouldbe prevented fromenteringthe compressed-air network(oilseparators shouldbe fitted) . For operationfit componentswhich can also operatewith non-lubricated compressed air . Oncea systemhas beenoperatedand run-inwith oil, the lubrication must be continued sincethe originallubrication will havebeen of the components flushedawayby the oil.
128
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Air generationand distribution
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the Compi'essed air filter -ne setectionof llrr; correctfriierplays an importantrcle in determrning of the workinqsystemwnich is to be suooliedwiti' ::ality and performance iilters:; 'he oore size ot comoressecl-air ::r'cressed iir One character-istic ^e oore siz: of lhe tir;erelelrent indicaiesthe minrmurnoarrtrcl€l size which ::.i be filteredout of the compressedair For examole.a S-micronfilterele-.:"i f;ltersout rr1oi:rticles'"vhose drametert,; greaierthan 0,005mm With a .rrr filtersare arso abie to seoarateccn,jerr;ai ;-:aore design.cornpressec :-::f the compressedair. Tire collectedcondensatemust be drainedbefoi,: - .,:c
rtta x i mt-tncro n d e n s a te n ri trk otherw i se tt w tl l be i -,6-l ni rorrrd = ) . . , ^^^A^ 'h ^ - A r . us ur r ^ r ne at r s l re a i |
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:cndensateaccumulatesit is advisableto fit art automatic =':= 1-:--:: - - : =-:ar' :": i'lanuallyoperateddrarncock The automaticdrainuses a '- :='z'^- -. :-3 e'rel of condersatein the bowl and when the lrmitis 3 S :t-r o o e n S :t /?
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The compressed air passesthroughthe filter from left to right and is fed througha baffleplatein the filterbowl. The effectof the baffleplateis that the air is causedto rotate,and the heavierdust particlesand waterdropletsare spunby centrifugal forceagainstthe innerwall of the filterbowl. They then run downthe wall of the housingand collectin the filterbowl, The air which hasbeenpre-cleaned in thiswaythenpassesthroughthe filterelement, which filtersoutthe smallerdirtparticles.Thefilterelementin thiscaseconsistsof a highly-porous sinteredmaterial. The degreeof separation dependson the poresize of the filterelementused. Insertswith differentporesizesare available. Theusualporesizesarebetween5 micronsand40 microns. A furtherimportantcharacteristic of compressed-air filtersis the degreeof separation, or efficiency, whichindicates the percentage of particlesof a particularsizewhichcan be separatedout. The efficiencyis quotedfor a particle size,e.g,efficiency of 99.99%for 5 microns. The filtrationactionof a compressed-air filter is retained,even after long serviceandwithheavycontamination. However, undertheseconditions, the pressuredropbecomesdisproportionately highand the filterbecomesan energywaster. In orderto recognise the correcttime to changethe filterelement,a visual inspection or a measurement of the pressuredifferenceacrossthe filtershould be carriedout. The cartridgeshouldbe changedor cleanedwhen the pressuredifferenceis 40 to 60 kPa (0.4to 0.6 bar). Maintenance
Dependingon the natureof the compressedair availableand the numberof components fitted,compressed-air filtersrequirea greateror lesseramountof maintenance work. Maintenance workmeansthe following: . .
Replacing or cleaning the filterelement Draining thecondensate
Whencleaningis required,the manufacturer's mustbe observed specifications concerning the cleaningagentsto be used. Manycleaningagentsare unsatisfactoryfor filterbowls(e.9.,trichloroethylene) sincetheyproducestresscracks in the plasticfilterbowls,whichmaythenburstwhenre-subjected to pressure. As a rule, it is sufficient to use lukewarmsoapywaterappliedwith a nonabrasivebrush. The filterelementsshouldthen be blownout in the opposite direction to the normalairflow. Compressed airregulators
The compressed will fluctuate. air generated by the compressor Centrallylocated regulatorsfittedto the compressedair networkensurethat there is a pressure)irrespective constantsupplypressure(secondary of the pressure pressure). fluctuations in the mainloop(primary Changesin the pressurelevelin the pipe systemcan adverselyaffectthe switchingcharacteristics of valves,the runningtimes of cylindersand the timingcharacteristics of flowcontroland memoryvalves.
130
A constantpressurelevelis ihus a prerequisite for the trouble{reeoperationof a pneumaticcontrol.[_[ ord9,1to provideconstantp-re9su-fe_cgndltio-ns, the pressurereduceror pressureregulatoris fitteddownstream of the compressed pressureconstant-rggard-_ air filter and has the task of keepingthe-operating less of pressurefluctuations or air consumptign. in the system.The air pressure requirements shouldbe matchedto individual upstream of eachplantsection. The systempressurewhich has provedin practiceto be the best economic generation compromise andtechnical betweencompressed-air andthe efficienis approximately: cy of the components . .
6 bar in the powersectionand 4 bar in the controlsection.
A higheroperatingpressurewouldleadto inefficientenergyutilisationand inpressure creasedwear,whereasa loweroperating wouldleadto poor,efficienin the powersection. cy, particularly Pressureregulatorwith vent hole
Pressureregulator:relieving
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presThe inputpressuremustalwaysbe higherthanthe outputpressure.'1-The The outputpressureactson one side of the g4e is regulatedby a diaphragm. diaphragmand a springacts on the otherside- The springforcecan be adI{ned by meansof an adjustingscrew.When the outputpressureincreases, the diaphragmmovesagainstthe springforcecausingthe outletcross-sectional areaat the vaMeseatto be reducedor elosedentirely.Thusthe pressureis regulatedby the volumeflowingthrough.
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pressuredropsand the spring Whenair consumption increases, the operating forceopensthe valve. Regulation of the presetoutputpressureis thusa continual openingand closingof the valve seat. To preventthe occurrenceof flutter,air or springclampingis providedabovethe valvedisc. The operation pressure is indicated on a gauge. for example lf the pressureon the secondaryside increasesconsiderably, duringcylinderload changes,the diaphragmis pressedagainstthe spring. The centrepiece of the diaphragm air canflow thenopensandthe compressed to atmosphere throughthe ventholesin the housing.Thisrelievesthe excessivesecondaryairpressure. Pressure regulator without venthole
PressureregulatingvalPressure regulator: non-relieving ves withno ventholeare available commercially. With these valves it is not possibleto exhaust the excessive compressedair producedby suddenloads, lf no air is drawnoff on the secondaryside, the pressurerisesand presses the diaphragm againstthe compression spring. Thus,the compression spring moves the plungerdownwards and the flow of air is closedoff at the sealing seat. The compressed air can continueto flow only when air is drawn off on the secondary side. movesagainstthe spring Whenthe outputpressureincreases, the diaphragm forcecausingthe outletcross-sectional areaat the valveseatto be reducedor closedentirely.Thusthe pressureis regulatedby the volumeof air flowing through.
Settingand adjusting
The pressureregulatorcan be adjustedbetweenthe limitsof zero and the supplypressureof the compressornetwork.The adjustmentto a higherpresWhenreducingpressure springcompression. sure is achievedby increasing it is necessary wellbelowthe limitrequiredto to relievethe pressure settings, relievethe air fromthe vent and then increasethe pressureup to the lower limitrequired.lt is not possibleto simplyadjustthe pressuredirectlydownto gauge. on the pressure the desiredvalueas indicated
132
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serviceunit:
Air serviceunit
of operation
The air serviceunit is a cornbination of elements: . . .
Compressed air filter Compressed air regulatorand gauge Compressed air lubricator Air serviceunitsymbols
Tle bllowingpointsshouldbe noted: .
The total air throughputin m3/hdeterminesthe size of the unit. lf the air throughputis too high,a largepressuredropoccursin the units.The values specifiedby the manufacturershouldbe qbserved . The workingpressuremay not exceedthe value statedon the serviceunit. The ambienttemperatureshould not exceed50oC (maximumvaluesfor flastic bowls).
133
Maintenance of air serviceunits Thefollowing routineservicemeasures are necessary on a regularbasis. .
levelmustbe checkedregularly,as Compressed air filter: The condensate on the sightglassmustnotbe exceeded.Theaccumuthe heightspecified lated condensatecould othenruise be drawn into the compressedair pipelines. The drainscrewon the bowl mustbe openedto drainthe condensate.The filtercartridgein the filtermustalsobe cleanedif it is dirty. . Compressedair regulator: This requires no servicing,providedit is preceded by a compressed airfilter. . Compressed : lf fittedcheckthe oil levelin the sightglassand air lubricator top up, if necessary, to the levelindicated.The plasticfilterand lubricator bowl mustnot be cleanedwith trichloroethylene. Only mineraloils may be usedfor the lubricator.
2.5 Air distribution
In orderto ensurereliableand trouble-free a numberof points air distribution, mustbe observed.Amongthesepointsthe correctsizingof the pipesystemis important. Also the pipe material,flow resistances,pipe layoutand maintenance.
Sizingpipesystems
ln the case of new installations, allowanceshouldbe"made in all casesfor network.The main line size determinedby extensionof the compressed-air currentrequirements shouldthereforebe increasedto includean appropriate safety margin. Plugsand shut-offvalvesallow extensionto be carriedout easilyat a latertime. are reprelggse,soccgr11all pipesdue to flow resistanqes.Flowresistances sentedby restrictions, bends,branchesand fittings. These lossesmust be The idealto aim for is a pressuredrop in the madeup by the compressor. 10 kPa(0.1bar). entirenetworkof approximately In orderto achievethisvalue,the totalpipelengthmustbe known. Forfittings, branchesand bends,equivalentpipe lengthsare determined.The choiceof the correctinternaldiameteris also dependenton the operatingpressureand delivery of the compressor. Selectionis best made with the aid of a nomograph:
Flowresistances
Any influenceor chan,ge of !i1gc!_o11.ryithin the pjpesystemmeansinterference qtI]he_air flow andthl,s gn increaseof the flow resistancb.This leadsto a pressuredrbp ?!gng_ gg@fLuggg the pipe syS-terii.Since 6iiniXg-s,b-"nq5, in all compressed-air network_s, adaptersand.fittings are_reqq!1ed this pressure red_uq,ed cannot be avoided but be consitlijrably .drop_ !y 1cglilg pipes ,c-?l materiqlsgnd assembli ng the fittingscorrectly. favourably,q[gqsing_ sqrltgbje
134
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The choiceof suitablepipematerialis determined placed Pipematerial by the requirements on a moderncompressed-air network: . Lowpressure losses . Freedom fiom leaks . Resistant to corrosion . Capability of systemexpansion lr selectinga suitablepipe material,consideration mustbe givennot onlyto crice per meterrun but also to anothermajorfactor,the installation costs. T,heseare lowestwith plastics ltaslic pipescan be joined100%airtightby or fittingsandcaneasilybe extended. -eans of adhesives 3opperand steelhavea lowerpurchasepricebut mustbe brazed,weldedor .: ned by means of threadedconnectors;if this work is not carriedout ::nectly,swarf,scale,weldingparticles or sealingmaterials maybe introduced -:c the system.This may leadto majormalfunctions. For smalland medium 3 ameters,plasticpipesare superiorto othermaterialsas regardsprice,asmaintenance sernbly, andeaseof extension. Pipinglayout
A,rdistribution system
1-2% gradient
Reservoir within a pneumatic system
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r;.ai from correctsizingof the pipingand the qualityof the pipe material, :,:-ect pipelayoutis the decisivefactorin determining the economic operation :' :e compressed-air system.Compressed air is fed intothe systemat inter, a s cy the compressor.lt is oftenthe casethat consumption at consuming :E.,3esrisesfor onlya shorttime. Thismayleadto unfavourable conditions in :-+ ccmpressed-air network. lt is thereforerecommended that the com:r:s.sed-airnetworkshouldbe producedin the form of a ring main. A ring -1 - ensureslargelyconstant pressure conditions. :-:ssure fluctuations in the networkmakeit necessary to ensurethatthe pipes a-: rounted securelyin orderto avoidleakagesat screwedand brazedconE--Jr
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135
Air ringmain
Production plont
Foreaseof maintenance, repairor extension of the networkwithoutinterfering with the overallair supply,it is advisableto sub-divide the networkinto individualsectionsby meansof shut-offvalves. Brancheswith T-piecesand manifolds withplug-incouplings makeit possibleto supplyadditional consumingdevicesas the needarises.In orderto protectthe consuming devicesfrom condensate fromthe mainline,branchlinesmustbe laidwith an upwardinclination. Ringmaincross-connected
P roduct io n pronr
Despitethe bestwaterseparation in the pressuregenerating system,pressure in the pipe system. In dropsand externalcoolingmay producecondensate orderto discharge thiscondensate, the pipesshouldbe inclined1-2%;thiscan fromthe alsobe carriedout in steps. Thecondensate canthenbe discharged at the lowestpoint. systemvia waterseparators It is advisableto fit the branchlineswith standardballvalvesor shut-offvalves.
