Fault recovery in control systems: a modular discrete-event approach *

The problem of fault recovery is studied in discreteevent systems (DES), assuming permanent failures. A diagnosis system is assumed to be available to detect and isolate faults with a bounded delay. Thus, the combination of the plant and diagnosis system can be thought of having three modes: nonnal, transient and recovery. Initially, the plant is in the normal mode. Once a failure occurs, the system enters the transient mode. After the failure is diagnosed by the diagnosis system, the system enters the recovery mode. This framework does not depend on the diagnosis technique used, us long as lower and upper bounds for diagnosis delay are available. As a result, the diagnosis and control problems are almost decoupled. A modular switching supervisory scheme for the control problem is proposed. The design consists of a normal-transient supervisor, and multiple recovery supervisors each for recovery from a particular failure mode. The issue of nonblocking is studied and it is shown that essentially if the system under supervision is nonblocking in the normal mode, then it will remain nonblocking during the recovery procedure. Furthermore, a procedure is provided to ensure that the proposed modular switching supervisor is admissible.