UNIT UNIT – V 5.1 STAGES IN DESIGNING MECHATRONICS SYSTEMS The design process consists of the following stages. Satgel: Need for design v The design process begins with a need. Needs usually arise from dissatisfaction with existing situation. v Needs may come from inputs of operating or service personal or from customer through sales or marketing representatives. v They may be to reduce cost, increase reliability or performance or just change because of public has become bored with the product. Stage2: Stage2: Analysi Analysiss of proble problem m v Probably the most critical step in design process is the analysis of the problem i.e., to find out the true nature of the problem. v The true problem is riot always what it seems to be at the first glance. because this stage requires such small small part v Its importance is often overlooked because of the total time to to relate the final design. design. ad vantageous to define the problem as broadly s possible. v It is advantageous problem is not accurately accurately defined, defined, it will will lead to waste of time on v If the problem designs and will not not fulfill the need. Stage3: Stage3: Preparation Preparation of specificatio specification n performance specifications. v The design must meet the required performance v Therefore, specification of the requirements can be prepared. v This will state the problem definition of special technical terms, any constraints placed on the solution, and the criteria that will be used to evaluate the design. Problem statement includes all the functions required of the design, together with any desirable features. The followings are the some of the statements about the problem: v v v v v v v v
Mass and dimensions of design. Type and range of motion required. Accuracy of the element. Input and output requirements of elements. Interfaces. Power requirements. Operating environment. Relevant standards and code of practice, etc.
Stage4: Generation Generation of possible solution solution v This stage is often called as conceptualisation stage. v The conceptulisation step is to detennine the elements. mechanisms, materials, process of configuration that in some combination or other result in a design that satisfies the need. v This is the key step for employing inventiveness and creativity. s ynthesis. v A vital aspect of this step is synthesis.
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Synthesis is the process of taking elements of the concept and arranging them in the proper order, sized and a nd dimensioned in the proper way. Outline solutions are prepared for various possible models which are worked out in sufficient details to indicate the means of obtaining each of the required functions. This stage involves a thorough analysis of the design. The evaluation stage involves detailed calculation, often computer calculation of the performance of the design de sign by using an analytical model. The various solutions obtained in stage 4 are analysed and the most suitable one is selected.
Stage6: Production of detailed design to be worked out. out. v The detail of selected design has to It might have required the extensive simulated service testing of an v experimental model or a full size prototype in order to determine the optimum details of design. Stage7: Production of working drawing v The finalised drawing must be properly communicated to the person who is going to manufacture. v The communication may be oral presentation or a design report. v Detailed engineering drawings of each components and the assembly of the machine with complete specification for the manufacturing process are written in the design report. The components as per the drawings are manufactured. 5.2. TRADITIONAL AND MECHATRONICS DESIGNS v v v v
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Engineering design is a complex process which involves interaction between many skills and discipline. In traditional design, the components are designed through mechanical, hydraulic or pneumatic components and principles. But in mechatronics approach, mechanical, electronics, computer technology and control engineering principles are included to design a system. For example design of weighing scale might be considered only in terms of the compression of springs and a mechanism used to convert the motion of spring into rotation of shaft and hence movements of a pointer across a scale. In this design measurement of weight is depended o n the position of weight on the scale. If we want to overcome foresaid problem, other possibilities can be considered. In mechatronics design, the spring might be replaced by load cells with strain gauges and output from them used with a microprocessor to provide a digital readout of the weight on an LED display. This scale might be mechanically simpler, involving fewer components and moving parts. But the software is somewhat complex. Similarly the traditional design of the temperature control for a central AC system involves a bimetallic thermostat in a closed loop control system. The basic principle behind this system is that the bending of the bimetallic strip changes as the temperature change and is used to operate an ON/OFF switch for the temperature control of the AC system.
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The same system can be modified by mechatronics approach. This system uses a microprocessor controlled thermo couple as the sensor. Such a system has may advantages over traditional system. The bimetallic thermostat is less sensitive compared to the thermodiode. Therefore the temperature is not accurately controlled. Also it is not suitable for having different temperature at different time of the day because it is very difficult to achieve. But the microprocessor controlled thermodiode system can overcome foresaid difficulties and is giving precision prec ision and programmed control. This system is much more flexible. This improvement in flexibility is a common characteristics of the mechatronics system when compared with traditional system.
5.3. POSSIBLE DESIGN SOLUTIONS v v v v
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Every mechanical design can be considered as a microprocessor subsystem as a solution. Some of the possible microprocessor based solutions for traditional systems are given in the following fo llowing article. 5.3.1. Timed switch: Consider a requirement for a device which is used to switch on a motor for some prescribed time.