136
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Directionalcontrolvafves
Directional controlvalvesare deviceswhichinfluencethe pathtakenby an air stream.Normallythis involvesone or all of the following:allowingthe passage it to particular of air anddirecting air lines,cancelling air signalsas required by passage blockingtheir and/orrelievingthe air to atmospherevia an exhaust port.The directionalcontrolvalveis characterised by its numberof controlled positions. connections or waysand by the numberof switching Additional intormationis given to define the methodsof actuationto achievethe various switchingpositions.The construction of the valveis importantwhen analysing the flowcharacteristics valve of the suchas the flow rate,pressurelosses,and switchingtimesfor a particularapplication. Generallythe symbolis adequateto representthe operationalcharacteristics of the valve in comparisonto other elements in the circuil.The samesymbolfor a directional controlvalvemaybe applicable for manydesigns,construction methodsand characteristics. 3.1 Configuration and construction
The designprincipieis a contributory factorwith regardto servicelife,actuating force,switchingtime,meansof actuation,meansof connection and size. Designsare categorised as follows: . .
Poppetvalves: - Ballseatvalve - Discseatvalve Slidevalves: - Longitudinal slidevalve - Longitudinalflat slidevalve - PlateslidevalvO
'
Poppetvalves
Withpoppetvalvesthe connections are openedand closedby meansof balls, discs,platesor cones. The valveseatsare usuallysealedsimplyusingelastic seals. Seatvalveshavefew partswhichare subjectto wear and hencethey have a long servicelife. They are insensitiveto dirt and are robust. The actuatingforce,however,is relativelyhigh as it is necessaryto overcomethe forceof the built-inresetspringandthe air pressure.
Slidevalves
In slide valves,the individual connections are linkedtogetheror closedby meansof spoolslides,spoolflatslidesor slidingdiscvalves.
3.2 2l?way valve
The 212way valvehas two portsand two positionS. lt is rarelyusedexceptas an on-offvalve,sinceits only functionis to enablesignalflow throughand cannotreleasethe air to atmosphere oncein the closedposition. lf air is to be releasedon closing,then the 312way valvemust be used.The 212way valve is normallyof the ball seat construction similarto the 312way valve.lt is generally manually operated or pneumatically operated.
138
Directionalcontrol valves
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Tirs J,2 way valve is a srgnalgeneratrng valve,with the charactei-isiic that a 1l.J J,2 'r,ravralve sicnai on the outputside of the valve can be generatedand also cancelled way valve has three portsand t'ryopositronsThe additionof the ex -ne 3,2 ^aLsi port 3(R) enablesthe signalgeneratedvia the passagethroughthe 3r2 ..jayvalve to be cancelledThe valve connectsthe outputsrgnal2(A; to ex' ^.r.st 3(R)and atrnosphei-e in the initialposition.
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: sc'ng forcesa ball againstthe valve seat preventingthe compressedair '-:- ilowrngfromthe air connection1(P)to the workingline21A). Actuationof :-e .,aiveplungercausesthe bail to be forcedaway from the seat. In doing :^ s i:e opposrngforce of the resetspringand that generatedfrom ine com:':sseo arr must be overcome The air supplyis then open to the outputside l':ie valveand a signalis generated.Once the plungeris releasedthe 1(P) :rr: is blockedand the output port 2(A) is exhaustedup the stem of the 3 !:lger and the signalremoved.The valve is actuatedmanuallyor mechani:a,i't in this case A separateactuationhead couldbe auxiliarymountedto the . arle top to operatethe plungerindirectlyby pushbutton.rolleror lever.The a:ruatronforcerequiredis dependenton the supplypressure,springforceand :^e frictionin the valve.The actuationforce limitsthe feasiblesize of the valve a^d the cross-sectional area of the valveseat must be small The construction :' ine ball seat valve is very simpleand henceis relativelyinexpensrve.The featureis the compactsize achievable. : slrnguishing
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139
a singleactingcylinder 3/2 wayvalvecontrolling \ 1
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In this circuitthe 3/2 way valve1.1 controlsa singleactingcylinder1.0.The pushbutton activatedvalveis at restwiththe 1(P)portblockedand the cylinder divertsthe via 3(R).The operationof the pushbutton exhaustedto atmosphere force. port spring and the cylinderextendsagainst 1(P) supplyto the 2(A) and the cylinder pushbutton returns by spring, is released, the valve Whenthe is returned to its initialpositionby the cylinderreturnspring.
140
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Elir6ctioEalcontrol valves
Festo Didactie
312way valve: disc seat normallyclosed, un-actuated
--e valve shown here is constructedon the disc seat principle.The sealingis s ltole but effective.The responsetime is short and a small movementresults - a large cross-sectional area being availablefor air flow. Like the ball seat .a\/es. they are insensitiveto dirt and thus have a long servicelife. The 3/2 'ray valves are used for controls employing single-actingcylinders or for signalssuppliedto processingelementsand finalcontrolelements. =:enerating
32 wayvalve:discseatnormally closed,actuated
141
1Directionalcontrol valves
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Valvesof the singledisc seat type are non-overlapping. When operatedslowly. thereis no loss of air Actuationof the plungerfirstcausesthe exhaustarr line from 2(A) to 3(R) to be closed,as the plungerrests on the disc On pressingfurlher,the disc is liftedfrom the seat allowingthe compressedair to flow from 1(P)to 2(A) Resettingis effectedby the resetspring.On releaseof the plunger,the 1(P; port is blockedand the supplyport is then openedto atmospherethroughthe 3(R) exhaustport.This valve.shownon the previous page.is referredto as normallyclosed,sincethe outputside 2(A) is ctosedotf from the 1(P) port at the initial,unactuatedposition The normallyopen configuration is shownbelow
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3l2way valve:discseat normallyopen,un-actuated
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A 3l2way valvein whichthe normalpositionis open to ftow from 1(P) to 2(A). is referredto as a normallyopen valve. Initiailythe i(P) Bortis connectedto the 2(A) port throughthe stem of the valveand the valvedisc seat is closedat the 3(R) port.when the plungeris operated,the 1(p) air is initiailyblockedby the stem seat and then the disc is pushedoff its seat to exhaustthe outputarr to atmospherevia the passage2(A) to 3(R). when the plungeris released the pistonwith the two sealingseats is returnedto the initialpositionby the returnspring.once againthe 3(R) port is blsckedand air is suppliedfrom 1(p) to 2(A). The valvescan be actuatedmanually,meehanically, electrically or pneumatically Differentactuationmethodscan be appliedto suitthe application.
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: = 3 2 way valve:disc seat normallyopen, actuated
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- iris circuitthe 3i2 way valve is normallyopen and at the rest condition the cylinderis initiallyextended. s-cpliesair to the cylinder1,0.Therefore dueto of the manual312wayvalve1.1, the cylinderretracts ;ocn opef?tion :-e release of airvia 2(A)to 3(R).
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Feslo Didactic
Rollerlevervaivewith idie ieturrt
The idle rollerlevervaive is a one way trip valve.lt is designedto operatein one directionof cam movementDastthe rollerhead.Thereforeihe valve must be siruatedjust beforethe limit of cylindertravel.The cylindercam overruns the limitswitch.The valvemust be fittedwith the one way trip operatingin the correctdirectionof motion.The stgnalgeneratedby the idle rolleris of relatively sherl duration The ,valveis used to generateshort durationsignalsto preventsignaloverlap.
3"2 way nand slidevalrre
The 3/2 way hand slide valve is used to supply air io a leg of the supply neiworkupstreamof the eonsumingdevices.The constructron oi this valve is simpleand it is uiilised as a shut-offvalve. The unit is compactand has two detent positions to hold the 'ralve open or closed. By moving the casing.line 1(P) is connected to the outlet 2(A) in one positionand 2(A1 with 3(R) in the other positionwhich exhausts air from the network.
312way hand slidevalve
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) Pneumatically actuated 3/2 way valve
The pneumatically actualedSl2way valveis operatedby an air signalat 12(Z). using no internalauxiliaryair assistance.This is referredto as single pilot operationsincethereis only one controlsignaland the valvehas springreturn. ln the initialposition,the vaiveis normallyclosedsincelhe 1(P)port is blocked by the disc seat and the 2(A) port is exhaustedto atmosphere. Circuitfor the 312way valve,normallyclosed
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triifrctloiral control vatuBs
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The pneumaticallyactuatedvalve can be used as a final controlelement,with indirectcontrol.The signalior extensionof the cylinderis initiatedindirectly by a pushbuttonvalve 1.2 which supoliesthe conirol signal to the final control to the element1 1 The'ralve1.1 can be of relativelylargesize in comparison pushbuttonsignalelementand the signalelementcan be fitted at a remote distancefromvaltre'1.1,
closed,un-actuated i 312wayvalvesinglepilot,normally
movesthe valveplungeragainstthe resetspring. Alr appliedat the '12(Z)por1. The connections generating 1(P)and 2(A)are connected a signalat po'12(A) ard the 3(R)exhaustportis blocked.Uponreleaseof the signalat port12(Z), to the initialpositionby the returnspring. The disc :,e giiotspoolis returned between1(P)and 2(A). The excessair in the working :$qsesthe connection rne2(A)is exhausted through3(R). closed,actuated 32 wayvalve,singlepilot,normally
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Directionalcontrol valves
Festo Didactic
The single piiot 3/2 way vairrecan be configuredas normallyclosed or as normallyopen.The pofis nave duarrores in whichfor the normailyopen func_ tion the 3(R) and 1(p)ports are interchanged. The head of the varvewith port 1212\can be rotated1gOu
3/,y^y vatve, singlepitot,normafu offi
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lf a normallyopen valveis usedat the position of valve1.1, initiallyextendedand upon operationoi tne pusnbufton. then the cylinderis the cyrinderretracts.
