Application to Estimated Disturbance Feedback Control

Abstract

The appeal of feedforward control is to a large extent related to the following properties: (i) the only stability concern in a feedforward controller design is its own stability whereas a feedback controller must additionally guarantee stability of the closed-loop system; (ii) the effect of a feedforward controller is restricted to the input channel to which it is added, not affecting other inputs and the related performance; and (iii) many applications can cope with limited noncausality of a feedforward controller, whereas a feedback controller must be causal. Unfortunately, feedforward control only applies to reference inputs or measurable disturbances, while unmeasurable disturbances, on the other hand, are widespread in engineering practice. Extending the advantages of feedforward control to unmeasurable disturbances is the rationale for disturbance observers, where the observer developed by Ohnishi [103] is most popular [82,95,104,116,155]. Disturbance observers provide an estimate of the unmeasurable disturbance, which can be used to compute an appropriate control input similar to feedforward control.