The nonrepeatable radial vibration (NRRV) of spindle motors used in magnetic disk memory devices was studied in detail. In particular a theoretical and experimental investigation on the comparison of NRRV in time and frequency domains was conducted. The random and statistical characteristics of NRRV were ascertained by analyzing the amplitude distributions of both asynchronous and synchronous vibration components in the frequency domain. It was found that the main part of NRRV is composed of asynchronous components in most of the spindles tested. Some of the asynchronous component amplitudes, varying randomly, can be approximated by Rayleigh distribution function. It was also found that the synchronous vibrations include nonrepeatable components. The variance of NRRV measured in the time domain was shown theoretically and experimentally to be equal to the sum of the total power of asynchronous components and the total variance of synchronous components. From this relation, the cause of tracking error in such memory devices can be analyzed quantitatively in frequency domain.

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