Abstract:
A claw-pole type stepping motor which includes a stator in which coils are arranged in association with a pair of inner and outer yokes and a pair of pole tooth sections thereby forming a magnetic circuit; and a rotor magnet which is rotatably disposed opposite to the pair of pole tooth sections, each of the pair of pole tooth sections being prepared discretely from each of the inner and outer yokes by punching out a soft magnetic steel plate so as to form a piece having a belt portion and a plurality of pole teeth integrally connected to the belt portion and thereafter rolling into a cylindrical shape. Thus, it becomes possible to set arbitrarily individual dimensions of each of the pole teeth as required by the characteristics of a motor, thereby providing an excellent high output and miniature motor.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention: 
     The present invention relates to a claw-pole type stepping motor, and particularly to a stator structure of a claw-pole type stepping motor for realizing the miniaturization and high efficiency of the motor and also the simple and easy manufacture of the motor. 
     2. Related Art: 
     In recent years, various devices have been coming out with miniaturization and high efficiency, and this trend keeps going on. However, although sufficient space a s a whole is provided in devices such as a laptop computer, the height space is limited, and high performance characteristics are required for the motor. 
     A claw-pole type stepping motor is capable of high precision position control with a comparatively simple structure, but it is difficult to reduce the size (diameter) of such a motor. This arises mainly from the formation of the teeth of claw poles. 
     FIG. 1A is a partially cut away perspective view showing a conventional claw-pole type stepping motor, and FIG. 1B is an exploded perspective view of yokes with pole teeth that are main portions of a stator. 
     In FIG. 1A, reference numeral  1  denotes a flange for motor attachment, and an outer yoke  2  and an inner yoke  3  are each manufactured by punching and drawing a soft magnetic steel plate, each of the yokes  2  and  3  having at its inner circumferential side a plurality of pole teeth  4  formed by bending. Coils  6  formed by winding magnetic wires around bobbins  5  are each arranged at an equal interval in an annular space between the outer yoke  2  and the inner yoke  3 . Respective pole teeth  4  of the outer and inner yokes  2  and  3  are displaced by an electrical angle of 180°, and from a magnetic circuit together with the coil  6 , thus constructing each of an A phase stator  15  and a B phase stator  16 . The A phase and B phase stators  15  and  16  thus constructed are coupled together back to back such that their respective pole teeth  4  are displaced by an electrical angle of 90°, thereby forming a stator assembly  17 . A lower flange  7  has a bearing (not shown) at its center and is attached to the outer yoke  2  of the B phase stator  16 . Reference numeral  9  represents a rotor magnet, and a sleeve  10  joins the rotor magnet  9  to a shaft  11 . 
     As can be seen in FIG. 1B, the plurality of pole teeth  4  are formed in the outer yoke  2  and inner yoke  3 , respectively, by bending the pole teeth after integrally forming them at the time of machining the respective yokes. Accordingly, since the pole teeth  4  are made of the same material of a section corresponding to a hole formed in the center of each of the yokes, it is extremely difficult to arbitrarily set the length and thickness of each pole tooth  4 . For example, if the motor diameter is made small, it is difficult to obtain each pole tooth of sufficient length. For this reason, a claw-pole type stepping motor tends not to be mounted in a thin device in which only a motor having a very small diameter can be used. 
     To solve this problem, a structure where pole teeth are parts manufactured discretely from the yokes has been proposed and an example is disclosed in Japanese Patent Application Laid-open No. 2000-050536. However, since this solution proposes that each individual pole tooth should be manufactured and assembled discretely from other pole teeth and from the yokes, it is not feasible method and structure to adapt for a practical motor, because it requires a great deal of labor and cost to manufacture this type of motor. 
     Therefore, with a claw-pole type stepping motor of the related art, each of the pole teeth  4  of the outer yoke  2  and inner yoke  3  are integrally formed with their respective yokes. However, in order to secure sufficient space for coils, the smaller a motor diameter becomes the thinner the plate has to be. As a result, from the point of view of strength as well as magnetic characteristics, it is very difficult to form satisfactory pole teeth. For this reason, instead of using a claw-pole type stepping motor, a brushless DC motor is used. However, in this case, it is necessary to add an encoder as a position detector just to control its position. As a result, the structure of the brushless DC motor becomes expensive. 
