Components and Instruments for Distributed Control Systems. Proceedings of the IFAC Symposium Paris, France, 9–11 December 1982

216 Pages·1983·18.093 MB·English

by  Z. Binder, R. Perret

Titles in the IF A C Proceedings Series
AKASHI: Control Science and Technology for the Progress of LANDAU: Adaptive Systems in Control and Signal Processing
Society, 7 Volumes LAUBER: Safety of Computer Control Systems (1979)
ALONSO CONCHEIRO: Real Time Digital Control Applications LEININGER: Computer Aided Design of Multivariable
ATHERTON: Multivariable Technological Systems Technological Systems
BABARY & LE LETTY: Control of Distributed Parameter Systems LEONHARD: Control in Power Electronics and Electrical Drives
(1982) (1977)
BANKS & PRITCHARD: Control of Distributed Parameter LESKIEWICZ & ZAREMBA: Pneumatic and Hydraulic
Systems Components and Instruments in Automatic Control
BAYLIS: Safety of Computer Control Systems (1983) MAHALANABIS: Theory and Application of Digital Control
BEKEY & SARIDIS: Identification and System Parameter MILLER: Distributed Computer Control Systems (1981)
Estimation (1982) MUNDAY: Automatic Control in Space
BINDER: Components and Instruments for Distributed Computer NAJIM & ABDEL-FATTAH: Systems Approach for Development
(1980)
Control Systems
BULL: Real Time Programming (1983) NIEMI: A Link Between Science and Applications of Automatic
CAMPBELL: Control Aspects of Prosthetics and Orthotics Control
Van CAUWENBERGHE: Instrumentation and Automation in the NOVAK: Software for Computer Control
Paper, Rubber, Plastics and Polymerisation Industries (1980) O'SHEA & POLIS: Automation in Mining, Mineral and Metal
CICHOCKI & STRASZAK: Systems Analysis Applications to Processing il980)
Complex Programs OSHIMA: Information Control Problems in Manufacturing
CRONHJORT: Real Time Programming (1978) Technology (1977)
CUENOD: Computer Aided Design of Control Systems PAU: Dynamic Modelling and Control of National Economies
(1983)
De GIORGO & ROVEDA: Criteria for Selecting Appropriate
Technologies under Different Cultural, Technical and Social RAUCH: Applications of Nonlinear Programming to
Conditions Optimization and Control
RAUCH: Control Applications of Nonlinear Programming
DUBUISSON: Information and Systems
ELLIS: Control Problems and Devices in Manufacturing Technolog y REMBOLD: Information Control Problems in Manufacturing
(1980) Technology (1979)
FERRATE & PUENTE: Software for Computer Control RIJNSDORP: Case Studies in Automation related to Humanization
FLEISSNER: Systems Approach to Appropriate Technology of Work
Transfer RIJNSDORP & PLOMP: Training for Tomorrow - Educational
Aspects of Computerised Automation
GELLIE & TAVAST: Distributed Computer Control Systems ( 1982)
RODD: Distributed Computer Control Systems (1983)
GHONAIMY: Systems Approach for Development (1977)
SANCHEZ & GUPTA: Fuzzy Information, Knowledge
HAASE: Real Time Programming (1980)
Representation and Decision Analysis
HAÏMES & KINDLER: Water and Related Land Resource Systems
SAWARAGI & AKASHI: Environmental Systems Planning, Design
HALME: Modelling and Control of Biotechnical Processes
and Control
HARDT: Information Control Problems in Manufacturing
SINGH & TITLI: Control and Management of Integrated Industrial
Technology (1982) Complexes
HARRISON: Distributed Computer Control Systems SMEDEMA: Real Time Programming (1977)
HASEGAWA: Real Time Programming (1981) STRASZAK: Large Scale Systems: Theory and Applications (1983)
HASEGAWA & INOUE: Urban, Regional and National Planning SUBRAMANYAM: Computer Applications in Large Scale Power
— Environmental Aspects Systems
HERBST: Automatic Control in Power Generation Distribution and TITLI & SINGH: Large Scale Systems: Theory and Applications
Protection (1980)
ISERMANN: Identification and System Parameter Estimation
WESTERLUND: Automation in Mining, Mineral and Metal
(1979)
Processing (1983)
ISERMANN & KALTENECKER: Digital Computer Applications to
Van WOERKOM: Automatic Control in Space (1982)
Process Control
ZWICKY: Control in Power Electronics and Electrical Drives
JANSSEN, PAU & STRASZAK: Dynamic Modelling and Control of
(1983)
National Economics (1980)
JOHANNSEN & RIJNSDORP: Analysis, Design, and Evaluation of
Man-Machine Systems
NOTICE TO READERS
Dear Reader
If your library is not already a standing/continuation order customer to this series, may we recommend that you place a
standing/continuation order to receive immediately upon publication all new volumes. Should you find that these volumes no
longer serve your needs, your order can be cancelled at any time without notice.
