Basic Symbols Line Continuous line (flow line (flow line)
Long dashes (flow dashes (flow line) Short dashes (flow dashes (flow line) Double mechanical connections(shafts,lev connections (shafts,lev Long chain thin Enclosure thin Enclosure for several components
Circle,Semi-Circle
Circle As Circle As a rule, energy conversion units(pump, compress Circle Measuring Circle Measuring instruments Circle Non-return Circle Non-return link, roller etc. Circle Mechanical Circle Mechanical link, roller etc. Circle Semi-rotary Circle Semi-rotary actuator
Square, Rectangle
Squares As Squares As a rule, control valves (valve) except for non-re
Diamond
Diamond Conditioning Diamond Conditioning apparatus(filter, separator, lubrica exchanger)
Miscellaneous Symbols
Flow line Flow line connection Spring Restriction (affected Restriction (affected by viscosity) Restriction (unaffected Restriction (unaffected by viscosity)
Functional Symbols
Triangle
Solid Direction Solid Direction of Hydraulic Fluid In outline only Direction only Direction of Pnematic flow or exhaust to atmospher
Arrow
Line Indication Line Indication of direction Curve Indication Curve Indication of rotation Other lines -path lines -path and direction of flow through valves(as a general rule the line perpendicular to the head of the arrow indicates
that when the arrow moves, the interior pathe always remains connected to the corresponding exterior path.) Sloping arrow -indication of the possibility of the regulation or a progressive(proportional)variability.
Pumps And Compressors
(convert mechanical energy into hydraulic or pneumatic energy.)
Fixed Capacity Hydraulic Pump
One direction of flow Two directions of flow Variable Displacement Hydraulic Pump
One direction of flow (uses flow (uses sloping arrow symbol) Two directions of flow (uses flow (uses sloping arrow symbol) Fixed Capacity Compressor
Always one Always one direction of flow
Motors
(convert hydraulic or pneumatic energy into rotary mechanical energy.)
Fixed Capacity Hydraulic Motor
One direction of flow Two directions of flow Variable Displacement Hydraulic Motor
One direction of flow (uses sloping arrow symbol) Two directions of flow (uses sloping arrow symbol) Fixed Displacement Pneumatic Motor
One direction of flow Two directions of flow Variable Displacement Pneumatic Motor
One direction of flow (uses sloping arrow symbol) Two directions of flow (uses sloping arrow symbol) Oscillating Motor
Hydraulic Pneumatic
Pump/Motor Units
(unit with two functions,either as a pump or as a rotary motor.)
Fixed Displacement Pump/Motor Unit
Reversal of direction of flow functions as pump or motor according to direction of flow Single direction of flow functions as pump or motor without change of direction of flow Two directions of flow functions as pump or motor with either direction of flow Variable Displacement Pump/Motor Unit
Reversal of direction of flow (uses sloping arrow symbol) Single direction of flow (uses sloping arrow symbol) Two directions of flow (uses sloping arrow symbol)
Variable Speed Drive Units
Torque converter Pump and/or motor are variable capacity. (relate to remote drive)
Cylinders
(equipment to convert hydraulic or pneumatic energy into l inear energy.)
Single Acting Cylinder
Returned by unspecified force cylinder in which the fluid pressure always acts in one and the same direction on the extension stroke. Returned by spring cylinder in which the fluid pressure always acts in one and the same direction on the extension stroke.(uses spring symbol) Double Acting Cylinder
Single piston rod cylinder in which the pressure fluid operates alternately in both directions(extend and retract strokes) Double ended piston rod cylinder in which the pressure fluid operates alternately in both directions(extend and retract strokes) Differential Cylinder
The action is dependent between the effective areas on each side of the piston. Cylinder With Cushion
Single fixed cushonCylinder incorporating fixed cushion acting in one direction only Double fixed cushionCylinder with fixed cushion acting in both directions Single adjustable cushion(uses slope arrow symbol)Cylinder incorporating adjustable cushion acting in one direction only Double adjustable cushion(uses slope arrow symbol)Cylinder incorporating adjustable cushions acting in both directions Telescopic Cylinder
Single actingfluid pressure always acts in one direction(on extend stroke) Double acting fluid pressure operates alternately in both directions(on extend and retract strokes)
Pressure Intensifiers
(equipment transforming a pressure x into a higher pressure y.)
One type of fluid ex.- a pneumatic pressure x is transformed into a higher pneumatic pressure y. Two types of fluid ex.- a pneumatic pressure x is transformed into a higher hydraulic pressure y.
Air-Oil Actuator
Equal Power tranferer equipment to transform a pnuematic pressure into a equal hydraulic pressure (or vice versa).
Control Valves
General Representation
Single square indicates a unit that controls flow or pressure (having an infinite number of possible positions that would vary the conditions of flow across one or more of its ports, ensuring the chosen pressure and/or flow depending on the operating conditions of the circuit) Two squares indicates a directional control valve with two positions (would show flow paths through ports in actual application) Three squares indicates a directional control valve with three positions (would show flow paths through ports in actual application)
Simplified symbol for valves in repetitionwould refer to a note on the diagram where symbol in full is shown
Directional Control Valves
Units that provide full or restricted flow by opening or closing of on or more paths(one or several squares)
Flow Paths
One flow path Two closed ports Two flow paths Two flow paths and one closed port Two flow paths with cross connection One flow path in a bypass position, two closed ports Non-Throttling Directional Control Valve
Two position would have distinct circuit conditions shown in each square Three position would have distinct circuit conditions shown in each square Transitory position significant position passed through while switching positions optionally shown by dashed lines Directional Control Valve 2/2
Manual control directional control valve with 2 ports and 2 distinct postions.
Controlled by pressure operating against a spring (ex. air unloadin valve)directional control valve with 2 ports and 2 distinct postions. Directional Control Valve 3/2
Controlled by pressure in both directions directional control valve with 3 ports and 2 distinct postions. Controlled by solenoid with spring return (showing transitory position)directional control valve with 3 ports and 2 distinct postions. Directional Control Valve 4/2
Detailed Pic- directional control valve with 4 ports and 2 distinct postions controlled by pressure in both directions by means of pilot valve (with single solenoid and spring return) Simplified Pic- directional control valve with 4 ports and 2 distinct postions controlled by pressure in both directions by means of pilot valve (with single solenoid and spring return) Directional Control Valve 5/2
Controlled by pressure in both directions- directional control valve with 5 ports and 2 distinct postions Throttling Directional Control
Two extreme positions- unit has 2 extreme positions and an infinite number of intermediate conditions with varying degrees of throttle) Shows the extreme positions and a central(neutral) positionTwo ports(one throttling orifice) ex. -tracer valve plunger operated against a return spring Three ports(two throttling orifices) ex. -directional control valve controlled by pressure against a spring return
Four ports(four throttling orifices)- ex. --tracer valve plunger operated against a return spring Electro-Hydraulic Servo Valve
Single-stage direct operation unit which accepts an analog signal and provides a similar analog fluid power output Two-stage with mechnical feedback indirect pilot operation unit which accepts an analog signal and provides a similar analog flui power output Two-stage with hydraulic feedback indirect pilot operation unit which accepts an analog signal and provides a similar analog flui power output
Non-Return Valves, Shuttle Valves, Rapid Exhaust Valves
Non-Return Valve
Free opens if the inlet pressure is higher than the outlet pressure Spring loaded opens if the inlet pressure is greater than the outlet pressure plus the spring pressure Pilot signal closes the valve
Pilot signal opens the valve With restriction unit allowing free flow in one direction but restricted flow in the other Shuttle Valve
The inlet port connected to the higher pressure is automatically connected to the outlet port while the other inlet port is closed. Rapid Exhaust Valve
When the inlet port is unloaded the outlet port is freely exhausted
Pressure Control Valves
Pressure Control Valve
One normally closed throttling orifice (general symbol) One normally open throttling orifice (general symbol) Two normally closed throttling orifices (general symbol) Pressure Relief Valve(Safety Valve)
Inlet pressure is controlled by opening the exhaust port to the reservoir or to atmosphere against an opposing force (for example a spring). With remote pilot control the pressure at the inlet port is limited to the setting of a pilot control Proportional Pressure Relief
Inlet pressure is limited to a value proportional to the pilot pressure
Sequence Valve
When the inlet pressure overcomes the opposing force of the spring, the valve opens permitting flow from the outlet port Pressure Regulator Or Reducing Valve
A unit which , with a pressure variable inlet pressure, gives substantially constant output pressure provided that the inlet pressure remains higher than the required outlet pressure
Without relief port Without relief port with remote control but the outlet pressure is dependent on the pressure control With relief port With relief port with remote control but the outlet pressure is dependent on the pressure control Differential Pressure Regulator
The outlet pressure is reduced by a fixed amount with respect to the inlet pressure Proportional Pressure Regulator
The outlet pressure is reduced by a fixed ratio with respect to the inlet pressure
Flow Control Valves
Throttle Valve
Simplified symbol (doesn't show the control method or state of valve) With manual control detailed(shows control method) With mechanical control against a spring return detailed(shows control method) ex.-(braking valve) Flow Control Valve
With fixed output (variations in inlet pressure do not affect rate of flow) With fixed output and relief port to reservoir with relief for excess flow (variations in inlet pressure do not affect rate of flow) With variable output
With variable output and relief port to reservoir Flow Dividing Valve
The flow is divided into two flow in a fixed ratio substanially independent or pressure variations
Shut-Off Valve
Simplified symbol
Energy Transmission And Conditioning
Sources Of Energy
Pressure source Simplified general symbol Hydraulic pressure source Simplified general symbol (to be used when the nature of the source should be indicated) Pneumatic pressure source Simplified general symbol (to be used when the nature of the source should be indicated) Electric motor Heat Engine
Flow Lines And Connections
Flow Lines And Connections
Working line, return line and feed line Pilot control line Drain or bleed line Flexible pipe flexible hose,usually connecting moving parts Electric line Pipeline Junction
Connected Crossed Piplines
Not connected Air Bleed
Exhaust Port
Plain with no provision for connection Threaded for connection Power Take-Off
Plugged on equipment or lines, for energy take-off or measurement With take-off line on equipment or lines, for energy takeoff or measurement Quick-Acting Coupling
Connected, without mechanically opened non-return valve Connected, with mechanically opened non-return valve Uncoupled, with open end Uncoupled, closed by free non-return valve Rotary Connection
One way line junction allowing angular movement in service Three way line junction allowing angular movement in service
Silencer
Reservoirs
Reservoir Open To Atmosphere
General symbol With inlet pipe above fluid level With inlet pipe below fluid level With a header line Pressurized Reservoir
Accumulators
The fluid is maintained under pressure by a spring, weight or compressed gas (air, nitrogen, etc.)
Filters, Water Traps, Lubricators And Miscellaneous Apparatus
Filter Or Strainer
Water Trap
With manual control drain
Automatically drained Filter With Water Trap
With manual control drain
Automatically drained Air Dryer
A unit drying air(for example by chemical means) Lubricator
Lubricates equipmentsmall quantities of oil are added to the air passing through the unit, in order to lubricate equipment receiving the air Conditioning Unit
Detailed consisting of filter, pressure regulator gage and lubricator Simplified consisting of filter, pressure regulator gage and lubricator
Heat Exchangers
Temperature Controller
Maintains temperature the fluid temperature is maintained between two predetermined values. The arrows indicate that heat may be introduced or dissipated. Cooler
Not showing coolant flow the arrows indicate the extraction of heat Showing coolant flow lines the arrows indicate the extraction of heat Heater
Arrow in diamond indicate the introduction of heat
Control Mechanisms
Mechanical Components
Detent a device for maintaining a given postion Locking Device (*) is the symbol for unlocking control Over-center device prevents the mechanism for stopping in the dead center position
Rotating Shaft
In one direction(arrows indicate direction) In either direction(arrows indicate direction) Pivoting Devices
Simple With traversing lever With fixed fulcrum
Control Methods
Muscular Control
General symbol (without showing the control type) Controlled by pushbutton Controlled by lever Controlled by pedal Mechanical Control
Controlled by plunger or tracer Controlled by spring Controlled by roller
Controlled by roller(one direction only) Electrical Control
Controlled by Solenoid (the one winding) Controlled by Solenoid (the two winding operating in opposite directions) Controlled by Solenoid (the two winding operating in a variable way progressively, operating in opposite direction) Controlled by Electric Motor Direct Acting Control
Controlled by application of pressure Controlled by release of pressure Controlled by different control areas (Larger rectangle represents the larger control area) Indirect Control, Pilot Actuated
Controlled by application of pressure Controlled by release of pressure Controlled by Interior control path (control paths are inside unit) Combined Control
Controlled by solenoid and pilot directional valve pilot directional valve is actuated by solenoid Controlled by solenoid or pilot directional valve either may actuat the control independently
HYDRAULIC AND PNEUMATIC CONTROL FLUID POWER GRAPHIC SYMBOLS The standard icons to graphically represent fluid power components are defined in the Australian standard AS 1101.1-1993 Graphic symbols for general engineering - Hydraulic and pneumatic systems. The following are some of the commonly used components:
The valve designation is as follows: p/n DCV = Directional Control Valve with n settings and p ports. It can be Normally Open (NO) or Normally Close (CO). The port numbers on DCVs have the following standard designation:
Port Number
Designation
1
Pressure port (coming from the pump)
3
Exhaust port (back to the tank)
2, 4
Output ports
VARIABLE FLOW VALVES
The great of majority of the valves used in fluid power applications are discrete valves as represented by the above valve symbols. The opening area for these valves are constant and can be either open or closed. In variable flow valves, the flow area is a function of the spool position. By adjusting the spool position, one can adjust the flow rate.
The flow rate obviously depends on the flow area as well as the pressure difference across the valve. Only the flow area can be controlled by the spool position. Therefore, if precise control of the flow rate is essential, then a control loop has to be implemented that modulates the spool position to keep the flow rate constant. Such valves are called the servovalves. Valve selection The choice of the valve is an important consideration in any fluid control application. This choice is a usually compromise between the control requirements and the system cost. In difficult environments (eg mining or construction industry), reliability also is a factor. Best control performance is usually offered by servo valves due to their good linearity and high bandwidth characteristics. However, these valves have a very low tolerance against contaminants in the hydraulic oil. The standard of cleanliness required by a servovalve may be difficult to achieve and maintain in "dirty" environments. In some instances, capacity may also be a problem. For large machines, it simply is not possible to find a servovalve with the required flow capacity. Regular directional control valves can be made extremely robust and are available at the desired size and they should be the first option for jobs where control requirements are not too demanding. Solenoid-driven proportional directional valves (SDPDV) provide a reasonable trade-off. SDPDVs are relatively new additions to control world. They were introduced in the 1980s and they have become very popular since then due to their simplicity and cost effectiveness. Their severe nonlinearity (in comparison with the servovalves), however, may offer a significant challenge for the control designer. EXAMPLES OF FLUID POWER CONTROL We will now provide some examples of how the fluid power components are connected and controlled to perform simple tasks. The examples all use simple DCVs. The treatment of proportional or servo-valves are beyond the scope of this course.
