Abstract:
To enable the thrust of a linear actuator comprised of a driving magnet and a drive coil to act in a fixed and stable manner, the present invention is a lens driving device that comprises a driven body supporting an optical lens capable of freely moving along an optical axis direction, a drive coil fitted to the driven body, and a magnet having a surface set so as to have a prescribed gap with the moving drive coil, wherein a gap between the surface of the magnet and the drive coil is set in such a manner as to be broader at a central part of the range of movement of the drive coil than at an end part.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority from Japanese Priority Document No. 2003-109777, filed on Apr. 15, 2003 with the Japanese Patent Office, which document is hereby incorporated by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a lens driving device for moving an optical lens in the direction of an optical axis and an imaging device using this lens driving device. 
   2. Description of Related Art 
   Typically, imaging devices such as video cameras equipped with an autofocus function and electric zoom function are provided with lens driving devices for moving a moving lens for focusing and a moving lens for zooming along the direction of an optical axis. Electromagnetically driven actuators having, for example, a coil and a magnet, such as electromagnetically driven devices, are commonly used as this type of lens driving device. 
   Conventionally, drive means employing a stepping motor or DC motor where rotational movement of the motor is converted to rectilinear movement using gears etc. so as to cause a zoom lens or focus lens to move along an optical axis direction are widely adopted as actuators used in these lens driving devices. However, rectilinear driving means using linear actuators that combine flat plate magnets and drive coils have also been adopted to accompany advancement of performance required in high-speed drive and high-precision drive. 
   As disclosed in patent document 1 and patent document 2 mentioned below, such an actuator is equipped with a magnet being flat and having an equal width in the drive direction, a yoke being flat and having an equal width in the drive direction, and a drive coil. A gap between the surface of the driving magnet and an opposing drive coil is always kept a fixed distance over the whole of a drive stroke region as a result of a moveable part being guided by a guide shaft. 
   As shown in  FIG. 15 , at the magnet that is flat and has the same width in the drive direction, flux density flowing from the surface thereof is such that the direction of the flux density tends towards the drive direction side at the upper surfaces at both ends of the magnet and thrust generated at the coil is therefore also at an angle with respect to the drive direction. The thrust in the drive direction generated by the actuator therefore becomes lower towards both ends in the drive direction and becomes higher at a central section, as shown in  FIG. 16 . 
   The moving lens is constrained in the drive direction by a guide shaft and there is a certain angle between the drive direction and the thrust direction. Load due to friction between the guide shaft and the moving lens therefore increases due to a thrust component being generated in a direction rectilinear with the drive direction, and the drive load at both ends of the magnet increases. 
   Patent Document 1: Japanese Patent Laid-open Publication No. Heisei. 7-239437. 
   Patent Document 2: Japanese Patent Laid-open Publication No. 2002-169073. 
   When the moving lens is driven, the thrust in the drive direction generated by the drive coil changes according to the position of the moving lens. This damages the linearity of the thrust, and the drive load also changes according to position, and this is detrimental to the drive performance for the moving lens. 
   In particular, in the case of a servo system lens driving device configured using feedback control using a position detector etc., the stability of the servo system fluctuates according to the position of the moving lens because of the problem described above, with this being detrimental to the servo characteristics. 
   Further, it is not possible to effectively utilize the entire length of the magnet because the thrust generated by the coil at the ends of the stroke direction of the magnet falls. In cases where, for example, the thrust generated at the ends is not sufficient for that required by design for an actuator, it is necessary for only portions where the required thrust is obtained to be utilized, and the entire length of the magnet is therefore not used effectively. This makes it necessary to make the drive stroke small, and prevents the device from becoming smaller. 
   SUMMARY OF THE INVENTION 
   The present invention sets out to resolve these problems. The present invention is a lens driving device which comprises a driven body supporting an optical lens capable of freely moving along an optical axis direction, a drive coil fitted to the driven body, and a magnet having a surface set so as to have a prescribed gap with the moving drive coil, wherein a gap between the surface of the magnet and the drive coil is set in such a manner as to be broader at a central part of a range of movement of the drive coil than at an end part. 
   The present invention is a lens driving device which comprises a driven body supporting an optical lens capable of freely moving along an optical axis direction, a drive coil fitted to the driven body, and a magnet having a surface set so as to have a prescribed gap with the moving drive coil, wherein a width of the surface of this magnet is set in such a manner as to be narrower at a central part of a range of movement of the drive coil than at an end part. 
