The present invention relates to an optical head device for an optical disk apparatus for optically reading and writing information by irradiating a light spot onto a information recording surface of an optical disk (optical recording medium)
An optical head device for an optical disk apparatus generally includes an objective lens drive unit having an objective lens, and an optical system for receiving and transmitting a beam via the optical lens, and has such construction that the objective lens drive unit is disposed on a mounting base for an optical system block.
The objective lens drive unit is generally constructed of a movable unit including an objective lens, a focus coil, and a tracking coil, and a stationary unit provided with a magnetic circuit. The movable unit is supported by the stationary unit via a plurality of elastic supporting members. The elastic supporting member is enclosed at least partly by a shock-absorbing material.
In accordance with recent increase in speed of the optical disk apparatus, there is a need for a highly sensitive objective lens drive unit. In JP-A-2001-229555, an objective lens drive unit and an optical head device using the same in which a layout of a magnet, a focus coil and a tracking coil, which can provide a highly sensitive objective lens drive unit, is disclosed.
Referring now to FIG. 8 and FIG. 9, an optical head device in the related art will be described. FIG. 8 is an exploded perspective view showing a construction of an objective lens drive unit of the optical head device in the related art. The objective lens drive unit shown in FIG. 8 has such construction that a coil and a magnet are disposed on both sides of an objective lens 210 respectively in tangential direction of an optical recording medium so as to be capable of driving the objective lens 210 with a high degree of sensitivity.
As shown in FIG. 8, the objective lens 210 is held by a lens holder 220 so as to oppose an information recording surface of an optical recording medium, not shown, for focusing an optical beam and irradiating the focused optical beam on the information recording surface of the optical recording medium to record or reproduce information.
The lens holder 220 shown in FIG. 8 has a coil unit 260a adhered on a left side surface S1, and a coil unit 260b adhered on a right side surface S2, which opposes the left side surface S1 in substantially parallel therewith. The lens holder 220 is formed substantially symmetrically with respect to a plane including an optical axis of the objective lens 210 and being parallel with the left and the right side surfaces S1 and S2, and is also formed substantially symmetrically with respect to a plane including the optical axis of the objective lens 210 and intersecting with the left and the right side surfaces S1 and S2. The coil units 260a and 260b are also formed symmetrically with respect to the planes described above. Therefore, the lens holder 220 and the coil units 260a and 260b adhered on both side surfaces of the lens holder 220 are integrated, and the center of gravity of a movable unit 200 including the objective lens 210 is located on the optical axis of the objective lens 210 in the lens holder 220.
The coil units 260a and 260b have focus coils 261a and 261b, which are wound into a rectangular shape and connected in series, respectively. The focus coils 261a and 261b are wound so as to generate the substantially same forces in the focusing direction when being energized. The coil unit 260a includes two tracking coils 262a and 262b connected in series are provided on both sides of the focus coil 261a. The coil unit 260b includes two tracking coils 263a and 263b connected in series on both sides of the focus coil 261b. The tracking coils 262a and 262b and the tracking coils 263a and 263b are connected in series. The tracking coils 262a, 262b, 263a, and 263b are wound so as to generate the substantially same forces in the tracking direction when being energized.
The lens holder 220 includes four wire wound coil-connecting portions 265. One of these wire wound coil-connecting portions 265 is connected to one terminal of the focus coils 261a and 261b connected in series via a leading portion (not shown) of a wire wound coil, and another wire wound coil-connecting portion 265 is connected to the other terminal of the focus coils 261a and 261b via a leading portion (not shown) of a wire wound coil.
Still another wire wound coil-connecting portion 265 is connected to one terminal of the tracking coils 262a, 262b, 263a, and 263b connected in series via a wire wound coil leading portion (not shown), and still another wire wound coil-connecting portion 265 is connected to the other terminal of the tracking coils 262a, 262b, 263a, 263b. Each wire wound coil-connecting portion is connected to one end of each of four conductive elastic bodies 270 by welding or the like. The lens holder 220 supported by four conductive elastic bodies 270 and the coil units 260a and 260b adhered on both side surfaces of the lens holder 220 are integrated and constitute the movable unit 200 including the objective lens 200.
