Patent Publication Number: US-2002006090-A1

Title: Objective lens drive apparatus for use in optical pickup

Description:
BACKGROUND OF THE INVENTION  
       [0001] The present invention relates to an objective lens drive apparatus for use in an optical pickup forming an optical disk unit which projects an optical spot onto a record medium to be able to read information out of the record medium optically.  
       [0002] An optical pickup forming an optical disk unit is generally composed of an objective lens drive apparatus including an objective lens and an optical system for transmitting the light to the objective lens and receiving the light therefrom, while the objective lens drive apparatus is disposed on an optical system block mounting table. The objective lens drive apparatus is composed of a movable part including an objective lens, a focus coil and a tracking coil, and a fixed part including a magnetic circuit; and, the movable part is supported on the fixed part by four wires each of which is in part surrounded and held by an elastic damper member such as a visco-elastic member.  
       [0003] As an objective lens drive apparatus which not only can drive an objective lens in a focus direction and in a tracking direction but also can correct the coma and astigmatism of a spot which is image formed on a disk, there is known an apparatus which is disclosed in Japanese Patent Publication No. Hei. 9-231595. This conventional device is characterized in that, as shown in FIGS. 24, 25 and  26 , on the surface of a lens holder  1101  that is situated opposed to an optical disk, there are disposed at least a pair of optical sensors  1301 ,  1302  which extend in the optical disk radial direction or tangential direction of an objective lens  1103 ; on one or both of the side surfaces of the lens holder  1101  in the optical disk radial direction, there are disposed coils  1105  which are used to correct the inclination of the objective lens; and, on a pair of yokes  1113  and  1114  which are disposed opposed to the side surface of the lens holder  1101 , there are disposed a pair of reversed-polarity magnet members  1106  and  1107  for correction of the inclination of the objective lens in such a manner that they correspond to the positions of the coils  1105 , whereby the inclination of the objective lens with respect to an optical disk  1100  is detected in accordance with the outputs of the optical sensors  1301  and  1302 . In accordance with the thus detected objective lens inclination angle and the calculated value of a shift between the optical axis of a collimator and the optical axis of the objective lens, currents are supplied to the coils  1105  for inclination correction to thereby drive the coils  1105  and, due to the electromagnetic mutual reaction between the coils  1105  and the reversed polarity magnet members  1106  and  1107 , the side surfaces of the lens holder  1101  are driven, so that the side surfaces of the lens holder  1101  can be servo controlled in a freely inclinable manner.  
       [0004] The pair of optical sensors  1301  and  1302  are respectively mounted on the two sides of the objective lens  1103  of the lens holder  1101  and, as shown in FIG. 25, are used to receive primary lights  1201 ,  1202  that are emitted from an optical head and diffracted by an optical disk groove. Electric signals from the optical sensors  1301 ,  1302 , as shown in FIG. 27, are amplified by amplifiers  1407 ,  1408  and are then differentially input to a differential amplifier  1403 . From the output of the differential amplifier  1403 , there is calculated an inclination angle between the optical disk  1100  and lens holder  1101 .  
       [0005] As shown in FIG. 27, from the thus calculated inclination angle and the shift between the objective lens optical axis and collimator optical axis, preferably, using a preset section  1404  set in a ROM (Read Only Memory), there is calculated a lens optimum inclination angle; and, based on the above two calculation results, the inclination correcting coils  1105  are driven through a phase compensation circuit  1405  and a drive amplifier  1406  for servo control.  
       [0006] Referring to the structure of the lens holder  1101 , in the plane surface thereof, there are formed two slits  1102  through which their associated yoke members  1109  can be inserted respectively; on the central portion of the lens holder  1101 , there is mounted the objective lens  1103 ; and, on a pair of mutually opposing side surfaces of the lens holder  1101 , there are disposed square-shaped flat coils  1104  for tracking drive by twos, a total of four coils  1104 . Also, on the two mutually opposing surfaces of the lens holder  1101  in the optical disk radial direction (R), as the coils  1105  for inclination correction, there are disposed a pair of square-shaped flat coils; and, above and below the coils  1105  for inclination correction, there are disposed printed circuit boards (not shown) which are supported through copper foil portions  1115 ,  1116 .  
       [0007] On an actuator base  1108 , there are projectingly provided yoke portions  1109 ,  1110 ; and, the yoke portions  1109 ,  1110 , through-magnets  1111 ,  1112 , form a substantially closed-magnetic circuit for focus-direction and tracking-direction driving. Also, on the two side surfaces of the actuator base  1108 , there are disposed two side yokes  1113 ,  1114  for lens holder inclination adjustment drive, the top plan views of which respectively show a horseshoe shape. And, in each of the side yokes  1113 ,  1114 , there are disposed long magnets  1106  and  1107  of mutually reversed polarities in such a manner that they correspond to the upper and lower sides of the coils  1105  for inclination correction.  
       [0008] Also, on the actuator base  1108 , similarly to the above, there are further disposed square-shaped printed circuit boards  1117 ,  1118  through copper foil portions  1119 ,  1120 . And, four spring wires  1121  of phosphor bronze are connected to the lens holder  1101  in such a manner that the spring wires  1121  are respectively fixed by printed circuit boards disposed on the two ends of the spring wires  1121 ; and thus, the lens holder  1101  is supported elastically by the spring wires  1121  (as for the fixation of the spring wires  1121 , see the plan view shown in FIG. 26).  
       [0009] In FIG. 24, reference character F designates a focus axis of a moving system of an objective lens actuator, R stands for a tracking axis thereof, and T represents an optical disk tangent axis thereof.  
       [0010] Next, description will be given below of the inclination drive of the lens holder  1101  according to the related art with reference to FIG. 25. In case where the current directions of the coils  1105  for right and left inclination correction respectively disposed on the optical-disk-radial-direction two side surfaces of the lens holder  1101  are set in the same direction and the magnetic field directions of the left and right magnets  1106  and  1107  disposed so as to correspond to the upper and lower sides of the coils  1105  for inclination correction are set symmetrical, the electromagnetic driving forces of the right and left coils are different in direction from each other according to Fleming&#39;s rule (see arrow marks F, F′ in FIG. 25). Therefore, while the center of gravity or center of support of the lens holder  1101  is present substantially at the same point, in case where the lens holder  1101  is rotated about this point, the inclination of the objective lens with respect to the optical disk  1100  can be corrected.  
       [0011] However, in the above-mentioned conventional technique, in order to correct the inclination of the objective lens, separately from the coils and magnets for tracking servo and focus servo, there must be further disposed the coils  1105  and magnets  1106 ,  1107  for inclination correction, which results in the increased cost of the objective lens drive apparatus. Also, in the conventional technique, the coils  1105  and magnets  1106 ,  1107  for inclination correction must be disposed on the optical-disk- 1100  radial direction side surfaces of the lens holder  1101  holding the objective lens  1103 , which results in the increased width and weight of the objective lens drive apparatus.  
       SUMMARY OF THE INVENTION  
       [0012] The present invention aims at solving the above problems found in the conventional technique.  
       [0013] Now, description will be given below of first aspect of the invention for solving the above problems with reference to Fig. 1 which corresponds to a first embodiment of the invention. According to the first aspect, within the same magnetic gap  5 g of a magnetic circuit having at least one magnet  5  magnetized in two polarities, there is disposed a coil unit  3  on which a focus coil  3 f, tracking coils  3 tr and tilt coils  3 ti are mounted.  
