Abstract:
A steering system includes a steering unit including a first actuator; a turning unit including a second actuator; and a backup mechanism configured to mechanically couple the steering unit and the turning unit, wherein a maximum torque that the first actuator is able to transmit to the second actuator via the backup mechanism is larger than or equal to a maximum torque that the second actuator is able to transmit to the first actuator via the backup mechanism.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to a steering system for a vehicle. More particularly, the invention relates to a steer-by-wire steering system for a vehicle. 
         [0003]    2. Description of Related Art 
         [0004]    In recent years, development of a steer-by-wire system (hereinafter, SBW system) in which a steering unit including a steering wheel and a turning unit including a turning mechanism are mechanically separated from each other has been proceeding. The SBW system may include a backup mechanism including a clutch that mechanically couples the steering unit to the turning unit for the purpose of fail-safe. In the thus configured SBW system, the turning mechanism is controlled by an electric signal in response to operation of the steering wheel (hereinafter, SBW control) when the system is normal. On the other hand, when there occurs an abnormality in the SBW system, the clutch is quickly connected, SBW control is stopped, and the system is switched into assist control for reducing a burden on driver&#39;s steering. 
         [0005]    However, if the clutch is connected although there is no abnormality in the SBW system, the steering wheel may rotate as a result of continuation of SBW control (in other words, operation of a turning actuator). In SBW control, the turning actuator is driven such that an actual steered angle of steered wheels follows a command steered angle. When the clutch is connected, the steering wheel and the turning mechanism are mechanically coupled to each other by the clutch. Therefore, when the turning actuator turns the steered wheels in SBW control, the steering wheel mechanically rotates in interlocking with the turn of the steered wheels, so the command steered angle changes (hereinafter, the phenomenon that the steering wheel is rotated through this operation may be simply described as that the steering wheel is rotated interlockingly). Therefore, the deviation between the command steered angle and the actual steered angle does not reduce, and self-steer may occur. 
         [0006]    Japanese Patent Application Publication No. 2007-137294 (JP 2007-137294 A) describes a technique switching from SBW control to assist control when an SBW system determines that a clutch is connected in process of SBW control. 
         [0007]    According to JP 2007-137294 A, because SBW control is executed until connection of the clutch is detected, there is an undeniable possibility that the steering wheel is rotated interlockingly in a period from when the clutch is connected to when SBW control is switched to assist control. 
       SUMMARY OF THE INVENTION 
       [0008]    The invention provides a technique for reducing occurrence of self-steer due to connection of a clutch in process of SBW control in an SBW system. 
         [0009]    An aspect of the invention provides a steering system. The steering system includes a steering unit including a first actuator; a turning unit including a second actuator; and a backup mechanism configured to mechanically couple the steering unit and the turning unit, wherein a maximum torque that the first actuator is able to transmit to the second actuator via the backup mechanism is larger than or equal to a maximum torque that the second actuator is able to transmit to the first actuator via the backup mechanism. 
         [0010]    According to the above aspect, it is possible to reduce interlocking rotation of a steering wheel. 
         [0011]    In the above aspect, the steering system may further include a control unit configured to set a generatable maximum torque of the second actuator on the basis of a vehicle state. Thus, it is possible to achieve steering control suitable for a vehicle state. 
         [0012]    In the above aspect, when a vehicle speed is higher than a first predetermined value, the control unit may be configured to set the generatable maximum torque of the second actuator such that the maximum torque that the second actuator is able to transmit via the backup mechanism becomes smaller than or equal to the maximum torque that the first actuator is able to transmit via the backup mechanism. Thus, for example, it is allowed to employ a small-sized motor as the first actuator. 
         [0013]    In the above aspect, when a steering angle is smaller than a second predetermined value, the control unit may be configured to set the generatable maximum torque of the second actuator such that the maximum torque that the second actuator is able to transmit via the backup mechanism becomes smaller than or equal to the maximum torque that the first actuator is able to transmit via the backup mechanism. Thus, for example, it is allowed to employ a small-sized motor as the first actuator. 
         [0014]    According to the above aspect, it is possible to reduce occurrence of self-steer due to connection of a clutch in process of SBW control in an SBW system. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
           [0016]      FIG. 1  is a view that shows the schematic configuration of a vehicle steering system according to an embodiment of the invention; 
           [0017]      FIG. 2  is a view that shows the relationship between torque that a steering reaction actuator transmits via a clutch and torque that a turning actuator transmits via the clutch; 
           [0018]      FIG. 3  is a view that shows an example of a flowchart for limiting a maximum value of turning torque; and 
           [0019]      FIG. 4  is a view that shows another example of a flowchart for limiting a maximum value of turning torque. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0020]      FIG. 1  shows the schematic configuration of a vehicle steering system  1  according to an embodiment of the invention. The vehicle steering system  1  includes a steering unit  20  and a turning unit  50 . The steering unit  20  is steered by a driver. The turning unit  50  turns right and left front wheels FW 2 , FW 1  in response to driver&#39;s steering. The right and left front wheels FW 2 , FW 1  are steered wheels. An SBW system is employed as the vehicle steering system  1 . When the system is normal, the steering unit  20  and the turning unit  50  are mechanically separated from each other by a clutch  30 . In the example shown in  FIG. 1 , a state where the clutch  30  is connected and the steering unit  20  and the turning unit  50  are mechanically coupled to each other is shown. 
