John O. Moody

Supervisory Control of Discrete Event Systems Using Petri Nets

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 books 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.

Petri net supervisors for DES with uncontrollable and unobservable transitions

A supervisor synthesis technique for Petri net plants with uncontrollable and unobservable transitions, that enforces the conjunction of a set of linear inequalities on the reachable markings of the plant, is presented. The approach is based on the concept of Petri net place invariants. Each step of the procedure is illustrated through a running example involving the supervision of a robotic assembly cell. The controller is described by an auxiliary Petri net connected to the plants transitions, providing a unified Petri net model of the closed-loop system. The synthesis technique is based on the concept of admissible constraints. Procedures are given for identifying all admissible linear constraints for a plant with uncontrollable and unobservable transitions, as well as methods for transforming inadmissible constraints into admissible ones. A technique is described for creating a modified Petri net controller that enforces the union of all of these control laws. The method is practical and computationally inexpensive in terms of size, design time, and implementation complexity.

Synthesis of deadlock prevention supervisors using Petri nets

Given an arbitrary Petri net (PN) structure, which may have uncontrollable and unobservable transitions, the deadlock prevention procedure presented here determines a set of linear inequalities on the PN markings. When the PN is supervised so that its markings satisfy these inequalities, the supervised net is proved to be deadlock-free for all initial markings that satisfy the supervision constraints. Deadlock-freedom implies that there will always be at least one transition that is enabled in the closed-loop (supervised) system. The method is not guaranteed to ensure liveness, as it can be applied to systems that cannot be made live under any circumstances. However, for controllable and observable PNs, it is shown that, when the method ensures liveness as well, the liveness-ensuring supervisor is least restrictive. Moreover, it is shown that the method is not restrictive even for PNs in which not all transitions can be made live. The procedure allows automated synthesis of the supervisors.

Feedback control of Petri nets based on place invariants

This paper describes a method for constructing a Petri net feedback controller for a discrete event system modeled by a Petri net. The controller enforces a set of linear constraints on the plant and consists of places and arcs. It is computed using the concept of Petri net place invariants. The size of the controller is proportional to the number of constraints which must be satisfied. The method is very attractive computationally, and it makes possible the systematic design of Petri net controllers for complex industrial systems.<<ETX>>

The dependence identification neural network construction algorithm

An algorithm for constructing and training multilayer neural networks, dependence identification, is presented in this paper. Its distinctive features are that (i) it transforms the training problem into a set of quadratic optimization problems that are solved by a number of linear equations, (ii) it constructs an appropriate network to meet the training specifications, and (iii) the resulting network architecture and weights can be further refined with standard training algorithms, like backpropagation, giving a significant speedup in the development time of the neural network and decreasing the amount of trial and error usually associated with network development.

Supervisory control of Petri nets with uncontrollable/unobservable transitions

This paper expands upon results of previous research dealing with the supervisory control of Petri net modeled discrete event systems that, contain uncontrollable transitions. The concept of unobservable plant transitions is introduced here and incorporated into the controller design procedure. New conditions are developed which govern the existence of controllers for these problems. Two procedures are presented for automatically generating controllers for plants that incorporate uncontrollable and unobservable events.

A method for the synthesis of liveness enforcing supervisors in Petri nets

Given an arbitrary Petri net structure, which may have uncontrollable and unobservable transitions and may be unbounded, the procedure described in this paper generates a supervisor for liveness enforcement. The supervisor is specified as a conjunction of linear marking inequalities. For all initial markings satisfying the linear marking inequalities, the supervised Petri net is live. Moreover, the supervision is least restrictive in the fully controllable and observable case.

Automated design of a Petri net feedback controller for a robotic assembly cell

This paper has presented an extended example demonstrating a particularly simple and computationally efficient method for constructing feedback controllers for untimed Petri nets, even in the face of uncontrollable and unobservable plant transitions. The method is based on the idea that specifications representing desired plant behaviors can be enforced by making them invariants of the controlled net, and that simple row operations on a matrix containing the uncontrollable/unobservable columns of the plant incidence matrix can be used to eliminate controller use of illegal transitions. The significance of this particular approach to Petri net controller design is that the control net can be computed efficiently and automatically based on the plant constraints. The method shows promise for controlling large, complex systems, or for recomputing control laws online due to some plant failure, such as the loss of a required resource, the break down of an actuator, or the corruption of a sensor.

Deadlock avoidance in Petri nets with uncontrollable transitions

Results in the literature have provided efficient control synthesis techniques for the problem of deadlock avoidance in Petri nets. These results are shown to fit within an established framework for the enforcement of linear constraints on the marking behavior of a net. Framing the problem in this way allows uncontrollable and of unobservable transitions to be included in the plant model when deadlock avoidance is performed.

A method for the synthesis of deadlock prevention controllers in systems modeled by Petri nets

Given an arbitrary Petri net structure, the deadlock prevention procedure presented here determines a set of linear inequalities on the marking of a Petri net. When the Petri net is supervised so that its markings satisfy these inequalities, the supervised net is proved to be deadlock-free for all initial markings that satisfy the supervision constraints. Deadlock-freedom implies that there will always be at least one transition that is enabled in the closed loop (supervised) system. The method is not guaranteed to insure liveness, as it can be applied to systems that cannot be made live under any circumstances. However, it is shown that when the method does insure liveness, it is at least as permissive as any other liveness-insuring supervisor. The procedure is illustrated using an example from flexible manufacturing.