Patent Publication Number: US-2007107974-A1

Title: Steering device for vehicle

Description:
TECHNICAL FIELD  
      The present invention relates to an automotive steering device for an automobile or the like.  
     BACKGROUND ART  
      Automotive steering devices are broadly classified into a link type in which a steering member such as a steering wheel is mechanically coupled to a steering mechanism via a transmission member such as a steering shaft or the like and a steer-by-wire type in which the steering member is not mechanically coupled to the steering mechanism.  
      Automotive steering devices of the former link type include an electric power steering device including an electric motor which generates a steering assist force according to operation of the steering member. A DC motor is typically used as the electric motor for the electric power steering device (see, for example, Japanese Unexamined Patent Publication No. 7-137644 laid open on May 30, 1995 and Japanese Unexamined Patent Publication No. 2002-153019 laid open on May 24, 2002).  
      Automotive steering devices of the latter steer-by-wire type include an automotive steering device which includes a steering shaft mechanically separated from a steering mechanism and an electric motor provided as a steering actuator in the middle of the steering mechanism. In this automotive steering device, a control section including a microprocessor controls steerable vehicle wheels by driving the electric motor on the basis of a difference between a steering angle of the steering mechanism and a target steering angle defined by the steering member so that the steering angle becomes close to the target steering angle.  
      In the automotive steering device described above, another electric motor is provided as a counter force actuator for applying an operation counter force to the steering member according to an operation torque. A DC motor is typically used as this electric motor.  
      Thus, the automotive steering devices of the former and latter types employ the DC motors. An exemplary DC motor of this kind is shown in  FIG. 8 .  
      Referring to  FIG. 8 , a rotor  93  is rotatably provided between a pair of magnets  91 ,  92  disposed in opposed relation. The rotor  93  is rotated about a rotation axis  94  extending perpendicularly to a direction along which the magnets  91 ,  92  are opposed to each other. Coils  95  (partly shown) are arranged circumferentially of the rotor  93 .  
      When the rotor  93  is rotated, the coils  95  move across magnetic fluxes  96  (partly shown) occurring between the magnets  91  and  92 . As a result, the direction of the magnetic fluxes  96  is changed with respect to the coils  95 , resulting in unevenness of the torque.  
      This brings about a problem such that a steering feeling is deteriorated in the automotive steering device of either of the aforesaid types.  
      It is therefore an object of the present invention to provide an automotive steering device which ensures an improved steering feeling.  
     DISCLOSURE OF THE INVENTION  
      According to one aspect of the present invention to achieve the aforesaid object, there is provided an automotive steering device including an electric motor which generates a steering assist force. The electric motor includes a rotation shaft, a stator surrounding the rotation shaft, and first and second rotors rotatable together with the rotation shaft. The stator includes a plurality of cores arranged circularly about the rotation shaft and elongated parallel to an axis of the rotation shaft, and coils respectively wound around the cores. An electric current is caused to flow through the coils around the respective cores, whereby the cores are each formed with first and second magnetic poles disposed opposite to each other longitudinally thereof and having opposite polarities. The first rotor has a third magnetic pole having a polarity opposite to the polarity of the first magnetic pole, and the second rotor has a fourth magnetic pole having a polarity opposite to the polarity of the second magnetic pole. As the rotation shaft is rotated, the third and fourth magnetic poles are respectively brought into opposed relation to the first and second magnetic poles of the cores in the same phase. As a result, magnetic fluxes interlink with the coils around the cores in predetermined directions longitudinally of the cores.  
