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Browse Prior Art Database

Track-Crossing Detection And Seek-Direction Sensing

IP.com Disclosure Number: IPCOM000119955D
Original Publication Date: 1991-Mar-01
Included in the Prior Art Database: 2005-Apr-02
Document File: 6 page(s) / 199K

Publishing Venue

IBM

Related People

Fiorino, BC: AUTHOR [+2]

Abstract

This article teaches a track-seeking servo scheme in an optical disk recorder having the ability to accurately count tracks. The optical signals retrieved from a "Quad" detector (four photo diodes arranged in a rectangular array) are summed in pairs, then differentially compared to provide a tracking error signal. This tracking error signal (TES) has a +TES signal and a -TES signal, one being the complement of the other, and are the inputs into a track-crossing detector. The output signals of the four photo diodes are scanned to create an amplitude modulated signal having phase quadrature to TES. The two signals are combined to reliably detect track crossing with an indication of relative radial motion of the disk to the optical head.

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This is the abbreviated version, containing approximately 40% of the total text.

Track-Crossing Detection And Seek-Direction Sensing

      This article teaches a track-seeking servo scheme in an
optical disk recorder having the ability to accurately count tracks.
The optical signals retrieved from a "Quad" detector (four photo
diodes arranged in a rectangular array) are summed in pairs, then
differentially compared to provide a tracking error signal.  This
tracking error signal (TES) has a +TES signal and a -TES signal, one
being the complement of the other, and are the inputs into a
track-crossing detector. The output signals of the four photo diodes
are scanned to create an amplitude modulated signal having phase
quadrature to TES.  The two signals are combined to reliably detect
track crossing with an indication of relative radial motion of the
disk to the optical head.

      The operation of the invention is described first with respect
to the detector circuit and its associated spatial diagram, and then
the details will be described referring to the block diagram and the
signal diagram.  In the detector circuit a "Quad" detector consists
of four photo diodes (A, B, C, and D) arranged in a rectangular array
with one center line of the array being aligned with the track center
line. A differential tracking error signal (TES) is generated by
subtracting the sum of the photo currents from the two photo cells on
one side of the track center line from the sum of the photo currents
supplied by the diodes on the other side of the track center line.
Current summing circuit L sums the current outputs of photo cells A
and D while summing circuit R sums the current amplitudes of photo
diodes B and C.  The differential amplifier (diff amp) provides a
differential comparison of the two summed currents to supply a
differential TES having the two signal lines A1 and C1. The effective
differential signal TES is shown in the spatial diagram (Fig. 4) with
the spatial relationships to a grooved optical disk wherein the
tracks are in the grooves and the mesas identify the areas between
the grooves.  The radiation beam (not shown) is in focus at least at
the track center lines.

      The detector circuit (Fig. 3) generates a second signal in
quadrature with TES, hereafter referred to as the quad signal.  Sum
circuit Q receives inputs from all four of the photo diodes A-D for
summing the currents therefrom into the quad signal which is supplied
as signal B1 to the block diagram circuits.  As seen in the spatial
diagram, the quad signal is in quadrature to TES.  The quad signal
will lag the TES in a first radial direction of motion and lead the
TES in a second radial direction of motion, which is transverse to
the longitudinal extent of the grooves and mesas on the optical disk.
This phase relationship gives an unambiguous indication of seek
direction.  Such indication is important in optical disk because the
run out or eccentricity of rotation of the disk can be substantially
greater than the track pitch or distance...