Showing papers on "Supervisory control published in 1998"

Supervisory Control of Discrete Event Systems Using Petri Nets

Abstract: From the Publisher: Supervisory Control of Discrete Event Systems Using Petri Nets presents a novel approach for the supervisory control of discrete event systems using Petri nets. The concepts of supervisory control and discrete event systems are explained, and the background material on general Petri net theory necessary for using the book's control techniques is provided. A large number of examples are used to illustrate the concepts and techniques presented in the text, and there are plenty of references for those interested in additional study or more information on a particular topic. Supervisory Control of Discrete Event Systems Using Petri Nets is intended for graduate students, advanced undergraduates, and practicing engineers who are interested in the control problems of manufacturing, communication and computer networks, chemical process plants, and other high level control applications.

Active diagnosis of discrete-event systems

Abstract: The need for accurate and timely diagnosis of system failures and the advantages of automated diagnostic systems are well appreciated. However, diagnosability considerations are often not explicitly taken into account in the system design. In particular, design of the controller and that of the diagnostic subsystem are decoupled, and this may significantly affect the diagnosability properties of a system. The authors present an integrated approach to control and diagnosis. More specifically, they present an approach for the design of diagnosable systems by appropriate design of the system controller. This problem, which they refer to as the active diagnosis problem, is studied in the framework of discrete-event systems (DESs); it is based on prior and new results on the theory of diagnosis for DESs and on existing results in supervisory control under partial observations. They formulate the active diagnosis problem as a supervisory control problem where the legal language is an "appropriate" regular sublanguage of the regular language generated by the system. They present an iterative procedure for determining the supremal controllable, observable, and diagnosable sublanguage of the legal language and for obtaining the supervisor that synthesizes this language. This procedure provides both a controller that ensures diagnosability of the closed-loop system and a diagnoser for online failure diagnosis. The procedure can be implemented using finite-state machines and is guaranteed to converge in a finite number of iterations. The authors illustrate their approach using a simple pump-valve system.

PLC-based implementation of supervisory control for discrete event systems

Abstract: The supervisory control theory is a general theory for automatic synthesis of controllers (supervisors) for discrete event systems, given a plant model and a specification for the controlled behavior. Though the theory has for over a decade received substantial attention in academics, still very few industrial applications exist. The main reason for this seems to be a discrepancy between the abstract supervisor and its physical implementation. This is specifically noticeable when the implementation is supposed to be based on programmable logic controllers (PLCs), as is the case with many manufacturing systems. The asynchronous event-driven nature of the supervisor is not straightforwardly implemented in the synchronous signal-based PLC. We point out the main problems of supervisor implementation on a PLC, and suggest procedures to alleviate the problems.

Discrete approximation and supervisory control of continuous systems

Abstract: Addresses the following hybrid control problem: a continuous plant (its state evolving in Euclidean space) is to be controlled via symbolic output feedback-both measurement and control signal "live" on finite sets of symbols. We adopt the following approach: the hybrid problem is first translated into a purely discrete problem by approximating the continuous plant model by a (nondeterministic) finite-state machine. By taking into account past measurement and control symbols, approximation accuracy can be improved and adjusted to the specification requirements. Supervisory control theory for discrete-event systems (DES) is then applied to find the optimal controller which enforces the specifications. As the behavior of the approximating automaton is guaranteed to contain the behavior of the underlying continuous plant model, the controller also forces the latter to obey the specifications.

Systems and methods for implementing a dynamic cache in a supervisory control system

Abstract: The present invention provides systems and methods for controlling processes of a process facility and, in particular, for distributing data among nodes of a real time process control system controlling such a facility. An exemplary process control system includes a plurality of sensors, controllable devices, and communication paths, and a computer system. The sensors and controllable devices are associated with various ones of the processes of the process facility, and the communication paths couple the sensors and controllable devices to the computer system. The computer system operates on data related to the process facility, and distributes the data among both client and process nodes. The client nodes desire data associated with processes of the facility and the process nodes control such process data. The computer system includes (i) a cache having a short access time for retrieving records, and (ii) a supervisory controller, associated with the client and process nodes, that controls storage of the process data in the cache and selectively transfers the process data from the cache to the client nodes.

