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The future of control
Autom,tica, Vol. 13, pp. 389-392.
Pergamon Press, 1977.
Printed in Great Britain
Opinions The Future of Control* H. H. ROSENBROCKt Key Word Index--Computer-aided design; control theory; control system synthesis; control system design; industrial control; social effects of automation; quality of working life. Summary--The development of control is briefly reviewed. It is suggested that 'modern' control has two aspects: a mathematical investigation of basic properties of dynamical systems, and the development of algorithmic methods of synthesis. Reasons are given for believing that the first of these will have more enduring value than the second. Algorithmic methods which try to eliminate the skill of the designer are contrasted with alternative methods which accept his skill and make it more productive. It is finally suggested that the impact of computers upon industry may give the opportunity for a similar development of production methods which accept and enhance the skill of manual workers.
changes, were taken into account implicitly, but not usually quantified: Optimisation was usually confined to choice of parameters in a fixed structure, and was not widely used. These methods had some important advantages for industrial applications. They gave a great deal of insight, showed when a simple solution existed, and allowed practical constraints to be taken into account. Mathematical foundations on the other hand were often shaky--for example no proof of the Nyquist theorem for systems with time delays was available at that time. Structural properties such as redundancy (e.g. uncontrollability or unobservability) were also not understood. From about 1960 new problems and new methods became prominent: this was the era of'modern' control. The source of the problems was aerospace, chiefly rocket guidance. The governing equations were nonlinear or time-dependent and realistic problems of minimising time or fuel or other quantities could be posed. These were attacked by time-domain methods, and similar methods were applied to filtering. Theoretical results were embodied in algorithms which could be run on a digital computer. At the same time a much more powerful attack was made on some of the fundamental problems of control. Liapunov theory and functional analysis were brought into use in the study of stability, structural properties such as controllability and observability were worked out in detail. The possibility of arbitrary pole assignment was first appreciated. New mathematical formulations of control theory were developed, based on module theory, geometric methods, polynomial matrix theory, etc. Special classes of nonlinear systems, such as bilinear systems, were attacked with success. Such a list could be extended to great lengths, but these examples will serve to show what is meant. It is important to distinguish these two strands in "modern control'. The term has often been identified with a formulation of the control problem which allows an algorithmic solution: usually via optimal control, but sometimes through pole assignment or diagonalisation or similar aims. This aspect of 'modern control' may in the end prove to be a historical accident. It suited the guidance problem, which could be formulated in a closed mathematical form. It also suited the computing facilities which were available at the time. On the other hand it did not sort too well with the common run of industrial control problems. We have to remember that the late sixties were pre-eminently the period of the 'gap between theory and practice'. There was a prevalent feeling that theory and practice were moving further apart, and the blame was variously placed on theorists, for failing to produce useful results, or on industry, for failing to use the results which were available. My own view is that many of the difficulties arose from the attempt to turn engineering problems into mathematical problems, which could be solved algorithmically. It may well happen that the most enduring part of 'modern control' will not be its algorithmic methods of solution, but the contribution which has been made in the last fifteen years to our understanding of the foundations of control. This is the area in which I would personally expect to see the most vigorous advance in the next few years.
Introduction TO ATTEMPT to write a paper on the future of control shows perhaps more rashness than good sense. I should therefore like to begin by saying what will not be attempted here. First, this is in no sense a review. To review the whole field of control would be impossible for any one man, and excellent reviews of specialised areas are already available. Secondly, it is not an attempt to foretell the future. The long history of prophetic utterances and their subsequent confutation gives little encouragement to that. What will be attempted is something more modest. Standing _at this moment of time, we can look back at the development of control. We can see its successes and its disappointments. We can see the areas of current activity and can guess at some future developments. In this perspective we can try to decide what are the most valuable contributions that our subject can make to society. We can also try to suggest where the most promising hopes lie for immediate advance. This is what I shall attempt to do, in a way which inevitably is coloured by my own interests and experience. Most of you will have thought deeply about such problems. If my thoughts agree with your own I shall be happy, and if not they may prompt a discussion out of which better agreement may result. Theory The easiest aspect of control to discuss is undoubtedly its theory. It is copious, well-documented (perhaps too well) and in an apparently vigorous condition. Let us look briefly at how it arrived at its present state. If we start from 1945, we can divide the subsequent ,development into two roughly equal periods. The first, the socalled 'classical' period, was concerned with frequency-response methods, with single-loop systems and largely with industrial applications. Random disturbances, and the effects of parameter
* Received 19 November 1976; revised 1 February 1977. The original version of this paper was presented at the 6th IFAC Congress on Control Technology in the Service of Man which was held in Boston/Cambridge. MA. U.S.A. during August 1975. The published Proceedings of this IFAC Meeting may be ordered from: Instrument Society of America, 400 Stanwix Street, Pittsburgh, PA 15222, or John Wiley, Baffins Lane, Chichester, Sussex, PO 19 IUD, UK. This paper was recommended for publication in revised form by associate editor S. J. Kahne. tProfessor of Control Engineering, Control Systems Centre, University of Manchester Institute of Science & Technology, Manchester, England.
