This paper presents an efficient test that uses abstractions to detect conflict in composed systems controlled by local supervisors. This test, called nonconflict test, is not applied over the languages generated by the supervisors, but over abstractions of the supervisors with some specific characteristics. The concept of observer and the definition of the set of relevant events are the basis for the approach. Two strategies to define the set of relevant events are presented, along with illustrative examples. The paper also introduces a combined strategy, which consists of applying the two strategies in sequence leading, in many cases, to better reductions than when applying each strategy independently. An example is presented to illustrate the combined strategy.

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Abstract:The observer property is an important condition to be satisfied by abstractions of Discrete Event Systems (DES) models. This paper presents a generalised version of a previous algorithm which tests if an abstraction of a DES obtained through natural projection has the observer property. The procedure called OP-verifier II overcomes the limitations of the previously proposed verifier while keeping its computational complexity. Results are illustrated by a case study of a transfer line system.

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This paper studies abstraction methods suitable to verify very large models of discrete-event systems to be nonconflicting. It compares the observer property to methods known from process algebra, namely to conflict equivalence and observation equivalence. The observer property is shown to be the property that corresponds to conflict equivalence in the case where natural projection is used for abstraction. In this case, the observer property turns out to be the least restrictive condition that can be imposed on natural projection to enable compositional reasoning about conflicts. The observer property is also shown to be closely related to observation equivalence. Several examples and propositions are presented to relate different aspects of these methods of abstraction. Copyright 2007 IFAC