United States Patent [19J
`Wakabayashi et al.
`
`lllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllllll
`
`US005905255A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,905,255
`May 18,1999
`
`[54] OBJECTIVE LENS DRIVER
`
`[75]
`
`Inventors: Kanji Wakabayashi, Kyoutosi; Hitoshi
`Fujii, Hirakatasi; Hiroshi Yamamoto,
`Yawatasi; JoujiAnzai, Osakashi;
`Yasumasa Shibata, Hirakatashi, all of
`Japan
`
`[73] Assignee: Matsushita Electric Industrial Co.,
`Ltd., Japan
`
`[21] Appl. No.: 09/007,145
`
`[22] Filed:
`
`Jan. 14, 1998
`
`[30]
`
`Foreign Application Priority Data
`
`Jan. 14, 1997
`Oct. 16, 1997
`
`[JP]
`[JP]
`
`Japan .................................... 9-004895
`Japan .................................... 9-283824
`
`Int. Cl.6
`....................................................... G02B 7/04
`[51]
`[52] U.S. CI ..................... 250/201.5; 359/824; 369/44.22
`[58] Field of Search .............................. 250/201.5, 201.1,
`250/201.2; 359/814, 822, 824; 369/44.15,
`44.21, 44.22
`
`[56]
`
`References Cited
`
`U.S. PAIENT DOCUMENTS
`
`6/1994 Wakabayashi eta!. .............. 250/201.5
`5,319,497
`5,703,730 12/1997 Yomoda ............................... 250/201.5
`
`FOREIGN PArENT DOCUMENTS
`
`4-366429 12/1992
`6-162540
`6/1994
`
`Japan .
`Japan .
`
`Primary Examiner-Que T. Le
`Assistant Examiner-Kevin Pyo
`Attorney, Agent, or Firm-Parkhurst & Wendel
`
`[57]
`
`ABSTRACT
`
`In a objective lens driver, at least one permanent magnet is
`mounted on a movable member, and plural yokes, each
`being wound with a coil having its winding axis in the
`focusing direction, are disposed facing the permanent mag(cid:173)
`net. The yokes are arranged almost symmetrically about a
`plane including the center of gravity of the movable member
`and being perpendicular to the tracking direction. In this
`structure, even when the movable member moves in the
`tracking direction, since the permanent magnet (a magnetic
`field source) also moves with the movable member, the
`distribution of magnetic flux density that contributes to the
`drive also shifts according to the movement of the movable
`member, so that the distribution of magnetic flux density
`maintains being almost symmetrical about the center of
`gravity of the movable member. Accordingly, in tilt drive, a
`constant sensitivity to tilt drive is obtained regardless of the
`position of the movable member. Further, since the rotation
`axis of tilt drive is fixed, there arises no crosstalk of
`displacement in the focusing direction and the tracking
`direction. Furthermore, since the point of action of driving
`force on the movable member is fixed, there arises no
`crosstalk of angular displacement in the tilt direction due to
`focusing drive and tracking drive. Consequently, stable
`focusing control, tracking control and tilt correction control
`are realized in an objective lens driver capable of correcting
`a tilt.
`
`18 Claims, 17 Drawing Sheets
`
`10
`
`25 22a
`
`sa 21a
`
`LG Electronics, Inc. et al.
`EXHIBIT 1007
`IPR Petition for
`U.S. Patent No. 6,785,065
`
`

`
`U.S. Patent
`
`May 18,1999
`
`Sheet 1 of 17
`
`5,905,255
`
`Fig.1
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`Ba
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`7c
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`5
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`

`
`U.S. Patent
`
`May 18,1999
`
`Sheet 2 of 17
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`5,905,255
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`U.S. Patent
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`Fig.3
`
`May 18,1999
`
`Sheet 3 of 17
`
`5,905,255
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`

`
`U.S. Patent
`
`May 18,1999
`
`Sheet 4 of 17
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`5,905,255
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`Fig.5
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`
`U.S. Patent
`
`May 18,1999
`
`Sheet 5 of 17
`
`5,905,255
`
`Fig 6
`
`tilt driving force
`
`position in
`trucking direction
`
`

`
`U.S. Patent
`
`May 18,1999
`
`Sheet 6 of 17
`
`5,905,255
`
`10
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`23c
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`

