Patent Publication Number: US-10331085-B2

Title: Stepping motor and timepiece

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation of U.S. Ser. No. 14/486,639, filed Sep. 15, 2014, which is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2013-201949, filed Sep. 27, 2013, the entire contents of both of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a stepping motor and a timepiece. 
     2. Description of Related Art 
     A stepping motor including a rotor, a stator, and a coil is used to drive various devices. 
     The stepping motor can rotate the rotor by a predetermined step angle through application of voltage to the coil. Such a stepping motor is widely applied to devices, such as a timepiece including hands (a second hand, a minute hand, and an hour hand), that need to accurately operate by a predetermined amount. 
     In the invention described in WO99/064937, the stator has a pair of outer notches on a line orthogonal to a linear portion of a coil core provided with a coil, such that the outer notches are disposed on the opposite sides of a rotor accommodating hole for accommodating the rotor. This configuration defines two positions of saturated magnetic fluxes. 
     In general, a motor should be further miniaturized to be installed in a small electronic device such as a wristwatch. 
     SUMMARY OF THE INVENTION 
     An object of the invention, which has been accomplished on the above background, is miniaturization. 
     In order to achieve the above objects, one aspect of the present invention is a stepping motor including a rotor, a stator which includes a rotor accommodating hole to accommodate the rotor, a pair of outer notches to determine positions of saturated magnetic fluxes and inner notches disposed on an inner periphery of the stator around the rotor accommodating hole, the inner notches determining stably stationary positions of the rotor, and a coil block configured by a coil wound around a coil core which is magnetically coupled to the stator, and the outer notches are disposed on an outer periphery of the stator on opposite sides of the rotor accommodating hole, and a line that connects narrowest portions between the outer notches and the rotor accommodating hole and extends through a center of the rotor accommodating hole shifts by a predetermined angle from a line that extends through the center of the rotor accommodating hole and is orthogonal to an extending direction of the stator. 
     In order to achieve the above objects, another aspect of the present invention is a timepiece including a stepping motor which includes a rotor, a stator having a rotor accommodating hole to accommodate the rotor, a pair of outer notches to determine positions of saturated magnetic fluxes and inner notches disposed on an inner periphery of the stator around the rotor accommodating hole, the inner notches determining stably stationary positions of the rotor, and a coil block configured by a coil wound around a coil core which is magnetically coupled to the stator, hands, and a gear train mechanism which is coupled with the hands and which is driven by rotation of the stepping motor, and in the stepping motor, the outer notches are disposed on an outer periphery of the stator on opposite sides of the rotor accommodating hole, and a line that connects narrowest portions between the outer notches and the rotor accommodating hole and extends through a center of the rotor accommodating hole shifts by a predetermined angle from a line that extends through the center of the rotor accommodating hole and is orthogonal to an extending direction of the stator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, advantages and features of the present invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention, and wherein: 
         FIG. 1A  is a front view of a stepping motor according to a first embodiment where a rotor thereof rotates clockwise, and  FIG. 1B  is a side view of the stepping motor when seen in the direction of an arrow b in  FIG. 1A ; 
         FIG. 2A  is a plan view of a stator according to the first embodiment, and  FIG. 2B  is a cross-sectional view of the stator cut along a line b-b in  FIG. 2A ; 
         FIG. 3A  is a front view of the stepping motor in  FIG. 1A  where the rotor thereof rotates counterclockwise, and  FIG. 3B  is a side view of the stepping motor when seen in the direction of an arrow b in  FIG. 3A ; 
         FIG. 4A  is a front view of a coil block according to the first embodiment, and  FIG. 4B  is a side view of the coil block when seen in the direction of an arrow b in  FIG. 4A ; 
         FIG. 5A  is a front view of a coil core according to the first embodiment,  FIG. 5B  is a side view of the coil core when seen in the direction of an arrow b in  FIG. 5A , and  FIG. 5C  is a cross-sectional view of the coil core cut along a line c-c in  FIG. 5A ; 
         FIG. 6  is a schematic plan view of a timepiece including stepping motors according to the first embodiment in an exemplary arrangement; 
         FIG. 7  is a cross-sectional view of the main configuration of a stepping motor shown in  FIG. 6  and periphery thereof; 
         FIG. 8  is a plan view of the timepiece in  FIG. 6  that excludes the stepping motors; 
         FIG. 9A  is a plan view of a stator according to a second embodiment, and  FIG. 9B  is a side view of the stator when seen in the direction of an arrow b in  FIG. 9A ; 
         FIG. 10A  is a front view of a coil core according to the second embodiment, and  FIG. 10B  is a side view of the coil core when seen in the direction of an arrow b in  FIG. 10A ; 
         FIG. 11  is a diagram illustrating an assembly of the stator having a cross section along a line IX-IX in  FIG. 9  and the coil core having a cross section along a line X-X in  FIG. 10 ; and 
         FIG. 12A  is a plan view of a stator and a rotor according to a third embodiment,  FIG. 12B  is a plan view of a coil block according to the third embodiment,  FIG. 12C  is a front view of a stepping motor according to the third embodiment where the rotor thereof rotates clockwise, and  FIG. 12D  is a front view of the stepping motor in  FIG. 12C  where the rotor thereof rotates counterclockwise. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
     A stepping motor according to a first embodiment of the invention and a timepiece including the stepping motor will now be described with reference to  FIGS. 1A, 1B  to  FIG. 8 . Although the following description includes various limitations to describe technically preferred embodiments of the invention, the invention should not be limited to the embodiments and the illustrated examples. 
       FIG. 1A  is a plan view of a stepping motor according to the first embodiment, and  FIG. 1B  is a side view of the stepping motor when seen in the direction of the arrow b in  FIG. 1A . 
     The stepping motor  100  according to the embodiment is a small motor to drive, for example, a date indicating mechanism or a hand driving mechanism for driving hands  501  (e.g., a second hand, a minute hand, and an hour hand) in a timepiece  500  (refer to  FIG. 6 ). 
