Abstract:
The invention discloses a motor device comprising a pair of motor units and a driven member meshed with rotary gears of the pair of motor units to drive the driven member, the motor units each comprising a rotary magnet that is cylindrical and magnetized so as to have different magnetic poles in turn along the circumferential direction, a rotary gear having a plurality of teeth, the rotary gear rotating about the rotation axis of the rotary magnet together with the rotary magnet, and a stator member having a plurality of outer magnetic poles that are arranged on the outer periphery of the rotary magnet and a plurality of inner magnetic poles that are arranged on the inner periphery of the rotary magnet and are opposed to the outer magnetic poles, the plural outer magnetic poles and the plural inner magnetic poles being excited by a coil.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a driving device composed of an ultra-small motor, and particularly to an aperture blade driving device using the same.  
           [0003]    2. Related Backgroud Art  
           [0004]    [0004]FIG. 18 shows one of small-sized cylindrical step motors in prior art. A stator coil  105  is wound around a bobbin  101  concentrically. The bobbin  101  is fixed by being axially held and fixed by two stator yokes  106  and  106 . Each stator yoke  106  has stator teeth  106   a  and  106   b  that are alternately placed on the inner circumferential surface and in the circumferential direction of the bobbin  101 . The stator yoke  106  integrated with the stator teeth  106   a  or  106   b  is fixed to case  103  and whereby composing a stator  102 .  
           [0005]    Of two cases  103  and  103 , one case  103  has a flange  115  and a bearing  108  that are fixed thereto whereas the other case  103  has another bearing  108  fixed thereto. A rotor  109  is composed of a rotor magnet  111  fixed to a rotor shaft  110 . A radial void is formed between the rotor magnet  111  and the stator teeth  106   a  of stator  102 . The rotor shaft  110  is rotatably supported between the two bearings  108  and  108 .  
           [0006]    [0006]FIG. 20 shows a step motor that is one of those driven by a single coil and is frequently used in watches. Denoted by  201  is a rotor composed of a permanent magnet, and  202  and  203  denotes stators while  204  denotes a coil.  
           [0007]    The conventional small-sized step motor shown in FIG. 18 has a drawback that the external dimension of the motor is large because the case  103 , the bobbin  101 , the stator coil  105  and the stator yoke  106  are arranged concentrically about the outer periphery of the rotor. This step motor has another drawback of low output, for, as shown in FIG. 19, flux of magnetic force generated upon energization of the stator coil  105  mainly passing between an end face  106   a   1  of the stator tooth  106   a  and an end face  106   b   1  of the stator tooth  106   b  and not acting on the rotor magnet  111  effectively.  
           [0008]    The step motor shown in FIG. 20 is also ineffective in making flux of magnetic force that is generated upon energization of the coil act on the magnet because the flux of magnetic force concentrates on a small gap between the stator  202  and the stator  203 . The inventors of the present invention have proposed a motor to solve these problems in Japanese Patent Application Laid-Open No. 9-331666 (U.S. Pat. No. 5,831,356).  
           [0009]    The motor proposed has a rotor formed into a cylinder and composed of a permanent magnet that is divided into equal parts along the circumferential direction and is magnetized so as to form different poles alternately. In the axial direction of the rotor, a first coil, the rotor and a second coil are arranged in this order. The first coil excites a first outer magnetic pole that faces the outer peripheral surface of the rotor as well as a first inner magnetic pole that faces the inner peripheral surface of the rotor. The second coil excites a second outer magnetic pole that faces the outer peripheral surface of the rotor as well as a second inner magnetic pole that faces the inner peripheral surface of the rotor. The rotation axis that is the rotor shaft is taken out the interior of the cylindrical permanent magnet.  
           [0010]    The motor structured as above has high output and small motor external dimension, but there is room to improve in facilitating joining of the rotor shaft to the permanent magnet. If a thin magnet is used in the structure above, the distance between the first outer magnetic pole and the first inner magnetic pole as well as the distance between the second outer magnetic pole and the second inner magnetic pole can be closed, resulting in reduction in magnetic resistance in the magnetic circuit. This makes it possible to generate more flux of magnetic force with a small amount of current flowing through the first coil and the second coil.  
           [0011]    However, the motor disclosed in Japanese Patent Application Laid-Open No. 9-331666 and other motors of that type are required to be long to a certain degree in the axial direction. For that reason, there has lately been a demand for a drive transmission device composed of a ultra-small motor whose axial length is very short. The drive transmission device composed of a ultra-small motor whose axial length is very short is particularly needed as a drive transmission device used in a lens barrel of a camera in order to drive an aperture blade, a shutter, a lens, etc.  
         SUMMARY OF THE INVENTION  
         [0012]    A first object of the present invention is to provide a drive transmission device composed of an ultra-small motor that is easy to assemble and has high output.  
           [0013]    A second object of the present invention is to provide a drive transmission device composed of an ultra-small motor whose axial length is very short.  
           [0014]    A third object of the present invention is to provide an aperture blade driving device as an actuator for driving an aperture blade to open and close the blade which does not form an obstacle in arranging a lens with respect the direction parallel to the principal axis.  
           [0015]    According to one aspect of the present invention, a motor device comprises:  
           [0016]    a first motor unit including:  
           [0017]    a first rotary magnet that is cylindrical and magnetized so as to have different magnetic poles in turn along the circumferential direction;  
           [0018]    a first rotary gear having a plurality of teeth, the rotary gear rotating about the rotation axis of the first rotary magnet together with the first rotary magnet; and  
           [0019]    a stator member having a plurality of outer magnetic poles that are arranged on the outer periphery of the first rotary magnet and a plurality of inner magnetic poles that are arranged on the inner periphery of the first rotary magnet and are opposed to the outer magnetic poles, the plural outer magnetic poles and the plural inner magnetic poles being excited by a first coil;  
           [0020]    a second motor unit including:  
           [0021]    a second rotary magnet that is cylindrical and magnetized so as to have different magnetic poles in turn along the circumferential direction;  
           [0022]    a second rotary gear having a plurality of teeth, the rotary gear rotating about the rotation axis of the second rotary magnet together with the second rotary magnet; and  
           [0023]    a stator member having a plurality of outer magnetic poles that are arranged on the outer periphery of the second rotary magnet and a plurality of inner magnetic poles that are arranged on the inner periphery of the second rotary magnet and are opposed to the outer magnetic poles, the plural outer magnetic poles and the plural inner magnetic poles being excited by a second coil; and  
           [0024]    a driven member meshed with the first rotary gear and the second rotary gear,  
           [0025]    wherein electric power having different phases are supplied to the first and second coils to rotate the first rotary gear and the second rotary gear, which then drive the driven member.  
           [0026]    The number of teeth of the first rotary gear is equal to the number of magnetic poles of the first rotary magnet, and the number of teeth of the second rotary gear is equal to the number of magnetic poles of the second rotary magnet.  
           [0027]    The number of magnetic poles of the first rotary magnet is equal to the number of magnetic poles of the second rotary magnet.  
           [0028]    The first and second motor units are arranged side by side so that their rotation axes are in parallel with each other.  
           [0029]    The number of the plural outer magnetic poles is half the number of the magnetic poles of the respective rotary magnets.  
           [0030]    The device further comprises an aperture blade that is put in motion when the driven member is driven.  
           [0031]    The first rotary gear is formed in the first rotary magnet and the second rotary gear is formed in the second rotary magnet.  
