An Adaptive Switching-Gain Sliding-Mode-Assisted Disturbance Observer for High-Precision Servo Control

Active disturbance compensation is crucial for realizing high-precision servo control. However, the suppression of rapidly changing disturbances is still a challenging issue. In this work, observer-based disturbance-rejection control is investigated, and an observer with adaptive switching gain is proposed for improved robustness against changing disturbances. First, a disturbance observer (DOB) is established, based on the tracking differentiator technique. Second, a sliding-mode-assisted observer with an adaptive law is designed to estimate the residual-disturbance estimation error of the DOB. The proposed scheme instantly increases the switching gain when the state deviates from a preset boundary, thus eliminating the tuning time. The switching control action with an immediately large switching gain is robust against suddenly changing disturbances. Moreover, the switching gain then decays to zero, reducing chattering in the steady state. The gain adjustment process is designed to have the same dynamic characteristics as the proposed DOB, which guarantees the convergence of the proposed scheme. This method is not only suitable for linear systems, but also extendable to nonlinear systems. The proposed scheme was evaluated experimentally; the results showed that it improves disturbance rejection and decreases servo error.