Lens moving apparatus preventing resonance without using yoke bridge and counterweight

A lens moving apparatus of the present invention is used in an optical pickup of, for example, an optical recording/reproducing system. The apparatus includes a moving assembly holding an objective lens and provided with a coil bobbin, elastic supporting devices which support the moving assembly along the focusing dimension and the tracking dimension, focusing coils and tracking coils wound on the coil bobbin, and a magnetic circuit formed by a yoke and a magnet. The center of gravity of the moving assembly is positioned so that the product of the distance from an effective section of the focusing coil to the center of gravity of the moving assembly and the force produced at the effective section is about equal to the product of the distance from a noneffective section of the focusing coil to the center of gravity of the moving assembly and the force produced at the noneffective section.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical pickup bi-axial actuator 
utilized for recording and reproducing a signal with respect to an optical 
information recording medium such as an optical disc or a magneto-optic 
disc. 
2. Description of the Related Art 
Hitherto, reproduction and recording of an information signal with respect 
to an information recording medium such as an optical disc, e.g. so-called 
compact disc (CD) and magneto-optic disc, have been performed using an 
optical pickup. The optical pickup includes a semiconductor laser as a 
light source, an objective lens, an optical system, and a light detector. 
In the optical pickup, the optical beam, emitted from the semiconductor 
laser, is focused onto an optical disc recording surface by the objective 
lens via an optical system. Then, the light beam reflected by the optical 
disc is separated with a light beam emitted from the semiconductor laser 
by the optical system and then introduced into the light detector. 
The position along the optical axis dimension of the objective lens is 
adjusted by an actuator described below so that the light beam emitted 
from the semiconductor laser is focused onto the optical disc recording 
surface in response to the displacement of the optical disc along the 
dimension perpendicular to the dimension of the surface of the optical 
disc, the displacement being caused by warping of the optical disc or the 
like. At the same time, the position along the dimension perpendicular to 
the optical axis of the objective lens is adjusted by an actuator 
described below so that the position of the spot of the light beam emitted 
from the semiconductor laser on the optical disc moves along the track 
formed on the eccentricity of the optical disc or on the optical disc. 
Adjustments of the position of focus of the light beam emitted from the 
semiconductor laser and the position of the spot on the optical disc 
recording surface are performed by adjusting the objective lens at a 
position along the optical axis dimension of the objective lens as well as 
the position along the dimension perpendicular to the optical axis of the 
objective lens. The objective lens position is adjusted using an 
electromagnetic drive type actuator. 
This actuator is called an objective lens actuator or a bi-axial actuator. 
A form of the actuator comprises a bobbin having mounted thereto an 
objective lens, a plurality of elastic supporting members, and a drive 
section which generates a driving force. The bobbin is supported by the 
plurality of elastic supporting members with respect to a fixing section 
in such a manner that allows it to adjust the position of the objective 
lens along the optical axis dimension, that is the focus position, and the 
position along the dimension perpendicular to the optical axis of the 
objective lens, that is the tracking position. A bi-axial actuator will be 
described below, with reference to FIG. 5. 
Such a bi-axial actuator is, for example, constructed as shown in FIG. 5. 
Referring to the figure, a bi-axial actuator 1 comprises a lens holder 2 
having an objective lens 2a mounted at its front end, and a coil bobbin 3 
mounted to the lens holder 2 by bonding or the like. 
The aforementioned lens holder 2 is supported by two pairs of elastic 
supporting members 5, one end of each pair being fixed to both sides of 
the lens holder 2 and the other end of each pair being fixed to the fixing 
section 4, so that the holder 2 can move vertically along two dimensions, 
that is along the tracking dimension indicated by Trk and the focusing 
dimension denoted by Fcs, with respect to the fixing section 4. 
The aforementioned coil bobbin 3 is constructed as illustrated in FIG. 6. 
The coil bobbin 3 has an opening 3a which extends along the focusing 
dimension which is denoted by Fcs right through the bobbin, focusing coils 
3b wound in such a manner as to surround the opening 3a, and tracking 
coils 3c provided at two places at the front side of the coil bobbin 3. 
Each of the ends of the focusing coils 3b and the tracking coils 3c are 
connected to a connecting pin (not shown) which is provided at the back 
side of the coil bobbin 3. 
