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Benefits from distributed control in the PRP-industry
Copyright
© IFAC
Session 16
PRP 4 Automation, Ghent, Belgium 1980
BENEFITS FROM DISTRIBUTED CONTROL IN THE PRP INDUSTRY B. Biornstad (Honeywell Europe, Brussels, Belgium) Panel: E. Jutila (University of Oulu, Finland), R. Koreman (Agfa-Gevaert, Mortsel, Belgium), H. H. Steuslo££ (Fraunhofer Institute for Information and Data Processing, Karlsruhe, Federal Republic of Germany), J. van Reusel (BP Chemicals, Chairman:
Zwijndrecht, Belgium)
Therefore it is extremely important to support the application programming of distributed computing systems. Programming distributed systems in assembler will enhance the software crisis. Higher-level languages with genuine real time statements, supporting parallel processes and synchronization of processes, have to be completed by language elements for the description of hard- and software structures and configurations, varying with the system status and error conditions. This will secure the benefits from distributed control.
INTRODUCTION by H.H. STEUSLOFF
Technical processes in PRP-as well as in other industries-show a growing tendency towards higher complexity, operation near technological limits and close to quality limits, increased safety regulations and reliability requirements. The answer to the corresponding dramatic quantitative increase of control loops and measuring equipment can only be a qualitative change of the automation systems in the sense of a structural change. This situation introduces and forces distributed computer control systems.
INTRODUCTION by J. VAN REUSEL What are the possible benefits in our industries? 1. Reduction in manpower In general, distributed digital control can only replace operators where there is scope for more centralisation of control but even these savings may be partially or totally outweighed by the requirement for more expensive manpower such as instrument and application engineers. 2. Support of the operating personnel to do their job more efficiently - through reliability of both hardware and software The operator is interested in reliability from a purely selfish and a company viewpoint. A distributed system offering some advanced control possibilities which gives perhaps 1 per cent more yield of a desirable product is very fine. However, this benefit can easily be lost if the system is unreliable. The first big question therefore is how much risk spreading bo you indulge in? The number of control loops per microprocessor seems to vary, according to the manufacturer from one to sixty. - Through ease of access for operational information and control This is the area of most concern to the operator. There is no doubt that a first class, experienced operator could cope with any operator/interface system currently on the market but unfortunately many of our operators are not in this category.
There are some important properties of distributed computing systems which have to be implemented in a way to support high efficiency as well as fault tolerance. The communication system should be controlled decentrally and should comprise redundant message ways. Each microcomputer should be equipped with errordetecting components for local error detection and global status supervision, i.e. supervision of the other microcomputers in the system. Dynamic redundancy may reduce the number - and costs - of redundant system components by sharing uniform system resources in a preplanned manner (function-sharing redundancy) . Assuming the availability of such a distributed computer control system, we will achieve some benefits concerning the automatic control of PRP-processes, availability of production installations, man/process-communication and costs of the automation system. In general we will yield improved control quality by improved control algorithms, utilizing more plant data via the communication system (e.g. observers). Data- and maniprocess-communication will improve the understanding of the processes by simulation and operator training. Distributed systems will save costs by reducing the cabling expenses and providing higher flexibility. Changes in the plant automation are performed by changing the software of standard and universal hardware components.
480
Benefits from Distributed Control in the PRP-Industry
What this amounts to is that the ideal operator interface will be one, such that the average operator of fairly limited experience can easily assess the plant situation, carry out normal control functions and, assisted by some automatic systems, handle any emergency which arises. I think that the Question of how well a distributed control system can help the operator in handling emergencies, should also be considered in our discussion. INTRODUCTION by R. KOREMAN Control by its very meaning is a centra1 function. Distributed means the relegation, and devolution of tasks to separate processors. Distributed control exists because of communication between processors. Logically in an information sense, we wish centralisation. Functionally we can distribute tasks and nevertheless obtain a much stronger central control if we have the right processes for communication of information. ----~ore
The advent of the digital computer made centralisation and coordination possible: - possibility of central supervision, steering (operator workstation) collection, processing and central display of all kinds of process information possibility of optimising the overall process of on-line identification of complex control algorithms introduction of greater accuracy by transforming signals into numbers central access point to the whole process central alarm reporting Because of cost, it also forced upon us an essential distributed function (regulators) to become centralised. The major contradiction to this is that a sequential processor must take care of an incredible number of parallel, cocurrent sub-processes. The failure of centralised control is the failure to manage properly DDC and supervisory control at the same time. Central functions were least developed because the management of control loops and control points took most of the computer capacity. The advent of the cheap microprocessor gives the possibility of turning back to individual loop control and gives also the possibility of relegation of separable central tasks to individual processors e.g. live graphics, alarms, operator interface. We now see the distribution of the lower end functions mainly via hardware and firmware and very attractive operator workstations. However, the upper end (supervisory, global control) functions are rather neglected. This is due to this being the domain of 'informatics ' . Providing turn-key hardware and firmware functions is easiest. Providing
