Stepping motor

Abstract: To provide a stepping motor having high precision torque formation and stepping angle at low production cost, cylindrical elastic magnetic members [6(a) through 6(e)], which cover yokes (1a-1, 1a-2, 1b-1 and 1b-2) and have an inner diameter smaller than the outer diameter of the yokes, readily come in tight contact with the yokes (1a-1, 1a-2, 1b-1 and 1b-2) and form strong magnetic circuits. The yokes (1a-1 and 1a-2) and the yokes (1b-1 and 1b-2) are integrally formed into a bobbin (1) through a nonmagnetic member 1c. Pole teeth (11a-1 and 11a-2) of the yokes (1a-1 and 1a-2) and pole teeth (11b-1 and 11b-2) of the yokes (1b-1 and 1b-2) are arranged by shifting at a given angle to each other in the peripheral direction.

Description: The invention relates to stepping motors used as actuators of various control devices, and in particular, a stepping motor suitable for smoothly driving an indicator of a speed meter or the like. Stepping motors are widely used in various industrial fields. Particularly, they have been increasingly applied to rotational driving devices which require delicate movement. Fig. 6 is an assembly view illustrating a configuration of a conventional stepping motor. This motor comprizes two cylindrical bobbins 31a and 31b and coils 32 wound on their peripheries. These bobbins 31a and 31b, on which coils 32 are wound, are clamped by circular outer yokes 33a and 33b and circular inner yokes 34a and 34b. On the inner peripheries of the outer yokes 33a and 33b and the inner yokes 34a and 34b, pole theeth 36a, 36b and 36c and 36d are protrusively provided, and these outer yokes 33a and 33b and these inner yokes 34a and 34b are placed parallel with a predetermined shift along the periphery so that the pole teeth 36a and 36b and the pole teeth 36c and 36d engage each other at a central space section 37. The pole teeth 36a and 36b and the pole teeth 36c and 36d form opposite magnetic poles, respectively, when the coils 32 are energized. The first yokes 33a and 34a and the second yokes 33b and 34b are stacked up through a non-permeable spacer (not shown in the drawing) so that the inner yokes 34a and 34b face each other with a given space therebetween, and a rotor 39 having a shaft 38 to transmit a rotational driving force is inserted in the central space section 37 of the first yokes 33a and 34a and the second yokes 33b and 34b. Multiple alternative polarization is performed on the outer periphery of the rotor 39, and the rotor 39 rotates by means of magnetic attraction force between the pole teeth 36a, 36b, 36c and 36d. The stepping motor is assembled by rotatably supporting an end of the shaft 38 of the rotor 39 on a bearing stand 40a in the center of a plate 40, by covering a permeable cup frame 41 on the other end of the shaft 38, and rotatably supporting the other end of the shaft 38 with a bearing stand 41a provided in the cup frame 41. The bobbins 31a and 31b have pole teeth inside their respective central cavities. In the stepping motor set forth in Fig. 6, the pole teeth of the bobbin 31a and the pole teeth of the bobbin 31b must be shifted at a given angle to each other when assembling them. In such a configuration, however, magnetic balance of the bobbins 31a and 31b cannot be readily achieved during assembly, resulting in stepping angle errors of the shaft 38. Since the cup frame 41 is formed by press working, its production cost is high and the cap tends to have a tapered shape. Therefore, the inside face of the cylindrical section comes in insufficient contact with the yokes 31a and 31b to achieve a precise torque generation and stepping angle. Further, an additional assembly step, such as cramping or welding, is required for contact of the cup frame with the yokes. In order to rotate the rotor 39, a current flow is conducted to the coils 32 to energize the outer yokes 33a and 33b and the inner yokes 34a and 34b and to generate magnetic attraction force between the pole teeth 36a, 36b, 36c and 36d. Thus, the outer yoke 33a and the inner yoke 34a, and the outer yoke 33b and the inner yoke 34b must be magnetically coupled to form respective magnetic circuits. The magnetic coupling has been conventionally achieved by a permeable cup frame 41. In this case, since the magnetic coupling is simultaneously performed between the first yokes 33a and 34b and second yokes 33b and 34b, a magnetic circuit of the first yokes 33a and 34a and a magnetic circuit of the second yokes 33b and 34b will lead each other through the cup frame to generate magnetic interference depending on the magnetizing patterns of the coils 32. In detail, as set forth in Fig. 7(a), when the polarized teeth 36b and 36c have the same polarity, they are repellent to each other and thus no magnetic interference occurs. When the pole teeth 36b and 36c have opposite polarities due to change in the magnetizing pattern as set forth in Fig. 7(b), a magnetic field from the pole teeth 36b to the