- Email: [email protected]
Discussions
SESSION 3 PROCESS AND PRODUCTION CONTROL (1) AUTOMAT ION
Chairman: Prof . ir . A. van Cauwenberghe, professor State University of Ghent, Belgium Secretary:
Ir. H . Claessens, I.B. M. of Belgium, Brussels, Belgium
Papers 3.1
PROCESS INDUSTRY PLANT DESIGN AND AUTOMATION SYSTEM IMPLEMENTATION USES FOR SIMULATION WI TH AN APPLICATION FROM THE PAPER INDUSTRY by R. M . Bakke, Consultant, Los Gatos, Cal. 95030, USA.
3.2
SYNTHESIS OF QUASI-OPTIMAL SWITCHING SURFACES BY MEANS OF LEARNING TECHNIQUES by dr. ir. J. van Reusel, systems engineer E. N. I., Aartselaar, Belgium
3.3
DIGITAL CONTROL OF A PROCESS USING ON-LINE AUTOTUNING by A. J . Pendlebury, R. F. Stedmon and Mrs. C. M. West, Kodak Ltd ., Wealdstone, U. K.
3.4
PROCESS AND PRODUCTION CONTROL PACKAGE by O. Alsholm, process computer manager Billeruds AB, Gruvon, Sweden, B. Morsk and G. Sangregorio, I BM Svenska, and A. Hempel, Elektrokemisk AB, Sweden
3.5
SIMPLIFICATION OF A MAN-MACHINE COMMUNICATION IN A FLEXIBLE DIGITAL COMPUTER PROCESS CONTROL SYSTEM by J. B. Williams, Ferranti Ltd., Gatley , U. K.
3.6
COMPUTER CONTROL OF BATCH AND DISCONTINUOUS PROCESSES by J . S. Noble and P. I. Burton, Kent Instruments Ltd. , Hitchin, U. K.
164
GENERAL REPORT The original impact of the computer on the dynamic control of PRP-processes has been considerable. In 1969 more than 50 proces s control computers were installed in the pulp and paper industry only . Particul arly the advent of DDC (direct digital control) and to a lesser extent of DDA (digitally directed analog or supervisory set-point) control has improved much the qual ity of process control. I t has also increased the possibilities to realize larger and better integrated production processes (e . g. pulp digester, bleaching unit , chemical recovery and paper mill) . This has improved horizontal integrations of production processes with in the plant. Also vertical integration of production plants will in the near future be improved by computer systems. Indeed , the realization of the final aim of the produc tion (producing with maximum profit and minimum harm to the environment) occurs on different levels : 1. on the lowest several processes are controlled by smalle r control computers e. g. in DDC or DDA mode with special purpose mini-computers performing simple regulatory or stabilizing control 2. this control may be optimized (static and / or dynamic optimization) 3. the process model used as well as the control and optimization policies may be adapted (adaptive control and self-learning systems) to the changing technical envi r onment 4. the whole control and operating policy is influenced strongly by the changing economic environment. Management decisions have to take this into account . Appropriate decision making needs much information from the lower levels In such a hierarchical system (see figure) information flows upwards and instruct ions or interventions downwards . The separation of all above mentioned control functions on different levels is not only opportunistic because it is modelled after de facto existing situations in real life (in nature as well as in industrial and social life), it allows also for flexible operation of the whole production process. Each level has its own characteristics : higher level tasks are more complicated but are less frequent than lower level tasks. Different computers (i . e. with different characteristics) can be used on each level. Simultaneously the difficul t control backup problem is alleviated because the master can always temporarily perform the job of its slave . For these and many other reasons the concept of multilevel or hierarchical control is bound to be successful in the futu re . The papers of thi5 session described different methods to reach the final production aim . These can be on-line or off-line . The latter can be in the design or in the production phase .
R. M . BAKKE (3 . 1) dealt mainly with the design of the overall plant (production p r ocess and its automat ion) for which realistic simulation methods are available. Also harmful (pollution) effects can and should be included to obtain a real istic overall picture . The other papers dealt with the on -l ine problem on different levels of the hierarchy pyramid: on the direct (regulatory) control level : - O . ALSHOLM, A. HEMPEL , B . MORSK and G . SANGREGORIO (3 .4) and - J. B. WI LLlAMS (3 .5) for continuous control - J . S. NOBLE and P. I. BURTON (3.6) for sequential (programmed) on controls the optimization level : - A . J . PENDLEBURY, R . F . STEDMON and C. M . WEST (3.3) on the adaptation level :
- J . VAN REUSEL (3 .2) Further evolution of computer control systems will strongly be determined by the ease of man - machine communication. Program packages, "fill in the blanks" procedures and push button control of algorithms contribute to alleviate this prob lem (3.4 ; 3 .5 and 3 .6) and will certainly help to resolve the magic still surrounding computer control projects and make it available to less reputed "apprentis sorciers" i. e. the common process (control) engineer. It was interesting to notice the differences between the three methods, as p resented by different control computer firms. Th E; attendance of this session by numer ous congress delegates and their active participation in the lively discussions proved the general interest that it has raised among congressists . May this in its own way be a guarantee for a better, more humane , though inevitably more computerized future .
