JPH07147021A - Objective lens driving device - Google Patents

Abstract

PURPOSE:To provide the objective lens driving device which is formable to a smaller size and thickness and is capable of stably driving the objective lens both in a focusing direction and a tracking direction. CONSTITUTION:This device 10 includes a moving section 13 provided with the objective lens, a focusing coil 11 and a tracking coil 12 and a stationary section 17 provided with a single magnetic circuit having a magnetic gap 18. Both coils 11, 12 are arranged in this magnetic gap 18. As a result, the formation of the device to the smaller size and thickness is possible. The device 10 is constituted by aligning the working point of tracking driving power, the working point of the composite force of the focus driving powers respectively generated inside and outside of the magnetic gap 18 and the centroid of the moving section 13. As a result, the generation of the resonation around the X-, Y- and Z-axes with a gravity position 13a as a center is suppressed and the generation of the unnecessary resonation is prevented.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an objective lens driving device applied to an optical pickup device for recording / reproducing information on / from an optical disk, and more specifically, to a compact and thin objective lens driving device and focusing. Stabilization of driving in both directions, the tracking direction and the tracking direction.

[0002]

2. Description of the Related Art In order to accurately record and reproduce information on an optical disc, it is necessary to prevent unnecessary resonance. In order to prevent the occurrence of such unnecessary resonance, FIG.
The conventional objective lens driving device 1A shown in the perspective view of FIG.
Tracking coil 3B for A and tracking direction Y
The position of the center of gravity of the movable portion 4 provided with is aligned with the optical axis 5, and the central axes of the focus coil 3A and the tracking coil 3B are aligned with the optical axis 5 (for example, Japanese Patent Laid-Open No. 2-2305)
No. 22).

In recent years, the size of optical pickup devices has been reduced.
As the device becomes thinner, the objective lens driving device is required to be smaller and thinner. In order to deal with such a request, FIG.
The conventional objective lens driving device 1B shown in the exploded perspective view of
The focus coil 8A and the tracking coil 8B are arranged such that the center axes of the focus coil 8A and the tracking coil 8B do not coincide with the optical axis 5 in the magnetic gap 7 of the single magnetic circuit 6 (for example, Japanese Patent Laid-Open No. Hei 4). −
No. 102235, JP-A-4-103038).

Further, the objective lens driving device disclosed in Japanese Patent Laid-Open No. 4-103038 discloses a focus driving force generated outside the magnetic gap in order to accurately drive the movable portion in the optical axis direction (focusing direction). Is minimized to prevent unnecessary force such as moment from acting on the movable part. The focus driving force, that is, the leakage magnetic flux density, generated outside the magnetic gap should be kept as low as possible in order to prevent interference with metal parts such as a motor arranged around the objective lens driving device. It was

[0005]

However, as shown in FIG.
The conventional objective lens driving device 1A shown in (1) can prevent the occurrence of unnecessary resonance, but has a drawback that it is difficult to make the device compact and thin.

In the conventional objective lens driving device 1B shown in FIG. 12, the device can be made smaller and thinner, but the center of gravity should be aligned with the driving point of either the focus coil 8A or the tracking coil 8B. In that case, there is a drawback that the other driving point deviates from the position of the center of gravity, and unwanted resonance occurs in the deviated direction.

The present invention has been made in view of the above circumstances, and provides an objective lens driving device which can be made compact and thin and which can stably drive the objective lens in both the focus direction and the tracking direction. The purpose is to

[0008]

In order to achieve the above object, an objective lens driving device according to claim 1 is provided with an objective lens,
A track tangential line in an objective lens driving device, comprising: a movable part having a focus coil and a tracking coil; and a fixed part having a single magnetic circuit having a magnetic gap, wherein both coils are arranged in the magnetic gap. in the direction, the distance between the point of action of the focus driving force generated in the magnetic gap and the working point of the tracking driving force L _1, into and out of the magnetic gap and the working point of the focus driving force generated in the magnetic gap Each part is configured so as to satisfy | L ₁ −L ₂ | <| L ₁ |, where L ₂ is the distance from the point of action of the combined focus driving force. is there.

In the objective lens driving device according to a second aspect of the present invention, the distance between the center of gravity of the movable portion and the point of action of the focus driving force generated in the magnetic gap in the track tangential direction is L _0. And L ₁ ≧ L ₀ ≧ L
₂ (when L ₁ ≧ L ₂ ) or L ₁ ≦ L ₀ ≦ L ₂ (L ₁ ≦
It is characterized in that each part is configured so as to satisfy (at L ₂ ).