Circuitfor lhe BIZwayvalvenorrnatty operr,lndirect
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To avoid a high actuatingforce. mechanically controlleddirectionalvalves can 0e equippedwith an internalpilot valve and servo piston to assist opening. The valve actuatingforce is oftenthe determining factorin appticationsand the servo assrstanceallowsfor largerbore valves to-be operat*o*itn small actuat_ ing forces.This increasesthe sensitivityof the system.
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Direetionaleontrol valves
j smatlhole connectsthe pressureconnection1(P) and the pilotvalve lf the -: er lever is operated.the pilot valve opens Compressedair flows to the s:rc pistonand actuatesthe main vaivedisc.The firsteffectis the closingof ^: cath 2(A)to 31R)followedby the seconddisc seat openingthe airwayfrom - r to 2(A1 This type of valvecan be used as eithera normallyclosedvalve valveby changingportsand rotatingthe head. :- -ormally-open 3 2 way valve,internalpilot,normallyclosed
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b 1(P)to 2(B) and 4(A) to Whenthe two plungersare actuatedsimultaneously, 3(R) are closedby the firstmovement.By pressingthe valveplungersfurther againstthe discs,opposingthe resetspringforce,the passagesbetween1(P) to 4(A)and from2(B)to 3(R)are opened.The plungerscan be operatedby an auxiliarymounteddevicesuchas a rollerarm or pushbutton. Thevalvehasa non-overlapping exhaustconnectionand is returnedto its start positionby the spring.The valvesare usedfor controlsemployingdouble-actirtgcylinders. Gircuitdiagramr412wayspringretut:nvalve
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GrqJitdiagram:5l2way springreturnvalve
Thereare otheractuating methods and types of construction availablefor the 412way valve including pushbutton, singleair pilot, double air pilot, roller lever actuated, spool and sliding plate. ln the main,the 412way valveis utilisedin similar roles as the 512 way valve.
ln general lhe 412 way valve is replacedby the 512way valve. The 5/2 way valve has advantagesin construction of passagesand allowing the exhaustof both extensionand retractionair for cylinders to be separately controlled. The 512way valve circuit carries out thei same primarycontrolfunctions as the 412way valvecircuit.
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Festo Didactie
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i 3.5 4/3way valve
The 4/3 way valve has four ports and three positions.An exampleof the 4/3 way valve is the plateslide valve with hand or foot actuation. lt is difficultto fit othermeansof actuationto these valves. By turningtwo discs,channelsare connectedwith one another.
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In thrs examplethe valve lines are closed in the middle position. This enablesthe prstonrod of a cylinderto be stopped in any positionover its rangeof stroke,although intermediatepositionsof the prstonrod cannotbe located with accuracy. Owing to the compressibility of air. another positionwill be assumed if the load on the piston rod changes
--: 5,2 way valvehas five portsand two positions. fhe 512way valveis used 3.6 5/2 way valve t;^^ l ^ . - ^ ,d, ^^ a,rqr ^^ .;r,, ,ry for cylinders.An exampleof vJ[tttol the control of ar element ''= 52 way valve,the longitudinal slidevalve,uses a pilot spool as a control lines by means of t:-oonent. This connectsor separatesthe corresponding :-giiudinalmovements.The requiredactuatingforce is lower becausethere air or spring.Sealingpresents a-: ,rinimalopposing{orcesdue to compressed =:'cblem in this type of slide valve The type of fit known in hydraulicsas -::al to metal,requiresthe spoolto fit preciselyin the bore of the housing. In :-=rmatic valves,the gap betweenspooland housingbore shouldnot exceed -:12-0.004 mm, as otherwisethe leakagelosseswill be too great.To save :-:se expensivefittingcosts,the spool is oftensealedwith 0-ringsor double:,: cackingsor the bore of the housingis sealed with 0-rings. To avoid portscan be distributed aroundthe circum:=-aging the seals,the connecting ';-:-ce or Ine spoornouslng. ^{
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All forms of actuationcan be used with longitudrnal slide valves,i.e manual. mechanrcal. electricalor pneumatic. These types of actuationcan also be used for resettingthe valve to its starting position. The actuationtravel is r;onsiderably largerthanwitlrseai valves.
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Anothermethodof sealingis to use a suspendeddisc seat with relatively small switchingmovement.The disc seat seal connectsthe 1(P) port to eitherthe 2(B) port or the 4(A) port.The secondaryseals on the spool pistonsconnect the exhaustpofts to the outletports.There is a manualoverridebuttonat each end to manuallyoperatethe valvespool.
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Tne 512way doubleair pilot valve has the characteristic of memorycontrol. The last switchedpositronis retaineduntila new switchingpositionis rnitiated oy a uniquepilot signal from the oppositeside to the last signal This new cosrtionis memorised untilanotheruniouesiqnaloccurs
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l,f:unringof rollerlevervalves: 3 . 7 Reliableoperationof ire reliabilityof a controlsequenceis heavilyoependentupon the correct valves 'ittingof the limit valves. For all designsof limit valvesthe mountingmust allowsrmpleadjustmentor readjustment of the limitvalve positionin orderio preciseco-ordination arr-ie','s of the cylindermotionswithina controlsequence. = :: rg of rralves: ioar from a careiulselectionof valves,correctfittingis a furtherprereqursite ':' :eliablesuritching characteristics, trouble-freeoperationand easy accessfor -ep.rirand nraintenance work.This appliesboth to valvesin the powersection in the controlsection. and rral,res vlanually-actuated valvesfor signalinputare generallyfittedon a controlpanel cr control desk. lt is thereforepracticaland convenientto use valves with actuatorsthat can be fitted onto the basic valve Variousactuatorsare availablefor a wide varietyof inputfunctions.
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Whenfittingcontrolvalves,particularcare shouldbe takento provideaccessibilityfor repair,extensionor modification work. Numberingthe components and utilisingvisual indicatorsfor the most importantcontrolsignalsreduces faultfindinganddowntimesconsiderably. Powervalveshavethe task of actuatingpneumaticdrivesin accordance with a specifiedcontrolsequence.A basicrequirement for powervalvesis to alloW rapidreversalof the actuatoronce the controlsignalhas beentriggered.The powervalveshouldthereforebe positionedas closelyas possibleto the actuator in order to keep line lengths,and thus switcfuing times, as short as possible.ldeally,the powervalveshouldbe fitteddirectlyto the drive.An additionaladvantageof this is that connectors, tubingand assemblytime can be saved.
154
Ghapter4 Valves
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-s Valves
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I valves 1.1 Non-return
Non-return valvesare deviceswhichpreferentially stopthe flow in one direction anci permii flow in the oppositedirection. The pressureon the downslream side acts againstthe restrictivecomponent,therebyassistingthe sealingeffect of the valve.
Oheckvalves
Check valves can stop the llow completelyin one direction.In the opposite directionthe flow is free with a minimalpressuredrop due to the resistanceof the valve. Blockingof the one directioncan be effectedby cones,balls,plates or diaphragms. Checkvalve
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Elementsfittedat a three way junctionwhrchhave non-returncharacteristics directthe movementof signalair The two valveshere referredto as junction whrchdeterminethe passageof two input elementshave logrccharacteristics signals The two pressurevalvetequirestwo srgnals(ANDfunction)to produce an outputand the shuttlevalverequiresat leastone signalinput(OR function) to producean output.Theseare processing elementswherebytwo signalsare processedinternally and the resultingsrgnalis outputat portA
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This non-return element Shuttlevalve:OR function has two inletsand one outlet. lf comoressed air is applied to the first inlet,the valveseatseals inlet.A sigthe opposing nal is generatedat the outlet.Whenthe airflow is reversed, i,e. a cylinderor valve is exhausted, the seat remainsin its previously assumed position because of the pressure conditions. This valveis also called lf a an OR comoonent. cylinderor controlvalve is to be actuatedfrom a two or morepositions, shuttlevalve shouldbe USEO .
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I'cuit: shuttlevalveandthedoubleactingcylinder
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valvesare usedto increase Quick-exhaust the pistonspeedof cylinders.This -d - | enableslengthyreturntimesto be avoided.particularly with single-acting cy|inders.T h e p rin c ip |e o f o p e ra t io n is t o a ||o wt h e c y |in d e rt o r e t r a c t o r e x t e n at its nearmaximumspeedby reducingthe resistance to flowof the exhaustingairduringmotionof thecylinder. To reduceresistance, theair is expelled to -J atmosphere via a largeorificeopening. closeto the cylinder The valvehas a (P) and an outlet(A). In this direction supplyconnection of flow the air is -l passedfreelyvia the openingof the checkvalvecomponent. PortR is blocked by the disc
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lf air is suppliedto the (A) port, the disc seals the (P) port and the air is expelledto atmospherethroughthe silencedlarge orifice(R). This increases the potentialspeed of exhaustcomparedwith the exhaustport of a final control valve. lt is advantageous to mount the quick-exhaust valve directlyon the cvlinderor as nearto it as oossible.
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Flow control valves
Throttlevalve,bi-directional
Flowcontrolvalvesinfluencethe volumetricflow of the comoressedair in both directions.lf a check valve is fittedto the flow controlvalve,the influenceof speedcontrolis in one directiononly.The valvecan be fittedas a valveblock in the circuitor attacheddirectlyto the cylinderport. Throttlevalves are normallyadjustableand the settingcan be locked in position. Due to the compressibility of air, the motioncharacteristics of a cylinder vary with load and air pressure.Thereforeflow control valves are used for speed controlof cylinderswithin a range of values. Care must be taken that the flow controlvalve is not closed fully, cutting off air from the system.The open flow settingshouldbe lockedin place.
Throttlevalve
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The throttlingdeviceshown here is commonlyknownas a one way flow control One way flow control valve.With this type of valve,the air flow is throttledin one directiononly.A VAIVE checkvalve blocksthe flow of air in the bypassleg and the air can flow only throughthe regulatedcross-sectionIn the oppositedirection, the air can flow freely through the opened check valve. These valves are used for speed regulationof actuatorsand if possible,should be mounteddirectlyon the cvlinder.
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Supplyairthrottling
Exhaustairthrottling
For supplyair throttling, Supplyair throttling one way flow controlvalves are installedso that the air entering the cylinderis throttled.The exhaustair can escaPe freelythroughthe check valveof the throttlevalve on the outletsideof the 1.02 cylinder. The slightest fluctuationsin the load on the piston rod, such as occur for examPle when passing a limit switch,leadto verYlarge in the feed irregularities air is speedif the suPPlY airthrotSupplY throttled. tling can be used for and small single-acting volumecylinders.