.......... GE ... ENT COOIPUTER
f -co~ TR-O~H-PL ~;~I~-G: 1-. _ ___ _ ..--I
I NSTRUCT I ON
FLOW
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1_
______
- - ..........ae"ENT
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INFORMAT I ON
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ADAPTATION
ADAPTING
REGULATORY CONTROL
HIER ... RCHICAL
165
SYSTE"
a. De gemakkelijke voorstellingswijze van de resultaten bekomen met het CMS algoritme . b . Ofschoon de door U vernoemde processen (warmtewisselaar enz.) meestal niet op "minimum-tijd" basis kunnen geregeld worden, kan dit soms wel het geval zijn voor bepaalde chemische batch-processen. Evolutie van de temperatuur : zie figuur.
DISCUSSIONS PAPER 3.2 : SYNTHESIS OF QUASI-OPTIMAL SWITCHING SURFACES BY MEANS OF LEARNING TECHNIQUES by dr. ir. J. van Reusel, systems engineer E. N . I., Aartselaar, Belgium.
PAPER 3.3 : DIGITAL CONTROL OF A PROCESS USING ON-LINE AUTOTUNING by A . J . Pendlebury, R . F. Stedmon and Mrs . C. M . West of Kodak Ltd ., Wealdstone, U . K .
Question by Cl. Foulard, Professeur Labo d'Automa· tique, Universite de Grenoble, France Vous utilisez un criter.e "Temps minimal". Si je comprends I'interet d'un tel critere, pour des problemes d'interception, par exemple, il me semble qu'il ne peut etre util ise tel quel pour des problemes industriels, tels que: echangeur de chaleur, machine 11 papier etc . Pouvez-vous donc justifier le choix d'un tel critere ? Pouvez-vous, par exemple indiquer ou montrer I'evolution de la temperature en fonction du temps sur I'exemple que vous avez traite ?
Questions by R . Bakke, Consultant, Los Gatos, Cal. 95030, USA Two key problems in allowing plant operators to tune using the computer without engineering supervision are: 1. Operators testing a loop for tuning interrupt their production. Generally operators are reluctant to do th is and are not particularly skilled in testing.
Answer
2. Different tuning objectives are appropriate for different loops. Fast response generally requires different tuning then least steady error. Operators usually do not know how to make this decision.
Het "minimum-tijd" foutkriterium werd hier toegepast om volgende redenen :
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100
.... ...........
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-20
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0.05 ,/,,,
0
200
300
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0.05 -0.1
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100
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166
Programs written by R. Bakke were written and implemented that made some progress on the above problem (ref. I FAC, Toronto, Canada 1968, I nstrument Engineers Handbook, Chilton Book, Vol. II 1970) Does autotune eliminate the first problem? How do you handle the second problem?
the hillclimber attempts to adjust a parameter outside its limit then the search is considered a failure . We are not concerned with the stability of the adjustable parameters during tuning except that key keep within the prescribed limits. We have not done any work with loops involving a very long time delay .
Answers
Question by A. G. Forton, S. I. R. A. Institute, U . K .
1. We have successfully tuned one loop at a time on -
Could the number of iterations required for reaching the best set of parameters be optimised by altering the sequence of changing the integral, proportional and derivative constants during the tuning process?
line without interrupting the process . However, this may not always be possible. We generally use the auto tune on a loop which has been taken off-line although the computer is at the same time still controlling the remaining loops.
Answer
2. This would be an engineer's decision to change the cost function to produce a different type of response if required . We have found that the I. T . A. E. cost function gives us a fast response and least steady error.
There is much room for improvement to the tuning technique to reduce the number of iterations required to reach the optimum. For example, when the step changes on one of the terms become insignificantly small the programme could then ignore th is term and continue the hill climb on the remaining terms .
Question by A . M . Burgess, I. C. I. Ltd., U. K. Questions by I. McFarlane, Wiggings Teape, Beaconsfield, U. K .
The auto - tu n ing procedu re has been used on worki ng plant . How big must the test disturbances be compared with the normal plant disturbances in order to provide a reasonably fast convergence? How is the rate of convergence affected by the step size? I n answer to a previous question Mr. Stedmon mentioned checks to present excessive disturbances to the measured value; do these checks help the program discriminate between the deliberate perturbation and process disturbances?