ROBERT MAXWELL
Publisher at Pergamon Press
IF AC Related Titles
BROADBENT & MASUBUCHI: Multilingual Glossary of Automatic Control Technology
EYKHOFF: Trends and Progress in System Identification
ISERMANN: System Identification Tutorials (Automatica Special Issue)
COMPONENTS AND
INSTRUMENTS FOR
DISTRIBUTED CONTROL
SYSTEMS
Proceedings of the IF A C Symposium
Paris, France, 9Ί1 December 1982
Edited by
Z. BINDER and R. PERRET
Laboratoire d'Automatique de Grenoble, France
Published for the
INTERNATIONAL FEDERATION OF AUTOMATIC CONTROL
by
PERGAMON PRESS
OXFORD NEW YORK TORONTO SYDNEY PARIS FRANKFURT
U.K. Pergamon Press Ltd., Headington Hill Hall,
Oxford OX3 OBW, England
U.S.A. Pergamon Press Inc., Maxwell House, Fairview Park,
Elmsford, New York 10523, U.S.A.
CANADA Pergamon Press Canada Ltd., Suite 104,
150 Consumers Road, Willowdale, Ontario M2J 1P9, Canada
AUSTRALIA Pergamon Press (Aust.) Pty. Ltd., P.O. Box 544,
Potts Point, N.S.W. 2011, Australia
FRANCE Pergamon Press SARL, 24 rue des Ecoles,
75240 Paris, Cedex 05, France
FEDERAL REPUBLIC Pergamon Press GmbH, Hammerweg 6,
OF GERMANY D-6242 Kronberg-Taunus, Federal Republic of Germany
Copyright © 1983 IFAC
All Rights Reserved. No part of this publication may be
reproduced, stored in a retrieval system or transmitted in any
form or by any means: electronic, electrostatic, magnetic tape,
mechanical, photocopying, recording or otherwise, without
permission in writing from the copyright holders.
First edition 1983
Library of Congress Cataloging in Publication Data
Main entry under title:
Components & instruments for distributed control systems.
(IFAC proceedings series)
Includes index.
1. Automatic control —Congresses. 2. Control theory —
Congresses. I. Binder, Ζ. II. Perret, R.
III. International Federation of Automatic Control.
IV. Title: Components and instruments for distributed
control systems.
V. Series.
TJ212.2.C65 1983 629.8'312 83-11448
British Library Cataloguing in Publication Data
Components & instruments for distributed control
systems. — (IFAC proceedings series)
1. Automatic control —Data processing —Congresses
2. Electronic data processing —Distributed processing
— Congresses
I. Binder, Z. II. Perret, R. III. Series
629.8*95 TJ212
ISBN 0-08-029991-1
These proceedings were reproduced by means of the
photo-offset process using the manuscripts supplied by the
authors of the different papers. The manuscripts have
been typed using different typewriters and typefaces. The
lay-out, figures and tables of some papers did not agree
completely with the standard requirements; consequently
the reproduction does not display complete uniformity.
To ensure rapid publication this discrepancy could not be
changed; nor could the English be checked completely.
Therefore, the readers are asked to excuse any deficiencies
of this publication which may be due to the above men-
tioned reasons.
