Active Fault Isolation for Discrete Event Systems

In practice, a supervisor can often not only disable some events, but also enforce the occurrence of some events prior to the occurrence of other events. In this article, we combine these two control mechanisms to synthesize a more powerful supervisor. The control goal is to design an isolation supervisor, which ensures in the closed-loop system, faults are fault isolatable in the sense that after a fault occurs, we can determine which type the fault belongs to by observing the output of the closed-loop system. The isolation supervisor starts to work when the occurrence of faults is detected. We solve the isolation supervisor synthesis problem as follows. For a given discrete event system, we first construct a bipartite transition system which includes all feasible isolation supervisors. An isolation supervisor is feasible if it enforces only events that are physically possible. We then develop an algorithm to check whether the synthesis problem is solvable or not. The algorithm can also be used to find a valid isolation supervisor if the synthesis problem is solvable. The method of combining two control mechanisms can be used to synthesize more powerful supervisors for other supervisory control problems of discrete event systems as well.