Estimating Bearing Coefficients of Fluid-Dynamic Bearing Spindle Motors: A Theoretical Treatment and Feasibility Study

In this paper, we present a theoretical framework and a feasibility study to extract bearing coefficients of a fluid-dynamic bearing (FDB) spindle motor used in hard disk drives (HDD). In this method, the spindle motor is first loaded with a known proof mass instead of disk media. An impact hammer and a laser Doppler vibrometer (or capacitance probe) are used to measure frequency response functions (FRF) of the spindle motor. Since the proof mass is very rigid, the FRF near and below the half-speed whirl is dominated by bearing stiffness and damping. By analyzing the FRF (both magnitude and phase) in this frequency range, one can extract bearing stiffness and damping coefficients from the experimental measurements. This method has two advantages. First, it can extract stiffness (both in-line and cross) and damping coefficients simultaneously. Second, it can extract stiffness coefficients of the upper and lower radial bearings separately by varying the proof mass and thus the centroid of the rotating part of the spindle motor.