Abstract:
In a motor comprising a stator assembly and a rotor assembly housed inside the stator assembly, a bobbin is structured to be oval-cylindrical in its radial cross section, rather than circular-cylindrical, such that a body section of the bobbin is composed of four continuous walls, two of which are formed of flat plates oriented perpendicular to the minor axis of the oval and disposed opposite to each other in parallel, and the other two of which are formed of outwardly arced plates oriented perpendicular to the flat walls and disposed opposite to each other in a symmetric manner. Pole teeth are provided only at portions corresponding to the arced walls to thereby reduce the dimension between the flat walls for downsizing while allowing an increased number of turns of a magnet wire wound around the bobbin.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a bobbin having a magnet wire wound therearound, and further to a motor incorporating such a bobbin.  
         [0003]     2. Description of the Related Art  
         [0004]      FIG. 6  is a partly cross-sectional view of a conventional general stepping motor. The conventional motor basically comprises a rotor assembly  10 , and a pair of stator units  20  and  30  which in combination constitute a stator assembly. The rotor assembly  10  is shaped substantially cylindrical, includes a permanent ring magnet having a plurality of magnetic poles  11  arranged on its circumferential surface, and has a rotary shaft  12  fixedly inserted through its center. The stator units  20  and  30  are both shaped substantially hollow-cylindrical like a doughnut, and are coupled to each other with their respective center openings coaxially aligned so as to rotatably house the rotor assembly  10  in the center openings.  
         [0005]     The stator unit  20  is composed of yoke members  21  and  22  disposed so as to oppose each other, and a magnetic wire  23  wound around a bobbin  24  sandwiched between the yoke members  21  and  22 . The yoke member  21  is shaped like a ring defining a center opening corresponding to the center opening of the stator unit  20 , and a plurality of pole teeth  21   a  are formed along the inner circumference of the yoke member  21  and bent up so as to have their distal ends pointing toward the yoke member  22  and to be positioned to oppose the magnetic poles  11  of the rotor assembly  10 . In a similar way, the yoke member  22  is shaped like a ring defining a center opening corresponding to the center opening of the stator unit  20 , and a plurality of pole teeth  22   a  are formed along the inner circumference of the yoke member  22  and bent up so as to have their distal ends pointing toward the yoke member  21  and to be positioned to oppose the magnetic poles  11  of the rotor assembly  10 . The bobbin  24  includes a body section shaped circular-cylindrical and two flanges disposed respectively at the ends of the body section, and the body section has an open hollow with a constant diameter. The pole teeth  21   a  and  22   a  of the yoke member  21  and  22  are inserted into the hollow of the body section of the bobbin  24  from respective ends thereof so as to intermesh with each other. An outer rim portion  22   b  of the yoke member  22  is bent, for example by drawing, toward the yoke member  21  so as to constitute an outer circumferential wall of the stator unit  20 .  
         [0006]     The stator unit  30  is composed of yoke members  31  and  32  structured and disposed like the above-described yoke members  21  and  22  of the stator unit  20 , and a magnetic wire  33  (identical with the magnet wire  23 ) wound around a bobbin  34  (identical with the bobbin  24 ) sandwiched between the yoke members  31  and  32 . An outer rim portion  32   b  of the yoke member  32  is bent up toward the yoke member  31  so as to constitute an outer circumferential wall of the stator unit  30 .  
         [0007]     With increasing demand for miniaturization of various devices, the above-described stepping motor incorporated therein is also requested to be downsized. If a motor is downsized in its axial direction for miniaturization, the distance between both bearings is inevitably decreased thus making it very difficult to control resulting problems such as wobbling of a rotor assembly, which hinders the productivity of reliable products. Consequently, there is a limit to the downsizing of a motor in the axial direction.  
         [0008]     In the meanwhile, a motor may be downsized in its radial direction for miniaturization such that a rotor assembly has its radial dimension decreased, but the downsizing of a motor in the radial direction is approaching the limit in view of angular resolution and also torque characteristic. Under the circumstances, Japanese Patent Application KOKAI Publication No. H07-123686 discloses a motor for downsizing in the radial direction. The motor, if explained by using reference numerals shown in  FIG. 6 , comprises a stator assembly composed of stator units  20  and  30 , in which an outer rim portions  22   b  and  32   b  of yoke members  22  and  32  constituting the outer circumferential walls have a reduced thickness thereby allowing magnet wires  23  and  33  to have an increased number of turns thus achieving an increased torque, or when the number of turns of the magnet wires  23  and  33  is not increased, the motor diameter can be reduced without detriment to the torque characteristic.  
