APPLICATION OF DISCRETE-EVENT SYSTEM-THEORY TO FLEXIBLE MANUFACTURING

The anticipated extensive use of flexible-manufacturing work-cells in the future has encouraged recent research efforts on the development o f automatic supervisory-control methodologies. However, despite intens ive research on the theoretical aspects of the control of manufacturin g workcells, modeled as discrete-event systems (DES), a limited amount of research has been reported on the implementation of DES-based supe rvisory controllers. In this article, such a generalized implementatio n methodology, that utilizes recent theoretical advances in conjunctio n with programmable-logic-controller (PLC) technology, is presented. T he two primary advantages of the proposed methodology are: (i) the uti lization of limited-size control strategies that can be efficiently ge nerated on-line, and which are conflict-and deadlock-free by construct ion (via controlled-automata DES theory); and (ii) the use of PLCs, wh ich are currently the most suitable and widely employed industrial pro cess-control technology. In our proposed methodology: a host personal computer (PC) possesses an on-line capability for the automatic genera tion of supervisory-control strategies; and their downloading to a PLC as required. The PLC, in rum, is responsible for monitoring the workc ell, reacting to events and enforcing device behavior based on the cur rent control strategy residing in its processor. A supervisory control ler developed based on this approach was successfully implemented for a manufacturing workcell in our laboratory.