Motor and linear moving device

Abstract: The present invention provides an ultra compact motor of a novel configuration, comprising a magnet (1) which is formed in a cylindrical shape and is circumferentially divided into n portions and alternately magnetized into different magnetic poles at least on the external periphery thereof, wherein a first coil (2), a rotor and a second coil (3) are arranged in this order along the axial direction of the rotor, and first external (18a,18b) and internal (18c,18d) magnetic poles magnetized by the first coil and so arranged as to oppose respectively to the external and internal peripheries at an end of the magnet (1) while second external (19a,19b) and internal (19c,19d) magnetic poles magnetized by the second coil are so arranged as to oppose respectively to the external and internal peripheries at the other end of the magnet (1).

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a motor of an ultra compact size and an advancing device utilizing the ultra compact motor for lens advancement. Related Background Art In the field of compact motors, there is for example known a compact cylindrical stepping motor as shown in Fig. 31. In such motor, stator coils are wound on bobbins 101, which are supported and fixed in the axial direction between two stator yokes 106. The stator yokes 106 are provided with stator teeth 106a and 106b which are alternately arranged along the internal circumference of the bobbins 101. The stator yokes 106 integral with the stator teeth 106a, 106b are fixed in cases 103 to constitute stators 102. In one of the two cases 103 there are fixed a flange 115 and a bearing 108, while another bearing 108 is fixed on the other case 103. A rotor 109 is composed of rotor magnets 111 fixed on a rotor shaft 110, and the rotor magnets 111 constitute radially extended gaps with the stator yoke 106a. The rotor shaft 110 is rotatably supported between the two bearings 108. The Japanese Patent Laid-open Application No. 3-180823 discloses the use of such compact stepping motor in driving a camera lens, by positioning an arc-shaped stepping motor alongside a phototaking lens to rotate a female screw by the output shaft of the stepping motor, thereby displacing, in the axial direction, a male screw fixed to a lens holder supporting the phototaking lens. However the conventional compact stepping motor mentioned above has been associated with a drawback of a large external dimension of the motor because the cases 103, the bobbins 101, the stator coils 105 and the stator yokes 106 are concentrically provided around the rotor. Also the output of such motor is relatively limited because the magnetic flux generated by the energization of the stator coils 105 principally passes between an end face 106a1 of the stator tooth 106a and that 106b1 of the stator tooth 106b as shown in Fig. 32 and it does not effectively applied to the rotor magnets 111. Also in the conventional lens advancing device, the arc-shaped stepping motor, positioned around the lens, occupies a large area in the circumferential direction, so that it becomes difficult to position other mechanisms, such as an actuator for driving the shutter, in a same plane. JP-A-62141955 discloses a motor comprising a cylindrical magnet implicitly divided into n alternately magnetized portions on at least the external periphery thereof; a first and a second stator coils for magnetizing respectively first internal and external magnetic stator poles and second external and internal magnetic stator poles, the first coil, the magnet and the second coil being provided in this order in the axial direction of said cylindrical magnet; wherein the first and second internal and the first and second external magnetic poles being respectively opposed to the external and internal peripheries of said magnet. JP-A-7015939 dislcoses a small light stepping motor for a camera lens with a conventional small magnet that can be magnetized minutely with multi-polarity. Therein, a first coil of a first stator, a second magnet and a second coil of a second stator are provided axially. Further, a cylindrical outer yoke covering the external periphery of the inner yokes are used. SUMMARY OF THE INVENTION It is an object of the present invention to provide an ultracompact motor having a high driving force. Further, it is an object of the present invention to provide a compact lens advancing device. These objects are solved by a motor according to claim 1 and an advancing device according to claim 10. Preferred embodiments are subject of the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an exploded perspective view of a stepping motor constituting an embodiment 1 of the present invention; Fig. 2 is a cross-sectional view, in the assembled state, of the stepping motor shown in Fig. 1; Figs. 3A, 3B, 3C, 3D, 3E, 3F, 3G and 3H are views explaining the rotating operation of the stepping motor shown in Fig. 2; Figs. 4A, 4B, 4C, 4D, 4E, 4F, 4G and 4H are views explaining the rotating operation of a stepping motor of an embodiment 2 of the present invention; Fig. 5 is an exploded perspective view of a stepping motor of an embodiment 3 of the present invention; Fig. 9 is an exploded perspective view of an advancing device for displacing the phototaking lens, employing the stepping motor of the foregoing embodiments; Fig. 16 is an exploded perspective view of a stepping motor of an embodiment 4 of the present invention; Fig. 