The other end of the conductive elastic body 270 is fixedly soldered to abase substrate 280. Accordingly, the movable unit 200 is cantilevered so as to be capable of moving with respect to the stationary unit including a yoke base 230, two yokes 231a and 231b, a wire base 240, two magnets 250a and 250b, and the base substrate 280.
The coil unit 260a is disposed in a magnetic circuit formed by a magnet 250a, which is adhered on the yoke 231a on the yoke base 230. The coil unit 260b is disposed in a magnetic circuit formed by the magnet 250b, which is adhered on the yoke 231b on the yoke base 230.
The coil surface of the coil unit 260a is disposed so as to face one magnetized surface 250as of the magnet 250a. The magnet 250a, which is substantially rectangular solid, is bipolarized; one in a recessed area and the other in a rectangular solid area to be fitted thereto, as shown by an image line 251a, which represents a magnetic boundary. The recessed area facing the coil unit 260a is magnetized in the S-Pole, and the rectangular solid area is magnetized in the N-Pole.
The coil surface of the coil unit 260b is disposed so as to face one magnetized surface 250bs of the magnet 250b. The magnet 250b, which is substantially rectangular solid, is bipolarized; one in a recessed area and the other in a rectangular solid area to be fitted thereto, as shown by an image line 251b, which represents a magnetic boundary. The recessed area facing the coil unit 260b is magnetized in the S-Pole, and the rectangular solid area is magnetized in the N-Pole. When the magnet 250a is rotated by 180° about an axis which extends in the direction of the optical axis of the objective lens 210 shown in FIG. 8, the same magnetized state as the magnet 250b is achieved.
Subsequently, referring to FIG. 9, magnetized areas of the magnets 250a and 250b in the optical head device and the positional relation among the focus coils 261a and 261b and the tracking coils 262a, 262b, 263a and 263b will be described. FIG. 9A shows a positional relation between the magnet 250a and the coil unit 260a when viewing the lens holder 220 in the direction indicated by V in FIG. 8. In FIG. 9A, the magnet 250a is positioned on the near side with respect to the coil unit 260a. As described before, the recessed area of the magnet 250a facing the coil unit 260a is magnetized in a S-Pole and the rectangular solid area is magnetized in a N-Pole. As shown in FIG. 9A, the focus coil 261a is wound into a rectangular shape. The two tracking coil 262a and 262b are disposed on both sides of the focus coil 261a. The tracking coils 262a and 262b are also wound into rectangular shapes.
As a matter of convenience, four sides of the rectangular of the focus coil 261a is supposedly divided into areas of A, B, C, and D as shown in FIG. 9A, the side B of the focus coil 261a is disposed at the position facing the S-Pole of the magnet 250a, so that a magnetic flux travels in the direction from the surface of the drawing toward the near side (shown by a double-circle in the drawing). The side D opposing the side Bis disposed at the position facing the N-Pole, so that a magnetic flux travels in the direction from the near side toward the surface of the drawing (shown by a cross in the drawing). The side A and the side C of the focus coil 261a are disposed at the positions crossing over the N-Pole and the S-Pole of the magnet 250a. 
As in the case of the focus coil 261a, when supposedly dividing the four sides of the tracking coils 262a and 262b into regions of A, B, C, and D as shown in FIG. 9A, the side A of the tracking coil 262a is disposed at the position facing the N-Pole of the magnet 250a, so that a magnetic flux travels in the direction from the near side toward the surface of the drawing (shown by a cross in the drawing). The side C opposing the side A is disposed at the position facing the S-Pole, so that a magnetic flux travels in the direction from the surface of the drawing toward the near side (shown by a double-circle in the drawing). The side B and the side D of the tracking coil 262a are disposed at the positions crossing over the N-Pole and the S-Pole of the magnet 250a. 
The side A of the tracking coil 262b is disposed at the position facing the S-Pole of the magnet 250a, so that a magnetic flux travels in the direction from the surface of the drawing toward the near side (shown by a double-circle in the drawing). The C side opposing the A side is disposed at the position facing the N-Pole, and a magnetic flux travels in the direction from the near side toward the surface of the drawing (shown by a cross in the drawing). The side B and the side D of the tracking coil 262b are disposed at the positions crossing over the N-Pole and the S-Pole of the magnet 250a. 
