Patent Publication Number: US-2021165183-A1

Title: Actuator and camera device

Description:
TECHNICAL FIELD 
     The present disclosure generally relates to an actuator and a camera device, and more particularly relates to an actuator and camera device configured to drive an object to be driven in rotation. 
     BACKGROUND ART 
     An actuator for rotating a camera has been known as an actuator for rotating an object to be driven in rotation. For example, Patent Literature 1 discloses a camera driver (camera device) with the ability to rotate a camera unit in three axis directions. The camera driver disclosed in Patent Literature 1 includes: a movable unit including, on its outer surface, a convex partial sphere; and a fixed unit which has a recess, in which the movable unit is loosely fitted at least partially, in which the surface of the convex partial sphere and the recess make point or line contact with each other, and which causes the movable unit to rotate by electromagnetic driving around the center of the convex partial sphere. 
     In the camera driver (actuator, camera device) of Patent Literature 1, the convex partial sphere of the movable unit is loosely fitted into the recess of the fixed unit to have the movable unit supported by the fixed unit. If the device is used so as to constantly rest and move repeatedly, while the movable unit is standing still with respect to the fixed unit, at least the loosely fitted part of the movable unit and the fixed unit are coupled together via static friction, thus letting the coupled parts behave as a rigid body. When the movable unit and the fixed unit start to move, a so-called “stick slip,” which is a self-excited vibration caused by a variation in static and sliding frictions, occurs. A torque pulsation caused by this stick slip has a saw-toothed sharp waveform, which excites (i.e., produced resonance of) the characteristic vibration that the rigid body coupled together during the static period owns, thus causing instability to the rotational control system temporarily. In addition, this phenomenon also arises in the process during which the object in motion is going to rest, thus constituting a factor eventually causing a decline in the positioning accuracy of the rotational control. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: WO 2012/004952 A1 
     SUMMARY OF INVENTION 
     In view of the foregoing background, it is therefore an object of the present disclosure to provide an actuator and camera device configured to allow the movable unit to start and stop moving smoothly at an initial stage of its rotary motion. 
     An actuator according to an aspect of the present disclosure includes: a movable unit configured to hold an object to be driven; a fixed unit configured to support the movable unit thereon to make the movable unit rotatable; and a structure for supporting the movable unit with respect to the fixed unit. The structure includes: a sphere; and a pair of holding members configured to clamp the sphere between themselves. A space is left to let the sphere roll while shifting a center position thereof with respect to at least one of the pair of holding members. 
     A camera device according to another aspect of the present disclosure includes: the actuator described above; and a camera module serving as the object to be driven. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a cross-sectional view of a camera device including an actuator according to an embodiment of the present invention; 
         FIG. 1B  illustrates a supporting structure for the camera device; 
         FIG. 2A  is a perspective view of the camera device; 
         FIG. 2B  is a plan view of the camera device; 
         FIG. 3  is an exploded perspective view of the camera device; 
         FIG. 4  is an exploded perspective view of the movable unit that the actuator includes; 
         FIGS. 5A-5C  illustrate a structure that allows the movable unit to rotate; 
         FIG. 6  illustrates a relation between the radius of a spherical surface of a fixed-end holding member and the radius of a sphere when the sphere rolls on the fixed-end holding member that the actuator includes; 
         FIG. 7  illustrates a relation between the radius of a spherical surface of a movable-end holding member and the radius of the sphere when the sphere rolls on the movable-end holding member that the actuator includes; 
         FIG. 8  illustrates a relation between the radius of the spherical surface of the fixed-end holding member, the radius of the spherical surface of the movable-end holding member, the radius of the sphere, and frictional force; 
         FIG. 9  shows a relation between the radius of the spherical surface of the fixed-end holding member, the radius of the spherical surface of the movable-end holding member, and the radius of the sphere when frictional force is taken into account by the camera device; 
         FIG. 10  shows a relation between the radius of the spherical surface of the fixed-end holding member, the radius of the spherical surface of the movable-end holding member, and the radius of the sphere when reduction in the deformation of the sphere is taken into account by the camera device; 
         FIG. 11  shows the magnitude of movement of a sphere when the magnitude of movement of the sphere is taken into account by the camera device; 
         FIG. 12  shows a relation between the radius of the spherical surface of the fixed-end holding member, the radius of the spherical surface of the movable-end holding member, and the radius of the sphere when the magnitude of movement of the sphere is taken into account by the camera device; and 
         FIG. 13  shows a relation between the radius of the spherical surface of the fixed-end holding member, the radius of the spherical surface of the movable-end holding member, and the radius of the sphere when the frictional force, reduction in the deformation of the sphere, and the magnitude of movement of the sphere are taken into account by the camera device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Note that embodiments and their variations to be described below are only examples of the present invention and should not be construed as limiting. Rather, those embodiments and variations may be readily modified in various manners depending on a design choice or any other factor without departing from a true spirit and scope of the present invention. The drawings to be referred to in the following description of the first embodiment are all schematic representations. That is to say, the ratio of the dimensions (including thicknesses) of respective constituent elements illustrated on the drawings does not always reflect their actual dimensional ratio. 
     First Embodiment 
     A camera device according to this embodiment will be described with reference to  FIGS. 1A-13 .  FIG. 1A  is a cross-sectional view taken along the plane X 1 -X 1  shown in  FIG. 2B .  FIG. 1B  is an enlarged view of the main part D 1  shown in  FIG. 1A . 
     The camera device  1  may be a portable camera, for example, and includes an actuator  2  and a camera module  3  as shown in  FIGS. 2A and 3 . 
     The camera module  3  includes an image sensor, a lens for forming a subject image on the image capturing plane of the image sensor, and a lens barrel for holding the lens. The camera module  3  converts video produced on the image capturing plane of the image sensor into an electrical signal. Also, a plurality of cables to transmit the electrical signal generated by the image sensor to an external image processor circuit (as an exemplary external circuit) are electrically connected to the camera module  3  via a connector. The camera module  3  transmits, by the low voltage differential signaling (LVDS) method, the electrical signal thus generated to the external image processor circuit via the plurality of cables. Note that in this embodiment, the plurality of cables includes coplanar waveguides or micro-strip lines. Alternatively, the plurality of cables may each include fine-line coaxial cables each having the same length. Note that the LVDS method is only an example and should not be construed as limiting. Those cables are grouped into two bundles of cables  11  so that each bundle of cables  11  consists of the same number of cables. The bundles of cables  11  may be implemented as flexible flat cables, for example. One end of the bundle of cables  11  is electrically connected to the camera module  3  and the other end of the bundle of cables  11  is electrically connected to the image processor circuit. 
