State space disturbance observer considering sliding mode and robustness improvement for mismatched disturbance

This paper proposes a novel state space disturbance observer (DOB) to improve the robustness against mismatched disturbances. The DOB is developed by using a sliding mode considering the elimination of mismatched disturbances. In the derivation of the novel DOB algorithm, the sliding function and the estimation error are considered together in a Lyapunov candidate function to guarantee the stability of the overall system, including the controller and DOB. The presented DOB can force the states to stay on the sliding surface by satisfying the hitting condition without an additional input requiring the maximum disturbance and sign function. The robust controller in conjunction with the DOB eliminates the effects of mismatched disturbance on the output in a steady-state. The controller structure in this paper is the same as that of the existing DOB-based control system, meaning the robustness improvement comes from the modified DOB algorithm. By contrast, in the existing DOB-based control systems, the control and disturbance estimation are separated in the stability analysis and as such, the DOB cannot contribute to control performance. © ICROS 2021.