Petri net based control for the dynamic scheduling of a flexible manufacturing cell

摘要

Deals with the dynamic scheduling of flexible manufacturing systems. The control hierarchy considers two levels. A higher level that provides parts and deadlines to the FMS and a lower (local) control that determines the sequence of the operations within the FMS. The local control uses as inputs a Petri net model of the system and of each of the products to be manufactured. The models of the products are deterministic place timed, and describe the possible sequences of operations to be executed in the cell. Assigning the time to the places allows a more compact representation of the system. The model of the system only describes its logical structure. The control, in order to determine the next steps to be carried out by the system, calculates the useful and possible work in the system by mapping the enabled transitions in the products into the system net. In order to avoid deadlocks the control must look ahead several transitions and at the same time the cost function is calculated which is used to select the transition to be fired. At this point, simulation results of a small FMS: three machines, one robot and three products are presented using Mathematica, although the system is capable of handling any number of them.