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DECENTRALISED MICROPROCESSOR PROCESS-CONTROL SYSTEM
Copyright © IF AC Digital Computer Applications to Process Control, Vienna, Austria, 1985
DECENTRALISED MICROPROCESSOR PROCESS-CONTROL SYSTEM I. V. Prangishvili Institute of Control Sciences, Moscowy USSR
Abstract, Process-control systems today are designed in decentralized form with connections provided by local computer network. Most important characteristics of the control system as end product are flexible and prompt control of processes, high reliability, and effective applied software. The paper discusses the distinctive features of a distributed microprocessor process-control system whose design was vectored to provision of the abovementioned characteristics via high degree of control decentralization, use of small local networks in industrial applications, and modularity of its hard - and software. Keywords. Process control, microprocessors, local area networks, distributed control systems.
INTRODUCTION The modern distributed microprocessor process-control system should at any rate meet the following minimum of requirements : - to be economical, i.e. perform its functions under minimal labour, logistics and power supply expenditures; - be reliable and survivable, and provide the desired mean time between failures under sufficiently low additional cost; - to be flexible and readily adjustable to a particular process; - to be open in order to enable connection of new facilities and software to the system without structural readjustment; - to have convenient facilities for usersystem interaction, system design, and for system testing and debugging. The system described in this paper was designed with due regard for these requirements. Presently, the design is approaching its completion, and the system has the following major features: - Hardware interaction at all the hierarchical levels of the system is supported by small local area networks (CSLAN). Thus, all the system components are united on the basis of a general principle which simplifies system design and production. - Control of interactions and data exchange in the system is completely decentralized, has high-speed, and allows for dynamic process priorities. Past delivery of broadcast and block messages is guaranteeed which means that the source quickly receives acknowledgements from all the receivers at times independent of the number of receivers. Operating system DMTCS is completely decentralized as well. 581
Control decentralization brings about high reliability and survivability at all system levels, simplifies its dynamic reconfiguration at failures of individual devices. - The system has a universal set of program modules enabling monitoring and control of a wide class of processes. - Interactive man-computer generation of a system fitting to a particular process is performed by the control-man without programmer's participation. - There are advanced facilities for dialogue between the system and human operator, including the possibility of modifying system characteristics through dialogue with control-man. - The system has advanced diagnostic hardand software facilities improving its operate reliability. Below, consideration will be given to the system architecture, and to the peculiarities of its small local area computer network, distributed operating system, applied software and design. 2. SYSTEM ARCHITECTURE Pig. 1 shows a simplified block-diagram of the system. The basic system hardware includes process stations (PS), control room stations (CRS) and process interface stations (PIS) for process communication connected into a single control system by small local area computer networks (SLAN) managed by decentralized priority control. SLAN structure, access and protocols for lower levels are discussed in the next section. Process stations All the process stations are connected to
582
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SLAN and control particular processes and plants. They have no I/O devices which enables their maximal possible unification. Data are processed by 8- and 16-bit microprocessors, number of bits being defined by the requirements of the control plant. It should be noted that SLAN with decentralized priority control enables very fast and flexible connections between microprocessors. This simplifies the task of global dynamic system reconfiguration and allows one to provide high system reliability quite economically. Control room stations Hardware for active dialogue of the human operator with microprocessor control systems and the process is referred to as control room station. These include microprocessors for communication of human operators with process systems, and various facilities of the operator console. Control room stations should perform data I/O and display, authorized start and stop of individual programs (tasks), print out, control of periodic process operations, changing of regulator settings, output of mneumonic diagrams and schedules. Man-computer interface is supported at control room stations by simple keyboards and black-and-white or colour displays. Data are displayed on the basis of the sequential selection principle. A system of signalization, monitoring and control built on this principle simplifies operation of process attendants: review is possible of individual process parameters as well as of all the process or its fragment. Control room stations may have also dispatcher mimic panels and printers. The range of these stations enables their use with any kind of technological equipment at any level of control (unit, group of units, bay, plant, workshop) and of servicing both dispatcher and operator. Video monitors display process data and accompanying texts for human operator, and results of diagnostics of system elements as well. Process interface stations Process communication devices are completely decentralized and are located as close to the process as possible. In order to enhance reliability, one must try to reduce the amount of I/O devices connected to the process interface device. The situation where along with stations each measuring unit or actuator is SLAN user would be an ideal one. In this case, one may completely eliminate concentrators that unite groups of I/O devices and are most vulnerable points of the system. At such a complete decentralization, data from an individual measuring unit are input into the network and may be routed to a group of monitoring and control stations located over the system which offers considerable possibilities
