Mechatronics - UNIT1-FH
Mechatronics - UNIT1-FH
Mechatronics - UNIT1-FH
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C3389
ME 1402 -- MECHATRONICS
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12.(a) Discuss about the following actuation systems:
(i) Self-excited wound field shunt configuration dc motor.
(ii) Self-excited wound field series configuration dc motor.
(iii) Stepper motor.
(iv) Induction motor.
or
(b) A flat belt, 7 mm thick and 95 mm wide transmits power between two pulleys
running at 1500m/min.
The mass of the belt is 0.85kg/m length. The angle of lap in the smaller pulley is
155deg and the
coefficient of friction between the belt and pulley is 0.25. If the maximum
permissible stress in
the belt is 2MN/m^2 find the maximum power transmitted and initial tension in the
belt.
13.(a) A hot object with capacitance C and temperature T, cools in a large room at
temperature Ta. If the thermal system has a resistance R, derive an equation
describing how the temperature of the hot object changes with time and give an
electrical analogue of the system.
or
(b) (i) Propose a model for a stepped shaft used to rotate a mass and derive an
equation relating
the input torque and the angular rotation. Neglect damping effect. (8)
(ii) Describe and compare the characteristics of
1) proportional controller
2) proportional plus integral controller (8)
14.(a) Devise a system, using a PLC that could be used with a conveyor belt which
is used to move an item to work station. The presence of the item at the work
station is detected by means of breaking a contact activated by a beam of light to a
photosensor. There it stops for 100sec for an operation to be carried out and then
starts moving. The motor for the belt is started by a normally open start switch and
stopped by a normally closed switch.
or
(b) Devise a circuit that could be used with a domestic washing machine to switch
on a pump to pump water for 100 sec into the machine, then switch off the pump
and switch on a heater for 50sec to heat the water. The heater is then switched off
and another pump is switched on for 100sec to empty the water from the machine.
15.(a) Explain the mechatronics systems used in an automatic camera with a neat
block diagram.
or
(b) Design a pick and place robot using mechatronics elements and explain about
the robot control.
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ME 1402 - MECHATRONICS
Unit – I
INTRODUCTION TO MECHATRONICS:
Consider the modern auto-focus, auto-exposure camera. To use
the camera all you need to do is point it at the subject and press the
button to take the picture. The camera automatically adjusts the
focus so that the subject is in focus and automatically adjusts the
aperture and shutter speed so that the correct exposure is given.
Consider a truck smart suspension. Such suspension adjusts
to uneven loading to maintain a level platform, 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 production processes which are all automatically
carried out in the correct sequence and in the correct way.
The automatic camera, the truck suspension and the automatic
production line are examples of a marriage between electronic
control systems and mechanical engineering. Such control systems
generally use microprocessors as controllers arid have electrical
sensors extracting information from the mechanical inputs and
outputs via electrical actuators to mechanical systems.
The term mechatronics is used for this integration of
microprocessor control systems, electrical systems and mechanical
system. A mechatronics system is not just a marriage of electrical
and mechanical systems and is more 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, and very many other machines, such an
integrated and interdisciplinary approach to engineering design is
increasingly being adopted Mechatronics has to involve a concurrent
approach to these disciplines rather than a sequential approach of
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developing, say, a mechanical system then designing the electrical
part and the microprocessor part.
Definition:
SYSTEMS:
Mechatronics involves what are termed as systems. A system can
be thought of as a box which has an input, and an output and where
we are not concerned with what goes on inside the box but only the
relationship between the output and the input. Thus for example, a
motor may be thought of as a system which has as input electric power
and as output the rotation of a shaft.
Example: A Motor.
A motor has input as electric power as input and rotation as
output. The following figure shows the representation.
Fig:
System
Basically in mechatronics we divide the systems in to 2 types
1. Measurement System.
2. Control System.
Now we will discuss in detail about these 2 systems.
MEASUREMENT SYSTEM:
A Measurement system can be defined as a black box which is used
for making measurements. It has an input the quantity being measured
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and its output the value of that quantity.
Example: A temperature measurement system. i.e. Thermometer
Fig: Measurement
system
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Elements of Measurement system
Measurement System can be considered to be made up of three elements
as shown in figure.
CONTROL SYSTEM:
A control system can be defined as a block box which can be used
to control its output to some particular value.
Example: a domestic central heating control system.
We can set the required temperature on the thermostat or
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controller and the pump can be adjusted to supply water through
radiators. So the required temperature can be maintained in the house.
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The heating system with the heating element could be made a closed
loop system if the person has a thermometer and switches the 1 kW
and 2 kW elements on or off, according to the difference between the
actual temperature and the required temperature, to maintain the
temperature of the room constant.
In this situation there is feedback, the input to the system being
adjusted according to whether its output is the required temperature.
This means that the input to the switch depends on the deviation of the
actual temperature from the required temperature.