481
software packages which can be customised and extended by the user is much more difficult: - non-standard reports - non-standard graphics - intelligent interrogation for troubleshooting and tuning the machine - intelligent alarm reporting and analyses - versatile and flexible man/machine communication. Its advantages include the fact that the system is a real computer control network. It has a high-order process control language. It is easy to process management language and has easy links to data processing systems. Computer control network - computer control because control is essentially an information processing task. Computers' distributed systems on the market ~ay satisfy the process engineer in terms of measurement and control but very likely not the production engineer or production management. - control network because distribution is natural and a mus t and control can only exist via communication. I have not yet seen commercially available systems which have realised a real network protocol. ll
II
Sophisticated man/machine interface (operator console, process engineer console) giving full and intelligent, non-standard access to the whole system via control devices (ball, buttons ... ), screens, languages. - centralised control - devolution towards microprocessors but not solving the communication problem such as twisted pairs or badly defined protocol. It is essentially a software solution with all its advantages. - no commercially available system for our next generation which could introduce distributed systems with the flexibility we are used to. - we are not prepared to do it ourselves, so there is a vacuum. We are waiting for a fully distributed process control system which is an information processing system as well, with all the versatilitya software solution can offer at a reasonable cost. DISCUSSION A lively discussion developed on many interesting points, such as the key ingredients and characteristics of good communic~tion software for distributed process control systems (dr. S. Sah, Univ. Alberta, CON). The higher efficiency of a distributed control system was discussed as it often implies a more expensive system, because of the increased redundancy (ir. M. De Coster, Essochem, B). Ir. C. Wilmering (Honeywell, NL) raised two points:
482
B.
Bi~rnstad
1. As the cost of integrated electronics comes down, Dr. Van Reusel might see the centralisation of the control functions as well - the cost would make this possible. I would like to make the point that risk distribution also involves risk with the software and as you make the units larger, the software becomes more complex and the chance that you introduce bugs becomes larger too and also the time it takes to get them out takes longer.
2. Ir. Koreman made a point for standardisation in the communication of a process control data network. This subject will be dealt with in another round-table discussion but ire Koreman mentioned languages as well. I think one should look at languages for the system developer (the vendor for instance) and languages to be used by the user who writes his own application software within the overall system. Therefore a system comes with firmware and languages to allow supervisory and higher level functions.
© IFAC
Session 16
PRP 4 Automation, Ghent, Belgium 1980
BENEFITS FROM DISTRIBUTED CONTROL IN THE PRP INDUSTRY B. Biornstad (Honeywell Europe, Brussels, Belgium) Panel: E. Jutila (University of Oulu, Finland), R. Koreman (Agfa-Gevaert, Mortsel, Belgium), H. H. Steuslo££ (Fraunhofer Institute for Information and Data Processing, Karlsruhe, Federal Republic of Germany), J. van Reusel (BP Chemicals, Chairman:
Zwijndrecht, Belgium)
Therefore it is extremely important to support the application programming of distributed computing systems. Programming distributed systems in assembler will enhance the software crisis. Higher-level languages with genuine real time statements, supporting parallel processes and synchronization of processes, have to be completed by language elements for the description of hard- and software structures and configurations, varying with the system status and error conditions. This will secure the benefits from distributed control.
INTRODUCTION by H.H. STEUSLOFF
Technical processes in PRP-as well as in other industries-show a growing tendency towards higher complexity, operation near technological limits and close to quality limits, increased safety regulations and reliability requirements. The answer to the corresponding dramatic quantitative increase of control loops and measuring equipment can only be a qualitative change of the automation systems in the sense of a structural change. This situation introduces and forces distributed computer control systems.
INTRODUCTION by J. VAN REUSEL What are the possible benefits in our industries? 1. Reduction in manpower In general, distributed digital control can only replace operators where there is scope for more centralisation of control but even these savings may be partially or totally outweighed by the requirement for more expensive manpower such as instrument and application engineers. 2. Support of the operating personnel to do their job more efficiently - through reliability of both hardware and software The operator is interested in reliability from a purely selfish and a company viewpoint. A distributed system offering some advanced control possibilities which gives perhaps 1 per cent more yield of a desirable product is very fine. However, this benefit can easily be lost if the system is unreliable. The first big question therefore is how much risk spreading bo you indulge in? The number of control loops per microprocessor seems to vary, according to the manufacturer from one to sixty. - Through ease of access for operational information and control This is the area of most concern to the operator. There is no doubt that a first class, experienced operator could cope with any operator/interface system currently on the market but unfortunately many of our operators are not in this category.
