Abstract:
A bobbin comprises three hollow-cylindrical sections, specifically a middle body section, and two lateral body sections. The middle body section has its diameter diminished compared to the two lateral body sections thus forming an annular recess which allows a magnet wire to be wound with an additional number of turns around the bobbin without increasing a motor size. With this bobbin structure, when the number of turns of a magnet wire is set to remain unchanged, the diameter of flanges of the bobbin can be reduced resulting in a downsized motor.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a bobbin around which a magnet wire is wound, to a motor in which the bobbin is incorporated, and further to a method of winding a magnet wire around the bobbin.  
         [0003]     2. Description of the Related Art  
         [0004]      FIG. 5  is a partly cross-sectioned axial view of a conventional stepping motor. The conventional stepping motor comprises a rotor assembly  10 , and a stator assembly composed of two stator units  20  and  30 .  
         [0005]     The rotor assembly  10  is shaped substantially cylindrical, and composed of a permanent magnet with a plurality of magnetic poles  11  arranged on its outer circumference, and a rotary shaft  12  passing through the center of the magnet. The stator units  20  and  30  have respective center openings and are attached to each other coaxially, and the rotor assembly  10  is rotatably housed in the center openings.  
         [0006]     The stator unit  20  includes yoke members  21  and  22  arranged to oppose each other, and a bobbin  24  having a magnet wire  23  wound therearound and sandwiched between the yoke members  21  and  22 . The yoke member  21  has an opening corresponding to the center opening of the stator unit  20 , and a plurality of pole teeth formed along its inner circumference and bent up toward the yoke member  22  so as to oppose the magnetic poles  11  of the rotor assembly  10 . In the same way, the yoke member  22  has an opening corresponding to the center opening of the stator unit  20 , and a plurality of pole teeth formed along its inner circumference and bet up toward the yoke member  21  so as to oppose the magnetic poles  11  of the rotor assembly  10 .  
         [0007]     The bobbin  24  includes a body section  24   a , and flanges  24   b  and  24   c  provided at the both ends of the body section  24   a , and a center hole with a constant diameter is formed through the body sections  24   a , and the flanges  24   b  and  24   c . The pole teeth of the yoke members  21  and  22  are inserted through the center hole of the bobbin  24  and intermesh with each other therein. An outer rim portion  22   a  of the yoke member  22  is bent, for example by drawing, toward the yoke member  21  for engagement therewith, thus constituting an outer circumferential wall of the stator unit  20 .  
         [0008]     The stator unit  30  includes yoke members  31  and  32  arranged to oppose each other, which are structured identical with the yoke members  21  and  22 , respectively, and which sandwich a bobbin  24  having a magnet wire  23  wound therearound. An outer rim portion  32   a  of the yoke member  32  is bent, for example by drawing, toward the yoke member  31  for engagement therewith, thus constituting an outer circumferential wall of the stator unit  30 .  
         [0009]     A front plate FP is attached to a side of the yoke member  22  opposite to a side facing the yoke member  21 , and one protruding end of the rotary shaft  12  of the rotor assembly  10  is rotatably supported by the front plate FP. A rear plate RP is attached to a side of the yoke member  32  opposite to a side facing the yoke member  31 , and the other protruding end of the rotary shaft  12  of the rotor assembly  10  is rotatably supported by the rear plate RP.  
         [0010]     As electronic devices are increasingly downsized, the stepping motor shown in  FIG. 5  is requested to be downsized. One approach for answering the request is to reduce the diameter of the rotor assembly  10 . This approach, however, is hitting a limit for the reason of angle resolution, and also torque characteristic.  
         [0011]     Another approach is disclosed in Unexamined Japanese Patent Application KOKAI Publication No. H07-123686, in which the thickness of the outer rim portions  22   a  and  32   a  of the yoke members  22  and  32  are diminished, whereby the number of turns of the magnet wire  23  can be increased for increased torque when the motor size remains unchanged, or the motor can be downsized when the number of turns of the magnet wire  23  remains unchanged. This approach, however, raises a problem with mechanical strength, such as degraded reliability in protection against unforeseeable external forces. Also, when increased torque is required, the diameter of the flanges  24   b ,  24   c  of the bobbin  24  must be increased making it difficult to downsize the motor, thus preventing concurrent achievement of the downsizing and the increased torque.  
       SUMMARY OF THE INVENTION  
       [0012]     The present invention has been made in light of the above circumstances, and it is an object of the present invention to achieve downsizing of a motor while retaining torque characteristic. In order to achieve the object, according to a first aspect of the present invention, a bobbin is provided which comprises: a middle body section shaped hollow-cylindrical; a first lateral body section shaped hollow-cylindrical and having a larger bore diameter and a larger outside diameter than the middle body section; a second lateral body section shaped hollow-cylindrical and having a larger bore diameter and a larger outside diameter than the middle body section; a first joining section to connect one end of the first lateral body section to one end of the middle body section in a coaxial manner; a second joining section to connect one end of the second lateral body section to the other end of the middle body section in a coaxial manner; a first flange formed at the other end of the first lateral body section so as to radially extend outward; and a second flange formed at the other end of the second lateral body section so as to radially extend outward.  
