Enhancing robustness by feedback loop gain adjustment

In the paper, we investigate a method to enhance the robustness of feedback loop by automatically adjusting the controller gain for keeping the open-loop gain in the presence of gain variation of the plant. Often the gain of the plant in practice differs from that of the nominal model so that the gain margin has been considered as one of measures for the robustness of controlled systems in view of stability. However, the gain margin does not guarantee the robust performance of the feedback system under the gain variation of the plant. Therefore, in order to improve the robust performance, we need to keep a priori the open loop gain as designed regardless of the system uncertainties. In the paper, considering the phase relationship between the signals in the feedback loop, it is shown that the controller's DC-gain can be effectively adjusted by introduction of the sinusoidal disturbance in the closed loop by force. Also, the effectiveness of the several approaches are compared in view of adjustment speed and accuracy.