The parametrization of all disturbance observers for plants with input disturbance

The disturbance observers have been used to estimate the disturbance in the plant. Several papers on design methods of disturbance observers have been published. The parametrization of all disturbance observers for plants with any output disturbance was clarified. However, no paper examines the parametrization of all disturbance observers for plants with any input disturbance. In this paper, we clarify existence conditions of a disturbance observer and a linear functional disturbance observers for plants with any input disturbance. Under these conditions, we propose the parametrizations of all disturbance observers and all linear functional disturbance observers for plants with any input disturbance.