Abstract:
When a stepping motor is used as a driving source for a shutter device, if the stepping motor loses synchronization because of variations in load during driving or the like, it becomes unable to rotate a driving ring at that time, and this disables an exposure operation. In a first zone where a driven member is driven, but a light shielding member remains in a closed state or an open state, the motor drives the driven member in open-loop driving mode. In a second zone where the driven member is driven, and thus the light shielding member moves to the closed state or the open state, the motor drives the driven member in feed-back driving mode.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 14/538,055 filed Nov. 11, 2014, which claims the benefit of International Patent Application No. PCT/JP2013/080757, filed Nov. 14, 2013, all of which are hereby incorporated by reference herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a shutter device and an image pickup apparatus including the shutter device. 
     BACKGROUND ART 
     Patent Literature 1 discloses a shutter device in which two shutter blades are made to open or close an opening portion by a stepping motor rotating a driving ring. 
     The shutter device disclosed in Patent Literature 1 has an acceleration region where the driving ring is rotated, but the two shutter blades do not open or close the opening portion and an exposure region where the two shutter blades are made to open or close the opening portion by rotation of the driving ring. In the shutter device disclosed in Patent Literature 1, after the stepping motor is accelerated in the acceleration region, the two shutter blades open or close the opening portion in the exposure region. 
     CITATION LIST 
     Patent Literature 
     PTL 1 Japanese Patent Laid-Open No. 7-56211 
     For the shutter device disclosed in Patent Literature 1, in the exposure region, a load for moving the two shutter blades may cause the stepping motor to lose synchronization. 
     That is, when the stepping motor is used as a driving source for the stepping motor, if the stepping motor loses synchronization because of variations in load during driving, it becomes unable to rotate the driving ring at that time, and this disables an exposure operation. 
     It is an object of the present invention to provide a shutter device in which, when a stepping motor drives a driven member and thus a light blocking member moves from a closed state to an open state or from the open state to the closed state, the stepping motor does not lose synchronization. 
     SUMMARY OF INVENTION 
     A shutter device according to an aspect of the present invention includes a stepping motor, a driven member, and a light shielding member. The stepping motor is configured to be driven in open-loop driving mode at which an energization state of a coil is switched at predetermined time intervals and in feed-back driving mode at which the energization state of the coil is switched in accordance with a rotation position of a rotor. The driven member is configured to be driven by the stepping motor. The light shielding member is configured to move to a closed state in which an aperture is closed and to an open state in which the aperture is open in coordination with driving the driven member. The driven member is configured to be driven in a first zone where the driven member is driven by the stepping motor, but the light shielding member remains in the closed state or the open state and in a second zone where the driven member is driven by the stepping motor, and thus the light shielding member moves from the closed state to the open state or from the open state to the closed state. The driven member is driven in the first zone by the stepping motor in one direction, and after the driven member is driven in the first zone, the driven member is driven in the second zone. In a case where the driven member is driven in the first zone, the stepping motor drives the driven member in the open-loop driving mode. In a case where the driven member is driven in the second zone, the stepping motor drives the driven member in the feed-back driving mode. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are illustrations for describing a shutter unit  20  as a shutter according to one embodiment of the present invention. 
         FIG. 2  is an illustration of a first rotor plate  107  (second rotor plate  117 ) as seen from a back surface side. 
         FIGS. 3A and 3B  are illustrations for describing a state of the shutter unit  20  in A status. 
         FIGS. 4A and 4B  are illustrations for describing a state of the shutter unit  20  in B status. 
         FIGS. 5A and 5B  are illustrations for describing a state of the shutter unit  20  in C status. 
         FIGS. 6A and 6B  are illustrations for describing a state of the shutter unit  20  in D status. 
         FIGS. 7A and 7B  are illustrations for describing a state of the shutter unit  20  in E status. 
         FIGS. 8A and 8B  are illustrations for describing a state of the shutter unit  20  in F status. 
         FIGS. 9A and 9B  are illustrations for describing a state of the shutter unit  20  in G status. 
         FIGS. 10A and 10B  are illustrations for describing a state of the shutter unit  20  in H status. 
         FIGS. 11A and 11B  are illustrations for describing a state of the shutter unit  20  in I status. 
         FIG. 12  is a timing chart for describing operations of the shutter unit  20  when a camera  100  is operating in continuous shooting mode. 
         FIG. 13  is a timing chart for describing operations of the shutter unit  20  when the camera  100  is operating in continuous shooting mode as a variation of the embodiment. 
         FIG. 14  illustrates a motor  1  used as each of a first motor Ma and a second motor Mb. 
         FIGS. 15A to 15I  are illustrations for describing operations of the motor. 
         FIGS. 16A to 16D  are illustrations for describing positions in which a first magnetic sensor  8 , a second magnetic sensor  9 , a third magnetic sensor  10 , and a fourth magnetic sensor  11  are arranged. 
         FIG. 17  is a central sectional view of the digital single-lens reflex camera body  100  as an image pickup apparatus according to one embodiment of the present invention and an interchangeable lens  50 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention are described in detail below with reference to the drawings. 
       FIG. 17  is a central sectional view of a digital single-lens reflex camera body (hereinafter referred to as camera)  100  according to one embodiment of the present invention and an interchangeable lens  50  as an image pickup apparatus. 
     The interchangeable lens  50  is detachably fixed on the camera  100  with a mount section  210  in the camera  100  and a mount section  51  in the interchangeable lens  50 . When the interchangeable lens  50  is attached to the camera  100 , a contact section  220  in the camera  100  and a contact section  52  in the interchangeable lens  50  are electrically connected to each other. 
     A light flux that has passed through focus lenses  53  in the interchangeable lens  50  enters a main mirror  130  in the camera  100 . The main mirror  130  is held on a main mirror holding frame  131  and is supported by a rotating shaft section  131   a  so as to be able to pivot between a mirror upper position and a mirror lower position. 
     The main mirror  130  is a semitransparent mirror. A light flux that has passed through the main mirror  130  is reflected downward by a sub mirror  140  and is guided to a focus detecting unit  150 . 
     The sub mirror  140  is held on a sub mirror holding frame  141 . The sub mirror holding frame  141  is supported by a hinge shaft (not illustrated) so as to be able to pivot with respect to the main mirror holding frame  131 . 
     The focus detecting unit  150  is configured to detect the amount of defocusing of the focus lenses  53  and calculate the amount of driving of the focus lenses  53  for achieving focus for the focus lenses  53 . The interchangeable lens  50  is configured to receive the calculated amount of driving through the contact sections  220  and  52 . The interchangeable lens  50  is configured to adjust the focus by controlling a motor (not illustrated) and driving the focus lenses  53  on the basis of the received amount of driving. 
     A light flux reflected by the main mirror  130  is guided to an optical viewfinder  160 . The optical viewfinder  160  includes a focusing plate  170 , a pentaprism  180 , and an eyepiece  190 . The light flux guided to the optical viewfinder  160  forms an object image on the focusing plate  170 . A user can observe the object image on the focusing plate  170  through the pentaprism  180  and the eyepiece  190 . 
     A shutter unit  20  is arranged behind the sub mirror  140 . An optical low-pass filter  21 , an image pickup element holder  22 , an image pickup element  23 , a cover member  24 , and a rubber member  25  are arranged behind the shutter unit  20 . In shooting, a light flux that has passed through the optical low-pass filter  21  enters the image pickup element  23 . The image pickup element holder  22  is fixed to the housing of the camera  100  with a screw (not illustrated). The image pickup element  23  is held by the image pickup element holder  22 . The cover member  24  protects the image pickup element  23 . The rubber member  25  holds the optical low-pass filter  21  and hermetically seals the gap between the optical low-pass filter  21  and the image pickup element  23 . 
     A display monitor  26  may be a liquid crystal display monitor and is configured to display a shot image and display various setting statuses of the camera  100 . 
       FIGS. 1A and 1B  are illustrations for describing the shutter unit  20  as a shutter according to one embodiment of the present invention.  FIG. 1A  is an exploded perspective view for describing a configuration of the shutter unit  20 .  FIG. 1B  is an exploded perspective view illustrating the shutter unit  20  further disassembled from the state illustrated in  FIG. 1A . 
     As illustrated in  FIG. 1A , the shutter unit  20  is driven by a first motor Ma and a second motor Mb. The first motor Ma is connected to a driving circuit  14   a . The second motor Mb is connected to a driving circuit  14   b . The driving circuit  14   a  and the driving circuit  14   b  are connected to a control circuit  13 . In the present embodiment, the first motor Ma and the second motor Mb are the same motors. A pinion gear  101  is press-fit to the output shaft of the first motor Ma. A pinion gear  111  is press-fit to the output shaft of the second motor Mb. 
     The first motor Ma is mounted to a motor mounting plate  102 . The motor mounting plate  102  is fixed to a cover plate  103 . The second motor Mb is mounted to a motor mounting plate  112 . The motor mounting plate  112  is fixed to a cover plate  113 . 
     A driving mechanism accommodating section  104  accommodates a first rotor plate  107  to which a weight  106  is bonded and a second rotor plate  117  to which a weight  116  is bonded. The first rotor plate  107  includes a protruding section  107   a . When the cover plate  103  is mounted on the driving mechanism accommodating section  104 , the protruding section  107   a  is exposed through the cover plate  103 . The second rotor plate  117  includes a protruding section  117   a . When the cover plate  113  is mounted on the driving mechanism accommodating section  104 , the protruding section  117   a  is exposed through the cover plate  113 . A first spring  108  is mounted to the cover plate  103 . A second spring  118  is mounted to the cover plate  113 . 
     The first rotor plate  107  includes a gear section  107   b . When the motor mounting plate  102  is fixed on the cover plate  103 , the pinion gear  101  and the gear section  107   b  engage with each other. The second rotor plate  117  includes a gear section  117   b . When the motor mounting plate  112  is fixed on the cover plate  113 , the pinion gear  111  and the gear section  117   b  engage each other. 
     Accordingly, when the first motor Ma is driven, the first rotor plate  107  rotates; when the second motor Mb is driven, the second rotor plate  117  rotates. 
     A blade accommodating section  105  has an aperture  105   a . The blade accommodating section  105  accommodates a first blade  110  and a second blade  120 . 
