Abstract:
In an objective lens actuating device, a magnetic field generating part generates a magnetic field with a focusing coil in a gap formed by an inner yoke and an outer yoke and an auxiliary magnetic field generating part generates an opposite magnetic field in respect to the magnetic field which is generated in the gap towards the focusing coil, at both ends of the inner yoke vertical to the focusing direction.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention generally relates to an objective lens actuating device applied to an optical recording/reproducing apparatus and controlling a drive of an objective lens in a focusing direction and/or a tracking direction on a recording surface of a recording medium.  
           [0003]    2. Description of the Related Art  
           [0004]    In conventional objective lens which is used to form a fine optical spot by condensing a laser beam on a track of an optical disc at a time of recording or reproducing information, an aberration occurs in the optical spot when an optical axis of the objective lens tilts in respect to the optical disc. Thus, problems may be caused during a recording/reproducing operation. Therefore, a tilt of the objective lens is required to be as small as possible. Especially recently, a DVD (Digital Versatile Disk) has been widely marketed to users. And, the DVD for recording is also practically used. In order to improve a higher density of recording information, the objective lens is needed to have a greater number of apertures and then it is more required to prevent the objective lens from being tilted. The main factor causing the tilt of the objective lens is an uneven magnetic field in a magnetic circuit used for a focus actuator.  
           [0005]    A conventional objective lens actuating device and focusing will now be described with reference to FIGS. 1A and 1B and FIGS. 2A and 2B. FIG. 1A and FIG. 2A are plan views of the conventional objective lens actuating device. FIG. 1B and FIG. 2B are diagrams showing a relationship between a actuating force and a magnetic flux density.  
           [0006]    Referring to FIG. 1A and FIG. 2A, a focusing coil  3  is wound around an outside of a holder  2  holding an objective lens  1 . The holder  2  is supported by two pairs of wires  5  so as to be movable in a focusing direction and in a tracking direction. That is, ends of the two pairs of wires  5  in one side are fixed to a wire supporting member  4  and other ends of the two pairs of wires  5  on another side are fixed to both sides of the holder  2 .  
           [0007]    Each of two magnetic circuits  6  is provided to both sides of the holder  2  in a direction “A” and each of air vents  8  is provided to the both sides of the holder  2  in order to insert inner yokes  7  each of which is a part of each of the magnetic circuits  6 . Each of the magnetic circuits  6  includes one inner yoke  7 , one outer yoke  9 , and one permanent magnet  10  fixed by the outer yoke  9 .  
           [0008]    A magnetic pole surface of each of the permanent magnets  10  is arranged to face to a side surface of the focusing coil  3  in the direction “A”. Both of the permanent magnets  10  are magnetized so as to face the same magnetic poles towards each other in the direction “A”. Each of the inner yokes  7  is arranged to face to the outer yokes  9  and the permanent magnets  10  inside the focusing coil  3 . And the focusing coil  3  is provided at each of gaps  11  between the inner yokes  7  and the permanent magnets  10 . A magnetic force as the focus actuating force occurs by a magnetic field within each of the gaps  11  and a current flowing through the focusing coil  3 .  
           [0009]    Also, two pairs of tracking coils  12  are adhered to both side surfaces of the focusing coil  3  in the direction “A”, in which focusing coil  3  is wound around the holder  2  in an optical axis direction of the objective lens  1  in a cylindrical shape, and totally four tracking coils  12  are fixed. In FIG. 1A and FIG. 2A, a gravity center is indicated as a gravity center G of a movable member  13 .  
           [0010]    As shown in FIG. 1A and FIG. 1B, in a case in which each component is properly arranged, the magnetic field (that is, the focus actuating force), which affects the focusing coil  3  between the gaps  11  of the magnetic circuits  6 , shows a symmetric state in that a center of the movable member  13  is stronger and the magnetic field becomes weaker towards each of both sides of the movable member  13 . However, as shown in FIG. 2A and FIG. 2B, when a relative displacement between the focusing coil  3  and each of the magnetic circuits  6  occurs, a moment occurs to the movable member  13  since an effect center of the focus actuating force is shifted from the gravity center G. Therefore, the objective lens  1  is tilted.  
           [0011]    As a factor of the relative displacement between the focusing coil  3  and the magnetic circuits  6 , which displacement causes the moment towards the movable member  13 , an error in an initial assembly stage or a move during a tracking operation can be considered.  