17 is a cross-sectional view, in the assembled state, of the stepping motor shown in Fig. 16; Fig. 18A, 18B, 18C, 18D, 18E, 18F, 18G and 18H are views explaining the rotating operation of the stepping motor shown in Fig. 17; Figs. 19A, 19B, 19C, 19D, 19E, 19F, 19G and 19H are views explaining the rotating operation of a stepping motor of an embodiment 5 of the present invention; Fig. 23 is an exploded perspective view of a stepping motor of an embodiment 6 of the present invention; Fig. 24 is a cross-sectional view, in the assembled state, of the stepping motor shown in Fig. 23; Fig. 25 is a cross sectional view of a motor not part of the invention, showing the comparison with the embodiment 6; Fig. 26 is a magnified perspective view of the rotor of the stepping motor shown in Fig. 24; Figs. 27A and 27B are views showing a first state of the relationship between the first and second magnetized layers of the rotor and the first and second yokes of the stepping motor shown in Fig. 24; Figs. 28A and 28B are views showing a second state of the relationship between the first and second magnetized layers of the rotor and the first and second yokes of the stepping motor shown in Fig. 24; Figs. 29A and 29B are views showing a third state of the relationship between the first and second magnetized layers of the rotor and the first and second yokes of the stepping motor shown in Fig. 24; Figs. 30A and 30B are views showing a fourth state of the relationship between the first and second magnetized layers of the rotor and the first and second yokes of the stepping motor shown in Fig. 24; Fig. 31 is a cross-sectional view of a conventional stepping motor; and Fig. 32 is a view showing the magnetic flux in the conventional stepping motor shown in Fig. 31. DESCRIPTION OF THE PREFERRED EMBODIMENTS Now the present invention will be clarified in detail by embodiments thereof, shown in the attached drawings. [Embodiment 1] Figs. 1 to 3H illustrate the stepping motor of an embodiment 1 of the present invention, and are respectively an exploded perspective view of the stepping motor, a cross-sectional view in the axial direction in the assembled state thereof, and cross-sectional views along lines A-A and B-B in Fig. 2. Referring to Figs. 1 to 3H, a hollow cylindrical magnet 1 constituting the rotor is circumferentially divided, on the external periphery thereof, into alternately magnetized in portions (4 portions 1a, 1b, 1c, 1d in the present embodiment), in which portions 1a and 1b are magnetized as S poles and portions 1c and 1d are magnetized as N poles. An output shaft 7 constituting the shaft of the rotor is fixed to the magnet 1. The rotor is constituted by the output shaft 7 and the magnet 1. Coils 2, 3 are provided concentrically with the above-mentioned magnet 1, in positions axially sandwiching the magnet 1, and have an external diameter approximately equal to that of the magnet 1. First and second stators 18, 19 composed of a soft magnetic material, are positioned with mutual displacement of 180°/n or 45°. Each of the first and second stators 18, 19 is composed of an outer tube and an inner tube. Between the outer and inner tubes of the first stator 18 there is provided a coil 2, which, under current supply, magnetizes the first stator 18. The outer and inner tubes of the first stator 18 constitute, at an end portion thereof, outer magnetic poles 18a, 18b and inner magnetic poles 18c, 18d. The inner magnetic poles 18c, 18d are mutually displaced by 360°/(n/2) or 180° in order to assume a same phase. The outer magnetic pole 18a is provided in a position opposed to the inner magnetic pole 18c, and the outer magnetic pole 18b is provided in a position opposed to the inner magnetic pole 18d. The outer magnetic poles 18a, 18b and the inner magnetic poles 18c, 18d of the first stator 18 are provided in positions respectively opposed to the external and internal peripheries at an end portion of the magnet 1, so as to sandwich such end portion of the magnet 1. An end of the rotary shaft 7 is rotatably fitted in a hole 18e of the first stator 18. Between the outer and inner tubes of the second stator 19 there is provided a coil 3, which, under current supply, magnetizes the second stator 19. The outer and inner tubes of the second stator 19 constitute, at an end portion thereof, outer magnetic poles 19a, 19b and inner magnetic poles 19c, 19d. The inner magnetic poles 19c, 19d are mutually displaced by 360°/(n/2) or 180° in order to assume a same phase. The outer magnetic pole 19a is provided in a position opposed to the inner magnetic pole 19c, and the outer magnetic pole 19b is provided in a position opposed to the inner magnetic pole 19d. The outer magnetic poles 19a, 19b and the inner magnetic poles 19c, 19d of the second stator 19 are provided in positions respectively opposed to the external and internal peripheries at the other end portion of the magnet 1, so as to sandwich such the other end portion of the magnet 1. The other end of the rotary shaft 7 is rotatably fitted in a hole 19e of the second stator 19. Consequently the magnetic flux generated by the coil 2 crosses the magnet 1 between the outer magnetic poles 18a, 18b and the inner ones 18c, 18d, thus effectively acting on the magnet 1 constituting the rotor. Also the magnetic flux generated by the coil 3 crosses the magnet 1 between the outer magnetic poles 19a, 19b and the inner ones 19c, 19d, thus effectively acting on the