     As illustrated in  FIG. 1B , a driving arm  110   a  is mounted to the first blade  110 . A driving arm  120   a  is mounted to the second blade  120 . 
     A first driving lever  109  and a second driving lever  119  are supported on the driving mechanism accommodating section  104 . The first driving lever  109  includes a cam pin  109   a  and an engagement pin  109   b . The cam pin  109   a  engages with a cam groove  107   c  in the first rotor plate  107 . The engagement pin  109   b  engages with the driving arm  110   a . When the first driving lever  109  pivots, the first blade  110  opens or closes the aperture  105   a . Similarly, the second driving lever  119  includes a cam pin  119   a  and an engagement pin  119   b . The cam pin  119   a  engages with a cam groove  117   c  in the second rotor plate  117 . The engagement pin  119   b  engages with the driving arm  120   a . When the second driving lever  119  pivots, the second blade  120  opens or closes the aperture  105   a . In the present embodiment, the first driving lever  109  and the second driving lever  119  are the same components. 
     The driving mechanism accommodating section  104  includes a shaft section  104   a  and a shaft section  104   b . The first rotor plate  107  is supported by the shaft section  104   a . The second rotor plate  117  is supported by the shaft section  104   b . The first rotor plate  107  includes the gear section  107   b  on its front surface. The weight  106  is bonded and fixed to the circumferential section of the first rotor plate  107 . The first rotor plate  107  includes the cam groove  107   c , with which the cam pin  109   a  engages, in its back surface. 
     Similarly, the second rotor plate  117  includes the gear section  117   b  on its front surface. The weight  116  is bonded and fixed to the circumferential section of the second rotor plate  117 . The second rotor plate  117  includes the cam groove  117   c , with which the cam pin  119   a  engages, in its back surface. In the present embodiment, the first rotor plate  107  and the second rotor plate  117  are the same components. The weight  106  and the weight  116  are the same components. 
     Each of the first rotor plate  107  and the second rotor plate  117  functions as a driven member. The first blade  110  and the first driving lever  109  function as a light shielding member capable of moving between a closed state where they closes the aperture  105   a  and an open state where they opens the aperture  105   a  in coordination with driving the first rotor plate  107 . The second blade  120  and the second driving lever  119  function as a light shielding member capable of moving between a closed state where they closes the aperture  105   a  and an open state where they opens the aperture  105   a  in coordination with driving the second rotor plate  117 . Each of the first spring  108  and the second spring  118  functions as an urging member. 
       FIG. 2  is an illustration of the first rotor plate  107  (second rotor plate  117 ) as seen from the back surface side. The cam groove  107   c  (cam groove  117   c ), with which the cam pin  109   a  (cam pin  119   a ) engages, are disposed in the back surface of the first rotor plate  107  (second rotor plate  117 ). As illustrated in  FIG. 2 , a first idle running driving region A, an exposure driving region B, and a second idle running driving region C are set in the cam groove  107   c  (cam groove  117   c ). In the first idle running driving region A and the second idle running driving region C in the cam groove  107   c  (cam groove  117   c ), the cam lift is substantially zero. 
     When the cam pin  109   a  (cam pin  119   a ) follows the first idle running driving region A or the second idle running driving region C, the first driving lever  109  (second driving lever  119 ) does not rotate and the first blade  110  (second blade  120 ) remains in a closed state or an open state. 
     When the cam pin  109   a  (cam pin  119   a ) follows the exposure driving region B, the first driving lever  109  (second driving lever  119 ) rotates and the first blade  110  (second blade  120 ) moves from the closed state to the open state or from the open state to the closed state. 
     When the first rotor plate  107  (second rotor plate  117 ) rotates clockwise, the cam pin  109   a  (cam pin  119   a ) follows the first idle running driving region A, the exposure driving region B, and the second idle running driving region C in this order. 
     The details of the clockwise rotation of the first rotor plate  107  (second rotor plate  117 ) are described below. 
     The first idle running driving region A is a first cam region. The zone where the cam pin  109   a  (cam pin  119   a ) follows the first idle running driving region A is a first zone. 
     The exposure driving region B is a second cam region. The zone where the cam pin  109   a  (cam pin  119   a ) follows the exposure driving region B is a second zone. 
     The second idle running driving region C is a third cam region. The zone where the cam pin  109   a  (cam pin  119   a ) follows the second idle running driving region C is a third zone. 
     In contrast, when the first rotor plate  107  (second rotor plate  117 ) rotates counterclockwise, the cam pin  109   a  (cam pin  119   a ) follows the second idle running driving region C, the exposure driving region B, and the first idle running driving region A in this order. 
     The details of the counterclockwise rotation of the first rotor plate  107  (second rotor plate  117 ) are described below. 
     The second idle running driving region C is the first cam region. The zone where the cam pin  109   a  (cam pin  119   a ) follows the second idle running driving region C is the first zone. 
     The exposure driving region B is the second cam region. The zone where the cam pin  109   a  (cam pin  119   a ) follows the exposure driving region B is the second zone. 
     The first idle running driving region A is the third cam region. The zone where the cam pin  109   a  (cam pin  119   a ) follows the first idle running driving region A is the third zone. 
     That is, the first rotor plate  107  (second rotor plate  117 ) is driven in one direction, and thus the first rotor plate  107  (second rotor plate  117 ) is driven in the first zone. After the first rotor plate  107  (second rotor plate  117 ) is driven in the first zone, the first rotor plate  107  (second rotor plate  117 ) is driven in the second zone. 
     As illustrated in  FIG. 1B , the cover plate  103  is provided with a hollow shaft section  103   a . When the cover plate  103  is mounted on the driving mechanism accommodating section  104 , the protruding section  107   a  in the first rotor plate  107  is exposed through the cover plate  103  and the shaft section  104   a  is fit into an inner section of the hollow shaft section  103   a . The first spring  108  is mounted on an outer section of the hollow shaft section  103   a.    
     Similarly, the cover plate  113  is provided with a hollow shaft section  113   a . When the cover plate  113  is mounted on the driving mechanism accommodating section  104 , the protruding section  117   a  in the second rotor plate  117  is exposed through the cover plate  113  and the shaft section  104   b  is fit into an inner section of the hollow shaft section  113   a . The second spring  118  is mounted on an outer section of the hollow shaft section  113   a.    
     When the motor mounting plate  102  with the first motor Ma mounted thereon is mounted on the cover plate  103 , the output shaft of the first motor Ma penetrates through an opening in the cover plate  103 , and the pinion gear  101  and the gear section  107   b  engage with each other. Similarly, when the motor mounting plate  112  with the second motor Mb mounted thereon is mounted on the cover plate  113 , the output shaft of the second motor Mb penetrates through an opening in cover plate  113 , and the pinion gear  111  and the gear section  117   b  engage with each other. 
     In the present embodiment, the first motor Ma, the first rotor plate  107 , the first spring  108 , the first driving lever  109 , and the first blade  110  constitute a first shutter driving mechanism. The second motor Mb, the second rotor plate  117 , the second spring  118 , the second driving lever  119 , and the second blade  120  constitute a second shutter driving mechanism. 
     Each of the first motor Ma and the second motor Mb is a stepping motor that can be driven in step-driving (open-loop driving) at which an energization state of the coil is switched at predetermined time intervals and in two types of feed-back driving with different advance angle values. To drive the first motor Ma and the second motor Mb in the step driving mode (open-loop driving mode), the energization state of the coil is switched at predetermined time intervals. To drive the first motor Ma and the second motor Mb in the feed-back driving mode, the energization state of the coil is switched in accordance with an output of a positional sensor. 
     The detailed configuration of each of the first motor Ma and the second motor Mb is described below. 
       FIG. 12  is a timing chart for describing operations of the shutter unit  20  when the camera  100  is operating in continuous shooting mode.  FIGS. 3 to 11  are illustrations for describing the states of the shutter unit  20  in A to I statuses illustrated in  FIG. 12 . 
     The shutter unit  20  according to the present embodiment performs a first-frame shooting operation from the A status to H status illustrated in  FIG. 12 . In the first-frame shooting operation, the first shutter driving mechanism functions as a leading blade, and the second shutter driving mechanism functions as a trailing blade. The shutter unit  20  according to the present embodiment performs a second-frame shooting operation from the H status to I status illustrated in  FIG. 12 . In the second-frame shooting operation, the second shutter driving mechanism functions as the leading blade, and the second shutter driving mechanism functions as the trailing blade. In a third-frame shooting operation, the first shutter driving mechanism functions as the leading blade, and the second shutter driving mechanism functions as the trailing blade. 
     When the camera  100  starts a shooting operation, it is in A status illustrated in  FIG. 12 .  FIGS. 3A and 3B  are illustrations for describing a state of the shutter unit  20  in A status.  FIG. 3A  is an illustration for describing the state of the first shutter driving mechanism.  FIG. 3B  is an illustration for describing the state of the second shutter driving mechanism. 
     As illustrated in  FIG. 3A , in A status, the first blade  110  closes the aperture  105   a . In the state illustrated in  FIG. 3A , the protruding section  107   a  in the first rotor plate  107  is in contact with the left arm section of the first spring  108 . However, in this state, the first spring  108  is not charged and is in its natural state. 
     As illustrated in  FIG. 3B , in A status, the second blade  120  opens the aperture  105   a . At this time, the protruding section  117   a  in the second rotor plate  117  is in contact with the right arm section of the second spring  118 . However, in this state, the second spring  118  is not charged and is in its natural state. 
     As illustrated in  FIG. 12 , in A status, the control circuit  13  controls the driving circuit  14   a  such that the first motor Ma is driven clockwise in feed-back driving mode with low advance angle. In A status, the control circuit  13  controls the driving circuit  14   b  such that the second motor Mb is not driven in any direction. Thus the shutter unit  20  moves to the B status illustrated in  FIG. 12 . 
       FIGS. 4A and 4B  are illustrations for describing a state of the shutter unit  20  in B status.  FIG. 4A  is an illustration for describing the state of the first shutter driving mechanism.  FIG. 4B  is an illustration for describing the state of the second shutter driving mechanism. 
