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ME 1402 – MECHATRONICS (UNIT – II)ACTUATORS
ACTUATION SYSTEM:
The actuation systems are the elements of the control system
and they are responsible for transforming the output of a
microprocessor into a controlling action on a machine or device.
Actuators produce physical changes such as linear and angular
displacement.
There are four types of actuators.1. Mechanical actuators.
2. Electrical actuators.
3. Hydraulic actuators.
4. Pneumatic actuators.
Example:
In a CNC milling machine, there may be an electrical signaloutput from the CNC controller to move the milling table in the x
direction for a certain length. There you need an actuation system.
PNEUMATIC AND HYDRAULIC SYSTEMS:
Power from one point to another point can also be transmitted
using air as medium called pneumatic transmission or liquid as
medium called hydraulic transmission. In case of hydraulic system,
liquid, which may be water or hydraulic oil is pressurized to 20 to 250
atm pressures and transmitted through pipe line. The pressurized
liquid is made to actuate rotary or linear actuator through control
valves to get required function. Hydraulic system of power
transmission is preferred over mechanical or electrical system on thefollowing grounds.
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1. Compact size.
2. Less moving parts.
3. Less wear and tear & self lubricating.
4. Controlled motion.
5. Adaptability for automatic control.
However, the initial cost of hydraulic transmission will be high.
Improperly filled hydraulic system will give maintenance problem and
cost of spares will be high. Some of the applications of hydraulic
system are hydraulic presses, fork lifts, hydraulic jacks and hydraulic
shaper etc.
In hydraulic actuation system, the hydraulic signals are used to
control device but are more expensive than pneumatic system. Oil leak
is another problem in hydraulic system.
Basic components of hydraulic system are
1. Reservoir to hold oil,
2. Hydraulic pump normally positive displacement type,
3. Electric motor to drive the pump,
4. Actuator, which may be rotary or linear,
5. Control valves for controlling flow, direction and pressure, and
6. Pipe lines and fittings to transmit oil power.
In pneumatic control system, the moister should be separated, to
avoid presence of free moisture during expansion. Besides, this
moisture will pose problems in line especially in pilot operator solenoid
valves. Pneumatic system is fast comparable to hydraulic system. But
positioning and speed control is difficult because of compressibility of
air.
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In the pneumatic actuation system pneumatic signals are used to
control the system. The pneumatic signals can be used to actuate
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large valves and other high power control device and so it can be used
to move heavy loads. Pneumatic system consists of a compressor,
control valves and actuators. Since air is used as medium, reservoir is
not required.
Power supplies:
Hydraulic power supply:
Hydraulic systems are design to move large loads by controlling a
high pressure fluid in distribution lines and piston with mechanical or
electromechanical valves.The basic components of a hydraulic system are,
In a hydraulic system, pressurized oil is provided by a hydraulic
pump driven by an electric motor.
The hydraulic pump pumps the oil from a sump through a non-
return valve and an accumulator to the system.
A pressure relief valve is circulated to release the pressure when
it rises above the safe level.
The non return valve is to prevent the oil returning back to the
pump.
The accumulator is a reservoir in which the oil is held under
pressure.The accumulator is used to store the oil and provides a smooth
drive during any short term fluctuation in the output oil pressure.
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Pneumatic System:
Fig. Hydraulic Power Supply
The basic components of a pneumatic system are,
In a pneumatic power supply an electric motor drives an air
compressor.Before the air enters the compressor, it passes through a filter
and a silencer.
In the filter all the dust particles present in the inlet air is
removed.
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Fig. Pneumatic power supply
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In the silencer the noise level is reduced.
A pressure relief valve is provided to protect the system in case
of pressure rises above the safe level.
Since the air compressor increases the temperature of the air, a
cooler is provided to reduce the temperature of air.
In the filter and water trap, the water from the air and other
unwanted particles in air are removed.
An air receiver increases the volume of air in the system and
smoothens out any short term pressure fluctuation.DIRECTION CONTROL VALVES:
The direction control valves are used in the pneumatic and
hydraulic system to direct the flow of liquid through a system. They are
used for varying the rate of flow of liquid. They are either completely
open or closed.
There are two types of direction control valves. They are.
1. Spool valve.
2. Poppet valve.
Spool Valve:
A spool moves horizontally within the valve body to control the
flow.In fig a, the air supply is connected to port 1. The port 3 is closed.
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The device is connected to port 2, and device is pressurized.
In fig. b when the spool is moved to the left, the air supply is cut
off.
Port 2 and port 3 are connected.
So the air in the system connected to port 2 is allowed to go out
to the atmosphere through port 3.
In fig. a air is allowed to flow into the system.
In fig. the air is allowed to flow out of the system.
Poppet Valve:
This valve is normally in the closed
condition.
The port 1 is connected to pressure
supply.
The Port 2 is connected to the system.
Initially there is no connection between
port 1 and port 2.
Here balls, discs, or cones are used as a
valve to be seated in the valve seat to
control the flow.
Here a ball is used as shown in fig.
When the push button is depressed, the ball is pushed out of its
seat.
This allows the flow from port 1 connected to port 2.
When the button is released, the spring forces the ball back to itsseat and so closes off the flow.
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Valve Symbols
The symbol used for control valves consists of a square for each
of its switching positions.
A two position valve will have two squares; a three position valve
will have three squares.
The arrow headed lines are used to indicate the direction of flow
in each of the position.
The blocked-off lines indicate the flow is closed.
In the fig the valve has four ports.
The ports are labeled by a number or a letter according to theirfunction.
The ports are labeled 1 (or P) for pressure supply.
