Optical pick-up actuator using a concentrated magnetization

Details Optical pick-up actuator using a concentrated magnetization Optical pick-up actuator using a concentrated magnetization

1998-12-30

2002-05-07

Young, W. R. (Department: 2651)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

C359S814000, C359S824000

Reexamination Certificate

active

06385146

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an actuator for an optical pick-up using the concentrated magnetization method that is used to drive an objective lens for converging a laser light beam onto an optical disc.
2. Description of the Related Art
Nowadays, there have been suggested various types of optical pick-up for recording and reproducing an information on and from an optical disc in accordance with the fast development of optical disc. Generally, the optical pick-up includes a laser diode used as a light source, an optical system for guiding a laser light beam into an optical disc, an objective lens for converging the laser light beam onto the optical disc in a spot shape and an actuator for moving the objective lens in the focusing direction of optical disc or in the width direction of track. The actuator for optical pick-up allows the light spot to trace tracks in an optical recording medium even though the recording medium occurs a surface vibration, an eccentricity and so on by driving the objective lens around two axes.
As shown in
FIG. 1
, such an optical pick-up actuator includes an objective lens
2
, a lens holder
4
, permanent magnets
6
, yokes
8
, tracking coils
10
, a focusing coil
12
and a wire spring
14
. The objective lens
2
converges a laser light beam generated at the laser diode onto the information recording surface of the optical disc in a spot shape. The lens holder
4
inserts and secures the objective lens
2
to support the objective lens
2
. To this end, the center of the lens holder
4
is provided with a circular hole for securing the objective lens
2
. The focusing coil
12
is wound around the circumference surface of the lens holder
4
. The tracking coils
10
are attached to the focusing coil
12
to thereby form a closed current loop perpendicular to a current flowing the focusing coil
12
. The permanent magnets
6
are arranged one by one at the upper portion and the lower portion of the lens holder
4
in such a manner to be opposite to the tracking coil
10
. Each permanent magnet
6
is attached to the yoke
8
made from a magnetic material such as still and the like so as to guide a magnetic flux. The permanent magnets
6
, the tracking coils
10
and the focusing coil
12
form a magnetic circuit generating a Lorentz force under the Fleming's left-hand rule. The objective lens
2
is moved in the focusing direction or the tracking direction by means of the Lorentz force generated at the magnetic circuit. The wire spring
14
forms a current path between the tracking coil
10
and the focusing coil
12
and, at the same time, acts as a driving axis of the lens holder
4
when the lens holder
4
is moved in the focusing direction(i.e., upward or downward direction) or the tracking direction(i.e., left or right direction). Also, the wire spring
14
serves to support the lens holder
4
by means of its elastic force.
The optical pick-up actuator generates Lorentz forces by means of the tracking coils
10
and the focusing coil
12
arranged within a magnetic space formed with the permanent magnets
6
and the yokes
8
. The objective lens
2
is moved in the upward, downward, left, or right direction along with the lens holder
4
by the Lorentz forces to thereby control a focusing and a tracking. As shown in
FIG. 2A
, a magnetic circuit for the focusing consists of a focusing coil
12
and a permanent magnet
6
magnetized into N pole or S pole. A magnetic flux generated at the permanent magnet
6
passes through the permanent magnet
6
and the yoke
8
to interlink the focusing coil
12
. In this case, the Lorentz force emerges in the vertical direction by a current applied to the focusing coil
12
, the horizontal surface of which is perpendicular to the magnetic flux. The objective lens
2
is moved in the vertical direction(i.e., upward or downward direction) along with the lens holder
4
by means of the Lorentz force, thereby controlling a size of the light spot on the optical recording medium. As shown in
FIG. 2B
, a magnetic circuit for the tracking consists of a permanent magnet
6
magnetized into N and S poles, and tracking coils
10
opposed to the magnetic surface of the permanent magnet
6
. In this case, the Lorentz force emerges in the horizontal direction by a current applied to the tracking coil
10
, the vertical surface of which is perpendicular to the magnetic flux. The objective lens
2
is moved in the horizontal direction(i.e., left or right direction) along with the lens holder
4
by the Lorentz force, thereby moving a light beam on the optical recording medium in the track direction. The permanent magnet
6
used in such a magnetic circuit is magnetized in such a manner that magnetic flux lines progress in parallel within itself. Since the magnetic flux lines progress in parallel within the permanent magnet
6
, as shown in
FIG. 3
, only magnetic flux lines generated at the center of the permanent magnet
6
are interlinked with the tracking coil
10
or the focusing coil
12
. In other words, many magnetic flux lines generated at the upper side and the lower side of the permanent magnet
6
are leaked. This leakage of magnetic flux lines causes a deterioration in the efficiency, that is, the sensitivity of the permanent magnet
6
. According to an experimental data in this regard, a magnetic flux density interlinked with the tracking coil or the focusing coil
12
is about 2000 to 3000 Gauss and a sensitivity in a frequency of 200 Hz is about 0.035 to 0.045 mm/V in the focusing direction and about 0.020 to 0.030 mm/V in the tracking direction in the case of the magnetic circuits as shown in FIG.
2
A and FIG.
2
B.
The optical pick-up actuator is preferable to have a great actuation amount with respect to a small signal because an allowance range in a size of the optical spot converged onto the optical disc becomes smaller as the optical disc has a higher density. Also, the optical pick-up actuator requires a wider servo band and a higher acceleration as a recording/reproducing apparatus has a high-multiple speed. In other words, the optical pick-up actuator is required to have a high sensitivity at a wide band including both the low frequency band and the high frequency band.
The sensitivity of the optical pick-up actuator can have a different value depending upon a magnetic flux density, an effective length of coil, a weight of actuating part and so on. Particularly, since a sensitivity in both the low frequency band and the high frequency band is considerably enhanced when a magnetic flux density of the permanent magnet
6
becomes high, a scheme for improving the permanent magnet has been raised as an important factor for making a high sensitivity of optical pick-up actuator. A high grade of neodymium sintered magnet capable of generating many magnetic flux lines is used as the permanent magnet to satisfy such a high sensitivity characteristic. This neodymium sintered magnet is an expensive material in itself. Furthermore, a higher grade of neodymium sintered magnet causes a higher rise in the material cost. Accordingly, it becomes necessary to enhance the efficiency of permanent magnet and the driving sensitivity in the optical pick-up actuator with a view to keeping up with a trend toward the high density and the high-multiple speed.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an actuator for optical pick-up using concentrated magnetization method wherein a driving sensitivity is improved in a wide frequency band.
In order to achieve these and other objects of the invention, an actuator for optical pick-up using the concentrated magnetization method according to one aspect of the present invention includes a lens holder attached to an objective lens; a coil wounded around the lens holder to receive a current; and permanent magnet means for constructing a magnetic circuit along with the coil so as to move the objective lens, the magnet means being magnetized in such a manner to concentrate a

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