     The actuator  2  includes an upper ring  4 , a movable unit  10 , a fixed unit  20 , a driving unit  30 , and a printed circuit board  90  as shown in  FIGS. 1A and 2A . 
     The upper ring  4  consists of a first ring  4   a  and a second ring  4   b . The upper ring  4  fixes first coil units  52  and second coil units  53  to be described later. 
     The movable unit  10  includes a camera holder  40 , a first movable base  41 , and a second movable base  42  (see  FIG. 4 ). The movable unit  10  is fitted into the fixed unit  20 . The movable unit  10  rotates (i.e., rolls) around the optical axis  1   a  of the lens of the camera module  3  with respect to the fixed unit  20 . The movable unit  10  also rotates around an X-axis and a Y-axis, which are both perpendicular to the optical axis  1   a , with respect to the fixed unit  20 . In this case, the X-axis and the Y-axis are both perpendicular to a fitting direction, in which the movable unit  10  is fitted into the fixed unit  20  while the movable unit  10  is not rotating. Furthermore, these X- and Y-axes intersect with each other at right angles. A detailed configuration for the movable unit  10  will be described later. The camera module  3  has been mounted on the camera holder  40 . The configuration of the first movable base  41  and the second movable base  42  will be described later. Rotating the movable unit  10  allows the camera module  3  to rotate. In this embodiment, when the optical axis  1   a  is perpendicular to both of the X- and Y-axes, the movable unit  10  (i.e., the camera module  3 ) is defined to be in a neutral position. In the following description, the direction in which the optical axis  1   a  extends when the movable unit  10  is in the neutral position is defined herein as a “Z-axis direction.” The direction of movement of the movable unit  10  in which the movable unit  10  rotates around the X-axis is defined herein as a “panning direction” and the direction of movement of the movable unit  10  in which the movable unit  10  rotates around the Y-axis is defined herein as a “tilting direction.” While the movable unit  10  is not driven by the driving unit  30  (i.e., in the state shown in  FIG. 3A  and other drawings), the optical axis  1   a  of the camera module  3 , the X-axis, and the Y-axis intersect with each other at right angles. 
     The fixed unit  20  includes a coupling member  50  and a body  51  (see  FIG. 3 ). 
     The coupling member  50  includes a linear coupling bar  501  and a fixed-end holding member  502 . The fixed-end holding member  502  is provided for a central portion of the coupling bar  501 . The fixed-end holding member  502  has a recessed spherical surface  503  at a central portion thereof. The fixed-end holding member  502  holds a resin-molded sphere  46  (see  FIG. 4 ). The radius of the recessed spherical surface  503  is larger than the radius of the sphere  46 . In other words, the recessed spherical surface  503  and the sphere  46  have mutually different curvatures. That is to say, when the fixed-end holding member  502  holds the sphere  46  (i.e., when the sphere  46  comes into contact with the recessed spherical surface  503 ), a space  504  is left (see  FIGS. 1B and 5A ). The space  504  left lets the sphere  46  roll on the recessed spherical surface  503  such that the center  460  of the sphere  46  shifts (see  FIGS. 1A and 1B ). The coupling member  50  is made of aluminum and the surface of the recessed spherical surface  503 , in particular, is subjected to alumite (anodized aluminum) treatment. 
     The body  51  includes a pair of protrusions  510 . The pair of protrusions  510  are provided so as to face each other in a direction perpendicular to the optical axis  1   a  of the movable unit  10  in the neutral position. The pair of protrusions  510  are also provided to be located in the gaps between the first coil units  52  and second coil units  53  arranged (to be described later). The coupling member  50  is screwed onto the body  51  with the second movable base  42  interposed between itself and the body  51 . Specifically, both ends of the coupling member  50  are respectively screwed onto the pair of protrusions  510  of the body  51 . 
     The body  51  is provided with two fixing portions  703  for fixing the two bundles of cables  11  thereto (see  FIGS. 2A-3 ). The two fixing portions  703  are arranged to face each other perpendicularly to the direction in which the pair of protrusions  510  are arranged. Each of the two fixing portions  703  includes a first member  704  and a second member  705  (see  FIG. 3 ). An associated bundle of cables  11  is partially clamped between the first member  704  and the second member  705  fitted into a cutout  512  of the body  51 . 
     The fixed unit  20  includes a pair of first coil units  52  and a pair of second coil units  53  to make the movable unit  10  electromagnetically drivable and rotatable (see  FIG. 3 ). The pair of first coil units  52  allows the movable unit  10  to rotate around the X-axis. The pair of second coil units  53  allows the movable unit  10  to rotate around the Y-axis. 
     The pair of first coil units  52  each include a first magnetic yoke  710  made of a magnetic material, drive coils  720  and  730 , and a magnetic yoke holder  740  (see  FIG. 3 ). Each of the first magnetic yokes  710  has the shape of an arc, of which the center is defined by the center of rotation. The drive coils  730  are each formed by winding a conductive wire around its associated first magnetic yoke  710  such that its winding direction is defined around the X-axis (i.e., the direction in which the second coil units  53  face each other) and that the pair of first drive magnets  620  (to be described later) are driven in rotation in the rolling direction. As used herein, the winding direction of the coil refers in this embodiment to a direction in which the number of turns increases. The respective first magnetic yokes  710  are arranged in their associated magnetic yoke holders  740 . The drive coils  720  are each formed by winding a conductive wire around its associated first magnetic yoke  710  arranged in its corresponding magnetic yoke holder  740 . The drive coils  720  have their winding direction defined around the Z-axis such that the pair of first drive magnets  620  are driven in rotation in the panning direction. Then, the pair of first coil units  52  are secured with screws onto the body  51  so as to face each other when viewed from the camera module  3 . Specifically, each of the first coil units  52  has one end thereof (i.e., the end opposite from the camera module  3 ) along the Z-axis secured with a screw onto the body  51 . Each of the first coil units  52  has the other end thereof along the Z-axis (i.e., the end facing the camera module  3 ) fitted into the upper ring  4 . 
     The pair of second coil units  53  each include a second magnetic yoke  711  made of a magnetic material, drive coils  721  and  731 , and a magnetic yoke holder  741  (see  FIG. 3 ). Each of the second magnetic yokes  711  has the shape of an arc, of which the center is defined by the center of rotation. The drive coils  731  are each formed by winding a conductive wire around its associated second magnetic yoke  711  such that its winding direction is defined around the Y-axis (i.e., the direction in which the first coil units  52  face each other) and that the pair of second drive magnets  621  (to be described later) are driven in rotation in the rolling direction. The respective second magnetic yokes  711  are arranged in their associated magnetic yoke holders  741 . The drive coils  721  are each formed by winding a conductive wire around its associated second magnetic yoke  711  arranged in its corresponding magnetic yoke holder  741 . The drive coils  721  have their winding direction defined around the Z-axis such that the pair of second drive magnets  621  are driven in rotation in the tilting direction. Then, the pair of second coil units  53  are secured with screws onto the body  51  so as to face each other when viewed from the camera module  3 . Specifically, each of the second coil units  53  has one end thereof (i.e., the end opposite from the camera module  3 ) along the Z-axis secured with a screw onto the body  51 . Each of the second coil units  53  has the other end thereof along the Z-axis (i.e., the end facing the camera module  3 ) fitted into the upper ring  4 . 
