Abstract:
A low-profile stepping motor has a stator assembly structured such that first and second coil units are disposed flush with each other horizontally and are sandwiched between upper and lower stator yoke pieces each comprising first and second stator yokes which are punched out integrally with each other, are shaped symmetrically, and which each have a plurality of pole teeth along its semi-circular inner circumference. The upper and lower stator yoke pieces are fixedly put together by means of two studs lodged fittingly inside respective bobbins of the first and second coil units such that the studs have their both ends tightly engaging with holes formed in the upper and lower stator yoke pieces, whereby the coil units are surely held between the upper and lower stator yoke pieces.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a stepping motor, and specifically to a low-profile stepping motor having its dimension reduced in an axial direction.  
           [0003]    2. Description of the Related Art  
           [0004]    Recently, in accordance with enhanced performance of various devices such as printers, facsimiles and floppy (Registered Trademark) disk drives, a stepping motor is required to be higher in torque, and lower in cogging torque and vibration. Under the circumstance, a PM (permanent magnet) stepping motor has been extensively used to meet the requirements.  
           [0005]    [0005]FIG. 1 is a partly cutaway perspective view of a conventional PM stepping motor. Referring to FIG. 1, the PM stepping motor generally comprises: a stator assembly  6  composed of two stator units  6 A and  6 B; a rotor assembly  13 ; a face plate  1  punched out of a stainless steel plate; and a front bearing  2  made of oil impregnated alloy. The stator units  6 A and  6 B each comprise: stator yokes  3   a  and  3   b  punched out of a soft magnetic steel plate and each having a plurality of pole teeth  10  bent up along its inner circumference; and a coil  4  formed such that a copper wire with polyurethane insulation is wound on a bobbin  5  housed in the stator yokes  3   a  and  3   b , and are attached back-to-back to each other. The rotor assembly  13  comprises: a rotary shaft  8 ; a cylindrical magnet  9  for magnetic field, having a plurality of magnetic poles at its circumferential surface, and having its circumferential surface opposing the pole teeth  10  of the stator yokes  3   a ,  3   b ; and a sleeve  12  to fasten the cylindrical magnet  9  and the rotary shaft  8  together such that the rotary shaft  8  is press fitted into the sleeve  12 , and the sleeve  12  with adhesive applied thereon is inserted into the cylindrical magnet  9 . The rotor assembly  13  thus structured has its outer circumference (of the magnet  9 ) magnetized for a predetermined number of magnetic poles, and is rotatably disposed inside the stator assembly  6  such that the rotary shaft  8  is supported by the front bearing  2  and a rear bearing (not shown in FIG. 1), and that the magnetic poles oppose the pole teeth  10  with a slight gap therebetween.  
           [0006]    [0006]FIG. 2 is a partly cross-sectioned view of the stator unit  6 A of the conventional PM stepping motor shown in FIG. 1. The stator unit  6 B attached to the stator unit  6 A is of the same structure as the stator unit  6 A and is omitted. The stator units  6 A and  6 B are each structured such that the pole teeth  10  of the stator yoke  3   a  mesh with the pole teeth  10  of the stator yoke  3   b  equidistantly from each other with a shift by an electrical angle of 180 degrees (half pitch). The stator units  6 A and  6 B thus structured are joined back-to-back to each other by molding with polymeric material or plasma welding such that their respective pole teeth  10  and  10  are shifted from each other by an electrical angle of 90 degrees, whereby the rotor assembly  13  (shown in FIG. 1) always starts its rotation in the same direction.  
           [0007]    [0007]FIG. 3 is a developed view of the stator yokes  3   a  and  3   b  of the conventional PM stepping motor. In the embodiment the stator units  6 A and  6 B each have ten pole teeth  10  in a spatial angle of 360 degrees, which are arrayed at an interval corresponding to that of the magnetic poles (not shown) formed on the cylindrical magnet  9 .  
           [0008]    Recently, since the devices incorporating PM stepping motors are required to be downsized and lower profiled as described above, the PM stepping motors are also required to be lower profiled. The conventional PM stepping motor, however, is composed of two stator units joined to each other back-to-back therefore making it extremely difficult or almost impossible to further reduce its profile. And the conventional PM stepping motor uses four discrete stator yokes, which inevitably increases the number of the components and makes alignment troublesome.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention has been made in view of the above circumstance, and it is an object of the present invention to provide a low-profile stepping motor which is of a simple structure and which has its dimension drastically reduced in the axial direction.  