The ports are labeled 3 (or T) for hydraulic return port, 3 or 5 (or
R or S) for pneumatic exhaust port and 2 or 5 (or B or A ) for
output ports.
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Example
The following are some of the illustrations of how these various
symbols can be combined to describe how a valve operates. The Fig.
is a 2/2 valve, because it has 2 ports and 2 positions. The first number
(numerator) indicates the number of ports. The second number
(denominator) indicates the number of .positions. The valve symbol in
Fig. is 2/2, solenoid operated, push button valve.
Fig. 2/2 Valve
The valve symbol in Fig. , is 3/2 because it has 3 ports and 2
positions.
Fig. 3/2 Valve
The valve symbol in Fig. is 4/2 valve because it has 4 port and 2
positions.
Fig. 4/2 Valve
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ft system
The following is an example for the application of valves in a
pneumatic li .
The push button 2/2 valves are used. When the up valve is pressed
the load is lifted. When the bottom valve is pressed the load is
lowered. An open arrow is used to indicate a vent to the
atmosphere.
Pilot Operated Valve:
The force required to move the ball or shuttle in a valve can
often be too large for manual or solenoid operation. To overcome this
problem a pilot operated system is used. Where one valve is used to
control second valve. Figure illustrates this. The pilot valve is small
capacity and can be operated manually or by a solenoid. It is used to
allow the main valve to be operated by the system pressure. The pilotpressure line is indicated by dashes. The pilot and main valve can be
operated by two separate valves but they are often combined in a
single housing.
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Direction Valves
Figure shows a simple direction control valve and its symbol.
Free flow can occur in one direction through the valve, that which
results in the ball being pressed against the spring. Flow in the other
direction is blocked by the spring forcing the ball against its seat.
PRESSURE CONTROL VALVE:
There are three types of pressure control valve.
1. Pressure regulating valves.
2. Pressure limiting valves.
3. Pressure sequence valves.1. Pressure regulating valves:
This is used to control the operating pressure in a circuit and
maintain it at a constant value. The compressed air produced by the
compressor may fluctuate. Changes in the pressure may affect. The
switching characteristics of the cylinder the running times of the
cylinders. The timing characteristics of flow control valve. Thus the
constant pressure level is required for the trouble free operation of a
pneumatic control. A pressure regulator is fitted downstream of the
compressed air filter. It provides a constant set pressure at the outlet
of the regulator. The pressure regulator is also called as pressure
reducing valve or pressure regulating valve. There are two types ofpressure regulators. They are
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1. Diaphragm type pressure regulator (with or without vent holes).
2. Piston spool type pressure regulator.
1. Diaphragm type pressure regulator:
Two types of pressure regulators with diaphragm are available.
(a)With vent holes and
(b) Without vent holes.
(a)Diaphragm type pressure regulator (with vent holes):
A diaphragm type pressure regulator is shown in figure (a) &
(b).In this type pressure is regulated by a diaphragm. The output
pressure acts on one side of the diaphragm. On the other side of the
diaphragm, a spring (set spring) force acts. The spring force can be
adjusted by an adjusting screw provided at the bottom of the
regulator.
When the pressure output increases:
The diaphragm moves against the spring force. Due to this, theoutlet area of cross-section at the valve seat reduces or closes
entirely. Thus the quantity of air flowing is regulated.
When the air drawn off on the outlet side:
The operation pressure drops. The spring force opens the valve.
Thus, the continual opening and closing of the valve seat
regulates the preset output pressure. a damper spring is provided
above the valve disc to avoid fluttering. A pressure gauge is fitted to
the outlet of the regulator for monitoring and setting of the circuit
pressure.
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If the pressure on the outlet side increases considerably:
The diaphragm is pushed down against the spring force. The
center piece of the diaphragm opens. The compressed air flows to the
atmosphere through the vent holes in the housing.
(b) Diaphragm type pressure regulator (without vent holes):
A diaphragm pressure gauge without vent holes is shown in the fig.
With these valves, it is not possible to exhaust the compressed air.
The spring is pre-stressed by means of adjusting screw. Thus the
diaphragm is also pre-stressed. The plunger is raised with the
diaphragm to a greater or lesser extent from the seat. Therefore, the
flow from the primary to the secondary side increases or decreases
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depending on the setting of the spring. If no air is drawn off on the
outlet side, the diaphragm moves down against the compression
spring. The damper spring moves the plunger downward to its seat.
Thus the flow of air is closed off at the sealing seat. The compressed
air can continue to flow only when the air is drawn off on the outlet
side.
Piston - spool type pressure regulator:
A piston type pressure regulator is shown in fig.
The valve is of piston type. It is kept on its seat by a spring force.
The spring force can be adjusted screw provided at the bottom. In the
normal piston, the valve is open and the compressed. Air freely flows
from inlet A and outlet B. The valve spool is kept in equilibrium by the
spring force on one side and air pressure on the other side through
secondary circuit. When the pressure in the secondary side rises, the
pressure on the spool face increases. The spool moves and partly
closes the outlet side. This reduces the volume of air going to thesecondary side and hence pressure is reduced. If the pressure in the
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secondary side increases considerably, the outlet port is completely
closed. The flow is completely closed.
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2. Pressure limiting valves:
These are used as safety devices to limit the pressure in a circuit
to below some safe value. The valve opens and vents to the
atmosphere or back to the sump, if the pressure rises above the set
safe value. A simple pressure relief valve is shown in the fig. it consists
of conical poppet valve, spring, adjusting screw. The force exerted by
the spring on the poppet can be varied by the pressure adjusting
screw.
Under normal conditions:
The spring presses the conical poppet valve in its seat. The oil flow
path is closed.
