Abstract:
An optical actuator according to this invention includes a focusing lens for focusing a laser beam on a information disk, a lens holder for holding the focusing lens, a focusing coil for driving the focusing lens in an axial direction of the laser beam, a tracking coil for driving the focusing lens in a radial direction of the information disk, a tilting coil for pivotally rotating the focusing lens on an axis along the tangential direction of the disk and a pair of supporting members each disposed on each opposing sides of the lens holder. Each of the supporting members having at least three fixing elements disposed in an approximately circular arc, and a plurality of linear elastic members are connected to each of the fixing elements.

Description:
This Nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application Ser. No(s). 2003-052554 filed in JAPAN on Feb. 28, 2003, the entire contents of which are hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical actuator for controlling position of a focusing lens that focuses a laser beam on a disk surface. The optical actuator drives the focusing lens in three directions including focusing, tracking and tilting directions. 
   2. Description of the Background Art 
   An Optical actuator drives a focusing lens which forms a beam spot on an information disk, such as a DVD (digital versatile disk) or a CD (compact disk). By driving a focusing lens in a focusing direction, which is orthogonal to the disk surface, and a tracking direction, which is parallel to a radial direction of the disk. The optical actuator controls position of the focusing lens, so that the beam spot is formed on a pit line of the disk accurately. Furthermore, when the disk surface tilts or wobbles with respect to an optical axis of the focusing lens due to disk warpage, the optical actuator performs tilt control by pivotally rotating the focusing lens on an axis along the tangential direction or on an axis-along the radial direction of the disk. 
     FIG. 10  is a diagram illustrating a conventional optical actuator described in Japanese Patent Laid-Open No. 2001-297460. A focusing lens  101  is fixed to a lens holder  102 . Six blade springs  103   a – 103   c , and  103   d – 103   f  ( 103   e  and  103   f  are not shown) are fixed to each side of the lens holder  102 . Print coils  104   a  and  104   b  are fixed to another side of the lens holder  102 . A suspension holder  106  and permanent magnets  107   a – 107   d  are fixed to a base  105 . The print coil  104   a  is disposed between the permanent magnets  107   a  and  107   b , and the print coil  104   b  is disposed between the permanent magnets  107   c  and  107   d . The blade springs  103   a – 103   c  and  103   d – 103   f  are connected to the suspension wires  108   a – 108   c  and  108   d – 108   f  ( 108   e  and  108   f  are not shown), each of which are fixed to the suspension holder  106 . 
   When a current is supplied to focusing coils built in the print coils  104   a  and  104   b  so as to generate electro-magnetic forces in a vertical condition, the lens holder  102  is controlled in the focusing direction Fo. When a current is supplied to tracking coils built in the print coils  104   a  and  104   b  so as to generate electro-magnetic forces in the radial direction, the lens holder  102  is controlled in a tracking direction Tk. When a current is supplied to the focusing coils so as to generate electro-magnetic forces having opposite direction in each of the print coils  104   a  and  104   b , rotational moment on an axis in the tracking direction Tk occurs to the lens holder  102 . As a result, the lens holder  102  is driven in the tilting direction Ti. 
   When the lens holder  102  moves in the tilting direction, each of the blade springs  103   a  and  103   c  is warped or stretched, and the tilt control becomes unstable. Furthermore, the structure using blade springs requires higher cost and many assembly steps. 
   It is therefore, a primary object of the invention to provide an optical actuator which is able to perform the tilt control stably and can be produced with low cost. 
   SUMMARY OF THE INVENTION 
   This object is achieved in accordance with one aspect of the present invention which is an optical actuator includes a focusing lens for focusing a laser beam on a information disk, a lens holder for holding the focusing lens, a focusing coil for driving the focusing lens in an axial direction of the laser beam, a tracking coil for driving the focusing lens in a radial direction of the information disk, a tilting coil for pivotally rotating the focusing lens on an axis along the tangential direction of the disk and a pair of supporting members each disposed on each opposing sides of the lens holder. Each of the supporting members having at least three fixing elements disposed in an approximately circular arc, and a plurality of linear elastic members are connected to each of the fixing elements. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1–3  are diagrams each illustrating an optical actuator according to the present invention; 
       FIG. 4  is a diagram illustrating a moving part of the optical actuator; 
       FIGS. 5 and 6  are diagrams each illustrating another example of the optical actuator according to the present invention; 
       FIG. 7  is a diagram illustrating another example of the optical actuator according to the present invention; 
       FIG. 8  is a diagram illustrating another example of the optical actuator according to the present invention; 
       FIG. 9  is a diagram illustrating a moving part of the optical actuator illustrated in  FIG. 9 ; 
       FIG. 10  is a diagrams each illustrating a conventional optical actuator. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  is a diagram illustrating an optical actuator according to the present invention.  FIG. 2  is a diagram illustrating a holding part  15  and a moving part  14  of the optical actuator illustrated in  FIG. 1 .  FIGS. 3A and 3B  are diagrams illustrating a top view and a side view of the optical actuator illustrated in  FIG. 1 . As illustrated in  FIG. 2 , the holding part  15  comprises a yoke  7 , permanent magnets  6   a  and  6   b , and a circuit board  8 . The permanent magnets  6   a  and  6   b  are attached on the yoke  7  so that their identical poles face each other. 
