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Discussion Session 4: Is Identification Obsolete?
DISCUSSION SESSION 4: IS IDENTIFICATION OBSOLETE? K. R. Godfrey Inter· University Institute of Engineering Control, University of Warwick, England
statistician, it means only deciding the structure of a model, but to most others, it has a much wider meaning; a good deal of the 90% listed in Richalet's example as other than identification would in fact be regarded as identification by participants at this symposium. The Chairman, K.R. Godfrey (University of Warwick, England) added that to design good PlO control, step responses are desirable and that these constitute identification. On the theme of trying to identify "identification", K. Henning (T.H. Aachen, FRG) considered that 80% of effort in designing a control system is in finding the structure of the process, (for example determining the Significant variables and deciding where to measure) and only 20% goes into regulator design.
For this Symposium, the fifth in the IFAC series on Identification and System Parameter Estimation, over 300 Abstracts were submitted and over 140 papers were accepted. 351 people registered. It may therefore seem at first sight unusual to organise a session to discuss whether identification is obsolete. Opening the discussion, D.W. Clarke (Oxford University, England) in fact put forward the more limited proposal that identification is obsolete as a method for designing controllers. The problems with identification are that it is expensive; plants are often time-varying and nonlinear; a well-designed experiment is untypical of normal plant operation and black-box identification provides no insight to the process. Few papers describe consistently successful control from an identification experiment; few industrial processes have such control. The alternative methods of control are PlO controllers which are very robust and can be re-tuned on-site to changing conditions and self-tuning regulators which are fairly cheap to implement using nicroprocessors, are capable of dealing with slowly time-varying plants and have the additional advantage of good interaction between theory and application.
R. Isermann (T.H. Darmstadt, FRG) said that in the experience of his group, 95% of processes in the chemical industry have no great control problems. In other industries, this figure is lower. Blast furnaces are controlled manually, while cement kilns usually have rather poor control. Both processes are complex and inert and a theoretical model would be very complicated. Identification could therefore prove useful as a prelude to improving plant control. Similarly, grinding processes, for example in the preparation of feed for a cement kiln, are very difficult to model theoretically. J. Richalet said that even in the chemical industry, there are problems due to the multivariable nature of processes. A distillation column typically has five inputs and four outputs; a plant manager would not allow a self -tuner to be implemented on site. Instead, a model has to be made and a con troller design tested in the laboratory before implementation on the plant. D. W. Clarke replied by saying that in his estimation, 95% of control in the chemical industry is by two-term control (the derivative is usually wound down), 2% is by self -tuning and 3% by identification and controller design.
J. Richalet. (ADERSA/GERBIOS, France), opposlng the motlon, said that one of the main problems is that while the benefit of going from no control to PlO is substantial that of going from PlO to optimal or self-' tuning control is considerably less. As more control people come into the design stages, particularly in new technology area~ identification will increase in importance. Control, identification and modelling should not be mixed. In a typical study undertaken rece~tly.by ADERSA/GERBIOS, the stage of conslderlng the purpose of modelling, model characterisation and selection of significam variables occupied about 30% of the total effort, planning the experimental work 20%, data collection, including visual inspection 20%, identification 10% and interpretation of the identification results 20%.
P. Wellstead (UMIST, England) considered that there is sti ll too little interaction between theory and practice, and that very few theoreticians had spent time in industry. Perhaps it should be made a condition of attendance at future Symposia that
P.C. Young (Australian National University) considered that it all depended on what is meant by identification. To a 1335
1336
K.R. Godfrey
participants should have had .experience of trying to implement theory in practice. In many areas, identification is not obsolete. Sinusoidal testing is still popular, even if it is not in the process industries due to difficulties of injecting the signal . R. Schumann (T.H . Darmstadt, FRG) was surprised that reference had not been made to feedforward control. It is important, widelyused and, to be effective, requires a model which needs to be identified. Several speakers from industry stated their views on identification and control design. Any control system must be robust this is the primary requirement. Sinusoidal techniques are popular in many areas, and least-squares identification methods are widely used. With the advent of the microprocessor, sophisticated methods are becoming more cost effective. P.C. Parks (Ruhr Universitat Bochum, FRG) predicted that with future energy problems, control and identification would become much more important. The processes of education and diffusion of knowledge are not particularly sati sfactory in many countries - a noteable exception being Sweden. CONCLUSIONS The main conclusion from the discussion was that identification is not obsolete. There was some support for~. Clarke's limited proposal that identification is
obsolete as a method for designing controllers, but even then, much depended on precisely what is meant by identification. On this, there was little agreement, except that engineers take a wider view than statisticians. What is totally obsolete is the philosophy, put forward in the early days of the subject, that one should go to a process, treat it as a black box, perturb it with a signal, record the response, process the data to give a z-transfer function and then design a controller based on the transfer function . All available plant knowledge mu~ be studied and if appropriate incorporated in the identification stage. The discussion showed clearly that identification requirements vary considerably from one industry to another. There is a complete contrast for example between the aerospace indu stry where measurements are usually good and dynamic s are rapid and frequently time-varying and the steel indu stry where some mea surements are poor (with noise often consisting of jumps and spikes) and dynamics are slow, but with infrequent time variations.
statistician, it means only deciding the structure of a model, but to most others, it has a much wider meaning; a good deal of the 90% listed in Richalet's example as other than identification would in fact be regarded as identification by participants at this symposium. The Chairman, K.R. Godfrey (University of Warwick, England) added that to design good PlO control, step responses are desirable and that these constitute identification. On the theme of trying to identify "identification", K. Henning (T.H. Aachen, FRG) considered that 80% of effort in designing a control system is in finding the structure of the process, (for example determining the Significant variables and deciding where to measure) and only 20% goes into regulator design.