1. Are your loops all non- interacting? 2. Have you used algorithms other than three-term control? Answers
1. Some of our loops are interacting but so far we have only tried the auto-tuning on non-interacting loops.
Answer
2. We have only used auto-tuning on one, two or three term control loops.
Test disturbances are about 2 % of full scale. Normal plant disturbances during the course of the tuning can be about 0 .2 % of full scale . The initial step size can influence the rate of convergence. We stated in the report that we took a step size of 1/4 of the parameter value . We have since found that using a step size of 1/2 of the parameter value gives a more rapid convergence. The checks are on the parameter values only and there is no discrimination between deliberate and process disturbances.
Question by R. Moore, The Foxboro Cy., Mass., USA I would like to mention a related application of adaptive tuning which was done by our company . The application was in the cement industry. The procedure was to perform a single experiment on a plant loop, and to use the inputoutput data to identify the parameters of a model. The tuning was then done off-line, using the hill climbing procedure on the identified model. The optimum tuning values were then displayed to the operator, so that he could implement them if he desired. This approach was used to successfully tune five critical loops at Meduse Cement Company.
Question by Cl. Foulard, professeur Labo d'Automatique, Universite de Grenoble, France La methode d'adaptation que vous employez ne contient aucune garantie de stabilite des parametres ajustes. Qu'avez-vous observe ce sujet? L'avez-vous testee, par exemple, dans le cas d'un retard pur proportionnellement tres important (cas d'une machine papier) ?
Answer
a
Our method does not rely on plant modelling since the tuning is done on-line . The final values are left in the control algorithm but the operator can still change them if he requ ires.
a
Answer
Question by G . Sangregorio, I BM Svenska AB, Stockholm, Sweden
The method we have used allows the parameters to be adjusted in any direction. We have built into the programme limits for each of the three terms and if
In non-linear systems optimal tuning parameters in the
167
control equation are not constant but depend on the working point. Can you use your automatic tuning technique in nonlinear loops? Answer At present we would use the auto-tune to hill climb at one point on the non -I inear curve at a time. The program could be made to do this automatically by tuning at a number of points and storing the optimum values. The appropriate values could then be used automatically by the control program.
PAPER 3.4 : PROCESS AND PRODUCTION CONTROL PACKAGE by O. Alsholm, process computer manager Billeruds AB, Gruvon, Sweden, B. Morsk and G. Sangregorio, IBM Svenska, and A. Hempel, Elektrokemisk AB, Sweden Question by B. W. Balls, Foxboro-Yoxall Ltd ., Redhill, U. K . Mr. Alsholm has made a plea for more simplicity in programming - aimed at cutting costs of putting a computer suppliers standard system to work. I agree. The Foxboro Cy have analysed experience gained with some 50 of our systems in operation, controlling and monitoring batch and continuous processes. We then conducted a wide survey of experienced users of process control systems and concluded that systems programming costs where what they most feared, also costs of putting systems to work. We also concluded that no system in operation met the total needs of process industry users - all were adaptations of standard systems using software and hardware designed for purposes other than process control. I n consequence, we designed a process software system and then set to work designing the systems hardware. The result is the new Fox I process control and monitoring system. Fox I uses C RT operator and control engineer display units, which provide alphanumeric and graphic data. Control engineers can use MAX, a new macro-processor so that special, problem-oriented languages can be developed, using process control statements. No special programming technique is required. I MPAC uses a 'fill - in -the-blanks' approach to control structuring and data handling. FORTRAN IV, augmented for process control enables great flexibility in developing control strategies and mathematically - oriented optimization schemes. This user-oriented software greatly simplifies the problem of building a system, as requested by Mr. Alsholm. However, much doubt still exists in the pulp and paper industry, as Mr. Alsholm has suggested and I would like to ask if he could be more specific about the financial gains to be obtained. Potential users would like to hear more. The systems suppliers often do not know, or are under secrecy agreements. Only the users can supply the vital statistics! Answer An elaboration here on statistics and economics of the
different process units of the Gruvon Mill, which today are under computer control, would lead to a second paper . 70 % of the process units of the Gruvon Mill are now computer controlled. The process computers are for us today natural bricks in the brickwork which makes Gruvon an efficient and an economical operation. Mr. Balls refers to the "fear" for computer control, especially the pre-installation and start-up costs. To me it is just as amazing that people go through all the trouble of learning all the problems of starting up a papermaking process with all the mechanical trouble, the sofisticated drive and speed control, the expensive instrumentation, etc. Truly enough there is a trend toward "key turn" projects, but still most papermaking companies rightly feel that they must know what they are going to run. Question by F. Church, Chief instrument engineer Reed Paper and Board, Aylesford, U . K. Having installed 2 complete systems what is the overall cost of installation one and what are the financial returns? What was the overall cost of computer two and the financial returns? Answer We have installed three complete computer control systems. To myself it feels more like installing four. To repeat the history we 1) 2) 3) 4)
installed installed doubled replaced dou bled
an IBM-1710 an I BM -1800 the existing IBM-1800 and the 1710 with a second I BM -1800 (of the size).