`
`U.S. Patent
`
`May 18,1999
`
`Sheet 7 of 17
`
`5,905,255
`
`Fig.8
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`4b
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`

`
`U.S. Patent
`
`May 18,1999
`
`Sheet 8 of 17
`
`5,905,255
`
`Fig.9 (a)
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`

`
`U.S. Patent
`
`May 18,1999
`
`Sheet 9 of 17
`
`5,905,255
`
`Fig.10
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`

`
`U.S. Patent
`
`May 18, 1999
`
`Sheet 10 of 17
`
`5,905,255
`
`Fig.11
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`

`
`U.S. Patent
`
`May 18,1999
`
`Sheet 11 of 17
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`5,905,255
`
`Fig.12
`
`magnetic flux density
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`
`U.S. Patent
`
`May 18, 1999
`
`Sheet 12 of 17
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`5,905,255
`
`Fig.13
`
`34
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`51 36
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`

`
`U.S. Patent
`
`May 18, 1999
`
`Sheet 13 of 17
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`5,905,255
`
`Fig.14
`
`35
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`104
`
`36
`
`3a
`
`33c
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`2a
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`6b
`
`

`
`U.S. Patent
`
`May 18, 1999
`
`Sheet 14 of 17
`
`5,905,255
`
`Fig.15
`
`tilt detector
`
`tilt error signal
`
`focus error signal
`
`?a
`
`?b
`
`

`
`U.S. Patent
`
`May 18, 1999
`
`Sheet 15 of 17
`
`5,905,255
`
`Fig.1s
`
`35
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`36
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`

`
`U.S. Patent
`
`May 18, 1999
`
`Sheet 16 of 17
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`5,905,255
`
`Fig.17
`
`53
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`

`
`U.S. Patent
`
`May 18, 1999
`
`Sheet 17 of 17
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`5,905,255
`
`Fig.18
`
`34a
`
`35
`
`104
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`52b
`
`

