Implementation of Modular Supervisory/Logic Control in an Assembly Cell: Petri net Agents in Discrete-Event Systems Approach

摘要

In the context of supervisory/logic control of Discrete Event Systems, modular control poses a number of difficult problems related to the required non-blocking property of the resulting control. Permissive control of the Supervisory Control Theory (SCT) can be synthesised and implemented in a modular fashion based on non-conflicting specification languages. However, implementation of logic control requires event forcing. Event forcing has been incorporated into the SCT, but decentralised implementation poses problems related to locally verified controllability conditions of modular specification languages. This has prompted layered supervisory/logic control synthesis approaches. There is no consensus on the manner in which this layering has to be done. In this paper we extend these ideas to synthesise and implement modular supervisory/logic control for a laboratory scale CIM system. We identify the difficulties in event forcing as partly due to the conjunctive fusion rule used for controllable events. Modular logic control is synthesised using interpreted Petri net modules: invariant analysis is used for verifying non-blocking behaviour. Marking graphs of the Petri net modules are then used as event generators for modular supervisory control synthesis. This approach facilitates the incorporation of more general fusion rules (disjunctive and conjunctive). An interesting simplification for a frequently occurring special case also is presented. Our approach provides the possibility of progressively incorporating SCT into accepted Petri net based design technologies for grafcet/SFC or ladder based implementation.