There are no extraordinary secrets about our costs and our return on investments. As a matter of fact though, it is my experience that our frankness of telling how much it did cost to install our first process computer has not helped the "art" of computer control. Firstly a joint effort between the computer manufacturer and a paper producing company on their research and development level is mostly more costly, because you are not doing just what you must do, but you are also doing several things to learn for the future. Secondly "no one" has really understood the difference between a limited control system controlling only the papermachine (or part of it) and an integrated computer system including not only process control. but also production planning, process supervision and quality control. The later installations are more or less devoted to new processes. To give specific figures between a process start-up rate without or with computer control is of course impossible. We have though e. g. experienced a second superimposed "I earning curve" on the top of the basic one for our fluting machine coinciding with the time when we put the NSSC continuous digester under computer control. Also it can be worth mentioning that on big processes small savings mean a lot of money, e. g. a 0,5 % increase in yield on our 1,000 ton per day continuous craft digester corresponds
168
to one million Skr pro year. For a mill of Gruvons size and complexibility one can easily understand that all means which can be used to assist the production management at reasonable costs are likely to prove themselves economical. To that sen se we also have an ex tensive reporting system and an advanced production control system co-ordinating the different subproces ses .
d) PPCP includes programs for dat a logging as well as programs for bllilrling and updating of datafiles for hourly, daily and weekly production and operation reports. PROSPRO 11 does not i nclude these feature s. e) PPCP cannot handle more than 960 measured and calculated variables. PROSPRO 11 does not have th is I im itation .
Questions by prof . ir . A . van Cauwenberghe, Professor State University Of Ghent, Belgium
f) PROSPRO 11 supports up to two standard IBM process operator consoles (mod . 1892) . PPCP supports up to fifteen non-standard , customer provided consoles of less sophisticated type .
1) What are the main differences between PPCP and PROSPRO? 2) Does PPCP allow for higher level optimization tasks?
g) PROSPRO 11 is clearly superior to PPCP i n petro chemical and chemical process control application s. PPCP is better suited in pulp - and-paper . In oth er industries there is surely an overlap but, due to the d ifferences between the two package s, the one or the other may be preferable in the various situations.
3) What about loop interactions? Does PPCP provide for noninteracting control? Answers 1) Before listing the differences between PPCP and PROSPRO 11,1 would like to explain the reasons for the PPCP development . When the Billerud's IBM 1800-project was started, IBM could offer two separate proces s- oriented packages; one for supe rviso ry control (PROSPRO 1) and one for DDC . We needed instead both supervisory control and DDC in the same system in order to apply different strategies on the very different types of processes and loops of a pulp and paper mill . Therefore, we started to develop PPCP, PROSPRO 11, which can perform both supervisory and direct digital control, was not available at that time. Although developed i ndependently , PPCP was somewhat inspired by the program and table structu re of the DDC package and by the fill - in-the-blanks idea of PROSPRO . The main differences between PROSPRO 11 and PPCP are the following :
2) PPCP does not include but allows to include optimization programs of any level (see point c, answer to Question 1) . 3) PPCP allows to set up multivariable control schemes, as it includes algorithms to interconnect process variables to each other to compensate for interactions .
PAPER 3_5: SIMPLIFICATION OF A MAN-MACHINE COMMUNICATION IN A FLEXIBLE DIGITAL COMPUTER PROCESS CONTROL SYSTEM by J. B . Williams, Ferranti Ltd ., Gatley, U . K . Question by prof. ir. A . van Cauwenberghe , prof. St ate University of Ghent, Belgium Can optimal control be included i n CONSUL? Is a different computer to be preferred for this tas k ?
a) PROSPRO 11 is an IBM "Program Product", i. e. it is hired at a monthly charge under full IBM responsibility regard i ng maintenance. PPCP is free of charge , but I BM does not take . full responsibility for its maintenance .
Answer On a CONSUL system, optimal control using a c omb i na tion of statistical and modelling techniques is performed via FORTRAN programs which are compiled on-line on to a disc backing store and scheduled by the DIRECTOR operating system . A FORTRAN-CONSUL Link Program provides full two-way communication between the FORTRAN optimisation programs and the CONSUL control loops. A dual computer system is to be prefe rred, but often is rejected because of cost. On a typical dual system one computer controls the process by CONSUL, whilst the other provides the FORTRAN facility whilst also maintaining a complete updated copy of the CONSUL loops so that it may take over control in the event of failure of the CONSUL computer.
b) PROSPRO 11 works under control of the operating system TSX (Time Sharing Executive) which allows for the use of an IBM 1800 with 32k of core or less _ PPCP works under MPX (Multiprogramming Ex ecutive) which allows 32 k of core or more_ c) PROSPRO 11 is more versatile for non-standard and complex optimization and supervisory control calcu lations . PPCP instead relies on FORTRAN for such a calculation . The user has to write his own programs which are easily integrated into the PPCP system thanks to special PPCP subroutines and communication tables.