       [0014] In the first aspect, the magnet  5  magnetized in two polarities is used to make a correction of the inclination of an objective lens, which can eliminate the need for provision of an exclusive magnet exclusively used to correct the above-mentioned objective lens inclination.  
       [0015] Also, description will be given below of second aspect of the invention for solving the above problems with reference to FIG. 11 which corresponds to a second embodiment of the invention. According to the second the second aspect, there are completed two magnetic circuits each having at least one magnet  105  magnetized in two polarities and, within the magnetic gap  105 g of each of the two magnetic circuits, there is disposed a coil unit  103  on which a focus coil  103 f, tracking coils  103 tr and tilt coils  103 ti are mounted.  
       [0016] In the second aspect, the magnet  105  magnetized in two polarities is used to make a correction of the inclination of an objective lens, which can eliminate the need for provision of an exclusive magnet exclusively used to correct the objective lens inclination.  
       [0017] Further, description will be given below of third aspect of the invention for solving the above problems with reference to FIG. 18 which corresponds to a third embodiment of the invention. According to the third aspect, there is provided an objective lens drive apparatus for use in an optical pickup which detects the inclination of an optical disk and adjusts the inclination of an objective lens in accordance with an optical disk inclination signal, wherein, within the same magnetic gap  205 g of a magnetic circuit having at least one magnet  205  magnetized in two polarities, there is disposed a coil unit  203  on which a plurality of focus coils  203 fl,  203 fr and tracking coils  203 t are mounted. Currents are respectively supplied to the plurality of focus coils  203 fl,  203 fr and, due to the sum of the driving forces of the focus coils  203 fl,  203 fr, focus servo is executed. Due to the difference between the above driving forces, there is produced moment around the center of gravity of a movable part and the inclination of the objective lens  202  can be thereby adjusted simultaneously with the focus servo operation.  
       [0018] In the third aspect, due to the operations of the plurality of focus coils  203 fl and  203 fr, not only the focus servo but also the adjustment of the inclination of the objective lens  202  can be executed. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0019]FIG. 1 is an exploded perspective view of a first embodiment of an objective lens drive apparatus for use in an optical pickup according to the invention;  
     [0020]FIG. 2 is a side view of a magnetic circuit employed in the first embodiment according to the invention;  
     [0021]FIG. 3 is an arrangement view of the first embodiment, showing the position relationship between magnets and focus coils/tracking coils at the self-weight position of the first embodiment in the focus direction;  
     [0022]FIG. 4 is an arrangement view of the first embodiment, showing the position relationship between magnets and tilt coils at the self-weight position of the first embodiment in the focus direction;  
     [0023]FIG. 5 is an arrangement view of a modification of the first embodiment, showing the position relationship between magnets and tilt coils at the self-weight position of the modification in the focus direction;  
     [0024]FIG. 6 is a plan view of a magnetic circuit employed in the modification of the first embodiment;  
     [0025]FIG. 7 is an arrangement view of the modification, showing the position relationship between magnets and focus coils/tracking coils at the self-weight position of the modification in the focus direction;  
     [0026]FIG. 8 shows of a modification of a coil unit of the first embodiment;  
     [0027]FIG. 9 is an exploded perspective view of another modification of the first embodiment;  
     [0028]FIG. 10 is a plan view of a magnetic circuit employed in the objective lens drive apparatus shown in FIG. 9;  
     [0029]FIG. 11 is an exploded perspective view of a second embodiment of an objective lens drive apparatus for use in an optical pickup according to the invention;  
     [0030]FIG. 12 is an exploded perspective view of a modification of the second embodiment;  
     [0031]FIGS. 13A and 13B are plan views of a magnetic circuit employed in the objective lens drive apparatus shown in FIG. 9;  
     [0032]FIG. 14 is an exploded perspective view of another modification of the second embodiment;  
     [0033]FIG. 15 is a front view of the objective lens drive apparatus shown in FIG. 14;  
     [0034]FIG. 16 is an arrangement view of the modification of the second embodiment shown in FIG. 14, showing the position relationship between magnets and tracking coils/tilt coils at the self-weight position of the first embodiment in the focus direction;  
     [0035]FIG. 17 is an arrangement view of the modification of the second embodiment shown in FIG. 14, showing the position relationship between magnets and tracking coils/tilt coils at the self-weight position of the first embodiment in the focus direction;  
     [0036] Fig. 18  is an exploded perspective view of a third embodiment of an objective lens drive apparatus for use in an optical pickup according to the invention;  
     [0037]FIG. 19 is an arrangement view of the third embodiment, showing the position relationship between magnets and focus coils/tracking coils at the self-weight position of the third embodiment in the focus direction;  
     [0038]FIG. 20 is a block diagram of a circuit configuration for focus servo and inclination drive employed in the third embodiment of the invention;  
     [0039]FIG. 21 is an explanatory view of the focus servo and inclination drive to be executed in the third embodiment; specifically, FIG. 16A shows a case where there are produced driving forces having the same direction; and, FIG. 16B shows a case where there are produced driving forces respectively having reversed directions;  
     [0040]FIG. 22 is an exploded perspective view of a modification of the third embodiment;  
     [0041]FIG. 23 is an arrangement view of the modification of the third embodiment, showing the position relationship between magnets and focus coils/tracking coils at the self-weight position of the modification in the focus direction;  
     [0042]FIG. 24 is an exploded perspective view of a conventional objective lens drive apparatus;  
     [0043]FIG. 25 is an explanatory view of an inclination correction driving operation to be executed in the conventional objective lens drive apparatus;  
     [0044]FIG. 26 is a plan view of an actuator employed in the conventional objective lens drive apparatus; and,  
     [0045]FIG. 27 is a block diagram of the configuration of a circuit employed in the conventional objective lens drive apparatus to execute the inclination correction driving operation. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0046] (Embodiment 1)  
     [0047] Now, FIG. 1 is an exploded perspective view of a first embodiment of an objective lens drive apparatus for use in an optical pickup according to the invention. In FIG. 1, reference character  1  designates a lens holder,  2  an objective lens,  3  a coil unit,  3 f a focus coil,  3 tr a tracking coil,  3 ti a tilt coil,  5  a magnet, and  5 g a magnetic gap, respectively.  
     [0048] The lens holder  1  is formed of light metal of high modulus of flexural elasticity, for example, magnesium alloy, or resin mixed with carbon fibers. Use of such material allows the lens holder  1  itself to have higher flexural elasticity modulus and thus have higher high-order resonance frequencies. Due to this, the lens holder  1  is able to cope with an increase in the speed of an optical disk unit.  
     [0049] In the lens holder  1 , there are formed two notch portions  1   a  which respectively extend in the tracking direction T. Also, an objective lens mounting portion  1   b , which is also formed in the lens holder  1 , is structured such that it is uniform in thickness.  
     [0050] Each of the two notch portions  1   a  has a surface on which there is formed an insulated protective film (not shown) for insulation reinforcement. The reason for provision of such insulated protective film is that, since light metal of high flexural elasticity modulus such as magnesium alloy or resin mixed with carbon fibers used as the material of the lens holder  1  is high in conductivity, the insulation of the coil unit  3  to be mounted on the notch portions  1   a  must be secured. In case where an insulated protective film for insulation reinforcement is not formed on the surfaces of the notch portions  1   a  of the lens holder  1 , an insulated protective film (not shown) for insulation reinforcement may be formed on the portions of the coil unit  3  that are to be mounted onto the notch portions  1   a  to thereby be able to secure the insulation of the coil unit  3 .  