     With reference to  FIGS. 1A and 1B , the stepping motor  100  includes a rotor  1 , a stator  2 , and a coil block  3  magnetically coupled to the stator  2 . 
     With reference to  FIG. 1A , the rotor  1  according to the embodiment is composed of a disk or cylindrical magnet having a substantially circular shape as viewed from above. 
     The rotor  1  is bipolarly-magnetized into the south pole and the north pole in the radial direction. 
     Although the rotor  1  should preferably be composed of a permanent magnet, such as a rare-earth magnet (e.g., a samarium-cobalt magnet), the rotor  1  may be composed of any other magnet. 
     The rotor  1  is provided with a rotary shaft  11  in the center. 
     The rotor  1  is accommodated in a rotor accommodating hole  23  (described below) of the stator  2  such that the rotor  1  is rotatable around the rotary shaft  11 . According to the embodiment, under driving pulses applied to a coil  32  (described below), the rotor  1  can rotate by a predetermined step angle inside the rotor accommodating hole  23 . 
     The rotary shaft  11  is provided with a pinion  12 . The pinion  12  engages with, for example, gears  502  (refer to  FIG. 6 ) constituting a gear train mechanism for driving the hands  501  of the timepiece  500 , such that the gears  502  engaging with the pinion  12  rotate with the rotor  1 . 
       FIG. 2A  is a plan view of the stator according to the embodiment, and  FIG. 2B  is a cross-sectional view of the stator cut along the line b-b in  FIG. 2A . 
     The stator  2  according to the embodiment is composed of a highly magnetically permeable material such as a permalloy. In specific, the stator  2  is composed of Permalloy B (PB), for example. 
     Permalloy B contains Ni (45%) and Fe (bal.), and has an initial magnetic permeability of 60,000 μi, a maximum magnetic permeability of 180,000 μm, a saturation flux density of 0.65 Bm(T), a magnetic coercive force of 1.2 Hc(A/m), and a specific resistance of 0.55 μΩ·m or higher. Permalloy B has a relatively low saturation flux density and thus is readily saturated with a magnetic flux. 
     The stator  2  may be composed of any material other than Permalloy B. 
     With reference to  FIGS. 1A and 2A , the stator  2  is a plate-like member extending in the extending direction of the stepping motor  100  (i.e., the lateral direction in  FIG. 1A or 2A , or the extending direction of a linear portion  311  of a coil core  31  provided with a coil  32  (described below)). 
     The stator  2  includes stator-side couplers  21  on both ends, which constitute couplers magnetically coupled to the coil core  31  (described below). The stator-side couplers  21  extend in the same direction orthogonal to the extending direction of the stator  2  (i.e., the lateral direction in  FIG. 1A or 2A , or the extending direction of the linear portion  311  of the coil core  31 ). The stator-side couplers  21  have shapes that substantially match the shapes of respective coil-side couplers  312  of the coil core  31 . 
     The stator-side couplers  21  each may have a screw hole  22 . The screw holes  22  are disposed at substantially the same distance from the center of the stator  2  in its extending direction (i.e., symmetrically in the lateral direction in  FIG. 1A or 2A ). One of the screw holes  22  should preferably be an elongated hole for precise assembly. 
     The stator  2  according to the embodiment has a substantially identical thickness at both sides of the stator  2  in its extending direction (i.e., the right and left in  FIG. 1A or 2A ). 
     With reference to  FIGS. 2A and 2B , the stator  2  has a rotor accommodating hole  23  for accommodating the rotor  1 , a pair of outer notches  24  for determining the positions of saturated magnetic fluxes, and inner notches  25  for determining stably stationary positions of the rotor  1 . 
     The rotor accommodating hole  23  according to the embodiment is a substantially circular opening disposed at the substantial center of the stator  2  in its extending direction (i.e., the lateral direction in  FIG. 1A or 2A , or the extending direction of the linear portion  311  of the coil core  31  in the embodiment). 
     In the stator  2  according to the embodiment, the stator-side couplers  21  and the screw holes  22  are substantially symmetrically disposed in the extending direction of the stator  2  (lateral direction in  FIG. 1A or 2A ) on both sides of the rotor accommodating hole  23 . The entire stator  2  has a substantially symmetrical shape and a substantially identical thickness at the right and left sides of the rotor accommodating hole  23  (the right and left in  FIG. 1A or 2A ). 
     The outer notches  24  are disposed on the outer periphery of the stator  2  on the opposite sides of the rotor accommodating hole  23 , such that the line that connects the respective narrowest portions between the outer notches  24  and the rotor accommodating hole  23  and extends through the center of the rotor accommodating hole  23  shifts by a predetermined angle (θ 1  in  FIG. 2A ) from the line that extends through the center of the rotor accommodating hole  23  and is orthogonal to the extending direction of the stator  2  (i.e., the lateral direction in  FIG. 1A or 2A , or the extending direction of the linear portion  311  of the coil core  31  in the embodiment). 
     The predetermined angle θ 1 , i.e., the angle defined by how much the line (La in  FIG. 2A ) that connects the respective narrowest portions between the outer notches  24  and the rotor accommodating hole  23  and extends through the center of the rotor accommodating hole  23  is shifted from the line (Lb in  FIG. 2A ) that extends through the center of the rotor accommodating hole  23  and is orthogonal to the extending direction of the stator  2  should preferably range from approximately 10° to 25°, depending on the size of the stepping motor  100  and the width of the stator  2 . According to the embodiment, the predetermined angle θ 1  is approximately 17°. 
     The outer notches  24  each may have any shape and any size other than the illustrated example. 
     Portions  26  of the stator  2  between the respective outer notches  24  and the rotor accommodating hole  23  have narrower widths than that of the other portions, so that the magnetic fluxes can be more readily saturated at the portions  26  compared to the other portions. 
     The portions  26  are configured so as to be at positions of saturated magnetic fluxes where they are not saturated with the magnetic fluxes from the rotor  1  but saturated with the excited coil  32  (described below) to have increased magnetic resistance. 
