Abstract:
An electromagnetic actuator includes a rotor rotatable within a predetermined angular range, a magnetizing coil, and a yoke having an arcuate surface facing the rotor, and first and second magnetic-pole parts that have mutually different magnetic poles upon energizing the coil. The rotor includes a magnetized rotor part defining the outer peripheral surface of the rotor and magnetized to have different magnetic poles, a non-magnetized driving pin rotatable with the magnetized rotor part, and a protrusion part protruding radially from the rotor and facing the first or second magnetic-pole part while being magnetized to have the same magnetic pole as the outer peripheral surface of the rotor. The surface of the rotor that faces the yoke and that exerts a magnetic action is increased due to the inclusion of the protrusion part, and hence a desired maintaining force and driving torque can be obtained.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an electromagnetic actuator that generates a driving force by an electromagnetic force. More specifically, the present invention relates to an electromagnetic actuator that includes a rotor having a magnetized outer peripheral surface and a yoke that forms a magnetic-pole part facing the outer peripheral surface of the rotor and that is used when a blade member of a camera, such as a shutter blade or a diaphragm blade, is driven, and relates to a camera blade driving device using the electromagnetic actuator. 
   2. Description of the Related Art 
   A conventionally known electromagnetic actuator mounted on, for example, a shutter device of a camera includes a cylindrical rotor that is supported rotatably with respect to a base plate having an exposure aperture and that is magnetized into N and S-poles by bisecting its outer peripheral surface in the circumferential direction, a nearly U-shaped yoke that has a magnetic-pole part disposed to face the outer peripheral surface of the rotor, and a magnetizing coil wound around the yoke (see Japanese Unexamined Patent Publication Nos. H9-152645 and 2001-327143, for example). 
   An other conventionally known electromagnetic actuator includes a cylindrical rotor that is supported rotatably with respect to a base plate having an exposure aperture and that is magnetized into N- and S-poles by bisecting its outer peripheral surface in the circumferential direction and a protrusion part that is protruded in the circumferential direction with respect to the rotor, in which the protrusion part is fixed so as to be rotatable together with the rotor and in which the rotational range of the rotor is restricted by bringing the protrusion part into contact with a stopper formed on the base plate (see Japanese Unexamined Patent Publication No. H7-13215, for example). 
   Meanwhile, correspondingly to a reduction in size of, for example, a digital camera, an electromagnetic actuator mounted on the digital camera is required to be reduced in size. If the electromagnetic actuator is merely reduced in size without changing the conventional structure, the magnetized cylindrical rotor becomes small in size, and it becomes difficult to sufficiently secure a driving torque generated by the rotor during energization and a magnetic attraction force generated during non-energization. Accordingly, when the electromagnetic actuator is used as a driving source that drives a shutter blade, etc., it becomes difficult to stably drive the shutter blade and hold this at a predetermined position. 
   SUMMARY OF THE INVENTION 
   The present invention has been made in consideration of these circumstances. It is therefore an object of the present invention to provide an electromagnetic actuator that is capable of securing a desired driving torque, a desired magnetic attraction force, etc., while being reduced in size and that is capable of exerting a stable driving force or a stable maintaining force when the electromagnetic actuator is used as a driving source that drives a blade member of a camera, such as a shutter blade, a diaphragm blade, an ND filter blade, or other filter blades, and provide a camera blade driving device using the electromagnetic actuator. 
   The electromagnetic actuator of the present invention that achieves the above-mentioned object includes a rotor that has a cylindrical outer peripheral surface and that is capable of rotating within a predetermined angular range, a magnetizing coil, and a yoke that has a circular arc surface facing the outer peripheral surface of the rotor, a first magnetic-pole part, and a second magnetic-pole part. The first and second magnetic-pole parts generate mutually different magnetic poles by energizing the coil. The rotor includes a magnetized rotor part that defines the outer peripheral surface of the rotor and that is magnetized to have different magnetic poles in a circumferential direction, a driving pin that is not magnetized so as to rotate together with the magnetized rotor part, and a protrusion part that is protruded in a radial direction from the outer peripheral surface of the rotor while being magnetized to have the same magnetic pole as the outer peripheral surface of the rotor and that faces the first magnetic-pole part or the second magnetic-pole part. 
