Secondary Controller with Additional Disturbance Rejection in H∞ Framework: Motivated by Disturbance Observer

In the paper, we propose a secondary controller design inspired by a disturbance observer in H-infinity framework. From the observation that the disturbance observer has a concept of an add-on controller, the secondary controller uses the reference and the system output to estimate and reject the external disturbances in form of input disturbance. The proposed approach has the advantage of allowing a wider range of low-frequency uncertainties than the disturbance observer approach. Also, it is shown that, using the stabilized nominal closed-loop system, the secondary controller deals with all linear systems, including unstable systems. Through examples, the effectiveness and advantages of the proposed approach will be demonstrated.