Discrete-time disturbance observers with multirate output state estimation

In this paper, a reduced order disturbance observer is newly proposed for estimating the unknown inputs from the partial measurements of states in the discrete-time domain. The multirate output state estimator is adopted for reconstructing the full state vector and combined with the disturbance observer. The asymptotic stability of the proposed disturbance observers is addressed, subject to the slowly time-varying disturbance. Also, the effectiveness of the proposed method is illustrated with a simple example which is a non-minimum phase system.