When the system pressure exceeds the set value:
The increased pressure presses the poppet against the spring
force. Oil flow through the exhaust port T to the reservoir. Thus the
excessive pressure is released. When the pressure drops below theset value, the poppet again closes.
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3. Pressure Sequence Valves:
The sequence valve helps two or more cylinders to work in a
particular sequence. It makes sure that the operation of one cylinder is
completed before the start of the operation of another cylinder. For
example, consider two hydraulic cylinders which operate in sequence.
The sequences of operations to be performed are
(i) Lifting the weight up to the floor level by the first cylinder
(ii) Pushing the weight into the floor by the Second Cylinder
The sequence valve is connected in the hydraulic circuit as shown
in the fig.
DCV is shifted in one extreme position The fluid from the pump enters into the inlet of the sequence valve
and comes out and enters into the first cylinder through a check valve
and causes the piston to rise up.
Now the load is lifted up to the floor level. During this operation fluid
on its top is going back to the reservoir. After lifting the load, the piston
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comes to rest. Thus the first operation is over. As soon as the piston
has come to rest, the oil does not find any passage for its flow.
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Thus the pressure in the sequence valve increases. The increase in
pressure lifts the valve piston and the oil is now entering to the second
cylinder. The piston of the second cylinder pushes the load into the
floor.
During this operation the fluid on the left side is discharged to the
reservoir. Thus the secondary operation is completed.
DCV is shifted in another extreme position
Now the outlet port in sequence valve is closed as the piston of the
sequence valve move down. The fluid now entering into the second
cylinder causes the piston to move from the left to right while the fluid
on the other side is connected to the reservoir through the check valve
After the second cylinder piston has come to rest, the pump supply
enters into the top of the first cylinder. The piston in the first cylinder
lower down while fluid at its bottom is flowing to the reservoir through
the check valve and sequence valve. The pressure setting of the
sequence valve is adjusted by adjustment screw.
CYLINDERS
The hydraulic or pneumatic cylinder is an example of a linear
actuator. The principles for both hydraulic and pneumatic versions are
the same. Only difference is big size cylinder are used in hydraulic due
to high pressure.
Construction
A cylinder consists of a cylindrical type along which a piston/ ram can
slide. There are two types of cylinder. They are.
1. Single acting cylinder.
2. Double acting cylinder.
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1. Single acting cylinder:
The simple level of control for the single acting cylinder involves
direct control signals. Direct control is used when. The flow rate
required to operate the cylinder is relatively small. The size of the
control valve is small with low actuating forces. The circuit for the direct
control of single acting cylinder is shown in the fig.
A 3/2 directional control valve is used. The cylinder is of small
capacity and the air consumption is low. Hence the operation can be
directly controlled by a push button 3/2 direction control valve with
spring return. When the push button is pressed, the air passes through
the valve from pressure port (P) the cylinder port (A). The piston rodextends against the force of the cylinder return spring. Thus the work
piece is clamped. When the push button is released, the valve spring
returns the 3/2 D.C.V. to its initial position. The cylinder retracts by the
return spring force. The air form the cylinder returns through the
exhaust port (R).Cylinder is the only working element or actuator.
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2. Double acting cylinder:
They are used when the control pressures are applied to each side
of the piston.
A difference in pressure between the two sides, result in motion
of the piston. The circuit includes a double acting cylinder with 5/2
D.C.V.
When the spool of the DCV is at extreme left: Air flows from pressure port (P) to the working port (B).Then air is
allowed to enter the right end of the cylinder. The piston moves from
right to left. At the same time air in the left end of the cylinder flows into
the valve through port 'A' and exhausted through exhaust port 'R'. The
other exhaust port 'S' is blocked.
When the spool of DCV is at extreme right:
Air flows from inlet pressure port (P) to the working port (A). Air is
allowed to enter the left end of the cylinder. The piston moves from left
to right .At the same time the air in the right end of the cylinder flows
into the valve through the exhaust port 'S'. The other exhaust port 'R' is
blocked. This is the principle of working of the double acting cylinder.
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CYLINDER SEQUENCING:
Many control system employ pneumatic (or) hydraulic cylinders as
actually elements and require a sequence of extensions and retraction
of the cylinder to occur.
For example,
There are two cylinders A & B. When the start button is pressed,
the piston of cylinder A extends. When it is fully extended, the piston of
cylinder B extends. The sequence of operation of these two cylinders
is explained.
Each cylinder is given a reference letter A & B. To indicate the
extension of the piston in cylinder. A, a+ sign is used. To indicate the
retraction of the piston in cylinder A a - sign is used. Similarly for
cylinder B, b+ (for extension) and b - (for retraction) are used.
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Sequence of operation:
Initially both the cylinder has retracted pistons. Start push button on
valve 1 is pressed. This applies pressure to valve 2 as the limit switch
b – is activated. So the supply is given to valve 3, and from valve 3 the
air enters the left side of the piston in cylinder A. As the pressure built
up on the left side, the piston starts extending towards right side till the
limit switch a+ operates. Once limit switch a+ is operated, it gives a
supply to valve 5 and then causes pressure to valve 6. Then air enters
in the left side the piston in the cylinder B through valve 6.So the pistonmoves towards right side, and then the piston actuates the limit switch
b+. Once limit switch b+ is actuated, it gives a supply to valve 4 and
then causes pressure to valve 3. So the air enters the cylinder A,
through the left end of the piston. It moves in the piston from left to
right through retraction and finally the switch a - is actuated. Once limit
switch a - is actuated, it gives a supply to valve 7, then gives a
pressure to valve 6. Now air enters the cylinder B, through the left end
of the piston and the piston moves from left to right end then it will
retract. This cycle can be started again by pushing the start button. If
we want to run the system continuously, then the last movement in the
sequence should be used to trigger the first movement.
