Control system principles and design

Control system principles and design

Book Reviews 111 Control System Principles and Design* Ernest O. Doebelin Reviewer: PROFESSOR G R A H A M GOODWIN Department of Electrical and Comp...

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Book Reviews

111

Control System Principles and Design* Ernest O. Doebelin

Reviewer: PROFESSOR G R A H A M GOODWIN Department of Electrical and Computer Engineering, University of Newcastle, New South Wales 2308, Australia. THE SUBJECT OF CONTROL is often thought of as a single discipline, however the reality is that control really consists of a number of different areas; in particular one can distinguish the topic of control theory as being largely distinct from the practice of control engineering. This distinction has been alluded to in the recent article by Edgar Bristol in the IEEE Control Systems Magazine, titled "An industrial point of view on control teaching and theory". In this article, Dr Bristol makes the following comment: "For the university, control is a single unified discipline, developed primarily in terms of formal and academic research interests and subdivided into courses along theoretical lines. As with most generalized academic subjects, this has the pedagogic advantage of building the broadest underlying background in the most time-efficient way, but it is midleading with a broad impact subject like control. For an industrial organization, control becomes an integration of many technologies that carry out the organization's commercial goals in terms of its own and its customer's technical needs and traditions. This totality of technologies, needs and traditions is likely to occupy the attention of the controlapplication engineer far more than the formal control material presently learned in the university." Many of the contemporary books on the subject of control aimed at undergraduate students adopt the traditional approach of presenting control theory principles without illustrating these principles with realistic industrial applications. Many of the examples presented in these books are quite unrealistic and could well mislead a student having little practical experience. In the reviewer's opinion, what is needed in an undergraduate text on control is a clear presentation of the underlying unifying theory, illustrated by appropriate laboratory demonstrations and relevent industrial case studies. The book by Professor Doebelin is characterized by an intense practical flavour with many examples taken from actual applications. The emphasis in the book is on classical control theory illustrated by practical engineering design examples• Mathematical techniques are minimized, instead emphasis is given to simulation of systems using such languages as CSMP and ACSL. The book contains fifteen chapters which cover the following topics. Chapter One presents an overview of control, listing various application areas. Chapter Two gives mathematical background. Chapter Three covers the important topic of component selection and several examples are presented including servo mechanism sizing to track a flying target and optimal selection of gears and control valve characteristics.

* Control System Principles and Design by E. O. Doebelin. John Wiley, Chichester (1985). 577 pp. £10.95.

Chapter Four is concerned with programmable logic controllers. This topic is often overlooked in courses on control yet a vast number of industrial control systems use these kinds of control components• Chapter Five discusses system performance measures including speed of response, relative stability and steady state errors. Various frequency and time domain criteria are developed• Chapter Six gives a rather traditional treatment of stabilitv theory including such topics as the Routh-Hurwitz criteria, Nyquist Stability Theory and a very brief treatment of Root Locus techniques. Chapter Seven is concerned with the important topic of feed-forward control. Here, both command feed-forward and disturbance feed-forward are considered. The techniques are illustrated by examples, including a heat exchanger. Chapter Eight is concerned with on/off feedback control• Describing functions are used to analyse the resulting control systems' performance. Again several applications are presented including an aircraft roll stabilization system and a residential heating system. Chapter Nine covers the traditional material of linear feedback design based on Nyquist diagrams, Nichols charts, Root Locus. Chapter Ten introduces integral action and shows how this can be achieved in both hardware and software. Anti-reset wind up is also discussed in detail. Chapter Eleven introduces the concept of derivative control or rate feedback. This leads to a discussion of cascaded control systems using intermediate variables for feedback; also a very brief discussion of state variable feedback is given. The author makes the following point about state variable feedback: "The main practical utility of state variable feedback for single input single output systems lies in its ability to suggest candidate feedback signals that can sometimes give performance improvements similar to those of derivative control without the need to actually perform differentiation. The designer should consciously examine system equation and block diagrams to discover promising intermediate or hidden signals (which may or may not be state variables) and formulate feedback schemes using various combinations of these and]or the state variables• Combining such an exploratory approach with our more definite knowledge of the basic control modes, we can come up with several alternative designs for trade-off studies." The reviewer has also found this more physical approach to state variable feedback to be of importance in practical control system design work. Chapter Twelve discusses combined feedback strategies including the ubiquitous PID controller. An interesting discussion of control difficulty is also included based on the ratio of dead time to storage time. Chapter Thirteen presents case studies and topics of special interest; the case studies include an airborne heliostat, an anti-aircraft gun director, a radio telescope pointing system and a nonlinear controller to stabilise a robot system having variable inertia arms. Chapter Fourteen gives an introduction to digital control including a brief discussion of difference equations and z

