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Data Head Self Servowrite

IP.com Disclosure Number: IPCOM000099537D
Original Publication Date: 1990-Feb-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 3 page(s) / 108K

Publishing Venue

IBM

Related People

Elliott, PJ: AUTHOR

Abstract

This article proposes that for disk files having dedi- cated servo information, further servo patterns can be written on the data surfaces by the data heads after assembly of the HDA by using the dedicated servo information for timing and positioning. This would enable wedge or hybrid servo patterns to be written outside of the cleanroom. The only extra electronics is a simple pattern generator. No access inside the file is required. Design Considerations

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Data Head Self Servowrite

       This article proposes that for disk files having dedi-
cated servo information, further servo patterns can be written on the
data surfaces by the data heads after assembly of the HDA by using
the dedicated servo information for timing and positioning.  This
would enable wedge or hybrid servo patterns to be written outside of
the cleanroom.  The only extra electronics is a simple pattern
generator.  No access inside the file is required.
Design Considerations

      Two main requirements for writing servo patterns on a disk are
the provision of position and timing information.

      On a conventional servowriter, position information is usually
supplied by a laser interferometer, controlling a move system of some
description, and the timing information by a clock head loaded onto
one of the disks.  Hence, access is required inside the file.  Thus,
the cover has to be removed and the servowriter must be inside a
clean area.

      Once the dedicated servo surface is written by the servowriter,
both position and timing information are available from this surface.
 Therefore, data head servo patterns can be written relative to the
dedicated surface. Since no access is required for a clock head or a
move system, it can be done outside the clean area, with relatively
simple equipment, and the cost is much lower. Also, it can take place
at any stage in the process, i.e, before or after surface analysis,
as required.

      For positioning, the normal file track-following servo is used
controlled by a file controller.  All that is required is the ability
to track-follow with an offset in the range  1/2 track, since the
servo pattern is built up with 1/2 track, or smaller steps.  Timing
information comes from the Servo ID pulses (SIDs).  Typically, these
are produced at 32 ms intervals.  Given a disk speed tolerance of
about 0.01%, the maximum error due to this is only a few nanoseconds.
 There is also an error due to the fact that SIDs are synchronized to
the file logic clock, which is asynchronous to the servo pattern
coming from the disk.  The clock is 18 MHz; hence, the error caused
is 27 nS.  A separate 36 MHz clock is needed to generate the
pattern which is also asynchronous, giving an error of 13.5 nS.

      At first sight, it may seem that these errors which limit the
phase coherence of the servo pattern from track to track will cause a
problem, especially with phase-encoded servo patterns.  However, the
critical part of the phase pattern is the average phase difference
between the three bursts.  When track-following, T...