In the objective lens driving device according to the third aspect of the invention, | L ₀ -L ₂ | ≤50 μm and | L ₀ -L ₁ | ≤2.
It is characterized in that each part is configured so as to satisfy even 00 μm.

An objective lens driving device according to a fourth aspect of the present invention comprises a movable part having an objective lens, a focus coil and a tracking coil, and a fixed part having a single magnetic circuit having a magnetic gap. In an objective lens driving device in which both of the coils are arranged in the magnetic gap, an action point of a tracking drive force, an action point of a combined force of focus drive forces generated inside and outside the magnetic gap, and a center of gravity of the movable portion. Is characterized in that they substantially coincide with each other.

An objective lens driving device according to a fifth aspect of the present invention comprises a movable part having an objective lens, a focus coil and a tracking coil, and a fixed part having a single magnetic circuit having a magnetic gap. In the objective lens driving device in which the both coils are arranged in the magnetic gap, the distance between the position of the center of gravity of the movable portion and the point of action of the focus driving force generated in the magnetic gap in the track tangential direction is L ₀ , and tracking is performed. The distance between the operating point of the driving force and the operating point of the focus driving force generated in the magnetic gap is L ₁ , and the operating point of the focus driving force generated in the magnetic gap and the focus generated inside and outside the magnetic gap, respectively. the distance between the point of application of the resultant force of the driving force L _2, action of the focus driving force generated respectively in and out of the magnetic gap Distance L ₃ between the points, the B ₁ magnetic flux density in the magnetic gap, the leakage magnetic flux density at the point of application of the focus driving force generated outside the magnetic gap B
In case of a _{2, L 2 = B 2 ·} L 3 / As (B ₁ -B ₂₎ and L ₂ and L ₁ and L ₀ which is expressed by a substantially equal,
It is characterized in that each value of L ₀ , L ₁ , L ₂ , L ₃ , B ₁ , and B ₂ is selected.

[0013]

According to the objective lens driving device of the first aspect,
By configuring each part so as to satisfy | L ₁ −L ₂ | <| L ₁ |, the action point of the combined force of the focus driving force generated inside and outside the magnetic gap approaches the action point of the tracking driving force. Therefore, it is possible to reduce both the distance from the center of gravity of the movable portion. This makes it easy to prevent unnecessary resonance from occurring in both the focus direction and the tracking direction.

According to the objective lens driving device of the second aspect, L ₁ ≧ L ₀ ≧ L ₂ (when L ₁ ≧ L ₂ ) or L ₁ ≦
By configuring each part so as to satisfy L ₀ ≦ L ₂ (when L ₁ ≦ L ₂ ), the position of the center of gravity of the movable part and the tracking point of the combined force of the focus driving force generated inside and outside the magnetic gap and tracking It can be set between the action point of the driving force and the distance between the center of gravity of the movable portion and the action point of the combined force and the action point of the tracking driving force. This makes it possible to prevent the occurrence of unnecessary resonance in both the focus direction and the tracking direction.

According to the objective lens driving device of the third aspect, | L ₀ -L ₂ | ≦ 50 μm and | L ₀ -L ₁ | ≦ 2
By configuring each part so that it also satisfies 00 μm,
Unwanted resonance can be reduced.

According to the objective lens driving device of the fourth aspect, since the focus coil and the tracking coil are arranged in the magnetic gap, the size and thickness can be reduced.
Further, by causing the action point of the tracking driving force and the position of the center of gravity of the movable portion to substantially coincide with each other, it is possible to prevent the occurrence of resonance around the optical axis. Resonance around the axis orthogonal to the optical axis can be prevented by matching the action point of the combined force of the focus driving force generated inside and outside the magnetic gap with the position of the center of gravity of the movable portion. This makes it possible to prevent the occurrence of unnecessary resonance in both the focus direction and the tracking direction.

According to the objective lens driving device of the fifth aspect, since the focus coil and the tracking coil are arranged in the magnetic gap as in the first aspect, the size and thickness can be reduced. Further, by selecting the respective values of L ₀ , L ₁ , L ₂ , L ₃ , B ₁ , and B ₂ so as to satisfy the above equation, the points of action of the tracking driving force and the inside and outside of the magnetic gap are generated. The point of action of the combined force of the focus driving force and the position of the center of gravity of the movable portion substantially match each other,
It is possible to prevent occurrence of resonance around the optical axis and about an axis orthogonal to the optical axis, and prevent occurrence of unnecessary resonance in both the focus direction and the tracking direction.

