Deadlock prevention policy based on elementary siphons for flexible manufacturing systems

This paper focuses on the problem of deadlocks in automated flexible manufacturing systems (FMS). Based on Petri nets, a deadlock prevention policy is proposed for a special class of Petri nets, S³PR. We apply the deadlock avoidance policy (DAP) of conjunctive/disjunctive resources upstream neighborhood (C/D RUN) to the deadlock prevention policy (DPP), and allocate the underlying (sequential) resource reasonably to guarantee the absence of deadlock states and processes. First, siphons in a net model are distinguished by elementary and dependent ones. From the set of elementary siphons, a set of linear inequality constraints expressed by the state vector can be formalized. After being modified by the proposed policy, a set of generalized mutual exclusion constraints (GMEC) expressed by the marking vector can be found. Then monitors based on the GMEC are added to the plant model such that the elementary siphons in the S³PR net are all invariant-controlled and no emptiable siphon is generated due to the addition of the monitors. This novel deadlock prevention policy can usually lead to a more permissive supervisor by adding a smaller number of monitors and arcs than the existing methods for the design of liveness-enforcing Petri net supervisors. Finally a manufacturing example is utilized to illustrate the proposed method and comparison with the existing ones.