When current flows through the focusing coils 3b and the tracking coils 3c 
via the aforementioned connecting pin, a magnetic flux is produced in the 
coils 3b and the coils 3c. The magnetic flux interacts with the magnetic 
flux in a yoke 6 integrally mounted to the fixing section 4 and that in a 
permanent magnet 7 attached to the yoke 6. In this case, the coils 3b and 
3c are such as to cause the magnetic flux developed in a coil portion 
inside the effective magnetic field between an inner yoke member 6a and a 
facing yoke member 6b of the yoke 6 to generate a driving force along the 
focusing dimension or the tracking dimension. 
More specifically, force F1 which moves the lens holder 3 along the 
focusing dimension Fcs is produced by current that flows through an 
effective section 3b-1 of the focusing coil 3b in accordance with 
Fleming's left-hand rule. The effective section 3b-1 exists between the 
inner yoke member 6a and the facing yoke member 6b and takes part in 
controlling the focusing operation. On the other hand, force F2 (not 
shown) which moves the lens holder 3 along the tracking dimension Trk is 
developed by current flowing through the tracking coils 3c which extend 
vertically at the inner side, in accordance with Fleming's left-hand rule. 
The inner yoke member 6a and the facing yoke member 6b have their upper 
ends linked by a yoke bridge 6c. The yoke bridge 6c, which is made of 
magnetic material, closes the magnetic path formed by the inner yoke 
member 6a and the facing yoke member 6b. This arrangement causes the 
magnetic flux, passing through a non-effective section 3b-2, which does 
not take part in controlling the focusing operation and faces the 
aforementioned effective section 3b-1 of the focusing coil 3c, to be 
almost completely blocked by the yoke bridge 6c. As a result, a force or 
opposing thrust F3 generated by the current which flows through the 
non-effective section 3b-2 becomes so small as to be negligible. 
The aforementioned elastic supporting members 5 are made of elastic 
material and are fixed between the lens holder 2 and the fixing section 4 
such that they are parallel therewith. 
Here, the elastic supporting members 5 have flexing displacement sections 9 
provided at the end portions 5a thereof, adjacent to the fixing section 4. 
In the bi-axial actuator 1 with such a construction, the magnetic flux, 
produced in each coil by externally supplied drive voltage to each coil, 
interacts with the magnetic flux of the yoke 6 and that of the permanent 
magnet 7 to move the coil bobbin 3 along the tracking dimension Trk and 
the focusing dimension Fcs. In this way, the objective lens 2a mounted 
onto the lens holder 2 is moved along the focusing dimension and the 
tracking dimension as required. 
In the bi-axial actuator 1 with such a construction, a so-called moving 
section assembly, comprising the lens holder 2, the objective lens 2a, and 
the coil bobbin 3 having wound thereon the coils 3b and 3c, is constructed 
such that its center of gravity G, as shown in FIG. 6, roughly coincides 
with where the aforementioned driving forces F1 and F2 are applied, 
without any phase lag with regard to the movement caused by the 
aforementioned driving forces F1 and F2 during focusing and tracking 
operations. When the center of gravity roughly coincides with where the 
driving forces are applied, the resonance mode of the elastic supporting 
member 5 is suppressed. 
As shown in FIG. 6, however, in the bi-axial actuator 1 having such a 
construction, a balance weight 8 is added to the rearmost end of the lens 
holder 2 in order to make the center of gravity G of the moving section 
assembly having a heavy objective lens 2a on its left side roughly 
coincide with the point of application of the driving forces F1 and F2 due 
to the focusing coils 3b and the tracking coils 3c. 
In addition, yoke bridge 6c is provided to link the upper ends of the inner 
yoke member 6a and the facing yoke member 6b so that the magnetic flux 
produced by the permanent magnet 7 and that due to the yoke 6 do not pass 
through the noneffective section 3b-2 opposite the effective section of 
the focusing coil 3b. 
Therefore, more parts must be used, which results in higher parts cost and 
assembly cost. In addition, the balance weight 8 added to the moving 
section assembly adds to the weight of the moving section assembly and 
thus makes the assembly and the objective lens less responsive to driving 
along the focusing dimension and the tracking dimension. 
SUMMARY OF THE INVENTION 
According to the present invention, there is provided a lens moving 
apparatus including a moving assembly which holds an objective lens and 
has a coil bobbin, elastic supporting means for supporting the moving 
assembly along the focusing dimension and the tracking dimension, focusing 
coils and tracking coils which are wound on the coil bobbin, and a 
magnetic circuit formed by a yoke and a magnet. 