     Nevertheless, in an environment surrounding the stepping motor, a high performance magnet such as a rare earth magnet has been developed as the material of a rotor magnet facing the stator, and only if a satisfactory structure of a stator can be found out, will it be possible to provide a motor with improved characteristics, and its appropriate structure is therefore expected. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-described circumstances in the prior art, and has for its aim to provide a claw-pole type stepping motor capable of being mounted in a thin device and being manufactured easily, and at the same time to provide a stator structure capable of dealing with a high output motor. 
     In order to achieve the above-described aim, according to a main aspect of the present invention, there is provided a claw-pole type stepping motor comprising: a stator in which coils are arranged in association with a yoke unit made of soft magnetic steel and consisting of an inner yoke and an outer yoke, each having a round hole at its center, and a of pole tooth sections made of soft magnetic steel thereby forming a magnetic circuit; and a rotor magnet which is rotatably disposed opposite to the pair of pole tooth sections of the stator, each of the pair of pole tooth sections being prepared discretely from each of the inner and outer yokes by punching out a soft magnetic strip steel plate so as to form a piece having a belt portion and a plurality of pole teeth integrally connected to the belt portion and thereafter rolling into a cylindrical shape. 
     The above-described stepping motor of the main aspect also has the following subsidiary aspects: 
     (a) In the stepping motor of the main aspect, each of the pole tooth sections is inserted into the round hole of each of the yokes and is fixed to an inner circumferential surface defining the round hole of each of the yokes. 
     (b) In the stepping motor of the main aspect, fixing each of the pole tooth sections to an inner circumferential surface defining the round hole of each of the yokes is achieved by press-fitting the pole tooth section into the round hole. 
     (c) In the stepping motor of the main aspect, fixing each of the pole tooth sections to an inner circumferential surface defining the round hole of each of the yokes is achieved by welding the pole tooth section to the yoke. 
     (d) In the stepping motor of the main aspect, fixing each of the pole tooth sections to an inner circumferential surface defining the round hole of each of the yokes is achieved by pouring mold resin into a gap in an assembled body of the yoke and the pole tooth section. 
     (e) In the stepping motor of the main aspect, protuberances facing the inside of the round hole are formed at appropriate places of an inner circumferential surface defining the round hole of each of the yokes, so that the protuberances engage fixedly with engaging portions of each of the pole tooth sections when joining the yokes with the pole tooth sections, respectively. 
     (f) In the stepping motor of the main aspect, protuberances facing the inside of the round hole are formed at appropriate places of an inner circumferential surface defining the round hole of each of the yokes, and notches are cut in appropriate places of a side of the belt portion of each of the pole tooth sections opposite to another side of the belt portion where the plurality of pole teeth are formed, so that the protuberances engage fixedly with the notches when joining the yokes with the pole tooth sections, respectively. 
     (g) In the stepping motor of the main aspect, notches are cut in appropriate places of an inner circumferential surface defining the round hole of each of the yokes, and protuberances are formed at appropriate places of a side of the belt portion of each of the pole tooth sections opposite to another side of the belt portion where the plurality of pole teeth are formed, so that the protuberances engage fixedly with the notches when joining the yokes with the pole tooth sections, respectively. 
     (h) In the stepping motor of the subsidiary aspects (f) and (g), the protuberances and notches are respectively formed at at least two places diametrically opposed so as to engage with each other. 
     As it is clear from the above aspects, according to the stepping motor of the present invention, the pole teeth of the soft magnetic steel facing the rotor magnet are prepared discretely from each of the inner and outer yokes, and the soft magnetic strip steel plate is punched out to form the pole tooth section having a belt portion and a plurality of pole teeth integrally connected to the belt portion, and the pole tooth section is rolled into a cylindrical shape. As a result, individual dimensions of each of the pole teeth (length, width, plate thickness, etc.) can be arbitrarily set as required by the characteristics of the motor, which can realize a miniature motor that develops high output. Moreover, the present invention makes it possible to reduce the number of parts and to easily assemble the motor. 