magnet 1 and increasing the output of the motor. A coupling ring 20, composed of a cylindrical member of a non-magnetic material, is provided, at an end on the internal periphery, with grooves 20a, 20b, and at the other end, with grooves 20c, 20d which is displaced in phase by 45° from the grooves 20a, 20b. The outer magnetic poles 18a, 18b of the first stator 18 are fitted in the grooves 20a, 20b while the outer magnetic poles 19a, 19b of the second stator 19 are fitted in the grooves 20c, 20d and such fitted portions are fixed with an adhesive material to mount the first and second stators 18, 19 on the coupling ring 20. The first and second stators 18, 19 are so fixed on the coupling ring 20 that the ends of the outer magnetic poles 18a, 18b and the inner magnetic poles 18c, 18d are respectively opposed to those of the outer magnetic poles 19a, 19b and the inner magnetic poles 19c, 19d and that the outer magnetic poles 18a, 18b and those 19a, 19b are mutually separated by the width of internal projections 20e, 20f of the coupling ring 20. Fig. 2 is a cross-sectional view of the stepping motor, and, in Figs. 3A, 3B, 3C and 3D are cross-sectional views along a line A-A in Fig. 2, while Figs. 3E, 3F, 3G and 3H are cross-sectional views along a line B-B in Fig. 2. In Figs. 3A and 3E are cross-sectional views at a same timing, also Figs. 3B and 3F are those of a same timing, Figs. 3C and 3G are those of a same timing, and Figs. 3D and 3H are also those of a same timing. In the following there will be explained the function of the stepping motor of the present embodiment. In a state shown in Figs. 3A and 3E, the coils 2 and 3 are so energized to magnetize the outer magnetic poles 18a, 18b of the first stator 18 as N poles, the inner magnetic poles 18c, 18d as S poles, the outer magnetic poles 19a, 19b of the second stator 19 as S poles and the inner magnetic poles 19c, 19d as N poles, whereby the magnet 1 constituting the rotor rotates counterclockwise by 45° to reach a state shown in Figs. 3B and 3F. Then the current supply to the coil 2 is inverted to magnetize the outer magnetic poles 18a, 18b of the first stator 18 as S poles, the inner magnetic poles 18c, 18d as N poles, the outer magnetic poles 19a, 19b of the second stator 19 as S poles and the inner magnetic poles 19c, 19d as N poles, whereby the magnet 1 constituting the rotor further rotates counterclockwise by 45° to reach a state shown in Figs. 3C and 3G. Then the current supply to the coil 3 is inverted to magnetize the outer magnetic poles 19a, 19b of the second stator 19 as S poles, the inner magnetic poles 19c, 19d as N poles, the outer magnetic poles 18a, 18b of the first stator 18 as S poles and the inner magnetic poles 18c, 18d as N poles, whereby the magnet 1 constituting the rotor further rotates counterclockwise by 45° to reach a state shown in Figs. 3D and 3H. Thereafter the current supplies to the coils 2, 3 are switched in succession whereby the magnet 1 of the rotor rotates to positions corresponding to the phases of the current supply. In the following there will be explained why the above-explained configuration is optimum for realizing an ultra compact stepping motor. The basic configuration of the stepping motor is featured by: 1) hollow cylindrical structure of the magnet; 2) alternate magnetization of n portions, formed by dividing the external periphery of the magnet; 3) arrangement of the first coil, the magnet and the second coil in the axial direction of the magnet; and 4) outer and inner magnetic poles of the first and second stators, respectively magnetized by the first and second coils, being so positioned as to respectively oppose to the external and internal peripheries of the magnet. Consequently the present stepping motor only requires a diameter allowing to position the magnetic poles of the stators outside the diameter of the magnet, and an axial length equal to the sum of the length of the magnet and those of the first and second coils. As the dimension of the stepping motor is thus determined by the diameter and the length of the magnet and the coils, the motor can be made ultra compact by minimizing such diameter and length. Such minimization of the diameter and the length of the magnet and the coils renders it difficult to maintain the accuracy of the output of the stepping motor, but this difficulty is avoided by a simple structure of forming the magnet into a hollow cylindrical shape and so positioning the outer and inner magnetic poles of the first and second stators as to be respectively opposed to the external and internal peripheries of the magnet of such hollow cylindrical shape. The output of the motor can be made more efficient by magnetizing not only the external periphery of the magnet but also the internal periphery thereof, as in the embodiment 2 to be explained later. [Embodiment 2] Figs. 4A to 4H illustrate an embodiment 2 of the present invention. In the foregoing embodiment 1 of the present invention, the magnet 1 constituting the rotor is circumferentially divided into n portions and alternately magnetized on the external periphery thereof, but, in the present embodiment 2, the magnet 1 constituting the rotor is circumferentially divided, not only on the external periphery thereof but also on the internal periphery, into n portions (4 portions in the present embodiment) and alternately magnetized into S and N, in order to improve the output efficiency of the motor. Each