Design and synthesis It may be well to elaborate on this theme, because although it is familiar enough to practising engineers it is often misunderstood by theorists. The typical class-room or textbook problem in engineering is completely formulated, to the point where a purely mathematical problem can be abstracted from it. The task is to 389
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obtain a mathematical solution and to interpret this as a solution of t he engineering problem. The practical situation is much less tidy. First, we cannot usually specify what we want in any exact way. There will be many desirable properties: speed of response, reliability, insensitivity to disturbances and parameter changes, and so on. If we ask for too much of all of these, there will be no feasible solution. How much of one we are prepared to give up depends on what we get in exchange. Consequently the solution has to be a compromise, and we need to know a good deal about the tradeoffs between different advantages before we can make a satisfactory choice. Secondly, there is a multitude of constraints which have to be obeyed by any satisfactory solution. Some of these arise from the limits of existing technology. Some may arise from the particular equipment which happens to be installed, or happens to be in stores, or happens to be available on suitable delivery. Other constraints may simply embody our unwillingness to investigate a situation in unlimited detail: for example if commercial d.p. transmitters are available only to absolute pressures of 40,000 p.s.i, we may accept this as a constraint; because we are not prepared to investigate the costs of developing transmitters for higher pressures, or believe that this would certainly be uneconomic. To write down all these constraints is usually impossible however long the engineer spends he will certainly omit some. Moreover it is a very burdensome and time-consuming process to attempt the construction of such a list. On the other hand it is usually quite easy for the engineer to say whether any proposed design satisfies the constraints. The h u m a n mind in fact is better adapted to such a task than to the preparation of exhaustive lists. Defects in a tentative solution stand out and are easily recognised. Thirdly, our knowledge of any practical system is always defective if we take a sufficiently detailed view. Finitedimensional models cannot truly represent any real situation. where a closer view will usually suggest that the system is distributed. A yet more detailed view will throw doubt on the distributed formulation: for example the heat conduction equation implies point temperatures which do not exist on a molecular view. Again, no real system is linear, and our knowledge of its nonlinear behaviour is at best approximate. These and other discrepancies between the real world and our formulation of the problem mean that we have to be very careful in interpreting the results of our calculations. Indeed, any engineer, given a mathematical model of a system, will be able if he wishes to draw from it conclusions which he knows to be false. The task of deciding which conclusions he can trust is one of the chief responsibilities of the engineer. Fourthly, engineering problems usually admit more than one satisfactory solution, depending on the particular compromise adopted. There is no "best' automobile because each achieves its outstanding virtues by accepting some limitations. We can have outstanding economy, or outstanding performance, or outstanding freedom from pollution, for example, but we cannot have the highest degree of all three at once. In such situations there will be a more or less wide range of acceptable compromises. The considerations which guide these compromises are partly quantitative, but partly they depend on value judgements. How much weight, for example, do we put on the damage caused by pollution '? These facts set severe limitations on what we can expect to achieve by synthesis procedures. These are procedures which ask the designer to specify what he wants in such detail that there is a unique solution. Then a computer is used to generate the solution by a suitable algorithm. Examples are optimal control with quadratic performance criterion, non-interacting control, pole assignment, and so on. Each of these techniques forces the designer to select a specific and restricted objective: for example to minimise a certain functional subject to certain constraints. The limited nature of the available objectives is due to the mathematical difficulties, and we may hope that continuing work will permit a wider choice of more realistic objectives. Even if it does, however, I suggest that this is not a satisfactory way of attacking most engineering problems. The designer is asked to select his objective, which implies a set
of compromises, without knowing what trade-oil; are availablc. He is asked to specify all the relevant constraints, which is more difficult than judging whether a proposed design transgresses the constraints. The details of the solution are hidden from him, and they may conceal assumptions which are not valid in some particular application. Finally, he is asked to define a unique answer, in a situation where technical requirements usually leave a choice to be settled by other considerations. My own conclusion is that engineering is an art rather than a science, and by saying this I imply a higher, not a lower status. Scientific knowledge and mathematical analysis enter into engineering in an indispensable way, and their role will continually increase. But engineering contains also elements of experience and judgement, and regard for social considerations and the most effective way of using h u m a n labour. These partly embody knowledge which has not yet been reduced to exact and mathematical form. They also embody value judgements which are not amenable to the scientific method. If such a view is correct, it suggests that the pursuit of synthesis techniques, though a legitimate objective, is likely to be relatively unrewarding. Increase in our basic knowledge of structural properties, on the other hand, can illuminate the engineering problem as well as the theory. To take just two examples, Popov's criterion and the related circle theorems reassure us that conclusions about stability based on a linear model are insensitive (in a precisely defined sense) to certain types of nonlinearity. Similarly the recently-discovered structural properties of multivariable systems have thrown new light on the problem of identification.
Interactive computing There is a further reason why I believe that the emphasis on synthesis methods may be reduced in the future. Until a few years ago, communication between the engineer and a digital computer was difficult and slow. We now have cheap and convenient terminals allowing graphical as well as alphanumeric input and output. We also have effective time-sharing systems which allow an immediate relation between the engineer and the computer. These offer a facility which is much better matched to many engineering tasks than the off-line batch computing which previously had to be used. Such facilities are of course widely used, but their impact in the future is likely to be much greater than at present. Current use is often still conceived in terms that were appropriate to batch computing, but do not use the abilities of the engineer in a fully effective way. In mechanical design, for example, the engineer may simply be asked to insert parameters, and the design may then be generated in an essentially synthetic way. The computer becomes an automated design manual, leaving only minor choices to the engineer. There are two related objections to this. First it fails to exploit the opportunity which interactive computing can offer. The computer and the h u m a n mind have quite different but complementary abilities. The computer excels in analysis and numerical computation. The h u m a n mind excels in pattern recognition, the assessment of complicated situations, and the intuitive leap to new solutions. If these different abilities can be combined, they a m o u n t to something much more powerful and effective than we have had before. What is needed in this situation is a technique which allows the computer to analyse a given situation and present the results graphically. This needs to be done in a way that allows the designer to assess the results easily, and that guides him in finding a better solution. These have been the aims of my own research team at Manchester, and I hope that our results are encouraging enough to lead others in the same direction. The second objection to the 'automated design manual" is a deeper one. It seems to me to represent a loss of nerve, a loss of belief in h u m a n abilities, and a further unthinking application of the doctrine of the division of labour. To explain this requires a digression, though one which will bring us back to the same point in a much wider context.
