A novel add-on compensator for cancellation of pivot nonlinearities in hard disk drives

This paper is concerned with the combined use of an accelerometer and a disturbance observer (DOB) to compensate for pivot nonlinearities that are subject to uncertainties in hard disk drives (HDDs). The DOB estimates the bias due to uncertain disturbances and nonlinearities such as pivot friction, and this estimate becomes a feedback signal for bias cancellation. Experiments have confirmed that the cancellation scheme is effective in the frequency range of from 0 to 700 Hz. Since the proposed cancellation scheme can be implemented as an analog system, it provides wider-bandwidth bias rejection and better settling performance than traditional bias compensation schemes such as integral action based on sampled position error signal. In a sequence of 0.5 msec track-to-track seeks, in which the bias was the primary source of modeling errors, the probability that the tracking error at the end of each seek would be within ±5% of track pitch was 0.35 with a conventional digital integral action; with the proposed compensation scheme, this probability was 0.80