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Distributed control systems—Their evaluation and design
294
Book Reviews
Distributed Control Systems Their Evaluation and Design* MICHAEL
Reviewer: M. G. RODD c/o Department of Electrical and Electronic Engineering, University College of Swansea, Singleton Park, Swansea SA2 8PP, U.K.
IT IS NOW WIDELY RECOGNIZED that the real problems of Distributed Computer Control Systems (DCCS) revolve around the issues of maintainability, reliability, expandability, fault-tolerance and real-time responses. For some years issues relating to input and output control etc., particularly via the use of programmable controllers and similar sub-systems, have been well developed, and widely discussed. Such sub-systems are, of course, freely available commercially. The real problems, however, lie in linking these sub-systems to each other and, subsequently, to the higher levels of control and management computing. Throughout all this integration two major problems have to be faced. On one hand, users are drawing on a variety of manufacturers' equipment with varying standards of hardware and software. On the other, there are serious problems which relate to the fact that in many cases it is absolutely vital for information to be received within well-defined strict limits. It is, for example now well understood that in real-time, control data and time form inseparable elements of information. As a result, the needs of very few users of Distributed Computer Control Systems have been fully satisfied, particularly in that, by the time plant data has been acquired, it is so out of date that there is very little it can be used for, except possibly for historical purposes. Only the shortest or tightest loops can be closed through real-time computer systems. The series of IFAC workshops on DCCS have aimed at tackling these critical issues. In earlier times the workshop was accused of being too highly theoretical--particularly when discussing issues of real-time data consistency, fault-tolerance, etc. Now t h e work, by for example, Professor Hermann Kopetz of Vienna has at last been recognized as being far ahead of its time and commercial interest has been shown in some of the philosophies that he and other researchers have been investigating. Much of this work has revolved around the problem of guaranteeing the response time of a computer control system when its components are interconnected via a computer network. Of importance is the distinction between so-called "hard" real-time (in which the response has to be guaranteed under all circumstances, including alarm occurrences), and "soft" real-time (when a small variation, albeit a degradation of the performance, can be tolerated). Undoubtedly, many of the problems which have to be faced in this area relate to the complete confusion shown in the use, by the "on-line" computer community, of the term real-time! When considering the text under review, it must be recognized that there are relatively few books in this field, and that any relevant contribution is therefore welcome. There is certainly an audience who will find this text of value, particularly when entering the field for the first time. There will, however, undoubtedly be major changes in this field in the very near future and already one is beginning to see signs of this in terms of new products being released by the major *Distributed Control Systems--Their Evaluation and Design by M. P. Lukas. Van Nostrand, Scarborough, CA (1986). US $44.95.
P. L U K A S
operators in the area. Reading the text in detail, one finds a degree of disparity between the overview given on the dust cover and the book's actual contents, which tends to provide rather a classical overview of a selected area of DCCS. After reading the book one is left feeling that the author lacked a clear objective when determining the contents and structure---one consistently finds certain philosophical aspects (which should clearly be considered at the early stage in the design phase) mixed in with the practical implementation details. It is also strange to find that very little attention has been paid to programmable controllers, or PLCs, as there can be little doubt that these form the major interface which most designers will use at the front end of a DCCS. Whilst a brief introduction to ladder diagrams has been included, that is virtually the only reference to this important class of computer-based components. Very few users, in reality, will be faced with the task of selecting a microprocessor--they will more probably be selecting a PLC, basing the choice primarily on its overall characteristics, and (possibly more important) on its supplier and manufacturer! Broadly, the text looks at the evolution of DCCS, but tends to cut off at a point some 3-5 years ago. It then moves through the various lower levels of architecture and interface components, considering, for example, the languages used to programme the local control units. It moves into discussing processes interfacing issues, albeit at a somewhat superficial level. A review of communication facilities follows, concentrating particularly on classical theoretical issues, such as media-access protocols, without really discussing the important aspects of systems currently available. The final section in the text is a useful, but again superficial, introduction tO operator and engineering interfaces, followed by the review of some of the key issues, including for example, control room environments etc. Whilst these aspects are extremely important, they simply cannot be covered in a mere few pages. Indeed, a topic such as the power supply requirements for industrial computer systems can warrant a book on its own, as do the areas of environmental and human factors. These latter stages of the book must, therefore, be regarded purely as alerting the reader to the fact that other aspects besides the computer hardware, software and interfacing should be considered. In summary, then, a somewhat confused text without a clearly defined purpose. In my opinion, it offers a very broad introductory approach to the state of the art as it was a few years ago. It will undoubtedly serve as a useful introduction to the subject or new-comers in the field, particularly as the referencing provided is good. There are yawning gaps, though, especially regarding the way the technology is developing, for example, the influences arising from the major advances being made in real-time systems, as well as the effect or (non-effects) of the whole MAP (Manufacturing Automation Protocol) exercise.