           [0010]    In order to achieve the object, according to a first aspect of the present invention, a low-profile stepping motor generally comprises a first stator unit, a second stator unit, and a rotor assembly including a cylindrical magnet. The first stator unit includes: a first upper stator yoke having a plurality of pole teeth which are formed along its semicircular inner circumference; a first lower stator yoke having a plurality of pole teeth which are formed along its semicircular inner circumference and which mesh with the pole teeth of the first upper stator yoke with a shift by an electrical angle of 180 degrees; and a first coil unit fixedly sandwiched between the first upper and lower stator yokes. The second stator unit includes: a second upper stator yoke formed as one piece integrally with the first upper stator yoke, and having a plurality of pole teeth which are formed along its semicircular inner circumference; a second lower stator yoke formed as one piece integrally with the first lower stator yoke, and having a plurality of pole teeth which are formed along its semicircular inner circumference and which mesh with the pole teeth of the second upper stator yoke with a shift by an electrical angle of 180 degrees; and a second coil unit fixedly sandwiched between the second upper and lower stator yokes and arranged horizontally flush with the first coil unit. Any two pole teeth adjacent to each other with one formed on the first upper stator yoke and the other formed on the second upper stator yoke are shifted from each other by an electrical angle of either 90 degrees or 270 degrees, and any two pole teeth adjacent to each other with one formed on the first lower stator yoke and the other formed on the second lower stator yoke are shifted from each other by an electrical angle of either 90 degrees or 270 degrees. The rotor assembly is rotatably disposed in a circular open space defined by the pole teeth of the first and second stator units. With this structure, the low-profile stepping motor has its axial dimension almost halved without sacrificing its performance, and reduces the number of components thereby achieving cost reduction.  
           [0011]    According to a second aspect of the present invention, in the low-profile stepping motor of the first aspect, the first upper and lower stator yokes are fixedly put together by means of a stud such that the stud has its top end squeezed into a hole formed at a predetermined place of the first upper stator yoke and has its bottom end squeezed into a hole formed at a predetermined place of the first lower stator yoke, and the second upper and lower stator yokes are fixedly put together by means of a stud such that the stud has its top end squeezed into a hole formed at a predetermined place of the second upper stator yoke and has its bottom end squeezed into a hole formed at a predetermined place of the second lower stator yoke. With this structure, the stator yokes can be easily and surely aligned to each other thereby reducing cost.  
           [0012]    According to a third aspect of the present invention, in the low-profile stepping motor of the second aspect, the coil unit is structured such that a winding is provided around the stud. Accordingly, the coil unit can be easily mounted thereby reducing cost.  
           [0013]    According to a fourth aspect of the present invention, in the low-profile stepping motor of the first aspect, a pair of slot cuts are provided at a midway portion between the first and second upper stator yokes, and a pair of slot cuts are provided at a midway portion between the first and second lower stator yokes. This structure keeps the first and second stator units from having respective magnetic influence on each other thereby achieving a smooth rotation.  
           [0014]    According to a fifth aspect of the present invention, in the low-profile stepping motor of the fourth aspect, the slot cuts each have a length measuring at least two-thirds of the width of the midway portion between the first and second stator yokes. When the slot cuts are arranged to have their lengths equal to two-thirds of the width of the midway portion, the magnetic influence can be surely minimized while an adequate strength can be maintained at the midway portion. And the lengths of the slot cuts can still be increased if so required, as long as the midway portion retains a minimum strength required.  
           [0015]    According to a sixth aspect of the present invention, in the low-profile stepping motor of the first aspect, the pole teeth taper off toward their distal ends so as to be shaped trapezoidal. This configuration of the pole teeth ensures an accurate assembly of the first and second upper stator yokes and the first and second lower stator yokes. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    [0016]FIG. 1 is a partly cutaway view of a conventional PM stepping motor;  
         [0017]    [0017]FIG. 2 is a partly cross-sectioned view of a stator unit of the conventional PM stepping motor of FIG. 1;  
         [0018]    [0018]FIG. 3 is a developed view of stator yokes of the conventional PM stepping motor of FIG. 1;  
         [0019]    [0019]FIG. 4 is an exploded perspective view of a low-profile stepping motor according to the present invention; and  
         [0020]    [0020]FIG. 5 is a cross sectional view of the low-profile stepping motor of FIG. 3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0021]    A preferred embodiment of the present invention will hereinafter be described with reference to the accompanying drawings.  
         [0022]    Referring to FIG. 4, a low-profile stepping motor of the present invention generally comprises: a stator assembly  30  including first and second stator units  30 A and  30 B assembled to be horizontally flush with each other; and a rotor assembly  50  rotatably disposed inside the stator assembly  30 .  