The difference between them determined by a comparison element. The
person in this case.
Illustration of a motor:
To illustrate further the differences between open and closed-loop
systems, consider a motor.
With an open-loop system the speed of rotation of the shaft might be
determined solely by the initial setting of a knob which affects the
voltage applied to the motor.
Any changes in the supply voltage, the characteristics of the motor as a
result of temperature changes, or the shaft load will change the shaft
speed but not be compensated for.
There is no feedback loop. With a closed-loop system, however, the
initial setting of the control knob will be for a particular shaft speed and
this will be maintained by feedback, regardless of any changes in supply
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voltage, motor characteristics or load.
In an open-loop control system the output from the system has no
effect on the input signal. In a closed-loop control system the output
does have an effect on the input signal, modifying it to maintain an
output signal at the required value.
OPEN-LOOP SYSTEMS have the advantage of being
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5. Measurement element.
Comparison element
This compares the required or reference value of the variable
condition being controlled with the measured value of what is being
achieved and produces an error signal.
It can be regarded as adding the reference signal, which is
positive, to the measured 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 a segmented circle, inputs going into segments.
The inputs are all added; hence the feedback input is marked as
negative and the reference signal positive so that the sum gives the
difference between the signals.
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 which switches on or off when here is an error, as in a room
thermostat or perhaps a signal which proportionally opens or closes a
valve according to the size of the error.
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
the temperature of the process or a valve which opens and allows more
liquid to enter the process.
The term actuator is used for the element of a correction unit that
provides the power to carry out the control action.
Process element
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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.
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 reached or a thermocouple which gives an e.m.f related to the
temperature.
FOR TEMPERATURE CONTROLLED CLOSED LOOP SYSTEM
With the closed-loop system illustrated in Fig. above, for a
person controlling the temperature of a room, the various elements
are:
Controlled variable - the room temperature
Reference value - the required room temperature
Comparison element - the person comparing the measured value with the required
value of temperature
Error signal - the difference between the measured and required
temperatures.
Control unit - the person
Correction unit - the switch on the fire
Process - the heating by the fire
Measuring device - a thermometer
AUTOMATIC WATER LEVEL CONTROLLER:
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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 suitable signal 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 different times of the
day.
The above figure shows an example of a simple control system
used to maintain a constant water level in a tank. The reference value is
the initial setting of the lever arm arrangement so that it just cuts off
the water supply at the required level. When water is drawn from the
tank the float moves downwards with the water level. This causes the
lever arrangement to rotate and so allows water to enter the tank.
This flow continues until the ball has risen to such a height that it has
moved the lever arrangement to cut off the 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 initial
settings 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
SEQUENTIAL CONTROLLERS:
When a controller operates in a sequence way i.e. Step by step, then
that type of controllers is called as sequence controllers.
In sequential controllers, step 2 is started only after completing step
1 and after completing step 2 step 3 will be started. In sequential
controllers, the control .actions are ordered in time, which is obtained by
an electrical circuit with sets, of relays or cam operated switches which
are wired up in such a way as to give the required sequence.
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Now-a-days hardwired circuits and relays are replaced by a
microprocessor controlled system, the sequencing are controlled by
software program
Example: Washing machine.
Consider a washing machine; the numbers of sequential operations
carried out are,
1. Pre wash cycle -the clothes in the drum are washed with cold
water.
2. Main wash cycle - the clothes are washed with hot water.
3. Rinse cycle - the washed clothes are rinsed with cold water number
of times.
4. Spinning - the rinsed clothes are spinned to remove water.
The above figure shows the basic washing machine system and
gives a rough idea of its constituent elements.
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The system that is used for the washing machine controller was a
mechanical system which involved a set of cam-operated switches, i.e
mechanical switches. Figure below 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 the controller cams so that each in turn operates
electrical switches and so switches on circuits in the correct sequence.
The contour of a cam determines the time at which it operates a switch.
The contours of the cams and the means by which the program is
specified and stored in the machine. The sequence of instructions and
the instructions used in a particular washing program are determined by
the set of cams chosen.
With modern washing machines the controller is a
microprocessor and the program is not supplied by the mechanical
arrangement of cams but by a software program.
For the pre-wash cycle an electrically operated valve is opened when a
current is supplied and switched off when it ceases. This valve allows
cold water into the drum for a period of time determined by the profile
of the cam or the output from the microprocessor used to operate its
switch.
However, since the requirement is a specific level of water
in the washing machine drum, there needs to be another mechanism
which will stop the water going into the tank, during the permitted time,
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when it reaches the required level.
A sensor is used to give a signal when the water level has reached
the preset level and give art output front the microprocessor which is
used to switch off the current to the valve. In the case of a cam-
controlled valve, the sensor actuates a switch 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 lie pre-wash part of the program is completed: in the
case of the cam-operated system the cam has a profile such that it
starts in operation when the pre-wash cycle is completed. It switches a
current into a circuit to open a valve to allow cold water into the drum.