     The camera holder  40  on which the camera module  3  has been mounted is secured with screws onto the first movable base  41 . The coupling member  50  is interposed between the first movable base  41  and the second movable base  42 . 
     The printed circuit board  90  includes a plurality of (e.g., four in this embodiment) magnetic sensors  92  for detecting rotational positions in the panning and tilting directions of the camera module  3 . In this embodiment, the magnetic sensors  92  may be implemented as Hall elements, for example. On the printed circuit board  90 , further assembled are a circuit for controlling the amount of a current allowed to flow through the drive coils  720 ,  721 ,  730 , and  731  and other circuits. 
     Next, detailed configurations for the first movable base  41  and the second movable base  42  will be described. 
     The first movable base  41  includes a body  43 , a pair of holding portions  44 , a movable-end holding member  45 , and a sphere  46  (see  FIG. 4 ). The body  43  sandwiches the rigid portion  12  between itself and the camera holder  40  to fix (hold) the rigid portion  12  thereon. The respective holding portions  44  are provided for the peripheral edge of the body  43  so as to face each other (see  FIG. 4 ). Each holding portion  44  clamps and holds an associated bundle of cables  11  between itself and a sidewall  431  of the body  43  (see  FIGS. 2A and 2B ). The movable-end holding member  45  has a recessed spherical surface  451  (see  FIG. 1B ). The movable-end holding member  45  holds the sphere  46 . The radius of the recessed spherical surface  451  is larger than the radius of the sphere  46  and as large as the radius of the recessed spherical surface  503 . In other words, although the recessed spherical surface  451  and the sphere  46  have different curvatures, the recessed spherical surface  451  and the recessed spherical surface  503  have the same curvature. As used herein, if the two curvatures are the same, the two curvatures may naturally be exactly the same as each other but may also be substantially the same as each other as long as their difference falls within a permissible tolerance range. When the movable-end holding member  45  holds the sphere  46  (i.e., when the sphere  46  comes into contact with the recessed spherical surface  451 ), a space  452  is left between them (see  FIGS. 1B and 5A ). The space  452  left lets the sphere  46  roll on the recessed spherical surface  451  such that the center  460  of the sphere  46  (see  FIGS. 1A and 1B ) shifts. In this case, the movable-end holding member  45  is formed of aluminum and the surface of the recessed spherical surface  451 , in particular, is subjected to alumite (anodized aluminum) treatment. 
     The fixed-end holding member  502  and the movable-end holding member  45  sandwich the sphere  46  between themselves, thus allowing the fixed unit  20  to pivotally support the movable unit  10  to make the movable unit  10  rotatable. 
     The second movable base  42  supports the first movable base  41 . The second movable base  42  includes a back yoke  610 , a pair of first drive magnets  620 , and a pair of second drive magnets  621  (see  FIG. 4 ). The second movable base  42  further includes a bottom plate  640 , a position detecting magnet  650 , a first stopper member  651 , and a second stopper member  652  (see  FIG. 4 ). 
     The back yoke  610  includes a disk portion and four fixing portions (arms) extending from the outer periphery of the disk portion toward the camera module  3  (i.e., upward). Two out of the four fixing portions face each other along the X-axis, while the other two fixing portions face each other along the Y-axis. The two fixing portions facing each other along the Y-axis face the pair of first coil units  52 . The two fixing portions facing each other along the X-axis face the pair of second coil units  53 . 
     The pair of first drive magnets  620  are respectively fixed onto two fixing portions, facing each other along the Y-axis, out of the four fixing portions of the back yoke  610 . The pair of second drive magnets  621  are respectively fixed onto two fixing portions, facing each other along the X-axis, out of the four fixing portions of the back yoke  610 . 
     Electromagnetic driving by the first drive magnets  620  and the first coil units  52  and electromagnetic driving by the second drive magnets  621  and the second coil units  53  allow the movable unit  10  (camera module  3 ) to rotate in the panning, tilting, and rolling directions. Specifically, electromagnetic driving by the two drive coils  720  and the two first drive magnets  620  allows the movable unit  10  to rotate in the panning direction. Electromagnetic driving by the two drive coils  721  and the two second drive magnets  621  allows the movable unit  10  to rotate in the tilting direction. Meanwhile, electromagnetic driving by the two drive coils  730  and the two first drive magnets  620  and electromagnetic driving by the two drive coils  731  and the two second drive magnets  621  allow the movable unit  10  to rotate in the rolling direction. 
     The bottom plate  640  is a non-magnetic member and may be made of brass, for example. The bottom plate  640  is attached to the back yoke  610  to define the bottom of the movable unit  10  (i.e., the bottom of the second movable base  42 ). The bottom plate  640  is secured with screws onto the back yoke  610  and the first movable base  41 . The bottom plate  640  serves as a counterweight. Having the bottom plate  640  serve as a counterweight allows the center of rotation to agree with the center of gravity of the movable unit  10 . That is why when external force is applied to the entire movable unit  10 , the moment of rotation of the movable unit  10  around the X-axis and the moment of rotation of the movable unit  10  around the Y-axis both decrease. This allows the movable unit  10  (or the camera module  3 ) to be held in the neutral position, or to rotate around the X- and Y-axes, with less driving force. 
     The back yoke  610  is fixed onto the surface, located closer to the camera module  3  (i.e., the upper surface), of the bottom plate  640 . 
     One surface, located more distant from the camera module  3  (i.e., the lower surface), of the bottom plate  640  is a spherical surface, a central portion of which has a recess. In the recess, arranged are the position detecting magnet  650  and the first stopper member  651  (see  FIG. 1A ). The first stopper member  651  prevents the position detecting magnet  650 , arranged in the recess of the bottom plate  640 , from falling off. 
     The second stopper member  652  prevents the sphere  46  from falling off. A central portion of the surface, located closer to the camera module  3  (i.e., the upper surface), of the second stopper member  652  has a curved recess  653  (see  FIGS. 1B and 4 ). A protrusion  654  protrudes from a central portion of the surface, located more distant from the camera module  3  (i.e., the lower surface), of the second stopper member  652  (see  FIGS. 1B and 4 ). 
     Inserting the protrusion  654  into a through hole  611  of the back yoke  610  allows the second stopper member  652  to be fixed onto the back yoke  610 . 
     A gap is left between the second stopper member  652  and the fixed-end holding member  502  of the coupling member  50  (see  FIG. 1B ). The surface, located more distant from the camera module  3 , of the fixed-end holding member  502  and the bottom surface of the recess  653  are curved surfaces that face each other. This gap is wide enough to prevent the sphere  46  from falling off even if the movable unit  10  has moved upward (i.e., even if the second stopper member  652  has moved toward the fixed-end holding member  502 ). 
     The four magnetic sensors  92  provided for the printed circuit board  90  detect the relative rotation (movement) of the movable unit  10  with respect to the fixed unit  20  based on the relative position of the position detecting magnet  650  with respect to the four magnetic sensors  92 . That is to say, as the movable unit  10  rotates (moves), the position detecting magnet  650  changes its position, thus causing a variation in the magnetic force applied to the four magnetic sensors  92 . The four magnetic sensors  92  detect this variation in the magnetic force, and calculate two-dimensional angles of rotation with respect to the X- and Y-axes. This allows the four magnetic sensors  92  to detect the angles of rotation of the movable unit  10  in the tilting and panning directions, respectively. In addition, the camera device  1  further includes, separately from the four magnetic sensors  92 , another magnetic sensor for detecting the rotation of the movable unit  10  (i.e., the rotation of the camera module  3 ) around the optical axis  1   a , i.e., the rotation of the movable unit  10  in the rolling direction. Note that the sensor for detecting the rotation of the movable unit  10  in the rolling direction does not have to be a magnetic sensor but may also be a gyrosensor, for example. 
