Stepping motor and method for driving the same

Abstract: In a stepping motor having two stator units (50,60) axially arranged, each of the stator units comprising an inner yoke (23,24) and an outer yoke (22,25) having a plurality of pole teeth circumferentially formed at a predetermined pitch around a rotor (18) having magnetic poles (19), the outer yoke of one of the stator units is asymmetrically arranged so that the pole teeth thereof are shifted from center between the pole teeth of the inner yoke in a specific direction. As a result, a cogging torque is reduced, constant speed rotation is attained and variation of an output torque is eliminated. Variation of generated torque is also reduced by controlling a drive pulse width, drive voltage or drive current during the drive in accordance with the number of excitation phases.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a structure of a stepping motor and a method for driving a stepping motor by switching the number of excitation phases between one and two. Related Background Art In a stepping motor, a method for driving the stepping motor by switching the number of excitation phases by a drive circuit is adopted. In a prior art method for driving the stepping motor by switching the number of excitation phases, a generated torque varies with the number of excitation phases so that vibration and noise are generated and ununiform rotation is caused. In a structure of a prior art stepping motor, plural phases (normally two phases) of stator unit comprising a yoke and a coil are axially arranged around a rotor having magnetic poles. In such a stepping motor, it is common to axially arrange two stator units each comprising an inner yoke and an outer yoke having a plurality of magnetic teeth circumferentially formed at a constant pitch around a rotor having magnetic poles, and a coil mounted between the yokes. In the prior art stepping motor, the inner and outer yokes of each of the stator units are constructed such that a circumferential spacing between the magnetic teeth thereof is uniform. In such a structure of the prior art stepping motor, however, a variation torque or cogging torque for each step angle increases by an affect of magnetic interference from other adjacent stator unit. This impedes a constant speed rotation. Further, because of a large cogging torque, an output torque is apt to vary. GB-A-2 211 030 discloses a stepping motor having this general structure and proposes adjustment of the phase deviation between the two complete stator units to reduce cogging torque. SUMMARY OF THE INVENTION In the light of the above, it is an object of the present invention to improve a rotation precision by eliminating a variation of torque. It is another object of the present invention to reduce ununiformity in rotational speed by controlling a drive pulse width in accordance with the number of excitation phases of a stepping motor. It is other object of the present invention to attain constant speed rotation by reducing a cogging torque. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a longitudinal sectional view of a stepping motor in accordance with one embodiment of the present invention, Fig. 2 shows a partial developed view of an arrangement of magnetic teeth of yokes of the stepping motor of Fig. 1, Fig. 3 shows measurements of a shift angle of an outer yoke and a cogging torque, Fig. 4 shows a partial developed view of another arrangement of magnetic poles of the yokes, Fig. 5 shows a drive circuit of a stepping motor of Fig. 1, Fig. 6 shows waveforms of drive voltages applied to excitation coils by the drive circuit of Fig. 5, Fig. 7 shows another drive circuit, Fig. 8 shows waveforms of voltages applied to excitation coils when the stepping motor shown in Fig. 7 is half-step driven, Fig. 9 shows a flow chart of a sequence of control process when a method for driving the motor shown in Fig. 7 is implemented. DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention is now explained with reference to the drawings. Fig. 1 shows a structure of a stepping motor. In Fig. 1, a motor case 10 comprises a drawn part of end-open drum shape. An opening 11 is formed at a center of a wall end plane of the motor case 10, and a bearing 15 is secured to the opening by press fitting. A mount