169
Chairman: Prof . ir . A. van Cauwenberghe, professor State University of Ghent, Belgium Secretary:
Ir. H . Claessens, I.B. M. of Belgium, Brussels, Belgium
Papers 3.1
PROCESS INDUSTRY PLANT DESIGN AND AUTOMATION SYSTEM IMPLEMENTATION USES FOR SIMULATION WI TH AN APPLICATION FROM THE PAPER INDUSTRY by R. M . Bakke, Consultant, Los Gatos, Cal. 95030, USA.
3.2
SYNTHESIS OF QUASI-OPTIMAL SWITCHING SURFACES BY MEANS OF LEARNING TECHNIQUES by dr. ir. J. van Reusel, systems engineer E. N. I., Aartselaar, Belgium
3.3
DIGITAL CONTROL OF A PROCESS USING ON-LINE AUTOTUNING by A. J . Pendlebury, R. F. Stedmon and Mrs. C. M. West, Kodak Ltd ., Wealdstone, U. K.
3.4
PROCESS AND PRODUCTION CONTROL PACKAGE by O. Alsholm, process computer manager Billeruds AB, Gruvon, Sweden, B. Morsk and G. Sangregorio, I BM Svenska, and A. Hempel, Elektrokemisk AB, Sweden
3.5
SIMPLIFICATION OF A MAN-MACHINE COMMUNICATION IN A FLEXIBLE DIGITAL COMPUTER PROCESS CONTROL SYSTEM by J. B. Williams, Ferranti Ltd., Gatley , U. K.
3.6
COMPUTER CONTROL OF BATCH AND DISCONTINUOUS PROCESSES by J . S. Noble and P. I. Burton, Kent Instruments Ltd. , Hitchin, U. K.
164
GENERAL REPORT The original impact of the computer on the dynamic control of PRP-processes has been considerable. In 1969 more than 50 proces s control computers were installed in the pulp and paper industry only . Particul arly the advent of DDC (direct digital control) and to a lesser extent of DDA (digitally directed analog or supervisory set-point) control has improved much the qual ity of process control. I t has also increased the possibilities to realize larger and better integrated production processes (e . g. pulp digester, bleaching unit , chemical recovery and paper mill) . This has improved horizontal integrations of production processes with in the plant. Also vertical integration of production plants will in the near future be improved by computer systems. Indeed , the realization of the final aim of the produc tion (producing with maximum profit and minimum harm to the environment) occurs on different levels : 1. on the lowest several processes are controlled by smalle r control computers e. g. in DDC or DDA mode with special purpose mini-computers performing simple regulatory or stabilizing control 2. this control may be optimized (static and / or dynamic optimization) 3. the process model used as well as the control and optimization policies may be adapted (adaptive control and self-learning systems) to the changing technical envi r onment 4. the whole control and operating policy is influenced strongly by the changing economic environment. Management decisions have to take this into account . Appropriate decision making needs much information from the lower levels In such a hierarchical system (see figure) information flows upwards and instruct ions or interventions downwards . The separation of all above mentioned control functions on different levels is not only opportunistic because it is modelled after de facto existing situations in real life (in nature as well as in industrial and social life), it allows also for flexible operation of the whole production process. Each level has its own characteristics : higher level tasks are more complicated but are less frequent than lower level tasks. Different computers (i . e. with different characteristics) can be used on each level. Simultaneously the difficul t control backup problem is alleviated because the master can always temporarily perform the job of its slave . For these and many other reasons the concept of multilevel or hierarchical control is bound to be successful in the futu re . The papers of thi5 session described different methods to reach the final production aim . These can be on-line or off-line . The latter can be in the design or in the production phase .
R. M . BAKKE (3 . 1) dealt mainly with the design of the overall plant (production p r ocess and its automat ion) for which realistic simulation methods are available. Also harmful (pollution) effects can and should be included to obtain a real istic overall picture . The other papers dealt with the on -l ine problem on different levels of the hierarchy pyramid: on the direct (regulatory) control level : - O . ALSHOLM, A. HEMPEL , B . MORSK and G . SANGREGORIO (3 .4) and - J. B. WI LLlAMS (3 .5) for continuous control - J . S. NOBLE and P. I. BURTON (3.6) for sequential (programmed) on controls the optimization level : - A . J . PENDLEBURY, R . F . STEDMON and C. M . WEST (3.3) on the adaptation level :
- J . VAN REUSEL (3 .2) Further evolution of computer control systems will strongly be determined by the ease of man - machine communication. Program packages, "fill in the blanks" procedures and push button control of algorithms contribute to alleviate this prob lem (3.4 ; 3 .5 and 3 .6) and will certainly help to resolve the magic still surrounding computer control projects and make it available to less reputed "apprentis sorciers" i. e. the common process (control) engineer. It was interesting to notice the differences between the three methods, as p resented by different control computer firms. Th E; attendance of this session by numer ous congress delegates and their active participation in the lively discussions proved the general interest that it has raised among congressists . May this in its own way be a guarantee for a better, more humane , though inevitably more computerized future .
