MengChu Zhou

Petri net synthesis for discrete event control of manufacturing systems

List of Figures. List of Tables. 1. Introduction. 2. Petri Nets and Manufacturing Systems. 3. Parallel Mutual Exclusions. 4. Sequential Mutual Exclusion. 5. Hybrid Synthesis of Petri Nets. 6. Illustration of Hybrid Synthesis. 7. Discrete Event Control of FMS. 8. Augmentation of Petri Nets. 9. Petri Nets: Past, Present and Future. Appendix A. Bibliography. Index.

Petri nets and industrial applications: A tutorial

Petri nets, as a graphical and mathematical tool, provide a uniform environment for modelling, formal analysis, and design of discrete event systems. The main objective of this paper is to introduce the fundamental concepts of Petri nets to researchers and practitioners, both from academia and industry, who are involved in the work in the areas of modelling and analysis of industrial types of systems, as well as those who may potentially be involved in these areas. The paper begins with an overview of applications of Petri nets, mostly industrial ones. Then, it proceeds with a description of Petri nets, properties, and analysis methods. The discussion of properties is put in the context of industrial applications. The analysis methods are illustrated using an example of a simple robotic assembly system. The performance analysis, using Petri nets, is discussed for deterministic and stochastic Petri nets. The presented techniques are illustrated by examples representing simple production systems. In addition, the paper introduces high-level Petri nets, fuzzy Petri nets, and temporal Petri nets. This is done in the context of application prospects. The paper also briefly discusses some of the reasons restricting the use of Petri nets, mostly, to academic institutions. >

A hybrid methodology for synthesis of Petri net models for manufacturing systems

Beginning with a bounded (safe), live, or reversible Petri net as a first-level net model for a system, Petri nets are synthesized by first refining operation places through basic design modules in top-down modular ways, then adding nonshared resource places stepwise, and finally adding shared resource places step by step in a bottom-up manner. Refinement theory is extended to include reversibility of Petri nets. Parallel and sequential mutual exclusions are used to model shared resources. Design of the first-level Petri nets is discussed, and two basic kinds of Petri nets, choice-free and choice-synchronization, are given to cope with different types of manufacturing systems. The major advantages of the method are that the modeling details can be introduced in incremental ways such that complexity can be handled, and the important properties of the resulting Petri net are guaranteed so that costly mathematical analysis for boundedness, liveness, and reversibility can be avoided. A manufacturing system consisting of four machines, one assembly cell, two shared robots, and two buffers is used to illustrate the design methodology. >

Parallel and sequential mutual exclusions for petri net modeling of manufacturing systems with shared resources

A theoretical basis for Petri net synthesis methods is provided that can be used to model systems with shared resources, and to make the resulting nets bounded, live, and reversible. Two resource-sharing concepts, parallel mutual exclusion (PME) and sequential mutual exclusion (SME), are formulated in the context of the Petri net theory. A PME models a resource shared by distinct independent processes, and an SME is a sequential composition of PMEs, modeling a resource shared by sequentially related processes. The conditions under which a net containing such structures remains bounded, live, and reversible are derived. >

Deadlock control methods in automated manufacturing systems

As more and more producers move to use flexible and agile manufacturing as a way to keep them with a competitive edge, the investigations on deadlock resolution in automated manufacturing have received significant attention for a decade. Deadlock and related blocking phenomena often lead to catastrophic results in automated manufacturing systems. Their efficient handling becomes a necessary condition for a system to gain high productivity. This paper intends to present a tutorial survey of state-of-the art modeling and deadlock control methods for discrete manufacturing systems. It presents the updated results in the areas of deadlock prevention, detection and recovery, and avoidance. It focuses on three modeling methods: digraphs, automata, and Petri nets. Moreover, for each approach, the main and relevant contributions are selected enlightening pros and cons. The paper concludes with the future research needs in this important area in order to bridge the gap between the academic research and industrial needs.

