Method of making a balance-compensated rotary actuator based upon track-follow performance of a rotatable test head stack assembly portion

1. Field of the Invention A method of making a balance-compensated (BC) rotary actuator for use in a rotatable BC head stack assembly (HSA) portion in a disk drive based upon track-follow characteristics of a rotatable test HSA portion is provided. The test HSA portion includes a test rotary actuator having a test actuator weight specification. The test rotary actuator has an actuator body portion, a test pivot axis extending through the actuator body portion and a head connected to the actuator body portion. The test rotary actuator is controlled to rotate about the test pivot axis for positioning the head over a selected disk track. The test rotary actuator has a test rotary actuator center-of-gravity torque vector associated therewith about the test pivot axis. The method provides for vibrating the disk drive at a vibration frequency in order to vibrate the test HSA portion. Position error information is read using the head while performing a track-follow operation during the vibrating of the test HSA portion. Head off-track error due to the vibration of the disk drive is derived based upon the read position error information representative of positioning of the head relative to the selected disk track. A resultant torque vector is derived based upon the head off-track error. The resultant torque vector has a magnitude equal to a mass of the test HSA portion times a distance between the test pivot axis and a test installed HSA portion center-of-gravity. The test installed HSA portion center-of-gravity takes into consideration forces acting upon the test rotary actuator due to installation with the test HSA portion. The resultant torque vector has a direction extending perpendicular from the test pivot axis towards the test installed HSA portion center-of-gravity. A balancing torque vector is derived having a magnitude equal to and a direction opposite of the resultant torque vector. The balancing torque vector is incorporated into the test actuator weight distribution specification to produce a BC actuator weight distribution specification. The BC actuator weight distribution specification is used to manufacture the BC rotary actuator having a BC pivot axis. The BC HSA portion has a BC HSA installation center-of-gravity at the BC pivot axis for mitigating external acceleration applied to the BC rotary actuator about the BC pivot axis due to external linear acceleration experienced by the disk drive during a track-follow operation.