Patent Publication Number: US-2018037254-A1

Title: Vehicle Steering System

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2016-154898 filed on Aug. 5, 2016 including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a vehicle steering system for a vehicle, such as an automobile. 
     2. Description of the Related Art 
     Japanese Patent Application Publication No. 2014-223862 (JP 2014-223862 A) describes a vehicle steering control system including a clutch, a reactive-force motor, and two steering motors. The clutch is provided on a power transmission path extending from a steering wheel to steered wheels. The reactive-force motor applies power to a portion of the power transmission path, which is closer to the steering wheel than the clutch is. The steering motors apply power to a portion of the power transmission path, which is closer to the steered wheels than the clutch is. 
     The vehicle steering control system described in JP 2014-223862 A includes the two steering motors. Thus, even when one of the steering motors is malfunctioning, the steered wheels can be steered by the other one of the steering motors. In view of this, the vehicle steering control system described in JP 2014-223862 A offers a high level of safety. However, the vehicle steering control system described in JP 2014-223862A is costly because the vehicle steering control system includes the two steering motors. 
     SUMMARY OF THE INVENTION 
     One object of the invention is to provide a vehicle steering system that is less costly and that ensures safety. 
     An aspect of the invention relates to a vehicle steering system including: a steering member configured to steer a vehicle; a steering operation mechanism configured to steer steered wheels; a clutch provided on a power transmission path extending from the steering member to the steered wheels; a reactive-force motor connected to a first portion of the power transmission path, the first portion being closer to the steering member than the clutch is; a steering motor connected to a second portion of the power transmission path, the second portion being closer to the steered wheels than the clutch is, the steering motor including a first motor coil and a second motor coil; a reactive-force motor driving circuit configured to supply electric power to the reactive-force motor; a first driving circuit configured to supply electric power to the first motor coil; a second driving circuit configured to supply electric power to the second motor coil; a reactive-force motor controller configured to control the reactive-force motor driving circuit; a steering motor controller configured to control the first driving circuit in a first system including the first motor coil and the first driving circuit and the second driving circuit in a second system including the second motor coil and the second driving circuit; and a clutch controller configured to control the clutch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
         FIG. 1  is a diagram illustrating the schematic configuration of a vehicle steering system according to an embodiment of the invention; 
         FIG. 2  is a block diagram illustrating the electrical configuration of an electronic control unit (ECU); 
         FIG. 3  is a functional block diagram illustrating an operation of a steering motor controller in a steer-by-wire (SBW) mode; 
         FIG. 4  is a functional block diagram illustrating an operation of a reactive-force motor controller in the SBW mode; 
         FIG. 5  is a functional block diagram illustrating an operation of the steering motor controller in a first electric power steering (EPS) mode; and 
         FIG. 6  is a functional block diagram illustrating an operation of the reactive-force motor controller in a second EPS mode. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, example embodiments of the invention will be described in detail with reference to the accompanying drawings.  FIG. 1  is a diagram illustrating the schematic configuration of a vehicle steering system according to an embodiment of the invention. A vehicle steering system  1  includes a steering wheel  2 , a steering operation mechanism  4 , a steering shaft  5 , and a clutch  6 . The steering wheel  2  is an example of a steering member for steering a vehicle. The steering operation mechanism  4  steers steered wheels  3 . The steering shaft  5  is connected to the steering wheel  2 . The clutch  6  mechanically connects the steering shaft  5  (steering wheel  2 ) to the steering operation mechanism  4  or mechanically disconnects the steering shaft  5  (steering wheel  2 ) from the steering operation mechanism  4 . In the present embodiment, the clutch  6  is an electromagnetic clutch. The clutch  6  includes an input shaft and an output shaft. The clutch  6  has a function of permitting or interrupting transmission of torque between the input shaft and the output shaft. 
     The steering shaft  5  includes a first shaft  7 , a second shaft  9 , and a torsion bar  8 . The first shaft  7  is connected to the steering wheel  2 . The second shaft  9  is connected to the input shaft of the clutch  6  in an integrated manner. The torsion bar  8  connects the first shaft  7  to the second shaft  9 . A steering angle sensor  10  for detecting a steering angle θh, which is a rotation angle of the first shaft  7 , is disposed around the first shaft  7 . In the present embodiment, the steering angle sensor  10  detects an amount of rotation (rotation angle) of the first shaft  7  in each of forward and reverse directions from the neutral position of the first shaft  7 . The steering angle sensor  10  outputs the amount of rotation to the right from the neutral position, for example, as a positive value, and outputs the amount of rotation to the left from the neutral position, for example, as a negative value. 
