Abstract:
The present invention has an object to provide an object lens actuator that reduces the amplitude of vibration of an object lens related to a torsional vibration of a lens holder and to enable a disc drive using the same to record information on a disc at high speeds. The above object has been achieved by placing the thickest one of the four walls, which are in an envelope of two opening portions provided in the lens holder and which are perpendicular to a flat plate between the two opening portions, in a position which is opposite to the object lens with respect to the center line between a first support member and a second support member which resiliently support the lens holder therebetween and which is opposite to the object lens with respect to the flat plate.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an object lens actuator which drives an object lens for focusing light on a recording surface of an optical disc in focusing and tracking directions so as to reproduce information recorded on the recording surface of the optical disc or record information thereon and to a disc drive using the same. 
     2. Background Art 
     A disc drive for recording information on a disc-like information recording medium or reproducing the recorded information is designed to increase the data transfer rate by rotating an optical disc at high speeds. The disc drive includes an object lens actuator which drives an object lens in focusing and tracking directions so as to accurately record or reproduce information by following the rotation of the optical disc. 
     A general object lens actuator comprises: a magnetic circuit having a yoke and a permanent magnet; a movable portion to which an object lens is attached; a fixing portion for holding the movable portion; and first and second support members which are connected to the fixing portion and resiliently support the movable portion. The object lens is disposed on a center line between the first support member and the second support member. Additionally, a focusing coil and a tracking coil are attached to a flat plate between the two opening portions disposed in the movable portion. When current is supplied to the focusing coil, the movable portion is driven in a focusing direction by an electromagnetic force generated by an effect of magnetic flux from a permanent magnet attached to the yoke. Likewise, when current is supplied to the tracking coil, the movable portion is driven in a tracking direction by an electromagnetic force generated by an effect of magnetic flux from a permanent magnet attached to the yoke (see JP Patent Publication (Kokai) No. 8-194962 (1996)(FIG. 1)). 
     Such an object lens actuator needs to enhance the control band for focusing and tracking with an increased number of rotations of the optical disc so as to position the object lens in focusing and tracking directions. To provide a stable control, the amplitude in a higher order resonance frequency of the object lens actuator needs to be small. 
     There is a torsional vibration of the lens holder as a higher order resonance of the object lens actuator. According to the conventional technique, when the object lens are placed symmetrically with respect to the center line between the first support member and the second support member which resiliently support the lens holder therebetween respectively, the object lens actuator is hard to be subject to torsional vibration. However, when the object lens are biased toward the inner periphery with respect to the center line, viewed from an optical axial direction of the object lens and between the support members which resiliently support the lens holder therebetween, so as to enable access to a data area provided at the inner periphery of an optical disc, the object lens actuator may have a problem in that when torsional vibration of the lens holder occurs, the position of the object lens serves as a vibration loop which may increase the amplitude thereof. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the aforementioned problems related to torsional vibration of a lens holder, and has an object to provide an object lens actuator that reduces the amplitude of vibration of an object lens and to enable a disc drive using the same to record information on a disc at high speeds. 
     The above object has been achieved by placing the thickest wall of the four walls, which are in an envelope of two opening portions provided in the lens holder and which are perpendicular to a flat plate between the two opening portions, in a position which is in the opposite side to the object lens with respect to the center line between a first support member and a second support member which resiliently support the lens holder therebetween and which is in the opposite side to the object lens with respect to the flat plate. 
     According to the present invention, in a mode of the torsional vibration of the lens holder, the mode components of each coil generating a driving force can be equal in magnitude and opposite in direction. As a result, the driving forces generated in each coil are the same, and thus the product of a driving force of each coil and a mode component of a torsional vibration is offset with each other, thereby minimizing the amplitude at a position of the object lens due to a torsional vibration of the lens holder. 
     As described above, the present invention can provide an object lens actuator that reduces the amplitude at the position of an object lens related to a torsional vibration of a lens holder, and a disc drive using the same capable of recording information on a disc at high speeds. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of the object lens actuator in accordance with an embodiment of the present invention; 
         FIG. 2  is a top view of the object lens actuator in accordance with an embodiment of the present invention; 
         FIG. 3  is a top view of the lens holder in accordance with an embodiment of the present invention; 
         FIG. 4  is a graph showing an advantage of an embodiment of the present invention; 
         FIG. 5  is a top view of the lens holder in accordance with an embodiment of the present invention; 
         FIG. 6  is a graph showing an advantage of an embodiment of the present invention; 
         FIG. 7  is a top view of the lens holder in accordance with an embodiment of the present invention; 
         FIG. 8  is a top view of a conventional lens holder; 
         FIG. 9  is a diagrammatic drawing showing a frequency response of displacement in a focusing direction of the object lens; 
         FIG. 10  is a perspective view of an optical head to which the object lens actuator of the present invention is applied; and 
         FIG. 11  is an exploded perspective view of a disc drive to which the object lens actuator of the present invention is applied. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, the preferred embodiments of the present invention will be described with reference to the drawings. 