         [0013]     In the bobbin structured as above, the winding space for a magnet wire can be increased compared to a conventional bobbin which has a constant body section diameter equal to the lateral body section diameter all the way between two flanges, whereby the number of turns of the magnet wire around the bobbin can be increased without increasing the diameter of the flanges, which results in an increased excitation force. If the number of turns of the magnet wire is set to remain unchanged, then the diameter of a circle defined by the outermost winding layer of the magnet wire can be reduced thus reducing the diameter of the flanges, whereby various electronic devices can be downsized. And, the downsizing and the increased excitation force can be concurrently achieved by appropriately setting the number of turns of the magnet wire.  
         [0014]     In the first aspect of the present invention, the first joining section may be shaped like an annular ring oriented perpendicular to the longitudinal direction of the middle body section and the first lateral body section so as to form a step configuration, and the second joining section may be shaped like an annular ring oriented perpendicular to the longitudinal direction of the middle body section and the second lateral body section so as to form a step configuration.  
         [0015]     In the first aspect of the present invention, the first joining section may be sloped with respect to outer surfaces of the middle body section and the first lateral body section, and the second joining section may be sloped with respect to outer surfaces of the middle body section and the second lateral body section.  
         [0016]     According to a second aspect of the present invention, there is provided a motor comprising: a rotor assembly shaped cylindrical, having a rotary shaft fixedly attached to its center, and having a plurality of magnetic poles arranged at its outer circumference; a first yoke formed of a soft-magnetic plate, having an opening for rotatably letting the rotor assembly through, and having a plurality of pole teeth formed at a circumference thereof defined by the opening so as to oppose the plurality of magnetic poles with a predetermined clearance in-between; a second yoke formed of a soft-magnetic plate, disposed to oppose the first yoke, having an opening for rotatably letting the rotor assembly through, and having a plurality of pole teeth formed at a circumference thereof defined by the opening so as to oppose the plurality of magnetic poles with a predetermined clearance in-between; a magnetic path means to magnetically connect the first and second yokes; and a bobbin which is structured as described in the first aspect, has a magnet wire wound around the first lateral body section, the middle body section, and the second lateral body section, and which is sandwiched between the first and second yokes so as to be coaxial to the openings thereof such that an inner circumferential surface of the middle body section opposes the outer circumference of the rotor assembly with a small clearance in-between, the pole teeth of the first magnetic yoke are housed in a space formed between an inner circumferential surface of the first lateral body section and the outer circumference of the rotor assembly, and such that the pole teeth of the second magnetic yoke are housed in a space formed between an inner circumferential surface of the second lateral body section and the outer circumference of the rotor assembly.  
         [0017]     Since the bobbin included in the motor described as above has an increased winding space for a magnet compared to a conventional bobbin which has a constant body section diameter equal to the lateral body section diameter all the way between two flanges, the number of turns of the magnet wire around the bobbin can be increased without increasing the diameter of the flanges thus without increasing the motor dimension. If the number of turns of the magnet wire is set to remain unchanged, then the diameter of a circle defined by the outermost winding layer of the magnet wire can be reduced thus reducing the diameter of the flanges, whereby the motor including the bobbin can be downsized. And, since the pole teeth of the first and second magnetic yokes are housed between respective inner circumferential surfaces of the first and second lateral body sections and the outer circumference of the rotor assembly, there is no impact on the diameter of the rotor assembly, thus the motor characteristic is not sacrificed.  
         [0018]     In the second aspect of the present invention, there may be provided a plurality of stator units, each of which includes the first and second yokes, and the bobbin.  
         [0019]     According to a third aspect of the present invention, there is provided a method of winding a magnet wire around a bobbin structured as described in the first aspect. The method comprises the steps of: winding a magnet wire for one layer around the first lateral body section from the other end thereof having the first flange to the one end; winding the magnet wire around the middle body section until a recess defined by difference in respective outside diameters of the middle body section and the first and second body sections is filled up; winding the magnet wire for one layer around the second lateral body section from the one end thereof to the other end having the second flange; and winding the magnet wire on top of respective outermost layers of the magnet wire wound around the first lateral body section, the middle body section, and the second lateral body section.  