    
    
     The above and many other aims, features and advantages of the present invention will become manifest to those skilled in the art upon making reference to the following detailed description and accompanying drawings in which the preferred embodiments incorporating the principle of the present invention are shown by way of illustrative examples. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A and 1B respectively show a conventional claw-pole type stepping motor and a stator structure of the related art, in which FIG. 1A is a partially cut away perspective view of the motor, and FIG. 1B is an exploded perspective view of a pair of inner and outer yokes and a pair of pole tooth sections that are main parts of the stator structure; 
     FIGS. 2A and 2B respectively show a claw-pole type stepping motor and a stator structure of the present invention, in which FIG. 2A is a partially cut away perspective view of the motor, and FIG. 2B is an exploded perspective view showing a pair of inner and outer yokes and a pair of pole tooth sections that are main parts of the stator structure; 
     FIGS. 3A and 3B respectively show a pole tooth section used in the stepping motor of the present invention, in which FIG. 3A is a plan view of a belt-shaped and toothed piece having pole teeth manufactured by punching out a soft magnetic strip steel plate, and FIG. 3B is a perspective view of a pole tooth section manufactured from the belt-shaped and toothed piece having pole teeth shown in FIG. 3A; 
     FIG. 4 is a plan view showing a first embodiment of a positioning method when fixing a pole tooth section to an inner yoke in the stator structure of the present invention; 
     FIG. 5 is a plan view showing a second embodiment of a positioning method when fixing the pole tooth section to an inner yoke in the stator structure of the present invention; 
     FIGS. 6A and 6B show a third embodiment of a positioning method in the stator structure of the present invention when fixing a pole tooth section shown in the expanded side elevation view of FIG. 6B to an inner yoke shown in the plan view of FIG. 6A; 
     FIGS. 7A and 7B show a fourth embodiment of a positioning method in the stator structure of the present invention when fixing a pole tooth section shown in the expanded side elevation view of FIG. 7B to a n inner yoke shown in the plan view of FIG. 7A; 
     FIGS. 8A to  8 D show a fifth embodiment of a positioning method in the stator structure of the present invention when fixing a pole tooth section to an inner yoke, in which FIG. 8A is a plan view of the inner yoke, FIG. 8B is a perspective view of a pole tooth section, FIG. 8C is an expanded si de elevation view of a belt-shaped piece having pole teeth, and FIG. 8D is a plan view showing an assembled state of the pole tooth section and the inner yoke; and 
     FIG. 9 is a cross-sectional view of a stator for explaining a manner of constructing the stator structure using an insert mold. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described hereinafter with reference to the accompanying drawings. 
     In the drawings, sections that are the same as or equivalent to the conventional embodiment shown in FIGS. 1A and 1B are shown by the same reference alphanumeric characters, and therefore detailed descriptions thereof are omitted. 
     FIGS. 2A and 2B respectively show an embodiment of a claw-pole type stepping motor of the present invention, in which FIG. 2A is a partially cut away perspective view of the motor, and FIG. 2B is an exploded perspective view showing a pair of pole tooth sections  41 ,  42  and a yoke unit consisting of outer and inner yokes  2  and  3 . 
     The schematic structure of the stator of the present invention is as described in the following. 
     As it is understood from FIG. 2B, pole teeth are not formed integrally with an outer yoke  2  and an inner yoke  3 , and instead only round holes  2   a  and  3   a  for the respective attachments of pole tooth sections  41  and  42  are formed in the yokes  2  and  3 , respectively. The pole tooth sections  41  and  42  each having pole teeth are manufactured independently of the yokes  2  and  3 . The pole tooth sections  41  and  42  of this embodiment are formed cylindrically after punching out a soft magnetic strip steel plate. That is, the cylindrical pole tooth sections  41  and  42  are made as follows. In the first step, as shown in FIG. 3A, the soft magnetic strip steel plate  40   a  such as a galvanized strip steel plate is punched out so as to form a belt-shaped and toothed piece  40  having a belt portion and a plurality of pole teeth  4   a  integrally connected to the belt portion and positioned at equal intervals in the first step. The shape of each of the pole teeth  4   a  is approximately triangular or trapezoidal. In the second step, as shown in FIG. 3B, the pole tooth sections  41  and  42  are obtained by rolling the belt-shaped and toothed piece  40  into a cylindrical shape with both ends of each piece joined. Reference numeral  21  represents a break (a joint). 