Control of a single-actuating ram Example: Draw the hydraulic circuit and the electrical diagram for a hydraulic ram. The piston is to be extended when a manual switch is closed. The piston should return back when the switch is released. Use a single-acting cylinder and a 3/2 valve. Answer :
Example: A hydraulic press is controlled by two manual switches placed 1 m apart (switches S1 and S2) and a third switch S3 representing the status of the protective cover. The switch S3 is automatically closed when the protective cover is in place. The press is to be activated whenever (a) both S1 and S2 are ON; or (b) either of the S1 or S2 and S3 are ON. Once the press is activated, it will stay down until a normally closed lift button switch (S4) is released and breaks the circuit. Design a hydraulic circuit and the electrical logic to drive this hydraulic press. Answer: There is a deliberate mistake in the PLC diagram. See if you can spot it.
Notes: 1. The symbol S1 appears twice on the PLC diagram. It refers to signals from the same switch on the hydraulic circuit. 2. ditto for S2. 3. K is the relay that engages the latch switch. The latch switch provides the signal for the solenoid relay X even if S1, S2, and S3 are broken. 4. S4 is the STOP button. When it is broken, K is to deenergise; the latch switch opened; the solenoid relay deenergised and, under the spring action, the valve returns to its normal position, causing the ram to retract. Is this happening in the above diagram? If not, how can you correct it? Regenerative Circuit
Extension at normal speed (piston moves to right)
Rapid retraction (piston moving to le
Centre position: The piston does not move. Right position: Retraction. The piston speed is given by
V pist = Q pump /(A pist - Arod ) where Vpist Piston Speed, m/s Qpump Pump Flow rate, m3/s Apist Piston area, m2 Arod Rod area, m2 Left Position: Rapid Extension Apist Vpist = Qpump + (Apist - Arod) Vpist
==> V pist = Q pump /Arod
A small rod area leads to very rapid extension.
OIL HEATING In a hydraulic system, the power is transmitted by pushing the working fluid (usually oil) through the circuit. During this process, some of the power is spent in heating the oil. For example, when oil is pushed through a valve, no external work is done but the pump still has to exert effort to push the oil through the pressure differential over the valve. You can think of this as work done on the oil and all of it is converted into heat:
The work is done on the oil is given by
This is converted into heat and the resultant temperature increase for the oil volume can be calculated by
Automated systems Course Content : • Introduction, Types of systems - mechanical, ele ctrical, electronics, fluidics; • Hydraulics Systems and components; • Pneumatic Systems Control;
• Sequence operation of more than two cylinders and motors; • Applications of relays/switches; • Measuring systems, Transducers; • Feed back control systems; • Programmable controllers; • Automatic orientation and assembly; • Design of components for assembly. • Cost considerations and case studies. • Laboratory work will be hands-on design and operation of automatic systems.
- Pneumatic Controls - Electropneumatic Controls. - Programmable Logic Controller (PLC) Books & Reference Material : Books : • Fluid Power with Applications.
- Anthony Esposito, Prentice Hall. • Fluid Power – Theory and Applications.
- James A. Sullivan, Prentice Hall.
Slides : • www.iitd.ac.in/~suniljha Manufacturing : • Latin Words : Manus (hand), Factus (make). • Manufacturing started by hand -> Factories. • Production System : rely on many separate but interacting functions. • Production systems automated & computerized. • Approaches & Technologies :
- Automation. - Material Handling Technologies.
- Manufacturing Systems. - Flexible Manufacturing. - Computer integrated manufacturing(CAD, CAM). Production Systems : • Production system is a collection of : - People. - Equipment. - Procedures to perform manufacturing. • Production divided into two levels :
(a) Facilities : - Factory, Equipment, Layout. (b) Manufacturing support systems : - Set of procedures used by the company to manage production. Facilities : • Manual Work Systems. • Worker-Machine Systems. • Automated Systems :
(a) System in which a process is performed by a machine without the direct participation of a human worker. (b) Two Levels : 1.Semi Automated : perform a portion of work cycle 2.Fully Automated : operate for extended period without human attention. Manufacturing Support Systems : • To operate the production facilities efficiently : - Design the processes and Equipments. - Plan and control production orders. - Satisfy product quality requirement. • Support systems do not directly contact t he product, but they plan and control its progress through
the factory. • Manufacturing support involves a cycle of information processing
activities.
Automation
in Production Systems : • Two levels : 1. Automation in Manufacturing Systems. 2. Computerization of the Manufacturing Support Systems. • In modern production system, two levels overlap. • Computer Integrated Manufacturing (CIM) indicates use of computers at 2 levels. Automated
Manufacturing Systems : • Operates in the factory on the physical product. • Automated operations : Processing, Assembly, Inspection, Material Handling. • Automated: Reduced level of Human intervention. • 3 Basic Types :
1. Fixed Automation. 2. Programmable Automation. 3. Flexible Automation. Human & Machines : • Relative Strength of Humans & Machines. • HUMANS :
1.Sense unexpected Stimuli. 2.Develop new solutions to problems. 3.Adapt to changes.
4.Cope with abstract problems. 5.Generalize from observations. 6.Learn from experience. 7.Make difficult decisions based on incomplete data. • MACHINES :
1.Perform repetitive tasks. 2.Store large amount of data. 3.Retrieve data from memory. 4.Perform multitasks simultaneously. 5.Apply high forces & power. 6.Perform simple computations quickly. 7.Make routine decisions quickly. Fixed Automation : • Sequence of processing operations is fixed by the equipment configuration. • Sequence of simple operations. • Integration and coordination of many operation in one equipment. • Typical features :
1. High initial cost. 2. Custom engineered equipment. 3. High production rates. 4. Inflexibility. Programmable Automation : • Production equipment is designed with capability to change sequence of operation to accommodate different product configurations. • Operation sequence controlled by PROGRAM. • Typical features :
1. High investment in general purpose equipment. 2. Low production rate. 3. Flexibility to deal with product change. 4. High suitability for batch production. • Typical cycle for a product :
1. Set up and reprogramming. 2. Batch production of parts.
• Examples :
1. NC Machine Tools. 2. Industrial Robots. 3. PLCs. Flexible Automation : • Extension of programmable automation. • Produce variety of parts with virtually no time lost for changeovers from one part style to next. • No production time lost in set up and reconfiguration. • System can produce various mixes of part s. • Example : FMS. • Typical Features :
- High Investment for custom engineered e quipments. - Continuous production of mix of products. - Medium production rates. - Flexible to deal with product design variations. Three Type of Automation
:
Automation Principles &
Strategies : • Automation is not always the right answer for a given production situation. • Three approaches for dealing with automation projects :
1.The USA Principle. 2.Ten strategies for Automation. 3.Automation Migration Strategy. The USA Principle : • It is common Sense approach to automation and
process improvement. • USA Stands for :
- U : understand the existing process. - S : simplify the process. - A : automate the process. U: Understanding the Process : • Study current process in detail. • Find answers :
1. What are inputs? 2. What are outputs? 3. What exactly happens between input & o utput? 4. What is function of process? 5. How does it add value to the product? 6. What are sequence of operations? Simplify & Automate : • Simplify the Process : (a) Simplify the existing process. (b) Generate answers to queries : 1. What is the purpose of each step and transport? 2. Is this step necessary? 3. Can this step be eliminated? 4. Does this step uses the most appropriate technology? 5. How can the step be simplified? 6. Can steps be combined? 7. Automate the steps in process. Ten Strategies for Automation : • Specialization of operations : Special purpose equipment to perform one operation with greatest possible efficiency. • Combined operations : Reducing number of distinct production machines. • Simultaneous Operations : Reducing total processing time. • Integration of operations : Linking several workstation into a single integrated mechanism. • Increased flexibility : To achieve max utilization of equipment. • Improved material handling and storage : Reducing non-productive time.
• On-line inspection : Corrections to the process during manufacturing. • Process control and optimization. • Plant operations and control. • CIM. Automation Migration Strategy : • Phase 1 : Manual Production using single station manned cells
operating independently. • Phase 2 : Automated Production using single-station automated ce lls operating independently. • Phase 3 : Automated integrated production using a multistation automated system with automated
material handling.
Hydraulic and Pneumatic ControlsFluid Power • Technology that deals with : 1. Generation. 2. Control. 3. Transmission. These all 3 of power using pressurized fluids. • Muscle that moves industry. • Used to Push, Pull, Regulate, or drive all machines of modern industry. • Fluid :
1. Liquid (Hydraulics). 2. Gas (Pneumatics). Advantages of Fluid Power : • Ease and Accuracy of Control : control using simple levers & buttons. • Multiplication of Force : multiply forces without gears, pulleys, levers. • Constant Force or Torque : only fluid power can provide this irrespective of speed changes. • Simplicity, Safety, Economy : Fewer moving parts, compact, reliable. Components of a Fluid Power
System : • A Tank (reservoir) – to hold fluid. • A Pump – to force liquid through system.
• Electric Motor – to drive pump. • Valves – to control liquid direction, pressure and flow r ate. • Actuator – to convert fluid energy to mechanical force or torque. • Piping – to carry fluid. Pascal’s Law : • Principle of how fluid transmit power : “Pressure applied to a confined fluid is transmitted undiminished in all
directions”
Hydraulic and Pneumatic ControlsFluid Power Actuators Fluid Power Actuators • Linear Hydraulic Actuators (Hydraulic Cylinders). • Limited Rotation Hydraulic Actuator. • Rotary
Actuator.
Hydraulic cylinders
: ylinder Mountings
C
:
:
Mechanical Linkages
C
ontrol Components : Three basic types of control devices : • Direction Control Valves (control fluid path) :
1. Check Valves. 2. Shuttle Valves. 3. 2-Way, 3-Way, 4-Way DCV. • Pressure Control Valves (protect system). • Flow Control Valves (speed control). Direction Control Valves : Control direction of flow in hydraulic
circuit, CHECK
VALVE.
CHECK VALVE : Pilot operated check
valve.
• Two-Way,
Four-Way : - Used to direct inlet flow to either of two outlet ports. - DCVs use sliding spool. - 2 or 3 position.
• Two-Way
Valves : - Used to direct pump flow to either of two paths of c ircuit. • Four-Way Valves :
- Used to control double acting hydraulic cylinders.
• Spool of a DCV can be positioned by :
- Manually. - Mechanically. - Using pilot pressure. - Using Electrical
Solenoid.
Two - way Valve
Four - way Valve Manually actuated, spring centered, 3-position, 4-way
valve. Two-position, spring-offset, four-way
valve. Mechanically actuated 4-way
valve. Air pilot-actuated four-way
valve.
Hydraulic and Pneumatic ControlsHydraulic Circuit Design Design Considerations : • Safety of Operation
: 1. Pressure and Temperature ratings. 2. Interlocks for sequential operations 3. Emergency shutdown features. 4. Power failure locks. 5. Operation speed. 6. Environment conditions. • Meet functional requirements :
1. Meet required performance specification. 2. Life expectancy same as machine. 3. Facilitate good maintenance practice. 4. Compatibility with electrical and mechanical components. 5. Withstand operational hazards. • Efficiency of Operation :
1. Keep system Simple, Safe and Functional. 2. Access to parts need repair or adjustment. 3. Design to keep min operational cost. 4. Design to prevent and remove contamination. Linear Circuits
:
Circuit Design Approach : • What are the
specifications of the job? - Force requirement – 8 kN - Length of work stroke – 15 cm - Speed of piston and rod assembly – 0.5 sec. • What Size of Cylinder is needed?
- Force known. Cylinder Area & Operating Pressure must be selected. F = PA - Standard bore and rod size cylinders. • Cylinder Selection Reasoning :
(a) Large Diameter Cylinder : - Operates at Low Pressure. - Requires Bigger pump for speed. (b) Small Diameter Cylinder : - Operates at High Pressure. - Smaller pump give speed. • Cylinder selected = 50 mm bore.
• Find Pressure and then select suitable pump. • What Capacity Pump is needed?
- Max Cylinder Speed required. - Flow rate = (Cyl Area)x(Stroke)/(time). - Pump selection. • What size of Electric Motor needed?
- Calculate power required to run Pump. - Consider efficiency of pump. - Shaft size, type and electric source. • What Size reservoir should be used?
- 2.5-3.0 times the pump capacity. • Size of pump inlet?
- Inlet flow velocity = 60-150 cm/sec. • Size of Discharge tubing?
- Outlet flow velocity = 200 – 450 cm/sec. • Relief valve selection :
- Pressure range. - Valve port size. • Direction Control valve :
- Based on function. Control of a Single Acting Hydraulic Cylinder
:
Cylinder : Circuit : • Pressurized fluid discharge returned to system. • Speed up extending speed. • Retraction bypass
Control of a Double Acting Hydraulic
Regenerative
DCV.
Drilling Machine
Application : • Spring centered position – Rapid spindle advance. • Left envelope – Slow feed. • Right envelope – Retracts
piston.
PUMP Unloading circuit
: • Unloading valve unloads the pump at the ends of extending and ret racting strokes.
• As well as in spring centered position of
DCV.
Double Pump Hydraulic
System : • Punch Press. • Initial Low Pressure high flow rate req. • When punching operation begins, increased pressure opens unloading valve to unload low pressure
pump.
Counterbalance
Valve : • To keep vertically mounted cylinder in upward position while pump is idling. • Counterbalance valve is set to open at slightly above the pressure required to hold the piston
up.
Hydraulic
Cylinder Sequence Circuit : • Left Env : Left Cyl extends completely and then Right Cyl extend. • Right Env : Right Cyl retracts fully and then L eft Cyl
retracts.
Automatic
Cylinder Reciprocating System : • Two sequence valve sensing strokes completion by corre sponding
pressure build up.
Locked
Cylinder using Pilot Check Valves : • Lock Cylinder so that piston can not move by external
load.
Hydraulic and Pneumatic Controls Cylinder Synchronizing Circuits
Cylinder Synchronizing Circuit
• Cylinders connected in parallel. • Loads identical – Moves in exact synchronization.
• Loads Not exactly Identical (practical situation). • Cylinders also not exactly identical ( packing Friction).
