Low-profile stepping motor with two coils arranged flush with each other horizontally

1. Field of the Invention

The present invention relates to a stepping motor, and specifically to a low-profile stepping motor having its dimension reduced in an axial direction.

2. Description of the Related Art

Recently, in accordance with enhanced performance of various devices such as printers, facsimiles and floppy (Registered Trademark) disk drives, a stepping motor is required to be higher in torque, and lower in cogging torque and vibration. Under the circumstance, a PM (permanent magnet) stepping motor such as is disclosed in Japanese Patent Publication No. Hei 10-127024 has been extensively used to meet the requirements.

FIG. 1 is a partly cutaway perspective view of a conventional PM stepping motor. As shown in FIG. 1, the PM stepping motor generally comprises: a stator assembly 6 composed of two stator units 6 A and 6 B; a rotor assembly 13 ; a face plate 1 punched out of a stainless steel plate; and a front bearing 2 made of oil impregnated alloy. The stator units 6 A and 6 B each comprise: stator yokes 3 a and 3 b each punched out of a soft magnetic steel plate and each having a plurality of pole teeth 10 bent up along its inner circumference; and a coil 4 formed such that a copper wire with polyurethane insulation is wound on a bobbin 5 housed in the stator yokes 3 a and 3 b , and are attached back-to-back to each other. The rotor assembly 13 comprises: a rotary shaft 8 ; a cylindrical magnet 9 for magnetic field, having a plurality of magnetic poles at its circumferential surface, and having the circumferential surface opposing the pole teeth 10 of the stator yokes 3 a , 3 b ; and a sleeve 12 to fasten the cylindrical magnet 9 and the rotary shaft 8 together such that the rotary shaft 8 is press-fitted into the sleeve 12 , and the sleeve 12 with adhesive applied thereon is inserted into the cylindrical magnet 9 . The rotor assembly 13 thus structured has its outer circumference (of the magnet 9 ) magnetized for a predetermined number of magnetic poles, and is rotatably disposed inside the stator assembly 6 such that the rotary shaft 8 is supported by the front bearing 2 and a rear bearing (not shown in FIG. 1 ), and that the magnetic poles oppose the pole teeth 10 with a slight gap therebetween.

The stator unit 6 A is structured such that the pole teeth 10 of the stator yoke 3 a mesh with the pole teeth 10 of the stator yoke 3 b equidistantly from each other with a shift by an electrical angle of 180 degrees (half pitch). The stator unit 6 B is of the same structure as the stator unit 6 A. The stator units 6 A and 6 B thus structured are joined back-to-back to each other by molding with polymeric material or by plasma welding such that their respective pole teeth 10 and 10 are shifted from each other by an electrical angle of 90 degrees, whereby the rotor assembly 13 always starts its rotation in the same direction.

Recently, since the devices incorporating PM stepping motors are required to be downsized and lower profiled, the PM stepping motors are also required to be further lower profiled. The conventional PM stepping motor, however, is composed of two stator units joined to each other back-to-back thus making it extremely difficult or almost impossible to further reduce its profile dimension. And the conventional PM stepping motor uses four discrete stator yokes, which inevitably increases the number of the components and makes alignment troublesome.

The present invention has been made in view of the above circumstance, and it is an object of the present invention to provide a low-profile stepping motor which is of a simple structure and which has its dimension drastically reduced in the axial direction.

In order to achieve the object, according to a first aspect of the present invention, a low-profile stepping motor comprises: a stator assembly composed of first and second stator units; a rotor assembly; and a spacer block. The first stator unit includes: a first upper stator yoke having a plurality of pole teeth which are formed along its semicircular inner circumference; a first lower stator yoke having a plurality of pole teeth which are formed along its semicircular inner circumference and which mesh with the pole teeth of the first upper stator yoke; and a first coil unit fixedly sandwiched between the first upper and lower stator yokes. The second stator unit includes: a second upper stator yoke formed as one piece integrally with the first upper stator yoke, and having a plurality of pole teeth which are formed along its semicircular inner circumference; a second lower stator yoke formed as one piece integrally with the first lower stator yoke, and having a plurality of pole teeth which are formed along its semicircular inner circumference and which mesh with the pole teeth of the second upper stator yoke; and a second coil unit fixedly sandwiched between the second upper and lower stator yokes and arranged horizontally flush with the first coil unit. The rotor assembly is rotatably disposed in a circular open space defined by the pole teeth of the first and second stator units. And the spacer block is disposed in the stator assembly and has openings shaped to surround the first and second coil units, and the pole teeth. With the structure described above, while the stepping motor can be significantly reduced in thickness, the entire assembly can be made rigid even if the stator yokes are made of a thin steel plate for downsizing, resin molding for a gap space between the intermeshing pole teeth can be performed using only a molding die set inside the pole teeth, and magnet wires of the coil units are prevented from breaking.

