Stepping motor

Abstract: A stepping motor comprises a rotor assembly (20) including a rotor shaft (22), a collar (23) coaxially fixed to the rotor shaft, and a cylindrical permanent magnet block (21) fixedly fitted on the collar, a yoke assembly including an inner yoke (26) having a plurality of upper pole pieces (27a) arrnaged at regular angular intervals corresponding to an electrical angle of 2π and a plurality of lower pole pieces (27b) arranged at regular angular intervals corresponding to an electrical angle of 2π, an upper outer yoke (24) having pole pieces (27) formed so as to extend between the upper pole pieces (27a) of the inner yoke, and a lower outer yoke (25) having a plurality of pole pieces (27) formed so as to extend between the lower pole pieces (27b) of the inner yoke, first and second driving coils (28, 29) received in a space defined by the inner yoke (26) and the upper outer yoke (24) and in a space defined by the inner yoke (26) and the lower outer yoke (25), respectively, a base plate (30) fixed to the upper end surface of the upper outer yoke, a flange (31) fixed to the lower end surface of the lower outer yoke, and bearing bushes (32, 33) respectively provided in the central portions of the base plate and the flange to support the rotor shaft therein for rotation. The inner yoke (26) is formed by bending the inner periphery of a single annular plate alternately in opposite directions to form the upper pole pieces and the lower pole pieces. The cylindrical permanent magnet block (21) is divided axially into an upper pole section and a lower pole section each having a plurality of magnetic poles arranged along the circumference thereof so that the phase difference between the magnetic poles of the upper pole section and those of the lower pole section corresponds to an electrical angle of π/2.

Description: The present invention relates to a stepping motor for printers or the like and, more particularly, to a PM stepping motor. Stepping motors, as is generally known, are commonly used as a driving source for moving the print head of a printer or as a driving source for feeding a printing sheet on a printer. Referring to Figure 5 of the accompanying drawings, there is shown a PM stepping motor for such purposes. The motor comprises a rotor shaft 2, a rotor 1 consisting of a collar 3 coaxially fixed to the rotor shaft 2, and a permanent magnet block 1a having magnetic poles arranged in divisions along the circumference thereof and fixedly fitted on the collar 3, inner yokes 5 and 6 and outer yokes 4 and 7. The inner periphery of each of the yokes 4, 5, 6, and 7 is bent inside to form n/4 pieces of pole pieces 8 (n is the total number of pole pieces of the yokes 4, 5, 6 and 7). The total number n of pole pieces 8 is the number of steps for one turn of the permanent magnet block 1a. Further shown in Figure 5 are a first driving coil 9, a second driving coil 10, a base plate 11 fixed to the outer yoke 4 by spot-welding or the like, a flange 12 fixed to the outer yoke 7 by spot-welding or the like, bushes 13 and 14 fitted in the respective central portions of the base plate 11 and the flange 12, respectively, for rotatably supporting the rotor shaft 2 therein, and a shim 15 mounted on the rotor shaft 2 to take up the axial play of the rotor shaft 2. As shown in Figure 6, the permanent magnet block 1a has magnetic poles formed about the periphery thereof in circumferential divisions. As shown in Figure 7, the inner yokes 5 and 6 are combined with positioning lugs (not shown) or positioning holes (not shown) formed in the respective inner circumference thereof coinciding with each other and are fixed together by spot-welding or the like so that the phase difference between the pole pieces 8 of the inner yoke 5 and the pole pieces 8 of the inner yoke 6 is an electrical angle of π/2 as shown in Figure 8. JP-A-6176059 describes a stepping motor having a pair of coils and an intermediate plate on both surfaces of which are provided pole elements. Each pole element is formed with a plurality of pole teeth. Likewise, pole teeth are formed on the outer circumference of each of the rotor magnets. These pole teeth are essential to the operation of the motor, but their presence makes the structure more complicated. Recent progressive reduction in cost, compaction and improvement in reliability of printers require the compaction, reduction in thickness and improvement in performance of stepping motors to be incorporated into printers. The performance of a PM stepping motor can readily be improved by employing a rare earth magnet instead of the conventional ferrite magnet. However, the employment of a rare earth magnet increases the cost of the motor. The performance of a PM motor can also be improved by driving the PM stepping motor by the bipolar driving system instead of the unipolar driving system. However, the employment of the bipolar driving system