Fault estimation in nonlinear uncertain systems using robust/sliding-mode observers

Under a single ´fault-type hypothesis´, either actuator/component faults or sensor faults, geometric conditions are given such that the original nonlinear system can be transformed into two different subsystems with uncertainty. The first is in the generalised observer canonical form, which is not affected by faults. The second, whose states can be measured, is affected by the faults. Constructing a nonlinear observer for the first subsystem allows estimation of the faults from the second subsystem. Robust (sliding mode) observers are proposed depending on different assumptions on the system uncertainties. The decision logic associated with the proposed bank of observers, and some extensions to simultaneous actuator and sensor faults case are also discussed. Two numerical examples are given to illustrate the design procedures and the efficiency of the proposed methods.