pole teeth 36c through the cup frame 41 is generated and magnetic interference occurs between the first and second yokes. Since the magnetic interference depends on the change in magnetizing pattern, irregular rotation occurs due to the change. When using this stepping motor for driving an indicator of a speed meter, the indicator does not move smoothly. An idea for solving such problems involves two upper and lower cup frames 41 with each cup frame independently covering the outer peripheries of the first yokes 33a and 34a and the second yokes 33b and 34b. In this case, two expensive cup frames must be prepared. The document GB-A-2245107 discloses a stepping motor comprising a rotor, having a column magnet, a bobbin with first and second yokes and first and second coils and a cylindrical magnetic member having elasticity, covering the bobbin and pressing the first and second yokes along their radial directions from the outside. Likewise document US-A-4695419 discloses a stepping motor having a cylindrical magnetic member covering the bobbin and pressing the first and second yokes radially. The present invention is achieved in view of such a background, and it is object of the invention to provide a stepping motor having a high precision torque formation and stepping angle, capable of low cost production. The present invention provides a stepping motor which does not have an irregular magnetic intensity of the magnetic circuit formed by each yoke, has a high precision torque generation and stepping angle by effectively preventing magnetic interference between the first yokes and the second yokes, and can be produced at low cost. In accordance with the invention the stepping motor comprises a rotor having a column magnet and a shaft attached along an axis of the column magnet; a bobbin comprising first yokes and second yokes, and first and second coils wound on these yokes, the bobbin covering the rotor; and an elastic plate magnetic member covering the bobbin, the plate magnetic member being formed into a cylinder having an inner diameter smaller than the outer diameter of the first and second yokes. In accordance with the present invention, a connecting section which magnetically connects one yoke with the other yoke is narrowed (the area decreases) due to holes formed in the cylindrical magnetic member, thus the magnetic flux flowing from one yoke to the other yoke is decreased and magnetic interference between the first and second yokes can be suppressed. In accordance with one aspect of the invention, the cylindrical magnetic member is divided into a first cylindrical magnetic member to cover the periphery of the first yoke and a second cylindrical magnetic member to cover the periphery of the second yoke, and these cylindrical magnetic members are placed with a given gap therebetween. Therefore, one yoke is not magnetically connected with the other yoke, and magnetic interference between both yokes of the bobbin can be suppressed. In accordance with a second aspect of the invention, the cylindrical magnetic member is provided with holes positioned along a given band between the inner yokes to decrease the side surface area of a connecting section between the first yoke and the second yoke. Embodiments of the present invention will now be described. The stepping motor exemplified in these embodiments is to be used for a speed meter. In the drawings Figs. 1(a) through 1(e) are assembly views illustrating stepping motor configurations in accordance with embodiments of the present invention, Fig. 2 is a schematic representation of pole teeth of the two yokes of the stepping motor of the present invention in an enlarged scale, Fig. 3 is a cross-sectional view illustrating cross-sections AA' and BB' in Fig. 1(a), Fig. 4 is an isometric view illustrating an appearance of the stepping motor set forth in Fig. 1 or Fig. 3 after assembly, Figs. 5(a) and 5(b) are isometric views illustrating stepping motors in accordance with other embodiments of the present invention, Fig. 6 is an assembly view of a conventional stepping motor, and Figs. 7(a) and 7(b) are schematic representations of pole teeth of the yokes of conventional stepping motors in an enlarged scale. In Figs. 1 and 3, a cylindrical bobbin 1 is composed of a cylindrical magnetic yoke 1a-1 having vertically protruding pole teeth 11a-1, a cylindrical magnetic yoke la-2 having vertically protruding pole teeth 11a-2, a cylindrical magnetic yoke 1b-1 having vertically protruding pole teeth 11b-1 and a cylindrical magnetic yoke 1b-2 having vertically protruding pole teeth 11b-2, and these are stacked with a nonmagnetic spacer 1c. These yokes 1a-1, 1a-2, 1b-1 and 1b-2 have the same diameter and are coaxially stacked. A coil 2a is wound on the bobbin 1 between the yokes 1a-1 and la-2, and a coil 2b is wound on the bobbin 1 between the yokes 1b-1 and 1b-2. In Fig. 2, the pole teeth 11a-1 and 11a-2 and the pole teeth 11b-1 and 11b-2 protrude so as to