VELOCITY PROFILE DEGRADATION DUE TO CONTROL LIMITATION
Original Publication Date: 1999-Dec-01
Included in the Prior Art Database: 2003-Jun-19
In most disk drives, torque factor varies across the stroke of the actuator. Even when this torque factor is measured and compensated for, seeks to targets where the torque factor is lower can be less robust because the lower torque factor results in a more constrained control. This is manifested as a saturation of the voice coil driver during the braking portion of a seek; it is most obvious using a velocity profile optimized for a target where the torque factor is high. This invention adjusts the maximum velocity of the seek based on the torque factor at the target location to improve robustness and performance. In prior art, the velocity profile specifies the desired velocity as a function of the distance to the target. The estimated or measured velocity is subtracted from the profile velocity to produce an error velocity. This error velocity is filtered and scaled to produce the commanded current to the voice coil driver. In addition, there is a maximum desired velocity for a given seek length. When the velocity profile is larger than this maximum velocity, the error velocity is the difference between the estimated or measured velocity and this maximum desired velocity. This invention is to add an adjustment to the maximum velocity based on the torque factor of the target: when the torque factor of the target is a certain fraction smaller than nominal, the maximum allowed velocity is reduced in proportion to that fraction. This torque factor adjustment can be most easily estimated by using existing data from the torque factor adaptation routine. In our implementation, this data is first written during the manufacturing process, personalized on a file basis, and is then adjusted during the recalibration process in the user's environment. A torque factor threshold is chosen (i.e. a nominal torque factor), and the adjustment as a result of torque factor is a mathematical function of the difference between the torque factor at the target and that threshold. This adjustment is then multiplied with the maximum velocity determined for the seek length, and the product becomes the adjusted maximum velocity. Alternatively, a default torque profile can be used in place of the personalized torque profile; this will reduce drive to drive variation introduced by the calibration and measurement processes.