Observer-based Fault Detection and Isolation for Nonlinear Systems

With the rise in automation the increase in fault detectionand isolation & reconfiguration is inevitable. Interest in fault detection and isolation (FDI) for nonlinear systems has grown significantly in recent years. The design of FDI is motivated by the need for knowledge about occurring faults in fault-tolerant control systems (FTC systems). The idea of FTC systems is to detect, isolate, and handle faults in such a way that the systems can still perform in a required manner. One prefers reduced performance after occurrence of a fault to the shut down of (sub-) systems. Hence, the idea of fault-tolerance can be applied to ordinary industrial processes that are not categorized as high risk applications, but where high availability is desirable. The quality of fault-tolerant control is totally dependent on the quality of the underlying algorithms. They detect possible faults, and later reconfigure control software to handle the effects of the particular fault event. In the past mainly linear FDI methods were developed, but as most industrial plants show nonlinear behavior, nonlinear methods for fault diagnosis could probably perform better. This thesis considers the design of FDI for nonlinear systems. It consists of four different contributions. First, it presents a review of the idea and the theory behind the geometric approach for FDI. Starting from the original solution for linear systems up to the latest results for input-affine systems the theory and solutions are described. Then the geometric approach is applied to a nonlinear ship propulsion system benchmark. The calculations and application results are presented in detail to give an illustrative example. The obtained subsystems are considered for the design of nonlinear observers in order to obtain FDI. Additionally, an adaptive nonlinear observer design is given for comparison. The simulation results are used to discuss different aspects of the geometric approach, e.g. the possibility to use it as a general approach. The third contribution considers stability analysis of observers used for FDI. It gives proofs of stability for the observers designed for the ship propulsion system. Furthermore, it stresses the importance of the time-variant character of the linearization along a trajectory. It leads to a different stability analysis than for linearization at one operation point. Finally, the preliminary concept of (actuator) fault-output decoupling is described. It is a new idea based on the solution of the input-output decoupling problem. The idea is to include FDI considerations already during the control design.