Application of discrete-event-system theory to flexible manufacturing

The anticipated extensive use of flexible-manufacturing workcells in the future has encouraged recent research efforts on the development of automatic supervisory-control methodologies. However, despite intensive research on the theoretical aspects of the control of manufacturing workcells, modeled as discrete-event systems (DES), a limited amount of research has been reported on the implementation of DES-based supervisory controllers. In this article, such a generalized implementation methodology, that utilizes theoretical advances in conjunction with programmable-logic-controller (PLC) technology, is presented. The two primary advantages of the proposed methodology are: (i) the utilization of limited-size control strategies that can be efficiently generated on-line, and which are conflict- and deadlock-free by construction (via controlled-automata DES theory); and (ii) the use of PLCs, which are currently the most suitable and widely employed industrial process-control technology. In the authors' proposed methodology, a host personal computer (PC) possesses an on-line capability for the automatic generation of supervisory-control strategies, and their downloading to a PLC as required. The PLC, in turn, is responsible for monitoring the workcell, reacting to events and enforcing device behavior based on the current control strategy residing in its processor. A supervisory controller developed based on this approach was successfully implemented for a manufacturing workcell in the authors' laboratory.