Bias in disk drive rotary actuators: characterization, prediction, and compensation

This paper examines bias, which is the average torque (or current) required to maintain a set actuator position in a disk drive. In this paper, we show that in a disk drive the value of bias depends on several factors including the actuator position, the length of time at the set position, the direction of the actuator motion, the distance traveled by the actuator, the proximity of the set position to locations at which the actuator changed direction, and the proximity of the set position to locations at which the actuator rested. With an adequate prediction of bias, the settling time of a seek can be reduced by properly selecting the initial conditions of the track following controller at the handoff between track seek and track settle. Two prediction algorithms for bias are shown: one based on a series of calibration tests and one based on Dahl's friction model. A method for choosing the initial condition of the integrator of an integral action controller to minimize settling time based on the bias prediction is developed.