for improvement of system reliability as a whole. 3.SMALL LOCAL AREA NETWORK OF THE SYSTEM The structure of SLAN designed for the system is shown in Pig. 2. User hardware is connected to the network through successvely connected network controller (NC) and repeater (R) that inputs signals from communication lines into network controller, outputs signals into the line and relay them. Neighbouring repeaters are connected by two lines coaxial cables, twisted-pair serial bus, optical cables - over which signals are transmitted in both directions in duplex mode at 4 Mbit/sec rate. As may be seen from the figure, the serial dataway consists of two-point connections only which improves network noise immunity and enables the use of different communication lines and signals in various segments of the network. Network controllers control interaction of users through the network by providing access to the dataway and hardwired support to the logic channel protocol. The dataway is accessed by means of decentralized spatial-temporal control which belongs to the class of decentralized priority control accesses developed in the 70*s at the Institute of Control Sciences for industrial networks (Prangishvili, Stetsyura, 1980; Prangishvili, Stetsyura, Podlazov, 1981). Having received a request for communication from a source user, the network controller waits for dataway release from current contact and begins competition with other network controllers for the dataway. As a result of competition, the dataway is occupied by a source with higher priority. Priority level is defined by the time of response to dataway release. At the same level, priority depends on the place where the source is connected to the dataway. Competition is won by a least-inertial network controller, and if there are several such controllers, it is the leftmost one that gets the dataway. At the i-th priority level dataway occupation time is 2 (i+1), where is time required for signal to pass through the dataway, i = 0, 1, ... In the designed SLAN, two-level priorities are used ensuring access to the dataway. As usually does not exceed several microseconds, access is performed rapidly. The existing LAN's are either sufficiently inertial, or deliver packets at guaranteed time. Priorities are dynamic, and source priority level may be promptly changed at its initiative. In the existing LAN's, dynamic priorities are not used. Logic channel control protocol defines the possibilities of resource sharing and process coordination in the distributed system, and provides means for generation of a distributed real-time operating system. As noted in the introduction, in order to ensure high effectiveness of the operating system the protocol should perform guaranteed delivery of data packets addressed to all the receivers or a group
Decentralised Microprocesor Process-control System
of receivers (broadcast and group packets). The protocol should also effectively perform such group transmissions as "multiple sources - one receiver" and "multiple sources - multiple receivers". These operations enable essential reduction of the transmitted control information and its fast processing by system facilities. The guaranteed delivery of group packets is performed by means of informational feedback (echo-checking) concurrently with packet transmission. Group transmissions from multiple sources are performed by means of common-channel computations (CGG) studied at the Institute of Gontrol Sciences in the 80's (Prangishvili, Stetsyura, Podlazov, 1981; Prangishvili, Podlazov, Stetsyura, 1984). In CCC, a packet sent into the dataway by one of the users is transformed in turns by other users as it moves along the dataway. Importantly, as the packet is not delayed by the users for processing, the GGG execution time practically does not gr0w with the number of SLAN users. GCG may significantly improve execution of a number of standard tasks of the distributed OS. During the time of packet transmission along the dataway, for example, one may indicate in the packet the total number of units of a free resource of certain type distributed among system users. The logic channel control protocol designed for SLAN has the following features: - packet delivery in the priority order; - detection of high-multiplicity packet errors; - minimization of the packet delivery time taking into account the acknowledgement wait time; - prompt checking of the correct delivery of group-addressing packets. Priority packet delivery is done by means of sending a request for reception of data packets and their reception according to priority-ordered request queues in the receivers (Fig. 3) which is equivalent to organizing a virtual connection for transmission of each data packet that cannot be received in one session with reception request. The protocol provides measures for minimization of the number of sessions required for packet delivery. Correctness of packet delivery is checked in the protocol by means of informational feedback (echo-checking) embracing all the data transmission path "source memory receiver memory" (Pig. 4) and thus enabling one to acknowledge packet reception in one session with its transmission. This reduces dramatically the total packet delivery time. Moreover, owing to the time shift between forward and back transmissions the echo-checking has high immunity to packet errors of any multiplicity. Finally, the echo-checking enables one to transmit a group packet and to receive acknowledgement of its correct delivery to all the receivers of the group (Pig. 5) in a single session. The possibilities offered by the duplex dataway are widely used for organization of the echo-checking. The protocol provides two types of group addressing of data packets, to all the
