Nonlinear observer/controller design for a class of nonlinear systems

We develop a global output-feedback stabilization and tracking method for a subclass of strict-feedback nonlinear systems. This subclass is characterized by nonlinearities that are affine functions of the derivative of the measured output, with coefficients that are smooth nonlinear functions of the measured output. The novelty of our method is a nonlinear transformation of the unmeasured states in the observer design, which eliminates the dependence of the nonlinearities on the derivative of the measured output. The combination of the nonlinear observer and the nonlinear controller stabilizes the system and guarantees exponential convergence of the tracking error to zero