The center of gravity of the moving assembly is positioned such that the 
product of a distance from the effective section of the focusing coil to 
the center of gravity of the moving assembly and the force produced at the 
effective section is about equal to the product of a distance from the 
noneffective section of the focusing coil to the center of gravity of the 
moving assembly and the force produced at the noneffective section. 
According to the present invention, a force F' opposite in direction to a 
force F produced at the effective section of the focusing coil is produced 
at the noneffective section of the focusing coil utilizing leakage 
magnetic flux from the magnetic circuit. The force F is balanced by the 
opposing force F' with respect to the center of gravity of the moving 
assembly, thereby making it possible to reduce the weight of the moving 
assembly as well as to suppress resonance produced during focusing. 
With such a construction, when the lens holder holding the objective lens 
is supported by the elastic supporting members, passing current through 
the focusing coils or the tracking coils causes the lens holder to move 
along the focusing dimension or the tracking dimension against the holding 
power of the elastic supporting member, so as to effect focusing or 
tracking of the objective lens. 
The focusing coil develops a thrust and an opposing thrust along the 
focusing dimension at the effective section disposed between the inner 
yoke member and the facing yoke member and at the noneffective section 
opposite the effective section, respectively. The product of the opposing 
thrust and the distance from the noneffective section to the center of 
gravity of the moving section assembly is equal to the product of the 
aforementioned thrust and the distance from the effective section to the 
center of gravity G of the moving section assembly. Therefore, the 
opposing thrust, produced at the noneffective section opposite the 
effective section of the focusing coil, suppresses resonance mode of the 
elastic supporting member caused by the movement of the moving section 
assembly during focusing. 
When a balance weight is not fixed to the rearmost end of the lens holder 
to position the center of gravity G forwardly of the tracking coils, the 
moving section assembly becomes lighter in weight. 
When the inner yoke member and the facing yoke member each have an open 
upper end in order to increase the opposing thrust produced at the 
noneffective section of the focusing coil, opposite the effective section, 
it becomes unnecessary to employ a yoke bridge that links the upper ends 
of the inner yoke member and the facing yoke member to form a magnetic 
path.

DESCRIPTION OF A PREFERRED EMBODIMENT 
A description will hereunder be given in detail of a preferred embodiment 
of the present invention, with reference to FIGS. 1 to 4. 
It is to be noted that although various preferred technical limitations 
have been described, since the embodiment to be described below is a 
preferred embodiment of the present invention, the scope of the present 
invention is not limited to the forms thereof, unless specific mention is 
made of limitations of the present invention in the following description. 
FIGS. 1 to 3 illustrate a bi-axial actuator of an embodiment in accordance 
with the present invention. Referring to FIGS. 1 to 3, a bi-axial actuator 
10 includes a lens holder 11, a coil bobbin 12, a plurality of elastic 
supporting members 13a, 13b, 13c, and 13d, a fixing section 14, and a yoke 
31. 
As illustrated in FIG. 3, the aforementioned lens holder 11 is divided into 
an upper section 11U and a lower section 11L by a horizontal dividing 
line, the sections being bonded together. The lens holder 11, as shown in 
FIG. 3, has an opening 11a used for fixing the coil bobbin as well as a 
recess 11b used for fixing the objective lens. 
A hole is formed in the bottom of the recess 11b, and is used for passing a 
light beam emitted from the semiconductor laser or a returning light beam 
from an optical disc recording surface. An objective lens 11c is mounted 
to the recess 11b of the lens holder by a bond or the like. 
Further the lens holder is supported by the elastic supporting members 13a, 
13b, 13c, and 13d in such a manner as to allow it to move along the 
focusing dimension Fcs, (which is parallel to the optical axis of the 
objective lens 11c); and the tracking dimension Trk, (which is 
perpendicular to the optical axis of the objective lens 11c). 
The coil bobbin,12 has formed therein an opening 12a, and is used for 
inserting a magnetic circuit including the yoke 31 formed integrally with 
a base 33 and a permanent magnet 32, the magnet being attached to the 
inner side of an inner yoke member 31a. It also has wound thereon focusing 
coils 12b and tracking coils 12c. 
The focusing coils 12b are wound on the coil bobbin 12 along an axis 
parallel to the optical axis of the objective lens 11c. On the other hand, 
the tracking coils 12c are elliptical-shaped or rectangular-shaped coils. 
The coil bobbin 12, with the focusing coils 12b and the tracking coils 12c 
wound thereon, is fitted into the opening 11a formed in the lens holder 
11. 
