Abstract:
A low-cost, small-sized and high-output driving device which makes it possible to produce two outputs separately and is easy to handle. The driving device has two driving units comprised of a coil, a stator, a magnet, and a rotor. The two driving units are arranged, side by side, along the axis of the driving device. In particular, a portion of the first rotor of one driving unit inserted in an inner periphery of the second coil of the other driving unit and the inner periphery of the second rotor of the other driving unit. The portion of the first rotor is magnetized by the second coil.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a driving device and a light amount controller using the driving device.  
         [0003]     2. Description of the Related Art  
         [0004]     Conventionally, a driving device which is reduced in diameter around the center of a rotation axis and at the same time increased in output has been proposed in Japanese Laid-Open Patent Publication (Kokai) No. 2002-49076.  
         [0005]      FIG. 10  is an exploded perspective view of the driving device disclosed in the above-mentioned publication, and  FIG. 11  is an axial cross-sectional view of the driving device shown in  FIG. 1 .  
         [0006]     In these figures, reference numeral  1001  designates a magnet,  1002  a coil,  1003  a stator,  1004  an auxiliary stator, and  1005  a base plate.  
         [0007]     The magnet  1001  is comprised of a bottomed hollow cylindrical magnet body  1001   a,  a drive pin  1001   b  formed integrally with the magnet body  1001   a  in a manner axially protruding from a portion of a peripheral wall of the magnet body  1001   a,  and shaft parts  1001   c  and  1001   d  axially protruding from the opposite sides of the center of the bottom wall of the magnet body  1001   a.  In the magnet  1001 , the peripheral wall of the magnet body  1001   a  is circumferentially divided into four sections which are magnetized such that they have alternately different S and N poles.  
         [0008]     The coil  1002  is formed by winding wire around an annular groove of a cylindrical bobbin. The coil  1002  is disposed within the stator  1003  in a manner extending along the axis of the magnet  1001 .  
         [0009]     The stator  1003  has a bottomed hollow cylindrical body, which has an outer peripheral wall thereof formed with a protruding piece-like outer magnetic pole part  1003   a  extending along the axis of the hollow cylindrical body, and a shaft-like protrusion  1003   b  extending along the axis of the hollow cylindrical body from the center of the inner bottom wall of the hollow cylindrical body. The shaft-like protrusion  1003   b  has a front end thereof formed therein with a shaft hole  1003   c,  in which the shaft part  1001   c  is rotatably fitted. Further, the coil  1002  is rigidly fitted on a base end portion of the shaft-like protrusion  1003   b.  The stator  1003  is magnetized by the coil  1002 .  
         [0010]     The auxiliary stator  1004  has a hollow cylindrical shape, and is rigidly fitted on the front end of the shaft-like protrusion  1003   b  of the stator  1003  in opposed relation to the coil  1002 . The auxiliary stator  1004  and the shaft-like protrusion  1003   b  cooperate to form an inner magnetic pole part.  
         [0011]     The base plate  1005  has a circular opening  1005   a  formed in the center thereof, an arcuate guide slot  1005   b  formed therein radially outward of the circular opening  1005   a,  and a shaft hole  1005   c  formed therein in the vicinity of the arcuate guide slot  1005   b.  The drive pin  1001   b  of the magnet  1001  is slidably engaged in the guide slot  1005   b  of the base plate  1005 . Further, the shaft part  1001   d  of the magnet  1001  is rotatably fitted in the shaft hole  1005   c  of the base plate  1005 .  
         [0012]     The outer magnetic pole part  1003   a  of the stator  1003  is opposed to the outer peripheral surface of the magnet body  1001   a  with a clearance therebetween, and the outer peripheral surface of the inner magnetic pole part formed by the auxiliary stator  1004  and the shaft-like protrusion  1003   b  of the stator  1003  is opposed to the inner peripheral surface of the magnet body  1001   a  with a clearance therebetween.  
         [0013]     In the driving device constructed as above, the magnet  1001  is angularly reciprocated about the shaft parts  1001   c  and  1001   d  within a limited range by switching the direction of energization of the coil  1002  and thereby changing the polarity of the outer magnetic pole part  1003   a  and that of the inner magnetic pole part (the protruding part  1003   b  and the auxiliary stator  1004 ).  
         [0014]     The angular reciprocation of the magnet  1001  is restricted by the guide hole  1005   b  formed in the base plate  1005  and the drive pin  1001   b  engaged in the guide hole  1005   b.    
         [0015]     In the driving device configured as above, magnetic flux generated by energization of the coil  1002  flows from the outer magnetic pole part  1003   a  to the opposed inner magnetic pole part, or from the inner magnetic pole part to the outer magnetic pole part  1003   a  opposed thereto, to effectively act on the magnet  1001  located between the outer magnetic pole part  1003   a  and the inner magnetic pole part.  
         [0016]     The distance between the outer magnetic pole part  1003   a  and the inner magnetic pole part is set to a value obtained by adding together the thickness of the hollow cylindrical magnet body  1001   a,  the clearance between the magnet body  1001   a  and the outer magnetic pole part  1003   a,  and the clearance between the magnet body  1001   a  and the inner magnetic pole part, i.e. to a minimum possible value, which makes it possible to reduce the resistance of a magnetic circuit formed by the outer magnetic pole part  1003   a  and the inner magnetic pole part. As the resistance of the magnetic circuit is smaller, a larger amount of magnetic flux can be generated by a small electric current, leading to an increase in the output of the driving device.  
         [0017]     In the above driving device disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2002-49076, the resistance of the magnetic circuit is reduced by setting the distance between the outer magnetic pole part  1003   a  and the inner magnetic pole part to the minimum possible value as stated above.  
         [0018]     However, in the driving device configured as above, the predetermined clearances are provided, respectively, between the magnet body  1001   a  and the outer magnetic pole part  1003   a  and between the magnet body  1001   a  and the inner magnetic pole part, and hence there is room for improvement in terms of reduction of the resistance of the magnet circuit. For example, if one of the clearances can be dispensed with, the distance between the outer magnetic pole part  1003   a  and the inner magnetic pole part can be shortened, and therefore reduction of the resistance of the magnet circuit can be expected.  
         [0019]     Further, in the driving device in which the predetermined clearance is provided between the magnet body  1001   a  and the opposed inner magnetic pole part, as stated above, it is necessary to control the clearance in course of manufacture, and hence the driving device still remains to be improved in terms of cost reduction as well. If the above-mentioned clearance can be omitted, the clearance control becomes unnecessary, which contributes to reduction of the costs.  