     [0051] The coil unit  3  is a laminated coil unit which comprises: a required number of printed circuit boards  31  each having a pattern in which a focus coil  3 f and four tracking coils  3 tr are formed; and, a required number of printed circuit boards  32  in each of which two tilt coils  3 ti are formed, whereby the two kinds of printed circuit boards  31  and  32  are alternately laminated one on top of another to thereby provide a pattern structure as a coil unit. The focus coil  3 f is disposed in the central portion of the printed circuit board  31 ; and, the tracking coils  3 tr are disposed right and left (in the tracking coil direction T) with respect to the position of the center of gravity of an objective-lens-optical-axis-direction movable part including the lens holder  1  holding the objective lens  2 , that is, on the right and left sides of the focus coil  3 f in two upper and lower stages. The four tracking coils  3 tr are connected in series. By the way, the tracking coils  3 tr may also be composed of two tracking coils. The two tilt coils  3 ti are disposed right and left (in the tracking coil direction T) with respect to the center of the printed circuit board  32 . The two tilt coils  3 ti are connected in series.  
     [0052] The printed circuit boards  31  and  32  can be laminated one on top of another, for example, by holding the two side surfaces of a printed circuit board  32  between two printed circuit boards  31  in such a manner that they are arranged symmetric when they are viewed from the tracking direction T. In this case, drive points in the respective directions can be made coincident, thereby being able to avoid resonance (pitching resonance, yawing resonance) which would be possibly caused when the drive points are not coincident.  
     [0053] The foregoing description relates to the structure where the focus coil  3 f and tracking coils  3 tr are formed in each printed circuit board  31 . However, the focus coil  3 f and tracking coils  3 tr may also be formed separately in two printed circuit boards. Further, as shown in FIG. 8, the coil unit  3 ′ may have a printed circuit board  31 ′ and a printed circuit board  32 ′, wherein the focus coil  3 f and tilt coils  3 ti are formed on the printed circuit board  31 ′, and the tracking coils  3 tr are formed on the printed circuit board  32 ′. FIG. 8 shows four tracking coil  3 ti are formed on the printed circuit board  32 ′, however, two tracking coil  3 ti may be formed on the printed circuit board  32 ′. In these structures as well, the printed circuit boards may be laid one on top of another so as to be symmetric right and left when they are viewed from the tracking direction T, thus being able to avoid resonance by possibly caused when the drive points are not coincident.  
     [0054] The coil unit  3  is inserted into and bonded to the notch portions  1   a  so that it is fixed to the lens holder  1 . In the two ends of the coil unit  3  in the tracking direction T, there are formed six V-grooves  3   v , while one-end portions of six conductive elastic members  4  are respectively fixed by solder (not shown) to the six V-grooves  3   v . In the case of the conductive elastic members  4  which serve as lead wires, two of them are used to drive the focus coils, two are used to drive the tracking coils, and two are used to drive the tilt coils: that is, a total of six conductive elastic members are provided. By the way, four conductive elastic members  4  are sufficient to elastically hold the lens holder  1  which serves as a movable part and, therefore, in case where four conductive elastic members  4  are used, lead wires (not shown) are to be connected to the remaining coils.  
     [0055] The magnet  5  is bonded to a yoke  7  disposed on a yoke base  6  in such a manner that the magnet  5  is magnetized in two polarities in the focus direction F by a boundary line  5 b between the N and S poles of the magnet  5 . As shown in FIG. 2, the boundary line  5 b between the N and S poles is positioned at the center of the magnet  5  in the focus direction F, the mutually opposing arrangement of two magnets  5  forms a magnetic gap  5 g between them, and, magnetic force lines B are reversed in direction in the focus direction F of the magnetic gap  5 g. By the way, as shown in FIG. 9, the magnetic circuit may include one magnet  5 ′, and the coil unit may be disposed in the magnetic gap  5 g′. FIG. 10 shows the magnetic circuit including the magnet  5 ′, similar operation of coils in the case of providing the magnetic circuit including two magnets  5  and gap  5 g described above can be obtained. Due to this, the whole objective lens drive apparatus can be made compact. Here, the magnetic gap means an air gap or an air path, in FIG. 9, the magnetic gap  5 g′ is formed by one magnet.  
     [0056] The width W of the magnet  5  is determined such that when the coil unit  3 , as shown in FIG. 3, is disposed in the magnetic gap  5 g at the movable neutral position of the movable part which is movably supported in a cantilevered manner by the conductive elastic members  4 , that is, at the self-weight position of the movable part in the focus direction F, of the vertical sides A and C (which extend in parallel to the focus direction F) of the four tracking coils  3 tr disposed right and left in the two upper and lower stages, the right and left inner vertical sides A and C can be disposed within the magnetic gap  5 g (which points out a gap existing within the width W of the two mutually opposing magnets  5 ); and also, as shown in FIG. 4, of the vertical sides a′ and c′ (which extend in parallel to the focus direction F) of the two tilt coils  3 ti disposed right and left in a row, the right and left outer vertical surfaces a′ and c′ can be disposed within the magnetic gap  5 g. Also, the height H of the magnet  5  is determined such that, as shown in FIG. 3, the horizontal sides b and d (which extend perpendicularly to the focus direction F) of the single focus coil  3 f disposed at the center of the printed circuit board  31  as well as the upper and lower outer horizontal sides B and D of the horizontal sides B and C (which extend perpendicularly to the focus direction F) of the tracking coils  3 tr can be disposed within the magnetic gap  5 g (which points out a gap existing within the height H of the two mutually opposing magnets  5 ); and also, as shown in FIG. 4, the horizontal sides b′ and d′ (which extend perpendicularly to the focus direction F) of the tilt coils  3 ti can be disposed within the magnetic gap  5 g.  
     [0057] The boundary line  5 b between the N and S poles of the magnet  5 , as shown in FIG. 3, is situated midway between the lower side b and upper side d of the horizontal sides b, d (which extend perpendicularly to the focus direction F) of the focus coil  3 f, midway between the lower side B of the horizontal sides B, D (which extend perpendicularly to the focus direction F) of the upper-stage tracking coil  3 tr and the upper side D of the horizontal sides B, D (which extend perpendicularly to the focus direction F) of the lower-stage tracking coil  3 tr, and, as shown in FIG. 4, midway between the lower side b′ and upper side d′ of the horizontal sides b′, d′ (which extend perpendicularly to the focus direction F) of the tilt coils  3 ti. The center of the magnet  5  is substantially coincident with the center of the coil unit  3 .  
     [0058] In FIG. 3, in case where currents are allowed to flow in the tracking coils  3 tr, due to the currents (shown by arrow marks) that flow in the vertical sides A, C which extend in parallel to the focus direction F) of the tracking coils  3 tr, in the four tracking coils  3 tr, there are generated driving forces in the same direction according to Fleming&#39;s left-hand rule. Also, in case where a current is allowed to flow in the focus coil  3 f, due to the currents that flow in the horizontal sides b, d (which extend perpendicularly to the focus direction F) of the focus coil  3 f, in the focus coil  3 f, there is generated a driving force in the focus direction F according to Fleming&#39;s left-hand rule.  
     [0059] In FIG. 4, in case where currents are allowed to flow in the tilt coils  3 ti, due to the currents (shown by arrow marks) that flow in the horizontal sides b′, d′ (which extend perpendicularly to the focus direction F) of the tilt coils  3 ti, in the two tilt coils  3 ti, there are generated driving forces F′ in the mutually reversed directions in the focus direction F according to Fleming&#39;s left-hand rule. Due to the mutually-reversed-direction driving forces F′, there is generated moment around the center of gravity of the movable part to thereby be able to adjust the inclination of the lens holder  1  and thus the inclination of the objective lens  2 .  
