Robust rejection of time-varying resonances in a hard disk drive

Accurate identification of resonance frequencies in hard disk drives (HDD) is required to increase the controller gain in future drives. High gain controllers result in accurate tracking but may excite mechanical resonance modes of the actuator. Mechanical resonances are usually attenuated by notch filters. However, the resonance frequencies of the modes can differ between drives due to manufacturing tolerances. Environmental factors such as temperature can cause time-varying resonances in a drive. Identification of the resonance frequencies during manufacturing and operation is therefore required to design accurate models and subsequently high gain controllers. A two step approach is proposed to deal with both manufacturing issues as well as time-varying resonances in HDD. First the nominal resonance frequency of each mode is identified during manufacturing. A adaptive notch filter is designed to attenuate the mechanical resonance. If during operation the error signal between the actual closed loop and the specified nominal closed loop model exceeds a predefined threshold value the centerfrequency of the adaptive notch filter is quickly adapted to the actual resonance frequency using a least mean squares (LMS) method. The performance of this approach is verified by simulation of a realistic hard disk drive model