     First Embodiment 
       FIG. 11  is an exploded perspective view of the disc drive  1  to which the object lens actuator  73  of the present invention is applied. In the figure, the disc drive  1  mainly includes a bottom case  10 , a disk tray  4  for retracting a disc serving as an information recording means into the apparatus or ejecting the disc outside the apparatus, and a circuit board  9  having semiconductor components for performing drive control and signal processing on electronic components mounted inside the disc drive. A top case  2  is installed on the top surface of the bottom case  10  and a front panel  3  is installed on the front surface of the bottom case  10 . As a result, the top surface and front surface of the bottom case  10  are covered with the top case  2  and the front panel  3 . 
     A unitized mechanical portion (hereinafter referred to as a unit mechanism)  6  is attached to the disc tray  4 , the undersurface of which is covered with an undercover  8 . The unit mechanism  6  includes a spindle motor  5  for rotating a disc, an optical head  7  for recording or reproducing information on and from the disc, and an optical head feeding mechanism for moving optical head  7  in a radial direction of the disc along a guide shaft (not shown). 
       FIG. 10  is a perspective view of an optical head  7  to which the object lens actuator  73  of the present invention is applied. In the figure, the optical head  7  is mainly composed of three components: an object lens actuator  73 , an optical system, and a flexible printed circuit board  74 . The object lens actuator  73  is configured to drive the object lens in focusing and tracking directions to accurately record or reproduce information by following a track on a disc. The optical system includes a semiconductor laser diode  71  for emitting a laser beam, a lens and a mirror (not shown) for branching light from the semiconductor laser diode and focusing light on the disc, a photo detector  72  for receiving reflected light from the disc. The flexible printed circuit board  74  electrically connects the object lens actuator  73  and the optical system to the circuit board  9  by a plurality of wiring patterns. 
       FIG. 1  is a perspective view of the object lens actuator  73  of the present invention. In the figure, the x-axis direction shows a tangential direction of an optical disc (not shown), the y-axis direction shows a tracking direction, i.e., a radial direction of the optical disc, and the z-axis direction shows a focusing direction, i.e., an optical axis direction of the object lens  731 . The object lens actuator  73  includes a movable portion to which the object lens  731  is attached, a fixing portion  733  for holding this movable portion, a first support member  734   a  and a second support member  734   b  which are connected to the fixing portion  733  and resiliently support the movable portion, and a magnetic circuit having a yoke  735  and a permanent magnet  736 . 
     One end of the wire shaped first support member  734   a  and the wire shaped second support member  734   b  is fixed near an end surface of the fixing portion  733  and the other end thereof is fixed by solder and the like to each of the projecting portions  739  which are disposed at both sides of the lens holder  732 . 
     The object lens  731  is disposed asymmetrically with respect to the center line L between the first support member  734   a  and the second support member  734   b . The lens holder  732  is provided with two opening portions. A tracking coil  737  and focusing coils  738   a ,  738   b ,  738   c  and  738   d  (not shown) are attached to the both ends of the flat plate  732   e  between the two opening portions. 
     Then, the object lens  731 , the lens holder  732 , the tracking coil  737 , and the focusing coils  738   a ,  738   b ,  738   c  and  738   d  are configured to form the movable portion. In other words, this movable portion is configured to move with respect to the fixing portion  733 . 
     Here, the torsional vibration of the lens holder  732  is apt to be excited when a driving force is generated in the focusing coils  738   a ,  738   b ,  738   c  and  738   d  of the movable portion.  FIG. 9  is a diagrammatic drawing showing a frequency response of displacement in a focusing direction of the object lens  731  when a driving force is generated in the focusing coils  738   a ,  738   b ,  738   c  and  738   d . In the figure, the symbol A denotes the amount of increase in amplitude of the object lens  731  due to a torsional vibration of the lens holder  732 . The amount of increase in amplitude A of the object lens  731  due to this torsional vibration is obtained by adding the product of the mode component of a torsional vibration of the object lens  731 , the driving force of a torsional vibration of the focusing coil, and the mode component of a torsional vibration of the focusing coil, each of which is calculated for each of the focusing coils  738   a ,  738   b ,  738   c  and  738   d.    
