Global output feedback stabilization of nonlinear systems with unstable nonlinear zero dynamics

This paper deals with global stabilization of a class of nonlinear dynamic systems using output feedback. The class of the systems have nonlinear functions of unmeasured state variables, in addition to nonlinear functions of the system outputs. As a consequence of the nonlinearity of unmeasured state variables, the systems have nonlinear zero dynamics in general, which may also be allowed to be unstable. A nonlinear reduced order observer is proposed to estimate unmeasured system states. Control design is then carried out using a transformed system starting with an estimated state variable. Certain conditions are identified for the proposed observer and control design. Under these conditions, the proposed control design ensures the global stability of the closed-loop control system.