Improved Equivalent-Input-Disturbance Approach Based on $H_\infty$ Control

This article presents an improved disturbance estimation and rejection method based on equivalent input disturbance (EID) with H
_∞
robust performance for a timevarying uncertain system. The analysis and synthesis are conducted in the frequency domain. Different from conventional two-degree-of-freedom (2-DOF) H
_∞
control, some related weighting functions are introduced into a new devised structure to separate the rejection of the parameter uncertainty and disturbance from reference tracking. Compared with the existent EID-based approaches designed in the time domain of which the robust performance is not addressed, H
_∞
robust performance of the closed-loop system is guaranteed in this design. Treating the difference between the real plant and an ideal plant, namely, the overall effect of the parameter uncertainty and disturbance, as an EID, an EID estimation and compensation strategy is then established to cancel out the effect of the EID in system output. Furthermore, a constant scaling matrix is introduced into system design to reduce the conservativeness of a quadratic stabilization condition. Finally, a comparison with conventional 2-DOF robust H
_∞
control illustrates the advantages of the developed method.