         [0020]     Furthermore, according to a light amount controller disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2002-49076 referred to above, when it is desired to carry out a plurality of driving operations for driving shutter blades, aperture blades, and the like, it is necessary to provide a number of driving devices corresponding to the number of the driving operations needed on the base plate  1005 . For example, most of the existing compact digital cameras have a plurality of driving devices provided on the base plate  1005 , and hence the base plate  1005  is almost covered with the driving devices.  
       SUMMARY OF THE INVENTION  
       [0021]     It is an object of the present invention to provide a low-cost, small-sized, and high-output driving device which is capable of producing two outputs separately, and a light amount controller using the driving device.  
         [0022]     To attain the above object, in a first aspect of the present invention, there is provided a driving device comprising a hollow cylindrical first magnet having a peripheral wall thereof circumferentially divided into sections magnetized to have alternately different poles, the first magnet having an outer peripheral surface and an inner periphery, a first coil disposed concentric with and adjacent the first magnet and extending axially of the first magnet, a first stator having a magnetic pole part opposed to the outer peripheral surface of the first magnet, the magnetic pole part of the first stator being magnetized by the first coil, a first rotor rigidly fitted in the inner periphery of the first magnet, the first rotor being magnetized by the first coil, a first output member disposed to be driven to a first position or a second position by rotation of the first magnet, a hollow cylindrical second magnet disposed concentric with the first magnet and having a peripheral wall thereof circumferentially divided into sections magnetized to have alternately different poles, the second magnet having an outer peripheral surface and an inner periphery, a second coil disposed concentric with and adjacent the second magnet and extending axially of the second magnet, the second coil having an inner periphery, a second stator having a magnetic pole part opposed to the outer peripheral-surface of the second magnet, the magnetic pole part of the second stator being magnetized by the second coil, a second rotor rigidly fitted in the inner periphery of the second magnet, the second rotor having an inner periphery, and a second output member disposed to be driven to a third position or a fourth position by rotation of the second magnet, wherein the first rotor has a portion thereof inserted in the inner periphery of the second coil and the inner periphery of the second rotor, the portion being magnetized by the second coil.  
         [0023]     With the arrangement of the first aspect of the present invention, in each of the two driving units comprised of a coil, a stator, a magnet, and a rotor, the rotor is configured such that a portion thereof rigidly fitted in the inner periphery of the magnet acts as an inner magnetic pole part, whereby the distance between an outer magnetic pole part opposed to the outer periphery of the magnet and the inner magnetic pole part opposed to the inner periphery of the same is reduced.  
         [0024]     The driving device according to the first aspect of the present invention is thus constructed by arranging the two driving units in which the resistance of a magnetic circuit is reduced by reducing the distance between the outer magnetic pole part and the inner magnetic pole part, side by side, along the axis of the driving device. As a result, the driving device, which is low in cost, small in size, and high in output, can produce two outputs separately. Moreover, the driving device is easy to handle.  
         [0025]     Further, the first rotor operating in a manner interlocked to the first output member is inserted in the inner periphery of the second coil. Therefore, this driving device is capable of causing the first output member and the second output member to produce respective outputs both in a direction from the first magnet to the second magnet, i.e. in the same direction.  
         [0026]     Furthermore, the first rotor causes not only a portion thereof rigidly fitted in the inner periphery of the first magnet to act on the first magnet as an inner magnetic pole part, but also a portion thereof inserted in the inner periphery of the second coil to act on the second magnet as an inner magnetic pole part, thereby performing component sharing so as to make the driving device low-cost and easy to assemble, and the magnetic circuit effective.  
         [0027]     Preferably, the first rotor and the second rotor are both formed of a soft magnetic material.  
         [0028]     Preferably, the driving device comprises a top plate sandwiched between the first coil and the second coil, the top plate having an outer peripheral edge, and the first magnet, the first coil,.the second coil, and the second magnet are arranged concentric with each other in an order mentioned, the first stator comprising at least one protruding piece-like magnetic pole part axially extending from the outer peripheral edge of the top plate toward the first magnet, and the second stator comprising at least one protruding piece-like magnetic pole part axially extending from the outer peripheral edge of the top plate toward the second magnet.  
         [0029]     With the arrangement of this preferred arrangement, since the first and second stators are formed integrally with the top plate, as protruding piece-like magnetic pole parts protruding from the top plate respectively, the number of component parts can be reduced. Further, since the magnetic pole parts of the first and second stators are configured to extend from the single top plate in the opposite axial directions, the axial length of the entire driving device can be reduced.  
         [0030]     Preferably, the driving device comprises a first top plate covering a surface of the first coil opposite to a surface thereof opposed to the first magnet, the first top plate having an outer peripheral edge, and a second top plate covering a surface of the second coil opposite to a surface thereof opposed to the second magnet, the second top plate having an outer peripheral edge and the first coil, the first magnet, the second magnet, and the second coil are arranged concentric with each other in an order mentioned, the first stator comprising at least one protruding piece-like magnetic pole part axially extending from the outer peripheral edge of the first top plate toward the first magnet, and the second stator comprising at least one protruding piece-like magnetic pole part axially extending from the outer peripheral edge of the second top plate toward the second magnet.  
         [0031]     With the arrangement of this preferred arrangement, since the opposite end faces of the driving device are closed by the respective top plates, it is not necessary to use special covers or the like to cover the opposite end faces, which simplifies the construction of the driving device. Further, since a magnetic path for passing magnetic flux generated by the first coil and a magnetic path for passing magnetic flux generated by the second coil are made separate from each other, each flow of magnetic flux effectively acts on the associated magnet without being disturbed. Furthermore, since the first stator and the second stator are independent of each other, it is possible to configure the shape of the outer magnetic pole part as desired. Moreover, it is unnecessary to cover the opposite end faces of the driving device with covers or the like as described above, and hence even if the first stator and the second stator are formed as separate members, demerits caused by an increase in the axial length of the driving device can be suppressed.  