Printed in Great Britain by A. Wheaton & Co Ltd., Exeter
IFAC SYMPOSIUM ON COMPONENTS AND INSTRUMENTS
FOR DISTRIBUTED CONTROL SYSTEMS
Sponsored by
The International Federation of Automatic Control
Technical Committee on Components and Instruments
Co-sponsored by
Technical Committee on Theory
International Programme Committee National Organizing Committee
H. J. Leskiewicz, Poland (Chairman) Z. Binder, France (Chairman)
Z. Binder, France (Vice-chairman) P. Andre
J. Clot, France M. Combe
M. Diaz, France M. Fournet
A. N. Hagras, Egypt C. Pourcel
T. Higuchi, Japan
M. Horejsi, Czechoslovakia
Β. V. Jayawant, GB
T. Kuusisto, Finland
F. Lhote, France
R. Mezencev, France
R. Perret, France
T. Pfeifer, Germany
I. V. Prangishvili, USSR
G. Ramos, Mexico
Κ. N. Reid, USA
F. Siwoff, Bulgaria
C. Sommer, South Africa
Wang Lian Mei, China
PREFACE
The first work on hierarchical control began about fifteen years ago.
Its industrial application, however, was put off for a long time because the
technical means available were ill-adapted.
With the enormous progress in micro-electronics that has taken place over
the past few years, intelligent instruments can now be created that integrate
processing once reserved for calculators.
The development of distributed micro-computing systems is linked to this
progress, and their use in industry and in service areas is becoming more and
more widespread.
Great progress has also been made in the design of sensors and other
components in the automatic control chain.
A combination of these four elements facilitates the design of distribu-
ted control system. Their performance does not depend only on the quality of
each element but also on the compatibility of these elements and the role
that they play in the system. Effective system design requires the collabo-
ration and co-operation of instrumentation specialists, control engineers,
distributed control system designers, and the system* s users.
This co-operation is the main objective of the symposium, and has been
in mind throughout its preparation.
I would like to thank all those who helped prepare this symposium, and
I hope that together the organizers and lecturers will have contributed to
a fruitful reflection on instrument development and its use in distributed
control.
Z. BINDER
Chairman of the National Organizing Committee.
vi
OPENING SPEECH
Professor Dr. Henryk J. Leskiewicz
Chairman of the IF AC Technical Committee on Components and Instruments
The Technical Committee on Components and reliability u/e are nou/ speaking about
Instruments of IFAC is one of a small group includes not only a reduction in the pro-
of Technical Committees established at the bability of failure, but also the ease and
first IFAC World Congress held in Moscou/ in speed in overcoming such a failure. Thus an
1960. The original name vi/as "The Technical automatic control system may be said to have
Committee on Components". The words "and a satisfactory reliability u/hen the probab-
Instruments" have been added later to ility of a failure is sufficiently small and
reflect the grou/ing field of interest of the u/hen difficulty in overcoming the failure is
Committee. This field is grou/ing and acceptably small.
changing rapidly, as can be seen by the
topic of the present Paris symposium on The distributed block structure of control
Components and Instruments for Distributed systems seems to be the best structure to
Control Systems, u/hich is sponsored by the ensure reliability using the above approach.
Committee. In this structure a certain number of com-
ponents and instruments u/hich form the
Many symposia in different countries have separate blocks of the system must be pro-
been sponsored or cosponsored by the grammable. We may also in the future see
Committee before, but this is the first one some intelligent components and instruments
in France. It is taking place as a result incorporated in such systems.
of the energetic efforts of AFCET, the
French member-organisation of IFAC. Commencing in 1960, an IFAC World Congress
Dr Ζ Binder from Grenoble, as the French has been held every three years in a
representative to the IFAC Technical different country. The next u/ill be held
Committee on Components and Instruments, has in Budapest, Hungary, in July 1984. It
contributed a major part in organising this u/ill be differently organised from the
Symposium; he is an internationally previous ones, as it u/ill be composed from
recognised specialist in distributed control separate symposia and u/orkshops sponsored
systems. and conducted by separate IFAC Technical
Committees. The Technical Committee on
The preprints of the Symposium illustrate the Components and Instruments u/ill sponsor in
state of the art in components and instru- this Budapest 1984 World Congress the
ments in Automatic Control, u/hich has been symposium on Programmable Components and
achieved partly due to the rapid improvement Instruments. It u/ill be related to and
in micro-electronics, particularly because influenced by this present IFAC Paris
miniaturisation has increased circuit Symposium on Components and Instruments for
reliability and reduced costs. In addition Distributed Control Systems.