     Accordingly, all that is needed to reduce the amount of increase in amplitude A of the object lens  731  due to this torsional vibration is to make the mode component of a torsional vibration of each of the focusing coils  738   a ,  738   b ,  738   c  and  738   d  equal in magnitude and opposite in direction. 
     Here, the positional relationship of the four walls  732   a ,  732   b ,  732   c , and  732   d  which are part of an envelope of the two opening portions of the lens holder  732  and which are perpendicular to the flat plate  732   e  between the two opening portions will be described with reference to  FIG. 2 . The wall  732   a  is positioned in the same side as the object lens  731  with respect to the center line L between the first support member  734   a  and the second support member  734   b  and is positioned in the same side as the object lens  731  with respect to the flat plate  732   e . The wall  732   b  is positioned in the opposite side to the object lens  731  with respect to the center line L and is positioned in the same side as the object lens  731  with respect to the flat plate  732   e . The wall  732   c  is positioned in the same side as the object lens  731  with respect to the center line L and is positioned in the opposite side to the object lens  731  with respect to the flat plate  732   e . The wall  732   d  is positioned in the opposite side to the object lens  731  with respect to the center line L and is positioned in the opposite side to the object lens  731  with respect to the flat plate  732   e.    
     Within the above guidelines, according to the present embodiment, as shown in  FIGS. 2 and 3 , the wall  732   d  is configured to be thickest of the four walls  732   a ,  732   b ,  732   c , and  732   d  of the lens holder  732 . In this case, the mode components of the torsional vibration of the two pairs of focusing coils  738   a ,  738   b ,  738   c , and  738   d  disposed symmetrically with respect to the center line L are equal in magnitude but the direction of the mode components of the torsional vibration of the focusing coils  738   a  and  738   c  and that of the focusing coils  738   b  and  738   d  are opposite to one another on either side of the center line L. Accordingly, the amount of increase in amplitude A of the object lens  731  due to a torsional vibration can be minimized. 
     This advantage is shown as the legend by a black filled circle in  FIG. 4 . 
     The legend of black filled circles indicate the amount of increase in amplitude A of the object lens  731  due to a torsional vibration of the lens holder  732  is plotted by changing from the state ( FIG. 8 ) in which the four walls  732   a ,  732   b ,  732   c , and  732   d  are as thick as 1.0 mm to the state ( FIG. 3 ) in which the thickness of the wall  732   d  of the four walls is changed from 0.8 mm to 1.6 mm. The abscissa indicates the thickness of the wall  732   d  as a ratio of thickness between the walls  732   d  and  732   c . In  FIG. 4 , it can be seen that the larger the value of the abscissa, the thicker the wall  732   d . When the ratio of thickness between the walls  732   d  and  732   c  is approximately 1.3, the amount of increase in amplitude A of the object lens  731  can be approximately zero. At this time, the ratio of thickness of the four walls (thickness of wall  732   a ): (thickness of wall  732   b ): (thickness of wall  732   c ): (thickness of wall  732   d ) becomes approximately 1:1:1:1.3, which means the wall  732   d  is the thickest. 
       FIG. 4  further includes the legend of an open triangle and an open box which indicate the results of a change in ratio of thickness between the wall  732   d  and the wall  732   c  by increasing or decreasing the thickness of the wall  732   d  in a state where the thickness of the wall  732   a  is increased in a range from 1.0 mm to 1.4 mm from the state of  FIG. 8  and the ratio of thickness between the wall  732   a  and the wall  732   b  is fixed as either 1.2 or 1.4. This results indicate that at least in a range where the ratio of thickness between the wall  732   a  and the wall  732   b  is between 1.0 and 1.2, making the ratio of thickness between the wall  732   d  and the wall  732   c  more than 1.2 and at the same time making the ratio of thickness between the wall  732   a  and the wall  732   b  more than 1.0 and equal to or less than 1.2 can suppress the amount of increase in amplitude A of the object lens  731  to approximate zero in a condition where the ratio of thickness between the wall  732   d  and the wall  732   c  is nearer to 1.0, as compared to increasing only the ratio of thickness between the wall  732   d  and the wall  732   c . In other words, as compared to increasing the thickness of the wall  732   d  only, increasing the thickness of the wall  732   a  as well can decrease the thickness of the wall  732   d . In this case, asymmetric diversity of the two opening portions with respect to the center line L and the flat plate  732   e  can be minimized, and thus the focusing coils  738   a ,  738   b ,  738   c , and  738   d  can be increased in width, thereby increasing the driving force of the object lens actuator  73 . 