           [0032]    According to another aspect of the present invention, a motor device comprises:  
           [0033]    a first motor unit including:  
           [0034]    a first rotary magnet that is cylindrical and magnetized so as to have different magnetic poles in turn along the circumferential direction, the first rotary magnet having a first gear portion that is formed therein and has a plurality of teeth; and  
           [0035]    a stator member having a plurality of outer magnetic poles that are arranged on the outer periphery of the first rotary magnet and a plurality of inner magnetic poles that are arranged on the inner periphery of the first rotary magnet and are opposed to the outer magnetic poles, the plural outer magnetic poles and the plural inner magnetic poles being excited by a first coil;  
           [0036]    a second motor unit including:  
           [0037]    a second rotary magnet that is cylindrical and magnetized so as to have different magnetic poles in turn along the circumferential direction, the second rotary magnet having a second gear portion that is formed therein and has a plurality of teeth; and  
           [0038]    a stator member having a plurality of outer magnetic poles that are arranged on the outer periphery of the second rotary magnet and a plurality of inner magnetic poles that are arranged on the inner periphery of the second rotary magnet and are opposed to the outer magnetic poles, the plural outer magnetic poles and the plural inner magnetic poles being excited by a second coil; and  
           [0039]    a driven member meshed with the first gear portion and the second gear portion,  
           [0040]    wherein electric power having different phases are supplied to the first and second coils to rotate the first gear portion and the second gear portion, which then drive the driven member. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0041]    [0041]FIG. 1 is an exploded perspective view of an aperture blade driving mechanism;  
         [0042]    [0042]FIG. 2 is an exploded perspective view of a first driving device;  
         [0043]    [0043]FIG. 3 is an exploded perspective view of a second driving device;  
         [0044]    [0044]FIG. 4 is a sectional view of the first driving device;  
         [0045]    [0045]FIG. 5 is a sectional view showing a relation in a motor composed of the first driving device, the second driving device and output means;  
         [0046]    [0046]FIG. 6 is a sectional view showing the relation in the motor composed of the first driving device, the second driving device and the output means;  
         [0047]    [0047]FIG. 7 is a sectional view showing the relation in the motor composed of the first driving device, the second driving device and the output means;  
         [0048]    [0048]FIG. 8 is a sectional view showing the relation in the motor composed of the first driving device, the second driving device and the output means;  
         [0049]    [0049]FIG. 9 is an exploded perspective view of an aperture blade driving mechanism according to the present invention;  
         [0050]    [0050]FIG. 10 is an exploded perspective view of a first driving device shown in FIG. 9;  
         [0051]    [0051]FIG. 11 is an exploded perspective view of a second driving device shown in FIG. 9;  
         [0052]    [0052]FIG. 12 is a sectional view of the first driving device;  
         [0053]    [0053]FIG. 13 is a sectional view showing a relation in a motor composed of the first driving device, the second driving device and output means;  
         [0054]    [0054]FIG. 14 is a sectional view showing the relation in the motor composed of the first driving device, the second driving device and the output means;  
         [0055]    [0055]FIG. 15 is a sectional view showing the relation in the motor composed of the first driving device, the second driving device and the output means;  
         [0056]    [0056]FIG. 16 is a sectional view showing the relation in the motor composed of the first driving device, the second driving device and the output means;  
         [0057]    [0057]FIG. 17 is a plan view showing the relation between a gear portion of a magnet and a magnetization phase in accordance with another embodiment;  
         [0058]    [0058]FIG. 18 is a sectional view of a step motor in prior art;  
         [0059]    [0059]FIG. 19 is a sectional view showing a part of a stator in the step motor of prior art; and  
         [0060]    [0060]FIG. 20 is a plan view of another step motor in prior art. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0061]    FIGS.  1  to  8  are diagrams showing an embodiment of the present invention. FIG. 1 is an exploded perspective view of an aperture blade driving mechanism. FIG. 2 is an exploded perspective view of a first driving device (motor A) and FIG. 3 is an exploded perspective view of a second driving device (motor B). FIG. 4 is a sectional view of the first driving device (motor A). FIGS.  5  to  8  are sectional views showing a relation in a motor composed of the first driving device (motor A), the second driving device (motor B) and output means. The second driving device has the same structure as the first driving device, and hence a sectional view thereof is identical with the sectional view of the first driving device shown in FIG. 4.  
         [0062]    In FIG. 1, aperture blades  32  and  33  are driven through driving forces given by the two motors A and B as will be described later.  
         [0063]    In FIG. 2, which is an exploded perspective view of the motor A, reference symbol  1  denotes a cylindrical magnet constituting a rotor. The magnet  1  serving as the rotor has its outer peripheral surface divided into n parts (10 parts in this embodiment) along the circumferential direction to form magnetized portions  1   a ,  1   b ,  1   c ,  1   d ,  1   e ,  1   f ,  1   g ,  1   h ,  1   i  and  1   j  such that the south poles and the north poles appear alternately. The magnetized portions  1   a ,  1   c ,  1   e ,  1   g  and  1   i  are the south poles whereas the magnetized portions  1   b ,  1   d ,  1   f ,  1   h  and  1   j  are the north poles. It is desirable to form the magnetic poles of the magnetized portions mainly on the outer surface of the cylinder. The magnet  1  is formed from a plastic magnet material through injection molding. This makes it possible to form the cylinder very thin in its radial direction.  
         [0064]    The magnet  1  has at the center in the axial direction an engagement portion  1   w  whose inner diameter is small. Denoted by  7  is a rotation shaft, which is fixed to the engagement portion  1   w  of the magnet  1  that is the rotor through press-fitting. The rotation shaft  7  has one end rotatably supported by a first stator  18  and, as shown in FIG. 4, the other end rotatably supported by a base plate  30 .  
         [0065]    Being formed from a plastic magnet through injection molding, the magnet  1  can be assembled by press-fitting without forming a crack and the manufacture is easy even when the magnet  1  has such a complicated shape as to have the small inner diameter engagement portion  1   w  at the center in the axial direction. The rotation shaft  7  is also incorporated in and fixed to the magnet  1  through press-fitting, and hence assemble of the rotor is easy and the manufacture thereof does not cost much. The rotor is comprised of the rotation shaft  7  and the magnet  1 .  
         [0066]    Reference numeral  2  indicates a fixed cylindrical coil. The coil  2  is concentric with the magnet  1  so that the coil  2  is piled on the magnet  1  in the axial direction. The outer diameter of the coil  2  is almost the same as the outer diameter of the magnet  1 .  
         [0067]    Numeral  18  indicates a first stator which is formed of a soft magnetic material. The first stator is composed of an outer tube and an inner tube whose interior is hollow. The front end of the outer tube of the first stator  18  forms outer magnetic poles  18   a ,  18   b ,  18   c ,  18   d  and  18   e . Denoted by  21  is a first auxiliary stator having an inner diameter portion  21   f  fitted and fixed to an inner tube  18   f  of the first stator  18 . The outer diameter portion of the first auxiliary stator has opposing portions  21   a ,  21   b ,  21   c ,  21   d  and  21   e  in the phase that makes them oppose to the outer magnetic poles  18   a ,  18   b ,  18   c ,  18   d  and  18   e  of the first stator. The opposing portions  21   a ,  21   b ,  21   c ,  21   d  and  21   e  are arranged with a displacement of 360/(n/2) degree, i.e., 72 degree put between every adjacent opposing portions so as to have the same phase as the magnetized portions of the magnet  1 . The outer magnetic poles  18   a ,  18   b ,  18   c ,  18   d  and  18   e  of the first stator  18  are arranged with a displacement of 360/(n/2) degree, i.e., 72 degree put between every adjacent outer magnetic poles so as to have the same phase as the magnetized portions of the magnet  1 . The hollow inner tube  18   f  of the first stator  18  and the auxiliary stator  21  constitute first inner magnetic poles.  
         [0068]    Reference numeral  23  indicates a first gear having teeth in a number equal to the number of magnetic poles of the magnetized portions of the magnet  1 , i.e, 10 teeth. The first gear  23  is integrated with the magnet  1 . As shown in FIGS.  5  to  8 , the teeth of the first gear  23  in this embodiment are formed to have a phase that makes their points coincide with the center of the respective magnetic poles of the magnet  1 .  
         [0069]    [0069]FIG. 3 is a perspective view of the motor B shown in FIG. 1. The basic structure thereof is the same as the motor A. Reference numeral  3  indicates a cylindrical magnet constituting a rotor. The magnet  3  serving as the rotor has its outer peripheral surface divided into n parts (10 parts in this embodiment) along the circumferential direction to form magnetized portions  3   a ,  3   b ,  3   c ,  3   d ,  3   e ,  3   f ,  3   g ,  3   h ,  3   i  and  3   j  such that the south poles and the north poles appear alternately. The magnetized portions  3   a ,  3   c ,  3   e ,  3   g  and  3   i  are the south poles whereas the magnetized portions  3   b ,  3   d ,  3   f ,  3   h  and  3   j  are the north poles. The magnet  3  is formed from a plastic magnet material through injection molding. This makes it possible to form the cylinder very thin in its radial direction.  
         [0070]    The magnet  3  has at the center in the axial direction an engagement portion  3   w  whose inner diameter is small. Indicated by numeral  8  is a rotation shaft, which is fixed to the engagement portion  3   w  of the magnet  3  that is the rotor through press-fitting. The rotation shaft  8  has one end rotatably supported by a second stator  19  and, as shown in FIG. 4, the other end rotatably supported by the base plate  30 .  
         [0071]    Being formed from a plastic magnet through injection molding, the magnet  3  can be assembled by press-fitting without forming a crack and the manufacture is easy even when the magnet  3  has such a complicated shape as to have the small inner diameter engagement portion  3   w  at the center in the axial direction. The rotation shaft  8  is also incorporated with and fixed to the magnet  3  through press-fitting, and hence assemble of the rotor is easy and the manufacture thereof does not cost much. The rotor is comprised of the rotation shaft  8  and the magnet  3 .  
         [0072]    Reference numeral  4  indicates a cylindrical coil. The coil  4  is concentric with the magnet  3  so that the coil  4  is piled on the magnet  3  in the axial direction. The outer diameter of the coil  4  is almost the same as the outer diameter of the magnet  3 .  