      According to this inventive aspect, the magnetic fluxes interlink with the coils in the predetermined directions, while changing the number of magnetic fluxes in the respective cores. Therefore, a loss of the magnetic fluxes can be suppressed as compared with the case where the direction of the interlinkage of the magnetic fluxes with the coils is changed. As a result, the unevenness of the torque of the electric motor can be suppressed. Therefore, unnatural fluctuation of the steering assist force applied from the electric motor is suppressed so that the steering feeling can be further improved. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic diagram illustrating the schematic construction of an automotive steering device according to one embodiment of the present invention;  
       FIG. 2  is a sectional view of an electric motor;  
       FIG. 3A  is a sectional view taken along a line  3 A- 3 A in  FIG. 2 , and  FIG. 3B  is a sectional view taken along a line  3 B- 3 B in  FIG. 2 ;  
       FIGS. 4A, 4B  and  4 C are schematic diagrams for explaining an operation of the electric motor in relation to a first rotor;  
       FIGS. 5A, 5B  and  5 C are schematic diagrams for explaining an operation of the electric motor in relation to a second rotor;  
       FIG. 6  is a graph illustrating changes of counter electromotive forces over time with the counter electromotive forces and the time being plotted as ordinate and abscissa, respectively;  
       FIG. 7  is a schematic sectional view illustrating another embodiment of the present invention; and  
       FIG. 8  is a schematic diagram illustrating a DC motor of a conventional automotive steering device. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      An automotive steering device according to one embodiment of the present invention will hereinafter be described with reference to the attached drawings.  FIG. 1  is a schematic diagram illustrating the schematic construction of this automotive steering device.  
      The automotive steering device  1  includes a steering shaft  3  coupled to a steering member  2  such as a steering wheel, and a steering column  4  rotatably supporting the steering shaft  3  therein.  
      The automotive steering device  1  includes an intermediate shaft  5  coupled to the steering shaft  3  via a universal joint  6   a , a pinion shaft  7  coupled to the intermediate shaft  5  via a universal joint  6   b , and a rack shaft  8  as a steerable shaft extending transversely of a motor vehicle and having rack teeth  8   a  meshed with pinion teeth  7   a  provided around the pinion shaft  7 . A steering mechanism A of a rack-and-pinion mechanism is constituted by the pinion shaft  7  and the rack shaft  8 .  
      The steering shaft  3  is divided into an input shaft  3   a  coupled to the steering member  2  and an output shaft  3   b  coupled to the pinion shaft  7 . The input and output shafts  3   a ,  3   b  are coaxially coupled to each other in a relatively rotatable manner via a torsion bar  3   c.    
      The rack shaft  8  is supported in a linearly reciprocally movable manner in a housing  9  fixed to a vehicle body via a plurality of bearings  10 . Both ends of the rack shaft  8  project laterally of the housing  9 , and are respectively coupled to tie rods  51 . The tie rods  51  are respectively coupled to corresponding vehicle wheels  52  via corresponding knuckle arms (not shown).  
      When the steering member  2  is operated to rotate the steering shaft  3 , the rotation of the steering shaft  3  is converted into linear movement of the rack shaft  8  extending transversely of the motor vehicle by the pinion teeth  7   a  and the rack teeth  8   a . Thus, the steering of the steerable vehicle wheels  52  is achieved.  
      The automotive steering device  1  is provided as an electric power steering device, which is adapted to provide a steering assist force according to a steering resistance occurring when a steering operation is performed.  
      That is, the automotive steering device  1  includes a torque sensor  11  provided in association with the steering shaft  3  for detecting a steering torque, an electric motor  12  for generating the steering assist force, and a reduction gear mechanism  14  which decelerates the rotation of a rotation shaft  13  of the electric motor  12  and transmits the rotation to the steering mechanism A. The reduction gear mechanism  14  includes a driving gear  14   a  such as a worm coupled to the rotation shaft  13  of the electric motor  12  via a joint  15 , and a driven gear  14   b  such as a worm wheel meshed with the driving gear  14   a  and coupled to the output shaft  3   b  of the steering shaft  3  in a co-rotatable manner.  
      The automotive steering device  1  includes a housing  16  which accommodates the torque sensor  11  and the reduction gear mechanism  14  and supports the electric motor  12 , and an ECU (electronic control unit)  17  which controls the driving of the electric motor  12 . The ECU  17  includes a control center  17   a.    
      The ECU  17  controls the driving of the electric motor  12  via a driving circuit  31  on the basis of a torque detection result and the like applied from the torque sensor  11  and a vehicle speed detection result applied from a vehicle sensor not shown. The output rotation of the electric motor  12  is decelerated and transmitted to the output shaft  3   b  of the steering shaft  3  by the reduction gear mechanism  14 , and then converted into the linear movement of the rack shaft  8  via the pinion shaft  7  to assist the steering operation.  
      In this embodiment, the electric motor  12  is a single opposed magnetic pole induction motor to be described later. When the rotation shaft  13  is rotated, magnetic fluxes generated between first permanent magnets  20 A,  20 B of a first rotor  18  and second permanent magnets  21 A,  21 B of a second rotor  19  are constantly directed in the same direction in a stator  22 , and continuously changed in flux interlinkage number. Thus, the electric motor  12  is very smoothly rotated without torque unevenness to provide a torque ranging from a low level to a very high level.  