     The two inner notches  25  are disposed on the inner periphery of the rotor accommodating hole  23  on the substantially opposite sides. 
     The inner notches  25  constitute position determiners for determining positions (stably stationary positions) where the rotor  1  stably stops. 
     The rotor  1  is attracted by the nearest metal; hence, the largest holding torque occurs in the stepping motor  100  when the two magnetic poles of the rotor  1  face the portions of the stator  2  other than the inner notches  25 , i.e., the pole boundary positions (the ends of the boundary between the magnetic poles) of the rotor  1  face the respective inner notches  25 . The rotor  1  thus stops its rotation at a magnetically stable position where the pole boundary positions of the rotor  1  face the respective inner notches  25 , under no driving pulse applied to the coil  32  (described below) in a non-energized state. 
     The inner notches  25  according to the embodiment are disposed such that the line that connects the bottoms of the respective inner notches  25  and extends through the center of the rotor accommodating hole  23  shifts by a predetermined angle (θ 2  in  FIG. 2A ) from the line that connects the respective narrowest portions between the outer notches  24  and the rotor accommodating hole  23  and extends through the center of the rotor accommodating hole  23 . 
     The predetermined angle θ 2 , i.e., the angle defined by how much the line (Lc in  FIG. 2A ) that connects the bottoms of the respective inner notches  25  and extends through the center of the rotor accommodating hole  23  is shifted from the line (La in  FIG. 2A ) that connects the respective narrowest portions between the outer notches  24  and the rotor accommodating hole  23  and extends through the center of the rotor accommodating hole  23  should preferably range from approximately 30° to 50°, depending on various conditions such as the specification of the stepping motor  100 . According to the embodiment, the predetermined angle θ 2  is approximately 45°. 
     The inner notches  25  each may have any shape and any size other than the illustrated example. 
     The stator  2  according to the embodiment has a substantially symmetrical shape and a substantially identical thickness at both sides of the rotor accommodating hole  23  in the extending direction of the stator  2  (the right and left in  FIG. 1A or 2A ), as described above. 
     The stator  2  can thus be provided with the coil block  3  even if the stator  2  is reversed front to back, as illustrated in  FIGS. 3A and 3B . 
     For example, in order to rotate the rotor  1  clockwise (as illustrated with an arrow in  FIG. 1A ), the stator  2  is disposed such that the outer notches  24  reside at upper-left and lower-right positions, and then is provided with the coil block  3 , as illustrated in  FIG. 1A . 
     In order to rotate the rotor  1  counterclockwise (as illustrated with an arrow in  FIG. 3A ), the stator  2  is disposed such that the outer notches  24  reside at upper-right and lower-left positions, and then is provided with the coil block  3 , as illustrated in  FIG. 3A . 
     According to the embodiment, the mere reversal of the identical stator  2  can reverse the rotational direction of the rotor  1 . 
       FIG. 4A  is a plan view of the coil block  3  according to the embodiment, and  FIG. 4B  is a side view of the coil block  3  when seen in the direction of the arrow b in  FIG. 4A . 
     With reference to  FIGS. 4A and 4B , the coil block  3  includes a coil core  31  and a coil  32  composed of a wire wound around the coil core  31 . 
     The coil core  31  is composed of a highly magnetically permeable material such as a permalloy. In specific, the coil core  31  is composed of Permalloy C (PC), for example. 
     Permalloy C contains Ni (77-78%), Mo (5%), Cu (4%), and Fe (bal.), and has an initial magnetic permeability of 4,500 μi, a maximum magnetic permeability of 45,000 μm, a saturation flux density of 1.50 Bm(T), a magnetic coercive force of 12 Hc(A/m), and a specific resistance of 0.45 μΩ·m or higher. Permalloy C is less readily saturated with a magnetic flux compared to Permalloy B constituting the stator  2 . 
     The coil core  31  may be composed of any material other than Permalloy C. In other words, the coil core  31  may be composed of any other highly magnetically permeable material such as ferrite. 
       FIG. 5A  is a plan view of the coil core  31  according to the embodiment,  FIG. 5B  is a side view of the coil core  31  when seen in the direction of the arrow b in  FIG. 5A , and  FIG. 5C  is a cross-sectional view of the coil core  31  cut along the line c-c in  FIG. 5A . 
     With reference to  FIGS. 5A to 5C , the coil core  31  has a linear portion  311  provided with the coil  32  composed of the wound wire, and coil-side couplers  312  disposed at both ends of the linear portion  311 . 
     The linear portion  311  extends in the extending direction of the stator  2  (i.e., the lateral direction in  FIG. 1A, 2A , or  5 A, or the extending direction of the stepping motor  100 ) in an assembled state of the stepping motor  100 . The linear portion  311  according to the embodiment has a thickness slightly larger than that of the coil-side couplers  312 . 
     The coil-side couplers  312  extend orthogonally to the extending direction of the linear portion  311 . The coil-side couplers  312  are disposed over the stator-side couplers  21  of the stator  2  in the assembled stepping motor  100 , to constitute the coupled parts magnetically coupled to the stator  2 . 
     The coil-side couplers  312  each have a screw hole  33  at a position corresponding to each of the screw holes  22  of the stator-side couplers  21 . One of the screw holes  33  should preferably be an elongated hole for precise assembly. 
     In the assembled stepping motor  100  according to the embodiment, the screw holes  22  of the stator-side couplers  21  and the screw holes  33  of the coil-side couplers  312  are respectively coupled with screws  507  (refer to  FIG. 6 ). 
     When the stepping motors  100  are installed into a timepiece case  510  as illustrated in  FIG. 6 , the screws  507  fix the stepping motors  100  onto a base board  505  (refer to  FIG. 6 ) or a substrate (not shown) provided inside the timepiece case  510  such that the screws  507  couple the respective coil cores  31  to the respective stators  2 . 