increases the number of electrical parts, such as transistors, of the driving circuit entailing an increase in the cost of the PM stepping motor. In DE3514895 a motor is described in which the rotor magnet is surrounded by rotor rims formed in a single strip out of which has been punched channels defining pole pieces. The pole pieces are arranged so that the inner pole pieces are at a phase angle corresponding to an electrical angle of 2π. This arrangement of the rotor rims is described as overcoming the problem of misalignment of pole pieces and is acknowledged as having the disadvantage of incurring undesirable magnetic losses since the pole pieces are connected by webs. In the technical report Stepping Motors-Construction, Operation and Application No. 211 of Berger Lahr GmbH a conventional motor having two separate stators connected together is described in which the motor has two separate permanent magnet rings which are offset to an electrical angle of π/2. The present invention seeks to provide a stepping motor which eliminates the foregoing problems of the conventional stepping motor and has an increased torque capacity attained by incorporating yokes of a new construction enabling an increase in the number of coil turns, and a permanent magnet rotor having a novel pole arrangement. The invention also seeks to provide a stepping motor comprising fewer parts, requiring less work for assembly and capable of being manufactured at a lower cost as compared with the conventional stepping motors. The present invention provides a stepping motor as defined in claim 1. Since the stator yoke assembly accommodating the two driving coils is formed of a single inner yoke and two outer yokes, spaces for accommodating the driving coils are increased to enable the number of turns of the driving coils to be increased, so that the torque capacity of the stepping motor is increased. Furthermore, since the stepping motor of the present invention has only a single inner yoke, the stepping motor of the present invention requires less parts and less work for assembly as compared with the conventional stepping motor having two inner yokes. In order that the invention may be better understood, an embodiment thereof will now be described by way of example only and with reference to the accompanying drawings in which:- Figure 1 is a longitudinal sectional view of a stepping motor, in a preferred embodiment, according to the present invention; Figure 2 is a perspective view of a permanent magnet rotor employed in the stepping motor of Figure 1, showing a mode of magnetisation; Figure 3 is a fragmentary enlarged perspective view of an inner yoke employed in the stepping motor of Figure 1; Figure 4 is a diagrammatic illustration showing the positional relation between the magnetic poles of the permanent magnet rotor and the pole pieces of the yoke assembly of the stepping motor of Figure 1; Figure 5 is a longitudinal sectional view of a conventional stepping motor; Figure 6 is a perspective view of a permanent magnet rotor employed in the stepping motor of Figure 5; Figure 7 is a fragmentary enlarged perspective view of a conventional inner yoke; and Figure 8 is a diagrammatic illustration showing the positional relation between the magnetic poles of the permanent magnet rotor and the pole pieces of the yoke assembly of the stepping motor of Figure 5. A stepping motor, in a preferred embodiment, according to the present invention will be described hereinafter with reference to Figures 1 to 4. Referring first to Figure 1, the stepping motor comprises a permanent magnet rotor 20 consisting of a rotor shaft 22, a collar 23 coaxially fixed to the rotor shaft 22, and a permanent magnet block 21 axially divided into two pole sections, namely, an upper pole section 21a and a lower pole section 21b, each having a plurality of magnetic poles arranged along the circumference thereof so that the phase difference between the magnetic poles of the upper pole section 21a and those of the lower pole section 21b is an electrical angle of π/2, and fixedly fitted on the collard 23, a pair of outer yokes 24 and 25 disposed one over the other each having n/4 pieces of pole pieces 27 formed along the inner circumference thereof, an inner yoke 26 having an alternate arrangement of n/4 pieces of upper pole pieces 27a and n/4 pieces of lower pole pieces 27b as shown in Figure 3 arranged so that the phase difference between the upper pole pieces 27a and the lower pole pieces 27b correspond to an electrical angle of 2π, a first driving coil 28 received in a space defined by the inner yoke 26 and the upper outer yoke 24, a second driving coil 29 received in a space defined by the inner yoke 26 and the lower outer yoke 25, a base plate 30 fixed to the upper surface of the upper outer yoke 24 by spot-welding or the like, a flange 31 fixed to the lower surface of the lower outer yoke 25 by spot-welding or the