Reinterpretation of disturbance observer as an add-on controller

In the paper, the reinterpretation of a disturbance observer formulated in the transfer function domain is considered. Recalling that the disturbance observer produces an additional disturbance suppresion performance to the existing closed-loop system, the idea of this paper is that the disturbance observer can be an add-on controller only to suppress the disturbances. By inducing the equivalent block diagram from generalized disturbance observer based control system [9], we obtain the equivalent feedback controller and the equivalent open-loop transfer function. Then, the role of a disturbance observer as an add-on controller is newly analyzed. Also, the reason of robust stability margin reduction phenomenon is explaned with the equivalent open-loop transfer function. The reinterpretation of a disturbance observer as an add-on controller is shown by a simple minimum-phase plant.