Fault Tolerance in Asynchronously Operated Machines Allowing Temporary Violation of Normal Specification

A novel scheme of fault tolerance for input/state asynchronous sequential machines is presented in this paper. The machine may undergo unauthorized state transitions caused by adversarial inputs. The considered faults have the feature of intermittency in their influences so that the adverse effect persists for some finite time after initial occurrence. Since strong fault tolerance is impossible, we define the notion of weak fault tolerance and propose a fault tolerant control law for which the closed-loop system recovers the nominal input/state behavior with a bounded delay. The existence condition and design procedure for a controller are addressed based on corrective control theory for asynchronous sequential machines. An illustrative example is provided for demonstrating the proposed fault tolerant control scheme.