Disk drive servo control

Abstract

In a disk storage device dedicated servo system, a reference track is written in the outer guardband of the servo surface and each of the data surfaces. Supplemental position error signal (PES) values are obtained from the difference of servo and data surface samples at a plurality of locations around the disk and stored for access during read/write operations. The supplemental PES values are combined with the dedicated surface PES values to yield a composite PES that more accurately follows the data track centerline of the disk surface being addressed. The supplemental PES values are also periodically updated. To provide a more accurate correction of the composite PES, two consecutive supplemental values are used and an interpolation is calculated to provide a continuously changing supplemental PES value for modification of each sector actuator drive pulse between the two current supplemental PES values. Provision is also made in the apparatus to make the correction technique tolerant of tangential skew that may make tracks unreadable because of early or late timing through the provision of sequences of alternative early and later sample sector sequences.