`
`5,905,255
`
`1
`OBJECTIVE LENS DRIVER
`
`FIELD OF THE INVENTION
`
`The present invention relates to an objective lens driver
`employed in an apparatus for optically recording and/or
`reproducing information in recording media.
`
`BACKGROUND OF THE INVENTION
`
`In order to correct focusing error due to vertical motion of
`a disk-type recording medium (hereinafter referred to simply
`as "disk") such as a compact disk, tracking error due to
`decentration, or a tilt of an objective lens relative to the disk,
`an objective lens driver drives the objective lens in two axes,
`i.e., the direction perpendicular to the disk (hereinafter
`referred to as "focusing direction") and the radial direction
`of the disk (hereinafter referred to as "tracking direction").
`It is to be noted that the focusing direction is the direction
`perpendicular to the disk and the tracking direction is the
`radial direction of the disk on the premise that the disk
`loaded on the driver is in ideal shape having no curvature
`and no decentration.
`In an optical recording/reproduction apparatus including
`such an objective lens driver, in order to increase the
`recording capacity, recording and reproduction are carried
`out with a converging spot of reduced diameter using an
`objective lens of high numerical aperture. In this case, since
`the degree of aberration attendant on a tilt of the objective
`lens relative to the disk increases in proportion to third
`power of the numerical aperture, positioning of the optical 30
`axis of the objective lens relative to the disk must be
`precisely performed to obtain satisfactory recording/
`reproduction signals.
`For this purpose, the conventional optical recording/
`reproduction apparatus is equipped with a tilt correction
`controller for correcting a tilt by inclining the whole optical
`head using a DC motor or the like.
`However, the tilt correction controller has the following
`drawbacks. Since this means can correct only angle errors of
`low frequencies, it is difficult to reduce aberration. In
`addition, since a mechanism for inclining the whole optical
`head is added, the size of the optical recording/reproduction
`apparatus is increased. Further, since it is very difficult to
`match the rotation center at the tilt correction with the
`principal point of the objective lens because of lack of space,
`the height of the objective lens significantly varies with the
`rotation and, therefore, some means for adjusting the height
`is needed, resulting in a further increase in the size of the
`apparatus. It is to be noted that the principal point of the
`objective lens is a point on the optical axis of the objective
`lens (shown by 100 in FIG. 9(b)), and a distance between
`this point and the focal point is the focal length.
`Meanwhile, Japanese Published Patent Application No.
`Hei. 4-366429 discloses an objective lens driver that detects 55
`a relative tilt of the disk and the optical axis of the objective
`lens and rapidly corrects the tilt.
`Hereinafter, the objective lens driver disclosed will be
`described in detail.
`This objective lens driver is equipped with an objective
`lens for focusing a light beam on an optical information
`recording medium; a holder for holding the objective lens;
`a supporter for supporting the holder movably in the focus(cid:173)
`ing direction and tiltably to the focusing direction; a plural(cid:173)
`ity of driving coils for moving the objective lens in the
`focusing direction and tilting the objective lens to the
`focusing direction by driving the holder in response to
`
`2
`voltages applied thereto, which coils are disposed on the
`holder individually and symmetrically about the center line
`of the objective lens; and a magnet for generating a force for
`driving the holder between itself and the driving coils, which
`5 magnet is fixed onto the supporter.
`The objective lens driver is so constructed operates as
`follows. It is assumed that the objective lens driver includes
`four driving coils, and the four driving coils are arranged,
`two by two, symmetrically about a plane including the
`10 center line of the objective lens and being perpendicular to
`the tracings direction. In this case, when the respective
`driving coils are supplied with currents of the same value in
`the same direction, the objective lens moves in the focusing
`direction without tilting to the focusing direction, whereby
`15 focusing control is carried out.
`When an arbitrary current is applied to two of the four
`driving coils performing focusing control in response to the
`currents of the same value in the same direction, which two
`coils are disposed on either side of the plane including the
`20 center line of the objective lens and being perpendicular to
`the tracking direction, a portion of the of the holder having
`the driving coils supplied with the arbitrary current moves
`up or down in response to the current, i.e., the holder tilts to
`the focusing direction. Since the objective lens can be tilted
`25 to the focusing direction as described above, it is possible to
`correct a tilt of the optical axis of the objective lens to the
`disk.
`Accordingly, a mechanism and a motor for tilting the
`whole optical head are dispensed with, resulting in a small(cid:173)
`sized optical recording/reproduction apparatus capable of
`high-speed tilt correction.
`In the objective lens driver described above, however,
`since the magnetic circuit for generating the operating force
`35 of the driving coils, i.e., the magnet and the supporter, is
`fixed, the current driving sensitivity in the tilt correction
`varies with the motion of thee holder in the focusing
`direction or the tracking direction. The reason is as follows.
`Usually, the magnetic flux density distribution in a void of
`40 the magnetic circuit has a shape like a mountain and,
`therefore, the amount of the magnetic flux interlinked with
`each driving coil varies when the position of the driving coil
`relative to the magnetic circuit varies.
`Accordingly, the driving sensitivity in the tilt correcting
`45 direction varies according to the position of the holder, so
`that the stability of tilt correction control is degraded.
`Furthermore, the rotation center in the tilt correction
`varies with the holder's moving in the focusing direction or
`the tracking direction. The reason is as follows. Since the
`50 magnetic flux density distribution in a magnetic void part of
`the magnetic circuit has a shape like a mountain as described
`above and, therefore, the relative position of the holder and
`the point of action of the driving force varies when the
`position of the driving coil relative to the magnetic circuit
`vanes.
`Since the rotation center in the tilt correction varies
`according to the position of the holder, the position of the
`objective lens in the focusing direction or the tracking
`direction varies due to the tilt correction, and the amount of
`60 the variation varies according to the position of the holder.
`Thereby, the control precision is degraded in focusing or
`tracking control.
`Furthermore, when the holder moves in the focusing
`direction or the tracking direction, i.e., when the relative
`65 position of the holder and the point of action of the driving
`force varies, the point of action of the driving force in the
`focusing direction or the tracking direction deviates from the
`
`