portion on the internal periphery of the magnet 1 is magnetized in opposite manner to the adjacent portion on the external periphery thereof. More specifically the internal periphery portions corresponding to the magnetized portions 1a, 1c are magnetized as N poles while those corresponding to the magnetized portions 1b, 1d are magnetized as S poles. In the present embodiment 2, since not only the external periphery of the magnet 1 of the rotor but also the internal periphery thereof are circumferentially divided into n portions and alternatly magnetized as N and S poles, the output of the motor can be increased by the relationship between the internal periphery of the magnet 1 and the inner magnetic poles 18c, 18d of the first stator 18 and those 19c, 19d of the second stator 19. [Embodiment 3] Fig. 5 illustrates an embodiment 3 of the present invention. In contract to the foregoing embodiment 1 where the outer and inner tubes are integrally formed in each of the first and second stators 18, 19, the outer and inner tubes in the present embodiment 3 are separately formed in each of the first and second stators 18, 19 as shown in Fig. 5. More specifically, the inner tube of the first stator 18 constitutes, together with the inner magnetic poles 18c, 18d at the end thereof, a first yoke 18 1 , while the outer tube of the first stator 18 constitutes, together with the outer magnetic poles 18a, 18b at the end thereof, a third yoke 18 2 . Also the inner tube of the second stator 19 constitutes, together with the inner magnetic poles 19c, 19d at the end thereof, a second yoke 19 1 , while the outer tube of the second stator 19 constitutes, together with the outer magnetic poles 19a, 19b at the end thereof, a forth yoke 19 2 . Also in the embodiment 3, the magnet 1 of the rotor may be circumferentially divided into n portions and alternately magnetized in N and S not only on the external periphery but also on the internal periphery as in the embodiment 2, thereby effectively increasing the output of the motor. Fig. 9 is an exploded perspective view of an advancing device, utilizing the stepping motor of any of the foregoing embodiments, for displacing the phototaking lens. In such advancing device, the stepping motor is provided in the lens barrel and the phototaking lens is moved in the axial direction by the linearly movable tube 8, through the output shaft of the motor. In Fig. 9, there are shown a phototaking lens 9 and a lens holder 10, which supports the phototaking lens 9 in the internal periphery 10a. The linearly movable tube 8 is mounted to an ear portion 10b of the lens holder 10, so as to be integrally movable therewith in the axial direction. A guide shaft 11 is fixed to a projecting portion 10c of the lens holder 10. A guide tube 12 is fixed to the lens barrel (not shown). The guide shaft 11 is slidably fitted in the internal periphery 12a of the guide tube 12 and guides the lens holder 10 movably in the axial direction. A coil spring 13 is provided between the lens holder 10 and a base plate (not shown) of the lens barrel, exerting a biasing force to separate the lens holder 10 and the base plate of the lens barrel and thereby eliminating the play between the aforementioned output shaft 7 and the first or second yoke 4, 5 and that between the male screw portion 7c of the output shaft 7 and the female screw portion of the linearly movable tube 8. Also the lens holder 10 is prevented from rotation about the guide shaft 11 of the lens holder 10, by slidable engagement of a pin 14, provided on the base plate (not shown), with a U-shaped groove 10d of the lens holder 10. In the above-explained configuration, the rotation of the output shaft 7 moves the linearly movable tube 8 in the direction of the optical axis, so that the phototaking lens 9 is driven in the axial direction according to the amount of rotation of the output shaft 7. The stepping motor serving as the drive source for the phototaking lens 9 occupies only a relatively small portion in the circumferential direction (indicated by an arrow D) of the lens barrel, so that another driving unit for the shutter may be positioned in the same plane. Similarly, the magnetic flux generated by energization of the coil 2 passes between the portions other than the above-mentioned apertures 16g, 16h and the second yoke, corresponding to the positions between the first yoke 4 and the portions 6e, 6f of the third yoke 6 in the embodiment 4. Thus the magnet ring 1 can be rotated by switching the direction of the currents supplied to the coils 2, 3 in succession as in the embodiments 4 and 5. As the third yoke 16 of the embodiment 6 lacks the protruding portions 6a, 6b, 6e, 6f in comparison with the third yoke 6 in the embodiments 4 and 5, the external diameter of the magnet ring 1 can be made correspondingly larger and the output of the motor can thus be increased. The apertures mentioned above may also be provided in the first and second yokes instead of the third yoke. [Embodiment 4] Figs. 16 to 18H illustrate the stepping motor of an embodiment 7 of the present invention, and are respectively an exploded perspective view of the stepping motor, a longitudinal cross-sectional view of the stepping motor in the assembled state, and cross-sectional views along lines A-A and B-B in Fig. 17. Referring to Fig. 16, a hollow cylindrical permanent magnet 1 constituting the rotor is provided with a first magnetized layer consisting of