On Deadlock Prevention In FMS Based On Petri Nets

This thesis focuses on 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. To find a solution to this problem, a novel deadlock prevention approach based on elementary siphons is proposed for a class of Petri nets, called ES3PR. In this approach, a monitor is added to each elementary siphon such that it is invariant-controlled without generating emptiable control-induced siphons. The controllability of a dependent siphon can be ensured by changing the control depth variables of its related elementary siphons. Hence we can get a structurally simple Petri net supervisor. Finally, the liveness of the supervisor is analized and demonstrated.Based on the theory of regions, an optimal liveness-enforcing Petri net supervisory can be obtained. However, when the initial marking of the original Petri net model changes, the supervisory may not be deadlock-free again. In the second approach, we can see that the net system will be deadlock-free if the initial marking of the supervisor satisfies a set of linear inequalities. Therefore, when changing the initial marking of the original Petri net model, a new supervisor can be obtained by adjusting the number of tokens in the monitors, which can alleviate the computation burden.FMS examples are used to illustrate the proposed concepts and approaches and show the significant advantages over the previous methods.