Deadlock prevention and avoidance in FMS: a Petri net based approach

摘要

The use of structure theory of Petri nets to develop efticient deadlock prevention and deadlock avoidance methods for flexible manufacturing systems (FMSs) modelled by S{sup}4R nets is demonstrated. Major synchronisation patterns, such asgeneralised parallel and sequential mutual exclusion, frequently observed in FMS contexts can be represented by this class. The liveness property of a given S{sup}4R net (deadlock freeness in the context of FMSs) is characterised in terms of structuralPetri net elements called siphons. An efficient method for controlling minimal siphons of a given S{sup}4R net is developed where local control places are added to the net A sufficient condition for liveness of the augmented net is provided. Thisconstitutes a deadlock prevention approach. When the net liveness condition is not satisfied, an on-line controller, using a dynamic resource allocation policy, is developed for the augmented net. The performance of the proposed approaches is illustratedusing several examples.