[0018]

Embodiments of the present invention will now be described in detail with reference to the drawings.

FIG. 1 shows the first objective lens driving device of the present invention.
3 is a perspective view of an essential part showing an embodiment of FIG.
FIG.

The apparatus 10 of the first embodiment has a movable portion 13 having an objective lens (not shown), a lens holder for holding the objective lens, a focus coil 11 and a tracking coil 12 fixed to the lens holder, and a U-shape. York 1
4, upper yoke 15 and a pair of magnets 16a,
16b and a fixed part 17 for supporting the movable part 13 by a spring member (not shown) which also serves as a current supply path to each coil 11, 12 of the movable part 13 and which includes a magnetic circuit and the like.

The U-shaped yoke 14 of the fixing portion 17 is formed in a U-shaped cross section so that the pair of first leg portion 14a and second leg portion 14b face each other. The upper yoke 15 is fixed to the tip surfaces of the first and second leg portions 14a and 14b of the U-shaped yoke 14. The pair of magnets 16a and 16b are arranged inside the first and second leg portions 14a and 14b of the U-shaped yoke 14 so that the different poles of the S pole and the N pole face each other. A magnetic gap 18 is formed between 16a and 16b.

Focus coil 11 of the movable part 13
Is wound around its central axis so that the coil member has a square cross section, and its central axis is parallel to the optical axis 19,
A gap is provided around the first leg portion 14a of the U-shaped yoke 14 so that a part of the gap passes through the magnetic gap 18. As a result, when a current is supplied to the focus coil 11, the lens holder (not shown) moves in the focus direction (optical axis direction) Z. The tracking coil 12 is wound so that a current flows in the focus direction Z, and the magnetic gap 1 of the focus coil 11 is increased.
It is fixed to the 8 side. As a result, when a current is supplied to the tracking coil 12, the lens holder (not shown) moves in the vertical direction Y with respect to the optical axis 19.

Further, in the apparatus 10 of the present embodiment, as shown in FIG. 4, the point of application of the tracking driving force generated by the tracking coil 12 in the track tangential direction (direction perpendicular to the focus direction Z and the tracking direction Y) X. 12t, a combined force (F ₁ + F) of the in-gap focus driving force F ₁ generated in the magnetic gap 18 and the out-of-gap focus driving force F ₂ generated outside the magnetic gap 18 (outside the first leg 14a). and the action point 12f of _2),
Each part is designed so that the position 13a of the center of gravity of the entire movable part 13 substantially coincides in the X direction.

Specifically, each part is designed by the following method. That is, in the track tangential direction X, the distance between the barycentric position 13a of the movable portion 13 and the action point 11a of the focus driving force F ₁ generated in the magnetic gap is L ₀ , and the action point 12t of the tracking driving force and the magnetic gap 18 are. distance L _1, magnetic working point of the focus driving force F ₁ generated in gap 18 11a and the gap in the focus drive force F ₁ and the gap outside the focus driving force F between the point 11a of the focus driving force F ₁ generated in ₂ synthetic power (F
₁ + F ₂ L ₂ the distance between the point 12f of), the distance between the action point 11b of the action point 11a and the gap outside focus drive force F ₂ of the gap focus drive force F ₁ L _3, in the magnetic gap 18 The generated magnetic flux density in the gap is B ₁ ,
When the out-gap magnetic flux density (hereinafter referred to as “leakage magnetic flux density”) generated at the portion where the out-gap focus driving force F ₂ acts is B ₂ , L ₂ = F ₂ · L ₃ / (F ₁ −F ₂ ) = B ₂ · L ₃ / (B ₁ −B ₂ ) ... (1)

This means that the driving force F is the effective width L and the current I.
This is because the magnetic flux density B is proportional to the magnetic flux density B when is constant.

[0026] Therefore, as the L ₂ and L ₁ and L ₀ is substantially equal, selecting _{L 0, L 1, L 2} , L 3, B 1, each value of B _2.

As a result, the acting point 12f of the combined force (F ₁ + F ₂ ), the acting point 12t of the tracking driving force generated by the tracking coil 12, and the center of gravity 13a of the entire movable portion 13 coincide with each other in the X direction. Therefore, generation of unnecessary resonance can be prevented.

