Compact stepping motor

Abstract: A motor is formed in such structure that a rotor of permanent magnet, equally divided in the circumferential direction and magnetized in alternately different poles, is formed in a cylindrical shape, that a first coil, the rotor, and a second coil are located in order in the axial direction, that first outside and first inside poles excited by the first coil are opposed to outside and inside peripheral surfaces of the rotor, and that second outside and second inside poles excited by the second coil are opposed to the outside and inside peripheral surfaces of the rotor, wherein yokes forming the first and second inside poles of this motor are formed so that a cross section thereof in a direction perpendicular to the axis of the rotor has alternately formed regions with a large outside dimension (5b,5c,6b,6c) and with a small outside dimension (5e,6f), thereby being formed in the shape with good workability. In addition, the working of a yoke for forming the outside poles is also made easy.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a motor that can be constructed in microstructure with enhanced workability. Related Background Art Among conventional compact motors, there is, for example, a compact, cylindrical step motor as shown in Fig. 13. A stator coil 105 is concentrically wound on each of bobbins 101, and each bobbin 101 is fixed as being axially sandwiched between two stator yokes 106. The stator yoke 106 have stator teeth 106a and stator teeth 106b arranged in alternating fashion along the circumferential direction of the bore surface of bobbin 101. The stator yokes 106 integral with the stator teeth 106a or 106b are fixed to cases 103, thus composing stators 102. Flange 115 and bearing 108 are fixed to one of two sets of cases 103 and another bearing 108 is fixed to the other case 103. Rotor 109 is comprised of a rotor magnet 111 fixed to rotor shaft 110, and the rotor magnet 111 and the stator yoke 106 of stator 102 form radial gap portions. The rotor shaft 110 is rotatably supported between the two bearings 108. The conventional compact step motor described above, however, had the drawback that the outside dimensions of motor were large, because the cases 103, bobbins 101, stator coils 105, stator yokes 106, etc. were concentrically located outside the rotor. In addition, it has another drawback that the output of motor is not high, because the magnetic flux generated by energizing the stator coils 105 mainly passes end faces 106a1 of the stator teeth 106a and end faces 106b1 of the stator teeth 106b, as shown in Fig. 14, and does not effectively act on the rotor magnet 111. The applicant proposed a motor solving such problems in United States Patent No . 5831356 (filed April 2, 1997) corresponding to european patent application EP 0 801 459 falling under Article 54(3) EPC. This proposed motor is constructed in such structure that the rotor of permanent magnet partitioned and magnetized in the alternately different poles at equal intervals in the circumferential direction is formed in a cylindrical shape, that a first coil, the rotor, and a second coil are located in order along the axial direction of the rotor, that the first outside pole and first inside pole excited by the first coil are opposed to outside and inside peripheral surfaces of the rotor, and that the second outside pole and second inside pole excited by the second coil are opposed to the outside and inside peripheral surfaces of the rotor. The motor of this structure was able to provide high output and had small outside dimensions of motor, but the radial dimension of the first inside pole and second inside pole which comprise a pair of magnetic tooth with separated tip ends was also small. When these pole teeth were processed by press working, there were problems of strength of mold, emission of chip, and so on. Therefore, the processing was not easy and it was rather difficult to achieve high strength of the teeth. There were desires for readily processing the first outside pole and second outside pole, too. SUMMARY OF THE INVENTION An object of the present invention is to overcome the above drawbacks and to provide a motor that can be constructed in microstructure wherein the inside poles are formed in a shape with high workability. Another object of the present invention is to provide a motor that can be constructed in microstructure wherein processing of the outside poles is easy. Other objects of the present invention will become more apparent by specific embodiments which will be described below. These objects are achieved with a motor as claimed in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an exploded, perspective view of a motor of the first embodiment according to the present invention; Fig. 2 is a cross-sectional view of the assembled state of the motor shown in Fig. 1; Figs. 3A, 3B, 3C, 3D, 3E, 3F, 3G, and 3H are drawings for explaining the rotor rotation operation of the motor shown in Fig. 2; Fig. 4 is an enlarged view of the yoke as an inside pole in the motor shown in Fig. 2; Fig. 5 is a drawing to show another example of the yoke as an inside pole in the motor shown in Fig. 2; Fig. 6 is a cross-sectional view of the yoke shown in Fig. 