         [0032]     To attain the above object, in a second aspect of the present invention, there is provided a light amount controller comprising a base plate having an opening, shutter blades for opening and closing the opening of the base plate, a light amount control member for controlling an amount of light passing through the opening of the base plate, and a driving device held on the base plate, for driving the shutter blades and the light amount control member, and the driving device comprises a hollow cylindrical first magnet having a peripheral wall thereof circumferentially divided into sections magnetized to have alternately different poles, the first magnet having an outer peripheral surface and an inner periphery, a first coil disposed concentric with and adjacent the first magnet and extending axially of the first magnet, a first stator having a magnetic pole part opposed to the outer peripheral surface of the first magnet, the magnetic pole part of the first stator being magnetized by the first coil, a first rotor rigidly fitted in the inner periphery of the first magnet, the first rotor being magnetized by the first coil, a first output member disposed to be driven to a first position or a second position by rotation of the first magnet, a hollow cylindrical second magnet disposed concentric with the first magnet and having a peripheral wall thereof circumferentially divided into sections magnetized to have alternately different poles, the second magnet having an outer peripheral surface and an inner periphery, a second coil disposed concentric with the second magnet and extending axially of the second magnet, the second coil having an inner periphery, a second stator having a magnetic pole part opposed to the outer peripheral surface of the second magnet, the magnetic pole part of the second stator being magnetized by the second coil, a second rotor rigidly fitted in the inner periphery of the second magnet, the second rotor having an inner periphery, and a second output member disposed to be driven to a third position or a fourth position by rotation of the second magnet, the first rotor of the driving device having a portion thereof inserted in the inner periphery of the second coil and the inner periphery of the second rotor, the portion being magnetized by the second coil, the first output member driving the light amount control member, and the second output member driving the shutter blades.  
         [0033]     The above and other objects, features, and advantages of the invention will become more apparent from the following detailed description taken in conjunction with the accompany drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0034]      FIG. 1  is an exploded perspective view of a driving device according to a first embodiment of the present invention;  
         [0035]      FIG. 2  is an axial cross-sectional view of the driving device in  FIG. 1  in an assembled state;  
         [0036]      FIG. 3A  is a cross-sectional view, taken on line A-A in  FIG. 2 , of a first magnet appearing in  FIG. 1  in a state having been driven to a first position;  
         [0037]      FIG. 3B  is a cross-sectional view, taken on line A-A in  FIG. 2 , of the first magnet in a state having been driven to a second position;  
         [0038]      FIG. 4A  is a cross-sectional view, taken on line B-B in  FIG. 2 , of a second magnet appearing in  FIG. 1  in a state having been driven to a third position;  
         [0039]      FIG. 4B  is a cross-sectional view, taken on line B-B in  FIG. 2 , of the second magnet in a state having been driven to a fourth position;  
         [0040]      FIG. 5A  is a perspective view showing the arrangement of a light amount controller equipped with the driving device according to the first embodiment;  
         [0041]      FIG. 5B  is a perspective view of the light amount controller as viewed from a shutter blade side;  
         [0042]      FIG. 6  is an exploded perspective view of the light amount controller having the driving device according to the first embodiment mounted incorporated therein;  
         [0043]      FIGS. 7A  to  7 D are plan views showing respective driven states of shutter blades and a light amount control blade of the light amount controller shown in  FIGS. 5A  to  6 ;  
         [0044]      FIG. 8  is an exploded perspective view of a driving device according to a second embodiment of the present invention;  
         [0045]      FIG. 9  is an axial cross-sectional view of the driving device in  FIG. 8  in an assembled state;  
         [0046]      FIG. 10  is an exploded perspective view of a conventional driving device; and  
         [0047]      FIG. 11  is an axial cross-sectional view of the driving device in  FIG. 10  in an assembled state. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0048]     The present invention will now be described in detail with reference to the drawings showing embodiments thereof.  
         [0049]     First, a driving device according to a first embodiment of the present invention will be described with reference to FIGS.  1  to  7 D.  
         [0050]      FIG. 1  is an exploded perspective view of the driving device, and  FIG. 2  is an axial cross-sectional view of the driving device in  FIG. 1  in an assembled state.  FIGS. 3A and 3B  and  4 A and  4 B are views useful in explaining angular reciprocation of the driving device through a predetermined rotational angle, in which  FIGS. 3A and 3B  are cross-sectional views taken on line A-A in  FIG. 2 , and  FIGS. 4A and 4B  cross-sectional views taken on line B-B in  FIG. 2 .  
         [0051]     In FIGS.  1  to  4 B, reference numeral  1  designates a first magnet in the form of a hollow cylinder having open opposite ends. The first magnet  1  has a peripheral wall thereof circumferentially divided into n sections (four sections in the present embodiment) magnetized such that they have alternately different S and N poles. More specifically, as shown e.g. in  FIG. 3A , a first section la and a third section  1   c  are N magnetized, and a second section  1   b  and a fourth section  1   d  are S magnetized. Further, the first magnet  1  has an inner periphery thereof rigidly fitted on the outer periphery of a disk-shaped part  8   a  of a first rotor  8 , described in detail hereinafter, such that the first magnet  1  can rotate in unison with the first rotor  8 .  
         [0052]     Reference numeral  2  designates a second magnet in the form of a hollow cylinder having open opposite ends. The second magnet  2  has a peripheral wall thereof circumferentially divided into n sections (four sections in the present embodiment) magnetized such that they have alternately different S and N poles. More specifically, as shown e.g. in  FIG. 4A , a first section  2   a  and a third section  2   c  are N magnetized, and a second section  2   b  and a fourth section  2   d  are S magnetized. The second magnet  2  has an inner periphery thereof rigidly fitted on the outer periphery of a second rotor  9 , described in detail hereinafter, such that the second magnet  2  can rotate in unison with the second rotor  9 .  
         [0053]     Reference numeral  3  designates a first coil in the form of a hollow cylinder having open opposite ends. The first coil  3  is formed by winding wire  3   a  around an annular groove  5   a  of a hollow cylindrical first bobbin  5  formed of an insulating material. The first coil  3  is disposed concentric with and adjacent the first magnet  1  and extends axially of the first magnet  1  on the second magnet  2  side with respect to the first magnet  1 . The outer diameter of the first coil  3  is approximately equal to that of the first magnet  1 . The first bobbin  5  has an inner periphery  5   b  thereof fitted on a base end side cylindrical portion  8   b  of the first rotor  8  to rotatably support the first rotor  8 .  