   According to this structure, the rotor is provided with the protrusion part that is protruded from the outer peripheral surface of the magnetized rotor part and that is magnetized into the same magnetic pole as the outer peripheral surface thereof, in addition to the magnetized rotor part and the driving pin that is not magnetized. This protrusion part can face the first magnetic-pole part or the second magnetic-pole part of the yoke. Therefore, the rotor is increased in the surface that faces the yoke and that exerts a magnetic action. 
   Accordingly, a great magnetic attraction force is generated between the protrusion part and the first magnetic-pole part or the second magnetic-pole part so that a stable maintaining force can be obtained when the coil is not energized, whereas a great repulsion force by an electromagnetic force is generated between the protrusion part and the first magnetic-pole part or the second magnetic-pole part so that a stable driving torque can be obtained when the coil is energized. On the other hand, since the driving pin is formed not to be magnetized, an excessive magnetic attraction force and an excessive driving torque can be prevented from being generated, so that a smooth, stable rotational operation can be performed. Therefore, it is possible to obtain an electromagnetic actuator that generates a desired maintaining force and a desired driving torque while the electromagnetic actuator is reduced in size in the direction of the rotational axis of the rotor. 
   Preferably, in the electromagnetic actuator structured as above, the magnetized rotor part has an N-pole outer peripheral surface and an S-pole outer peripheral surface that are obtained by being bisected in the circumferential direction, and the protrusion part is protruded from one of the N-pole outer peripheral surface and the S-pole outer peripheral surface. 
   According to this structure, what is required is to provide the conventional rotor with the protrusion part formed on the outer peripheral surface and magnetize the protrusion part so that the protrusion part has the same magnetic pole as the outer peripheral surface. Therefore, it is possible to easily set a necessary driving torque and a necessary magnetic attraction force while simplifying the structure. 
   The camera blade driving device of the present invention that achieves the above-mentioned object includes a base plate having an exposure aperture, a blade member provided so as to be movable between a position facing the aperture and a position retreating from the aperture, and a driving source that drives the blade member. The driving source is one of the electromagnetic actuators structured as above. 
   According to this structure, since the electromagnetic actuator described above is employed as a driving source, a sufficient driving force is outputted from the rotor, so that the blade member is driven stably and reliably at a desired timing, and is held at a predetermined position (for example, a position facing the aperture or a position retreating from the aperture) while the device is reduced in size in the direction of the rotational axis of the rotor. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an exploded perspective view showing an embodiment of an electromagnetic actuator according to the present invention. 
       FIG. 2  is a plan view showing an embodiment of a camera blade driving device using the electromagnetic actuator according to the present invention. 
       FIG. 3  is an expanded sectional view showing a part of the camera blade driving device of  FIG. 2 . 
       FIG. 4A ,  FIG. 4B ,  FIG. 4C , and  FIG. 4D  are plan views for explaining the operation of the electromagnetic actuator of  FIG. 1 , each showing an operational state. 
       FIG. 5  is a plan view for explaining the operation of the camera blade driving device, showing a state in which a blade member occupies a position facing an aperture. 
       FIG. 6  is a plan view for explaining the operation of the camera blade driving device, showing a state in which the blade member occupies a position retreating from the aperture. 
       FIG. 7  is a plan view showing another embodiment of the electromagnetic actuator according to the present invention. 
       FIG. 8A ,  FIG. 8B ,  FIG. 8C , and  FIG. 8D  are plan views for explaining the operation of the electromagnetic actuator of  FIG. 7 , each showing an operational state. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention will be herein after described with reference to the accompanying drawings. 
     FIG. 1  to  FIG. 6  show an embodiment in which an electromagnetic actuator according to the present invention is applied to a camera blade driving device. 
