A discrete event system (DES) is a dynamic system that evolves in accordance with the abrupt occurrence, at possibly unknown irregular intervals, of physical events. Such systems arise in a variety of contexts ranging from computer operating systems to the control of complex multimode processes. A control theory for the logical aspects of such DESs is surveyed. The focus is on the qualitative aspects of control, but computation and the related issue of computational complexity are also considered. Automata and formal language models for DESs are surveyed. >

The control of discrete event systems

Fault detection and isolation is a crucial and challenging task in the automatic control of large complex systems We propose a discrete-event system (DES) approach to the problem of failure diagnosis We introduce two related notions of diagnosability of DES's in the framework of formal languages and compare diagnosability with the related notions of observability and invertibility We present a systematic procedure for detection and isolation of failure events using diagnosers and provide necessary and sufficient conditions for a language to be diagnosable The diagnoser performs diagnostics using online observations of the system behavior; it is also used to state and verify off-line the necessary and sufficient conditions for diagnosability These conditions are stated on the diagnoser or variations thereof The approach to failure diagnosis presented in this paper is applicable to systems that fall naturally in the class of DES's; moreover, for the purpose of diagnosis, most continuous variable dynamic systems can be viewed as DES's at a higher level of abstraction >

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Diagnosability of discrete-event systems

The observability of discrete-event systems is investigated. A discrete-event system G is modeled as the controlled generator of a formal language L m ( G ) in the framework of Ramadge and Wonham. To control G, a supervisor S is developed whose action is to enable and disable the controllable events of G according to a record of occurrences of the observable events of G, in such a way that the resulting closed-loop system obeys some prespecified operating rules embodied in a given language K. A necessary and sufficient condition is found for the existence of a supervisor S such that L m ( S/G ) = K . Based on this condition, a solution of the supervisory control and observation problem (SCOP) is obtained. Two examples are provided.

On observability of discrete-event systems

Decentralized supervisory control is investigated by considering problem formulations that model systems whose specifications are given as global constraints, but whose solution is described by local controllers. A necessary and sufficient condition is given for the existence of a solution to the problem of finding decentralized supervisors that ensure that the behavior of the closed-loop system lies in a given range. Where the range of behavior can be described by regular languages, it can be effectively tested whether the decentralized control problem is solvable; in this case, a procedure is given to compute the associated supervisors. >

Think globally, act locally: decentralized supervisory control

We examine a modular approach to the synthesis of supervisory controls for discrete-event systems. Discrete-event systems are modelled by automata together with a mechanism for enabling and disabling a subset of state transitions. The basic problem of interest is to ensure by appropriate supervision that the closed loop behavior of the system lies within a given legal behavior. Assuming this behavior can be decomposed into an intersection of component restrictions we determine conditions under which it is possible to synthesize the appropriate control in a modular fashion.

Modular Supervisory Control of Discrete Event Systems

Extends certain aspects of the work of P.J. Ramadge and W.M. Wonham (see SIAM J. Control Optimiz., vol.25, Jan. 1987) on the control of a class of discrete-event processes. The controlled process is described by a language L having strings that specify the sequences of events sigma /sub 1/ . . . sigma /sub n/ that the process can execute. The controller makes partial observations on the process events. Based on these observations the controller must enable or disable certain process events so that the resulting language generated by the closed-loop process is the specified sublanguage K contained in/implied by L. The case of decentralized control in which there are several controllers each of which makes partial observations and controls a subset of the process events is also studied. The results are illustrated for an example of communication protocols. >

Supervisory control of discrete-event processes with partial observations

A successful approach to problems of protocol specification, implementation, and verification is presented. Protocols are specified in a language called LIFP (Language for Implementing Fast Protocols) which is based on a model of interacting state machines. A compiler translates LIFP specifications into code that can be executed on PNPS (programmable network prototyping system), an experimental network emulator developed at the University of California at Berkeley. Verification of a LIFP control is facilitated by a debugger that simulates both PNPS hardware and the network environment in which the protocol is supposed to operate. This approach is compared with others (SPANNER and ESTELLE). Some aspects of the problem of protocol design allow it to be usefully viewed as one of supervisory control of a discrete event system. However, other aspects of protocol design show that current theories of supervisory control are inadequate. >

A method for specifying, implementing, and verifying media access control protocols

The programmable network prototyping system (PNPS) uses a collection of reusable hardware modules that implement generic communications functions such as transmission, reception, signal propagation, and pattern matching. These modules are interconnected and configured to emulate a variety of communication networks whose behavior can be monitored under different load conditions. The user specifies a network as a set of interacting components using available software tools. These tools are accessible within a prototyping environment that includes a control system for configuring the hardware modules and interconnecting them according to the component specifications. Previously designed components are stored in a library and can be used to specify new networks. Although PNPS is designed to provide a prototyping environment for communication networks, some of the basic ideas can be useful in other contexts. >

The programmable network prototyping system

Some important aspects of the behavior and control of physical systems can be studied using discrete event system models. One such model developed by Ramadge and Wonham is presented here with extensions to allow for partial observations and uncertainty in the state transitions. Although most of the paper is concerned with mathematical analysis, in particular the problem of controller synthesis, some remarks are devoted to clarify the important issues concerning the relations between the model and the real system. Finally, some suggestions are made concerning fruitful directions for further research.

Modeling and control of discrete event systems

The Programmable Network Prototyping System (PNPS), which is a rapid-prototyping tool used for emulating a wide variety of communication networks, is discussed. There are two parts to the PNPS hardware: the reusable hardware modules and the control and observation system. The reusable hardware modules consist of a channel emulator and a collection of node emulators. The channel emulator can be programmed to behave like a variety of network topologies, and each node emulator is configured to behave like a node on a network under study. The control and observation system links these modules together for configuration, experimentation, and monitoring purposes. There are three phases to using PNPS. first, the various components of a proposed network are designed. Next, the components are combined and run on the hardware while its behavior is monitored. Finally, the monitoring data is analyzed in order to evaluate the network design. A menu-driven user interface guides the user through the various tools of the system. >

R. Cieslak

Papers

Supervisory control of discrete-event processes with partial observations

A method for specifying, implementing, and verifying media access control protocols

The programmable network prototyping system

Modeling and control of discrete event systems

The Programmable Network Prototyping System