   In the present invention, the gap between the magnet surface and the drive coil is made to be broader at a central part of the range of movement of the drive coil than at the ends, and the width of the surface of the magnet is made narrower at the central part of the range of movement of the drive coils than at the ends. The magnetic field formed by the magnet is therefore uniform so that, for example, in the case of driving using a fixed voltage, drive thrust generated by the coil can be made fixed over the whole region of the moving stroke of the body being moved. 
   As described above, according to the present invention, the following effects are obtained. Linearity of the thrust generated by the actuator due to the unusual shape of the drive magnet is ensured, fluctuation in thrust when driving the focus lens and the zoom lens is suppressed, and it is possible to obtain good drive characteristics and servo characteristics. Drive load of the actuator due to the unusual shape of the drive magnet is fixed, and superior drive characteristics and servo characteristics can be obtained when driving the focus lens and the zoom lens. Further, the entire length of the magnets used with the lens driving device can be utilized, and inefficient use of space in the imaging device can be kept to a minimum. This makes it possible to make the lens barrel small due to it being possible to make the drive stroke large. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded perspective view illustrating a first embodiment of the present invention; 
       FIG. 2  is a front view illustrating the first embodiment; 
       FIG. 3  is a view illustrating linearity of thrust generated with respect to the drive direction; 
       FIG. 4  is an exploded perspective view illustrating a second embodiment; 
       FIG. 5  is an exploded perspective view illustrating a third embodiment; 
       FIG. 6  is an exploded perspective view illustrating a fourth embodiment; 
       FIG. 7  is an exploded perspective view illustrating a fifth embodiment. 
       FIG. 8  is a front view illustrating the fifth embodiment; 
       FIG. 9  is a view illustrating linearity of thrust generated with respect to the drive direction; 
       FIG. 10  is an exploded perspective view illustrating a sixth embodiment; 
       FIG. 11  is an exploded perspective view illustrating a seventh embodiment; 
       FIG. 12  is an exploded perspective view illustrating an eighth embodiment; 
       FIG. 13  is a view illustrating change in thrust in the lengthwise direction of a magnet; 
       FIG. 14  is a view showing aspect ratio of a magnet and proportion of decrease in drive coil thrust; 
       FIG. 15  is a view illustrating magnetic flux density flowing from a surface of a magnet; and 
       FIG. 16  is a view illustrating thrust generated with respect to the drive direction. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following is a description based on the drawings of the embodiments of the present invention. First, a description of a fist embodiment is given. The lens driving device of this embodiment is applied to lens driving of a focus lens and a zoom lens in a lens barrel used in an imaging device such as a video camera. 
     FIG. 1  is an exploded perspective view of a lens driving device of this embodiment according to the present invention, and  FIG. 2  is a front view of the lens driving device of this embodiment. Namely, a driven body  110  has an optical lens  111 , a sleeve  114  on one side, and a slot  115  on the opposite side so as to sandwich the optical axis together with the sleeve  114 . 
   Further, a guide shaft  150  is a shaft fitting into the sleeve  114  with no rattle in order to cause the driven body  110  to move along the direction of the optical axis. The guide shaft  160  is a shaft inserted as a brace at the slot  115  and is for preventing the driven body  110  from rotating the guide shaft  150  as center. 
   The driven body  110  is guided in the direction of movement without rattling by the guide shaft  150  inserted into the sleeve  114 , and rotation of the driven body  110  is prevented by the guide shaft  160  inserted into the slot  115  so as to decide the optical axis. Further, the drive coil  112  is fixed to the driven body  110  using adhesive etc. 
   The magnet  121  is for causing the driven body  110  to move in the direction of the optical axis, and the shape of the magnet  121  is such that the surface (top surface) on the side of the drive coil  112  is curved in such a manner that the magnet is thinner towards a central part in the drive direction and thicker towards the ends. As a result, a gap between the surface of the magnet  121  and the opposing drive coil  112  is configured so as to be broad at the central part in the drive direction and narrower at the ends in the drive direction. 
   Further, an earthing yoke  120  is connected to earth at the opposite surface to the drive coil  112  with respect to the magnet  121 , and an opposing yoke  130  is passed through the toroidally wound drive coil  112 . 
   The position of the driven body  110  is detected by a position detection magnet  113  fitted to the driven body  110  and a magnetoresistive element  140  arranged in a non-contact manner spaced from the magnet. The position detection magnet  113  is a magnet where magnetic poles alternately change along the direction of movement, while on the other hand, the magnetoresistive element  140  is an element for which a resistance changes with changes in the magnetic field. Therefore, when the position detection magnet  113  moves in accompaniment with movement of the driven body  110 , the magnetic field reaching the magnetoresistive element  140  arranged opposite changes, and the resistance of the magnetoresistive element  140  changes. It is then possible to accurately detect the position of the driven body  110  by observing changes in this resistance value. In the above, the magnetoresistive element  140  and the position detection magnet  113  are used as means for detecting the position of the driven body  110 , but existing non-contact position detecting means may also be employed. 