     [0060] As described above, in case where not only the focus coil  3 f and tracking coils  3 tr but also the tilt coils  3 ti are arranged within the same magnetic gap  5 g of the magnetic circuit including at least one magnet, not only focus servo and tracking servo but also tilt servo (that is, the adjustment of the inclination of the objective lens  2 ) can be carried out. This can eliminate the need for provision of a magnet which is exclusively used to adjust the inclination of the objective lens  2 . Due to this, the number of parts can be reduced, the adjustment of the inclination of the objective lens  2  can be made at a low cost, and the whole objective lens drive apparatus can be made compact.  
     [0061] The foregoing description relates to the structure in which the two tilt coils  3 ti are arranged right and left (in the tracking direction T) with respect to the center of the printed circuit board  32 . However, a similar effect can also be obtained even in a structure in which, as shown in FIG. 5, two tilt coils  3 ti are arranged upward and downward (in the focus direction F) with respect to the center of the printed circuit board  32 .  
     [0062] In this case, the coil unit  3  is structured such that, as shown in FIG. 7, a required number of printed circuit boards (not shown) each having a pattern including a tracking coil  3 tr and four focus coils  3 f and, as shown in FIG. 5, a required number of printed circuit boards (not shown) each having a pattern including two tilt coils  3 ti are alternately laid one on top of another.  
     [0063] The foregoing description relates to the structure in which the focus coil  3 f and tracking coils  3 tr are disposed on the same printed circuit board. However, there can also be employed a structure in which focus coils  3 f and tracking coils  3 tr are separately disposed on two printed circuit boards. In this case as well, the printed circuit boards are laid one on top of another so as to be symmetric right and left when they are viewed from the tracking direction T.  
     [0064] In this structure, the magnet  5 , as shown in FIG. 6, is magnetized in two polarities in the tracking direction T by the boundary line  5 b between the N and S poles of the magnet  5 , and is bonded to the yoke  7  on the yoke base  6 . As shown in FIG. 6, the boundary line  5 b between the N and S poles is situated at the center of the magnet  5  in the tracking direction T, the magnetic gap  5 g is formed due to the mutually opposing arrangement of the two magnets  5  and, in the magnetic gap  5 g, the direction of a magnetic line of force B is reversed in the tracking direction T. By the way, alternatively, as shown in FIGS. 9 and 10, instead of the two magnets  5 , there may be used a single magnet  5 . In this case, the boundary line  5 b′ between the N and S poles is situated at the center of the magnet  5  in the tracking direction T. Due to this, the whole objective lens drive apparatus can be made compact.  
     [0065] The width W of the magnet  5  is determined such that, as shown in FIG. 7, when the coil unit  3  is arranged in the magnetic gap  5 g at the movable neutral position of the movable part movably supported in a cantilevered manner by the conductive elastic members  4 , that is, at the self-weight position thereof in the focus direction F, not only the right and left outer vertical sides a and c of the vertical sides a and c (which extend in parallel to the focus direction F) of the four focus coils  3 f arranged right and left in two upper and lower stages but also, as shown in FIG. 5, the vertical sides a′ and c′ (which extend in parallel to the focus direction F) of the two tilt coils  3 ti arranged in two upper and lower stages can be respectively disposed within the magnetic gap  5 g (which points out a gap existing within the width W of the mutually opposing magnets  5 ). Also, the height H of the magnets  5  is determined such that not only, as shown in FIG. 7, the lower sides b of the horizontal sides b, d (which extend perpendicularly to the focus direction F) of the upper-stage focus coils  3 f, the upper sides d of the horizontal sides b, d (which extend perpendicularly to the focus direction F) of the lower-stage focus coils  3 f, and the horizontal sides B and D (which extend perpendicularly to the focus direction F) of the tracking coil  3 tr but also, as shown in FIG. 5, the upper sides d′ of the horizontal sides b′, d′ (which extend perpendicularly to the focus direction F) of the upper-stage tilt coil  3 ti and the lower sides b′ of the horizontal sides b′, d′ (which extend perpendicularly to the focus direction F) of the lower-stage tilt coil  3 ti can be respectively disposed within the magnetic gap  5 g (which points out a gap existing within the height H of the mutually opposing magnets  5 ).  
     [0066] The boundary line  5 b between the N and S poles of the magnet  5  is situated not only, as shown in FIG. 7, midway between the left sides c of the vertical sides a, c (which extend in parallel to the focus direction F) of the right focus coil  3 f and the right sides a of the vertical sides a, c (which extend in parallel to the focus direction F) of the left focus coil  3 f, and midway between the right side A and left side C of the vertical sides A, C (which extend in parallel to the focus direction F) of the tracking coil  3 tr, but also with, as shown in FIG. 5, midway between the right side a′ and left c′ of vertical sides a′ , c′ (which extend in parallel to the focus direction F) of the tilt coil  3 ti. The center of the magnet  5  is substantially coincident with the center of the coil unit  3 .  
     [0067] In FIG. 7, in case where a current is allowed to flow in the tracking coil  3 tr, due to the current (shown by an arrow mark) that flows in the vertical sides A, C (which extend in parallel to the focus direction F) of the tracking coil  3 tr, in the tracking coil  3 tr, there is generated a driving force in the tracking direction T according to Fleming&#39;s left-hand rule; and, in case where currents are allowed to flow in the focus coils  3 f, due to the currents (shown by arrow marks) that flow in the horizontal sides b, d (which extend perpendicularly to the focus direction F) of the focus coils  3 f, in the four focus coils  3 f, there are generated driving forces respectively having the same direction in the tracking direction T according to Fleming&#39;s left-hand rule.  
     [0068] In FIG. 5, in case where currents are allowed to flow in the tilt coils  3 ti, due to the currents (shown by arrow marks) that flow in the vertical sides a′, c′ (which extend in parallel to the focus direction F) of the tilt coils  3 ti, in the two tilt coils  3 ti, there are generated driving forces in the mutually reversed directions in the tracking direction T according to Fleming&#39;s left-hand rule. Due to the reversed-direction driving forces, there is generated moment around the center of gravity of the movable part to thereby be able to adjust the inclination of the lens holder  1  and thus the inclination of the objective lens  2 .  
     [0069] (Embodiment 2)  
     [0070] Now, FIG. 11 is a perspective view of a second embodiment of an objective lens drive apparatus according to the invention. In FIG. 11, reference character  101  designates a lens holder,  102  an objective lens,  103  a coil unit,  103 f a focus coil,  103 tr a tracking coil,  103 ti a tilt coil,  105  a magnet, and  105 g a magnetic gap, respectively.  
     [0071] The lens holder  101  is made of light metal of high modulus of flexural elasticity, for example, magnesium alloy, or resin mixed with carbon fibers. Use of such material allows the lens holder  101  itself to have higher flexural elasticity modulus and thus have higher high-order resonance frequencies. Due to this, the lens holder  101  is able to cope with an increase in the speed of an optical disk unit.  
     [0072] Referring further to the structure of the lens holder  101 , on the plane surface thereof, there are formed two slits  111  through which a magnet  105  and a yoke  107  (both of which will be discussed later) can be inserted; on the central portion of the lens holder  101 , there is mounted the objective lens  102 ; on each of a pair of side surfaces of the lens holder  101  which extend at right angles to the tracking direction T, there are projectingly disposed two upper and lower support pieces  112  to which the one-end portions of conductive elastic members  104  (which will also be discussed later) can be fixed; and, to a pair of side surfaces of the lens holder  101  which extend in parallel to the tracking direction T, there are fixed coil units  103  (which will also be discussed later).  
