Global output regulation for strict-feedback nonlinear systems with mismatched nonvanishing disturbances

This paper concerns the problem of global output regulation for a class of strict-feedback nonlinear systems subject to mismatched nonvanishing disturbances. A composite control scheme is developed using a nonlinear disturbance observer-based control approach. A novel idea is that the disturbance estimation is introduced into the design of virtual control laws in each step. Global stability analysis for the closed-loop system is presented by the direct Lyapunov function method. It is shown that the system output asymptotically converges to zero in the presence of mismatched nonvanishing disturbances without the requirement of solving any partial differential equations involved with the traditional output regulation theory. An application design example of a single-machine infinite-bus system with static var compensator is presented with simulation results to demonstrate the effectiveness of the proposed method.