         [0021]    The steering unit  20  includes a steering wheel  10 , a steering shaft  12 , a steering angle sensor  14 , a steering torque sensor  16  and a steering reaction actuator  18 . The steering wheel  10  is a steering member that is rotationally operated by the driver, and is fixed to the top of the steering shaft  12 . The steering reaction actuator  18  includes a steering reaction motor for generating steering reaction, and is assembled to the lower portion of the steering shaft  12 . The steering reaction motor rotationally drives the steering shaft  12  around the axis of the steering shaft  12 . The steering angle sensor  14  detects a steering angle of the steering wheel  10  that is rotationally operated by the driver. The steering torque sensor  16  detects a steering torque that is applied to the steering wheel  10 . 
         [0022]    The turning unit  50  includes a rack bar  42  and a turning actuator  44 . The rack bar  42  extends in the transverse direction of a vehicle. The right and left front wheels FW 2 , FW 1  are turnably connected to both ends of the rack bar  42  via corresponding tie rods and knuckle arms (not shown). The right and left front wheels FW 2 , FW 1  are turned right and left by displacement of the rack bar  42  in, the axial direction. The turning actuator  44  includes a turning motor provided around the rack bar  42 . Rotation of the turning motor is converted to displacement of the rack bar  42  in the axial direction. A rotation angle sensor  40  detects a rotation angle of the turning motor. 
         [0023]    The turning unit  50  further includes a turning shaft  34 , a pinion gear  36  and rack teeth  38 . The turning shaft  34  is rotatable around its axis. The pinion gear  36  is fixed to the lower end of the turning shaft  34 . The pinion gear  36  is in mesh with the rack teeth  38  of the rack bar  42 . Thus, the turning shaft  34  rotates around its axis in interlocking with displacement of the rack bar  42  in the axial direction. 
         [0024]    The vehicle steering system  1  includes a backup mechanism  22  between the steering unit  20  and the turning unit  50  for the purpose of fail-safe. The backup mechanism  22  includes the clutch  30  and a backup cable  32 . The backup mechanism  22  has the function of mechanically coupling or separating the steering unit  20  to or from the turning unit  50 . The backup cable  32  has flexibility. The upper end of the backup cable  32  is connected to the clutch  30 , and the lower end of the backup cable  32  is connected to the turning shaft  34 . Thus, the turning shaft  34  integrally rotates with the backup cable  32 . The clutch  30  is an electromagnetic clutch, such as a two-way clutch and a multi-disk clutch. The clutch  30  is supplied with current and is set to a disconnected state (separated state) when the vehicle steering system  1  is normal; whereas supply of current to the clutch  30  is stopped and the clutch  30  is set to a connected state by spring force, or the like, when the vehicle steering system  1  is abnormal. Therefore, when supply of current is stopped, the steering shaft  12  of the steering unit  20  and the turning shaft  34  of the turning unit  50  integrally rotate via the connected clutch  30 . 
         [0025]    A control unit  60  is control means that is formed of a steering reaction ECU  62  and a turning ECU  64 . The steering reaction ECU  62  executes drive control over the steering reaction actuator  18 . The turning ECU  64  executes drive control over the turning actuator  44 . The steering reaction ECU  62  receives a detected steering angle from the steering angle sensor  14 , and receives a detected steering torque from the steering torque sensor  16 . The turning ECU  64  receives a detected rotation angle of the turning motor in the turning actuator  44  from the rotation angle sensor  40 . 
         [0026]    When the system is normal, the turning ECU  64  sets the clutch  30  to the disconnected state by supplying current to the clutch  30 . When the driver rotationally operates the steering wheel  10 , the steering reaction ECU  62  receives the detected steering angle from the steering angle sensor  14 , and transmits the detected steering angle to the turning ECU  64 . The turning ECU  64  obtains a command steered angle of the right and left front wheels FW 2 , FW 1  based on the operating state of the steering wheel  10 , and drives the turning actuator  44  such that the command steered angle is attained. Specifically, the turning ECU  64  derives an actual steered angle of the right and left front wheels FW 2 , FW 1  on the basis of the detected rotation angle received from the rotation angle sensor  40 , and drives the turning actuator  44  such that the actual steered angle follows the command steered angle. The steering reaction ECU  62  drives the steering reaction actuator  18  such that a command steering reaction torque based on the steering angle of the steering wheel  10  and the steered state of the right and left front wheels FW 2 , FW 1  is applied to the steering wheel  10 . In this way, when the system is normal, in a state where the steering unit  20  and the turning unit  50  are separated from each other, the control unit  60  executes SBW control in which the steered wheels are turned while steering reaction is applied to the steering wheel  10 . 