`
`5,905,255
`
`3
`center of gravity of the holder, so that tilting of the holder
`may occur outside of the tilt correction.
`Consequently, since the tilt angle at tilt correction varies
`according to the position of the holder, the control precision
`is degraded in controlling the tilt correction.
`
`SUMMARY OF THE INVENTION
`
`10
`
`4
`field source) also moves with the movable member, the
`distribution of magnetic flux density between the permanent
`magnet and the yoke shifts by the same amount as the
`movement of the movable member. So, the density of
`5 magnetic flux orthogonally crossing the focusing coil, the
`tracking coil and the tilt driving coil becomes constant, and
`there is no variation in the driving sensitivity in the tilt
`correcting direction. As a result, stable tilt correction control
`is realized.
`Further, since the distribution of magnetic flux density
`between the permanent magnet and the yoke shifts by the
`same amount as the movement of the movable member, the
`relative position of the movable member and the point of
`action of focusing drive, tracking drive or tilt drive does not
`vary. Therefore, the rotation center at tilt correction does not
`15 vary, so that the position of the objective lens in the focusing
`direction or the tracking direction does not vary.
`Consequently, control precision is not degraded in the focus(cid:173)
`ing or tracking control.
`Furthermore, since the distribution of magnetic flux den-
`20 sity between the permanent magnet and the yoke shifts by
`the same amount as the movement of the movable member,
`the relative position of the movable member and the point of
`action of driving force in the focusing direction or the
`tracking direction does not vary. Therefore, tilting of the
`25 movable member does not occur outside of tilt correction,
`resulting in highly precise tilt correction control.
`
`It is an object of the present invention to provide an
`objective lens driver capable of controlling tilt correction,
`that can correct tilts of low to high frequencies, that can
`perform stable tilt driving independent of the position of a
`movable member in the focusing direction or the tracking
`direction, and that does not degrade the control precision in
`the focusing direction and the tracking direction.
`Other objects and advantages of the invention will
`become apparent from the detailed description that follows.
`The detailed description and specific embodiments
`described are provided only for illustration since various
`additions and modifications within the scope of the inven(cid:173)
`tion will be apparent to those of skill in the art from the
`detailed description.
`According to an aspect of the present invention, there is
`provided an objective lens driver used for recording and/or
`reproduction of optical information on a disk type recording
`medium, and the objective lens driver comprises:
`a movable member comprising:
`an objective lens having, when being not driven, an
`optical axis in a focusing direction that is perpen(cid:173)
`dicular to the disk type recording medium;
`a lens holder holding the objective lens; and
`at least one permanent magnet fixed onto the lens
`holder, having a direction of magnetization in a
`tangent direction that is perpendicular to the focusing
`direction and to a tracking direction parallel to the 35
`radius of the disk type recording medium;
`a plurality of rod-shaped elastic supporting members,
`each supporting the movable member at an end thereof
`so that the movable member can move in the focusing
`direction and the tracking direction and rotate in a tilt
`direction that is a rotative direction around the tangent
`direction;
`a fixed base to which the other ends of the rod-shaped
`elastic supporting members are connected;
`means for driving the movable member in the focusing
`direction, the tracking direction, and the tilt direction
`by interaction with the permanent magnet, the driving
`means comprising:
`at least two yokes comprising a magnetic material,
`disposed on the fixed base and facing the permanent
`magnet in the tangent direction;
`at least one focusing coil winding round each yoke with
`its winding axis in the focusing direction;
`at least one tracking coil winding round each yoke with
`its winding axis in the tracking direction; and
`at least one tilt driving coil winding round each yoke
`with its winding axis in the focusing direction;
`wherein the two yokes are arranged almost symmetrically
`about a plane including the center of gravity to the 60
`movable member in its not-driven state and being
`perpendicular to the tracking direction, and the tilt
`driving coils around the two yokes are arranged almost
`symmetrically about the plane.
`In the objective lens driver so constructed, even when the
`movable member moves in the focusing direction or the
`tracking direction, since the permanent magnet (a magnetic
`
`40
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`FIG. 1 is a perspective view illustrating an objective lens
`30 driver according to a first embodiment of the present inven-
`tion.
`FIG. 2 is a block diagram illustrating a tilt control circuit
`employed for the objective lens driver according to the first
`embodiment.
`FIG. 3 is a schematic diagram illustrating a tilt detecting
`means included in the objective lens driver according to the
`first embodiment, in the case where no tilt occurs.
`FIG. 4 is a schematic diagram illustrating the tilt detecting
`means in the case where tilt occurs.
`FIG. 5 is a schematic diagram for explaining the operation
`of the objective lens driver according to the first embodi(cid:173)
`ment.
`FIG. 6 is a graph showing distribution of tilt driving force
`in the tracking direction, applied to a movable member in the
`45 objective lens driver according to the first embodiment.
`FIG. 7 is a perspective view illustrating an objective lens
`driver according to a second embodiment of the present
`invention.
`FIG. 8 is a block diagram illustrating a tilt correction
`control circuit employed for the objective lens driver accord(cid:173)
`ing to the second embodiment.
`FIG. 9(a) is a schematic diagram for explaining the
`operation of the objective lens driver according to the
`55 second embodiment, and FIG. 9(b) is a cross-sectional view
`taken along a ling A-A in FIG. 9(a).
`FIG.10 is a schematic diagram of the objective lens driver
`according to the second embodiment, in the state where the
`movable member is in the neutral position.
`FIG.ll is a schematic diagram of the objective lens driver
`according to the second embodiment, in the state where the
`movable member has moved in the tracking direction from
`the neutral position.
`FIG. 12 is a graph showing distribution of magnetic flux
`65 density in the tracking direction, for explaining the operation
`of the objective lens driver according to the second embodi-
`ment.
`
`50
`
`

