Dynamic information storage or retrieval – With servo positioning of transducer assembly over track... – Optical servo system
Reexamination Certificate
1999-03-31
2002-01-15
Hudspeth, David (Department: 2753)
Dynamic information storage or retrieval
With servo positioning of transducer assembly over track...
Optical servo system
C369S044350, C369S044410
Reexamination Certificate
active
06339565
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to optical disk play-back systems, and more particularly to center-error detection for a lens and control for a sledge in the optical pickup.
BACKGROUND OF THE INVENTION
Optical disks such as compact disk (CD) and digital-versatile disk (DVD) have been quite popular in computer and consumer devices. Data is stored on the disk by pits on the surface or on an internal layer within the disk. When a laser or other light source is shined on the spinning disk, the light is scattered by the pits but reflected back to a photosensor when no pit is present.
The pits are arranged on the disk into a continuous spiral track. The laser and pickup head can be moved along the radius of the disk to select different portions of the continuous spiral track. Indeed, along any radius of the disk, the disk appears to have many separate tracks of increasing distance from the first track at the inner diameter (ID) to the last track at the outer diameter (OD) of the disk.
FIG. 1
shows a pickup assembly for an optical disk. During operation, optical disk
98
is mounted on spindle
92
, which fits into the hole at the center of optical disk
98
. Spindle
92
has a motor (not shown) that spins the disk. Spindle
92
is mounted on or formed in pickup frame
100
.
Pickup frame
100
is a frame around a hollow center. Sledge
90
slides along rods
94
, which are metal rods mounted to pickup frame
100
. Sledge
90
slides along the radius of optical disk
98
mounted on spindle
92
, allowing any track on optical disk
98
to be selected for reading.
Sledge
90
contains electronics and optics for reading optical disk
98
. Lens
12
receives a laser beam reflected from the surface of optical disk
98
and focuses the light onto photodiodes
10
. A prism (not shown) is used to bend the light beam by ninety degrees.
Lens
12
fits inside cavity
96
on sledge
90
. As optical disk
98
is being read, the position of lens
12
within cavity
96
is altered slightly by a tracking control loop. The signals from photodiodes
10
are used by the electronics in the tracking control loop to adjust the position of lens
12
within cavity
96
to keep lens
12
on the track. The rapid rotational speed of optical disk
98
causes the tracking adjustment signals to have a high frequency, allowing the position of lens
12
to be adjusted quickly. A focusing control loop also uses the signals from photodiodes
10
to rapidly adjust the vertical distance that lens
12
is above the reading surface of optical disk
98
.
Since the tracks on optical disk
98
are actually one long spiral track, as optical disk
98
is continuously read, lens
12
gradually shifts to the outer diameter of optical disk
98
, toward the left of FIG.
1
. Eventually lens
12
would reach the left edge of cavity
96
if sledge
90
did not move. Motor
21
uses gears
13
which engage sledge
90
, moving sledge
90
to also follow the track being read on optical disk
98
.
In prior-art CD readers, motor
21
could simply be driven by a counter or other periodic signal, causing sledge
90
to gradually move to the OD at a constant rate. The tracking motion of lens
12
within cavity
96
was sufficiently large to maintain tracking with sledge
90
moving at a constant rate. The rate could be adjusted as the rotation speed of spindle
92
changed, or when an error in reading occurred.
However, newer DVD optical disks have a much higher recording density. Over 40,000 tracks are present from ID to OD on a DVD disk. Cavity
96
limits the tracking motion of lens
12
to about 500 tracks. Thus an additional control loop is always used to control motor
21
, adjusting the position of sledge
90
more precisely. This is known as a center-error (CE) control loop.
FIG. 2
illustrates a photodiode pickup moving among tracks on an optical disk. Photodiodes
10
contains four photodiodes, once for each quadrant. When photodiodes are exactly lined up along a track, photodiodes A and D pickup the same signal as photodiodes B and C. When mis-aligned, photodiodes A, D pickup more or less signal than photodiodes B,C. The difference in signal can be used to adjust the tracking, moving the lens focused onto photodiodes
10
either toward the outer diameter (OD) of the disk or toward the inner diameter (ID) of the disk.
Since any track on the disk can be selected, the sledge motion is rather large, spanning most of the radius of the disk. In contrast, the tracking motion of the lens within the cavity of the sledge is rather limited. High-frequency tracking-error circuits are used to rapidly adjust the lens position. Such tracking error generators using quadrant photodiodes are common. See for example, U.S. Pat. No. 5,859,816 by Yamamoto, and assigned to Toshiba, which uses phase comparison method. Focus control, where the optical distance to the disk is altered, is also known, such as described by Shimizume et al. in U.S. Pat. No. 5,475,664, and assigned to Sony Corp.
Another error is introduced when the lens is moved within the cavity by the tracking-error control loop. The lens position must be continuously adjusted within the cavity to maintain optical alignment with the track being read. However, if the sledge motion does not exactly match the track position, the lens may not be directly over the track. The light beam deviates from the perpendicular and a parallax error occurs. When the track is not directly under the lens, the alignment of the lens relative to the photodiodes and the disk surface is skewed from the perpendicular.
This parallax or center error can be corrected by shifting the position of the lens back to the center of the cavity in the sledge. However, the position of the sledge must change so that optical alignment to the track can be maintained. Sledge-centering errors are usually ignored in CD's since the larger track spacing and lower recording density make the error relatively insignificant. However, the sledge-centering error is more significant for higher-density DVD optical disks.
The lens may be shifted in position by a small tracking motion relative to the rest of the photodiode pickup assembly once the sledge is repositioned over the track being read. The range of motion of the sledge must be large, since during seek operations, the gears must quickly move the sledge to the desired track. During a continuous read operation, this sledge movement is very slight because the track spacing is very small.
FIG. 2
shows the tracking movement of the lens within the sledge's cavity as being slight, while overall sledge movement is large.
CENTER-ERROR FEEDBACK LOOP—FIG.
3
FIG. 3
shows a feedback loop that detects a sledge-center error and shifts the sledge position, allowing the tracking loop to shift the relative position of the lens within the sledge. Lens
12
receives a light beam reflected from a recording layer within an optical disk. Lens
12
projects this beam to photodiodes
10
. The four photodiodes
10
each send a signal to preamplifier
11
, which amplifies the four signals A, B, C, D from the four photodiodes
10
. These four amplified signals are input to center-error detector
15
. Center-error detector
15
compares the relative signal strengths and generates a center-error signal CE.
The center-error signal from detector
15
is amplified by amplifier
17
before being input to motor driver
19
. Motor driver
19
controls motor
21
. The output of motor
21
is geared down by gears
13
and then controls the position of sledge
18
that holds lens
12
. As the position of sledge
18
is changed, a tracking control loop (not shown) rapidly changes the position of lens
12
Motor
21
thus controls the position of sledge
18
. The relative position of lens
12
to photodiodes
10
is then adjusted by the tracking loop.
As the relative position of lens
12
within sledge
18
changes, the signal strengths from photodiodes
10
changes. This change is amplified and compared to generate the center-error signal CE that controls motor
21
and the position of sledge
18
Auvinen Stuart
Chu Kim-Kwok
Hudspeth David
LSI Logic Corporation
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