         [0054]     Reference numeral  4  designates a second coil in the form of a hollow cylinder having open opposite ends. The second coil  4  is formed by winding wire  4   a  around an annular groove  6   a  of a hollow cylindrical second bobbin  6  formed of an insulating material. The second coil  4  is disposed concentric with and adjacent the second magnet  2  and extends axially of the second magnet  2  on the first magnet  1  side with respect to the second magnet  2 . The outer diameter of the second coil  4  is approximately equal to that of the second magnet  2 . The second bobbin  6  has an inner periphery  6   b  thereof fitted on an intermediate cylindrical portion  8   c  of the first rotor  8  to rotatably support the first rotor  8 .  
         [0055]     Reference numeral  7  designates a stator (first and second stators formed integrally with each other) formed of a soft magnetic material. The stator  7  has a ring-shaped top plate  7   a,  and (n+N)/2 (four in the present embodiment) protruding parts  7   b,    7   c,    7   d  and  7   e  formed integrally with the top plate  7   a  and axially extending from the outer peripheral edge of the top plate  7   a.  The first protruding part  7   b  and the second protruding part  7   c  are bent through 90 degrees relative to the top plate  7   a  and extend in one axial direction, while the third protruding part  7   d  and the fourth protruding part  7   e  are bent through 90 degrees relative to the top plate  7   a  and extend in the opposite axial direction. The first protruding part  7   b  and the second protruding part  7   c  form first outer magnetic pole parts, and the third protruding part  7   d  and the fourth protruding part  7   e  form second outer magnetic pole parts. Thus, the top plate  7   a  and the first and second protruding parts  7   b  and  7   c  constitute a first stator, and the top plate  7   a  and the third and fourth protruding parts.  7   d  and  7   e  constitute a second stator.  
         [0056]     The first outer magnetic pole parts  7   b  and  7   c  are opposed to the outer peripheral surface of the first magnet  1  with a predetermined clearance therebetween. Further, the first outer magnetic pole parts  7   b  and  7   c  are offset from each other by an angle of 720/n degrees (180 degrees in the present embodiment) in the circumferential direction of the top plate  7   a.  The first outer magnetic pole parts  7   b  and  7   c  are magnetized by energization of the first coil  3  to act on the first magnet  1  together with a first inner magnetic pole part, described hereinafter.  
         [0057]     The second outer magnetic pole parts  7   d  and  7   e  are opposed to the outer peripheral surface of the second magnet  2  with a predetermined clearance therebetween. Further, the second outer magnetic pole parts  7   d  and  7   e  are offset from each other by an angle of 720/n degrees (180 degrees in the present embodiment) in the circumferential direction of the top plate  7   a.    
         [0058]     Since the first outer magnetic pole parts  7   b  and  7   c  and the second outer magnetic pole parts  7   d  and  7   e  are parts of the same member, it is necessary to form these parts at respective circumferential locations which do not overlap each other, as viewed in the axial direction. The second outer magnetic pole parts  7   d  and  7   e  are magnetized by energization of the second coil  4  to act on the second magnet  2  together with a second inner magnetic pole part, described hereinafter.  
         [0059]     The first rotor  8  is a rod-like rotor formed of a soft magnetic material. The first rotor  8  has a front end (lower end as viewed in  FIGS. 1 and 2 ) formed as a first output part (first output member)  8   e  and a base end (upper end as viewed in  FIGS. 1 and 2 ) formed as the disk-shaped part  8   a.  For example, a light amount control blade  15  (see  FIGS. 5B and 6 ), described in detail hereinafter, is driven by output from the first output part  8   e.  The disk-shaped part  8   a  is rigidly fitted in the inner periphery of the first magnet  1 , and the first rotor  8  performs angular reciprocation along with reciprocating motion of the first magnet  1  to drive, for example, the light amount control blade  15 .  
         [0060]     The base end side cylindrical portion  8   b  (a portion indicated by a double-headed arrow C in  FIG. 2 ) of the first rotor  8  is inserted in the inner periphery of the first coil  3 , and when the first coil  3  is energized, the disk-shaped part  8   a  and the base end side cylindrical portion  8   b  are magnetized. The disk-shaped part  8   a  of the first rotor  8  is opposed to the first outer magnetic pole parts  7   b  and  7   c  of the stator  7  opposed to the first magnet  1 , such that it sandwiches the first magnet  1  between the same and the first outer magnetic pole parts  7   b  and  7   c,  and forms the first inner magnetic pole part. The first inner magnetic pole part  8   a  is magnetized by the first coil  3  such that it has an opposite pole to the pole of the first outer magnetic pole parts  7   b  and  7   c  of the stator  7 . As a result, a magnetic circuit is formed by the top plate  7   a,  and the first outer magnetic pole parts  7   b  and  7   c  of the stator  7  and the first inner magnetic pole part  8   a  and the base end side cylindrical portion  8   b  of the first rotor  8 . The distance between the first outer magnetic pole parts  7   b  and  7   c  and the first inner magnetic pole part  8   a  of the first rotor  8  is controlled only by the thickness of the first magnet  1  and a gap between the first magnet  1  and the first outer magnetic pole parts  7   b  and  7   c,  which gap is set to a small value that prevents contact between these parts, so that the distance can be set to a required minimum value, which makes it possible to reduce the resistance of the magnetic circuit and cause magnetic flux to effectively act on the first magnet  1  sandwiched between the first outer magnetic pole parts  7   b  and  7   c  and the first inner magnetic pole part  8   a.    
         [0061]     The intermediate cylindrical portion  8   c  (a portion indicated by a double-headed arrow D in  FIG. 2 ) of the first rotor  8  is inserted in the inner periphery of the second coil  4 , and when the second coil  4  is energized, the intermediate cylindrical portion  8   c  and a front end-side cylindrical portion  8   d  of the first rotor  8  are magnetized. Further, since the front end-side cylindrical portion  8   d  is fitted in the second rotor  9 , magnetic flux flows through the front end-side cylindrical portion  8   d.  As a result, the intermediate cylindrical portion  8   c,  the front end-side cylindrical portion  8   d,  and the second rotor  9  form the second inner magnetic pole part, and the magnetic flux acts on the second magnet  2 . The second inner magnetic pole part (the intermediate cylindrical portion  8   c  and the front end-side cylindrical portion  8   d  of the first rotor  8 , and the second-rotor  9 ) is magnetized by the second coil  4  such that it has an opposite pole to the pole of the second outer magnetic pole parts  7   d  and  7   e.  As a result, a magnetic circuit is formed by the top plate  7   a  and the second outer magnetic pole parts  7   d  and  7   e,  and the second inner magnetic pole part. The distance between the second outer magnetic pole parts  7   d  and  7   e  and the second inner magnetic pole part (second rotor  9 ) is controlled only by the thickness of the second magnet  2  and a gap between the second magnet  2  and the second outer-magnetic pole parts  7   d  and  7   e,  which gap is set to a small value that prevents contact between these parts, so that the distance can be set to a required minimum value, which makes it possible to reduce the resistance of the magnetic circuit and cause magnetic flux to effectively act on the second magnet  2  sandwiched between the second outer magnetic pole parts  7   d  and  7   e  and the second inner magnetic pole part. The first rotor  8  is rigidly fitted in the first magnet  1 , and functions not only as the first inner magnetic pole part while rotating in unison with the first magnet  1 , but also as the second inner magnetic pole part.  
