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Off Track Gain Calibration of Position Error Signal

IP.com Disclosure Number: IPCOM000088520D
Original Publication Date: 1977-Jun-01
Included in the Prior Art Database: 2005-Mar-04
Document File: 3 page(s) / 39K

Publishing Venue

IBM

Related People

Palmer, RS: AUTHOR

Abstract

In a servo track following head-positioning system for a disk file, variations in individual servo head widths and variations in individual servo channel circuitry can cause off track gain variations in the position error signal which are not corrected by automatic gain control. For example, in the servo pattern of Fig. 1A, magnetic signals occurring in four possible positions, A, B, C and D between commonly aligned synchronizing signals define data track centers, shown as dotted lines. Position error signals are formed from the A-B signal and the C-D signal. Automatic gain control is employed to keep the servo A+B+C+D constant. If a servo head is not exactly equal in width to twice the track spacing then the off-track position error signal response will differ from the nominal.

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Off Track Gain Calibration of Position Error Signal

In a servo track following head-positioning system for a disk file, variations in individual servo head widths and variations in individual servo channel circuitry can cause off track gain variations in the position error signal which are not corrected by automatic gain control. For example, in the servo pattern of Fig. 1A, magnetic signals occurring in four possible positions, A, B, C and D between commonly aligned synchronizing signals define data track centers, shown as dotted lines. Position error signals are formed from the A-B signal and the C-D signal. Automatic gain control is employed to keep the servo A+B+C+D constant. If a servo head is not exactly equal in width to twice the track spacing then the off-track position error signal response will differ from the nominal.

To calibrate the off-track gain of the system, a group of three special servo tracks are written, as shown in Figs. 1B and 2. By moving the A and B signals away from the track center line, the read back A and B signals are effectively decreased in the same manner as if the head were to move off track in a normal pattern. In this special pattern, however, the head continues to follow the track centerline since A and B are modulated equally and the position error signal A-B remains zero. To compensate for the effect on AGC, an additional signal must be recorded in the D position but on the C track which is exactly equal to the missing portions of the A and B signals.

In practice, the position of the A and B signals is slowly varied by oscillating a write head radially as the servo pattern is written. By moving the head between extreme positions once per half revolution, a low frequency modulation effect is achieved. To write the special pattern, track C is written with signals in both the C and D positions and no position modulation. Next, the A signals...