With exhaust air throt- Exhaustair throttling tling,the supplyair llows freelyto the cylinderand the exhaustair is throttled. In.,this case, the pistonis loadedbetween two cushionsof air. The first cushioneffectis the supply pressureto the cylinderand the second cushionis theexhausting at the air beingrestricted one way flow control valve orifice.Arranging throttle relief valves in subthis way contributes stantiallyto the imProvement of feed behaviour. Exhaust air throttling should be used for doubleactingcYlinders.
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Pressurecontrolvalvesare elementswhichpredominarTtly influencethe pres- 4.3 Pressurevalves sureor are controlledby the magnitude of the pressure.Theyare dividedinto thethreegroups: . Pressure regulating valve . Pressure limiting valve . Sequence valve The pressureregulatingvalve is dealt with underthe section"Air Service The role of this unit is to maintainconstantpressureeven with Equipment". fluctuating supply.The inputpressuremustbe greaterthanthe requiredoutput Dressure. Pressureregulating valve
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ih-e pressurelimitingvalvesare usedmainlyas safetyvalves(pressurerelief Pressurelimitingvalve ',ralves).They preventthe maximumpermissible pressurein a systemfrom ceingexceeded.lf the maximumpressurehasbeenreachedat the valveinlet, ine valve outletis openedand the excessair pressureexhauststo atmoser'Iere.The valveremainsopenuntilit is closedby the buih-inspringafter -eaching thepresetsystempressure.
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valve Seouence
The principle on whichthis valveactsis the sameas for the pressurelimiting valve. lf the pressure exceedsthatset on the spring,the valveopens.Theair flowsfrom 1(P)to 2(A). Outlet2(A) is openedonly if a presetpressurehas builtup in pilotline12(Z).A pilotspoolopensthe passage1(P)to 2(A). pressure Adjustable valve sequence
2(A) 1(P) 12(z)
Sequence valvesare installed in pneumatic controlswherea specificpressure (pressure-dependent is required for a switching operation controls).The signal is transmitted pressure onlyafterthe required operating hasbeenreached. Circuit:sequencevalveoperation
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3omponentsof differentcontrolgroupscan be combinedinto the body of one . 4.4 Combinationalvalves and construction of a combination of val-rit with the features.characteristics .:s Theseare referredto as combinational valvesand theirsymbolsrepresent :-e variouscomponentsthat make up the combinedunrt.The followingunits ::r be definedas combinational valves: Timedelayvalves: for the delayof signals Air controlblocks: for reversingor oscillating cycles 5 4 way valve : consisting of four212way valves Air operated8 way valve: two 412way valvecombinations rpulse generator: multr-vibrator cycles ,'acuumgeneratorwith ejector. for pick and placeapplications S:eppermodules: ior sequential controltasks lommand memorymodules: for startupwith signalinputconditions -::Te delayvalveis a combined3l2way valve,one way flow controlvalve Timers =-: ?'reservoir. The 3/2 way valvecan be a valvewith normalpositionopen :- : :sed The delay time is generally0-30 secondsfor both types of valves the time can be extended.An accurateswitch- -s rg additionalreservoirs, : :- : Te is assured,if the air is cleanand the pressurerelatively constant.
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Referringto the previousdiagram,the compressedair is suppliedto the valve at connection1(P). The controlair flows into the valve al 12(Z). lt flows througha one way flow controlvalve and dependingon the settingof the screw,a greateror lesseramountof arrflowsper unit of time intothe throttling air reservoir. When the necessarycontrolBressurehas built up In the air reservoir,the pilot spool of the 3i2 way vaiire is moved downwards. This blocksthe passagefrom 2(A) to 3(R). The valvedisc is liftedfrom its seat and thus air can flow from 1(P)to 2(A). The time requrredfor pressureto buildup in the air reservoiris equalto the controltime delayof the valve. c
lf the time delayvalveis to switchto its initialposition.the pilotline'12(Z)must throughthe be exhausted.The air flowsfrom the air reservoirto atmosphere bypassof the one way flow controlvalve and then to the-exhaustline. The valvespringreturnsthe pilotspooland the valvedisc seat to their initialpositions.Workingl ine2(A)exhauststo 3(R) and 1(P)is blocked
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The normallyopen time delay valve includesa 312way valve which is open Initiallythe output 2(A) is active.When the valve is switchedby 10(Z) the output2(A) is exhausted.The resultis that the outputsignalis turnedoff afte: a sei time delay.
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The circuitbelowutilisestwo time delayvalves,one a normallyclosedvalve (1.5)and the othera normally openvalve(1.4).Uponoperation of the start passesthroughthe valve1.4and initiatesthe button1.2,the signalgenerated movementof cylinderextensionvia the 14(Z)port of the memoryvalve 1.1. The time delayvalve1.4 has a shorttime delayset of 0.5 seconds.This is longenoughto initiatethe startsignalbutthenthe 14(Z)signalis cancelled by the timer 10(Z)pilot signal.The cylinderoperateslimit valve 1.3. The time delayvalve1.5 receivesa pilotsignalwhichafterthe presettime opensthe timer. Thissuppliesthe 12(Y)signalwhichreversesvalve1.1and retractsthe cylinder.The new cyclecan only startif the startbuttonhas been released. resetsthe timer 1.4 by exhausting The releaseof the pushbutton the 10(Z) signal. Circuit:timedelayvalves
169
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Actuatorsand output devices
171
Actuatorsand outputdevices
Festo Didactic
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An actuatoris an outputdevicefor the conversionof supplyenergyinto useful work. The outputsignalis controlledby the controlsystem,and the actuator respondsto the controlsignalsvia the final controlelement.Other types of output devices are used to indicatethe status of the control system or actuators.
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The pneumaticactuatorcan be describedundertwo groups,linearand rotary: .
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Linearmotion - Singleactingcylinders - Doubleactingcylinders Rotarymotion - Air motor - Rotaryactuator
Withsingleactingcylinders compressed air is apptiedon onlyone sideof.the pistonface.The otherside is opento atmosphere.The cylindercan produce workin onlyone direction.The returnmovement of the pistonis effectedby a built-in springor by theapplication of an external force.Thespringforceof ihe built-inspringis designedto returnthe pistonto its start positionwith a reasonably highspeedunderno loadconditions. Singteactingcylinder
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with built-inspring,the strokeis limitedby the For singleactingcylinders naturallengthof the spring.Singleactingcylinders aretherefore onlyavailable in strokelengthsof up to approximately 80 mm. of thesingleactingcylinder makes Theconstruction andsimplicity of operation it particularly suitablefor compact,shortstrokelengthcylinderstor the followingtypesof applications: . . . . .
of workpieces Clamping Cuttingoperations parts Ejecting Pressing operations Feeding andlifting
The singleactingcylinderhas a singlepistonsealwhichis fittedto the air Construction supplyside.Theexhaustair on the pistonrodsideof the cylinderis ventedto atmosphere throughan exhaustport.lf this port is not protectedby a gauze coveror filter,then it is possiblethat the entryof dirt particlesmay damage internal ventwillrestrict seals.Additionally a blocked or stopthe exhausting air andthemotionwillbe jerkyor maystop.Sealingis by a duringforwardmotion, flexiblematerialthat is embeddedin a metalor plasticpiston(Perbunan). Duringmotion, thesealingedgesslideoverthecylinder bearingsurface. Therearevaryingdesigns of singleactingcylinders including:
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. Diaphragm cylinder . Rolling diaphragm cylinder
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' irre examplebelowthe pistonrod of the singleactingcylinderadvances Controlof a singleacting ,r-e^ a buttonis operated.On releaseof the buttonthe pistonreturnsto the cylinder for thisdirectconA normally closed3l2wayvalveis required --.ld position.
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Whenthe 3/2 way valve is actuated,compressed air flows from 1(P) to 2(A) and the exhaust port3(R)is blocked. The cylinderextends. When the button is released the valve return spring operates andthecylinder chamber is exhausted through the 3(R) port with the comoressed air 1(P)blocked, connection The cylinder retracts underspringforce.
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With indirectcontrol of a cylinderthe 3/2 way control valve is piloted by the 312 way pushbutton valve1.2.In thisway the final controlelementcan be of large orifice size. The control valve then matches the cylinder bore size and flow rate requirements.The pushbuttonvalve 1.2 indirectly extends the cylinder 1.0 via the final control element 1.1. When the pushbutton is pressed, the signal 12(Z) pilots the final controlelement to extend the cylinder against spring force. lf the button is released, the signal12(Z)exhausts and the control element returnsto the initialposiretracting the tion cylinder.
lndirect normally control, closedvalve
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In the case of a normally open 3/2 way valve being used, the cylinder is initiallyextendedand when the pushbuttonis operated the cylinder retractsunder the spring force. The final control element 1.1 is not switchedat the rest position.The cylinderis pressurised whilst at rest in the initial position.The circuit is drawn with the valve 1.2 unactuatedand with the cylinderinitially extendeddue to the signal 2(A) being active at valve1.1.
Indirect control, normally openvalve
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j' i- ic iiilrpl rrri i :i p i eo i .i d o rl i ti e a c ti rg cyl i nc-dii s si n-:i i arr to tnat oi i he F { o w e v e r. i h e r= i s rro i e turn spri ng. ai rd ngi c y lir rc re r, the fw o pori s .t1s Tre doi;Dle ilctiirg cylinCei :trrri =.riraltist r ierrtitrveiv srrDDiv*crts i n oui ,ttork both c ;i i i rc l e r i = .rb i rt c:rr;-y di r' ecti cns te - r ' ii' t ir ir e i i i a i l l -re i i ul; , i ri s i a l l a ti o nr-ro s s i b i i i ti easre u i l vei sati fhe i t.,r' i ;e transl erre, s sor:t,l,r'inaiq13aiei io, li-reioi-v,,rtrei siroKe iiran foi the r'etL,ri :-ir- r,.1.-i : , . iiec i i v + o rs i o rrs i i ri i l c e ;s i ' e c u c e rl :rn i i e ri si oi -t i ' cd si de bv i ne ,:yl i i '-il er i s r-j r col i rol of i he - , ii' ai ar e a o i i h e c i s to n i ,f,r: T re -or : - l i t.,,-' l i r.ti :S -ri -::'.-: -],)i :O t. bL.i ci ' .l tnq.rrtd ; : : ia s ii' c i (el e n g th o i l h e i y i i rrc e r i s u ni i i i ri i erl ..rl t!roL:gtr b e al l ow ed i or A s w rth i he si ngi e - ' - ' r e : x t e i rc l e L 1p rs i o r; rc c l -i rti , _- : . . , s e a i i n g i s ' c y me a r-:s o fs tp i s l cns fi tterj w i tl t seal i nE ri ngs oi '
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q u ri e m e n i s - i re n c e i i l e i j se of magnets on pi si ons 3g1 1 1 -i i irlerJ
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- . : - : . - r ' r , ,' tc a ,1 sth ro u g h c l a m o i n g u * i ts and a:< i ernaishock absorbers :, : ae i - ' r;i te i ' 3:D a c e ]s i i ml i e d '. = --,ti'rr.;iifcir-if irtgiliaieri.tis such a:-iplastic a gsa i ri s i n a rs l i + rl v i ro n l l l ents.i e. arcrd-r' esi starrt - ' . - . la. r , 1 :rrrg : : - - a r ad r l a rry i n gc a p a c i ty ' : : ii , i. 1 tro n s' rv ri hs p e c i a l fe a tu te s s Li cn i s i i on-roi ai i ng pi si on rods ' : i' : t . r c o s l o r v a c l u m s u c i to n c u p s
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doubleactingcylinder Cushioned
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-J J J J J ii iarge 1e-.'=s ai'+ moved by a eytinder,cushioirrngis used rn the erc posiii=ns to s'rer,r-=udden danraginginr+acts.Beforereachingthe end pos;iion : :ushioir,,g pislci-:ii:terruptsthe dlieet flow path of th+ air to the cutsiee. nstead a very sn-alland often adjusiableexhaustapertureis open. F6i:hc. l;151 =ai'i of the siicke the cylinderspeeciis progressivelyreduced.lf the pass?Qc adjustmentis toc small,the cylindermay not reachthe end positiondue to rhe biockageof air. ';l";henlhe pistcn ieverses,air ficws wiihout resisianceihroughthe r;tui:. valve ini,r the cylindersoace. With very large forces and high accelerationsextra measuresmusi be taken such as externalshock absorbersto assist the load deceleration. When cushioningadjustmentis being carried out, it is recomn":endedthat in order to avoid damage, the reguiatlngscrew should first be ;eiewed ;, fuii,vafid then back+i off in order to allow the adjustmentto b inilreaqcd Slc,,,lri to the Opt,nUm+aiuc it rs iinporlanl ts .cirsroei trtilne a magner l0 tne cylrnder piston. Once manufacturedthe cylindercannoi rlrmally be fitted with sensor magnetsdue to tne differencein construction.
