Design of a tracking system using n-delay two-degree-of-freedom control and its application to hard disk drives

So far, two-degree-of-freedom control systems have been used to realize desirable transient response in the tracking system of a hard disk drive (HDD). It is easy to realize a high speed seek, if the actuator of the HDD has a resonance mode at a much higher frequency domain than the desired control domain. However, it is generally difficult to design an actuator that has a resonance mode in a high frequency domain. So, in order to realize a high speed seek, it is important to design a digital control system in consideration of the resonance mode of the actuator. In this case, if the Nyquist frequency of the control system is higher than the resonance mode of the actuator, the control system can be designed by using a method such as a H_∞ control theory in consideration of the resonance mode. However, if the Nyquist frequency isn´t higher than the resonance mode of the actuator, it is very difficult to realize a high speed seek without mechanical vibration using a conventional method. In the paper the method of applying the n-delay control theory to the design of the feedforward controller in a two-degree-of-freedom control system, and searching for the optimum time allocation of the n-delay controller based on an evaluation function are proposed. Using these methods, the frequency component of the feedforward control signal can be diminished at the desired frequency point higher than the Nyquist frequency. The validity of the proposed method is confirmed by the numerical and experimental results with a miniature 2.5-inch hard disk drive