     As illustrated in  FIG. 4A , in B status, the first blade  110  closes the aperture  105   a . As illustrated in  FIG. 12 , in the period from the A status to B status, the first motor Ma is driven clockwise in feed-back driving mode with low advance angle. Thus the first rotor plate  107  rotates counterclockwise from the state illustrated in  FIG. 3A . Here, because the pinion gear  101  in the first motor Ma and the gear section  107   b  in the first rotor plate  107  engage with each other, the rotation direction of the first motor Ma and that of the first rotor plate  107  are opposite. 
     When the first rotor plate  107  rotates counterclockwise from the state illustrated in  FIG. 3A  (A status), the first rotor plate  107  rotates while charging the first spring  108 . In this period, the first rotor plate  107  rotates counterclockwise while charging the first spring  108 , and thus variations in load during the driving of the first motor Ma are large. However, because the first motor Ma is driven in feed-back driving mode with low advance angle, the first motor Ma does not lose synchronization. 
     In the state illustrated in  FIG. 4A  (B status), because the first spring  108  is charged, the first rotor plate  107  is urged in a clockwise direction by the first spring  108 . 
     When the first rotor plate  107  rotates counterclockwise from the state illustrated in  FIG. 3A  (A status), the cam pin  109   a  in the first driving lever  109  follows the first idle running driving region A in the cam groove  107   c  in this period. Accordingly, the position of the first driving lever  109  in the state illustrated in  FIG. 4A  (B status) is substantially the same as the position of the first driving lever  109  in the state illustrated in  FIG. 3A  (A status). 
     The B status of the second shutter driving mechanism illustrated in  FIG. 4B  is the same as the A status of the second shutter driving mechanism illustrated in  FIG. 3B . When the state moves from the A status to B status, the second motor Mb is not driven, and thus the second rotor plate  117  remains unchanged from the state illustrated in  FIG. 3B  (A status). 
     As illustrated in  FIG. 12 , in B status, the control circuit  13  controls the driving circuit  14   a  such that the first motor Ma is driven counterclockwise in step driving mode. In B status, the control circuit  13  controls the driving circuit  14   b  such that the second motor Mb is driven clockwise in feed-back driving mode with low advance angle. Thus the shutter unit  20  moves to the C status illustrated in  FIG. 12 . That is, in the present embodiment, the start of driving for an approach run in the second shutter driving mechanism lags behind the start of driving for an approach run in the first shutter driving mechanism by an exposure time t 1 . 
     The first shutter driving mechanism starts driving for an approach run in step driving mode in B status. In driving for the approach run, the control circuit  13  gradually increases the rotational speed of the first motor Ma by gradually reducing the width of a driving pulse. In driving for the approach run, the cam pin  109   a  follows the first idle running driving region A in the cam groove  107   c , where the cum lift is substantially zero. Accordingly, in this period, because the first driving lever  109  does not virtually rotate even when the first rotor plate  107  is driven, variations in load during the driving of the first motor Ma are small. Thus when the first motor Ma is driven in step driving mode, the first motor Ma does not lose synchronization. 
       FIGS. 5A and 5B  are illustrations for describing a state of the shutter unit  20  in C status.  FIG. 5A  is an illustration for describing the state of the first shutter driving mechanism.  FIG. 5B  is an illustration for describing the state of the second shutter driving mechanism. 
     As illustrated in  FIG. 5A , in C status, the first blade  110  closes the aperture  105   a . Because the first motor Ma is driven counterclockwise in the period from the B status to C status, the first rotor plate  107  is rotated clockwise by a combined force of the driving force of the first motor Ma and the urging force of the first spring  108 . The urging force of the first spring  108  is provided to the first rotor plate  107  up to the C status illustrated in  FIG. 5A . 
     When the first rotor plate  107  rotates clockwise from the state illustrated in  FIG. 4A  (B status), the cam pin  109   a  in the first driving lever  109  follows the first idle running driving region A in the cam groove  107   c  in this period. Accordingly, the position of the first driving lever  109  in the state illustrated in  FIG. 5A  (C status) is substantially the same as the position of the first driving lever  109  in the state illustrated in  FIG. 4A  (B status). 
     As illustrated in  FIG. 5B , in C status, the second blade  120  opens the aperture  105   a . In the period from the B status to C status, because the second motor Mb is driven clockwise in feed-back driving mode with low advance angle, the second rotor plate  117  rotates counterclockwise from the state illustrated in  FIG. 4B . Here, because the pinion gear  111  in the second motor Mb and the gear section  117   b  in the second rotor plate  117  engage with each other, the rotation direction of the second motor Mb and that of the second rotor plate  117  are opposite. 
     When the second rotor plate  117  rotates clockwise from the state illustrated in  FIG. 4B  (B status), the second rotor plate  117  rotates while charging the second spring  118 . In this period, the second rotor plate  117  rotates clockwise while charging the second spring  118 , and thus variations in load during the driving of the second motor Mb are large. However, because the second motor Mb is driven in feed-back driving mode with low advance angle, the second motor Mb does not lose synchronization. 
     In the state illustrated in  FIG. 5B  (C status), because the second spring  118  is charged, the second rotor plate  117  is urged in a clockwise direction by the second spring  118 . 
     When the second rotor plate  117  rotates clockwise from the state illustrated in  FIG. 4B  (B status), the cam pin  119   a  in the second driving lever  119  also follows the first idle running driving region A in the cam groove  117   c  in this period. Accordingly, the position of the second driving lever  119  in the state illustrated in  FIG. 5B  (C status) is substantially the same as the position of the second driving lever  119  in the state illustrated in  FIG. 4B  (B status). 
     As illustrated in  FIG. 12 , in C status, the control circuit  13  also controls the driving circuit  14   a  such that the first motor Ma is driven counterclockwise in step driving mode. In C status, the control circuit  13  controls the driving circuit  14   b  such that the second motor Mb is driven counterclockwise in step driving mode. Thus the shutter unit  20  moves to the D status illustrated in  FIG. 12 . The second shutter driving mechanism starts driving for an approach run in step driving mode in C status. In driving for the approach run, the control circuit  13  gradually increases the rotational speed of the second motor Mb by gradually reducing the width of a driving pulse. In driving for the approach run, the cam pin  119   a  follows the first idle running driving region A in the cam groove  117   c , where the cum lift is substantially zero. Thus when the second motor Mb is driven in step driving mode, the second motor Mb does not lose synchronization. 
       FIGS. 6A and 6B  are illustrations for describing a state of the shutter unit  20  in D status.  FIG. 6A  is an illustration for describing the state of the first shutter driving mechanism.  FIG. 6B  is an illustration for describing the state of the second shutter driving mechanism. 
     As illustrated in  FIG. 6A , the D status is a state immediately before the first blade  110  starts opening the aperture  105   a . Because the first motor Ma is driven counterclockwise in the period from the C status to D status, the first rotor plate  107  is rotated clockwise by the driving force of the first motor Ma. 
     When the first rotor plate  107  rotates clockwise from the state illustrated in  FIG. 5A  (C status), the cam pin  109   a  in the first driving lever  109  follows the first idle running driving region A in the cam groove  107   c  in this period. Accordingly, the position of the first driving lever  109  in the state illustrated in  FIG. 6A  (D status) is substantially the same as the position of the first driving lever  109  in the state illustrated in  FIG. 5A  (C status). 
     As illustrated in  FIG. 6B , in D status, the second blade  120  opens the aperture  105   a . In the period from the C status to a state before the D status, because the second motor Mb is driven counterclockwise, the second rotor plate  117  is rotated clockwise by a combined force of the driving force of the second motor Mb and the urging force of the second spring  118 . The urging force of the second spring  118  is provided to the second rotor plate  117  up to the state before the D status illustrated in  FIG. 6B . That is, in D status illustrated in  FIG. 6B , the urging force of the second spring  118  is not provided to the second rotor plate  117 , and the second rotor plate  117  is rotated clockwise by only the driving force of the second motor Mb. 
     When the second rotor plate  117  rotates clockwise from the state illustrated in  FIG. 5B  (C status), the cam pin  119   a  in the second driving lever  119  also follows the first idle running driving region A in the cam groove  117   c  in this period. Accordingly, the position of the second driving lever  119  in the state illustrated in  FIG. 6B  (D status) is substantially the same as the position of the second driving lever  119  in the state illustrated in  FIG. 5B  (C status). 
     As illustrated in  FIG. 12 , in D status, the control circuit  13  controls the driving circuit  14   a  such that the first motor Ma is driven counterclockwise in feed-back driving mode with high advance angle. In D status, the control circuit  13  also controls the driving circuit  14   b  such that the second motor Mb is driven counterclockwise in step driving mode. Thus the shutter unit  20  moves to the E status illustrated in  FIG. 12 . The first shutter driving mechanism starts driving for exposure in feed-back driving mode with high advance angle in D status. 
       FIGS. 7A and 7B  are illustrations for describing a state of the shutter unit  20  in E status.  FIG. 7A  is an illustration for describing the state of the first shutter driving mechanism.  FIG. 7B  is an illustration for describing the state of the second shutter driving mechanism. 
     As illustrated in  FIG. 7A , in E status, the first blade  110  opens the aperture  105   a . Because the first motor Ma is driven counterclockwise in the period from the D status to E status, the first rotor plate  107  is rotated clockwise by the driving force of the first motor Ma. 
     When the first rotor plate  107  rotates clockwise from the state illustrated in  FIG. 6A  (D status), the cam pin  109   a  in the first driving lever  109  follows the exposure driving region B in the cam groove  107   c  in this period. This causes the first blade  110  to open the closed aperture  105   a . Accordingly, in exposure driving, it is necessary to drive the first motor Ma at high speeds, and this leads to large variations in load during the driving of the first motor Ma. At this time, because the first motor Ma is driven in feed-back driving mode with high advance angle, the high-speed driving and the load variations do not cause the first motor Ma to lose synchronization. Because the rotation speed of the first motor Ma is sufficiently high due to the driving for the approach run, the first motor Ma can be driven in feed-back driving mode with high advance angle. 
     As illustrated in  FIG. 7B , the E status is a state immediately before the second blade  120  starts closing the aperture  105   a . In the period from the D status to E status, because the second motor Mb is driven counterclockwise, the second rotor plate  117  is rotated clockwise by the driving force of the second motor Mb. 