176
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L-. L. LThe cylinderconsistsof a cylinderbarrel,bearingand end cap, pistonwith seal Construction (double-cup packing),pistonrod, bearingbush,scraperring,connectingoarts and seals. The cylinderbarrelis usuallymadeof seamlessdrav;nsteeltLri:e.To increase the life of the sealingcomponents, the bearingsurfacesof the cylinderbari.el are precision-machined. For specialapplications, the cylinderbarrelcan be made of aluminium,brass or steel tube with hard-chromed bearingsurface. These specialdesignsare used where operationis in{requentcr ,,,;l'ierc; lfr+r'i,r are corroiiveinfluences. The end cap and the bearingcap are, for the most part, made of cast material (aluminiumor malleablecast iron). The two caps can be fastenedto the cylinderbanel by tie rods,threadsor flanges. The piston rod is preferablymade from lreat-treateci steel. l\ r:eriarnt-.rercenlage of chromein the steelprotectsagainstrusting. Generallythe threa.cis arre rolledto reducethe dangerof fracture. A sealingring is fittedin the bearingcap to seal the pistonrod. The bearing bush guidesthe pistonrod and may be made of sinieredbronzeor plasticcoatedmetal. In front of this bearingbush is a scraperring. lt preventsdust and dirt panicles fromenteringthe cylinderspace. Bellowsare thereforenot normailyrequired. The materialsfor the double-cup packingsealsare: Perbunan Viton Teflon O-ringsare normallyusedfor staticsealing.
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O-ring
Shapedring
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Grooveringson both sides
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Cup packing
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Double-cup packing
Supportedgrooveringswith slide ring
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174
Festo Didactic
Actuatorsand oulPut devices
TandemdoubleactingcYlinder
'=:-ired but the cylinderdiameteris restricted. andemdoubleactingcYlinder
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The cylindermountingand the pistonrod couplingmust be matchedcarefully to the relevantapplicationsince cylindersmust be loaded only in the axial direction. As soon as force is transmittedto a machine,stressesoccurat the cylinder. lf shaft mismatchingand misalignmentsare present, bearing stresses at the cylinderbarreland pistonrod can also be expected.The consequences are : . . .
High edge pressureson the cylinderbearingbushes leadingto increased wear Highedgepressures on the pistonrod guidebearings Increasedand unevenstresseson pistonrod sealsand pistonseals.
These stresseslead to a reductionin the servicelife of the cylinderwhich is ofien considerable.The fittingof supportbearingsadjustablein three dimensions makes it possibleto avoid this excessivebearingstresson the cylinder almostcompletely.The onlybendingmomentwhichthen occursis determined by the slidingfrictionin the bearings.This meansthat the cylinderis subjected only to the stress for which it was designed. lt can thereforereach its full designservicelife.
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Circuit:controlof a doubleactingcylinder
The 412way valve or the Control of a doubleactino 512 way valve can be cylinder 412wayvalve used to control the doubleactingcylinder.In both casesthe air is initially suppliedfrom 1(P) to 2(B) and the 4(A) port is exhausted. The cylinder is initiallyheld under pressure in the retractedposition.When the manual valve is operatedthe 4(A) port is active and the 2(B) port is exhausted. The cylinderis extended,and remains extended until the valve is released.ln the 412 way valve a singleexhaustport 3(R) is used.
Circuit:controlof a doubleactingcylinder
In the case of the 5/2 Controlof a doubleacting way valve, the exhaust cylinder5/2 way valve air is separately exhausted to atmosphere from one of the two ports 3(S) or 5(R). lt is more commonfor the 5/2 way valve to be used for the controlof the double actingcylinder.
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To regulatethe speedof the cylinder,the flow controlvalves1.01and 1.02arc fitted to the exhaust side of the cylinder. In this control circuit a 5/2 way memoryvalve is used. The signallingelements1-2 and 1.3 need only operate for a shortdurationto achieveswitching.
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Exhaustair lhrottlingof a doubleactingcylinder
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5.3 Rodlesscylinder
Rodlesscylinder
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This doubleactingpneumaticlinearactuator(cylinderwithoutpistonrod)consists of a cylindricalbarreland rodlesspiston.The pistonin the cylinderis freelymovableaccordingto pneumaticactuation, but thereis no positiveexternal connection. The pistonis fittedwith a set of annularpermanentmagnets. Thus,a magneticcouplingis producedbetweenslideand the piston.As soon as the pistonis movedby compressed air the slidemovessynchronously with it. The machinecomponentto be moved is mountedon the carriage.This designof cylinderis specifically usedfor extremestrokelengthsof up to 10m. featureof the rodlessdesignis the flat bed mountingavailableon An additional the carriageas opposedto the threadedpistonrod type of construction.
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Rodlesscylindercircuitfor positioning
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For the accurate Controlof a rodlesscvlinder positioningof the carriage,the circuitfor the rodless cylinder uses check valves to prevent the carriagefrom creeping. Referringto the circuit, the pushbuttonfor the carriageto move to the rightis the righthand valve 1.2. In this case the valve that exhausts the air controlsthe motion of the cylinder.
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183
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5.4 Cylinderperformance characteristics
Cylinderperformancecharacteristics can be determinedtheoreticallyor by the use of manufacturersdata. Both methodsare acceptable,but in generalthe manufacturers datais morerelevantto a particular designand application.
Pistonforce
The pistonforce exertedby the cylinderis dependentupon the air pressure, the cylinderdiameter,and the frictionalresistance of the sealingcomponents. The theoreticalpistonforceis calculatedby the formula:
Strokelength
Ftn
= A.p
Ftn
= TheoreticalpistonForce(N)
A
= UsefulpistonArea(m2)
p
= OperatingPressure(Pa)
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The strokelengthsof pneumaticcylindersshouldnot be greaterthan 2m and for rodlesscylinders10m. With excessivestrokelengthsthe mechanicalstresson the pistonrod and on the nose bearingwould be too great. To avoid the danger of buckling,a somewhatlarger piston rod diametershould be selected for longer stroke lengths.
Pistonspeed
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The pistonspeedof pneumaticcylindersis dependenton the load,the prevailing air pressure,the lengthof pipe,the cross-sectional area betweenthe final controlelementand the workingelementand also the flow rate throughthe finalcontrolelement. In addition,the speedis influenced by the end position cushioning. The averagepistonspeedof standardcylindersis about0.1-1.5m/sec. With specialcylinders(impactcylinders), speedsof up to 10 m/secare,attained.
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The pistonspeed can be regulatedby one way flow controlvalvesand speed increasedby the use of quickexhaustvalves. For the preparationof the air, and to obtainfacts concerningpowercosts,it is importantto knowthe air consumptionof the system.For a particularoperating pressure,pistondiameter,and stroke,the air consumptionis calculatedby : Air consumption= Compressionratio. Pistonarea . Strokelength ratio = The compression \,
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Axialpiston
Deviceswhichtransform 5.5 pneumaticenergy into mechanical rotaryrnovementwith the possibility of continuous motionare Known as pneumatic motors. The pneumatic motor with unlimited angleof rotationhas become one of the most widelyusedworkingelementsoperating on compressedair. Pneumatic motorSare categorised according to design:
Motors
. Pistonmotors . Sliding-vane motors . Gearmotors . Turbines (highflow)
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Pistonmotors
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-^e workingprinciple of the axialpistonmotoris similarto thatof the radial : ston motor The forcefrom 5 axiallyarrangedcylindersis convertedinto a '::ary motionvia a swashplate.compressedair is appliedto two pistons : :rultaneously, thebalanced torqueproviding smoothrunning of themoior. --ese pneumatic motorsare availablein clockwise or anti-clockwise rotation. -^e maximum speedis around5000rpm,the powerrangeat normalpressure 3 eng 1.5- 19 kW (2 - 25 hp).
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3ecauseof theirsimpleconstruction and the lowweight,slidingvane.motors Slidingvane motors :'e usedfor handtools.Theprinciple of operation is similar to theslidinq-vane I0 r'l'rpressor. Ar eccentric rotoris contained in bearings in a cylindrical chamber.Slotsare arranged in the rotor.Thevanesareguidedin the slotsof the rotorandforced lutwardsagainstthe innerwallof the cylinder by centrifugal force. Thisen_ s"resthattheindividual chambers aresealed. Tre rotorspeedis between3000and g500rpm. Heretoo,clockwise or anti:lockwise unitsareavailable as wellas reversible units. powerranqe0.1- 17 {W (0.1- 2a hfi.
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185
Actuators alid o,ulpu!:devices
Gearmotors
Feqto Didaclic
H::il:J"3:1ilf,:?1,"JH:ii'flHJ*1""3[ ff3s'J,*",:"iJ:$Jli:,';iiil motor shaft. These gear motorsare used in applications with a very high power rating (44 kW60 hp). The directionof rotationis also reversiblewhen spuror helicalgearingis used.
Turbines(flowmotors)
Turbinemotorscan be used only where a low poweris required.The speed range is very high. For example,the Dentists'air drillsoperatesat 500,000 rpm.The workingprincipleis the reverseof the flowcompressor. l
Characteristics of oneumatic motorsare: . . . . . . . . 5.6 Rotary actuators
Smoothregulation of speedand torque Smallsize(weight) Overloadsafe Insensitive to dust,water,heat,cold proof Explosion Largespeedselection Maintenance minimal Direction of rotationeasilvreversed
Rotaryactuator
-
-
Designfeaturesof pneumaticrotaryactuators: . Smalland robust . Precision machinedand henceveryefficient . Availablewithcontactless sensing . Adjustable for angulardisplacement . Constructed fromlightweight material . Easyto install The compactrotaryactuatoris suitedto roboticsand materialshandlingapplications wheretherers limitedspace.