     A first torque sensor  11  and a second torque sensor  12  are disposed around the steering shaft  5 . The torque sensors  11 ,  12  respectively detect steering torques Th 1 , Th 2  applied to the steering wheel  2 , based on an amount of relative rotational displacement between the first shaft  7  and the second shaft  9 , that is, a torsion angle of the torsion bar  8 . In the present embodiment, regarding the steering torques Th 1 , Th 2  are respectively detected by the respective torque sensors  11 ,  12  as follows. A torque for steering to the right is detected as a positive value, and a torque for steering to the left is detected as a negative value. As the absolute value of the torque is greater, the magnitude of the steering torque is higher. 
     A reactive-force motor  14  is connected to the second shaft  9  via a speed reducer  13 . The reactive-force motor  14  serves as an electric motor that applies a steering reactive force (torque applied in a direction opposite to a steering direction) to the steering wheel  2  in a normal state. In the present embodiment, the reactive-force motor  14  is a three-phase brushless motor. The speed reducer  13  is a worm gear mechanism including a worm shaft (not illustrated) and a worm wheel (not illustrated). The worm shaft is connected to an output shaft of the reactive-force motor  14  so as to be rotatable together with the output shaft thereof in an integrated manner. The worm wheel is meshed with the worm shaft. The worm wheel is connected to the second shaft  9  so as to be rotatable together with the second shaft  9  in an integrated manner. The reactive-force motor  14  is provided with a rotation angle sensor  15  for detecting a rotation angle of the reactive-force motor  14 . 
     The steering operation mechanism  4  includes a first pinion shaft  16 , a rack shaft  17  serving as a steered shaft, and a steering actuator  30 . The first pinion shaft  16  is connected to the output shaft of the clutch  6 . The steering actuator  30  applies a steering force to the rack shaft  17 . The steered wheels  3  are connected to respective end portions of the rack shaft  17  via tie rods  18  and knuckle arms (not illustrated). A first pinion  19  is connected to a distal end of the first pinion shaft  16 . The rack shaft  17  extends linearly in the lateral direction of the vehicle. A first rack  20  meshed with the first pinion  19  is provided at a first end portion of the rack shaft  17  in its axial direction. 
     The steering actuator  30  includes a steering motor  31 , a speed reducer  32 , a second pinion shaft  33 , a second pinion  34 , and a second rack  35 . In the present embodiment, the steering motor  31  is a three-phase brushless motor including one rotor and two sets of three-phase motor coils (three-phase stator coils)  31 A,  31 B (see  FIG. 2 ). The rotor includes a plurality of magnets provided at an outer peripheral portion thereof. The second pinion shaft  33  is disposed separately from the steering shaft  5 . The speed reducer  32  is a worm gear mechanism including a worm shaft (not illustrated) and a worm wheel (not illustrated). The worm shaft is connected an output shaft of the steering motor  31  so as to be rotatable together with the output shaft thereof in an integrated manner. The worm wheel (not illustrated) is meshed with the worm shaft. The worm wheel is connected to the second pinion shaft  33  so as to be rotatable together with the second pinion shaft  33  in an integrated manner. 
     The second pinion  34  is connected to a distal end of the second pinion shaft  33 . The second rack  35  is provided at a second end portion of the rack shaft  17 . The second end portion is on the opposite side of the rack shaft  17  from the first end portion in the axial direction. The second pinion  34  is meshed with the second rack  35 . The steering motor  31  is provided with a rotation angle sensor  37  for detecting a rotation angle of the steering motor  31 . A stroke sensor  38  for detecting an axial travel distance of the rack shaft  17  is disposed in the vicinity of the rack shaft  17 . A steered angle θt of the steered wheels  3  is detected based on the axial travel distance of the rack shaft  17  detected by the stroke sensor  38 . 