magnetized portions 1a, 1b, 1c, 1d formed by circumferentially dividing the external periphery of the magnet into n portions (4 portions in this embodiment) and alternately magnetizing such portions into S and N poles, and a second magnetized layer consisting of magnetized portions 1e, 1f, 1g, 1h formed, in an axially adjacent position, by circumferentially dividing the external periphery into n portions (4 portions in the present embodiment) and magnetizing such portions alternately into S and N poles. The first and second magnetized layers are magnetized with a mutual phase displacement of 180°/n or 45°. In the first magnetized layer, the magnetized portions 1a, 1c are magnetized as S poles while those 1b, 1d are magnetized as N poles, and, in the second magnetized layer, the magnetized portions 1e, 1g are magnetized as S poles while those 1f, 1h are magnetized as N poles. A rotary shaft 7, constituting the rotor shaft, is fixed to the permanent magnet 1. The rotor is constituted by the rotary shaft and the permanent magnet. Cylindrical coils 2, 3 are positioned concentrically with and on both sides in the axial direction of the permanent magnet 1, and have an external diameter substantially equal to that of the permanent magnet 1. First and second stators 18, 19 composed of a soft magnetic material are respectively composed of an outer tube and an inner tube. Between the outer and inner tubes of the first stator 18, there is provided the coil 2, which upon energization magnetizes the first stator 18. The outer and inner tubes of the first stator 18 are formed, at an end thereof, as outer magnetic poles 18a, 18b and inner magnetic poles 18c, 18d. The inner magnetic poles 18c, 18d are mutually displaced by 360°/(n/2) or 180° so as to assume a same phase, and the outer magnetic pole 18a is provided in a position opposed to the inner magnetic pole 18c while the outer magnetic pole 18b is opposed to the inner magnetic pole 18d. The outer magnetic poles 18a, 18b and the inner magnetic poles 18c, 18d of the first stator 18 are opposed respectively to the external and internal peripheries of an end portion of the permanent magnet 1, so as to sandwich such end portion. An end portion of the rotary shaft 7 is rotatably fitted in a hole 18e of the first stator 18. Between the outer and inner tubes of the second stator 19, there is provided the coil 3, which upon energization magnetizes the second stator 19. The outer and inner tubes of the second stator 19 are formed, at an end thereof, as outer magnetic poles 19a, 19b and inner magnetic poles 19c, 19d. The inner magnetic poles 19c, 19d are mutually displaced by 360°/(n/2) or 180° so as to assume a same phase, and the outer magnetic pole 19a is provided in a position opposed to the inner magnetic pole 19c while the outer magnetic pole 19b is opposed to the inner magnetic pole 19d. The outer magnetic poles 19a, 19b and the inner magnetic poles 19c, 19d of the second stator 19 are opposed respectively to the external and internal peripheries of an end portion of the permanent magnet 1, so as to sandwich such end portion. An end portion of the rotary shaft 7 is rotatably fitted in a hole 19e of the second stator 19. A coupling ring 20, composed of a cylindrical member of a non-magnetic material, is provided, at an end on the internal periphery, with grooves 20a, 20b, and, at the other end, with grooves 20c, 20d which is displaced in phase by 45° from the grooves 20a, 20b. The outer magnetic poles 18a, 18b of the first stator 18 are fitted in the grooves 20a, 20b while the outer magnetic poles 19a, 19b of the second stator 19 are fitted in the grooves 20c, 20d and such fitted portions are fixed with an adhesive material to mount the first and second stators 18, 19 on the coupling ring 20. The first and second stators 18, 19 are so fixed on the coupling ring 20 that the ends of the outer magnetic poles 18a, 18b and the inner magnetic poles 18c, 18d are respectively opposed to those of the outer magnetic poles 19a, 19b and the inner magnetic poles 19c, 19d and that the outer magnetic poles 18a, 18b and those 19a, 19b are mutually separated by the width of internal projections 20e, 20f of the coupling ring 20. Fig. 17 is a cross-sectional view of the stepping motor, and, in Figs. 18A, 18B, 18C and 18D are cross-sectional views along a line A-A in Fig. 17, while 18E, 18F, 18G and 18H are cross-sectional views along a line B-B in Fig. 17. In Figs. 18A and 18E are cross-sectional views at a same timing, also Figs. 18B and 18F are those of a same timing, Figs. 18C and 18G are those of a same timing, and Figs. 18D and 18H are also those of a same timing. In the following there will be explained the function of the stepping motor of the present embodiment. In a state shown in Figs. 18A and 18E, the coils 2 and 3 are so energized to magnetize the outer magnetic poles 18a, 18b of the first stator 18 as N poles, the inner magnetic poles 18c, 18d as S poles, the outer magnetic poles 19a, 19b of the second stator 19 as S poles and the inner magnetic poles 19c, 19d as N poles, whereby the permanent magnet 1 rotates counterclockwise by 45° to reach a state shown in Figs. 18B and 18F. Then the current supply to the coil 2 is inverted to magnetize the outer magnetic poles 18a, 18b of the first stator 18 as S poles, the inner magnetic poles 18c, 18d as N poles, the outer magnetic poles 19a, 19b of the second stator 19 as S poles and the inner magnetic poles 19c, 