Synthesis of supervisory controllers for hybrid systems based on approximating automata

Abstract: The paper concerns the synthesis of supervisory controllers for a class of continuous-time hybrid systems with discrete-valued input signals that select differential inclusions for continuous-valued state trajectories and event-valued output signals generated by threshold crossings in the continuous state space, the supervisor is allowed to switch the input signal value when threshold events are observed. The objective is to synthesize a nonblocking supervisor such that the set of possible sequences of control and threshold event pairs for the closed-loop system lies between given upper and lower bounds in the sense of set containment. We show how this problem can be converted into a supervisor synthesis problem for a standard controlled discrete-event system (DES). A finite representation may not exist for the exact DES model of the hybrid system, however. To circumvent this difficulty, we present an algorithm for constructing finite-state Muller automata that accept outer approximations to the exact controlled threshold-event language, and we demonstrate that supervisors that solve the synthesis problem for the approximating automata achieve the control specifications when applied to the original hybrid system.

Modular Control and Coordination of Discrete-Event Systems

Abstract: In the supervisory control of discrete-event systems based on controllable languages, a standard way to handle state explosion in large systems is by modular supervision: either horizontal (decentralized) or vertical (hierarchical). However, unless all the relevant languages are prefix-closed, a well-known potential hazard with modularity is that of conflict. In decentralized control, modular supervisors that are individually nonblocking for the plant may nevertheless produce blocking, or even deadlock, when operating on-line concurrently. Similarly, a high-level hierarchical supervisor that predicts nonblocking at its aggregated level of abstraction may inadvertently admit blocking in a low-level implementation. In two previous papers, the authors showed that nonblocking hierarchical control can be guaranteed provided high-level aggregation is sufficiently fine; the appropriate conditions were formalized in terms of ’control structures‘ and ’observers‘. In this paper we apply the same technique to decentralized control, when specifications are imposed on ’local‘ models of the ’global‘ process; in this way we remove the restriction in some earlier work that the plant and specification (marked) languages be prefix-closed. We then solve a more general problem of ’coordination‘: namely how to determine a high level ’coordinator‘ that forestalls conflict in a decentralized architecture when it potentially arises, but is otherwise minimally ’intrusive‘ on low-level control action. Coordination thus combines both vertical and horizontal modularity. The example of a simple production process is provided as a practical illustration. We conclude with an appraisal of the computational effort involved.

Controller switching based on output prediction errors

Abstract: We consider a switched nonlinear feedback control strategy for controlling a plant with unknown parameters so that the output asymptotically tracks a reference signal. The controller is selected online from a given set of controllers according to a switching rule based on output prediction errors. For control problems requiring asymptotic tracking of a reference input we provide sufficient conditions under which the switched closed-loop control system is exponentially stable and asymptotically achieves good control even if the switching does not stop, Our results are illustrated with three examples.

The challenge of modelling supervisory systems using fuzzy cognitive maps

Abstract: This paper examines fuzzy cognitive map (FCM) theory and its use in supervisory control systems. An FCM is a graph used to depict cause and effect between concepts that stand for the states and variables of the system. An FCM represents the whole system in a symbolic manner, just as humans have stored the operation of the system in their brains, thus it is possible to help man's intention for more intelligent and autonomous systems. FCM representation, construction and a mathematical model are examined; a generic system is proposed and the implementation of FCM in a process control problem is illustrated and a model for supervisors of manufacturing systems is discussed. Although an FCM seems to be a simple model of system behaviour, it appears to be a powerful and effective tool describing the behaviour of a system and representing the accumulated knowledge of a system.

Differential Petri nets: representing continuous systems in a discrete-event world

Abstract: Differential Petri nets are a new extension of Petri nets. Through the introduction of the differential place, the differential transition, and suitable evolution rules, it is possible to model concurrently discrete-event processes and continuous-time dynamic processes, represented by systems of linear ordinary differential equations. This model can contribute to the performance analysis and design of industrial supervisory control systems and of hybrid control systems in general.

