Robustness analysis of a disturbance-observer based PI control

This paper deals with robustness analysis of disturbance observer (DO) based filtered PI (DO FPI) control for the first order plants based on performance portrait method. For loop robustification and noise attenuation the loop is augmented by n-th order filters used in the inner DO loop, as well as in the outer loop with the stabilizing P controller. It shows results derived by means of simple "ultra-local" integral plant models that under some conditions may be of use in a broader context originating in control of plants with a dominant first order dynamics. In this paper, they are applied to a robust velocity servo control with a variable inertia moment.