         [0073]    Numeral  19  is indicates the second stator which is formed of a soft magnetic material. The second stator is composed of an outer tube and an inner tube whose interior is hollow. The front end of the outer tube of the second stator  19  forms outer magnetic poles  19   a ,  19   b ,  19   c ,  19   d  and  19   e . Denoted by  22  is a second auxiliary stator having an inner diameter portion  22   f  fitted and fixed to an inner tube  19   f  of the second stator  19 . The outer diameter portion of the second auxiliary stator has opposing portions  22   a ,  22   b ,  22   c ,  22   d  and  22   e  in the phase that makes them oppose to the outer magnetic poles  19   a ,  19   b ,  19   c ,  19   d  and  19   e  of the second stator. The opposing portions  22   a ,  22   b ,  22   c ,  22   d  and  22   e  are arranged with a displacement of 360/(n/2) degree, i.e., 72 degree put between every adjacent opposing portions so as to have the same phase as the magnetized portions of the magnet  3 . The second outer magnetic poles  19   a ,  19   b ,  19   c ,  19   d  and  19   e  of the second stator  19  are arranged with a gap of 360/(n/2) degree, i.e., 72 degree put between every adjacent outer magnetic poles so as to have the same phase as the magnetized portions of the magnet  3 . The hollow inner tube  19   f  of the second stator  19  and the auxiliary stator  22  constitute second inner magnetic poles.  
         [0074]    Reference numeral  24  indicates a second gear having teeth in a number equal to the number of magnetic poles of the magnetized portions of the magnet  3 , i.e, 10 teeth. The second gear  24  is integrated with the magnet  3 . As shown in FIGS.  5  to  8 , the teeth of the second gear  24  in this embodiment are formed to have a phase that makes their points coincide with the center of the respective magnetic poles of the magnet  3 .  
         [0075]    Back to FIG. 1, the base plate  30  has an engagement portion  30 A. Denoted by  31  is an output ring attached to the engagement portion  30 A in a rotatable manner. The base plate also has an opening  30 D. The first stator  18  of the first driving device A and the second stator  19  of the second driving device B are fixed to the base plate  30  using an adhesive, a screw, or the like.  
         [0076]    The first driving device and the second driving device are arranged on the base plate  30  so as to be axially parallel with each other. The output ring  31  has a gear portion  31 A. The gear portion  31 A is meshed and coupled with the first gear  23  and the second gear  24 , the first gear  23  being integrated with the magnet of the first driving device and the second gear  24  being integrated with the magnet of the second driving device.  
         [0077]    Thus the magnet  1  of the first driving device and the magnet  3  of the second driving device can drive in association with each other through the output ring  31 . In this case, the first and second driving devices are arranged such that there is a displacement of 180/n degree, i.e., 18 degree between the positional relation of the magnetization phase of the magnet  1  to the outer magnetic poles  18   a ,  18   b ,  18   c ,  18   d  and  18   e  of the first stator and the positional relation of the magnetization phase of the magnet  3  to the outer magnetic poles  19   a ,  19   b ,  19   c ,  19   d  and  19   e  of the second stator.  
         [0078]    The output ring  31  is driven and rotated by the first driving device and the second driving device. Details thereof will be described later. The aperture blades  32  and  33  have dowels  32 A and  33 A, respectively. The dowels  32 A and  32 B are respectively fitted to cam grooves  30 A and  30 B formed in the base plate  30  but are slidable along the grooves. Dowels  31 B and  31 C of the output ring  31  are rotatably fitted to a hole  32 B of the aperture blade  32  and a hole  33 B of the aperture blade  33 , respectively. Rotation of the output ring  31  rotates the aperture blades  32  and  33  about the optical axis to change the stretch of opening. (This aperture mechanism is a known in the art.)  
         [0079]    The outer magnetic poles  18   a ,  18   b ,  18   c ,  18   d  and  18   e  of the first stator and the outer magnetic poles  19   a ,  19   b ,  19   c ,  19   d  and  19   e  of the second stator are both composed of a combination of cut-out holes and teeth extending in the direction parallel to the axis. This structure makes it possible to form magnetic poles while limiting the diameter of the motor to a minimum. If the outer magnetic poles are formed as radially extending concave and convex instead, the diameter of the motor is increased that much. This embodiment employs the outer magnetic poles formed as a combination of cut-out holes and teeth extending in the direction parallel to the axis, and hence the diameter of the first driving device and the diameter of the second driving device can be limited to a minimum. The first driving device and the second driving device are arranged such that the opening  30 D is not blocked. Because a diameter D of each of the driving devices is limited to a minimum, a width W of a portion excluding the opening is small and the diameter of the aperture blade driving device itself can also be small. This is illustrated in FIGS.  1  to  4 .  
         [0080]    The required length for each of the first driving device and the second driving device is equal to the sum of the length of their respective magnets and the length of the first or second coil. Therefore the first and second driving devices can be formed as a very short actuator. The first driving device and the second driving device are arranged side by side so as to be axially parallel to each other, whereby they can constitute an actuator for driving an aperture blade which is short in the direction parallel to the optical axis and which does not form an obstacle to a lens and other components (not shown).  
         [0081]    The outer magnetic poles  18   a ,  18   b ,  18   c ,  18   d  and  18   e  of the first stator  18  face the outer peripheral surface of the magnet  1  on one end. The outer diameter portions (opposing portions)  21   a ,  21   b ,  21   c ,  21   d  and  21   e  of the first auxiliary stator which constitute a part of the first inner magnetic poles face the inner peripheral surface of the magnet  1  on one end. That one end of the magnet  1  is sandwiched between the outer magnetic poles of the first stator and the outer diameter portions (opposing portions) of the first auxiliary stator.  
         [0082]    The outer magnetic poles  19   a ,  19   b ,  19   c ,  19   d  and  19   e  of the second stator  19  face the outer peripheral surface of the magnet  3  on one end. The outer diameter portions (opposing portions)  22   a ,  22   b ,  22   c ,  22   d  and  22   e  of the second auxiliary stator which constitute a part of the second inner magnetic poles face the inner peripheral surface of the magnet  3  on one end. That one end of the magnet  3  is sandwiched between the outer magnetic poles of the second stator and the outer diameter portions (opposing portions) of the second auxiliary stator.  
         [0083]    The coil  2  is provided between the outer tube of the first stator  18  and the inner tube thereof. Energization of the coil  2  excites the first stator  18  and the first auxiliary yoke (auxiliary stator)  21 .  
         [0084]    The coil  4  is provided between the outer tube of the second stator  19  and the inner tube thereof. Energization of the coil  4  excites the second stator  19  and the second auxiliary yoke (auxiliary stator)  22 .  
         [0085]    Therefore, flux of magnetic force generated from the coil  2  crosses the magnet  1  that is the rotor placed between the outer magnetic poles  18   a ,  18   b ,  18   c ,  18   d  and  18   e  and the opposing portions  21   a ,  21   b ,  21   c ,  21   d  and  21   e  that constitute a part of the inner magnetic poles. The flux of magnetic force thus effectively acts on the magnet  1  that is the rotor. On the other hand, flux of magnetic force generated from the coil  4  crosses the magnet  3  that is the rotor placed between the outer magnetic poles  19   a ,  19   b ,  19   c ,  19   d  and  19   e  and the opposing portions  22   a ,  22   b ,  22   c ,  22   d  and  22   e  that constitute a part of the inner magnetic poles. The flux of magnetic force thus effectively acts on the magnet  3  that is the rotor. The output of the motor can therefore be increased.  
         [0086]    The first inner magnetic poles form a circle that has an outer diameter larger than the inner diameter of the first coil and the second inner magnetic poles form a circle that has an outer diameter larger than the inner diameter of the second coil. Therefore, the distance between the first outer magnetic poles and the first inner magnetic poles, as well as the distance between the second outer magnetic poles and the second inner magnetic poles can be close even when the inner diameter of the coils are reduced to increase the volume of the coils. This makes the magnetic resistance viewed from the coil side small, and hence more flux of magnetic force can be produced with a small electric power. The output of the motor thus can be increased.  
         [0087]    FIGS.  5  to  8  are sectional views showing a relation in the motor composed of the first driving device, the second driving device and output means.  
         [0088]    The first and second driving devices are arranged such that there is a displacement of 180/n degree, i.e., 18 degree between the positional relation of the magnetization phase of the magnet  1  to the outer magnetic poles  18   a ,  18   b ,  18   c ,  18   d  and  18   e  of the first stator and the positional relation of the magnetization phase of the magnet  3  to the outer magnetic poles  19   a ,  19   b ,  19   c ,  19   d  and  19   e  of the second stator.  