The tracking coils 262a and 262b are wound in such a manner that the direction of a current flowing along the side A of one tracking coil 262a is opposite from the direction of a current flowing along the side A of the other tracking coil 262b. 
FIG. 9B shows a positional relationship between the magnet 250b and the coil unit 260b when viewing the lens holder 220 in the direction indicated by an arrow V in FIG. 8. In FIG. 9B, the magnet 250b is positioned on the further side with respect to the coil unit 260b. As described above, the recessed area facing the coil unit 260b is magnetized in the S-Pole, and the rectangular solid area is magnetized in the N-Pole. As shown in FIG. 9B, the focus coil 261b is wound into a rectangular shape. The two tracking coil 263a and 263b are disposed on both sides of the focus coil 261b. The tracking coils 263a and 263b are also wound in a rectangular shape, respectively.
The side B of the focus coil 261b is disposed at the position facing the S-Pole of the magnet 250b, so that a magnetic flux travels in the direction from the near side toward the surface of the drawing (shown by a cross in the drawing). The side D opposing the side B is disposed at the position facing the N-Pole, so that a magnetic flux travels in the direction from the surface of the drawing toward the near side (shown by a double-circle in the drawing). The side A and the side C of the focus coil 261b are disposed at the positions crossing over the N-Pole and the S-Pole of the magnet 250b. 
The side A of the tracking coil 263a is disposed at the position facing the N-Pole of the magnet 250b, so that a magnetic flux travels in the direction from the surface of the drawing toward the near side (shown by a double-circles in the drawing). The side C opposing the side A is disposed at the position facing the S-Pole, so that a magnetic flux travels from the near side toward the surface of the drawing (shown by a cross in the drawing). The side B and the side D of the tracking coil 263a are disposed at the positions crossing over the N-Pole and the S-Pole of the magnet 250b. 
The side A of the tracking coil 263b is disposed at the position facing the S-Pole of the magnet 250b, so that a magnetic flux travels in the direction from the near side toward the surface of the drawing (shown by a cross in the drawing). The side C opposing the side A is disposed at the position facing the N-Pole, so that a magnetic flux travels in the direction from the surface of the drawing to the near side (shown by a double-circles in the drawing). The side B and the side D of the tracking coil 263b are disposed at the positions crossing over the N-Pole and the S-Pole of the magnet 250b. 
The tracking coils 263a and 263b are wound in such a manner that the direction of a current flowing along the side A of one tracking coil 263a is opposite from the direction of a current flowing along the side A of the other tracking coil 263b. 
When the coil surface of the optical head device having the objective lens drive unit constructed as described above is disposed in substantially parallel with the radial direction of the optical disk, and the focus coils 261a and 261b and the tracking coils 262a, 262b, 263a, and 263b in the coil units 260a and 260b are energized, a force acting on the coil according to a Fleming's left hand rule is generated, and thus the lens holder 220 can be moved in desired directions. When the focus coils 261a and 261b are energized, a driving force for moving the side B and the side D in the focusing direction (vertical direction in FIG. 9), and when the tracking coils 262a, 262b, 263a, and 263b are energized, a driving force for moving the side A and side C in the tracking direction (lateral directions in FIG. 9) is generated.
For example, when a current is flown in the focus coils 261a and 261b in the directions indicated by an arrow if as shown in FIG. 9, a force Ff acting in the upward direction in the drawing is generated. Accordingly, the objective lens 210 can be moved correspondingly to the fluctuation of the surface of the optical disk. For example, the objective lens 210 can be moved by the focus coils 261a and 262a in the direction substantially vertical to the information recording surface of the optical disk to adjust a focusing position.
As shown in FIG. 9, when a current is flown in the tracking coils 262a, 262b, 263a, and 263b in the direction indicated by an arrow it, a force Ft acting toward the right in the drawing is generated. Accordingly, the objective lens 210 can be moved corresponding to eccentricity of the optical disk. For example, the tracking position can be adjusted by moving the objective lens 210 radially of the optical disk by the tracking coils 262a, 262b, 263a, and 263b. 