The effects of the first embodiment thus constructed will be described with reference to FIGS. 5 and 6.

FIG. 5 is a graph showing the relationship between the thickness of the upper yoke 15 and the magnetic flux density B _{1 in the} gap and the leakage magnetic flux density (magnetic flux density outside the gap) B ₂ . FIG. 6 is a graph showing the effect of the thickness of the U-shaped yoke 14 on the pitching resonance, and when the center of gravity 13a of the movable portion 13 is substantially aligned with the tracking drive point 12t and the thickness of the upper yoke 15 is changed, the Y-axis rotation 3 is a graph showing the magnitude (gain) and phase of resonance (pitching resonance) in the rotation mode of FIG.

According to the first embodiment, since the focus coil 11 and the tracking coil 12 are arranged in one magnetic gap 18, the size and thickness can be reduced.

As is clear from FIG. 5, the magnetic flux density B ₁ in the gap and the leakage magnetic flux density B ₂ change with the thickness of the upper yoke 15, and the magnetic flux density in the gap increases as the thickness of the upper yoke 15 increases. B ₁ increases and leakage magnetic flux density B ₂ decreases. Therefore, as the thickness of the upper yoke 15 increases at an arbitrary distance L ₃ ,
L ₂ becomes smaller, and the action point 12 f of the combined force (F ₁ + F ₂ ) approaches the action point 11 a of the focus drive force F ₁ in the gap. Therefore, when L ₁ and L ₃ are fixed in size, if the thickness of the upper yoke 15 is set to an appropriate value so that the values of B ₁ and B ₂ are L ₁ = L ₂ , the action of the tracking driving force The point 12t, the action point 12f of the combined force (F ₁ + F ₂ ) of the focus driving force F ₁ inside the gap and the focus driving force F ₂ outside the gap, and the center of gravity position 13a of the entire movable portion 13 coincide with each other in the X direction. . By making the action point 12t of the tracking driving force and the center of gravity 13a of the movable portion 13 coincide with each other, it is possible to prevent the occurrence of resonance around the Z axis about the center of gravity 13a. Synthetic power (F ₁
+ F ₂ ) point of action 12f and center of gravity 13a of movable part 13
By matching and, it is possible to prevent the occurrence of resonance around the Y axis around the center of gravity 13a. As described above, by positively utilizing the out-gap focus driving force F ₂ which should be made as small as possible, it is possible to prevent the occurrence of unnecessary resonance in both the tracking direction Y and the focus direction Z.

Further, as apparent from FIG. 6, if the thickness of the upper yoke 15 is set to 0.55 mm, the occurrence of pitching resonance can be prevented.

Further, the objective lens driving device disclosed in Japanese Patent Laid-Open No. 4-102235, which is a publicly known example, takes measures on the focus side, but this embodiment has an advantage that it can also deal with the tracking side.

FIG. 7 shows the second objective lens driving device of the present invention.
FIG. 9 is a diagram showing a positional relationship in the track tangential direction X of the respective action points 11a, 12f, 12t and the center of gravity position 13a showing the embodiment of FIG. The second embodiment is different from the first embodiment in the relation of the action points 11a, 12f, 12t and the center of gravity position 13a, and is otherwise common to the first embodiment.

In the second embodiment, | L ₁ -L ₂ | <
Each part is configured so that | L ₁ |, and L ₁ ≧ L ₀
≧ L ₂ (when L ₁ ≧ L ₂ ) or L ₁ ≦ L ₀ ≦ L ₂ (L
L ₀ is determined so that ₁ ≦ L ₂ ).

The effects of the second embodiment will be described with reference to FIGS. 8 to 10.

FIG. 8 shows L ₀ = 200 μm and L ₁ = 400 μm.
m, L ₂ = 150 μm, S ₂ = 50 μm, S ₁ = 2
FIG. 9 is a transmission characteristic diagram of the actuator in the focus direction Z when it is set to 00 μm.
The positions of and are arranged opposite to those in FIG. 8, and the value of S ₂ is set to 0.
The transfer characteristic diagram of the actuator when it is set to 05 mm, and FIG. 10 shows L ₀ = 200 μm, L ₁ = 400 μm, L ₂ = 2
FIG. 10 is a transfer characteristic diagram of the actuator in the tracking direction Y when 50 μm, S ₂ = 50 μm, and S ₁ = 200 μm. Further, in FIGS. 8 to 10, a portion indicated by a dashed-dotted circle indicates a resonance point.