         [0009]     However, the motor disclosed by the aforementioned Japanese Patent Application KOKAI Publication No. H07-123686 has a problem that since the outer circumference walls of the stator units  20  and  30  constituted by the outer rim portions  22   b  and  32   b  of the yoke members  22  and  32  have a decreased thickness, the motor is weakened in mechanical strength thus resulting in deteriorated reliability against accidental external force. Thus, there still lies a limit to effort for downsizing.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention has been made in light of the circumstances described above, and it is an object of the present invention to downsize a motor while maintaining its torque characteristic.  
         [0011]     In order to achieve the object, according to a first aspect of the present invention, a bobbin is provided which comprises: 
        a) a body section shaped hollow-cylindrical, having a predetermined length, and composed of 
            a first wall formed of a flat rectangular plate,     a second wall formed of a flat rectangular plate, and disposed to oppose the first wall in parallel,     a third wall formed of a rectangular plate curved circumferentially outward, and having its both ends connected respectively to one end of the first wall and one end of the second wall, and     a fourth wall formed of a rectangular plate curved circumferentially outward, disposed to symmetrically oppose the third wall, and having its both ends connected respectively to other end of the first wall and other end of the second wall;    
            b) a first flange disposed at one end of the body section so as to extend perpendicularly to the walls for a substantially uniform dimension, and     c) a second flange disposed at other end of the body section so as to extend perpendicularly to the walls for a substantially uniform dimension.        
 
         [0019]     In the first aspect of the present invention, the third and fourth walls of the body section may have a circular arc radial cross-section, and a distance between the first and second walls may be smaller than a distance between the third and fourth walls.  
         [0020]     Consequently, the bobbin according to the first aspect of the present invention has a reduced dimension at the first and second walls compared with the conventional structure, thus allowing dimensional reduction.  
         [0021]     According to a second aspect of the present invention, a motor is provided which comprises 
        a) a rotor assembly shaped cylindrical, and having a ring magnet disposed at its outer circumference and provided with a predetermined number of magnetic poles, and a rotary shaft fixedly inserted through its center,     b) a stator assembly having 
            a bobbin including 
                a body section shaped hollow-cylindrical, having a predetermined length, and composed of: a first wall formed of a flat rectangular plate; a second wall formed of a flat rectangular plate, and disposed to oppose the first wall in parallel; a third wall formed of a rectangular plate curved circumferentially outward, and having its both ends connected respectively to one end of the first wall and one end of the second wall; and a fourth wall formed of a rectangular plate curved circumferentially outward, disposed to symmetrically oppose the third wall, and having its both ends connected respectively to other end of the first wall and other end of the second wall,     a first flange disposed at one end of the body section so as to extend perpendicularly to the walls for a substantially uniform dimension, and     a second flange disposed at other end of the body section so as to extend perpendicularly to the walls for a substantially uniform dimension,    
                a magnet wire wound around the body section of the bobbin,     a first yoke made of a soft magnetic steel plate, having at a center thereof an opening which defines an inner circumference of the first yoke, has a dimension larger than a diameter of the rotor assembly, corresponds to a hollow of the bobbin, and in which the rotor assembly is inserted, the first yoke having a plurality of pole teeth formed along the inner circumference thereof respectively at two portions corresponding to the third and fourth walls of the bobbin, the plurality of pole teeth being inserted in the hollow of the bobbin so as to oppose the magnetic poles of the rotor assembly,     a second yoke made of a soft magnetic steel plate, disposed to oppose the first yoke so as to sandwich the bobbin therebetween, and having at a center thereof an opening which defines an inner circumference of the second yoke, has a dimension larger than the diameter of the rotor assembly, corresponds to the hollow of the bobbin, and in which the rotor assembly is inserted, the second yoke having a plurality of pole teeth formed along the inner circumference thereof respectively at two portions corresponding to the third and fourth walls of the bobbin, the plurality of pole teeth being inserted in the hollow of the bobbin so as to oppose the magnetic poles of the rotor assembly, and     a magnetic path means to magnetically connect the first and second yokes, and    
            c) bearing means to rotatably support the rotary shaft of the rotor assembly such that a predetermined gap is kept between the magnetic poles of the rotor assembly and the pole teeth of the first and second yokes.        
 
         [0033]     In the second aspect of the present invention, the third and fourth walls of the body section of the bobbin may have a circular arc radial cross-section, and a distance between the first and second walls may be smaller than a distance between the third and fourth walls.  
         [0034]     In the second aspect of the present invention, the stator assembly may be composed of a plurality of stator units each constituted by the bobbin, the magnet wire, the first and second yokes, and the magnetic path means.  
