Diagnosis of Intermittent Faults

The diagnosis of``intermittent'' faults in dynamic systems modeled as discrete event systems is considered. In many systems, faulty behavior often occurs intermittently, with fault events followed by corresponding``reset'' events for these faults, followed by new occurrences of fault events, and so forth. Since these events are usually unobservable, it is necessary to develop diagnostic methodologies for intermittent faults. Prior methodologies for detection and isolation of permanent faults are no longer adequate in the context of intermittent faults, since they do not account explicitly for the dynamic behavior of these faults. This paper addresses this issue by: (i) proposing a modeling methodology for discrete event systems with intermittent faults; (ii) introducing new notions of diagnosability associated with fault and reset events; and (iii) developing necessary and suf®cient conditions, in terms of the system model and the set of observable events, for these notions of diagnosability. The de®nitions of diagnosability are complementary and capture desired objectives regarding the detection and identi®cation of faults, resets, and the current system status (namely, is the fault present or absent). The associated necessary and suf®cient conditions are based upon the technique of``diagnosers'' introduced in earlier work, albeit the structure of the diagnosers needs to be enhanced to capture the dynamic nature of faults in the system model. The diagnosability conditions are veri®able in polynomial time in the number of states of the diagnosers.