         [0020]     Since the entire body section surface is leveled all the way between the flanges before the magnet wire is wound for the entire length, there is no variation in the amount of magnet wire to be reeled in when the magnet wire is wound. This enables the magnet wire to be wound in a preferred manner without a slacked wire and variation in tension.  
         [0021]     According to a fourth aspect of the present invention, there is provided a method of winding a magnet wire around a bobbin structured as described in the first aspect. The method comprises the steps of: winding a magnet wire around the middle body section until a recess defined by difference in respective outside diameters of the middle body section and the first and second body sections is filled up; and winding the magnet wire around the first lateral body section, an outermost layer of the magnet wire wound around the middle body section, and the second lateral body section.  
         [0022]     Since the recess around the middle body section is first filled up with the magnet wire wound so as to level the entire body section surface all the way between the flanges, there is no variation in the amount of magnet wire to be reeled in when the magnet wire is wound, thus the magnet wire can be wound in a preferred manner without a slacked wire and variation in tension. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]      FIG. 1  is a partly cross-sectioned axial view of a stepping motor including a bobbin according to a first embodiment of the present invention;  
         [0024]      FIG. 2  is a cross-sectional view of the bobbin shown in  FIG. 1 ;  
         [0025]      FIG. 3  is a cross-sectional view of a bobbin according to a second embodiment of the present invention; and  
         [0026]      FIG. 4  is an explanatory view of a method of winding a magnet wire according to a third embodiment of the present invention;  
         [0027]      FIG. 5  is a partly cross-sectioned axial view of a conventional stepping motor. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]     A first embodiment of the present invention will be described with reference to  FIGS. 1 and 2 .  
         [0029]     A stepping motor shown in  FIG. 1  is capable of two-phase driving, and comprises a rotor assembly  40 , and two stator units  50  and  60 .  
         [0030]     The rotor assembly  10  is shaped substantially cylindrical, and is composed of a permanent magnet with a plurality of magnetic poles  42 , and a rotary shaft  41  passing through the center of the magnet.  
         [0031]     The stator unit  50  includes first and second yokes  51  and  52  arranged to oppose each other, and a bobbin  53 . The bobbin  53  has a magnet wire  54  wound therearound, and an insulator  55  is disposed around the magnet wire  54  wound. The bobbin  53  is sandwiched between the first and second yokes  51  and  52 .  
         [0032]     The first yoke  51  is punched out of a soft-magnetic plate into a disk having at its center a circular opening for letting the rotor assembly  40  through. A plurality of pole teeth  51   a  are formed along an inner circumference of the first yoke  51  defined by the circular opening and are bent up perpendicularly toward the second yoke  52  so as to closely oppose the magnetic poles  42  of the rotor assembly  40 .  
         [0033]     The second yoke  52  is punched out of a soft-magnetic plate into a disk having at its center a circular opening for letting the rotor assembly  40  through, and disposed to oppose the first yoke  51  so as to sandwich the bobbin  53  therebetween. A plurality of pole teeth  52   a  are formed along an inner circumference of the second yoke  52  defined by the circular opening and are bent up perpendicularly toward the first yoke  51  so as to closely oppose the magnetic poles  42  of the rotor assembly  40 . An outer rim portion  52   b  of the second yoke  52  is bent, for example by drawing, toward the first yoke  51  so as to engage therewith. The outer rim portion  52   b  of the second yoke  52  constitutes a magnetic path means to magnetically connect the first and second yokes  51  and  52 . The pole teeth  52   a  of the second yoke  52  are shifted in phase by 180 degrees from the pole teeth  51   a  of the first yoke  51 .  
         [0034]     The bobbin  53  is formed of synthetic resin or the like, and includes, as shown in  FIG. 2 , a middle body section  53   a  shaped hollow-cylindrical, and two lateral body sections  53   b  and  53   c  shaped hollow-cylindrical and disposed coaxially to the middle body section  53   a . One end of the middle body section  53   a  is connected to one end of the lateral body section  53   b  via a joining section  53   d , and the other end of the middle body section  53   a  is connected to one end of the lateral body section  53   c  via a joining section  53   e . Respective hollows of the middle body section  53   a , and the lateral body sections  53   d  and  53   c  are coaxial to one another. The joining section  53   d  is oriented perpendicular to the outer surface of the middle body section  53   a  and the inner surface of the lateral body section  53   b . The joining section  53   e  is oriented perpendicular to the outer surface of the middle body section  53   a  and the inner surface of the lateral body section  53   c.    