there is a considerable improvement in the
reliability of an automatic control u/hen I u/ish this Symposium great success, which
modern components and instruments are used is so u/ell deserved in recognition of the
to form a distributed control system. The skill and enthusiasm of the organisers.
vu
Copyright © IFAC Components and Instruments for PLENARY PAPERS
Distributed Control Systems, Paris, France 1982
DISTRIBUTED CONTROL: STATUS AND
OPPORTUNITIES
I. Lefkowitz and M. R. Büchner
Department of Systems Engineering, Case Western Reserve University, Cleveland, Ohio,
USA
Abstract. Goals of improved productivity, efficiency, and product quality
have motivated a continuing development over the years of automatic control
theory and practice in industrial applications. A brief historical account
of this development leads to a discussion of the current status of distribu-
ted control as a natural stage in the evolution toward complete integrated
control of an industrial plant. The historical perspective also provides
background for an examination of trends and opportunities in distributed
control.
Present generation microprocessor-based distributed control systems are des-
cribed in terms of their generic features and attributes. Reference to
limitations of these systems with respect to the goal of achieving implemen-
table, cost effective and reliable integrated systems control motivates dis-
cussion of some current research areas and expected future developments.
Finally, a review of the hierarchical control approach is presented. This
provides a conceptual framework for organizing the elements of the distribu-
ted system for integration of the many diverse information processing, de-
cision-making and control functions that are involved in a total plant
control.
Keywords. Hierarchical control; computer control; distributed control systems;
real-time systems; integrated systems control.
INTRODUCTION It was soon recognized that some of the more
elementary supervisory functions could be
The control of industrial processes has carried out automatically. For example, con-
evolved very considerably over the past half trollers were introduced that could maintain
century. Early objectives of automatic con- a fixed functional relationship among several
trol were to relieve human operators of the process variables so as to improve process
tedium and drudgery of maintaining certain performance (e.g. yield, efficiency, product
key variables of the plant at desired values quality). At the same time, sophisticated
through feedback actions. At the same time, monitoring and alarm systems were developed
control devices provided the means of better that automatically sensed the status of all
accuracy and more consistent and dependable the plant variables and alerted the operator
results. The introduction of electronic in- if any of them exceeded preset limits.
strumentation enabled remote sensing and ac-
tuation which led to the development of cen- The advent of the digital control computer in
tral control rooms where the operator could the 19501 s initiated a revolution in the con-
keep track of a large number of control loops trol of industrial plants. The computer made
His role became increasingly one of super- it possible to store and process large quan-
vision - to adjust controller setpoints when- tities of data and to implement complex al-
ever required by a change in the mode of op- gorithms in real-time so that we could ad-
eration or in the specifications of the vance from simple control objectives of main-
product. The operator also had the responsi- taining process variables at fixed desired
bility of monitoring the performance of the values to the more interesting objectives of
various control loops to make sure that the determining how these variables should be
plant was operating properly, to make changes changed with time or in relationship to other
whenever product quality or production effi- variables in order to optimize plant perfor-
ciency fell below tolerance limits and to mance. The control computer also provided
respond to contingency events (e.g. a mal- the capability of rapid switching from one
function of a piece of equipment) with proper computational task to another. Thus, one
emergency actions. machine could handle a large number of
1
2 I. Lefkowitz and M.R. Büchner
control loops as well as various auxiliary- control capabilities had been accompanied by
tasks such as monitoring, start-up sequences, a growing perception in industry of the need
operational control, etc. for applying more advanced and effective con-
trol, spurred on by such factors as: (a) the
Unfortunately, the early process control com- need for more efficient utilization of re-
puters were costly and had limited speed, sources (e.g. energy, water, labor, materials)
memory capacity and software capabilities. because of increasing cost, limited avail-
Reliability was another problem, i.e. assur- ability, or both; (b) demands for higher pro-
ing safe and dependable performance of the ductivity to meet more intense competition;
system over months and even years of contin- and (c) more stringent requirements concern-
uous plant operation. As a result, many of ing product quality, environment impact, and
the initial attempts at computer control, human safety because of government regula-
while boldly conceived and implemented with tions and greater consumer awareness,
great fervor and effort, fell dismally short
of expectations. The problems of realization and implementation
of an integrated systems control are quite
Developments in computer technology over the formidable because of the complexity of pro-
past fifteen years have resulted in tremen- duction processes, dynamic interactions among
dous reductions in hardware costs while the production units, time-varying aspects of
computation speeds and storage capacities the system, various constraints to be satis-
have increased dramatically. User-oriented fied, etc. Additional factors to be consid-
programming languages have greatly eased the ered include multiobjective decision-making
man-machine interaction problem, e.g. in pro- under uncertainty and man-machine interactions.