• Cylinders connected in Series. • For two cylinder to be synchronized. • Piston Area of Cyl 2 = Piston Area of Cyl 1-Rod area.
• Pump pressure should overcome load acting on both cylinders : P1Ap1 - P2(Ap1 - Ar1) = F1 P2Ap2 - P3(Ap2 - Ar2) = F2 • Question : For the Cylinder Synchronizing Circuit, what pump pressure is required if the cylinder loads are 22kN each and cylinder 1 has a piston area of 65 cm2?
Ap2 = Ap1 - Ar1. P3 = 0. P1Ap1 = F1 + F2. Fail Safe Circuit : • Designed to prevent injury to operator or damage to equipment. • Prevent Cylinder from accidentally falling on an operator in the event of : - Hydraulic line ruptures. - Person inadvertently operates manual override on Pilot actuated DCV when pump not operating.
Fail Safe Circuit with Overload Protection : • DCV-1 controlled by Push button valve-2. • When Overload Valve-3 is in spring offset mode, it drains the pilot line of valve 1. • If Cyl experience excessive resistance, Valve-4 actuates overload valve-3. This drains pilot li ne of Valve1, causing it to return to spring offset mode. • Nothing happen if push button 2 pressed unless overload valve shifted manually into blocked configuration.
Two hand Safety Circuit : • Designed to protect an operator from injury. • For circuit to function, operator must depress both manually actuated valves. • Any one button prevents operation.
Speed Control of a Hydraulic Motor : • Hydraulic Motor – Rotary Motion. • Spring Center Position - Motor hydraulically locked. • Left Envelop - Rotates Clockwise. • Right Envelop - Rotates Anticlockwise.
Hydraulic Motor Braking System : • Hydraulic motor may be driving Machine having a large inertia. • Creates Flywheel effect. • Stopping motor, acts as a pump. • Circuit is designed to provide fluid to the motor while it is pumping. • Provisions should be made for discharge fluid from motor to be returned to Tank. • This would stop motor without damaging. Hydraulic Motor Braking System :
QUIZ Q1. Give the sequence of operation of Cylinders 1 and 2 when pump is turned ON. Assume both Cylinders are initially fully retracted.?
Q2. What Safety feature does the below shown circuit posses in addition to the pressure relief valve.?
Q3. Assuming two double-rodded cylinders are identical, what unique feature does this circuit possess? What is the difference if they are single rodded cylinders?
Hydrostatic Transmission : • Open Circuit Drives : - Pump draws fluid from reservoir. - Pump output directed to Hydraulic Motor. - Discharge from Motor into reservoir. • Closed Circuit Drive : - Exhaust oil from t he motor returned directly to pump inlet. Closed Circuit One-Direction Hydrostatic Transmission : • Closed Circuit that all ows only one direction of motor rotation.
• Motor speed varied by changing pump displacement. • Torque capacity of motor adjusted by pressure setting of the relief valve.
Closed Circuit Reversible Direction Hydrostatic Transmission :
Hydraulic and Pneumatic Controls Hydraulic Pumps Introduction : • Pump – Converts Mechanical Energy to Hydraulic Energy. • Pump pushes the fluid into the hydraulic system. • Broad Classification : Positive Displacement Pump & Non-positive Displacement Pump Positive Displacement Pumps : • Universally used for Fluid Power. • It push a fixed amount of fluid i nto the Hydraulic System per revolution of shaft. • Overcome pressure from Mechanical loads as well as friction resistance to flow. • Main types – Gear, Vane, Piston. Non Positive Displacement Pumps : • Used for Low Pressure, High Volume Flow applications. • Not withstand high pressure, of little use in Fluid Power. • Used for transporting fluid from one location to another. Advantages of Positive Displacement Pumps : 1. High Pressure capabilities. 2. Small, compact size. 3. High volumetric efficiency. 4. Great Flexibility of performance. 5. Constant efficiency on all pressure. Pumping Theory : • A partial vacuum is created at pump inlet due to internal operation of pump. • Atmospheric pressure push the fluid out of oil tank. • The pump mechanically pushes the fluid out of the discharge line Pumping Theory – Simple Piston Pump :
Gear Pump :
Lobe Pump : • Both lobes driven externally. • Lobes don’t contact each other. • Quieter than gear pump. • Volume discharge is more. • Less mating elements, and more pulsation.
Screw Pump : • Axial flow positive displacement pump. Precision ground screw meshing within closed fitti ng. • Non pulsating flow, Idler act as rotating seal.
Vane Pumps : • Rotor contains radial slots rotates inside a cam ring. • Each slot contains Vane designed to mate with surface of cam ring. • First half cycle suction, second half delivery.
Variable Displacement Pump :
• Provision for mechanically varying Eccentricity. • Pressure compensation by spring force. • Constant pressure variable delivery pump.
Cavitation Problem : • Noise problem occurred due to entrained air bubbles. • Occurs when suction lift is excessive and the inlet pressure falls below vapour pressure of fluid. • Air bubbles formed in low pressure inlet region collapsed when reached high pressure
discharge region. • This produce high velocity explosive forces eroded metallic pump components. Rules to Eliminate Cavitation : • Keep suction line velocities below 10 m/min. • Keep pump inlet lines as short as possible. • Minimize number of fittings in the inlet line. • Mount pump close to the reservoir. • Use low pressure drop inlet filters or strainers. • Use proper oil as recommended. Pump Selection : • Select the actuator (Hydraulic cylinder or motor) based on load. • Determine flow rate requirements. • Determine pump speed and select the prime mover. • Select pump type based on application. • Select System pressure. • Select reservoir and other components. • Calculate overall cost of the system.
Hydraulic and Pneumatic Controls Hydraulic Circuit Analysis Introduction : • Energy Losses in Fluid P ower Systems. • Bernoulli‘s equation & Continuity equation used to perform analysis of Fluid Power System. • Calculating Pressure drops, flow rates, HP losses for all components. Laminar or Turbulent Flow : • Reynolds number. • <2000 : LAMINAR.
• >4000 : TURBULENT. • CRITICAL ZONE. • Greater losses in Turbulent flow. • Fluid power system designed to operate in LAMINAR fl ow region. Friction Losses : • Friction – Main cause of losses. • Loss in Pressure head. • Head Loss : Losses in Pipes and Losses in Fittings. • Head loss in Pipes – DARCY’s Equation. DARCY’s Equation : • HL = f (L/D)(v2 /2g) : f = friction factor (dimensionless). L = length of pipe. D = pipe inside diameter. v = avg. fluid velocity. g = Acceleration due to gravity. • Used for Laminar as well as Turbulent flow. • Difference lies in evaluation of friction factor. Frictional Losses in Laminar Flow : • Friction Factor f = 64/(Reynolds Number). • Hagen Poiseuille Equation. • HL = (64/NR)(L/D)(v2 /2g). Frictional Losses in Turbulent Flow : • Relative roughness = ε / D : ε = absolute roughness. D = Pipe inside diameter. • Typical values of Roughness (μ m) : Drawn tubing = 1.524 Commercial steel = 45.72 Cast Iron = 121.92 Galvanized Iron = 152.4 Riveted Steel = 1828.8
• Moody diagram used to calculate friction factor. Moody Diagram :
Losses in Valves & Fittings : • Energy losses in valves and fittings such as Tees, Elbows, and Bends. • Nature of flow through Valves and Fittings is very complex. • Head Loss HL = Kv2 /2g. • K factor for Valves and Fittings : - Gate Valve (wide open) = 0.19 - 3/4 open = 0.90, 1/2 open = 4.5, 1/4 open = 24.0 K-Factor : • Return Bend = 2.2 • Standard Tee = 1.8 • 90 deg Elbow = 0.75 • 45 deg Elbow = 0.42 • Ball Check valve = 4.0
Equivalent Length Technique : • Darcy’s Equation : -> Friction head loss in pipes proportional to : - square of Fluid Velocity. - Length of Pipe. • Head Loss in valves & fitting : - Proportional to square of fluid velocity. • Possible to find a length of pipe that for the same flow rate would produce same head loss as a valve or fitting. • HL(valve or Fitting) = HL(Pipe). • K(v2 /2g) = f(L/D)(v2 /2g). • Since velocities are equal. • Equivalent Length Le = KD/f.
• Convenient method for analyzing hydraulic circuits where frictional energy losses are to be taken in to account. Problem : • For the Hydraulic System shown, following data are given : -
Pump is adding 5 hp (3730 W) to fluid. Pump flow is 0.001896 m3 /s. Pipe has 0.0254 m inside dia. Sp. Gravity of oil = 0.9 Kinematic viscosity of oil is 100 cS. Elevation difference between station 1 & 2 is 6.096 m. Pipe lengths : 1 ft = 0.305 m, 4 ft = 1.22 m, 16 ft = 4.88 m.
• Find pressure available at inlet to hydraulic motor. The pressure at the oil top surface level in the hydraulic tank is atmospheric (0 MPa). The head loss HL due to friction between stations 1 & 2 is not given.
Solution : • Bernoulli’s equation between stations 1 & 2. 2 • Z1 + P 1/ρ + v1 /2g + Hp - Hm - HL = Z2 + P 2/ρ + v22 /2g :
- Hp = Head added by pump. - Hm = Head removed by Hydraulic Motor. - HL = Head lost in friction. • Hm = 0, No Hyd motor between 1 & 2. • V1 = 0, P1/ρ = 0. • Z2 – Z1 = ?.
• Solve for v2. • Evaluate velocity head at station 2. • Find Reynolds Number : - NR ρ vD/μ = vD/(Kinematic Viscosity). - SI unit Kinematic Viscosity = m2 /s. - Stokes = cm 2 /s. • Laminar or Turbulent ? • Find friction factor. • Determine Head loss due to friction : - Darcy’s equation. - Calculate L. - Use Leq for standard elbow, K = 0.9 • Substitute in Bernoulli’s equation to solve for Pressure head at 2. • Calculate Pump Head : - Hp = 0.762 (HP)/Q(m3 /s).Sg • Solve for P2 , use ρwater = 9797 N/m2
Electropneumatics Signals : • A Signal is the representation of information by means of the value or value pattern of physical variable. • Analog Signal :
- Information is assigned point by point to a continuous value range of the signal parameter. - Example : pressure gauge –each pressure value (information parameter) is assigned a particular display value (information). • Digital Signal :
- A digital signal is a signal with a finite number of value ranges of the information parameter. Each value range is assigned a specific item o f information. - Example: A pressure measuring system with a digital display shows the pressure in increments of 1 bar. There are 8 possible display values (0 to 7 bar) for a pressure range of 7 bar. • Binary Signal :
- A binary signal is a digital signal with only two value ranges for the information parameter. These are normally designated 0 and 1. A control lamp indicates whether a pneumatic system is being correctly supplied with compressed air. -> If the supply pressure (= signal) is below 5 bar, the control lamp is off (0 status). -> If the pressure is above 5 bar, the control lamp is on (1 status). Controllers : • Controllers can be divided into different categories according to the type of information representation, into analogue, digital and binary
controllers.
Signal Flow in a
Control System : • A controller can be divided into the functions: signal input, signal processing, signal output and command execution. The components in the circuit diagram of a purely pneumatic controller are arranged so that the signal flow is clear. Bottom up : Input elements(such as manually operated valves), Logical association elements(such as two-pressure valves), Signal output elements(power valves, such as 5/2-way valves), Command execution(such as
cylinders).
• The components in the circuit
diagram of a purely pneumatic controller are arranged so that the signal flow is clear : Bottom up : - Input elements(such as manually operated valves) - Logical association elements(such as two-pressure valves) - Signal output elements(power valves, such as 5/2 -way valves) - Command execution(such as cylinders). Pneumatic and Eletropneumatic Systems : Both pneumatic and
electropneumatic controllers have a pneumatic power section. The signal control section varies according to type. • In a pneumatic control pneumatic components are used, that is, various types of valves, sequencers,
air barriers, etc. • In an electro-pneumatic control the signal control section is made up of a ele ctrical components, for
example with electrical input buttons, proximity switches, re lays, or a programmable logic controller. • The directional control valves form the interface between the signal control section and the pneumatic
power section in both types of c ontroller. Pneumatic control system
: Pneumatic control system
Electro-
:
Operation
of Electro-pneumatic Controller : • In contrast to a purely pneumatic control system, electropneumatic controllers are not shown in any single overall c ircuit diagram, but in two separate circ uit diagrams - one for the electrical part and one for the pneumatic
part.
Components
of Electrical Signal Control : Power Supply Unit : • The signal control section of an electr opneumatic controller is supplied with power via the elect rical
mains. The controller has a power supply unit for this
purpose.
Push
button and control switches : • Switches are installed in circuits to apply a current to a load or to interrupt the circuit. These switches
are divided into pushbuttons and control switches. • Control switches are mechanically detented in the selected position. The switch position remains
unchanged until a new switch position is selected. Example : L ight switches in the home.
• Push button switches only maintain the selected position as long as the switch is actuated (pressed).
Example : Bell push.
Normally Open Contact : o f a normally open contact, the circuit is o pen if the switch is in its initial position (not • In the case of actuated). The circuit is closed by pressing the push button – current flows to the load. When the plunger is released, the spring returns the switch to its initial position, interrupting the
circuit.
Normally Close
Contact : • In this case, the circuit is closed when the switch is in its initial position. The circuit is interrupted by
pressing the
pushbutton.
Changeover
Contact : • The changeover contact combines the functions of the normally open and normally closed contacts in
one device.
• Changeover contacts are used to close one circuit and open another in one switching operation. • The circuits are momentarily interrupted during
changeover.
Sensor
s for Displacement and Pressure : • Sensors have the task of measuring information and passing this on to the signal processing part in a form that can easily be processed. • In electropneumatic controllers, sensors are primarily used for the following purposes :
- To detect the advanced and retracted end position of the piston rod in cylinder drives. - To detect the presence and position of workpieces. - To measure and monitor pressure. • Limit Switch. • A limit switch is actuated when a machine part or workpiece is in a certain position.
Normally, actuation is effected by a cam. • Limit switches are normally changeover contacts. They can then be connected – as required :
- As a normally open contact. - Normally closed contact or - Changeover
contact.
Proximity
Switches : • In contrast to limit switches, proximity switches operated contactlessly (non-contact switching) and
without an external mechanical actuating force. • As a result, proximity switches have a long service life and high switching reliability. The following
types of proximity switch are differentiated : 1. Reed switch. 2. Inductive proximity switch. 3. Capacitive proximity switch. 4. Optical proximity switch.
Reed Switch : • Reed switches are magnetically actuated proximity switches. They consist of two contact reeds in a
glass tube filled with inert gas.