   As shown in  FIG. 1  to  FIG. 3 , the camera blade driving device includes a main plate  10  and a back plate  20  that constitute a base plate having exposure apertures  10   a  and  20   a , a blade member  30  supported by the main plate  10  movably between a position facing the apertures  10   a  and  20   a  and a position retreating from the apertures  10   a  and  20   a , and an electromagnetic actuator  40  serving as a driving source that drives the blade member  30 . 
   As shown in  FIG. 1  to  FIG. 3 , the main plate  10  has the circular exposure aperture  10   a , a supporting shaft  11  that rotatably supports a rotor  41  described later, a substantially fan-shaped through-hole lob, a positioning pin  12  and a positioning projection  13  both of which position a yoke  42  described later, connection parts  14  each of which has a screw hole  14   a  into which a screw B is screwed, and a supporting shaft  15  that rotatably supports the blade member  30 . 
   As shown in  FIG. 3 , the back plate  20  is connected to an end surface on the reverse side of the main plate  10  with a predetermined gap there between by means of the screws B, and defines a blade chamber W that rotatably contains the blade member  30 . 
   As shown in  FIG. 5  and  FIG. 6 , the blade member  30  has a circular hole  31  into which the supporting shaft  15  is inserted and a long hole  32  into which a driving pin  41   b  described later is inserted. 
   The blade member  30  is reciprocated between a position facing the apertures  10   a  and  20   a  shown in  FIG. 5  and a position retreating from the apertures  10   a  and  20   a  shown in  FIG. 6  by reciprocating the driving pin  41   b.    
   Various blade members, such as a shutter blade made of a shading plate, an ND (Neutral Density) filter blade by which the quantity of light passing therethrough is reduced, or a filter blade that cuts off infrared light, can be used as the blade member  30 . 
   As shown in  FIG. 1  to  FIG. 3 , the electromagnetic actuator  40  is made up of the rotor  41  rotatably supported by the main plate  10 , the substantially U-shaped yoke  42 , a magnetizing coil  43 , and a presser plate  44 . 
   As shown in  FIG. 1  to  FIG. 3 , the rotor  41  is shaped like a cylinder, and is made up of a magnetized rotor part  41   a  that defines an N-pole outer peripheral surface  41   a ″ and an S-pole outer peripheral surface  41   a ′″ that are magnetized to have mutually different magnetic poles by being bisected with a boundary plane passing through a through-hole  41   a ′, through which the supporting shaft  11  passes, and a rotational axis L as the boundary there between, the driving pin  41   b  that is not magnetized and that is rotated together with the magnetized rotor part  41   a , and a protrusion part  41   c  that is magnetized to have an N-pole and that is protruded from the N-pole outer peripheral surface  41   a ″ radially outwardly. The driving pin  41   b  includes not only a part that is connected to the blade member  30  but also an arm part extending in a horizontal direction (i.e., radially outwardly) from the bottom of the magnetized rotor part  41   a.    
   The magnetized rotor part  41   a  is formed to define a cylindrical, outer peripheral surface. The outer peripheral surface thereof is bisected in the circumferential direction by the plane passing through the rotational axis L, thereby forming the N-pole outer peripheral surface  41   a ″ magnetized to have an N-pole and the S-pole outer peripheral surface  41   a ′″ magnetized to have an S-pole. 
   The driving pin  41   b  is molded integrally with the magnetized rotor part  41   a , and serves to transmit a rotational driving force of the rotor  41  to the outside. The driving pin  41   b  is made of, for example, a resinous material so as not to be magnetized. 
   The protrusion part  41   c  is protruded in a radial direction from the N-pole outer peripheral surface  41   a ″, and is magnetized into an N-pole that is the same as the N-pole outer peripheral surface  41   a ″. Both end surfaces  41   c ′ and  41   c ″ in the circumferential direction thereof are formed to face a first magnetic-pole part  42   a  (end surface  42   a ′) and a second magnetic-pole part  42   b  (end surface  42   b ″), which are described later, of the yoke  42 , respectively. 