Discrete-event control of nondeterministic systems

Abstract: Nondeterminism in discrete-event systems occurs in many practical situations and often as a result of partial observability of events. For the adequate description of nondeterministic systems and nondeterministic phenomena, the trajectory-model formalism was introduced by Heymann (1991) and Heymann et al. (1991). This formalism has been used in by Shayman et al. (1995) for obtaining various results on supervisory control of nondeterministic systems subject to language specifications. In the present paper we develop a theory of supervisory control for nondeterministic discrete-event systems subject to both language and trajectory-model specifications. We further show how well-known algorithms for supervisory control (of deterministic systems) under partial observation can be adapted for synthesis of supervisors for nondeterministic systems subject to both language and trajectory-model specifications.

Bisimulation, the Supervisory Control Problem and StrongModel Matching for Finite State Machines

Abstract: A fundamental relationship between the controllability of a language with respect to another language and a set of uncontrollable events in the Supervisory Control Theory initiated by (Ramadge and Wonham, 1989) and bisimulation of automata models is derived. The theoretical results relating bisimulation to controllability support an efficient solution to the Basic Supervisory Control Problem. Using (Fernandez, 1990) generalization of the partition refinement algorithm of (Paige and Tarjan, 1987), it is possible to find a partition which represents the supremal controllable sublanguage of an automaton with respect to the language of another automaton and a set of events in a worst-case running time of O( m\log(n)), where m is the number of transitions and n is the number of states. Utilizing the bisimulation property of language controllability and derived relationships between automata languages and input/output finite-state machine behaviors, a precise relationship is formally derived between Supervisory Control Theory and the system-theoretic problem posed by (DiBenedetto et al., 1994) called Strong Input/Output FSM Model Matching. Specifically, it is proven that in deterministic settings instances of each problem can be mapped to the other framework and solved.

Canal control algorithms currently in use

Abstract: Many canal control methods and algorithms have been developed, but only some of them are being used on operating canal projects. As a part of the ASCE task committee on canal automation algorithms, this paper discusses field application of automatic control algorithms. Based on available data, information is presented on the implementation of canal algorithms. These algorithms are categorized as implicit algorithms in self-regulating gates, local automatic feedback controllers, and supervisory control algorithms. For each algorithm, brief information is provided on water projects that are using the algorithm, the type of application, implementation history, and algorithm performance.

Monitoring and control of semiconductor manufacturing processes

Abstract: Concerns optical measurement techniques for semiconductor manufacturing process monitoring and control. They can provide previously impossible real-time monitoring of several process variables, in-situ or ex-situ. This further enables the applications of more sophisticated real-time control algorithms, other than SPC. The SPC-based run-to-run (RtR) control, on the other hand, is still an instrumental part of the control algorithm, whenever there is a lack of real-time sensors for measuring critical process metrics. An emerging practice is to integrate RtR with the real-time control design to provide a comprehensive control design algorithm for semiconductor manufacture. The continued trend of semiconductor industry is toward an bigger wafers and smaller devices. This requires more integrated supervisory control, able to provide even tighter control, especially for photolithography, CVD, and etch processes. A unified framework needs to be established, at least for these critical processes, to facilitate and expedite systematic control design and development. In addition, as more sophisticated algorithms are being implemented, the control-related software and hardware should be user-friendly so that it can be operated by nonexpert personnel. Finally, since chip manufacturing consists of various processes, a comprehensive control algorithm on a factory-wide basis should utilize information (process-state, wafer-state, and tool-state data) from the current process as well as upstream and downstream processes.

Synthesis of fault-tolerant supervisor for automated manufacturing systems: a case study on photolithographic process

Abstract: A discrete event dynamic system (DEDS) approach is utilized to improve the reliability of a system from the fault-tolerance viewpoint. We propose a systematic way to classify faults and failures quantitatively and to find tolerable fault event sequences embedded in DEDSs. After this, the synthesis of a fault-tolerant supervisory control system is investigated. A case study of a photolithographic process in a semiconductor manufacturing system is provided to illustrate these techniques.