         [0035]     The middle body section  53   a  has a bore diameter D 1  larger than the diameter of the rotor assembly  40  and substantially equal to the diameter of the circular openings of the first and second yokes  51  and  52 . The lateral body sections  53   b  and  53   c  have a bore diameter D 3  larger than the bore diameter D 1  of the middle body section  53   a  so as to form respective spaces to house the pole teeth  51   a  and  52   a . The middle body section  53   a  has an outside diameter D 2  smaller than an outside diameter D 4  of the lateral body sections  53   b  and  53   c , thus the bobbin  53 , unlike a conventional bobbin, has an annular recess  53   h  formed around the middle body section  53   a.    
         [0036]     A flange  53   f  is formed at the distal end of the lateral body section  53   b  so as to radially extend outward, and a flange  53   g  is formed at the distal end of the lateral body section  53   c  so as to radially extend outward. The flanges  53   f  and  53   g  are parallel to each other and have a same outside diameter D 5 . The distance between the outer face of the flange  53   f  and the joining section  53   d  is larger than the length of the pole teeth  52   a , and the distance between the outer face of the flange  53   g  and the joining section  53   e  is larger than the length of the pole teeth  51   a.    
         [0037]     The stator unit  60  includes first and second yokes  61  and  62  arranged to oppose each other, and a bobbin  53 . The first yoke  61  is structured and made identically with the first yoke  51 , and has a circular opening at its center, and a plurality of pole teeth  61   a  are formed along an inner circumference of the first yoke  61  defined by the circular opening and are bent up perpendicularly toward the second yoke  62 . The second yoke  62  is structured and made identically with the second yoke  52 , and has a circular opening at its center, and a plurality of pole teeth  62   a  are formed along an inner circumference of the second yoke  62  define by the circular opening a and are bent up perpendicularly toward the first yoke  61 . The first and second yokes  61  and  62  sandwich the bobbin  53  which has the same structure as described above in explaining the stator unit  50 , and which has a magnet wire  54  wound therearound, and an insulator  55  is disposed around the magnet wire  54  wound.  
         [0038]     The stator units  50  and  60  structured as described above are coaxially put together such that the first yoke  51  of the stator unit  50  is welded to the first yoke  61  of the stator unit  60 . A front plate  71  is attached to a side of the second yoke  52  opposite to a side facing the first yoke  51 , and a bearing  72  to rotatably support the rotary shaft  41  of the rotor assembly  40  is attached to the front plate  71 . A rear plate  73  is attached to a side of the second yoke  62  opposite to a side facing the first yoke  61 , and a bearing  74  to rotatably support the rotary shaft  41  is attached to the rear plate  73 . The rotor assembly  40  with the rotary shaft  41  is held coaxial to the stator units  50  and  60  by means of the bearings  72  and  74 .  
         [0039]     When the rotor assembly  40  with the rotary shaft  41  is held coaxial to the stator units  50  and  60  as described above, the inner surfaces of respective middle body sections  53  of the two bobbins  53  are positioned close to the outer circumferential surface of the rotor assembly  40  such that the pole teeth  51   a  are housed in a space defined by the rotor assembly  40 , and the lateral body section  53   c  and the joining section  53   e  of the bobbin  53  sandwiched between the first and second yokes  51  and  52  so as to oppose the magnetic poles  42  with a small air gap in-between, and such that the pole teeth  52   a  are housed in a space defined by the rotor assembly  40 , and the lateral body section  53   b  and the joining section  53   d  of the bobbin  53  sandwiched between the first and second yokes  51  and  52  so as to oppose the magnetic poles  42  with a small air gap in-between. And in the same way, the pole teeth  61   a  are housed in a space defined by the rotor assembly  40 , and the lateral body section  53   c  and the joining section  53   e  of the bobbin sandwiched between the first and second yokes  61  and  62  so as to oppose the magnetic poles  42  with a small air gap in-between, and the pole teeth  62  are housed in a space defined by the rotor assembly, and the lateral body section  53   b  and the joining section  53   d  of the bobbin  53  sandwiched between the first and second yokes  61  and  62  so as to oppose the magnetic poles  42  with a small air gap in-between.  
         [0040]     In the stepping motor structured above, when current is applied to respective magnet wires  54 , the pole teeth  51   a ,  52   a ,  61   a  and  62   a  of the yokes  51 ,  52 ,  61  and  62  are excited, whereby attraction and repulsion forces are generated between the magnetic poles  42  and the pole teeth  51   a ,  52   a ,  61   a  and  62   a  causing the rotor assembly with the rotary shaft  41  to rotate.  
         [0041]     The bobbin  53  is provided with the annular recess  53   h  formed around the middle body section  53   a , and the magnet wire  54  is wound also at the recess  53   h , which means provision of an additional winding space compared to a conventional structure. Accordingly, the stepping motor shown in  FIG. 1  has following advantages:  
         [0042]     (1) The total length L of the magnet wire  54  allowed to be wound around the bobbin  53  is obtained as follows (the thickness of the insulator  55  is ignored).  
         [0043]     The number of rows N 1  per layer of the magnet wire  54  allowed to be wound in the space of the recess  53   h  is given by 
 