     In this case, the pair of first drive magnets  620  serves as attracting magnets, thus producing first magnetic attraction forces between the pair of first drive magnets  620  and the first magnetic yokes  710  that face the first drive magnets  620 . Likewise, the pair of second drive magnets  621  also serves as attracting magnets, thus producing second magnetic attraction forces between the pair of second drive magnets  621  and the second magnetic yokes  711  that face the second drive magnets  621 . The vector direction of each of the first magnetic attraction forces is parallel to a centerline that connects together the center of rotation, the center of mass of an associated one of the first magnetic yokes  710 , and the center of mass of an associated one of the first drive magnets  620 . The vector direction of each of the second magnetic attraction forces is parallel to a centerline that connects together the center of rotation, the center of mass of an associated one of the second magnetic yokes  711 , and the center of mass of an associated one of the second drive magnets  621 . 
     The first and second magnetic attraction forces become normal forces produced by the fixed unit  20  with respect to the sphere  46  of the fixed-end holding member  502 . Also, when the movable unit  10  is in the neutral position, the magnetic attraction forces of the movable unit  10  define a synthetic vector along the Z-axis. This force balance between the first magnetic attraction forces, the second magnetic attraction forces, and the synthetic vector resembles the dynamic configuration of a balancing toy, and allows the movable unit  10  to rotate in three axis directions with good stability. 
     In this embodiment, the pair of first coil units  52 , the pair of second coil units  53 , the pair of first drive magnets  620 , and the pair of second drive magnets  621  together form the driving unit  30 . 
     The camera device  1  of this embodiment allows the movable unit  10  to rotate two-dimensionally (i.e., pan and tilt) by supplying electricity to the pair of drive coils  720  and the pair of drive coils  721  simultaneously. In addition, the camera device  1  also allows the movable unit  10  to rotate (i.e., to roll) around the optical axis  1   a  by supplying electricity to the pair of drive coils  730  and the pair of drive coils  731  simultaneously. 
     Next, a supporting structure for supporting the movable unit  10  with respect to the fixed unit  20  will be described. The supporting structure includes the sphere  46  and a pair of holding members (namely, the fixed-end holding member  502  and the movable-end holding member  45 ) that clamp the sphere  46  between themselves. In this embodiment, there is a space  504  that lets the sphere  46  roll so that the center  460  (i.e., the center of mass) of the sphere  46  shifts with respect to the fixed-end holding member  502 . In addition, there is another space  452  that lets the sphere  46  roll so that the center  460  (i.e., the center of mass) of the sphere  46  shifts with respect to the movable-end holding member  45 . 
     In this supporting structure, when the movable unit  10  is going to rotate in the panning direction from the neutral position (see  FIG. 5A ), the sphere  46  rolls through the spaces  452  and  504  first. As a result, the movable unit  10  rotates in the panning direction (see  FIG. 5B ). Supply of electricity to the pair of drive coils  720  causes the movable unit  10  to further rotate in the panning direction (see  FIG. 5C ). Note that in  FIGS. 5A-5C , the shapes of the spherical surfaces  451  and  503  are not actual ones but exaggerated to make this description more easily understandable. 
     Likewise, when the movable unit  10  is going to rotate in the tilting direction from the neutral position, the sphere  46  also rolls through the spaces  452  and  504  to cause the movable unit  10  to rotate in the tilting direction. Thereafter, supply of electricity to the pair of drive coils  721  causes the movable unit  10  to further rotate in the tilting direction. 
     In the following description, the operation of causing the movable unit  10  to rotate in either the panning direction or the tilting direction by letting the sphere  46  roll through the spaces  452  and  504  will be hereinafter referred to as a “first mode,” and the operation of causing the movable unit  10  to further rotate in the same direction by supplying electricity to the pair of drive coils after having rotated in either the panning direction or the tilting direction in the first mode will be hereinafter referred to as a “second mode.” In the first mode, the position of the sphere  46  relative to the fixed-end holding member  502  changes (i.e., the position where the sphere  46  makes contact with the fixed-end holding member  502  changes) but the position where the sphere  46  makes contact with the movable-end holding member  45  does not change. In the second mode, on the other hand, the position of the sphere  46  does not change but the position of the movable-end holding member  45  changes relatively (i.e., the position where the sphere  46  makes contact with the movable-end holding member  45  changes). In other words, it can be said that in the second mode, considering from the standpoint of the movable-end holding member  45  (i.e., considering with the movable-end holding member  45  fixed), the position of the sphere  46  changes relative to the movable-end holding member  45 . 
     Next, the relation in magnitude between the respective radii R of the spherical surface  503  of the fixed-end holding member  502  and the spherical surface  451  of the movable-end holding member  45  and the radius r of the sphere will be described with reference to  FIGS. 6-13 . Note that in  FIGS. 6-8 and 11 , the shapes of the spherical surfaces  451  and  503  are not actual ones but exaggerated to make this description more easily understandable. In this embodiment, the center of the spherical surface  503  of the fixed-end holding member  502  is designated by A 1  and the center of the spherical surface  451  of the movable-end holding member  45  is designated by A 2 . The respective centers A 1  and A 2  of the spherical surfaces  503  and  451  may be either the same position or two different positions. 
     In a situation where the sphere  46  has rolled in the first mode on the spherical surface  503  of the fixed-end holding member  502 , the angle of movement of the sphere  46  with respect to a vertical line drawn to the center A 1  of the spherical surface  503  is supposed to be θ 01  and the tilt angle defined by the sphere  46  with respect to the vertical line is supposed to be φ 1  (see  FIG. 6 ). In this case, in  FIG. 6 , the sphere  46  before rotating in the first mode is indicated by the two-dot chain circle and the sphere  46  that has rotated (i.e., after the first mode is over) is indicated by the solid circle. The tilt angle φ 1  is an angle defined, with respect to the vertical line, by a line segment connecting a point P 1  where the sphere  46  contacted with the spherical surface  503  before the first mode (i.e., before the rotation) (i.e., the point P 1  of the sphere  46  indicated by the two-dot chain circle), or a point P 1  after the rotation (i.e., the point P 1  of the sphere  46  indicated by the solid circle), to the center  460  of the sphere  46  that has rotated. Furthermore, the angle of rotation of the sphere  46  is supposed to be θ 1  (see  FIG. 6 ). In that case, the following Equations (1) and (2) are satisfied, and Equation (3) is derived from Equations (1) and (2). In these Equations (1), (2), and (3), the angle of rotation θ 1  is the angle formed between the line segment connecting the point of contact C 1  of the sphere  46  that has rotated with the spherical surface  503  to the center  460  of the sphere  46  that has rotated and the line segment connecting the point P 1  of the sphere  46  that has rotated to the center  460  of the sphere  46  that has rotated. 
         rθ   1   =Rθ   01   [Equation 1]
 