For this problem, a traditional mechanical system consists of rotating cam and pivoted flexible flexible arm as shown in fig 5.1.
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The cam is rotated at constant rate and the pivoted flexible arm which acts as cam follower used to actuate a switch. The amount of time for which the switch is closed depending on the shape of the cam and speed of the revolution of cam. Some of the possible solution for this problem using mechatronics approach is explained below. Possible Possible solution: solution: The above problem can be solved by b y PLC arrangement as shown fig 5.2. The ladder program is also shown in this fig. To start the timer the following requirements should be satisfied.
Start the pulse applied. Check the timer whether it is ON or OFF condition. The timer should be in OFF condition before triggering. The above 3 steps have been carried out in ladder program.
Apply the start pulse which energies the IR Switch on timer A by IR. Set the timer for which the timer is in ON Get the output from timer A Timer B started when timer A switched ON and determines the time at which output switched OFF
Fig 5.2. PLC timer switch Duration of the timer A and timer B can be varied by using a microprocessor with a memory chip and inputloutput interfaces. The memory chip can be activated by a assembly language program. The program is written in such a manner that the timer is turned ON by a start pulse, after a particular duration (i.e., delay time) the timer is turned OFF. The delay can be varied using a simple program given below:
BNE LOOP RT S DELAY LOOP LDX DATA DEX In this program LDX represents loading a particular particular “DATA”. This This DATA represents the number of loops DEX-decrements the index register. BNE-branch to loop if if not equal to zero. zero. RTS - return to main program. program. Advantages of PLC system over traditional mechanical system: v
In PLC system, the time duration can be easily adjusted by changing the timer preset values (i.e., DATA) in the program whereas the traditional system requires various sizes of the cams.
Possible soltuion2: v The alternative for this problem is to use timer IC. v The well- known IC for this purpose is 555. v The external resister and capacitor are used to set the timing intervals in 555 timer. Fig 5.3 shows a internal circuit diagram of 555 timer with external circuits. v When applying the trigger, the output is turned ON and the time duration of ON output being 1.1 R. C. where R = resistance in ohms and C = capacitors in farads. Large times need larger value of R and C. R value varies from 1 kO to I MO. The C value ranges from 0.1 to l0 v The accuracy of 555 timer is the best within the foresaid values of R and C otherwise leakage capacitance becomes a problem. duration is required, required, 555 timer can be replaced with ZN! 034E v If more time duration timer. The circuit shown in fig 5.3 is limited to times less than about 11 seconds.
Fig 5.3. Circuits of 555 timer Possible solution 3: v Another alternative for this problem is to use the micro controller timer such as 8051, 8096 and MC68HCI 1. v Here we are using MC68HCI 1 micro controller. Fig 5.4 shows clock diagram of MC68HC1I micro controller timer. v This circuit consists of a 8MHz crystal, two 22PF capacitor, a 1 OOMQ resister, and 2MHz E-clock. v The timer is controlled by 16-bit count register (TCNT). When a system is reset, TCNT is set set to $0000. It counts continuously up to $FFFF. On the application o f next pulse it overflows and reset to $0000. v v When it overflows, it will set timer flag TOF. Seventh bit of timer interrupt flag register 2 (TFLG 2) is TOF. v The system E-clock can be prescaled by setting bits in the time r interrupt mask register2 (TMSK2). v The prescaling factor is given in fig 5.5.
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Fig 5.4. E-clock generation While using this timer, the TOF status is watched by polling. When the flag is set, the program resets the flag in the TFLG2. This timing operation consists of program duration of the timer number of overflow flag settings.
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Port A of the micro controller can be used for general-purpose input output functions for timing. The timer port consists of output pins OC 1, 0C2, 0C3, OC4 and 0C5 with their corresponding internal registers TOC1. TOC2, TOC3, TOC4 and TOC5. Fig 5.6 shows the output comparator. This comparator compares the value in the free timing counter (TCNT) with the preset value in TOC and sets OC according to the preset condition. The output-compare function can generate timing delays with much higher accuracy than the timer overflow flag. For example, with a prescale factor of 4 and E-code of 2MHz can generate time delay up to 13 1.072 milliseconds (ms). The longest delay of 32.768ms with the prescale factor of I. In order to generate longer time delays, multiple output compare functions are required. To get the total time delay of O.5s, we may use output compare operation producing a delay of 25ms and repeating for 20 times.