         [0035]     Consequently, the motor according to the second aspect of the present invention is allowed to have an increased number of turns of the magnet wire wound around the bobbin, whereby the dimensional reduction can be achieved without detriment to the motor characteristic. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0036]      FIG. 1  is an axial partly cross-sectional view of a stepping motor according to an embodiment of the present invention;  
         [0037]      FIG. 2  is a perspective view of a bobbin of the stepping motor of  FIG. 1 ;  
         [0038]      FIG. 3  is an exploded perspective view of a stator unit of the stepping motor of  FIG. 1 ;  
         [0039]      FIG. 4A  is an explanatory view of the bobbin and pole teeth in the stepping motor of  FIG. 1 ;  
         [0040]      FIG. 4B  is an explanatory view of a bobbin and pole teeth in a conventional stepping motor;  
         [0041]      FIG. 5  is a perspective view of a modification of a yoke; and  
         [0042]      FIG. 6  is an axial partly cross-sectional view of a conventional stepping motor. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0043]     A preferred embodiment of the present invention will hereinafter be described with reference to  FIG. 1  to  FIGS. 4A and 4B .  
         [0044]     Referring to  FIG. 1 , a stepping motor according to the embodiment is capable of two-phase driving, and comprises a rotor assembly  40  and two stator units  50  and  60  which in combination constitute a stator assembly.  
         [0045]     The rotor assembly  40  is shaped cylindrical, and has a rotary shaft  41  fixedly inserted at its center and a ring magnet with a plurality of magnetic poles  42  at its outer circumference.  
         [0046]     The stator unit  50  is composed of first and second yokes  51  and  52  disposed to oppose each other, and a magnet wire  54  wound around a bobbin  53  sandwiched between the first and second yokes  51  and  52 , and an insulator  55  is disposed around the magnet wire  54 .  
         [0047]     The bobbin  53  is formed of plastic resin or the like, and includes a body section  53   a , and first and second flanges  53   b  and  53   c  as shown in  FIG. 2 . The body section  53   a  is composed of first, second, third and fourth walls  53   a   1 ,  53   a   2 ,  53   a   3  and  53   a   4  having a uniform and equal thickness, and shaped oval-cylindrical in its radial view rather than circular-cylindrical as conventionally, specifically such that the first and second walls  53   a   1  and  53   a   2  are rectangular and flat, and disposed to oppose each other in parallel, and that the third and fourth walls  53   a   3  and  53   a   4  are rectangular and arced outward, disposed so as to symmetrically oppose each other, have their one ends connected respectively to both ends of the first wall  53   a   1 , and have their respective other ends connected respectively to both ends of the second wall  53   a   2 . The first and second flanges  53   b  and  53   c  extend radially outward from respective ends of the body section  53   a  so as to form a radial cross-sectional outer configuration similar to and larger than that of the body section  53   a.    
         [0048]     Referring to  FIG. 3 , the first yoke  51  is punched out of a soft magnetic steel plate, and has an opening at its center corresponding to the hollow of the bobbin  53 . The opening of the first yoke  51  in the figure is shaped oval, or may alternatively be shaped circular, and the rotor assembly  40  is inserted through the opening of the first yoke  51 . Four pole teeth  51   a  are formed respectively at two portions of the inner circumference of the first yoke  51  corresponding to the third and fourth walls  53   a   3  and  53   a   4  and are bent up toward the second yoke  52  so as to be inserted through the hollow of the bobbin  53  thus opposing the magnetic poles  42  of the rotor assembly  40 . The first yoke  51  has an outer configuration similar to that of the first and second flanges  53   b  and  53   c  of the bobbin  53 .  
         [0049]     The second yoke  52  is punched out of a soft magnetic steel plate, and has an opening at its center corresponding to the hollow of the bobbin  53 . The opening of the second yoke  52 , like that of the first yoke  51 , is shaped oval as shown in  FIG. 3 , or may alternatively be shaped circular, and the rotor assembly  40  is inserted through the opening of the second yoke  52 . Four pole teeth  52   a  are formed respectively at two portions of the inner circumference of the second yoke  52  corresponding to the third and fourth walls  53   a   3  and  53   a   4  and are bent up toward the first yoke  51  so as to be inserted through the hollow of the bobbin  53  and to intermesh with the pole teeth  51   a  of the first yoke  51  thus opposing 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 with an outer circumference of the first yoke  51 , and functions as a magnetic path to magnetically connect the first and second yokes  51  and  52 . The second yoke  52  has an outer configuration similar to that of the first and second flanges  53   b  and  53   c  of the bobbin  53 . 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 .  
