Characteristics of servo system using high order disturbance observer

It is desirable to realize a nominal system which can control acceleration in order to realize a fast and precise servo system, even if the servo system has parameter variation and suffers from disturbance. We have shown previously that compensating disturbance and parameter variation by using a disturbance observer can realize an ideal linear system which can control acceleration. In this paper, first, we transform a plant with a disturbance compensation loop by disturbance observer into a two-degrees-of-freedom control system with a feedforward controller, which indicates that the disturbance observer system includes a series compensator such as P, PI, etc. depending on the order of the disturbance observer. By clarifying the feature of the series compensator, we describe the bandwidth and the stability (the attenuation) of the system and show some simulation results, considering the influence of the measurement noise.