   As shown in  FIG. 5 , when the rotor  41  is positioned at a counterclockwise rotational end, the driving pin  41   b  comes into contact with a stopper (not shown) at a rotational end, and, as a result, the end surface  41   c ′ frontally faces the end surface  42   a ″ of the first magnetic-pole part  42   a  described later without being in contact therewith. On the other hand, as shown in  FIG. 6 , when the rotor  41  is positioned at a clockwise rotational end, the driving pin  41   b  comes into contact with a stopper (not shown) at an opposite rotational end, and, as a result, the end surface  41   c ″ frontally faces the end surface  42   b ″ of the second magnetic-pole part  42   b  described later without being in contact therewith. 
   As shown in  FIG. 1 , the yoke  42  is bent substantially in the shape of the letter U, and is made up of the first magnetic-pole part  42   a  that is formed at an end of the yoke  42  and that defines a circular arc surface  42   a ′ and the end surface  42   a ″, the second magnetic-pole part  42   b  that is formed at the other end of the yoke  42  and that defines a circular arc surface  42   b ′ and the end surface  42   b ″, and a positioning hole  42   c  that is formed in a bent part of the yoke  42 . 
   The end surface  42   a ″ of the first magnetic-pole part  42   a  and the end surface  42   b ″ of the second magnetic-pole part  42   b  serve to generate a magnetic attraction force and a repulsion force at the protrusion part  41   c  of the rotor  41  (i.e., between the end surfaces  41   c ′ and  41   c ″). 
   As shown in  FIG. 1 , the coil  43  is wound around a bobbin part  44   a  of the presser plate  44  described later. 
   As shown in  FIG. 1 , the presser plate  44  is formed like a flat plate, and is formed integrally with the bobbin part  44   a . The presser plate  44  has, at both sides thereof, fitting holes  44   b  and  44   c  through which the supporting shaft  11  and the positioning pin  12  of the main plate  10  pass and holes  44   d  through each of which a screw B passes. 
   A description will be given of the operation of the thus structured electromagnetic actuator  40  and the operation of the camera blade driving device with reference to  FIG. 4A  to  FIG. 6 . 
   First, when the rotor  41  is situated at a counterclockwise rotational end in a state of not energizing the coil  43  as shown in  FIG. 4A , the rotor  41  is in contact with the stopper at a position where the boundary line between the magnetic poles of the magnetized rotor part  41   a  deviates from the intermediate position of each of the circular arc surfaces  42   a ′ and  42   b ′. Therefore, a magnetic attraction force is generated between the N-pole outer peripheral surface  41   a ″ and the first magnetic-pole part  42   a  (the circular arc surface  42   a ′), between the S-pole outer peripheral surface  41   a ′″ and the second magnetic-pole part  42   b  (the circular arc surface  42   b ′), and between the protrusion part  41   c  (the end surface  41   c ′) and the end surface  42   a ″ of the first magnetic-pole part  42   a . Therefore, the rotor  41  is positioned by the stopper (not shown) at the counterclockwise rotational end, and is reliably held thereby. 
   This state corresponds to a state in which the blade member  30  is set at a position facing the apertures  10   a  and  20   a , as shown in  FIG. 5 , in the camera blade driving device. 
   When an electric current is passed through the coil  43  in a predetermined direction in this state, an N-pole is generated in the first magnetic-pole part  42   a , and an S-pole is generated in the second magnetic-pole part  42   b  as shown in  FIG. 4B . 
   Accordingly, a repulsion force is generated by an electromagnetic force between the N-pole outer peripheral surface  41   a ″ and the first magnetic-pole part  42   a  (the circular arc surface  42   a ′), between the S-pole outer peripheral surface  41   a ′″ and the second magnetic-pole part  42   b  (the circular arc surface  42   b ′), and between the protrusion part  41   c  (the end surface  41   c ′) and the end surface  42   a ″ of the first magnetic-pole part  42   a , so that the rotor  41  starts to rotate clockwise. 
