Pivot friction compensation using an accelerometer and a disturbance observer for hard disk drives

This paper considers a pivot friction compensator using the combination of an accelerometer and a disturbance observer, Noting that the pivot torque bias due to uncertain disturbances and nonlinearities such as pivot friction is approximately the difference between the scaled voice-coil motor (VCM) current and the scaled carriage angular acceleration, a disturbance observer to estimate the bias can be constructed using the above two continuously measured signals. By feeding back the bias estimate into the VCM amplifier, the frequency response from the VCM current to the carriage angular acceleration is linearized to be a constant gain within the observer bandwidth. This allows an assumption to be made in the controller design process that the plant dynamics are fixed in the subsequent design stage, for example, the design of model-based controllers. Since the proposed cancellation scheme can be implemented as an analog system, it provides wider bandwidth bias rejection and better settling performance than traditional bias compensation schemes, such as an integral action based on sampled position error signal. Experiments have confirmed that the cancellation scheme is effective in the frequency range 0-700 Hz, Furthermore, in a sequence of 0.5-ms track-to-track seeks, in which the bias was the primary source of modeling errors, the probability of being within +/-5% track pitch error was 0.35 with a digital integral action and 0.80 with the proposed compensation scheme.