     It is noted that the coil core  31  may also be coupled to the stator  2  by any other means. For example, the respective coil cores  31  may be coupled to the respective stators  2  with screws or by welding to assemble the stepping motor  100  unit before the mounting of the finished stepping motor  100  unit onto the base board  505  in the timepiece case  510  with the screws  507 . 
     With reference to  FIGS. 4A and 4B , one of the coil-side couplers  312  (the left coil-side coupler  312  in  FIG. 4A  in the embodiment) is covered with a substrate  5  via a spacer  5   a . The spacer  5   a  adjusts the height of the substrate  5  to that of the coil  32 . The thickness of the spacer  5   a  is appropriately determined depending on the height of the coil  32 . The spacer  5   a  is an optional component and may be omitted. 
     The substrate  5  is provided with a first coil terminal  51  and a second coil terminal  52  thereon. The first coil terminal  51  and the second coil terminal  52  are connected to wire ends  34  of the coil  32 . The coil  32  is connected to a driving pulse supplying circuit (not shown) via the first coil terminal  51  and the second coil terminal  52 . Under driving pulses applied from the driving pulse supplying circuit to the coil  32 , the coil  32  generates a magnetic flux. The magnetic flux generated in the coil  32  flows from the coil core  31  to the stator  2 . This operation appropriately switches the magnetic poles around the rotor accommodating hole  23 , so that the rotor  1  rotates by a predetermined step angle (e.g., 180°). 
     The substrate  5  has a screw hole  53  at a position corresponding to one of the screw holes  22  of the stator-side couplers  21  and one of the screw holes  33  of the coil-side couplers  312 . The screws  507  couple the respective stator  2 , the respective coil core  31  and the substrate  5  together. 
       FIG. 6  is a schematic view of a timepiece (e.g., a wristwatch) including the stepping motors according to the embodiment in an exemplary arrangement.  FIG. 7  is a cross-sectional view of the main configuration of a stepping motor and periphery thereof installed in the timepiece. 
     The timepiece  500  according to the embodiment is, for example, an analog timepiece that indicates time with the rotary hands  501  (e.g., a second hand, a minute hand, and an hour hand). The hands  501  are illustrated with broken lines in  FIG. 6 . 
     With reference to  FIG. 6 , the timepiece  500  according to the embodiment includes the timepiece case  510 , which is composed of a metal and/or a synthetic resin. 
     The timepiece case  510  accommodates a timepiece module (not shown) including the stepping motors  100  and the gear train mechanisms connected thereto, and the hands  501  to be rotated by the timepiece module. The timepiece module is covered with a dial and a windshield composed of a transparent glass, for example. In  FIG. 6 , the dial and the windshield are not depicted to show the arrangement of the stepping motors  100 . 
       FIG. 6  illustrates an exemplary timepiece case  510  accommodating three stepping motors  100  ( 100   a ,  100   b , and  100   c ). It is noted that the number of stepping motors  100  installed in a single timepiece case  510  should not be limited to three, i.e., may be one, two, four, or more. 
     With reference to  FIG. 7 , each stepping motor  100  and the gear train mechanism including the gears  502  connected thereto are disposed between the base board  505  and a gear receiver  506 . The gear receiver  506  holds the stepping motor  100  mounted on the base board  505  against the base board  505 . 
     The stepping motors  100  are fixed onto the base board  505  with the screws  507 , as described above. 
       FIG. 8  is a plan view of the base board  505  from which the stepping motors  100  are removed. 
     With reference to  FIG. 8 , the surface of the base board  505  has, for example, shallow recesses, each defining the outline of the corresponding stepping motor  100  to indicate a proper position of the stepping motor  100 . 
     The base board  505  according to the embodiment further has positioners  508  corresponding to the outer notches  24  of the respective stators  2  of the stepping motors  100 . 
     The positioners  508  are composed of protrusions that can fit in the respective outer notches  24 . 
     The number and the arrangement of the positioners  508  should not be limited to the illustrated example, provided that each positioner  508  corresponds to either one of the outer notches  24  and the inner notches  25 . For example, the positioner  508  may have a shape fittable in one of the inner notches  25 . Alternatively, the base board  505  may have a pair of positioners  508  for a single stepping motor  100  such that the positioners  508  can fit in the two respective outer notches  24  or the two respective inner notches  25 . Each positioner  508  may have any shape other than the illustrated example, provided that the positioner  508  can fit in one of the outer notches  24  and the inner notches  25  to position the stepping motor  100 . For example, the positioner  508  may have a thin shape like a pin. 
     The stator  2  according to the embodiment is reversible, and the reversal of the stator  2  can reverse the rotational direction of the rotor  1 , as described above. If the positioners  508  are disposed on the base board  505  onto which the stepping motors  100  are to be mounted, as illustrated in  FIGS. 6 and 8 , the reversed stator  2  (i.e., the stepping motor  100  including the reversed stator  2 ) is blocked by each positioner  508  and cannot be mounted onto the base board  505 . The positioner  508  determines the orientation (front side up or back side up) of the mounted stator  2  and thus can avoid erroneous assembly. 
     In an example illustrated in  FIG. 6 , for the stepping motors  100   a  and  100   b  in the three stepping motors  100  ( 100   a ,  100   b , and  100   c ) accommodated in the timepiece case  510 , the positioners  508  position the stators  2  in the orientation as illustrated in  FIG. 1A , so that the rotors  1  rotate clockwise. In contrast, for the stepping motor  100   c , the positioner  508  positions the stator  2  in the orientation as illustrated in  FIG. 3A , so that the rotor  1  rotates counterclockwise. 
     The base board  505  further has shaft holes  509  to receive the rotary shafts  11  of the stepping motors  100  and shafts of the gears  502  of the gear train mechanisms. 
     The operations of the stepping motor  100  according to the embodiment and the timepiece  500  including the stepping motor  100  will now be explained. 