PROCESS CONTROL VALVES:
Process control valves are used to control the rate of fluid flow. For
example it can be used to control the rate of flow of a liquid into a tank.
Principle:
The basis of such valve is an actuator used to move a plug into the
flow pipe and alter the cross section of the pipe. Therefore the liquidflow through. The cross section can be increased or decreased. A
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common type of pneumatic actuator used with the process control
valves is the diaphragm actuator. The diaphragm is made of rubber
which is sand winched in its centre between two circular steel disc, as
shown in the fig.
The effect of changes in the input pressure results in the movement
of the central part of the diaphragm. This movement is communicated
to the final control element by a shaft which is attached to the
diaphragm.The force F acting on the shaft is the force that is acting on the
diaphragm and it is equal to the gauge pressure P multiplied by the
diagram area.
F= PA 1
Restoring force is provided by the spring. Assume shaft moves
through a distance x and the compression of the spring is proportional
to the force F,
(ie) F = kx. 2
Where k is a constant.
Therefore comparing equation (1) and (2)
kx = PA
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Thus the displacement of the shaft is proportional to the gauge
pressure.
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The above fig. shows the cross-section of a valve for the control of
rate of flow of a fluid. The pressure change in the actuator causes the
diaphragm to move and results in the movement of the stem. When
the stem moves, it results in the movement of the inner valve plug
within the valve body. The plug restricts the fluid flow and the position
of the plug determines the flow rate.
Valve bodies
There are many types of valve bodies. They are
i) Single seated valve.
ii) Double seated value.
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through the valve and so just one plug is needed to control the
flow. In double seated valve, the fluid after entering the valve splits into
two streams and this needs two plugs.
Valve plugs:
The shape of the valve plug determines the relationship between
the stem movement and the effect on the flow rate. There are three
commonly used types:
a) Quick opening type - A large change in flow rate occurs for a small
movement of the valve stem. Such a plug is used where on/off control
of flow rate is required.
Linear - Contoured type - The change in flow rate is proportional to
the change in displacement of the valve stem
Equal Percentage type - Equal percentage changes in flow rate
occur for equal changes in the valve stem position.
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ROTARY ACTUATORS:
There are many methods to achieve the rotary motion. Three
types of Rotary activators are discussed below.
A linear cylinder to produce rotation.
As shown in the fig. 3.25 a linear cylinder with the help of suitable
mechanical linkages can be used to produce rotary movement throughangles less than 360° .
Semi - Rotary Actuator
It has two ports clockwise port and anticlockwise Port, and also
has a vane attached with rotor of the actuator. The pressure difference27
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between the two ports causes the vane to rotate. When the vane
rotates, the shaft attached to the vane also rotates. Therefore the shaft
rotation is a measure of the pressure difference between the two ports.
Depending on the pressures, the vane can be rotated clockwise (or)
anticlockwise.
Vane motor:
The vane motor is a pneumatic motor through which a rotation
angle greater than 360° can be achieved. It has an eccentric rotor with
slots in which vanes are forced outwards against the wall of the
cylinder by the rotation. The vane divides the chamber
In to separate compartments which increase in size from the inletport round to the outlet port. The air entering each compartment exerts
a force and the vane and causes the rotor rotates. The motor can be
made to rotate in both clockwise and anticlockwise directions.
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ELECTRICAL ACTUATION SYSTEMS
Any discussion of electrical systems used as actuators for
control. The discussion has to include:
1. Switching devices such as mechanical switches. e.g. relays, or
solid-state switches, e.g. diodes, thyristors, and transistors, where the
control signal switches on or off some electrical device, perhaps a
heater or a motor.
2. Solenoid type devices where a current through a solenoid is used to
actuate a soft iron core. as, for example, the solenoid operatedhydraulic/pneumatic valve where a control current through a solenoid
is used to actuate a hydraulic/pneumatic flow.
3. Drive systems, such as d.c. and a.c. motors, where a current
through a motor is used to produce rotation. This chapter is an
overview of such devices and their characteristics.
Mechanical switches
Mechanical switches are elements which are often used as sensors
to give inputs to systems. e.g. keyboards. In this chapter we are
concerned with their use as actuators to perhaps switch on electric
motors or kiting elements, or switch on the current to actuate solenoid
valves controlling hydraulic or pneumatic cylinders. The electrical relayis an example of a mechanical switch used in control systems as an
actuator.
Relays
The electrical relay offers a simple on/off switching action in
response to a control signal. Figure illustrates the principle. When a
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current flows through the coil of wire a magnetic field is produced. This
pulls a movable arm, the armature that forces the contacts to open or
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dose; usually there are two sets of contacts with one being opened
and the other closed by the action. This action might then be used to
supply a current to a motor or perhaps an electric heater in a
temperature control system.
As an illustration of the ways relays can be used in control systems,
Fig. shows how two relays might be used to control the operation of
pneumatic valves which in turn control the movement of pistons in
three cylinders A, B and C. The sequence of operation is:
1. When the start switch is closed, current is applied to the A and B
solenoids and results in both A and B extending, i.e. A+ and B+.
2.The limit switches a+ and b+ are then closed, the a+ closure results
in a current flowing through relay coil 1 which then closes its contacts
and so supplies current to the C solenoid and results in it extending,
i.e. C+.
3. Its extension causes limit switch c+ to close and so current to switchthe A and B control valves and hence retraction of cylinders A and B,
i.e. A- and B-.
4. Closing limit switch a- passes a current through relay coil 2; its
contacts close and allows a current to valve C and cylinder C to
retract, i.e. C-. ,
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The sequence thus given by this system is A+ and B+ concurrently,
then C+, followed by A- and concurrently and finally C-.Time-delay
relays are control relays that have a delayed switching action. The time
delay is usually adjustable and can be initiated when a current flows
through the relay coil or when it ceases to flow through the coil.