           [0012]    In order to prevent the tilt of the objective lens  1  caused by the moment occurred by the relative displacement between the focusing coil  3  and the magnetic circuits  6 , the Japanese Laid-open Patent Application No. 7-320278 discloses an objective lens actuating device in which a magnetic substance is arranged at a location facing a side surface of the focusing coil  3  in the tracking direction. The Japanese Laid-Open Patent Application No. 9-231593 discloses another objective lens actuating device in that a magnet instead of the magnetic substance is arranged.  
           [0013]    However, in such configurations of conventional technologies described above, the magnetic substance or the like is required to be arranged outside the side surface of the focusing coil  3 . Thus, these configurations prevent minimizing the objective lens actuating device.  
           [0014]    Especially, in a configuration described in the Japanese Laid-Open Patent Application No. 7-320278, the magnetic-field distribution is leveled. However, the moment at the side surface facing the magnetic substance of the focusing coil  3  is increased. Thus, a practical effect cannot be expected in the conventional objective lens actuating device.  
         SUMMARY OF THE INVENTION  
         [0015]    It is a general object of the present invention to provide an objective lens actuating device in which the above-mentioned problems are eliminated.  
           [0016]    A more specific object of the present invention is to provide the objective lens actuating device, instead of a large-sized objective-lens actuating device, that prevents an objective lens from tilting by canceling a shift from a center of a movable member including the objective lens, and a holder, which shift is caused by a displacement between a focusing coil and a magnetic circuit.  
           [0017]    Another specific object of the present invention is to provide the objective lens actuating device, instead of the large-sized objective lens actuating device, that prevents the objective lens from tilting by canceling the shift from the center of the movable member including the objective lens, and the holder, which shift is caused by the displacement caused by a movement during tracking between the focusing coil and the magnetic circuit.  
           [0018]    The above objects of the present invention are achieved by an objective lens actuating device including: a holder holding an objective lens; a supporting part movably supporting the holder at least in a focusing direction; a focusing coil being cylindrically wound in an axis parallel with an optical axis of the objective lens and fixed on the holder; an inner yoke located inside the focusing coil and having a flat surface extending in the focusing direction and a direction perpendicularly crossing the focusing direction; an outer yoke arranged so as to partially sandwich the focusing coil and face the inner yoke; a connecting member connecting the inner yoke and the outer yoke; a magnetic field generating part generating a magnetic field with the focusing coil in a gap formed by the inner yoke and the outer yoke; and an auxiliary magnetic field generating part generating an opposite magnetic field in respect to the magnetic field which is generated in the gap towards the focusing coil, at both ends of the inner yoke vertical to the focusing direction.  
           [0019]    According to the present invention, a special member is not required to mount outside the objective lens actuating device and it is possible to cancel the moment caused by the shift of the focus actuating force by a displacement caused when the focusing coil and the magnetic circuits are assembled, and by the shift of the focus actuating force by a tracking movement in the focusing coil and the magnetic circuits. Also, it is not required to enlarge the objective lens actuating device and it is possible to prevent the objective lens from being tilted.  
           [0020]    In the objective lens actuating device, the magnetic field generating part may include a permanent magnet that is fixed on a flat surface of the outer yoke, in which the flat surface is faced to the inner yoke, and that is magnetized in a vertical direction in respect to the flat surface and the auxiliary magnetic field generating part may include both ends of the inner yoke, in which the both ends are formed to be thinner so as to saturate the magnetic field caused by the permanent magnet at the both ends of the inner yoke in a direction perpendicularly crossing the focusing direction. In this configuration, it is not required to provide an additional member such as an auxiliary permanent magnet. Thus, an additional expense is not required to achieve the objects.  
           [0021]    Moreover, the above objects of the present invention are achieved by an objective lens actuating device including: a holder holding an objective lens; a supporting part movably supporting the holder in a focusing direction and in a tracking direction; a focusing coil being cylindrically wound in an axis parallel with an optical axis of the objective lens and fixed on the holder; an inner yoke located inside the focusing coil and having a flat surface extending in the focusing direction and a direction perpendicularly crossing the focusing direction; an outer yoke arranged so as to partially sandwich the focusing coil and face the inner yoke; a connecting member connecting the inner yoke and the outer yoke; a magnetic field generating part generating a magnetic field with the focusing coil in a gap formed by the inner yoke and the outer yoke; and an auxiliary magnetic field generating part generating an opposite magnetic field in respect to the magnetic field which is generated in the gap towards the focusing coil, at both ends of the inner yoke in the focusing direction.  