In the objective lens drive unit of the optical head device having the construction described thus far, a driving action in a case in which part of any sides of the tracking coils 262a, 262b, 263a, and 263b are included in neutral areas n in the vicinity of the magnetic boundaries 251a and 251b, will be described. In the neutral areas n, the magnetic flux is not present, or is present but in a very low density.
As shown in FIG. 9A, a right portion of the side B of the tracking coil 262a and a left position of the side B of the tracking coil 262b are included in the neutral area n in the vicinity of the magnetic boundary 251a of the magnet 250. Therefore, when an attempt is made to move the movable portion 200 toward the right in FIG. 9, for example, by energizing the tracking coils 262a, 262b, 263a, and 263b, a downward force, which is generated at the right portion of the side B of the tracking coil 262a (included in the neutral area n), is smaller than the upward force, which is generated at the right portion of the side D of the tracking coil 262a. Therefore, in FIG. 9, when the tracking coil 262a is energized, an upward force Fe1, as well as force to move the lens holder 220 rightward, is generated.
Likewise, an upward force, which is generated at the left portion of the side B of the tracking coil 262b (included in the neutral area n), is smaller than a downward force, which is generated at the left portion of the side D of the tracking coil 262b. Therefore, in FIG. 9, when the tracking coil 262b is energized, a downward force Fe2, as well as a force to move the lens holder 220 rightward, is generated. Therefore, from the coil unit 260a, a moment, which rotates the lens holder 220 clockwise when viewing in the direction indicated by an arrow V in FIG. 8, is generated.
On the other hand, as shown in FIG. 9B, the right portion of the side B of the tracking coil 263a and the left portion of the side B of the tracking coil 263b are included in the neutral area n in the vicinity of the magnetic boundary 251b of the magnet 250b. Therefore, for example, when an attempt is made to move the movable portion 200 toward the right in FIG. 9 by energizing the tracking coils 262a, 262b, 263a, and 263b, a downward force, which is generated at the right portion of the side B of the tracking coil 263a (included in the neutral area n), is smaller than an upward force, which is generated on the right portion of the side D of the tracking coil 263a. Therefore, in FIG. 9B, when the tracking coil 263a is energized, an upward force Fe3, as well as a force to move the lens holder 220 rightward, is generated.
Likewise, an upward force, which is generated on the left portion of the side B of the tracking coil 263b (included in the neutral area n) is smaller than a downward force, which is generated on the left portion of the side D of the tracking coil 263b. Therefore, in FIG. 9B, when the tracking coil 263b is energized, a downward force Fe4, as well as a force to move the lens holder 220 rightward is generated. Therefore, a moment for rotating the lens holder 220 clockwise when viewing in the direction indicated by the arrow V in FIG. 8 is generated from the coil unit 260b. 
In order to increase the sensitivity in the tracking direction, it is preferable to secure the lengths of the sides A and the sides C of the tracking coils 262a, 262b, 263a, and 263b as long as possible. However, the objective lens drive unit disclosed in the aforementioned publication, when the lengths of the sides A and the sides C of the tracking coils 262a, 262b, 263a, and 263b are increased, the sides B partly overlap the neutral areas n along the magnetic boundaries 251a and 251b. When energized for moving the lens holder 220 in the direction indicated by the arrow Ft in this state, a moment is generated by unnecessary forces Fe1 to Fe4. Therefore, there arises a problem in that a rolling about the tangential direction occurs and thus the optical axis of the objective lens 210, which is mounted to the lens holder 220 is inclined into the direction radially of the optical recording medium when an attempt is made to move the movable portion 220 only in the tracking direction by a predetermined extent.
In addition, there arises a problem in that when the widths of the magnets 250a and 250b in the focusing direction so as to avoid overlapping of the sides B of the tracking coils 262a, 262b, 263a, and 263b and the neutral areas n along the magnetic boundaries 251a and 251b, the thickness of the movable portion 200 must be increased, and thus reduction of size and weight of the apparatus is hindered.