.......... GE ... ENT COOIPUTER
f -co~ TR-O~H-PL ~;~I~-G: 1-. _ ___ _ ..--I
I NSTRUCT I ON
FLOW
1~ :
1_
______
- - ..........ae"ENT
~
INFORMAT I ON
FLOW
ADAPTATION
ADAPTING
REGULATORY CONTROL
HIER ... RCHICAL
165
SYSTE"
a. De gemakkelijke voorstellingswijze van de resultaten bekomen met het CMS algoritme . b . Ofschoon de door U vernoemde processen (warmtewisselaar enz.) meestal niet op "minimum-tijd" basis kunnen geregeld worden, kan dit soms wel het geval zijn voor bepaalde chemische batch-processen. Evolutie van de temperatuur : zie figuur.
DISCUSSIONS PAPER 3.2 : SYNTHESIS OF QUASI-OPTIMAL SWITCHING SURFACES BY MEANS OF LEARNING TECHNIQUES by dr. ir. J. van Reusel, systems engineer E. N . I., Aartselaar, Belgium.
PAPER 3.3 : DIGITAL CONTROL OF A PROCESS USING ON-LINE AUTOTUNING by A . J . Pendlebury, R . F. Stedmon and Mrs . C. M . West of Kodak Ltd ., Wealdstone, U . K .
Question by Cl. Foulard, Professeur Labo d'Automa· tique, Universite de Grenoble, France Vous utilisez un criter.e "Temps minimal". Si je comprends I'interet d'un tel critere, pour des problemes d'interception, par exemple, il me semble qu'il ne peut etre util ise tel quel pour des problemes industriels, tels que: echangeur de chaleur, machine 11 papier etc . Pouvez-vous donc justifier le choix d'un tel critere ? Pouvez-vous, par exemple indiquer ou montrer I'evolution de la temperature en fonction du temps sur I'exemple que vous avez traite ?
Questions by R . Bakke, Consultant, Los Gatos, Cal. 95030, USA Two key problems in allowing plant operators to tune using the computer without engineering supervision are: 1. Operators testing a loop for tuning interrupt their production. Generally operators are reluctant to do th is and are not particularly skilled in testing.
Answer
2. Different tuning objectives are appropriate for different loops. Fast response generally requires different tuning then least steady error. Operators usually do not know how to make this decision.
Het "minimum-tijd" foutkriterium werd hier toegepast om volgende redenen :
)(,
+5
"".,,--_ ........ - ... /
0
100
.... ...........
600
_---_...... ,..,..
900
-5 -10
-15
-20
X2
0.1
0.05 ,/,,,
0
200
300
',,~o
,,/
800
............ _,,'
0.05 -0.1
u
+1 t
100
•
200
-1
166
Programs written by R. Bakke were written and implemented that made some progress on the above problem (ref. I FAC, Toronto, Canada 1968, I nstrument Engineers Handbook, Chilton Book, Vol. II 1970) Does autotune eliminate the first problem? How do you handle the second problem?
the hillclimber attempts to adjust a parameter outside its limit then the search is considered a failure . We are not concerned with the stability of the adjustable parameters during tuning except that key keep within the prescribed limits. We have not done any work with loops involving a very long time delay .
Answers
Question by A. G. Forton, S. I. R. A. Institute, U . K .
1. We have successfully tuned one loop at a time on -
Could the number of iterations required for reaching the best set of parameters be optimised by altering the sequence of changing the integral, proportional and derivative constants during the tuning process?
line without interrupting the process . However, this may not always be possible. We generally use the auto tune on a loop which has been taken off-line although the computer is at the same time still controlling the remaining loops.
Answer
2. This would be an engineer's decision to change the cost function to produce a different type of response if required . We have found that the I. T . A. E. cost function gives us a fast response and least steady error.
There is much room for improvement to the tuning technique to reduce the number of iterations required to reach the optimum. For example, when the step changes on one of the terms become insignificantly small the programme could then ignore th is term and continue the hill climb on the remaining terms .
Question by A . M . Burgess, I. C. I. Ltd., U. K. Questions by I. McFarlane, Wiggings Teape, Beaconsfield, U. K .
The auto - tu n ing procedu re has been used on worki ng plant . How big must the test disturbances be compared with the normal plant disturbances in order to provide a reasonably fast convergence? How is the rate of convergence affected by the step size? I n answer to a previous question Mr. Stedmon mentioned checks to present excessive disturbances to the measured value; do these checks help the program discriminate between the deliberate perturbation and process disturbances?