     [0073] Insulated protective films (not shown) for reinforcement are respectively formed on the surfaces of the pair of side surfaces (which extend in parallel to the tracking direction T) of the lens holder  101 . The reason for provision of such insulated protective films is to secure the insulation of the coil units  103  to be mounted onto the lens holder  101  because light metal of high modulus of flexural elasticity, for example, magnesium alloy, or resin mixed with carbon fibers used as the material of the lens holder  101  is high in conductivity. In case where such insulated protective films for reinforcement are not formed on the surfaces of the pair of side surfaces (which extend in parallel to the tracking direction T) of the lens holder  101 , insulated protective films (not shown) for reinforcement may be formed on the portions of the coil units  103  that are to be mounted onto the lens holder  101 , thereby securing the insulation of the coil units  103 .  
     [0074] Referring now to the coil unit  103 , a required number of printed circuit plates  131  each having a pattern composed of a focus coil  103 f and four tracking coils  103 tr and a required number of printed circuit plates  132  each having a pattern composed of two tilt coils  103 ti are laminated or laid one on top of another to thereby form the coil unit  103 . The focus coil  103 f is disposed in the central portion of the printed circuit board  131 ; and, the tracking coils  103 tr are disposed in the right and left directions (in the tracking direction T) with respect to the position of the center of gravity of an objective-lens-optical-axis-direction movable part including the lens holder  101  holding the objective lens  102 , that is, on the right and left sides of the focus coil  103 f in two upper and lower stages. The four tracking coils  103 tr are connected in series. By the way, the four tracking coils  103 tr may also be replaced with two tracking coils. The two tilt coils  103 ti are disposed in a row right and left (in the tracking coil direction T) with respect to the center of the printed circuit board  32 . The two tilt coils  103 ti are connected in series.  
     [0075] The printed circuit boards  131  and  132  may be laminated in such a manner that the two side surfaces (which extend in parallel to the tracking direction T) of the printed circuit board  131  and the two side surfaces (which extend in parallel to the tracking direction T) of the printed circuit board  132  are arranged symmetric when they are viewed from the tracking direction T, for example, the printed circuit board  131  is arranged inside on the objective lens  102  side and the printed circuit board  132  is arranged outside on the objective lens  102  side. In this case, drive points in the respective directions can be made coincident with each other, thereby being able to avoid resonance (pitching resonance, yawing resonance) which would be possibly caused when the drive points are not coincident.  
     [0076] The foregoing description relates to the structure in which the focus coil  103 f and tracking coils  103 tr are formed in the same printed circuit board  131 . However, the focus coil  3 f and tracking coils  3 tr may also be formed separately in two different printed circuit boards. Further, as shown in FIG. 8, the coil unit  3 ′ may have a printed circuit board  31 ′ and a printed circuit board  32 ′, wherein the focus coil  3 f and tilt coils  3 ti are formed on the printed circuit board  31 ′, and the tracking coils  3 tr are formed on the printed circuit board  32 ′. FIG. 8 shows four tracking coil  3 ti are formed on the printed circuit board  32 ′, however, two tracking coil  3 ti may be formed on the printed circuit board  32 ′. In these case as well, the printed circuit boards may be laid one on top of another symmetrically right and left when they are viewed from the tracking direction T, thus being able to avoid resonance by possibly caused when the drive points are not coincident.  
     [0077] The one-end portions of the four conductive elastic members  104  are respectively fixed by solder (not shown) to the support pieces  112  of the lens holder  101  with the coil units  103  fixed thereto. Two lead wires are necessary to drive the focus coils, two lead wires are necessary to drive the tracking coils, and two lead wires are necessary to drive the tilt coils, that is, a total of six lead wires are necessary. Here, four units of such conductive elastic member  104  are enough to elastically support the lens holder  101  serving as the movable part. Here, the conductive elastic members  104  can also be used as lead wires. Therefore, the four conductive elastic members  104  are used as four of the six lead wires, while other lead wires (not shown) are connected to the remaining coils.  
     [0078] The two coil units  103  are respectively arranged in the two magnetic gaps  105 g, while the other-end portions of the conductive elastic members  104  are respectively penetrated through a wire base  108  and are fixed to a base plate  109  by soldering. Due to this, the focus coil  103 f, tracking coils  103 tr and tilt coils  103 ti mounted on the coil unit  103  can be disposed within the magnetic gap  105 g and, at the same time, the movable part including the lens holder  101  holding the objective lens  2  is supported in a cantilevered manner so as to be movable with respect to the fixed part which includes the magnet  105 , yoke base  106 , yoke  7 , wire base  108  and base plate  109 .  
     [0079] The structures of the magnetic circuits employed in the apparatus shown in FIG. 11 as well as the arrangements and operations of the focus coils, tracking coils and tilt coils used in the coil units of the apparatus shown in FIG. 11 are similar to the previously described first embodiment and thus the description thereof is omitted here (see FIGS.  2  to  4 ).  
     [0080] As described above, according to the present embodiment, there are completed two magnetic circuits each including at least one magnet  105  magnetized in two polarities, and, within the magnetic gap  105 g of each of the two magnetic circuits  105 , there are disposed not only the focus coil  103 f and tracking coils  103 tr but also the tilt coils  103 ti. Thanks to this, not only focus servo and tracking servo but also tilt servo (that is, the adjustment of the inclination of the objective lens  102 ) can be attained. Therefore, there is eliminated the need for provision of a magnet which is exclusively used to adjust the inclination of the objective lens  102 . This can reduce the number of parts, can adjust the inclination of the objective lens  102  at a low cost, and can reduce the size of the whole objective lens drive apparatus.  
     [0081] The above description relates to the structure in which the two tilt coils  103 ti are respectively disposed right and left (in the tracking direction T) with respect to the center of the printed circuit board  132 . However, similarly to the first embodiment, even in case where the two tilt coils  103 ti are respectively disposed upwardly and downwardly (in the focus direction F) of the center of the printed circuit board  132 , there can be obtained a similar effect. In this case, the structure of the magnetic circuit and the operation of the coil unit are similar to the first embodiment and thus the description thereof is omitted here (see FIGS.  5  to  7 ). By the way, as shown in FIG. 12, two magnetic circuits may respectively include one magnet  105 ′. In this case, magnets  105 ′ and yokes  107 ′ are respectively provided outside of a lens holder  101 ′ with respect to the center of the lens holder  101 ′. In this structure, the slit  111  need not be provided in the lens holder  101 ′, therefore, the whole objective lens drive apparatus can be made compact. The magnetic circuit in this case is shown in FIGS. 13A or  13 B. Here, the magnetic gap means an air gap or air path. In FIG. 13A, the magnetic gap  105 g′ is formed by two magnets, and in FIG. 13B, the magnetic gaps  105 g′ are respectively formed by each magnet.  
     [0082] In the above structure, the coil units  103  are bonded and fixed to the pair of side surfaces of the lens holder  101  that extend in parallel to the tracking direction. However, a similar effect can also be obtained even in another structure in which, as shown in FIG. 14, there are completed two magnetic circuits each including at least one magnet  105  magnetized in two polarities in the focus direction F and, within each of the magnetic gaps  105 g of the magnetic circuits, there are disposed focus coils  130 f respectively wound around the side surfaces of the lens holder  101  as well as the tracking coils  130 tr and tilt coils  130 ti respectively mounted on the two side surfaces (which extend in parallel to the tracking direction T) of the lens holder  101 . By the way, two magnetic circuit may be respectively include one magnet, as shown in FIG. 12.  