       ϕ 1 =θ 1 −θ 01   [Equation 2]
 
       ϕ 1 =θ 1 ×( R−r )/ R   [Equation 3]
 
     Next, a situation where the movable-end holding member  45  has rolled on the sphere  46  in the second mode (i.e., a situation where the sphere  46  has rolled on the spherical surface  451  of the movable-end holding member  45 ) will be described. In this case, it can be said that in the second mode, considering from the standpoint of the movable-end holding member  45  (i.e., considering with the movable-end holding member  45  fixed), the position of the sphere  46  changes relatively to the movable-end holding member  45 , as described above. Thus, in  FIG. 7 , the sphere  46  before rotating in the second mode is indicated by the two-dot chain circle and the sphere  46  that has rotated (i.e., after the second mode is over) and has moved relatively to the movable-end holding member  45  is indicated by the solid circle. In the sphere  46  at the beginning of the second mode, the point P 2  where the sphere  46  contacted with the movable-end holding member  45  (i.e., the point P 2  of the sphere  46  indicated by the two-dot chain circle) shifts to a region where the sphere  46  does not contact with the movable-end holding member  45  (see the point P 2  of the sphere  46  indicated by the solid circle) as a result of the relative movement of the sphere  46  with respect to the movable-end holding member  45 . In the following description, the point P 2  of the sphere  46  at the beginning of the second mode will be hereinafter referred to as “point P 2   a .” Also, the respective angles shown in  FIG. 7  and to be described later are the angles defined with respect to the movable-end holding member  45  on the supposition that the sphere  46  has moved relatively to the movable-end holding member  45 . 
     In the following description, the angle of movement formed by the sphere  46  with respect to the line segment that connects the center A 2  of the spherical surface  503  to the point P 2   a  is designated by θ 02 , and the tilt angle of the sphere  46  is designated by φ 2  (see  FIG. 7 ). The tilt angle φ 2  is an angle defined, with respect to the vertical line, by a line segment connecting a point P 2  where the sphere  46  contacted with the spherical surface  451  right after the first mode was over (i.e., the point P 2  of the sphere  46  indicated by the two-dot chain circle), or a point P 2  after the rotation (i.e., the point P 2  of the sphere  46  indicated by the solid circle), to the center  460  of the sphere  46  that has rotated. Furthermore, the angle of rotation of the sphere  46  is supposed to be θ 2  (see  FIG. 7 ). In that case, the following Equations (4) and (5) are satisfied, and Equation (6) is derived from Equations (4) and (5). Since the tilt angle of the barrel of the camera module  3  is φ 1 +φ 2 , Equation (7) is derived from Equations (3) and (6). In these Equations (4), (5), (6), and (7), the angle of rotation θ 2  is the angle formed between the line segment connecting the point of contact B 1  of the sphere  46  that has rotated with the spherical surface  451  to the center  460  of the sphere  46  that has rotated and the line segment connecting the point P 2  of the sphere  46  that has rotated to the center  460  of the sphere  46  that has rotated. 
         rθ   2   =Rθ   02   [Equation 4]
 
       ϕ 2 =θ 2 −θ 02   [Equation 5]
 
       ϕ 2 =θ 2 ×( R−r )/ R   [Equation 6]
 
       ϕ 1 +ϕ 2 (θ 1 +θ 2 )×( R−r )/ R   [Equation 7]
 
     Also, although it depends on the angle of view of a given optical lens, the smallest angle that causes a sensible camera shake at the telephoto end (i.e., at the largest zoom power) is about 0.5 degrees. Therefore, control needs to be performed so as to converge the residual toward this angle or less. In this case, in a range of very small angles from −0.5 degrees to 0.5 degrees, the self-excited vibration caused by the stick slip due to a variation in friction causes a decline in the positioning performance of rotational control. Thus, a rolling friction is applied to the range of very small angles. In that case, the following Inequality (8) is satisfied. The inequality “(θ 1 +θ 2 )×(R−r)/R≥0.5” is obtained based on Equation (7) and Inequality (8) and may be modified into the following Inequality (9): 
       ϕ 1 +ϕ 2 ≥0.5  [Inequality 8]
 
       θ 1 +θ 2 ≥0.5× R /( R−r )  [Inequality 9]
 
     A condition for preventing the sphere  46  from sliding at any of two points of contact B 1  and C 1  in a situation where a vertical load N has been produced in the camera module  3  may be represented by the following Inequalities (10) and (11), where μ is the coefficient of static friction. Inequality (10) may be modified into the following Inequality (12). Furthermore, the following Inequality (13) is obtained by substituting Equation (2) for Inequality (12). Furthermore, Inequality (11) may be modified into the following Inequality (14). The following Inequality (15) is derived from Inequalities (13) and (14). 
         N  sin(θ 2 +ϕ 1 )≤μ N  cos(θ 2 +ϕ 1 )  [Inequality 10]
 
         N  sin θ 01   ≤μN  cos θ 01   [Inequality 11]
 
       θ 2 +ϕ 1 ≤tan −1 μ  [Inequality 12]
 
       θ 2 +θ 1 −θ 01 ≤tan −1 μ  [Inequality 13]
 
       θ 01 ≤tan −1 μ  [Inequality 14]
 
       θ 1 +θ 2 ≤2 tan −1 μ  [Inequality 15]
 