         [0023]    The first stator unit  30 A comprises: first upper and lower stator yokes  31   a  and  34   a ; and a first coil unit  40 A which is composed of a bobbin  41   a  and a winding  42   a  provided around the bobbin  41   a , is sandwiched between the first upper and lower stator yokes  31   a  and  34   a , and which is provided with a terminal pin  43   a . The first upper stator yoke  31   a  is punched out of a soft magnetic steel plate, includes a plurality of pole teeth  32   a   1  to  32   an  punched out of the soft magnetic steel plate and bent up inward (downward in FIG. 4) at its semicircular inner circumference, and has a hole  33   a  for engaging with a top protrusion  44   a   1  of the first coil unit  40 A. The first lower stator yoke  34   a  is punched out of a soft magnetic steel plate, includes a plurality of pole teeth  35   a   1  to  35   an  punched out of the soft magnetic steel plate and bent up inward (upward in FIG. 4) at its semicircular inner circumference, and has a hole  36   a  for engaging with a bottom protrusion  44   a   2  (see FIG. 5) of the first coil unit  40 A.  
         [0024]    The second stator unit  30 B comprises: second upper and lower stator yokes  31   b  and  34   b  punched out as one piece integrally with the first upper and lower stator yokes  31   a  and  34   a , respectively; and a second coil unit  40 B which is composed of a bobbin  41   b  and a winding  42   b  provided around the bobbin  41   b , is sandwiched between the second upper and lower stator yokes  31   b  and  34   b , and which is provided with a terminal pin  43   b . The second upper stator yoke  31   b  is punched out of a soft magnetic steel plate, includes a plurality of pole teeth  32   b   1  to  32   bn  punched out of the soft magnetic steel plate and bent up inward (downward in FIG. 4) at its semicircular inner circumference, and has a hole  33   b  for accepting a top protrusion  44   b   1  of the second coil unit  40 B. The second lower stator yoke  34   b  is punched out of a soft magnetic steel plate, includes a plurality of pole teeth  35   b   1  to  35   bn  punched out of the soft magnetic steel plate and bent up inward (upward in FIG. 4) at its semicircular inner circumference, and has a hole  36   b  for receiving a bottom protrusion  44   b   2  (see FIG. 5) of the second coil unit  40 B.  
         [0025]    The plurality of pole teeth  32   a   1  to  32   an  of the first upper stator yoke  31   a  and the plurality of pole teeth  32   b   1  to  32   bn  of the second upper stator yoke  31   b  are punched out simultaneously, and the plurality of pole teeth  35   a   1  to  35   an  of the first lower stator yoke  34   a  and the plurality of pole teeth  35   b   1  to  35   bn  of the second lower stator yoke  34   b  are punched out simultaneously.  
         [0026]    Each stator yoke has pole teeth provided in number corresponding to the required number of steps per rotation of a stepping motor and arranged equidistant from one another. The pole teeth  32   a   1  to  32   an  of the first upper stator yoke  31   a  and the pole teeth  35   a   1  to  35   an  of the first lower stator yoke  34   a  mesh with each other with a shift by an electrical angle of 180 degrees, and the pole teeth  32   b   1  to  32   bn  of the second upper stator yoke  31   b  and the pole teeth  35   b   1  to  35   bn  of the second lower stator yoke  34   b  mesh with each other with a shift by an electrical angle of 180 degrees.  
         [0027]    In each of the upper and lower stator yokes of the first and second stator units  30 A and  30 B thus structured, two pole teeth adjacent to each other sandwiching one of a pair of slot cuts  37  or  38  (detailed hereinafter) are shifted from each other by an electrical angle of 90 degrees or 270 degrees. Specifically, in the first and second upper stator yokes  31   a  and  31   b , the pole tooth  32   a   1  is shifted by an electrical angle of 90 degrees or 270 degrees from the pole tooth  32   b   1  adjacent to the pole tooth  32   a   1  sandwiching one slot cut  37 , and the pole tooth  32   an  is shifted by an electrical angle of 90 degrees or 270 degrees from the pole tooth  32   bn  adjacent to the pole tooth  32   an  sandwiching the other slot cut  37 , and in the first and second lower stator yokes  34   a  and  34   b , the pole tooth  35   a   1  is shifted by an electrical angle of 90 degrees or 270 degrees from the pole tooth  35   b   1  adjacent to the  35   a   1  sandwiching one slot cut  38 , and the pole tooth  35   an  is shifted by an electrical angle of 90 degrees or 270 degrees from the pole tooth  35   bn  adjacent to the pole tooth  35   an  sandwiching the other slot cut  38 .  