     When the second rotor plate  117  rotates clockwise from the state illustrated in  FIG. 6B  (D status), the cam pin  119   a  in the second driving lever  119  follows the first idle running driving region A in the cam groove  117   c  in this period. Accordingly, the position of the second driving lever  119  in the state illustrated in  FIG. 7B  (E status) is substantially the same as the position of the second driving lever  119  in the state illustrated in  FIG. 6B  (D status). 
     As illustrated in  FIG. 12 , in E status, the control circuit  13  controls the driving circuit  14   a  such that the first motor Ma is driven counterclockwise in feed-back driving mode with high advance angle. In E status, the control circuit  13  also controls the driving circuit  14   b  such that the second motor Mb is driven counterclockwise in feed-back driving mode with high advance angle. Thus the shutter unit  20  moves to the F status illustrated in  FIG. 12 . The second shutter driving mechanism starts driving for exposure in feed-back driving mode with high advance angle in E status. 
       FIGS. 8A and 8B  are illustrations for describing a state of the shutter unit  20  in F status.  FIG. 8A  is an illustration for describing the state of the first shutter driving mechanism.  FIG. 8B  is an illustration for describing the state of the second shutter driving mechanism. 
     As illustrated in  FIG. 8A , in F status, the first blade  110  opens the aperture  105   a . Because the first motor Ma is driven counterclockwise in the period from the D status to E status, the first rotor plate  107  is rotated clockwise by the driving force of the first motor Ma. 
     When the first rotor plate  107  rotates clockwise from the state illustrated in  FIG. 7A  (E status), the cam pin  109   a  in the first driving lever  109  follows the second idle running driving region C in the cam groove  107   c  in this period. Accordingly, the position of the first driving lever  109  in the state illustrated in  FIG. 8A  (F status) is substantially the same as the position of the first driving lever  109  in the state illustrated in  FIG. 7A  (E status). 
     As illustrated in  FIG. 8B , in F status, the second blade  120  closes the aperture  105   a . In the period from the E status to F status, because the second motor Mb is driven counterclockwise, the second rotor plate  117  is rotated clockwise by the driving force of the second motor Mb. 
     When the second rotor plate  117  rotates clockwise from the state illustrated in  FIG. 7B  (E status), the cam pin  119   a  in the second driving lever  119  follows the exposure driving region B in the cam groove  117   c  in this period. This causes the second blade  120  to close the opened aperture  105   a . Accordingly, in exposure driving, it is necessary to drive the second motor Mb at high speeds, and this leads to large variations in load during the driving of the second motor Mb. At this time, because the second motor Mb is driven in feed-back driving mode with high advance angle, the high-speed driving and the load variations do not cause the second motor Mb to lose synchronization. Because the rotation speed of the second motor Mb is sufficiently high due to the driving for the approach run, the second motor Mb can be driven in feed-back driving mode with high advance angle. 
     As illustrated in  FIG. 12 , in F status, the control circuit  13  controls the driving circuit  14   a  such that the first motor Ma is driven counterclockwise in feed-back driving mode with high advance angle. In F status, the control circuit  13  also controls the driving circuit  14   b  such that the second motor Mb is driven counterclockwise in feed-back driving mode with high advance angle. Thus the shutter unit  20  moves to the G status illustrated in  FIG. 12 . 
       FIGS. 9A and 9B  are illustrations for describing a state of the shutter unit  20  in G status.  FIG. 9A  is an illustration for describing the state of the first shutter driving mechanism.  FIG. 9B  is an illustration for describing the state of the second shutter driving mechanism. 
     As illustrated in  FIG. 9A , in G status, the first blade  110  opens the aperture  105   a . The first motor Ma is driven counterclockwise in the period from the F status to G status. In the period from the F status to G status, the protruding section  107   a  in the first rotor plate  107  is in contact with the right arm section of the first spring  108 , and the first rotor plate  107  rotates clockwise while charging the first spring  108 . That is, the first spring  108  acts to apply a break to the clockwise rotation of the first rotor plate  107 . In the state illustrated in  FIG. 9A , the first spring  108  is charged, and the first rotor plate  107  is urged in a counterclockwise direction by the first spring  108 . 
     When the first rotor plate  107  rotates clockwise from the state illustrated in  FIG. 8A  (F status), the cam pin  109   a  in the first driving lever  109  follows the second idle running driving region C in the cam groove  107   c  in this period. Accordingly, the position of the first driving lever  109  in the state illustrated in  FIG. 9A  (G status) is substantially the same as the position of the first driving lever  109  in the state illustrated in  FIG. 8A  (F status). In this period, the first rotor plate  107  rotates clockwise while charging the first spring  108 , and thus variations in load during the driving of the first motor Ma are large. However, because the first motor Ma is driven in feed-back driving mode with high advance angle, the first motor Ma does not lose synchronization. 
     As illustrated in  FIG. 9B , in G status, the second blade  120  closes the aperture  105   a . In the period from the F status to G status, because the second motor Mb is driven counterclockwise, the second rotor plate  117  is rotated clockwise by the driving force of the second motor Mb. In the state illustrated in  FIG. 9B , the protruding section  117   a  in the second rotor plate  117  is in contact with the left arm section of the second spring  118 . However, in this state, the second spring  118  is not charged and is in its natural state. 
     When the second rotor plate  117  rotates clockwise from the state illustrated in  FIG. 8B  (F status), the cam pin  119   a  in the second driving lever  119  also follows the second idle running driving region C in the cam groove  117   c  in this period. Accordingly, the position of the second driving lever  119  in the state illustrated in  FIG. 9B  (G status) is substantially the same as the position of the second driving lever  119  in the state illustrated in  FIG. 8B  (F status). 
     As illustrated in  FIG. 12 , in G status, the control circuit  13  controls the driving circuit  14   a  such that current supply to the first motor Ma is held. Here, holding the current supply indicates maintaining the phase of the current supply to the coil of the first motor Ma in G status. In G status, the control circuit  13  also controls the driving circuit  14   b  such that the second motor Mb is driven counterclockwise in feed-back driving mode with high advance angle. Thus the shutter unit  20  moves to the H status illustrated in  FIG. 12 . 
       FIGS. 10A and 10B  are illustrations for describing a state of the shutter unit  20  in H status.  FIG. 10A  is an illustration for describing the state of the first shutter driving mechanism.  FIG. 10B  is an illustration for describing the state of the second shutter driving mechanism. 
     As illustrated in  FIG. 10A , in H status, the first blade  110  opens the aperture  105   a . Because the current supply to the first motor Ma is held in G status, the first motor Ma and the first rotor plate  107  remain in G status. That is, the state illustrated in  FIG. 10A  (H status) is the same as the state illustrated in  FIG. 9A  (G status). 
     As illustrated in  FIG. 10B , in H status, the second blade  120  closes the aperture  105   a . In the period from the G status to H status, the second motor Mb is driven counterclockwise. In the period from the G status to H status, the protruding section  117   a  in the second rotor plate  117  is in contact with the left arm section of the second spring  118 , and the second rotor plate  117  rotates clockwise while charging the second spring  118 . That is, the second spring  118  acts to apply a break to the clockwise rotation of the second rotor plate  117 . In the state illustrated in  FIG. 10B , the second spring  118  is charged, and the second rotor plate  117  is urged in a counterclockwise direction by the second spring  118 . 
     When the second rotor plate  117  rotates clockwise from the state illustrated in  FIG. 9B  (G status), the cam pin  119   a  in the second driving lever  119  follows the second idle running driving region C in the cam groove  117   c  in this period. Accordingly, the position of the second driving lever  119  in the state illustrated in  FIG. 10B  (H status) is substantially the same as the position of the second driving lever  119  in the state illustrated in  FIG. 9B  (G status). In this period, the second rotor plate  117  rotates clockwise while charging the second spring  118 , and thus variations in load during the driving of the second motor Mb are large. However, because the second motor Mb is driven in feed-back driving mode with high advance angle, the second motor Mb does not lose synchronization. 
     As described above, the shutter unit  20  according to the present embodiment performs the first-frame shooting operation from the A status to H status illustrated in  FIG. 12 . In the first-frame shooting operation, the first shutter driving mechanism functions as the leading blade, and the second shutter driving mechanism functions as the trailing blade. In the second-frame shooting operation, the second shutter driving mechanism functions as the leading blade, and the first shutter driving mechanism functions as the trailing blade. That is, in the first-frame shooting operation, the first shutter driving mechanism performs an exposure operation ahead of the second shutter driving mechanism. In the second-frame shooting operation, the second shutter driving mechanism performs an exposure operation ahead of the first shutter driving mechanism. 
     In the present embodiment, the start of driving for an approach run in the first shutter driving mechanism is caused to lag behind the start of driving for an approach run in the second shutter driving mechanism by an exposure time t 2  for the second frame by adjustment of the period of time for which the current supply to the first motor Ma is held. 
     As illustrated in  FIG. 12 , in H status, the control circuit  13  controls the driving circuit  14   a  such that the current supply to the first motor Ma is held. In H status, the control circuit  13  also controls the driving circuit  14   b  such that the second motor Mb is driven clockwise in step driving mode. Thus the second rotor plate  117  is rotated counterclockwise by the driving force of the second motor Mb and the urging force of the second spring  118 . The second shutter driving mechanism starts driving for an approach run in step driving mode in H status. Thus the shutter unit  20  moves to the G′ status illustrated in  FIG. 12 . 
     The state of the shutter unit  20  in G′ status illustrated in  FIG. 12  is the same as the state illustrated in  FIGS. 9A and 9B . 
     As illustrated in  FIG. 12 , in G′ status, the control circuit  13  controls the driving circuit  14   a  such that the first motor Ma is driven clockwise in step driving mode. Thus the first rotor plate  107  is rotated counterclockwise by the driving force of the first motor Ma and the urging force of the first spring  108 . The first shutter driving mechanism starts driving for an approach run in step driving mode in G′ status. In G′ status, the control circuit  13  controls the driving circuit  14   b  such that the second motor Mb is driven clockwise in step driving mode. Thus the shutter unit  20  moves to the F′ status illustrated in  FIG. 12 . 
     The state of the shutter unit  20  in F′ status illustrated in  FIG. 12  is the same as the state illustrated in  FIGS. 8A and 8B . 