   When the rotor  41  rotates clockwise, the repulsion force generated between the protrusion part  41   c  (the end surface  41   c ′) and the end surface  42   a ″ of the first magnetic-pole part  42   a  greatly acts until the rotor  41  reaches an intermediate position of a rotational range (a working angle) thereof. After the rotor  41  goes beyond the intermediate position of the rotational range, an attraction force generated between the protrusion part  41   c  (the end surface  41   c ″) and the end surface  42   b ″ of the second magnetic-pole part  42   b  greatly acts. Therefore, the rotor  41  continues to rotate while maintaining a stable rotational force, and, as shown in  FIG. 4C , the rotor  41  is positioned and stopped by the stopper (not shown) at a clockwise rotational end in a state in which the boundary line between the magnetic poles of the magnetized rotor part  41   a  deviates from the intermediate position of each of the circular arc surfaces  42   a ′ and  42   b′.    
   When an electric current stops being passed through the coil  43  in this state, the magnetic attraction force generated between the protrusion part  41   c  (the end surface  41   c ″) and the end surface  42   b ″ of the second magnetic-pole part  42   b  acts, and the magnetic attraction force generated between the N-pole outer peripheral surface  41   a ″ and the second magnetic-pole part  42   b  (the circular arc surface  42   b ′) and between the S-pole outer peripheral surface  41   a ′″ and the first magnetic-pole part  42   a  (the circular arc surface  42   a ′) acts. As a result, the rotor  41  is reliably held at the clockwise rotational end as shown in  FIG. 4D . 
   This state corresponds to a state in which the blade member  30  is set at the position retreating from the apertures  10   a  and  20   a , as shown in  FIG. 6 , in the camera blade driving device. 
   On the other hand, when an electric current is passed through the coil  43  in an opposite direction, an opposite magnetic pole is generated in each of the first and second magnetic-pole parts  42   a  and  42   b . Accordingly, the rotor  41  stably rotates in the counterclockwise direction while following an opposite path from the state of  FIG. 4D , and is positioned and held at the counterclockwise rotational end shown in  FIG. 4A . 
   At this time, in the camera blade driving device, the blade member  30  is moved from the position retreating from the apertures  10   a  and  20   a  shown in  FIG. 6  to the position facing the apertures  10   a  and  20   a  shown in  FIG. 5 , and is positioned. 
   Since the rotor  41  is provided with the protrusion part  41   c  magnetized into the same magnetic pole as the N-pole outer peripheral surface  41   a ″ as described above, the rotor  41  can obtain a stable rotational force and a stable driving torque by means of the driving pin  41   b . Additionally, the rotor  41  can generate a desired maintaining force (i.e., a magnetic attraction force) at the rotational ends on both sides thereof. Still additionally, when the electromagnetic actuator  40  is used as a driving source for the camera blade driving device, the blade member  30  can be driven smoothly and stably. 
   Still additionally, even if the driving pin  41   b  is disposed near the first magnetic-pole part  42   a  or the second magnetic-pole part  42   b  in the direction of the rotational axis of the rotor  41 , neither an excessive magnetic attraction force nor an excessive driving torque is generated between the driving pin  41   b  and the magnetic-pole part  42   a  or  42   b , because the driving pin  41   b  is not magnetized. Therefore, the electromagnetic actuator can perform a smooth, stable rotational operation, and can be reduced in size. The protrusion part  41   c  may be formed on the S-pole outer peripheral surface  41   a′″.    
     FIG. 7  to  FIG. 8D  show another embodiment of the electromagnetic actuator according to the present invention. This embodiment is the same as the above-described embodiment, except that the rotor and the yoke are partially modified. Therefore, the same reference character is given to the same structure as in the above-described embodiment, and a description of the same structure is omitted. 
   As shown in  FIG. 7 , in this electromagnetic actuator  40 ′, the rotor  41  is additionally provided with a protrusion part  41   d  magnetized into an S-pole on the S-pole outer peripheral surface  41   a ′″, and the yoke  42  additionally has end surfaces  42   b ′″ and  42   a ′″ that face both end surfaces  41   d ′ and  41   d ″, respectively, of the protrusion part  41   d.    
   The operation of the electromagnetic actuator  40 ′ will be described with reference to  FIG. 8A  to  FIG. 8D . 