• The field of a magnet causes the two reeds to close, allowing current to flow. • In reed switches that act as normally closed contacts, the contact reeds are closed by small magnets.
This magnetic field is overcome by the c onsiderably stronger magnetic field of the switching magnets. • Reed switches have a long service life and a very short switching time (approx. 0.2 ms). They are
maintenance-free, but must not be used in environments subject to strong magnetic fields (for example in the vicinity of resistance
welders).
Electronic
Sensors : • Inductive, optical and capacitive proximity switches are e lectronic sensors. They normally have three
electrical contacts : - Contact for supply voltage. - Contact for ground. - Contact for output signal. • In these sensors, no movable contact is switched. Instead, the output is e ither electrically connected to
the supply voltage or to ground(=output voltage 0 V).
Positive and negative switching sensors : There are two types of electronic sensor with regard to the polarity of the output voltage :
• In positive switching sensors, the output voltage is zero if no part is detected in the proximity. The
approach of a workpiece or machine part leads to switchover of the o utput, applying the supply voltage. • In negative switching sensors, the supply voltage is applied to the output if no part is detected in the
proximity. The approach of a workpiece or machine part leads to switchover of the output, switching the output voltage to 0 V. Inductive Proximity Sensor : • An inductive proximity sensor consists of : - an electrical oscillator(1) - a flip-flop(2) and - an amplifier(3). • When a voltage is applied, the oscillator generates a high-frequency alternating magnetic field that is
emitted from the front of the sensor. If an conducting substance is introduced into this field, the oscillator is attenuated. • The downstream circuitry, consisting of a flip-flop and an amplifier, evaluates t he behavior of the
oscillator and actuates the output. • Inductive proximity sensors can be used for the detection of all good electrical conductors (materials).
In addition to metals, these include, for example,
graphite.
Capacitive
Proximity Sensor : • A capacitive proximity sensor consists of a capacitor and an electrical resistance that together form an RC oscillator, and a circuit for evaluation of the frequency.
• An electrostatic field is generated between the anode and the cathode of the capacitor. A stray field
forms at the front of the sensor. • If an object is introduced into this stray field, the capacitance of the capacitor changes. The oscillator is
attenuated. The circuitry switches the output. • Capacitive proximity sensors not only react to highly conductive materials (such as metals) but also to
insulators of high dielectric strength (such as plastics, glass, ceramics, fluids and
wood).
Optical Proximity
Sensors : • Optical proximity sensors use optical and electronic means for object detection. Red or infrared light is used. • Semiconductor light-emitting diodes (LEDs) are particularly reliable sources of red o r infrared light.
They are small and rugged, have a long service life and can be simply modulated. • Photodiodes or phototransistors are used as a re ceiver. • Red light has the advantage that the light beam can be seen during adjustment of the optical axes of
the proximity switch. • Three different types of optical prox imity switch are differentiated :
1. One-way light barrier. 2. Reflective light barrier.
3. Diffuse reflective optical sensor.
One - way light barrier : • The one-way light barrier has spatially separate tr ansmitter and receiver units. The parts are mounted
in such a way that the transmitter beam is directed at the receiver. The output is switched if the beam is
interrupted.
Reflectiv
e Light barrier : • In the reflective light barrier, the transmitter and receiver are mounted together in one housing. • The reflector is mounted in such a way that the light beam transmitted by t he transmitter is practically
completely reflected to the receiver. The output is switched if the beam is
interrupted.
Diffuse
Reflective Optical Sensor : • In the diffuse reflective optical se nsor, the transmitter and receiver are mounted together in one unit. • If the light hits a reflective object, it is redirected to the receiver and causes the output of the sensor
to switch.
Pressure
Sensors : There are various types of pressure-sensitive sensors : • Pressure switch with mechanical contact(binary output signal). • Pressure switch with electronic switching(binary output signal). • Electronic pressure sensor with analogue output signal.
Mechanical Pressure Switch : • In the mechanically actuated pressure switch, the pressure acts on a cylinder surface. • If the pressure exerted exceeds the spring force of the return spring, the piston moves and operates
the contact set.
Electronic
Pressure Switch : • Diaphragm pressure switches are of incre asing importance. Instead of actuating a mechanical contact,
the output is switched electronically.
• Pressure or force sensitive sensors are attached to the diaphragm. The sensor signal is evaluated by an
electronic circuit. • As soon as the pressure exceeds a certain value, the output is switched.
Analog Pressure Sensor : • Variable resistor 1 changes its v alue when pressure is applied to the diaphragm. Via the co ntacts 2, the
resistor is connected to the electronic evaluating device, which generates the output
signal.
Relays : • A relay is an electromagnetically actuated switch. • When a voltage is applied to the solenoid coil, an electromagnet field results. This causes the armature
to be attracted to the coil core. • The armature actuates the relay contacts, either closing or opening them, depending on the design.
• A return spring returns the armature to its initial position when the current to the coil is
interrupted.
• A relay
coil can switch one or more contacts. In addition to the type of relay described above, there are other types of electromagnetically actuated switch, such as the retentive relay, the time relay, and the contactor. Retentive Relay : • The retentive relay responds to current pulses :
- The armature is energised when a positive pulse is applied. - The armature is de-energised w hen a negative pulse is applied. - If no input signal is applied, the previously set switch position is retained (retention). • The behavior of a retentive relay is analogous to that of a pneumatic double pilot valve, which
responds to pressure pulses.
Time Relay : • There are two types of time relay : –
- Pull-in delay and - Drop-out delay. • With pull-in delay, the armature is e nergized after a set delay; drop-out however, is effected without
delay. • The reverse applies in the case of the drop-out delay relay. Pull in Delay Time Relay
:
Relay :
Drop out Delay Time
Contactors : • A
contactor has multiple switching elements, normally four to ten contacts. • For contactors – as for relays – there are various types with combinations of
- Normally open contact. - Normally closed contact. - Changeover contact. - Delayed normally closed contact etc. • Contactors that only switch auxiliary contacts (control contacts) are called contactor relays. • Contactors with main and auxiliary contacts are called main or power
contactors. • Contactors are used for the following applications :
1. Currents of 4 to 30 kW are switched via the main contacts of power contactors. 2. Control functions and logical associations are switched by auxiliary contacts. • In electropneumatic controllers, electrical c urrents and power are low. For t his reason, they can be
implemented with auxiliary contactors. Main or power contactors are not required.
Electropneumatics Electrically Actuated DCV • An electropneumatic control system works with two forms of energy : 1. Electrical energy in the signal control section. 2. Compressed air in the power section. • Electrically actuated directional control valves form the interface between the two parts of an electropneumatic control. • They are switched by the output signals of the signal control section and open or close connections in the power section. • The most important tasks of electrically actuated directional control valves include : 1. Switching supply air on or off. 2. Extension and retraction of cylinder drives.
Actuation of a Single Acting Cylinder :
Actuation of a Double Acting Cylinder :
Electrical Circuit Diagram : • The electrical circuit diagram of a control system shows how the electrical control components are interconnected and how they interact. • Depending on the task definition, the following types of circuit diagram are used : 1. Overview diagram. 2. Function diagram. 3. Circuit diagram. Overview diagram : • An overview diagram provides an overview of the electrical apparatus of a rel atively large system.
• It shows only the most important interdependencies. • The various subsystems are shown in greater detail i n other diagrams. Function diagram : • A function diagram illustrates the individual functions of a system. • No account is taken of how these functions are executed. Circuit diagram : A Circuit diagram shows the details of the design of systems, installations, apparatus etc. It contains : • Graphical symbols for the items of equipment. • Connections between these items. • Equipment identifiers. • Terminal identifiers. • Other details necessary for tracing the paths (signal identifiers, notes on the representation location). Consolidated & Distributed Representation : • If consolidated representation is used for a circuit diagram, each device is represented as a single coherent symbol, i.e. for example even a relay t hat has more than one normally open and normally closed contact. • If distributed representation is used for a circuit diagram, the various components of a device may be drawn at different locations. • The normally closed and normally open contacts of a relay, for example, can be distributed throughout the circuit diagram as appropriate. • A circuit diagram with distributed representation is used to represent the signal control section in electropneumatics. Electrical Symbols :
• Graphical symbols for contacts : basic functions.
• Graphical symbols for contacts : delayed actuation.
• Graphical symbols for Manually operated switches.
Electrical Symbols (Relays & Contractor) :
Electrical Symbols (Sensors) :
Circuit Diagram Guidelines : • In the circuit diagram of an electropneumatic control system the graphical symbols of the components required to implement logic circuits and sequences are entered consecutively from the top to the bottom and from left to right. • Relay coils and valve coils are always drawn beneath the contacts. • Other measures to ensure that a circuit diagram is easy to read include : -
Division into individual current paths. Identification of devices and contacts by l etters and numbers. Subdivision into a control circuit and main circuit. Preparation of tables of contact elements.
Current Paths : • The individual current paths of an electropneumatic control system are drawn alongside each other in the circuit diagram and numbered consecutively. • The circuit diagram of an electropneumatic control system shown in Figure on next slide has 10 current paths. • Current paths 1 to 8 belong to the control circuit, current paths 9 and 10 to the main circuit.
Identification of Components : • The components in the circuit diagram of a control system are identified by a letter in accordance with following Table. • Components with identical identifying letters are assigned consecutive numbers (for example 1S1, 1S2 etc.).
The components shown in the circuit diagram are identified as follows : 1. Manually operated switches S1, S2 and S3. 2. Limit switches 1S1 and 1S2.
3. 4. 5. 6.
Pressure switch 1B1. Relays K1, K2, K3 and K4. Solenoid coil 1Y1. Lamp H1.
Terminal Designation of Contacts : • In order to ensure error-free wiring of contacts, all connections on a component and in the circuit diagram are identified in the same way. • Each connection of a contact is assigned a function number. The function numbers for different types of contact are listed in Table on next slide. • If a switch, relay or contactor has more than one contact, they are numbered by means of sequence numbers prefixed to the function number.
Terminal Designation of a Relay :
In the example circuit diagram, the terminals of relay K1 are identified as follows : 1. Coil (current path 2): A1, A2. 2. Normally open contact (current path 3): 11, 14. 3. Normally open contact (current path 10): 21, 24. Contact Element Table : • All contacts actuated by a relay coil or contactor coil are listed in a contact element table. • The contact element table is placed beneath the current path containing the relay coil.
There are a total of 4 contact element tables in the Example circuit diagram : • Current path 2: contact element table for relay K1. • Current path 4: contact element table for relay K2.
• Current path 5: contact element table for relay K3. • Current path 8: contact element table for relay K4.
Actuated Limit Switch : • The electrical circuit diagram is shown in the de-energized state ( electric power supply switched off). • If limit switches are actuated in this position, they are identified by an arrow. • The associated contacts are also shown in the actuated position.
Terminal connection diagram :
• In an electropneumatic control system, sensors, control elements, signal processing units and solenoid coils have to be wired up t o each other. • The large number of components and the distances between them make wiring a significant cost factor in an electropneumatic control system. Particular attention needs to be paid to the arrangement of the control components : • Sensors are frequently mounted in parts of an installation that are difficult to access. • Signal processing equipment (relays, programmable logic controllers) are usually located in a control cabinet. To an increasing extent, however, PLCs are also now being integrated into valve terminals. • Control elements are either mounted directly in t he front of the control cabinet or the system is operated via a separate control console. • Electrically actuated directional control valves are mounted in blocks in the control cabinet, in blocks on valve terminals or individually in the vicinity of the drive units.
The wiring of an electropneumatic control system must satisfy the following requirements : Cost-effective design : • Use of components which allow speedy wiring while maintaining a good price/performance ratio, optimization of the circuit diagram i n terms of wiring expense, use of components with reduced number of terminals. Simple troubleshooting : • Clear wiring which is accurately documented and i s easy to follow.
Swift repair : • Simple replacement of components by means of terminal or plug-in connections, no solderedon components. • Terminal allocations are documented in a terminal connection diagram. This consists of two parts : - a circuit diagram and - a terminal allocation list. • In the circuit diagram, each terminal is represented by a circle. The terminals are identified by the letter X, and are numbered consecutively in sequence within the terminal strip (terminal designation X1, X2 etc. for example). • If there is more than one terminal strip, each strip is also assigned a sequence number (terminal designation X2.6, for example, for the 6th t erminal of terminal strip 2). • The basis to produce the terminal connection diagram is the circuit diagram with no terminal allocations shown. • The terminal connection diagram is drawn up in two stages : 1. Allocation of terminal numbers and drawing the t erminals in the circuit diagram. 2. Compilation of the terminal allocation list(s).
Circuit
Allocation of Terminal Number : • The terminal numbers are allocated i n ascending order and marked on the circuit diagram. • The allocation procedure between the circuit diagram and terminals comprises three stages : 1. Power supply for all current paths (t erminals X1-1 to X1-4 in the circuit diagram). 2. Ground connection for all current paths (terminals X1-5 to X1-8 in t he circuit diagram). 3. Connection of all components situated outside the control cabinet, according to the following system : - In the order of the current paths. - From top to bottom within each current path. - In the case of contacts, in the order of the function numbers. - In the case of electronic components, in the order of supply voltage connection, signal connection (if applicable), ground connection. • In the circuit diagram, the components are allocated to terminals X1-9 to X1-17.
Terminal Allocation List : Entries are made in the terminal allocation list in the following steps : 1. Enter the component and connection designations of the components outside the control cabinet (on the left-hand side of the terminal allocation list). 2. Enter the component and connection designations of the components inside the control cabinet (on the right-hand side of the terminal allocation list). 3. Draw any required jumpers (in the example: terminals X1-1 t o X1-4 for 24 V supply voltage, X1-5 to X1-8 for supply ground). 4. Enter the terminal-terminal connections that cannot be implemented with jumpers.
Wiring Electropneumatic System : The structure of a terminal allocation list is based on the design of the terminal strip. Accordingly, an electropneumatic control system can largely be wired up on the basis of the terminal allocation list : 1. All lines running to components outside the control cabinet are connected in accordance with the list on the left-hand side of the terminal strip. 2. All lines running to components inside the control cabinet are connected in accordance with the list on the right-hand side of the terminal strip.
ElectropneumaticsRelay control system Applications of relay control systems in electro pneumatics : • The entire signal processing needs of an e lectropneumatic control system can be implemented with relays. • Now a days programmable logic controllers(PLCs) are commonly used for signal processing instead of
relay control systems. • Relays are still used in modern control systems however, for example in an EMERGENCY STOP
switching device. • The principal advantages of relay control systems are the clarity of their design and the ease of
understanding their mode of operation. Direct and Indirect Control : Direct Control :
• The piston rod of a single-acting cylinder is to be extended when pushbutton S1 is pressed and
retracted when the pushbutton is released.