583
receivers of the group or to an arbitrary receiver. Transmission of group addressing packets is implemented as an indivisible operation over the common dataway which enable the operating system to perform fast resource sharing. Heavier demands are imposed on the SLAN reliability. Therefore, the network controller and repeater have means for diagnostics and switching off the faulty hardware. Most stringent demands are placed on reliability of the dataway because it should support network integrity at failure of one of its components. The following measures are provided to this end. If a repeater fails, it is disconnected from the dataway and a bypass is generated. If a cable line fails, standby lines are used. Repeaters have facilities for checking communication lines. If a failed line is detected, the faulty dataway segment is disconnected and standby lines are put into operation. As a result, a dataway is generated between the ends of the faulty segment. Dataway integrity is restored in a decentralized manner. 4. DISTRIBUTED REAL-TIME OPERATING SYSTEM The design of the architecture of distributed real-time operating system (DOS-RT) was primarily oriented to: - fast response of DOS-RT; - better fault-tolerance of DOS-RT; - simplification of connection to DMTCS of processors with different instruction sets; and - simplification of remote task execution. The DOS-RT uses extensively the abovementioned abilities of SLAN. The main feature which enabled complete decentralization of DOS-RT is its common memory consisting of identical copies distributed between SLAN users. At reading, each user reads data out of its own copy. Thus, reading may be done by all the users concurrently. At writing, the common memory content is updated by a broadcast message with fast guaranteed delivery as described above. The distributed common memory significantly simplifies system control, improves speed and fault-tolerance of DOS-RT. The common memory may store descriptors of global problems, common flags of events, descriptors of distributed resource state, etc. The common memory enables application in DOS-RT of the well known centralized control algorithms at preservation of system decentralization on the whole. Moreover, such a memory allows one to solve effectively a number of classical problems of the distributed control such as updating of multiple data copies, addressing to an object dynamically travelling over the system, multi-user protection, etc. In order to facilitate connection of different processors to the system and remote execution of tasks, a so called mutual distributed programming is used that organizes remote supervisor call (SVG-servicing) under hitherto unknown in advance number of simultaneously served processes. The use of this technique allows one to distribute expenses on
I. V. Prangishvili
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remote service among the served users. The mutual distributed programming is performed in the following manner. A remote user sends a service request through SLAN to where the desired task may be executed. If the task may be executed completely without interruption and if the remote user does not envision any requests for execution of such a task in the future, the traditional solution is used. Otherwise, the user executing the request re~. turns to the customer for storage a copy of the servicing process state. For the customer, it is only a set of data which it ought not to examine. When the customer again requires a remote servicing, it returns the stored copy, and the remote servicing commences. Since the servicing user does not need to store process copies, it may serve an unlimited number of requests which is very important for realtime systems. In addition, this scheme evidently enables interaction of processors of various types if there is a common language for formulation of service requests. 5. SYSTEM APPLIED SOFTWARE The described above architecture, operation of the local area microprocessor network and system software are rather universal and independent of the nature of industrial processes. Therefore, the system may be employed as a basic computing facility for monitoring and control of processes in any industry. The system applied software, however, is oriented to particular processes, to continuous and periodic chemical plants which are the core of chemical, oil-processing, food, mineral fertilizer, construction material, etc. enterpirses. Within this class, the software allows a sufficiently wide range of dynamic characteristics, of individual processes, parameter non-stationarity, model nonlinearity. The system applied software is oriented to generation of formalized monitoring and control functions for the above class of processes in various modes: normal operation, emergency states, start and stop, transition from one mode to another. It is envisioned that in all the modes some control functions should by done automatically by the system and the rest be performed according to the commands of human operator. The basic system functions are as follows: - data collection and preprocessing; - accounting and calculation of technicaland-economic indices; - programmed control; - logic control of mechanisms; - optimal control with respect to given criteria; - processing of input data for display to the operator; and - processing of operatorfs commands into control signals. All these functions are controlled by various versions of algorithms and programs differing in accuracy, execution time, memory space required. Along with standard algorithms used in systems of this kind such as exponential smoothing for filtration, PID regulation, etc., a series of new algorithms is used significantly ex-
