ESSENTIALLY DECISION FREE PETRI NETS FOR REAL-TIME RESOURCE ALLOCATION.

Qualitative properties of Petri nets (such as liveness and safeness) are useful in discrete manufacturing applications since they correspond to consistent, deadlock-free control logic. The authors introduce the qualitative property of essentially decision-free (EDF) places in Petri nets (PNs) to represent the absence of unresolved (nonunique) resource allocation conditions. It is shown that a previously developed method for PN synthesis of online control logic may lead to ambiguous resource allocation conditions which are exhibited in the PN model as non-EDF places. Using the theory of place-invariants, a procedure for identifying non-EDF places in the synthesized logic is developed. The procedure is illustrated for an example of assigning robots to tasks in a flexible assembly cell. For this example, allocation ambiguities are resolved by introducing logical NOT conditions in the PN model.