Design of a robust disturbance observer for track-following control system of high-speed rotational optical disk drives

Abstract

This dissertation deals with design problems for the track-following control system of optical disk drives. A disturbance observer (DOB) is utilized to attenuate disturbances in the high frequency region, and the stability of the overall system should be preserved after the DOB is added on. In addition, we present a compensator design method for a dual-stage actuator that is a general mechanism to track a spiral data track on an optical disk. The validity of the proposed design methods is proved using simulations and experiments. First, a design procedure of a DOB for a track-following control system is presented. A robust stability condition under uncertain plant dynamics is introduced with intuitive graphical representation. A stability measure is defined using structured singular value of the overall control system with respect to the cutoff frequency of the Q-filter in the DOB structure. Because the relationship between robust stability and cutoff frequency is easily represented, the design procedure results in one dimensional search of the maximum cutoff frequency that satisfies the rotust stability condition. Because the performance of DOB is enhanced as the cutoff frequency increases, we can achieve the best disturbance attenuation performance without destabilizing the overall system. Secondly, we propose a new DOB structure for asymptotic disturbance rejection in a track-following control system. Due to the increase of disk rotational speed for the improvement of data transfer rate, the resultant disturbance frequency is also increased. Conventional DOBs are no longer effective in disk drive systems with a high-speed rotation mechanism because the performance of conventional DOBs is severely degraded as the disk rotational frequency increases. We can achieve an asymptotic tracking property against an external disturbance of a very high frequency by adopting a band-pass filter in the DOB structure, which is tuned based on the information on the disturban...