     Inequality (9) needs to be satisfied due to a constraint on the tilt angle of the camera module  3 . Inequality (15) needs to be satisfied to prevent the sphere  46  from sliding at any of the two points of contact B 1  and C 1 . 
     If “2 tan −1 μ&lt;0.5×R/(R−r)” is satisfied, then there is no optimum condition for θ 1 +θ 2 . Therefore, the relation between the radius R, the radius r of the sphere, and the coefficient of static friction μ is represented by “2 tan −1 μ≥0.5×R/(R−r).” This inequality may be modified into the following Inequality (16) as a relational expression representing the relation between the radius R, the radius r of the sphere, and the coefficient of static friction μ. 
         R≥r× 4 tan −l ρ/(4 tan −1 μ−1)  [Inequality 16]
 
     As can be seen from the foregoing description, when the rolling friction is taken into account, the relation between the respective radii R of the spherical surfaces  503  and  451 , the radius r of the sphere, and the coefficient of static friction μ needs to satisfy Inequality (16). For example, supposing the coefficient of static friction μ is 0.1, the line L 1  shown in  FIG. 9  is obtained from Inequality (16). In that case, the range of values that the radius R may assume according to the radius r should fall within the range R 1  indicated by the oblique lines in  FIG. 9 . 
     Also, the sphere  46  is molded out of resin, and the movable-end holding member  45  and the fixed-end holding member  502  are formed out of aluminum. Therefore, the sphere  46  has different hardness from (i.e., lower hardness than) the movable-end holding member  45  and the fixed-end holding member  502 . In addition, the vertical load N has been produced in the sphere  46 . Thus, the sphere  46  is compressed by the vertical load N, and therefore, may be deformed. That is why to reduce the deformation of the sphere  46 , the relation between the respective radii R of the spherical surfaces  503  and  451  and the radius r of the sphere needs to be taken into account. 
     According to the Hertz contact theory, the maximum contact pressure in the case of point contact is given by the following Equation (17), where E 1  is the Young&#39;s modulus of the sphere  46 , E 2  is the Young&#39;s modulus of the fixed-end holding member  502  (in particular, at the spherical surface  503 ), u 1  is the Poisson ratio of the sphere  46 , and u 2  is the Poisson ratio of the fixed-end holding member  502  (in particular, at the spherical surface  503 ). 
     
       
         
           
             
               
                 
                   
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     To prevent the sphere  46  from being deformed, P max  needs to be less than the compressive strength P c . That is to say, the inequality “P max &lt;P c ” needs to be satisfied. In this case, supposing N=3 [N], P c =100 [MPa], E 1 =3000 [MPa], E 2 =68.3 [GPa], u 1 =0.38, and u 2 =0.34, the following Equation (18) is obtained based on Equation (17) and the inequality “P max &lt;P c .” 
       2.56 r &gt;(2.56− r ) R   [Inequality 18]
 
     If the radius r of the sphere  46  is greater than 2.56 [mm], then Inequality (18) may be modified into the following Inequality (19) (hereinafter referred to as “Case 1”). If the radius r of the sphere  46  is less than 2.56, then Inequality (18) may be modified into the following Inequality (20) (hereinafter referred to as “Case 2”). If the radius r of the sphere  46  is equal to 2.56, then Inequality (18) is always satisfied, no matter what value the radius R assumes (hereinafter referred to as “Case 3”). 
         R&gt; 2.56 r /(2.56− r )  [Inequality 19]
 
         R&lt; 2.56 r /(2.56− r )  [Inequality 20]
 
     The relations between the respective radii R of the spherical surfaces  503  and  451  and the radius r of the sphere, which are obtained based on these Inequalities (18) to (20), are shown in  FIG. 10 . The curve L 11  is obtained from the right side of Inequality (19). The curve L 12  is obtained from the right side of Inequality (20). The line L 13  represents Case 3. According to Inequalities (18) to (20) and these curves L 11  and L 12  and the line L 13 , the range of the values that the radii R may assume according to the radius r becomes the range R 2  indicated by the oblique lines in  FIG. 10 . 
     Furthermore, when the relation between the respective radii R of the spherical surfaces  503  and  451  and the radius r of the sphere is taken into account, the magnitude of movement of the sphere  46  needs to be taken into account. This is because if the magnitude of movement of the sphere  46  is significant, then the sphere  46  rotates by just rolling in the first mode, and therefore, is no longer controllable by electromagnetic driving or supportable with good stability. 
     Thus, in a situation where the sphere  46  has rolled on the spherical surface  503  of the fixed-end holding member  502  in the first mode as described above, the angle of movement of the sphere  46  with respect to a vertical line drawn to the center of the spherical surface  503  is supposed to be θ 01 , the tilt angle of the sphere  46  is supposed to be φ 1 , and the angle of rotation of the sphere  46  is supposed to be θ 1  (see  FIG. 11 ). In  FIG. 11 , the sphere  46  before rotating in the first mode is indicated by the two-dot chain circle and the sphere  46  that has rotated (i.e., after the first mode is over) is indicated by the solid circle. 
     The magnitude of movement of the sphere  46  that has moved from the center  460  before the first mode began (i.e., before the rotation) (i.e., the center  460  of the two-dot chain circle) to the center  460  after the rotation (i.e., the center of the solid circle) is supposed to be “c x ” with respect to the horizontal direction and “c y ” with respect to the vertical direction (see  FIG. 11 ). In that case, the magnitude of movement c x  of the center  460  of the sphere  46  with respect to the horizontal direction is given by the following Equation (21) and the magnitude of movement c y  of the center  460  of the sphere  46  with respect to the vertical direction is given by the following Equation (22): 
         c   x =( R−r )×sin θ 01   [Inequality 21]
 
         c   y =( R−r )×(1−cos θ 01 )  [Inequality 22]
 
     In this case, if the value of the coefficient of static friction μ is 0.1, then the value of θ 01  is calculated 5.71 [deg] by Inequality (14). Also, if the tolerance of the magnitude of movement of the center  460  of the sphere  46  is 0.15 [mm], then the inequalities c x &lt;0.15 and c y &lt;0.15 are satisfied. The following Inequality (23) is obtained by substituting a value of 5.71 for Ow in Equation (21), and the following Inequality (24) is obtained by substituting a value of 5.71 for θ 01  in Equation (22). 
         R≤ 1.51+ r   [Inequality 23]
 
         R≤ 30.2+ r   [Inequality 24]
 