     Detection signals from the steering angle sensor  10 , the torque sensors  11 ,  12 , the rotation angle sensors  15 ,  37 , the stroke sensor  38 , and the vehicle speed sensor  39 , and a state detection signal indicating the state of an ignition key are input into an electronic control unit (ECU)  40 . Based on the input signals, the ECU  40  controls the clutch  6 , the reactive-force motor  14 , and the steering motor  31 . 
       FIG. 2  is a block diagram illustrating the electrical configuration of the ECU  40 . The ECU  40  includes a microcomputer  41 . The ECU  40  includes a first driving circuit (inverter circuit)  42 A and a second driving circuit (inverter circuit)  42 B. The first driving circuit  42 A and the second driving circuit  42 B are controlled by the microcomputer  41 , and respectively supply electric power to the first motor coil  31 A and the second motor coil  31 B of the steering motor  31 . The ECU  40  includes a first current detector  43 A and a second current detector  43 B that respectively detect motor currents to be supplied to the first motor coil  31 A and the second motor coil  31 B. The ECU  40  includes a driving circuit (inverter circuit)  44  and a current detector  45 . The driving circuit  44  is controlled by the microcomputer  41 , and supplies electric power to the reactive-force motor  14 . The current detector  45  detects a motor current to be supplied to the reactive-force motor  14 . The ECU  40  includes a driving circuit  46  that is controlled by the microcomputer  41  and that drives the clutch  6 . 
     The microcomputer  41  includes a central processing unit (CPU) and memories (e.g., a read-only memory (ROM), a random-access memory (RAM), and a nonvolatile memory), and functions as a plurality of function processing units by executing prescribed programs. The function processing units include a steering motor controller  50 , a reactive-force motor controller  70 , and a clutch controller  90 . The steering motor controller  50  controls the steering motor  31 . The reactive-force motor controller  70  controls the reactive-force motor  14 . The clutch controller  90  controls the clutch  6 . 
     The clutch  6  is, for example, normally engaged, and is disengaged by being energized. The clutch  6  is controlled as follows depending on the state of the ignition key. When the ignition key is operated to an on-position, the clutch  6  is energized to be disengaged. When the ignition key is operated from the on-position to an off-position, the clutch  6  is de-energized to be engaged. 
     The steering motor controller  50  has a function of notifying the reactive-force motor controller  70  and the clutch controller  90  of the occurrence of a malfunction when the malfunction has occurred in a first system including the first motor coil  31 A of the steering motor  31  and the first driving circuit  42 A or a second system including the second motor coil  31 B of the steering motor  31  and the second driving circuit  42 B. The reactive-force motor controller  70  has a function of notifying the steering motor controller  50  and the clutch controller  90  of the occurrence of a malfunction when the malfunction has occurred in the reactive-force motor  14  or the driving circuit  44 . 
     When the reactive-force motor  14  and the driving circuit  44  are functioning normally and at least one of the first system ( 31 A,  42 A) and the second system ( 31 B,  42 B) is functioning normally, the reactive-force motor controller  70 , the steering motor controller  50 , and the clutch controller  90  operate in a steer-by-wire mode (hereinafter, referred to as “SBW mode”). That is, the operation mode of each of the controllers  50 ,  70 ,  90  is set to the SBW mode. In the SBW mode, the clutch  6  is disengaged under the control of the clutch controller  90 . A steering reactive force is applied to the steering wheel  2  from the reactive-force motor  14  under the control of the reactive-force motor controller  70 . The steered wheels  3  are steered by the steering motor  31  under the control of the steering motor controller  50 . 
     In the SBW mode, when a malfunction has occurred in one of the first system ( 31 A,  42 A) and the second system ( 31 B,  42 B), the steering motor controller  50  controls the steering motor  31  by using the system that is functioning normally, thereby steering the steered wheels  3 . When a malfunction has occurred in the reactive-force motor  14  or the driving circuit  44  and at least one of the first system ( 31 A,  42 A) and the second system ( 31 B,  42 B) is functioning normally, the steering motor controller  50  and the clutch controller  90  operate in a first power steering mode (EPS mode). That is, the operation mode of each of the controllers  50 ,  70 ,  90  is set to the first EPS mode. In the first EPS mode, the clutch  6  is engaged under the control of the clutch controller  90 . Further, a steering assist force is generated by the steering motor  31  under the control of the steering motor controller  50 . 
