Analysis of Residual Vibrations above the Nyquist Frequency in Head-Positioning Control of Hard Disk Drives

Intersampling vibrations of the head of hard disk drives may lead to destruction of user data. Such vibrations are caused by mechanical resonances with frequencies above the Nyquist frequency of the control system. To address this problem, we analyze residual vibrations using the SRS (Shock Response Spectrum) analysis employed for characterizing transient characteristics of mechanical resonances. We show that the acceleration input in sampled-data control systems excites the mechanical resonance near the sampling frequency in the settling characteristics after the track-seeking control. 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 the 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.