Nonlinear output feedback almost disturbance decoupling

The contribution of this paper is to design output feedback controls for a class of nonlinear single-input single-output systems affected by bounded unknown disturbances with unknown bounds. The proposed controls guarantee arbitrary disturbance attenuation on the output tracking error for smooth reference signals. Disturbance attenuation by output feedback for nonlinear systems was previously addressed by the authors (1993) under the assumption of known disturbance bounds; however such an assumption is not required in this paper