      Therefore, the automotive steering device  1  utilizes a higher torque of the electric motor  12  to accelerate the steering start-up operation of the vehicle wheels  52 . For example, it is possible to generate a steering assist force sufficient to follow a driver&#39;s abrupt steering operation for sharp steering. This improves the steering feeling.  
      Since a higher torque can be provided, the reduction gear ratio of the reduction gear mechanism  14  can be reduced, or the provision of the reduction gear mechanism  14  can be omitted. In this respect, the steering start-up operation of the vehicle wheels  52  can be accelerated. Further, a higher torque can be provided even if the rotation shaft  13  of the electric motor  12  is rotated at a relatively low speed. Therefore, the steering assist force can be provided in a short period of time after the standstill of the electric motor  12 . Thus, the steering feeling can be further improved.  
      Since the torque unevenness is reduced, it is possible to suppress the unnatural fluctuation of the steering assist force generated from the electric motor  12  and, hence, the fluctuation of the counter force received by the driver during the steering operation. Therefore, the steering feeling can be further improved.  
      Reference is next made to  FIGS. 2, 3A  and  3 B. The electric motor  12  includes a casing  23  as a stationary member, the rotation shaft  13 , the stator  22 , and a rotor  13   a . The casing  23  is tubular and supported by the housing  16  (see  FIG. 1 ). The rotation shaft  13  is rotatably supported by the casing  23  via a plurality of bearings  24 . The stator  22  is annular, and surrounds the rotation shaft  13 . The rotor  13   a  includes the first and second rotors  18 ,  19  which are fixed to the rotation shaft  13  in a co-rotatable manner. A major part of the rotation shaft  13 , the stator  22  and the first and second rotors  18 ,  19  are accommodated in the casing  23 .  
      The rotation shaft  13  is composed of a nonmagnetic material. One end portion of the rotation shaft  13  extends out of the casing  23 , and is connected to the steering shaft  3  in a drivable manner via the joint  15 . The casing  23  has a pair of flanges  23   a ,  23   b , and a hollow cylindrical cover  23   c.    
      The stator  22  includes a plurality of cores (e.g., four cores  25 A,  25 B,  25 C,  25 D) arranged circularly about the rotation shaft  13 , and a plurality of coils (e.g., four coils  26 A,  26 B,  26 C,  26 D).  
      The cores  25 A,  25 B,  25 C,  25 D are fixedly supported by the casing  23 . The cores  25 A,  25 B,  25 C,  25 D are arranged equidistantly circularly about the rotation shaft  13 , and equally slightly spaced from each other.  
      The cores  25 A,  25 B,  25 C,  25 D are elongate members having the same shape and each extending parallel to an axial direction S of the rotation shaft  13 . As shown in  FIGS. 3A and 3B , the cores  25 A,  25 B,  25 C,  25 D each have an arcuate cross section extending about the rotation shaft  13 . For example, the cores  25 A,  25 B,  25 C,  25 D are each composed of a single metal bar.  
      Coils  26 A,  26 B,  26 C,  26 D are respectively wound around longitudinally (L) middle portions of the cores  25 A,  25 B,  25 C,  25 D.  
      Referring to  FIG. 2 , the core  25 A has first and second magnetic poles  27 A and  28 A which are respectively formed at longitudinally (L) opposite ends  251  and  252  thereof. The core  25 C has first and second magnetic poles  27 C and  28 C which are respectively formed at longitudinally (L) opposite ends  251  and  252  thereof.  
      Similarly, the core  25 B has first and second magnetic poles  27 B and  28 B (see  FIGS. 3A and 3B ) which are respectively formed at longitudinally (L) opposite ends  251  and  252  (not shown) thereof. The core  25 D has first and second magnetic poles  27 D and  28 D (see  FIGS. 3A and 3B ) which are respectively formed at longitudinally (L) opposite ends  251  and  252  (not shown) thereof.  
      In the respective cores  25 A,  25 B,  25 C,  25 D, the first magnetic poles  27 A,  27 B,  27 C,  27 D and the second magnetic poles  28 A,  28 B,  28 C,  28 D are magnetized with opposite magnetic polarities.  