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`5,905,255
`
`5
`FIG. 13 is an exploded perspective diagram illustrating an
`objective lens driver according to a third embodiment of the
`invention.
`FIG. 14 is a plan view schematically illustrating the
`objective lens driver according to the third embodiment.
`FIG. 1S is a block diagram illustrating a circuit for driving
`the movable member in the focusing direction and the tilt
`direction, employed for the objective lens driver according
`to the third embodiment.
`FIG. 16 is a plan view for explaining the operation of
`driving the movable member in the tracking direction in the
`objective lens driver according to the third embodiment.
`FIG. 17 is a plan view schematically illustrating an
`objective lens driver according to a fourth embodiment of
`the invention.
`FIG. 1S is a plan view for explaining the operation of
`driving the movable member in the tracking direction in the
`objective lens driver according to the fourth embodiment.
`
`DETAILED DESCRIPTION OF 1HE
`PREFERRED EMBODIMENTS
`Embodiment 1
`An objective lens driver according to a first embodiment
`of the present invention will be described with reference to
`FIGS. 1 to 6.
`FIG. 1 is a perspective view illustrating an objective lens
`driver according to the first embodiment. FIG. 1 shows the
`state where a movable member is not driven.
`In FIG. 1, reference numeral 10 designates a disk, and
`reference characters T, F, and K designate the tracking
`direction parallel to the radius of the disk 10, the focusing
`direction perpendicular to the disk 10, and the tangent
`direction perpendicular to the focusing direction F and the
`tracking direction T, respectively. The focusing direction F,
`the tracking direction T, and the tangent direction K cross
`each other at right angles, and respectively correspond to
`directions of coordinate axes in a three-dimensional rectan(cid:173)
`gular coordinate.
`The objective lens driver comprises an objective lens 1; a
`lens holder 2 for holding the objective lens 1; permanent
`magnets 3a and 3b fixed on the lens holder 2 along the
`tangent direction K, each magnet being a rectangular plate
`extending in the focusing direction F and the tracking
`direction; a tilt detector 4 mounted on the lens holder 2; a
`fixed baseS; tracking coils 6a to 6d; focusing coils 7a to 7d;
`wire members (rod-shaped elastic supporting members) Sa
`to Sd (Sd is not shown because it is located on the rear side
`of the lens holder 2); and a supporting member 9. The
`objective lens 1, the lens holder 2, the permanent magnets 3a
`and 3b, and the tilt detector 4 constitute a movable member
`20. Further, reference character 1 shows the optical axis of
`the objective lens 1, which corresponds to the focusing
`direction F in the state where the movable member 20 is not
`driven. The shape of the lens holder 2 is almost symmetrical
`about its center in the focusing direction F, the tracking
`direction T, and the tangent direction K. The objective lens
`1 is located in the center of the lend holder 2 in the focusing
`direction F, and the permanent magnets 3a to 3d are arranged
`symmetrically about the center of the lens holder 2 in the
`tangent direction, so that the center of gravity of the movable
`member 2 is approximately in the center of the lens holder
`2. Accordingly, the optical axis 1 of the objective lens 2
`passes almost the center of gravity of the movable member
`20.
`The tilt detector 4 comprises a light emitting part 4a, and
`light responsive parts 4b and 4c arranged in the tracking
`direction T with the light emitting part 4a between them. The
`
`5
`
`10
`
`6
`light emitting part 4a emits light toward the disk 10. The
`light responsive parts 4b and 4c receive light reflected at the
`disk 10 and generate voltages in response to the quantities of
