Retrieval and rejection of unknown sinusoidal disturbances in some output feedback non-minimum phase nonlinear uncertain systems

We address the disturbance rejection problem of some parameter-perturbed non-minimum phase nonlinear systems. The disturbance is assumed to be a combination of an unknown constant and sinusoids with unknown magnitudes, phases and frequencies. A novel three-stage output feedback control strategy is proposed, which seems the first theoretical solution of this problem. The system states are first led to and kept within a high order sliding manifold for a moment; simultaneously the disturbance coefficients can be precisely extracted from an constructed auxiliary signal. Subsequently the disturbance may be completely rejected by feedforward compensation. We analyze the significance the proposed strategy, as well as its problems in practice. We also explain how it benefits further controller design.