The qualit) of working life The division of labour occurred naturally in the early industrial revolution. It was already highly developed in 1776
Opinions when Adam Smith published his famous account of pin-making in The Wealth of Nations. "One m a n draws out the wire, another straights it, a third cuts it, a fourth points it, a fifth grinds it at the top for receiving the head; to make the head requires two or three distinct operations; to put it on, is a peculiar business, to whiten the pins is another: it is even a trade by itself to put them into the paper; and the important business of making a pin is, in this manner, divided into about eighteen distinct operations, which, in some manufactories, are all performed by distinct hands, .... Each person, therefore, ... might be considered as making four thousand eight hundred pins a day. But if they had all wrought separately and i n d e p e n d e n t l y , . . , they certainly could not each of them have made twenty[ 1] . . . . " Despite the economic advantages, Adam Smith saw the damage which might be caused to the workman. "In the progress of the division of labour, the employment of the far greater part of those who live by labour, that is, of the great body of the people, comes to be confined to a few very simple operations; frequently to one or two. But the understandings of the greater part of men are necessarily formed by their ordinary employments. The m a n whose whole life is spent in performing a few simple operations, . . . has no occasion to exert his understanding .... He naturally loses, therefore, the habit of such exertion, and generally becomes as stupid and ignorant as it is possible for a h u m a n creature to become[21." The process of subdivision and regulation was given a further refinement in the early twentieth century by the exponents of 'scientific management', Taylor and Gilbreth and their followers. it was in 1899 that Taylor achieved fame when he taught a D u t c h m a n named Schmidt to shovel forty-seven tons instread of twelve and a half tons of pig iron a day.* Every detail of the man's job was specified: the size of the shovel, the bite into the pile, the weight of the scoop, the distance to walk, the arc of the swing, and the rest periods that Schmidt would take. By systematically varying each factor, Taylor got the optimum a m o u n t of barrow load. By exact calculation, he got the correct response[3]." Taylor, too, was not unaware of the implications: "'Now one of the very first requirements for a m a n who is fit to handle pig iron as a regular occupation is that he shall be so stupid and so phlegmatic that he more nearly resembles in his mental make-up the ox than any other type[4]." Whatever relation 'scientific management' may bear to science, it leads to a direction diametrically opposite to the origins of modern industry. The marriage of intellectual inquiry and practical skill, which resulted in the industrial revolution, came about in England in a society where craftsmanship had an honourable if lowly place. The blacksmith and the carpenter and their like were respected members of the community, and Robert Hooke could write, of the founder members of the Royal Society, "And the ends of all these Inquiries they intend to be the Pleasure of Contemplative minds, but above all, the ease and dispatch of the labours of mens hands[7]." Such words could not have been written in India at that time, where the carpenter-blacksmith was untouchable. Not only was the craftsman respected, but he derived from his craft a basic satisfaction and self-respect. It is vain to look for written records to confirm this, but it is well enough documented in the lines of a simple chair, or the carving of a church pew, or the wrought iron of a gate or screen. The division of labour, in its modern extreme, is destructive of this satisfaction in work. It destroys also the workman's selfrespect, and the respect of society for 'the labours of mens hands'. This destruction began with manual work, but Cooley[8] points out that it has now progressed to intellectual work. Gilbreth defined 'therbligs', elementary physical movements which could be timed and optimised. A recent paper[9] defines 'yalcs' [the • . .
* Confusion has run riot here. The pig iron was not shovelled: a pig weighed 92 pounds and was lifted and carried. Bell appears to have run together two different stories[4, 5] related by Taylor. Moreover, recent research[6] suggests, even if one does not fully accept its conclusions, that Taylor's own account of the experiments with a 'little Pennsylvania D u t c h m a n ' had been considerably improved by imagination•
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acute reader will deduce that the author, perhaps only partly serious, was Clay] which are basic mental operations amenable to the same techniques of work study. Such a development was in fact foreshadowed by Babbage.* "We have already mentioned what may, perhaps, appear paradoxical to some of our readers--that the division of labour can be applied with equal success to mental as to mechanical operations, and that it ensures in both the same economy of time[10]." This is the direction in which the 'automated design manual' leads. The decisions left to the operator of the system are reduced to routine choices between fixed alternatives. His skill as a designer is not used, and decays. He is subject to pressure to match his speed of working to that of the machine, and may be asked to work shifts in order to utilise the expensive capital equipment. His task ceases to be that of a designer in the proper sense, and comes to resemble that of the assembly-line worker.
Is technology neutral? At this point some will object that if technology produces such adverse effects, whether for the factory worker or the designer, it is not technology which is at fault but the social organisation which misuses it. But is this true? I think not. T h r o u g h o u t the world, wherever goods are produced in large quantities, there is only one technology. However different the social systems, an automobile factory in the USA, or the UK, or Sweden or Japan or Poland uses similar machines and similar processes. Attempts have indeed been made to reorganise work in a way which is more satisfactory to the worker, notably by Volvo. The basic character of the work, however, is unchanged. T h r o u g h o u t the world, m a n as consumer exploits m a n as producer, however nearly identical these categories may be. Division of labour, with its consequent trivialisation of work, has become embodied in the machinery of industry. So much is this so, that reorganisation of work, using the same machinery, can have little impact upon working conditions. Moreover, the responsibility for the technical developments which have led to this situation lies upon us as engineers. It lies particularly heavily upon us as control engineers: certainly control and automation have improved the working conditions in industries such as paper, glass and chemicals. Equally certainly they have contributed to a deterioration in other industries. This would hardly be a useful or opportune point to make but for one fact. Over the next twenty years it seems certain that the whole technology of production will change beyond recognition. Mini- and micro-computers are now becoming so cheap that computation will cease to be a significant problem. We shall be able to implement control algorithms of almost any desired complexity. Machinery, and small parts of machines, can have computers built in to control them. Each joint of a manipulator (robot) can have a computer associated with it. O u r chief difficulty, in envisaging where such changes may take us, is in being sufficiently audacious in our conjectures. There is here, I believe, an opportunity for the present generation of engineers which is unlikely to recur. The enhancement of skill Let us return to the discussion of synthesis and design as applied to control systems. We have in this an example of the evolution of technology, and there are two distinct paths that might be followed. The first is to accept the skill and knowledge of designers, and to attempt to give them improved techniques and improved facilities for exercising their knowledge and skill. This demands a truly interactive use of computers in a way that allows the very different capabilities of the computer and the h u m a n mind to be used to the full. Such a route will lead to a continually evolving ability in the designer, and a continued interaction between his ability and the techniques and equipment he uses.