 N   1 = W   1 / d  (decimals omitted) 
 
 where W 1  is the axial dimension of the recess  53   h , and d is the diameter of the magnet wire  54 . 
 
         [0045]     The number of layers M 1  of the magnet wire  54  allowed to be wound in the space of the recess  53   h  is given by 
 
 M   1 =( D   4   −D   2 )/2 /d  (decimals omitted). 
 
         [0046]     So, the number of turns T 1  of the magnet wire  54  allowed to be wound in the space of the recess  53   h  is given by 
 
 T   1 = N   1 × M   1 . 
 
         [0047]     Therefore, the length L 1  of the magnet wire  54  allowed to be wound in the space of the recess  53   h  is calculated by the following formula A:  
             L1   =       ⁢       N1   ×     (     D2   +     1   ×   d       )     ⁢   •     +     N1   ×     (     D2   +     2   ×   d       )     ⁢   •     +   …   +     N1   ×     (     D2   +     M1   ×   d       )     ⁢   •                   =       ⁢     N1   ×   M1   ×   •   ×     (     D2   +       (     M1   +   1     )     ×     d   /   2         )                 
 
         [0048]     Now, the number of rows N 2  per layer of the magnet wire  54  allowed to be wound around the bobbin  53  including the area covering the space of the recess  53   h  is given by 
 
 N   2 = W 2 /d  (decimals omitted) 
 
 where W2 is the distance between the inner faces of the flanges  53   f  and  53   g.  
 