   First, when the rotor  41  is positioned at the counterclockwise rotational end in a state of not energizing the coil  43  as shown in  FIG. 8A , a magnetic attraction force is generated between the N-pole outer peripheral surface  41   a ″ and the first magnetic-pole part  42   a  (the circular arc surface  42   a ′), between the S-pole outer peripheral surface  41   a ′″ and the second magnetic-pole part  42   b  (the circular arc surface  42   b ′), between the protrusion part  41   c  (the end surface  41   c ′) and the end surface  42   a ″ of the first magnetic-pole part  42   a , and between the protrusion part  41   d  (the end surface  41   d ′) and the end surface  42   b ′″ of the second magnetic-pole part  42   b . Therefore, the rotor  41  is positioned by the stopper (not shown) at the counterclockwise rotational end, and is reliably held thereby. 
   This state corresponds to a state in which the blade member  30  is set at the position facing the apertures  10   a  and  20   a , as shown in  FIG. 5 , in the camera blade driving device. 
   When an electric current is passed through the coil  43  in a predetermined direction in this state, an N-pole is generated in the first magnetic-pole part  42   a , and an S-pole is generated in the second magnetic-pole part  42   b  as shown in  FIG. 8B . 
   Accordingly, a repulsion force is generated by an electromagnetic force between the N-pole outer peripheral surface  41   a ″ and the first magnetic-pole part  42   a  (the circular arc surface  42   a ′), between the S-pole outer peripheral surface  41   a ′″ and the second magnetic-pole part  42   b  (the circular arc surface  42   b ′), between the protrusion part  41   c  (the end surface  41   c ′) and the end surface  42   a ″ of the first magnetic-pole part  42   a , and between the protrusion part  41   d  (the end surface  41   d ′) and the end surface  42   b ′″ of the second magnetic-pole part  42   b , so that the rotor  41  starts to rotate clockwise. 
   When the rotor  41  rotates clockwise, a repulsion force generated between the protrusion part  41   c  (the end surface  41   c ′) and the end surface  42   a ″ of the first magnetic-pole part  42   a  and between the protrusion part  41   d  (the end surface  41   d ′) and the end surface  42   b ′″ of the second magnetic-pole part  42   b  greatly acts until the rotor  41  reaches an intermediate position of a rotational range (a working angle) thereof. After the rotor  41  goes beyond the intermediate position of the rotational range, an attraction force generated between the protrusion part  41   c  (the end surface  41   c ″) and the end surface  42   b ″ of the second magnetic-pole part  42   b  and between the protrusion part  41   d  (the end surface  41   d ″) and the end surface  42   a ′″ of the first magnetic-pole part  42   a  greatly acts. Therefore, the rotor  41  continues to rotate while maintaining a stable rotational force, and, as shown in  FIG. 8C , the rotor  41  is positioned and stopped by the stopper (not shown) at the clockwise rotational end. 
   When an electric current stops being passed through the coil  43  in this state, a magnetic attraction force generated between the protrusion part  41   c  (the end surface  41   c ″) and the end surface  42   b ″ of the second magnetic-pole part  42   b  and between the protrusion part  41   d  (the end surface  41   d ″) and the end surface  42   a ′″ of the first magnetic-pole part  42   a  acts, and a magnetic attraction force generated between the N-pole outer peripheral surface  41   a ″ and the second magnetic-pole part  42   b  (the circular arc surface  42   b ′) and between the S-pole outer peripheral surface  41   a ′″ and the first magnetic-pole part  42   a  (the circular arc surface  42   a ′) acts as shown in  FIG. 8D . As a result, the rotor  41  is reliably held at the clockwise rotational end. 
   This state corresponds to a state in which the blade member  30  is set at the position retreating from the apertures  10   a  and  20   a , as shown in  FIG. 6 , in the camera blade driving device. 
   On the other hand, when an electric current is passed through the coil  43  in an opposite direction, an opposite magnetic pole is generated in each of the first and second magnetic-pole parts  42   a  and  42   b . Accordingly, the rotor  41  stably rotates in the counterclockwise direction while following an opposite path from the state of  FIG. 8D , and is positioned and held at the counterclockwise rotational end shown in  FIG. 8A . 
