Hierarchical Control System

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HIERARCHICAL CONTROL SYSTEM

J.

Pinker* and

J.

Bejvl**

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Abstract. The paper describes the hierarchical control system based on identical universal building blocks. Their universality and reliability are achieved by using microprocessors. Applications in control of complex technological processes and particularly in turbine control are considered. Keywords. Process control system; hierarchical system; controller; microprocessor; steam turbine; reliability. INTRODUCTION Automation equipnent is undergoing presently

a system. In some cases its redundancy may be required. The higher level, on the contrary, determines mainly the quality of automatic control and its economy. High flexibility at this level is desirable.

rapid development. At the same time criteria characterizing the quality of technological process control are subject to changes, reliability and economy being the most important features.

The hierarchical system described below is based on a unified build~ block - "universal controller" (henceforth abbreviated UC) - that was developed specially for this purpose utilizing microelectronic

Complex technological systems are composed of many subsystems operati~ more or less independently,being, however, coordinated so that the final result is optimal in some respect. This is in good correspondence with the control system composed of

circuits. It offers the following features: - High reliability, autodiagnostiCS, posSibility of parallel redundant Connection of UCs. - Flexible interconnection between UCs in higher and lower levels as

independent ( o r almost independent) c o ntrollers that form the lowest - "basic" - level in a hierarchical system. A higher-level controller is responsible for c oo peration of controllers in a lower level.

a consequence of "inteligence" of an on-board microprocessor. Information concerning the status of a UC is always readily available. - Easier design, development, repa-

The basic level of controllers, being very tightly connected to the technological process, strongly influences the overall reliability of

rations and trimming of an automa-

133

J. Pinker and J. Bejvl

134

tion system. The number of spare parts is held low. The control alg~ rithm is determined by program memory (EPROM) and can easily be changed in the field. - 80th digital and analog signals are processed by this UC. Analog circuitry in a technological process will not be completely replaced by an all-digital one in the near future.

and outputs can be used arbitrarily; additional signals are available to facilitate the cooperation of multiple UCs as described below. The board also contains an internal connector for an individual plug-in module with additional circuitry in case of special tasks. The UC can operate as an entirely independent unit (Fig.l.) in simple

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UNIVERSAL CONTROLLER

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Z-SO), two sockets for EPROM memories (2716 or 2732), C-MOS RAM memory with a back-up battery (256 byt~~ two programmable timers !counters/ S253/, a specially designed interrupt contr oller, one D/A and two A/D converters, two input and two output digital (S-bit) gates, diagnostical circuits and other parts all of them on a single board. Digital gates have sufficient output power (S212 type) to drive long lines. Inputs and outputs of timers and interrupt inputs can be interconnected by an on-board programming pin array t o enable performing diverse timing ana test functions, eve~ts -. counting, frequency and time measurement, frequency generation etc. Similarly, the mode of operation of I/O gates can be widely modified by the pin array, which contributes to the universality of UCs and their interconnections.

log and digital (S-bit) form; eight additional outputs are used to drive switches, signal lamps etc . . In more complex applications, all inputs

,TEST INPUTS

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Fig. 1.

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systems (less important signals are omitted for the sake of simplicity in this figure). If additional I/O gates, converters and other circuits are needed (which is considered a rare case), extension boards can be attached carrying auxiliary circuits addressable up to 32 K addresses. ~ig. 2. illustrates this. r--'- -- ---- '- -- -- '--- '- -- -- '- -- -- '- - , INT, TEST

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The number of input and output signals is low yet sufficient. In simple applications they are treated as an input variable, output variable and feedback variable, both in ana-

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The controller fully utilizes mod~ microelectronic parts. It contains an S-bit microprocessor (SOSO or

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To increase the reliability of UCs, internal diagnostic circuits and diagnostic programs are included. The diagnostic circuits are based on retriggerable monostables supervising periodicity and regularity of

Hierarchical Control System

135

application programs. The diagnostic Slave structure is probably the program is periodically started and best choice for this type of conis designed as a partial test of a trollers (see Fig. 4.). The units controller board, as the full-length in a lower level are independent and _._-- ---, test consumes too much time to be r - - - - - - - - ~ - ------- . - - --- . - . --- - - . - . practical. External boards (if any) also contain their own diagnostics. If any fault is detected in a UC, the 1 output "FAULT" is generated and all output gates are blocked (forced to the high-impedance status). A parallel redundant structure can be built to increase reliability in special 1_ _ cases (Fig. 3.). The faulty unit automatically disables itself and .------.

