There exist two basic approaches for solving fault detection problems, passive approaches and active methods. Traditionally most fault detection approaches were passive in that system performance was monitored and decisions made. Recently, there
has been more interest in active approaches. The term ‘‘active’’ here is not the same as ‘‘active fault tolerant’’ where the controller is modified to compensate for a fault. Here, active refers to acting upon the system on a periodic basis or at critical times using a test signal, called the auxiliary signal, in order to exhibit abnormal behaviors which would otherwise remain undetected during normal operation. For more information see (Campbell & Nikoukhah, 2004) and (Esna Ashari et. al. 2011).
We initiated research on active fault detection of closed-loop systems, where the problem of masked faults is more serious. The reason is that controllers usually make the closed-loop system robust to the fault. For more details see (A. Esna Ashari et. al. 2012) and (A. Esna Ashari et. al. 2012)