     According to the embodiment, in order to assemble the stepping motor  100  that includes a rotor  1  rotating clockwise, the stator  2  is disposed at a predetermined position on the base board  505  such that the outer notches  24  reside at the upper-left and lower-right positions whereas the inner notches  25  reside at upper-left and lower-right positions, like the stepping motor  100   a  or  100   b  in  FIG. 6 . The positioner  508  fits in one of the outer notches  24  to position the stator  2  with a proper orientation. 
     In order to assemble the stepping motor  100  that includes a rotor  1  rotating counterclockwise, the stator  2  is disposed at a predetermined position on the base board  505  such that the outer notches  24  reside at the upper-right and lower-left positions whereas the inner notches  25  reside at upper-right and lower-left positions, like the stepping motor  100   c  in  FIG. 6 . The positioner  508  fits in one of the outer notches  24  to position the stator  2  with a proper orientation. 
     The coil block  3  is disposed on the positioned stator  2 . The substrate  5  is then placed on one of the coil-side couplers  312  of the coil block  3  (the left coil-side coupler  312  in  FIG. 6  in the embodiment) with the spacer  5   a  therebetween. 
     The substrate  5 , the coil block  3 , and the stator  2  are fixed onto the base board  505  with the screws  507 . This process completes the assembly of the stepping motor  100  and the installation of the stepping motor  100  into the timepiece case  510 . 
     For the stepping motor  100  rotating a hand  501  of a timepiece (e.g., the timepiece  500  in  FIG. 6 ), the gears  502  of the gear train mechanism are interposed between the rotary shaft  11  of the stepping motor  100  and the hand  501  to transmit the torque of the rotor  1  to the hand  501 . 
     Under driving pulses applied to the coil  32  of the stepping motor  100  (e.g., the stepping motor  100   a  or  100   b  in  FIG. 6 ) that includes the stator  2  having the upper-left and lower-right outer notches  24  and the upper-left and lower-right inner notches  25 , the rotor  1  rotates clockwise by a predetermined step angle inside the rotor accommodating hole  23 . Under driving pulses applied to the coil  32  of the stepping motor  100  (e.g., the stepping motor  100   c  in  FIG. 6 ) that includes the stator  2  having the upper-right and lower-left outer notches  24  and the upper-right and lower-left inner notches  25 , the rotor  1  rotates counterclockwise by a predetermined step angle inside the rotor accommodating hole  23 . 
     The gears  502  engaging with the pinion  12  provided at the rotary shaft  11  rotate in response to the rotation of the rotor  1 . The torque of the rotor  1  is transmitted to the hand  501 , and the hand  501  rotates by a predetermined angle. 
     The outer notches  24  according to the embodiment are disposed on the outer periphery of the stator  2  on the opposite sides of the rotor accommodating hole  23 , such that the line that connects the respective narrowest portions between the outer notches  24  and the rotor accommodating hole  23  and extends through the center of the rotor accommodating hole  23  shifts by a predetermined angle (e.g., 17°) from the line that extends through the center of the rotor accommodating hole  23  and is orthogonal to the extending direction of the stator  2 , as described above. 
     If the outer notches  24  were disposed on the line that extends through the center of the rotor accommodating hole  23  and is orthogonal to the extending direction of the stator  2 , the stator  2  might have narrow portions adjacent to the positions of saturated magnetic fluxes (portions  26 ) depending on the positions of the inner notches  25 . The magnetic fluxes generated from the coil core  31  would be readily saturated at the narrow portions before reaching the positions of saturated magnetic fluxes (portions  26 ) in the stator  2 . This configuration would cause energy loss. If the width of the stator  2  (the width orthogonal to the extending direction of the stator  2 ) was narrowed by the miniaturization of the stepping motor  100 , the magnetic fluxes would be more readily saturated before reaching the positions of saturated magnetic fluxes. The energy loss would accordingly increase. In contrast, the positions of the outer notches  24  according to the embodiment are shifted from the line that extends through the center of the rotor accommodating hole  23  and is orthogonal to the extending direction of the stator  2 , such that the outer notches  24  are not disposed at the narrowest portions between the rotor accommodating hole  23  and the outer periphery of the stator  2 . The stator  2  through which the magnetic fluxes generated from the coil core  31  flow has a cross section gently decreasing to the positions of saturated magnetic fluxes in the stator  2 , regardless of the narrowed width of the stator  2  because of the miniaturization of the stepping motor  100 . Thus, the magnetic fluxes are barely saturated before reaching the positions of saturated magnetic fluxes, resulting in less energy loss. This configuration can effectively utilize the magnetic force generated from the coil  32  for the 360° rotation of the rotor  1 . 
     According to the embodiment, the rotor accommodating hole  23  is disposed at the center of the stator  2  in its extending direction, and the stator  2  has an identical thickness at both sides in the extending direction. The stator  2  has a substantially symmetrical shape and a substantially identical thickness at both sides in the extending direction of the stator  2  (the right and left in  FIG. 1A ), so that the reversed stator  2  can accept the coil block  3 , as illustrated in  FIGS. 1A, 3A , and  6 . The side of the stator  2  to accept the coil block  3  can determine the clockwise or counterclockwise rotation of the rotor  1 . In other words, the mere reversal of the stator  2  having the same shape can reverse the rotational direction of the rotor  1 . This configuration can reduce the types of components to be manufactured, leading to increased productivity and reduced costs for the production and stock of the components. 
     In the stepping motor  100 , if the thickness or the shape of the stator  2  was asymmetric in its extending direction, the rotor  1  (magnet) and the right and left connections (couplers) between the stator  2  and the coil core  31  (i.e., the thickest magnetically permeable portions in the stepping motor  100 ) would generate asymmetric magnetic force between the right and left. The rotary shaft  11  of the rotor  1  would thus be attracted by one side of the stator  2 , leading to an increase in the friction. This phenomenon would increase the holding torque (index torque) and lead to asymmetric holding force on the rotary shaft  11  upon the rotation of the rotor  1 . The friction occurring at the rotary shaft  11  would thus increase, resulting in more energy loss. 
