Pickup actuator

Details Pickup actuator Pickup actuator

1999-11-18

2003-06-24

Hindi, Nabil (Department: 2655)

Dynamic information storage or retrieval

With servo positioning of transducer assembly over track...

Optical servo system

Reexamination Certificate

active

06584047

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pickup actuator of an optical information recording/reproducing apparatus for recording and/or reproducing information on/from a disc by using optical means.
2. Description of the Related Art
The field of optical means for recording/reproducing information by using an optical disc, or the like, is expected to be the most demanded in the future. In such an optical recording/reproducing apparatus a pickup actuator is an important element since it seriously affects the information recording/reproducing speed. The pickup actuator includes a pickup base, a lens holder movably disposed with respect to the pickup base and has an objective lens mounted thereon, and lens holder driving means for moving the lens holder toward the directions for focusing and tracking.
An ordinary example of the pickup actuator is shown in FIG.
1
. which will be described in greater detail below.
FIG. 1
is a perspective view of a conventional pickup actuator in which a reference numeral
10
refers to the pickup base. As shown in
FIG. 1
, the pickup base
10
has a body
12
and a holder support
14
. The body
12
has a pair of yokes
20
and
20
′ of a certain height disposed thereon while keeping a predetermined distance therebetween. Further, the body
12
is attached to the holder support
14
by a plurality of screws
16
a
and
16
b.
Further, a magnet
30
is attached to the inner side of the yoke
20
. The magnet
30
forms a magnetic circuit, while the yokes
20
and
20
′ function to concentrate and maximize the magnetic flux density of the magnet
30
toward the desired direction.
In
FIG. 1
, the reference numeral
40
refers to the lens holder. The lens holder
40
has the objective lens
42
, and is movably suspended to the holder support
14
of the pickup base
10
by two pairs of wire springs
44
a
and
44
b
. Further, the lens holder
40
has a driving coil
50
, which constitutes the lens holder driving means in cooperation with the magnet
30
.
The driving coil
50
has a focusing coil
52
wound in a horizontal relation with the lens holder
40
, and a pair of tracking coils
54
a
and
54
b
wound in a perpendicular relation with respect to the focusing coil
52
. The focusing and tracking coils
52
,
54
a
, and
54
b
cooperate with the magnet
30
to move the lens holder
40
in the directions for the focusing and tracking.
In such a pickup actuator, the objective lens
42
performs the focusing and tracking operations as the lens holder
40
is moved in the directions for the focusing and tracking by the mutual electromagnetic interaction of the magnet
30
and the focusing and tracking coils
52
,
54
a
, and
54
b.
Here, as the objective lens
42
is moved in the directions for focusing and tracking, an undesired rolling in addition to a translational motion occurs, resulting in a degradation of optical signals and thus, causing the inaccurate reading of information or failure to record the information.
It has been known that the main cause of the rolling is due to inconsistency of following three centers, i.e., the center of gravity of the lens holder
40
, the center of force of the electromagnetic circuit, and the center of support of the wiring springs
44
a
and
44
b
supporting the lens holder
40
. In order to lower the possibility of rolling, these three centers are designed to be aligned with each other, to prevent the rolling when the lens holder is moved in the directions for focusing and tracking from its initial position, i.e., the neutral position.
However, when the lens holder
40
is moved in the focusing direction after having been moved in the tracking direction, or vice versa, the inconsistency of the centers of gravity and force may occur, resulting in the rolling.
The above will be described with reference to
FIGS. 2 and 3
.
FIG. 2
is a view showing the positions of the coils of the lens holder
40
of the conventional pickup actuator relative to the effective area
60
of the magnetic flux distribution when the lens holder
40
is at its initial position, i.e., at the neutral position, and
FIG. 3
is a view showing the positions of the coils of the lens holder
40
of the conventional pickup actuator relative to the effective area
60
of the magnetic flux distribution when the lens holder
40
is moved in the focusing direction after having been moved in the tracking direction.
When the lens holder
40
is moved in the focusing direction after having been moved in the tracking direction, the portions of the tracking coils
54
a
and
54
b
which intersect the effective area of the magnetic flux distribution
60
in
FIG. 2
, cause unbalanced focusing directional forces of the lens holder
40
as shown in
FIG. 3
(since the lens holder
40
is hidden by the focusing and tracking coils which are attached to the lens holder
40
in the figures, the lens holder
40
would be described referring to the coil area of the figures). As shown in
FIG. 3
, it is noticeable that a portion area X of the tracking coil
54
a
intersecting the effective area
60
is smaller than another portion area Y of the tracking coil
54
b
. This is due to a narrow width B between the tracking coils
54
a
and
54
b
. Accordingly, the unbalanced forces of the tracking coils
54
a
and
54
b
impedes to reduce the rolling of the pickup actuator.
Further, according to the conventional pickup actuator, since the magnet
30
and the yoke
20
are made to have widths C and A at a rate of 1:0.5-0.8, the width A of the yoke
20
is relatively narrower than the width C of the magnet
30
. Accordingly, as shown in
FIG. 4
, the width of the uniform magnetic flux area is narrowed, again impeding the rolling control of the pickup actuator.
Meanwhile, to solve the above-mentioned problems, there have been several approaches such that the unbalance forces of the tracking coils in the focusing direction are balanced by laying a center line of the coils on a center line of the magnet, and such that the uniform magnetic flux area is expanded by shielding the leakage of magnetic flux using a cover covering the yoke, etc. The approaches, however, have shortcomings in that it can hardly balance the force just by coinciding the center lines of the coils and the magnet. Also, employing the cover to cover the yoke would result in increased number of parts, and thus, it inevitably increases the manufacturing cost.
SUMMARY OF THE INVENTION
The present invention has been developed to overcome the above-mentioned problems of the prior art, and accordingly, it is an object of the present invention to provide a pickup actuator having lower possibility of unbalanced forces of tracking coils in the focusing direction by minimizing the forces in the focusing direction of the tracking coil when the lens holder is moved in the focusing direction after having been moved in the tracking direction so that the rolling of the pickup actuator can be reduced.
Another object of the present invention is to provide a pickup actuator having wider uniform magnetic flux area of an electromagnetic circuit without using a separate part to decrease the rolling thereof.
Yet another object of the present invention is to provide a pickup actuator having a significantly reduced rolling of the area thereof by coinciding the center lines of the focusing coil and the magnetic flux of the magnet.
The above object is accomplished by a pickup actuator, including a pickup base, a lens holder having an objective lens mounted thereon, and lens holder driving means for driving the lens holder on the pickup base in the direction for focusing and tracking. The lens holder driving means includes at least one magnet, a pair of yokes attached to the magnet for concentrating and maximizing the magnetic flux density of the magnet toward the desired direction, and a driving coil having a focusing coil and a pair of tracking coils disposed between the pair of yokes.
Here, the magnet is attached to an inner side of one of the yokes, and the yo

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