Head-positioning control using resonant modes in hard disk drives

The best way to enhance the IO performance of a hard disk drive is by increasing the spindle speed. Therefore, the effect of windage vibrations caused by the airflow grows with spindle speed. The servo bandwidth is limited by the first-order resonant frequency of the mechanical system. However, the frequencies of the windage vibrations are higher than the first-order resonant frequency. Accordingly, these frequencies are also above the servo bandwidth and are too high to be controlled by a conventional control system. In response to this problem, we have developed two methods for designing a servo control system that can suppress windage vibrations. One method uses a stable mechanical resonant mode, and the other uses a stable resonant mode created by a digital filter. By using these methods, the head-positioning system can control the vibrations above the frequency of the first-order mechanical resonance and the servo bandwidth. Application of these methods to actual hard disk drives showed that they can greatly decrease the windage vibrations in which the peak frequency is about six times the open-loop gain 0-dB crossover frequency.