   With the above configuration, when current flows in the drive coil  112 , thrust parallel with the optical axis direction is generated (Fleming&#39;s left hand rule) at the drive coil  112  due to magnetic flux passing between the opposing yoke  130  and the drive magnet  121 , and the driven body  110  is moved integrally together with the drive coil  112  in the optical axis direction as a result of this thrust. 
   On the other hand, at the lens driving device of the above configuration, within a gap formed between the magnet  121  and the opposing yoke  130 , a rectilinear component in the drive direction of magnetic flux density received by the drive coil is substantially the same at both the center and the ends of the magnet. As a result, a parallel and fixed thrust is generated in the drive direction over the entire drive stroke region at the drive coil  112 , and linearity of thrust can be ensured as shown by the solid lines in  FIG. 3 . 
   Further, frictional resistance generated between the driven body  110  and the guide shafts becomes fixed because the thrust direction and the drive direction are always parallel over the entire region of the drive stroke. The thrust and drive load generated at the time of driving for focusing and zooming of the driven body  110  can therefore be kept fixed, and drive characteristics and servo characteristics can be made superior. 
   Next, a description is given of a second embodiment. The lens driving device of this embodiment is applied to lens driving of a focus lens and a zoom lens in a lens barrel used in an imaging device such as a video camera. 
     FIG. 4  is an exploded perspective view of a lens driving device of this embodiment. In this embodiment, portions that are the same as portions in  FIG. 1  and  FIG. 2  are given the same numerals, and detailed description thereof is omitted. 
   In  FIG. 4 , the lens driving device of this embodiment is such that the surface (top surface) of the magnet  122  is bent (inclined) on the side of the drive coil  112  in such a manner as to be thinner at a central part in the drive direction of the magnet  122  and thinner at the end parts. As a result, a gap between the surface of the magnet  122  and the opposing drive coil  112  is configured so as to be broad at the central part in the drive direction and narrower at the ends in the drive direction. 
   In this configuration also, as with the lens driving device of the first embodiment described above, the thrust and driving load occurring when driving for focusing and zooming can be kept fixed, and superior drive and servo characteristics can be attained. 
   Further, the lens driving device of the present invention is by no means limited to the first and second embodiments described above. For example, the shapes of the magnets  121  and  122  are by no means limited to that shown in the drawings, and various shapes obtaining the same results as for each of the embodiments described above may be considered. Further, the front and back surfaces of the magnets  121  and  122  may be curved or bent. Moreover, the magnets  121  and  122  are taken to have uniform thickness but the same results may also be obtained if the magnets  121  and  122  are curved or bent. 
   Next, a description is given of a third embodiment. The lens driving device of this embodiment is applied to lens driving of a focus lens and a zoom lens in a lens barrel used in an imaging device such as a video camera. 
     FIG. 5  is an exploded perspective view of a lens driving device of this embodiment. In this embodiment, portions that are the same as portions in  FIG. 1  and  FIG. 2  are given the same numerals, and detailed description thereof is omitted. 
   In  FIG. 5 , the lens driving device of this embodiment is such that the magnet  123  is bent in such a manner as to have a narrower width at a central part in the drive direction and broader width at the end parts. In this configuration also, as with the lens driving device of the first embodiment described above, the thrust and driving load occurring when driving for focusing and zooming can be kept fixed, and superior drive and servo characteristics can be obtained. 
   Next, a description is given of a fourth embodiment. The lens driving device of this embodiment is applied to lens driving of a focus lens and a zoom lens in a lens barrel used in an imaging device such as a video camera. 
     FIG. 6  is an exploded perspective view of a lens driving device of this embodiment. In this embodiment, portions that are the same as portions in  FIG. 1  and  FIG. 2  are given the same numerals, and detailed description thereof is omitted. 
   In  FIG. 6 , the lens driving device of this embodiment is such that the magnet  124  is bent in such a manner as to have a narrower width at a central part in the drive direction and broader width at the end parts. In this configuration also, as with the lens driving device of the first embodiment described above, the thrust and driving load occurring when driving for focusing and zooming can be kept fixed, and superior drive and servo characteristics can be obtained. 