         [0089]    The description given next is of the output ring  31  being driven by the first driving device and the second driving device. Under the state shown in FIG. 5, the coil  2  is energized for excitation to set the outer magnetic poles  18   a ,  18   b ,  18   c ,  18   d  and  18   e  of the first stator  18  to the south poles and to set the opposing portions  21   a ,  21   b ,  21   c ,  21   d  and  21   e  of the first auxiliary yoke (auxiliary stator)  21  which constitute a part of the first inner magnetic poles to the north poles. On the other hand, the coil  4  is energized for excitation to set the outer magnetic poles  19   a ,  19   b ,  19   c ,  19   d  and  19   e  of the second stator  19  to the north poles and to set the opposing portions  22   a ,  22   b ,  22   c ,  22   d  and  22   e  of the second auxiliary yoke (auxiliary stator)  22  which constitute a part of the second inner magnetic poles to the south poles. As a result, the magnet  1  and the magnet  3  coupled to each other by the output ring  31  rotate by 18 degree counterclockwise. The output ring  31  rotates simultaneously and is moved to the corresponding position to reach the state shown in FIG. 6, and then rotations of the motors are stopped at the state shown in FIG. 6.  
         [0090]    While keeping the energization of the coil  2  as it is, the energization of the coil  4  is then inverted. To be specific, the outer magnetic poles  18   a ,  18   b ,  18   c ,  18   d  and  18   e  of the first stator  18  are remained set to the south poles and the opposing portions  21   a ,  21   b ,  21   c ,  21   d  and  21   e  of the first auxiliary yoke (auxiliary stator)  21  which constitute a part of the first inner magnetic poles are remained set to the north poles, whereas the excitation sets the outer magnetic poles  19   a ,  19   b ,  19   c ,  19   d  and  19   e  of the second stator  19  to the south poles and sets the opposing portions  22   a ,  22   b ,  22   c ,  22   d  and  22   e  of the second auxiliary yoke (auxiliary stator)  22  which constitute a part of the second inner magnetic poles to the north poles. As a result, the magnet  1  and the magnet  3  coupled to each other by the output ring  31  rotate by 18 degree counterclockwise. The output ring  31  rotates simultaneously and is moved to the corresponding position to reach the state shown in FIG. 7, and then rotations of motors are stopped at the state shown in FIG. 7.  
         [0091]    Next, while keeping the energization of the coil  4  as it is, the energization of the coil  2  is inverted. To be specific, the outer magnetic poles  18   a ,  18   b ,  18   c ,  18   d  and  18   e  of the first stator  18  are remained set to the north poles and the opposing portions  21   a ,  21   b ,  21   c ,  21   d  and  21   e  of the first auxiliary yoke (auxiliary stator)  21  which constitute a part of the first inner magnetic poles are remained set to the south poles, whereas the excitation sets the outer magnetic poles  19   a ,  19   b ,  19   c ,  19   d  and  19   e  of the second stator  19  to the north poles and sets the opposing portions  22   a ,  22   b ,  22   c ,  22   d  and  22   e  of the second auxiliary yoke (auxiliary stator)  22  which constitute a part of the second inner magnetic poles to the south poles. As a result, the magnet  1  and the magnet  3  coupled to each other by the output ring  31  rotate by 18 degree counterclockwise. The output ring  31  rotates simultaneously and is moved to the corresponding position to reach the state shown in FIG. 8, and then rotations of motors are stopped at the state shown in FIG. 8.  
         [0092]    In this way, the direction of energization of the coil  4  is successively switched, whereby the magnet  1 , the magnet  3  and the output ring  31  are simultaneously rotated and moved to the position corresponding to the energization phase. That is, a power supply in FIG. 1 supplies electric power to the coils  2  and  4  with the phase shifted by 90 degree each time. If the motor is to be rotated backward, the coils  2  and  4  are energized while inverting the phase.  
         [0093]    Now, the optimum structure for the thus structured actuator to have a ultra-small size will be described. The basic structure of the actuator includes:  
         [0094]    1) shaping the magnet of each of the first driving device and the second driving device into a hollow cylinder;  
         [0095]    2) dividing the outer peripheral surface of the magnet of each of the first driving device and the second driving device into n parts along the circumferential direction and magnetizing it so as to form different magnetic poles alternately;  
         [0096]    3) piling the coil on the magnet in the axial direction of the magnet in each of the first driving device and the second driving device;  
         [0097]    4) opposing the outer magnetic poles of the first stator excited by the first coil to the outer peripheral surface of the magnet of the first driving device whereas opposing the inner magnetic poles thereof to the inner peripheral surface of the magnet, and opposing the outer magnetic poles of the second stator excited by the second coil to the outer peripheral surface of the magnet of the second driving device whereas opposing the inner magnetic poles thereof to the inner peripheral surface of the magnet;  
         [0098]    5) using a combination of cut-out holes and teeth extending in the direction parallel to the axis as the outer magnetic poles of each of the first driving device and the second driving device; and  
         [0099]    6) arranging the first driving device and the second driving device in parallel to each other.  
         [0100]    The required diameter for each of the first driving device and the second driving device is substantially equal to the sum of the diameter of the magnet and the thickness of the magnetic poles of the stator which face the peripheral surfaces of the magnet. The required length for each of the first driving device and the second driving device is substantially equal to the sum of the length of their respective magnets and the length of the first or second coil. Therefore the diameter and the length of the magnet and the coil determine the size of the first driving device and the second driving device. When the diameter and the length of the magnet and the coil are very small, ultra-small size first driving device and second driving device can be obtained.  
         [0101]    With a very short diameter and a very short length for the magnet and the coil, it is difficult to maintain the precision as the first driving device and the second driving device. However, this problem regarding the precision of the first driving device and the second driving device is solved by a simple structure in which the magnet is formed into a hollow cylinder and the outer magnetic poles of the first (second) stator face the outer peripheral surface of the magnet shaped into a hollow cylinder whereas the inner magnetic poles thereof face the inner peripheral surface of the magnet. If the inner peripheral surface of the magnet is divided along the peripheral surface and magnetized in addition to the outer peripheral surface of the magnet, the output of the motor can be enhanced even more.  
         [0102]    The first driving device and the second driving device are arranged such that the opening  30 D is not blocked. The diameter of the respective driving devices is thus limited to a minimum, thereby making the diameter of the aperture blade driving device itself small.  
         [0103]    The first driving device and the second driving device are arranged side by side so as to be axially parallel to each other, whereby they can constitute an actuator for driving an aperture blade which is short in the direction parallel to the optical axis and which does not form an obstacle to a lens and other components.  
         [0104]    As described above, the first and second driving devices have to be arranged such that there is a displacement of 180/n degree, i.e., 18 degree between the positional relation of the magnetization phase of the magnet  1  to the outer magnetic poles  18   a ,  18   b ,  18   c ,  18   d  and  18   e  of the first stator and the positional relation of the magnetization phase of the magnet  3  to the outer magnetic poles  19   a ,  19   b ,  19   c ,  19   d  and  19   e  of the second stator. The first gear  23  has teeth in a number equal to the number of magnetic poles of the magnetized portions of the magnet  1 , i.e, 10 teeth and is integrated with the magnet  1 . The second gear  24  has teeth in a number equal to the number of magnetic poles of the magnetized portions of the magnet  3 , i.e, 10 teeth and is integrated with the magnet  3 .  
         [0105]    Therefore, an assembling worker can tell the magnetization phase of the magnets by looking at the phase of the teeth of the first gear and the second gear. The teeth of the gears in this embodiment are formed to have a phase that makes their points coincide with the center of the respective magnetic poles of the magnets.  
         [0106]    Since the magnetization phase of the magnet can be confirmed visually by looking at the position of the teeth of the gears, it is easy to set the relative rotational position, through the output means, of the magnet of the first driving device and the magnet of the second driving device to a predetermined position for the assembling. The efficiency in assembling work is therefore improved.  
         [0107]    The magnet  1  and the magnet  3  in this embodiment are coupled to each other with the gear of output ring by the gears having teeth in a number corresponding to the number of magnetic poles of the magnetized portions. However, the number of teeth of the first gear and the second gear may not be the same as the number of magnetic poles of the magnetized portions of the magnet  1  and the magnet  3  in the case where facilitating the assembling is not a particularly pursued object.  
         [0108]    The magnet  1  and the magnet  3  are coupled to each other by the first gear  23  and the second gear  24  and by the gear portion  31 A of the output ring  31 . However, other coupling methods can also be employed. For instance, pins are provided in the magnet  1  and the magnet  3  and grooves are formed in the output ring so that the pins are fitted to the grooves. The structure as such will be sufficient if the magnet  1  and the magnet  3  rotate less.  
         [0109]    In this embodiment, the drive transmission device composed of the first driving device, the second driving device and the output ring that is a transmission mechanism is used as an actuator for driving an aperture blade to open and close the blade. However, the drive transmission device is also useful in other applications as a device that is advantageous in its high output, small diameter and short axial length. For example, the device can be used to rotate a cam barrel for driving a lens.  