     The relation between the respective radii R of the spherical surfaces  503  and  451  and the radius r of the sphere  46  needs to satisfy both of Inequalities (23) and (24). In that case, when Inequality (23) is satisfied, then Inequality (24) is also satisfied. 
     The relation between the respective radii R of the spherical surfaces  503  and  451  and the radius r of the sphere  46 , which is based on Inequality (23), is shown in  FIG. 12 . The line L 21  is obtained from the right side of Inequality (23). In that case, the range of the values that the radii R may assume according to the radius r should fall within the range R 3  indicated by the oblique lines in  FIG. 12 . 
     As can be seen from the foregoing description, the relation between the respective radii R of the spherical surfaces  503  and  451  and the radius r of the sphere  46  needs to be determined so as to reduce the rolling friction and the deformation of the sphere  46  and to constrain the magnitude of movement of the center  460  of the sphere  46 . Taking all of these factors into consideration, the relation between the respective radii R of the spherical surfaces  503  and  451  and the radius r of the sphere  46  needs to satisfy all of Inequalities (16), (18), and (23). If a region that satisfies all of Inequalities (16), (18), and (23) is designated by R 10 , then the region R 10  is indicated by the oblique lines in  FIG. 13 . The respective radii R of the spherical surfaces  503  and  451  and the radius r of the sphere  46  are suitably picked from the region R 10 . For example, the radius r of the sphere  46  may be 1.9 [mm] and the respective radii R of the spherical surfaces  503  and  451  may be 2.05 [mm]. 
     Note that the relation between the respective radii R of the spherical surfaces  503  and  451  and the radius r of the sphere  46  is most suitably determined with reduction of the rolling friction and the deformation of the sphere  46  and constraint on the magnitude of movement of the center  460  of the sphere  46  both taken into account. However, the scope of the present disclosure also covers a situation where at least one of these conditions is satisfied. 
     As already described in the Background Art section, in the known actuator (actuator as a comparative example), the known movable unit is supported by the known fixed unit so as to be loosely fitted into the fixed unit. Thus, in a state where the known movable unit stands still with respect to the fixed unit, the known movable unit and the known fixed unit together behave as a coupled rigid body due to the static friction produced between them. When the known movable unit is going to be rotated from this state, the stick slip occurs due to a variation in friction during the transition from the resting state to the kinetic state. Then, a saw-toothed torque pulsation caused by this stick slip excites the characteristic vibration of the rigid body that is temporarily coupled together due to the static friction. 
     In that case, the frequency will be a relatively high frequency (of 300 Hz, for example). Once the known movable unit starts its rotary motion, the coupling between the movable unit and the fixed unit due to the static friction is canceled, and thereafter, the movable unit behaves as an object with a characteristic vibration (with a frequency of 30 Hz, for example) as a single pendulum. That is to say, in the known movable unit, if a relatively low voltage is applied during the initial stage of rotation to let the movable unit start rotating smoothly, then a characteristic vibration with a high frequency would be temporarily excited due to the stick slip phenomenon to cause instability to the rotational control system during that period, and eventually produce oscillation in a worst-case scenario. To avoid such a scenario, it has been considered an effective measure to decrease the gain of the rotational control. However, this would prevent the movable unit from start or stop moving smoothly. In short, in the actuator as a comparative example, the known movable unit causes so much frictional variation during the transition from the resting state to the kinetic state that its own peculiar, characteristic vibration would be produced only during the initial stage of rotation, thus causing a decline in stability of control and posing an obstacle to the improvement of positioning performance of the rotational control. 
     On the other hand, the actuator  2  according to this embodiment sets the respective radii R of the spherical surfaces  503  and  451  at a value larger than the radius r of the sphere  46  to leave spaces  452  and  504 , thus allowing the sphere  46  to roll freely. Thus, the actuator  2  according to this embodiment reduces the stick slip phenomenon by letting the sphere move due to the rolling friction during the initial stage of the rotary motion of the movable unit  10 , and allows only the characteristic vibration (with a frequency of 30 Hz, for example) as a pendulum to be set up without exciting the characteristic vibration with a relatively high frequency as is observed in the actuator as a comparative example. That is to say, in the actuator  2  according to this embodiment, the magnitude of the frictional variation during the transition from the resting state to the kinetic state is so much smaller than the magnitude of the frictional variation in the actuator as a comparative example as to reduce the occurrence of the special characteristic vibration only during the initial stage of the rotary motion, improve the stability of control, and eventually improve the positioning performance of the rotational control. 
     This embodiment compensates for a shake of the camera module  3  by controlling the rotation of the camera module  3  by electromagnetic driving. In this case, the camera device  1  according to this embodiment determines the respective radii R of the spherical surfaces  503  and  451  and the radius r of the sphere  46  such that the sphere  46  rolling defines a tilt angle of −0.5 to 0.5 degrees with respect to the Z-axis of the camera module  3 . That is why when the tilt angle defined by the sphere  46  with respect to the Z-axis of the camera module  3  falls within the range from −0.5 to 0.5 degrees through the electromagnetic driving in the second mode, the camera device  1  is allowed to make a transition to the first mode as a mode for controlling the rotation of the camera module  3  (movable unit  10 ). Compared to the situation where control is performed only through electromagnetic driving, the camera device  1  is easily controllable at an angle which is even smaller than the smallest angle (of 0.5 degrees) at which a camera shake is sensible on the video. 
     (Variations) 
     Note that the embodiment described above is only an exemplary one of various embodiments of the present invention and should not be construed as limiting. Rather, the exemplary embodiment described above may be readily modified in various manners depending on a design choice or any other factor without departing from a true spirit and scope of the present invention. 
     In the embodiment described above, a grease pool may be provided by injecting grease into the space  452  left between the sphere  46  and the movable-end holding member  45  and the space  504  left between the sphere  46  and the fixed-end holding member  502  to let the sphere  46  roll smoothly. Note that the grease pool does not have to be provided in both of these spaces  452  and  504  but may be provided in only one of these spaces  452  and  504 . 
     Also, in the embodiment described above, the sphere  46  is not fixed to the pair of holding members (namely, the fixed-end holding member  502  and the movable-end holding member  45 ). However, this configuration is only an example and should not be construed as limiting. Alternatively, the sphere  46  may be fixed to one of the pair of holding members. 
     Furthermore, in the embodiment described above, the pair of holding members (namely, the fixed-end holding member  502  and the movable-end holding member  45 ) is configured to have a recessed spherical surface. However, this configuration is only an example and should not be construed as limiting. Alternatively, one of the pair of holding members does not have to have such a recessed spherical surface, as long as the surface is recessed. For example, the recessed surfaces may be curved surfaces with two different radii of curvature or tapered surfaces (in the shape of a mortar, for example). In that case, the sphere  46  may be fixed onto the holding member that has the recessed non-spherical surface. 