*Babbage, with his difference engine in mind, bases his c o m m e n t also on the methods used by Prony in France to compute mathematical tables; these methods in turn being inspired, so it is said[10], by Adam Smith's description of pinmaking.
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The alternative is to subdivide and codify the design process. incorporating the knowledge of existing designers, so that it is reduced to a sequence of simple choices. Thus 'de-skilled' the job can be done by men with much less training and much less experience. In the areas affected, design skill will gradually die and there ,,~ill be no effective dialogue between the operator of the system and the m a n (in the Research Department ?) who devised it and will modify it. Given equal effort applied to the two possibilities it seems most likely, for reasons given earlier, that the first would be more effective, cheaper, and more satisfying to the user. If contrariwise, almost all research effort is devoted to the second approach, then however inherently less promising, that will eventua!lv become the only effective method. Once tl.,.~ has happened, it x~ill become progr~.;s~vely more expensive and unrewarding to go back and develop the first alternative. What is at this moment an open choice will have been closed off. The reasons which could lead to an cxc',usive concentration on the second approach might include a conviction that this was likely to be the most effective route to pursue. They might also, however, include a belief that exercise: of h u m a n judgement and skill is in some way 'unscientific'. A belief that engineering reaches its highest development when it can be explained as a sequence of logical steps. A belief that engineering problems can be expressed in a closed form, then solved by an algorithmic procedure, and that these two steps are independent and consecutive. A belief, finally, that the engineer's view should bc bounded by technology and mathematics, and should stop short of social and h u m a n questions. It is this which I have referred to as a loss of nerve. Algorithmic, mathematical, 'scientific' techniques have great power and intellectual appeal, but they are only one aspect of engineering. We should pay equal regard to the cultivation of its other aspects. This does not mean that we should neglect the algorithmic aspects in design; but they can be used either in a way that eliminates the designer's skills, or in a way that assists those skills and makes them more productive. This debate over the future of the designer is still open. Was there a similar choice of alternatives a hundred years ago in the role of the manual worker'? Was an alternative technology possible in which machines would be used not to eliminate the skill of the workman, but to enhance it and make it more effective ? We shall never know, because if that choice existed it was closed off by the path actually followed. We cannot now return and develop the alternative because so much effort would be needed simply to equal existing technology. Once a choice has been made, even if unconsciously, even if the alternative was potentially better, we are bound by the consequences that follow. It is this fact which makes the present moment so important. The on-line computer has been much slower to develop than we might have expected, but do any of us doubt that its impact on technology will be overwhelming? If it will, then perhaps we are released from the consequences of past decisions. Perhaps it is open to us to guide this transformation towards a technology which does not seek to eliminate the skill of the workman: a technology which cooperates with his skill and interacts with it to make it more productive. Such a possibility may seem far-fetched, hut we have the example of the designer in front of us to show that there are
indeed two possible paths. It is certainly open to us to develop computer systems which collaborate with the designeffs skill, augment it~ and make it more productive. Is it beyond our abilities to achieve this also for the man who works with his hands ? I leave the question with you, suggesting only that the answer to it will affect decisively the view which is taken of our subject by society.
Acknowledgements--The views expressed here are my own, but 1 have received great help in formulating them from friends and colleagues, a m o n g them in particular from Mike Cooley, Dr. B. Moores, and Professor A. T. M. Wilson. Re[erences [ l ] ADAM SMITH: The IWealth oj Nations, (Ed. CANNAN) VOI. 1, pp. 6-7. Methuen, London (1904). [2q ibid., vol. 2. p. 267. [3] DANIEL BELL: The End of Ideology, p. 232, Free Press (1965). I-4] FREDERICK W. TAYLOR: Principles of scientific management. In Scientific Management p. 59, (1947), Harper & Row. New York. [53 ibid., pp, 67-72. [6J C. D. WREGE and A. G. PERRONI: Taylor's pig-tale: a historical analysis of Frederick W. Taylor's pig-iron exoeriments. Acad. Mangmt Jl. 17, 6-27 (1974). [7] ROB[RT HOOKE: Micrographia. Preface, Dover, London (1965~. [8] MIKE COOLEY: Mental therbligs. New Scientist 20 March, p. 711 (1975). [9] A classification and terminology of mental work, Work Study 23, 23-29 (1974). [103 CHARLES BABBAGE: On the Economy of Machinery and Manufactures, p. 191, p. 193. Kelley, New York. 1832, fourth edition 1835, reprinted 1963. BIOGRAPHY Howard Rosenbrock was educated at Slough G r a m m a r School and University College, London, taking a B.Sc. in Electrical Engineering in 1941. After 5 years in the Royal Air Force he worked in various industrial jobs until 1962, the last six years of this period being spent as Research Manager of CJB, running an industrial R & D Laboratory concerned with chemical processes and control. In 1962 he joined the control group under Professor J. F. Coales at Cambridge, where he remained until the end of 1965, with one year in 1963-64 in the Electronic Systems Laboratory at MIT. From 1966 he has been Professor of Control Engineering at UMIST, and head of the Control Systems Centre there. Publications include about 85 papers and four books, and interests are multivariable control systems, computer-aided design, and quality of working life. He received his Ph.D. in 1955 while working in industry, D.Sc. in 1963, and was elected Fellow of the Royal Society in 1976. Institution membership includes Fellow and Vice-President, IEE, F.I.Chem.E., and Fellow and Past-President of the Institute of Measurement and Control. He received the Moulton Medal of the l.Chem.E, in 1957, the Heaviside Premium of the IEE in 1968, and the Sir Harold Hartley medal of the Inst. M.C. in 1970.