         [0062]     A hemispheric protrusion  8   f  protrudes from the center of the upper surface of the disk-shaped part  8   a  of the first rotor  8 . The protrusion  8   f  is held in point-contact with the inner surface of the closed end of a cover  11 , referred to hereinafter, whereby contact resistance of the cover  11  to the rotation of the first rotor  8  is reduced.  
         [0063]     The second rotor  9  is a rod-like rotor formed of a soft magnetic material. The second rotor  9  is rigidly fitted in the inner periphery of the second magnet  2 , and performs angular reciprocation along with angular reciprocation of the second magnet  2 . Further, the second rotor  9  is rotatably fitted on the front end-side cylindrical portion  8   d  of the first rotor  8 .  
         [0064]     Reference numeral  10  designates a second output member e.g. for driving shutter blades  13  and  14  (see  FIGS. 5B and 6 ), referred to hereinafter. The second output member  10  is comprised of a ring-shaped member  10   a,  an output pin lob axially extending from the outer peripheral edge of the ring-shaped member  10   a,  an engaging pin  10   c  axially extending from the upper surface of the ring-shaped member  10   a  in the opposite direction to the direction in which the output pin  10   b  extends. The ring-shaped member  10   a  has a central opening thereof fitted on the first output part  8   e  of the first rotor  8 , and the engaging pin  10   c  is fitted in an engaging hole  9   a  in the second rotor  9 , whereby the second output member  10  is fixedly attached to the lower surface of the second rotor  9  for angular reciprocation in unison with the second rotor  9  through a predetermined rotational angle.  
         [0065]     The second output member  10  may be formed integrally with the second magnet  2  or the second rotor  9 .  
         [0066]     The cover  11  in the form of an inverted bottomed hollow cylinder covers the entire driving device to prevent an external force from being applied to the rotating magnets  1  and  2  and the magnetic pole parts of the stator  7  to deform these, and dust from entering the driving device.  
         [0067]     Next, a description will be given of a light amount controller equipped with the driving device configured as above, according to the present embodiment, with reference to  FIGS. 5A  to  7 D.  
         [0068]      FIG. 5A  is a perspective view showing the arrangement of the light amount controller equipped with the driving device of the present embodiment, and  FIG. 5B  is a perspective view of the light amount controller as viewed from the shutter blade side. Further,  FIG. 6  is an exploded perspective view of the light amount controller, and  FIGS. 7A  to  7 D are plan views showing driven states of the shutter blades and the light amount control blade.  
         [0069]     In  FIGS. 5A  to  7 D, reference numeral  12  designates a disk-shaped base plate. The base plate  12  has an opening  12   a  formed in its center, and holds the driving device of the present embodiment, the shutter blades  13  and  14 , and the light amount control blade  15 , referred to hereinafter, and so forth.  
         [0070]     The above-described driving device drives the shutter blades  13  and  14  to open and close the opening  12   a  of the base plate  12 , and sets the light amount control blade  15  in the optical path to reduce the amount of light or out of the same.  
         [0071]     The two shutter blades  13  and  14  can be driven by the output pin  10   b  of the second output member  10  between a position for closing the opening  12   a  of the base plate  12  and a position for opening the opening  12   a.  More specifically, the output pin  10   b  is slidably engaged in an arcuate guide slot  13   b  in the shutter blade  13  and an arcuate guide slot  14   b  in the shutter blade  14 . Further, a shaft hole  13   a  in the shutter blade  13  is rotatably fitted on a shaft part  12   b  protruding from the base plate  12 , and a shaft hole  14   a  in the shutter blade  14  is rotatably fitted on the first output part  8   e  of the first rotor  8 . Thus, the shutter blade  13  rotates about the shaft hole  13   a,  while the shutter blade  14  rotates about the shaft hole  14   a.    
         [0072]      FIGS. 7A and 7D  show states in which the shutter blades  13  and  14  have been driven to the respective positions for opening the opening  12   a  of the base plate  12 , and  FIGS. 7B and 7C  show states in which the shutter blades  13  and  14  have been driven to the respective positions for closing the opening  12   a  of the base plate  12 .  
         [0073]     The second magnet  2  can be driven for rotation between two positions by switching the direction of energization of the second coil  4 , and in accordance with this rotation, the shutter blade  14  is driven between the positions shown in  FIGS. 7A and 7B , respectively.  
         [0074]     An operation for driving the second magnet  2  will be described in detail hereinafter.  
         [0075]     As shown in  FIGS. 5B  to  7 D, the light amount control blade (light amount control member)  15  has one end thereof formed with an opening  15   a  smaller in diameter than the opening  12   a  of the base plate  12 . Further, the light amount control blade  15  has the other end thereof formed with a shaft hole  15   b  rigidly fitted on the first output part  8   e  of the first rotor  8 . The light amount control blade  15  serves as a member for reducing the opening area of the opening  12   a  of the base plate  12  to thereby limit the amount of exposure, and is brought into an aligned position or a retreated position with respect to the opening  12   a  according to brightness (exposure amount). The light amount control blade  15  is driven for rotation about the shaft hole  15   b  by rotation of the first output part  8   e  of the first rotor  8 . The aligned position of the light amount control blade  15  with respect to the opening  12   a  is shown in  FIGS. 7A and 7B , while the retreated position of the light amount control blade  15  with respect to the opening  12   a  is shown in  FIGS. 7C and 7D . The first magnet  1  can be driven for rotation to two positions by switching the direction of energization of the first coil  3 , and the light amount control blade  15  is driven in accordance with the rotation of the first magnet  1 .  
         [0076]     Next, a description will be given of the operation for driving the first magnet  1  between the two positions (first and second positions) by energizing the first coil  3 , with reference to  FIGS. 3A and 3B .  
