Suboptimal Deadlock Prevention Policies Based On FMS

A variety of important Petri net-based methods to prevent deadlocks arising in flexible manufacturing systems (FMS) are to add some control places and related arcs to strict minimal siphons (SMS) such that no siphon can be emptied. Since the number of siphons grows in general exponentially with respect to a Petri net size, their disadvantages lie in that they often add too many additional places to the net, thereby making the resulting net supervisor more complex than the originally built one. Based on our previous work, we distinguish emptiable siphons in a plant Petri net model by elementary and dependent ones. By adding a monitor for each elementary siphon, all dependent siphons can be controlled by adjusting control depth variables of elementary siphons. Furthermore, by checking the control redundancy of the additional monitors, we can remove some monitors added, which leads to a more permissive and structurally simple liveness-enforcing Petri net supervisor.