Design of unknown input functional observers for nonlinear systems with application to fault diagnosis

• Design of reduced order unknown input functional observers for nonlinear Lipschitz systems.
• Structured residuals are generated to detect and isolate actuator faults.
• Lower dynamical order than the system dimension and simplifies design procedures.
• Stability convergence of observer is analysed using linear matrix inequalities formulation.

This paper considers the design of low-order unknown input functional observers for robust fault detection and isolation of a class of nonlinear Lipschitz systems subject to unknown inputs. The proposed functional observers can be used to generate residual signals to detect and isolate actuator faults. By using the generalized inverse approach, the effect of the unknown inputs can be decoupled completely from the residual signals. Conditions for the existence and stability of reduced-order unknown input functional observer are derived. A design procedure for the generation of residual signals to detect and isolate actuator faults is presented using the proposed unknown-input observer-based approach. A numerical example is given to illustrate the proposed fault diagnosis scheme in nonlinear systems subject to unknown inputs.