         [0077]      FIG. 3A  is a cross-sectional view, taken on line A-A in  FIG. 2 , of the first magnet  1  in a state having been driven to the first position (e.g. the state shown in  FIG. 7A ), and  FIG. 3B  is a cross-sectional view, taken on line A-A in  FIG. 2 , of the first magnet  1  in a state having been driven to the second position (e.g. the state shown in  FIG. 7C ).  
         [0078]     When the first outer magnetic pole parts  7   b  and  7   c  of the stator  7  are S magnetized and the first inner magnetic pole part  8   a  of the first rotor  8  is N magnetized, by energizing the first coil  3  in the state shown in  FIG. 3A , the first magnet  1  is rotated clockwise to reach the state shown in  FIG. 3B , in which the first magnet  1  abuts a stopper, not shown, to stop.  
         [0079]     As the first magnet  1  is rotated, the first output part  8 e of the first rotor  8  rigidly fitted in the first magnet  1  is rotated along with the first magnet  1 , whereby the light amount control blade  15  interlocked to the first rotor  8  is also rotated. In the state shown in  FIG. 3B , if the first outer magnetic pole parts  7   b  and  7   c  of the stator  7  are N magnetized and the first inner magnetic pole part  8   a  of the first rotor  8  is S magnetized by energizing the first coil  3  in the opposite direction to that of the above energization in the state shown in  FIG. 3A , the first magnet  1  is rotated counterclockwise to reach the state shown in  FIG. 3A , in which the first magnet  1  abuts a stopper, not shown, to stop.  
         [0080]     As is apparent from the above description, the first magnet  1  can be driven between the first and second positions through a predetermined rotational angle by switching the direction of energization of the first coil  3 .  
         [0081]     Next, a description will be given of the operation for driving the second magnet  2  between the two positions (third and fourth positions) by energizing the second coil  4 , with reference to  FIGS. 4A and 4B .  
         [0082]      FIG. 4A  is a cross-sectional view, taken on line B-B in  FIG. 2 , of the second magnet  2  in a state having been driven to the third position (e.g. the state shown in  FIG. 7B ), and  FIG. 4B  is a cross-sectional view, taken on line B-B in  FIG. 2 , of the second magnet  2  in a state having been driven to the fourth position (e.g. the state shown in  FIG. 7D ).  
         [0083]     When the second outer magnetic pole parts  7   d  and  7   e  of the stator  7  are N magnetized and the second rotor  9  and the second inner magnetic pole part of the first rotor  8  are S magnetized, by energizing the second coil  4  in the state shown in  FIG. 4A , the second magnet  2  is rotated clockwise to reach the state shown in  FIG. 4B , in which the second magnet  2  abuts a stopper, not shown, to stop. As the second magnet  2  is rotated, the output pin  10   b  of the second output member  10  is rotated along with the second magnet  2 , whereby the shutter blades  13  and  14  interlocked to the output pin  10   b  are also rotated.  
         [0084]     In the state shown in  FIG. 4B , if the second outer magnetic pole parts  7   d  and  7   e  of the stator  7  are S magnetized and the second rotor  9  and the second inner magnetic pole part of the first rotor  8  are N magnetized by energizing the second coil  4  in the opposite direction to that of the energization in the state shown in  FIG. 4A , the second magnet  2  is rotated counterclockwise to reach the state shown in  FIG. 4A , in which the second magnet  2  abuts a stopper, not shown, to stop.  
         [0085]     As is apparent from the above description, the second magnet  2  can be driven between the third and fourth positions through a predetermined rotational angle by switching the direction of energization of the second coil  4 .  
         [0086]     With the arrangement described above, magnetic flux generated by energization of the first coil  3  forms the magnetic circuit extending from the top plate  7   a  through the first outer magnetic pole parts  7   b  and  7   c  to the first inner magnetic pole part  8   a,  so that leakage of magnetic flux into the second outer magnetic pole parts  7   d  and  7   e  and the second inner magnetic pole part, which causes an increase in magnetic resistance, can be almost reduced to zero. Thus, the energization of the first coil  3  only drives the first magnet  1  without adversely affecting the second magnet  2 .  
         [0087]     Similarly, magnetic flux generated by energization of the second coil  4  forms the magnetic circuit extending from the top plate  7   a  through the second outer magnetic pole parts  7   d  and  7   e  to the second inner magnetic pole part, so that leakage of magnetic flux into the first outer magnetic pole parts  7   b  and  7   c  and the first inner magnetic pole part  8   a,  which causes an increase in magnetic resistance, can be almost reduced to zero. Thus, the energization of the second coil  4  only drives the second magnet  2  without adversely affecting the first magnet  1 .  
         [0088]     As is apparent from the above description, the rotations of the two magnets  1  and  2  can be controlled as desired by switching the two coils  3  and  4  for energization, as desired, which makes it possible to produce two outputs separately.  
         [0089]     If two driving devices as disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2002-49076, referred to hereinbefore, are combined back-to-back, the combined devices extend over a doubled axial length, and the number of component parts is also doubled. Further, one of the two output pins extends from one end of the combined devices, and the other extends from the other end of the same. Therefore, it is impossible to have both of the two output parts extended in the same direction as in the present embodiment.  
         [0090]     In contrast, in the driving device of the present embodiment, the top plate  7   a  connecting between the inner magnetic pole part and the outer magnetic pole parts is shared by the first and second stators, which makes it possible to reduce the axial length of the entire driving device. Further, the first outer magnetic pole parts and the second outer magnetic pole parts are integrally formed by bending the former and the latter in opposite directions and in a manner offset from each other in the circumferential direction of the top plate  7   a,  which contributes to reduction of the number of component parts and manufacturing costs. Furthermore, the first rotor  8  is a rod-like component part extending through the center of the driving device, and functions not only as the first output member but also as the first and second inner magnetic pole parts. Thus, the first rotor  8  has a simple shape and the number of component parts can be reduced, eliminating the necessity to use such a complicated stator as is used in the driving device disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 2002-49076.  
         [0091]     Moreover, as is apparent from  FIG. 2  showing the driving device of the present embodiment in cross section, most of the component parts are formed of soft magnetic materials and used to form a magnetic circuit, magnetic resistance is reduced so that magnetic flux effectively acts on the magnets  1  and  2 . Therefore, the driving device can produce a high output despite its small size.  
