Vibration suppression beyond Nyquist frequency in hard disk drives

In conventional hard disk drives, a control system compensates for the vibration in which frequency is higher than the Nyquist frequency by using a multi-rate filter that can decrease the gain above the Nyquist frequency. However, such a control system can only avoid instability that results from mechanical vibrations and cannot suppress the vibrations. In response to these problems, we proposed a design method for a sampled-data control system that decreases vibrations in which frequencies are higher than the Nyquist frequency. Our method can easily design a control system and can calculate the norm of the sensitivity function in a sampled-data system by using frequency responses of a controlled object. When the method was applied to the head-positioning system of a hard disk drive, we found that the control system can suppress the disturbance in which the frequency was higher than the Nyquist frequency