like, bearing bushes 32 and 33 respectively provided in the central portions of the base plate 30 and the flange 31 to support the rotor shaft 22 therein for rotation, and a shim 34 mounted on the rotor shaft 22 to take up the axial play of the rotor 20. It will be seen that the phase difference between the upper pole pieces 27a of the inner yoke 26 and the pole pieces 27 of the upper outer yoke 24 corresponds to an electrical angle of π, and the phase difference between the lower pole pieces 27b of the inner yoke 26 and the pole pieces 27 of the lower outer yoke 25 corresponds to an electrical angle of π. Theoretically, such an arrangement of pole pieces 27a, 27b and 27 is unable to drive a rotor having a conventional construction. Accordingly, the permanent magnet block 21 is divided axially into two pole sections and each pole section is provided with a plurality of magnetic poles arranged along the circumference thereof so that the phase difference between the magnetic poles of the upper pole section and those of the lower pole section corresponds to an electrical angle of π/2 as shown in Figure 4 to enable the first driving coil 28 and the second driving coil 20 to drive the rotor 20. Thus, the present invention reduces the component parts of a stepping motor and work for assembling the stepping motor to further reduce the manufacturing cost of the stepping motor by employing a single integral inner yoke instead of a conventional yoke which is formed by combining two inner yoke pieces together by spot-welding or the like. Furthermore, since the employment of such an integral inner yoke increases the spaces for receiving the first and second driving coils, the number of turns of the first and second driving coils can be increased to increase the torque capacity and performance of the stepping motor.

Claims: A stepping motor comprising: a rotor assembly (20) comprising a rotor shaft (22); a collar (23) coaxially fixed to the rotor shaft (22); a cylindrical permanent magnet block (21) fixedly fitted on the collar; a yoke assembly formed by fixedly combining an inner yoke (26), an upper outer yoke (24) and a lower outer yoke (25), the inner yoke (26) having n/4 (where n is the number of steps for one turn of the rotor assembly) pieces of upper pole pieces (27a) arranged at regular angular intervals around the inner circumference thereof at a phase difference corresponding to an electrical angle of 2π, and n/4 pieces of lower pole pieces (27b) arranged at regular angular intervals around the inner circumference thereof at a phase difference corresponding to an electrical angle of 2π, the upper outer yoke (24) having n/4 pieces of pole pieces (27) arranged at regular angular intervals around the inner circumference thereof at a phase difference corresponding to an electrical angle of 2π so as to extend between the upper pole pieces of the inner yoke, respectively, and the lower yoke (25) having n/4 pieces of pole pieces (27) arranged at regular angular intervals around the inner circumference thereof at a phase difference corresponding to an electrical angle of 2π so as to extend between the lower pole pieces (27b) of the inner yoke (26); a first driving coil (28) accommodated in a space defined by the inner yoke (26) and the upper outer yoke (24); a second driving coil (29) accommodated in a space defined by the inner yoke (26) and the lower outer yoke (25); a base plate (30) fixed to the upper surface of the upper outer yoke (24); a flange (31) fixed to the lower surface of the lower outer yoke (25); bearing means (32, 33) respectively fixedly provided in the respective central portions of the base plate (30) and the flange (31) to support the rotor shaft (22) therein for rotation; and a shim (34) mounted on the rotor shaft (22) between said collar (23) and either one of said bearing means (32, 33) to take up the axial play of said rotor assembly (20), characterised in that the phase difference between the upper pole pieces (27a) and lower pole pieces (27b) of the inner yoke (26) corresponds to an electrical angle of π and the inner yoke (26) is integral with the upper and lower pole pieces (27a, 27b), and that the cylindrical permanent magnet block (21) is divided axially into an upper pole section and a lower pole section each having different magnetic poles alternately arranged around the circumference thereof so that the phase difference between the magnetic poles of the upper pole section and those of the lower pole section corresponds to an electrical angle of π/2.

Inventors: CHIBA NAOKI, HAYASHI KUNIHARU C O OKI ELECT, KIKUCHI HIROSHI C O OKI ELECTR, MIZUTANI MINORU C O OKI ELECTR, CHIBA, NAOKI, HAYASHI, KUNIHARU C/O OKI ELECTRIC, KIKUCHI, HIROSHI C/O OKI ELECTRIC, MIZUTANI, MINORU C/O OKI ELECTRIC
Applicants: OKI ELECTRIC IND CO LTD, OKI ELECTRIC INDUSTRY COMPANY, LIMITED
Application year: 1988
Main classification: H02K37
IPC codes: H02K37, H02K1