Context-free Forbidden Path Control of Net Condition/Event Systems

This work proposes a context-free forbidden path control method for manufacturing systems based on net condition/event systems (NCESs) under the firing rule 'maximal single spontaneous transition steps'. The NCES formalism is a modular extension of Petri nets. A context free path is defined by a sequence of discrete events and it is independent from the starting, ending, and intermediate states. The aim of the control is that the system is constrained to not follow a forbidden path, specified by the corresponding sequence of events, no matter from which state it starts (i.e. context-free). To this end, a novel control method based on NCESs is proposed, where a forbidden path tracker (FPC) tracks the occurrence status of events on a forbidden path and a last event controller (LEC) restrains the occurrence of the last event on the forbidden path. As a result, a context-free forbidden path controller is obtained without searching the state space and the controlled system meets the context-free forbidden path control specification if a few pre-conditions are. met. A chemical plant is employed to illustrate the proposed method.