SOLID STATE SWITCHES:
Here are number of solid-state devices which can be used to
electronically switch circuits. These include:
1. Diode.2 .Thyistors and triacs.
3 .Bipolar transistors.
4. Power MOSFETs
DIODES
The diode has the characteristic shown in Fig. and so allows a
significant current in one direction only. A diode can thus be regarded
as a 'directional element', only passing a current with forward biased.
i.e. with the anode being positive with respect to the cathode. If the
diode is sufficiently reversed biased, i.e. a very high voltage, it will
break down. If an alternating voltage- is applied across diode, it can be
regarded as only switching on when the direction of the voltage is suchas to forward biased it and being off in the reverse biased direction.
The result is that the current through the diode is half – rectified to
become just the current due to the positive halves of the input voltage
(Fig.).
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Thyristors and triacs
The thyristor, or silicon-controlled rectifier (SCR), can be regarded
as a diode which has a gate controlling the conditions under which the
diode can be switched on. Fig shows the thyristor characteristic. With
the gate current zero. The thyristor passes negligible current when
reverse biased (unless sufficiently reverse biased, hundreds of volts,
when it breaks down).
When forward biased the current is also negligible until the forward
breakdown voltage is exceeded. When this occurs the voltage acrossthe diode falls, to a low level, about 1 to 2 V, and the current is then
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only limited by the external resistance in a circuit. Thus, for example, if
the forward breakdown is at 300 V then. When this voltage is reached
the thyristor switches on and the voltage across it drops to I or 2 V. If
the thyristor is in series with a resistance of. say. 20 ohm (Fig.)
then before breakdown we have a very high resistance in series
with the 20 and so virtually all the 300 V is across the thyristor and
there is negligible current. When forward breakdown occurs, the
voltage across the thyristor drops to, say, 2 V and so there is now 300
– 2 = 298 V across the 20 Ω resistor, hence the current rises to 298/20
= 14.9 A. When once switched on the thyristor remains on until theforward current is reduced to below a level of a few milliamps. The
voltage at which forward breakdown occurs is determined by the
current entering the gate, the higher the current the lower the
breakdown voltage. The power-handling capability of a thyristor is high
and thus it is widely used for switching high power applications.
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Triac
The triac is similar to the thyristor and is equivalent to a pair of
thyristors connected in reverse parallel on the same chip. The triac can
be turned on in either the forward or reverse direction. Figure showsthe characteristic.
Bipolar transistors
Bipolar transistors come in two forms, the npn and the pnp. Figure
shows the symbol for each. For the npn transistor, the main current
flows in at the collector and out at the emitter, a controlling signal being
applied to the base. The pnp transistor has the main current flowing in
at the emitter and out at the collector, a controlling signal being applied
to the base.
npn Transistor pnp Transistor
For an npn transistor connected as shown in Fig.(a) These so
termed common-emitter circuit, the relationship between the collector
current I C and the potential difference between the collector and emitterVCE is described by the series of graphs shown in Fig.(b)
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When the base current I s is zero the transistor is cut off. In this state
both the base emitter and the base collector junctions are reverse
biased. When the base current is increased, the collector current
increases and VCE decreases as a result of more of the voltage being
dropped across R c. When V CE reaches a value VC the base-collector
junction becomes forward biased and the collector current can
increase no further, even if the base current is further increased. This
is termed saturation. By switching the base current between 0 and a
value that drives the transistor into saturation, bipolar transistors can
be used as switches. When there is no input voltage V, then virtually
the entire V EC voltage appears at the output. When the input voltage is
made sufficiently high the transistor switches so that very little of the
VCC voltage appears at the output (Fig. (c)).
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MOSFETs
MOSFETs (metal-oxide field-effect transistors) come in two
types the n-channel and the p-channel. Figure shows the symbols. The
main difference between the use of a MOSFET for switching and abipolar transistor is that no current flows into the gate to exercise the
control. The gate voltage is the controlling signal. Thus drive circuitry
can be simplified in that there is no need to be concerned about the
size of the current.
n- Channel p- channel
SOLENOID
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It is used to provide electrically operated actuator. Example:-
solenoid valves are used in hydraulic and pneumatic valves. A current
passes through a coil. Due to this current, a soft iron core is pulled into
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the coil. In doing so it can open or close the ports to allow the flow of a
fluid.
D.C Motor
Electric motors are frequently used as the final control element inpositional or speed-control systems. Motors can be classified into two
main categories: d.c. motors and a.c. motors. Most motors used in
modern control systems being d.c. motors
Construction ofd.c motor
* YOKE:
1. It is the outermost covering of the machine. 2. It provides
mechanical support for the poles. 3. It is a stationary part. 4. It carries
magnetic flux produced by the poles. 5. It is made of cast iron.
* FIELD SYSTEM:(a) Pole core
(b) Pole shoe
(c) Field coil.
(a) Pole core:
1. They are fabricated by laminations of steel.
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2. They are laminated so as to avoid eddy current loss.
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(b) Pole shoe:
1. They act as a mechanical support to the field coil.
2. They reduce the reluctance of the magnetic flux.
3. They spread out the flux in the air gap uniformly.(c) Field coil:
1. These coils are wound on the pole core. 2. When current is passed
through this coil, they electromagnetic.