      The coils  26 A,  26 B,  26 C,  26 D arranged circularly (C) about the rotation shaft  13  are alternately wound in opposite directions. For example, the two coils  26 A,  26 C are wound clockwise, and the two coils  26 B,  26 D are wound counterclockwise in  FIG. 3A .  
      The first rotor  18  includes the plural first permanent magnets  20 A,  20 B and an intervention member  29 . The first permanent magnets  20 A,  20 B are fixed to the rotation shaft  13  via the intervention member  29 , and arranged circumferentially (C) evenly about the rotation shaft  13 . The first permanent magnets  20 A,  20 B have the same arcuate shape. The first permanent magnets  20 A,  20 B each have a third magnetic pole  20   n  and a fifth magnetic pole  20   s  which have opposite magnetic polarities. The third magnetic poles  20   n  have an N-polarity, and are located radially (R) outward of the rotation shaft  13 .  
      The second rotor  19  includes the plural second permanent magnets  21 A,  21 B and an intervention member  30 . The second permanent magnets  21 A,  21 B are fixed to the rotation shaft  13  via the intervention member  30 , and arranged circumferentially (C) evenly about the rotation shaft  13 . The second permanent magnets  21 A,  21 B have the same arcuate shape. The second permanent magnets  21 A,  21 B each have a fourth magnetic pole  21   s  and a sixth magnetic pole  21   n  which have opposite magnetic polarities. The fourth magnetic poles  21   s  each have a polarity opposite to the polarity of the third magnetic poles  20   n . That is, the fourth magnetic poles  21   s  each have an S-polarity, and are located radially (R) outward of the rotation shaft  13 . The number of the second permanent magnets  21 A,  21 B is equal to the number of the first permanent magnets  20 A,  20 B of the first rotor  18 .  
      The third magnetic poles  20   n  of the first rotor  18  are arranged so as to be sequentially brought into opposed relation to the first magnetic poles  27 A,  27 B,  27 C,  27 D of the stator  22  as the rotation shaft  13  is rotated. Similarly, the fourth magnetic poles  21   s  of the second rotor  19  are arranged so as to be sequentially brought into opposed relation to the second magnetic poles  28 A,  28 B,  28 C,  28 D of the stator  22  as the rotation shaft  13  is rotated.  
      The third magnetic poles  20   n  of the first rotor  18  and the fourth magnetic poles  21   s  of the second rotor  19  are located at the same angular positions circumferentially (C) of the rotation shaft  13  (i.e., in the same phase as during the rotation of the rotation shaft  13 ).  
      When the third magnetic poles  20   n  of the first rotor  18  are respectively brought into radially (R) opposed relation to the first magnetic poles  27 A,  27 C of the stator  22  with the intervention of small air gaps during the rotation of the rotation shaft  13  as shown in  FIG. 4A , the fourth magnetic poles  21   s  of the second rotor  19  are respectively brought into radially (R) opposed relation to the second magnetic poles  28 A,  28 C of the stator  22  with the intervention of small air gaps as shown in  FIG. 5A .  
      Similarly, when the third magnetic poles  20   n  of the first rotor  18  are respectively brought into radially (R) opposed relation to the first magnetic poles  27 B,  27 D of the stator  22  with the intervention of small air gaps during the rotation of the rotation shaft  13  as shown in  FIG. 4C , the fourth magnetic poles  21   s  of the second rotor  19  are respectively brought into radially (R) opposed relation to the second magnetic poles  28 B,  28 D of the stator  22  with the intervention of small air gaps as shown in  FIG. 5C .  
      Referring again to  FIG. 1 , the control section  17  functions as driving means for driving the electric motor  12 , and includes a driving circuit  31  for properly energizing the coils  26 A,  26 B,  26 C,  26 D of the stator  22  under control of the control center  17   a , and a single position sensor  32  which detects rotation angular positions of the first and second rotors  18 ,  19  with respect to the stator  22 . Alternatively, a plurality of position sensors  32  may be provided.  
      The driving circuit  31  includes a first energization circuit not shown which causes an electric current to flow through the coils  26 A,  26 B,  26 C,  26 D in a first direction, a second energization circuit not shown which causes an electric current to flow through the coils  26 A,  26 B,  26 C,  26 D in a second direction opposite to the first direction, and at least one switch functioning as switching means for switching between the first and second energization circuits on the basis of a position detection signal applied from the position sensor  32 .  
