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Control System Technology PDF

460 Pages·1982·21.268 MB·English
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ram ηηππηπππ U üulJJLi LDÜUUU s u s l ik "ΓΡΓ^ΓΡππππΓίΠπ m r öw ULiÜDÜÜUuUüiiLyiilu 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.

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