Disturbance Suppression beyond Nyquist Frequency in Hard Disk Drives

In conventional hard disk drives, a control system compensates for vibration in which the frequency is higher than the Nyquist frequency by using a multi-rate filter that decreases the gain above the Nyquist frequency. However, such a control system can only avoid instability and cannot suppress disturbances above the Nyquist frequency. In response to this problem, a control system design method that suppresses disturbances beyond the Nyquist frequency is proposed. This method uses frequency responses of a controlled object and a digital controller to calculate the gain of the sensitivity function in a sampled-data system without requiring complex calculations involving matrices, and realizes a stable resonant filter that decreases the gain of the sensitivity function above the Nyquist frequency. When the method was applied to the head-positioning system of a hard disk drive, the experimental results showed that the control system suppressed disturbances above the Nyquist frequency.