MECHATRONICSUnit – I

1.0 Introduction to Mechatronics:

Consider the modern auto-focus, auto-exposure camera. To use the camera all youneed to do is point it at the subject and press the button to take the picture. The cameraautomatically adjusts the focus so that the subject is in focus and automatically adjuststhe aperture and shutter speed so that the correct exposure is given. Consider a trucksmart suspension. Such suspension adjusts to uneven loading to maintain a levelplatform, adjusts to cornering, moving across rough ground. etc. to maintain smooth ride.Consider an automated production line. Such a line may involve a number of productionprocesses which are all automatically carried out in the correct sequence and in thecorrect way. The automatic camera, the truck suspension and the automatic productionline are examples of a marriage between electronic control systems and mechanicalengineering. Such control systems generally use microprocessors as controllers arid haveelectrical sensors extracting information from the mechanical inputs and outputs viaelectrical actuators to mechanical systems. The term mechatronics is used for thisintegration of microprocessor control systems, electrical systems and mechanical system.A mechatronics system is not just a marriage of electrical and mechanical systems and ismore than just a control system; it is a complete integration of all of them.

In the design now of cars, robots, machine tools, washing machines, cameras, andvery many other machines. Such an integrated and interdisciplinary approach toengineering design is increasingly being adopted the integration across the traditionalboundaries of mechanical engineering, electrical engineering, electronics and controlengineering has to occur at the earliest stages of the design process if cheaper morereliable, more flexible systems are to be developed. Mechatronics has to involve aconcurrent approach to these disciplines rather than a sequential approach of developing,say, a mechanical system then designing the electrical part and the microprocessor part.

Mechatronics involves what are termed as systems. A system can be thought of as a boxwhich has an input, and an output and where we are not concerned with what goes oninside the box but only the relationship between the output and the input. Thus forexample, a motor may be thought of as a system which has as input electric power and asoutput the rotation of a shaft.

2.0 Systems:A system can be defined as a box which has an input and an output where we

concentrate about the relationship between the input and output, not with the input.

Example: a motor.

A motor has input as electric power as input and rotation as output. The followingfigure shows the representation.

Fig: 1.1 System

3.0 Measurement System Definition:

A measurement system can be defined as a black box which is used for makingmeasurements. It has an input the quantity being measured and its output the value of thatquantity.

Example: A temperature measurement system. i.e. Thermometer

Fig: 1.2 Measurement System

4.0 Control System:

A control system can be defined as a block box which can be used to control itsoutput to some particular value.

Example: a domestic central heating control system.

We can set the required temperature on the thermostat or controller and the pumpcan be adjusted to supply water through radiators. So the required temperature can bemaintained in the house.

Fig: 1.3 Control System

5.0 Measurement Systems:

Measurement System can be considered to be made up of three elements asshown in figure.

Fig: 1.4 Measurement System and its Elements

1. A sensor which responds to the quantity being measured by giving as its output asignal which is related to the quantity. Ex. a thermocouple is a temperature sensor.

2. A signal conditioner takes the signal from the sensor and manipulates it into acondition which is suitable for either display or in the case of a control system, for use toexercise control. Thus for example the output from a thermocouple is a rather small e.m.fand might be fed through an amplifier to obtain a bigger signal. The amplifier is thesignal conditioner.

3. A display system where the output from the signal conditioner is displayed. Thismight, for example be a pointer moving across a scale or a digital readout.

As an example, consider a digital thermometer. This has an input of temperature to asensor probably a semiconductor diode. The potential difference across the sensor is aconstant current.

6.0 Open and closed-loop systems:

• There are two basic forms of control system one being called and Open loop andother closed-loop systems. The difference between these can be illustrated by asimple example.

• Consider an electric fire which has a selection switch which allows a 1 KW or a 2kW heating element to be selected. If a person used the heating element to heat aroom, he or she might just switch on the 1 kW element if the room is not requiredto be at too high a temperature. The room will heat up and reach a temperaturewhich is only determined by the fact the 1 kW element was switched on, and notthe 2 kW elements. If there are changes in the conditions perhaps someoneopening a window, there is no way the heat output is adjusted to compensate.

Fig:1.5 Open and Closed Loop System

• This is an example of open loop control in that there is no information fed back tothe element to adjust it and maintain a constant temperature.

