Robust position control of DC motor using a low-order disturbance observer against biased harmonic disturbances

Differences between nominal and real plants often arise from parametric uncertainties and external disturbances in practical control systems. Disturbance observer-based controllers have been widely used in various control applications to compensate plant uncertainties and reject disturbances. This paper addresses a model-based low-order disturbance observer design that can estimate a biased harmonic disturbance. By using the disturbance model, the proposed method successfully estimates the disturbance with unknown amplitude and phase but known frequency. A sufficient condition is presented for the stability of the proposed low-order observer based on output measurement information only. Based on the proposed observer, the position control problem of a dc motor is solved in the presence of parametric uncertainties and the harmonic disturbance. The conventional integral state feedback controller is combined with the proposed observer. The proposed observer estimates unmeasurable system states as well as the disturbance. Comparative computer simulations for a numerical example and a dc motor system are performed to test the performance of the proposed method. The simulation results show the enhanced performance of the proposed disturbance observer-based scheme.