         [0110]    In the structure above, the magnetization phase of the magnet corresponds to the teeth of the gear on one on one basis. Therefore it is easy to set the relative rotational position, through the transmission mechanism, of the magnet of the first driving device and the magnet of the second driving device to a predetermined position for the assembling. The efficiency in assembling work is therefore improved.  
         [0111]    The diameter of each of the first driving device and the second driving device is determined by the outer magnetic poles that face the outer peripheral surface of the magnet. The axial length of each of the first driving device and the second driving device is determined by arranging the coil and the magnet in this order. The first driving device and the second driving device thus can be made very small. The flux of magnetic force generated from the coil crosses the magnet placed between the outer magnetic poles and the inner magnetic poles to act on the magnet effectively. The transmission mechanism coupled to the magnet of the first driving device and the magnet of the second driving device makes the drive transmission device bidirectionally rotatable.  
         [0112]    The first driving device and the second driving device are arranged side by side so as to be axially parallel to each other. With the first driving device and the second driving device being arranged axially parallel to each other, a drive transmission device having a short axial length can be obtained.  
         [0113]    The transmission mechanism coupled to the magnet of the first driving device and the magnet of the second driving device drives the aperture blade to open and close the blade.  
         [0114]    The aperture blade driving device thus can serve as an actuator for driving an aperture blade to open and close the blade which does not form an obstacle in arranging a lens with respect the direction parallel to the optical axis.  
         [0115]    Next, a more improved embodiment will be described.  
         [0116]    FIGS.  9  to  16  are diagrams showing the improved embodiment of the present invention. FIG. 9 is an exploded perspective view of an aperture blade driving mechanism. FIG. 10 is an exploded perspective view of a first driving device (motor A) and FIG. 11 is an exploded perspective view of a second driving device (motor B). FIG. 12 is a sectional view of the first driving device. FIGS.  13  to  16  are sectional views showing a relation in a motor composed of the first driving device, the second driving device and output means. The second driving device has the same structure as the first driving device, and hence a sectional view thereof is identical with the sectional view of the first driving device shown in FIG. 12.  
         [0117]    In FIG. 10 that shows an exploded perspective view of the motor A, reference symbol  101  denotes a cylindrical magnet constituting a rotor. The magnet  101  serving as the rotor has its outer peripheral surface divided into n parts (10 parts in this embodiment) along the circumferential direction to form magnetized portions  1   a ,  1   b ,  1   c ,  1   d ,  1   e ,  1   f ,  1   g ,  1   h ,  1   i  and  1   j  such that the south poles and the north poles appear alternately. The magnetized portions  1   a ,  1   c ,  1   e ,  1   g  and  1   i  are the south poles whereas the magnetized portions  1   b ,  1   d ,  1   f ,  1   h  and  1   j  are the north poles. The magnet  101  is formed from a plastic magnet material through injection molding. This makes it possible to form the cylinder very thin in its radial direction.  
         [0118]    The magnet  101  has at the center in the axial direction an engagement portion  1   w  whose inner diameter is small. Reference symbol  1   p  denotes a gear portion formed in the magnet and has teeth in a number corresponding to the number of the magnetic poles of the magnetized portions of the magnet  101 , i.e., 10 teeth. The teeth of the gear portion are magnetized, of course. The phase of the teeth and the magnetization phase in this embodiment are such that the points of the teeth coincide with the center of the respective magnetic poles of the magnet as shown in FIGS.  13  to  16 . The number of teeth can be set at discretion as long as it is any one of integral multiples of the number of magnetic poles of the magnetized portions. For instance, the number of teeth may be twice the number of magnetic poles of the magnetized portions (10 poles) and twenty teeth are formed. In this case, the teeth are arranged so as to be symmetrical with respect to the center of the respective magnetic poles of the magnetized portions as shown in FIG. 17. Whether the number of teeth is 10 or 20, the teeth are arranged so as to be symmetrical with respect to the center of the respective magnetic poles of the magnetized portions and the external shape is also symmetrical with respect to the center of the respective magnetic poles of the magnetized portions. The magnet thus has magnetization intensity symmetrical with respect to the center of the respective magnetic poles of the magnetized portions, thereby providing a stable output.  
         [0119]    Denoted by  7  is a rotation shaft, which is fixed to the engagement portion  1   w  of the magnet  101  that is the rotor through press-fitting. The rotation shaft  7  has one end rotatably supported by a first stator  18  and the other end rotatably supported by a base plate  30 .  
         [0120]    Being formed from a plastic magnet through injection molding, the magnet  101  can be assembled by press-fitting without forming a crack and the manufacture is easy even when the magnet  101  has such a complicated shape as to have the small inner diameter engagement portion  1   w  at the center in the axial direction. The rotation shaft  7  is also incorporated with and fixed to the magnet  101  through press-fitting, and hence assemble of the rotor is easy and the manufacture thereof does not cost much. The rotor is comprised of the rotation shaft  7  and the magnet  101 . Reference symbol  2  denotes a cylindrical coil. The coil  2  is concentric with the magnet  101  so that the coil  2  is piled on the magnet  101  in the axial direction. The outer diameter of the coil  2  is almost the same as the outer diameter of the magnet  101 .  
         [0121]    The first stator  18  is formed from a soft magnetic material. The first stator is composed of an outer tube and an inner tube whose interior is hollow. The front end of the outer tube of the first stator  18  forms (N/2−1), i.e., four outer magnetic poles  18   a ,  18   b ,  18   c , and  18   d . Denoted by  21  is a first auxiliary stator having an inner diameter portion  21   f  fitted and fixed to an inner tube  18   f  of the first stator  18 . The outer diameter portion of the first auxiliary stator has opposing portions  21   a ,  21   b ,  21   c , and  21   d  in the phase that makes them oppose to the outer magnetic poles  18   a ,  18   b ,  18   c  and  18   d  of the first stator. The opposing portions  21   a ,  21   b ,  21   c , and  21   d  are arranged with a displacement of one of integral multiples of 360/(n/2) degree, i.e., one of integral multiples of 72 degree put between every adjacent opposing portions so as to have the same phase as the magnetized portions of the magnet  101 . The outer magnetic poles  18   a ,  18   b ,  18   c , and  18   d  of the first stator  18  are arranged with a displacement of one of integral multiples of 360/(n/2) degree, i.e., one of integral multiples of 72 degree put between every adjacent outer magnetic poles so as to have the same phase as the magnetized portions of the magnet  101 . The gap between the outer magnetic poles  18   a  and  18   b , the gap between the outer magnetic poles  18   b  and  18   c , and the gap between the outer magnetic poles  18   c  and  18   d  are each 360/(n/2) degree, i.e., 72 degree. Only the gap between the outer magnetic poles,  18   a  and  18   d  is twice the 360/(n/2) degree, i.e., 144 degree.  
         [0122]    The hollow inner tube  18   f  of the first stator  18  and the auxiliary stator  21  constitute first inner magnetic poles.  
         [0123]    [0123]FIG. 11 is a perspective view of the motor B. Reference symbol  103  denotes a cylindrical magnet constituting a rotor. The magnet  103  serving as the rotor has its outer peripheral surface divided into n parts (10 parts in this embodiment) along the circumferential direction to form magnetized portions  3   a ,  3   b ,  3   c ,  3   d ,  3   e ,  3   f ,  3   g ,  3   h ,  3   i  and  3   j  such that the south poles and the north poles appear alternately. The magnetized portions  3   a ,  3   c ,  3   e ,  3   g  and  3   i  are the south poles whereas the magnetized portions  3   b ,  3   d ,  3   f ,  3   h  and  3   j  are the north poles. The magnet  103  is formed from a plastic magnet material through injection molding. This makes it possible to form the cylinder very thin in its radial direction.  
         [0124]    The magnet  103  has at the center in the axial direction an engagement portion  3   w  whose inner diameter is small. Reference symbol  3   p  denotes a gear portion formed in the magnet and has teeth in a number corresponding to the number of the magnetic poles of the magnetized portions of the magnet  103 , i.e., 10 teeth. The teeth of the gear portion are magnetized, of course. The phase of the teeth and the magnetization phase in this embodiment are such that the points of the teeth coincide with the center of the respective magnetic poles of the magnet as shown in FIGS.  13  to  16 . The number of teeth can be set at discretion as long as it is any one of integral multiples of the number of magnetic poles of the magnetized portions. For instance, the number of teeth may be twice the number of magnetic poles of the magnetized portions (10 poles) and twenty teeth are formed. In this case, the teeth are arranged so as to be symmetrical with respect to the center of the respective magnetic poles of the magnetized portions as shown in FIG. 17. Whether the number of teeth is 10 or 20, the teeth are arranged so as to be symmetrical with respect to the center of the respective magnetic poles of the magnetized portions and the external shape is also symmetrical with respect to the center of the respective magnetic poles of the magnetized portions. The magnet thus has magnetization intensity symmetrical with respect to the center of the respective magnetic poles of the magnetized portions, thereby providing a stable output.  
