Fault detection in closed loop adaptive systems

This paper introduces a new fault indicator for closed loop systems. Although this new indicator is applicable for all systems, open loop or closed loop, it is especially useful for closed loop adaptive systems, because in such systems the controller is designed to compensate for the fault, thereby making direct detection of the fault more difficult once the compensation has taken place. The fault indicator proposed in this paper is meant for detecting the existence of faults, regardless of whether they have been compensated for, and to what extent. It is shown that the total measurable fault information in a system can be formally divided into two parts: the uncompensated part, and the compensated part. For linear time-invariant systems, a simple way to calculate the total measurable fault information is derived. The fault indicator developed in this paper is particularly useful for detecting sub-threshold faults, which is a new concept introduced in this paper, defined as faults that are below performance-based detection thresholds but would exceed that threshold in the absence of a controller.