Abstract
This paper addresses the deadlock problems in flexible manufacturing systems (FMS) by using a Petri net siphon control method and the theory of regions. The proposed policy consists of two stages. The first one, called siphons control, is to add, for every siphon that we identify, a monitor to the original net model such that it is optimally invariant controlled. In the second stage, the theory of regions is utilized to derive the net supervisors such that deadlocks can be prevented. The first-stage work significantly lowers the computational cost compared with the approach where the theory of regions is used alone. An FMS example is presented to illustrate the technique. By varying the markings of given net structures, this paper shows its computational advantages.
Note to Practitioners-Deadlock is a constant problem in flexible manufacturing systems (FMS) with shared resources, which often offsets the advantages of these systems since deadlock can cause unnecessary cost, such as long downtime and low use of some critical and expensive resources, and may lead to catastrophic results in highly automated FMS. Behavior permissiveness has been an important criterion in designing the liveness-enforcing supervisor for an uncontrolled system. The theory of region is an effective method to derive a maximally permissive supervisor from a plant net model. However, it is rather inefficient. In this particular research, we develop a hybrid approach that combines siphon control and the theory of regions to derive a maximally permissive liveness-enforcing Petri net supervisor for a large class of FMS.