Timed Petri Nets in Hybrid Systems: Stability and SupervisoryControl

Abstract: In this paper, timed Petri nets are used to model and control hybrid systems. Petri nets are used instead of finite automata primarily because of the advantages they offer in dealing with concurrency and complexity issues. A brief overview of existing results on hybrid systems that are based on Petri nets is first presented. A class of timed Petri nets named programmable timed Petri nets (PTPN) is then used to model hybrid systems. Using the PTPN, the stability and supervisory control of hybrid systems are addressed and efficient algorithms are introduced. In particular, we present sufficient conditions for the uniform ultimate boundness of hybrid systems composed of multiple linear time invariant plants which are switched between using a logical rule described by a Petri net. This paper also examines the supervisory control of a hybrid system in which the continuous state is transfered to a region of the state space in a way that respects safety specifications on the plant‘s discrete and continuous dynamics.

Haptic interface for virtual reality based minimally invasive surgery simulation

Abstract: This paper deals with the mechanical design and control of a novel 4-DOF force-reflecting manual man-machine interface for virtual reality based minimally invasive surgery simulation for training purposes. The realized force feedback manipulator is based on a novel spherical remote-center-of-motion mechanical structure, and features compactness, high sustained output force capability, low friction, zero backlash, and a particularly large, singularity-free workspace. An implicit force control scheme is employed for virtual object impedance simulations. Active compensation of manipulator gravity and actuator friction allows one to simulate static and dynamic object attributes including weight, elasticity, and viscosity over a wide dynamic range of approximately 1:100. Finally, a novel 'supervisory control' concept is proposed which allows one to connect the haptic feedback system to the corresponding graphical simulation environment with highly differing bandwidth requirements.

A method for the synthesis of controllers to handle safety, liveness, and real-time constraints

Abstract: Describes a synthesis method that automatically derives controllers for timed discrete-event systems with nonterminating behavior modeled by timed transition graphs and specifications of control requirements expressed by metric temporal logic (MTL) formulas. Synthesis is performed by using: 1) a forward-chaining search that evaluates the satisfiability of MTL formulas over sequences of states generated by occurrences of actions and 2) a control-directed backtracking technique that takes into consideration the controllability of actions. This method has several interesting features. First, the issues of controllability, safety, liveness, and real time are integrated in a single framework. Second, the synthesis process does not require explicit storage of an entire transition structure over which formulas are checked and can be stopped at any moment, giving an approximate but useful result. Third, search and control mechanisms allow circumvention of the state explosion problem.

Petri-net-based supervisory control of discrete event systems and their ladder logic diagram implementations

Abstract: The last decade has witnessed rapid developments in computer technology, which in return, has found widespread applications in manufacturing systems, communication networks, robots etc. Such systems are called Discrete Event Systems (DESs), in which properties such as non-determinism, conflict and parallelism are exhibited. As DESs become more complex, the need for an effective design tool and its implementation becomes more important. Supervisory control theory, based on finite state machines (FSM) and formal languages, is a well established framework for the study of DESs. In supervisory control, given a model of the system and the desired system behaviour specifications, the objective is to find a supervisor (controller) such that the controlled behaviour of the system does not contradict the specifications given and does not unnecessarily constrain the behaviour of the system. In general, the classes of specifications that have been considered within the supervisory control fall into two categories: the forbidden state problem, in which the control specifications are expressed as forbidden conditions that must be avoided, and the desired string problem, in which the control specifications are expressed as sequence of activities that must be provided. In supervisory control, there are some problems when using FSMs as an underlying modelling tool. Firstly, the number of states grows exponentially as the system becomes bigger. Secondly, FMSs lack from graphical visivalisation. To overcome these problems Petri nets have been considered as an alternative modelling tool for the analysis, design and implementation of such DESs, because of their easily understood graphical representation in addition to their well formed mathematical formalism. The thesis investigates the use of Petri nets in supervisory control. Both the forbidden state problem and the desired string problem are solved. In other words, this work presents systematic approaches to the synthesis of Petri-nets-based supervisors (controllers) for both the forbidden state problem and the desired string problem and introduces the details of supervisory design procedures. The supervisors obtained are the form of a net structure as oppose to supervisors given as a feedback fiinction. This means that a controlled model of the system can be constructed and analysed using the techniques regarding to Petri net models. In particular the thesis considers discrete manufacturing systems. The results obtained can be applied to high level control of manufacturing systems, where the role of the supervisor is to coordinate the control of machines, robots, etc. and to low-level control of manufacturing systems, where the role of the supervisor is to arrange low-level interactions between the control devices, such as motors, actuators, etc. An approach to the conversion from the supervisors to ladder logic diagrams (LLDs) for implementation on a programmable logic controller (PLC) is proposed. A discrete manufacturing system example is then considered. The aim of this is to illustrate the applicability, strengths and drawbacks of the design techniques proposed.

