Exam (elaborations)
Automatic Control System
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Automatic Control System
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- 71 -CHAPTER – 5
CONTROL SYSTEM COMPONENTS AND DEVICES
5.1. INTRODUCTION:
A closed loop control system can be represented by the general block diagram shown in fig 5.1. Such a
system is composed of three basic elements; the feedback element, controller and controlled system.
Error detector
Controller
Output
Input (controlled)
(command)
Actuating Control
Control Controlled
+ Signal, e
elements system
signal
Signal, r - Variable, C
Feedback
element
Feedback signal, b
Fig.5.1 General Block diagram of a closed-loop control system.
The feedback element is a device which converts the output (controlled) variable c into another suitable
variable, the feedback signal b which then is compared with the input (command) signal.
The controlled consists of an error detector and control elements. The error detector compares the
feedback signal obtained from the plant output with the input (command) signal and determines there from the
deviation known as the actuating signal. The actuating signal is usually at low power level. It is suitable
manipulated by the control elements to produce a control signal. The manipulation may involve amplification,
generation of a suitable function of the actuating signal and a power stage. The power stage in control elements is
essential so that the control signal can drive the controlled system (a plant or process) to produce the desired
output variable. Large power amplification may be involved in the plant or process being controlled.
5.2. CONTROLLER COMPONENTS:
1. Sensors: These are low-power transducers which produce output signal as a measure of the controlled
variable; a linear (proportional) relationship is generally preferred though it could be a suitable functional
relationship. Sensors are employed for a variety of measurements – position, velocity, acceleration, pressure,
temperature pr a quantity representing the chemical state of a reactor, neutron flux level in an atomic reactor etc.
The output signal of the sensor is invariably in electrical form; analog or digital.
2. Differencing and amplification: Differencing to get the error signal and its amplification to suitable level in
magnitude and power are most conveniently carried out electronically. Availability of OPAMs for differencing
and stable amplification over a wide frequency range and large range of voltage (or current) and power levels by
use of power transistors or SCR’s at the final (power) stage are the chief advantages offered by the electronic
systems.
3. Actuators: These are devices whose output is mechanical motion (translator/rotary; though rotary motion is
convenient and is preferred). The actuators are characterized by power output and speed-torque relationship to
match the load. These could be electrical, hydraulic or pneumatic. An actuator in a control system performs a
variety of tasks to manipulate the controlled process or plant. For example, it may open/close a valve in a
hydraulic/steam/chemical process (plant), turn a robot link with respect to its neighboring link, move a
transformer tap (up or down), move up/ down the control rods of a nuclear reactor etc.
4. Electric system: DC and AC motors are the two kinds of electric actuators; in low power ratings these are
known as servomotors. DC motors are costlier than AC motors because of the additional cost of commutation
, - 72 -
gear. These have, however, the important advantages of linearity of characteristic and higher stalled torque /
inertia ratio; this being an important figure of merit for a servomotor. Stalled torque is the torque developed by
motor when stationary with full applied voltage (and full field in case of a DC motor). It may be pointed out here
that high torque / inertia ratio means lower motor time constant and so faster dynamic response. With advanced
manufacturing techniques, low brush commutator friction and still higher torque / inertia ratios have been
achieved in dc servomotors, such that these have practically taken over from ac servomotors in most control
applications.
5.3. CONTROL SYSTEM DEVICES:
Control system devices which make changes from one form to another are termed transducers. Actually,
the control system devices convert process variables in one form into variables in another form. Suppose a
potentiometer converts the angular position of a shaft to an output voltage, then the potentiometer will be termed
transducer. Some of the devices that work as transducers are listed below:
1. Potentiometer
2. Synchro.
3. Differential transformer.
4. Servomotor.
5. Tachogenerator.
6. Gyroscope.
7. Power amplifier.
8. Magnetic amplifier.
9. Stepper motor.
5.3.1. SERVOMOTORS:
Servomotors usually drive a final control element (or elements). When a control input is applied to the
servomotor it converts it into a mechanical displacement, the velocity, the torque etc, being desired outputs. The
servomotors may be either AC or DC and constructed through a wide range of power from fraction of a watt to
kilowatts. The most commonly used motors are separately excited dc motors and squirrel cage and drag cup type
induction motors which can meet the starting and regulating characteristics required for control operation. The
most important characteristic of the servomotor is the maximum acceleration. For a given available torque, the
rotor moment of inertia must be a minimum.
