Modeling of dispatching-rules for job shop scheduling in manufacturing systems - a Petri net approach

There are many advantages for a company to get information previously by making simulations if a new manufacturing system shall be built up or an existing one must be adjusted to new circumstances. For that both a model of the plant and models of the control systems are needed to reach the desired behavior. One way for controlling the system are rules to determine the sequence of orders at certain points in the production process. There are a large number of such dispatching-rules used today. Models of some of them are introduced in this paper whereas they are divided into a few classes. The modeling is done by colored Petri nets which makes it possible to analyze the manufacturing system and the dispatching-rules and helps to write a control program.

1 Introduction

There are several possibilities for controlling a given plant which is modeled as a discrete event dynamic system (DEDS). One way is by supervisor synthesis, introduced by Wonham [1]. This method is based on formal languages and automata theory and its intention is to prevent the plant to reach undesired states. If the system is modeled by Petri Nets [5], [6] and the reachability graph is determined, this method can be used to inhibit undesired behavior [2]. Another way is the supervisor synthesis using place invariants, presented by Moody and Antsaklis [3]. This method is used to avoid unwanted markings of a Petri Net, for instance buffer overflow.