         [0028]    The first and second upper stator yokes  31   a  and  31   b  are punched out as one piece such that the pair of slot cuts  37  above mentioned are formed right midway between the first and second upper stator yokes  31   a  and  31   b  thereby minimizing magnetic interflow tehrebetween so as not to have magnetic influence on each other, and the first and second lower stator yokes  34   a  and  34   b  are punched out as one piece such that the pair of slot cuts  38  above mentioned are formed right midway between the first and second lower stator yokes  34   a  and  34   b  thereby minimizing magnetic interflow therebetween so as not to have magnetic influence on each other. The slot cuts  37  and  38  should preferably be as long as possible without detriment to the strength. Specifically, the slot cuts  37  and  38  with their length measuring at least two-thirds of the width of the midway portion work satisfactorily for a smooth rotation.  
         [0029]    The rotor assembly  50  comprises: a cylindrical magnet  51  formed of ferrite magnet, rear-earth magnet and so forth and magnetized at regular intervals; a rotary shaft  55  made of non-magnetic stainless steel; and a sleeve  52  made of aluminum, and is structured such that the cylindrical magnet  51  is adhesively fixed to the sleeve  52  which has the rotary shaft  55  press-fitted thereinto. The rotor assembly  50  is rotatably disposed inside the stator assembly  30  with a slight gap between the cylindrical magnet  51  and the pole teeth of the stator assembly  30 .  
         [0030]    Referring to FIG. 5, the first coil unit  40 A includes: the aforementioned bobbin  41   a ; the aforementioned winding  42   a  provided around the bobbin  41   a ; and a hollow-cylindrical stud  45   a  made of metal, disposed inside the bobbin  41   a , and provided with the top and bottom protrusions  44   a   1  and  44   a   2 , and the second coil unit  40 B includes: the aforementioned bobbin  41   b ; the aforementioned winding  42   b  provided around the bobbin  41   b ; and a hollow-cylindrical stud  45   b  made of metal, disposed inside the bobbin  41   b , and provided with the top and bottom protrusions  44   b   1  and  44   b   2 .  
         [0031]    The first and second coil units  40 A and  40 B are put together respectively with the first and second stator units  30 A and  30 B such that the top and bottom protrusions  44   a   1  and  44   a   2  of the first coil unit  40 A are inserted respectively into the holes  33   a  and  36   a  of the first upper and lower stator yokes  31   a  and  34   a  and that the top and bottom protrusions  44   b   1  and  44   b   2  of the second coil unit  40 B are inserted respectively into the holes  33   b  and  36   b  of the second upper and lower stator yokes  31   b  and  34   b . In the process of putting together, adhesive may be applied between the protrusions and the holes for reinforcement. The stud  45   a , together with the winding  42   a  and the first upper and lower stator yokes  31   a  and  34   a , constitutes a part of a closed magnetic path, and the stud  45   b , together with the winding  42   b  and the second upper and lower stator yokes  31   b  and  34   b , constitutes a part of a closed magnetic path. Thanks to the studs  45   a  and  45   b , the first and second coil units  40 A and  40 B can be easily put together with the first and second stator units  30 A and  30 B, and also the first and second upper stator yokes  31   a  and  31   b  can be easily and surely aligned with the first and second lower stator yokes  34   a  and  34   b.    
         [0032]    A pole tooth  32   a  represents one pole tooth of the first upper stator yoke  31   a , and a pole tooth  35   b  represents one pole tooth of the second lower stator yoke  34   b . Resin is filled in a gap space between the meshing pole teeth for reinforcement thereby forming a resin portion  63 , which develops a vibration-proof structure.  
         [0033]    A face plate  61 , which is punched out of a non-magnetic steel plate and has a front bearing  53  attached thereto, is fixed to the first and second upper stator yokes  31   a  and  31   b  by welding, bonding or other methods. A resin wall  62  is formed by resin-molding while the rotor assembly  50  is disposed between the stator units  30 A and  30 B with the rotary shaft  55  inserted through the front bearing  53 . A rear bearing  54  is attached to the resin wall  62 , and the rotor assembly  50  is rotatably disposed with the rotary shaft  55  supported by the front and rear bearings  53  and  54 . The resin wall  62  may be replaced by a plate which, like the face plate  61 , is punched out of a non-magnetic steel plate, and be fixed to the first and second lower stator yokes  34   a  and  34   b.    
         [0034]    Thus, since the low-profile stepping motor of the present invention is composed of the stator assembly  30  which is structured such that the first and second stator units  30 A and  30 B are flush with each other horizontally and have their respective pole teeth shifted from each other by an electrical angle predetermined, the profile thereof is reduced approximately to half without sacrificing its performance, compared with the conventional PM stepping motor which has two stator units stacked vertically and concentrically. Further, the low-profile stepping motor of the present invention uses two stator yoke pieces each comprising two stator yokes, which not only reduces the number of components but also eases dramatically the alignment of the stator yokes, thereby lowering the cost compared with the conventional PM stepping motor which uses four discrete stator yokes resulting in increasing the number of components and complicating the alignment of the stator yokes.