tending systemfs possibilities and improving its operation quality. Of these algorithms one should mention versions of adaptive regulation with active and passive identification of control plant parameters. They can operate in two modes: single adjustment of regulator parametes, and continuous tracking of non-stationary process parameters. A sliding-mode regulation algorithm is next to them by its parameters. All these algorithms operate without human intervention within a wide range of process parameter variations and non-linearities which not only improves their effectiveness, but also simplifies and makes cheaper system operation in one industrial environment. The system may implement several versions of optimal control of static process operation according to given technical-and-economic criteria. These algorithms improve process indices by varying regulation algorithm settings. This is done either through forecasting process response to current disturbances, or through some version of search of the best mode by means of an adjustable process model. Algorithms for detection of variations in the propeties of signals and systems that are used for detection of disturbances and failures in the process, and in the measuring and actuating parts of the process system were significantly evolved. With addition of some logical algorithms, they enable a kind of automatic diagnostics of detected disturbances and failures, i.e. detection of their causes. 6. COMPUTER-AIDED DESIGN OP SYSTEM APPLIED SOFTWARE The system applied software for a particular application is designed by a special mini-computer system. Design agencies automating processes should be equipped with this system. The system is intended for operation in the man-computer dialogue mode and is designer's expert system aiding him to identify control plant and its distirbancies; to select the algorithmic structure of monitoring, control and communication with human operator; to determine the configuration of program allocation to system stations; to calculate rational values of algorithm parameters; to check all the applied software on a simulational model of the process and its disturbances; to generate programs for system stations. All these jobs are done without programmer's participation. To this end, the computer-aided design system has the following basic facilities: - designer-system dialogue in a language near to natural; - data base storing libraries of applied programs and information about the process ; - simulation means enabling simulational experiment with the aim of verification of various software versions on models of process and its disturbancies; and - documentation of all the stages of system design. The GAD system data base stores libraries from which the designer selects modules of system applied software and models for simulational experiment: - standard process models;
Decentralised Microprocesor Process-control System - standard disturbance generators; - identification algorithms; - standard monitoring and control modules; - algorithms for calculation of parameters of monitoring and control modules; - algorithms for synthesis of programmed logical control; - standard program means for data representation; and - algorithm for analysis of operational indices of monitoring and control loops. The use of GAD system enables the designer to use extensively standard algorithms, models and structures in the design of a particular applied software, to carry out multi-version design and verify it on a simulational model; to analyse at the design stage operational idices of the system for various number and composition of stations and various configurations of their applied software as well as for various versions of network reconfiguration at faults in individual system stations.
585
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Prangishvili, I.V., G. G. Stetsyura (1980). Microprocessor systems, Moscow, Nauka Publ. (in Russian) Prangishvili I. V., G.G. Stetsyura, V.S.Podlazov (1981). Decentralized control of processor interactions in concentrated and distributed multi-microprocessor systems.- Microprocessing and Microprogramming, No.J7, pp. 220-228. Prangishvili, I.V., v.S. Podlazov, G.G. Stetsyura (1984), Local area microprocessor computer networks, Moscow, Nauka Publ. (in Russian).
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CONCLUSION The design of the described above system relies upon original results obtained in the USSR in such areas as fault-tolerant distributed control systems built around small local area microprocessor networks with decentralized priority control, distributed system program software, applied software including, in particular, original algorithms for diagnostics of processes, measuring loops and control systems, adaptive regulation systems, sliding-mode regulation systems, and for optimization of installations in terms of technicaland-economic indices.