     In order to rotate the rotor  1  with less energy loss as efficiently as possible, the right and left connections (couplers) between the stator  2  and the coil core  31  (i.e., the thickest magnetically permeable portions in the stepping motor  100 ) should be equally distant from the rotor  1  (magnet) for equalization of the generated magnetic force between the right and left. In specific, the further miniaturization of the stepping motor  100  would remarkably reduce the distances between the rotor  1  (magnet) and the respective connections (couplers) between the stator  2  and the coil core  31 , so that the magnetic relationships between the rotor  1  and the respective couplers and the resulting energy loss could not be ignored. In contrast, the thickness and the shape of the stator  2  according to the embodiment are symmetric in its extending direction. This configuration can reduce the friction occurring at the rotary shaft  11  and reduce the holding torque (index torque). The holding force on the rotary shaft  11  upon the rotation of the rotor  1  is equalized between the right and left, so that the friction occurring at the rotary shaft  11  is cancelled between the right and left. This configuration can effectively rotate the rotor  1  with less energy loss. 
     The connections (couplers) between the stator  2  and the coil core  31  have the same magnetic force as that of the rotor accommodating hole  23  of the stator  2 ; hence, the rotor  1  (magnet) may have the same thickness as those of the connections (couplers) between the stator  2  and the coil core  31 . 
     The stepping motor  100  according to the embodiment can effectively operate with less energy loss even if the stepping motor  100  is miniaturized, as described above. A small timepiece  500 , such as a wristwatch, including the stepping motors  100  can thus be reduced in size and weight. 
     According to the embodiment, the base board  505  of the timepiece  500  has the positioners  508  corresponding to the outer notches  24  or the inner notches  25  of the stators  2  of the stepping motors  100 . The stepping motors  100  each include the reversible stator  2 , and the reversal of the stator  2  can reverse the rotational direction of the rotor  1 . The positioners  508  can readily and certainly position such stepping motors  100  with a desired orientation, to improve the efficiency of the assembly while maintaining its accuracy. 
     Second Embodiment 
     A stepping motor according to a second embodiment of the invention and a timepiece including the stepping motor will now be described with reference to  FIGS. 9A and 9B ,  FIGS. 10A and 10B  and  FIG. 11 . The second embodiment differs from the first embodiment only in the configurations of the couplers of the stator and the coil block. The following description focuses on the differences from the first embodiment. 
       FIG. 9A  is a plan view of a stator according to the second embodiment, and  FIG. 9B  is a side view of the stator when seen in the direction of the arrow b in  FIG. 9A .  FIG. 10A  is a front view of a coil core according to the embodiment, and  FIG. 10B  is a side view of the coil core when seen in the direction of the arrow b in  FIG. 10A .  FIG. 11  is a diagram illustrating an assembly of the stator having a cross section along the line IX-IX in  FIG. 9  and the coil core having a cross section along the line X-X in  FIG. 10 . 
     With reference to  FIGS. 9A and 9B , a stator  4  according to the second embodiment includes stator-side couplers  41  having screw holes  42 , a rotor accommodating hole  43 , outer notches  44 , inner notches  45 , and portions  46  of saturated magnetic fluxes, just like the first embodiment. 
     With reference to  FIGS. 10A and 10B , a coil core  61  according to the second embodiment includes a linear portion  611  and coil-side couplers  612  having screw holes  63 , just like the first embodiment. 
     The stator  4  adjoins the coil core  61  through the stator-side couplers  41  and the coil-side couplers  612 . 
     According to the second embodiment, free ends of the stator-side couplers  41  are thinned from the front and rear surfaces into thin portions  411  thinner than the rest of the portions of the stator-side couplers  41 , as illustrated in  FIG. 9B  and the lower right of  FIG. 11 . The stator  4  according to the embodiment also has a substantially symmetrical shape and a substantially identical thickness at both sides in the extending direction of the stator  4  (the right and left in  FIG. 9A ). The free ends of the stator-side couplers  41  are thinned by substantially the same depth from the front and rear surfaces, such that the front and rear surfaces of the stator  4  have substantially the same shape. 
     With reference to  FIG. 10B  and the upper right of  FIG. 11 , free ends of the coil-side couplers  612  are thinned from the front surface into thin portions  613  thinner than the rest of the portions of the coil-side couplers  612 . 
     With reference to  FIG. 11 , the coil-side couplers  612  are provided on the stator-side couplers  41 , such that the thin portions  411  of the stator-side couplers  41  adjoin the respective rest of the portions of the coil-side couplers  612  whereas the thin portions  613  of the coil-side couplers  612  adjoin the respective rest of the portions of the stator-side couplers  41 . 
     Such stacked couplers have a smaller thickness compared to that of the stacked couplers having no thin portion. 
     For example, if each stator-side coupler  41  and each coil-side coupler  612  have the same thickness and have no thin portion, the stacked couplers have a thickness of the sum of the stator-side coupler  41  and the coil-side coupler  612 , i.e., twice (six-thirds) of the original thickness of a single coupler. 
     In another example illustrated in  FIG. 11 , each stator-side coupler  41  is thinned by a depth of one-third of the original thickness from the front and rear surfaces into the thin portion  411  having a thickness of one-third of that of the rest of the portion, whereas each coil-side coupler  612  is thinned by a depth of one-third of the original thickness from the front surface into the thin portion  613  having a thickness of two-thirds of that of the rest of the portion. In this case, with reference to the left of  FIG. 11 , the thin portion  411  of the stator-side coupler  41  combined with the rest of the portion of the coil-side coupler  612  has a thickness of four-thirds of the original thickness, whereas the thin portion  613  of the coil-side coupler  612  combined with the rest of the portion of the stator-side coupler  41  has a thickness of five-thirds of the original thickness. In other words, both of the resulting thicknesses are smaller than the thickness of the sum of the original stator-side coupler  41  and coil-side coupler  612 , i.e., six-thirds of the original thickness of a single coupler. 