face each other, and are arranged while shifting the pole teeth 11a-2 and llb-2 by a given angle α along the circumferential direction of the yokes 1a-1, 1a-2, 1b-1 and 1b-2. The angle α is determined by the magnetizing interval of the magnet 3. The pole teeth 11a-1 and 11b-1 and pole teeth 11a-2 and 11b-2 are also arranged while shifting the pole teeth 11b-1 and 11b-2 by a given angle β along the circumferential direction of the yokes 1a-1, 1a-2, 1b-1 and 1b-2. The bobbin 1 has a hole bearing 1f near the center of the bottom cover 1e. The magnet 3 is a column, and is magnetized at a given interval along the circumferential direction (not shown in the drawings). A shaft 4 is provided to transmit the rotational force of the magnet 3, and is coaxially assembled with the magnet 3. The rotor consists of these components. A cover 5 has a flange 5a and a hole bearing 5b near the center. Elastic cylindrical magnetic members 6a and 6b press the bobbin 1 except a terminal 1d in the radial direction from outside. The cylindrical magnetic members 6a and 6b formed of an elastic plate magnetic member so as to have a diameter smaller than the outer diameter of the yokes 1a-1, 1a-2, 1b-1, and 1b-2. When assembling the stepping motor set forth in Figs. 1 and 3, one end 4a of the shaft 4 is inserted into the hole bearing if to rotatably support the magnet 3 in the cavity of the bobbin 1. Also, the other end 4b of the shaft 4 is inserted into the hole bearing 5b to rotatably support the magnet 3. The opening of the bobbin 1 is closed with a cover 5. Then, the peripheries of the yokes 1a-1, 1a-2, 1b-1, and 1b-2 are covered with the elastic cylindrical magnetic members 6a and 6b which presses the yokes along the radial direction. A blade spring 7 is provided in the cavity in the bobbin center to suppress shaking of the shaft 4 which occurs after the rotor comprising the magnet 3 and the shaft 4 is assembled. The magnet 3 is pressed toward the bottom 1e of the bobbin 1 by means of the blade spring 7. Since the cylindrical magnetic members 6a and 6b have a diameter smaller than the outer diameter of the yokes 1a-1, 1a-2, 1b-1 and 1b-2, it comes in tight contact with the peripheries of these yokes. Strong magnetic circuits are generated between the cylindrical magnetic members 6a and 6b and the yokes 1a-1, 1a-2, 1b-1 and lb-2. Therefore, precise torque formation and step angle can be achieved. Further, the cylindrical magnetic member 6b can form magnetic circuits just above and below the terminal 1d because they have a cutout 8 to pass a terminal of the bobbin through. Figs. 5(a) and 5(b) are isometric views illustrating the appearance of stepping motors in accordance with other embodiments of the present invention. A stepping motor in accordance with the present invention may have a configuration in which the stepping motor has a terminal 1d-1 to supply magnetizing current to the coil 2a and a terminal ld-2 to supply magnetizing current to the coil 2b, with the terminal pins protruding from the bottom, as set forth in Fig. 5(a). Alternatively, terminal 1d may be provided in the center along the axis, as set forth in Fig. 4, or at a position on the bottom so that the terminal pins protrude from the bottom, as set forth in Fig. 5(b). A stepping motor having a terminal set forth in Fig. 5(a) or 5(b) can be directly attached to a printed circuit board. Therefore, the stepping motor can be readily assembled without wiring and with a decreased number of fixing parts and assembly steps. The cylindrical magnetic members 6c and 6d cover the periphery of the bobbin 1, and comes in contact with the yokes la-1, 1a-2, 1b-1 and 1b-2 to form independent magnetic circuits between the first yokes 1a-1 and 1a-2, and between the second yokes 1b-1 and 1b-2. The cylindrical magnetic members 6c and 6d are provided with holes 12 near the center along the axis, that is, at a position, in which the inner yokes 1a-2 and 1b-1 face each other through a spacer 1c, so as to decrease the cross-sectional area of the connecting section 3 of the first yokes 1a-1 and 1a-2 with the second yokes 1b-1 and 1b-2 after assembling the stepping motor. The holes 12 can decrease the amount of magnetic flux from the first yokes 1a-1 and 1a-2 to the respective second yokes 1b-1 and 1b-2, even if the pole teeth 11a-2 and 11b-1 are polarized with different polarities, due to change in the polarization pattern and a magnetic field is generated from pole teeth 11a-2 to the pole teeth 11b-1 through the cylindrical magnetic members 6c and 6d. Therefore, magnetic insulation between the first yokes and the second yokes can be enhanced, resulting in suppression of magnetic interference. Accordingly, irregular rotation due to magnetic interference caused by change in the magnetizing pattern is prevented. Further, the cylindrical magnetic member 6d can form magnetic circuits just above and below the terminal because they have a cutout 8 to pass a terminal 1d of the bobbin 