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enables the next good one. Two identical units with identical programs are quite sufficient for all practical cases. External boards can be parallelled in a similar manner. Neither input gates nor internal circuits are blocked and all the parallelled boards are therefore in the same status thus eliminating the severe transients arising when the boards are switched over. The faulty board can be removed and a new one inserted under fully operational status of the system. UNIVERSAL CONTROLLERS IN NEH"JORKS Several UCs can be grouped into networks in a hierarchical structure. Of many possibilities, the Master -

UCs in a hierarchical structure

each operates in several possible states, i.e. on one of several possible programs. A higher-level UC transmits the control word (or numerical data) accompanied by the request signal /RO/via the common command bus /"C"/ t o the lower-level UC. All UCs regularly test the request lines, the selected one accepts the control word, decodes it and starts the appropriate program. The beginning of a new program is signalled to the higher-level UC by transmitting the corresponding status word via the common status bus /"S"/. The acknowledge /ACK/signal synchronizes this transmission. Then the correct answer and timing are checked, which serves as powerful diagnostic mechanism. The selected control program is repeated periodically from this time on without further interference of the higher-level UC. The general structure of programs in a lower-level UC is illustrated in Fig. 5 . . The diagnostic program is

J. Pinker and J. Bejvl

136

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General structure of programs in UCs

permanently executed while the control program is entered in regular periods (usually tens of milliseconcS) due to interrupts from a programmable timer. Thus complete diagnostic information is available after a few control-program cycles have been executed. More detailed information about control programs is given in Fig. 6 .. The UCs of the lowest level are directly connected with the technological system while the higher-level UC selects appropriate algorithms for each lower-level UC and changes their mode of operation according to the present operating status of the whole technological system. At this level the UC processes information from several possible sources: important pOints of measurement, human operator, controllers at yet higher levels and status words from lower-level UCs. Of great importance

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Control programs on the basic level

is the fact that if any UC fails, the operation of neighbouring controller/s/ can be augmented to take over the most crucial activities of the failed one so that the corresponding technological subsystem is kept going even though non-optimally. All the controllers can be parallelled or extended (see Fig. 2. - 3.). To illustrate the principles mentioned above, a practical example of a steam-turbine control system will now be presented. PRACTICAL EXAMPLE A simplified example of a steam turbine control system of two levels (Fig. 7.) is in fact a lower-level part of a four-level hierarchical system in a fully automated power plant. Decentralized basic-level controllers performing as a speed controller (n), steam-pressure controller (P) and power controller (N) are designed as universal controllers. A higher-level controller go-

Hierarchical Control System

verns the basic UCs so that the whole system operates in one of four principal states (I - "generator disconnected, turbine start", 11 - "generator connected, high-pressure bypass station open", III - "generator HIC-HER LE~E L !!I

connected, HPBS closed", IV - "generator disconnected"); possible transitions are presented in Fig.7., marked by arrows. Conditions for these transitions are determined by evaluating data from the technolo-

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gical process, the control desk and superior control levels. Tab. 1.

Control programs in basic UCs are explained in Tab. 1 . . It should be

Mode of operation Constant parameter control (even t. adajJt ive con t roll

Zero output

States of UCs in turbine control

Algorithm selected by higher-level Input value UC for PI control 1 A Adjusted by operator 1 B Time dependent 1 C Dependent on other variable 2 A Adjusted by operator 2 B Dependent on other variable 2 C Constant,reached before 2C 2 D Zero 3

138

J. Pinker and J. Bejvl

emphasized that a contr o l algorithm in the usual sense (PI o r other) represents here only a minor part of the whole computation while most of the time and of the pr ogram memory are consumed by such functions as nonlinear corrections, limiting, comparation, transition-less switching after the change o f programs and many others. CONCLUSION The micropr o cessor-based universal controller as a unified bUilding block of a control system is a very promising alternative on account of its excellent reliability and flexibility. This is particularly true in a hierarchical system. The control system outlined in Fig. 7. has been utilized in a 500 MW turbine control system. The universal controller was designed as a hybrid one (analog circuits plus a microprogrammed controller) and at present is being replaced by a microprocessor UC board described above. It is supposed to be used in all future designs of Czechoslovak energy blocks. The necessary amount o f program memory is surprisingly small - around 3 K-bytes including diagnostics. The l ongest program does n o t take more than 10 milliseconds which is more than satisfactory for this purpose. The reliability aspect comes forth in a turbine speedometer that was developed using two parallelled UCs. The calculated mean time between failures is as long as 10 years.

REFERENCES 8oussin,J.L. Synthesis and analysis of large automation systems. IFAC, 1978, Helsinki, pp.15271535, Pergamon Press. Hermance,F. and Farwell,D. Singlelo o p pr oc ess controller adapts gain t o o perating le v el. Electronics, Nov. 1978, pp. 150 - 153. Hirayama,H. et al. Devel o pment o f Mitsubitshi digital electrohydraulic g ov ernor. Technical Revue, June 1982, Mitsubitshi heavy industries Ltd., pp.100108. King,K.L. A building bl ock approach t o ad vanced control techniques. Contr o l Engineering, Aug.1981, pp. 17-20. Williams,T.J. Hierarchical contr o l for large scale systems a survey. IFAC, 1978, Helsinki, pp. 1393-1406 , Pergamon Press.