1. Are your loops all non- interacting? 2. Have you used algorithms other than three-term control? Answers
1. Some of our loops are interacting but so far we have only tried the auto-tuning on non-interacting loops.
Answer
2. We have only used auto-tuning on one, two or three term control loops.
Test disturbances are about 2 % of full scale. Normal plant disturbances during the course of the tuning can be about 0 .2 % of full scale . The initial step size can influence the rate of convergence. We stated in the report that we took a step size of 1/4 of the parameter value . We have since found that using a step size of 1/2 of the parameter value gives a more rapid convergence. The checks are on the parameter values only and there is no discrimination between deliberate and process disturbances.
Question by R. Moore, The Foxboro Cy., Mass., USA I would like to mention a related application of adaptive tuning which was done by our company . The application was in the cement industry. The procedure was to perform a single experiment on a plant loop, and to use the inputoutput data to identify the parameters of a model. The tuning was then done off-line, using the hill climbing procedure on the identified model. The optimum tuning values were then displayed to the operator, so that he could implement them if he desired. This approach was used to successfully tune five critical loops at Meduse Cement Company.
Question by Cl. Foulard, professeur Labo d'Automatique, Universite de Grenoble, France La methode d'adaptation que vous employez ne contient aucune garantie de stabilite des parametres ajustes. Qu'avez-vous observe ce sujet? L'avez-vous testee, par exemple, dans le cas d'un retard pur proportionnellement tres important (cas d'une machine papier) ?
Answer
a
Our method does not rely on plant modelling since the tuning is done on-line . The final values are left in the control algorithm but the operator can still change them if he requ ires.
a
Answer
Question by G . Sangregorio, I BM Svenska AB, Stockholm, Sweden
The method we have used allows the parameters to be adjusted in any direction. We have built into the programme limits for each of the three terms and if
In non-linear systems optimal tuning parameters in the
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control equation are not constant but depend on the working point. Can you use your automatic tuning technique in nonlinear loops? Answer At present we would use the auto-tune to hill climb at one point on the non -I inear curve at a time. The program could be made to do this automatically by tuning at a number of points and storing the optimum values. The appropriate values could then be used automatically by the control program.
PAPER 3.4 : PROCESS AND PRODUCTION CONTROL PACKAGE by O. Alsholm, process computer manager Billeruds AB, Gruvon, Sweden, B. Morsk and G. Sangregorio, IBM Svenska, and A. Hempel, Elektrokemisk AB, Sweden Question by B. W. Balls, Foxboro-Yoxall Ltd ., Redhill, U. K . Mr. Alsholm has made a plea for more simplicity in programming - aimed at cutting costs of putting a computer suppliers standard system to work. I agree. The Foxboro Cy have analysed experience gained with some 50 of our systems in operation, controlling and monitoring batch and continuous processes. We then conducted a wide survey of experienced users of process control systems and concluded that systems programming costs where what they most feared, also costs of putting systems to work. We also concluded that no system in operation met the total needs of process industry users - all were adaptations of standard systems using software and hardware designed for purposes other than process control. I n consequence, we designed a process software system and then set to work designing the systems hardware. The result is the new Fox I process control and monitoring system. Fox I uses C RT operator and control engineer display units, which provide alphanumeric and graphic data. Control engineers can use MAX, a new macro-processor so that special, problem-oriented languages can be developed, using process control statements. No special programming technique is required. I MPAC uses a 'fill - in -the-blanks' approach to control structuring and data handling. FORTRAN IV, augmented for process control enables great flexibility in developing control strategies and mathematically - oriented optimization schemes. This user-oriented software greatly simplifies the problem of building a system, as requested by Mr. Alsholm. However, much doubt still exists in the pulp and paper industry, as Mr. Alsholm has suggested and I would like to ask if he could be more specific about the financial gains to be obtained. Potential users would like to hear more. The systems suppliers often do not know, or are under secrecy agreements. Only the users can supply the vital statistics! Answer An elaboration here on statistics and economics of the
different process units of the Gruvon Mill, which today are under computer control, would lead to a second paper . 70 % of the process units of the Gruvon Mill are now computer controlled. The process computers are for us today natural bricks in the brickwork which makes Gruvon an efficient and an economical operation. Mr. Balls refers to the "fear" for computer control, especially the pre-installation and start-up costs. To me it is just as amazing that people go through all the trouble of learning all the problems of starting up a papermaking process with all the mechanical trouble, the sofisticated drive and speed control, the expensive instrumentation, etc. Truly enough there is a trend toward "key turn" projects, but still most papermaking companies rightly feel that they must know what they are going to run. Question by F. Church, Chief instrument engineer Reed Paper and Board, Aylesford, U . K. Having installed 2 complete systems what is the overall cost of installation one and what are the financial returns? What was the overall cost of computer two and the financial returns? Answer We have installed three complete computer control systems. To myself it feels more like installing four. To repeat the history we 1) 2) 3) 4)
installed installed doubled replaced dou bled
an IBM-1710 an I BM -1800 the existing IBM-1800 and the 1710 with a second I BM -1800 (of the size).