* Inter poles:
1. They are fixed between the main poles.
2. They are in-line with the neutral axis.
3. They are smaller in size than main poles.
4. They are used for spark less commutation.
* Armature:
1. It is the rotating part of the machine.
2. It is cylindrical in shape.3. It is fabricated by means of steel laminations.
4. It is laminated to avoid the eddy current loss.
5. The periphery of the armature is cut into slots and teeth's.
6. The conductors are placed in the slots.
7. Due to loss, heat is developed in the armature.
8. Therefore to dissipate heat a fan is provided at one end of thearmature.
* Commutator:
1. It is made up of copper segments insulated from each other by mica
sheets.
2. The armature conductors are soldered to Commutator.
3. It is used to convert bidirectional current to unidirectional current.
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* Brushes:
1. These are made of carbon. It is rectangular in shape.
2. The brush holders are kept passed against the Commutator.
3. It collects current from the line to the Commutator.
Principle of Working:
The armature is made up of magnetic material with coils of core
wound on it. The armature is mounted on bearings and is free to
rotate. They are field coils wound and permanent magnet W
electromagnets fixed to the carrying [w] starter. The ends of the
armature coil are connected to Commutator. When the current is
applied to the field coil it cuts the magnetic flux near to armature, and
armature start rotating. The direction of rotation of the D.C motor can
be changed by reversing either the armature current (or) the field
current.Types of D.C. motor
1. Series wound motor (fig. a)
With the series wound motor the armature and fields coils are in
series. Such a motor exerts the highest starting toque and has the
greatest no-load speed. With light loads there is a danger that a series
wound motor might run at too high a speed. Reversing the polarity of
the supply to the coils has no effect on the direction of rotation of the
motor. It will continue rotating in the same direction since both the field
and armature currents have been reversed.
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2. Shunt wound motor (fig.b)
With the shunt wound motor the armature and field coils are in
parallel. It provides the lowest starling toque, a much lower no-load
speed and has good speed regulation. Because of this almost constant
speed regardless of load, shunt wound motors are very widely .used.
To reversed the direction of rotation. either the armature or field
supplied must be reversed. For this reason, the separately excited
windings are preferable for such a situation.
3. Compound motor (fig. c)
The compound motor has two field windings. One in series with the
armature and one in parallel. Compound wound motors aim to get thebest features of the series and shunt wound motors, namely a high
starting torque and good sped regulation.
4. Separately excited motor (fig. d)
The separately excited motor has separate control of the armature
and field currents and can be considered to be a special case of the
shunt bound motor.
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Control of D.C. Motor:
1. The speed of the permanent magnet motor depends upon the
current through the armature coil.
2. In a field coil D.C motor, the speed can be changed by varying the
armature current or the field current.
3. Generally it is the armature current that is varied.
4. To obtain a variable voltage at the armature an electric circuit is
used.
5. Usually the D.C motors are controlled by the signals coming frommicroprocessors.
6. In such cases the technique knows as pulse width modulation
(PWM) is used, to obtain a variable voltage.
7. This PWM can be obtained by means of a basic transistor circuit.
8. This technique can be used to drive the motor in one direction only.
9. By involving four transistors which is know as H - circuit, the
direction change in rotation of motor can be obtained
In a closed loop control system, the feed back signals are
used to modify the motor speed. There are three methods for doing it.
Method - I
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1. Here the feed back signal is provided by a tachometer.
2. The analogue signal from the tachometer is converted into digital
signal by using ADC.
3. This digital signal is given as input to the microprocessor isconverted into analogue by using DAC.
4. This signal is used to vary the voltage applied to the armature of the
D.C. motor.
Method – II
1. The feed back signal is coded using a encoder.
2. In the code converter, the digital output is obtained.
3. This digital signal is given as an input to the microprocessor.
Method – III
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1. The system is completely digital.
2. PWM is used to control the average voltage applied to the armature.
A.C MOTOR
Types of A.C motor:1. Squirrel cage induction rotor.
2. Slip ring (or) wound rotor.
Construction of A.C Motor
STATOR:
1. It is the stationary part of the machine.
2. It is made of high grade silicon steel laminations.
3. it is laminated so as to avoid eddy current loss.
4. The stator windings are placed in the slots on the inner surface of
stator core.
5. The windings are wound for a particular number of poles.
6. The three phase stator windings are fed from three phase supply.
7. The stator windings are sometimes known as primary windings.
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ROTOR:
Squirrel cage rotor:
1. The construction is very simple.2. It is the rotating part of the
machine.3. It is cylindrical in shape with slots on its outer surface.
4. The rotor conductors are heavy bars of copper or aluminum.
5. All the ends of the bars are short circuited by means offend rings on
both sides.
6. The slots are slightly angled to prevent hum noise and locking of
stator and rotor.
Slip ring or wound rotor:
1. The rotor is wound for the same numbers of poles as that of the
stator.
2. The rotor is made up of silicon steel laminations.
3. The open ends of the rotor windings are brought out and they are
connected to three slip rings.
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4. The three slip rings are mounted on the shaft.
5. The slip rings are insulated from each other.
6. The slip rings are made of phosphor bronze.
Principle of Operation: 1. A three phase supply is given to the stator winding.
2. A rotating magnetic field is produced in the stator which rotates in
synchronous speed.
3. Synchronous speed depends upon supply frequency and number ofpoles.
4. In rotor short circuited copper bars are provided.
5. The rotating magnetic field cuts the short circuited copper
conductors, thereby inducing an emf in the rotor conductors.
6. Hence a magnetic field is setup in the rotor.
7. Due to the interaction between the stator and rotor magnetic flux,
the rotor rotates.
8. The direction of rotation of rotor is same as that of rotating magnetic
field but with the speed lesser than the synchronous speed.
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Control of A.C Motor:
1. The speed control of A.C motor is more complex than the D.C
motors.
2. The speed of the A.C motor is determined by the frequency of
supply.