           [0022]    According to the present invention, a special member is not required to mount outside the objective lens actuating device and it is possible to cancel the moment caused by the shift of the focus actuating force by a displacement caused when the focusing coil and the magnetic circuits are assembled, and by the shift of the focus actuating force by a tracking movement in the focusing coil and the magnetic circuits. Also, it is not required to enlarge the objective lens actuating device and it is possible to prevent the objective lens from being tilted. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings, in which:  
         [0024]    [0024]FIG. 1A is a plan view of a conventional objective lens actuating apparatus and FIG. 1B is a diagram showing a relationship between a focus actuating force and a magnetic flux density;  
         [0025]    [0025]FIG. 2A is a plan view of the conventional objective lens actuating apparatus and FIG. 2B is a diagram showing a relationship between the focus actuating force and the magnetic flux density;  
         [0026]    [0026]FIG. 3 is a perspective view of an entire objective lens actuating device according to a first embodiment of the present invention;  
         [0027]    [0027]FIG. 4 is a diagram for explaining an occurrence state of the focus actuating force in a case of placing the objective lens actuating device on a flat surface;  
         [0028]    [0028]FIG. 5 is a diagram for explaining an occurrence state of a force in a case of placing an objective lens actuating device on a flat surface, according to a second embodiment of the present invention;  
         [0029]    [0029]FIG. 6 is a perspective view of an entire of an objective lens actuating device according to a third embodiment of the present invention;  
         [0030]    [0030]FIG. 7 is a diagram for explaining an occurrence state of a force in a case of placing the objective lens actuating device on a flat surface, according to the third embodiment of the present invention;  
         [0031]    [0031]FIG. 8 is a magnified view for explaining the occurrence state of the force in the case of placing the objective lens actuating device with an opposite actuating force on the flat surface, according to the third embodiment of the present invention; and  
         [0032]    [0032]FIG. 9 is a diagram for explaining the occurrence state of the force in the case of placing the objective lens actuating device without an opposite actuating force to the focus actuating force on the flat surface. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    Preferred embodiments of the present invention will now be described. [First Embodiment] 
         [0034]    [0034]FIG. 3 is a perspective view of an entire objective lens actuating device according to a first embodiment of the present invention. FIG. 4 is a diagram for explaining an occurrence state of a focus actuating force in a case of placing the objective lens actuating device on a flat surface. Hereinafter, parts that are the same as the ones in FIG. 1 and FIG. 2 are indicated by the same reference numerals and the explanation thereof will be omitted.  
         [0035]    A base  15  shown in FIG. 3 includes a mechanism as a connecting member for connecting an outer yoke  9  and an inner yoke  7 . A gravity center G of a movable member  113  including an objective lens  1 , a holder  2 , a focusing coil  3 , and a tracking coil  12  passes an optical axis of the objective lens  1 . And the objective lens  1 , the holder  2 , the focusing coil  3 , and the tracking coil  12  are arranged to be symmetrical to a surface extending to the tracking direction and including the optical axis of the objective lens  1  and also to be symmetrical to a surface extending to a direction “A”. Also, both magnetic circuits  6  are arranged to be symmetrical to two surfaces above, so as that a center of a actuating force affecting the movable member  113  passes the gravity center G of the movable member  113 .  
         [0036]    As describe in the Description of the Related Art, in a case in which components of the objective lens actuating device are initially displaced or the displacement between the focusing coil  3  and the magnetic circuit  6  occurs in the tracking direction when the movable member  113  moves in the tracking direction to follow an information track of the optical disc, a moment is additionally applied to the movable member  113  by a shift between an effect center of the focus actuating force and the gravity center G of the movable member  113 . However, in the first embodiment, each of auxiliary permanent magnets  16  are provided to both ends of each of the inner yokes  7  in the tracking direction perpendicularly crossing the focusing direction, so that the moment is offset by occurring a magnetic force at both ends of each of the inner yokes  7 . Therefore, tilt of the movable member  113  caused in the conventional objective lens actuating device can be suppressed.  
         [0037]    Details will now be described. As shown in FIG. 4, it is assumed that the focusing coil  3  is set to be a N pole in a magnetized direction of the permanent magnets  10  fixed on surfaces facing the focusing coil  3  on the outer yoke  9 . And the auxiliary permanent magnets  16  are fixed to both ends  7   a  of the tracking direction in the inner yoke  7  so that magnetized directions of the auxiliary permanent magnets  16  are opposite to magnetized directions of the permanent magnets  10  (magnetic force lines of magnetic fields occurred at the permanent magnets  10  of the outer yokes  9  are directed from an outside to an inside of the focusing coil  3 , on the other hand, magnetic force lines of the auxiliary permanent magnets  16  are directed from the inside to the outside of the focusing coil  3 ), that is, an outside of the tracking direction is set to be the N pole.  