     For the respective pole tooth sections to be formed, as shown in FIG. 2B, the pole tooth section  41  for the outer yoke  2  and the pole tooth section  42  for the inner yoke  3  are prepared, and then they are respectively inserted into a round hole  2   a  formed in the outer yoke  2  and a round hole  3   a  formed in the inner yoke  3 , and are respectively fixed to an inner circumferential surface forming the round hole  2   a  of the outer yoke  2  and an inner circumferential surface forming the round hole  3   a  of the inner yoke  2 . 
     FIG. 4 is a plan view of the pole tooth section  42  mounted to the inner yoke  3  through the round hole  3   a . The pole tooth section  42  can be fixed to the inner yoke  3  by an elastic return force due to the inherent springiness of the material, or fixed by forcing the pole tooth section into the round holes  3 , depending on the intended use of the motor. In this connection, it is more effective to weld them in order to increase fixing reliability (the symbol character “x” in FIGS. 4 and 5 represents each welding point). 
     Referring to FIG. 2B again, the pole tooth section  41  is also fixed to the inner circumferential surface forming the round hole  2   a  of the outer yoke  2  in a similar manner. In order to make a magnetic circuit, a bobbin  5  (not shown in FIG. 2B) on which a coil is wound is mounted onto the periphery of the pole tooth section  41 . Further, protuberances  3   b  formed at certain points on the edge of the inner yoke  3  and indentations  2   b  formed at the corresponding points on an upper edge of the outer yoke  2  are positioned so that they fit right in with each other. Still further, the outer yoke  2  is superposed on the inner yoke  3 . Thus, a stator can be assembled. 
     Next, a positioning method for fitting the pole tooth sections on the inner and outer yokes, respectively, will be described using the inner yoke  3  as an example. 
     FIG. 5 shows a first embodiment of a positioning method of the present invention. In FIG. 5, in the same way as shown in FIG. 4, the state where the pole tooth section  42  is fitted in the yoke  3  through the round hole  3   a  is shown in plan view. 
     A protuberance  3 c is provided at a certain point of the circumferential surface forming the round hole  3   a  of the inner yoke  3 , and when the pole tooth section  42  is fitted in the yoke  3  through the round hole  3   a , it is possible to position the pole tooth section  42  at the right point along the circumference of the round hole  3  in order to cause the protuberance  3   c  to bite into a break  21  of the pole tooth section  42 . 
     As described above, pole teeth of the pole tooth section  42  fitted in the inner yoke  3  and those of the pole tooth section  41  fitted in the outer yoke  2  must be alternately aligned back to back at intervals of an electrical angle of 180° along the respective circumferential directions of the yokes. At this time, as shown in FIG. 2B, positioning of the outer yoke  2  and the inner yoke  3  in the circumferential direction is achieved by causing the protuberance  3   b  formed on the inner yoke  3  to engage with the indentation  2   b  formed in the outer yoke  2 . Accordingly, by appropriately determining the position of the protuberance  3   c  provided at a certain point on the inner yoke  3 , when seeking to cause this protuberance  3   c  to engage with the break  21  of the pole tooth section  42 , it becomes easy to just position both the pole tooth section  41  of the outer yoke  2  and the pole tooth section  42  of the inner yoke  3 , each at an electrical angle of 180°. 
     A second embodiment of a positioning method when fixing the pole tooth section to the inner yoke is shown in FIGS. 6A and 6B. 
     In this second embodiment, two protuberances  3   d  and  3   e  are provided along an inner circumferential surface forming the round hole  3   a  of the inner yoke  3  at positions diametrically opposite to each other (opposite with the center of the round hole  3   a  as a center), while two notches  4   d  and  4   e  are formed in a belt-shaped and toothed piece  40  (which is shown in FIG. 6B as an expanded view so as to facilitate the understanding), each at an interval equal to the interval between the two protuberances  3   d  and  3   e . When seeking to fit the pole tooth section  42  in the inner yoke  3 , it is possible to always keep the same positional relationship between the inner yoke  3  and the pole tooth section  42  in the circumferential direction of the round hole  3   a  by causing the protuberances  3   d  and  3   e  of the inner yoke  3  to engage with the notches  4   d  and  4   e  of the pole tooth section  42 . At the same time, it is also possible to keep constant the positional relationship between the inner yoke  3  and the pole tooth section  42  in the axial direction thereof by causing the protuberances  3   d  and  3   e  to engage with the notches  4   d  and  4   e . 
