Deadlock Prevention Policies For FMS Based On Petri Net

A variety of important Petri net-based methods to prevent deadlocks arising inflexible manufacturing systems (FMS) are to add some control places and related arcs tostrict minimal siphons (SMS) such that no siphon can be emptied. Since the number ofsiphons grows in general exponentially with respect to a Petri net size, theirdisadvantages lie in that they often add too many additional places to the net, therebymaking the resulting net supervisor more complex than the originally bulit one. Thisthesis explores ways to minimize the new additions of places while achieving the samecontrol purpose. Based on the existing policies of P-invariants and the theory of regions, twodeadlock prevention policies are developed. We finally can get a deadlock-free Petri netsupervisor using the first method in which the flowing arcs of the control places arebrought forward. In the second method, we add places for each SMS that can beemptied to prevent it from being unmarked and the places added can also control othersSMS. Finally all SMS in the net cannot be emptied. We do not have to generatereachability graph in this method. A small number of control places and related arcs areadded in the first approach. The behavior of the net is far less restricted using the secondapproach. Examples of flexible manufacturing systems are given to illustrate theadvantages of our approaches over the existing work.