Decomposition Of Deadlock-free Petri Net Supervisors For Flexible Manufacturing Systems

This thesis deals with the deadlock prevention problems in flexible manufacturing systems (FMS). The major disadvantage of the existing approaches is that too many monitors and arcs have to be added, leading to a much more structurally complex Petri net supervisor for an FMS. An effective control method is developed to prevent deadlocks from occurring in a class of nets, ES3PR, where deadlocks are tied to unmarked siphons. Strict minimal siphons are divided into elementary and dependent ones. For each elementary siphon, a monitor is added to ensure it sufficiently marked without generating emptiable control-induced siphons. The controllability of a dependent siphon is ensured by changing the control depth variables of its related elementary siphons, which leads to a liveness-enforcing supervisor for a plant model. Furthermore, by checking the redundancy of the additional monitors, some control-redundant monitors can be removed, which leads to a more permissive and structurally simple net supervisor.The other deadlock prevention plociy, which combines MIP (Mixed Interger Programming) with the concept of elementary siphons, is presented, which can avoid both explicit enumeration of siphons and the redundancy of the monitors for a plant model. This policy can lead to a more structurally simple net supervisor. However, the resultant supervisor usually becomes a generalized net after controlling the emptiable siphons in ES3PR nets, which is difficult to conduct further analysis. Therefore, it is necessary to transform a generalized net supervisor into an ordinary one without changing its behavior properties such as liveness.An FMS example is used to illustrate the above-mentioned concepts and approaches, which shows the significant advantages over the previous methods.