         [0125]    Denoted by  8  is a rotation shaft, which is fixed to the engagement portion  3   w  of the magnet  103  that is the rotor through press-fitting. The rotation shaft  8  has one end rotatably supported by a second stator  19  and the other end rotatably supported by the base plate  30 . Being formed from a plastic magnet through injection molding, the magnet  103  can be assembled by press-fitting without forming a crack and the manufacture is easy even when the magnet  103  has such a complicated shape as to have the small inner diameter engagement portion  3   w  at the center in the axial direction. The rotation shaft  8  is also incorporated with and fixed to the magnet  103  through press-fitting, and hence assemble of the rotor is easy and the manufacture thereof does not cost much. The rotor is comprised of the rotation shaft  8  and the magnet  103 . Reference symbol  4  denotes a cylindrical coil. The coil  4  is concentric with the magnet  103  so that the coil  4  is piled on the magnet  103  in the axial direction. The outer diameter of the coil  4  is almost the same as the outer diameter of the magnet  103 .  
         [0126]    The second stator  19  is formed from a soft magnetic material. The second stator is composed of an outer tube and an inner tube whose interior is hollow. The front end of the outer tube of the second stator  19  forms outer magnetic poles  19   a ,  19   b ,  19   c  and  19   d . Denoted by  22  is a second auxiliary stator having an inner diameter portion  22   f  fitted and fixed to an inner tube  19   f  of the second stator  19 . The outer diameter portion of the second auxiliary stator has opposing portions  22   a ,  22   b ,  22   c  and  22   d  in the phase that makes them oppose to the outer magnetic poles  19   a ,  19   b ,  19   c  and  19   d  of the second stator. The opposing portions  22   a ,  22   b ,  22   c , and  22   d  are arranged with a displacement of one of integral multiples of 360/(n/2) degree, i.e., one of integral multiples of 72 degree put between every adjacent opposing portions so as to have the same phase as the magnetized portions of the magnet  103 . The second outer magnetic poles  19   a ,  19   b ,  19   c , and  19   d  of the second stator  19  are arranged with a displacement of one of integral multiples of 360/(n/2) degree, i.e., one of integral multiples of 72 degree put between every adjacent outer magnetic poles so as to have the same phase as the magnetized portions of the magnet  103 . The gap between the outer magnetic poles  19   a  and  19   b , the gap between the outer magnetic poles  19   b  and  19   c , and the gap between the outer magnetic poles  19   c  and  19   d  are each 360/(n/2) degree, i.e., 72 degree. Only the gap between the outer magnetic poles  19   a  and  19   d  is twice the 360/(n/2) degree, i.e., 144 degree.  
         [0127]    The hollow inner tube  19   f  of the second stator  19  and the auxiliary stator  22  constitute second inner magnetic poles.  
         [0128]    In this embodiment, the phase of the teeth is such that the points of the teeth coincide with the center of the respective magnetic poles of the magnet as shown in FIGS.  13  to  16 .  
         [0129]    The base plate  30  has an engagement portion  30 A. Denoted by  31  is an output ring attached to the engagement portion  30 A in a rotatable manner. The base plate also has an opening  30 D. The first stator  18  of the first driving device A and the second stator  19  of the second driving device B are fixed to the base plate  30  using a known method such as bonding or fastening a screw.  
         [0130]    The first driving device and the second driving device are arranged on the base plate  30  so as to be axially parallel with each other. The output ring  31  has a gear portion  31 A. The gear portion  31 A is meshed and coupled with the gear portion  1   p  and the gear portion  3   p , the gear portion  1   p  being integrated with the magnet of the first driving device and the gear portion  3   p  being integrated with the magnet of the second driving device.  
         [0131]    Thus the magnet  101  of the first driving device and the magnet  103  of the second driving device can drive in association with each other through the output ring  31 . In this case, the first and second driving devices are arranged such that there is a displacement of 180/n degree, i.e., 18 degree between the positional relation of the magnetization phase of the magnet  101  to the outer magnetic poles  18   a ,  18   b ,  18   c  and  18   d  of the first stator and the positional relation of the magnetization phase of the magnet  103  to the outer magnetic poles  19   a ,  19   b ,  19   c  and  19   d  of the second stator. The output ring  31  meshes with the gear portion of the magnet  101  at a position in the wide gap between the magnetic poles  18   a  and  18   d  which is wider than the gaps between the other outer magnetic poles of the first stator. The output ring  31  meshes with the gear portion of the magnet  103  at a position in the wide gap between the magnetic poles  19   a  and  19   d  which is wider than the gaps between the other outer magnetic poles of the second stator. Therefore, the output ring can be placed in a position that axially overlaps the outer magnetic poles without touching any outer magnetic pole. This helps the motor reducing its axial dimension, namely a dimension H in FIG. 12.  
         [0132]    The output ring  31  is driven and rotated by the first driving device and the second driving device. Details thereof will be described later. Aperture blades  32  and  33  have dowels  32 A and  33 A, respectively. The dowels  32 A and  32 B are respectively fitted to cam grooves  30 A and  30 B formed in the base plate  30  but are slidable along the grooves. Dowels  31 B and  31 C of the output ring  31  are rotatably fitted to a hole  32 B of the aperture blade  32  and a hole  33 B of the aperture blade  33 , respectively. Rotation of the output ring  31  rotates the aperture blades  32  and  33  about the optical axis to change the stretch of opening.  
         [0133]    The outer magnetic poles  18   a ,  18   b ,  18   c  and  18   d  of the first stator and the outer magnetic poles  19   a ,  19   b ,  19   c  and  19   d  of the second stator are both composed of a combination of cut-out holes and teeth extending in the direction parallel to the axis. This structure makes it possible to form magnetic poles while limiting the diameter of the motor to a minimum. If the outer magnetic poles are formed as radially extending concave and convex instead, the diameter of the motor is increased that much. This embodiment employs the outer magnetic poles formed as a combination of cut-out holes and teeth extending in the direction parallel to the axis, and hence the diameter of the first driving device and the diameter of the second driving device can be limited to a minimum. The first driving device and the second driving device are arranged such that the opening  30 D is not blocked. Because a diameter D of each of the driving devices is limited to a minimum, a width W of a portion excluding the opening is small and the diameter of the aperture blade driving device itself can also be small. This is illustrated in FIGS. 9 and 12.  
         [0134]    The required length for each of the first driving device and the second driving device is equal to the sum of the length of their respective magnets and the length of the first or second coil. Therefore the first and second driving devices can be formed as a very short actuator. The first driving device and the second driving device are arranged side by side so as to be axially parallel to each other, whereby they can constitute an actuator for driving an aperture blade which is short in the direction parallel to the optical axis and which does not form an obstacle to a lens and other components.  
         [0135]    The outer magnetic poles  18   a ,  18   b ,  18   c  and  18   d  of the first stator  18  face the outer peripheral surface of the magnet  101  on one end. The outer diameter portions (opposing portions)  21   a ,  21   b ,  21   c  and  21   d  of the first auxiliary stator which constitute a part of the first inner magnetic poles face the inner peripheral surface of the magnet  101  on one end. That one end of the magnet  101  is sandwiched between the outer magnetic poles of the first stator and the outer diameter portions (opposing portions) of the first auxiliary stator.  
         [0136]    The outer magnetic poles  19   a ,  19   b ,  19   c  and  19   d  of the second stator  19  face the outer peripheral surface of the magnet  103  on one end. The outer diameter portions (opposing portions)  22   a ,  22   b ,  22   c  and  22   d  of the second auxiliary stator which constitute a part of the second inner magnetic poles face the inner peripheral surface of the magnet  103  on one end. That one end of the magnet  103  is sandwiched between the outer magnetic poles of the second stator and the outer diameter portions (opposing portions) of the second auxiliary stator.  
         [0137]    The coil  2  is provided between the outer tube of the first stator  18  and the inner tube thereof. Energization of the coil  2  excites the first stator  18  and the first auxiliary yoke (auxiliary stator)  21 .  
         [0138]    The coil  4  is provided between the outer tube of the second stator  19  and the inner tube thereof. Energization of the coil  4  excites the second stator  19  and the second auxiliary yoke (auxiliary stator)  22 .  