N= 120 F/P Where, N = speed in rpm.
F = frequency
P = no. of poles.
3. Therefore the control of A.C motor is based on the variable
frequency supply.
4. The change in the frequency can be achieved by two methods.
A. using a converter and an inverter.
B. using cyclo converter.
Using a converter and an inverter:
1. The three phase A.C is rectified to D.C by a converter.2. Then it is inverted back to A.C. again but at a frequency that can be
selected.
Cyclo converter:1. It is used to operate slow speed motors.
2. This converts A.C at one frequency directly to A.C at another
frequency without the intermediate D.C conversion.
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STEPPER MOTORS:
1. It is a device that produces rotation through equal angle called steps
for each digital input.
2. for example :-Suppose say one phase input produces 6O of rotation,
then60 phase of input produces 360° of rotation.
Construction and working principle:
1. Stepper motor is a special type synchronous motor.
2. It converts electrical pulses applied to it into discrete rotor
movements called steps.
3. A 30 ° per step motor will require 12 pulses to move through one
revolution.
4. From diagram, A, B and C are the three stator coils placed at 120°
apart around the circumference of the stator.
5. A four pole rotor made of soft iron is placed in between stator coilsso as to rotate.
6. When coil A is excited, the rotor teeth 1 & 3 are aligned along 'A'
axis as shown in figure (i).
7. When excitation of coil A is removed and if coil B is excited, the
rotor teeth 2 is attracted by coil B and thus rotor teeth2 and 4 are
aligned 'B' axis as shown in figure (ii).
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8. Now when the coil C is excited after removing the excitation of coil
B, the rotor teeth 3 gets aligned along 'C' axis as shown in figure
(iii).
9. Hence a clockwise motion of the motor is produced when pulsesare given in the order of A, B, c, A, B, C.... etc.
10. For each pulse, the rotor moves 30° per step.
11. If no coil is excited, then the rotor stands at any position.
12. When pulses are given for the coils in the order A, C, B, A, C, B....
etc., the rotor rotates in anticlockwise direction.
Application:
1. Used in X - Y plotters.
2. Used in machine tools.
3. Used in robots.
4. Used in computer peripherals like floppy disc drives line printers etc.
5. Used in watches.
Types of stepper motor:
1. Variable reluctance of stepper motor.
The rotor is made of soft steel and is cylindrical in the four poles.
The rotor will rotate until rotor and stator poles line up. This is termed
as position of minimum reluctance. This form of stepper gives step
angle of 7.5 0(or) 15 0
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2. Permanent magnet stepper motor:
1. The rotor is a permanent magnet.
2. The motor has a stator with four poles.
3. Each pole is wound with a field winding.
4. The coils in position pair of poles are connected in series.5. When the current is supplied to the stator windings, the rotor which
is a permanent magnet will move to line up with the stator poles.
6. From the figure shown, the rotor will move to the 45 0osition.
7. When the polarity is reversed for the current supply to stator coil,
thus the rotor will move on the reversed side to45 0positions.
8. Thus by switching through the coils the rotor rotates in 45 0teps.
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9. The usual step angles are 1.8 0 7.5 0 15 0 30°, or 90°.
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3. Hybrid stepper
1. It combines feature of both the variable reluctance and permanent
magnet motor.
2. It has a permanent magnet encased in iron caps.
3. The iron caps are cut to have teeth.
4. The typical steps angles are 0.9 0to1.8 0
5. Used in high accuracy positioning application.
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CONTROLLERS
Open-loop control is essentially just a switch on-switch off form
of control, e.g. an electric fire is either switched on or off in order to
heat a room. With closed-loop control systems, a controller is used
to compare the output of a system with the required condition and
convert the error into a control action designed to reduce the error.
In this chapter we are concerned with the ways in which controllers
can react to error signals, i.e. the control modes as they are termed,
which occur with continuous processes.C o n t r o l m o d e s :
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TWO – STEP MODE
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\
Switchoff
p y On Dead ben
Switcon
Off
,n : ·
An example of the two- st e p mod e of control is the bimetallicthermostat that might be used with a simpletemperature control system . This is just a switch which isswitched on or off according to the temperature . If the roomtemperature is above the required temperature then the bimetallicstrip is in an off position and the heater is off . If the roomtemperature falls below the required temperature then the
bimetallic strip moves into an on position and the heater isswitched fully on. The controller in this case can be in only two
positions, onor of( as indicated by Fig. · With the two-step mode the control action is discontinuous. A
consequence of this is that oscillations of the controlled variableoccur about the required condition . This is because of lags in thetime ~ control system and the process take to respond. For ...example , in the case of t)le temperature control for a domesticcentral heating system, when the room temperature drops belowthe required level the time that elapses before the control systemresponds and switches the heater on might be very small incomparison with the time that elapses before the heater begins tohave an effect on the room temperature . In the meantime thetemperature has fallen even more. The reverse situation occurswhen the temperature has risen to the required temperature . Sincetime elapses before the control system reacts and switches theheater off, and yet more time while the heater cools and stops
Oscillations with two step mode Two step control with two
I
I-
Cont r oler swlch positions controller switch pointsHeater su
d
I ITime
!o"H .:I I
h
\ / Temperatu r eI Off ._ ~ _ ._ _, Controller
switch points45
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I
I
I I
L-1
1 I I
---1--- i °"
0
heating the room, the room temperature goes beyond the requiredvalue. The result is that the room temperature oscillates abo veand below the required temperature
With the simple two-step system described above ther e is the . problem that when the room temperature is hovering about the setvalue the thermostat might be almost continually switching on oroff, reacting to very slight changes in temperature . This can beavoided if, instead of just a single temperature value at which thecontroller switches the heater on or off, two values are used andthe heater is switched on at a lower temperature than the one atwhich it is switched · off (Fig . · . The term dead band is used
for the values between the on and off values. A large dead bandresults in large fluctuations of the temperature about the settemperature; a small dead band will result in an increasedfrequency of switc~ng .-· The bimetallic element shown in Fig.
has a permanent magnet for a switch contact; this has theeffect of producing a dead band .