19d as N poles, whereby the permanent magnet 1 further rotates counterclockwise by 45° to reach a state shown in Figs. 18C and 18G. Then the current supply to the coil 3 is inverted to magnetize the outer magnetic poles 19a, 19b of the second stator 19 as S poles, the inner magnetic poles 19c, 19d as N poles, the outer magnetic poles 18a, 18b of the first stator 18 as S poles and the inner magnetic poles 18c, 18d as N poles, whereby the permanent magnet 1 further rotates counterclockwise by 45° to reach a state shown in Figs. 18D and 18H. Thereafter the current supplies to the coils 2, 3 are switched in succession whereby the permanent magnet 1 rotates to positions corresponding to the phases of the current supply. In the following there will be explained why the above-explained configuration is optimum for realizing an ultra compact stepping motor. The basic configuration of the stepping motor is featured by: 1) hollow cylindrical structure of the permanent magnet; 2) alternate magnetization of n portions, formed by dividing the external periphery of the permanent magnet; 3) arrangement of the first coil, the permanent magnet and the second coil in the axial direction of the magnet; and 4) outer and inner magnetic poles of the first and second stators, respectively magnetized by the first and second coils, being so positioned as to respectively oppose to the external and internal peripheries of the permanent magnet. Consequently the present stepping motor only requires a diameter allowing to position the magnetic poles of the stators outside the diameter of the magnet, and an axial length equal to the sum of the length of the permanent magnet and those of the first and second coils. As the dimension of the stepping motor is thus determined by the diameter and the length of the magnet and the coils, the motor can be made ultra compact by minimizing such diameter and length. Such minimization of the diameter and the length of the magnet and the coils renders it difficult to maintain the accuracy of the output of the stepping motor, but his difficulty is avoided by a simple structure of forming the permanent magnet into a hollow cylindrical shape and so positioning the outer and inner magnetic poles of the first and second stators as to be respectively opposed to the external and internal peripheries of the permanent magnet of such hollow cylindrical shape. The output of the motor can be made more efficient by magnetizing not only the external periphery of the permanent magnet but also the internal periphery thereof, as in the embodiment 5 to be explained later. [Embodiment 5] Figs. 19A to 19H illustrate an embodiment 5 of the present invention and shows the rotating operation of the rotor of the stepping motor. In the foregoing embodiment 4 of the present invention, the permanent magnet 1 constituting the rotor is circumferentially divided into n portions and alternately magnetized in N and S poles on the external periphery thereof, but, in the present embodiment, the permanent magnet 1 is circumferentially divided, not only on the external periphery thereof but also on the internal periphery, into n portions (4 portions in the present embodiment) and alternately magnetized into S and N. Each portion on the internal periphery of the permanent magnet 1 is magnetized in opposite manner to the adjacent portion on the external periphery thereof. More specifically the internal periphery portions corresponding to the magnetized portions 1a, 1c are magnetized as N poles while those corresponding to the magnetized portions 1b, 1d are magnetized as S poles. In the present embodiment 5, since not only the external periphery of the permanent magnet 1 but also the internal periphery thereof are circumferentially divided into n portions and alternately magnetized as N and S poles, the output of the motor can be increased by the relationship between the internal periphery of the permanent magnet 1 and the inner magnetic poles 18c, 18d of the first stator 18 and those 19c, 19d of the second stator 19. [Embodiment 6] Figs. 23 and 24 illustrate the stepping motor of an embodiment 6 of the present invention, wherein Fig. 23 is an exploded perspective view of the stepping motor and Fig. 24 is a cross-sectional view thereof. In contrast to the foregoing embodiment 4 where the outer and inner tubes are integrally formed in each of the first and second stators 18, 19, the outer and inner tubes in the present embodiment 6 are separately formed in each of the first and second stators 18, 19 as shown in Figs. 23 and 24. More specifically, the inner tube of the first stator 18 constitutes, together with the inner magnetic poles 18c, 18d at the end thereof, a first yoke 18 1 , while the outer tube of the first stator 18 constitutes, together with the outer magnetic poles 18a, 18b at the end thereof, a third yoke 18 2 . Also the inner tube of the second stator 19 constitutes, together with the inner magnetic poles 19c, 19d at the end thereof, a second yoke 19 1 , while the outer tube of the second stator 19 constitutes, together with the outer magnetic poles 19a, 19b at the end thereof, a fourth yoke 19 2 . Also in the embodiment 6, the magnet 1 of the rotor may be circumferentially divided into n portions and alternately magnetized in N and S poles not only on the external periphery but also on the internal periphery as in the embodiment 4, thereby effectively increasing the output of the motor. Fig. 26 is