Research On Net Redundant For Flexible Manufacturing Systems Based On Petri Nets

The misallocation of shared resources is the main cause leads to deadlocks in flexible manufacturing systems. Once deadlock occurs, the flexible manufacturing system will be halted, resulting in low productivity of expensive equipment, sometimes even cause significant economic losses and catastrophic consequences. Since dealing efficiently with deadlock problem becomes important for high productivity, finding an effective way to control deadlocks turn out to be a hot research spot in academic and engineering circles.Petri net is an important modeling tool to describe the sharing of resources, mutual exclusion, concurrency and conflict in a system. Compared with other modeling tools, Petri net has obvious efficiency and superior performance in calculation. Therefore, it has become the main modeling tools for flexible manufacturing systems. S3PR is an important subset of Petri nets, which is commonly used for handling deadlock problems. In general, deadlock prevention policies are evaluated with their performances in terms of structural complexity, behavior permissiveness and computional efficiency. In this paper, two efficient mothods are proposed for improving the performance of a liveness-enforcing Petri net supervisor, they are summarized as follows: 1. So far, the existing deadlock prevention policies are mainly based on siphons. In this paper, the characteristics of the monitor in siphon-based deadlock control policies are analyzed, and an effective method is proposed to test the redundancy of the monitors based on the number of resources in siphons. The existing deadlock prevention policies usually contain redundant monitors, but the proposed mothod can leads to the maximum number of redundant monitors, and ultimately leads to a liveness-enforcing supervisor with simpler structure and more permissive behaviors.2. In this paper, a method is proposed to obtain the optimized supervisor based on necessary monitors. In most cases, liveness-enforcing supervisors obtained by this methodare with simpler structure and higher behavior permissiveness. It is remarkable that the proposed method proforms better when there are multiset of necessary monitors in the original liveness-enforcing supervisors. The way to design an optimal liveness-enforcing Petri net supervisor has been studied in the past two decades. Two flexible manufacturing examples in this paper show that the optimized supervisor generated by the proposed method is close to the optimal supervisor in behavior permissiveness and its structure is simplified.Finally, conclusions and future research are provided.