     As illustrated in  FIG. 12 , in F′ status, the control circuit  13  controls the driving circuit  14   a  such that the first motor Ma is driven clockwise in step driving mode. In F′ status, the control circuit  13  controls the driving circuit  14   b  such that the second motor Mb is driven clockwise in feed-back driving mode with high advance angle. Thus the shutter unit  20  moves to the E′ status illustrated in  FIG. 12 . The second shutter driving mechanism starts driving for exposure in feed-back driving mode with high advance angle in F′ status. Because the rotation speed of the second motor Mb is sufficiently high due to the driving for the approach run, the second motor Mb can be driven in feed-back driving mode with high advance angle. 
     The state of the shutter unit  20  in F′ status illustrated in  FIG. 12  is the same as the state illustrated in  FIG. 8 . 
     As illustrated in  FIG. 12 , in E′ status, the control circuit  13  controls the driving circuit  14   a  such that the first motor Ma is driven clockwise in feed-back driving mode with high advance angle. In E′ status, the control circuit  13  controls the driving circuit  14   b  such that the second motor Mb is driven clockwise in feed-back driving mode with high advance angle. Thus the shutter unit  20  moves to the D′ status illustrated in  FIG. 12 . The first shutter driving mechanism starts driving for exposure in feed-back driving mode with high advance angle in E′ status. Because the rotation speed of the first motor Ma is sufficiently high due to the driving for the approach run, the first motor Ma can be driven in feed-back driving mode with high advance angle. 
     The state of the shutter unit  20  in E′ status illustrated in  FIG. 12  is the same as the state illustrated in  FIGS. 7A and 7B . 
     As illustrated in  FIG. 12 , in D′ status, the control circuit  13  controls the driving circuit  14   a  such that the first motor Ma is driven clockwise in feed-back driving mode with high advance angle. In D′ status, the control circuit  13  controls the driving circuit  14   b  such that the second motor Mb is driven clockwise in feed-back driving mode with high advance angle. Thus the shutter unit  20  moves to the C′ status illustrated in  FIG. 12 . 
     The state of the shutter unit  20  in C′ status illustrated in  FIG. 12  is the same as the state illustrated in  FIGS. 6A and 6B . 
     As illustrated in  FIG. 12 , in C′ status, the control circuit  13  controls the driving circuit  14   a  such that the first motor Ma is driven clockwise in feed-back driving mode with high advance angle. In C′ status, the control circuit  13  controls the driving circuit  14   b  such that the current supply to the second motor Mb is held. Here, holding the current supply indicates maintaining the phase of the current supply to the second motor Mb in D status. Thus the shutter unit  20  moves to the I status illustrated in  FIG. 12 . 
       FIGS. 11A and 11B  are illustrations for describing a state of the shutter unit  20  in I status.  FIG. 11A  is an illustration for describing the state of the first shutter driving mechanism.  FIG. 11B  is an illustration for describing the state of the second shutter driving mechanism. 
     As illustrated in  FIG. 11A , in I status, the first blade  110  closes the aperture  105   a . As illustrated in FIG.  12 , because the first motor Ma is driven clockwise in the period from the C′ status to I status, the first rotor plate  107  is rotated counterclockwise from the state illustrated in  FIG. 5A . In the period from the C′ status to I status, the protruding section  107   a  in the first rotor plate  107  is in contact with the left arm section of the first spring  108 , and the first rotor plate  107  rotates counterclockwise while charging the first spring  108 . That is, the first spring  108  acts to apply a break to the counterclockwise rotation of the first rotor plate  107 . In the state illustrated in  FIG. 11A , the first spring  108  is charged, and the first rotor plate  107  is urged in a clockwise direction by the first spring  108 . 
     As illustrated in  FIG. 11B , in I status, the second blade  120  opens the aperture  105   a . Because the current supply to the second motor Mb is held in C′ status, the second motor Mb and the second rotor plate  117  remain in C′ status. That is, the state illustrated in  FIG. 5B  is the same as the state illustrated in  FIG. 11B . 
     As described above, the shutter unit  20  according to the present embodiment performs the second-frame shooting operation from the H status to I status illustrated in  FIG. 12 . In the second-frame shooting operation, the second shutter driving mechanism functions as the leading blade, and the first shutter driving mechanism functions as the trailing blade. In the third-frame shooting operation, the first shutter driving mechanism functions as the leading blade, and the second shutter driving mechanism functions as the trailing blade. In the present embodiment, the start of driving for an approach run in the second shutter driving mechanism is caused to lag behind the start of driving for an approach run in the first shutter driving mechanism by an exposure time t 3  for the third frame by adjustment of the period of time for which the current supply to the second motor Mb is held. 
     As illustrated in  FIG. 12 , in I status, the control circuit  13  controls the driving circuit  14   a  such that the first motor Ma is driven counterclockwise in step driving mode. The control circuit  13  controls the driving circuit  14   b  such that the current supply to the second motor Mb is held. Thus the shutter unit  20  moves to the C status illustrated in  FIG. 12 . 
     After that, the same shooting operation as that for the first frame is performed. 
     (Variation) 
       FIG. 13  is a timing chart for describing operations of the shutter unit  20  when the camera  100  is operating in continuous shooting mode as a variation of the present embodiment. 
     In the above-described embodiment, a lag between the leading blade and the trailing blade is produced by making the timing for starting the driving for the approach run in the shutter driving mechanism functioning as the leading blade and the timing for starting the driving for the approach run in the shutter driving mechanism functioning as the trailing blade different. 
     In contrast, in the variation, a lag between the leading blade and the trailing blade is produced by making a pulse rate for the driving for the approach run in the shutter driving mechanism functioning as the leading blade and a pulse rate for the driving for the approach run in the shutter driving mechanism functioning as the trailing blade different. That is, the pulse rate for the driving for the approach run in the shutter driving mechanism functioning as the leading blade is set at a value larger than the pulse rate for the driving for the approach run in the shutter driving mechanism functioning as the trailing blade. Thus even in the same approach run period, the time required for the driving for the approach run in the shutter driving mechanism functioning as the trailing blade is longer than the time required for the driving for the approach run in the shutter driving mechanism functioning as the leading blade. 
     In the variation, in A status illustrated in  FIG. 13 , the control circuit  13  controls the driving circuit  14   a  such that the first motor Ma is driven clockwise in feed-back driving mode with low advance angle. In A status illustrated in  FIG. 13 , the control circuit  13  controls the driving circuit  14   b  such that the second motor Mb is driven clockwise in feed-back driving mode with low advance angle. Thus the shutter unit  20  moves to the I status illustrated in  FIG. 13 . 
     In I status illustrated in  FIG. 13 , the control circuit  13  controls the driving circuit  14   a  such that the first motor Ma is driven counterclockwise in step driving mode. In I status illustrated in  FIG. 13 , the control circuit  13  controls the driving circuit  14   b  such that the second motor Mb is driven counterclockwise in step driving mode. Thus the shutter unit  20  moves to the D status illustrated in  FIG. 13 . 
     The state from the D status to G status illustrated in  FIG. 13  is the same as that from the D status to G status illustrated in  FIG. 12  in the embodiment described above. 
     In G status illustrated in  FIG. 13 , the control circuit  13  also controls the driving circuit  14   a  such that the first motor Ma is driven counterclockwise in feed-back driving mode with high advance angle. In D status illustrated in  FIG. 13 , the control circuit  13  also controls the driving circuit  14   b  such that the second motor Mb is driven counterclockwise in feed-back driving mode with high advance angle. 
     In the above-described embodiment, in G status illustrated in  FIG. 12 , the control circuit  13  controls the driving circuit  14   a  such that the current supply to the first motor Ma is held. In the variation, the control circuit  13  controls the driving circuit  14   a  such that the first motor Ma is driven counterclockwise in feed-back driving mode with high advance angle. Accordingly, although the first rotor plate  107  tries to rotate clockwise, because the protruding section  107   a  in the first rotor plate  107  comes into contact with the stopper on the cover plate  103 , the clockwise rotation of the first rotor plate  107  is blocked. 
     The characteristics in the variation are substantially the same as those in the above-described embodiment, except for the method of producing a lag between the leading blade and the trailing blade and the respect in which holding the current supply is not performed. 
     Next, the details of the first motor Ma and the second motor Mb are described with reference to  FIGS. 14 to 16 . 
       FIG. 14  illustrates a motor  1  used as each of the first motor Ma and the second motor Mb. For the sake of the description, parts of some components are removed in the illustration. 
     As illustrated in  FIG. 14 , a rotor  3  includes a magnet  2  and is rotatably controlled by the control circuit (controller)  13  and the driving circuit  14 . The magnet  2  is cylindrical, has a circumferential surface divided in its circumferential direction, and is multipole-magnetized in different poles in an alternatingly manner. In the present embodiment, the magnet  2  is divided in eight elements, that is, magnetized in eight poles. The number of divisions is not limited to eight. The magnet  2  may be magnetized in four or twelve poles. 
     A first coil  4  is arranged on a first end of the magnet  2  in its axial direction. 
     A first yoke  6  is made of a soft magnetic material and is opposed to the circumferential surface of the magnet  2  such that a gap is present therebetween. The first yoke  6  axially extends from an annular main body portion and includes a plurality of first magnetic pole sections  6   a  arranged at predetermined intervals in its circumferential direction. The first magnetic pole sections  6   a  are excited by energization of the first coil  4 . 
     The first coil  4 , the first yoke  6 , and the magnet  2  opposed to the plurality of first magnetic pole sections  6   a  constitute a first stator unit. 
     A second coil  5  is arranged on a second end of the magnet  2  in its axial direction, and the second end is opposite to the first end on which the first coil  4  is arranged. 
     A second yoke  7  is made of a soft magnetic material and is opposed to the circumferential surface of the magnet  2  such that a gap is present therebetween. The second yoke  7  axially extends from the annular main body portion and includes a plurality of second magnetic pole sections  7   a  arranged at predetermined intervals in its circumferential direction. The second magnetic pole sections  7   a  are excited by energization of the second coil  5 . 
     The second coil  5 , the second yoke  7 , and the magnet  2  opposed to the plurality of second magnetic pole sections  7   a  constitute a second stator unit. 
     A torque provided to the rotor  3  can be changed by switching the magnetized polarity (north pole, south pole) of each of the first magnetic pole sections  6   a  and the second magnetic pole sections  7   a.    