plate 12 is secured to an open end plane of the motor case 10 by calking 13, and another bearing 16 is secured by press fitting to an opening 14 formed in the mount plate 12 at a center of shaft of a motor. The mount plate 12 also serves as an end plate to close the open end plane of the motor case 10. A motor shaft 17 is rotatably supported by the two bearings 15 and 16. A rotor 18 is secured by press fitting to the motor shaft 17 within the motor case 10, and a rotor magnet 19 circumferentially magnetized to N and S poles at a predetermined pitch is formed on an outer periphery of the rotor 18. Two stator units 50 and 60 having magnetic teeth facing the rotor magnet 19 are axially arranged in the motor interior formed by the motor case 10 and the mount plate 12. The stator units 50 and 60 have excitation coils 20 and 21 housed between inner and outer yokes having the magnetic teeth to form excitation phases of the motor. Thus, the motor shown has two excitation phases. The excitation coil 20 and the inner and outer yokes 22 and 23 form the first phase stator unit 50, and the excitation coil 21 and the inner and outer yokes 24 and 25 form the second phase stator unit 60. In the illustrated embodiment, the first phase outer yoke 22 is secured to the end plane of the motor case 10, and the first phase inner yoke 23 and the second phase inner yoke 24 are joined back to back, and the second phase outer yoke 25 is secured to the mount plate 12. The first stator unit 50 is constructed by mounting the first excitation coil 20 wound on a bobbin 26 into the first inner and outer yokes 22 and 23, and the second phase stator unit 60 is constructed by mounting the second excitation coil 21 wound on a bobbin 27 into the second inner and outer yokes 24 and 25. A plurality of magnetic teeth 22A, 23A, 24A and 25A circumferentially formed at a predetermined pitch are provided on the yokes 22, 23, 24 and 25, respectively to face the rotor magnet 19 with a predetermined gap. Rotational force output means 30 such as a pulley is secured to an output projection of the motor shaft 17. The bobbins 26 and 27 are made of an insulating material such as plastic, and portions 26A and 27A thereof extend from a notch 31 formed on a side plane of the motor case 10 slightly beyond the surface of the motor case. Bobbin terminals (or motor terminals) 28 and 29 on which winding ends of the excitation coils 20 and 21 are wound are secured by embedding to the projection. In case of unifilar winding, two bobbin terminals are provided for each of the bobbin terminals 28 and 29, and in case of bifilar winding, three are provided. The winding ends of the excitation coils 20 and 21 are wound on roots of the bobbin terminals 28 and 29 and secured by soldering. Leading ends of the bobbin terminals 28 and 29 are secured by soldering in an electrical connection manner to a circuit board 35 for connection to a motor control unit. Fig. 2 shows a partial developed view of an arrangement of pole teeth 22A ∿ 25A of the yokes 22 ∿ 25. In Fig. 2, a plurality of (for example, twelve) pole teeth 22A ∿ 25A are formed in the yokes 22 ∿ 25 at a constant circumferential pitch (for example, 30 degrees in center angle). In assembling the motor, the first phase inner and outer yokes 22 and 23 are assembled such that the pole teeth 22A and 23A thereof are arranged in a zig-zag pattern between the pole teeth, and the second phase inner and outer yokes 24 and 25 are assembled such that the pole teeth 24A and 25A thereof are arranged in a zig-zag pattern between the pole teeth. Thus, in the first phase stator unit 50, the pole teeth 22A and 23A of the inner and outer yokes 22 and 23 are at the centers of the spacings of the pole teeth so that they are symmetric, and in the second phase stator unit 60, the pole teeth 25A of the outer yoke 25 are shifted from the centers of the spacings of the pole teeth 24 of the inner yoke as viewed from the inner yoke 24 so that they are asymmetric. A shift angle of the pole teeth 25A of the outer yoke 25 is set to approximately 30′ ∿ 1°30′. In Fig. 2, the following relations are met; a = b ≠ c ≠ d d > a = b > c g > e = f > h where a and b are intervals between the center of the pole teeth 22A of the outer yoke 22 in the first phase and the pole teeth 23A of the inner yoke 23 on