For this Symposium, the fifth in the IFAC series on Identification and System Parameter Estimation, over 300 Abstracts were submitted and over 140 papers were accepted. 351 people registered. It may therefore seem at first sight unusual to organise a session to discuss whether identification is obsolete. Opening the discussion, D.W. Clarke (Oxford University, England) in fact put forward the more limited proposal that identification is obsolete as a method for designing controllers. The problems with identification are that it is expensive; plants are often time-varying and nonlinear; a well-designed experiment is untypical of normal plant operation and black-box identification provides no insight to the process. Few papers describe consistently successful control from an identification experiment; few industrial processes have such control. The alternative methods of control are PlO controllers which are very robust and can be re-tuned on-site to changing conditions and self-tuning regulators which are fairly cheap to implement using nicroprocessors, are capable of dealing with slowly time-varying plants and have the additional advantage of good interaction between theory and application.
R. Isermann (T.H. Darmstadt, FRG) said that in the experience of his group, 95% of processes in the chemical industry have no great control problems. In other industries, this figure is lower. Blast furnaces are controlled manually, while cement kilns usually have rather poor control. Both processes are complex and inert and a theoretical model would be very complicated. Identification could therefore prove useful as a prelude to improving plant control. Similarly, grinding processes, for example in the preparation of feed for a cement kiln, are very difficult to model theoretically. J. Richalet said that even in the chemical industry, there are problems due to the multivariable nature of processes. A distillation column typically has five inputs and four outputs; a plant manager would not allow a self -tuner to be implemented on site. Instead, a model has to be made and a con troller design tested in the laboratory before implementation on the plant. D. W. Clarke replied by saying that in his estimation, 95% of control in the chemical industry is by two-term control (the derivative is usually wound down), 2% is by self -tuning and 3% by identification and controller design.
J. Richalet. (ADERSA/GERBIOS, France), opposlng the motlon, said that one of the main problems is that while the benefit of going from no control to PlO is substantial that of going from PlO to optimal or self-' tuning control is considerably less. As more control people come into the design stages, particularly in new technology area~ identification will increase in importance. Control, identification and modelling should not be mixed. In a typical study undertaken rece~tly.by ADERSA/GERBIOS, the stage of conslderlng the purpose of modelling, model characterisation and selection of significam variables occupied about 30% of the total effort, planning the experimental work 20%, data collection, including visual inspection 20%, identification 10% and interpretation of the identification results 20%.
P. Wellstead (UMIST, England) considered that there is sti ll too little interaction between theory and practice, and that very few theoreticians had spent time in industry. Perhaps it should be made a condition of attendance at future Symposia that
P.C. Young (Australian National University) considered that it all depended on what is meant by identification. To a 1335
1336
K.R. Godfrey
participants should have had .experience of trying to implement theory in practice. In many areas, identification is not obsolete. Sinusoidal testing is still popular, even if it is not in the process industries due to difficulties of injecting the signal . R. Schumann (T.H . Darmstadt, FRG) was surprised that reference had not been made to feedforward control. It is important, widelyused and, to be effective, requires a model which needs to be identified. Several speakers from industry stated their views on identification and control design. Any control system must be robust this is the primary requirement. Sinusoidal techniques are popular in many areas, and least-squares identification methods are widely used. With the advent of the microprocessor, sophisticated methods are becoming more cost effective. P.C. Parks (Ruhr Universitat Bochum, FRG) predicted that with future energy problems, control and identification would become much more important. The processes of education and diffusion of knowledge are not particularly sati sfactory in many countries - a noteable exception being Sweden. CONCLUSIONS The main conclusion from the discussion was that identification is not obsolete. There was some support for~. Clarke's limited proposal that identification is
obsolete as a method for designing controllers, but even then, much depended on precisely what is meant by identification. On this, there was little agreement, except that engineers take a wider view than statisticians. What is totally obsolete is the philosophy, put forward in the early days of the subject, that one should go to a process, treat it as a black box, perturb it with a signal, record the response, process the data to give a z-transfer function and then design a controller based on the transfer function . All available plant knowledge mu~ be studied and if appropriate incorporated in the identification stage. The discussion showed clearly that identification requirements vary considerably from one industry to another. There is a complete contrast for example between the aerospace indu stry where measurements are usually good and dynamic s are rapid and frequently time-varying and the steel indu stry where some mea surements are poor (with noise often consisting of jumps and spikes) and dynamics are slow, but with infrequent time variations.