     A first magnetic sensor (first detecting element)  8 , a second magnetic sensor (second detecting element)  9 , a third magnetic sensor (third detecting element)  10 , and a fourth magnetic sensor (fourth detecting element)  11  constitute detecting means. Each of the magnetic sensors is a Hall element configured to detect a magnetic flux of the magnet  2  and is fixed to a motor cover  12 . 
     The motor cover  12  fixes and retains the first yoke  6  and the second yoke  7  such that the first magnetic pole sections  6   a  and the second magnetic pole sections  7   a  are displaced with respect to a magnetization phase of the magnet  2  by approximately 90 degrees in electrical angle. 
     Here, the electrical angle is an angle represented based on the assumption that one cycle of the magnetic force of the magnet is 360°. The electrical angle θ can be expressed by the following equation:
 
θ=θ0× M/ 2
 
where M is the number of poles of the rotor, and the mechanical angle is θ0.
 
     In the present embodiment, the magnet  2  is magnetized in eight poles, and 90 degrees in electrical angle is 22.5 degrees in mechanical angle. 
     The control circuit  13  can switch the driving among the step driving and the two kinds of feed-back driving with different amounts of the advance angle. In step driving, the control circuit  13  controls the driving circuit  14  such that the energization state of the first coil  4  and the second coil  5  is switched at predetermined time intervals. That is, in step driving, none of outputs of the first magnetic sensor  8 , the second magnetic sensor  9 , the third magnetic sensor  10 , and the fourth magnetic sensor  11  are used. 
     A case where the control circuit  13  performs the feed-back driving is described below. When the control circuit  13  performs the two kinds of feed-back driving, outputs of the first magnetic sensor  8 , the second magnetic sensor  9 , the third magnetic sensor  10 , and the fourth magnetic sensor  11  are used. 
     In the present embodiment, even in switching the energization direction, a large rotational driving force is obtainable by arranging each magnetic sensor in a positional relationship with respect to each yoke described below. 
       FIGS. 15A to 15I  are illustrations for describing operations of the motor  1 . Actual operations of the motor  1  are described with reference to  FIGS. 15A to 15I . The state in  FIG. 15A  is described as an initial state in driving. 
     (1) Clockwise Driving 
     (1-i) Low Advance Angle Driving (First Energization Mode) 
     The clockwise driving mode with low advance angle is described. The driving mode with low advance angle can achieve larger torque than that in the driving mode with high advance angle described below. 
     In the clockwise driving mode with low advance angle, the rotor  3  is rotated clockwise by switching excitation of each of the first magnetic pole sections  6   a  in response to an output signal of the first magnetic sensor  8  and switching excitation of each of the second magnetic pole sections  7   a  in response to an output signal of the second magnetic sensor  9 . The direction of the clockwise rotation of the rotor  3  corresponds to a first rotation direction. 
     In this driving mode, the energization direction of each of the first coil  4  and the second coil  5  is switched using combinations described below. 
     When the first magnetic sensor  8  detects the south pole of the magnet  2  (switching from the north pole to south pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the first magnetic pole section  6   a  is magnetized with the north pole. When the first magnetic sensor  8  detects the north pole of the magnet  2  (switching from the south pole to north pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the first magnetic pole section  6   a  is magnetized with the south pole. 
     When the second magnetic sensor  9  detects the south pole of the magnet  2  (switching from the north pole to south pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the second magnetic pole section  7   a  is magnetized with the south pole. When the second magnetic sensor  9  detects the north pole of the magnet  2  (switching from the south pole to north pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the second magnetic pole section  7   a  is magnetized with the north pole. 
     In the state illustrated in  FIG. 15A , both the first magnetic sensor  8  and the second magnetic sensor  9  detect the south pole of the magnet  2 . At this time, the control circuit  13  controls the driving circuit  14  such that the first magnetic pole section  6   a  is magnetized with the north pole and the second magnetic pole section  7   a  is magnetized with the south pole. This produces a clockwise rotation force in the rotor  3  and the magnet  2 . 
     When the rotor  3  rotates clockwise from the state illustrated in  FIG. 15A , the center Q 1  of each of the south poles of the magnet  2  and the center of the corresponding first magnetic pole section  6   a  are opposed to each other, as illustrated in  FIG. 15B . 
     When the rotor  3  rotates clockwise from the state illustrated in  FIG. 15B , the distance between the center Q 1  of the south pole of the magnet  2  and the first magnetic pole section  6   a  is the same as the distance between the center Q 2  of each of the north poles of the magnet  2  and the corresponding second magnetic pole section  7   a , as illustrated in  FIG. 15C . 
     The first magnetic sensor  8  is arranged such that when the magnetized polarity of the first magnetic pole section  6   a  is switched on the basis of the output of the first magnetic sensor  8 , the excitation switching timing for the first magnetic pole section  6   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between angle 0 degree to 45 degrees. 
     The first magnetic sensor  8  detects the north pole of the magnet  2  (switching from the south pole to north pole) between the state illustrated in  FIG. 15B  and the state illustrated in  FIG. 15C . At this time, the driving circuit  14  energizes the first coil  4  such that the first magnetic pole section  6   a  is magnetized with the south pole. Because the second magnetic sensor  9  detects the south pole of the magnet  2  between the state illustrated in  FIG. 15B  and the state illustrated in  FIG. 15C , the driving circuit  14  energizes the second coil  5  such that the second magnetic pole section  7   a  is magnetized with the south pole. This produces the clockwise rotation force in the rotor  3  and the magnet  2 . 
     When the rotor  3  rotates clockwise from the state illustrated in  FIG. 15C , the center Q 2  of the north pole of the magnet  2  and the center of the second magnetic pole section  7   a  are opposed to each other, as illustrated in  FIG. 15D . 
     When the rotor  3  rotates clockwise from the state illustrated in  FIG. 15D , the distance between the center Q 2  of the north pole of the magnet  2  and the first magnetic pole section  6   a  is the same as the distance between the center Q 2  of the north pole of the magnet  2  and the second magnetic pole section  7   a , as illustrated in  FIG. 15E . 
     The second magnetic sensor  9  is arranged such that when the magnetized polarity of the second magnetic pole section  7   a  is switched on the basis of the output of the second magnetic sensor  9 , the excitation switching timing for the second magnetic pole section  7   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between 0 degree to 45 degrees. 
     The second magnetic sensor  9  detects the north pole of the magnet  2  (switching from the south pole to north pole) between the state illustrated in  FIG. 15D  and the state illustrated in  FIG. 15E . At this time, the driving circuit  14  energizes the second coil  5  such that the second magnetic pole section  7   a  is magnetized with the north pole. Because the first magnetic sensor  8  detects the north pole of the magnet  2  between the state illustrated in  FIG. 15D  and the state illustrated in  FIG. 15E , the driving circuit  14  energizes the first coil  4  such that the first magnetic pole section  6   a  is magnetized with the south pole. This produces the clockwise rotation force in the rotor  3  and the magnet  2 . 
     As described above, in the clockwise driving mode with low advance angle, the energization of the first coil  4  and the second coil  5  is sequentially switched by the outputs of the first magnetic sensor  8  and the second magnetic sensor  9 , and the rotor  3  and the magnet  2  rotate in a clockwise direction. 
     When the rotor  3  rotates clockwise and the magnetized polarity of the first magnetic pole section  6   a  is switched on the basis of the output of the first magnetic sensor  8 , the excitation switching timing for the first magnetic pole section  6   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between angle 0 degree to 45 degrees. That is, the first magnetic sensor  8  is arranged in a position where the amount of the advance angle from the position of the electrical advance angle 0 degree from the excitation switching timing at the first magnetic pole section  6   a  is smaller than the amount of the lag angle from the position of the electrical advance angle 90 degrees from the excitation switching timing at the first magnetic pole section  6   a.    
     When the rotor  3  rotates clockwise and the magnetized polarity of the second magnetic pole section  7   a  is switched on the basis of the output of the second magnetic sensor  9 , the excitation switching timing for the second magnetic pole section  7   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between angle 0 degree to 45 degrees. That is, the second magnetic sensor  9  is arranged in a position where the amount of the advance angle from the position of the electrical advance angle 0 degree from the excitation switching timing at the second magnetic pole section  7   a  is smaller than the amount of the lag angle from the position of the electrical advance angle 90 degrees from the excitation switching timing at the second magnetic pole section  7   a.    
     (1-ii) High Advance Angle Driving (Second Energization Mode) 
     The clockwise driving mode with high advance angle is described. The driving mode with high advance angle can achieve higher speed rotation than that in the above-described driving mode with low advance angle. 
     In the clockwise driving mode with high advance angle, the rotor  3  is rotated clockwise by switching the magnetized polarity of the first magnetic pole section  6   a  in response to the output of the third magnetic sensor  10  and switching the magnetized polarity of the second magnetic pole section  7   a  in response to the output of the fourth magnetic sensor  11 . 
     In this driving mode, the energization direction of each of the first coil  4  and the second coil  5  is switched using combinations described below. 
     When the third magnetic sensor  10  detects the south pole of the magnet  2  (switching from the north pole to south pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the first magnetic pole section  6   a  is magnetized with the north pole. When the third magnetic sensor  10  detects the north pole of the magnet  2  (switching from the south pole to north pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the first magnetic pole section  6   a  is magnetized with the south pole. 
     When the fourth magnetic sensor  11  detects the south pole of the magnet  2  (switching from the north pole to south pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the second magnetic pole section  7   a  is magnetized with the south pole. When the fourth magnetic sensor  11  detects the north pole of the magnet  2  (switching from the south pole to north pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the second magnetic pole section  7   a  is magnetized with the north pole. 
     In the state illustrated in  FIG. 15A , both the third magnetic sensor  10  and the fourth magnetic sensor  11  detect the south pole of the magnet  2 . Accordingly, when the first magnetic pole section  6   a  is magnetized with the north pole and the second magnetic pole section  7   a  is magnetized with the south pole, a clockwise rotation force is produced in the rotor  3  and the magnet  2 . 
     When the rotor  3  rotates clockwise from the state illustrated in  FIG. 15A , the center Q 1  of each of the south poles of the magnet  2  and the center of the corresponding first magnetic pole section  6   a  are opposed to each other, as illustrated in  FIG. 15B . 