1 through. The cylindrical magnetic member 6e covers the periphery of the bobbin 1 so that a given space is provided in the center along the axis, that is, at a position in which the inner yokes la-2 and 1b-1 face each other through the spacer 1c. Since the yokes 1a-1 and 1a-2 are not magnetically connected to the yokes 1b-1 and 1b-2, no magnetic interference occurs when the pole teeth 11a-2 of the yoke 1a-2 and the pole teeth 11b-1 of the yoke 1b-1 are polarized with opposite polarities due to change in the polarizing pattern. Accordingly, irregular rotation due to magnetic interference caused by change in the magnetizing pattern is prevented. As set forth above, the present invention has the following advantages: a) An elastic plate magnetic member, which covers the first and the second yokes and is formed in a cylindrical shape having an inner diameter smaller than the outer diameter of the first and second yokes, readily comes in tight contact with the first and second yokes to form a strong magnetic circuit. The first yokes and the second yokes are integrally formed into a bobbin and a first pole teeth array of a first yoke and a second pole teeth array of a second yoke are arranged so as to have a given angle shift in the circumferential direction. Therefore, a stepping motor having high precision torque formation and stepping angle can be achieved at low production cost. b) The cylindrical magnetic member is proved with a cutout to pass a terminal of the bobbin through. Therefore, the cylindrical magnetic member can form magnetic circuits just above and below the terminal, and a stepping motor having high precision torque formation and stepping angle can be achieved. c) The connecting section which magnetically leads between the first yokes and the second yokes is narrowed since the cylindrical magnetic member is provided with holes to decrease the side surface area of the connecting section. Therefore, magnetic flux flow from one side of the yoke to the other side of the yoke is decreased, even if two facing inner yokes have opposite polarities due to change in the magnetizing pattern, thus magnetic interference between the first and second yokes can be suppressed, and irregular rotation can be reduced d) The magnetic member covering the yoke periphery is divided into two magnetic members, i.e., a first magnetic member covering the peripheries of the first yokes and a second magnetic member covering the peripheries of the second yokes, and these magnetic members are provided with a given space therebetween. Therefore, the first yokes and the second yokes are not magnetically connected to each other, magnetic interference between the first and second yokes can be suppressed, and irregular rotation can be reduced.

Claims: Stepping motor comprising a rotor, having a column magnet (3) and a shaft (4) attached along an axis of the column magnet; a bobbin (1) comprising first yokes (1a-1 and 1a-2), and second yokes (1b-1 and 1b-2), and first and second coils (2a and 2b) wound on these yokes, said bobbin covering said rotor; and a cylindrical magnetic member 6(a) having elasticity covering the bobbin and pressing the first and second yokes along their radial directions from the outside. characterised in that the cylindrical magnetic member is a magnetic member [6(c)] provided with holes (12) positioned along a given band between the inner yokes to decrease the side surface area of a connecting section between the first yoke (1a) and the second yoke (2a). Stepping motor comprising a rotor, having a column magnet (3) and a shaft (4) attached along an axis of the column magnet; a bobbin (1) comprising first yokes (1a-1 and la-2), and second yokes (1b-1 and lb-2), and first and second coils (2a and 2b) wound on these yokes, said bobbin covering said rotor; and a cylindrical magnetic member 6(a) having elasticity covering the bobbin and pressing the first and second yokes along their radial directions from the outside. charakterised in that the cylindrical magnetic member is a magnetic member [6(e)] divided into a first cylindrical magnetic member to cover the periphery of the first yoke (1a) and a second cylindrical magnetic member to cover the periphery of the second yoke (2a) with a given gap therebetween. Stepping motor according to claim 1 or 2, wherin the cylindrical magnetig member is a magnetic member [6(b)] having a cutout (8) to pass a terminal (1d) of the bobbin (1) through. Stepping motor according to one of the claims 1 to 3, wherin the cylindrical magnetic member is a magnetic member 6(d) having a cutout (8) to pass a terminal (1d) of the bobbin (1) through.

Inventors: HANAZUMI HIROSHI, YOSHIDA KAZUMASA, HANAZUMI, HIROSHI, YOSHIDA, KAZUMASA, Hanazumi, Hiroshi, c/o Mitsubishi Materials Corp., Yoshida, Kazumasa, c/o Mitsubishi Materials Corp.
Applicants: MITSUBISHI MATERIALS CORP, MITSUBISHI MATERIALS CORPORATION
Application year: 1997
Main classification: H02K1
IPC codes: H02K1, H02K37