Abstract

This paper considers a pivot friction compensator using the combination of an accelerometer and a disturbance observer. Noting that the pivot friction torque is approximately the difference between the scaled voice coil motor (VCM) current and the scaled carriage angular acceleration, a disturbance observer to estimate the pivot friction torque can be constructed using the above two signals. By feeding back the friction estimate into a VCM amplifier, the dynamics between the VCM current and the carriage angular acceleration are linearized to be a constant gain within an observer bandwidth. This allows the design to assume that the plant dynamics are fixed in the subsequent stage of the controller design, for example, the design of model-based controllers. The disturbance observer is designed using stochastic estimation theory. Furthermore, because the two signals are both measured continuously, the disturbance observer can be implemented experimentally as an analog system. This enlarges the observer bandwidth and the effect of the friction compensation, and reduces the reliance on the lower sampling rate of the positioning loop. To show the validity of the proposed method, experiments were conducted using a 3-1/2 inch disk drive. It has been shown that the mechanical nonlinearity, i.e., the gain reduction and the variation in the dynamics resulting from the pivot friction can be eliminated. The loop gain reduction resulting from the variation in the VCM current-to-torque constant can be recovered to be a nominal value up to 500Hz.

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