gramming, debugging and updating computer The hierarchical control approach provides a
control algorithms. Also, reliability has rational and systematic design procedure for
improved substantially as a result of more addressing the problems of complexity and un-
reliable components and the increased feasi- certainty. Modern distributed intelligence/
bility of fault-tolerant designs, redundancy computer control systems provide the hardware
and diagnostic routines — enhanced by low and software capabilities for cost effective
hardware costs and more sophisticated design realization of the integrated system and for
techniques. effective incorporation of the human (operator/
decision-maker) as an integral part of the
More recently, advances in real-time applica- system.
tions of minicomputers and microprocessors
have had a profound effect on the directions With respect to decision-making and control,
of current effort in industrial systems con- we distinguish the following basic elements:
trol. Specifically, these have opened up new
opportunities for system configuration based 1. The plant denotes the controlled system
on (i) distributed data acquisition and con- and may refer variously to a production unit,
trol and (ii) hierarchical computer control a processing complex or even an entire company
where each computer performs selected tasks depending on the level of control being con-
appropriate to its position in the hierarchy. sidered. The plant is subject to a variety of
These approaches (in contrast with the ini- disturbance inputs, i.e. the effects of inter-
tial idea of lumping all tasks in one giant actions of the plant with other plant units
control computer) have contributed to design and with the environment that cause the system
flexibility, improved reliability and secu- to deviate from desired or predicted behavior.
rity, better performance, etc. A special type of disturbance is the discrete
event or "contingency" occurrence, e.g., fail-
Integrated Systems Control (See [1,2]) ure of a component or taking a unit off line,
A contingency event usually implies that the
A consequence of these developments has been system is no longer operating according to
a vast broadening of the domain of what is assumptions imbedded in the current control
technologically and economically feasible to model and hence, it is necessary to modify the
achieve in the application of computers to structure of the system, go into a new control
control of industrial systems. Now, all as- mode or develop some other corrective action.
pects of information processing, data gather-
ing, process control, on-line optimization, 2. The controller generates the controlled in-
operations control - advancing even to real- puts to the plant for the purpose of achieving
time scheduling and production planning func- a desired behavior or performance consistent
tions may be included in the range of tasks with system constraints. The constraints define
to be carried out by the computer control the regionsof feasible or acceptable plant
system. This has made possible the realiza- operation; they are imposed to ensure safety
tion of integrated systems control in which of operating personnel, security of the pro-
all factors influencing plant performance are duction means, and that various "quality" re-
taken into account in an integrated fashion— quirements are met, e.g. product specifications,
recognizing the couplings, interactions and effluent pollution restrictions, etc.