As is apparent from FIG. 8, the resonance of the rotation mode around the Y axis about the center of gravity 13a appears in the portion indicated by the dashed line circle, but it is sufficiently small. Therefore,
As shown in FIG. 7, the distance S between the action point 12t of the tracking drive force and the action point 12f of the combined force (F ₁ + F ₂ ) which is the actual focus drive force is shortened, and further, during that period (S), the entire movable portion 13 is moved. If the center of gravity position 13a is arranged, the center of gravity position 1
Both the distance S ₁ between the point 3a and the point of action 12t and the distance S ₂ between the center of gravity 13a and the point of action 12f can be made smaller than in the conventional case, so that the occurrence of unnecessary resonance in both directions can be made sufficiently small.

As is clear from FIG. 9, the resonance of the rotational mode about the Y axis around the center of gravity 13a appears in the portion indicated by the chain line, and the phases are opposite to those in FIG. However, the resonance value is sufficiently small. As is clear from FIG. 10, the position of the center of gravity 1
Resonance of the rotation mode around the Z axis centering on 3a appears, but it is sufficiently small. Therefore, without placing the movable portion 13 of the center of gravity of the whole position 13a between the point 12f of the actual focus driving force and action points 12t of the tracking drive force (resultant force) required if S _1, S ₂ is sufficiently small The occurrence of resonance can be made sufficiently small. As described above, not only in the above arrangement, but by reducing S, it is possible to sufficiently reduce unnecessary resonance by reducing S ₁ and S ₂ .

The present invention is not limited to the above embodiment, but can be modified in various ways. For example, in order to control the values of the magnetic flux density B ₁ in the gap and the leakage magnetic flux density B ₂ , the thickness or shape of one leg portion 14a of the yoke 14 may be devised, and L ₁ and L ₃ may be included. Each value may be determined so that L ₁ = L ₂ .

[0041]

According to the present invention described in detail above, the following effects can be obtained.

According to the first aspect of the present invention, the action point of the combined force of the focus driving force generated inside and outside the magnetic gap is brought close to the action point of the tracking driving force, and the distance from the position of the center of gravity of the entire movable portion is set. Since it is possible to reduce both, it becomes easy to prevent the occurrence of unnecessary resonance in both the focus direction and the tracking direction.

According to the second aspect of the present invention, the position of the center of gravity of the entire movable portion is set between the action point of the combined force of the focus driving force generated inside and outside the magnetic gap and the action point of the tracking driving force. Since the distance between the center of gravity of the entire movable portion and the point of application of the combined force and the point of application of the tracking driving force is shortened, it is possible to prevent the occurrence of unnecessary resonance in both the focus direction and the tracking direction.

According to the invention of claim 3, | L ₀ -L
Unnecessary resonance can be reduced by configuring each part so that ₂ | ≦ 50 μm and | L ₀ −L ₁ | ≦ 200 μm are also satisfied.

According to the fourth aspect of the present invention, the focus coil and the tracking coil are arranged in a single magnetic circuit, and the combined force of the focus driving force generated inside and outside the action point of the tracking driving force and the magnetic gap, respectively. Since the point of action and the position of the center of gravity of the movable portion are made to substantially coincide with each other, the size and thickness can be reduced, and stable servo can be achieved in which unnecessary resonance does not occur in the focus direction and the tracking direction.

According to the invention of claim 5, the focus coil and the tracking coil are arranged in a single magnetic circuit, and L ₀ , L ₁ , L ₂ , L ₃ and
By selecting the respective values of B ₁ and B ₂ , the action point of the tracking drive force, the action point of the combined force of the focus drive force generated inside and outside the magnetic gap, and the position of the center of gravity of the movable portion are substantially mutually Because it is configured to match, small size
Thinning is possible, and stable servo can be achieved in which unnecessary resonance does not occur in both the focus and tracking directions. If necessary, L ₀ , L ₁ , L ₂ , L ₃ ,
Since each value of B ₁ and B ₂ can be selected, a highly flexible design is possible. Moreover, since the present invention can be configured based on the magnetic flux density, which is easier to measure than the driving force, the design of each part is facilitated.

[Brief description of drawings]

FIG. 1 is a perspective view of an essential part showing an embodiment of an objective lens driving device of the present invention.

FIG. 2 is a vertical sectional view of FIG.

FIG. 3 is a cross-sectional view of FIG.

