This paper proposes and ev aluates two improved Petri net (PN)-based hybrid sea rch strategies and their applications to flexible ma nufacturing system (FMS) scheduling.The algorithms prop osed in some previous papers,which combine PN si mulation capabilities with A* heuristic search within the PN reachability graph,may not find an opti mum solution even with an admissible heuristic funct ion.To remedy the defects an improved heuristic search strategy is proposed,which adopts a different method for selecting the promising markings and reserves the admissibility of the algorithm.To spe ed up the search process,another algorithm is al so proposed which invokes faster termination conditions and still guarantees that the solution found is optimum.The scheduling results are compared through a simple FMS between our algorithms and the previous methods.They are also applied and evaluat ed in a set of randomly-generated FMSs with such characteristics as multiple resources and alternative routes.
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