complex feedback paths existing in the sys-
tem — to achieve an overall optimum perfor- The control functions may be performed by man,
mance . by machine (computer), or more generally, by
an integration of human operators, schedulers,
The analytical and technological advances in and planners with the computer control and data
Status and Opportunities 3
management system. The functions performed 1st Layer: The first or direct layer func-
by man include those based on judgment or ex- tion constitutes the interface between the
periences whose subleties or nonquantifiable controlled plant and the decision-making and
attributes defy computer implementation. The control aspects of the system. An important
functions performed by computers are essen- characteristic of the first layer, therefore,
tially those where the tasks are routine and is its ability to interact directly with the
well-defined and where the operating standards plant and in the same time scale. Typically,
are quantified and established. this layer incorporates the functions of data
acquisition, event monitoring and direct con-
3. The underlying assumption in the achieve- trol.
ment of integrated control is that the con-
troller acts on the basis of (real-time) in- 2nd Layer : The second-layer or supervisory
formation concerning the state of the plant, function is concerned with the problem of de-
external inputs, etc. Major functions of the fining the immediate target or task to be im-
information processor include: plemented by the first layer. In the normal
mode, the objective may be control of the
(a) data gathering and processing, e.g. data- plant for optimum performance according to
smoothing, noise filtering, prediction and ex- the assumed mathematical model. Under emer-
trapolation, etc. gency conditions, different objectives may
take precedence through implementation of the
(b) the monitoring of system status for con- appropriate contingency plan. In general,
tingency events to determine whether diag- there may be a number of operating modes or
nostic and/or corrective responses are to be topologies identified for the system with
initiated. each having a different mathematical model
through which information describing the
(c) the storage and retrieval of operating current state of the system is transformed
instructions, standards, parameter values, into directives applied to the first layer
and other information required for the func- function. In the conventional process con-
tioning of the system. trol application, the second layer interven-
tion takes the form of defining the set-point
A block diagram of the relationship of the values for the first-layer controllers. In
controller to the plant is given in Fig. 1. the discrete formulation, the output of the
Current values of the output variables y supervisory function may be a specified tar-
(feedback action) and some of the disturbance get or "next state" to be implemented by the
variables ζ (for feedforward actions) are direct controller through a predetermined se-
transmitted to the controller by means of sen- quence of actions.
sors or measuring devices. The raw informa-
tion set χ may be further processed (filtered, 3rd Layer: The third-layer or adaptive func-
smoothed, transformed, etc.) by the informa- tion is concerned with the problem of updating
tion processor. The controller generates its algorithms employed at the first and second
outputs according to current information con- layers. The adaptive layer may intervene in
cerning y and z, in relation to the input r, the operation of the lower layers in the fol-
which defines the desired behavior of the lowing ways :
plant, e.g. provides the setpoint values at
which certain output variables are to be main- (a) updating of parameter values associated
tained. Finally, the controller must communi- with the first and second layer control al-
cate its decisions/actions to the plant — gorithms, say by least-squares fitting of the
and this is the role of the actuator. The underlying mathematical models to observed
elements represented in Fig. 1 are generic to plant behavior.
all control and decision-making functions and
are embedded within each of the hierarchical (b) updating of parameters associated with
control structures described below. the event monitoring function. Of particular
interest here is the sensing of the transition
Hierarchical Control ([2,3]) of the plant from one operating mode to an-
other .
The overall complex problem may be decomposed
according to various criteria; these include: (c) development of contingency plans, i.e.
alternative procedures for second-layer imple-
(a) decomposition according to control func- mentation, to be invoked when the plant de-
tion: functional multilayer control hierarchy generates to an emergency mode.
(b) decomposition according to subsystem A common and distinguishing feature of the
classification or system structure: multilevel third-layer function is that its actions are
control hierarchy a reflection of operating experience over a
period of time. The actions are discrete,
(c) decomposition according to time scale: taking place in response to event occurrences
temporal multilayer control hierarchy. (e.g. operator inputs).