         [0038]    When the movable member  113  is moved to a right direction for a tracking operation, the magnetic circuits  6  are shifted relatively to the movable member  113  towards a left direction. In this case, when the movable member  113  supplies current to the focusing coil  3  in a counterclockwise rotation so as to actuate the movable member  113  up in the focusing direction, the effect center of the actuating force of the focusing coil  3  is shifted from the gravity center G of the movable member  113  to a left side by an uneven force of the magnetic field distribution at the gaps  11  of the magnetic circuits  6 . Thus, a moment is additionally applied to the movable member  113   
         [0039]    However, in the first embodiment, by providing the auxiliary permanent magnets  16 , a magnet field causing a downward force in the focusing direction simultaneously affects a side surface of the focusing coil  3  in the tracking direction. In a case in which the movable member  113  is moved right, the auxiliary permanent magnets  16  on a left side approach the focusing coil  3  and magnetic flux on the left side crosses the focusing coil  3  more than the right side. Accordingly, the downward force on the left side is stronger and a moment from the downward force is opposite to the moment occurred by the magnetic flux at the gaps  11  of the magnetic circuit  6 . Consequently, the moment of the permanent magnets  10  of the outer yokes  9  and the moment of the auxiliary permanent magnet  16  offset each other. Therefore, it is possible to prevent the movable member  113  from tilting. [Second Embodiment] 
         [0040]    [0040]FIG. 5 is a diagram for explaining an occurrence state of a force in a case of placing an objective lens actuating device on a flat surface, according to a second embodiment of the present invention. Differently from the first embodiment, in the second embodiment, a thickness of both ends  7   a  of each of the inner yokes  7  is made thinner in the tracking direction to form thinner parts  7   b,  instead of the auxiliary permanent magnets  16  of the first embodiment.  
         [0041]    In the second embodiment, the thinner parts  7   b  are formed on the inner yokes  7  to form both ends  7   a  to be thinner. Then, leaking magnetic flux can be utilized. That is, magnetic force lines occurred from N poles of the permanent magnets  10  fixed to the outer yokes  9  cross from the outside to the inside of the focusing coil  3  and are directed to the inner yoke  7 . The both ends  7   a  of each of the inner yokes  7  is made thinner and then saturated magnetic flux leaks and directs from the inside to the outside of the focusing coil  3 . This magnetic flux leak is has the same effect as the magnetic flux occurred by the auxiliary permanent magnets  16  of the first embodiment of the present invention. [Third Embodiment] 
         [0042]    [0042]FIG. 6 is a perspective view of an entire of an objective lens actuating device according to a third embodiment of the present invention. FIG. 7 is a diagram for explaining an occurrence state of a force in a case of placing the objective lens actuating device  313  on a flat surface, according to the third embodiment of the present invention.  
         [0043]    In FIG. 6, an objective lens  21  of the objective lens actuating device  313 , the tracking coil  22 , and the focusing coil  23  are arranged in order in the direction “A” perpendicularly crossing the focusing direction and the tracking direction in a holder  20  and are fixed to the holder  20 . The tracking coil  22  including two flat coils is adhered and fixed on a surface facing of the focusing coil  23  at a side of an objective lens  21  in the direction “A”, which tracking coil  22  is wound in a cylindrical shape in the optical axis direction of the objective lens  21 .  
         [0044]    The focusing coil  23  is adhered and fixed at an opposite side to the objective lens  21  in the direction “A” in an inner surface of an inside through hole  24  in the focusing direction of the holder  20 . Therefore, in a movable member  25  including the holder  20 , the objective lens  21 , the tracking coil  22 , and the focusing coil  23 , two through holes are formed in the focusing direction. That is, the inside through hole  24  of the focusing coil  23  and a through hole  26  between the focusing coil  23 , the tracking coil  22 , and the objective lens  21 . In the movable member  25 , the holder  20  is movably supported by a wire supporting member  28  with four wires  27  in the focusing direction and the tracking direction, similarly to the first embodiment and the second embodiment of the present invention.  
         [0045]    Magnetic circuit  29  includes an inner yoke  30  arranged inside the inside through hole  24  of the focusing coil  23 , an outer yoke  31  arranged at through hole  26  between the focusing coil  23 , the tracking coil  22 , and the objective lens  21 , a base  32  connecting the inner yoke  30  and the outer yoke  31 , permanent magnets  33  and  34  that are fixed faced surfaces of the inner yoke  30  and the outer yoke  31 , respectively, and that are magnetized so as that different magnetic poles face each other. And, actuating forces in the focusing direction and in the tracking direction are obtained by a magnetic field at a gap  35  provided between the permanent magnet  33  of the inner yoke  30  and the permanent magnet  34  of the outer yoke  31 , and current flowing to the focusing coil  23  and the tracking coil  22 .  