• The heating system with the heating element could be made a closed loop systemif the person has a thermometer and switches the 1 kW and 2 kW elements on oroff, according to the difference between the actual temperature and the requiredtemperature, to maintain the temperature of the room constant.

• In this situation there is feedback, the input to the system being adjustedaccording to whether its output is the required temperature. This means that theinput to the switch depends on the deviation of the actual temperature from therequired temperature.

• The difference between them determined by a comparison element. The person inthis case.

To illustrate further the differences between open and closed-loop systems, consider amotor.

• With an open-loop system the speed of rotation of the shaft might be determinedsolely by the initial setting of a knob which affects the voltage applied to themotor.

• Any changes in the supply voltage, the characteristics of the motor as a result oftemperature changes, or the shaft load will change the shaft speed but not becompensated for.

• There is no feedback loop. With a closed-loop system, however, the initial settingof the control knob will be for a particular shaft speed and this will be maintainedby feedback, regardless of any changes in supply voltage, motor characteristics orload.

• In an open-loop control system the output from the system has no effect on theinput signal. In a closed-loop control system the output does have an effect on theinput signal, modifying it to maintain an output signal at the required value.

Open-loop systems have the advantage of being

• relatively simple and• consequently low cost with generally good reliability.

However, they are disadvantages like• inaccurate since there is no correction for error.

Closed-loop systems have the advantage of being• relatively accurate in matching the actual to the required values.

They are, however, having disadvantages like,

• more complex and• so more costly and a• greater chance of breakdown as a consequence of the greater number of

components.

7.0 Basic elements of a closed-loop system:The following figure shows the general form of a basic closed-loop system.

Fig.1.6 Basic elements of closed loop

It consists or the following elements:

1 Comparison element

• This compares the required or reference value of the variable condition beingcontrolled with the measured value of what is being achieved and produces anerror signal.

• It can be regarded as adding the reference signal, which is positive, to themeasured value signal, which is negative in this case:

• Error signal = reference value signal - measured value signal

• The symbol used, in, general, for an element at which signals are summed is asegmented circle, inputs going into segments.

• The inputs are all added; hence the feedback input is marked as negative and thereference signal positive so that the sum gives the difference between the signals.

• A feedback loop is a means whereby a signal related to the actual condition beingachieved is fed back to modify the Input signal to a process. The feedback is saidto be negative feedback when the signal which is fed back subtracts from theinput value. It is negative feedback that is required to control a system. Positivefeedback occurs when the signal fed back adds to the input signal.

2 Control element• This decides what action to take when it receives an error signal.

• It may be for example, a signal to operate a switch or open a valve.

• The control plan being used by the element may be just to supply a signal whichswitches on or off when here is an error, as in a room thermostat or perhaps asignal which proportionally opens or closes a valve according to the size of theerror.

• Control plans may be hard-wired systems in which the control plan ispermanently fixed by the way the elements are connected together orprogrammable systems where the control plan is stored within a memory unit andmay be altered by reprogramming it Controllers.

3. Correction element• The correction element produces a change in the process to correct or change the

controlled condition.

• Thus it might be a switch which switches on a heater and so increases thetemperature of the process or a valve which opens and allows more liquid to enterthe process.

• The term actuator is used for the element of a correction unit that provides thepower to carry out the control action.

4 Process element• The process is what is being controlled. It could be a room in a house with its

temperature being controlled or a tank of water with its level being controlled.

5 measurement element• The measurement element produces a signal related to the variable condition of

the process that is being controlled.

• For example, a switch which is switched on when a particular position is reachedor a thermocouple which gives an e.m.f related to the temperature.

With the closed-loop system illustrated in Fig.1.6 for a person controlling thetemperature of a room, the various elements are:

Controlled variable - the room temperatureReference value - the required room temperatureComparison element - the person comparing the measured value with the

required value of temperatureError signal - the difference between the measured and required

temperatures.Control unit - the personCorrection unit - the switch on the fireProcess - the heating by the fireMeasuring device - a thermometer

An automatic control system for the control of the room temperature could involve atemperature sensor, after Suitable signal conditioning, feeding an electrical signal to theinput of a computer where it is compared with the set value and an error signal generated.This is then acted on by the computer to give at its output a signal, which, after suitablesignal conditioning, might be used to control a heater and hence the room temperature.Such a system can readily be programmed to give different temperatures al differenttimes of the day.