     The thin portion  411  of the stator-side coupler  41  and the thin portion  613  of the coil-side coupler  612  each may have any thickness other than the illustrated example. One or both of the thin portions may have an even smaller thickness. 
     The other components are identical to those in the first embodiment, and the redundant description thereof is omitted. 
     The operations of the stepping motor according to the second embodiment and the timepiece including the stepping motor will now be explained. 
     In order to assemble the stepping motor according to the embodiment that includes a rotor rotating clockwise, the stator  4  is disposed at a predetermined position on the base board such that the outer notches  44  reside at upper-left and lower-right positions whereas the inner notches  45  reside at upper-left and lower-right positions. The positioner fits in one of the outer notches  44  to position the stator  4  with a proper orientation. 
     In order to assemble the stepping motor that includes a rotor rotating counterclockwise, the stator  4  is disposed at a predetermined position on the base board such that the outer notches  44  reside at upper-right and lower-left positions whereas the inner notches  45  reside at upper-right and lower-left positions. The positioner fits in one of the outer notches  44  to position the stator  4  with a proper orientation. 
     The coil block is disposed on the positioned stator  4 . In specific, the coil-side couplers  612  are provided on the stator-side couplers  41 , such that the thin portions  411  of the stator-side couplers  41  adjoin the rest of the portions of the coil-side couplers  612  whereas the thin portions  613  of the coil-side couplers  612  adjoin the rest of the portions of the stator-side couplers  41 . 
     The substrate is then placed on one of the coil-side couplers  612  of the coil block. The substrate, the coil block, and the stator  4  are fixed onto the base board with screws. This process completes the assembly of the stepping motor and the installation of the stepping motor into a timepiece case. 
     The other operations are identical to those in the first embodiment, and the redundant explanation thereof is omitted. 
     As described above, the second embodiment can provide the same advantageous effects as the first embodiment and additional advantageous effects below. 
     According to the second embodiment, the stator  4  adjoins the coil core  61  through the stator-side couplers  41  and the coil-side couplers  612 . At least part of the stator-side couplers  41  of the stator  4  constitutes the thin portions  411  thinner than the rest of the stator-side couplers  41 , whereas at least part of the coil-side couplers  612  constitutes the thin portions  613  thinner than the rest of the coil-side couplers  612 . 
     This configuration can reduce the thickness of the stacked couplers of the stator  4  and the coil core  61 , leading to miniaturization of the stepping motor. 
     The reduction in the thickness of the stacked couplers of the stator  4  and the coil core  61  can reduce the influence of the connections (couplers) between the stator  4  and the coil core  61  (i.e., the thickest magnetically permeable portions in the stepping motor) on the rotor (magnet), so that the rotor can more effectively rotate. 
     According to the embodiment, the stator-side couplers  41  and the coil-side couplers  612  both have thin portions ( 411  and  613 ). This configuration can further reduce the thickness of the couplers. 
     If the stator-side couplers  41  are thinned by the same depth from the front and rear surfaces as in the embodiment, the front and rear surfaces of the stator  4  have the same shape; hence, the mere reversal of the stator  4  in the stepping motor can reverse the rotational direction of the rotor, just like the first embodiment. 
     Although the stator-side couplers  41  and the coil-side couplers  612  have thin portions ( 411  and  613 ) in the second embodiment, the thin portions are not essential for both the stator-side couplers  41  and the coil-side couplers  612 . For example, the thin portions may be provided to only the coil-side couplers  612 . 
     If the stator-side couplers  41  have no thin portion, the stator  4  that has the same shape on the front and rear surfaces and thus is reversible can be readily manufactured. 
     Although the process (thinning process) for thinning part of the couplers may impair the motor characteristics of the stepping motor, the impairment can be minimized because the portions wider than the linear portion  611  wound with a wire in the coil core  61  are thinned into the thin portions  613 . In addition, if the stator  4  composed of Permalloy C having a low saturation flux density is not thinned (squeeze processing), the impairment of the motor characteristics can be further minimized. 
     Third Embodiment 
     A stepping motor according to a third embodiment of the invention and a timepiece including the stepping motor will now be described with reference to  FIGS. 12A to 12D . The third embodiment differs from the first embodiment mainly in the configuration of the coil core. The following description focuses on differences from the first embodiment. 
       FIG. 12A  is a plan view of a stator and a rotor according to the third embodiment,  FIG. 12B  is a plan view of a coil block according to the embodiment,  FIG. 12C  is a front view of a stepping motor according to the embodiment that includes the rotor rotating clockwise, and  FIG. 12D  is a front view of the stepping motor in  FIG. 12C  that includes the rotor rotating counterclockwise. 
     With reference to  FIG. 12A , a stator  7  according to the third embodiment includes stator-side couplers  71  having screw holes  72 , a rotor accommodating hole  73 , outer notches  74 , inner notches  75 , and portions  76  of saturated magnetic fluxes, just like the first embodiment. 
     With reference to  FIG. 12B , a coil block  8  according to the third embodiment includes a coil core  81  including a linear portion  811  and coil-side couplers  812  having screw holes  83 , and a coil  82  composed of a wire wound around the linear portion  811  of the coil core  81 , just like the first embodiment. 
     According to the embodiment, the coil core  81  has notches  85  at positions corresponding to the respective inner notches  75  of the stator  7 . 
     In specific, the coil core  81  has four notches  85  ( 85   a ,  85   b ,  85   c , and  85   d ) on the inner sides of the coil-side couplers  812 , such that two of the notches  85  are disposed over the inner notches  75  when the coil-side couplers  812  are provided on the stator  7 , according to the embodiment. The notches  85  each may have any shape other than the illustrated example. The shape of the notch  85  does not necessarily need to completely match the corresponding inner notch  75 . For example, the notch  85  may be slightly larger than the corresponding inner notch  75 . 
     The stator  7  according to the embodiment is reversible, just like the first embodiment. The notches  85  of the coil core  81  are effective regardless of the orientation of the mounted stator  7 . 
