Abstract:
A claw-pole type stepping motor comprises: a rotor assembly shaped substantially cylindrical, and having a center shaft; and a stator assembly composed of two cup-shaped stator units which are coupled to each other coaxially so as to axially sandwich the rotor assembly, and which each include a bobbin having a magnet wire wound therearound, and two pole tooth arrays magnetically connected to each other and shifted in phase from each other by an electrical angle of 180 degrees. In the motor, each stator unit further includes a cover ring which protects the magnet wire wound around the bobbin against resin injected when the stator unit is resin-molded for an integrated solid structure. The motor structured as described above can be successfully resin-molded so as to enable elimination of a motor case, thus achieving downsizing for the dimension of the eliminated motor case while maintaining a sufficient mechanical strength.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a structure of a claw-pole type stepping motor, and particularly to a structure thereof for achieving reduction in dimension and offering technical advantages.  
         [0003]     2. Description of the Related Art  
         [0004]     As various electronic devices are increasingly requested to be downsized, motors incorporated in the devices are also requested to be downsized. In a conventional claw-pole type stepping motor with an inner rotor, A-phase and B-phase driving coils are typically disposed so as to surround the outer circumference of a rotor. In such a claw-pole type stepping motor, the diameter of a rotor magnet is restricted by the inner diameter of a stator, so a motor with a smaller dimension is forced to have a rotor magnet with a smaller diameter thus resulting in significant deterioration in motor characteristic. This makes it difficult for a claw-pole type stepping motor with a small radial dimension to ensure performance characteristic required for poisoning control. In such a case, a brushless DC motor equipped with an encoder as a position detector must be used in place of a claw-pole type stepping motor, which inevitably pushes up production cost.  
         [0005]     Under the aforementioned circumstances, Japanese Patent Application Laid-Open No. 2003-009497 discloses a motor structured such that driving coils are disposed so as to axially sandwich a rotor magnet. This motor structure helps reduction of its radial dimension, but since the motor is entirely covered by a case, the motor size has to be increased for the thickness of the case in all directional dimensions including the radial dimension. In order to reduce the radial dimension of this motor, the thickness of the case must be decreased. A case with a small thickness is technically difficult to fabricate, and also is inferior in mechanical strength, thus effort in reducing the thickness of the case has its limit. The case may be eliminated by, for example, resin-molding a stator for an integral structure, and such a solution is mentioned in the aforementioned Japanese Patent Application Laid-Open No. 2003-009497 (refer to Paragraph [0004]). However, the solution is negatively discussed therein, because when bobbins having respective exciting coils wound therearound are resin-molded integrally with yokes, it happens occasionally that the terminal wires of coils are broken due to molding pressure. Specifically, resin injected for molding gets in direct touch with the surface of magnet wires thereby deteriorating the coating of the magnet wires or even deforming and breaking the magnet wires.  
       SUMMARY OF THE INVENTION  
       [0006]     The present invention has been made in light of the above circumstances, and it is an object of the present invention to overcome the difficulty in resin-molding a stator to thereby eliminate a motor case for a claw-pole type stepping motor in order to downsize its radial dimension while maintaining its mechanical strength.  
         [0007]     In order to achieve the object, according to an aspect of the present invention, a claw-pole type stepping motor comprises: a rotor assembly shaped substantially cylindrical, and having a center shaft; and a stator assembly composed of two cup-shaped stator units which are coupled to each other coaxially so as to axially sandwich the rotor assembly, and which each include a bobbin having a magnet wire wound therearound for excitation, and two pole tooth arrays magnetically connected to each other and shifted in phase from each other by an electrical angle of 180 degrees. In the motor, each of the stator units further includes a cover ring which protects the magnet wire wound around the bobbin against resin injected when the stator unit is resin-molded for an integrated solid structure. Consequently, the difficulty with resin-molding the stator unit for an integrated solid structure is overcome, and a motor case for entirely covering a motor can be eliminated thus downsizing the motor with a sufficient mechanical strength ensured.  
         [0008]     In the aspect of the present invention, the bobbin may include two flanges, the magnet wire may be wound between the two flanges, and the cover ring may be in contact with the two flanges such that the cover ring touches an outer circumference of at least one of the two flanges so as to protect the magnet wire wound between the two flanges against the resin injected. Consequently, the resin injected is prevented from getting to the magnet wire wound around the bobbin, and also the bobbin and the cover ring are held coaxial to each other when positioned.  
