Minimum-Order Exosystem-Model-Based Disturbance Observer

This paper considers a minimum-order observer for the augmented system consisting of the nominal plant model and disturbance generating model (exosystem model). This paper shows that this minimum-order exosystem-model-based observer acts as a disturbance observer. Shikada and Sebe showed that the inverse-model-based disturbance observer can be realized as a full-order Luenberger-type observer for the nominal model. On the other hand, the exosystem-model-based disturbance observer is a full-order observer for the augmented system consisting of the nominal plant model and the exosystem model. Accordingly, the order of the exosystem-model-based disturbance observer is higher than that of the inverse-model-based disturbance observer. The minimum-order exosystem-model-based disturbance observer considered in this paper fills the gap of the difference in order. Furthermore, this paper also clarifies the one-to-one correspondence to the Luenberger-type disturbance observer for the nominal plant model. This correspondence shows that the minimum-order exosystem-model-based disturbance observer can achieve the same characteristics as the inverse-model-based disturbance observer.