The functional multilayer control hierarchy is 4th Layer: The fourth-layer or self-organi-
characterized by the diagram in Fig. 2 where zing function is concerned with decisions
four layers of control are identified: relevant to the choice of structure of the
4 I. Lefkowitz and M.R. Büchner
algorithms associated with the lower layers In effect, the subsystem problems are solved
of the hierarchy. These decisions are based at the first level of control. However,
on overall considerations of performance ob- since the subsystems are coupled and inter-
jectives, assumptions of the nature of the acting, these solutions have no meaning un-
system relationships, coordination with other less the interaction constraints are simul-
systems, etc. taneously satisfied. This is the coordina-
tion problem that is solved at the second
An example of the application of the foregoing level of the hierarchy.
structure is provided by a catalytic reactor
process. The process inputs are controlled The decomposition of the overall system into
as continuous functions of time; however, at subsystems may be based on geographical con-
discrete points in time, the "normal" opera- siderations (e.g. relative proximity of
ting mode is disrupted to go into a "regenera- different units), lines of managerial res-
tion" mode for the purpose of restoring cata- ponsibility (e.g. steel-making shop and
lyst activity. rolling mill in a steel works), or on the type
of equipment (e.g. distillation tower and re-
The direct control function is concerned with actor in a chemical plant). In general, how-
the task of controlling the process variables, ever, the plants are designed so that these
e.g. pressures, temperatures and flow rates divisions correspond to lines of weak inter-
to the set-point values defined by the super- action, i.e. through the incorporation of
visory control function. This function is various "buffer" or control mechanisms, the
implemented by means of conventional feedback resulting subsystems are partially decoupled
control loops, with perhaps some feed forward so that interaction effects tend to be small
considerations. and/or only slowly varying with time.
The determination of set-points at the second Advantages of multilevel decomposition in-
layer depends on the mode of operation. In clude (i) reduction in computational effort
the normal operating mode the set-points may and data transmission requirements because the
be determined to maximize product yield con- more complex coordination tasks are handled at
sistent with system constraints and specifi- a higher level at lower frequency, (ii) oppor-
cations on product quality. In the catalyst tunities for increased system reliability be-
regeneration mode, the objective may be to cause most of the control tasks are designed
minimize the regeneration period, i.e. the to be handled locally with short-term inde-
time during which product is not being pro- pendence of the other subsystems, and (iii)
duced. reductions in maintenance and system develop-
ment costs by virtue of the fact that models,
There are at least two distinct tasks that may control algorithms and computer software may
be assigned to the third layer. The first re- be developed in a step-by-step, semi-indepen-
lates to the updating of selected parameters dent fashion.
of the lower-layer control algorithms to take
care of the effects of normal variations in A particularly relevant application of the
operating conditions, catalyst activity, etc. multilevel approach is in the electric power
The second task updates the criterion function industry where the power generation and dis-
for switching between operating and regenera- tribution system is designed as an inter-
ting modes. connection of semi-independent subsystems.
Thus, there is a natural decomposition induce<
The fourth control layer has the responsibili- by technological considerations at the genera-
ty of selecting the operating mode and, con- ting unit level, geographical considerations
sequently, the programs to be used by the at the generating station level, ownership
lower-layer control functions. In general, a boundaries at the company level, etc.
transfer of mode requires extensive changes in
the control structure which are to be coordi- A second application is suggested by the sys-
nated by fourth-layer intervention. tem shown in Fig. 4 comprising several inter-
connected production operations of a steel
In the multilevel control hierarchy, the over- mill. In order to maximize his performance
all plant system is decomposed into subsystems, (and still meet delivery commitments), rolling
each with its own local controller. In this mill operator may call for slabs of different
scheme (see Fig. 3) : sizes and grades. However, the steel shop
scheduler wants to minimize the number of
(a) The first-level controllers compensate grade changes because of the increased likel:
for local effects of the disturbances, e.g. hood of off-standard product during transiti<
maintain local performance close to the opti- from one grade to another. Similarly, there
mum while ensuring that local constraints are is a significant set-up cost associated with
not violated. changing slab dimensions on the continuous
casting machine, hence, the slabbing depart-
(b) The second-level controller modifies the ment wants to minimize the frequency of slab
criteria and/or the constraints for the first- changes. An alternative is to provide more
level controllers in response to changing re- storage of slabs in the slab yard but this m
quirements on the system so that actions of increase slab yard costs. Thus, we have a r
the local controllers are consistent with the for a higher level production scheduler that
overall objectives of the system. reconciles the various conflicting local