Materials such as phosphor bronze, beryllium copper, titanium copper, 
tin-nickel alloy, stainless steel, etc., are used to make the elastic 
supporting members 13a, 13b, 13c, and 13d because it is preferable that 
they are conductive and resilient. In the embodiment, these are formed, 
for example, as sheet metal suspensions by thin leaf springs. These 
suspensions are fixed between the lens holder 11 and the fixing section 14 
in such a manner as to be parallel therewith. 
Accordingly, the elastic supporting members 13a, 13b, 13c, and 13d may be 
constructed such that the driving current from an external current supply 
means (not shown) is supplied to the focusing coils 12b and the tracking 
coils 12c. 
Viscous members 16, or dampers, are applied to the end areas 15 of the 
elastic supporting members 13a, 13b, 13c, and 13d, and hardened. 
With the lens holder 11 and the fixing section 14 linked to the four 
elastic supporting members 13a, 13b, 13c, and 13d, the fixing section 14 
is mounted onto an adjusting plate 30. The adjusting plate 30 is used for 
adjusting the fixing position of the fixing section 14 during assembly of 
the bi-axial actuator 10, and is fixed onto the base 33 formed integrally 
with the yoke 31 by soldering or the like. 
Mounting of the adjusting plate 30 onto the base 33 is performed by 
soldering rise sections 30a with respect rise sections 31c. The rise 
sections 30a extend upward from both sides of around the back end of the 
adjusting plate 30. The rise sections 31c extend upward from both sides of 
the back end of the base 33. 
The base 33 has the pair of yoke members 31a and 31b forming the 
above-described magnetic circuit, which bend upward from the edges of the 
base 33 at the objective lens side, and the permanent magnet 32 mounted to 
the inner side of the inner yoke member 31a which is opposite the facing 
yoke member 31b. Accordingly, the magnetic circuit is formed by the pair 
of yoke members 31a and 31b, and the permanent magnet 32. 
As illustrated above, when the fixing section 14 is mounted onto the base 
33, the focusing coils 12b and the tracking coils 12c wound on the coil 
bobbin 12 are inserted into the gap between the facing yoke member 31b and 
the permanent magnet 32. At the same time, the inner yoke member 31a and 
the permanent magnet 32 are inserted into the opening 12a in the coil 
bobbin 12. 
The coil bobbin 12 of the bi-axial actuator 10 may be constructed as 
illustrated in FIG. 4. 
The coil bobbin 12 includes focusing coils 12b wound thereon such that they 
surround the opening 12a. The focusing coils 12b have an effective section 
12b-1, which exists between the yoke members 31a and 31b, and a 
noneffective section 12b-2, which exists opposite the effective section 
12b-1. Passing current through the focusing coils 12b causes drive current 
to flow through the aforementioned effective section 12b-1 and 
noneffective section 12b-2 of the focusing coil 12b. The current which 
flows through the effective section 12b-1 of the focusing coils 12b, 
disposed between the inner yoke member 31a and the facing yoke member 31b, 
reacts with the magnetic flux due to the yoke members 31a, 31b, and the 
permanent magnet 32 so as to generate, in accordance with Fleming's 
left-hand rule, a thrust F1 which moves the lens holder 11 along the 
focusing dimension Fcs. 
Unlike conventional yokes, in this case, the upper ends of the inner yoke 
member 31a and the facing yoke member 31b are not linked by a yoke bridge, 
but are open. This means that leakage magnetic flux from the yoke members 
31a, 31b, and the permanent magnet 32 flows through the noneffective 
section 12b-2 of the focusing coils 12b, opposite the effective section 
12b-1 . When this occurs, the current flowing through the noneffective 
section 12b-2 of the focusing coils 12b, opposite the effective section 
12b-1 , reacts with the leakage magnetic flux due to the yoke 31a, 31b, 
and the permanent magnet 32, so as to generate, in accordance with 
Fleming's left-hand rule, an opposing thrust F3 which moves the lens 
holder 3 along the focusing dimension Fcs. 
The moving section assembly, including the coil bobbin 12 having wound 
thereon the focusing coils 12b and the tracking coils 12c, the objective 
lens 11c, and the lens holder 11, has its center of gravity G positioned 
forwardly of the front focusing coils 12b-1 and the tracking coils 12c, as 
illustrated in FIG. 4. Here, the position of the center of gravity is 
selected such that when the distances from the centers of gravity G to the 
effective section 12b-1 and the noneffective section 12b-2 of the focusing 
coils 12b toward the back side of the bi-axial actuator are represented by 
L1 and L3, respectively, the following Equation 1 is satisfied: 
EQU F1.times.L1=F3.times.L3 (1) 
In the bi-axial actuator 1 such as that illustrated in FIG. 5, the center 
of gravity G of the moving section assembly is positioned between the 
effective section 3b-1 of the focusing coil 3b and the tracking coil 3c, 
whereas in the bi-axial actuator 10 of the embodiment, the center of 
gravity G is shifted toward the front. The difference in the center of 
gravity position occurs because the balance weight 8, mounted to the 
rearmost end of the conventional moving section assembly or the lens 
holder 2, is not employed in the bi-axial actuator of the embodiment. 