         [0050]     The number of layers M 2  of the magnet wire  54  allowed to be wound around the bobbin  53  except the area covering the space of the recess  53   h  is given by 
 
 M   2 =( D   5   −D   4 )/2 /d  (decimals omitted). 
 
         [0051]     So, the number of turns T 2  of the magnet wire  54  allowed to be wound around the bobbin  53  excluding the space of the recess  53   h  is given by 
 
 T   2 = N   2 × M   2 . 
 
         [0052]     Therefore, the length L 2  of the magnet wire  54  allowed to be wound around the bobbin  53  excluding the space of the recess  53   h  is calculated by the following formula B:  
             L2   =       ⁢       N2   ×     (     D4   +     1   ×   d       )     ⁢   •     +     N2   ×     (     D4   +     2   ×   d       )     ⁢   •     +   …   +     N2   ×     (     D2   +     M2   ×   d       )     ⁢   •                   =       ⁢     N2   ×   M2   ×   •   ×     (     D4   +       (     M2   +   1     )     ×     d   /   2         )                 
 
         [0053]     Accordingly, the total length L of the magnet wire  54  allowed to be wound around the bobbin  53  including the space of the recess  53  is roughly obtained based on the length L 1  and the length L 2  as follows 
 
 L=L   1   +L   2  
 
         [0054]     Here, since the length L 2  obtained by the formula B is equivalent to the winding length of a magnet wire allowed to be wound around a conventional bobbin, the length L 1  obtained by the formula A constitutes an additional winding length and therefore an additional number of turns when compared to the conventional bobbin. Thus, the excitation force generated can be increased.  
         [0055]     (2) The advantage (1) described above is derived from the case where the outside diameter D 5  of the flanges  53   f  and  53   g  is set to match that of a conventionally structured bobbin. Now, in case if the number of turns of the magnet wire  54  is set to match that allowed on a conventional bobbin, the outside diameter D 5  of the flanges  53   f  and  53   g  can be reduced, whereby the radial dimension of a motor can be reduced.  
         [0056]     (3) Considering the above-described advantages (1) and (2), both increase in the excitation force of the magnet wire  54  and reduction in the outside diameter D 5  of the flanges  53  and  53   g  can be concurrently achieved by appropriately determining the dimension of the recess  53   h.    
         [0057]     A second embodiment of the present invention will be described with reference to  FIG. 3 .  
         [0058]     A bobbin  81  shown in  FIG. 3  is formed of synthetic resin, and comprises a middle body section  81   a  shaped hollow-cylindrical, and two lateral body sections  81   b  and  81   c  shaped hollow-cylindrical and having a larger diameter than the middle body section  81   a . One end of the middle body section  81   a  is connected to one end of the lateral body section  81   b  via a joining section  81   d  with a gradual change in diametrical dimension thus not forming a step configuration, and the other end of the middle body section  81   a  is connected to one end of the lateral body section  81   c  via a joining section  81   e  with a gradual change in diametrical dimension thus not forming a step configuration. The hollows of the middle body section  81   a , and the lateral body sections  81   b  and  81   c  are set coaxial to one another. A flange  81   f  is formed at the distal end of the lateral body section  81   b  so as to radially extend outward, and a flange  81   g  is formed at the distal end of the lateral body section  81   c  so as to radially extend outward.  
         [0059]     In the bobbin  81  structured above, when a magnet wire is wound, for example from the flange  81   f  to the flange  83   g , unbalance with respect to amounts of the magnet wire to be reeled out and reeled in, which is caused due to the difference in length of one turn of the magnet wire between around the middle body section  81   a  and around the lateral body sections  81   b  and  81   c , does not appear drastically thanks to the diametrical dimension changing gradually at the joining portions between the middle body section  81   a  and the lateral body sections  81   b  and  81   c  and therefore can be absorbed, whereby the magnet wire can be wound in a preferable manner.  
         [0060]     A third embodiment of the present invention will be described with reference to  FIG. 4 . The third embodiment relates to a method for preferably winding a magnet wire around such a bobbin as having a structure described above in the first embodiment, and the description will be made with reference to the bobbin  53 .  
