The Supervisory Control of Automated Manufacturing Systems: A Discrete Event Systems Approach

Bertil A. Brandin

Automated manufacturing systems are generally composed of a number of interconnected material processing stations, a material transport system, a communication system, and a supervisory control system. The flexible manufacturing concept advocates that, as one of the options, manufacturing operations be carried out within workcells, each workcell being responsible for the production of a specific part family. Cells are usually interconnected by a transport system. Although workcell configurations may vary, these typically incorporate the following systems: numerically controlled (NC) part processing machines, material handling devices, part inspection/testing devices, in-process part storage systems, and a supervisory control system. The latter performs the following three tasks: supervisory control, communication and housekeeping. Supervisory control consists of (i) the monitoring of the workcell behavior via sensory feedback, (ii) control evaluation in accordance with a supervisory control law which maps the workcel! behavior to corresponding controls, and (iii) control enforcement via the downloading and execution of the appropriate device programs. Communication allows sensory feedback and control enforcement to be performed. Housekeeping is the set of tasks related to supervisory control and communication which are necessary to their implementation, e.g. data-base management. Supervisory control system development consists in the procedure generating supervisors and control laws which specify how the supervisory control system is to react to the manufacturing system behavior in order to satisfy given behavioral specifications (routing, sequencing, safety, ...). To develop supervisory control systems, different techniques such as knowledge engineering, Petri nets and controlled automata may be exploited. The work presented is based on controlled automata, since these provide important advantages over other approaches.

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