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186
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optical indicatorsvisuallyrepresentthe operatingstatusof the pneumaticsystem and serveas diagnosticaids.
5.7Indicators
Someof the visualdevicesare : . Opticalindicators,singleand multiplecolouredunits . Pin type opticalindicators,for visualdisplayand tactile sensing . Counters,for displayingcountingcycles . Pressuregauges, to indicateair pressurevalues . Timers,with visualindicationof time delay With the opticalindicatorsthe colour codes representcertainfunctionsin the Opticalindicators cycle. The visualindicatorsare mountedon the controlpanelto indicatestatus of controlfunctionsand the sequentialsteps currenilyactive.The coloursfor visualindicatorsin accordancewith VDI/VDE0113/57113 are: Colour
Meaning
Notes
Red
lmmediate danger,alarm
Yellow
Caution
Green
Safety
Blue
Special information
Whiteor Clear
General information
Machinestatusor situations requiring immediate intervention. No entry. Changeor imminentchangeof conditions. Normaloperation,safe situation, free entry. Specialmeaningwhichcannot be madeclearby red,yellowor green. Withoutspecialmeaning. Can also be used in caseswhere there is doubtas to the suitabilityof the threecolours red,yellowor green.
187
Chapter6
Systems
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Systems
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6.1 Selection and comparisonof media
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To selectthe controlmediaconsideration mustbe givento the following: . . . .
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The workor outputrequirements The preferredcontrolmethods The resourcesand expertiseavailableto suppoftthe project The systemscurrentlyinstalledwhich are to be integratedwith the new project
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The total projectmay requiremixturesof mediumboth on the controlside and the work side. Therefore,the interfaceor conversiondevicewill be an important elementof the processto ensurecontinuityand uniformityof signalsand data.
:-\
Firstly, the individualadvantagesand disadvantages of the mediumsavailable must be considered,both as a controlmediumand as a workingmedium. Thenthe selectionscan be developedtowardsa solution. 6.2 Gontrolsystem
development
-
The development of the controlsystemsolutionrequiresthat the problemis definedclearly.Thereare manyways of representing the problemin a descriptive or graphicalform. The methodsof representing the controlprobleminclude: . . . . . . .
-
Positionalsketch Circuitdiagram Displacement-stepdiagram Displacement-timediagram Controldiagram Flowchart Functionchart
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At the fundamentallevel of pneumaticcontrolthe most commonlyused representationsare the positionalsketch,circuitdiagram,.displacement-step diagram andthe displacement-time diagram.
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190
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Festo Didactic
The positionalsketch shows the relationshipbetweenthe actuatorsand the Positionalsketch machinefixture.The actuatorsare shown in the correctorientation.The positional sketch is not normallyto scale and should not be too detailed.The diagramwill be used in conjunctionwith the descriptionof the machineoperation and the motiondiagrams.
sketchexamole Positional
191
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Circuitdiagram
The circuitdiagramshows signalflow, relationshipbetweencomponentsand the air connections.There is no mechanicallayoutrepresentation with the circuitdiagram.
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Circuitdiagram
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The circuitis drawnwith the energyflow from the bottomto the top. The variouslevelsof a circuitincludethe energysource,signalinputs,signal processing, final controlelementand the actuator.The positionof the limit valvesare markedat the actuator. components and linesare identified by the component numbering systemandthe port(way)connection numbers. These allowcrossreference to the components on the actualmachineand makethe circuitreadable.
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Thedisplacement-step diagramis usedfor motionsequences in the pneumatic Displacement-step diagram and hydraulicfields,The dlagramrepresents the operatingsequenceof the is recordedin relationto the sequencestep. lf a actuators;the displacement a numberof actuators, controlsystemincorporates theyareshownin the same way and are drawnone belowthe other.Theirinterrelation can be seenby the steps. comparing Displacement-step diagram
In thiscasetherearetwo cylinders1.0and2.0.In step1 cylinder1.0extends and thencylinder2.0 extendsin step2. Step3 retractscylinder2.0 and step4 rcfractscylinder1.0.Stepnumber5 is equivalent to step1.
193
Systerns
Functionchad
Festo,Didaptic
The flow chartand the functionchartare morecommonin the development of electricaland electronic controlsystems,althoughthe functionchartdoes give a clearpictureof action,and reactionsin pneumatic sequences In this casethe sequenceis describedusingthe cylinderdesignations A and B insteadof 1.0 and 2.0. The extensionsignalis represented as a + and the retractionas a - . The limit valvesare given the same letteras the cylinderin lowercase,with the designation of 0 for the retractedpositionand 1 for the extendedposition.The clampcylinderis extended(A+)and the limitvalvethen operatedis a1. This limital initiatesthe extensionof B cylinder(B+)whichis the rivetingprocess.The rivetingcylinderfully extendsand operatesthe limit b1. The limitb1 initiatesthe retraction of the rivetingcylinder(B-).The limitb0 is then operatedwhich initiatesthe movementof cylinderA unclamping(A-). The full retraction of cylinderA is indicatedby the limita0 and this is the initial conditionrequiredin conjunctionwith the start conditionfor a new cycle to commence.
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Functionchart:rivetingprocess Stort position Clompin
-
Rivetin
Rivetin \rr
Unclompin
6.3 Field systems (actoric)
An importantcomponentin the transferof powerfrom the processorto the linearor rotaryactuatoris the directionalcontrolvalve (DCV). The selectionof the size and type of valve determinesmany of the operatingcharacteristics of the actuator. The development in directional is towards: controlvalves . . . . . .
Sub-baseand manifoldmountingwithcommonsupplyand exhaust Low powerrequirements for pilotor solenoidoperation Multiplefunctionvalves where characteristicsare changedvia wafer and sealvariations plasticand die caslingmethods Materialchangesand in particular Multiplevalvesin singleunitconstruction Mountingof the DCVon the cylinder
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Manifoldmountingof valves
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The manifold mounted valves utilisea common supply port (centre)and exhaust ports (outside). The exhausts can be tubed away separatelyor locally silenced as required. The compact and rigid mounting is suitable for a control cabinetconstruction.
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In pneumatics, specialunit is a term used to describea combinationof ac-
6.4 Specialunits and assemblies
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Rotaryindexingtable
In many manufacturing Rotaryindexingtable processes,it is necessary to performfeed motions in a circularpath. Rotary indexing tables are used for this purpose. The powering device in a rotary indexing table is a pneumatic cylinder in conjunction with an air controlblock whichcontrolsthe movementcycles.
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195
i
Systems
Festo Didactic
Rotaryindexingtableapplication
The rotaryindexingtable can be used for the one-offproductionof machine toolsfor pitchcircledrillingpatterns,shiftholes,gearsand so on. In seriesproductionthe rotaryindexingtable is used on drillingand tapping machinesor on rotarycyclingmachines. lt can also be used for testing,assembling,drilling,riveting,spot welding,punching,in otherwords anywhere whererotarycyclicproduction is required. Pneumatic feedunit
This unitis a gripperfeed unit.lt is usedfor feedingstripmaterialto machines or presses.Forthe mostpart,feedersare usedfor transporting tapesor strips. By usingdifferentclampingand feed grippersit is possibleto clampand feed rods,tubesand also profiledmaterial. Diaphragmcylindersalternately clamp and opento gripthe material. All movements madeby the unit,the feed movements as well as the clamping movementsare controlledby two 412way valves. 196
L
Systems
operation Stripfeederunitin synchronous
The widthof the materialcan be up to 200mm. lf particular attentionis paidto certainvalues (high cyclingspeed,dead weightof the material)a feed accuracyof 0.02to 0.05mmcan be attained. Hydro-pneumatic feedunit
mentioned, Theseunits,likethosepreviously are mainlyusedwherea uniform workingspeedis required. The pneumaticcylinders,hydrauliccheckcylinderand air controlblockform a compactunit. The two cylindersare connectedby a cross-tie.The pneumatic cylinderis retainedas the workingelement.
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When compressed air is appliedto the pneumaticcylinder,the pistonin the hydrauliccheck cylinderis carriedalong with it. This pistondisplacesthe oil througha throttlereliefvalve to the otherside of the piston. The throttlevalve can be adjusted,therebyregulatingthe feed speed. Heretoo, the oil prevents the feedfrombeingunevenwhenthe workingresistance changes. On the returnstroke,the oil can pass quicklyto the otherside of the piston throughthe returnvalve and thus the returnstrokecan also be made in rapid traverse.
\
{ \ \-
An adjustablestop on the piston rod of the hydrauliccheck cylinder also enablesthe forward stroke to be divided into rapid traverse and working feed. The piston is carriedforwardonly when the cross-tiehas moved againstthe stop. The speedof the workingstrokecan be regulateci beiweenapproximately 30 and 6000mm/min. Specialunits are availablewhich also performa workingstrokeon the return movement. A second throttlerelief valve providesthe brakingeffect on the returnmovement.
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SectionC Solutions
s Festo Didactic
R
\ Exercise1: Directcontrol of a double actingcyllnder Problemdefinition
A double acting cylinderis to extendwhen a pushbuttonis operated. Upon releaseof the pushbuttonthe cylinderis to retract.The cylinderis of smallbore (25mmdiameter)requiringa smallflow rateto operateat the correctspeed. Circuitdiagramwith 5/2 way valve
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Knowledge
The controlvalvefor a doubleactingcylindercan be selectedasa4/2 way or a 512way valve.In this case sincethe cylinderhas a smallcapacitythe operation can be directlycontrolledby a pushbuttoncontrolvalvewith springreturn.
-
On operatingthe pushbutton, the air passesthroughthe valvefrom 1(P)to the 4(A) port and extendsthe piston rod. On releaseof the pushbutton,the valve spring returnsthe controlvalve to its initialpositionand the cylinderretracts. Air returnsfromthe cylindervia the exhaustport. -
\ \ \ !
200
i
Sincethe cylinderis the onlyworkingelementor actuatorin the circuit,it is J"sign"t"Oi.O. fne finalcohtrolelementthat extendsthe cylinderis designated1.1.
ll the pushbuttonis pressedfor a very shortperiod,the cylinderonly partlally solutlon extendsandthen retracts,sinCethe springresetsthe Controlvalveas soonas is released.The cylinderand valvewill returnto their initial fre pusnOutton posiiion.Thereforewith this circuit,it is possiblethat the cylindernever To try and achievefull extensionin this case,the pushieachesfull extension. buttonmustbe helddownuntilthecylindermovssfullyforward.
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C\, \ Exercise2: Indirect control of a doubleacting cylinder Problemdefinition
cy.tinder is to extendwhena pushbutton l,lgl9l".acting rs operated.Upon reteaseof the pushbutton the cylinderis to retract.The cylinderis 250mm diameter andconsumes a largeuotur" oi"ii.
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Knowledge
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The valve 1.2 when operatedsuppliesa pilot signal to'the 14(Z) port of the Solution controlvalve 1.1. This generatesa signal at the outlet 4(A) and the cylinder extends until the pushbuttonis released.lf the pushbuttonis releasedthe returnsignal is suppliedfrom the 2(B) port of valve 1.1 and the air is vented fromthe unpressurised sideof the cylindervia the valve1.1exhaustport5(R).
