Method and apparatus for tracking radially-dependent repeatable run-out

A disc drive comprising:

• an actuator configured to move at least one head responsive to a received servo control signal;
• a sensor configured to sense servo information and to produce a servo signal therefrom, the servo signal is combined with a reference signal to produce a position error signal;
• a servo controller which receives the position error signal and generates the servo control signal in response to the received position error signal, the servo controller comprising:
• a repeatable run-out tracking module adapted to track coherent repeatable run-out in the servo loop by:
• (a) storing several repeatable run-out control components determined from a plurality of tracks on the disc surface;
• (b) receiving a command initiating a seek to a selected target track on the disc surface;
• (c) before settling on the target track, retrieving the control components stored in the storing step (a);
• and
• (d) using the control components retrieved in the retrieving step (c) to follow the target track.