Showing papers by "W. M. Wonham published in 2012"

Supervisor localization of discrete-event systems based on State Tree Structures

Abstract: Recently we developed supervisor localization, a top-down approach to distributed control of discrete-event systems in the Ramadge-Wonham supervisory control framework. Its essence is the decomposition of monolithic (global) control action into local control strategies for the individual agents. In this paper, we establish a counterpart supervisor localization theory in the framework of State Tree Structures, known to be efficient for control design of very large systems. As before, we prove that the collective localized control behavior is identical to the monolithic optimal (i.e. maximally permissive) and nonblocking controlled behavior. Further, we propose a new and more efficient localization algorithm which exploits BDD computation.

Distributed Supervisory Control of Discrete-Event Systems with Communication Delay

Abstract: This paper identifies a property of delay-robustness in distributed supervisory control of discrete-event systems (DES) with communication delays. In previous work a distributed supervisory control problem has been investigated on the assumption that inter-agent communications take place with negligible delay. From an applications viewpoint it is desirable to relax this constraint and identify communicating distributed controllers which are delay-robust, namely logically equivalent to their delay-free counterparts. For this we introduce inter-agent channels modeled as 2-state automata, compute the overall system behavior, and present an effective computational test for delay-robustness. From the test it typically results that the given delay-free distributed control is delay-robust with respect to certain communicated events, but not for all, thus distinguishing events which are not delay-critical from those that are. The approach is illustrated by a workcell model with three communicating agents.

Checking delay-robustness of distributed supervisors of discrete-event systems

Abstract: This paper identifies a property of delay-robustness in distributed supervisory control by supervisor localization of discrete-event systems (DES) with communication delays. In previous work a distributed supervisory control problem has been investigated on the assumption that inter-agent communications take place with negligible delay. From an applications viewpoint it is desirable to relax this constraint and identify communicating distributed controllers which are delay-robust, namely logically equivalent to their delay-free counterparts. For this we introduce interagent channels modelled as 2-state automata, compute the overall system behaviour, and then present an effective computational test for delay-robustness. From the test it typically results that the given delay-free distributed control is delay-robust with respect to certain communicated events, but not for all, thus distinguishing events which are not delay-critical from those that are. The approach is illustrated by a work-cell model with three communicating agents.

Decentralized supervisory control of discrete-event systems based on State Tree Structures

Abstract: Decentralized supervisory control of discrete-event systems (DES) is developed, by use of State Tree Structures (STS) to manage state explosion. Assuming the specification of the controlled system can be decomposed into several sub-specifications, we design a separate optimal (maximally permissive) supervisor for each. If the decentralized supervisory controlled system is blocking, an additional coordinator is adjoined. The result guarantees that the controlled behavior of the decentralized supervisory controlled system is both nonblocking and optimal.

Representation of supervisory controls using state tree structures, binary decision diagrams, automata, and supervisor reduction

Abstract: In the synthesis of an optimal nonblocking supervisor for a discrete-event system (DES), the problem of state explosion is a well-known computational obstacle. This problem can often be managed successfully by the use of state-tree structures (STS) and binary decision diagrams (BDD). Unfortunately BDD control functions may become quite large, and as such difficult to represent and interpret. In some cases it may, therefore, be convenient to convert an STS/BDD based controller to automaton form, and then apply a well known algorithm for supervisor reduction. In this paper we illustrate the advantage of this approach with a concrete example.

On computation of distributed supervisory controllers in discrete-event systems

Abstract: This paper refines an earlier localization procedure used to design optimal nonblocking distributed supervisory controllers in discrete-event systems (DES). The earlier algorithm needed a control consistency relation involving two conditions, and introduced superfluous selfloops. We propose a refined procedure, including a newly defined weak control consistency relation, and a cleanup algorithm, which results in simpler and cleaner controllers. It is proved to be correct, and illustrated by a Transfer Line plant.