Two-step control action tends to be used where changes aretaking place very slowly, i.e . with a process with a large capacit •ance. Thus , in the case of heating a room, the effect of switchingthe heater on or off on the room temperature is only a . slow change . The result of this is an oscillation with a long periodic time . Two-step control is thus not very precise. but it docs involve simple devices and is thus fairly cheap. On-off control is not restricted to mechanical switches such as bimetallic strips or
relays; rapid switching can be achieved with the use of thyristorcircuits such a circuit might be used forcontrolling the speed of a n,ntnr _ and oeerational amplifiers.
PROPORTIONAL MODE (P) 100%
i ) Set - -
- point
--• I I I I
0 + 46 «rol'
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With the two-step method of control, the controller output iseither an on or an off signal, regardless of th e magnitud e of theerror . With the pr o port ional mode , the si ze of the controlleroutput is pr o portional to th e size of the error . This mean s thecorrection element of the control system , e.g. a valve, will r eceivea signal which is proportional to the size of th e correctionrequired .
Figure shows how the output of such a controller varies with the size and sign of the error . The linear re lationship between controller output and error tends to exist only o
.ver a
certain range of errors , this range being called the pr o porti ona l band . Within the proportional band the equation of the straight line can be represented by
Change in controller output from set point = K,e
where e is the error and KPa constant. K, is thus the gradient of the straight line in Fig . The controll er output is general ly expressed as a per centage of
the full rang e of possi ble outputs within the pr o portional band.
Thi s out put can th en correspond to, say, a correctio n valvechanging fr om fully closed 'to fully o pen. 'Similarly, the erro r isexpressed as a per centage of the full-range value, i.e. the err orrange corr espondingto the Oto 100% contr oller output. Thu s
% change in contr oller output fr om set point = K»x % change in error
Hence, since 1 OOo/o contr oller .output corr esponds to an err or percentage equal to the pro portional band
K _ 100 P - pr o portional band
We can r ewrite the equation as
change in output= I ~ - 10 = Kpe
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DERIVATIVE CONTROL (D)
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PROPORTIONAL PLUS DERIVATIVE CONTROL (PD)
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50
Hence
lou • K,.e+ Ko di +l o
where Io is the output at tbe set point, I ou tbe output when theerror is e, K» is tbe proportionality constant and K o the derivativeconstant. de/dJ is tbe rate of change of error . The system bas atransfer function giv en by
(J-. - loXs) = K.,E(s) + KoSE(s)
Hence the transfer function is K, + Kor . This is often written as:
transferfunct.ioo • K o(s+ T~)
where To= K r/ KP and isthe deri vative lime con stant . Figure shows how the controller output can vary when
there is a constantly changing error . There is an initial quickchange in controller output because o f the derivative actionfollowed by the gradual change due to proportional action . Thisfonn of control can thus deal with fast process changes ; however ,a change in set value will require an offset err or (see earli erdiscussion of proportional control) .
INTEGRAL CONTROL (I)
The int e gral mode of control is one wher e the rate of change ofthe contr ol output I is proportional to th e input error signal e.
K 1 is the constant of pr o portionality and , whe n th e contr olleroutput is expressed as a percentage and the error as a percentage ,has units of s-•. Integrating the above equation giv es
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0
llof
0KaedJ
K, is the consta nt of pr o portionality and, when th e co ntro ller i out put is expressed as a percentage and the erro r as a percentage. o i--- ---. --- --has unit& of s" . Integrating the above equation give s
Jr,'.•
di= J' I N - 1, = J : K ,edt
1~ is the coauolle r output at zero time. J .,. is the outpUt at ume r, The transfer function is obtained by taking ~ La place
transf'onn. Thus \
( / ... - 1,X s) • tK aE(s )
and so
Transfer function = tK1 Figure 1llustralcs the action of an integral controller when
there is a constant error inpu t lo the controller .' We can conside r
the gra phs intwo ways.
Whenthe
con troller outpu t is constantthe error is zero; when the controller output varies at a constantrate the error has a co nstan t value. The alternative way ofconsidering the graphs is in _ terms of the area unde r the erro rgra ph.
Ar ca under the error gra ph between t • 0 and t • J~ e di
Thus up to the time w hen the err or occurs the va lue of th e integral is zero. Hence / Otll. = / o. When the err or occ ur s it maintains a constan t value. Thus the area und er the grap h is increasing as the time increases . Since the area incr eases at aconstant r ate the contr oller output increases at a constant rate.
lnlegr
v.
Tlme
PROPORTIONAL PLUS INTEGRAL CONTROL (PD)
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PID CONTROLLERS
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DIGITAL CONTROLLERS
The digital controller requiring inputs which are digital, process
the information in digital form and give an output in digital form. The
controller performs the following functions:1) Receives input from sensors.
2) Executes control programs
3) Provides the output to the correction elements.
As several control systems have analog measurements an analog
– to digital converters (ADC) is used for the inputs. The fig shows thedigital closed – loop control system which can be used with a
continuous process.
The clock supplies a pulse at regular time intervals, and dictateswhen samples of controlled variables are taken by ADC.
These samples are then converted into digital signals which are
compared by the microprocessor with the set point value to give the
error signal. The error signal is processed by a control mode and
digital output is produced.