According to a second aspect of the present invention, in the low-profile stepping motor of the first aspect, the spacer block is of a single piece structure. Consequently, the spacer block keeps readily and surely the upper and lower stator yokes equidistant from and in parallel to each other.

According to a third aspect of the present invention, in the low-profile stepping motor of the first or second aspect, the spacer block is formed of resin. Consequently, an insulating spacer block can be produced easily.

According to a fourth aspect of the present invention, in the low-profile stepping motor of any one of the first to third aspects, the spacer block has, on its side, a plurality of hooks for securely retaining lead wires leading out from the first and second coil units. Consequently, the lead wires are prevented from accidentally getting unhooked, thereby providing high reliability.

According to a fifth aspect of the present invention, in the low-profile stepping motor of the fourth aspect, the hooks are shaped like an L-letter in cross-section with their shorter bar sections joined to the side of the spacer block and are disposed in a line such that the shorter bar sections are positioned alternately at the opposite ends to one another. Consequently, the lead wires are surely prevented from getting unhooked, thereby ensuring high reliability.

FIG. 1 is a partly cutaway view of a conventional PM stepping motor;

FIG. 2 is an exploded perspective view of a low-profile stepping motor according to an embodiment of the present invention;

FIG. 3 is a cross sectional view of the low-profile stepping motor of FIG. 1; and

FIG. 4 is a side view of the low-profile stepping motor of FIG. 2 .

Preferred embodiments of the present invention will hereinafter be described with reference to the accompanying drawings.

Referring to FIG. 2, a low-profile stepping motor of the present invention generally comprises: a stator assembly 30 including first and second stator units 30 A and 30 B assembled to be horizontally flush with each other; a rotor assembly 50 rotatably disposed inside the stator assembly 30 ; and a spacer block 70 set in the stator assembly 30 .

The first stator unit 30 A comprises: first upper and lower stator yokes 31 a and 34 a ; and a first coil unit 40 A which is composed of a bobbin 41 a and a winding 42 a provided around the bobbin 41 a , is sandwiched between the first upper and lower stator yokes 31 a and 34 a , and which is provided with terminal pins 43 a 1 and 43 a 2 . The first upper stator yoke 31 a is punched out of a soft magnetic steel plate, includes a plurality of pole teeth 32 a 1 to 32 an punched out of the soft magnetic steel plate and bent up inward (downward in FIG. 2) at its semicircular inner circumference, and has a hole 33 a for engaging with a top protrusion 44 a of the first coil unit 40 A. The first lower stator yoke 34 a is punched out of a soft magnetic steel plate, includes a plurality of pole teeth 35 a 1 to 35 an punched out of the soft magnetic steel plate and bent up inward (upward in FIG. 2) at its semicircular inner circumference, and has a hole 36 a for engaging with a bottom protrusion 44 a (see FIG. 3) of the first coil unit 40 A.

The second stator unit 30 B comprises: second upper and lower stator yokes 31 b and 34 b which are punched out as one piece integrally with the first upper and lower stator yokes 31 a and 34 a , respectively; and a second coil unit 40 B which is composed of a bobbin 41 b and a winding 42 b provided around the bobbin 41 b , is sandwiched between the second upper and lower stator yokes 31 b and 34 b , and which is provided with terminal pins 43 b 1 and 43 b 2 . The second upper stator yoke 31 b is punched out of a soft magnetic steel plate, includes a plurality of pole teeth 32 b 1 to 32 bn punched out of the soft magnetic steel plate and bent up inward (downward in FIG. 2) at its semicircular inner circumference, and has a hole 33 b for engaging with a top protrusion 44 b of the second coil unit 40 B. The second lower stator yoke 34 b is punched out of a soft magnetic steel plate, includes a plurality of pole teeth 35 b 1 to 35 bn punched out of the soft magnetic steel plate and bent up inward (upward in FIG. 2) at its semicircular inner circumference, and has a hole 36 b for engaging with a bottom protrusion 44 b (see FIG. 3) of the second coil unit 40 B.

The plurality of pole teeth 32 a 1 to 32 an of the first upper stator yoke 31 a and the plurality of pole teeth 32 b 1 to 32 bn of the second upper stator yoke 31 b are punched out simultaneously, and the plurality of pole teeth 35 a 1 to 35 an of the first lower stator yoke 34 a and the plurality of pole teeth 35 b 1 to 35 bn of the second lower stator yoke 34 b are punched out simultaneously.

Each stator yoke has pole teeth provided in number corresponding to the required number of steps per rotation of a stepping motor and arranged equidistant from one another. The pole teeth 32 a 1 to 32 an of the first upper stator yoke 31 a and the pole teeth 35 a 1 to 35 an of the first lower stator yoke 34 a mesh with each other with a shift by an electrical angle of 180 degrees, and the pole teeth 32 b 1 to 32 bn of the second upper stator yoke 31 b and the pole teeth 35 b 1 to 35 bn of the second lower stator yoke 34 b mesh with each other with a shift by an electrical angle of 180 degrees.