         [0046]    In the third embodiment, a holder  20  mounting the objective lens  21  is projected in the holder  20 , so that a standing mirror (not shown) can be arranged directly under the objective lens  21 . Therefore, a thinner-sized objective lens actuating device can be realized. Differently from the first and the second embodiments, instead of arranging two magnetic circuits  6 , the magnetic circuit  29  as a single magnetic circuit is arranged and the permanent magnets  33  and  34  are arranged to both the inner yoke  30  and the outer yoke  31  in order to obtain a desired actuating force by the magnetic circuit  29  alone.  
         [0047]    In such as a configuration of the objective lens actuating device  313 , generally, the gravity center G of the movable member  25  is positioned so that the tracking direction corresponds to the optical axis of the objective lens  21  and the direction “A” is arranged in the middle between the focusing coil  23  and the tracking coil  22 . However, in this case of positioning the gravity center G of the movable member  25 , as shown in FIG. 9, a location where the focus actuating force occurs in the direction “A” (a direction of a thickness of the focusing coil  23  in a gap  35  of the magnetic circuit  29  equals a center of the direction “A”) is shifted from the gravity center G, and also a location where the track actuating force occurs in the direction “A” (a direction of a thickness of the tracking coil  22  equals a center of the direction “A”) is shifted from the gravity center G. Thus, when the focus actuating force and the track actuating force are shifted from the gravity center G, an unwanted resonance is easily caused.  
         [0048]    However, according to the third embodiment, as shown in FIG. 7 and FIG. 8, since opposite actuating forces are occurred in the gap  35  at a side surface of the focusing coil  23  in the tracking direction, an effect center of the focus actuating force is shifted towards an outer yoke  31 . Accordingly, even if the track actuating force is corresponded so that the gravity center G is positioned at a center of the tracking coil  22 , a shift from the focus actuating force can be suppressed.  
         [0049]    In detail, as shown in FIG. 7, it is assumed that a surface, which faces the focusing coil  23 , of the permanent magnet  34  fixed to the outer yoke  31  is a N pole (a surface fixed to the outer yoke  31  is a S pole) and a surface, which faces the focusing coil  23 , of the permanent magnet  33  fixed to the inner yoke  30  is the S pole (a surface fixed to the inner yoke  30  is the N pole). Magnetic force lines emitted from a N pole surface of the permanent magnet  34  fixed to outer yoke  31  crosses a wire rod of the focusing coil  23  from an outside to an inside of the focusing coil  23  and enters a S pole surface of the inner yoke  30 .  
         [0050]    In a configuration shown in FIG. 7, a width in the tracking direction is narrower than that of the permanent magnet  33  fixed to the inner yoke  30  and the permanent magnet  33  is exposed in the tracking direction. The magnetic force lines output from the N pole surface, which is exposed in the tracking direction, crosses the wire rod of the focusing coil  23  from the inside to the outside of the focusing coil  23  in the tracking direction. This magnetic flux affects similarly to the magnetic flux output from an auxiliary permanent magnet  16  in the first embodiments and then cancels the moment. Therefore, it is possible to prevent the movable member  25  from being tilted.  
         [0051]    In the embodiments described above, objective lens actuating devices  113 ,  213 , and  313  which are a two-axes electromagnetic-actuator type for conducting a actuating control in two directions for the focusing operation and the tracking operation, are described. Alternatively, in an objective lens actuating device of a single-axis electromagnetic-actuating type in which only the focusing operation is conducted by electromagnetic means, the tracking operation can be conducted by other means such as a Galvano mirror. And by applying any one of similar configurations described in the embodiments, it is possible to prevent the focus actuating force from acting as the moment tilting the movable member by the shift caused by an assembly error related to the focusing coil and the magnetic circuits.  
         [0052]    As described above, in the objective lens actuating device according to the present invention, a special member is not required to mount outside the objective lens actuating device and it is possible to cancel the moment caused by the shift of the focus actuating force by a displacement caused when the focusing coil and the magnetic circuits are assembled, and by the shift of the focus actuating force by a tracking movement in the focusing coil and the magnetic circuits. Also, it is not required to enlarge the objective lens actuating device and it is possible to prevent the objective lens from being tilted.  
         [0053]    The present invention is not limited to the specifically disclosed embodiments, variations and modifications, and other variations and modifications may be made without departing from the scope of the present invention.  
         [0054]    The present application is based on Japanese Priority Application No. 2000-378492 filed on Dec. 13, 2000, the entire contents of which are hereby incorporated by reference.