         [0027]    On the other hand, when the system is abnormal, the turning ECU  64  sets the clutch  30  to the connected state by stopping supply of current to the clutch  30 . Thus, the steering shaft  12  and the turning shaft  34  are mechanically coupled. The control unit  60  stops SBW control, and executes assist control. In assist control, at least one of the steering reaction actuator  18  and the turning actuator  44  is driven such that assist torque is applied to driver&#39;s operation on the basis of the detected steering torque from the steering torque sensor  16 . For example, the control unit  60  executes assist control for driving the turning actuator  44  when the steering reaction actuator  18  is abnormal, and executes assist control for driving the steering reaction actuator  18  when the turning actuator  44  is abnormal. In this way, when the system is abnormal, in a state where the steering unit  20  and the turning unit  50  are coupled, the control unit  60  executes assist control for adding assist torque to the steering wheel  10 . The control unit  60  desirably immediately switches from SBW control to assist control at the timing at which occurrence of a system abnormality is detected. 
         [0028]    Incidentally, the clutch  30  has such a structure that spring force acts on the clutch  30  in the connecting direction as described above. When current is supplied, the clutch  30  should be released and keep the disconnected state. However, the clutch  30  may be erroneously connected by the action of spring force. When the clutch  30  is erroneously connected, the control unit  60  desirably detects occurrence of a system abnormality and switches from SBW control to assist control. However, in a period from the timing at which the clutch  30  is actually erroneously connected to the timing at which SBW control is stopped, SBW control is executed for driving the turning actuator  44  such that the actual steered angle of the steered wheels follows the command steered angle. 
         [0029]    The vehicle steering system  1  includes clutch state detecting means (not shown) for detecting whether the clutch  30  is in the connected state or the separated state. When the connected state is detected by the clutch state detecting means although current is supplied to the clutch  30 , the control unit  60  may determine that the clutch  30  is erroneously connected and detect that there occurs an abnormality in the system. 
         [0030]      FIG. 2  shows the relationship between torque that the steering reaction actuator  18  transmits via the clutch  30  and torque that the turning actuator  44  transmits via the clutch  30 . The steering reaction motor of the steering reaction actuator  18  serves to apply steering reaction to the steering wheel  10 . The turning motor of the turning actuator  44  serves to turn the right and left front wheels FW 2 , FW 1 . 
         [0031]    Because the turning motor needs to generate large turning torque particularly at the time of stationary steering, a maximum torque outputtable by the turning motor is considerably larger than a maximum torque outputtable by the steering reaction motor in an existing steering system. Therefore, in the existing steering system, when the clutch  30  is erroneously connected in process of SBW control, torque generated by the turning actuator  44  is transmitted to the steering wheel  10  via the backup mechanism  22 , and the steering wheel  10  is rotated interlockingly. 
         [0032]    Therefore, in the vehicle steering system  1  according to the present embodiment, a maximum torque A that the steering reaction actuator  18  is able to transmit to the turning actuator  44  via the backup mechanism  22  is larger than or equal to a maximum torque B that the turning actuator  44  is able to transmit to the steering reaction actuator  18  via the backup mechanism  22 . By setting driving conditions of the steering reaction actuator  18  and turning actuator  44  such that the maximum torque A becomes larger than or equal to the maximum torque B, torque generated by the turning actuator  44  is allowed to be cancelled by torque generated by the steering reaction actuator  18  in a torque transmission path between the steering reaction actuator  18  and the turning actuator  44 . 
         [0033]    During SBW control, the steering reaction ECU  62  obtains a command steering reaction torque that is applied to the steering wheel  10  on the basis of at least the steering angle of the steering wheel  10 , detected by the steering angle sensor  14 , and drives the steering reaction actuator  18 . In the case where the clutch  30  is in the connected state, when the turning ECU  64  drives the turning actuator  44  such that the actual steered angle of the steered wheels follows the command steered angle, the driver adds force to the steering wheel  10  in the return direction such that the steering wheel  10  is not rotated interlockingly. The steering reaction ECU  62  derives an actual steering reaction torque that is actually applied to the steering wheel  10  from the detected steering torque received from the steering torque sensor  16 . When the steering reaction ECU  62  detects a deviation between the actual steering reaction torque and the command steering reaction torque, the steering reaction ECU  62  drives the steering reaction actuator  18  such that the deviation is reduced. 