In each of the upper and lower stator yokes of the first and second stator units 30 A and 30 B thus structured, two pole teeth located adjacent to each other sandwiching one of a pair of slot cuts 37 or 38 (detailed hereinafter) are shifted from each other by an electrical angle of 90 degrees or 270 degrees. Specifically, in the first and second upper stator yokes 31 a and 31 b , the pole tooth 32 a 1 is shifted by an electrical angle of 90 degrees or 270 degrees from the pole tooth 32 b 1 located adjacent to the pole tooth 32 a 1 sandwiching one slot cut 37 , and the pole tooth 32 an is shifted by an electrical angle of 90 degrees or 270 degrees from the pole tooth 32 bn located adjacent to the pole tooth 32 an sandwiching the other slot cut 37 , and in the first and second lower stator yokes 34 a and 34 b , the pole tooth 35 a 1 is shifted by an electrical angle of 90 degrees or 270 degrees from the pole tooth 35 b 1 located adjacent to the 35 a 1 sandwiching one slot cut 38 , and the pole tooth 35 an is shifted by an electrical angle of 90 degrees or 270 degrees from the pole tooth 35 bn located adjacent to the pole tooth 35 an sandwiching the other slot cut 38 .

The first and second upper stator yokes 31 a and 31 b are punched out as one piece with the pair of slot cuts 37 above mentioned being formed right midway between the first and second upper stator yokes 31 a and 31 b thereby minimizing magnetic interflow therebetween so as not to have magnetic influence on each other, and the first and second lower stator yokes 34 a and 34 b are punched out as one piece with the pair of slot cuts 38 above mentioned being formed right midway between the first and second lower stator yokes 34 a and 34 b thereby minimizing magnetic interflow therebetween so as not to have magnetic influence on each other. The first and second upper stator yokes 31 a and 31 b , and the first and second lower stator yokes 34 a and 34 b are formed of a soft magnetic steel plate, such as silicon steel plate, SECC (galvanized steel plate) and SUY (electromagnetic soft iron).

The rotor assembly 50 comprises: a cylindrical magnet 51 formed of ferrite magnet, rare-earth magnet and so forth, and magnetized at regular intervals; a rotary shaft 55 made of non-magnetic stainless steel; and a sleeve 52 made of aluminum, and is structured such that the cylindrical magnet 51 is adhesively fixed to the sleeve 52 which has the rotary shaft 55 press-fitted thereinto. The rotor assembly 50 is rotatably disposed inside the stator assembly 30 with a slight gap between the cylindrical magnet 51 and the pole teeth of the stator assembly 30 , and always starts rotating in the same direction by means of the first and second stator units 30 A and 30 B.

The spacer block 70 has openings 71 a , 71 b and 72 shaped to surround and lodge respectively the first coil unit 40 A, the second coil unit 40 B, and the pole teeth 32 a 1 to 32 an , 32 b 1 to 32 bn , 35 a 1 to 35 an , and 35 b 1 to 35 bn . The opening 71 a is continuous with a cut 73 a for letting out the terminal pins 43 a 1 and 43 a 2 therethrough, and the opening 71 b is continuous with a cut 73 b for letting out the terminal pins 43 b 1 and 43 b 2 therethrough. The spacer block 70 has hooks 74 , which retain securely lead wires leading out from the terminal pins 43 a 1 , 43 a 2 , 43 b 1 and 43 b 2 , provided at a side having the cuts 73 and 74 b . The spacer block 70 is formed of resin such as PBT (polybutylene terephthalate), which is highly insulative and excellent in heat resistance.

Referring now to FIG. 3, the first coil unit 40 A includes a cylindrical stud 45 a which is formed of metal, has the aforementioned top and bottom protrusions 44 a , 44 a , and which has the aforementioned winding 42 a therearound, and the second coil unit 40 B includes a cylindrical stud 45 b which is formed of metal, has the aforementioned top and bottom protrusions 44 b , 44 b , and which has the aforementioned winding 42 b therearound.

The first and second coil units 40 A and 40 B are lodged respectively in the openings 71 a and 71 b (see FIG. 2) of the spacer block 70 , and the pole teeth 32 a 1 to 32 an , 32 b 1 to 32 bn , 35 a 1 to 35 an , and 35 b 1 to 35 bn , which are represented by 32 a , 32 b (not shown), 35 a (not shown), and 35 b , respectively, are lodged in the opening 72 (see FIG. 2) of the spacer block 70 . The first and second upper stator yokes 31 a and 31 b and the first and second lower stator yokes 34 a and 34 b are duly positioned and fixedly put together such that the holes 33 a and 33 b of the first and second upper stator yokes 31 a and 31 b engage with the top protrusions 44 a and 44 b of the cylindrical studs 45 a and 45 b , and the holes 36 a and 36 b of the first and second lower stator yokes 34 a and 34 b engage with the bottom protrusions 44 a and 44 b of the cylindrical studs 45 a and 45 b . The engagement may be reinforced by bonding or welding as required.