5; Fig. 13 is a cross-sectional view to show a conventional step motor; and Fig. 14 is an explanatory drawing to show the magnetic flux in the conventional step motor shown in Fig. 13. DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be described by reference to the drawings. (Embodiment 1) Fig. 1 to Fig. 4 are drawings to show Embodiment 1. Reference numeral 1 designates a permanent magnet of a cylindrical shape, which is composed of a first magnetic layer having four portions 1a, 1b, 1c, 1d obtained by quartering the circumference and alternately magnetized in the S and N poles, and a second magnetic layer having four portions 1e, 1f, 1g, 1h obtained similarly by quartering the circumference and alternately magnetized in the S and N poles. The first magnetic layer and the second magnetic layer are magnetized in respective phases shifted by 45° from each other. The outside peripheral surfaces of the portions 1a, 1c of the first magnetic layer and the portions 1e, 1g of the second magnetic layer are magnetized in the S pole and the inside peripheral surfaces thereof in the N pole; the outside peripheral surfaces of the portions 1b, 1d of first magnetic layer and the portions 1f, 1h of the second magnetic layer are magnetized in the N pole and the inside peripheral surfaces thereof in the S pole. Numeral 2 denotes a rotational shaft, to which the permanent magnet 1 is fixed. The rotational shaft 2 and permanent magnet 1 compose a rotor. Numerals 3, 4 are coils which are located so as to be concentric with the permanent magnet 1 and to sandwich the permanent magnet 1 in the axial direction. Numeral 5 represents a first yoke made of a soft magnetic material, which has a portion 5d set in a bore portion 3a of the coil 3 and a pole section opposed to a bore portion of the first magnetic layer of the permanent magnet 1. In the pole section, as shown in Fig. 4, portions 5b, 5c with a large outside diameter and portions 5e, 5f with a small outside diameter are alternately formed in the cross section taken in the direction perpendicular to the axis. The portions 5b, 5c are formed in the same phase, relative to the poles of the first magnetic layer. Namely, the portion 5b and the portion 5c are located as shifted by 180° from each other. A portion 2a of the rotational shaft 2 is fitted in a hole 5a of the first yoke so as to be rotatable. Numeral 6 denotes a second yoke made of a soft magnetic material, which has a portion 6d set in a bore portion 4a of the coil 4 and a pole section opposed to a bore porion of the second magnetic layer of the permanent magnet 1. In the pole section, portions 6b, 6c with a large outside diameter and portions 6e, 6f with a small outside diameter are alternately formed, as in the first yoke 5. The portions 6b, 6c are formed in the same phase, relative to the poles of the second magnetic layer. Namely, the portion 6b and the portion 6c are located as shifted by 180° from each other. A portion 2b of the rotational shaft 2 is fitted in a hole 6a of the second yoke so as to be rotatable. The large diameter portions 5b, 5c of the first yoke 5 and the large diameter portions 6b, 6c of the second yoke 6 are located in the same phase, i.e., opposed to each other with respect to the axial direction. Numeral 8 stands for a third yoke made of a soft magnetic material, in which teeth 8a, 8b are formed at positions where they sandwich the portions 5b, 5c of the first yoke 5 and the first magnetic layer of the permanent magnet 1. Numeral 9 stands for a fourth outer yoke made of a soft magnetic material, in which teeth 9a, 9b are formed at positions where they sandwich the portions 6b, 6c of the second yoke 6 and the second magnetic layer of the permanent magnet 1. Numeral 10 designates a coupling ring made of a non-magnetic material, which has grooves 10a, 10b engaged with the teeth 8a, 8b of the third yoke 8 and grooves 10c, 10d engaged with the teeth 9a, 9b of the fourth yoke. The third yoke 8 and the fourth yoke 9 are fixed to the coupling ring 10 by a conventional method, for example, by bonding or the like. The third yoke 8 and the fourth yoke 9 are fixed with a predetermined space through portions 10e, 10f of the coupling ring 10. The teeth 8a, 8b of the third yoke 8 are so arranged as to face the teeth 9a, 9b of the fourth yoke 9. The third yoke 8 is so arranged, as shown in Fig. 2, that one end thereof is connected to the first yoke 5 and covers the outside-diameter portion of coil 3 and that the teeth 8a, 8b at the other end are opposed with a predetermined gap to the outside peripheral portion of the permanent magnet 1. The fourth yoke 9 is so arranged, as shown in Fig. 2, that one end thereof is connected to the second yoke 6 and covers the outside-diameter portion of coil 4 and that the teeth 9a, 9b at the other end are opposed with a predetermined gap to the outside peripheral portion of the permanent magnet 1. Fig. 2 is a cross-sectional view of the assembled state, Figs. 3A, 3B, 3C, and 3D are cross-sectional views along A-A in Fig. 2, and Figs. 3E, 3F, 3G, and 3H are cross-sectional views along B-B in Fig. 2. Figs. 3A and 3E are cross-sectional views at a moment, Figs. 3B and 3F those at another moment, Figs. 3C and 3G those at still another moment, and Figs. 