Unobservable Settling Vibration on Head-Positioning Control in Hard Disk Drives

In this paper, we study the unobserved vibrations of the head in hard disk drives which may lead to destruction of user data. We show that the acceleration input excites the mechanical resonance near the sampling frequency of the hold in the settling characteristics after the track-seeking control. We also show that the degree to which these oscillations are unobservable depends on the sampling frequency of the sampler. Then we define the index of unobservable position based on its RMS amplitude to estimate effect of the unobservable oscillations of mechanical resonances in the positioning control system. These phenomena are confirmed through the experiments using the actual head-positioning system of the hard disk drive. Consequently the vibrations caused by the mechanical resonances around sampling frequency are not only difficult to observe but readily excited by the control input. Therefore, the head-positioning control system must be designed such that the sampling frequency is away from mechanical resonances.