The spacer block 70 lodges the first and second coil units 40 A and 40 B and the pole teeth 32 a , 32 b , 35 a and 35 b , and are sandwiched firmly between the first and second upper stator yokes 31 a and 31 b and the first and second lower stator yokes 34 a and 34 b , whereby the entire assembly can be kept rigid and the upper and lower stator yokes can be kept equidistant from each other in parallel. Also, the problem that the wires of the coil units 40 A and 40 B, when having a small diameter, can be broken from damages incurred during and after assembly can be prevented by the spacer block 70 lodging the coil units 40 a and 40 B.

The cylindrical studs 45 a , together with the first upper and lower stator yokes 31 a and 34 a , forms a part of a closed magnetic path, and the cylindrical studs 45 b , together with the second upper and lower stator yokes 31 b and 34 b , forms a part of a closed magnetic path. Thanks to the cylindrical studs 45 a and 45 b , the first and second coil units 40 A and 40 B can be attached easily, and the first and second upper stator yokes 31 a and 31 b and the first and second lower stator yokes 34 a and 34 b can be set in position easily and surely.

Resin is filled in the gap space between the intermeshing pole teeth thereby forming a resin portion 63 for reinforcement. For the process of filling resin in the gap space, molding dies are required previously both outside and inside the pole teeth 32 a , 32 b , 35 a and 35 b , but now with the spacer block 70 covering the outside of the pole teeth 32 a , 32 b , 35 a and 35 b the outside die is not required. In this process, a resin plate 62 is formed simultaneously. A bearing 54 is attached to the resin plate 62 , and the rotary shaft 55 of the rotor assembly is let through the bearing 54 .

And, a plate 61 punched out of a non-magnetic steel plate and having a bearing 53 attached thereto is fixed to the first and second lower stator yokes 34 a and 34 b by welding, bonding, crimping and the like. The rotor assembly is rotatably supported by the bearings 53 and 54 .

Referring to FIG. 4, hooks 74 a 1 , 74 a 2 , 74 b 1 and 74 b 2 for securing the lead wires leading out from the coil units 40 A and 40 B are formed in a line at one side of the spacer block 70 . A connection circuit board 76 is disposed at the cut 73 a shown in FIG. 2 . The terminal pins 43 a 1 and 43 a 2 are soldered to patterns 76 a 1 and 76 a 2 , respectively, formed on the connection circuit board 76 , and the lead wires 75 a 1 and 75 a 2 are also soldered to the patterns 761 and 76 a 2 , respectively, whereby the terminal pins 43 a 1 and 43 a 2 are electrically connected to the lead wires 75 a 1 and 75 a 2 , respectively. In the same way, the terminal pins 43 b 1 and 43 b 2 shown in FIG. 2 are electrically connected to the lead wires 75 b 1 and 75 b 2 , respectively.

The hooks 74 a 1 , 74 a 2 , 74 b 1 and 74 b 2 are square in plan view, shaped like an L-letter in cross section, and each provided at the side of the spacer block 70 such that its shorter bar section is joined to the side and that its longer bar section is oriented substantially parallel to the side, and the lead wires are secured between the side of the spacer block 70 and respective longer bar sections of the hooks 74 . The hooks 74 a 1 and 74 a 2 are formed in a line such that their respective shorter bar sections which are joined to the side of the spacer block 70 are positioned on the opposite ends to each other. The hooks 74 b 1 and 74 b 2 are provided in the same way as the hooks 74 a 1 and 74 a 2 .

The lead wires 75 a 1 and 75 a 2 are securely retained by the hooks 74 a 1 and 74 ab , and the lead wires 75 b 1 and 75 b 2 are securely retained by the hooks 74 b 1 and 74 b 2 . Since the hooks 74 a 1 and 74 a 2 are formed such that their shorter bar sections joined to the side of the spacer block 70 are positioned at the opposite ends to each other, the lead wires 75 a 1 and 75 a 2 are prevented from accidentally getting unhooked. The hooks 75 b 1 and 74 b 2 are formed in the same way thus preventing the lead wires 75 b 1 and 75 b 2 from accidentally getting unhooked.

The plate 61 is attached to the lower side of the low-profile stepping motor in the embodiment discussed above, but may alternatively be attached to the upper side thereof.

While the present invention has been illustrated and explained with respect to specific embodiment thereof, it is to be understood that the present invention is by no means limited thereto but encompasses all changes and modifications which will become possible within the scope of the appended claims.