     [0083] Each focus coil  130 f is a winding coil with the lens holder  101  as its winding frame and thus, when compared with a focus coil which is pattern formed on a printed circuit board, the focus coil  130 f is easy to manufacture.  
     [0084] The tracking coil  130 tr and tilt coil  130 ti are respectively a coreless coil which is mounted on top of the focus coil  130 f. However, the tracking coil  130 tr and tilt coil  130 ti may also be pattern formed on a printed circuit board. Also, the tracking coils  130 tr and tilt coils  130 ti may also be winding coils in which, as shown in FIG. 15, coil winding frames  113  are provided on and projected from the side surfaces (which extend in parallel to the tracking direction T) of the lens holder  101  and coils are respectively wound around these coil winding frames  113 . Further, one of the tracking coil  130 tr and tilt coil  130 ti may be mounted on the focus coil  130 f and the other may be wound around the coil winding frame  113 .  
     [0085] The magnet  105  is magnetized in two polarities in the focus direction F by the boundary line  105 b between the N and S poles of the magnet  105  and is bonded to the yoke  107  which is disposed on a yoke base  106 .  
     [0086] The width W of the magnet  105  is determined such that, when, at the movable neutral position of the movable part movably supported in a cantilevered manner by the conductive elastic members  104 , that is, at the self-weight position of the movable part in the focus direction F, as shown in FIG. 16, the lens holder  101  is arranged in the magnetic gap  105 g, not only the right and left inner vertical sides A and C of the vertical sides A and C (which extend in parallel to the focus direction F) of the two tracking coils  130 tr, which are disposed in the upper stage in the focus direction F as well as are disposed right and left in a row in the tracking direction T, but also the right and left outer vertical sides a′ and c′ of the vertical sides a′ and c′ (which extend in parallel to the focus direction F) of the two tilt coils  130 tr which are disposed in the lower stage in the focus direction F as well as are disposed right and left in a row in the tracking direction T can be respectively arranged within the magnetic gap  105 g (which points out a gap existing within the width W of the two mutually opposing magnets  105 ). Also, the height H of the magnet  105  is determined such that, as shown in FIG. 16, the horizontal sides B and D (which extend perpendicularly to the focus direction F) of the tracking coils  130 tr as well as the horizontal sides b′ and d′ (which extend perpendicularly to the focus direction F) of the tilt coils  130 ti can be respectively disposed within the magnetic gap  105 g (which points out a gap existing within the height H of the two mutually opposing magnets  105 ).  
     [0087] The boundary line  105 b between the N and S poles of the magnet  105 , as shown in FIG. 16, is situated downwardly of the lower sides B of the horizontal sides B and D (which extend perpendicularly to the focus direction F) of the tracking coils  130 tr as well as midway between the lower sides b′ and upper sides d′ of the horizontal sides b′ and d′ (which extend perpendicularly to the focus direction F) of the tilt coils  130 ti. The center of the magnet  105  is substantially coincident with the center of the lens holder  101 .  
     [0088] The focus coils  130 f are disposed upwardly and downwardly with the boundary line  105 b between the N and S poles of the magnet  105  as the boundary line thereof. The upper and lower focus coils  130 f are connected in series, while the directions of the currents of the upper and lower focus coils  130 f are reversed. The directions of magnetic lines of force in the two magnetic gaps  105 g are reversed.  
     [0089] In FIGS. 14 and 15, all sides of the tracking coils  130 tr and tilt coils  130 ti are mounted on one side surface (which extends in parallel to the tracking direction T) of the lens holder  1 . However, this is not limitative but it is also possible to employ another structure; that is, the sides that are arranged within the magnetic gap  105 g and are able to generate drive forces, for example, the vertical sides A, C (see FIG. 16) (which extend in parallel to the focus direction F) of the tracking coils  130 tr, which, in case where currents are allowed to flow in the tracking coils  130 tr, can generate drive forces in the same direction in the tracking direction T, are mounted on one side surface of the lens holder  1 .  
     [0090] The lens holder  101  is disposed in the two magnetic gaps  105 g and the other-side ends of the conductive elastic members  104  are penetrated through a wire base  108  and are fixed to a base plate  109  by soldering. Thanks to this, the focus coils  130 f, tracking coils  130 tr and tilt coils  130 ti can be disposed within the magnetic gap  105 g and, at the same time, the movable part including the lens holder  101  holding the objective lens  102  can be supported in a cantilevered manner so as to be movable with respect to the fixed part which includes the magnet  5 , yoke base  106 , yoke  107 , wire base  108  and base plate  109 .  
     [0091] In FIG. 14, in case where currents are allowed to flow in the focus coils  130 f, due to the current that flows in the magnetic gap  105 g, in the focus coils  130 f, there are generated drive forces in the focus direction F according to Fleming&#39;s left-hand rule.  
     [0092] In FIG. 16, in case where current are allowed to flow in the tracking coils  130 tr, due to the currents (shown by arrow marks) that flow in the vertical sides A and C (which extend in parallel to the focus direction F) of the tracking coils  130 tr, in the two tracking coils  130 tr, there are generated drive forces in the same direction in the tracking direction T according to Fleming&#39;s left-hand rule; and, in case where currents are allowed to flow in the tilt coils  130 ti, due to the currents (shown by arrow marks) that flow in the horizontal sides b′ and d′ (which extend perpendicularly to the focus direction F) of the tilt coils  130 ti, in the two tilt coils  130 ti, there are generated drive forces F′ in the mutually reversed directions in the focus direction F according to Fleming&#39;s left-hand rule. Due to the reversed-direction drive forces F′, there is generated moment around the center of gravity of the movable part, thereby being able to adjust the inclination of the lens holder  101  and thus the inclination of the objective lens  102 .  
     [0093] The above description relates to the structure in which the two tracking coils  130 tr and two tilt coils  130 ti are arranged right and left symmetrically in the tracking direction T, while there are generated the drive forces in the same direction in the two tracking coils  130 tr and there are generated drive forces in the reversed directions in the two tilt coils  130 ti. However, as shown in FIG. 17, the vertical side A (which extends in parallel to the focus direction F) of a tracking coil  130 tr may be disposed within the width W of the magnet  105 , and the vertical side C (which extends in parallel to the focus direction F) of the tracking coil  130 tr may be disposed outside the width W of the magnet  105 ; and, at the same time, a tilt coil  130 ti may be disposed shifted outside with respect to the center of the magnet  105  in the tracking direction T. Also, instead of the tracking coil  130 tr, as shown in FIG. 16, there may be used two tracking coils  130 tr; and, instead of the tilt coil  130 ti, as shown in FIG. 17, there may be used two tilt coils  130 ti. Further, the tracking coil  130 tr, as shown in FIG. 17, may be one in number and the tilt coil  130 ti, as shown in FIG. 16, may be two in number. In any of these structures, the weight of the objective lens drive apparatus can be reduced.  
     [0094] (Embodiment 3) Now, FIG. 18 is a perspective view of a third embodiment of an objective lens drive apparatus according to the invention. In FIG. 18, reference character  201  designates a lens holder,  202  an objective lens,  203  a coil unit, and  205  a magnet, respectively.  