Indirect Control : • If the pushbutton is pressed in an indirect control system, current flows through the relay coil. Contact
K1 of the relay closes, and the directional control valve switches. The piston rod advances. Direct Control of Single Acting Cylinder : • The electrical circuit diagram for direct control of a single-acting c ylinder is
shown in Figure.
Indirect
Control of Single Acting Cylinder : • The electrical circuit diagram for indirect control of a single-acting cylinder is shown in
Figure.
Direct Control of
Double Acting Cylinder : • The electrical circuit diagram for direct control of a Double-acting cylinder is
shown in Figure.
Indirect
Control of Double Acting Cylinder : • The electrical circuit diagram for indirect control of a Double-acting cylinder is shown in
Figure.
Logic Operations
: • Combine signals from several control elements through logic operations. • Parallel Connection (OR Circuit) :
- The aim is to be able to trigger extend of the piston rod of a cylinder with two different input elements, pushbuttons S1 and S2. - The contacts of the two pushbuttons S1 and S2 are arranged in parallel in the circuit diagram. Parallel
(OR Circuit) :
Logic
Operations : • Series Connection (AND Circuit) : - In this case the piston rod of a cylinder is to be advanced only if both pushbuttons, S1 and S2, are pressed. - The contacts of the two pushbuttons are arranged in series in the circuit diagram Series (AND Circuit)
:
Representation of Logic in
Tabular Form : • The OR and AND operations are shown in summarized form in Tables. • The following values are assigned to the signals in the three right-hand columns :
0: Pushbutton not pressed or piston rod does not advance. 1: Pushbutton pressed or piston rod
advances.
Signal Storage : • In the circuits that we have looked at so far, the piston rod o nly advances as long as the input pushbutton is actuated. If the pushbutton is released during the advancing movement, the piston rod is retr acted without having reached the forward end position. • In practice it is usually necessary for the piston rod to be fully advanced even if t he pushbutton is
pressed only briefly. • To achieve this, the directional control valve must remain in the actuated position when the
pushbutton is released; in other words, actuation of the pushbutton must be stored. • A double solenoid valve maintains its switching position even when the associated solenoid coil is no
longer energized. It is used as a storage element. • The piston rod of a cylinder is to be controlled by brief actuation of two pushbuttons (S1: advance, S2:
retract).
Automatic Return
Stroke : • The aim is for the piston rod of a double-acting cylinder to be advanced when pushbutton S1 is actuated. • When the forward end position is reache d, the piston rod is to return automatically. • Using Double Solenoid and Limit Switch. • The prerequisite for the return movement is that pushbutton S1 must first have been
released.
Oscillating
Movement : • The piston rod of a cylinder is to advance and retract automatically as soon as control switch S1 is actuated. • When the control switch is reset, the piston rod is to occupy the retracted end
position.
Relay
Circuit with Latching : • When the "ON" pushbutton is actuated in the circuit, the relay coil is energized. The relay is energised, and contact K1 closes. • After the "ON" pushbutton is released, current continues to flow via contact K1 through the coil, and
the relay remains in the actuated position. • The "ON" signal is stored. This is therefore a relay circuit with latching
function.
Dominant ON :
• When the "OFF" pushbutton is pressed the flow of current is interrupted and the r elay becomes
deenergised. • If the "ON" and "OFF" pushbuttons are both pressed at the same time, the relay coil is energized. • This circuit is referred to as a dominant ON latching circuit.
Dominant OFF : • When the "OFF" pushbutton is pressed the flow of current is interrupted and the relay becomes
deenergised. • If the "ON" and "OFF" pushbuttons are both pressed at the same time, the relay coil is energized. • This circuit is referred to as a dominant ON latching
circuit.
Cylinder Control with Latching Circuit
:
Comparison Of Signal Storage Circuit : • Signal storage can be effected by means of : - A double solenoid valve in the power section.
- Alternatively in the signal control section by means of a relay with latching function. • The various circuits behave differently in response to the simultaneous presence of a setting and
resetting signal, and in the event of failure of the electrical power supply or a wire
break
Time Delay : • In many applications it is necessary for the piston rod of a pneumatic cylinder to remain at a certain position for a set length of time. • This is the case for the drive of a pressing device, for e xample, which presses two workpieces together
until the adhesive has set. • Time relays with delayed switch-on or switch-off are used for tasks such as these. Control of Cylinder
with Timing : • When pushbutton S1 is pressed momentarily, the piston rod of a cylinder is to advance, subsequently remain at the forward end position for ten se conds and then automatically
return.
Sequenc
e control with signal storage by double solenoid valves : • The positional sketch of a feeding device is shown in Figure. • The end positions of the two cylinder drives 1A and 2A are detected by the positive switching inductive
proximity switches 1B1 and
2B2.
Feeding Device : The
program-controlled sequence is triggered when the operator presses the "START" pushbutton. The sequence comprises the following steps :
Step 1 : The piston rod of cylinder 1A advances. The workpiece is pushed out of the m agazine.
Step 2 : The piston rod of cylinder 2A advances. The workpiece is fed to the mac hining station. Step 3 : The piston rod of cylinder 1A retracts. Step 4 : The piston rod of cylinder 2A retracts. Displacement Step Diagram
:
Pneumatic circuit : • The
control system is implemented using double-acting cylinders and 5/2-way double solenoid
valves. Circuit Diagram
Design of Relay
:
• In a relay
circuit the signals are combined with each other by the contacts of control switches, pushbuttons and relays. • The electronic proximity switches used here do not have contacts; instead they generate an o utput
signal by means of an electronic circ uit. • Each sensor output signal therefore acts o n the coil of a relay, which in turn switches the necessary
contact or contacts. Sensor Evaluation
:
First Sequence Step : • The
following preconditions must be satisfied before the sequence is started : 1. Piston rod of cylinder 1A in re tracted end position (proximity switch 1B1 and relay K1 actuated). 2. Piston rod of cylinder 2A in re tracted end position (proximity switch 2B1 and relay K3 actuated). 3. START pushbutton (S5) actuated. • If all of these conditions are met, relay coil K6 is e nergised. Solenoid coil 1Y1 is actuated, and the
piston rod of cylinder 1A
advances.
Second
Sequence Step : • When the piston rod of cylinder 1A reaches the forward end position, sensor 1B2
responds. The second step of the sequence is activated. Solenoid coil 2Y1 is actuated, and the piston rod of drive 2A
advances.
Third
Sequence Step : • When the piston rod of cylinder 2A reaches the forward end position, sensor 2B2 responds. The third step of the sequence is activated. Solenoid coil 1Y2 is actuated, and the piston rod of drive 1A
retracts.
Fourth
Sequence Step : • When the piston rod of cylinder 1A reaches the retracted end position, sensor 1B1 responds. The fourth step of the sequence is activated. Solenoid coil 2Y2 is actuated, and the piston rod of drive 2A
retracts.
Cir
cuit for Control Elements : • Important control elements such as a main switch and EMERGENCY STOP switch are missing. • Control switches(latching-type switches) must be used for switching on elec trical power and for the
EMERGENCY STOP function. • All other control elements may take the form of either pushbuttons (momentary-contact switches) or
control switches. • In the Feeding Device Circuit, the control elements for :
"Manual" "Reset" "Automatic" "Continuous cycle ON" "Continuous cycle OFF" "Single cycle Start" are implemented as pushbuttons. Feeding Device with Control Elements
:
Main
Switch : • When the main switch is closed, r elay K1 is energised. Voltage is supplied to the signal control section
and the entire system via contact K1.
Emergency Stop : • If the EMERGENCY STOP switch is act uated, relay K2 is de-energized and the associated contacts
switch to the normal position. • The EMERGENCY STOP line is connected to t he supply voltage via the normally closed contact of K2.
Warning lamps can be actuated via this line, for example. • The "EMERGENCY STOP released" line is de-energized, causing t he voltage supply to the signal control
section to be interrupted. As long as EMERGENCY STOP applies, all control elements except the main switch are rendered inoperative.
Manual Operation : • When the "Manual" pushbutton is actuated, relay K4 picks up and latches. The line marked "Manual"
in the circuit diagram is connected to the supply voltage.
• If relay K3 is latched, the latching is released. The line marked "Automatic" is disconnected from the
supply voltage.
Automatic Operation : • When the "Automatic" pushbutton is actuated, relay K3 is energised and latches. The line marked
"Automatic" in the circuit diagram is connected to the supply voltage. • If relay K4 is latched, the latching is released, and the line marked "Manual" is disconnected from the
supply voltage. • Continuous cycle ON • Continuous cycle OFF • Single cycle Start :
- These functions are only possible in automatic mode. Electrical power is therefore supplied to the associated contacts and relays via the line marked "Automatic". Automatic Operation : • If "Automatic" mode is selected (relay K3 latched) and "Continuous cycle ON" is active (relay K5 latched), the control system runs in continuous operation. • Actuating the "Continuous cycle OFF" pushbutton releases the latching of relay K5. The program-
controlled sequence stops as soon as the last step in the sequence is completed. • When the "Single cycle Start" pushbutton is actuated, the sequence (movement c ycle) is executed
once only.
Design of a
Lifting Device : • A lifting device transfers workpieces from o ne roller conveyor to another at a different height. • A positional sketch of the lifting device is shown in Figure. There are three pneumatic drives :
1. Drive 1A lifts the workpieces. 2. Drive 2A pushes the workpieces o nto the upper roller conveyor. 3. Drive 3A is used as a stopper, for releasing and interrupting the supply of
workpieces.
• Safety
Requirement : - To prevent the possibility of secondary damage, in the event of an electrical power failure the piston rods of cylinders 1A and 2A are to be braked immediately and remain at a standstill. - The piston rod of the stopper cylinder 3A is meant to extend in t hese circumstances. Movement Cycle
:
Operator
Control : • The control system of the lifting device m ust enable the device to be run in a continuous cycle (continuous operation). • A single cycle operating mode is also necessary in which the sequence is processed precisely
once. Operator Control Panel
:
Operating Functions
: The following operating functions are specified for lifting device : • "EMERGENCY STOP" : When this is actuated, not only the electrical power supply, also the pneumatic
power supply must be shut down. t he initial position, i.e. the piston rods of cylinders 1A and 2A • "Reset" : This returns the system to the retract, the piston rod of cylinder 3A extends. • "Continuous cycle OFF" : This stops the continuous cycle process. If ther e is already a workpiece in the
device, it is transferred to the upper roller conveyor. The piston rods of cylinders 1A and 2A retract. The device is subsequently in its initial position. Electrical Circuit : • Design of a relay control system with clearly defined requirements as to o perator control, operational performance and behavior in the event of a fault. • The relay control system is designed in the following order :
1. Power supply. 2. Sensor evaluation. 3. Operator control. 4. Program-controlled sequence. 5. Wiring of solenoids. Flow Chart
: EMERGENCY Stop : • Simplified Emergency Control. • EMERGENCY STOP signal is required in inverted form
Main Switch &
only. Circuit
Pneumatic
:
S
ensor Evaluation: • Electrical power is supplied to the sensors as long as the EMERGENCY STOP device is not actuated. Relays K6 to K11 are assigned to sensors 1B1 to 3B1 and
B5 and Reset Control
Manual
:
• If "Manual"
mode is selected (relay K4 latched) and the "Reset" pushbutton (S5) is pressed, relay K12 is then latched. The reset process is ended when the piston rods of the cylinders assume the following positions : Cylinder 1A: retracted end position(sensor 1B1 responds, re lay K6 actuated). Cylinder 2A: retracted end position(sensor 2B1 responds, re lay K8 actuated). Cylinder 3A: forward end position(sensor 3B1 responds, relay K11 act uated). • When all three of these conditions are met, the latching of relay K12 is released via normally closed
contacts K6, K8 and K11. Automatic Cycle : Automatic : • Continuous cycle ON • Continuous cycle
OFF
Reset
Sequencer with Latching Relays : Stepped Sequence implementation : • The movement process is made up of four steps. Relays K13 (step 1) to K16 (step 4) are assigned to
these four steps.
Sc hematic Design of Step Sequence
:
Interlocki
ng of Steps : • If the preceding step is set (in this case: step 1, normally open contact of relay K13 closed) and the other setting conditions for step 2 are satisfied : - Relay K14 switches to the latched position. - The latching of relay K13 is released via the normally closed contact of relay K14. • The second step in the sequence is now set, and the first step deactivated Step Enabling
Conditions: • None of the step can be set unless the relay in the preceding step
actuated. elay Circuit Step -1 : • To allow the first movement step to be activated, the following conditions must be satisfied : -> Piston rod of cylinder 1A in ret racted end position(relay K6 actuated). -> Piston rod of cylinder 2A in ret racted end position(relay K8 actuated). -> Piston rod of cylinder 3A in forward e nd position(relay K11 actuated). -> Step 4 active(relay K16 actuated). -> Either continuous cycle active(relay K5 latched) or "Single cycle Start" (pushbutton S7) actuated. • If all of these conditions are satisfied, relay K13 is latched and the first step is
R
active.
First to Second Step : • If optical
sensor B5 responds while the first step is active, the setting condition for the second step is satisfied. • The step is activated by actuation of relay K14. Relay K14 is latched, and the latching of relay K13 is
released by the normally closed contact K14. Third to Fourth Step : • If proximity switch 1B2 responds while the second step is active, relay K15 is latched. • The latching of relay K14 is
released. responds while the third step is active, r elay K16 is latched. • The latching of relay K15 is
• If proximity switch 2B2
released.
Start
Condition : • To enable the sequence to be started, the fourth step of the sequence (relay K16) must be activated. • When the system is switched to automatic mode, therefore, relay c oil K16 is actuated via the
"Automatic" line and normally closed contact K17. • Relay K16 is latched. Current flows through the coil of relay K17 via a normally open contact of K16,
and relay K17 is also latched. • No more current flows through the normally closed contact of K17. Relay Circuit Diagram
:
Main Circuit
: • The solenoid coils of the directional control valves are actuated with the main circuits. • There are 6 coils altogether. To allow power to be supplied to the coils, the main switch must be in
position 1 and the EMERGENCY STOP device must not have been actuated. • The compressed air is connected via relay K18 in order to prevent the pneumatic drives from moving
before the relays have assumed a defined position. • Conditions for actuation of the solenoid
coils
Pneumatic
Circuit
:
Solenoid
Wiring :
List of
Relays :
List of
Control Elements
: Elements
Control
: ensor Evaluation
S
:
Sequ
ence Step Switching
:
S
olenoid Circuit
:
Programmable Logic ControllersProgrammable Logic Controllers(PLC`S) Introduction : • A PLC (i.e. Programmable Logic Controller) is a device that was invented to replace the necessary se quential relay circuits for machine control. • The PLC works by looking at its inputs and depending upon their state, turning on/off its outputs. The
user enters a program, usually via software, that gives the desired results. PLC Example : • Let's assume that when a switch turns on we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on for. • We can do this with a simple external timer. But what if the process included 10 switches and
solenoids? We would need 10 external timers. W hat if the process also needed to count how many times the switches individually turned on? We need a lot of external counters. PLC - need : • The bigger the process the more is need for a PLC. • Simply program the PLC to count its inputs and turn the solenoids on for the specified time. • The primary reason for designing PLC w as eliminating the large cost involved in replacing the
complicated relay based machine control systems. Inside PLC : • The PLC mainly consists of :
- A CPU, - Memory areas, and - Appropriate circuits to receive input/output data. • We can actually consider the PLC to be a box full of hundreds or thousands of separate relays,
counters, timers and data storage
locations.