About the reviewer Mike Rodd was born in Johannesburg, South Africa, on 21 June 1946 and grew up in Bulawayo, Zimbabwe. He received the B.Sc. Eng., M.Sc. Eng. and Ph.D. degrees from the University of Cape Town, South Africa. He worked, initially, as a research scientist in computer systems for the Council for Scientific and Industrial Research in South Africa, before taking up an appointment at the University of Cape Town in the Department of Electrical Engineering. He
Book Reviews was appointed as the Professor of Electronics at the University of the Witwatersrand in 1978 and assumed the Headship of that Department in 1982. He founded the MechaTronics Research Programme of that University in 1983 and has been its Director until taking up his present appointment as Head of the Department of Electrical and
295
Electronic Engineering at the University College of Swansea, Wales. His IFAC involvements have been extensive and he is currently Vice-Chairman of the Computers Committee. He has published widely in the areas of distributed computer control systems, real-time operating systems, computer architecture and the social impacts of automation.
Book Reviews
Distributed Control Systems Their Evaluation and Design* MICHAEL
Reviewer: M. G. RODD c/o Department of Electrical and Electronic Engineering, University College of Swansea, Singleton Park, Swansea SA2 8PP, U.K.
IT IS NOW WIDELY RECOGNIZED that the real problems of Distributed Computer Control Systems (DCCS) revolve around the issues of maintainability, reliability, expandability, fault-tolerance and real-time responses. For some years issues relating to input and output control etc., particularly via the use of programmable controllers and similar sub-systems, have been well developed, and widely discussed. Such sub-systems are, of course, freely available commercially. The real problems, however, lie in linking these sub-systems to each other and, subsequently, to the higher levels of control and management computing. Throughout all this integration two major problems have to be faced. On one hand, users are drawing on a variety of manufacturers' equipment with varying standards of hardware and software. On the other, there are serious problems which relate to the fact that in many cases it is absolutely vital for information to be received within well-defined strict limits. It is, for example now well understood that in real-time, control data and time form inseparable elements of information. As a result, the needs of very few users of Distributed Computer Control Systems have been fully satisfied, particularly in that, by the time plant data has been acquired, it is so out of date that there is very little it can be used for, except possibly for historical purposes. Only the shortest or tightest loops can be closed through real-time computer systems. The series of IFAC workshops on DCCS have aimed at tackling these critical issues. In earlier times the workshop was accused of being too highly theoretical--particularly when discussing issues of real-time data consistency, fault-tolerance, etc. Now t h e work, by for example, Professor Hermann Kopetz of Vienna has at last been recognized as being far ahead of its time and commercial interest has been shown in some of the philosophies that he and other researchers have been investigating. Much of this work has revolved around the problem of guaranteeing the response time of a computer control system when its components are interconnected via a computer network. Of importance is the distinction between so-called "hard" real-time (in which the response has to be guaranteed under all circumstances, including alarm occurrences), and "soft" real-time (when a small variation, albeit a degradation of the performance, can be tolerated). Undoubtedly, many of the problems which have to be faced in this area relate to the complete confusion shown in the use, by the "on-line" computer community, of the term real-time! When considering the text under review, it must be recognized that there are relatively few books in this field, and that any relevant contribution is therefore welcome. There is certainly an audience who will find this text of value, particularly when entering the field for the first time. There will, however, undoubtedly be major changes in this field in the very near future and already one is beginning to see signs of this in terms of new products being released by the major *Distributed Control Systems--Their Evaluation and Design by M. P. Lukas. Van Nostrand, Scarborough, CA (1986). US $44.95.
