A note on global robust output regulation for time-varying output-feedback systems

The problem of global robust output regulation was recently studied for uncertain time-varying nonlinear systems in output-feedback form with unity relative degree. In this note, we further investigate the case of such systems with the general relative degree. By augmenting a time-varying internal model to the plant dynamics, the problem is first converted into a global robust stabilization problem for a transformed time-varying augmented system. Then by employing a reduced-order observer for the augmented system, the stabilization problem is solved by a recursive design.