There are some important properties of distributed computing systems which have to be implemented in a way to support high efficiency as well as fault tolerance. The communication system should be controlled decentrally and should comprise redundant message ways. Each microcomputer should be equipped with errordetecting components for local error detection and global status supervision, i.e. supervision of the other microcomputers in the system. Dynamic redundancy may reduce the number - and costs - of redundant system components by sharing uniform system resources in a preplanned manner (function-sharing redundancy) . Assuming the availability of such a distributed computer control system, we will achieve some benefits concerning the automatic control of PRP-processes, availability of production installations, man/process-communication and costs of the automation system. In general we will yield improved control quality by improved control algorithms, utilizing more plant data via the communication system (e.g. observers). Data- and maniprocess-communication will improve the understanding of the processes by simulation and operator training. Distributed systems will save costs by reducing the cabling expenses and providing higher flexibility. Changes in the plant automation are performed by changing the software of standard and universal hardware components.
480
Benefits from Distributed Control in the PRP-Industry
What this amounts to is that the ideal operator interface will be one, such that the average operator of fairly limited experience can easily assess the plant situation, carry out normal control functions and, assisted by some automatic systems, handle any emergency which arises. I think that the Question of how well a distributed control system can help the operator in handling emergencies, should also be considered in our discussion. INTRODUCTION by R. KOREMAN Control by its very meaning is a centra1 function. Distributed means the relegation, and devolution of tasks to separate processors. Distributed control exists because of communication between processors. Logically in an information sense, we wish centralisation. Functionally we can distribute tasks and nevertheless obtain a much stronger central control if we have the right processes for communication of information. ----~ore
The advent of the digital computer made centralisation and coordination possible: - possibility of central supervision, steering (operator workstation) collection, processing and central display of all kinds of process information possibility of optimising the overall process of on-line identification of complex control algorithms introduction of greater accuracy by transforming signals into numbers central access point to the whole process central alarm reporting Because of cost, it also forced upon us an essential distributed function (regulators) to become centralised. The major contradiction to this is that a sequential processor must take care of an incredible number of parallel, cocurrent sub-processes. The failure of centralised control is the failure to manage properly DDC and supervisory control at the same time. Central functions were least developed because the management of control loops and control points took most of the computer capacity. The advent of the cheap microprocessor gives the possibility of turning back to individual loop control and gives also the possibility of relegation of separable central tasks to individual processors e.g. live graphics, alarms, operator interface. We now see the distribution of the lower end functions mainly via hardware and firmware and very attractive operator workstations. However, the upper end (supervisory, global control) functions are rather neglected. This is due to this being the domain of 'informatics ' . Providing turn-key hardware and firmware functions is easiest. Providing
481
software packages which can be customised and extended by the user is much more difficult: - non-standard reports - non-standard graphics - intelligent interrogation for troubleshooting and tuning the machine - intelligent alarm reporting and analyses - versatile and flexible man/machine communication. Its advantages include the fact that the system is a real computer control network. It has a high-order process control language. It is easy to process management language and has easy links to data processing systems. Computer control network - computer control because control is essentially an information processing task. Computers' distributed systems on the market ~ay satisfy the process engineer in terms of measurement and control but very likely not the production engineer or production management. - control network because distribution is natural and a mus t and control can only exist via communication. I have not yet seen commercially available systems which have realised a real network protocol. ll
II
Sophisticated man/machine interface (operator console, process engineer console) giving full and intelligent, non-standard access to the whole system via control devices (ball, buttons ... ), screens, languages. - centralised control - devolution towards microprocessors but not solving the communication problem such as twisted pairs or badly defined protocol. It is essentially a software solution with all its advantages. - no commercially available system for our next generation which could introduce distributed systems with the flexibility we are used to. - we are not prepared to do it ourselves, so there is a vacuum. We are waiting for a fully distributed process control system which is an information processing system as well, with all the versatilitya software solution can offer at a reasonable cost. DISCUSSION A lively discussion developed on many interesting points, such as the key ingredients and characteristics of good communic~tion software for distributed process control systems (dr. S. Sah, Univ. Alberta, CON). The higher efficiency of a distributed control system was discussed as it often implies a more expensive system, because of the increased redundancy (ir. M. De Coster, Essochem, B). Ir. C. Wilmering (Honeywell, NL) raised two points:
482
B.
Bi~rnstad
1. As the cost of integrated electronics comes down, Dr. Van Reusel might see the centralisation of the control functions as well - the cost would make this possible. I would like to make the point that risk distribution also involves risk with the software and as you make the units larger, the software becomes more complex and the chance that you introduce bugs becomes larger too and also the time it takes to get them out takes longer.
2. Ir. Koreman made a point for standardisation in the communication of a process control data network. This subject will be dealt with in another round-table discussion but ire Koreman mentioned languages as well. I think one should look at languages for the system developer (the vendor for instance) and languages to be used by the user who writes his own application software within the overall system. Therefore a system comes with firmware and languages to allow supervisory and higher level functions.