Control System Technology

460 Pages·1982·21.268 MB·English

by  C. J. Chesmond (Auth.)

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C.J. CHESMOND
Senior Lecturer in Control Engineering,
Queensland Institute of Technology.
Edward Arnold
©C.J. Chesmond1982
First published by Q Search, Queensland Institute of Technology, Brisbane, Australia
First published in Great Britain 1984 by
Edward Arnold (Publishers) Ltd, 41 Bedford Square, London WC1 B 3DQ
Edward Arnold, 300 North Charles Street, Baltimore, Maryland 21201, U.S.A.
Edward Arnold (Australia) Pty Ltd, 80 Waverley Road, Caulfield East, Victoria 3145, Australia
ISBN 0 7131 3508 5
All rights reserved. No part of this publication may be reproduced,
stored in a retrieval system, or transmitted in any form or by any means,
electronic, mechanical, photocopying, recording or otherwise, without
the prior permission of Edward Arnold (Publishers) Ltd.
Printed in Great Britain by Butler & Tanner Ltd, Frome and London
PREFACE
Having taught Control Engineering for many years, I have formed the strong opinion
that an engineering student, of whatever category, should be instructed how to create
a practical control system before being expected to learn how to analyse, in detail, its
behaviour. Certainly, when the student eventually becomes either an engineer or an
engineering associate, the bulk of his working life will be devoted to practical engineering
with analysis, of whatever complexity, occupying only a small proportion of his time.
Thus, the educator has a responsibility to introduce the student to the principles involved
in the creation of practical engineering systems.
As the bibliography appended to this volume testifies, there is a wealth of reference
material available in the fields of instrumentation and automatic control. What, to my
mind, has been significantly lacking has been a comprehensive textbook dealing with
control system technology, and this volume is an attempt to fill this void. The book
has evolved from a far more modest publication entitled Control System Hardware.
Control System Technology has been written with professional and para-professional
engineering students in mind, be they enrolled in courses in electrical, mechanical,
chemical, mining, aeronautical, nuclear, or production engineering. To satisfy such
a wide market, I have tried to present the material in as readily understood a form as
possible; however, in order to limit the book to a reasonable size, it has been necessary
to assume that the reader has a prior knowledge of such fundamental fields as basic
electrical and electronic circuits and components, elementary mechanics, fluid dynamics,
thermodynamics, system analysis, etc.
This book is intended to complement the multiplicity of texts available for the study of
control system analysis, many of which are listed in the bibliography: the two types of
book should be used concurrently. In order to avoid producing an encyclopaedic type
of reference work, it has been necessary to limit the coverage devoted to each topic:
the book at least indicates all of the alternatives available to the practising engineer, and
the bibliography provides sources for further, more detailed, information.
I have tried to provide a balanced coverage of the field. The book includes a compre-
hensive survey of transducers; servomechanisms and process control systems have been
given equal emphasis; there is a comprehensive treatment of signal conditioning and
data conversion; construction, testing, and commissioning of control systems have all
been covered; computer interfacing and on-stream analysers have been included in order
to reflect their increasing importance in control engineering.
I have yet to read a text which has not contained omissions or errors of fact and have,
alas, little reason to expect this volume to be flawless. I should be grateful if the reader
would draw my attention to those areas deserving of alteration or elaboration, so that
any subsequent edition may be an improvement.
I am deeply indebted to many of my colleagues at the Queensland Institute of Technology
for the assistance which they have given me in the preparation of this material: they are
too numerous for it to be practical to list their names here.
Colin Chesmond
Brisbane
October 1982
(iv)
1
CLASSIFICATION,
TERMINOLOGY
AND DEFINITIONS
2 CLASSIFICA TION, TERMINOLOGY AND DEFINITIONS
1.1 NATURAL CONTROL SYSTEMS
Feedback control systems exist in nature, to a considerable extent. The simple
action of a human picking up a pencil typically involves two feedback paths:
firstly, visual feedback data enable the current positions of the fingers to be
signalled to the optical system and hence to the brain; secondly, having located
the pencil, feedback data are transmitted to the brain, via the nervous system,
to signal the amount of pressure currently being applied by the fingertips to
the pencil.
The objective of the brain is to establish the desired positions for the fingers
and the desired degrees of finger pressure, to compare these desired values with
the actual values being transmitted back to the brain, and to use the results of
these comparisons to compute an appropriate course of action which will then
be implemented, by appropriate body muscles, upon receipt of suitable signals
from the brain.
