Optimal plant shaping for high bandwidth disturbance rejection in discrete disturbance observers

The Q-filter cut-off frequency in a Disturbance Observer (DOB) is restricted by the model mismatch between the plant and its nominal model. DOB Robustness is usually poor when the cut-off frequency of Q-filter increases. In this paper, robustness of the DOB is improved by shaping the true plant with a cascaded frequency shaping filter, while the nominal plant model remains fixed. Once the shaping filter is optimally designed based on DOB robust stability, the cut-off frequency of Q-filter can be greatly increased without loss of stability. The proposed algorithm is implemented on a typical tracking problem, i.e., the spiral writing in hard disk drives.