• INPUT RELAYS-(contacts) : These
are connected to the outside world. They physically exist and receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors. • INTERNAL UTILITY RELAYS-(contacts) : These do not receive signals from the outside world nor do
they physically exist. They are simulated relays and are what enables a PLC to eliminate exte rnal relays. There are also some special relays that are dedicated to performing only one task. Some are always on while some are always off. Some are on only once during power-on and are typically used for initializing data that was stored. • COUNTERS : These again do not physically exist. They are simulated counters and they can be
programmed to count pulses. Typically these counters can count up, down or both up and down. • TIMERS : These also do not physically exist. They come in many varieties and increments. The most
common type is an on-delay type. Others include off-delay and both ret entive and non-retentive types. Increments vary from 1ms through 1s. • OUTPUT RELAYS-(coils): These are connected to the outside world. They physically exist and send
on/off signals to solenoids, lights, etc. They can be transistors, re lays, or triacs depending upon the model chosen. • DATA STORAGE : Typically there are registers assigned to simply store data. They are usually used as
temporary storage for math or data manipulation. They can also typically be used to store data when power is removed from the PLC. Upon power-up they will still have the same contents as before power was removed. PLC Operation : • A PLC works by continually scanning a program. We can think of this
scan cycle as consisting of 3 important steps.
• Step 1-CHECK
INPUT STATUS : First the PLC takes a look at each input to deter mine if it is on or off. I n other words, is the sensor connected to the first input on? How about the second input? How about the third... It records this data into its memory to be used during the next step. • Step 2-EXECUTE PROGRAM : Next the PLC executes program one instruction at a time. Maybe
program said that if the first input was on then it should turn on the first o utput. Since it already knows which inputs are on/off from the previous step it will be able to decide whether the first output should be turned on based on the stat e of the first input. It will sto re the execution results for use later during the next step. • Step 3-UPDATE OUTPUT STATUS : Finally the PLC updates the status of t he outputs. It updates the
outputs based on which inputs were on during the first step and the results of executing your program during the second step. Based on the example in step 2 it would now t urn on the first output because the first input was on and your progr am said to turn on the first output when this co ndition is true. After the third step the PLC goes back to step one and repeats the steps continuously. One scan time is defined as the time it takes to execute the 3 steps listed above. PLC - Response : • The total response time of the PLC is a fact we have to consider when purchasing a PLC. • PLC takes a certain amount of time to react to changes. In many applications speed is not a concer n, in
others though...
PLC –
Response Time Concern :
• Input 1 is
not seen until Scan 2. • Input 2 is not seen until Scan 3.
• Input 3 never seen by PLC.
• To avoid this we say
that the input should be ON for at least 1 input delay time + one scan time . • But what if it was not possible for the input to be on this long? Then the plc doesn't see the input
turn on. Pulse Stretch Function : • This function extends the length of t he input signal until the plc looks at the inputs during the next
scan.( i.e. it stretches the duration of the pulse.)
Interrupt
Function : • This function interrupts the scan to process a special routine i.e. As soon as the input turns on,
regardless of where the scan currently is, the PLC immediately stops what its doing and executes an interrupt routine. • An interrupt routine can be thought of as a mini program outside of the main program. After its done
executing the interrupt routine, it goes back to the point it left off at and continues on with the normal
scan process.
Relays : • We understand how the PLC proce sses
inputs, outputs, and the actual program. • Now lets see How a relay actually works . After all, the main purpose of a PLC is to replace "real-world"
relays. • We can think of a relay as an electromagnetic switch. • Apply a voltage to the coil and a magnetic field is generated. This magnetic field sucks the contacts of
the relay in, causing them to make a connection. • These contacts can be considered to be a switch. They allow current t o flow between 2 points thereby
closing the circuit. Relays – a real example :
• Here we simply turn on
a bell whenever a switch is closed. • We have 3 real-world parts : A switch, a relay and a bell. Whenever the switch closes we apply a
current to a bell causing it to sound. Replacing Relays : • Lets use a PLC in place of the relay. • The first thing that's necessary is to create what's called a Ladder Diagram.
• We have to create one of these because, unfortunately, a PLC doesn't understand a schematic diagram
it only recognizes code. • Most PLCs have software which convert ladder diagrams into code. Ladder Diagram : • First Step
: Translate all of the items we're using into symbols the PLC understands. • Second step : We must tell the PLC where everything is located. In other words we have to give all the
devices an address. • Final step : We have to convert the schematic into a logical sequence of events.
First Step : • The PLC doesn't understand terms like switch, relay, bell, etc. • It prefers input, output, coil, contact, etc. • It doesn't care what the actual input or output device actually is. It only cares that its an input or an
output. • First we replace the battery with a symbol. This symbol is common to all ladder diagrams. We draw
what are called bus bars. • These simply look like two vertical bars. One o n each side of the diagram. Think of the left one as being
+ voltage and the right one as being ground. Further think of the current (logic) flow as being from left to right. • Next we give the inputs a symbol. In this basic example we have one real world input. (i.e. the switch). • We give the input that the switch will be connected to the symbol shown below. This symbol can also
be used as the contact of a relay .
• Next we give the outputs a symbol. In this
example we use one output (i.e. t he bell). • We give the output that the bell will be physically connected to the symbol shown below. This symbol
is used as the coil of a relay.
• The AC supply is an external supply so we don't
put it in our ladder. The PLC only cares about which output it turns on and not what's physically connected to it. Second Step : • We must tell the PLC where everything is located. In other words we
have to give all the devices an address. • Where is the switch going to be physically connected to the PLC? How about the bell? We start with a
blank road map in the PLCs town and give each item an address. • Could you find your friends if you didn't know their address? You know they live in the same town but
which house? The plc town has a lot of houses (inputs and outputs) but we have to figure out who lives where (what device is connected where). • We'll get further into the addressing scheme later. The PLC manufacturers e ach do it a different way!
For now let's say that our input will be called "0000". The output will be called "500". Final Step : • Convert the schematic into a logical sequence of events. • The program we're going to write tells the PLC what to do when certain events take place. • In our example we have to tell the plc what to do when the operator turns on the
switch.
• Final converted diagram.
• We eliminated the real world relay from needing a symbol. Basic Instructions : Load : • The load(LD) instruction is a normally open contact . It is sometimes also called examine if on ( XIO).(as
in examine the input to see if its physically on). The symbol for a load instruction is shown
below.
• This is used when an input signal is needed to be
present for the symbol to turn on. • When the physical input is on we c an say that the instruction is True. • We examine the input for an on signal. If the input is physically on then the symbol is on. • An on condition is also referred to as a logic 1 state. Load Bar :
• The Load bar instruction is a normally closed contact . It is sometimes also called LoaDNot or examine
if closed(XIC)(as in examine the input to see if its physically closed) The symbol for a loadbar instruction
is shown below.
• This is used when an input
signal does not need to be prese nt for the symbol to turn on. • When the physical input is off we can say that the instruction is True. • We examine the input for an off signal. If the input is physically off then t he symbol is
on.
• With most PLCs this
instruction(Load or Loadbar) MUST be the first symbol on the left of the ladder. Out : • The Out instruction is sometimes also called an Output Energize instruction . The output instruction is
like a relay coil. Its symbol looks as shown below.
• When there is
a path of True instructions preceding this on t he ladder rung, it will also be True. • When the instruction is True it is physically ON . • We can think of this instruction as a normally open output. Out Bar : • The Outbar instruction is sometimes also called an OutNot instruction. • The Outbar instruction is like a normally closed relay coil . Its symbol looks like that shown
below.
A Simple Example
:
• In the above circuit, the coil will be energized
when there is a closed loop between the + and - terminals of the battery.
• We can simulate this same circuit with a ladder diagram.
ladder diagram consists of individual rungs just like on a real ladder. • Each rung must contain one or more inputs and one or more outputs. • The first instruction on a rung must always be an input instruction and the last instruction on a rung
should always be an output (or its equivalent). • Notice in this simple one rung ladder diagram w e have recreated the external circuit above with a
ladder diagram. • Here we used the Load and Out instructions. • Some manufacturers require that ever y ladder diagram include an END instruction on the last rung.
Some PLCs also require an ENDH instruction on the rung after the END rung.
• A
• Next we'll trace the registers PLC Registers :
• In Previous
example change switch 2 (SW2) to a normally closed symbol (loadbar instruction). • SW1 will be physically OFF and SW2 will be physically ON initially . The ladder diagram now looks like
this :
• We gave each
symbol (or instruction) an address. • This address sets aside a certain storage area in the PLCs data files so that the status of the instruction
(i.e. true/false) can be
stored.
A Level
Application : • We are controlling lubricating oil being dispensed from a tank. • This is possible by using two sensors. • We put one near the bottom and one near the top, as shown in the picture.
Here, we want the fill motor to pump lubricating oil into the tank until the high level sensor turns on. At that point we want to turn off the motor until the level falls below the low level sensor. Then we should turn on the fill motor and repeat the
process.
Here we have a need for 3 I/O (i.e.
Inputs/Outputs) : • 2 are inputs (the sensors) and 1 is an output (the fill motor). • Both of our inputs will be NC (normally closed) fiber-optic level se nsors. When they are NOT immersed
in liquid they will be ON. When they are immersed in liquid they will be
OFF.
Program Scan
: • Initially the tank is empty. Therefore, input 0000 is TRUE and input 0001 is also
TRUE.
• Gradually the tank
fills because 500(fill motor) is on. • After 100 scans the oil level rises above the low level sensor and it becomes open. (i.e.
FALSE).
• Even when the
low level sensor is false there is still a path of true logic from left to right. This is why we used an internal relay. Relay 1000 is latching the output (500) on. It will stay this way until there is no true logic path from left to right.(i.e. when 0001 be comes
false).
• Since there is
no more true logic path, output 500 is no longer energized (true) and there fore the motor turns
off.
• Even though the
high level sensor became true there still is NO continuous true logic path and therefore coil 1000 remains false!. • After 2000 scans the oil level falls below the low level sensor and it will also become true again. • At this point the logic will appear the same as SCAN 1 above and the logic will repeat as illustrated
above.
Latch
Instruction : • The latching instructions let us use momentary switches and program the plc so that when we push one the output turns on and when we push another the output turns off. • Picture the remote control for your TV. It has a button for ON and another for OFF :
- When I push the ON button the TV turns on. - When I push the OFF button the TV turns off. • I don't have to keep pushing the ON button to keep the TV o n. This would be the function of a latching
instruction. • The latch instruction is often called a SET or OTL (output latch). • The unlatch instruction is often called a RES (reset), OUT (output unlatch) or RST (reset). The diagram
below shows how to use them in a program.
Here we are using 2
momentary push button switches. One is physically connected to input 0000 while the other is physically connected to input 0001. When the operator pushes switch 0000 the instruction "set 0500" will become true and output 0500 physically turns on. Even after the operator stops pushing the switch, the output (0500) will remain on. It is latched on. The only way to turn off output 0 500 is turn on input 0001. This will cause the instruction "res 0500" t o become true thereby unlatching or resetting output
0500.
• What would
happen if input 0000 and 0001 both t urn on at the exact same time. • Will output 0500 be latched or unlatched? • To answer this question we have to think about the scanning sequence. The ladder is always scanned
from top to bottom, left to right. • The first thing in the scan is to physically look at the inputs. • 0000 and 0001 are both physically on.
• Next the PLC executes the program. • Starting from the top left, input 0000 is true therefore it should set 0500. • Next it goes to the next rung and since input 0001 is true it should reset 0500. • The last thing it said was to reset 0500. Therefore on the last part of the scan when it updates the
outputs it will keep 0500 off. (i.e. reset 0500). Counters : • A counter is a simple device intended to do one simple thing - count. • There are up-counters(they only count up 1,2,3...). These are called CTU,(count up) CNT,C, or CTR. • There are down counters(they only count down 9,8,7,...). These are typically called CTD (count down).
To use Counters we must know 3 things : • Where the pulses that we want to count are coming from. Typically this is from one of the inputs.(a
sensor connected to input 0000 for example). • How many pulses we want to count before we react. Let's count 5 widgets before we box them, for
example. • When/how we will reset the counter so it can count again. After we count 5 widgets lets reset the
counter, for example.
• In this counter we need 2 inputs :
- One goes before the reset line. When this input turns on t he current (accumulated) count value will return to zero. - The second input is the address where the pulses we are counting are coming from. • Cxxx is the name of the counter. If we want to call it counter 000 then we would put "C000" here. • yyyyy is the number of pulses we want to count before doing
something.
Timers : • TIMER : It is an instruction that waits a set amount of time before doing something. • Type of Timers : On-Delay Timer and Off-Delay Timer.
On-Delay Timer : • Simply "delays turning on". • After sensor (input) turns ON, wait x -seconds before activating a solenoid valve(output). • This is the most common timer. It is often called TON(timer on-delay), TIM(timer) or TMR(timer).
Off-Delay Timer : • Simply "delays turning off". • After sensor (input) sees a target it turn on a solenoid (output). • When the sensor no longer sees the target it hold the solenoid on for x-seconds before turning it off. • It is called a TOF (timer off-delay).
Need to know 2 things :
• What will enable the timer :
- Typically this is one of the inputs,(a sensor co nnected to input 0000 for example). • How long we want to delay before we react.
- Let's wait 5 seconds before we turn on a solenoid, for example.
On-Delay
Timer : • Txxx : Timer Name. • When the enable input is ON t he timer starts to tick. • When it ticks yyyyy (the preset value) times, it will turn on its contacts that we will use later in the
program.
Timer in Ladder Diagram :
• Wait for input 0001 to turn on.
