Experimental modeling and compensation of pivot nonlinearity in hard disk drives

This paper considers a new model-based friction compensator for servo control of hard disk drives (HDDs). Using measured and simulated responses in both time and frequency domains as a guide, an HDD designer can iteratively improve the friction model of the HDD system and verify the correctness of the HDD measurement through this new model. By feeding back the model-based estimated friction forces into the voice coil motor (VCM), frequency response between the read/write head position and the VCM current can be linearized. This allows the cancellation scheme to be developed and implemented through a model-based compensation. Experiments have confirmed that the cancellation scheme is indeed effective in the frequency range of 0-300 Hz. Furthermore, by using a sequence of square waves as references for track-to-track seeking, the overshooting in the time responses can be compared between servo controls with and without compensators for friction. The effectiveness of the proposed friction compensation results also can be compared with the inner-loop compensation scheme using an accelerometer as a disturbance, observer. From those comparisons, we have found that our proposed model-based friction compensation scheme is practically useful.