Extraction of Bearing Coefficients of Fluid-Dynamic Bearing Spindle Motors Using a Proof Mass and a Hammer—A Refined Approach

In this paper, we report significant theoretical and experimental improvements over the method developed by Shen et al. (“Estimating bearing coefficients of fluid-dynamic bearing spindle motors: A theoretical treatment and feasibility study” in IEEE Trans. on Magnetics, vol. 47, no. 7, 2011) to extract bearing coefficients of a fluid-dynamic bearing (FDB) spindle motor used in hard disk drives (HDD). On the theoretical side, many assumptions made in the prior mathematical model are removed. For example, effects of radial displacements, inertia, and gyroscopic moments are now accounted for. Governing equations to extract the bearing coefficients are exact thus enhancing extraction accuracy. Besides, two new schemes are developed to extract the bearing coefficients from the governing equations. On the experimental side, we use a dual-beam differential laser Doppler vibrometer (LDV) to directly measure angular displacements, thus significantly reducing measurement uncertainties. Radial displacements are also measured to increase accuracy of extracted bearing coefficients. With these improvements, three rounds of experiments were conducted to extract in-line stiffness, cross stiffness and damping coefficients of the lower radial bearing of a 15 000 r/min spindle motor.