Some of the important and desirable requirements of a control motor are listed below:
1. The rotor should have a low moment of inertia.
dT
2. The speed torque curve should have a negative slope, i.e. < 0, and have the same slope over the
dN
entire range of voltage regulation.
3. The motor should withstand frequent starting operations.
5.3.1.1. AC SERVOMOTORS:
AC servomotors are solely squirrel cage or drag cup induction motors. These are usually two phase type
for power applications generally accomplished by a control signal via saturable reactors, magnetic amplifiers and
S.C.R. circuits. Possibilities with bi-directional S.C.R. are explored at the present time. The reason for these
motors being preferred particularly for low power application is because they are light weight, rugged and there
are no brush contacts to maintain.
A two phase induction motor consists of a stator with two distributed windings displaced 90 electrical
degrees apart. Figure 5.2(a) shows the schematic diagram for balanced operation of the motor, i.e. voltages of
equal rms magnitude and 90 0 phase difference are applied to the two stator phases, thus making their respective
fields 90 0 apart in both time and space, resulting in a magnetic field of constant magnitude rotating at
synchronous speed. The direction of rotation depends upon phase relationship of voltages V1 and V2 . As the field
sweeps over the rotor, voltages are induced in it producing current in the short circuited rotor. The rotating
magnetic field interacts with these currents producing a torque on the rotor in the direction of the field rotation.
The general shape of the torque speed characteristics of a two phase induction motor is shown in figure 5.2(b).
, - 73 -
Torque
V1
High
X/R
Stall
Torque 1
Rotor 2
Small
V2 X/R
Fig .5.2(a) Two phase induction motor. Fig.5.2(b) Torque – Speed characteristics.
It is seen from that the shape of the characteristic depends upon the ratio of the rotor reactance X to the
X
rotor resistance R. In normal induction motors, ratio is generally kept high so as to obtain the maximum torque
R
close to the operating region which is usually around 5% slip.
A two phase servomotor differs in two ways from a normal induction motor.
X
1. The rotor of the servomotor is built with high resistance so that its ratio is small and the
R
torque speed characteristic as shown by the curve 2 of fig.5.2(b) is nearly linear in contrast to the highly nonlinear
X
characteristic when large ratio is used for servo application then because of the positive slope for part of the
R
characteristic, the system using such a motor becomes unstable.
The rotor construction is usually squirrel cage or drag cup type. The diameter of the rotor is kept small
in order to reduce inertia and thus to obtain good accelerating characteristics. Drag cup construction is used for
very low inertia applications.
2. In servo applications the voltages applied to the two stator windings are seldom balanced. As
shown in fig. 5.3 one of the phase known as the reference phase is excited by a constant voltage and the other
phase, known as the control phase is energized by a voltage which is 90 0 out of phase with respect to the voltage
of the reference phase. The control phase voltage is supplied from a servo amplifier and it has a variable
magnitude and polarity ( 90 0 phase angle with respect to the reference phase). The direction of rotation of the
motor reverses as the polarity of the control phase signal changes sign.
It can be proved using symmetrical components that starting torque of servomotor under unbalanced
operation, is proportional to E, the rms value of the sinusoidal control voltage e(t).
A family of torque-speed curves with variable rms control voltage is shown in fig 5.4(a). All these curves
have negative slope. Note that the curve for zero control voltage goes through the origin and the motor develops a
decelerating torque. The linearised torque-speed characteristics of a two phase induction motor is shown in
fig.5.4(b).
Reference Phase, V2=Vr
Actuating Servo
Amplifier TM J, f
signal
Control Rotor
phase
Figure 5.3 Schematic diagram of a two phase servomotor.
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