Fig:1.7 The automatic control of water level

The above figure shows an example of a simple control system used to maintain aconstant water level in a tank. The reference value is the initial setting of the lever armarrangement so that it just cuts off the water supply at the required level. When water isdrawn from the tank the float moves downwards with the water level. This causes thelever arrangement to rotate and so allows water to enter the tank. This flow continuesuntil the ball has risen to such a height that it has moved the lever arrangement to cut offthe water supply. It is closed loop control system with the elements being:

Controlled variable - the water level in the tank

Reference value - initial setting of the float and lever position

Comparison clement - the lever

Error signal - the difference between the actual and initialsettings of the lever positions

Control unit - the pivoted lever

Correction unit - the flap opening or closing the water supply

Process - the water level in the tank

Measuring device - the floating ball and lever

7.0 Sequential Controllers:

Fig.1.8 Washing Machine System• The above figure shows the basic washing machine system and gives a rough idea

of its constituent elements.

• The system that used to be used for the washing machine controller was amechanical system which involved a set of cam-operated switches, i.e mechanicalswitches. Figure 1.9 show the basic principle of one such switch.

• When, the machine is switched on, a signal electric motor slowly rotates its shaft,giving an amount of rotation proportional no tune. The rotation turns thecontroller cams so that each in turn operates electrical switches and so switcheson circuits in the correct sequence. The contour of a cam determines the time atwhich it operates a switch.

• The contours of the cams and the means by which the program is specified andstored in the machine. The sequence of instructions and the instructions used in aparticular washing program are determined by the set of cams chosen.

• With modern washing machines the controller is a microprocessor and theprogram is not supplied by the mechanical arrangement of cams but by a softwareprogram.

Fig.1.9 Cam Operated Switch

• For the pre-wash cycle an electrically operated valve is opened when a current issupplied and switched off when it ceases. This valve allows cold water into thedrum for a period of time determined by the profile of the cam or the output fromthe microprocessor used to operate its switch.

• However, since the requirement is a specific level of water in the washingmachine drum, there needs to be another mechanism which will stop the watergoing into the tank, during the permitted time, when it reaches the required level.

• A sensor is used to give a signal when the water level has reached the preset leveland give art output front the microprocessor which is used to switch off thecurrent to the valve. In the case of a cam-controlled valve, the sensor actuates aswitch which closes the valve admitting water to the washing machine drum.

• When this event is completed die microprocessor, or the rotation of the cams,initiates a pump to empty the drum.

• For the main wash cycle, the microprocessor gives an output which starts when liepre-wash part of the program is completed: in the case of the cam-operatedsystem the cam has a profile such that it starts in operation when the pre-washcycle is completed. It switches a current into a circuit to open a valve to allowcold water into the drum. This level is sensed and the water shut off when tinerequired level is reached.

• The microprocessor or cam then supplies a current to activate a switch whichapplies a larger current to an electric heater to heat the water. A temperaturesensor is used to switch off the current when the water temperature reaches thepreset value.

• The microprocessor or cams then switch on the drum motor to rotate the drum.This will continue for the time determined by the microprocessor or cam profilebefore switching off. Then the microprocessor or a cam switches on the current toa discharge pump to empty the water from the drum.

• The rinse part of the operation is now switched as a sequence of signals to openvalves which allow cold water into the machine. Switch it off, operate the motorto rotate the drum, operate a pump to empty the water from the drum, and repeatthis sequence a number of times.

8.0 Microprocessor based Controllers:

8.1 The automatic camera:

Fig.1.10 Elements of Automatic Camera

The modern camera is likely to have automatic focusing and exposure. Figure 1.10illustrates the basic aspects of a microprocessor-based system that can’ t be used tocontrol the focusing and exposure.

• When the switch is operated to activate the system and the camera pointed at theobject being photographed, the microprocessor takes the input from the rangesensor and sends an output to the lens position drive to move the lens to achievefocusing. The lens position is fed back to the microprocessor so that the feedbacksignal can’ t be used to modify the lens position according to the inputs from therange sensor.

• The light sensor gives an input to the microprocessor which then gives an outputto determine, if the photographer has selected the shutter controlled rather thanaperture controlled mode, the time for which the shutter will be opened. When thephotograph has been taken, the microprocessor gives an output to the motor driveto advance the film ready for the next photograph.

