DESIGN AND IMPLEMENTATION OF A PETRI NET BASED SUPERVISOR FOR A FLEXIBLE MANUFACTURING SYSTEM

This paper presents the design, generation and implementation of coordinating discrete-event control code using Petri nets (PNs) for an operating flexible manufacturing system (FMS). The modeling and control of manufacturing systems using PNs is discussed. PN synthesis methods for manufacturing control are briefly reviewed, including top-down, bottom-up, and a hybrid methodology combining the preceding two. The hybrid methodology and mutual exclusion theory is summarized. This synthesis method guarantees that the desirable properties of liveness, boundedness, and reversibility are present in the resulting net. A description of the FMS and the network of computers controlling the components is given. The automated production system controlled is a one-sixth scale physical model of a shop which performs both machining operations and assembly. The system contains two different machining workstations with robotic loading and unloading, a robotic assembly workstation, a materials movement system, raw material and final product inventory storage, and an automated storage and retrieval system. The hybrid methodology is illustrated to design a PN model for this manufacturing system. Examples of top-down and bottom-up synthesis and choice-synchronization structures are given. A PN description language and a PN execution algorithm that allow supervisory control are discussed. This paper shows that it is possible to synthesize a live, bounded, and reversible PN and to use that net description for the control of a medium sized FMS.