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Phase Demodulator Velocity Extension

IP.com Disclosure Number: IPCOM000117266D
Original Publication Date: 1996-Jan-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 2 page(s) / 55K

Publishing Venue

IBM

Related People

DeRemer, R: AUTHOR [+4]

Abstract

Position demodulators using phase shift patterns are sensitive to velocity effects. With the Increasing Track Pitch (ITP) and velocity (M/sec) found in hard disk drives, the design limit has been reached.

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Phase Demodulator Velocity Extension

      Position demodulators using phase shift patterns are sensitive
to velocity effects.  With the Increasing Track Pitch (ITP) and
velocity (M/sec) found in hard disk drives, the design limit has been
reached.

      Velocity related problems can be eliminated by modifying
control timing as a function of velocity, thus maintaining the full
dynamic range of the demodulator.

      The basic intent of a phase demodulator as used in Hard Disk
Drives (HDD) is to convert physical position to a numeric
representation.  Accuracy is based on the linearity of this
conversion process.

      The effect of velocity on phase shift patterns (as it applies
to this design) is the growth of an error term caused by the normal
delay between differential measurements.  This error term is
monotonic with velocity and can become of such magnitude so as to
cause saturation of the demodulator.  Once saturation occurs the
numeric representation of position is worthless.

      The error term can be reduced in magnitude by adding an offset
timing value that effectively centers the dynamic range of the
demodulator.

      The technique used was to burden the servo control
microprocessor with maintaining a register value as a function of the
velocity profile.  The registered value was used by the demodulator
implementation to center the timing windows and thus the dynamic
range of the device.

      See the Figure for a pictorial...