     In the first EPS mode, when a malfunction has occurred in one of the first system ( 31 A,  42 A) and the second system ( 31 B,  42 B), the steering motor controller  50  controls the steering motor  31  by using the system functioning normally, thereby causing the steering motor  31  to generate a steering assist force. When a malfunction has occurred in each of both the first system ( 31 A,  42 A) and the second system ( 31 B,  42 B) and the reactive-force motor  14  and the driving circuit  44  are functioning normally, the reactive-force motor controller  70  and the clutch controller  90  operate in a second power steering mode (EPS mode). That is, the operation mode of each of the control units  50 ,  70 ,  90  is set to the second EPS mode. In the second EPS mode, the clutch  6  is engaged under the control of the clutch controller  90 . Further, a steering assist force is generated by the reactive-force motor  14  under the control of the reactive-force motor controller  70 . 
     When a malfunction has occurred in the reactive-force motor  14  or the driving circuit  44  and a malfunction has occurred in each of both the first system of the steering motor  31  and the second system of the steering motor  31 , the clutch  6  is engaged under the control of the clutch controller  90 .  FIG. 3  is a functional block diagram illustrating an operation of the steering motor controller  50  in the SBW mode. 
     In the SBW mode, the clutch  6  is disengaged. Thus, the steering shaft  5  and the first pinion shaft  16  are disconnected from each other. In the SBW mode, the steering motor controller  50  includes, as function processing units thereof, a target steered angle setting unit  51 , an angle deviation calculation unit  52 , a current command value generation unit  53 , a command value distribution unit  54 , a first current feedback control unit  55 A, and a second current feedback control unit  55 B. 
     The target steered angle setting unit  51  sets a target steered angle θt* based on the vehicle speed V detected by the vehicle speed sensor  39  and the steering angle θh detected by the steering angle sensor  10 . For example, the target steered angle setting unit  51  sets the target steered angle θt* corresponding to the vehicle speed V and the steering angle θh, using a prescribed transfer function. The angle deviation calculation unit  52  calculates a deviation between the target steered angle θt* set by the target steered angle setting unit  51  and an actual steered angle θt detected by the stroke sensor  38 . 
     The current command value generation unit  53  executes, for example, PI calculation on the angle deviation calculated by the angle deviation calculation unit  52 . Thus, a motor current command value It* (a two-phase current command value (a d-axis current command value and a q-axis current command value)) is generated. The command value distribution unit  54  distributes the motor current command value It* generated by the current command value generation unit  53  to the first current feedback control unit  55 A and the second current feedback control unit  55 B. In the present embodiment, when the first system and the second system of the steering motor  31  are both functioning normally, the command value distribution unit  54  distributes half of the motor current command value It* to each of the first current feedback control unit  55 A and the second current feedback control unit  55 B. That is, a distribution ratio of the motor current command value It* for the first current feedback control unit  55 A and a distribution ratio of the motor current command value It* for the second current feedback control unit  55 B are both 50%. 
     On the other hand, when a malfunction (failure) has occurred in one of the first system and the second system of the steering motor  31 , the distribution ratio for the control unit corresponding to the normally functioning system, among the first current feedback control unit  55 A and the second current feedback control unit  55 B, is set to 100%, and the distribution ratio for the control unit corresponding to the malfunctioning system, among the first current feedback control unit  55 A and the second current feedback control unit  55 B, is set to 0%. The motor current command value distributed to the first current feedback control unit  55 A will be referred to as “first motor current command value It 1 *”. The motor current command value distributed to the second current feedback control unit  55 B will be referred to as “second motor current command value It 2 *”. 
     The first current feedback control unit  55 A controls the first driving circuit  42 A through known vector control using an output signal from the rotation angle sensor  37 , such that a motor current (a d-axis current and a q-axis current) detected by the first current detector  43 A and subjected to the three-phase-to-two-phase conversion is equal to the first motor current command value It 1 *. The second current feedback control unit  55 B controls the second driving circuit  42 B through known vector control using an output signal from the rotation angle sensor  37 , such that a motor current (a d-axis current and a q-axis current) detected by the second current detector  43 B and subjected to the three-phase-to-two-phase conversion is equal to the second motor current command value It 2 *. Thus, the steering motor  31  is controlled such that the actual steered angle θt detected by the stroke sensor  38  is equal to the target steered angle θt* that is set by the target steered angle setting unit  51 . 