`the received light.
`The fixed base S comprises a magnetic material. Yokes Sa
`to Sd are vertically disposed on the fixed base S along the
`tangent direction K, facing the permanent magnets 3a and
`3b. More specifically, the yokes Sa and Sb are located facing
`the permanent magnet 3a while the yokes Sc and Sd are
`located facing the permanent magnet 3b. The yokes Sa and
`Sb are located almost symmetrically about a plane including
`the center of gravity of the movable member 20 and being
`perpendicular to the tracking direction T, and the yokes Sc
`and Sd are similarly located.
`The tracking coils 6a to 6d, each having the winding axis
`15 in the tracking direction T, are wound round the yokes Sa to
`Sd, respectively. The focusing coils 7a to 7d, each having the
`winding axis in the focusing direction F, are wound round
`the yokes Sa to Sd, respectively.
`The wire members Sa to Sd are rod-shaped elastic sup-
`20 porting members comprising a conductive material, having
`their axes in the tangent direction K, and being almost
`parallel to each other. An end of each wire member is
`connected to the lens holder 2, and the other end thereof is
`connected to the supporting member 9 that is vertically
`25 disposed on the fixed base S, whereby the wire members Sa
`to Sa are fixed to the fixed base S. Accordingly, by the wire
`members Sa to Sd, the movable member 20 is supported
`movably in the focusing direction F and the tracking direc(cid:173)
`tion T and rotatably in the rotative direction around the
`30 tangent direction K.
`Furthermore, the centroid (center of figure) of a quadri(cid:173)
`lateral made by four points at which the four wire members
`Sa to Sd support the movable member 20, a middle point
`(driving center) of a segment connecting the centroids of
`35 pole faces of the permanent magnets 3a and 3b, and the
`center of gravity of the movable member 20 are in approxi(cid:173)
`mate agreement with each other.
`FIG. 2 is a schematic diagram showing a tilt control
`circuit (tilt control means) employed for the objective lens
`40 driver according to the first embodiment. As shown in FIG.
`2, the tilt control circuit comprises a differential amplifier 11
`that outputs a difference in detected optical signals from the
`light responsive parts 4b and 4c of the tilt detector 4; a
`normal rotation driving amplifier 12 that outputs the sum of
`45 the differential output from the differential amplifier 11 and
`a focus error signal (focus driving signal); and a reverse
`rotation driving amplifier 13 that outputs a difference
`between the differential output from the differential ampli(cid:173)
`fier 11 and the focus error signal. The output from the normal
`50 rotation driving amplifier 12 is applied to the focusing coils
`7a and 7c while the output from the reverse rotation driving
`amplifier 13 is applied to the focusing coils 7b and 7d.
`A description is now given of the operation of the objec(cid:173)
`tive lens driver so constructed. When driving the objective
`55 lens in the tracking direction T, the magnetic fluxes gener(cid:173)
`ated by the permanent magnets 3a and 3b orthogonally cross
`the currents flowing through the tracking coils 6a to 6d,
`whereby an electromagnetic force is generated. Since the
`tracking coils 6a to 6d are fixed onto the fixed base S, the
`60 movable member 20 moves almost translationally in the
`tracking direction T. On the other hand, when driving the
`objective lens in the focusing direction F, the magnetic
`fluxes generated by the permanent magnets 3a and 3b
`orthogonally cross the currents flowing through the focusing
`65 coils 7a to 7d, whereby an electromagnetic force is gener(cid:173)
`ated and the movable member 20 moves almost translation(cid:173)
`ally in the focusing direction F.
`
`