The following program is used to control the micro controller to get the delay of 0.55. REGBAS EQU $1000 TFLG1 EQU $23 TCNT EQU $OE TDC2 EQU $18 OC1 EQtJ $40 CLEAR EQU $40 D25MS E 50000 NTIMES EQU 20 ORG $1000 COUNT RMB 1 LDX #RE GBAS LDAP LDAP #NTI #NTIME MES S STAA STAA COUN COUNT T LDAA LDAA #OC1 #OC1 STAA TFLG1, X LDD TCNT, X AIT: ADDD#D2 D#D25MS STD TOC2, X BRCLR TFLG1, X, OC1 v
LDAA LDAA #OC1 #OC1 STAA TFLG1, DEC COUNT BEQ OTHER LDD TOC2, X BRPi WAIT
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5.3.2. Windscreen wiper motion: v Windscreen wiper is a device which is used to clear the front glass of the cars, buses, train etc., during raining days. v It will oscillate an arm back and forth in an arc like a windscreen wiper. A traditional mechanical solution for this problem is sho wn in fig 5.7. v It uses a principle of four-bar mechanisms. It consists of a crank which rotates about its center and a nd a connecting rod. v An end of a connecting rod is connected to the crank and other with wiper arm. Rotation of crank causes connecting rod to impart an oscillatory motion to wiper arm.
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An alternative mechatronics approach for this problem is to use a stepper motor. For operating a stepper motor motor a microprocessor with a PIA, or a micro controller can be used. Fig 5.8 shows a circuit for interfacing stepper motor with micro controller or PIA. The input to the stepper motor is required to cause it to rotate a number of steps in one direction and reversing the same number of steps in other direction while the data is reversed. In the fig 5.8 isolating diodes are used to prevent the current flow into the micro controller from interfacing circuits. Transistors are used as a switch to control stepper motor. Data in the data bus causes the transistors to turn ON/OFF according to the data conditions 1 or 0 respectively.
Stepper motor can be operated in two configurations 1. Fu ll ll-step co co nf nfigura ti tion. 2. Half-step co configuration. 1. Full step configuration: configuration: If the stepper motor is in the full-step configuration needed from micro controller is shown in table 5.1. Table 5.1
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To rotate the stepper motor in forward direction the output sequence from the micro controller is A-9-5-6-1. To rotate the stepper motor in the reverse direction the sequence from the micro controller is 6-5-9-A.
2. Half-step configuration: v If the stepper motor is in half step configuration then the data needed from micro controller are shown on table 5.2.
Step!: Advance a step by applying applying a data. Step2: Call time delay routine to complete a step.
To rotate the stepper motor in forward direction the output sequence from the micro controller is A-8-9-1-5-4-6-2-A. to to rotate stepper motor motor in the reverse reverse direction the output sequence from the micro controller is 2-6-4-5-1-9-8-A. The basic steps of the program to run a stepper motor is given as follows. follows. Step3: Repeat step and step2 until the required number of steps completed in forward direction. Step4: To reverse the direction of stepper motor, the same steps given above are repeated in the reverse order o f data. The C program can be written for three step forward and three step backward rotation of stepper motor in full-step configuration as fo llows: 5.3.3. Weighing scales: weighing machine which is is used to indicate the weight of v Consider the simple weighing a person standing on it. requirement of this this weighing machine is to to indicate the weight v The important requirement of the person with reasonable and accuracy accuracy and be independ independent ent of where where on the platform platform the person person v speed and stand. mechanical system for this problem problem is to use the weight of the the v The traditional mechanical person on the platform to deflect deflect an arrangement of two parallel leaf springs as shown in fig 5.10(a).
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The deflection deflection of the the leaf spring spring is transferred transferred to the rack and and pinion pinion arrangement where the linear movement of rack is converted into rotary motion of the pinion about the horizontal axis. The rotary motion is transferred to the movement of a pointer across a scale through bevel gears. With this arrangement the deflection is independent of where on the platform the person stands. The mechatronics solution for this problem involves the use of a microprocessor. The basic principle behind this approach is to use load cells employing electrical resistance strain gauges. Fig 5.11 shows a Wheatstone bridge arrangement with ADC. Similarly for displaying number of 6 of 168 in the second LED are given as follows:
5.4 CASE STUDIES OF MECHATRONIC M ECHATRONIC SYSTEM Mechatronics systems are widely used now a days in many industries Some of the examples are explained here as per our university syllabus. 5.4.1. Pick and Place Robot: The basic form of a pick and place robot is shown in fig 5.13. The robot has three axis a xis about which motion can occur. The following movements are required for this robot. i. clockwise and anticlockwise rotation of the ro robot un unit on its ba base. 2. Linear move ovement ent of the arm arm hori orizontall ally i.e. .e., ext extens ension or cont ontracti ction of arm.