• Timer T000 (a 100ms increment timer) starts ticking. It will tick 100 times. Each tick (increment) is
100ms so the timer will be a 10000ms (i.e. 10 second) timer. • 100ticks X 100ms = 10,000ms. • When 10 seconds have elapsed, t he T000 contacts close and 500 turns on. • When input 0001 turns off (false) the timer T000 will reset back to 0 causing its contacts to turn off
(become false) thereby making output 500 t urn back off. One Shot : • A one-shot is used to make something happen for ONLY 1 SCAN .
• Most manufacturers have one-shots that react to an off to on transition and a different type that
reacts to an on to off transition. • Some names for the instructions could be DIFU/DIFD (differentiate up/down), SOTU/SOTD(single
output up/down), OSR (one-shot rising) and others. Master Controls
:
• Master controls can be thought of as "emergency stop
switches". • An emergency stop switch typically is a big red button on a machine that will shut it off in cases of
emergency. • The master control instruction typically is used in pairs with a master control reset. • It is commonly abbreviated as MC/MCR(master control/master control reset). • Timers should not be used inside the MC/MCR block. MCR in Ladder Diagram
:
• In this example, rungs 2 and 3 are only executed when input
0000 is on (true). • If input 0000 is not true the plc pretends that the logic between the MC and MCR instructions does not
exist. Shift Registers : • To store many previous events and act upon them later. • We use a register or group of registers to form a train of bits (cars) to store the previous on/off status.
Each new change in status gets stored in the first bit and the remaining bits get shifted down the train. • SFT (ShiFT), BSL (Bit Shift Left), SFR (Shift Forward Register).
• BSR (Bit Shift Right) and SFRN (Shift Forward Register Not).
Symbol needs 3 inputs and has some data inside the symbol : 1000: First Bit, 1003 : Last Bit. • Data : The data input gathers the true/false statuses that will be shifted down the train. When the data input is true the first bit (car) in the register (train) will be a 1. This data is only entered into t he register (train) on the rising edge of the clock input. • Clock : The clock input tells the shift re gister to "do its thing". On the rising edge o f this input, the shift
register shifts the data one location over inside the register and enters the status of the data input into the first bit. On each rising edge of this input the process will repeat. • Reset : The reset input does just what it says. It clears all the bits inside the register we're using to
0. Shift Registers - Application : • Imagine an ice-cream cone machine. We have 4 steps : 1. First we verify the cone is not broken. 2. Next we put ice cream inside the cone.(turn on output 500). 3. Next we add peanuts.(turn on output 501). 4. Finally we add sprinkles.(turn on output 502). • If the cone is broken we obviously don't want to add ice cream and the other items. Therefore we
have to track the bad cone down our process line so that we can tell the machine not to add each item. • We use a sensor to look at the bottom of the cone (input 0000). If its on t hen the cone is perfect and if
its off then the cone is broken. • An encoder tracks the cone going down the conveyor. (input 0001). A push button on the machine will
clear the register. (input
0002).
A good cone comes in front of the sensor (input 0000). The sensor (data input) turns on. 1000 will not turn on until the rising edge of the encoder (input 0001). Finally the encoder now generates a pulse and the status of the data input (cone sensor input 0000) is transferred to bit 1000. The register now looks like
:
As the conveying
system moves on, another cone comes in front of the sensor. This time it's a broken cone and the sensor remains off. Now the encoder gener ates another pulse. The old status of bit 1000 is transferred to bit 1001. The old status of 1001 shifts to 1002. The old status o f 1002 shifts to 1003. And the new status of the data input (cone sensor) is tr ansferred to bit 1000. The register now looks like
:
Since the register shows
that 1001 is now on, the ladder says that output 0500 will turn on and ice cream is put in the cone. As the conveying system continues to move on, another cone comes in front of the sensor . This time it's a good cone and the sensor turns o n. Now the encoder generates another pulse. The old status of bit
1000 is transferred to bit 1001. The old status of 1001 shifts to 1002. The old status of 1002 shifts to 1003. And the new status of the data input (cone sensor) is transferred to bit 1000. The register now looks like
:
Since the register shows that 1002 is now on the ladder says that output 0501 will turn on and peanuts are put on the cone. Since 1001 now holds the status of a broken cone, 500 remains off in the ladder above and no ice-cream is inserted into this cone. As the conveying system continues to move on, another cone comes in front of the sensor. This time it's also a good cone and the sensor turns on. Now the encoder generates another pulse. - The old status of bit 1000 is transferred to bit 1001. - The old status of 1001 shifts to 1002. - The old status of 1002 shifts to 1003. - And the new status of the data input(cone sensor) is transferred to bit 1000. The register now looks like
:
• Since the register shows
that 1003 is now on the ladder says that output 0502 will turn on and sprinkles are put on the cone. • Since 1002 now holds the st atus of a broken cone, 501 remains off in the ladder above and no peanuts
are put onto this cone. • Since the register shows that 1001 is now on the ladder says that output 0500 will turn on and ice
cream is put in that cone. As the conveying system continues to move on, another c one comes in front of the sensor. This time it's another broken cone and the sensor turns off. Now the encode r generates another pulse.
The old status of bit 1000 is transferred to bit 1001. The old status of 1001 shifts to 10 02. The old status of 1002 shifts to 10 03. And the new status of the data input (cone sensor) is transferred to bit 1000. The register now looks like
:
As the conveying system
continues to move on, another cone comes in front of the sensor. This time it's another broken cone and the sensor turns off. Now the e ncoder generates another pulse. The old status of bit 1000 is transferred to bit 1001. The old status of 1001 shifts to 10 02. The old status of 1002 shifts to 10 03. And the new status of the data input (cone sensor) is transferred to bit 1000. The register now looks like
:
Notice that the status of
our first cone has disappeared. • The shift register is most commonly used in conveyor systems, labeling or bottling applications,
etc. Programming Languages : • IEC 61131-3 defines FIVE programming languages for PLC. • The languages can be mixed in any way within a PLC project. • Machining process involved in Valve Production :
- Two sensors are used to establish whether a work piece with correctly drilled holes is available at the machining position. - If the valve to be machined is of type A or type B (set via two selector switches). - The cylinder advances and presses the sleeve in to the drilled hole. L adder Diagram (LD) : • Graphical Programming language derived from the circuit diagram of directly wired relay
controls.
Functional
Block Diagram (FBD) : • Functions and function blocks are represented graphically and interconnected into networks. • Originates from Logic
diagrams.
Instructio
n List (IL) : • Textual Assembler-type language. • Formulated from control instructions consisting of an operator and an
operand.
Structured Text (ST) : • Structured text is high-level
language based on Pascal, which consists of expressions and instructions. Sequential Functional Chart (SFC) : • Language resource for the structuring of sequence oriented control programs. The elements o f SFC are steps, transitions, alternative and parallel branching. Common Elements of Programming Languages : • Resources of a PLC : -> Inputs, outputs and the memory. -> Directly addressed variables. • Variables and data types :
-> Representation of data. -> Data types. -> Variable declaration.
-> Initialization. • Program :
-> Functions. -> Function blocks. Inputs, outputs and memory : • Designation of resources :
-> Inputs I. -> Outputs Q. -> Memory M. • Data types :
-> BOOL (X) 1 bit. -> BYTE (B) 8 bit. -> WORD (W) 16 bit. • IX, QX, MX, IB, QB, MB, IW, QW, MW. • Multiple resources :
-> I1 Input 1. -> I15 Input 15. -> QW3 Output Word 3. -> MB5 Memory Byte 5. • Directly addressed variables :
-> Resource in control program addressed directly. -> Prefixed with the sign %. -> %I12 – Input bit 12, %QB8 – Output byte 8. Variables and Data Types : • Representation of Numerical
Data : -> Integers: 12, -8, 123_456, +751. -> Floating point: 0.123_4, -12.0, -8.0. -> Numbers to base 2: 2#1101_0011.
-> Numbers to base 8: 8#323. -> Numbers to base 16: 16#D3. -> Boolean: 0, 1.
• Representation of Time Data :
-> Time duration: T#18ms, TIME#18ms, t#3.5s. -> Date: D#2008-04-21, DATE#2008-04-21. -> Time of day: TOD#13:18:42.55. -> Date and Time: DT#2008-04-21:13:18:42.55.
• Representation of Strings :
->‘B’ character. -> ‘warning’ string. • Data Types :
-> BOOL -> SINT, INT, DINT, UINT -> REAL -> TIME -> STRING -> BYTE -> WORD
• Controller program constructed into individual organization units :
-> Configuration. -> Resources. -> Programs. -> Function blocks. -> Functions. • All variables have specific position. • All variable declaration starts with a keywor d, which designate position of variable and end with
keyword END_VAR
VAR Temp : INT; Hand : BOOL;
END_VAR • Keywords for variable declaration :
-> Input variables VAR_INPUT -> Output variables VAR_OUTPUT -> In/Output variables VAR_IN_OUT -> Local variables VAR -> Global variables VAR_GLOBAL -> External variables VAR_EXTERN • Keyword AT is used to assign variables to the inputs and outputs of the controller.
VAR Stop_button AT %I2 : BOOL; Temperature AT %IW3 : BOOL;
END_VAR PROGRAM : • Program for a controller is divided into individual organization units at the programming level :
-> Programs. -> Function blocks. -> Functions. Functions : • Function are software modules which, when invoked provide exactly o ne result. • E.g. Addition of INT values or logic
OR.
• Standard Functions :
-> AND, OR, XOR, NOT. -> SHL, SHR, ROR, ROL. -> GT, GE, EQ, LE, LT, NE. -> ADD, MUL, SUB, DIV, MOVE. Function Blocks : • Function blocks are software modules, which supply one or several result parameters. • One important characteristic is possibility of instantiation. • To use, a copy or instance must be created. • The status information of the function block copy remains intact from one processing to the next.
• Example : Counters, Timers. • CTU : incremental
counter.
Standard Function
Blocks : • CTU – Incremental counter. • CTD – Decremental counter. • TP – Pulse. • TON – Switch-ON delay signal. • TOF – Switch-OFF signal delay. • R_TRIG – Edge detection: rising edge. • F_TRIG – Edge detection: falling edge. Programs : • Consists of any language elements. • Constructs necessary instructions to achieve the desired machine or process behavior by PLC. • Stairwell Light Control Program :
-> The stairwell light is switched ON for 3 min, if one of the two light switches on the apartment door or the front door is
activated.
Timers
: • IEC 61131-3 defines 3 types of timer function blocks : -> TP Pulse Timing. -> TON On-delay timing. -> TOF Off-delay timing. • Time duration is specified by means of a defined character format :
-> T#2h15m, T#20s. -> T#10m25s, t#3h_40m_20s. Timers – TP Pulse Timer : • TP is started by a shorter or longer 1-signal at input IN. A 1-signal now applies at output Q for the time specified at its input PT (preset t ime). • The output signal Q has a fixed
duration.
TP Pulse Timer Example : • Pressing of the start button S2 S 2 is to cause the piston of a cylinder to advance. This mechanism is used to clamp workpieces. • When the piston advance fully, it is to remain in this position for 20 seconds. • The cylinder then returns to its initial
position.
Timers – Switch-
On signal delay : • TON is used to generate switch ON signal delays. • After the start via a 1-signal at input IN, output Q does not assume value 1 until the time specified at
input PT has expired, and retains this until input signal IN returns to 0. • If the duration of input signal IN is shorter than the PT, the value o f output remains at
0.
TON Example : • Cylinder 1A1 extends, if start button S1 is actuated. • Once this has been extended for 2s, a second cylinder 2A1 move to its forward end position. • Sensors 1B1 and 1B2 indicates the retracted and the forward e nd positions of cylinder
1A1. Off signal delay : • TOF is used to generate switch OFF signal delays.
Timers – Switch-
• Timer start via a 1-signal at input IN, at the same time, output Q assume value 1. • After the input signal IN has re verted to the value 0, the output remains at 1 for the duration PT and
does not return to 0 until it
expired.
Incremental counter CTU : • Counter is set at the initial value 0 by a signal at reset input R. • The value in the counter is incremented by 1 with each positive edge at counter input CU (count up). • As soon as current value is equal to or greater than the preset value, the output value assumes value
1. Prior to reaching this value Q = 0. example : • Parts are to be ejected from a gravity-feed magazine via a cylinder.
CTU
• If Push button S1 is actuated, the cylinder is to advance, eject a workpiece and then retract again. • 15 parts are to be ejected in this way. When 15 parts have been ejected, it should no longer be
possible to trigger a cylinder movement via push button S1. First the counter must be reset by actuating push button
S2.
Decremental counter
CTD : • The decremental counter with preselect value PV is loaded with a 1 -signal at input LD (load). • During normal operation, each positive edge at input C D (count down) reduces the counter reading. • Output Q of function block CTD is 0, until the counter reading CV becomes less than or =
0.
CTD example : • A cylinder is moved via a valve
1Y1. • The position of cylinder is signaled via the sensors 1B1 (retracted) and 1B2 (extended). • The cylinder is to advance, if push button S1 is pressed. • When 10 strokes have been executed this way, lamp H1 is illuminated and the counter has expired. • The counter must be re-loaded with the preset value, before any cylinder movements can be executed
further. This is affected by means of actuating push button S2. Automation Control Systems : Automation control systems : • These control systems are used in manufacturing plants of all types, and some other applications that
you may not have considered. • The control systems are built around special devices, designed to operate industrial machines, and
processes. We call these devices Programmable Logic Controllers (PLC) and Programmable Automation
Controllers (PAC).
PLC and PAC Systems : • PLCs
were introduced in the early 1970s. The term “PAC”, was developed to differentiate those older systems from today’s much more pow erful, and flexible devices. • An analogy can be made betwee n the VHS video tape, and a DVD. Both systems allow viewers to
record TV programs for viewing at a latter time, but the DVD also can also be used to record music, data, and more. • PLCs were designed to control machinery. PACs can be used for m achine control, process, motion
control, and other applications.
• We will use the
term PLC generically to refer to both PLCs, and PACs. • This section will explore the various components that comprise a PLC
system.
Basic Components of a PLC System
: There are five basic components in a PLC system : • The PLC processor, or controller. • I/O (Input /Output) modules. • Chassis or backplane. • Power supply. • Programming software that runs in a PC .
In addition to these 5, most PLCs also have : A network interface. Let’s look at each in more detail..
Processor, Controller, or CPU : • Stores the control program and data in its memory. • Reads the status of connected input devices. • Executes the control program.
• Commands connected outputs to change state based o n program execution.
- For example : Turn a light on, start a fan, adjust a speed, or temperature. • Comes in various physical
forms.
