Can a fast disturbance observer work under unmodeled actuators?

Abstract

In this paper, we study robust stability of a closed-loop system with disturbance observer (DOB) when the relative degree of the plant P is greater than or equal to one, but is unknown to the control designer so that he/she chooses the nominal plant P n of relative degree one. This situation typically happens when the actuator dynamics is ignored at the control design stage. Based on the standing assumption that the bandwidth of the low-pass Q-filter is sufficiently large, our study reveals the following results: 1) if the relative degree of P is equal to, or bigger than the relative degree of P n by one, robust stabilization may be achieved by carefully designing the Q-filter and the nominal model P n. It is emphasized that, when the relative degrees are not the same, not only the Q-filter but also the nominal model P n should be carefully chosen to achieve the goal. 2) When the difference of relative degrees is greater than one, robust stabilization is not possible no matter how Q-filter and P n are chosen. Therefore, in order to achieve robust stabilization, it is suggested to estimate the relative degree of the plant as close as possible. © 2011 ICROS.