         [0050]     Referring back to  FIG. 1 , the stator unit  60  is composed of first and second yokes  61  and  62  disposed to oppose each other, and a magnet wire  64  (identical with the magnet wire  54 ) wound around a bobbin  63  (identical with the bobbin  53 ) sandwiched between the first and second yokes  61  and  62 , and an insulator  65  (identical with the insulator  55 ) is disposed around the magnet wire  64 . The first yokes  61  and  62  are structured identically with the first and second yokes  51  and  52  of the stator unit  50 , and have respective pole teeth  61   a  and  62   a.    
         [0051]     The first yoke  51  of the stator unit  50  and the first yoke  61  of the stator unit  60  are coaxially welded to each other. A front plate  71  is attached to the second yoke  52 , and a front bearing  72  to rotatably support the rotary shaft  41  is attached to the front plate  71 . A rear plate  73  is attached to the second yoke  62 , and a rear bearing  74  to support rotatably the rotary shaft  41  is attached to the rear plate  73 .  
         [0052]     In the stepping motor structured as above, when the magnet wire  54  disposed between the first and second yokes  51  and  52  is supplied with electricity, the pole teeth  51   a  and  52   a  are magnetized, whereby attraction and repulsion are generated against the magnetic poles  42  of the rotor assembly  40 . Also, when the magnet wire  64  disposed between the first and second yokes  61  and  62  is supplied with electricity, the pole teeth  61   a  and  62   a  are magnetized, whereby attraction and repulsion are generated against the magnetic poles  42  of the rotor assembly  40 . The attraction and repulsion thus generated cause the rotor assembly  40  to rotate.  
         [0053]     Next, an advantage of this stepping motor will be explained.  
         [0054]     In a case where the pole teeth  51   a  of the yoke  51  and the pole teeth  52   a  of the yoke  52  are formed everywhere along a circular hole as conventional, the body section  53   a  of the bobbin  53  has to be circular-cylindrical as shown in  FIG. 4B .  
         [0055]     On the other hand, according to the above-described embodiment, since the pole teeth  51   a  and  52   a  of the yokes  51  and  52  are formed only at the portions corresponding to the walls  51   a   3  and  53   a   4  of the bobbin  53  and not at the portions corresponding to the walls  53   a   1  and  53   a   2  as shown in  FIG. 4A , the yokes  51  and  52  can be formed oval, and the body section  53   a  of the bobbin  53  can be shaped oval-cylindrical.  
         [0056]     When the body section  53   a  of the bobbin  53  is shaped oval-cylindrical as described above, the dimension between the first and second walls  53   a   1  and  53   a   2  can be reduced, and accordingly the flanges  53   b  and  53   c  can be shaped with a reduced dimension therebetween, which allows the number of turns for the magnet wire  54  to be increased to such an extent as to ensure the torque characteristic conventionally achieved while realizing dimensional reduction.  
         [0057]     The present invention is not limited to the embodiment described above but may alternatively be embodied as follows.  
         [0058]     (1) The numbers of the pole teeth  51   a ,  52   a ,  61   a  and  62   a  may be changed according to angular resolution of the stepping motor. And, all the pole teeth do not necessarily have a complete shape as shown in  FIGS. 3 and 5  (symmetric trapezoid in the figures), but alternatively some thereof may be half-formed, which may be defined by, for example, (4+½).  
         [0059]     (2) The present invention may be applied to a stepping motor of single-phase or three or more-phase driving, instead of two-phase driving.  
         [0060]     (3) The second yoke  52  may be modified, for example, as shown in  FIG. 5 , where the bent outer rim portion  52   b  thereof is interrupted by two voids  52   c  (only one is shown in the figure) to be separated into two sections such that the magnet wire  54  wound around the bobbin  53 , and the insulator  55  are not entirely covered, while the bent outer rims portion  52   b  in the embodiment described above is uninterrupted so as to entirely cover the magnet wire  54  and the insulator  55 . This modification can be applied when the motor is used in an environment free from exterior force, and can contribute to further downsizing, reduction in weight, and efficient heat dissipation. Also, this enables the outer rim portion  52   b  sectioned into two to be bent without drawing process thus making the bending work easier.  
         [0061]     The scope of the invention is, therefore, indicated by the appended claims rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.  
         [0062]     This application is based on Japanese Patent Application No. 2003-278652 filed on Jul. 23, 2003 and including specification, claims, drawings and summary. The disclosure of the above Japanese Patent Application is incorporated herein by reference in its entirety.