         [0092]     Next, a description will be given of a second embodiment of the present invention, with reference to  FIGS. 8 and 9 .  
         [0093]      FIG. 8  is an exploded perspective view of a driving device according to the second embodiment, and  FIG. 9  is an axial cross-sectional view of the driving device in  FIG. 8  in an assembled state.  
         [0094]     In  FIGS. 8 and 9 , reference numeral  21  designates a first magnet in the form of a hollow cylinder having open opposite ends. The first magnet  21  has a peripheral wall thereof circumferentially divided into n sections (four sections in the present embodiment) magnetized such that they have alternately different S and N poles. The first magnet  21  has an inner periphery thereof rigidly fitted on an outer periphery of an increased diameter part  28   c  of a first rotor  28 , described in detail hereinafter, such that the first magnet  21  can rotate in unison with the first rotor  28 .  
         [0095]     Reference numeral  22  designates a second magnet in the form of a hollow cylinder having open-opposite ends. The second magnet  22  has a peripheral wall thereof circumferentially divided into n sections (four sections in the present embodiment) magnetized such that they have alternately different S and N poles. The second magnet  22  has an inner periphery thereof rigidly fitted on an outer periphery of a second rotor  29 , described in detail hereinafter, such that the second magnet  22  can rotate in unison with the second rotor  29 .  
         [0096]     Reference numeral  23  designates a first coil in the form of a hollow cylinder. The first coil  23  is formed by winding wire  23   a  around an annular groove  25   a  of a hollow cylindrical first bobbin  25  formed of an insulating material. The first coil  23  is disposed concentric with and adjacent the first magnet  21  and extends axially of the first magnet  21  on a side opposite to the second magnet  22  with respect to the first magnet  21 . The outer diameter of the first coil  23  is approximately equal to that of the first magnet  21 .  
         [0097]     Reference numeral  24  designates a second coil in the form of a hollow cylinder. The second coil  24  is formed by winding wire  24   a  around an annular groove  26   a  of a hollow cylindrical second bobbin  26  formed of an insulating material. The second coil  24  is disposed concentric with and adjacent the second magnet  22  and extends axially of the second magnet  22  on a side opposite to the first magnet  21  with respect to the second magnet  22 . The outer diameter of the second coil  24  is approximately equal to that of the second magnet  22 .  
         [0098]     Reference numeral  32  designates a first stator formed of a soft magnetic material. The first stator  32  has a circular top plate  32   b  formed with a hole  32   a  in its center and protruding parts  32   c  and  32   d  axially extending from the outer peripheral edge of the top plate  32   b.  The protruding parts  32   c  and  32   d  are opposed to the outer peripheral surface of the first magnet  21  with a predetermined clearance therebetween. The protruding parts  32   c  and  32   d,  which function as outer magnetic pole parts, are circumferentially offset from each other by 720/n degrees (180 degrees in the present embodiment). When the first coil  23  is energized, the first outer magnetic pole parts  32   c  and  32   d  are magnetized to act on the first magnet  21  together with a first inner magnetic pole part, described in detail hereinafter.  
         [0099]     Reference numeral  33  designates a second stator formed of a soft magnetic material. The second stator  33  has a circular top plate  33   b  formed with a hole  33   a  in its center and protruding parts  33   c  and  33   d  axially extending from the outer peripheral edge of the top plate  33   b.  The protruding parts  33   c  and  33   d  are opposed to the outer peripheral surface of the second magnet  22  with a predetermined clearance therebetween. The protruding parts  33   c  and  33   d,  which function as outer magnetic pole parts, are circumferentially offset from each other by 720/n degrees (180 degrees in the present embodiment). When the second coil  24  is energized, the second outer magnetic pole parts  33   c  and  33   d  are magnetized to act on the second magnet  22  together with a second inner magnetic pole part, described in detail hereinafter.  
         [0100]     The second rotor  29 , which is formed of a soft magnetic material, is rigidly fitted in the inner periphery of the second magnet  22 . The second rotor  29  performs angular reciprocation along with angular reciprocation of the second magnet  22 . Further, the second rotor  29  is rotatably fitted on a cylindrical part  28   d,  described in detail hereinbelow, of the first rotor  28 . The second rotor  29  is magnetized together with the cylindrical part  28   d  of the first rotor  28  and a cylindrical part  28   e,  referred to hereinafter, of the same, to act on the second magnet  22  as the second inner magnetic pole part.  
         [0101]     The rod-like first rotor  28 , which is formed of a soft magnetic material, has a first output part (first output member)  28   a  on a front end side (lower end side as viewed in  FIGS. 8 and 9 ) thereof. Further, the first rotor  28  has a cylindrical part  28   b  on a base end side (upper end side as viewed in  FIGS. 8 and 9 ) thereof, and the increased diameter part  28   c  lies between the cylindrical part  28   b  and the first output part  28   a.  The increased diameter part  28   c  of the first rotor  28  is rigidly fitted in the inner periphery of the first magnet  21 . The first rotor  28  performs angular reciprocation along with angular reciprocation of the first magnet  21  to drive the light amount control blade  15  similarly to the first embodiment, e.g. as described hereinabove with reference to  FIGS. 5A  to  7 D.  
         [0102]     Further, the cylindrical part  28   b  of the first rotor  28  is inserted in the inner periphery of the first coil  23 . When the first coil  23  is energized, the cylindrical part  28   b  and the increased diameter part  28   c  are magnetized. The increased diameter part  28   c  of the first rotor  28  is opposed to the first outer magnetic pole parts  32   c  and  32   d  of the first stator  32  which is opposed to the first magnet  21 , such that it sandwiches the first magnet  21  between the same and the first outer magnetic pole parts  32   c  and  32   d,  and forms the first inner magnetic pole part. The first inner magnetic pole part  28   c  is magnetized by the first coil  23  such that it has an opposite pole to the pole of the first outer magnetic pole parts  32   c  and  32   d.  As a result, a magnetic circuit is formed by the first outer magnetic pole parts  32   c  and  32   d,  the top plate  32   b,  and the first inner magnetic pole part. The distance between the first outer magnetic pole parts  32   c  and  32   d  and the first inner magnetic pole part is controlled only by the thickness of the first magnet  21  and a gap between the first magnet  21  and the first outer magnetic pole parts  32   c  and  32   d,  which gap is set to a small value that prevents contact between these parts, so that the distance can be set to a required minimum value, which makes it possible to reduce the resistance of the magnetic circuit and cause magnetic flux to effectively act-on the first magnet  21  sandwiched between the first outer magnetic pole parts  32   c  and  32   d  and the first inner magnetic pole part.  
