A time-controlled Petri net model and deadlock detouring algorithm for real-time FMC control

One of the fundamental problems in modelling the flexible manufacturing cell (FMC) is the representation of variable state-transition relations of the system over time. A new Petri net-based approach, i.e. time-controlled Petri nets (TC-PN), is proposed to model and control the FMC. The major contribution of this paper is that it develops a method of real-time control while avoiding some time-dependent deadlock situations. The structure and execution of TC-PN are formally defined. A look forward method is suggested to predict possible deadlock situations, and a backtrack-and-delay algorithm is developed to drive the system into non-deadlock conditions. An example is provided to demonstrate the workability of the algorithm. The TC-PN approach has several advantages. First, the varieties of part types and routes can be taken into account. Second, the TC-PN representation is compact, so that the complexity resulted from the variety of parts and routes can be readily managed. Third, the algorithm for detouring deadlock is very efficient in terms of computation time. These advantages become especially important for realtime control of FMC.