      The coils  26 A,  26 B,  26 C,  26 D are connected in series in this order, and further connected to a DC power source via the switch, whereby the first and second energization circuits are selectively established.  
      With the switch being switched to the first energization circuit, the electric current from the DC power source flows through the coils  26 A,  26 C in one rotational direction around the cores  25 A,  25 C, and flows through the coils  26 B,  26 D in the other rotational direction around the cores  25 B,  25 D in  FIG. 4A .  
      With the switch being switched to the second energization circuit, the electric current from the DC power source flows through the coils  26 A,  26 C in the other rotational direction around the cores  25 A,  25 C, and flows through the coils  26 B,  26 D in the one rotational direction around the cores  25 B,  25 D.  
      Referring to  FIGS. 4A  to  4 C and  FIGS. 5A  to  5 C, an operation to be performed by the control section  17  when the rotation shaft  13  is rotated clockwise will hereinafter be described.  
      As shown in  FIGS. 4A and 4B , the third magnetic poles  20   n  of the first permanent magnets  20 A,  20 B of the first rotor  18  are respectively moved away from the first magnetic poles  27 A,  27 C of the stator  22  toward the first magnetic poles  27 B,  27 D circumferentially (C) adjacent to the first magnetic poles  27 A,  27 C about the rotation shaft  13  during the rotation of the rotation shaft  13 .  
      At the same time, the fourth magnetic poles  21   s  of the second permanent magnets  21 A,  21 B of the second rotor  19  are respectively moved away from the second magnetic poles  28 A,  28 C of the stator  22  toward the second magnetic poles  28 B,  28 D circumferentially (C) adjacent to the second magnetic poles  28 A,  28 C about the rotation shaft  13  as shown in  FIGS. 5A and 5B .  
      That is, the electric current is supplied to the coil  26 A around the core  25 A so as to generate a repulsive force F 1  between the first magnetic pole  27 A of the core  25 A and the third magnetic pole  20   n  of the first permanent magnet  20 A of the first rotor  18  as shown in  FIGS. 4A and 4B  and to generate a repulsive force F 1  between the second magnetic pole  28 A of the core  25 A and the fourth magnetic pole  21   s  of the second permanent magnet  21 A of the second rotor  19  as shown in  FIGS. 5A and 5B . At the same time, the electric current is, supplied to the coil  26 B around the core  25 B so as to generate an attractive force F 2  between the first magnetic pole  27 B of the core  25 B and the third magnetic pole  20   n  of the first permanent magnet  20 A of the first rotor  18  and to generate an attractive force F 2  between the second magnetic pole  28 B of the core  25 B and the fourth magnetic pole  21   s  of the second permanent magnet  21 B of the second rotor  19  as shown in  FIGS. 4A and 5A .  
      Further, the electric current is supplied to the coil  26 C around the core  25 C so as to generate a repulsive force F 3  between the first magnetic pole  27 C of the core  25 C and the third magnetic pole  20   n  of the first permanent magnet  20 B of the first rotor  18  and to generate a repulsive force F 3  between the second magnetic pole  28 C of the core  25 C and the fourth magnetic pole  21   s  of the second permanent magnet  21 B of the second rotor  19 . At the same time, the electric current is supplied to the coil  26 D around the core  25 D so as to generate an attractive force F 4  between the first magnetic pole  27 D of the core  25 D and the third magnetic pole  20   n  of the first permanent magnet  20 B of the first rotor  18  and to generate an attractive force F 4  between the second magnetic pole  28 D of the core  25 D and the fourth magnetic pole  21   s  of the second permanent magnet  21 B of the second rotor  19 .  
      More specifically, the electric current is supplied to the respective coils  26 A,  26 B,  26 C,  26 D in the first direction from the DC power source with the switch being switched to the first energization circuit. The first magnetic poles  27 A,  27 C and the second magnetic poles  28 B,  28 D are magnetized with an N-polarity, while the first magnetic poles  27 B,  27 D and the second magnetic poles  28 A,  28 C are magnetized with an S-polarity. The repulsive forces F 1 , F 3  and the attractive forces F 2 , F 4  are simultaneously generated. Thus, the rotation shaft  13  is rotatively driven.  