I/O Modules
: • Physically connect to field devices. • Input modules convert electrical signals coming in from input field devices such as pushbuttons, to
electrical signals that the PLC can understand. • Output modules take information coming from the PLC and convert it to electrical signals the output
field devices can understand, such as a motor starter, or a hydraulic solenoid valve. • I/O comes in various
forms.
• Input
modules interface directly to devices such as switches and temperature sensors. • Input modules convert many different types of e lectrical signals such as 120VAC, 24VDC, or 4-20mA, to
signals which the controller can
understand.
• Input
modules convert real world voltage and currents to signals the PLC can understand. Since there are different types of input devices, there is a wide variety of input modules available, including both digital and analog
modules.
Discrete vs.
Analog Modules : • Discrete Modules use only a single bit to represent the state of the device. For example, a switch is either open or closed. Therefore, the bit is either a 0 (switch is open) or a 1 (switch is closed). Discrete modules are also known as Digital modules. • Analog Modules use words to represent t he state of a device. An analog signal repre sents a value.. For
example, the temperature could be 5, 9, 20, 100, etc degrees. Analog modules use a value, such as 52, rather than a 0 or 1 to represent the state of the
device.
Discrete Modules :
• Devices that are either on or off , such as a pushbutton, get wired to discrete modules. Discrete
modules come in a variety of types, such as 24VDC or 120VAC. You can buy discrete modules that allow you to typically connect anywhere from 2 to 32 devices, with the most popular being 16 devices. • Since it takes only 1 bit to represent the state of a device, a 16 point discrete module only requires 16
bits of memory in the controller to store the states of all the points on the module. Analog Modules : • Devices that have a number associated with them, such as a temperature sensor, get wired
to analog modules. Analog modules come in a variety of types, such as 4 to 20 mA or 0 to 10 VDC. You can buy analog modules that allow you to connect anywhere from 2 to 16 devices. • Since it takes 1 word to represent a number, a 16 point analog module requires 16 words of memory
in the controller to store the value of all the numbers on the module. Each word in a PLC takes 16 or 32 bits (depending on the PLC), therefore it takes 16 or 32 times the amount of PLC memory to store analog points vs. digital points. Output Modules : • Output modules interface directly to devices such as motor starters and lights. • Output modules take digital signals from the PLC and convert them to electrical signals such as 2 4VDC
and 4 mA that field devices can
understand.
• Outp
ut modules take a signal from a PLC and convert it to a signal that a field device needs to operate. Since there are different types of output devices, there is a wide variety of output cards available, including both digital and analog
cards.
Basic Components of
a PLC System : Chassis/Backplane : All PLCs need some method of communicating between the controller, I/O and communications modules. Here are three ways used to accomplish this communications between the various components that make up the PLC system. • Modules are installed in the same chassis as the PLC and communicate over the chassis backplane. • Modules are designed to “plug” into each other. The interconnecting plugs form a backplane there is
no chassis.
• Modules are built into the PLC and modules come together in one physical block. The backplane in this
case is transparent to the user. Below is an example of a backplane in a chassis based system. You can see the backplane in t he area where the modules are not insert ed. The modules have connectors that plug into the black connectors on the backplane. All of the connectors on the backplane are connected together
electrically.
Chassis
and Backplane Examples
:
Benefits of the
Different Forms : • Great flexibility in choice of modules. Modules can be easily installed or removed without affecting other modules. • Great flexibility in choice of modules. In some cases modules cannot be removed without “breaking the chain” and affecting all modules downstream no chassis cost. • Low cost solution but limited flexibility, generally used in smaller, simpler
systems.
Basic Components of a PLC System : Power
Supply : A power supply is needed to provide power to the PLC and any other modules. Power supplies come in various forms : • Power supply modules that fit into one of the slots in a chassis. • External power supplies that mount to the outside of a chassis. • Stand alone power supplies that connect to the PLC or I/O through a power cable. • Embedded power supplies that come as part of the PLC
block.
ramming Software : Software that runs on a PC is required to configure and program PL Cs. • Different products may require different progr amming software.
Prog
,br> • Software allows programs to be written in several different
languages
Network Interface : Most PLCs have the ability to communicate with other devices. These devices include computers running programming software, or collecting data about the manufacturing process, a terminal that lets an operator enter commands into the PLC, or I/O that is located in a remote location from the PLC. The PLC will communicate to the other devices through a network
interface.
Network
Interface :
PLC
Control Panel : • Typically, PLCs are installed in enclosures, on a
“panel”.
PLCs are part of a
Control System : The PLC system is the center of a control system, but it is not the entire control system. There are several other key pieces that must be added to a PLC system to make a complete control system. Examples are :
• Operator terminals. • Networks. • Distributed I/O devices (I/O that is in a different location then the
PLC).
Programmable Logic ControllersPLC programming languages Programming a P LC : Every PLC has associated programming software that allows the user to enter a program into the PLC. Software used today is Windows based, and can be run on any PC.
•
Different products may require different software: PLC5, SLC, and ControlLogix each require their own programming •
software. Bef ore a PLC can perform any control task, it must be programmed to do so. The most popular language used to program a PLC is ladder •
logic.
In a conveyor system, we have several “requirements” to accomp lish; for example, timing and counting parts on the conveyor. Each of these requirements must be programmed into the PLC so that it knows how to respond to different •
events. The programmer develops the program, and connects their personal computer to the PLC through a network or cable and then downloads the program to the PLC. Ladder Logic Example •
: Here, we can see an example of ladder logic. Each line of code is known as a “rung”. In this example there are 4 rungs, numbered 0, 1 and 2, and the end rung marking the end of the program. •
The PLC executes the program 1 rung at a time, starting with the first rung and then working
•
down. Ladder logic rungs are basically IF-THEN statements. Each individual rung is e xecuted from the left to the right. The outputs at the right side of each rung is set to a condition that reflects the status of the permissive contacts in a particular rung. This is a very simple rung of logic, from a PLC •
•
program :
The rung is read as :
•
-> If the Start Button is on, turn the Motor on. If the Start Button is off, then turn the Motor off. -> Let’s take a look at this simple program in
detail… In this example, if the actual Start Button is on, then the value of all the XICs named Start Button, in the program will be true(also known as a ‘one”, or “closed”). •
If the start button is off (not on) then the value of the Start Button XICs will be false (also known as a “zero”, or •
“open”). This instruction turns on if the logic to the left of the OTE is true. If the logic is false, then the output will be turned off . The OTE commands a physical output located on a output module to turn either on or off. •
•
If there are multiple XIC’s on the rung, then all would have to be on for the rung to be
true. If there are multiple OTE’s on the rung, then all would be turned on or off based on the rung condition (true or •
false).
The text above the XIC and OTE is the address associated with the instruction. PLC addresses may appear in many different ways depending on the PLC being used : •
-> Start_Button -> Local:2:I.Data[0].1 -> I:020/2 The address is used by the PLC to tell exactly which input to read or which output to command. I/O Addressing : Since a PLC will be controlling real devices down on a plant floor, it has to have some way of communicating to the correct device. All P LCs use some sort of method of I/O Addressing to perform this function. •
I/O addresses are a means to tie a physical I/O point to a location in PLC memory.
•
An input address will represent the state o f an input device, i.e. the switch is on or off.
•
An output address will represent the commanded state for a device. i.e., turn the motor on or
•
off. Often, a descriptive name of the device connected to the I/O point is used in addition to, or in place of the base I/O address which describes the physical location of the module in the •
rack. I/O Addressing vs. Data Addressing : I/O addresses are a means to tie a physical I/O point to a location in PLC memory. There are other addresses that do not connect to physical I/O, but are used to hold a value. •
Data addresses store a value used for functions like timers, counters, or
•
calculations. this example, parts on a conveyor are counted. The input, “Part_detected” looks at the I/O address of the actual sensor counting the parts.
In
•
The counter references a data address, and accumulates the counts in that location in memory.
•
Each time the “Part_detected” switch closes, the counter adds one more count to the area of memory called •
“part_count”.
Ladder
Logic Example : Next, we will take a look at an actual segment of ladder logic code. The code will be the code used to control our conveyor from the first lesson. Before we do that, let’s take a look at a few more ladder logic instructions : OTL - Output Latch - turns on the output and keeps it even if the rung goes false. OTU - Output Unlatch - turns off the output when the rung is true. TON - Timer On Delay - when the rung is true the timer will run.
• • •
It will store the elapsed time in the “Accum” field (accumulator). As long as the rung remains true it will count until it reaches the preset value. If the preset value is hit the DN bit will go on (Don bit). When the rung goes false the timer will be reset. Programming a PLC – Conveyor
example : take a look at a simple program for a conveyor application.
Now let’s
•
When a box is placed on the conveyor in front of Photoeye 1, Light 1,and Motor 1 will turn on,
•
causing the box to move down the conveyor to the left. When the box passes in front of Photoeye 2, Motor 1 and Light 1 will turn off, stopping the conveyor. Relay Ladder Logic Example •
: Here’s the program for the conveyor: The first line of code turns on the motor and the light when a box is detected by photoeye1. Likewise, the motor and light are turned off when photoeye2 detects the box in the second line of
code. The third line begins a timer when the box passes by photoeye1, and if the box does not pass by photoeye2 in 30 seconds (the timer counts in milliseconds), the motor and light are shut off by line 4. This is the indication of a jam condition. PLC Addressing Examples : Logix Controller sample addresses : •
1. Motor_start - Binary Tag 2. Tank_temp - Integer Tag 3. Local:5:I.Data.0 - Input Tag (Local - same chassis as processor, in slot 5, is an input, data bit 0)
•
PLC-5 sample addresses :
1. B3:0/2 - Binary File 2. N7:0 - Integer File 3. I:012/3 - Input file (Describes: Rack#, Group, Bit) •
SLC sample addresses :
1. B3:0/2 - Binary File 2. N7:0 - Integer File 3. I:3.0/2 - Input file (Describes: Slot.word/bit) Other Programming Languages : While ladd er logic is the oldest and most popular language used in PLCs today, man y other languages are gaining in popularity and are in wide use. Examples are : Sequential Function Chart(SFC). •
Function Block.
•
Structured Text.
•
Higher level languages such as C.
•
Architecture Cont rol System Architectures : A complete control system is made up of a combination of PLCs, networks, I/O, terminals and software. All the components work together to form a complete control
system. Within a control system architecture there are many subsystems and terms used to de scribe them. This section will go over some of the popularly used terms to describe parts of control systems. Local I/O
•
Distributed I/O
•
Centralized I/O
•
Centralized Control
•
Distributed Control
•
Control System
•
Data Acquisition System
•
Safety System I/O Systems : I/O systems are often referred to as local or distributed :
•
Local refers to the I/O being attached directly to the controller or on the same backplane as the Controller. •
Distributed refers to I/O which is not on the same backplane as the Controller. Distributed I/O is connected using a •
network. The distributed input module sends the inputs across the backplane to the adapter. The adapter sends them over the I/O network to the PLC (Controller). I/O Architectures : I/O architectures are made up of I/O systems. The architectures are referred to as Centralized and Distributed : Centralized refers to the I/O being located near or in the same cabinet as the processor. Wires from field devices are brought back to the I/O, and can be quite long. •
Distributed refers to I/O that is located near the field devices. Th e wires from the field devices are short. The network cable is run out to the Distributed I/O instead of running the field wires back to the •
I/O. Systems Vs. Architectures : I/O systems are part of an I/O architecture : -> If all the I/O is located near or in the same cabinet as the processor, it is a centralized architecture. Within the centralized architecture could be either local or distributed I/O, or •
both. I/O systems are part of an I/O architecture : -> If some of the I/O is located remotely from the processor it is a distributed architecture. Within the distributed architecture is distributed I/O, or both local and distributed •
I/O. Distributed I/O System : Why use Local I/O? :
Local vs.
Faster than distributed I/O.
•
Easy to install - add a module to the chassis : In some cases the I/O is already attached to the processor. •
Less expensive than adding distributed : Use the I/O on the processor or simply add a module to the backplane. Don’t need to add a chassis, power supply, etc. •
Why use Distributed I/O? :
Field devices distributed around the machine - too much wiring to take back to one chassis.
•
Out of local I/O : local I/O limited by number of slots in the backplane or fixed I/O attached to the processor. •
Local I/O does not meet your needs module type, current capability, etc. Centralized vs. Distributed I/O Architecture : Why use a Centralized Architecture? : •
If using local only, then it is faster than distributed I/O.
•
If the only centralized I/O is local I/O, then you don’t have to buy additional chassis, power supply, etc. •
The field devices are near the cabinet or processor so there is no wiring cost savings of going distributed. •
Why use a Distributed Architecture? :
Field devices distributed around the machine - too much wiring to take back to one chassis.
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I/O mounted to machine - becomes a part of it instead of in a central cabinet. Centralized Control : Centralized control refers to a control system where a single (usually large) PLC controls all of the I/O and performs all the control for the •
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system. disadvantages of centralized control :
Advantages and
Advantages :
1. The control program is all in one place. 2. Easier to troubleshoot system problems. 3. I/O performance throughout entire system. Disadvantages :
1. Programs can get quite large. 2. Related I/O performance slower compared to distributed control. 3. Single PLC failure shuts down entire
system. Distributed Control : Distributed control refers to a control system where multiple PLC controllers share the responsibility of controlling the system. The PLC’s usually communicate frequently with each •
other. and disadvantages of distributed control :
Advantages
Advantages :
1. Segmented programs to specific tasks. 2. Easier to troubleshoot local problems. 3. Performance on local network. Disadvantages :
1. System problems harder to troubleshoot. 2. Performance from I/O across PLC’s. 3. Cost. 4. Maintain multiple
programs. The Control System : The control system is the system that is responsible for the control of the process. This is the system that includes the PLC, all of the I/O and any Human Machine Interfaces •
(HMI). Data Acquisition System : The Data Acquisition system is generally responsible for collecting data about the control system, and storing it on master computers or servers, or displaying it on terminals. The data is often used later for reporting or charting •
purposes. Ma de up of devices and networks which are responsible for acquiring data about the process . Not responsible for direct control of the process. •
The network used for data acquisition is often Ethernet. While data acquisition devices can exist directly on the control network, a gateway is often used to separate network traffic between the data acquisition system and the control system. Safety System : The Function of a Safety System is to monitor and control conditions on a machine or process that are hazardous in themselves or, if no action were taken, may give rise to hazardous situations. •
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The Safety System runs in parallel with the Control System. The Control S ystem and Safety System may share components : •
-> Focus of Control System is Throughput. -> Focus of Safety System is Protection. A Safety system is designed to protect :
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-> People. -> Environment.