   At this time, in the camera blade driving device, the blade member  30  is moved from the position retreating from the apertures  10   a  and  20   a  shown in  FIG. 6  to the position facing the apertures  10   a  and  20   a  shown in  FIG. 5 , and is positioned. 
   Since the rotor  41  is provided with the protrusion parts  41   c  and  41   d  magnetized into the same magnetic poles as the N-pole and S-pole outer peripheral surfaces  41   a ″ and  41   a ′″, respectively, as described above, the rotor  41  can obtain a more stable rotational force and a more stable and greater driving torque by means of the driving pin  41   b . Additionally, the rotor  41  can generate a desired maintaining force (i.e., a magnetic attraction force) at the rotational ends on both sides thereof. Still additionally, when the electromagnetic actuator  40 ′ is used as a driving source for the camera blade driving device, the blade member  30  can be driven smoothly and stably. 
     FIG. 9  to  FIG. 10D  show still another embodiment of the electromagnetic actuator according to the present invention. This embodiment is the same as the above-described embodiment shown in  FIG. 4A  to  FIG. 4D , except that the rotor and the yoke are partially modified. Therefore, the same reference character is given to the same structure as in the above-described embodiment, and a description of the same structure is omitted. 
   As shown in  FIG. 9 , in this electromagnetic actuator  40 ″, the rotor  41  is provided with two protrusion parts  41   e  and  41   f  protruded from two boundary areas of the N-pole outer peripheral surface  41   a ″ and the S-pole outer peripheral surface  41   a′″.    
   The protrusion part  41   e  is formed to face the circular arc surface  42   a ′ of the first magnetic-pole part  42   a , and the protrusion part  41   f  is formed to face the circular arc surface  42   b ′ of the second magnetic-pole part  42   b . When the rotor  41  is positioned at the center of the working angle, the protrusion part  41   e  is positioned at the center of the circular arc surface  42   a ′, and the protrusion part  41   f  is positioned at the center of the circular arc surface  42   b ′. Each of the protrusion parts  41   e  and  41   f  is magnetized to have two magnetic poles (i.e., N-pole and S-pole). 
   The operation of the electromagnetic actuator  40 ″ will be described with reference to  FIG. 10A  or  FIG. 10D . 
   First, when the rotor  41  is situated at the counterclockwise rotational end in a state of not energizing the coil  43  as shown in  FIG. 10A , the rotor  41  is in contact with the stopper at a position where the boundary line between the magnetic poles of the magnetized rotor part  41   a  deviates from the intermediate position of each of the circular arc surfaces  42   a ′ and  42   b ′. Therefore, a magnetic attraction force is generated between the N-pole outer peripheral surface  41   a ″ and the first magnetic-pole part  42   a  (the circular arc surface  42   a ′), between the S-pole outer peripheral surface  41   a ′″ and the second magnetic-pole part  42   b  (the circular arc surface  42   b ′), between the protrusion part  41   e  and the circular arc surface  42   a ′ of the first magnetic-pole part  42   a , and between the protrusion part  41   f  and the circular arc surface  42   b ′ of the second magnetic-pole part  42   b . Therefore, the rotor  41  is positioned by the stopper (not shown) at the counterclockwise rotational end, and is reliably held thereby. 
   This state corresponds to a state in which the blade member  30  is set at a position facing the apertures  10   a  and  20   a , as shown in  FIG. 5 , in the camera blade driving device. 
   When an electric current is passed through the coil  43  in a predetermined direction in this state, an N-pole is generated in the first magnetic-pole part  42   a , and an S-pole is generated in the second magnetic-pole part  42   b  as shown in  FIG. 10B . 
   Accordingly, a repulsion force is generated by an electromagnetic force between the N-pole outer peripheral surface  41   a ″ and the first magnetic-pole part  42   a  (the circular arc surface  42   a ′) and between the S-pole outer peripheral surface  41   a ′″ and the second magnetic-pole part  42   b  (the circular arc surface  42   b ′), so that the rotor  41  starts to rotate clockwise. 