         [0061]     Referring to  FIG. 4 , the bobbin  53  is set on a rotating mechanism  91  of a winding apparatus, and a starting end  54 S of the magnet wire  54  is fixed to the flange  53   f  by a tape (not shown), or the like. The rotating mechanism  91  is such as to rotate the bobbin  53  about its axis. A traverse roll  92  is positioned to the inner face of the flange  53   f  so that the magnet wire  54  can be wound starting from the distal end of the lateral body section  53   b . The traverse roll  92  is made to work in conjunction with the rotating mechanism  91  via a mechanism (not shown) so as to reciprocate the magnet wire  54  in a direction along the axis of the bobbin  53 . The winding apparatus has various sensors (not shown) as well as the rotating mechanism  91  and the traverse roll  92 . Among the sensors are a sensor to detect the position of the traverse roll  92  to thereby change the traveling direction thereof, and a sensor to detect the amount of the magnet wire  54  wound in the recess  53   h  of the bobbin  53  (specifically the diameter of a circle defined by the outermost layer of the magnet wire  54  wound around the middle body section  53   a ).  
         [0062]     After the traverse roll  92  is duly positioned as described above, the rotating mechanism  92  is driven. When the rotating mechanism  91  is driven, the bobbin  53  rotates about its axis, and the magnet wire  54  is wound around the bobbin  53  starting from the distal end of the lateral body section  53   b . The traverse roll  92  travels to the one end of the middle body section  53   a  thereby completing the first winding layer around the lateral body section  53   b , and then the magnet wire  54  is wound around the middle body section  53   a  for one layer while the traverse roll  92  travels to the other end of the middle body section  53   a . When the traverse roll  92  reaches the other end of the middle body section  53   a  and has its position sensed, the traveling direction of the traverse roll  92  is reversed, and then the magnet wire  54  is wound around the middle body section  53   a  from the other end thereof back to the one end for the second winding layer. When the traverse roll  92  reaches the one end of the middle body section  53  and has its position sensed, the traveling direction is reversed. Thus, the magnet wire  54  is wound in the recess  53   h  of the bobbin  53  in a reciprocating manner.  
         [0063]     When it is sensed that the diameter of a circle defined by the outermost layer of the magnet wire  54  wound around the middle body section  53   a  is equal to the diameter of a circle defined by the one layer of the magnet wire  54  wound around the lateral body section  53   b , which means that the recess  53   h  is filled up with the magnet wire  54  wound, the traverse roll  92  has its traveling direction no longer reversed at the other end of the middle body section  53   a  and continues to travel toward the flange  53   g , whereby the magnet wire  54  is wound around the lateral body section  53   c . When the traverse rolls  92  reaches the inner face of the flange  53   g  and has its position sensed, the traveling direction of the traverse roll  92  is reversed toward the flange  53   f . Thereafter, the traverse roll  92  is adapted to have its traveling direction reversed at the inner faces of the flanges  53   f  and  53   g  so that the magnet wire  54  is wound around the bobbin  53  all the way between the flanges  53   f  and  53   g.    
         [0064]     Thus, since the recess  53   h  around the middle body section  53   a  is first filled up with the magnet wire  54  thereby eliminating the step configurations formed at the joining sections  53   d  and  53   e , the magnet wire  54  can then be wound in a level manner all the way from the flange  53   f  to the flange  53   g , which enhances winding quality.  
         [0065]     It should be understood that the present invention is not limited to the specific embodiments described in this specification, and various modifications are possible in light of the above teaching. For example, the present invention may be applied to a sing-phase driving stepping motor, and also a three or more-phase driving stepping motor. Also, in the third embodiment, the magnet wire  54  is wound around the lateral body section  53   b  for one layer before filling up the recess  53   h , but the magnet wire  54  may alternatively be wound around the middle body section  53  from the start to fill up the recess  53   h . Therefore, it is intended that the scope of the present invention be defined by the following claims.  
         [0066]     This application is based on Japanese Patent Application No. 2003-297993 filed on Aug. 21, 2003 and including specification, claims, drawings and summary. The disclosure of the above Japanese Patent Application is incorporated herein by reference in its entirety.