   In the third and fourth embodiments described above, the thickness of the magnets  123  and  124  is fixed, and this is advantageous in the manufacture of magnets  123  and  124  employing molds. Further, the lens device of the present invention is by no means limited to the third and fourth embodiments described above. For example, the shapes of the magnets  123  and  124  are by no means limited to that shown in the drawings, and various shapes obtaining the same results as for each of the embodiments described above may be considered. 
   The following is a description of a fifth embodiment. The lens driving device of this embodiment is applied to lens driving of a focus lens and a zoom lens in a lens barrel used in an imaging device such as a video camera. 
     FIG. 7  is an exploded perspective view of a lens driving device of this embodiment, and  FIG. 8  is a front view of the lens driving device of this embodiment. Namely, a driven body  210  has an optical lens  211 , a sleeve  214  on one side, and a slot  215  on the opposite side so as to sandwich the light axis together with the sleeve  214 . 
   Further, a guide shaft  250  is a shaft fitting into the sleeve  214  with no rattle in order to cause the driven body  210  to move along the direction of the optical axis. The guide shaft  260  is a shaft inserted as a brace at the slot  215  and is for preventing the driven body  210  from rotating taking the guide shaft  250  as center. 
   The driven body  210  is guided in the direction of movement without rattling by the guide shaft  250  inserted into the sleeve  214  and rotation of the driven body  210  is prevented by the guide shaft  260  inserted into the slot  215  so as to decide the optical axis. 
   A flat coil  212  is wound in a flat manner and arranged in such a manner that a normal direction of the flat plane is at right angles with the drive direction. The flat coil  212  is fixed using adhesive etc. to the driven body  210 . 
   The magnet  221  is such that a region  221 A and a region  221 B magnetized in mutually opposite directions are arranged next to each other along the direction of movement of the driven body  210 . The shapes of the regions  221 A and  221 B are such that the surfaces (top surfaces) are curved so that the regions  221 A and  221 B are thin at central parts in the direction of movement and thick at the ends. As a result, a gap between the magnet  221  and the opposing drive coil  212  is configured so as to be broad at the central part in the drive direction and narrow at the ends in the drive direction. 
   Further, an earthing yoke  220  is connected to earth at a surface on the opposite side to the flat coil  212  with respect to the magnet  221 , and an opposing yoke  230  is arranged so as to sandwich the flat coil  212  in parallel with the earthing yoke  220 . 
   The position of the driven body  210  is detected by a position detection magnet  213  fitted to the driven body  110  and a magnetoresistive element  240  arranged in a non-contact manner spaced from the magnet. The position detection magnet  213  is magnet where magnetic poles alternately change along the direction of movement, while on the other hand, the magnetoresistive element  240  is an element for which a resistance changes with changes in the magnetic field. Therefore, when the position detection magnet  213  moves in accompaniment with movement of the driven body  210 , the magnetic field reaching the magnetoresistive element  240  arranged opposite changes, and the resistance of the magnetoresistive element  240  changes. It is then possible to accurately detect the position of the driven body  210  by observing changes in resistance. In the above, the magnetoresistive element  240  and the position detection magnet  213  are used as means for detection the position of the driven body  210 , but existing non-contact position detecting means may also be employed. 
   With the above configuration, when current flows in the drive coil  212 , thrust parallel with the optical axis direction is generated (Fleming&#39;s left hand rule) at the drive coil  212  due to magnetic flux passing between the facing yoke  230  and the drive magnet  221 , and the driven body  210  is moved integrally together with the drive coil  212  in the optical axis direction as a result of this thrust. 
   On the other hand, at the lens driving device of the above configuration, within a gap formed between the magnet  221  and the facing yoke  230 , a rectilinear component in the drive direction of magnetic flux density acting on the drive coil is substantially the same at both the center and the ends of the magnet. As a result, a parallel and fixed thrust is generated in the drive direction over the entire drive stroke region at the drive coil  212 , and linearity of thrust can be ensured as shown by the solid lines in  FIG. 9 . 
   Further, frictional resistance generated between the driven body  210  and the guide shafts becomes fixed because the thrust direction and the drive direction are always parallel over the entire region of the drive stroke. The thrust and drive load generated at the time of driving for focusing and zooming of the driven body  210  can therefore be kept fixed, and drive characteristics and servo characteristics can be made superior. 
   Next, a description is given of a sixth embodiment. The lens driving device of this embodiment is applied to lens driving of a focus lens and a zoom lens in a lens barrel used in an imaging device such as a video camera. 
     FIG. 10  is an exploded perspective view of a lens driving device of this embodiment. In this embodiment, portions that are the same as portions in  FIG. 7  and  FIG. 8  are given the same numerals, and detailed description thereof is omitted. 
