On Intrinsically Live Structure And Deadlock Control Of Generalized Petri Nets Modeling Flexible Manufacturing Systems

As an indispensable component of contemporary advanced manufacturing systems, flexible manufacturing systems (FMSs) possess flexibility and agility that traditional manufacturing systems lack. An FMS usually consists of picking and placing robots, machining centers, logistic systems, and advanced control systems. Some of them can be recognized as its shared resources, which result in its flexibility but may lead to its deadlocks. As a classic problem in resource allocation systems, deadlocks may arise in a fully automated FMS and bring about a series of disturbing issues, from degraded and deteriorated system productivity and performance to low utilization of some crit-ical and expensive resources and even long system downtime. Therefore, the analysis of and solution to deadlock problems are imperative for both a theoretical investigation and practical application of FMSs. Deadlock-freedom means that concurrent produc-tion processes in an FMS will never stagnate. Furthermore, liveness, another significant behavioral property, means that every production process can always be finished. Live-ness implies deadlock-freedom, but not vice versa. The liveness-enforcement is a higher requirement than deadlock-freedom.From the perspective of the behavioral logic, the thesis focuses on the intrinsically live structures and deadlock control of generalized Petri nets modeling flexible manu-facturing systems. Being different from the existing siphon-based methods, a concept of intrinsically live structures becomes the starting point to design, analyze, and op-timize a series of novel deadlock control and liveness-enforcing methods in the work. The characteristics and essence of intrinsically live structures are identified and derived from subclasses of generalized Petri nets modeling FMSs with complex resource usage styles. In addition, the numerical relationship between initial markings and weights of connecting arcs is investigated and used to design restrictions that ensure the intrin-sical liveness of global or local structures. With the structural theory, graph theory, and number theory, the thesis work achieves the goals of deadlock control and liveness-enforcement. The proposed methods are superior over the traditional siphon-based ones with a lower computational complexity (or a higher computational efficiency), a lower structural complexity, and a better behavioral permissiveness of the controlled system.