A Framework for Fault-Tolerant Control of Discrete Event Systems

We introduce a framework for fault-tolerant supervisory control of discrete-event systems. Given a plant, possessing both faulty and nonfaulty behavior, and a submodel for just the nonfaulty part, the goal of fault-tolerant supervisory control is to enforce a certain specification for the nonfaulty plant and another (perhaps more liberal) specification for the overall plant, and further to ensure that the plant recovers from any fault within a bounded delay so that following the recovery the system state is equivalent to a nonfaulty state (as if no fault ever happened). The specification for the overall plant is more liberal compared to the one for the nonfaulty part since a degraded performance may be allowed after a fault has occurred. We formulate this notion of fault-tolerant supervisory control and provide a necessary and sufficient condition for the existence of such a supervisor. The condition involves the usual notions of controllability, observability and relative-closure, together with the notion of stability. An example of a power system is provided to illustrate the framework. We also propose a weaker notion of fault-tolerance where following the recovery, the system state is simulated by some nonfaulty state, i.e., behaviors following the recovery are also the behaviors from some faulty state. Also, we formulate the corresponding notion of weakly fault-tolerant supervisory control and present a necessary and sufficient condition (involving the notion of language-stability) for the its existence. We also introduce the notion of nonuniformly-bounded fault-tolerance (and its weak version) where the delay-bound for recovery is not uniformly bounded over the set of faulty traces, and show that when the plant model has finitely many states, this more general notion of fault-tolerance coincides with the one in which the delay-bound for recovery is uniformly bounded.