Petri net models for message manipulation and event monitoring in a flexible manufacturing system cell

A shop floor control system (SFCS), the central part of a computer integrated manufacturing system, performs the production activities required to fill orders received from the factory level control system. In order to effectively control these activities, a hierarchically decomposed SFCS (equipment, workstation, and shop) can be devised and implemented. The controller at each level is functionally decomposed into planner, scheduler, and executor which perform various tasks such as part dispatching, resource selection and assignment, message passing, event monitoring, communication, machine code downloading, etc. In particular, the executor is responsible for (1) monitoring and interpreting input messages, (2) making execution-based decisions, and (3) broadcasting messages and down-loading machine instruction. The purpose of the paper is to present the interpreted Petri net models of the executors embedded in the workstation and equipment level controller. Each event is captured and processed and then some actions associated with the event are taken. These series of activities are modelled using interpreted Petri nets in which a circle node represents the set of actions (e.g. message sending, state variables and database updating, etc.), a bar node represents an event (e.g. message capturing), and an edge represents the transition between two nodes. The execution models have been implemented in an FMS cell with dumb planner and schedulers (meaning that planning and scheduling problems are randomly solved). The form description of the executors presented in this paper will provide the shop floor control engineer with the basis of the rapid generation of shop floor control software. Since the Petri nets detect and process all the events, including even planning and scheduling events, an FMS cell can be intelligently controlled by plugging a planner and scheduler into the Petri net models.