     The third magnetic sensor  10  is arranged such that when the magnetized polarity of the first magnetic pole section  6   a  is switched on the basis of the output of the third magnetic sensor  10 , the excitation switching timing for the first magnetic pole section  6   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between angle 45 degrees to 90 degrees. 
     The third magnetic sensor  10  detects the north pole of the magnet  2  (switching from the south pole to north pole) between the state illustrated in  FIG. 15A  and the state illustrated in  FIG. 15B . At this time, the driving circuit  14  energizes the first coil  4  such that the first magnetic pole section  6   a  is magnetized with the south pole. Because the fourth magnetic sensor  11  detects the south pole of the magnet  2  between the state illustrated in  FIG. 15A  and the state illustrated in  FIG. 15B , the driving circuit  14  energizes the second coil  5  such that the second magnetic pole section  7   a  is magnetized with the south pole. This produces the clockwise rotation force in the rotor  3  and the magnet  2 . 
     When the rotor  3  rotates clockwise from the state illustrated in  FIG. 15B , the state moves to the state illustrated in  FIG. 15C , and then the center Q 2  of the north pole of the magnet  2  and the center of the second magnetic pole section  7   a  are opposed to each other, as illustrated in  FIG. 15D . 
     The fourth magnetic sensor  11  is arranged such that when the magnetized polarity of the second magnetic pole section  7   a  is switched on the basis of the output of the fourth magnetic sensor  11 , the excitation switching timing for the second magnetic pole section  7   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between 45 degrees to 90 degrees. 
     The fourth magnetic sensor  11  detects the north pole of the magnet  2  (switching from the south pole to north pole) between the state illustrated in  FIG. 15C  and the state illustrated in  FIG. 15D . At this time, the driving circuit  14  energizes the second coil  5  such that the second magnetic pole section  7   a  is magnetized with the north pole. Because the third magnetic sensor  10  detects the north pole of the magnet  2  between the state illustrated in  FIG. 15C  and the state illustrated in  FIG. 15D , the driving circuit  14  energizes the first coil  4  such that the first magnetic pole section  6   a  is magnetized with the south pole. This produces the clockwise rotation force in the rotor  3  and the magnet  2 . 
     As described above, in the clockwise driving mode with high advance angle, the energization of the first coil  4  and the second coil  5  is sequentially switched by the outputs of the third magnetic sensor  10  and the fourth magnetic sensor  11 , and the rotor  3  and the magnet  2  rotate in a clockwise direction. 
     When the rotor  3  rotates clockwise and the magnetized polarity of the first magnetic pole section  6   a  is switched on the basis of the output of the third magnetic sensor  10 , the excitation switching timing for the first magnetic pole section  6   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between angle 45 degrees to 90 degrees. That is, the third magnetic sensor  10  is arranged in a position where the amount of the advance angle from the position of the electrical advance angle 0 degree from the excitation switching timing at the first magnetic pole section  6   a  is larger than the amount of the lag angle from the position of the electrical advance angle 90 degrees from the excitation switching timing at the first magnetic pole section  6   a.    
     When the rotor  3  rotates clockwise and the magnetized polarity of the second magnetic pole section  7   a  is switched on the basis of the output of the fourth magnetic sensor  11 , the excitation switching timing for the second magnetic pole section  7   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between angle 45 degrees to 90 degrees. That is, the fourth magnetic sensor  11  is arranged in a position where the amount of the advance angle from the position of the electrical advance angle 0 degree from the excitation switching timing at the second magnetic pole section  7   a  is larger than the amount of the lag angle from the position of the electrical advance angle 90 degrees from the excitation switching timing at the second magnetic pole section  7   a.    
     (2) Counterclockwise Driving 
     (2-i) Low Advance Angle Driving (Third Energization Mode) 
     The counterclockwise driving mode with low advance angle is described. Even for the counterclockwise rotation, the driving mode with low advance angle can achieve larger torque than that in the driving mode with high advance angle. 
     In the counterclockwise driving mode with low advance angle, the rotor  3  is rotated counterclockwise by switching excitation of each of the first magnetic pole sections  6   a  in response to an output signal of the third magnetic sensor  10  and switching excitation of each of the second magnetic pole sections  7   a  in response to an output signal of the fourth magnetic sensor  11 . The direction of the counterclockwise rotation of the rotor  3  corresponds to a second rotation direction opposite to the first rotation direction. 
     In this driving mode, the energization direction of each of the first coil  4  and the second coil  5  is switched using combinations described below. 
     When the third magnetic sensor  10  detects the south pole of the magnet  2  (switching from the north pole to south pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the first magnetic pole section  6   a  is magnetized with the south pole. When the third magnetic sensor  10  detects the north pole of the magnet  2  (switching from the south pole to north pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the first magnetic pole section  6   a  is magnetized with the north pole. 
     When the fourth magnetic sensor  11  detects the south pole of the magnet  2  (switching from the north pole to south pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the second magnetic pole section  7   a  is magnetized with the north pole. When the fourth magnetic sensor  11  detects the north pole of the magnet  2  (switching from the south pole to north pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the second magnetic pole section  7   a  is magnetized with the south pole. 
     In the state illustrated in  FIG. 15A , both the third magnetic sensor  10  and the fourth magnetic sensor  11  detect the south pole of the magnet  2 . At this time, the control circuit  13  controls the driving circuit  14  such that the first magnetic pole section  6   a  is magnetized with the south pole and the second magnetic pole section  7   a  is magnetized with the north pole. This produces a counterclockwise rotation force in the rotor  3  and the magnet  2 . 
     When the rotor  3  rotates counterclockwise from the state illustrated in  FIG. 15A , the center Q 1  of the south pole of the magnet  2  and the center of the second magnetic pole section  7   a  are opposed to each other, as illustrated in  FIG. 15F . 
     When the rotor  3  rotates counterclockwise from the state illustrated in  FIG. 15F , the distance between the center Q 1  of the south pole of the magnet  2  and the second magnetic pole section  7   a  is the same as the distance between the center Q 3  of the north pole of the magnet  2  and the first magnetic pole section  6   a , as illustrated in  FIG. 15G . 
     The fourth magnetic sensor  11  is arranged such that when the magnetized polarity of the second magnetic pole section  7   a  is switched on the basis of the output of the fourth magnetic sensor  11 , the excitation switching timing for the second magnetic pole section  7   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between 0 degree to 45 degrees. 
     The north pole of the magnet  2  (switching from the south pole to north pole) is detected between the state illustrated in  FIG. 15F  and the state illustrated in  FIG. 15G . At this time, the driving circuit  14  energizes the second coil  5  such that the second magnetic pole section  7   a  is magnetized with the south pole. Because the third magnetic sensor  10  detects the south pole of the magnet  2  between the state illustrated in  FIG. 15F  and the state illustrated in  FIG. 15G , the driving circuit  14  energizes the first coil  4  such that the first magnetic pole section  6   a  is magnetized with the south pole. This produces the counterclockwise rotation force in the rotor  3  and the magnet  2 . 
     When the rotor  3  rotates counterclockwise from the state illustrated in  FIG. 15G , the center Q 3  of the north pole of the magnet  2  and the center of the first magnetic pole section  6   a  are opposed to each other, as illustrated in  FIG. 15H . 
     When the rotor  3  rotates counterclockwise from the state illustrated in  FIG. 15H , the distance between the center Q 3  of the north pole of the magnet  2  and the first magnetic pole section  6   a  is the same as the distance between the center Q 3  of the north pole of the magnet  2  and the second magnetic pole section  7   a , as illustrated in  FIG. 15I . 
     The third magnetic sensor  10  is arranged such that when the magnetized polarity of the first magnetic pole section  6   a  is switched on the basis of the output of the third magnetic sensor  10 , the excitation switching timing for the first magnetic pole section  6   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between angle 0 degree to 45 degrees. 
     The third magnetic sensor  10  detects the north pole of the magnet  2  (switching from the south pole to north pole) between the state illustrated in  FIG. 15H  and the state illustrated in  FIG. 15I . At this time, the driving circuit  14  energizes the first coil  4  such that the first magnetic pole section  6   a  is magnetized with the north pole. Because the fourth magnetic sensor  11  detects the north pole of the magnet  2  between the state illustrated in  FIG. 15H  and the state illustrated in  FIG. 15I , the driving circuit  14  energizes the second coil  5  such that the second magnetic pole section  7   a  is magnetized with the south pole. This produces the counterclockwise rotation force in the rotor  3  and the magnet  2 . 
     As described above, in the counterclockwise driving mode with low advance angle, the energization of the first coil  4  and the second coil  5  is sequentially switched by the outputs of the third magnetic sensor  10  and the fourth magnetic sensor  11 , and the rotor  3  and the magnet  2  rotate in a counterclockwise direction. 
     When the rotor  3  rotates counterclockwise and the magnetized polarity of the first magnetic pole section  6   a  is switched on the basis of the output of the third magnetic sensor  10 , the excitation switching timing for the first magnetic pole section  6   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between angle 0 degree to 45 degrees. 
     When the rotor  3  rotates counterclockwise and the magnetized polarity of the second magnetic pole section  7   a  is switched on the basis of the output of the fourth magnetic sensor  11 , the excitation switching timing for the second magnetic pole section  7   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between angle 0 degree to 45 degrees. 
     (2-ii) High Advance Angle Driving (Fourth Energization Mode) 
     The counterclockwise driving mode with high advance angle is described. Even for the counterclockwise rotation, the driving mode with high advance angle can achieve higher speed rotation than that in the above-described driving mode with low advance angle. 
     In the counterclockwise driving mode with high advance angle, the rotor  3  is rotated counterclockwise by switching excitation of each of the first magnetic pole sections  6   a  in response to an output signal of the first magnetic sensor  8  and switching excitation of each of the second magnetic pole sections  7   a  in response to an output signal of the second magnetic sensor  9 . 
     In this driving mode, the energization direction of each of the first coil  4  and the second coil  5  is switched using combinations described below. 
     When the first magnetic sensor  8  detects the south pole of the magnet  2  (switching from the north pole to south pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the first magnetic pole section  6   a  is magnetized with the south pole. When the first magnetic sensor  8  detects the north pole of the magnet  2  (switching from the south pole to north pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the first magnetic pole section  6   a  is magnetized with the north pole. 