         [0139]    Therefore, flux of magnetic force generated from the coil  2  crosses the magnet  101  that is the rotor placed between the outer magnetic poles  18   a ,  18   b ,  18   c  and  18   d  and the opposing portions  21   a ,  21   b ,  21   c  and  21   d  that constitute a part of the inner magnetic poles. The flux of magnetic force thus effectively acts on the magnet  101  that is the rotor. On the other hand, flux of magnetic force generated from the coil  4  crosses the magnet  103  that is the rotor placed between the outer magnetic poles  19   a ,  19   b ,  19   c  and  19   d  and the opposing portions  22   a ,  22   b ,  22   c  and  22   d  that constitute a part of the inner magnetic poles. The flux of magnetic force thus effectively acts on the magnet  103  that is the rotor. The output of the motor can therefore be increased.  
         [0140]    The first inner magnetic poles form a circle that has an outer diameter larger than the inner diameter of the first coil and the second inner magnetic poles form a circle that has an outer diameter larger than the inner diameter of the second coil. Therefore, the distance between the first outer magnetic poles and the first inner magnetic poles, as well as the distance between the second outer magnetic poles and the second inner magnetic poles can be close even when the inner diameter of the coils are reduced to increase the volume of the coils. This makes the magnetic resistance viewed from the coil side small, and hence more flux of magnetic force can be produced with a small electric power. The output of the motor thus can be increased.  
         [0141]    FIGS.  13  to  16  are sectional views showing a relation in the motor composed of the first driving device, the second driving device and output means.  
         [0142]    The first and second driving devices are arranged such that there is a displacement of 180/n degree, i.e., 18 degree between the positional relation of the magnetization phase of the magnet  101  to the outer magnetic poles  18   a ,  18   b ,  18   c  and  18   d  of the first stator and the positional relation of the magnetization phase of the magnet  103  to the outer magnetic poles  19   a ,  19   b ,  19   c  and  19   d  of the second stator.  
         [0143]    The description given next is of the output ring  31  being driven by the first driving device and the second driving device. FIG. 13 illustrates a state in which only the coil  2  is energized for excitation to set the outer magnetic poles  18   a ,  18   b ,  18   c  and  18   d  of the first stator  18  to the north poles and to set the opposing portions  21   a ,  21   b ,  21   c  and  21   d  of the first auxiliary yoke (auxiliary stator)  21  which constitute a part of the first inner magnetic poles to the south poles. As a result, the magnetized portions of the magnet  101  which are the south poles are positioned such that each magnetized portion points the center of the respective outer magnetic poles of the first stator  18 . In this case, the magnetized portion  1   c  of the magnet  101  is positioned so as to face the center of the outer magnetic pole  18   a  of the first stator  18 . The magnetized portion le of the magnet  101  is positioned so as to face the center of the outer magnetic pole  18   b  of the first stator  18 . The magnetized portion  1   g  of the magnet  101  is positioned so as to face the center of the outer magnetic pole  18   c  of the first stator  18 . The magnetized portion  1   i  of the magnet  101  is positioned so as to face the center of the outer magnetic pole  18   d  of the first stator  18 .  
         [0144]    The state shown in FIG. 13 is changed by terminating the energization of the coil  2  and energizing the coil  4 . The energization of the coil  4  excites the outer magnetic poles  19   a ,  19   b ,  19   c  and  19   d  of the second stator  19  to set them to the south poles and excites the opposing portions  22   a ,  22   b ,  22   c  and  22   d  of the second auxiliary yoke (auxiliary stator)  22  which constitute a part of the second inner magnetic poles to set them to the north poles. As a result, the magnetized portions of the magnet  103  which are the north poles are positioned such that each magnetized portion points the center of the respective outer magnetic poles of the second stator  19 . In this case, the magnetized portion  3   b  of the magnet  103  is positioned so as to face the center of the outer magnetic pole  19   a  of the second stator  19 . The magnetized portion  3   d  of the magnet  103  is positioned so as to face the center of the outer magnetic pole  19   b  of the second stator  19 . The magnetized portion  3   f  of the magnet  103  is positioned so as to face the center of the outer magnetic pole  19   c  of the second stator  19 . The magnetized portion  3   h  of the magnet  103  is positioned so as to face the center of the outer magnetic pole  19   d  of the second stator  19 .  
         [0145]    The magnet  101  and the magnet  103  coupled to each other by the output ring  31  rotate by 18 degree counterclockwise. The output ring  31  rotates simultaneously and is moved to the corresponding position to reach the state shown in FIG. 14.  
         [0146]    Next, the energization of the coil  4  is terminated and the coil  2  is energized in the direction reverse to the energization direction in FIG. 13.  
         [0147]    The excitation sets the outer magnetic poles  18   a ,  18   b ,  18   c  and  18   d  of the first stator  18  to the south poles and sets the opposing portions  21   a ,  21   b ,  21   c  and  21   d  of the first auxiliary yoke (auxiliary stator)  21  which constitute a part of the first inner magnetic poles to the north poles. As a result, the magnetized portions of the magnet  101  which are the north poles are positioned such that each magnetized portion points the center of the respective outer magnetic poles of the first stator  18 . In this case, the magnetized portion  1   b  of the magnet  101  is positioned so as to face the center of the outer magnetic pole  18   a  of the first stator  18 . The magnetized portion  1   d  of the magnet  101  is positioned so as to face the center of the outer magnetic pole  18   b  of the first stator  18 . The magnetized portion  1   f  of the magnet  101  is positioned so as to face the center of the outer magnetic pole  18   c  of the first stator  18 . The magnetized portion  1   h  of the magnet  101  is positioned so as to face the center of the outer magnetic pole  18   d  of the first stator  18 .  
         [0148]    The magnet  101  and the magnet  103  coupled to each other by the output ring  31  rotate by 18 degree counterclockwise. The output ring  31  rotates simultaneously and is moved to the corresponding position to reach the state shown in FIG. 15.  
         [0149]    Next, the energization of the coil  2  is terminated and the coil  4  is energized in the direction reverse to the energization direction in FIG. 14.  
         [0150]    The excitation sets the outer magnetic poles  19   a ,  19   b ,  19   c  and  19   d  of the second stator  19  to the north poles and sets the opposing portions  22   a ,  22   b ,  22   c  and  22   d  of the second auxiliary yoke (auxiliary stator)  22  which constitute a part of the second inner magnetic poles to the south poles. As a result, the magnetized portions of the magnet  103  which are the south poles are positioned such that each magnetized portion points the center of the respective outer magnetic poles of the second stator  19 . In this case, the magnetized portion  3   a  of the magnet  103  is positioned so as to face the center of the outer magnetic pole  19   a  of the second stator  19 . The magnetized portion  3   c  of the magnet  103  is positioned so as to face the center of the outer magnetic pole  19   b  of the second stator  19 . The magnetized portion  3   e  of the magnet  103  is positioned so as to face the center of the outer magnetic pole  19   c  of the second stator  19 . The magnetized portion  3   g  of the magnet  103  is positioned so as to face the center of the outer magnetic pole  19   d  of the second stator  19 .  
         [0151]    The magnet  101  and the magnet  103  coupled to each other by the output ring  31  rotate by 18 degree counterclockwise. The output ring  31  rotates simultaneously and is moved to the corresponding position to reach the state shown in FIG. 16.  
         [0152]    In this way, the direction of energization of the coil  2  and the coil  4  is successively switched, whereby the magnet  101 , the magnet  103  and the output ring  31  are simultaneously rotated and moved to the position corresponding to the energization phase.  
         [0153]    Now, the optimum structure for the thus structured actuator to have a ultra-small size will be described. The basic structure of the actuator includes:  
         [0154]    1) shaping the magnet of each of the first driving device and the second driving device into a hollow cylinder;  
         [0155]    2) dividing the outer peripheral surface of the magnet of each of the first driving device and the second driving device into n parts along the circumferential direction and magnetizing it so as to form different magnetic poles alternately;  
         [0156]    3) placing the coil and the magnet successively in the axial direction of the magnet in each of the first driving device and the second driving device;  
         [0157]    4) opposing the outer magnetic poles of the first stator excited by the first coil to the outer peripheral surface of the magnet of the first driving device whereas opposing the inner magnetic poles thereof to the inner peripheral surface of the magnet, and opposing the outer magnetic poles of the second stator excited by the second coil to the outer peripheral surface of the magnet of the second driving device whereas opposing the inner magnetic poles thereof to the inner peripheral surface of the magnet;  
         [0158]    5) using a combination of cut-out holes and teeth extending in the direction parallel to the axis as the outer magnetic poles of each of the first driving device and the second driving device;  
         [0159]    6) arranging the first driving device and the second driving device in parallel to each other;  
         [0160]    7) forming gear portions in the magnets of the first and second driving devices which face the outer magnetic poles and coupling the magnet of the first driving device to the magnet of the second driving device at their gear portions through coupling means;  
         [0161]    8) forming teeth of the gear portion of the respective magnets in a number that corresponds to one of integral multiples of the number of magnetic poles of the magnetized portions, and giving the teeth a phase that makes the teeth symmetrical with respect to the center of the respective magnetic poles of the magnetized portions; and  
         [0162]    9) forming (N/2−1) outer magnetic poles that face the outer peripheral surface of the respective magnets when the magnets each have N magnetic poles of the magnetized portions, selecting one gap out of the gaps between the outer magnetic poles to set it to a pitch of (360×4/N) degree, in this embodiment, 144 degree, and meshing a coupling ring with the gear portions in this gap.  