   The rotor  41  rotates clockwise, and is positioned and stopped by the stopper (not shown) at the clockwise rotational end in a state in which the boundary line between the magnetic poles of the magnetized rotor part  41   a  deviates from the intermediate position of each of the circular arc surfaces  42   a ′ and  42   b ′ as shown in  FIG. 10C . 
   When an electric current stops being passed through the coil  43  in this state, a magnetic attraction force generated between the protrusion part  41   e  and the circular arc surface  42   a ′ of the first magnetic-pole part  42   a  and between the protrusion part  41   f  and the circular arc surface  42   b ′ of the second magnetic-pole part  42   b  acts, and a magnetic attraction force generated between the N-pole outer peripheral surface  41   a ″ and the second magnetic-pole part  42   b  (the circular arc surface  42   b ′) and between the S-pole outer peripheral surface  41   a ′″ and the first magnetic-pole part  42   a  (the circular arc surface  42   a ′) acts. As a result, the rotor  41  is reliably held at the clockwise rotational end as shown in  FIG. 10D . 
   This state corresponds to a state in which the blade member  30  is set at the position retreating from the apertures  10   a  and  20   a , as shown in  FIG. 6 , in the camera blade driving device. 
   On the other hand, when an electric current is passed through the coil  43  in an opposite direction, an opposite magnetic pole is generated in each of the first and second magnetic-pole parts  42   a  and  42   b . Accordingly, the rotor  41  stably rotates in the counterclockwise direction while following an opposite path from the state of  FIG. 10D , and is positioned and held at the counterclockwise rotational end shown in  FIG. 10A . 
   At this time, in the camera blade driving device, the blade member  30  is moved from the position retreating from the apertures  10   a  and  20   a  shown in  FIG. 6  to the position facing the apertures  10   a  and  20   a  shown in  FIG. 5 , and is positioned. 
   As in the above-described embodiments, the rotor  41  is provided with the two protrusion parts  41   e  and  41   f  on the circumferential surface thereof in order to enlarge the facing areas of the magnetic-pole parts  42   a  and  42   b  in this embodiment, and hence the rotor  41  especially can generate a desired maintaining force (i.e., a magnetic attraction force) at the rotational ends on both sides. Therefore, when the electromagnetic actuator  40 ″ is used as a driving source for the camera blade driving device, the blade member  30  can be reliably held and set at a predetermined stop position. 
   Additionally, since the driving pin  41   b  is not magnetized, the rotor  41  can be reduced in size in the direction of the rotational axis as in the above-described embodiments. 
   In the above-described embodiments, the camera blade driving device that employs the electromagnetic actuators  40 ,  40 ′, and  40 ″ according to the present invention is used as a driving source for driving the single blade member  30 . However, without being limited to this, the electromagnetic actuator according to the present invention may be employed as a driving source for driving a plurality of blade members. 
   As described above, according to the electromagnetic actuator of the present invention, the surface of the rotor facing the yoke is increased, as a whole, by providing the protrusion parts that are protruded radially in the outward direction from the outer peripheral surface of the rotor, that are magnetized to have the same magnetic pole as the outer peripheral surface of the rotor, and that face the first magnetic-pole part and the second magnetic-pole part of the yoke. Therefore, it is possible to obtain an electromagnetic actuator that generates a desired maintaining force and a driving torque, in spite of the fact that the electromagnetic actuator is reduced in size. 
   Additionally, according to the camera blade driving device of the present invention, the above-described electromagnetic actuator is employed as a driving source, and hence a sufficient driving torque can be obtained by the rotor, in spite of being reduced in size. Therefore, the blade member (for example, a shutter blade, a diaphragm blade, an ND filter blade, or other filter blades) can be reliably and stably driven at a desired timing, and can be held at a desired position (i.e., a position facing the apertures or a position retreating from the apertures). 
   As described above, the electromagnetic actuator of the present invention can be reduced in size, and, in addition, can generate a desired maintaining force and a desired driving torque. Therefore, the electromagnetic actuator of the present invention can, of course, be used as a driving source for the camera blade driving device, and is useful as a driving source for other optical devices or electronic devices that are required to reciprocate a driven member.