the opposite sides of the center, c and d are intervals between the center of the pole teeth 24A of the inner yoke 24 in the second phase and the pole teeth 25A of the outer yoke 25 on the opposite sides of the center, e is the interval of the pole teeth of the first phase outer yoke 22 and the second phase inner yoke 24, f is the interval of the pole teeth of the second phase inner yoke 24 and the first phase inner yoke 23, g is the interval of the pole teeth of the first phase inner yoke 23 and the second phase outer yoke 25, and h is the interval of the pole teeth of the second phase outer yoke 25 and the first phase outer yoke 22. As described above, in the stepping motor having two stator units axially arranged, each of the stator units comprising the inner yoke and the outer yoke having a plurality of pole teeth formed circumferentially at the predetermined pitch around the rotor 18 having magnetic poles 19, and the coil mounted between the yokes, the outer yoke 25 of one of the stator units 60 has the pole teeth 25A of the outer yoke arranged asymmetrically so that they are shifted clockwise from the centers between the pole teeth 24A of the inner yoke 24 as viewed from the inner yoke. In the above arrangement, the shift angle of the pole teeth 25A of the outer yoke 25 is preferably approximately 30′ ∿ 1°30′. In the present embodiment, the outer yoke 25 of the second phase stator unit 60 is arranged asymmetrically. Alternatively, the outer yoke 22 of the first phase stator unit 50 may be asymmetrically arranged so that the pole teeth 22A of the outer yoke 22 are shifted clockwise from the center between the pole teeth 23A of the inner yoke 23 as viewed from the inner yoke 23. Fig. 3 shows a graph of measurement of a cogging torque when the second phase outer yoke 25 is shifted from the symmetric position (shift angle = 0) by a predetermined angle in a specific direction (clockwise as viewed from the inner yoke). In Fig. 3, an abscissa represents the shift angle of the outer yoke 25, an ordinate represents the cogging torque (g-cm), a curve A represents the cogging torque when the motor is driven clockwise, and a curve B represents the cogging torque when the motor is driven counterclockwise. As seen from the graph, by arranging the outer yoke of one of the stator units shifted to the asymmetric position, the cogging torque is reduced compared to that of the symmetric arrangement (shift angle = 0). Particularly when the shift angle is approximately 30′ ∿ 1°30′, the cogging torque can be significantly reduced. In the present embodiment, in the stepping motor having the two stator units 50 and 60 axially arranged, the outer yoke 25 or 22 of one of the stator units is asymmetrically arranged so that the pole teeth 25A or 22A of the outer yoke are shifted clockwise from the center between the pole teeth 24A or 23A of the inner yoke 24 or 23 as viewed from the inner yoke. Accordingly, the magnetic interference from the other stator unit which would be caused when the inner and outer units of the two stator units are symmetrically arranged in the prior art motor is mechanically compensated so that magnetically symmetric stator units are provided. Thus, a stepping motor having a small cogging torque and a smooth constant speed rotation is attained. In the present embodiment, the outer yoke 25 or 22 of one of the stator units is shifted clockwise as viewed from the inner yoke 24 or 23. Depending on the status of the magnetic interference between the stator units 50 and 60, it may be asymmetrically shifted in the opposite direction to that in the present embodiment. Namely, depending on a specific magnetic interference of the stepping motor, one of the outer yoke is shifted counterclockwise by a predetermined angle as viewed from the inner yoke so that the cogging torque may be reduced compared to that in the symmetric arrangement and a variation of an output torque can be eliminated (or reduced). Fig. 4 shows a partial developed view of an arrangement of pole teeth when the outer yoke 25 is arranged asymmetrically so that the pole teeth 25A thereof are shifted counter clockwise from the center between the pole teeth 24A of the inner yoke 24 as viewed from the inner yoke. Fig. 4 differs from Fig. 1 only in that the pole teeth 25A are shifted from the pole