     Second Embodiment 
       FIG. 5  is a top view of the lens holder  732  of the object lens actuator  73  in accordance with a second embodiment of the present invention. According to the second embodiment, the wall  732   a  is configured to be the thickest of the four walls  732   a ,  732   b ,  732   c , and  732   d . The detailed proofs thereof are omitted since the other configurations of the second embodiment are the same as those of the first embodiment. In these configurations, like the first embodiment, the mode components of the torsional vibration of the two pairs of the focusing coils  738   a ,  738   b ,  738   c , and  738   d  disposed symmetrically with respect to the center line L are equal in magnitude, but the direction of the mode components of the torsional vibration of the focusing coils  738   a  and  738   c  and that of the focusing coils  738   b  and  738   d  are opposite to one another on either side of the center line L. Accordingly, the amount of increase in amplitude A of the object lens  731  due to a torsional vibration can be minimized. This advantage is shown as the legend by a black filled circle in  FIG. 6 . 
     The legend of black filled circles indicate the amount of increase in amplitude A of the object lens  731  due to a torsional vibration of the lens holder  732  is plotted by changing from the state ( FIG. 8 ) in which the four walls  732   a ,  732   b ,  732   c , and  732   d  are as thick as 1.0 mm to the state ( FIG. 5 ) in which the thickness of the wall  732   a  of the four walls is changed from 0.6 mm to 1.8 mm. The abscissa indicates the thickness of the wall  732   a  as a ratio of thickness between the walls  732   a  and  732   b . In  FIG. 6 , the larger the value of the horizontal axis, the thicker the wall  732   a . When the ratio of thickness between the walls  732   a  and  732   b  is approximately 1.6, the amount of increase in amplitude A of the object lens  731  can be approximately zero. At this time, the ratio of thickness of the four walls (thickness of wall  732   a ): (thickness of wall  732   b ): (thickness of wall  732   c ): (thickness of wall  732   d ) becomes approximately 1.6:1:1:1, which means the wall  732   a  is the thickest. 
       FIG. 6  further includes the legend of an open triangle, a filled rhombus, and an open box which indicate the results of a change in ratio of thickness between the wall  732   a  and the wall  732   b  by increasing or decreasing the thickness of the wall  732   a  in a state where the thickness of the wall  732   d  is increased in a range from 1.0 mm to 1.4 mm starting with  FIG. 8  and the ratio of thickness between the wall  732   d  and the wall  732   c  is fixed as any one of 1.2, 1.3, and 1.4. This results indicate that at least in a range where the thickness of the wall  732   d  is in a range from 1.0 to 1.2, making the ratio of thickness between the wall  732   a  and the wall  732   b  more than 1.2 and at the same time making the ratio of thickness between the wall  732   d  and the wall  732   c  more than 1.0 or equal to or less than 1.2 can suppress the amount of increase in amplitude A of the object lens  731  to approximately zero in a condition where the ratio of thickness between the wall  732   a  and the wall  732   b  is nearer to 1.0, as compared to increasing only the ratio of thickness between the wall  732   a  and the wall  732   b . In other words, as compared to increasing the thickness of the wall  732   a  only, increasing the thickness of the wall  732   d  as well can decrease the thickness of the wall  732   a . Accordingly, asymmetric diversity of the two opening portions with respect to the center line L and the flat plate  732   e  can be minimized, and thus the focusing coils  738   a ,  738   b ,  738   c , and  738   d  can be widened, thereby increasing the driving force of the object lens actuator  73 . 
     Third Embodiment 
       FIG. 7  is a top view of the lens holder  732  of the object lens actuator  73  in accordance with a third embodiment of the present invention. According to the third embodiment, of the four walls  732   a ,  732   b ,  732   c , and  732   d , the wall  732   a  is as thick as the wall  732   d , the walls  732   a  and  732   d  are thicker than the remaining two walls  732   b  and  732   c.    
     The detailed proofs thereof are omitted since the other configurations of the third embodiment are the same as those of the first and the second embodiments. In these configurations, like the first and second embodiments, the mode components of the torsional vibration of the two pairs of the focusing coils  738   a ,  738   b ,  738   c , and  738   d  disposed symmetrically with respect to the center line L can be equal in magnitude, but the direction of the mode components of the torsional vibration of the focusing coils  738   a  and  738   c  and that of the focusing coils  738   b  and  738   d  can be opposite to one another on either side of the center line L. Accordingly, the amount of increase in amplitude A of the object lens  731  due to a torsional vibration can be minimized. Further, according to the present embodiment, the two opening portions can be equal in size. Accordingly, the permanent magnet  736  disposed in the opening portion, focusing coils  738   a ,  738   b ,  738   c ,  738   d , and the tracking coil  737  attached to the flat plate  732   e  can be the same in shape and magnitude. 
     Accordingly, the present invention allows an object lens actuator and disc drive using the same to increase the speed of recording information on a disc.