Systems such as this, which employ feedback data, are termed Closed Loop
Control Systems. The converse of these are termed Open Loop Control Systems,
such as would result, in the example under consideration, if the person involved
were blind and had finger tips insensitive to skin pressure: it is still conceivable
that the pencil could be picked up, but the probabilities would be high for missing
the pencil altogether, failing to grip it, subsequently dropping it, or snapping it!
Thus, with Closed Loop action there is the potential for considerably improved
quality of control, compared with Open Loop action.
The contents of this volume are concerned with automatic control systems, in
which the functions alluded to so far are implemented by hardware, with the
function of the human operator reduced to the task of establishing desired
values, or goals, for the automatic systems.
1.2 HISTORY
One of the first automatic systems to be documented was constructed in pre-
Christian times, to open the doors of an ancient Greek temple. The lighting
of the fire on an altar caused water to be driven by pressure into a bucket and
the resulting additional weight was used to actuate the door opening mechanism.
This was inherently an open loop system, because there were no feedback data
supplied to the hardware to indicate the actual position of the doors.
The first significant closed loop control system was James Watt's flyball governor,
developed in 1788 for the speed control of a steam engine. Minor developments
occurred from time to time (for example, in windmills and machine looms),
but the real watershed for control engineering was triggered by the Second World
War and has continued ever since, accelerated until recently by the space programs.
Initial progress was made in single loop systems, which contain a single feedback
channel, but the technology has been extended to embrace multi-loop systems,
which contain two or more feedback paths: thus, in a modern transport aircraft,
systems will be present to automatically control such variables as altitude, rate
of climb and descent, Mach number, airspeed, airport approach trajectory, cruise
1.3 THE FUTURE 3
flight path, etc., in addition to the more straightforward variables such as attitude
and rotational velocity, and these are implemented with a hierarchy of feedback
control loops.
1.3 THE FUTURE
In the aerospace field, it can be anticipated that there will be progressive improve-
ment in the degree of sophistication of the control systems used. Precision guidance
of space probes, the wide range of operational modes of the space shuttle, and
the ability, remotely from Earth, to manoeuvre vehicles traversing areas of the
surface of planets all require a high level of complexity for the control systems
involved.
In the industrial field, repetitive production line operations are increasingly
being taken over by robots, which can be designed to operate in the most hostile
of environments and which can function for twenty four hours per day without
exhibiting fatigue. Progressive improvement in the control of product quality
can be expected to result from improvements in instrumentation hardware.
Control engineering techniques are being adapted to be applied to fields such
as environments, economies, company management, and resource management.
In many instances, it is becoming necessary to interface the control systems to
on-line digital computers, in order to cope with the complexity of the systems
under control. Indeed, the application of computers is often rendering it logical
to employ digital hardware for the control systems themselves: note that, although
the digital hardware is very different in nature from its analog counterpart,
the control principles employed are changed little by this modified approach.
1.4 GENERALISEDSINGLE LOOP CONTINUOUS FEEDBACKCONTROLSYSTEM
CONTROLLER error signal manipulated variable
l / FORWARD ^] PATH- /
lii Hi
FINAL
Reference Error Signal Power PLANT
CONTROL *"►
Transducer Detector Amplifier Amplifier PROCESS
ELEMENT
I L_
"C
THE LOOP
reference variable — -
the desired value for
the plant property
being controlled
FEEDBACK
reference signal
TRANSDUCER
feedback signal controlled variable-
the actual value of
the plant property
FEEDBACK PATH being controlled
FIG. 1
4 CLASSIFICATION, TERMINOLOGY AND DEFINITIONS
Figure 1 is a generalised representation which is valid for any single closed loop
continuous control system. Notice that the plant process forms an integral part
of the control loop: the implications from this are, firstly, that the performance
of the control loop is heavily influenced by the performance of the plant process
and,secondly, that the control engineer will need to have a fairly intimate know-
ledge of the details of the plant process being controlled.
A transducer is a device which is capable of converting the value of a data variable
into a signal whose magnitude and sense are representative of the magnitude
and sense of the data variable. Typically, the feedback transducer, which is
measuring the actual value of the variable being controlled, is mounted directly
on the plant under control, whereas the reference transducer is mounted on a
control station (for example, a console) which may be remote from the plant;
the reference transducer indicates the desired value (which typically is selected
by an operator) of the variable under control.
