An event based supervisor for avoiding deadlock in semiconductor manufacturing systems

Modern semiconductor fabrication is carried out in highly automated and flexible manufacturing systems. In these systems, due to shared equipment usage and concurrent processing of multiple jobs of different types, deadlock can occur. The occurrence of deadlock can stall the automatic operation of the system, thus reduce productivity. In this paper, we extend our previous deadlock avoidance supervisor for regular systems to general systems which may have the so-called key resources and exhibit impending deadlocks - a type of deadlock more difficult to detect. New concepts such as, critical circuit, weight of the operating string and impending deadlock are introduced to help to establish the new supervisor. Major features of this supervisor includes: i) capability of avoiding both immediate deadlocks and impending deadlocks that are arbitrary steps away from an immediate one, whereas only second level deadlocks have been studied in the literature; ii) real time executability (fast online computation) provided that the set of deadlock strings is calculated offline. Simulation results show that the average permissiveness of our supervisor is very high. Examples are provided to illustrate our method.