• The program for the microprocessor is a number of steps where themicroprocessor is making simple decisions of the form: is there an input signal ofa particular input line or not and if there is output a signal on a particular outputline. The decisions are logic decisions with the input and output signals eitherbeing low or high to give on-off states. A few steps of the program for theautomatic camera might be of the form:

beginif battery check input OK

then continueotherwise stop

loopread input from range sensor calculate lens movementoutput signal to lens position driveinput data from lens position encodercompare calculated output with actual output stop output when lens in correctpositionsend in-focus signal to viewfinder displayetc.

8.2 The Engine Management System:

• The engine management system of a car is responsible for managing the ignitionand fuelling requirements of the engine.

• With a four-stroke internal combustion engine there are several cylinders, each ofwhich has a piston connected to a common crankshaft and each of which carriesout a four-stroke sequence of operations (fig. 1.11).

• When the piston moves down a valve opens and the air —fuel mixture is drawninto the cylinder.

• When the piston moves up again the valve closes and the air —fuel mixture iscompressed.

• When the piston is near the top of the cylinder the spark plug ignites the mixturewith a resulting expansion of the hot gases. This expansion causes the piston tomove back down again and so the cycle is repeated.

• The pistons of cacti cylinder are connected to a common crankshaft and theirpower strokes occur at different times so that here is continuous power forrotating the crankshaft.

Fig.1.11 Four Stroke Sequence

Fig1.12 Elements of Engine Management System

• The power and speed of the engine are controlled by varying the ignition timingand the air —fuel mixture.

• With modem car engines this is done by a microprocessor. Figure 1.12 shows thebasic elements of a microprocessor control system.

• For ignition timing, the crankshaft drives a distributor which makes electricalcontact for each spark plug in turn and a timing wheel. This timing wheelgenerates pulses to indicate he crankshaft position.

• The microprocessor then adjusts the timing at which high voltage pulses are sentto the distributor so they occur at the right moments of time.

• To control the amount of air —fuel mixture entering a cylinder during the intakestrokes, the microprocessor varies the time for which a solenoid is activated toopen the intake on the basis of inputs received of the engine temperature and thethrottle position.

• The amount of fuel to be injected into the air stream can be determined by aninput from a sensor of the mass rate of air flow, or computed from othermeasurements, and the microprocessor hen gives an output to control a fuelinjection valve.

Mechatronics Approach:

• The domestic washing machine is used cam-operated switches in order to controlthe washing cycle is now out-of-date. Such mechanical switches are beingreplaced by microprocessors.

• A microprocessor can be just considered as being essentially a collection of logicgates and memory elements that are not wired up as individual components butwhose logical functions are implemented by means of software.

• The microprocessor-controlled washing machine can be considered an example ofa mechatronics approach in that a mechanical system has become integrated withelectronic controls.

• As a consequence, a bulky mechanical system is replaced by a much morecompact microprocessor system which is readily adjustable to give a greatervariety of programs.

PART – A –TWO MARK QUESTIONS1. Write about Mechatronics?

2. What are the components in a Mechatronics system?3. What is the use of actuators and sensors?

4. What is the use of digital devices?5. What is the function of conditioning and interfacing Circuits and graphical displays?

6. Give some examples of Mechatronics systems?7. What are the important sub-systems involved in Mechatronic system?

8. What is the use of control system?9. What are the important elements of measurement system?

10. What is the function of sensor?11. What is the function of signal conditioner?

12. What is the use of Display system?13. How the control system is classified?

14. What is meant by open loop control system?15. What is meant by closed loop control system in CNC machine?

16. What are the import elements of a closed loop control system?17. What is the use of comparison element?

18. What is meant by error signal?19. What is the use of control element?

20. What is the function of the correction element?21. What is meant by process element?

22. What is meant by sequence control?23. Why mechatronic systems are also known as smart devices?

PART – B QUESTIONS

1. Explain the closed loop system with example.

2. What are the basic components of closed loop system? Explain.

3. Describe the sequential controllers.

4. Explain the microprocessor controlled automatic camera.

5. Explain the microprocessor controlled engine management system.

6. Explain the mechatronics approach with its advantages.