a magnified perspective view of the rotor, and Figs. 27A, 27B, 28A, 28B, 29A, 29B, 30A and 30B are views showing rotational phases of the permanent magnet 1, wherein A indicates the first magnetized layer and B indicates the second magnetized layer. If the third yoke 6 is integrally formed as in the embodiment shown in Fig. 25, there results flows of magnetic flux between the first magnetized layer C and the second magnetized layer D of the permanent magnet 1, across the third yoke 6. Consequently a cogging force becomes stronger in the rotational positions of the permanent magnet 1 shown in Figs. 27A, 27B, 29A and 29B than in those shown in Figs. 28A, 28B, 30A and 30B. Such phase with stronger cogging appears 4 times in a rotation, at a pitch of 90°. Since the present motor achieved a full turn by 8 switchings of the current supplies to the coils 2, 3, the electromagnetic force generated by the energization of the coils 2, 3 does not necessarily coincide with the cogging force. For this reason the generated driving force shows significant fluctuation and the obtained rotation does not become smooth. In the embodiment 6, since the third yoke 18 2 and the fourth yoke 19 2 are magnetically separated by the coupling ring 20 of the non-magnetic material, there is scarcely generated the follow of magnetic flux between the first magnetized layer and the second magnetized layer across the third and fourth yokes 18 2 , 19 2 , so that the cogging is generated 8 times, namely 4 times at a pitch of 90° by the first magnetized layer and 4 times of a pitch of 90°, displaced by 45°, by the second magnetized layer. As the cogging is generated at a pitch of 45°, the generated driving force shows smaller fluctuation to provide smoother rotation of the motor. Though the foregoing embodiments have been explained by stepping motors, the present invention is not limited to such embodiments and is naturally applicable to a brushless motor by switching the current supply according to the position of the rotor for example by a hall element. As detailedly explained in the foregoing, the present invention provides a motor comprising a magnet which is formed in a cylindrical shape and is circumferentially divided into n portions and magnetized into different magnetic poles at least on the external periphery thereof, wherein a first coil, the magnet and a second coil are arranged in this order along the axial direction to the magnet, and first external and internal magnetic poles magnetized by the first coil are so arranged as to oppose respectively to the external and internal peripheries at an end of the magnet while second external and internal magnetic poles magnetized by the second coil are so arranged as to oppose respectively to the external and internal peripheries at the other end of the magnet. Thus there can be provided the motor of a novel configuration, which is different from the conventional configuration and is optimum for obtaining an ultra compact motor. Also according to the present invention, a similar effect can be obtained by providing the magnet with a magnetized layer or with two magnetized layers. More specifically, the motor with such two magnetized layers comprises a permanent magnet which is formed in a cylindrical shape and is provided with a first magnetized layer formed by circumferentially dividing at least the external periphery thereof into n portions and alternately magnetizing such portions into different magnetic poles, and with a second magnetized layer formed by circumferentially dividing at least the external periphery into n portions with a phase displacement by 180°/n with respect to the first magnetized layer and alternately magnetizing such portions into different magnetic poles, wherein a first coil, the permanent magnet and a second coil are arranged in this order along the axial direction of the permanent magnet, and first external and internal magnetic poles magnetized by the first coil are so arranged as to oppose respectively to the external and internal peripheries of the first magnetized layer of the permanent magnet while second external and internal magnetic poles magnetized by the second coil are so arranged as to oppose respectively to the external and internal peripheries of the second magnetized layer of the permanent magnet. In such motor of one-layer or two-layer structure, the diameter of the motor is determined by positioning the first and second external magnetic poles in mutually opposed manner on the external periphery of the permanent magnet, while the axial length of the motor is determined by successive positioning of the first coil, the permanent magnet and the second coil in the axial direction of the permanent magnet. Consequently the motor can be realized in an ultra compact form, and such configuration is optimum for realizing an ultra compact motor. Also an effective motor output can be obtained by forming the permanent magnet into a hollow cylindrical shape and positioning the first and second external and internal magnetic poles respectively on the external and internal peripheries of such hollow cylindrical permanent magnet. Furthermore, since the first and second stators can be composed of a same component, there can be provided a motor which allows easy assembling and is reduced in the fluctuation of the performance. Such motor, when applied to a lens advancing device of the camera, allows to provide a compact advancing device.