         [0009]     In the aspect of the present invention, the bobbin may include a terminal block provided with terminals to conduct supply current to the magnet wire, and the cover ring may include a guide block which is at least partly in touch with the terminal block so as to protect the end portions of the magnet wire leading out to the terminals against the resin injected. Consequently, the bobbin is stopped from moving in the axial direction due to molding pressure at the time of resin-molding the stator unit for an integrated structure.  
         [0010]     In the aspect of the present invention, the guide block of the cover ring may have a groove which allows the magnet wire to lead out to the terminals. Consequently, the stator unit is resin-molded with a guide passage ensured for leading out the magnet wire to the terminals.  
         [0011]     Further, since the two stator units structured as described above are coupled to each other such that pins are inserted though respective holes formed on the stator units when resin-molded and riveted for fixation, a sufficient strength is provided for the motor assembled. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is an exploded perspective view of one stator unit for a claw-pole type stepping motor according to an embodiment of the present invention;  
         [0013]      FIG. 2  is a perspective view of a framed state of the one stator unit shown in  FIG. 1 ;  
         [0014]      FIG. 3  is an exploded perspective view of the claw-pole type stepping motor according to the embodiment of the present invention;  
         [0015]      FIG. 4  is a sectioned perspective view of the motor shown in  FIG. 3 ;  
         [0016]      FIG. 5  is a perspective view of the motor of  FIG. 4  seen from a rear side, omitting a rear plate  37 , a B-phase stator unit  10 , a rotor assembly  30 , and washers  32 ;  
         [0017]      FIG. 6  is a perspective view of the motor of  FIG. 4  seen from a front side;  
         [0018]      FIG. 7  is a perspective view of the motor of  FIG. 4  seen from the rear side;  
         [0019]      FIGS. 8A and 8B  are perspective views of a bobbin  14  shown in  FIG. 1  seen from a side opposite to a side as seen in  FIG. 1 , wherein  FIG. 8A  omits a magnet wire  14   d  and terminal pins  14   e;    
         [0020]      FIG. 9  is a perspective view of a cover ring  13  shown in  FIG. 1 ; and  
         [0021]      FIGS. 10A and 10B  are perspective views of the bobbin  14  of  FIG. 8B  with the cover ring  13  of  FIG. 9  put thereon, seen from both sides, respectively. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0022]     An embodiment of the present invention will hereinafter be described with reference to the accompanying drawings.  
         [0023]     A stepping motor according to the present invention comprises two stator units, specifically an A-phase stator unit and a B-phase stator unit arranged coaxially to each other. The B-phase stator unit is shown in  FIG. 1  in an exploded manner so as to explain its constituent parts. The A-phase stator unit uses common constituent parts to the B-phase stator unit and an explanation thereof will be omitted. Since the A-phase and B-phase stator units use common constituent parts, the production cost of the parts can be reduced.  
         [0024]     Referring to  FIG. 1 , the B-phase stator includes a bearing  11 , a first stator yoke  12 , a cover ring  13 , a bobbin  14 , a core  15 , and a second stator yoke  16 .  
         [0025]     The bearing  11  is, for example, a sintered sleeve bearing to rotatably support a rotary shaft of a rotor assembly to be described later.  
         [0026]     The first and second stator yokes  12  and  16  are punched out of a soft magnetic plate, such as a galvanized steel plate (SECC), a silicon steel plate, and an electromagnetic soft steel (SUY), have respective pole teeth  12   a  and  16   a , are magnetically connected to each other via the core  15 , and are coupled to each other with the respective pole teeth  12   a  and  16   a  shifted in phase from each other by an electrical angle of 180 degrees. The second stator yoke  16  has a pit  16   b  which engages with a boss  14   b  provided on the bobbin  14 , whereby the second stator yoke  16  and the bobbin  14  are duly positioned with respect to each other in a circumferential direction for resin-molding process.  
         [0027]     The core  15  is formed of a soft magnetic plate, such as an SECC, a silicon plate steel, and an SUY, and has a center hole for allowing the rotary shaft of the rotor assembly to pass through.  