There are no extraordinary secrets about our costs and our return on investments. As a matter of fact though, it is my experience that our frankness of telling how much it did cost to install our first process computer has not helped the "art" of computer control. Firstly a joint effort between the computer manufacturer and a paper producing company on their research and development level is mostly more costly, because you are not doing just what you must do, but you are also doing several things to learn for the future. Secondly "no one" has really understood the difference between a limited control system controlling only the papermachine (or part of it) and an integrated computer system including not only process control. but also production planning, process supervision and quality control. The later installations are more or less devoted to new processes. To give specific figures between a process start-up rate without or with computer control is of course impossible. We have though e. g. experienced a second superimposed "I earning curve" on the top of the basic one for our fluting machine coinciding with the time when we put the NSSC continuous digester under computer control. Also it can be worth mentioning that on big processes small savings mean a lot of money, e. g. a 0,5 % increase in yield on our 1,000 ton per day continuous craft digester corresponds
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to one million Skr pro year. For a mill of Gruvons size and complexibility one can easily understand that all means which can be used to assist the production management at reasonable costs are likely to prove themselves economical. To that sen se we also have an ex tensive reporting system and an advanced production control system co-ordinating the different subproces ses .
d) PPCP includes programs for dat a logging as well as programs for bllilrling and updating of datafiles for hourly, daily and weekly production and operation reports. PROSPRO 11 does not i nclude these feature s. e) PPCP cannot handle more than 960 measured and calculated variables. PROSPRO 11 does not have th is I im itation .
Questions by prof . ir . A . van Cauwenberghe, Professor State University Of Ghent, Belgium
f) PROSPRO 11 supports up to two standard IBM process operator consoles (mod . 1892) . PPCP supports up to fifteen non-standard , customer provided consoles of less sophisticated type .
1) What are the main differences between PPCP and PROSPRO? 2) Does PPCP allow for higher level optimization tasks?
g) PROSPRO 11 is clearly superior to PPCP i n petro chemical and chemical process control application s. PPCP is better suited in pulp - and-paper . In oth er industries there is surely an overlap but, due to the d ifferences between the two package s, the one or the other may be preferable in the various situations.
3) What about loop interactions? Does PPCP provide for noninteracting control? Answers 1) Before listing the differences between PPCP and PROSPRO 11,1 would like to explain the reasons for the PPCP development . When the Billerud's IBM 1800-project was started, IBM could offer two separate proces s- oriented packages; one for supe rviso ry control (PROSPRO 1) and one for DDC . We needed instead both supervisory control and DDC in the same system in order to apply different strategies on the very different types of processes and loops of a pulp and paper mill . Therefore, we started to develop PPCP, PROSPRO 11, which can perform both supervisory and direct digital control, was not available at that time. Although developed i ndependently , PPCP was somewhat inspired by the program and table structu re of the DDC package and by the fill - in-the-blanks idea of PROSPRO . The main differences between PROSPRO 11 and PPCP are the following :
2) PPCP does not include but allows to include optimization programs of any level (see point c, answer to Question 1) . 3) PPCP allows to set up multivariable control schemes, as it includes algorithms to interconnect process variables to each other to compensate for interactions .
PAPER 3_5: SIMPLIFICATION OF A MAN-MACHINE COMMUNICATION IN A FLEXIBLE DIGITAL COMPUTER PROCESS CONTROL SYSTEM by J. B . Williams, Ferranti Ltd ., Gatley, U . K . Question by prof. ir. A . van Cauwenberghe , prof. St ate University of Ghent, Belgium Can optimal control be included i n CONSUL? Is a different computer to be preferred for this tas k ?
a) PROSPRO 11 is an IBM "Program Product", i. e. it is hired at a monthly charge under full IBM responsibility regard i ng maintenance. PPCP is free of charge , but I BM does not take . full responsibility for its maintenance .
Answer On a CONSUL system, optimal control using a c omb i na tion of statistical and modelling techniques is performed via FORTRAN programs which are compiled on-line on to a disc backing store and scheduled by the DIRECTOR operating system . A FORTRAN-CONSUL Link Program provides full two-way communication between the FORTRAN optimisation programs and the CONSUL control loops. A dual computer system is to be prefe rred, but often is rejected because of cost. On a typical dual system one computer controls the process by CONSUL, whilst the other provides the FORTRAN facility whilst also maintaining a complete updated copy of the CONSUL loops so that it may take over control in the event of failure of the CONSUL computer.
b) PROSPRO 11 works under control of the operating system TSX (Time Sharing Executive) which allows for the use of an IBM 1800 with 32k of core or less _ PPCP works under MPX (Multiprogramming Ex ecutive) which allows 32 k of core or more_ c) PROSPRO 11 is more versatile for non-standard and complex optimization and supervisory control calcu lations . PPCP instead relies on FORTRAN for such a calculation . The user has to write his own programs which are easily integrated into the PPCP system thanks to special PPCP subroutines and communication tables.
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