FIG. 4 is a diagram showing the relationship between each driving force and the position of the center of gravity.

FIG. 5 is a graph showing the relationship between the thickness of the upper yoke and the gap magnetic flux density and the leakage magnetic flux density of this embodiment.

FIG. 6 is a graph showing the effect of the thickness of the U-shaped yoke on the pitching resonance of the present embodiment.

FIG. 7 is a diagram showing the relationship between each action point and the position of the center of gravity.

FIG. 8 is a transfer characteristic diagram of the present embodiment.

FIG. 9 is a transfer characteristic diagram of the present embodiment.

FIG. 10 is a transfer characteristic diagram of the present embodiment.

FIG. 11 is a perspective view of a main part of a conventional objective lens driving device.

FIG. 12 is a perspective view of a main part of a conventional objective lens driving device.

[Explanation of symbols]

10 Objective Lens Driving Device 11 Focus Coil 11a Point of Action of Focus Driving Force in Gap 11b Point of Action of Focus Driving Force Outside the Gap 12 Tracking Coil 12f Point of Action of Combined Force of Focus Driving Force 12t Point of Action of Tracking Driving Force 13 Moving Part 13a Center of gravity of movable part 17 Fixed part 18 Magnetic gap F ₁ Focus drive force inside gap F ₂ Focus drive force outside gap

 ─────────────────────────────────────────────────── --Continued front page (72) Inventor Mitsuru Kinouchi 1-13-1 Nihonbashi, Chuo-ku, Tokyo TDC Corporation

Claims (5)

[Claims] 1. A movable part having an objective lens, a focus coil and a tracking coil, and a fixed part having a single magnetic circuit having a magnetic gap, wherein both coils are arranged in the magnetic gap. In the objective lens driving device, the distance between the action point of the tracking drive force and the action point of the focus drive force generated in the magnetic gap in the track tangential direction is L ₁ , and the action of the focus drive force generated in the magnetic gap Let L ₂ be the distance between the point and the point of action of the combined force of the focus driving force generated inside and outside the magnetic gap, | L ₁ −L ₂
An objective lens driving device in which each part is configured so as to satisfy | <| L ₁ |. 2. L ₁ ≧ L, where L ₀ is the distance between the position of the center of gravity of the movable portion and the point of application of the focus driving force generated in the magnetic gap in the track tangential direction.
₀ ≧ L ₂ (when L ₁ ≧ L ₂ ) or L ₁ ≦ L ₀ ≦ L
_2. The objective lens driving device according to claim 1, wherein each part is configured so as to satisfy _{2 as well} (when L ₁ ≦ L ₂ ). 3. | L ₀ −L ₂ | ≦ 50 μm and | L ₀ −
3. The objective lens driving device according to claim 2, wherein each part is configured so as to satisfy L ₁ | ≦ 200 μm. 4. A movable part having an objective lens, a focus coil and a tracking coil, and a fixed part having a single magnetic circuit having a magnetic gap, wherein both coils are arranged in the magnetic gap. In the objective lens driving device, the action point of the tracking drive force and the action point of the combined force of the focus drive force generated inside and outside the magnetic gap and the position of the center of gravity of the movable portion are substantially matched with each other. Objective lens driving device. 5. A movable part provided with an objective lens, a focus coil and a tracking coil, and a fixed part provided with a single magnetic circuit having a magnetic gap, and both coils are arranged in the magnetic gap. In the objective lens driving device, the distance between the position of the center of gravity of the movable portion and the operating point of the focus driving force generated in the magnetic gap in the track tangential direction is L ₀ , and the operating point of the tracking driving force is in the magnetic gap. The distance from the point of action of the focus driving force generated is L ₁ , and the distance between the point of action of the focus driving force generated inside the magnetic gap and the point of application of the combined force of the focus driving force generated inside and outside the magnetic gap. L _{2 is} the distance between the action points of the focus driving force generated inside and outside the magnetic gap, and L _{3 is} the magnetic field in the magnetic gap. When the flux density is B ₁ and the leakage magnetic flux density at the point of action of the focus driving force generated outside the magnetic gap is B ₂ , L ₂ = B ₂ · L ₃ / (B ₁ −
Each of the values of L ₀ , L ₁ , L ₂ , L ₃ , B ₁ , and B ₂ is selected so that L ₂ represented by B ₂ ) and L ₁ and L ₀ are substantially the same. Objective lens drive device.