Modeling And Deadlock Prevention Policies For FMS Based On Petri Net In Case Of Equipment Failure

With high complexity, a flexible manufactuirng system (FMS) is liable to conk out. Petri net models describing equipment failure need to be built for analyzing properties of the system and choosing suitable control methods. A lot of FMS can be modeled by a subclass of Petri nets, ES~3PR net which is representative. Equipment failure may take place either during processing or in processing. Based on ES~3PR net, two Petri net models for above cases are modeled respectively, which can describe the behavior of malfunctioned equipment such as failure, recovery and rework with a failure place, a failure transition and a recovery transition. The two Petri nets are named by ES~3PRF-I nets and ES~3PRF-II nets (ES~3PR net with equipment failure) respectively. Combined the two cases, an ES~3PRF-III net is got.Equipment failure decreasing shared resources leads to deadlock situation in FMS. Based on above model, a new deadlock prevention policy is presented. First, a control place is added to each elementary siphon that can be emptied, which makes it P-invariant controlled. Second we prevent independent siphons from being unmarked by adjusting the control depth of elementary siphons. Such policy does not give rise to new SMS, then all SMS in the net cannot be emptied, i.e., the net is deadlock-free. Control places are added to elementary siphons only, which simplifies the resulting net supervisor by adding less control places and related arcs. The behavior of the net is far less restricted and able to reach more system states. The policy does not need iteration, avoiding the problem of state explosion.