`
`5,905,255
`
`7
`Next, a description is given of rotation drive around the
`tangent direction K. First of all, tilt detection is carried out
`as follows. When the optical axis 1 of the objective lens 1 is
`perpendicular to the disk 10 as shown in FIG. 3, light
`emitted from the light emitting part 4a of the tilt detector 4
`is reflected at the disk 10 and received by the light respon(cid:173)
`sive parts 4b and 4c. The quantity of light received by the
`light responsive part 4b is equal to the quantity of light
`received by the light responsive part 4c. On the other hand,
`when the optical axis 1 of the objective lens 1 is not
`perpendicular to the disk 10 as shown in FIG. 4, a portion of
`light reflected by the disk 10 is not received by the light
`responsive part 4b or 4c, so that a difference arises between
`the quantity of light received by the light responsive part 4b
`and the quantity of light received by the light responsive part
`4c. Accordingly, a tilt can be detected by detecting this
`difference. In this first embodiment, signals generated in the
`light responsive parts 4b and 4c are Input to the differential
`amplifier 11, and a difference between these signals is
`detected to generate a tilt error signal (tilt driving signal).
`Tilt drive, i.e., rotation drive around the tangent direction
`K, is performed as follows. First of all, the tilt error signal
`is divided into two, and one is input to the normal rotation
`driving amplifier 12 while the other is input to the reverse
`rotation driving amplifier 13. The output from the normal
`rotation driving amplifier 12 and the focus error signal are
`applied to the focusing coils 7a and 7c, and the output from
`the reverse rotation driving amplifier 13 and the focus error
`signal are applied to the focusing coils 7b and 7d.
`Accordingly, a moment according to the tilt error signal acts 30
`on the permanent magnets 3a and 3b, whereby the angle
`error between the disk 10 and the optical axis 1 of the
`objective lens 1 is corrected.
`Further, tilt drive when the movable member 20 moves in
`the tracking direction T will be described with reference to
`FIGS. 5 and 6. It is assumed that a tilt driving current
`according to the angle error between the disk 10 and the
`optical axis 1 of the objective lens 1, i.e., the tilt error signal,
`is applied through the normal rotation driving amplifier 12
`and the reverse rotation driving amplifier 13 to the focusing
`coils 7a and 7c and the focusing coils 7b and 7d, respec(cid:173)
`tively. When the movable member 20 is located in the
`neutral position shown by the solid line in FIG. 5, i.e., when
`it is not driven, reaction forces of electromagnetic forces
`generated by the focusing coils 7a to 7d are generated in the
`permanent magnets 3a and 3b. Among these forces, the tilt
`driving force in the tracking direction T is distributed as
`shown by the solid line in FIG. 6. Assuming that an axis
`passing the center of gravity of the movable member 20 and
`being parallel to the tangent direction K is called "a center(cid:173)
`of-gravity axis", the distribution of the tilt driving force is
`almost symmetrical about the center-of-gravity axis of the
`movable member 20, and the movable member 20 rotates on
`the center-of-gravity axis by an angle according to the tilt
`error signal.
`When the movable member 20 has moved in the tracking
`direction T and is located in the position shown by the dotted
`line in FIG. 5, since the permanent magnets 3a and 3b
`serving as magnetic field sources have moved with the
`movable member 20, the magnetic flux density distribution 60
`between the permanent magnet 3a and the yokes Sa and 5b
`or between the permanent magnet 3b and the yokes 5c and
`5d changes as well, so that the tilt driving force in the
`tracking direction T is distributed as shown by the dotted
`line in FIG. 6. That is, since the distribution of the tilt driving 65
`force moves with the move of the permanent magnets 3a and
`3b, this distribution is almost symmetrical about the center-
`
`8
`of-gravity axis of the movable member 20. Accordingly, the
`movable member 20 rotates on the center-of-gravity axis by
`an angle according to the tilt error signal. Consequently,
`constant sensitivity to tilt drive is obtained regardless of the
`5 position of the movable member 20.
`In this first embodiment, the focusing coils 7a and 7b (7c
`and 7d) are located almost symmetrically about a plane
`including the center of gravity of the movable member 20
`and being perpendicular to the tracking direction T, thereby
`10 to supply the movable member 20 with a rotation driving
`force round the center-of-gravity axis thereof. However, the
`present invention is not restricted thereto. For example, two
`tracking coils may be located almost symmetrically about a
`plane including the center of gravity of the movable member
`15 20 and being perpendicular to the focusing direction F, and
`a signal obtained by superposing the tilt error signal on the
`tracking error signal may be output toward the tracking
`coils.
`Although in this first embodiment the direction of gravity
`20 is not specifically discussed, the same effects as mentioned
`above are obtained regardless of the direction of gravity.
`Further, although an emphasis has been placed on the
`sensitivity to tilt drive in the case where the movable
`member 20 moves in the tracking direction T