Decentralized diagnosis of discrete event systems subject to permanent sensor failures

In this paper, we consider a decentralized failure diagnosis problem for discrete event systems. Each local diagnoser makes a diagnosis decision based on local event observations. A sensor that detects the occurrence of an event may possibly fail due to, for example, aging degradation. It is desirable that the occurrence of any failure string should be correctly detected in the presence of sensor failures. We introduce a new notion of codiagnosability subject to permanent sensor failures, which is defined with respect to not only the set of nondeterministic local observation masks but also the global nondeterministic observation mask. Although the global observation mask is necessary to define codiagnosability, it is not used for performing decentralized diagnosis. The introduced notion of codiagnosability guarantees that the occurrence of any failure string can be correctly detected by a decentralized diagnoser within a bounded number of steps even if permanent sensor failures occur. We develop a method for verifying the codiagnosability property subject to permanent sensor failures. In addition, we compute the delay bound within which the occurrence of any failure string can be detected.