3D and 3H those at yet still another moment. When from the state of Figs. 3A and 3E the coil 3 and coil 4 are energized to excite the pole portions 5b, 5c, 5e, 5f of the first yoke 5 to the S pole, the portions 8a, 8b of the third yoke opposed to the portions 5b, 5c to the N pole, the pole portions 6b, 6c, 6e, 6f of the second yoke 6 to the S pole, and the portions 9a, 9b of the fourth yoke 9 opposed to the portions 6b, 6c to the N pole, the permanent magnet 1 rotates 45° counterclockwise so as to go into the state shown in Figs. 3B and 3F. Since the small-diameter portions 5e, 5f in the pole section of the first yoke 5 are more distant from the inside peripheral surface of the first magnetic layer than the portions 5b, 5c, the pole occurring in the small-diameter portions 5e, 5f little acts on the first magnetic layer and thus little affects the driving force generated. Accordingly, the first yoke 5 can function in the same manner as in the case of applicant's prior application No. EP-A-0 801 459 which comprises inner magnetic pole with a pair of separated magnetic teeth. The shape of the first yoke 5 forms no chip when produced by press working. Therefore, no consideration is necessary for emission of chip and the production is thus easy. Since the portions 5b, 5e, 5c, 5f are joined throughout the entire circumference, the strength is high and there is no possibility of deformation during assembly. The above is also the case for the second yoke 6. Since the small-diameter portions 6e, 6f in the pole section of the second yoke 6 are more distant from the inside peripheral surface of the second magnetic layer than the portions 6b, 6c, the pole occurring in the small-diameter portions 6e, 6f little acts on the second magnetic layer and thus little affects the driving force generated. Accordingly, the second yoke 6 can function in the same manner as in the case of above-mentioned application EP-A-0 801 459. The shape of the second yoke 6 forms no chip when produced by press working. Therefore, no consideration is necessary for emission of chip and the production is thus easy. Since the portions 6b, 6e, 6c, 6f are joined throughout the entire circumference, the strength is high and there is no possibility of deformation during assembly. Then the power supply to the coil 3 is inverted to excite the pole portions 5b, 5c, 5e, 5f of the first yoke 5 to the N pole, the portions 8a, 8b of the third yoke opposed to the portions 5b, 5c to the S pole, the pole portions 6b, 6c, 6e, 6f of the second yoke 6 to the S pole, and the portions 9a, 9b of the fourth yoke opposed to the portions 6b, 6c to the N pole, whereupon the permanent magnet 1 is rotated further 45° counterclockwise to go into the state shown in Figs. 3C and 3G. Then the power supply to the coil 4 is inverted to excite the pole portions 6b, 6c, 6e, 6f of the second yoke 6 to the N pole and the portions 9a, 9b of the fourth yoke opposed to the 6b, 6c to the S pole, whereupon the permanent magnet 1 is rotated further 45° counterclockwise. By successively switching the directions of power supply to the coil 3 and coil 4 in this manner, the rotor composed of the permanent magnet 1 and rotational shaft 2 rotates to the positions according to the phases of power supply. Described below is the reason why the step motor of the above structure is optimum for microminiaturization of motor. Features of the fundamental structure of the step motor are as follows. First, the magnet is formed in the hollow cylinder shape. Second, the outside peripheral surface of the magnet is divided circumferentially into n portions and the n portions are magnetized alternately in the different poles. Third, the first coil, the magnet, and the second coil are located in order in the axial direction of the magnet. Fourth, the outside pole and the inside pole of the first and second stators excited by the first and second coils are opposed to the outside peripheral surface and to the inside peripheral surface, respectively, of the magnet. Accordingly, a necessary diameter of this step motor is a size enough to oppose the poles of the stators to the diameter of the magnet; and a necessary axial length of the step motor is the sum of the length of the magnet and the lengths of the first and second coils. Therefore, the dimensions of the step motor are determined by the diameters and lengths of the magnet and coils, and the step motor can be microminiaturized by decreasing the diameters and lengths of the magnet and coils to very small values. When the diameters and lengths of the magnet and coils are made very small, it becomes difficult to maintain the output accuracy of the step motor. This problem of the output accuracy of the step motor is solved by the simple structure wherein the magnet is formed in the hollow cylinder shape and the outside pole and inside pole of the first and second stators are opposed to the outside peripheral surface and to the inside peripheral surface of the magnet formed in the hollow cylinder shape. Next described is another example in which the shape of the first and second yokes is modified. Fig. 5 and Fig. 6 are drawings to show another example of the first embodiment. Fig. 5 is a perspective view of the first yoke, in which the