   As illustrated in  FIG. 2 , the yoke  7  has projecting portions  7   a  and  7   b , and a hole  7   h  for passing a laser beam. The moving part  14  includes a focusing lens  1 , a lens holder  2 , a focusing-control coil  3 , tracking-control coils  4   a ,  4   b ,  4   c  and  4   d , and tilting-control coils  5   a  and  5   b . The focusing lens  1  focuses the laser beam passing through the hole  7   h  onto a information disk. The focusing-control coil  3  is wound around the lens holder  2 , and the tilting-control coils  5   a  and  5   b  are fixed to a pair of poles  20   a  and  20   b  formed on the bottom of the lens holder  2 . The focusing-control coil  3 , and the tilting-control coils  5   a  and  5   b  are arranged so that their respective axes will be parallel to an optical axis of the focusing lens  1 . As illustrated in  FIG. 2 , positioning members  12   a  and  12   b  for positioning the tracking-control coils  4   a  and  4   b  in appropriate positions are provided on a side of the lens holder  2 . The tracking-control coils  4   c  and  4   d  are arranged on the opposite side of the lens holder  2  by positioning members  12   c  and  12   d  (not shown). The lens holder  2  has holes  2   a  and  2   b  where each of the projecting member  7   a  and  7   b  passes through. 
   A pair of supporting members  2   c  and  2   d  is fixed on both sides of the lens holder  2 . Each of the supporting members  2   c  and  2   d  is provided with fixing elements  29   a – 29   c  and  29   d – 29   f . The linear elastic members  9   a – 9   c  and  9   d – 9   f  are fixed to the supporting members  2   c  and  2   d  via the fixing elements  29   a – 29   c  and  29   d – 9   f . The other ends (not shown) of the linear elastic members  9   a – 9   c , and  9   d – 9   f  are connected to the circuit board  8  via fixing elements disposed on the side of the yoke  7 . With these linear elastic members  9   a – 9   f , the moving part  14  is supported movably in each of three control directions including the focusing direction Fo, the tracking direction Tk, and the tilting direction Ti. The linear elastic members  9   a – 9   c  and  9   d – 9   e  also provide electric connection to the respective control coils. 
     FIG. 4  is a diagram illustrating a side fiew of the moving part  14 . As illustrated in  FIG. 4 , the fixing elements  29   a – 29   c , and  29   d – 29   f  are arranged in the circular arcs indicated as dotted lines. In other words, the linear elastic members  9   a – 9   c  and  9   d – 9   f  are arranged on cylindrical surfaces. Each cylindrical surface may have the same center. 
   Focusing error and tracking error corresponding to displacement of the beam spot in the focusing and tracking directions are detected by a well-known method such as the astigmatic method or DPD (differential phase detection). In this method, currents proportional to the focusing error and the tracking error are supplied to the focusing coil  3  and the tracking coils  4   a  and  4   b , respectively. Then, the respective coils are driven in the focusing direction Fo and the tracking direction Tk, with forces generated by interaction between currents of the coils and the magnetic field formed by the permanent magnets  6   a  and  6   b  to perform the focusing control and the tracking control. 
   With those controls, when the disk surface tilts with respect to an optical axis of the focusing lens  1 , an amount of the tilt is detected using a well-known method. Using the detected amount of the tilt, a tilt control signal for adjusting the optical axis of the focusing lens  1  is generated, and a current corresponding to the tilt control signal is supplied to the tilt control coils  5   a  and  5   b . Accordingly, the tilting-control coil  5   a  is driven in the direction +Fo (or −Fo), and the tilting-control coil  5   b  is driven in the opposite direction −Fo (or +Fo), thereby rotating the lens holder  2  in the tilting direction Ti illustrated in  FIG. 2 . Since the linear elastic members  9   a – 9   c  and  9   d – 9   f  are arranged on the respective cylindrical surfaces, they keep almost same length when they are twisted in the tilting direction Ti. Thus, stress (bending force) that generated in the linear elastic materials  9   b  and  9   e  in its longitudinal direction during the tilt control can be decreased. As a result, the lens holder  2  is rotated in the tilting direction Ti stably. Furthermore, since the distortion of the linear elastic members  9   a – 9   f  in the tilt control is limited to only bending distortion, each of three controls, the focusing control, the tracking control and the tilt control, can be stably performed. 