      Then, the third magnetic poles  20   n  of the first rotor  18  are respectively brought into opposed relation to the first magnetic poles  27 B,  27 D of the stator  22  as shown in  FIG. 4C , and the fourth magnetic poles  21   s  of the second rotor  19  are respectively brought into opposed relation to the second magnetic poles  28 B,  28 D of the stator  22  as shown in  FIG. 5C .  
      In this state, the switch is switched to the second energization circuit to supply the electric current to the coils  26 A,  26 B,  26 C,  26 D from the DC power source in the second direction opposite to the first direction, whereby the rotation shaft  13  is further rotated. Thereafter, the direction of the supply of the electric current to the coils  26 A,  26 B,  26 C,  26 D is alternately switched between the first and second directions, whereby the rotation shaft  13  is continuously rotated.  
      On the other hand, as the rotation shaft  13  is rotated, the first and second permanent magnets  20 A,  20 B;  21 A,  21 B are rotated together with the rotation shaft  13 , whereby the magnetic fluxes interlinking with the respective coils  26 A,  26 B,  26 C,  26 D are fluctuated. As a result, an inductive counter electromotive force is generated.  
      The fluctuation of the interlinkage of the magnetic fluxes from the first permanent magnet  20 A observed when the rotation shaft  13  is rotated clockwise at a constant rotation speed will hereinafter be described. In  FIG. 6  which illustrates changes of the counter electromotive forces, a time point t 1  which corresponds to a state shown in  FIGS. 4A and 5A  and time points t 2 , t 3 , t 4  and t 5  which respectively correspond to states where the rotation shaft  13  is rotated by 90 degrees, 180 degrees, 270 degrees and 360 degrees from the state shown in  FIGS. 4A and 5A  are plotted on the abscissa.  
      Referring to  FIGS. 4A and 5A , the first permanent magnet  20 A and the second permanent magnet  21 A associated with each other are respectively opposed directly to the first magnetic pole  27 A and the second magnetic pole  28 A of the core  25 A. In this state, a magnetic path extending from the first permanent magnet  20 A to the second permanent magnet  21 A through the core  25 A is established, so that magnetic fluxes J from the first permanent magnet  20 A entirely interlink with the coil  26 A (see  FIG. 2 ).  
      Referring next to  FIG. 4B , the first permanent magnet  20 A is moved away from the first magnetic pole  27 A toward the first magnetic pole  27 B-adjacent to the first magnetic pole  27 A as the rotation shaft  13  is rotated. Thus, the number of the magnetic fluxes interlinking with the coil  26 A is reduced, whereby a counter electromotive force V 1  is generated as decreasing in a triangular waveform by the coil  26 A as shown in  FIG. 6 . At the same time, the number of magnetic fluxes interlinking with the coil  26 B is increased, whereby a counter electromotive force V 2  is generated as increasing in a triangular waveform by the coil  26 B.  
      These counter electromotive forces V 1 , V 2  each occur in a direction opposite to the direction of the electric current supplied by the first energization circuit. In the first energization circuit, a counter electromotive force obtained by combining these two triangular waveforms is generated. The combined counter electromotive force is in a generally constant rectangular waveform, and no torque unevenness occurs.  
      With the first permanent magnet  20 A being directly opposed to the first magnetic pole  27 B as shown in  FIG. 4C , the magnetic fluxes entirely interlink with the coil  26 B (as corresponding to the state at the time point t 2  in  FIG. 6 ).  
      Referring to  FIG. 6 , during a period from the time point t 2  to the time point t 3 , the number of the magnetic fluxes interlinking with the coil  26 B is reduced, while the number of magnetic fluxes interlinking with the coil  26 C adjacent to the coil  26 B is increased. During this period, the second energization circuit is actuated, so that the direction of the electric current supplied to the respective coils is reversed from that observed during the period from the time point t 1  to the time point t 2 . In the coils  26 B,  26 C, a counter electromotive force V 3  increasing in a triangular waveform and a counter electromotive force V 4  decreasing in a triangular waveform are each generated in a direction opposite to the direction of the electric current supplied to the respective coils in the same manner as described above, thereby providing a constant combined counter electromotive force.  