   In  FIG. 10 , the lens driving device of this embodiment is such that the surfaces on the sides of the drive coil  212  are bent in such a manner as to be thinner at centrals part in the drive direction of the magnet  122  and thinner at the end parts. 
   In this configuration also, as with the lens driving device of the fifth embodiment described above, the thrust and driving load occurring when driving for focusing and zooming can be kept fixed, and superior drive and servo characteristics can be attained. 
   Further, the lens device of the present invention is by no means limited to that of the fifth and sixth embodiments. For example, the shapes of the magnets  221  and  222  are by no means limited to that shown in the drawings, and various shapes obtaining the same results as for each of the embodiments described above may be considered. The front and back surfaces of the magnets  221  and  222  may be curved or bent. Moreover, the magnets  221  and  222  are taken to have uniform thickness but the same results may also be obtained if the magnets  221  and  222  are curved or bent. 
   Next, a description is given of a seventh embodiment. The lens driving device of this embodiment is applied to lens driving of a focus lens and a zoom lens in a lens barrel used in an imaging device such as a video camera. 
     FIG. 11  is an exploded perspective view of a lens driving device of this embodiment. In this embodiment, portions that are the same as portions in  FIG. 7  and  FIG. 8  are given the same numerals, and detailed description thereof is omitted. 
   In  FIG. 11 , the lens driving device of this embodiment is such that respective regions of the magnets  223 A and  223 B are bent in such a manner as to have a narrower widths at a central part in the drive direction and broader widths at the end parts. In this configuration also, as with the lens driving device of the fifth embodiment described above, the thrust and driving load occurring when driving for focusing and zooming can be kept fixed, and superior drive and servo characteristics can be attained. 
   Next, a description is given of an eighth embodiment. The lens driving device of this embodiment is applied to lens driving of a focus lens and a zoom lens in a lens barrel used in an imaging device such as a video camera. 
     FIG. 12  is an exploded perspective view of a lens driving device of this embodiment. In this embodiment, portions that are the same as portions in  FIG. 7  and  FIG. 8  are given the same numerals, and detailed description thereof is omitted. 
   In  FIG. 12 , the lens driving device of this embodiment is such that respective regions of the magnets  224 A and  224 B are bent in such a manner as to have narrower widths at central parts in the drive direction and broader widths at the end parts. In this configuration also, as with the lens driving device of the fifth embodiment described above, the thrust and driving load occurring when driving for focusing and zooming can be kept fixed, and superior drive and servo characteristics can be attained. 
   In the seventh and eighth embodiments described above, the thickness of the magnets  223  and  224  is fixed, and this is advantageous in the manufacture of magnets  223  and  224  employing molds. Further, the lens device of the present invention is by no means limited to the seventh and eighth embodiments described above. For example, the shapes of the magnets  223  and  224  are by no means limited to that shown in the drawings, and various shapes obtaining the same results as for each of the embodiments described above may be considered. 
   Next, a description is given of the bent or curved parts of the magnets in each embodiment. In this embodiment, by providing bent or curved parts at both ends of the magnets as described above, the rectilinear component of the magnetic flux density in the drive direction incurred by the drive coil is substantially the same at the center of the magnets and the ends of the magnets and a fixed thrust is therefore generated parallel to the drive direction over the entire drive stroke region of the drive coil. 
   When this kind of magnet is constructed, the size of the bent parts or curved parts provided at the ends of the magnet is obtained by examining changes in the drive coil occurring in the length (length along the direction of movement of the drive coil) direction of the magnet. 
     FIG. 13  is a view showing a situation where change in thrust along the length direction of a magnet is calculated taking the aspect ratio (length/thickness) as a parameter. Further,  FIG. 14  is a view showing aspect ratio of a magnet and proportion of decrease in drive coil thrust. 
   From the relationship of  FIG. 13 , it can be understood that the proportion of a decrease in thrust part of the drive coil corresponding to the parts at both end of the magnet is smaller for a large aspect ratio for the magnet, and that the proportion of a decrease in thrust part of the drive coil corresponding to parts at both ends of the magnet is larger for a large aspect ratio. 
   Further, from the relationship shown in  FIG. 13 ,  FIG. 14  shows the proportion with respect to overall magnet length for portions of 100% (no fall in thrust), 90% (10% fall in thrust), and 80% (20% fall in thrust) of the maximum thrust. According to  FIG. 14 , if the range (proportion of overall length) of bent portions or curved portions at both ends of the magnet is decided, the thrust of the drive coils can be made uniform with respect to the overall length of the magnet.