Pergamon Press, 1977.
Printed in Great Britain
Opinions The Future of Control* H. H. ROSENBROCKt Key Word Index--Computer-aided design; control theory; control system synthesis; control system design; industrial control; social effects of automation; quality of working life. Summary--The development of control is briefly reviewed. It is suggested that 'modern' control has two aspects: a mathematical investigation of basic properties of dynamical systems, and the development of algorithmic methods of synthesis. Reasons are given for believing that the first of these will have more enduring value than the second. Algorithmic methods which try to eliminate the skill of the designer are contrasted with alternative methods which accept his skill and make it more productive. It is finally suggested that the impact of computers upon industry may give the opportunity for a similar development of production methods which accept and enhance the skill of manual workers.
changes, were taken into account implicitly, but not usually quantified: Optimisation was usually confined to choice of parameters in a fixed structure, and was not widely used. These methods had some important advantages for industrial applications. They gave a great deal of insight, showed when a simple solution existed, and allowed practical constraints to be taken into account. Mathematical foundations on the other hand were often shaky--for example no proof of the Nyquist theorem for systems with time delays was available at that time. Structural properties such as redundancy (e.g. uncontrollability or unobservability) were also not understood. From about 1960 new problems and new methods became prominent: this was the era of'modern' control. The source of the problems was aerospace, chiefly rocket guidance. The governing equations were nonlinear or time-dependent and realistic problems of minimising time or fuel or other quantities could be posed. These were attacked by time-domain methods, and similar methods were applied to filtering. Theoretical results were embodied in algorithms which could be run on a digital computer. At the same time a much more powerful attack was made on some of the fundamental problems of control. Liapunov theory and functional analysis were brought into use in the study of stability, structural properties such as controllability and observability were worked out in detail. The possibility of arbitrary pole assignment was first appreciated. New mathematical formulations of control theory were developed, based on module theory, geometric methods, polynomial matrix theory, etc. Special classes of nonlinear systems, such as bilinear systems, were attacked with success. Such a list could be extended to great lengths, but these examples will serve to show what is meant. It is important to distinguish these two strands in "modern control'. The term has often been identified with a formulation of the control problem which allows an algorithmic solution: usually via optimal control, but sometimes through pole assignment or diagonalisation or similar aims. This aspect of 'modern control' may in the end prove to be a historical accident. It suited the guidance problem, which could be formulated in a closed mathematical form. It also suited the computing facilities which were available at the time. On the other hand it did not sort too well with the common run of industrial control problems. We have to remember that the late sixties were pre-eminently the period of the 'gap between theory and practice'. There was a prevalent feeling that theory and practice were moving further apart, and the blame was variously placed on theorists, for failing to produce useful results, or on industry, for failing to use the results which were available. My own view is that many of the difficulties arose from the attempt to turn engineering problems into mathematical problems, which could be solved algorithmically. It may well happen that the most enduring part of 'modern control' will not be its algorithmic methods of solution, but the contribution which has been made in the last fifteen years to our understanding of the foundations of control. This is the area in which I would personally expect to see the most vigorous advance in the next few years.
Introduction TO ATTEMPT to write a paper on the future of control shows perhaps more rashness than good sense. I should therefore like to begin by saying what will not be attempted here. First, this is in no sense a review. To review the whole field of control would be impossible for any one man, and excellent reviews of specialised areas are already available. Secondly, it is not an attempt to foretell the future. The long history of prophetic utterances and their subsequent confutation gives little encouragement to that. What will be attempted is something more modest. Standing _at this moment of time, we can look back at the development of control. We can see its successes and its disappointments. We can see the areas of current activity and can guess at some future developments. In this perspective we can try to decide what are the most valuable contributions that our subject can make to society. We can also try to suggest where the most promising hopes lie for immediate advance. This is what I shall attempt to do, in a way which inevitably is coloured by my own interests and experience. Most of you will have thought deeply about such problems. If my thoughts agree with your own I shall be happy, and if not they may prompt a discussion out of which better agreement may result. Theory The easiest aspect of control to discuss is undoubtedly its theory. It is copious, well-documented (perhaps too well) and in an apparently vigorous condition. Let us look briefly at how it arrived at its present state. If we start from 1945, we can divide the subsequent ,development into two roughly equal periods. The first, the socalled 'classical' period, was concerned with frequency-response methods, with single-loop systems and largely with industrial applications. Random disturbances, and the effects of parameter
* Received 19 November 1976; revised 1 February 1977. The original version of this paper was presented at the 6th IFAC Congress on Control Technology in the Service of Man which was held in Boston/Cambridge. MA. U.S.A. during August 1975. The published Proceedings of this IFAC Meeting may be ordered from: Instrument Society of America, 400 Stanwix Street, Pittsburgh, PA 15222, or John Wiley, Baffins Lane, Chichester, Sussex, PO 19 IUD, UK. This paper was recommended for publication in revised form by associate editor S. J. Kahne. tProfessor of Control Engineering, Control Systems Centre, University of Manchester Institute of Science & Technology, Manchester, England.