      As the rotation shaft  13  is further rotated during a period from the time point t 3  to the time point t 5 , magnetic fluxes sequentially interlink with the coils  26 C,  26 D,  26 A, and the numbers of the magnetic fluxes interlinking with the respective coils are changed. During this period, counter electromotive forces in serrate waveforms occur in the respective coils  26 C,  26 D,  26 A. Further, the magnetic fluxes from the first permanent magnet  20 B also cause counter electromotive forces like the aforesaid counter electromotive forces. Thus, a combined counter electromotive force is provided in a rectangular waveform in the stator  22 , and no torque unevenness occurs.  
      In this embodiment, the third magnetic poles  20   n  of the first permanent magnets  20 A,  20 B of the first rotor  18  each having an N-polarity are sequentially brought into opposed relation to the first magnetic poles  27 A to  27 D of the stator  22 , and the fourth magnetic poles  21   s  of the second permanent magnets  21 A,  21 B of the second rotor  19  each having an S-polarity are sequentially brought into opposed relation to the second magnetic poles  28 A to  28 D of the stator  22  during the rotation of the rotation shaft  13 .  
      Note a relationship between the third magnetic pole  20   n  of the first permanent magnet  20 A of the first rotor  18  and the first magnetic pole  27 A of the core  25 A which are opposed to each other. When the third magnetic pole  20   n  and the first magnetic pole  27 A are moved relative to each other in substantially opposed relation during the rotation of the rotation shaft  13 , the magnetic fluxes J from the first permanent magnet  20 A of the first rotor  18  are introduced into the core  25 A having the first magnetic pole  27 A opposed to the third magnetic pole  20   n  of the first permanent magnet  20 A.  
      The magnetic fluxes J are generated so as to interlink with the coil  26 A in a predetermined direction irrespective of the rotation of the rotation shaft  13 , and the number of the magnetic fluxes in the core  25 A is changed during the rotation of the rotation shaft  13 . Therefore, the loss of the magnetic fluxes is suppressed as compared with the case where the direction of the magnetic fluxes with respect to the coil is changed. As a result, the unevenness of the torque of the electric motor  12  can be suppressed.  
      The first magnetic poles  27 A to  27 D of the stator  22  are brought into opposed relation to only the third magnetic poles  20   n  of the first permanent magnets  20 A,  20 B of the first rotor  18  each having an N-polarity. The second magnetic poles  28 A to  28 D of the stator  22  are brought into opposed relation to only the fourth magnetic poles  21   s  of the second permanent magnets  21 A,  21 B of the second rotor  19  each having an S-polarity. Therefore, the changes in the number of the magnetic fluxes during the rotation of the rotation shaft  13  are reduced as compared with a case where the first magnetic poles  27 A to  27 D are alternately brought into opposed relation to an N-pole and an S-pole or a case where the second magnetic poles  28 A to  28 D are alternately brought into opposed relation to an N-pole and an S-pole. As a result, the unevenness of the torque of the electric motor  12  can be further reduced. Therefore, the steering feeling can be further improved.  
      In this embodiment, the repulsive forces F 1 , F 3  and the attractive forces F 2 , F 4  simultaneously act on the magnetic poles  20   n ,  21   s  of the first and second rotors  18 ,  19  to accelerate the rotation of the rotation shaft  13 , for example, as shown in  FIGS. 4B and 5B , and a higher torque can be efficiently generated.  
      Since the magnetic poles  20   n ,  21   s  of the first and second rotors  18 ,  19  each have one of the opposite polarities, inter-leakage of the magnetic fluxes between the magnetic poles  20   n  and  21   s  of the rotors  18  and  19  is suppressed, thereby contributing to the generation of the higher torque. Therefore, the steering feeling can be further improved with the higher torque thus provided.  
      The number of the magnetic poles of the stator  22  is preferably twice the number of the magnetic poles of each of the rotors  18 ,  19 . Thus, the magnetic poles  27 A to  27 D,  28 A to  28 D of the stator  22  can continuously apply the repulsive forces F 1 , F 3  and the attractive forces F 2 , F 4  to the magnetic poles  20   n ,  21   s  of the corresponding rotors  18 ,  19 . This is advantageous for providing a higher torque.  