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DECENTRALISED MICROPROCESSOR PROCESS-CONTROL SYSTEM I. V. Prangishvili Institute of Control Sciences, Moscowy USSR
Abstract, Process-control systems today are designed in decentralized form with connections provided by local computer network. Most important characteristics of the control system as end product are flexible and prompt control of processes, high reliability, and effective applied software. The paper discusses the distinctive features of a distributed microprocessor process-control system whose design was vectored to provision of the abovementioned characteristics via high degree of control decentralization, use of small local networks in industrial applications, and modularity of its hard - and software. Keywords. Process control, microprocessors, local area networks, distributed control systems.
INTRODUCTION The modern distributed microprocessor process-control system should at any rate meet the following minimum of requirements : - to be economical, i.e. perform its functions under minimal labour, logistics and power supply expenditures; - be reliable and survivable, and provide the desired mean time between failures under sufficiently low additional cost; - to be flexible and readily adjustable to a particular process; - to be open in order to enable connection of new facilities and software to the system without structural readjustment; - to have convenient facilities for usersystem interaction, system design, and for system testing and debugging. The system described in this paper was designed with due regard for these requirements. Presently, the design is approaching its completion, and the system has the following major features: - Hardware interaction at all the hierarchical levels of the system is supported by small local area networks (CSLAN). Thus, all the system components are united on the basis of a general principle which simplifies system design and production. - Control of interactions and data exchange in the system is completely decentralized, has high-speed, and allows for dynamic process priorities. Past delivery of broadcast and block messages is guaranteeed which means that the source quickly receives acknowledgements from all the receivers at times independent of the number of receivers. Operating system DMTCS is completely decentralized as well. 581
Control decentralization brings about high reliability and survivability at all system levels, simplifies its dynamic reconfiguration at failures of individual devices. - The system has a universal set of program modules enabling monitoring and control of a wide class of processes. - Interactive man-computer generation of a system fitting to a particular process is performed by the control-man without programmer's participation. - There are advanced facilities for dialogue between the system and human operator, including the possibility of modifying system characteristics through dialogue with control-man. - The system has advanced diagnostic hardand software facilities improving its operate reliability. Below, consideration will be given to the system architecture, and to the peculiarities of its small local area computer network, distributed operating system, applied software and design. 2. SYSTEM ARCHITECTURE Pig. 1 shows a simplified block-diagram of the system. The basic system hardware includes process stations (PS), control room stations (CRS) and process interface stations (PIS) for process communication connected into a single control system by small local area computer networks (SLAN) managed by decentralized priority control. SLAN structure, access and protocols for lower levels are discussed in the next section. Process stations All the process stations are connected to
582
I. V. Prangishvili
SLAN and control particular processes and plants. They have no I/O devices which enables their maximal possible unification. Data are processed by 8- and 16-bit microprocessors, number of bits being defined by the requirements of the control plant. It should be noted that SLAN with decentralized priority control enables very fast and flexible connections between microprocessors. This simplifies the task of global dynamic system reconfiguration and allows one to provide high system reliability quite economically. Control room stations Hardware for active dialogue of the human operator with microprocessor control systems and the process is referred to as control room station. These include microprocessors for communication of human operators with process systems, and various facilities of the operator console. Control room stations should perform data I/O and display, authorized start and stop of individual programs (tasks), print out, control of periodic process operations, changing of regulator settings, output of mneumonic diagrams and schedules. Man-computer interface is supported at control room stations by simple keyboards and black-and-white or colour displays. Data are displayed on the basis of the sequential selection principle. A system of signalization, monitoring and control built on this principle simplifies operation of process attendants: review is possible of individual process parameters as well as of all the process or its fragment. Control room stations may have also dispatcher mimic panels and printers. The range of these stations enables their use with any kind of technological equipment at any level of control (unit, group of units, bay, plant, workshop) and of servicing both dispatcher and operator. Video monitors display process data and accompanying texts for human operator, and results of diagnostics of system elements as well. Process interface stations Process communication devices are completely decentralized and are located as close to the process as possible. In order to enhance reliability, one must try to reduce the amount of I/O devices connected to the process interface device. The situation where along with stations each measuring unit or actuator is SLAN user would be an ideal one. In this case, one may completely eliminate concentrators that unite groups of I/O devices and are most vulnerable points of the system. At such a complete decentralization, data from an individual measuring unit are input into the network and may be routed to a group of monitoring and control stations located over the system which offers considerable possibilities