         [0163]    The required diameter for each of the first driving device and the second driving device is equal to the sum of the diameter of the magnet and the thickness of the magnetic poles of the stator which face the peripheral surfaces of the magnet. The required length for each of the first driving device and the second driving device is substantially equal to the sum of the length of their respective magnets and the length of the first or second coil. Therefore the diameter and the length of the magnet and the coil determine the size of the first driving device and the second driving device. When the diameter and the length of the magnet and the coil are very small, ultra-small size first driving device and second driving device can be obtained.  
         [0164]    With a very short diameter and a very short length for the magnet and the coil, it is difficult to maintain the precision as the first driving device and the second driving device. However, this problem regarding the precision of the first driving device and the second driving device is solved by a simple structure in which the magnet is formed into a hollow cylinder and the outer magnetic poles of the first (second) stator face the outer peripheral surface of the magnet shaped into a hollow cylinder whereas the inner magnetic poles thereof face the inner peripheral surface of the magnet. If the inner peripheral surface of the magnet is divided along the peripheral surface and magnetized in addition to the outer peripheral surface of the magnet, the output of the motor can be enhanced even more.  
         [0165]    The first driving device and the second driving device are arranged such that the opening  30 D is not blocked. The diameter of the respective driving devices is thus limited to a minimum, thereby making the diameter of the aperture blade driving device itself small.  
         [0166]    The first driving device and the second driving device are arranged side by side so as to be axially parallel to each other, whereby they can constitute an actuator for driving an aperture blade which is short in the direction parallel to the optical axis and which does not form an obstacle to a lens and other components.  
         [0167]    As described above, the first and second driving devices have to be arranged such that there is a displacement of 180/n degree, i.e., 18 degree between the positional relation of the magnetization phase of the magnet  101  to the outer magnetic poles  18   a ,  18   b ,  18   c  and  18   d  of the first stator and the positional relation of the magnetization phase of the magnet  103  to the outer magnetic poles  19   a ,  19   b ,  19   c  and  19   d  of the second stator. The gear portion  1   p  has teeth in a number equal to the number of magnetic poles of the magnetized portions of the magnet  101 , i.e, 10 teeth and the teeth has a phase that makes them symmetrical with respect to the center of the respective magnetic poles of the magnetized portions of the magnet  101 . The gear portion  3   p  has teeth in a number equal to the number of magnetic poles of the magnetized portions of the magnet  103 , i.e, 10 teeth and the teeth has a phase that makes them symmetrical with respect to the center of the respective magnetic poles of the magnetized portions of the magnet  103 . The magnets thus have balanced magnetization intensity to provide a motor of stable output.  
         [0168]    The output ring  31  meshes with the gear portion of the magnet  101  at a position in the wide gap between the magnetic poles  18   a  and  18   d  which is wider than the gaps between the other outer magnetic poles of the first stator. The output ring  31  meshes with the gear portion of the magnet  103  at a position in the wide gap between the magnetic poles  19   a  and  19   d  which is wider than the gaps between the other outer magnetic poles of the second stator. Therefore, the output ring  31  can be placed in a position that axially overlaps the outer magnetic poles without touching any outer magnetic pole. This helps the motor reducing its axial dimension (namely, a dimension H in FIG. 12).  
         [0169]    In this embodiment, the number of magnetic poles of the magnetized portions of the respective magnets is the same as the number of teeth of the respective gear portions, and the teeth are formed to have a phase that makes their points coincide with the center of the respective magnetic poles of the magnets. Therefore, an assembling worker can tell the magnetization phase of the magnets  101  and  103  by looking at the phase of the teeth of the gear portions  1   p  and  3   p , respectively.  
         [0170]    Since the magnetization phase of the magnets can be confirmed visually by looking at the position of the teeth of the gear portions, it is easy to set the relative rotational position, through the output means, of the magnet of the first driving device and the magnet of the second driving device to a predetermined position for the assembling. The efficiency in assembling work is therefore improved.  
         [0171]    The magnet  101  and the magnet  103  in this embodiment are coupled to each other by the gear of the output ring, by the gear portions each having teeth in a number corresponding to the number of magnetic poles of the magnetized portions. However, the number of teeth of the first gear portion  1   p  and the second gear portion  3   p  may not be the same as the number of magnetic poles of the magnetized portions of the magnet  101  and the magnet  103  in the case where facilitating the assembling is not a particularly pursued object. As long as the number of teeth corresponds to one of integral multiples of the number of magnetic poles of the magnetized portions, the teeth can have a phase that makes them symmetrical with respect to the center of the respective magnetic poles of the magnetized portions of the magnets  101  and  103 . The magnets thus have balanced magnetization intensity to provide a motor of stable output. FIG. 17 shows the case where the number of teeth is twice the number of magnetic poles of the magnetized portions. In FIG. 17, the center of the respective magnetized portions coincides with the middle between two teeth so that the shape of the magnet is also symmetrical with respect to the center of the respective magnetized portions. Therefore magnetic distribution is also symmetrical with respect to the center of the respective magnetized portions.  
         [0172]    In this embodiment, the motor composed of the first driving device, the second driving device and the output ring that is the output means is used as an actuator for driving an aperture blade. However, the motor is also useful in other applications as a motor that is advantageous in its high output, small diameter and short axial length. For example, the motor can be used to rotate a cam barrel for driving a lens.  
         [0173]    As detailed in the above, the drive transmission device comprises:  
         [0174]    a first driving device comprised of:  
         [0175]    a magnet having a gear portion at least on its outer peripheral surface, the gear portion being divided along the circumferential direction and magnetized to form different magnetic poles alternately, the magnet being rotatable about a rotation axis;  
         [0176]    a coil arranged in the axial direction of the magnet; and  
         [0177]    a stator having outer magnetic poles and inner magnetic poles that are excited by the coil, the outer magnetic poles facing the outer peripheral surface of the magnet whereas the inner magnetic poles facing the inner peripheral surface of the magnet;  
         [0178]    a second driving device comprised of:  
         [0179]    a magnet having a gear portion at least on its outer peripheral surface, the gear portion being divided along the circumferential direction and magnetized to form different magnetic poles alternately, the magnet being rotatable about a rotation axis;  
         [0180]    a coil arranged in the axial direction of the magnet; and  
         [0181]    a stator having outer magnetic poles and inner magnetic poles that are excited by the coil, the outer magnetic poles facing the outer peripheral surface of the magnet whereas the inner magnetic poles facing the inner peripheral surface of the magnet; and  
         [0182]    output means having a gear portion that is meshed with the gear portion of the magnet of the first driving device and the gear portion of the magnet of the second driving device,  
         [0183]    wherein the first driving device and the second driving device are arranged so as to be axially parallel to each other.  
         [0184]    Therefore the present invention can provide a drive transmission device which can rotate bidirectionally, which is short in the axial direction, and which has high output.  
         [0185]    The number of teeth of the gear portion formed in the respective magnets of the first driving device and the second driving device corresponds to one of integral multiples of the number of magnetic poles of the magnetized portions. The teeth has a phase that makes them symmetrical with respect to the center of the respective magnetic poles of the magnetized portions. The magnetization intensity distribution is thus symmetrical with respect to the center of the respective magnetic poles of the magnetized portions of the magnets. This makes the magnetized portions balanced and gives the drive transmission device a stable output.  
         [0186]    (N/2−1) outer magnetic poles that face the outer peripheral surface of the respective magnets are formed when the magnets each have N magnetic poles of the magnetized portions. In this case, (N/2−1) gaps between the outer magnetic poles are each set to a pitch of (2×360/N) degree whereas one gap between the outer magnetic poles is set to a pitch of 360/N degree. In the gap having a relatively wide pitch of 360/N degree, coupling means couples the gear portion of the magnet of the first driving device to the gear portion of the magnet of the second driving device. The coupling means thus can be arranged in a position axially overlapping the outer magnetic poles, thereby reducing the axial dimension of the drive transmission device.  
         [0187]    The aperture blade driving mechanism comprises the motor described above, is capable of rotating about the principal axis to open and close the aperture blade by the rotation, and is driven by the output means coupled to the magnet of the first driving device and the magnet of the second driving device. Therefore, the mechanism can form an aperture blade driving device which is provided with a high-power, inexpensive, small-sized motor. The device can serve as an actuator for driving an aperture blade which is short in the direction parallel to the optical axis and which does not form an obstacle to a lens and other components.