       FIG. 4  is a functional block diagram illustrating an operation of the reactive-force motor controller  70  in the SBW mode. In the SBW mode, the reactive-force motor controller  70  includes, as function processing units thereof, a target reactive torque setting unit  71 , a current command value generation unit  72 , and a current feedback control unit  73 . 
     The target reactive torque setting unit  71  sets a target reactive torque Tf* based on the vehicle speed V detected by the vehicle speed sensor  39 , the steering angle θh detected by the steering angle sensor  10 , and the steering torque Th 1  detected by the first torque sensor  11 . For example, the target reactive torque setting unit  71  obtains a target reactive torque basic value based on the steering angle θh and the vehicle speed V. The target reactive torque setting unit  71  sets the target reactive torque Tf* by multiplying the target reactive torque basic value by a gain corresponding to the steering torque Th 1 . 
     The current command value generation unit  72  generates a motor current command value If* (a d-axis current command value and a q-axis current command value) corresponding to the target reactive torque Tf* calculated by the target reactive torque setting unit  71 . The current feedback control unit  73  controls the driving circuit  44  through known vector control using an output signal from the rotation angle sensor  15 , such that a motor current (a d-axis current and a q-axis current) detected by the current detector  45  and subjected to the three-phase-to-two-phase conversion is equal to the motor current command value If* generated by the current command value generation unit  72 . 
     Thus, the reactive-force motor  14  is controlled such that the motor torque corresponding to the target reactive torque Tf* is generated from the reactive-force motor  14 . The first torque sensor  11  detects a steering torque that is applied to the steering shaft  5  that is a power transmission path extending from the steering wheel  2  to the clutch  6 . In this way, it is possible to detect a torque substantially equal to the steering torque that is applied to the steering wheel  2  by a driver. Thus, it is possible to set the target reactive torque Tf* corresponding to the steering torque that is applied to the steering wheel  2  by the driver. As a result, it is possible to achieve a better steering feel than that in a case where a torque that is applied to the power transmission path extending from the clutch  6  to the steered wheels  3  is detected and the target reactive torque is set based on the detected value. 
       FIG. 5  is a functional block diagram illustrating an operation of the steering motor controller  50  in the first EPS mode. In the first EPS mode, the clutch  6  is engaged. Thus, the steering shaft  5  and the first pinion shaft  16  are connected to each other. In the first EPS mode, the steering motor controller  50  includes, as function processing units thereof, a target assist torque calculation unit  61 , a current command value generation unit  62 , a command value distribution unit  63 , a first current feedback control unit  64 A, and a second current feedback control unit  64 B. 
     The target assist torque calculation unit  61  sets a target assist torque Ta* based on the steering torque Th 2  detected by the second torque sensor  12  and the vehicle speed V detected by the vehicle speed sensor  39 . The target assist torque calculation unit  61 , for example, calculates the target assist torque Ta* by multiplying the steering torque Th 2  by a vehicle speed gain corresponding to the vehicle speed V. The vehicle speed gain is a smaller value as the vehicle speed V is higher. Thus, the absolute value of the target assist torque Ta* is greater as the absolute value of the steering torque Th 2  is greater, and the absolute value of the target assist torque Ta* is smaller as the vehicle speed V is higher. 
     The current command value generation unit  62  generates a motor current command value It* (a d-axis current command value and a q-axis current command value) corresponding to the target assist torque Ta* calculated by the target assist torque calculation unit  61 . The command value distribution unit  63  distributes the motor current command value It* generated by the current command value generation unit  62  to the first current feedback control unit  64 A and the second current feedback control unit  64 B. In the present embodiment, when the first system and the second system of the steering motor  31  are both functioning normally, the command value distribution unit  63  distributes half of the motor current command value It* to each of the first current feedback control unit  64 A and the second current feedback control unit  64 B. That is, a distribution ratio of the motor current command value It* for the first current feedback control unit  64 A and a distribution ratio of the motor current command value It* for the second current feedback control unit  64 B are both 50%. 
