Effect of disk drive actuator unbalance on track following response to external vibration and shock

A two-disk, 2 1/2" drive with a multirate observer-based controller is tested for operating vibration and shock, and the off-track performance of the servo system is measured. Measured data are compared to analytical predictions in each case. Actuator pivot unbalance is computed by comparing analysis results to measurement for 0.5 G sine sweep excitation. Four shock excitations are considered from 20 G to 100 G in amplitude, with 11 ms and 2 ms durations. Predicted off-track displacements are converted to corresponding position error quadrature signals for comparison to measurement. Measured acceleration excitation is also used as input to the analysis. Predicted response compares favorably with measurement for operating vibration, as well as for shock excitations of 20 G (11 ms), 50 G (11 ms), and 25 G (2 ms). For 100 G (2 ms) shock, predicted response varies significantly from measurement. The paper also includes a discussion of the control algorithm typically used for head positioning in Seagate small disk drives.