         [0028]     The bobbin  14  is formed of, for example, liquid crystal polymer, and made up of a body section  14   a , flanges  14   a ′ and  14   a ″ sandwiching the body section  14   a , and a terminal block  14   c . A magnet wire  14   d  is wound around the body section  14   a  between the flanges  14   a ′ and  14   a ′, and terminal pins  14   e  for supplying current to the magnet wire  14   d  are attached to the terminal block  14   c . The bobbin  14  is provided with the aforementioned boss  14   b  which fits engagingly into the aforementioned pit  16   b  of the second stator yoke  16  for positioning function as mentioned above. The mechanism for positioning the bobbin  14  with respect to the second stator yoke  16  may alternatively be structured such that the second stator yoke  16  is provided with a boss while the bobbin  14  is provided with a pit, or may be constituted by means of a positioning marker, such as a notch, provided appropriately. The terminal block  14   c  has a groove  14   f  which functions mainly as a mechanism for hooking the magnet wire  14   d  when the magnet wire  14   d  is wound around the bobbin  14 .  
         [0029]     The cover ring  13  is formed of, for example, liquid crystal polymer, made up of a body section  13   a  and a guide block  13   b , and protects the magnet wire  14   d  wound around the bobbin  14  against resin injected for molding the B-phase stator unit for integrated structure. The guide block  13   b  has a groove  13   c  which allows the terminal ends of the magnet wire  14   d  wound around the bobbin  14  to lead out to the terminal pins  14   e  when the cover ring  13  is attached onto the bobbin  14  thereby bringing the guide block  13   b  in contact with the terminal block  14   c.    
         [0030]     The above-described constituent parts are put together in an axial direction in reference-numerical order, and the B-phase stator unit is framed as shown in  FIG. 2 . When the B-phase stator unit is duly framed, the guide block  13   b  of the cover ring  13  and the terminal block  14   c  of the bobbin  14  are engagingly sandwiched between two adjacent pole teeth out of the pole teeth  12   a  of the first stator yoke  12 , whereby the cover ring  13  and the bobbin  14  can be duly positioned with respect to the first stator yoke  12  in the circumferential direction. And, the bobbin  14  can be duly positioned with respect to the second stator yoke  16  in the circumferential direction by means of the boss  14   b  of the bobbin  14  engagingly fitting into the pit  16   b  of the second stator yoke  16  as described above. The B-phase stator units framed as shown in  FIG. 2  is resin-molded by, for example, injection molding to be integrated for solidification.  
         [0031]     Referring to  FIG. 3 , a claw-pole type stepping motor  1  according to the present invention comprises the above-described B-phase stator unit  10  resin-molded, the aforementioned A-phase stator unit  20  resin-molded, a rotor assembly  30 , a front plate  36 , a rear plate  37 , and pins  34  and  35  for securely holding together the parts in place.  
         [0032]     The B-phase stator unit  10  is resin-molded as described above so as to have a resin portion  17 , and holes  17   a  for passing the pins  34  and  35  are formed in the resin portion  17  at the time of resin-molding. The A-phase stator unit  20  is structured in the same way as the B-phase stator unit  10  with only difference found in the location of the holes for passing the pins  34  and  35 , and therefore a detailed explanation thereof is omitted.  
         [0033]     The rotor assembly  30  includes a rotor magnet  31  constituted by, for example, a rare-earth cylindrical magnet and having a outer diameter of some 5 mm, and a rotary shaft  33  formed of, for example, stainless steel, and washers  32  formed of, for example, stainless steel are put on the rotary shaft  33 .  
         [0034]     The front plate  36 , which may be formed of a magnetic or nonmagnetic material, has holes  36   a  for passing the pins  34  and  35 , and the pins  34  and  35  are, for example, riveted to be fixed at the holes  36   a.    
         [0035]     The rear plate  37 , which may also be formed of a magnetic or nonmagnetic material, has holes  37   a  for passing the pins  34  and  35 , and the pins  34  and  35  are, for example, riveted to be fixed at the holes  37   a.    
         [0036]     The motor  1  is assembled such that the constituent parts described above are put together in an axial direction in reference-numerical order. In the assembling process, the pins  34  and  35  are inserted through respective holes  36   a  of the front plate  36 , the A-phase stator unit  20 , the B-phase stator unit  10 , and the rear plate  37 , and have their respective both ends, for example, riveted for fixation, whereby the parts are duly and fixedly positioned with respect to one another in the circumferential direction.  