Modeling, analysis, simulation, scheduling, and control of semiconductor manufacturing systems: A Petri net approach

This paper presents a Petri net approach to modeling, analysis, simulation, scheduling, and control of semiconductor manufacturing systems. These systems can be characterized as discrete event systems that exhibit sequential, concurrent, and conflicting relations among the events and operations. Their evolution is dynamic over time. The system complexity is tremendous owing to the complex semiconductor manufacturing processes and test procedures. A formal approach such as Petri nets enables one to describe such complex discrete event systems precisely and thus allows one to perform both qualitative and quantitative analysis, scheduling and discrete-event control of them. This paper also serves as a tutorial paper. It briefly reviews applications of Petri nets in semiconductor manufacturing automation. It then introduces definitions and concepts of Petri nets. It proceeds with a discussion of basic Petri net modules in system modeling, a modeling method and a practical systems modeling example. Next, the paper presents their properties and their implications in manufacturing systems, as well as their analysis methods. Timed Petri nets are introduced for system simulation, performance evaluation, and scheduling purposes. An application-oriented case study is presented. Finally, the paper concludes with the active research areas in applying Petri nets to design of semiconductor manufacturing systems.

A Survey and Comparison of Petri Net-Based Deadlock Prevention Policies for Flexible Manufacturing Systems

Over the last two decades, a great deal of research has been focused on solving deadlock problems in resource allocation systems such as computer communication systems, workflow systems, and flexible manufacturing systems, resulting in a wide variety of approaches. As a well-defined problem in resource allocation systems, deadlock prevention based on a Petri net formalism has received an enormous amount of attention in the literature. This paper intends to review and compare a variety of Petri net-based deadlock prevention policies reported in the literature. Their comparison is done in terms of structural complexity, behavior permissiveness, and computational complexity. This paper should facilitate engineers in choosing a suited method for their industrial application cases.

An Iterative Synthesis Approach to Petri Net-Based Deadlock Prevention Policy for Flexible Manufacturing Systems

This paper proposes an iterative synthesis approach to Petri net (PN)-based deadlock prevention policy for flexible manufacturing systems (FMS). Given the PN model (PNM) of an FMS prone to deadlock, the goal is to synthesize a live controlled PNM. Its use for FMS control guarantees its deadlock-free operation and high performance in terms of resource utilization and system throughput. The proposed method is an iterative approach. At each iteration, a first-met bad marking is singled out from the reachability graph of a given PNM. The objective is to prevent this marking from being reached via a place invariant of the PN. A well-established invariant-based control method is used to derive a control place. This process is carried out until the net model becomes live. The proposed method is generally applicable, easy to use, effective, and straightforward although its off-line computation is of exponential complexity. Two FMS are used to show its effectiveness and applicability

A Maximally Permissive Deadlock Prevention Policy for FMS Based on Petri Net Siphon Control and the Theory of Regions

This paper addresses the deadlock problems in flexible manufacturing systems (FMS) by using a Petri net siphon control method and the theory of regions. The proposed policy consists of two stages. The first one, called siphons control, is to add, for every siphon that we identify, a monitor to the original net model such that it is optimally invariant controlled. In the second stage, the theory of regions is utilized to derive the net supervisors such that deadlocks can be prevented. The first-stage work significantly lowers the computational cost compared with the approach where the theory of regions is used alone. An FMS example is presented to illustrate the technique. By varying the markings of given net structures, this paper shows its computational advantages.

Avoiding deadlock and reducing starvation and blocking in automated manufacturing systems

Deadlock-free operations of automated manufacturing systems (AMS) are essential for high machine utilization and productivity. Based on the resource-oriented Petri net models of AMS and our previous work (2000) on a necessary and sufficient condition for deadlock-free operation, this paper proposes a new control policy such that it can avoid deadlock completely, and reduce starvation and blocking situations significantly. It attempts to release an appropriate number of jobs into the system and control the order of resource usage based on the state information in the net model. The theoretical results for the correctness of this policy are presented. An AMS allowing routing flexibility and varying operation times is used to demonstrate the potential of the proposed policy.