Design and synthesis It may be well to elaborate on this theme, because although it is familiar enough to practising engineers it is often misunderstood by theorists. The typical class-room or textbook problem in engineering is completely formulated, to the point where a purely mathematical problem can be abstracted from it. The task is to 389
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obtain a mathematical solution and to interpret this as a solution of t he engineering problem. The practical situation is much less tidy. First, we cannot usually specify what we want in any exact way. There will be many desirable properties: speed of response, reliability, insensitivity to disturbances and parameter changes, and so on. If we ask for too much of all of these, there will be no feasible solution. How much of one we are prepared to give up depends on what we get in exchange. Consequently the solution has to be a compromise, and we need to know a good deal about the tradeoffs between different advantages before we can make a satisfactory choice. Secondly, there is a multitude of constraints which have to be obeyed by any satisfactory solution. Some of these arise from the limits of existing technology. Some may arise from the particular equipment which happens to be installed, or happens to be in stores, or happens to be available on suitable delivery. Other constraints may simply embody our unwillingness to investigate a situation in unlimited detail: for example if commercial d.p. transmitters are available only to absolute pressures of 40,000 p.s.i, we may accept this as a constraint; because we are not prepared to investigate the costs of developing transmitters for higher pressures, or believe that this would certainly be uneconomic. To write down all these constraints is usually impossible however long the engineer spends he will certainly omit some. Moreover it is a very burdensome and time-consuming process to attempt the construction of such a list. On the other hand it is usually quite easy for the engineer to say whether any proposed design satisfies the constraints. The h u m a n mind in fact is better adapted to such a task than to the preparation of exhaustive lists. Defects in a tentative solution stand out and are easily recognised. Thirdly, our knowledge of any practical system is always defective if we take a sufficiently detailed view. Finitedimensional models cannot truly represent any real situation. where a closer view will usually suggest that the system is distributed. A yet more detailed view will throw doubt on the distributed formulation: for example the heat conduction equation implies point temperatures which do not exist on a molecular view. Again, no real system is linear, and our knowledge of its nonlinear behaviour is at best approximate. These and other discrepancies between the real world and our formulation of the problem mean that we have to be very careful in interpreting the results of our calculations. Indeed, any engineer, given a mathematical model of a system, will be able if he wishes to draw from it conclusions which he knows to be false. The task of deciding which conclusions he can trust is one of the chief responsibilities of the engineer. Fourthly, engineering problems usually admit more than one satisfactory solution, depending on the particular compromise adopted. There is no "best' automobile because each achieves its outstanding virtues by accepting some limitations. We can have outstanding economy, or outstanding performance, or outstanding freedom from pollution, for example, but we cannot have the highest degree of all three at once. In such situations there will be a more or less wide range of acceptable compromises. The considerations which guide these compromises are partly quantitative, but partly they depend on value judgements. How much weight, for example, do we put on the damage caused by pollution '? These facts set severe limitations on what we can expect to achieve by synthesis procedures. These are procedures which ask the designer to specify what he wants in such detail that there is a unique solution. Then a computer is used to generate the solution by a suitable algorithm. Examples are optimal control with quadratic performance criterion, non-interacting control, pole assignment, and so on. Each of these techniques forces the designer to select a specific and restricted objective: for example to minimise a certain functional subject to certain constraints. The limited nature of the available objectives is due to the mathematical difficulties, and we may hope that continuing work will permit a wider choice of more realistic objectives. Even if it does, however, I suggest that this is not a satisfactory way of attacking most engineering problems. The designer is asked to select his objective, which implies a set
of compromises, without knowing what trade-oil; are availablc. He is asked to specify all the relevant constraints, which is more difficult than judging whether a proposed design transgresses the constraints. The details of the solution are hidden from him, and they may conceal assumptions which are not valid in some particular application. Finally, he is asked to define a unique answer, in a situation where technical requirements usually leave a choice to be settled by other considerations. My own conclusion is that engineering is an art rather than a science, and by saying this I imply a higher, not a lower status. Scientific knowledge and mathematical analysis enter into engineering in an indispensable way, and their role will continually increase. But engineering contains also elements of experience and judgement, and regard for social considerations and the most effective way of using h u m a n labour. These partly embody knowledge which has not yet been reduced to exact and mathematical form. They also embody value judgements which are not amenable to the scientific method. If such a view is correct, it suggests that the pursuit of synthesis techniques, though a legitimate objective, is likely to be relatively unrewarding. Increase in our basic knowledge of structural properties, on the other hand, can illuminate the engineering problem as well as the theory. To take just two examples, Popov's criterion and the related circle theorems reassure us that conclusions about stability based on a linear model are insensitive (in a precisely defined sense) to certain types of nonlinearity. Similarly the recently-discovered structural properties of multivariable systems have thrown new light on the problem of identification.
Interactive computing There is a further reason why I believe that the emphasis on synthesis methods may be reduced in the future. Until a few years ago, communication between the engineer and a digital computer was difficult and slow. We now have cheap and convenient terminals allowing graphical as well as alphanumeric input and output. We also have effective time-sharing systems which allow an immediate relation between the engineer and the computer. These offer a facility which is much better matched to many engineering tasks than the off-line batch computing which previously had to be used. Such facilities are of course widely used, but their impact in the future is likely to be much greater than at present. Current use is often still conceived in terms that were appropriate to batch computing, but do not use the abilities of the engineer in a fully effective way. In mechanical design, for example, the engineer may simply be asked to insert parameters, and the design may then be generated in an essentially synthetic way. The computer becomes an automated design manual, leaving only minor choices to the engineer. There are two related objections to this. First it fails to exploit the opportunity which interactive computing can offer. The computer and the h u m a n mind have quite different but complementary abilities. The computer excels in analysis and numerical computation. The h u m a n mind excels in pattern recognition, the assessment of complicated situations, and the intuitive leap to new solutions. If these different abilities can be combined, they a m o u n t to something much more powerful and effective than we have had before. What is needed in this situation is a technique which allows the computer to analyse a given situation and present the results graphically. This needs to be done in a way that allows the designer to assess the results easily, and that guides him in finding a better solution. These have been the aims of my own research team at Manchester, and I hope that our results are encouraging enough to lead others in the same direction. The second objection to the 'automated design manual" is a deeper one. It seems to me to represent a loss of nerve, a loss of belief in h u m a n abilities, and a further unthinking application of the doctrine of the division of labour. To explain this requires a digression, though one which will bring us back to the same point in a much wider context.
