Shouguang Wang

A Method to Compute Strict Minimal Siphons in a Class of Petri Nets Based on Loop Resource Subsets

Strict minimal siphons (SMS) play an important role in the development of deadlock control policies for flexible manufacturing systems (FMS). For a class of Petri nets called Systems of Simple Sequential Processes with Resources (S3PR), the resource circuit-based method is an effective way to compute SMS. In this paper, a more effective one to compute SMS is proposed. First, the concepts of loop resource subsets and their characteristic resource subnets are proposed. Next, sufficient and necessary conditions for loop resource subsets to generate SMS are established. Finally, an algorithm is given to find all the SMS based on loop resource subsets. Since the number of loop resource subsets is much less than that of resource circuits and their combinations, the computational efficiency of the SMS enumeration task is significantly improved by the proposed method. An FMS example is used to illustrate the application of the proposed method, and computational time comparisons are provided on several S3PRs to show its superior efficiency.

Controllability Conditions of Resultant Siphons in a Class of Petri Nets

Strict minimal siphons (SMSs) play a key role in the development of deadlock prevention policies by using Petri nets for flexible manufacturing systems (FMSs). In this paper, given two SMSs and their resultant siphon, the concept of key resource subsets is proposed which is shown to be the critical factor in deciding the controllability conditions of the latter. A necessary and sufficient condition is then proposed under which the resultant siphon can be always marked if its two SMSs are M-controlled, i.e., invariant controlled via the method proposed by Moody and Antsaklis. As for a resultant siphon that is composed by more than two composable SMSs, a sufficient condition is proposed under which it can be always marked if its SMSs are M-controlled. They are established by analyzing the structural characteristics and markings of the resource subnets in a class of Petri nets called L-S3PR. When they are applied to some classes of S3PR nets, i.e., those whose controlled ones are maximally permissive and live once their original SMSs are M-controlled, the number of monitors may be reduced. An FMS example is used to illustrate the application of the results.

Design of Optimal Monitor-Based Supervisors for a Class of Petri Nets With Uncontrollable Transitions

For a class of Petri nets whose uncontrollable influence subnets are forward synchronization and backward conflict-free nets, this paper proposes an algorithm to transform a given generalized mutual exclusion constraint (GMEC) into an optimal admissible one. Based on the proposed constraint transformation, a method is given to synthesize an optimal monitor-based supervisors. The proposed method utilizes an optimal monitor-based supervisor to enforce a GMEC containing infinite elements, which has not been seen in any prior research. Moreover, it has higher computational efficiency at the expense of application scope than some existing methods. An example is used to illustrate the application of the proposed method.

A New Modified Reachability Tree Approach and Its Applications to Unbounded Petri Nets

This paper proposes a new modified reachability tree (NMRT) approach for a class of unbounded generalized Petri nets called ω-independent nets. The NMRT of an ω-independent net consists of all and only reachable markings from its initial marking. Moreover, the applications of the NMRT to deadlock analysis for ω-independent nets are developed. The proposed method has a larger application scope than all the existing methods. Several examples are provided to show its superiority over them.

Comments on "Siphon-Based Deadlock Prevention Policy for Flexible Manufacturing Systems

A siphon-based algorithm for deadlock prevention of Petri nets called Algorithm H2 is proposed in the above paper. However, the illustrative examples for Algorithm H2 are incorrect because of the redundant weighted control (WC) places which control some marked siphons. This correspondence modifies the illustrative examples by removing the redundant control places and getting a maximally permissive control for the original net. As no WC place is presented in the modified examples, the examples cannot well illustrate Algorithm H2.

Macro liveness graph and liveness of ω-independent unbounded nets

Liveness is a basic property of a system and the liveness issue of unbounded Petri nets remains one of the most difficult problems in this field. This work proposes a novel method to decide the liveness of a class of unbounded generalized Petri nets called ω-independent unbounded nets, breaking the existing limits to one-place-unbounded nets. An algorithm to construct a macro liveness graph (MLG) is developed and a critical condition based on MLG deciding the liveness of ω-independent unbounded nets is proposed. Examples are provided to demonstrate its effectiveness.

Simultaneous Reduction of Petri Nets and Linear Constraints for Efficient Supervisor Synthesis

Due to state-space explosion and uncontrollable events in discrete-event systems, it is very difficult to design supervisors to enforce user-defined linear-constraints and ensure the liveness of their Petri-net (PN) models with complex structures. Different from all the existing methods, which are to transform original constraints into admissible or weakly admissible ones, the method proposed in this work aims to reduce linear-constraints and PN models simultaneously. As a result, an original PN control problem is equivalently reduced to a simpler one, i.e., the optimal supervisors for them make the same restriction on the behavior of a discrete-event system. Moreover, it can be guaranteed that the original PN system is live if and only if the reduced one is so. Since the state space of a PN may grow exponentially with its size, and the sizes of real discrete-event systems are often too large to handle, the proposed method is useful to greatly reduce the computational complexity of both property-analysis and supervisor-synthesis of discrete-event systems.

A reduced reachability tree for a class of unbounded petri nets

As a powerful analysis tool of Petri nets, reachability trees are fundamental for systematically investigating many characteristics such as boundedness, liveness and reversibility. This work proposes a method to generate a reachability tree, called ωRT for short, for a class of unbounded generalized nets called ω-independent nets based on new modified reachability trees (NMRTs). ωRT can effectively decrease the number of nodes by removing duplicate and ω-duplicate nodes in the tree, and verify properties such as reachability, liveness and deadlocks. Two examples are provided to show its superiority over NMRTs in terms of tree size.

Design of Liveness-Enforcing Supervisors for S3PR Based on Complementary Places

In this article, an algorithm is proposed to design liveness-enforcing supervisors for systems of simple sequential processes with resources (S3PR) based on complementary places. Firstly, a mixed integer programming (MIP) based deadlock detection method is used to find unmarked strict minimal siphons from an infinite-capacity net. Next, the finite-capacity net, in which liveness can be enforced, is obtained by adding capacity function to the infinite-capacity net. Finally, complementary-place transformation is used to transform the finite-capacity net into an infinite-capacity net. This article focuses on adding a complementary place to each operation place that is related to unmarked siphons, deals with the deadlock problem from a new view point, and hence advances the deadlock control theory. Compared with the existing methods, the new policy is easier to implement for real industrial systems. More importantly, design of a complementary-place supervisor is very easy. Finally, in some cases, the new policy can obtain a structurally simpler supervisor with more permissive behavior than the existing methods do. A flexible manufacturing systems (FMS) example is used to compare the proposed policy with some other methods.

Characterization of Admissible Marking Sets in Petri Nets with Uncontrollable Transitions

This work studies the equivalent transformation from a legal marking set to its admissible marking set. First, the concepts of escaping-marking set and transforming marking set are defined, and two algorithms are provided to compute the transforming marking set and the admissible marking set. Second, the equivalent transformation of a disjunction of linear constraints expressed in terms of generalized mutual exclusion constraints (GMECs) with non-negative weights via uncontrollable transitions is established.