The error detector performs the function of computing the error signal, which
is the difference between the reference and feedback signals and therefore
represents a measure of the difference between the desired and actual values
for the controlled variable. Obviously, then, a perfect control system is one in
which the error signal is held at zero at all times, so that any departure from
this condition represents a performance degradation from the ideal.
The final control element is some actuating device physically integral with the
plant and which is capable of manipulating the plant in such a way that the
controlled variable is, in fact, capable of being adjusted. Because of the input
signal requirements of the final control element, stages of signal and power
amplification generally are necessary, in order to boost the error signal strength
appropriately.
Typically, the amplifiers and the error detector join the reference transducer
in the control station; when mounted together as a single unit, they generally
are referred to as a Controller. The Controller may be remote from the remainder
of the loop elements, even to the extent of being connected by telemetry link,
in some cases.
Because of the nature of the hardware, the power level at the output from the
plant typically will be many orders of magnitude greater than the power level at
the input to the reference transducer, so that it may be possible to control MW
of output power with mW of input power, for example.
1.5 CLASSIFICATION OF CONTROL SYSTEMS
Automatic control systems may be classified in many different ways and these
are outlined below. Any one control system obviously will relate to several of
the categories listed.
A. Open Loop versus Closed Loop Continuous Control
Figure 2 represents an elementary example of a simple open loop speed control
1.5 CLASSIFICA TION OF CONTROL SYSTEMS 5
system, using a separately excited DC motor.
DC armature
supply voltage
Q^ 0
DC field ^^^. speed N
supply voltage
( MOTOR P ■ ■ ■)■ LOAD
FIG. 2
The speed of the output shaft will be set manually, by adjustment of the field
rheostat, so that, in theory at least, there will be a given specific speed for each
position of the rheostat slider, which therefore may be calibrated in terms of
equivalent shaft rpm.
The accuracy with which a particular desired speed is actually attained will be
impaired by the following factors:
• changes in the supply voltages;
• variations in the resistances of the rheostat, field winding, and armature
winding, resulting from temperature changes due to self heating or fluc-
tuations in ambient temperature;
• variations in the characteristics of the load;
• magnetic hysteresis in the motor, which will cause the value of speed
attained to depend upon the recent past history of variations in desired
speed setting.
The degree of precision with which the speed is obtained may be improved
considerably, by monitoring the shaft speed with a suitable instrument (a velocity
transducer) and requiring a human operator to make appropriate adjustments,
to the rheostat setting, in order to correct for any drift in the measured speed
away from the desired value. This arrangement represents an elementary form
of closed loop control, with the operator serving as the error correcting part
of the loop, so that the quality of control will depend largely upon the manual
dexterity and mental concentration of the operator. The system may be automated,
by employing hardware based on the block diagram of Figure 1, so that the quality
of control should now be consistent. Figure 3 represents a typical closed loop
arrangement developed from the open loop configuration of Figure 2.
6 CLASSIFICATION, TERMINOLOGY AND DEFINITIONS
DC armature
supply voltage
set
Q ^ O
desired Λ^
speed //III
o- Jr
POWER
AMPLIFIER
VELOCITY
TRANSDUCER
DC ft
supply LOAD
voltage
speed
DIFFERENTIAL
AMPLIFIER
X7
FIG. 3
Generally speaking, the properties of an open loop system are as follows:
Advantages Disadvantages
Relatively simple, resulting in cost, Relatively slow in response to demanded
reliability and maintainability changes.
advantages. Inaccurate, due to lack of corrective
Inherently stable. action for error.
Generally speaking, the properties of a closed loop system are as follows:
Advantages Disadvantages
Relatively fast in response to Relatively complex.
demanded changes. Potentially unstable, under fault
Relatively accurate in matching conditions.
actual to desired value.
Instability is a condition, with feedback systems, whereby control is lost and
the actual value ceases to track the desired value. Typically, but not inevitably,
the output oscillates and these oscillations progressively increase in magnitude.
With adequate design, instability can only develop after an equipment failure
has occurred. Obviously, the effects can be catastrophic (for example, in aircraft
or nuclear reactors) so that, in these situations, it is necessary to detect failure
and to disable the control system as rapidly as possible: inevitably, this will
introduce further complexity.
B. Classification by Type of Plant being Controlled
This is self explanatory: examples are power generator control, boiler control,
air conditioning control, aircraft control, ship control, space probe control,
etc.