pole section opposed to the inside peripheral surface of the first magnetic layer is formed in the shape of an ellipse 5g (A > B where A is the major axis and B is the minor axis) in the cross section perpendicular to the axis. Fig. 6 is a lateral, cross-sectional view of the motor in which the first yoke 5 is incorporated. The first yoke 5 is located so that the major axis A is directed toward the positions opposed to the teeth 8a, 8b of the third yoke 8. Similarly, though not illustrated, the second yoke is also formed in the elliptic shape with the major axis A and the minor axis B (A > B) at least in the outside peripheral surface of the pole section, and the major axis is arranged so as to be directed toward the positions opposed to the teeth 9a, 9b of the fourth yoke. The outside peripheral surfaces of the first yoke 5 and second yoke 6 in the minor-axis direction are apart from the circumferential surfaces of the first magnetic layer and the second magnetic layer, respectively, and thus they do not negatively affect the output of motor, similar to the portions 5e, 5f, 6e, 6f in the first example. Since the pole section of the first yoke and second yoke is continuous throughout the entire circumference, the strength is high and no chip is produced during press working, thus achieving high workability. In the first embodiment described above the first to fourth yokes 5, 6, 8, 9 were separate members, but they may be formed in such structure that the first and third yokes 5, 8 are integrated into a first stator yoke and that the second and fourth yokes 6, 9 are integrated into a second stator yoke. The first magnet 1 was constructed in the double layer structure consisting of the first magnetic layer and the second magnetic layer the phases of which were shifted by 180/n degrees (n is the number of magnetic poles of the permanent magnet), i.e., 45°, but, without having to be limited to this, the present invention can also be applied to the single layer structure of a magnetic layer in the permanent magnet 1 wherein the phase of the second and fourth yokes 6, 9 is shifted by 180/n degrees, i.e., 45°, relative to that of the first and third yokes 5, 8.

Claims: A motor having a magnet (1) for forming a rotor, and a stator comprising a first pole (5, 8) and a second pole (6, 9) magnetized by a first coil (3) and a second coil (4), wherein said magnet (1) is formed in a hollow, cylindrical shape, in which at least an outside peripheral surface thereof is divided circumferentially into n portions and said n portions are magnetized in alternately different poles; said first coil (3), said magnet (1), and the second coil (4) are located in order in the axial direction of said magnet; the first pole (5, 8) excited by said first coil has a first outside pole (8a, 8b) and a first inside pole (5b, 5c), said first outside pole being opposed to the outside peripheral surface of said magnet on the side of one end thereof, said first inside pole being opposed to an inside peripheral surface of said magnet on the side of the one end thereof; the second pole (6, 9) excited by said second coil has a second outside pole (9a, 9b) and a second inside pole (6b, 6c), said second outside pole being opposed to the outside peripheral surface of said magnet on the side of the other end thereof, said second inside pole being opposed to the inside peripheral surface of said magnet on the side of the other end thereof, said first inside pole is formed as a first yoke and said second inside pole is formed as a second yoke, said first yoke has such a shape that a cross section thereof in a direction perpendicular to the axis of said magnet comprises alternately formed regions with a large outside dimension and with a small outside dimension, and said second yoke has such a shape that a cross section thereof in the direction perpendicular to the axis of said magnet comprises alternately formed regions with a large outside dimension and with a small outside dimension, characterized in that said first outside pole and said second outside pole being spaced and connected through a connecting member (10) made of non-magnetic material. A motor according to claim 1, wherein said magnet (1) is magnetized so that the inside peripheral surface thereof is divided circumferentially into n portions and said n portions are magnetized in alternately different poles that are also different from those in the outside peripheral surface adjacent thereto. A motor according to claim 1, wherein said first outside pole (8) is formed as a third yoke, said second outside pole (9) is formed as a fourth yoke, said first yoke and said third yoke form a first stator, and said second yoke and said fourth yoke form a second stator. A motor according to claim 3, wherein said first yoke or/and said second yoke are formed so that a cross section thereof in the direction perpendicular to the axis of said magnet is an elliptic shape with the major axis A and the minor axis B (A > B), wherein the major axis A is arranged so as to be located opposite to the poles of the third yoke and the fourth yoke.

Inventors: AOSHIMA CHIKARA, AOSHIMA, CHIKARA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1997
Main classification: H02K37
IPC codes: H02K37