This level is sensed and the water shut off when tine required level is
reached.
The microprocessor or cam then supplies a current to activate a
switch which applies a larger current to an electric heater to heat the
water. A temperature sensor is used to switch off the current when the
water temperature reaches the preset 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 profile before switching off. Then the
microprocessor or a cam switches on the current to a discharge pump to
empty the water from the drum.
The rinse part of the operation is now switched as a sequence of
signals to open valves which allow cold water into the machine. Switch
it off, operate the motor to rotate the drum, operate a pump to empty
the water from the drum, and repeat this sequence a number of times.
MICROPROCESSOR BASED CONTROLLERS:
Microprocessors are rapidly replacing the mechanical cam operated
controllers. These microprocessors are used to control the function. In
many simple systems, an embedded micro controller is used to control or
perform the particular task.
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A more adaptable form of controller is the programmable logic
controller. The programmable logic controller is defined as a sequential
logic device that generates output signals according to logic operations
performed on the input signals. The PLC is a microprocessor based
controller which uses programmable memory to store instructions and to
implement functions such as logic sequence, timing counting and
arithmetic to control events. This PLC can be easily reprogrammed for
different tasks. The PLC is shown below.
The example for input devices are switches relays and limit switches. The
examples for output devices are motor to be controlled, Lamp, relay and
solenoid. The controller monitors the inputs and outputs according to the
program stored in the PLC by the operator. PLC are similar to computers
but have certain features which are specific to their use of controllers.
These are,
1. They are rugged and designed to withstand vibrations, temperature,
humidity and noise.
2. The interfacing for inputs and outputs is inside the controllers.
3. They are easily programmed and have an easily understood
programming language. The Programming is primari1y concerned with
logic and switching operations.
Note:
Micro controller: - Microprocessor with integrated peripherals is called as
micro controller
Some of the microprocessor based control system is discussed below.
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AUTOMATIC CAMERA:
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where the microprocessor is making simple decisions of the form: is
there an input signal of a particular input line or not and if there is
output a signal on a particular output line. The decisions are logic
decisions with the input and output signals either being low or high
to give on-off states.
A few steps of the program for the automatic camera might be of
the form:
begin
if battery check input OK
then continue
otherwise stop
loop
read input from range sensor calculate lens
movement output signal to lens position drive
input data from lens position
encoder
compare calculated output with actual output stop output when lens in
correct
position
send in-focus signal to viewfinder
display
etc.
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THE ENGINE MANAGEMENT SYSTEM:
The engine management system of a car is responsible for managing
the ignition and fuelling requirements of the engine.
With a four-stroke internal combustion engine there are several
cylinders, each of which has a piston connected to a common crankshaft
and each of which carries out a four-stroke sequence of operations.
When the piston moves down a valve opens and the air-fuel
mixture is drawn into the cylinder.
When the piston moves up again the valve closes and the air-fuel
mixture is compressed.
When the piston is near the top of the cylinder the spark plug ignites
the mixture with a resulting expansion of the hot gases. This expansion
causes the piston to move back down again and so the cycle is repeated.
The pistons of cacti cylinder are connected to a common crankshaft
and their power strokes occur at different times so that here is
continuous power for rotating the crankshaft.
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For ignition timing, the crankshaft drives a distributor which makes
electrical contact for each spark plug in turn and a timing wheel.
This timing wheel generates pulses to indicate he crankshaft position.
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MECHATRONICS APPROACH:
The domestic washing machine that used cam operated switches in
order to control the washing cycle is out of date. Such mechanical switches
are being replaced by microprocessor. A microprocessor may be considered
as being essentially a collection of logic gates and memory elements that are
not wired up is individual components but whose logical functions are
implemented by means of software.
The microprocessor- controlled washing machine can be considered an
example of a mechatronics approach in that a mechanical system has
become integrated with electronic controls. As a consequence, a bulky
mechanical system is replaced by a much more compact microprocessor
system which is readily adjustable to give a greater variety of programs.
Mechatronics brings together a number of technologies like,
mechanical engineering, Electronic Engineering, electrical engineering,
information technology, computer technology and control engineering. This
can be considered as the application of Computer based digital control
techniques, through electronic and electric interfaces to mechanical
engineering problems.
There are many applications of mechatronics in the mass produced
products used in home. Microprocessor based controllers are to be found in
domestic washing machines, dish washers, microwave ovens, cameras,
camcorders, watches, hi-fi and video recorder systems, central heating
controls, sewing machines, etc.. They are to be found in cars in the active
suspension, antiskid brakes, engine control, speedometer display,
transmission etc. A large scale application of mechatronics is a flexible
manufacturing engineering system (FMS) involving computer controlled
machines, robots, automatic material conveying and overall supervisory
control.
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PART – A –TWO MARK
QUESTIONS
1. 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
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