Circuitdiagram
lf the pushbuttonis releasedbefore the cylinderfully extends,the cylinder immediatelyreturnsto the initialposition.Eventhoughindirectcontrolis used, the final controlelementis a singlepilot valve and does not have the characteristicof memory.Thereforethe controlvalve requiresa sustainedsignalfor it to remain operated,Once the pushbuttonis released,this pilot signal is exhaustedthroughlhe 312way valveexhaustport 3(R) and the cylinderretracts.
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s Exercise3: The logic AND function;the two pressure valve
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from a conveyorbelt. lf the productis )r pressesthe pushbutton,the pick_up ensed by a 3/2 way roller lever valve. e cylinder1.0 is to retractto the initial
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Problemdefinition
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Circuitdiagram
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I for the pick-upcylinderto extendis a logicANDfunc- Solutlon Theoperating condition tion betweenthe productsensorand the operatorpushbutton. Thereforeif a two pressure valveis usedto combinethe signalsfromthe sensorand pushbuttonthe logicconditionscan be met.The two pressurevalveis connected betweenthe outlet lines of the two 312way valves.Upon operationof the pushbutton at the X sideof the two pressure valve.This a signalis generated valve.Oncethe partis sensedas signalcannotpassthroughthetwopressure present,the 3/2 way rollervalvegeneratesa secondsignal,thistimeat portY of the two pressurevalve.A signalis passedthroughto pofi A. This signal opelatesthe controlvalvepilotsignal14(Z)againstthe springreturnand the cyfinderextends.The controlvalvecan be a 4/2 way or a 512wayvalveand forthecylinderspeed.lf eitherof the canbe sizedto suittheflowraterequired two signalscreatedby the 312way valvesis removed,the two pressurevalve will releasethe 14(Z)signalbackthroughthe exhaustportof one of the 312 way valves.The returnspringin the controlvalvethen switchesthe control Thecontrolvalveoutlet2(B)is activewiththe outlet valveto the initialposition; 4(A)exhausted to atmosphere andthe cylinderretracts.
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or a A cylinderis usedto transferpartsfroma magazine.lf eithera pushbutton iootpedalis operateci, then the cylinderis to extend.Once the cylinderis fully extended,it is to retractto the initialposition.A 3/2 way rollerlevervalve is to be usedto detectthe full extensionof the cylinder.
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Theshuttlevalveis connected to thejunctionbetween thetwo manual3/2 way Solution vafves.Upon operationof one of the manual312way valves,a signalis generated at the X or Y sideof the shuttlevalve.Thissignalpassesthrough the shuttlevalveand is emittedat A. Thisoperates the controlvalvevia pilot porl14(Z),andthe cylinderextends. A limitvalve1.3sensesthefullextension positionof the cylinder.The pilotsignal2(A)fromvalve1.3pilotsthe 5/2way valveat the 12(Y)port and the cylinderretracts.The signalat port 12(Y)is onlyeffective,if the opposingsignalat port 14(Z)is released.lf bothof the viathe pushbutton valvesareremoved, signalsproduced thentheshuttlevalve will releasethe pilotsignal14(Z)backthroughthe exhaustportof one of the 312way valves. ln otherwordsboththe pushbuttons and the foot pedalmust be inactivefor retraction to occur.The controlvalvecan be a 412wayor a 5/2 way valve and can be sizedto suit the flow rate requiredfor the cylinder speed.
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Exercise 5: Memoryclrcuit and speed control of a cylinder
A doubleactingcylinderis to extendfully when a pushbuttonis operated.The cylinderis not to retractuntil full extensionis reached.Extensionis confirmed by a rollerlevervalve.The cylinderis to continueforwardeven if the pushbutton is releasedbeforefull extensionis reached.The speedof the cylinderis to be adjustablein bothdirectionsof motion.
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Solutions 1. The memorycontrolvalve 1.1 when firstfittedcouldbe in eitherof the two Question1 positions14(Z)or 12(Y),whichmeansthat it is not easyto predictthe position thenthe valve of the valvewhenfitted.lf a manualoverridebuttonis available, shouldbe manuallyset to the 12(Y)positionbeforeturningon the air supplyto ensurethatthe cylinderremainsretracledinitially. 2. For normaloperationof the circuit,the valve1.1 shouldbe initialised to the Question2 12(Y)positionbeloreair is applied.The air can be turnedon and the pushbutton valve 1.2 then operatedto extendthe cylinder1.0. Operationof valve 1.1 producesa signalal 14(Z)of valve 1.1 whichswitchesthe air to the 4(A) port of the controlvalve.At the sametime air is exhaustedfrom the unpressurised side of the cylinder,via the 3(S) port. Once the cylindertravelsto the limit valve1.3,a pilotsignalis sentto the 12(Y)portof the controlvalve.Thissignal will switchthe controlvalveif the pushbutton valveis released.The valve '1.1 then suppliesair to the returnside of the cylindervia port2(B) and the unpresvia port5(R)of valve1.1. surisedsideof the cylinderis exhausted
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The speedof extensionand retraction are controlled by the throttlevalves1.01 The and 1.02and in both casesthe speedcontrolis by exhaustair throttling. valve 1.01 controlsthe returnsoeedand the valve 1.02controlsthe advance speed.The check valvesfitted in the throttleact as by-passvalvesfor the Onlythe exhaustedair is throttled. supplyair to the cylinders.
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3. lf the pushbuttonis held operatedeven afterfull extensionis reached,the Question3 cylinderwill remainextendeduntil the pushbuttonvalve 1.2 is released.The finalcontrolelement1.1 is a memoryvalvewith the characteristic that the last positionwill be retaineduntila uniqueopposingsignalis received.Thereby,if to the valve'1.1, thena signalapplied the signal14(Z)is appliedcontinuously at 12(Y)by the limitvalve1.3cannothaveany effectuntilthepushbutton valve 1.2 is released.
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positionof the cylinder,the Question4 4. lf the ro]lerlevervalveis fittedat the mid-stroke cylinderwill extendup to the limit switchand retract(if the pushbuttonis alheld downthen the cylinder readyreleased).lf the pushbuttonis inadvertently will extendup to the limitswitchand then overrunto the fully extendedposition. In this case the cylinderwill not returnsincethe only signalavailableto returnthe cylinderhas been bypassed(rollervalve 1.3).Thereforeit wouldbe necessaryto manuallyresetthe controlvalve 1.1 and this would retractthe cylinder.A returnstrokeis only possibleif the rollerlever valve is manually ooeratedor withthe heloof the manualoverrideof the finalcontrolelement.
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Exercise6: Thequickexhaustvalve Problemdefinition
The operationof two identicalpushbuttonvalves advancesa formingtool on an edge-foldingdevice. For rapid forward travel, the circuit utilises a quick movementfoldsthe edge of a flat sheet.lf eitherof exhaustvalve.The fonruard the two pushbuttonsare released,the doubleactingcylinderis to returnslowly to the initialposition.
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Initial posltlon: ln the initialstate,the cylinderassumesthe retractedposition. Solution ff both ot the 3/2 way valvesare actuated,a signalis presentat the outputport A of the two pressurevalve 1.6. This reversesthe 5/2 way controlvalve.The passagethrough cylinderextendswith air beingsuppliedvia an unrestricted the one way flow controlvalve 1.03.The actuatortravelsrapidlyto its forward end positionsince the pressurespace on the piston rod side is rapidly exhaustedthroughthe quick exhaustvalve. lf both 3/2 way valves remain actuated,the cylinderremainsin the forwardend position. lf at least one of the two pushbuttonsis released.the actuatoris no longer pressurised, sincethe controlvalvereversesvia the returnspring.The actuator travelsto its initialpositionunderconditionsof restrictedflow (valve1.03)and thereforeat a reducedspeed. The two pressurevalve in combinationwith the hrvopushbuttonvalves does not fulfilthe functionof a safetystart unit for a bendingapplication.In practice, must not be used.lnsteada safetystartcircuitor control this startconfiguration unit mustbe usedto meet localsafetyregulations.
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A plasticcomponentis embossedusing a die poweredby a doubleacting cylinder.The die is to advanceand embossthe plasticwhen a pushbuttonis operated.The returnof the die is to be effectedwhen the cylinderrod has fully extendedto the embossingpositionand the preset pressureis reached.A rollerlimitvalve is to be used to confirmfull extension.The pressurein the pisionchamberis indicatedon the pressuregauge. Circuitdiagram
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The power circuitmust firstlybe initialisedby operatingthe 5/2 way memory valvemanually(viathe manualoverrides) with the air off. The air can then be turnedon.
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The cylinderextendsif the 5/2 way directionalcontrolvalve 1.1 is switchedvia Solution the 14(Z) port by the operationof the pushbuttonvalve 1.2. The plasticcomponentis embossedunderpressureby the die untilthe presetpressureset on the sequencevalve is achieved.The pressureon the advancingside of the cylinderis fed from a junctionto the limitvalve 1.3 and then in seriesto the sequencevalve.The signalport 12(Z)al the sequencevalve acts againstthe presetcompression of the adjustablespring.lf the limitvalve 1.3 is operated due to full extensionof the cylinderand the presetvalueis reached,then the sequencevalveopensfrom 1(P)to 2(A) and sendsa pilotsignalto port 12(Y) of the controlvalve 1.1. lf there is no signalal14(Z), then the memoryvalve switchesand air is suppliedfrom the 2(B) port to retractthe cylinder.At the same time the air in the 4(A) port is exhaustedand the pilot signalat the sequencevalveis thereforerelievedthroughthe exhaustportof the limitvalve. Thereforethe sequencevalvecancelsthe outputsignal2(A) and thus the pilot signal 12(Y).The cylinderretractsto the initialposition.The pilot signalsat 14(Z) and 12(Y)need only to be very short pulsesto effectthe positionof the 512way controlvalve.
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lf the pressureat the sequencevalve pilot line does not reachthe presetlimit of the springadjustment,then the cylinderwill remainextended.lf the cylinder encountersan obstacleor obstructionto the die movementduringextensionto the forwardposition,the cylinderwill not retractsince the sequencevalve is dependentuponthe operationof the limitvalve1.3.
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A doubleactingcylinderis used to presstogethergrued com'ponents. Upon operationof a pushbutton, the.clamping cylind6rexterids trip. u rollerlever valve.once the fullyextendedposiiion-is'reached, "rJ the cylinderisto remaintor a time of r = 6 secondsand then immediatery retractto ttre initiatposition.A new startcycleis only.possible afterthe cylinderhas fullyretracteJand aftera delay of 5 seconds.The cylinderextensionis to be slow and the retraclion adjustable,but relativelvfast.
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Initialiythe cylindershouldbe at the rest positionbut this is dependenton the positionof the 5r2 way varve 1.1. This memory varve must be positioned manuallybeforeair is suppliedto the circuitto ensure that the cylinderwill be retractedinitially.