teeth 24A in the opposite direction, and other construction is identical. Accordingly, the corresponding elements in Fig. 4 to those in Fig. 1 are designated by the like numerals and the detailed explanation thereof is omitted. In the embodiment of Fig. 4, like that of Fig. 1, a stator motor which can reduce the cogging torque and eliminate (or reduce) the variation of the output torque is provided. In accordance with the present invention, in the stepping motor having the two stator units axially arranged, each of the stator units comprising the inner yoke and the outer yoke having a plurality of pole teeth formed circumferentially at the predetermined pitch around the rotor having the magnetic poles, and the coil mounted between the yokes, the outer yoke of one of the stator units is asymmetrically arranged with respect to the corresponding inner yoke so that the pole teeth of the outer yoke are shifted in the specific direction from the center between the pole teeth of the inner yoke. Accordingly, the stepping motor which can reduce the cogging torque, permit the constant speed rotation and eliminate the variation of the output torque is provided. Fig. 5 shows a drive circuit of the stepping motor shown in Fig. 1. In Fig. 5, PWM (pulse width modulation) generator 71 generates pulse width signals of two duty factors, which are set to 30 % and 40 %, as will be explained later. The pulse width signal from the PWM generator 71 is supplied to four gates 76, 77, 78 and 79 through parallel gates 72 and 73 and a serial gate 74. The gates 72 and 73 switch the PWM value for one-phase excitation and the PWM for two-phase excitation by a pattern signal from a drive pattern generator 75. In the present embodiment, the PWM generator 71 uses a pulse source of a carrier frequency of 20 KHz and it generates two pulse signals, one having a 30 % duty factor for driving the motor by a pulse width equal to 30 % of a period (a reciprocal of a frequency) and the other having a 40 % duty factor for driving the motor by a pulse width equal to 40 %. In the one-phase excitation mode, the PWM value is switched to the 40 % duty factor, and in the two-phase excitation mode, the PWM value is switched to the 30 % duty factor by the gates 72 and 73. The pattern signal from the drive pattern generator 75 is also supplied to second input terminals of the four gates 76 ∿ 79. The drive pulse signal of the frequency switched in accordance with the number of excitation phases is supplied to a motor driver 80 from the four gates 76 ∿ 79. A motor such as a stepping motor to be driven is constructed such that a rotor 83 on a motor shaft is rotated when two coils 81 and 82 which form two excitation phases are energized. When the coils 81 and 82 are to be energized in the one-phase excitation mode, the driver 80 drives by the 40 % duty factor pulse, and when they are to be energized in the two-phase excitation mode, it drives by the 30 % duty factor pulse. Fig. 6 shows waveforms of drive voltages applied to the excitation coils 81 and 82 when the motor is driven in the 1-2 phase excitation or half-step excitation mode. In Fig. 8, the 40 % duty factor pulse is applied in the one-phase excitation mode in which one of the coils 81 and 82 is energized, and the 30 % duty factor pulse is applied in the two-phase excitation mode in which both coils 81 and 82 are energized. A torque generated by the motor is proportional to a time integration of an applied voltage (or current), and when a motor such as a stepping motor is driven by a constant voltage, the generated torque (combined torque) in the two-phase excitation mode is 2 times as high as that in the one-phase excitation mode. Accordingly, assuming that a torque T₁ generated in the one-phase excitation mode is 0.4 x K₁ZI, a torque T₂ generated in the two-phase excitation mode is equal to 0.3 x 2 x K₁ZI. Since 0.3 x 2 = 0.4, the generated torques T₁ and T₂ in the respective modes are substantially equal and the torques generated by the motor can be generally unified. In the present embodiment, the ununiformity in the rotation of the motor and the vibration and the noise can be effectively reduced. The motor noise reduction of approximately 2 ∿ 3 dB was attained. Another drive circuit which is suitable to implement the method for driving the