The qualit) of working life The division of labour occurred naturally in the early industrial revolution. It was already highly developed in 1776
Opinions when Adam Smith published his famous account of pin-making in The Wealth of Nations. "One m a n draws out the wire, another straights it, a third cuts it, a fourth points it, a fifth grinds it at the top for receiving the head; to make the head requires two or three distinct operations; to put it on, is a peculiar business, to whiten the pins is another: it is even a trade by itself to put them into the paper; and the important business of making a pin is, in this manner, divided into about eighteen distinct operations, which, in some manufactories, are all performed by distinct hands, .... Each person, therefore, ... might be considered as making four thousand eight hundred pins a day. But if they had all wrought separately and i n d e p e n d e n t l y , . . , they certainly could not each of them have made twenty[ 1] . . . . " Despite the economic advantages, Adam Smith saw the damage which might be caused to the workman. "In the progress of the division of labour, the employment of the far greater part of those who live by labour, that is, of the great body of the people, comes to be confined to a few very simple operations; frequently to one or two. But the understandings of the greater part of men are necessarily formed by their ordinary employments. The m a n whose whole life is spent in performing a few simple operations, . . . has no occasion to exert his understanding .... He naturally loses, therefore, the habit of such exertion, and generally becomes as stupid and ignorant as it is possible for a h u m a n creature to become[21." The process of subdivision and regulation was given a further refinement in the early twentieth century by the exponents of 'scientific management', Taylor and Gilbreth and their followers. it was in 1899 that Taylor achieved fame when he taught a D u t c h m a n named Schmidt to shovel forty-seven tons instread of twelve and a half tons of pig iron a day.* Every detail of the man's job was specified: the size of the shovel, the bite into the pile, the weight of the scoop, the distance to walk, the arc of the swing, and the rest periods that Schmidt would take. By systematically varying each factor, Taylor got the optimum a m o u n t of barrow load. By exact calculation, he got the correct response[3]." Taylor, too, was not unaware of the implications: "'Now one of the very first requirements for a m a n who is fit to handle pig iron as a regular occupation is that he shall be so stupid and so phlegmatic that he more nearly resembles in his mental make-up the ox than any other type[4]." Whatever relation 'scientific management' may bear to science, it leads to a direction diametrically opposite to the origins of modern industry. The marriage of intellectual inquiry and practical skill, which resulted in the industrial revolution, came about in England in a society where craftsmanship had an honourable if lowly place. The blacksmith and the carpenter and their like were respected members of the community, and Robert Hooke could write, of the founder members of the Royal Society, "And the ends of all these Inquiries they intend to be the Pleasure of Contemplative minds, but above all, the ease and dispatch of the labours of mens hands[7]." Such words could not have been written in India at that time, where the carpenter-blacksmith was untouchable. Not only was the craftsman respected, but he derived from his craft a basic satisfaction and self-respect. It is vain to look for written records to confirm this, but it is well enough documented in the lines of a simple chair, or the carving of a church pew, or the wrought iron of a gate or screen. The division of labour, in its modern extreme, is destructive of this satisfaction in work. It destroys also the workman's selfrespect, and the respect of society for 'the labours of mens hands'. This destruction began with manual work, but Cooley[8] points out that it has now progressed to intellectual work. Gilbreth defined 'therbligs', elementary physical movements which could be timed and optimised. A recent paper[9] defines 'yalcs' [the • . .
* Confusion has run riot here. The pig iron was not shovelled: a pig weighed 92 pounds and was lifted and carried. Bell appears to have run together two different stories[4, 5] related by Taylor. Moreover, recent research[6] suggests, even if one does not fully accept its conclusions, that Taylor's own account of the experiments with a 'little Pennsylvania D u t c h m a n ' had been considerably improved by imagination•
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acute reader will deduce that the author, perhaps only partly serious, was Clay] which are basic mental operations amenable to the same techniques of work study. Such a development was in fact foreshadowed by Babbage.* "We have already mentioned what may, perhaps, appear paradoxical to some of our readers--that the division of labour can be applied with equal success to mental as to mechanical operations, and that it ensures in both the same economy of time[10]." This is the direction in which the 'automated design manual' leads. The decisions left to the operator of the system are reduced to routine choices between fixed alternatives. His skill as a designer is not used, and decays. He is subject to pressure to match his speed of working to that of the machine, and may be asked to work shifts in order to utilise the expensive capital equipment. His task ceases to be that of a designer in the proper sense, and comes to resemble that of the assembly-line worker.
Is technology neutral? At this point some will object that if technology produces such adverse effects, whether for the factory worker or the designer, it is not technology which is at fault but the social organisation which misuses it. But is this true? I think not. T h r o u g h o u t the world, wherever goods are produced in large quantities, there is only one technology. However different the social systems, an automobile factory in the USA, or the UK, or Sweden or Japan or Poland uses similar machines and similar processes. Attempts have indeed been made to reorganise work in a way which is more satisfactory to the worker, notably by Volvo. The basic character of the work, however, is unchanged. T h r o u g h o u t the world, m a n as consumer exploits m a n as producer, however nearly identical these categories may be. Division of labour, with its consequent trivialisation of work, has become embodied in the machinery of industry. So much is this so, that reorganisation of work, using the same machinery, can have little impact upon working conditions. Moreover, the responsibility for the technical developments which have led to this situation lies upon us as engineers. It lies particularly heavily upon us as control engineers: certainly control and automation have improved the working conditions in industries such as paper, glass and chemicals. Equally certainly they have contributed to a deterioration in other industries. This would hardly be a useful or opportune point to make but for one fact. Over the next twenty years it seems certain that the whole technology of production will change beyond recognition. Mini- and micro-computers are now becoming so cheap that computation will cease to be a significant problem. We shall be able to implement control algorithms of almost any desired complexity. Machinery, and small parts of machines, can have computers built in to control them. Each joint of a manipulator (robot) can have a computer associated with it. O u r chief difficulty, in envisaging where such changes may take us, is in being sufficiently audacious in our conjectures. There is here, I believe, an opportunity for the present generation of engineers which is unlikely to recur. The enhancement of skill Let us return to the discussion of synthesis and design as applied to control systems. We have in this an example of the evolution of technology, and there are two distinct paths that might be followed. The first is to accept the skill and knowledge of designers, and to attempt to give them improved techniques and improved facilities for exercising their knowledge and skill. This demands a truly interactive use of computers in a way that allows the very different capabilities of the computer and the h u m a n mind to be used to the full. Such a route will lead to a continually evolving ability in the designer, and a continued interaction between his ability and the techniques and equipment he uses.