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The startconditions for the extensionof the doubleactingcylinder1.0 are the Solution of the retractedposition(rollerlimitvalve'1.4)a delayof five acknowledgement secondsafterthe end of cycledue to timer 1.6 and the operationof the staft buttonvalve 1.2. The outputsignalA at the two pressurevalve 1.8 pilotsthe 512way memoryvalve at 1aQ). The signal4(A) extendsthe cylinderat a preset speed via the flow controlvalve 1.02 (exhaustthrottling).The limit switch 1.4 is deactivatedand thereforeqven if the start button is still held down,the signalat 14(Z)is exhaustedby the removalof the limitswitchsignal, whichresetsthe timer1.6untilthe cylinderhas retracted again. The cylinderreachesthe limitvalve1.3and producesa pilotsignalfor the time delayvalve 1.5. The time delayvalveis normallyclosedand only opensport 2(A) if the presettime, as determinedby the adjustablethrottle,is reached. The air reservoirin the time delayvalvefills,and a pressureis reachedthat is sufficientto operatethe timer againstthe spring return.A pilot signal is produced6 secondsafterthe limitvalve1.3 is operatedand that signalis then sentto the 5/2 way valveport 12(Y).The 512way valveswitchesto the initial positionwith 2(B)activeand 4(A)exhausted. The air to the cylinderis supplied to the returnside and the speedcontrolledby the valve 1.01. The rollerlimit 1.3 is deactivated and the pilotsignalto the timer'1.5is cut-off,therebyremoving the 12(Y)signalfrom the 5l2way valve.The cylinderretractsto the limit valve 1.4.A new startsignalcan only occurif the rollerof valve1.4 is aciive. The timer 1.6 activatesafter 5 secondsand the start buttonmust be pressed for a new cycleto commence.
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List of standards and references
217
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Fluidpowersystemsandcomponents; graphicsymbols
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for controlsystems Terminology
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concepts Controltechnology,
tso 5599
fluidpower;S-portdirectional Pneumatic controlvalvemountingsurfaces;general
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N. andMeixner, H.: Bissinger,
SimpleConlroland LogicCircuits, 1978 FestoDidactic,Esslingen,
References
B.: Bocksnick,
Fundamentals of ControlTechnology, FestoDidactic,Esslingen1988
Deppert,W. and Stoll,K.:
Pneumatics in Control,VogelVerlag,1985
W. andStoll,K.: Deppert,
'Pneumatics Applications, VogelVerlag, 1983
W. andStoll,K.: Deppert,
Pneumatics in Packaging, VogelVerlag, 1983
W. andStoll,K.: Deppert,
Pneumatics in Woodworking, VogelVerlag, 1979
FestoKG.:
TechnicalInformation. Pneumatics
Hasebrink and KoblerR.:
Fundamentals of Pneumatic Control Engineering, FestoDidactic, Esslingen, 1979
Meixner,H. and KoblerR.:
Introduction to Pneumatics, FestoDidactic, Esslingen, 1979
H. and KoblerR.: Meixner,
Serviceand Maintenance of Pneumatic Systems, Esslingen, 1984
219
lndex
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2l2wayvalve .....;....83.2 3/2wayvalve ....83.3 4l2wayvalve ....83.4 4/3wayvalve ....83.5 S /2wayvalve ....83.6 A bsorptiondrying .B2.3 A dsorptiondryers ,B2.3 A ctoric ....86.3 A ctuatingdevice ..81.3 A ctuators ..A2.5 Advantageouscharacteristics of pneumatics .... A 1.1 Aircompressor .. .8 2.1 A irdistribution... ..A2.2.82.5 A irdryeis ..82.3 A irfilter . . . .A 2.2.82.4 A irlubricator .... ..A2.2.B 2.4 Airmotor . . .A 3.1.B 5.5 A irreceiver ......82.2 Airservice equipment . . . .8 2.4 A irsupply ....A2.2,82 A mplifier ... B 1.3 A naloguecontrol ..81.3 A nalysis of problem ...... A4.4 A pplications .,......A 1 A synchronouscon t ro l .... B 1.3 A uxiliaryenerE y ... B 1.3 B inarycontrol ....81.3 Cascade ..A6.1 Cylinderperforma n c e c h a ra c t e. .ris . t ic s .85.4 Characteristics of air .....81.2 Checkvalves ......A3.1, B 4.1 Circuit diagram . . . . . A 4 . 1 ,A 4 . 3 ,A 7 . 1 , B 6 . 2 Clampingdevices .A3.2 Combinationalval v; .e. .s. . .....A 2.3.84.4 Comparison of mediums . . .A 1.1,B 6.1 Components.... ...A2 Compressor ......82.1 Compressedairreg u la t o rs . . . . ..82.4 Condensate .. A7.82.5 Oontrolcircuits- Systems Controldiagram ...86.2 Control mediumcrit e ria ... A 1.1 Controlsystemdev e lo p me n t . . . . . .. A4, B 6.2 Controltheory ....B1.3 Coordinated motion .. A6.86.2 Derivedelements .A3.1 Derivedquantities . B 1.1 Designprocess ...A4.4 Designating theelements.,, . Desigriations.... ,A3.1 Dewpoint .8 2.4 Diagnostics ........A7 Diaphragm . B 2.4 Diaphragm . . .8 2.1 compressor
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..84.2 ...... D i ap hragmvalve. . . . . . . B 1.3 Digitalcontrol ....:. ..... A5.1 apneumaticcyli n d e r D i r ectcontrolof .... A2.3,4 3.1,83.1 D i r ectionalcontrolvalves ...83.3 D i scseat A 6.2,B 6.2 stepdiagram Displacement ,... A 1.1 D i stinguishingfeatures ...47.1,86.2 D ocu mentation... . . . 8 4 . 1, 4 . 2 , 8 4 . 3 ,B 4 . 4 , 8 5 . 2 Do u b feacting cylinder ..85.2 D ou b lepistonrod . . .B 2 Drycompressed air .,...82.3 Dr ye r s ..A3.2 Em e r gencystop.. ....:. ....85.2 En d p ositioncushioning .... A3.2 En vir onmentalpollution . A 3.1,B 2 Energy 1:Directcontrol of asinglea c t in g c y lin d e r ..... A5.2 Exa m ple . . . . A 5.5 Example2: Indirect controlof a singleactingcylinder valve . . . . . A 5.8 Example3: The logicANDfunction, the two pressure the shuttlevalve . . . . A 5.10 Example4: The logicOB function, Exa m ple 5: Memorycircuitandsp e e d c oonftarocly lin d e r. . . . . A5.12 control;embossing of components. . . A 5.15 Example7: Pressure dependent Example8: Thetimedelayvalve . A 5J7 Exe r cise 1: Directcontrol of adoub lea c t in gc y lin d e r ..... A5.3 2: lndirectcontrolof a doubleactingcylinder Exercise . . . A 5.6 valve . Exercise 3: The logicANDfunction; the twopressure . . . . A 5.9 4: The logicOR function; Exercise the shuttlevalve . . ,.. A 5.11 Exercise 5: Memofu circuitandspeedcontrol of a cylinder ...'.. A s.tg Exe r cise 6: Thequickexhaustva lv e ..... A5.14 7: Pressure Exercise dependent control;embossing of components . . . . A 5.16 Exercise8: Thetimedelayvalve . A 5.18 Exh a ustsilencers ... A3.2 ..44.4 Evalu ation Fa ilu re of control ... A3.2 Fa u ltfinding ...47 Filter s ......82.4 . . . .8 3.7 Fittingof valves . .8 2.1 Ffowcompressors ...... A3.1 Flo wcontrol elements . .A 2.3, B 4.2 Flowcontrolvalves ..82.5 Flo wr esistances. ...85.6 Ge a r motors..:. ......83.3 ld ler ollerlevervalve ....44.4 l m ple mentation ...85.7 Ind icators A 5.4,B 3.3 Indirect control ..A5.4 Ind ir ectcontrol of acylinder.... ...81.3 Inp u telement... ....A3.1 tso5 5 99 ..84.1 Ju n ctionelements ,.. A3.1 Le tte ringsystem .... A3.1 L in e a ractuators ....,,..B5 L in e a rmotion ..81.3 L o o iccontrol....
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...83.6 Longitudinalslidevalv e ...82.4.87 Lubrication ..A7.3,82.4 Maintenanc€..; ......A 7.1,47.2 Malfunctions Manifoldmounting of valves Memorycontrolsystem ..... A3.1 Methodsof actuation ..86.3 Modularsystems ....85.5 Motors . . B 3.7 Mounting of rollervalves .... B 5.2 Mountingof cylinders . A 6.1 Multipleactuators .A3,2 Noisepollution... Non-returnelements ..... A3.1 valves Non-return .. A2.3,A 3.1,B 4.1 ... Ag.2 Oil mist .....82.4 Oil removal . A3.2 Operationalsafety ..85.7 Opticalindicators ...... B2.1 Optimumpressures .82.4 Over-lubrication.. P ilotcontrolsystem ..:... ......81.3 P ipematerial .... .82.5 Pipelayout .8 2.5 : P istonforce. .....85.4 P istonmotors ....85.5 P lasticpipes .....82.5 P neumatics ..... A'1.2 andcontrolsy s t edm e v e lo p me n. .t. . P neumatic elements . ... .42.1 P neumatics in review .... A 1.1 P oppetvalves ....83.1' P ositionalsketch ...... ........86.2 P owervalves ...... .....83.7 P ressure . . . . . A 2 . 3 ,A 3 . 1 ,B 2 . 4 , 8 4 . 3 controlvalves .....82 P ressurelevel . .42.2 Pressure regulator . B 2.4 Pressureregulatorwith/without venthole P ressuresequencevalv e . ......A2.3 ..81.1 P rinciplesof air. V alvesandlog ice le me n t s ...:.. ..42.4 P rocessors: ......A1.2 P roductdevelopment. . . . Pushbutton .A 2.6,A 3.1 ....84.1 Quickexhaustvalve. Reciprocatingcompres s o r. . . . ..'.......82.1 .....83.7 Reliableoperation of valve s .82.2 i. . . . . Reservoirs .84.2 ....:. Resfiictors .8 2.5 Ringmain ..86.2 Rivetingprocess ..85.3 Rodlesscylinder Rollerlevervalve . . .A 3.1,B 5.6 Rotaryactuators ,..:.. ....B6.4 Rotaryindexingtable .,....B5 Rotarymotion
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rdgulator Settingpressure Sh iftr egister..... valves Sh u t- off Sh u ttleORvalve Sig n a lconverter Sig n a lflow. Sig n a loverlap Singleactingcylinder Sizin g pipe. Slid e valves Slid in g-vanemotors...:.. Sp e cialdesignsandassemblies . . Stan d ards Stati coutputdevices Str i pfeeder Structure Supplyair throttling Syn ch ronouscontrol Sym b ols anddescriptions of compon e n t s forworking System characteristics medium .i.... Systemupgrade Systerns:Controlcircuits Systems:Hardware Tandemdoubleacting,cylinder ... Thr o ttfevalve,..:.... Timedelayvalvd . Ti m e- dependentsequence... i. . . . . Timers Trouble-shooting in pneumatic systems Tu r b in emotors Two p r essurevalve:A NDfunctio ,. . . .n. . ; . . U tilisationfactor. Ve n tho le Wayvalve.s
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