     [0095] The lens holder  201  is similar in structure to the lens holder  1  employed in the previously described first embodiment.  
     [0096] The coil unit  203  comprises a required number of printed circuit boards  203 p which are laminated one on top of another, while each of the printed circuit board  203 p comprises a tracking coil  203 t and four focus coils  203 fl and  203 fr. The tracking coil  203 t is situated at the center of the printed circuit board  203 p, while the focus coils  203 fl and  203 fr are arranged in two upper and lower stages and are disposed right and left with respect to the position of the objective-lens-optical-axis-direction center of gravity of a movable part including the lens holder  201  holding the objective lens  202 , that is, on the right and left sides of the tracking coil  203 t. The number of the focus coils  203 fl and the number of the focus coils  203 fr may also be one respectively. And, since currents are supplied to the left and right focus coils  203 fl and  203 fr individually, the left and right focus coils  203 fl and  203 fr are not connected in series but they are independent of each other.  
     [0097] The foregoing description relates to the structure in which the left and right focus coils  203 fl,  203 fr and tracking coil  203 t are disposed on the same printed circuit board  203 p. However, as a modification of the third embodiment, the left and right focus coils  203 fl,  203 fr and tracking coil  203 t may also be disposed separately on two printed circuit boards. In this modidification as well, the number of focus coils to be disposed on a printed circuit board is even and the number of tracking coils to be disposed on a printed circuit board is one.  
     [0098] The coil unit  203  is inserted into and bonded to the notch portions  201   a  of the lens holder  201  and is thereby fixed to the lens holder  201 . In the two ends (in the tracking direction T) of the coil unit  203 , there are formed six V-grooves  203   v , while the one-side ends of six conductive elastic members  204  are respectively fixed to the six V-grooves  203   v  by solders  203   h . The conductive elastic members  204 , which are used as lead wires, consist of four members  204  (2×2) for focus coil driving and two members  204  for tracking coil driving, a total of six members  204 .  
     [0099] By the way, four conductive elastic members  204  are enough to elastically hold the lens holder  201  serving as the movable part and, therefore, in case where four conductive elastic members  204  are employed to hold the lens holder  201 , lead wires (not shown) are to be connected to the remaining coils.  
     [0100] The magnetic circuit employed in the present embodiment is similar to the magnetic circuit employed in the first embodiment and shown in FIG. 6. Further, the magnetic circuit may be include one magnet, as shown in FIGS. 9 and 10. In this case, the boundary line between the N and S poles is situated at the center of the magnet  5  in the tracking direction T as well as FIG. 6. Due to this, the whole objective lens drive apparatus can be made compact. The width W of the magnet  205  is determined such that, at the movable neutral position of the movable part movably supported in a cantilevered manner by the conductive elastic members  204 , that is, at the self-weight position of the movable part in the focus direction F, as shown in FIG. 19, when the coil unit  203  is arranged within the magnetic gap  205 g, the right and left outer vertical sides c and a (which extend in parallel to the focus direction F) of the vertical sides a and c of the left focus coils  203 fl in the two upper and lower stages as well as the right focus coils  203 fr in the two upper and lower stages can be respectively disposed within the magnetic gap  205 g (which points out a gap existing within the width W of the mutually opposing magnets  205 ). Also, the height H of the magnet  205  is determined such that, as shown in FIG. 19, the lower sides b of the horizontal sides b and d (which extend perpendicularly to the focus direction F) of the upper-stage focus coils  203 fl and  203 fr, the upper sides d ( which extend perpendicular to the focus direction F) of the lower-stage focus coils  203 fl and  203 fr, and the horizontal sides B and D (which extend perpendicularly to the focus direction F) of the tracking coil  203 t can be respectively disposed within the magnetic gap  205 g (which points out a gap existing within the height W of the mutually opposing magnets  205 ).  
     [0101] The boundary line  205 b between the N and S poles of the magnet  205 , as shown in FIG. 19, is situated midway not only between the vertical sides A and C (which extend in parallel to the focus direction F) of the tracking coil  203 t but also between the right sides a of the vertical sides a, c (which extend in parallel to the focus direction F) of the left focus coils  203 fl and the left sides c of the vertical sides a, c (which extend in parallel to the focus direction F) of the right focus coils  203 fr. The center of the magnet  205  is substantially coincident with the center of the coil unit  203 .  
     [0102] The coil units  203  are respectively disposed within the magnetic gap  205 g and the other-side ends of the conductive elastic members  204  are respectively penetrated through the wire base  208  and are fixed to the base plate  209  by soldering. In this manner, the focus coils  203 fl,  203 fr and tracking coil  203 t mounted on the coil unit  203  are disposed with in the magnetic gap  205 g and, at the same time, the movable part including the lens holder  201  supporting the objective lens  202  is supported in a cantilevered manner so as to be movable with respect to the fixed part which includes the magnets  205 , yoke base  206 , yokes  207 , wire base  208 , and base plate  209 .  
     [0103] The inclination of the optical disk can be detected using an inclination detect sensor which is separately prepared, or using a reproduction signal given by an optical pickup.  
     [0104] A tilt error signal and a focus error signal, which have been obtained using the inclination detect sensor or using the reproduction signal of the optical pickup, are input to a control circuit shown in FIG. 20; and, the control circuit calculates the optimum currents Il and Ir which can urge the focus coils  203 fl and  203 fr shown in FIG. 19 to thereby correct the focus error and tilt error at the same time, and then the control circuit outputs the thus calculated currents Il and Ir. The objective lens drive apparatus, which is a controlled object, not only executes a focus driving operation due to a force which is the sum of drive forces Fl and Fr generated in response to the currents Il and Ir and shown in FIG. 21A and also which moves in the focus direction F, but also executes a tilt driving operation due to the moment M=Fl×d−Fr×d that is generated around the center of gravity G of the lens holder  201  due to the difference between the drive forces Fl and Fr. Here, d expresses the distance between the center of gravity G of the lens holder  201  and the focus coils  203 fl,  203 fr.  
     [0105] Now, FIG. 21B shows a case in which, differently from FIG. 21A, the drive forces Fl and Fr are generated in the mutually opposite directions. In this case, a force, which is going to move in the focus direction F, is Fl+(−Fr), while a tilt is Fl×d−(−Fr×d). At any rate, the objective lens drive apparatus executes a focus driving operation with a function of (Fl+Fr) and executes a tilt driving operation with a function of (Fl−Fr).  
     [0106] The left and right focus coils  203 fl and  203 fr not only can execute focus servo but also can adjust the inclination of the objective lens  202 . Therefore, there is eliminated the need for provision of a coil and a magnet which are exclusively used to adjust the inclination of the objective lens  202 . This can reduce the number of parts, can adjust the inclination of the objective lens  202  at a low cost, and can reduce the size of the whole objective lens drive apparatus.  
     [0107] In case where the tracking coil  203  is urged, due to the currents (shown by arrow marks in FIG. 19) that flow in the vertical sides A and C (which extend in parallel to the focus direction F) of the tracking coil  203 t, there are generated drive forces in the same direction in the tracking direction T, so that the objective lens  202  can be moved in the tracking direction T according to the eccentricity of a record medium.  
     [0108] In case where the coil unit  203  is inserted into and bonded to the notch portions  201   a  of the lens holder  201 , the number of magnetic gaps  205 g can be reduced down to one. This can also reduce the number of parts, can adjust the inclination of the objective lens  202  at a low cost, and can reduce the size of the whole objective lens drive apparatus.  
