Reduced-order observer design for nonlinear systems with unknown inputs

This paper deals with observer design for a class of nonlinear systems with unknown inputs. Conditions are identified for the existence of such observers, and design procedures are proposed. The relations of the proposed design to some recent results on reduced-order observer design are also analyzed in some details. Furthermore, it is explored in the paper that under what conditions a reduced-order observer has linear observer error dynamics. An example is included to demonstrate the proposed analytical and observer design procedures.