for improvement of system reliability as a whole. 3.SMALL LOCAL AREA NETWORK OF THE SYSTEM The structure of SLAN designed for the system is shown in Pig. 2. User hardware is connected to the network through successvely connected network controller (NC) and repeater (R) that inputs signals from communication lines into network controller, outputs signals into the line and relay them. Neighbouring repeaters are connected by two lines coaxial cables, twisted-pair serial bus, optical cables - over which signals are transmitted in both directions in duplex mode at 4 Mbit/sec rate. As may be seen from the figure, the serial dataway consists of two-point connections only which improves network noise immunity and enables the use of different communication lines and signals in various segments of the network. Network controllers control interaction of users through the network by providing access to the dataway and hardwired support to the logic channel protocol. The dataway is accessed by means of decentralized spatial-temporal control which belongs to the class of decentralized priority control accesses developed in the 70*s at the Institute of Control Sciences for industrial networks (Prangishvili, Stetsyura, 1980; Prangishvili, Stetsyura, Podlazov, 1981). Having received a request for communication from a source user, the network controller waits for dataway release from current contact and begins competition with other network controllers for the dataway. As a result of competition, the dataway is occupied by a source with higher priority. Priority level is defined by the time of response to dataway release. At the same level, priority depends on the place where the source is connected to the dataway. Competition is won by a least-inertial network controller, and if there are several such controllers, it is the leftmost one that gets the dataway. At the i-th priority level dataway occupation time is 2 (i+1), where is time required for signal to pass through the dataway, i = 0, 1, ... In the designed SLAN, two-level priorities are used ensuring access to the dataway. As usually does not exceed several microseconds, access is performed rapidly. The existing LAN's are either sufficiently inertial, or deliver packets at guaranteed time. Priorities are dynamic, and source priority level may be promptly changed at its initiative. In the existing LAN's, dynamic priorities are not used. Logic channel control protocol defines the possibilities of resource sharing and process coordination in the distributed system, and provides means for generation of a distributed real-time operating system. As noted in the introduction, in order to ensure high effectiveness of the operating system the protocol should perform guaranteed delivery of data packets addressed to all the receivers or a group
Decentralised Microprocesor Process-control System
of receivers (broadcast and group packets). The protocol should also effectively perform such group transmissions as "multiple sources - one receiver" and "multiple sources - multiple receivers". These operations enable essential reduction of the transmitted control information and its fast processing by system facilities. The guaranteed delivery of group packets is performed by means of informational feedback (echo-checking) concurrently with packet transmission. Group transmissions from multiple sources are performed by means of common-channel computations (CGG) studied at the Institute of Gontrol Sciences in the 80's (Prangishvili, Stetsyura, Podlazov, 1981; Prangishvili, Podlazov, Stetsyura, 1984). In CCC, a packet sent into the dataway by one of the users is transformed in turns by other users as it moves along the dataway. Importantly, as the packet is not delayed by the users for processing, the GGG execution time practically does not gr0w with the number of SLAN users. GCG may significantly improve execution of a number of standard tasks of the distributed OS. During the time of packet transmission along the dataway, for example, one may indicate in the packet the total number of units of a free resource of certain type distributed among system users. The logic channel control protocol designed for SLAN has the following features: - packet delivery in the priority order; - detection of high-multiplicity packet errors; - minimization of the packet delivery time taking into account the acknowledgement wait time; - prompt checking of the correct delivery of group-addressing packets. Priority packet delivery is done by means of sending a request for reception of data packets and their reception according to priority-ordered request queues in the receivers (Fig. 3) which is equivalent to organizing a virtual connection for transmission of each data packet that cannot be received in one session with reception request. The protocol provides measures for minimization of the number of sessions required for packet delivery. Correctness of packet delivery is checked in the protocol by means of informational feedback (echo-checking) embracing all the data transmission path "source memory receiver memory" (Pig. 4) and thus enabling one to acknowledge packet reception in one session with its transmission. This reduces dramatically the total packet delivery time. Moreover, owing to the time shift between forward and back transmissions the echo-checking has high immunity to packet errors of any multiplicity. Finally, the echo-checking enables one to transmit a group packet and to receive acknowledgement of its correct delivery to all the receivers of the group (Pig. 5) in a single session. The possibilities offered by the duplex dataway are widely used for organization of the echo-checking. The protocol provides two types of group addressing of data packets, to all the