Automatic synthesis of controllers from formal specifications

Abstract: Many safety critical reactive systems are indeed embedded control systems. Usually a control system can be partitioned into two main subsystems: a controller and a plant. Roughly speaking: the controller observes the state of the plant and sends commands (stimulus) to the plant to achieve predefined goals. We show that when the plant can be modeled as a deterministic finite state system (FSS) it is possible to effectively use formal methods to automatically synthesize the program implementing the controller from the plant model and the given formal specifications for the closed loop system (plant+controller). This guarantees that the controller program is correct by construction. To the best of our knowledge there is no previously published effective algorithm to extract executable code for the controller from closed loop formal specifications. We show practical usefulness of our techniques by giving experimental results on their use to synthesize C programs implementing optimal controllers (OCs) for plants with more than 10/sup 9/ states.

On the synthesis of communicating controllers with decentralized information structures for discrete-event systems

Abstract: The decentralized control problem addressed in the paper is that of several communicating supervisory controllers, each with different information, working in concert to exactly achieve a given legal sublanguage of the uncontrolled system's language model. A general model is reviewed for dealing with this class of problems. Existence, non-uniqueness, and some synthesis results to the above-mentioned problem are given.

Information Flow and Development of Coordination in Distributed Supervisory Control Teams

Abstract: This article presents results of a study to examine the flow of information between members of a new task team conducting a distributed supervisory control task. The emphasis of this project was on the effects of information presentation and message transmission delays on the development of effective information flow among human operators. The project focused on the earliest stages of team performance to explore how teams begin to refine distributed task coordination. The task simulation used in this project was a distributed navigation task based on a commercially available computer game (Spectre VRTM). Teams of three ("out the window" [OTW] observer, "long-range radar" observer, and a driver without direct visual information) were required to navigate a vehicle in a dynamic and potentially hostile environment containing obstacles and moving hazards. The goal of the task was to accumulate points through capturing flags. Information presentation was manipulated through standard game selections of wirefram...

Extending cognitive work analysis to manufacturing scheduling

Abstract: This paper discusses the challenges of applying tools associated with cognitive work analysis to a domain that is quite different from the process control domain in which cognitive work analysis was developed. The context of the paper is the design of a decision support system to aid scheduling of tasks within a manufacturing domain typical of job shops. The constraints in the supervisory control of a discrete manufacturing system are more intentional than physical. The paper explores the problems that arise with an intentional system and discusses how they might be overcome.

Supervisory control of distributed systems: conflict resolution

Abstract: In distributed synthesis and control, one well-known potential hazard is conflict between modular designs. In a modular approach to the supervisory control of discrete-event systems, modular supervisors that are individually nonblocking may nevertheless conflict and thus produce blocking, when operating concurrently. A scheme of resolving this potential conflict between the modular supervisors would be to accord priorities to the conflicting supervisors. We formalize this scheme with reporter maps from a hierarchical approach to the supervisory control of discrete-event systems. Sufficient conditions on these reporter maps for conflict resolution are obtained. With these conditions, a constructive solution is developed.

Supervisory control of hybrid systems via l-complete approximations

Abstract: This contribution deals with the synthesis of supervisory control for hybrid systems Σ with discrete external signals. Such systems are in general neither lcomplete nor representable by finite state machines. We find the strongest l-complete approximation (abstraction) Σl for Σ, represent it by a finite state machine, and investigate the control problem for the approximation. If a solution exists, we synthesize the maximally permissive supervisor for Σl. We show that it also solves the control problem for the hybrid system Σ. If no solution exists, approximation accuracy can be increased by computing the strongest k-complete abstraction Σk, k > l. Most of this paper is set within the framework of Willems’ behavioural systems theory.