   As shown in  FIG. 3B  and  FIG. 4 , the height of the tracking-control coils  4   a  and  4   b  ( 4   c  and  4   d ) are narrower than the distance between the linear elastic members  9   a  and  9   c , and  9   d  and  9   f , and outer sides of the tracking-control coils  4   a  and  4   b  ( 4   c  and  4   d ) indicated as A 1 ′ and A 2 ′ in  FIG. 4  are disposed outside of the lens holder  2 . While the tracking control, an opposite force to the tracking direction occurs inner sides of the tracking control coils  4   a – 4   d  indicated as A in  FIG. 4 . At the same time, a force in the opposite direction occurs in the outer sides (A 1 ′ and A 2 ′). In order to perform the tracking control effectively, it is preferable to keep enough distance between the outer side of the tracing control coils  4   a – 4   d  (A 1 ′ and A 2 ′) and the permanent magnets  6   a  and  6   b , so as to reduce the opposite force occurring in the outer side of the tracking coils  4   a – 4   d.    
   As illustrated in  FIG. 4 , the linear elastic members  9   b  and  9   e  are arranged outward of the other linear elastic members  9   a – 9   c , and  9   d – 9   f , and the height of the tracking-control coils  4   a  and  4   b  is narrower than the distance between the linear elastic members  9   a  and  9   c , and  9   d  and  9   f . Thus, the outer sides of the tracking control coils  4   a – 4   d  can be disposed outside of the lens holder  2 , thereby securing enough distance from the permanent magnets  6   a  and  6   b . This configuration enables to drive the lens holder  2  in the tracking direction effectively. 
     FIG. 5  is a diagram illustrating another example of the optical actuator according to this invention.  FIGS. 6A and 6B  are diagrams illustrating a top view and a side view of the optical actuator in  FIG. 5 . The optical actuator illustrated in  FIGS. 5 ,  6 A and  6 B use single permanent magnet  6  attached on the yoke  7  to form magnetic filed, which is necessary to generate force to drive the lens holder  2 . 
   A structure using single permanent magnet  6  to produce magnetic field can reduce the cost, however, in this structure, each center of the focusing control, the tracking control, and the tilt control deviates from the focusing lens  1 . As a result, the control become unstable. This problem can be solved by adjusting the position of the supporting member  2   c  and  2   d . The supporting members  2   c  and  2   d  are arranged in the suitable position between each center of the focusing control, the tracking control, and the tilt control and the center of the focusing lens  1 . A counterweight may be provided so that the center of gravity of the lens holder  2  locates in suitable position for preventing unnecessary wobble. Using one permanent magnet  6  can decrease the cost. 
     FIG. 7  is a diagram illustrating another example of a moving part  14 ′. As illustrated in  FIG. 7 , the moving member  14 ′ has single tracking coil  4   c . By using single tracking coil, the cost can be decreased. 
     FIG. 8  is a diagram illustrating another example of the optical actuator according to this invention. The width of a permanent magnet  6   c  of the optical actuator illustrated in  FIG. 8  is made wider than the width of the permanent magnets  6   a  and  6  of the optical actuator illustrated in  FIGS. 1 and 5 .  FIG. 9  is a diagram illustrating the lens holder  2  of the optical actuator in  FIG. 8 . As illustrated in  FIG. 9 , outer sides of the tracking-control coils  4   f – 4   g  are disposed outside of the linear elastic members  9   b  and  9   e , in order to keep enough distance from the permanent magnet  6   c.    
   By increasing the width of the permanent magnet  6   c , the magnet flux acting each of the focusing coils can be increased, and sufficient force for driving the lens holder  2  can be obtained. Furthermore, by disposing the outer sides of the tracking coils  4   f – 4   g , where the opposite forces to the tracking direction occur, stable tracking control can be performed. 
   In the optical actuator according to this invention, the linear elastic members  9   a – 9   c , and  9   d – 9   f  for supporting the lens holder  2  are disposed on cylindrical surface. Thus, the stress generated in its longitudinal direction of the linear elastic materials  9   b  and  9   e  can be decreased. As a result, the tilt control can be performed stably. 
   Furthermore, by disposing the linear elastic members  9   a – 9   c  and  9   d – 9   f  on cylindrical surfaces, the outer sides of the tracking coils  4   a – 4   d , where the opposite forces to the tracking direction occur, can be disposed outside of the holder to secure enough distance from the permanent magnets  6   a  and  6   b . This configuration enables to drive the lens holder  2  in the tracking direction effectively. 
   Also, as illustrated in  FIGS. 5 through 8 , reduction of the permanent magnets and the tracking coils decreases the cost.