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transforms. Hardware considerations are again introduced including digital fluid actuators, digital valves and stepping motor systems. Chapter Fifteen gives a brief introduction to multivariable control systems. The widely used technique of relative gain array is discussed in detail, as are more formal techniques for decoupling multivariable systems. It is clear from the book that the author has a wealth of real experience. To give two examples, his discussion of shrink and swell effects in feedwater control for boilers is most illuminating. Also members of the reviewer's own department (led by Professor Rob Evans) have recently been undertaking the design of the servo systems for the new Australian synthesis radio telescope. In this regard, the reviewer was most interested in the case study concerned with radio telescope servo systems. Here again, the discussion focuses on the relevant practical issues including such matters as the choice of motors, gear boxes and the architecture of the control system. The author correctly points out that the principal design consideration is to achieve high pointing accuracy and fast dynamic response without exciting resonant modes in either the structure or the control apparatus. There are a few minor criticisms which could be levelled at the book. Firstly the book contains so many examples that a student, being introduced to control for the first time, might have difficulty seeing the wood for the trees. Also, some of the examples are a little outdated since they relate to the 1960s and early 1970s. Finally, perhaps the book goes a little too far towards the practical side without capitalizing on the unifications made possible by appropriate theory.

However, it must be said that, on balance, this is an excellent book. It is highly recommended as a text book for a first course in control engineering. In the reviewer's opinion this would give a more balanced treatment than is contained in some of the more academically orientated books, the latter being more appropriate for follow-on courses. The book will serve as a valuable reference for practising control engineers. The young control engineer is much more likely to be asked "What size motor should we use?", or "Which gear box do we need?" rather than "Where are you going to place the closed loop poles?" or "What weighting are you going to give to the input in this quadratic criterion?" The challenge for the future might well be to write a book which brings these ideas together.

About the reviewer Professor Graham C. Goodwin was born in Broken Hill, Australia in 1945. He obtained a B.Sc. (physics), B.E. (electrical engineering) and Ph.D. from the University of New South Wales. From 1970 until 1974 he was a lecturer in the Department of Computing and Control, Imperial College, London. Since 1974 he has been with the Department of Electrical and Computer Engineering, University of Newcastle, Australia. He is the coauthor of three books: Control Theory (Oliver and Boyd, 1970), Dynamic System Identification (Academic Press, 1977) and Adaptive Filtering, Prediction and Control (Prentice-Hall, 1984). He is currently Professor of Electrical Engineering and a member of the Australian Research Grants Committee.

Competent Expert Systems: A Case Study in Fault Diagnosis* E. T. Keravnou and L. Johnson

Reviewer: ACADEMICIAN T. V.~.MOS Computer and Automation Institute, Hungarian Academy of Sciences, 1132 Budapest, Victor H.u. 18, Hungary. THE READER is always in trouble if the title of a book promises too much. Maybe this is not the fault of the authors but of editorial business interest. Nevertheless, the readers start with high expectations and page by page wonder if they will discover revolutionary ideas----or if it is their fault, the overlooking of a major novelty. This was my case reviewing the book on competent expert systems. If understood properly, the authors claim that until now expert systems have not had competency in their field and the authors' field is the first to achieve this level. The book is a report on a technological diagnostic system started by a British government program and aimed at being tested by the diagnostic procedures of an ICL computer. The funding stopped and the company could not continue the support. This means that although relevant parts of the general system are ready and coded as demonstrated within the text and in an annex, on 72 slightly readable, reduced font pages (a quarter of the whole book), no data or other indication can be given about the efficiency of the projected system, or its superiority to other systems of recent less

* Competent Expert Systems: A Case Study in Fault Diagnosis by E. T. Keravnou and L. Johnson. Kogan Page, London (1986).

intelligent solutions. This is the more intriguing because the authors start with an overview of related works and systems in existence or reported elsewhere, the review of which is much less a logical guidance of evolving ideas--just as it is the usual progress in science but with a very superficial criticism of most pioneering efforts. These have real shortcomings, several of them are constantly revised and improved. On the one hand, though, these were important and pathfinding antecedents for the whole artificial intelligence community and on the other hand there is no final solution yet; moreover, such solution cannot be discerned on the present horizon as a promise for the near future. We learn at last that "competent expert systems a r e . . , that contain explicitly representations of strategies and structure" (p. 231). This is fine; however, this more or less is the objective of any expert system. The exaggerated claim is to be regretted. Should this and many superfluous tautologies have been omitted, we could have had an interesting report with some good ideas on a relevant topic. Perhaps this would not make up the material for a book, yet sut~ce for a very comprehensive and valuable paper. Some statements of the introduction can be underlined: " A methodology that focuses on the characteristics of the human expert and not on the knowledge representation level" and further: "in software engineering the first step is to gain an understanding of the particular environment into which the technology is to be i n t r o d u c e d . . . " . The expert system builders must, therefore, construct models of human