583
receivers of the group or to an arbitrary receiver. Transmission of group addressing packets is implemented as an indivisible operation over the common dataway which enable the operating system to perform fast resource sharing. Heavier demands are imposed on the SLAN reliability. Therefore, the network controller and repeater have means for diagnostics and switching off the faulty hardware. Most stringent demands are placed on reliability of the dataway because it should support network integrity at failure of one of its components. The following measures are provided to this end. If a repeater fails, it is disconnected from the dataway and a bypass is generated. If a cable line fails, standby lines are used. Repeaters have facilities for checking communication lines. If a failed line is detected, the faulty dataway segment is disconnected and standby lines are put into operation. As a result, a dataway is generated between the ends of the faulty segment. Dataway integrity is restored in a decentralized manner. 4. DISTRIBUTED REAL-TIME OPERATING SYSTEM The design of the architecture of distributed real-time operating system (DOS-RT) was primarily oriented to: - fast response of DOS-RT; - better fault-tolerance of DOS-RT; - simplification of connection to DMTCS of processors with different instruction sets; and - simplification of remote task execution. The DOS-RT uses extensively the abovementioned abilities of SLAN. The main feature which enabled complete decentralization of DOS-RT is its common memory consisting of identical copies distributed between SLAN users. At reading, each user reads data out of its own copy. Thus, reading may be done by all the users concurrently. At writing, the common memory content is updated by a broadcast message with fast guaranteed delivery as described above. The distributed common memory significantly simplifies system control, improves speed and fault-tolerance of DOS-RT. The common memory may store descriptors of global problems, common flags of events, descriptors of distributed resource state, etc. The common memory enables application in DOS-RT of the well known centralized control algorithms at preservation of system decentralization on the whole. Moreover, such a memory allows one to solve effectively a number of classical problems of the distributed control such as updating of multiple data copies, addressing to an object dynamically travelling over the system, multi-user protection, etc. In order to facilitate connection of different processors to the system and remote execution of tasks, a so called mutual distributed programming is used that organizes remote supervisor call (SVG-servicing) under hitherto unknown in advance number of simultaneously served processes. The use of this technique allows one to distribute expenses on
I. V. Prangishvili
584
remote service among the served users. The mutual distributed programming is performed in the following manner. A remote user sends a service request through SLAN to where the desired task may be executed. If the task may be executed completely without interruption and if the remote user does not envision any requests for execution of such a task in the future, the traditional solution is used. Otherwise, the user executing the request re~. turns to the customer for storage a copy of the servicing process state. For the customer, it is only a set of data which it ought not to examine. When the customer again requires a remote servicing, it returns the stored copy, and the remote servicing commences. Since the servicing user does not need to store process copies, it may serve an unlimited number of requests which is very important for realtime systems. In addition, this scheme evidently enables interaction of processors of various types if there is a common language for formulation of service requests. 5. SYSTEM APPLIED SOFTWARE The described above architecture, operation of the local area microprocessor network and system software are rather universal and independent of the nature of industrial processes. Therefore, the system may be employed as a basic computing facility for monitoring and control of processes in any industry. The system applied software, however, is oriented to particular processes, to continuous and periodic chemical plants which are the core of chemical, oil-processing, food, mineral fertilizer, construction material, etc. enterpirses. Within this class, the software allows a sufficiently wide range of dynamic characteristics, of individual processes, parameter non-stationarity, model nonlinearity. The system applied software is oriented to generation of formalized monitoring and control functions for the above class of processes in various modes: normal operation, emergency states, start and stop, transition from one mode to another. It is envisioned that in all the modes some control functions should by done automatically by the system and the rest be performed according to the commands of human operator. The basic system functions are as follows: - data collection and preprocessing; - accounting and calculation of technicaland-economic indices; - programmed control; - logic control of mechanisms; - optimal control with respect to given criteria; - processing of input data for display to the operator; and - processing of operatorfs commands into control signals. All these functions are controlled by various versions of algorithms and programs differing in accuracy, execution time, memory space required. Along with standard algorithms used in systems of this kind such as exponential smoothing for filtration, PID regulation, etc., a series of new algorithms is used significantly ex-