         [0037]     Referring now to  FIG. 4 , the motor  1  has the rotary shaft  33  disposed at the center, and the rotor magnet  31  is mounted on the rotary shaft  33 . The body section  13   a  of the cover ring  13  is disposed so as to be in contact with the flanges  14   a ′ and  14   a ″ of the bobbin  14  which sandwich the magnet wire  14   d  wound around the bobbin  14 , and prevents injected resin from getting to the magnet wire  14   d . This way, the magnet wire  14   d  is protected against the injected resin and therefore prevented from deteriorating or breaking when the stator units are resin-molded for integration. The cover ring  13  is put on the bobbin  14  preferably with the body section  13   a  set in touch with the outer circumference of at least one of the flanges  14   a ′ and  14   a ″, so that the cover ring  13  is held coaxial to the bobbin  14 . Also, the guide block  13   b  of the cover ring  13  is at least partly in touch with the terminal block  14   c  of the bobbin  14 , whereby the injected resin is prevented from getting to the end portions of the magnet wire  14   d  leading out to the terminal pins  14   e , and whereby the terminal block  14   c  is stopped from moving in the axial direction due to molding pressure at the process of resin-molding. And, it is realized from  FIG. 4  that the groove  13   c  provided at the guide block  13   b  allows the magnet wire  14   d  wound around the bobbin  14  to lead out to the terminal pins  14   e.    
         [0038]     Referring to  FIG. 5 , the motor  1  shown in  FIG. 4  is perspectively viewed from its rear side, omitting the rear plate  37 , the B-phase stator unit  10 , the rotor assembly  30 , and the washers  32 . The boss  14   b  of the bobbin  14  fits engagingly into the pit  16   b  of the second stator yoke  16  so as to fixedly position the bobbin  14  with respect to the second stator yoke  16  in the circumferential direction. The positioning may alternatively be implemented, for example, such that a notch provided on the second stator yoke  16  in place of the pit  16   b  is set to a predetermined marker at the process of resin-molding.  
         [0039]     Referring to  FIGS. 6 and 7 , it is known that the groove  14   f , which is formed on the terminal block  14   c  of the bobbin  14 , and which is intended to serve as a hooking mechanism for the magnet wire  14   d  at the process of winding, works, together with the groove  13   c  of the guide block  13   b  of the cover ring  13 , also as a guide passage for the end portions of the magnet wire  14   d  leading to the terminal pins  14   e , thus the magnet wire  14   d  is surely and reliably allowed to lead out to the terminal pins  14   e .  FIG. 7  shows that the motor  1  structured according to the present invention is downsized so as to measure as small as 6 mm in height.  
         [0040]     Referring to  FIGS. 8A and 8B , the bridging portion between the flange  14   a ′ and the terminal block  14   c  on the bobbin  14  has a width adapted to fit engagingly between two adjacent pole teeth of the pole teeth  12   a  of the first stator yoke  12  thereby fixedly positioning the bobbin  14  with respect to the first stator yoke  12  in the circumferential direction.  
         [0041]     Referring to  FIG. 9 , the bridging portion between the body section  13   a  and the guide block  13   b  on the ring cover  13  has a width adapted to fit engagingly between the two adjacent pole teeth of the pole teeth  12   a  of the first stator yoke  12  thereby fixedly positioning the cover ring  13  with respect to the first stator yoke  12  in the circumferential direction.  
         [0042]     Referring to  FIGS. 10A and 10B , the cover ring  13  is put on the bobbin  14  such that the body section  13   a  covers the outer circumferences of the flanges  14   a ′ and  14   a ″, whereby the magnet wire  14   d  wound around the body section  14   a  between the flanges  14   a ′ and  14   a ″ is protected.  
         [0043]     In the above description of the embodiment according to the present invention, a conventionally required motor case for entirely covering a motor is eliminated thereby achieving downsizing, but if the motor cover is eliminated with the motor size remaining unchanged, then the dimension of its components can be increased thereby enhancing its motor characteristic. Also, the motor according to the present invention may have a round, rectangular or otherwise configured radial cross section according to a housing space configuration.  
         [0044]     While the present invention has been illustrated and explained with respect to a specific embodiment thereof, it is to be understood that the present invention is by no means limited thereto but encompasses all changes and modifications that will become possible within the scope of the appended claims.