motor of the present invention is shown in Fig. 7. In the drive circuit of Fig. 7, the drive pattern generator 75 and the PWM generator 1 of Fig. 5 have been substituted by one CPU 84. In Fig. 7, the duty factor of the pulse width for driving the motor is changed by software (control circuit) in the CPU 84. The CPU 84 has the control circuit which switches the pulse width for driving the motor between two duty factors, that is, 30 % duty factor and 40 % duty factor in accordance with the number of excitation phases. The CPU 84 also has a control circuit for the drive pattern of the motor. The CPU 84 selects the 40 % duty factor for the PWM to be applied in the one-phase excitation mode, and the 30 % duty factor for the PWM to be applied in the two-phase excitation mode, in accordance with the pattern signal from the drive pattern control circuit. The PWM value from the CPU 84 is supplied to four gates 86, 87, 88 and 89, and outputs of those four gates are supplied to a motor driver 85. A motor such as a stepping motor is constructed such that a rotor 92 on a motor shaft is rotated when coils 90 and 91 of the two excitation phases are energized. Thus, when the coils 90 and 91 are to be energized in one-phase excitation mode, the driver 85 drives by the 40 % duty factor pulse width, and when they are to be energized in the two-phase excitation mode, it drives by the 30 % duty factor pulse width. Accordingly, like in the previous embodiment, when the coils 90 and 91 are driven by the voltage waveform shown in Fig. 6, the torque T₁ generated in the one-phase excitation mode is equal to 0.4 x K₁ZI, and the torque T₂ generated in the two-phase excitation mode is equal to 0.3 x 2 x K₁ZI. Accordingly, the torques generated in the respective modes are unified. Accordingly, like the drive circuit of Fig. 5, the drive circuit of Fig. 7 can unify the torques generated by the motor so that the method for driving the motor which can reduce the ununiformity in the rotation and reduce the vibration and noise is attained. In the present embodiment, the value of the PWM is changed for each mode to unify the generated torques. Alternatively, the same unification of the generated torques can be attained by changing the drive voltage or the drive current for each mode and the same effect as that of the present embodiment is attained. Fig. 8 shows waveforms of drive voltage (or current) applied to the excitation coils when the motor is driven in the 1-2 phase or half-step excitation mode. In Fig. 8, where a voltage of 40 volts is applied in the one-phase excitation mode in which one of the coils is energized, a voltage of 30 volts is applied in the two-phase excitation mode in which both coils are energized. The torque generated by the motor is proportional to a time integration of an applied voltage (or current). When a motor such as a stepping motor is driven by a constant voltage, the torque (combined torque) generated in the two-phase excitation mode is 2 times as high as that in the one-phase excitation mode. Thus, assuming that the torque T₁ generated in the one-phase excitation mode is 0.4 x K₁ZI, the torque T₂ generated in the two-phase excitation mode is equal to 0.3 x 2 x K₁ZI. Thus, the torques T₁ and T₂ generated in the respective modes are substantially equal. Accordingly, like in the previous embodiment, the torques generated by the motor are substantially unified and the ununiformity in the rotation is eliminated and the vibration and noise are reduced. Fig. 9 shows a flow chart of a sequence of control in the implementation of the method for driving the motor of the present invention by controlling the PWM value in the half-step excitation mode or 1-2 phase excitation mode. In Fig. 9, when the drive of the motor is started, the control system is initialized in a step S1. In a step S2, an encoder for detecting a rotational angle of the motor is initialized. In a step S3, the direction and the speed of the rotation of the motor are set. In a step S4, whether a stop of drive command has been issued or not is checked. If the stop of drive command has been issued, the process proceeds to a step S5 to determine whether the drive is to be terminated or not. If the drive is not to be terminated, the process returns to the step S3 to repeat the above steps. If the