     Furthermore, in the embodiment described above, the coupling member  50  and the movable-end holding member  45  are formed out of aluminum. In particular, both of the spherical surfaces  503  and  451  with the recessed shape are subjected to alumite treatment, while the sphere  46  is molded out of resin. However, this configuration is only an example and should not be construed as limiting. Alternatively, the sphere  46  may be formed out of aluminum, of which the surface has been subjected to alumite treatment, and the coupling member  50  and the movable-end holding member  45  may be molded out of resin. In that case, a vertical load N will be produced between the sphere  46  and the pair of holding members (namely, the movable-end holding member  45  and the fixed-end holding member  502 ), and the pair of holding members will be compressed under the vertical load N, thus possibly deforming the pair of holding members. Thus, to reduce the deformation of the pair of holding members, the relation between the respective radii R of the spherical surfaces  503  and  451  and the radius r of the sphere  46  needs to be considered. In that case, the relation between the radii R and the radius r of the sphere  46  is the same as expressed by Inequality (18). Note that not both of the pair of holding members (namely, the movable-end holding member  45  and the fixed-end holding member  502 ) have to be molded out of resin, but at least one of the pair of holding members may be molded out of resin. 
     Furthermore, the actuator  2  according to the embodiment described above is applied to the camera device  1 . However, this configuration is only an example and should not be construed as limiting. Alternatively, the actuator  2  is also applicable for use in a laser pointer, a haptic device, or any other appropriate device. For example, when the actuator  2  is applied to a laser pointer, a module for emitting a laser beam is provided for the movable unit  10 . When the actuator  2  is provided for a haptic device, a lever is provided for the movable unit  10 . 
     (Resume) 
     As can be seen from the foregoing description, an actuator ( 2 ) according to a first aspect includes: a movable unit ( 10 ) configured to hold an object to be driven; a fixed unit ( 20 ) configured to support the movable unit ( 10 ) thereon to make the movable unit ( 10 ) rotatable; and a structure for supporting the movable unit ( 10 ) with respect to the fixed unit ( 20 ). The structure includes: a sphere ( 46 ); and a pair of holding members (namely, a fixed-end holding member  502  and a movable-end holding member  45 ) configured to clamp the sphere ( 46 ) between themselves. A space is left to let the sphere ( 46 ) roll while shifting a center position thereof with respect to at least one of the pair of holding members. 
     This configuration leaves a space that lets the sphere ( 46 ) roll with respect to at least one of the pair of holding members, thus allowing the sphere ( 46 ) to move freely. This allows the movable unit ( 10 ) to be supported like a balancing toy. Therefore, this actuator ( 2 ) reduces a variation in the friction when the movable unit ( 10 ) starts moving, thus reducing the stick slip and the self-excited vibration caused by the stick slip and stabilizing the rotational control. Consequently, this allows the movable unit ( 10 ) to start and stop moving smoothly. 
     In an actuator ( 2 ) according to a second aspect, which may be implemented in conjunction with the first aspect, the sphere ( 46 ) is not fixed to any of the pair of holding members. 
     This configuration reduces the difference between the static frictional force and kinetic frictional force in the movable unit ( 10 ). This allows the movable unit ( 10 ) to start rotating smoothly during the initial stage of its rotary motion. 
     In an actuator ( 2 ) according to a third aspect, which may be implemented in conjunction with the first or second aspect, at least one of two contact surfaces between the pair of holding members and the sphere ( 46 ) is a recessed spherical surface (the spherical surface  503  or the spherical surface  451 ). 
     According to this configuration, making at least one of the two contact surfaces between the pair of holding members and the sphere ( 46 ) a recessed spherical surface allows the movable unit ( 10 ) to rotate smoothly. 
     In an actuator ( 2 ) according to a fourth aspect, which may be implemented in conjunction with the first or second aspect, both of two contact surfaces between the pair of holding members and the sphere ( 46 ) are recessed spherical surfaces. 
     According to this configuration, making both of the two contact surfaces between the pair of holding members and the sphere ( 46 ) recessed spherical surfaces allows the movable unit ( 10 ) to rotate even more smoothly. 
     In an actuator ( 2 ) according to a fifth aspect, which may be implemented in conjunction with the third or fourth aspect, the contact surface between at least one of the pair of holding members and the sphere ( 46 ) is the recessed spherical surface having a radius (R) larger than the radius (r) of the sphere ( 46 ). 
     This configuration allows a space that lets the sphere ( 46 ) roll while shifting its center position to be left with reliability when the pair of holding members holds the sphere ( 46 ). 
     In an actuator ( 2 ) according to a sixth aspect, which may be implemented in conjunction with the fifth aspect, the radius (R) of the recessed spherical surface of the holding member is larger than the product of the radius (r) of the sphere ( 46 ) and (4×tan −1  (coefficient of static friction of the spherical surface)/(4× tan −1  (coefficient of static friction of the spherical surface)−1). 
     This configuration allows the radius (R) of the recessed spherical surface of the holding member and the radius of the sphere ( 46 ) to be determined with the rolling friction taken into consideration. 
     In an actuator ( 2 ) according to a seventh aspect, which may be implemented in conjunction with the sixth aspect, the movable unit ( 10 ) is configured to rotate by electromagnetic driving. The radius (R) of the recessed spherical surface of the holding member is defined so as to prevent pushing force applied to the sphere ( 46 ) by magnetic force for use to control rotation of the movable unit by electromagnetic driving from deforming the sphere ( 46 ) or at least one of the pair of holding members. 
     This configuration allows the radius of the recessed spherical surface of the holding member and the radius of the sphere ( 46 ) to be defined so as to reduce the deformation of the sphere ( 46 ) or at least one of the pair of holding members. 
     In an actuator ( 2 ) according to an eighth aspect, which may be implemented in conjunction with the seventh aspect, the radius (R) of the recessed spherical surface of the holding member is defined such that magnitude of movement of a center of the sphere ( 46 ) is equal to or less than a prescribed value. 
     This configuration allows the radius of the recessed spherical surface of the holding member and the radius of the sphere ( 46 ) to be defined with the magnitude of movement of the sphere ( 46 ) taken into account. 
     In an actuator ( 2 ) according to a ninth aspect, which may be implemented in conjunction with any one of the first to eighth aspects, a grease pool is provided for the space. 
     This configuration allows the sphere ( 46 ) to roll even more smoothly. 
     A camera device according to a tenth aspect includes: the actuator ( 2 ) according to any one of the first to ninth aspects; and a camera module ( 3 ) serving as the object to be driven. 
     This configuration allows the camera device ( 1 ) to reduce a variation in the friction when the movable unit ( 10 ) starts moving, thus reducing the self-excited vibration caused by the stick slip and stabilizing the rotational control. Consequently, this allows the movable unit ( 10 ) to start and stop moving smoothly. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  Camera Device 
               2  Actuator 
               3  Camera Module 
               10  Movable Unit 
               20  Fixed Unit 
               45  Movable-End Holding Member 
               46  Sphere 
               451 ,  503  Spherical Surface 
               452 ,  504  Space 
               502  Fixed-End Holding Member