3. Up and down movement of the arm and 4. Open and close movement of the gripper. v The foresaid movements can be obtained by pneumatic cylinder which is operated by solenoid valves with limit switches. indicate when a motion motion is completed. v Limits switches are used to indicate obtained from a v The clockwise rotation of the robot unit on its base can be obtained piston and cylinder arrangement during pistons forward movement Similarly counter counter clockwise clockwise rotation rotation can be be obtained obtained during during backward backward v Similarly movement of the piston in cylinder.
5.4.2. Automatic car park system: v Consider an automatic car park system with barriers operated by coin inserts. The system uses a PLC for its operation. v There are two barriers used namely in barrier and out barrier. In barrier is used to open when the correct money is inserted inserted while out barrier open when the car is detected in front of it. v Fig 5.16 shows a schematic arrangement of an automatic car park barrier. It consists of a barrier which is pivoted at one end, two Solenoid valves A and B and a piston cylinder arrangement
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A connecting rod connects connects piston and barrier as shown in fig 5.16. 5.16. Solenoid valves are used to control co ntrol the movement of the piston. Solenoid A is used to move the piston upward inturn barrier whereas solenoid B is used to move the piston downward. Limit switches are used to detect the foremost position of the barrier. When current flows through solenoid A, the, piston in the cylinder moves upward and causes the barrier to rotate about its pivot and raises raises to let a car through.
When the barrier hits the limit switch, it will turns on the timer I give a required time delay. After that time delay, the solenoid B activated which brings the barrier downward by operating piston in th cylinder. This principle is used for both the barriers. Fig 5.17 shows a PLC arrangement to operate the barrier. Fig 5.1 shows the ladder program for that PLC system.
5.4.3. Engine management system: v Engine management system is, now-a-days, used in many of the modern cars such as Benz, Mitsuibisi, and Toyota etc. v This system uses many elec tronic control system involving micro controllers. system is shown in fig 5.19. v The generalljse block diagram of this system
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The objective of the system being to ensure that the engine is operated at its optimum settings. The system consists of many sensors for observing vehicle speed, engine temperature, oil and fuel pressure, airflow etc. These sensors are supplying input signals to the micro controller after suitable signal conditioning and providing output signals via drivers to actuate corresponding actuators. A single cylinder engine consists of some of these elements in relation to an engine is shown in fig 5.20. The engine sensor is an inductive type s It consists of a coil and sensor wheel. The inductance inductance of the coil changes as the teeth of the sensor wheel pass it and so results in an oscillating voltage.
The engine temperature sensor is generally thermocouple which is made of bimetallic strip or a thermister. The resistance of the thermistor changes with change in engine temperature this results in voltage vo ltage variation. Hot wire anemometer is used as a sensor for measuring mass airflow rate. The basic principle is that the heated wire will be cooled as air passes over it. The amount of cooling is depending on the mass rate of flow. The oil and pressure sensors are diaphragm type sensors. According to the pressure variation, the diaphragm may contract or expand and activates strain gauges which produces voltage variation in the circuit. The oxygen oxygen sensor is usually usually a close end tube which is made of ZirConium ZirConium oxide with porous platinum electrode on the inner and and outer Surfaces The sensor becomes permeable to oxygen o xygen ions at about 300°C. This results in generation of voltage between the electrodes. The various drivers such as fuel injector drivers, ignition coil drivers. solenoid drivers and used to actuate actuation according to the signal by various sensors. Analog signals given by sensors are converted into digital signal by using analog to digital converters (ADC) and a nd sent it to micro controllers. The various output digital signals are converted into analog signals by DAC (i.e., Digital to Analog Converter) and shown in various recorders or meters.
TWO MARKS QUESTIONS AND ANSWERS 1. Name the various stages in designing mechatronics system? 2. Mention any four statements about the problem definition. 3. Generation of possible solution stage is called as___________ 4. List the various sensors contained in engine management system. 5. What are the requirements satisfied before starting the timer? 6. How can be delay varied in a simple program? Z What are the advantages of PLC system? 8. What is a windscreen wiper? 9. What are the configurations con figurations in operating stepper motor? 10. Write the basic steps of the program to run a stepper motor. 11. What is the function of decoder? 12. What are the various movements of robots? 13. Name the two barriers used in automatic car parking system and state its uses.
REVIEW QUESTIONS 1. Explain the various stages sta ges in designing mechatronics system. 2. Briefly explain traditional and mechatronics designs. 3. Compare the traditional and mechatromcs design. 4. Explain possible design solutions. solu tions. 5. Describe the two t wo configurations of stepper motor in operation. 6. How simple weighing weighing works using traditional mechanical system? 7. Explain the working of a weighing scale using mechatronic solution compare this over traditional mechanical system. 8. Describe pick and place robot with its movements. 9. Explain the working of an automatic car parking system with neat sketch. 10. Describe engine management system.