*Babbage, with his difference engine in mind, bases his c o m m e n t also on the methods used by Prony in France to compute mathematical tables; these methods in turn being inspired, so it is said[10], by Adam Smith's description of pinmaking.
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The alternative is to subdivide and codify the design process. incorporating the knowledge of existing designers, so that it is reduced to a sequence of simple choices. Thus 'de-skilled' the job can be done by men with much less training and much less experience. In the areas affected, design skill will gradually die and there ,,~ill be no effective dialogue between the operator of the system and the m a n (in the Research Department ?) who devised it and will modify it. Given equal effort applied to the two possibilities it seems most likely, for reasons given earlier, that the first would be more effective, cheaper, and more satisfying to the user. If contrariwise, almost all research effort is devoted to the second approach, then however inherently less promising, that will eventua!lv become the only effective method. Once tl.,.~ has happened, it x~ill become progr~.;s~vely more expensive and unrewarding to go back and develop the first alternative. What is at this moment an open choice will have been closed off. The reasons which could lead to an cxc',usive concentration on the second approach might include a conviction that this was likely to be the most effective route to pursue. They might also, however, include a belief that exercise: of h u m a n judgement and skill is in some way 'unscientific'. A belief that engineering reaches its highest development when it can be explained as a sequence of logical steps. A belief that engineering problems can be expressed in a closed form, then solved by an algorithmic procedure, and that these two steps are independent and consecutive. A belief, finally, that the engineer's view should bc bounded by technology and mathematics, and should stop short of social and h u m a n questions. It is this which I have referred to as a loss of nerve. Algorithmic, mathematical, 'scientific' techniques have great power and intellectual appeal, but they are only one aspect of engineering. We should pay equal regard to the cultivation of its other aspects. This does not mean that we should neglect the algorithmic aspects in design; but they can be used either in a way that eliminates the designer's skills, or in a way that assists those skills and makes them more productive. This debate over the future of the designer is still open. Was there a similar choice of alternatives a hundred years ago in the role of the manual worker'? Was an alternative technology possible in which machines would be used not to eliminate the skill of the workman, but to enhance it and make it more effective ? We shall never know, because if that choice existed it was closed off by the path actually followed. We cannot now return and develop the alternative because so much effort would be needed simply to equal existing technology. Once a choice has been made, even if unconsciously, even if the alternative was potentially better, we are bound by the consequences that follow. It is this fact which makes the present moment so important. The on-line computer has been much slower to develop than we might have expected, but do any of us doubt that its impact on technology will be overwhelming? If it will, then perhaps we are released from the consequences of past decisions. Perhaps it is open to us to guide this transformation towards a technology which does not seek to eliminate the skill of the workman: a technology which cooperates with his skill and interacts with it to make it more productive. Such a possibility may seem far-fetched, hut we have the example of the designer in front of us to show that there are
indeed two possible paths. It is certainly open to us to develop computer systems which collaborate with the designeffs skill, augment it~ and make it more productive. Is it beyond our abilities to achieve this also for the man who works with his hands ? I leave the question with you, suggesting only that the answer to it will affect decisively the view which is taken of our subject by society.
Acknowledgements--The views expressed here are my own, but 1 have received great help in formulating them from friends and colleagues, a m o n g them in particular from Mike Cooley, Dr. B. Moores, and Professor A. T. M. Wilson. Re[erences [ l ] ADAM SMITH: The IWealth oj Nations, (Ed. CANNAN) VOI. 1, pp. 6-7. Methuen, London (1904). [2q ibid., vol. 2. p. 267. [3] DANIEL BELL: The End of Ideology, p. 232, Free Press (1965). I-4] FREDERICK W. TAYLOR: Principles of scientific management. In Scientific Management p. 59, (1947), Harper & Row. New York. [53 ibid., pp, 67-72. [6J C. D. WREGE and A. G. PERRONI: Taylor's pig-tale: a historical analysis of Frederick W. Taylor's pig-iron exoeriments. Acad. Mangmt Jl. 17, 6-27 (1974). [7] ROB[RT HOOKE: Micrographia. Preface, Dover, London (1965~. [8] MIKE COOLEY: Mental therbligs. New Scientist 20 March, p. 711 (1975). [9] A classification and terminology of mental work, Work Study 23, 23-29 (1974). [103 CHARLES BABBAGE: On the Economy of Machinery and Manufactures, p. 191, p. 193. Kelley, New York. 1832, fourth edition 1835, reprinted 1963. BIOGRAPHY Howard Rosenbrock was educated at Slough G r a m m a r School and University College, London, taking a B.Sc. in Electrical Engineering in 1941. After 5 years in the Royal Air Force he worked in various industrial jobs until 1962, the last six years of this period being spent as Research Manager of CJB, running an industrial R & D Laboratory concerned with chemical processes and control. In 1962 he joined the control group under Professor J. F. Coales at Cambridge, where he remained until the end of 1965, with one year in 1963-64 in the Electronic Systems Laboratory at MIT. From 1966 he has been Professor of Control Engineering at UMIST, and head of the Control Systems Centre there. Publications include about 85 papers and four books, and interests are multivariable control systems, computer-aided design, and quality of working life. He received his Ph.D. in 1955 while working in industry, D.Sc. in 1963, and was elected Fellow of the Royal Society in 1976. Institution membership includes Fellow and Vice-President, IEE, F.I.Chem.E., and Fellow and Past-President of the Institute of Measurement and Control. He received the Moulton Medal of the l.Chem.E, in 1957, the Heaviside Premium of the IEE in 1968, and the Sir Harold Hartley medal of the Inst. M.C. in 1970.