drive is to be terminated in the step S5, the process proceeds to a step S6 to terminate the control process. If the stop of drive command has not been issued in the step S4, the counting by the encoder is started in a step S7. In a step S8, the 1-2 phase excitation drive pattern and the drive pulse PWM pattern are outputted. As the drive is started, whether the motor rotation is out of synchronization or not is determined in a step S9. If it is out of synchronization, the process proceeds to a step S10 to reset the count of the encoder. In a step S11, the PWM pattern is changed. Then, the process returns to the step S7 to repeat the subsequent steps. If the out-of-synchronization is not detected in the step S9, the process proceeds to a step S12 to determine whether the motor drive mode has been changed or not. If it has been changed, the process returns to the step S3 to repeat the above steps. If the drive mode has not been changed in the step S12, the process proceeds to a step S13 to determine whether the motor has been rotated by a predetermined number of steps. If it has not yet been so rotated, the process returns to the step S7 to repeat the subsequent steps. If it has been so rotated, the process proceeds to a step S14 to terminate the drive of the motor. In the above embodiments, the present invention is applied to the 1-2 phase excitation drive method although the present invention is equally applicable to other drive method in which the number of excitation phases is switched such as 1-1 phase excitation or 1-2-1 phase excitation, and similar effect may be attained. In accordance with the present invention, in the method for driving the motor by switching the number of excitation phases by the motor drive circuit, the drive pulse width, drive voltage or drive current is controlled during the drive in accordance with the number of excitation phases. Accordingly, the variation of the generated torque is reduced, the ununiformity in the rotation is reduced, and the vibration and noise are reduced.

Claims: A stepping motor comprising: a rotor (18) having magnetic poles; a first stator unit (50) including: an inner yoke (23,24) having a plurality of pole teeth (23A,24A) circumferentially formed at a predetermined pitch around said rotor (18); an outer yoke (22,25) having a plurality of pole teeth (22A,25A) circumferentially formed at a predetermined pitch around said rotor (18); the pole teeth (22A,25A) of said outer yoke (22,25) being located between the pole teeth (23A,24A) of said inner yoke (23,24); and a coil (20,21) mounted between said inner yoke (23,24) and said outer yoke (22,25); and a second stator unit (60) constructed similarly to said first stator unit (50) and axially arranged with said first stator unit (50); CHARACTERISED BY said outer yoke (22,25) of one of said first and second stator units (50,60) being asymmetrically arranged with respect to the corresponding inner yoke (23,24) so that the pole teeth (22A,25A) of said outer yoke (22,25) are shifted from centers between the pole teeth (23A,24A) of said inner yoke (23,24) in a specific direction. A stepping motor according to Claim 1 wherein a shift angle of the pole teeth (22A,25A) of said outer yoke (22,25) from the centres between the pole teeth (23A,24A) of said inner yoke (23,24) is 30′ to 1°30′. A stepping motor according to Claim 1 further comprising: a drive circuit for driving said stepping motor; and a circuit for controlling a drive pulse width of said drive circuit in accordance with the number of excitation phases. A stepping motor according to Claim 3 wherein said control circuit sets a duty factor of the drive pulse to 30 % in a two-phase excitation mode and to 40 % in a one-phase excitation mode.

Inventors: FURUKAWA MASAMICHI C O CANON S, KANAYA SHINICHI C O CANON SEIK, SANBE SHINGO C O CANON SEIKI K, SHIOTSUKI SEIKI C O CANON SEIK, TANEICHI SHIGERU C O CANON SEI, FURUKAWA, MASAMICHI, C/O CANON SEIKI K.K., KANAYA, SHINICHI, C/O CANON SEIKI KABUSHIKI KAISHA, SANBE, SHINGO, C/O CANON SEIKI KABUSHIKI KAISHA, SHIOTSUKI, SEIKI, C/O CANON SEIKI KABUSHIKI KAISHA, TANEICHI, SHIGERU, C/O CANON SEIKI K.K.
Applicants: CANON KK, CANON PRECISION INC, CANON KABUSHIKI KAISHA, CANON SEIKI KABUSHIKI KAISHA
Application year: 1991
Main classification: H02K37
IPC codes: H02K37