     [0109] In the above-mentioned embodiment, not only the focus servo but also the adjustment of the inclination of the objective lens  202  are carried out using the left and right focus coils  203 fl and  203 fr. However, a similar effect can also be obtained in the following structure: that is, as shown in FIG. 22, a coil unit  203  comprises a required number of printed circuit boards  203 p which are laid one on top of another, while each of the printed circuit boards  203 p includes a focus coil  203 f and four tracking coils  203 t; the focus coil  203 f is arranged at the center of each printed circuit board  203 p; the tracking coils  203 tu and  203 td are respectively disposed upwardly and downwardly of the objective-lens-optical-axi s-direction center of gravity of a movable part including a lens holder  201  holding an objective lens  202 , that is, the tracking coils  203 tu and  203 td are disposed in two right and left rows respectively extending upwardly and downwardly of the focus coil  203 f; and, a magnet  205  is magnetized in two polarities in the focus direction by the boundary line  205 b between the N and S poles of the magnet  205 . In this structure, a magnetic circuit is similar to the magnetic circuit that is employed in the first embodiment and is shown in FIG. 2. By the way, the magnetic circuit may include one magnet  51  as shown in FIG. 9, and the coil unit may be disposed in the air gap  5 g′. The magnetic circuit is shown in FIG. 10, similar operation of coils when the magnetic circuit includes two magnets and the magnetic gap as described above can be obtained.  
     [0110] Here, alternatively, the tracking coils  203 tu and  203 td may also be one in number respectively. Since currents are supplied to the upper and lower tracking coils  203 tu and  203 td individually, they are not connected in series but are connected independent of each other.  
     [0111] In this structure, the focus coil  203 f and tracking coils  203 tu,  203 td are disposed on the same printed circuit board  203 p. However, the focus coil  203 f and tracking coils  203 tu,  203 td may also be disposed separately on two different printed circuit boards. In this case as well, the numbers of focus coils and tracking coils to be disposed on a printed circuit board are respectively one and even.  
     [0112] The width W of the magnet  205  is determined such that, at the movable neutral position of the movable part movably supported in a cantilevered manner by the conductive elastic members  204 , that is, at the self-weight position of the movable part in the focus direction F, as shown in FIG. 23, when the coil unit  203  is arranged within the magnetic gap  205 g, the right and left inner vertical sides C and A of the vertical sides A and C (which extend in parallel to the focus direction) of the two upper-stage right and left tracking coils  203 tu as well as the two lower-stage right and left tracking coils  203 td can be respectively disposed within the magnetic gap  205 g (which points out a gap existing within the width W of the mutually opposing magnets  205 ). Also, the height H of the magnet  205  is determined such that, as shown in FIG. 23, not only the horizontal sides b and d (which extend perpendicularly to the focus direction F) of the focus coil  203 f situated at the center of the print circuit board  203 p, but also the upper sides D of the horizontal sides B and D (which extend perpendicularly to the focus direction F) of the upper-stage tracking coils  203 tu and the lower sides B of the horizontal sides B and D (which extend perpendicularly to the focus direction F) of the lower-stage tracking coil  203 td can be respectively disposed within the magnetic gap  205 g (which points out a gap existing within the height W of the mutually opposing magnets  205 ).  
     [0113] The boundary line  205 b between the N and S poles of the magnet  205 , as shown in FIG. 23, is situated midway not only between the lower side b and upper side d of the horizontal sides b and d (which extend perpendicularly to the focus direction F) of the focus coil  203 f but also between the lower sides B of the horizontal sides B and D (which extend perpendicularly to the focus direction F) of the upper-stage tracking coils  203 tu and the upper sides D of the horizontal sides B and D (which extend perpendicularly to the focus direction F) of the lower-stage tracking coils  203 td. And, the center of the magnet  205  is substantially coincident with the center of the coil unit  203 .  
     [0114] A tilt error signal and a tracking error signal, which have been obtained using an inclination detect sensor or using the reproduction signal of an optical pickup, are input to a control circuit which is similar to the control circuit shown in FIG. 20; and, the control circuit calculates the optimum currents Iu and Id which can urge the tracking coils  203 tu and  203 td shown in FIG. 23 to thereby correct the tracking error and tilt error at the same time, and then the control circuit outputs the thus calculated currents Iu and Id. The objective lens drive apparatus, which is an controlled object, not only executes a tracking driving operation due to a force which is the sum of driving forces (not shown) generated in response to the currents Iu and Id and also which moves in the tracking direction F, but also executes a tilt driving operation due to the moment that is generated around the center of gravity of the lens holder  201  due to the difference between the driving forces.  
     [0115] In case where the focus coil  203 f is urged, due to the currents (shown by arrow marks in FIG. 18) that flow in the horizontal sides b and d which extend perpendicularly to the focus direction F of the focus coil  203 f in FIG. 18, there are generated driving forces in the same direction in the focus direction F, so that the objective lens  202  can be moved in the focus direction F according to the surface vibration of the record medium.  
     [0116] Further, the system that can execute the tilt driving by the control unit having focus coil and tracking coil in the third embodiment can be applied to the objective lens drive apparatus according the second embodiment shown in FIGS. 11 and 12.  
     [0117] As has been described heretofore, according to the first aspect of the invention, there is provided an objective lens drive apparatus in which a coil unit with a focus coil, a tracking coil and a tilt coil mounted thereon is disposed within the same magnetic gap of a magnetic circuit including at least one magnet. In the present objective lens drive apparatus, the inclination of an objective lens can be adjusted using the magnet for focus and tracking driving, which eliminates the need for provision of a magnet which is exclusively used to adjust the inclination of the objective lens. Therefore, according to the first aspect of the invention, it is possible to prevent an increase in the cost as well as an increase in the size of the objective lens drive apparatus, which are otherwise caused by the adjustment of the inclination of the objective lens.  
     [0118] Also, according to the second aspect of the invention, there is provided an objective lens drive apparatus in which there are completed two magnetic circuits each including at least one magnet magnetized in two polarities and, within the magnetic gap of each of the two magnetic circuits, there is disposed a coil unit with a focus coil, a tracking coil and a tilt coil mounted thereon. In the present objective lens drive apparatus, the inclination of an objective lens can be adjusted using the magnets for focus and tracking driving, which eliminates the need for provision of a magnet which is exclusively used to adjust the inclination of the objective lens. Therefore, according to the second aspect of the invention, it is possible to prevent an increase in the cost of the objective lens drive apparatus as well as an increase in the size thereof, which are otherwise caused by the adjustment of the inclination of the objective lens.  
     [0119] Further, according to the third aspect of the invention, there is provided an objective lens drive apparatus in which a coil unit with a plurality of focus coils and a tracking coil mounted thereon is disposed within the same magnetic gap of a magnetic circuit including at least one magnet magnetized in two polarities, currents are supplied respectively to the plurality of focus coils included in the coil unit to thereby be able to execute focus servo due to the sum of drive forces generated in response to the supply of the currents, and moment is generated around the center of gravity of a movable part due to the difference between the drive forces to thereby be able to adjust the inclination of an objective lens simultaneously with the focus servo. In the present objective lens drive apparatus, using the right and left focus coils, not only the focus servo but also the adjustment of the inclination of the objective lens can be carried out, which eliminates the need for provision of a coil and a magnet which are exclusively used to adjust the inclination of the objective lens. Therefore, according to the third aspect of the invention, it is possible to prevent an increase in the cost of the objective lens drive apparatus as well as an increase in the size thereof, which are otherwise caused by the adjustment of the inclination of the objective lens.