Claims: A motor provided with a rotor constituted by a magnet (1), and a stator constituted by a first magnetic pole (4, 6, 18) and a second magnetic pole (5, 6, 19) magnetized by a first coil (2) and a second coil (3), said magnet (1) being formed in a cylindrical shape, circumferentially divided into n portions at least on the external periphery thereof and alternately magnetized into different magnetic poles; said first coil (2), said magnet (1) and said second coil being provided in this order in the axial direction of said magnet; the first magnetic pole (4, 6, 18) magnetized by said first coil comprising a first external magnetic pole (6, 18a, 18b) and a first internal magnetic pole (4, 18c, 18d); the first external and internal magnetic poles being respectively opposed to the external and internal peripheries of an end of said magnet; the second magnetic pole (5, 6, 10) magnetized by said second coil comprising a second external magnetic pole (6, 19a, 19b) and a second internal magnetic pole (4, 19c, 19d); and the second external and internal magnetic poles being respectively opposed to the external and internal peripheries of the other end of said magnet, characterized in that the first external magnetic pole and the second external magnetic pole are formed by cutting out a cylindrical member, are magnetically separated and are connected to each other through a connecting member (20) made of a non-magnetic material. A motor according to claim 1, wherein the internal periphery of said magnet (1) is circumferentially divided into n portions and alternately magnetized into different magnetic poles, which are different from those of the adjacent portions on the external periphery. A motor according to claim 1, wherein said first external magnetic pole (6, 18a, 18b) and said first internal magnetic pole (4, 18c, 18d) are displaced by 180°/n with respect to said second external magnetic pole (6, 19a, 19b) and said second internal magnetic pole (5, 19c, 19d). A motor according to claim 1, wherein said magnet (1) is provided with a first magnetized layer (1a, 1b, 1c, 1d) divided in the circumferential direction at least on the external periphery into n portions and alternately magnetized in different magnetic poles, and a second magnetized layer (1e, 1f, 1g, 1h) divided in the circumferential direction at least on the external periphery, in an axially adjacent position, into n portions and alternately magnetized in different magnetic poles; said first external magnetic pole (6, 18a, 18b) is opposed to the external periphery of the first magnetized layer (1a, 1b, 1c, 1d) of said magnet; said first internal magnetic pole (4, 18c, 18d) is opposed to the internal periphery of the first magnetized layer (1a, 1b, 1c, 1d) of said magnet; said second external magnetic pole (6, 18c, 18d) is opposed to the external periphery of the second magnetized layer (1e, 1f, 1g, 1h) of said magnet; and said second internal magnetic pole (5, 19c, 19d) is opposed to the internal periphery of the second magnetized layer (1e, 1f, 1g, 1h) of said magnet. A motor according to claim 4, wherein the internal peripheries of the first magnetized layer (1a, 1b, 1c, 1d) and the second magnetized layer (1e, 1f, 1g, 1h) of said magnet are circumferentially divided into n portions and alternately magnetized into different magnetic poles, which are different from those of the adjacent portions on the external peripheries. A motor according to claim 4, wherein said first external magnetic pole (6, 18a, 18b) and said first internal magnetic pole (4, 18c, 18d) are provided respectively opposed to said second external magnetic pole (6, 19a, 19b) and said second internal magnetic pole (5, 19c, 19d). A motor according to claim 1 or 4, wherein said first external magnetic pole (6, 18a, 18b) and said first internal magnetic pole (4, 18c, 18d) constitute a first stator (4, 6, 18), and said second external magnetic pole (6, 19a, 19b) and said second internal magnetic pole (5, 19c, 19d) constitute a second stator (5, 6, 19). A motor according to claim 1 or 4, wherein said first coil (2) and said second coil (3) are formed with a diameter approximately equal to that of said magnet (1). A motor according to claim 1 or 4, wherein said first internal magnetic pole is formed by a first yoke (18 1 ), said second internal magnetic pole is formed by a second yoke (19 1 ), said first external magnetic pole is formed by a third yoke (18 2 ), and said second external magnetic pole is formed by a fourth yoke (19 2 ). An advancing device with motor according to any one of the preceding claims, wherein said magnet has an output shaft; and there are further provided conversion means (7d, 8d) for converting the rotary motion of the output shaft of the magnet of the motor into a linear motion; and linearly movable means (8) to be linearly driven by the linear motion from said conversion means. An advancing device with motor according to claim 10, wherein said conversion means includes first screw means (7d) provided on the output shaft of said magnet, second screw means (8b) provided on said linearly movable means (8) and engaging with said first screw means, and limiting means (4d, 8c) so provided between said linearly movable means and said first or second yoke as to enable linear motion of said linearly movable means (8) and to inhibit the rotation thereof. An advancing device with motor according to claim 10 or 11, wherein said linearly movable means is fixed to a lens holder (10) for supporting a lens.

Inventors: AOSHIMA CHIKARA, AOSHIMA, CHIKARA
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1997
Main classification: H02K37
IPC codes: H02K37, H02K7