A new observer for SISO minimum phase linear systems with unknown inputs

The paper presents a new and general method for the design of an unknown input observer for single input single output time-invariant minimum phase linear systems. The proposed method is applicable to systems of any relative degree. In the case of systems with a relative degree of one, the observer reduces to those proposed in the literature. Since some derivatives of the output signal may be required, a variable structure filter is used to smoothly recover these derivatives. An illustrative example is presented and simulation results clearly demonstrate that all estimation errors converge to zero.