     When the second magnetic sensor  9  detects the south pole of the magnet  2  (switching from the north pole to south pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the second magnetic pole section  7   a  is magnetized with the north pole. When the second magnetic sensor  9  detects the north pole of the magnet  2  (switching from the south pole to north pole), its detection signal is input into the control circuit  13 . The control circuit  13  controls the driving circuit  14  such that the second magnetic pole section  7   a  is magnetized with the south pole. 
     In the state illustrated in  FIG. 15A , both the first magnetic sensor  8  and the second magnetic sensor  9  detect the south pole of the magnet  2 . Accordingly, when the first magnetic pole section  6   a  is magnetized with the south pole and the second magnetic pole section  7   a  is magnetized with the north pole, the counterclockwise rotation force is produced in the rotor  3  and the magnet  2 . 
     When the rotor  3  rotates counterclockwise from the state illustrated in  FIG. 15A , the center Q 1  of the south pole of the magnet  2  and the center of the second magnetic pole section  7   a  are opposed to each other, as illustrated in  FIG. 15F . 
     The second magnetic sensor  9  is arranged such that when the magnetized polarity of the second magnetic pole section  7   a  is switched on the basis of the output of the second magnetic sensor  9 , the excitation switching timing for the second magnetic pole section  7   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between 45 degrees to 90 degrees. 
     The second magnetic sensor  9  detects the north pole of the magnet  2  (switching from the south pole to north pole) between the state illustrated in  FIG. 15A  and the state illustrated in  FIG. 15F . At this time, the driving circuit  14  energizes the second coil  5  such that the second magnetic pole section  7   a  is magnetized with the north pole. Because the first magnetic sensor  8  detects the south pole of the magnet  2  between the state illustrated in  FIG. 15A  and the state illustrated in  FIG. 15F , the driving circuit  14  energizes the first coil  4  such that the first magnetic pole section  6   a  is magnetized with the south pole. This produces the counterclockwise rotation force in the rotor  3  and the magnet  2 . 
     When the rotor  3  rotates counterclockwise from the state illustrated in  FIG. 15F , the state moves to the state illustrated in  FIG. 15G , and then the center Q 3  of the north pole of the magnet  2  and the center of the first magnetic pole section  6   a  are opposed to each other, as illustrated in  FIG. 15H . 
     The first magnetic sensor  8  is arranged such that when the magnetized polarity of the first magnetic pole section  6   a  is switched on the basis of the output of the first magnetic sensor  8 , the excitation switching timing for the first magnetic pole section  6   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between 45 degrees to 90 degrees. 
     The first magnetic sensor  8  detects the north pole of the magnet  2  (switching from the south pole to north pole) between the state illustrated in  FIG. 15G  and the state illustrated in  FIG. 15H . At this time, the driving circuit  14  energizes the first coil  4  such that the first magnetic pole section  6   a  is magnetized with the north pole. Because the second magnetic sensor  9  detects the north pole of the magnet  2  between the state illustrated in  FIG. 15G  and the state illustrated in  FIG. 15H , the driving circuit  14  energizes the second coil  5  such that the second magnetic pole section  7   a  is magnetized with the south pole. This produces the counterclockwise rotation force in the rotor  3  and the magnet  2 . 
     As described above, in the counterclockwise driving mode with high advance angle, the energization of the first coil  4  and the second coil  5  is sequentially switched by the outputs of the first magnetic sensor  8  and the second magnetic sensor  9 , and the rotor  3  and the magnet  2  rotate in a counterclockwise direction. 
     When the rotor  3  rotates counterclockwise and the magnetized polarity of the first magnetic pole section  6   a  is switched on the basis of the output of the first magnetic sensor  8 , the excitation switching timing for the first magnetic pole section  6   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between angle 45 degrees to 90 degrees. 
     When the rotor  3  rotates counterclockwise and the magnetized polarity of the second magnetic pole section  7   a  is switched on the basis of the output of the second magnetic sensor  9 , the excitation switching timing for the second magnetic pole section  7   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between angle 45 degrees to 90 degrees. 
       FIGS. 16A to 16D  are illustrations for describing positions in which the first magnetic sensor  8 , the second magnetic sensor  9 , the third magnetic sensor  10 , and the fourth magnetic sensor  11  are arranged. As illustrated in  FIGS. 16A to 16D , the first magnetic sensor  8  in the motor  1  according to the present embodiment is arranged in a position that satisfies the following conditions. 
     (a) In the clockwise driving, when the magnetized polarity of the first magnetic pole section  6   a  is switched on the basis of the output of the first magnetic sensor  8 , the excitation switching timing for the first magnetic pole section  6   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between 0 degree and 45 degrees (see  FIG. 16A ). 
     (b) In the counterclockwise driving, when the magnetized polarity of the first magnetic pole section  6   a  is switched on the basis of the output of the first magnetic sensor  8 , the excitation switching timing for the first magnetic pole section  6   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between 45 degrees and 90 degrees (see  FIG. 16C ). 
     The second magnetic sensor  9  in the motor  1  according to the present embodiment is arranged in a position that satisfies the following conditions. 
     (c) In the clockwise driving, when the magnetized polarity of the second magnetic pole section  7   a  is switched on the basis of the output of the second magnetic sensor  9 , the excitation switching timing for the second magnetic pole section  7   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between 0 degree and 45 degrees (see  FIG. 16B ). 
     (d) In the counterclockwise driving, when the magnetized polarity of the second magnetic pole section  7   a  is switched on the basis of the output of the second magnetic sensor  9 , the excitation switching timing for the second magnetic pole section  7   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between 45 degrees and 90 degrees (see  FIG. 16D ). 
     The third magnetic sensor  10  in the motor  1  according to the present embodiment is arranged in a position that satisfies the following conditions. 
     (e) In the clockwise driving, when the magnetized polarity of the first magnetic pole section  6   a  is switched on the basis of the output of the third magnetic sensor  10 , the excitation switching timing for the first magnetic pole section  6   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between 45 degrees and 90 degrees (see  FIG. 16A ). 
     (f) In the counterclockwise driving, when the magnetized polarity of the first magnetic pole section  6   a  is switched on the basis of the output of the third magnetic sensor  10 , the excitation switching timing for the first magnetic pole section  6   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between 0 degree and 45 degrees (see  FIG. 16C ). 
     The fourth magnetic sensor  11  in the motor  1  according to the present embodiment is arranged in a position that satisfies the following conditions. 
     (g) In the clockwise driving, when the magnetized polarity of the second magnetic pole section  7   a  is switched on the basis of the output of the fourth magnetic sensor  11 , the excitation switching timing for the second magnetic pole section  7   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between 45 degrees and 90 degrees (see  FIG. 16B ). 
     (h) In the counterclockwise driving, when the magnetized polarity of the second magnetic pole section  7   a  is switched on the basis of the output of the fourth magnetic sensor  11 , the excitation switching timing for the second magnetic pole section  7   a  with respect to the rotation position of the rotor  3  corresponds to an electrical advance angle between 0 degree and 45 degrees (see  FIG. 16D ). 
     In the present embodiment, in consideration of errors in magnetization of magnets, errors in dimensions of sensors, errors of yokes, each magnetic sensor is arranged in a range described below. 
     The first magnetic sensor  8  is arranged in a range where the excitation switching timing for the first magnetic pole section  6   a  in the clockwise driving corresponds to an electrical advance angle between 14.4 degrees and 33.6 degrees and the excitation switching timing for the first magnetic pole section  6   a  in the counterclockwise driving corresponds to an electrical advance angle between 56.4 degrees and 75.6 degrees. 
     The second magnetic sensor  9  is arranged in a range where the excitation switching timing for the second magnetic pole section  7   a  in the clockwise driving corresponds to an electrical advance angle between 14.4 degrees and 33.6 degrees and the excitation switching timing for the second magnetic pole section  7   a  in the counterclockwise driving corresponds to an electrical advance angle between 56.4 degrees and 75.6 degrees. 
     The third magnetic sensor  10  is arranged in a range where the excitation switching timing for the first magnetic pole section  6   a  in the clockwise driving corresponds to an electrical advance angle between 56.4 degrees and 75.6 degrees and the excitation switching timing for the first magnetic pole section  6   a  in the counterclockwise driving corresponds to an electrical advance angle between 14.4 degrees and 33.6 degrees. 
     The fourth magnetic sensor  11  is arranged in a range where the excitation switching timing for the second magnetic pole section  7   a  in the clockwise driving corresponds to an electrical advance angle between 56.4 degrees and 75.6 degrees and the excitation switching timing for the second magnetic pole section  7   a  in the counterclockwise driving corresponds to an electrical advance angle between 14.4 degrees and 33.6 degrees. 
     The midpoint of a line segment connecting the first magnetic sensor  8  and the third magnetic sensor  10  corresponds to the electrical advance angle 45 degrees at the excitation switching timing for the first magnetic pole section  6   a . The midpoint of a line segment connecting the second magnetic sensor  9  and the fourth magnetic sensor  11  corresponds to the electrical advance angle 45 degrees at the excitation switching timing for the second magnetic pole section  7   a . This reduces variations in driving characteristics between the clockwise driving and the counterclockwise driving in the present embodiment. 
     The present embodiment uses a sensor unit in which the first magnetic sensor  8  and the third magnetic sensor  10  constitute a single unit and the second magnetic sensor  9  and the fourth magnetic sensor  11  constitute a single unit. In this case, in the clockwise driving, the first magnetic sensor  8  is in the position where the excitation switching timing for the first magnetic pole section  6   a  corresponds to the electrical advance angle 21 degrees, and the third magnetic sensor  10  is in the position where the excitation switching timing for the first magnetic pole section  6   a  corresponds to the electrical advance angle 69 degrees. In the clockwise driving, the second magnetic sensor  9  is in the position where the excitation switching timing for the second magnetic pole section  7   a  corresponds to the electrical advance angle 21 degrees, and the fourth magnetic sensor  11  is in the position where the excitation switching timing for the second magnetic pole section  7   a  corresponds to the electrical advance angle 69 degrees. 
     The present invention can provide a shutter device in which, when a driven member is driven by a stepping motor and thus a light shielding member moves from a closed state to an open state or from the open state to the closed state, a stepping motor does not lose synchronization. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.