     In order to rotate the rotor  1  clockwise, as illustrated in  FIG. 12C , the stator  7  is disposed such that the outer notches  74  reside at upper-left and lower-right positions whereas the inner notches  75  reside at upper-left and lower-right positions. In this case, the notches  85   a  and  85   d  of the four notches  85  in the coil core  81  correspond to the respective inner notches  75 . In other words, when the coil block  8  is provided on the stator  7 , the notches  85   a  and  85   d  are disposed over the respective inner notches  75  of the stator  7 . 
     In order to rotate the rotor  1  counterclockwise, as illustrated in  FIG. 12D , the stator  7  is disposed such that the outer notches  74  reside at upper-right and lower-left positions whereas the inner notches  75  reside at upper-right and lower-left positions. In this case, the notches  85   b  and  85   c  of the four notches  85  in the coil core  81  correspond to the respective inner notches  75 . In other words, when the coil block  8  is provided on the stator  7 , the notches  85   b  and  85   c  are disposed over the respective inner notches  75  of the stator  7 . 
     The other components are identical to those in the first embodiment, and the redundant description thereof is omitted. 
     The operations of the stepping motor  200  according to the third embodiment and the timepiece including the stepping motor  200  will now be explained. 
     In order to assemble the stepping motor  200  according to the embodiment that includes a rotor  1  rotating clockwise, the stator  7  is disposed at a predetermined position on the base board such that the outer notches  74  reside at the upper-left and lower-right positions whereas the inner notches  75  reside at the upper-left and lower-right positions. The positioner fits in one of the outer notches  74  to position the stator  7  with a proper orientation. 
     The coil block  8  is provided on the positioned stator  7 . In specific, the coil block  8  is disposed such that the notches  85   a  and  85   d  of the four notches  85  in the coil core  81  reside over the respective inner notches  75  of the stator  7 . 
     In order to assemble the stepping motor  200  that includes a rotor  1  rotating counterclockwise, the stator  7  is disposed at a predetermined position on the base board such that the outer notches  74  reside at the upper-right and lower-left positions whereas the inner notches  75  reside at the upper-right and lower-left positions. The positioner fits in one of the outer notches  74  to position the stator  7  with a proper orientation. 
     The coil block  8  is provided on the positioned stator  7 . In specific, the coil block  8  is disposed such that the notches  85   b  and  85   c  of the four notches  85  in the coil core  81  reside over the respective inner notches  75  of the stator  7 . 
     The substrate is then placed on one of the coil-side couplers  812  of the coil block  8 . The substrate, the coil block  8 , and the stator  7  are fixed onto the base board with screws. This process completes the assembly of the stepping motor  200  and the installation of the stepping motor  200  into a timepiece case. 
     The other operations are identical to those in the first embodiment, and the redundant explanation thereof is omitted. 
     As described above, the third embodiment can provide the same advantageous effects as the first embodiment and additional advantageous effects below. 
     According to the third embodiment, the coil core  81  has the notches  85  ( 85   a ,  85   b ,  85   c , and  85   d ) at positions corresponding to the respective inner notches  75  of the stator  7 . 
     In general, if the width (in the lateral direction in FIG.  12 C) of the stepping motor  200  is reduced by the miniaturization of the stepping motor  200 , the coil-side couplers  812  of the coil core  81  approach the rotor accommodating hole  73  of the stator  7 , so that the coil-side couplers  812  overlap the inner notches  75  of the stator  7 . 
     In this case, the rotor  1  (magnet) is significantly close to the connections (couplers) between the stator  7  and the coil core  81 . The connections (couplers) between the stator  7  and the coil core  81  are the thickest magnetically permeable portions; hence, the connections close to the rotor  1  significantly influence the rotor  1 . 
     If the coil core  81  also has notches  85  corresponding to the respective inner notches  75  as in the embodiment, the rotor  1  can stably stop at a certain stationary position. 
     The coil-side couplers  812  of the coil core  81  are close to the rotor accommodating hole  73  of the stator  7  such that the coil-side couplers  812  overlap the inner notches  75  of the stator  7  in the embodiment. Alternatively, the stator  7  and the coil core  81  may have any positional relationship other than the illustrated example. 
     Even if the coil-side couplers  812  do not overlap the inner notches  75  of the stator  7 , the connections (couplers) between the stator  7  and the coil core  81  (i.e., the thickest magnetically permeable portions in the stepping motor  200 ) may influence the rotor  1 . The influence of the connections (couplers) between the stator  7  and the coil core  81  on the rotor  1  can be effectively reduced with notches at positions corresponding to the inner notches  75 . 
     The invention should not be limited to the embodiments described above, and the embodiments may be modified in various manners within the gist of the invention. 
     For example, although the timepiece is an analogue timepiece  500  that indicates time with rotary hands in the embodiments, the timepiece may also be of any type other than the analogue type. 
     For example, the timepiece may be capable of both digital and analog display. 
     The stepping motor  100  may drive any functional indicator, such as a date indicator, other than the hands. If the stepping motor  100  drives a functional indicator, the timepiece may be a digital timepiece including a display (e.g., a liquid crystal display) for displaying various pieces of information, such as time and calendar. 
     The timepiece may be a pocket watch or a watch with a key chain attachable to and detachable from a bag, other than the wristwatch. 
     Although the stepping motor  100  is applied to the timepiece  500  (e.g., a wristwatch) in the embodiments, the stepping motor  100  may also be applied to any device other than the timepiece. 
     For example, the stepping motor  100  or  200  according to the embodiments may drive hands and/or functional indicators of a pedometer, pulse rate meter, altimeter, or barometer. 
     The invention is not intended to be limited to the embodiments described above but rather is defined by the following claims and equivalents thereof. 
     The entire disclosure of Japanese Patent Application No. 2013-201949 filed on Sep. 27, 2013 including description, claims, drawings, and abstract are incorporated herein by reference in its entirety.