UC Berkeley

The hard disk drive (HDD) industry strives for higher storage densities and capacities. The critical factor for the performance of HDDs in this regard is the track mis-registration (TMR) which is the statistical number to indicate the performance of track-following control. After traditional track-following servo control, several visible frequency components remain in the spectrum of non-repeatable position error signal (PES). The dominant ones among these frequency components are a main contributor to the TMR. The rejection of these dominant components via servo control is difficult due to the fact that the frequency of the dominant component is not exactly known.

This dissertation introduces several adaptive control schemes to reject the dominant frequency component (narrow band disturbance with the largest magnitude) to reduce TMR for higher achievable areal density of HDDs.

A natural approach to narrow band disturbance rejection is indirect adaptive control, which involves two steps both performed in real time. At the first step, the frequency of the dominant component is estimated. Two frequency estimation methods are investigated in this dissertation. The discrete Fourier transform (DFT) method results in fast and accurate frequency estimation, but its large computational amount makes it an impratical approach for on-line identification. The least mean squares (LMS) algorithm is a computationally simple method for frequency identification. Carefully choosing the step size profile, the frequency estimate converges within one revolution and the resulting bias is small. The second step of the indirect adaptive control is to apply an add-on compensator based on the frequency estimate to reject the dominant component. Two choices for the add-on compensator are discussed in this dissertation. One is to identify the magnitude and phase of the dominant component. With the identified frequency, magnitude and phase, an estimate of the dominant component is constructed and then canceled by the control signal. This scheme is further extended to rejecting multiple frequency components. Another proposed compensator adopts the structure of a disturbance observer (DOB). The Q filter in DOB is selected to be a narrow band-pass filter centered at the estimated frequency. A deep notch in the error rejection function is introduced by the DOB with such a Q filter to reject the dominant component.

Two direct adaptive control schemes, which adapt compensator parameters directly, are also applied to compensate for the dominant component. One scheme applies a finite-impulse-response (FIR) Q filter built around the baseline servo controller to reject the dominant component based on Youla-Kucera parameterization. The coefficients of the Q filter are updated in such a way that the resulting controller incorporates the internal model of the narrow-band disturbance. To make the scheme suited for HDD systems, two modifications are proposed: 1) adding a pre-specified term to the Q filter to avoid large transient oscillation, and 2) cascading a bandpass filter to the Q filter to deal with inaccurate HDD plant model as well as to limit the waterbed effect to a certain frequency range. Another direct adaptive controller adopts the disturbance observer (DOB) loop with a narrow bandpass Q filter. The frequency parameter of the Q filter is directly adapted to the optimal value in the sense of minimizing the track-following TMR.

Realistic simulation tools are used to show that all adaptive control schemes described in this dissertation are effective in terms of rejecting narrow band disturbances to achieve smaller TMR. The advantages and disadvantages of each scheme are also discussed.