     FIGS. 7A and 7B show a third embodiment of a positioning method which is a further development of the positioning method. Although this embodiment is the same as the second one shown in FIGS. 6A and 6B in that two protuberances  3   d  and  3   e  are provided on an inner circumferential surface of the round hole  3   a  of the inner yoke  3 , it is different from the second embodiment in that one notch  4   f  out of the two notches provided in the band-shaped and toothed piece  40  is made at the break  21  of the pole tooth section  42 , while the other notch  4   e  is made in the center of a common base line surface of the band-shaped and toothed piece  40 . 
     In this third embodiment, since the notch  4   f  is made at the break  21  of the pole tooth section  42  which is an unstable portion, the whole of the pole tooth section  42  having the notch  4   f  at its break  21  becomes more stable by causing the protuberance  3   d  to engage with the notch  4   f . 
     A fourth embodiment of the positioning method for fixing the pole tooth section to the inner yoke is shown in FIGS. 8A to  8 D. 
     FIG. BA is a plan view of the inner yoke  3 . Notches  3   f  and  3   g  are made in an inner circumferential surface forming the round hole  3   a  of the inner yoke  3  at positions diametrically opposite to each other (opposite with the center of the round hole  3   a  as a center), and protuberances  4   g  and  4   h , as shown in FIG. 8B, are formed on a common base line surface of the pole tooth section  42  at positions diametrically opposite to each other. Firstly, this pole tooth section  42  can be manufactured by punching out a soft magnetic strip steel plate to form a belt-shaped and toothed piece  40 , as shown in FIG. 8 c , having protuberances  4   g  and  4   h  separated from each other on a common base line surface. Successively, as shown in FIG. 8B, this belt-shaped and toothed piece  40  is made into a cylindrical shape by joining both ends thereof. After that, the protuberances  4   g  and  4   f  are bent outside, so that the pole tooth section  42  with the protuberances  4   g  and  4   h  can be manufactured. 
     FIG. 8D shows the state where the pole tooth section  42  having the above-described structure is fitted in the inner yoke  3  through the round hole  3 . It is possible to achieve positioning between the inner yoke  3  and the pole tooth section  42  in the circumferential direction of the round hole  3   a  by engaging the protuberances  4   g  and  4   h  of the pole tooth section  42  with the notches  3   g  and  3   f  of the inner yoke  3 . 
     The above description is in connection with some embodiments, in each of which the pole tooth section  42  is fitted in the inner yoke  3 , and the description can apply also to the pole tooth section  41  of the outer yoke  2  in the same way. 
     FIG. 9 is a cross-sectional view showing a stator structure of the present invention for explaining another method of assembling the stator according to the present invention. 
     In FIG. 9, a bobbin  5  wound with a coil  6 , an inner yoke  3 , an outer yoke  2  and pole tooth sections  41  and  42  are assembled into a unit of stator as illustrated. In this stator, mold resin  28  is poured into the pole tooth sections  41  and  42  with an insert molding machine using a press jig or the like (not shown). In this way, it makes it possible to fix the pole tooth sections  41  and  42  to the outer yoke  2  and the inner yoke  3 . 
     If such an insert mold is used, it is possible to fix the pole tooth sections to the yokes, respectively, without mechanical attachment or welding. 
     Furthermore, with reference to the shape of the stator structure, the stator structure does not have to be always cylindrical in order to function as a miniature motor. As long as a rotor magnet fits into a limited gap, it also makes it possible for the stator structure to be rectangular or elliptical. Accordingly, it is possible to try any scheme such as making the shape rectangular to secure more space inside the stator, and injecting mold resin into this more secured space to fix an A-phase stator  15  to a B-phase stator  16 , in order to improve the heat radiating characteristics.