tending systemfs possibilities and improving its operation quality. Of these algorithms one should mention versions of adaptive regulation with active and passive identification of control plant parameters. They can operate in two modes: single adjustment of regulator parametes, and continuous tracking of non-stationary process parameters. A sliding-mode regulation algorithm is next to them by its parameters. All these algorithms operate without human intervention within a wide range of process parameter variations and non-linearities which not only improves their effectiveness, but also simplifies and makes cheaper system operation in one industrial environment. The system may implement several versions of optimal control of static process operation according to given technical-and-economic criteria. These algorithms improve process indices by varying regulation algorithm settings. This is done either through forecasting process response to current disturbances, or through some version of search of the best mode by means of an adjustable process model. Algorithms for detection of variations in the propeties of signals and systems that are used for detection of disturbances and failures in the process, and in the measuring and actuating parts of the process system were significantly evolved. With addition of some logical algorithms, they enable a kind of automatic diagnostics of detected disturbances and failures, i.e. detection of their causes. 6. COMPUTER-AIDED DESIGN OP SYSTEM APPLIED SOFTWARE The system applied software for a particular application is designed by a special mini-computer system. Design agencies automating processes should be equipped with this system. The system is intended for operation in the man-computer dialogue mode and is designer's expert system aiding him to identify control plant and its distirbancies; to select the algorithmic structure of monitoring, control and communication with human operator; to determine the configuration of program allocation to system stations; to calculate rational values of algorithm parameters; to check all the applied software on a simulational model of the process and its disturbances; to generate programs for system stations. All these jobs are done without programmer's participation. To this end, the computer-aided design system has the following basic facilities: - designer-system dialogue in a language near to natural; - data base storing libraries of applied programs and information about the process ; - simulation means enabling simulational experiment with the aim of verification of various software versions on models of process and its disturbancies; and - documentation of all the stages of system design. The GAD system data base stores libraries from which the designer selects modules of system applied software and models for simulational experiment: - standard process models;
Decentralised Microprocesor Process-control System - standard disturbance generators; - identification algorithms; - standard monitoring and control modules; - algorithms for calculation of parameters of monitoring and control modules; - algorithms for synthesis of programmed logical control; - standard program means for data representation; and - algorithm for analysis of operational indices of monitoring and control loops. The use of GAD system enables the designer to use extensively standard algorithms, models and structures in the design of a particular applied software, to carry out multi-version design and verify it on a simulational model; to analyse at the design stage operational idices of the system for various number and composition of stations and various configurations of their applied software as well as for various versions of network reconfiguration at faults in individual system stations.
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Prangishvili, I.V., G. G. Stetsyura (1980). Microprocessor systems, Moscow, Nauka Publ. (in Russian) Prangishvili I. V., G.G. Stetsyura, V.S.Podlazov (1981). Decentralized control of processor interactions in concentrated and distributed multi-microprocessor systems.- Microprocessing and Microprogramming, No.J7, pp. 220-228. Prangishvili, I.V., v.S. Podlazov, G.G. Stetsyura (1984), Local area microprocessor computer networks, Moscow, Nauka Publ. (in Russian).
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CONCLUSION The design of the described above system relies upon original results obtained in the USSR in such areas as fault-tolerant distributed control systems built around small local area microprocessor networks with decentralized priority control, distributed system program software, applied software including, in particular, original algorithms for diagnostics of processes, measuring loops and control systems, adaptive regulation systems, sliding-mode regulation systems, and for optimization of installations in terms of technicaland-economic indices.
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