The bi-axial actuator 10 of the embodiment is constructed in the 
above-described way, wherein current is supplied to each of the focusing 
coils 12b and the tracking coils 12c, both of which are wound on the coil 
bobbin 12, in accordance with the focus servo signal and the tracking 
servo signal, respectively. 
This causes the direct current magnetic field of the magnetic circuit and 
the alternating magnetic field developed from the focusing coils 12b and 
the tracking coils 12c to drive the lens holder 11, that is the objective 
lens 11c, along the focus dimension Fcs and the tracking dimension Trk. 
The viscous members 16, or dampers, are applied onto the end areas 15 of 
the elastic supporting members 13a, 13b, 13c, and 13d, the end areas being 
adjacent to the fixing section 14, and are hardened, so that the desired 
damping characteristics can be obtained. The dampers dampen the vibrations 
of the elastic supporting members 13a, 13b, 13c, and 13d, during focusing 
or tracking. 
Here, as shown in FIG. 4, the center of gravity G of the moving section 
assembly is positioned forwardly of the tracking coils 12c, so that the 
Equation 1 is satisfied, as stated above. Accordingly, when the opposing 
thrust F3 due to the leakage magnetic flux that is developed in the 
noneffective section 12b-2, opposite the effective section of the focusing 
coils 12b, is utilized to balance it with the thrust F1 developed at the 
effective section 12b-1 with respect to the center of gravity, the 
opposing thrust F3 suppresses resonance mode resulting from the thrust F1, 
during focusing, at the elastic supporting members 13a, 13b, 13c, and 13d. 
In this way, the resonance mode of the elastic supporting members 13a, 13b, 
13c, and 13d, during focusing, is suppressed by the opposing thrust F3 at 
the noneffective section 12b-2 of the focusing coils 12b, opposite the 
effective section 12b-1. Here, the opposing thrust F3 at the noneffective 
section 12b-2, opposite the effective section of the focusing coils 12b, 
is used, thereby making it unnecessary to block the magnetic flux that 
passes through the noneffective section 12b-2. This in turn makes it 
unnecessary to use a yoke bridge which links the upper ends of the yoke 
members 31a and 31b to form a closed magnetic circuit. 
In this way, in the focusing coil of the embodiment, a thrust and an 
opposing thrust along the focusing dimension are produced, respectively, 
at the effective section between the yoke members and the noneffective 
section which is opposite the effective section. Resonance mode of the 
elastic supporting members caused by movement of the moving section 
assembly during focusing is suppressed by the opposing thrust produced at 
the noneffective section, opposite the effective section of the focusing 
coil. Therefore, to reduce the opposing thrust at the noneffective 
section, opposite the effective section of the focusing coil, it is no 
longer necessary to block the passage of the leakage magnetic flux to the 
noneffective section. 
The moving section assembly can be made lighter in weight when the balance 
weight is not mounted to the rearmost end of the lens holder in order to 
position the center of gravity forwardly of the tracking coil. Therefore, 
fewer parts need to be used, the parts cost and the assembly cost are 
reduced, and, in addition, the responsiveness of the moving section 
assembly, during focusing and tracking, is improved because of its lighter 
weight. 
To increase the opposing thrust produced at the noneffective section, 
opposite the effective section of the focusing coil, it is not necessary 
to use a yoke bridge to link the open upper ends of the inner yoke member 
and the facing yoke member, which results in the use of fewer parts, and 
reduced parts cost and assembly cost. 
It is obvious that the elastic supporting members 13a, 13b, 13c, and 13d in 
the aforementioned embodiment can be inserted into the lens holder 11 and 
the fixing section 14 to form an integral structure therewith, although in 
the foregoing description the elastic supporting members were merely fixed 
with respect to the lens holder and the fixing section 14. It is also 
obvious that the lens holder 11 can be integrally formed, although in the 
foregoing description it was divided into the upper section 11U and the 
lower section 11L.