      Referring to  FIGS. 3A and 3B , the magnetic poles of the rotors  18 ,  19  and the stator  22  to be brought into opposed relation have substantially the same arcuate length. With this arrangement, rotational magnetic fields are induced sequentially in the cores  25 A,  25 B,  25 C,  25 D of the stator  22  according to the positions of the first and second permanent magnets  20 A,  20 B;  21 A,  21 B of the first and second rotors  18 ,  19  during the rotation of the rotation shaft  13 . Thus, it is possible to repeatedly cause such a phenomenon that, for example, the number of the magnetic fluxes interlinking with the coil  26 A is increased, and at the same time, the number of the magnetic fluxes interlinking with the coil  26 B adjacent to the coil  26 A is reduced.  
      As a result, the inductive counter electromotive force caused in the entire stator  22  by the changes in the number of the magnetic fluxes can be kept constant as shown in  FIG. 6 , and the unevenness of the torque of the rotation shaft  13  can be further reduced. Therefore, the steering feeling can be further improved. In addition, the numbers of the magnetic fluxes occurring in the respective cores  25 A,  25 B,  25 C,  25 D are changed in serrate waveforms during the rotation of the rotation shaft  13 . Therefore, the numbers of the magnetic fluxes are changed substantially equivalently in the respective coils  26 A to  26 D, and the unevenness of the torque can be further reduced.  
      In this embodiment, the problem associated with the conventional electric power steering device using the ordinary DC motor can be solved by employing the single opposed magnetic pole induction motor as the electric motor  12 . In this embodiment, since a higher torque can be provided even at a lower speed, the value of the electric current required for the driving of the electric motor  12  can be reduced. This makes it possible to reduce the costs of the driving circuit  31  with the use of less expensive components. Further, the size reduction of the reduction gear mechanism  14  can be achieved by reducing the reduction gear ratio.  
      Since the torque unevenness can be reduced, there is no need for a control for suppressing the torque unevenness, thereby simplifying the construction of the control section  17 . Further, the construction of the electric motor  12  can be simplified by reducing the number of the magnetic poles.  
      The cores  25 A,  25 B,  25 C,  25 D are each composed of the single elongate bar. Therefore, the number of components of the electric motor  12  can be reduced, for example, without a need for the use of a multiplicity of laminate iron plates in the stator  22 .  
      The coil winding method can be simplified by winding the coils  26 A,  26 B,  26 C,  26 D around the longitudinal (L) axes of the cores  25 A,  25 B,  25 C,  25 D of the elongate bars. Therefore, a copper loss, an iron loss and a circuit loss of the electric motor  12  can be drastically reduced, thereby increasing the efficiency.  
      As shown in  FIG. 7 , it is possible to employ at least one of an arrangement such that third magnetic poles  200   n  of the first rotor  18  are brought into axially (S) opposed relation to the first magnetic poles  27 A to  27 D of the cores  25 A to  25 D of the stator  22  and an arrangement such that fourth magnetic poles  210   s  of the second rotor  19  are brought into axially (S) opposed relation to the second magnetic poles  28 A to  28 D of the cores  25 A to  25 D of the stator  22 .  FIG. 7  illustrates a construction in which the third and fourth magnetic poles  200   n ,  210   s  are respectively brought into axially (S) opposed relation to the first and second magnetic poles  27 A to  27 D and  28 A to  28 D. Further, fifth magnetic poles  200   s  each having a polarity opposite to the polarity of the third magnetic poles  200   n  and sixth magnetic poles  210   n  each having a polarity opposite to the polarity of the fourth magnetic poles  210   s  are provided.  
      The numbers of the third and fourth magnetic poles  20   n ,  21   s ;  200   n ,  210   s  may each be increased, for example, to three or four. In this case, the number of the cores  25 A to  25 D of the stator  22  is preferably twice the number of the third or fourth magnetic poles  20   n ,  21   s ;  200   n ,  210   s.    
      The present invention is applicable not only to the column-type electric power steering device but also to an electric power steering device of a type in which an electric motor  12  is disposed coaxially with or in the vicinity of the rack shaft  8  for axially driving the rack shaft  8 , and a steer-by-wire steering device employing an electric motor  12 .  
      While the present invention has thus been described in detail by way of the specific embodiments thereof, those skilled in the art who have understood the foregoing will easily come up with alterations, modifications and equivalents of the embodiments. Therefore, the scope of the present invention is to be defined by the following claims and their equivalents.  
      This application corresponds to Japanese Patent Application No. 2003-200687 filed with the Japanese Patent Office on Jul. 23, 2003, the disclosure of which is incorporated herein by reference in its entirety.