Fault Detection of the Cascade Control System based on Output Performance

Based on the achievable variance of the control outputs, a fault diagnosis of cascade control systems is developed. Without any prior knowledge of the complex operating processes and/or a prior external input to perturb the operating system, the accurate fault identification can be achieved by a series of the statistical hypothesis procedure that is applied to the current measured data. To isolate the possible faults, the output variances of the primary and the secondary loop can be separated into the cascade-invariant (CI) and cascade-dependent (CD) terms, respectively, by the diophantine decompositions. After a sequence of the hypotheses testing is performed on the CI and DI terms of the current control and the achievable performance conditions, the hierarchy diagnostic trees for the primary and the secondary outputs are respectively established to explore the possible faults. The final faults can be inferred by merging both diagnosis trees