     On the other hand, when a malfunction (failure) has occurred in one of the first system and the second system of the steering motor  31 , the distribution ratio for the control unit corresponding to the normally functioning system, among the first current feedback control unit  64 A and the second current feedback control unit  64 B, is set to 100%, and the distribution ratio for the control unit corresponding to the malfunctioning system, among the first current feedback control unit  64 A and the second current feedback control unit  64 B, is set to 0%. The motor current command value distributed to the first current feedback control unit  64 A will be referred to as “first motor current command value It 1 *”. The motor current command value distributed to the second current feedback control unit  64 B will be referred to as “second motor current command value It 2 *”. 
     The first current feedback control unit  64 A controls the first driving circuit  42 A through known vector control using an output signal from the rotation angle sensor  37 , such that a motor current (a d-axis current and a q-axis current) detected by the first current detector  43 A and subjected to the three-phase-to-two-phase conversion is equal to the first motor current command value It 1 *. The second current feedback control unit  64 B controls the second driving circuit  42 B through known vector control using an output signal from the rotation angle sensor  37 , such that a motor current (a d-axis current and a q-axis current) detected by the second current detector  43 B and subjected to the three-phase-to-two-phase conversion is equal to the second motor current command value It 2 *. 
     Thus, the steering motor  31  is controlled such that the motor torque corresponding to the target assist torque Ta* is generated by the steering motor  31 . As a result, a steering assist force corresponding to the steering torque is generated.  FIG. 6  is a functional block diagram illustrating an operation of the reactive-force motor controller  70  in the second EPS mode. 
     In the second EPS mode, the clutch  6  is engaged. Thus, the steering shaft  5  and the first pinion shaft  16  are connected to each other. In the second EPS mode, the reactive-force motor controller  70  includes, as function processing units thereof, a target assist torque calculation unit  81 , a current command value generation unit  82 , and a current feedback control unit  83 . 
     The target assist torque calculation unit  81  sets a target assist torque Ta* based on the vehicle speed V detected by the vehicle speed sensor  39  and the steering torque Th 1  detected by the first torque sensor  11 . The target assist torque calculation unit  81  calculates, for example, the target assist torque Ta* by multiplying the steering torque Th 1  by a vehicle speed gain corresponding to the vehicle speed V. The vehicle speed gain is a smaller value as the vehicle speed V is higher. Thus, the absolute value of the target assist torque Ta* is greater as the absolute value of the steering torque Th 1  is greater, and the absolute value of the target assist torque Ta* is smaller as the vehicle speed V is higher. 
     The current command value generation unit  82  generates a motor current command value If* (a d-axis current command value and a q-axis current command value) corresponding to the target assist torque Ta* calculated by the target assist torque calculation unit  81 . The current feedback control unit  83  controls the driving circuit  44  through known vector control using an output signal from the rotation angle sensor  15 , such that a motor current (a d-axis current and a q-axis current) detected by the current detector  45  and subjected to the three-phase-to-two-phase conversion is equal to the motor current command value if* generated by the current command value generation unit  82 . 
     Thus, the reactive-force motor  14  is controlled such that the motor torque corresponding to the target assist torque Ta* is generated by the reactive-force motor  14 . As a result, a steering assist force corresponding to the steering torque is generated. The vehicle steering system according to the foregoing embodiment is less costly than the vehicle steering control system described in JP 2014-223862 A in which two steering motors are provided. The steering motor  31  includes the motor coils  31 A,  31 B respectively corresponding to two systems. Thus, the steering motor  31  offers a higher level of safety than the steering motor including a motor coil corresponding to only one system. That is, according to the foregoing embodiment, it is possible to achieve cost reduction while ensuring safety. 
     While one example embodiment of the invention has been described above, the invention may be implemented in various other embodiments. For example, in the foregoing embodiment, the steered angle θt of the steered wheels  3  is detected based on the axial travel distance of the rack shaft  17  detected by the stroke sensor  38 . However, the steered angle θt of the steered wheels  3  may be detected based on the rotation angle of the steering motor  31  detected by the rotation angle sensor  37 . 
     The first torque sensor  11  and the second torque sensor  12  are provided in the foregoing embodiment. Alternatively, a single torque sensor may be provided. In this case, the steering torque detected by the single torque sensor is provided to both the steering motor controller  50  and the reactive-force motor controller  70 . Further, various design changes may be made within the technical scope defined in the appended claims.