         [0103]     The cylindrical parts  28   d  and  28   e  of the first rotor  28  between the increased diameter part  28   c  and the first output part  28   a  are inserted in the inner periphery of the second magnet  22  and the inner periphery of the second coil  24 , respectively, and when the second coil  24  is energized, the cylindrical parts  28   d  and  28   e  are magnetized. Further, since the cylindrical part  28   d  is in contact with the second rotor  29 , magnetic flux flows into the cylindrical part  28   d.  As a result, the second inner magnetic pole part is formed by the cylindrical parts  28   d  and  28   e  and the second rotor  29 , whereby the magnetic flux acts on the second magnet  22 .  
         [0104]     The second inner magnetic pole part (cylindrical parts  28   d  and  28   e  and second rotor  29 ) is magnetized by the second coil  24  such that it has an opposite pole to the pole of the second outer magnetic pole parts  33   c  and  33   d  of the second stator  33 . As a result, a magnetic circuit is formed by the second outer magnetic pole parts  33   c  and  33   d,  the top plate  33   b,  and the second inner magnetic pole part. The distance between the second outer magnetic pole parts  33   c  and  33   d  and the second inner magnetic pole part is controlled only by the thickness of the second magnet  22  and a gap between the second magnet  22  and the second outer magnetic pole parts  33   c  and  33   d,  which gap is set to a small value that prevents contact between these parts, so that the distance can be set to a required minimum value, which makes it possible to reduce the resistance of the magnetic circuit and cause magnetic flux to effectively act on the second magnet  22  sandwiched between the second outer magnetic pole parts  33   c  and  33   d  and the second inner magnetic pole part.  
         [0105]     The first rotor  28  is rigidly fitted in the first magnet  21 , and acts not only as the inner magnetic pole part for rotation in unison with the first magnet  21 , but also as the second inner magnetic pole part.  
         [0106]     Reference numeral  30  designates a second output member for driving the shutter blades  13  and  14  similarly to the first embodiment, e.g. as described hereinabove with reference to  FIGS. 5A  to  7 D. The second output member  30  is comprised of a ring-shaped member  30   a,  an output pin  30   b  axially extending from the outer peripheral edge of the ring-shaped member  30   a,  and an engaging pin  30   c  axially extending from the upper surface of the ring-shaped member  30   a  in the opposite direction to the direction in which the output pin  30   b  extends. The central opening of the ring-shaped member  30   a  is fitted on the cylindrical part  28   d  of the first rotor  28 , and the engaging pin  30   c  is fitted in an engaging hole  29   a  in the second rotor  29 , whereby the second output member  30  is fixedly attached to the lower surface of the second rotor  29  for angular reciprocation in unison with the second rotor  29  through a predetermined rotational angle. Further, the second output member  30  drives the shutter blades  13  and  14 , by the axially extending output pin  30   b.    
         [0107]     The second output member  30  may be formed integrally with the second magnet  22  or the second rotor  29 .  
         [0108]     Reference numeral  31  designates a cover in the form of a hollow cylinder having open opposite ends, which covers the driving device at the entire circumferential side thereof and holds the first stator  32  and the second stator  33  in concentricity with each other.  
         [0109]     As is distinct from the driving device of the first embodiment in which the first output part  8 e side end thereof is fully exposed, in the present embodiment, the driving device has opposite axial ends thereof both covered by the top plate  32   b  of the first stator  32  and the top plate  33   b  of the second stator  33 , respectively. Therefore, it suffices to cover the circumferential side of the driving device to protect the driving device from external influence of dust and the like.  
         [0110]     The operation of the driving device of the present embodiment is the same as that of the driving device of the first embodiment, and therefore description thereof is omitted.  
         [0111]     The driving device of the present embodiment has the opposite axial ends thereof covered, respectively, by the top plate  32   b  of the first stator  32  and the top plate  33   b  of the second stator  33 , as described above, so that it is unnecessary to provide special cover members to cover the opposite axial ends of the driving device.  
         [0112]     In the first embodiment, the top plate  7   a  of the stator  7  functioning as a magnetic path passes not only magnetic flux generated by the first coil  3 , but also magnetic flux generated by the second coil  4 .  
         [0113]     On the other hand, in the second embodiment, a magnetic path for passing magnetic flux generated by the first coil  23  and a magnetic path for passing magnetic flux generated by the second coil  24  are completely separated from each other. Therefore, disturbance of magnetic flux is prevented, and each flow of magnetic flux effectively acts on a corresponding one of the magnets  21  and  22 .  
         [0114]     Further, in the first embodiment in which the first stator and the second stator ate integrally formed as the stator  7 , the first outer magnetic pole parts and the second outer magnetic pole parts should be axially bent from the top plate  7   a  by press working such that the first outer magnetic pole parts and the second outer magnetic pole parts are disposed at respective circumferential locations which do not overlap each other.  
         [0115]     In contrast, in the second embodiment, since the first stator  32  and the second stator  33  are independent of each other, the number of protruding parts (i.e. the number of magnetic poles of each magnet) and the shape thereof are not limited, and therefore the shape of the outer magnetic pole parts can be configured as desired.  
         [0116]     In the first embodiment, in which the first stator and the second stator are integrally formed as the stator  7 , and the magnetic path for passing magnetic flux generated by the first coil  3  and the magnetic path for passing magnetic flux generated by the second coil  4  are commonly provided by the top plate  7   a  of the stator  7 , the axial length of the driving device can be reduced, but the driving device has one axial end thereof open (i.e. the first magnet  1  side end portion is exposed), which necessitates covering the open end by the cover  11 . The axial length, including the thickness of the cover  11 , of the driving device of the first embodiment is substantially equal to that of the driving device of the second embodiment, and therefore the driving device of the second embodiment which is configured to have the first and second stators  32  and  33  as separate members is hardly disadvantageous in respect of the axial length thereof.  
         [0117]     It should be noted that the present invention is not limited to the above-described embodiments, but can be modified in various manners based on the subject matter of the present invention, which should not be excluded from the scope of the present invention.  
       CROSS REFERENCE TO RELATED APPLICATION  
       [0118]     This application claims priority from Japanese Patent Application No. 2004-175937 filed Jun. 14, 2004, which is hereby incorporated by reference.