P. L U K A S
operators in the area. Reading the text in detail, one finds a degree of disparity between the overview given on the dust cover and the book's actual contents, which tends to provide rather a classical overview of a selected area of DCCS. After reading the book one is left feeling that the author lacked a clear objective when determining the contents and structure---one consistently finds certain philosophical aspects (which should clearly be considered at the early stage in the design phase) mixed in with the practical implementation details. It is also strange to find that very little attention has been paid to programmable controllers, or PLCs, as there can be little doubt that these form the major interface which most designers will use at the front end of a DCCS. Whilst a brief introduction to ladder diagrams has been included, that is virtually the only reference to this important class of computer-based components. Very few users, in reality, will be faced with the task of selecting a microprocessor--they will more probably be selecting a PLC, basing the choice primarily on its overall characteristics, and (possibly more important) on its supplier and manufacturer! Broadly, the text looks at the evolution of DCCS, but tends to cut off at a point some 3-5 years ago. It then moves through the various lower levels of architecture and interface components, considering, for example, the languages used to programme the local control units. It moves into discussing processes interfacing issues, albeit at a somewhat superficial level. A review of communication facilities follows, concentrating particularly on classical theoretical issues, such as media-access protocols, without really discussing the important aspects of systems currently available. The final section in the text is a useful, but again superficial, introduction tO operator and engineering interfaces, followed by the review of some of the key issues, including for example, control room environments etc. Whilst these aspects are extremely important, they simply cannot be covered in a mere few pages. Indeed, a topic such as the power supply requirements for industrial computer systems can warrant a book on its own, as do the areas of environmental and human factors. These latter stages of the book must, therefore, be regarded purely as alerting the reader to the fact that other aspects besides the computer hardware, software and interfacing should be considered. In summary, then, a somewhat confused text without a clearly defined purpose. In my opinion, it offers a very broad introductory approach to the state of the art as it was a few years ago. It will undoubtedly serve as a useful introduction to the subject or new-comers in the field, particularly as the referencing provided is good. There are yawning gaps, though, especially regarding the way the technology is developing, for example, the influences arising from the major advances being made in real-time systems, as well as the effect or (non-effects) of the whole MAP (Manufacturing Automation Protocol) exercise.
About the reviewer Mike Rodd was born in Johannesburg, South Africa, on 21 June 1946 and grew up in Bulawayo, Zimbabwe. He received the B.Sc. Eng., M.Sc. Eng. and Ph.D. degrees from the University of Cape Town, South Africa. He worked, initially, as a research scientist in computer systems for the Council for Scientific and Industrial Research in South Africa, before taking up an appointment at the University of Cape Town in the Department of Electrical Engineering. He
Book Reviews was appointed as the Professor of Electronics at the University of the Witwatersrand in 1978 and assumed the Headship of that Department in 1982. He founded the MechaTronics Research Programme of that University in 1983 and has been its Director until taking up his present appointment as Head of the Department of Electrical and
295
Electronic Engineering at the University College of Swansea, Wales. His IFAC involvements have been extensive and he is currently Vice-Chairman of the Computers Committee. He has published widely in the areas of distributed computer control systems, real-time operating systems, computer architecture and the social impacts of automation.