         [0034]    In this way, when the clutch  30  is connected in process of SBW control, the steering reaction actuator  18  and the turning actuator  44  each generate torque in a direction to cancel torque from the other side. At this time, in the vehicle steering system  1 , because the maximum torque A that the steering reaction actuator  18  is able to transmit via the backup mechanism  22  is set so as to be larger than or equal to the maximum torque B that the turning actuator  44  is able to transmit via the backup mechanism  22 , even if the maximum torque B is transmitted from the turning actuator  44 , the steering reaction actuator  18  is able to generate torque that cancels the maximum torque B. Therefore, it is possible to reduce occurrence of self-steer, that is, rotation of the steering wheel  10  interlockingly. When the maximum torque A is larger than the maximum torque B, not only it is allowed to cancel torque that is transmitted from the turning actuator  44  via the backup mechanism  22  but also the steering reaction actuator  18  is allowed to apply appropriate steering reaction torque to the steering wheel  10 . In this way, by using the steering reaction motor having a higher rating than the existing one, it is possible to reduce occurrence of self-steer by effectively utilizing the feature of SBW control at the time of erroneous connection of the clutch  30  without necessity to determine whether the clutch  30  is erroneously connected. As described above, the control unit  60  may immediately switch from SBW control to assist control after a system abnormality due to erroneous connection of the clutch  30  is detected. 
         [0035]    Generally, a motor having a high rating is larger and more expensive than a motor having a lower rating. Therefore, a technical idea for making it possible to reduce occurrence of self-steer due to connection of the clutch  30  even when a steering reaction motor having a lower rating than a turning motor is used as in the case of the existing steering system will be described below. In the following example, the control unit  60  sets a generatable maximum torque of the turning actuator  44  on the basis of a vehicle state. An example in which the vehicle state includes a vehicle speed and a steering angle (steered angle) will be described below; however, a generatable maximum torque of the turning actuator  44  may be set on the basis of another state. 
         [0036]    As described above, in the turning actuator  44 , a turning motor having a large outputtable maximum torque is used so as to be able to sufficiently turn the steered wheels at the time of stationary steering. Considering a steered torque required for turning in terms of a vehicle speed range, a turning torque required while traveling at a vehicle speed of 0 to a minute low speed (0 to 5 km/h) is larger than a turning torque required while traveling at a low speed, an intermediate speed or a high speed. In other words, the turning actuator  44  does not need to generate a large turning torque in a vehicle speed range over a predetermined speed (for example, 5 km/h), and limiting the maximum torque of the turning actuator  44  does not impair turning during traveling. That is, in a vehicle speed range over the predetermined speed, the generatable maximum torque of the turning actuator  44  is allowed to be set so as to be lower than the rating. 
         [0037]      FIG. 3  shows an example of a flowchart for limiting the maximum value of turning torque. Here, a current value required at the time of generating the rated maximum torque of the turning motor is Ia. The turning ECU  64  receives a detected vehicle speed from a vehicle speed sensor  52 , and obtains a vehicle speed. When the vehicle speed is higher than a threshold (for example, 5 km/h) (Y in S 10 ), the turning ECU  64  limits the maximum value of turning torque that is generated by the turning actuator  44  (S 12 ). Specifically, the turning ECU  64  sets Ib (&lt;Ia) for the maximum current value that is supplied to the turning motor, thus setting the generatable maximum torque such that the generatable maximum torque is smaller than the rated maximum torque. On the other hand, when the vehicle speed is lower than or equal to the threshold (N in S 10 ), the turning ECU  64  sets the generatable maximum torque to the rated maximum torque, thus setting the supplied maximum current value to Ia. 
         [0038]    The maximum value of turning torque, limited in S 12 , will be described. Initially, the maximum value of turning torque needs to be a value enough to turn the steered wheels when the vehicle speed is higher than the threshold. Subsequently, as described with reference to  FIG. 2 , in order to reduce occurrence of self-steer due to erroneous connection of the clutch  30 , the maximum torque A that the steering reaction actuator  18  is able to transmit via the backup mechanism  22  needs to be larger than or equal to the maximum torque B that the turning actuator  44  is able to transmit via the backup mechanism  22 . 
         [0039]    Thus, when the vehicle speed is higher than the threshold, the, turning ECU  64  sets the generatable maximum torque of the turning actuator  44  such that the maximum torque that the turning actuator,  44  is able to transmit via the backup mechanism  22  is smaller than or equal to the maximum torque that the steering reaction actuator  18  is able to transmit via the backup mechanism  22 . Thus, even when turning torque generated by the turning actuator  44  is transmitted via the backup mechanism  22 , the turning torque is allowed to be cancelled by torque that is generated by the steering reaction actuator  18 . In this way, by changing the generatable maximum torque of the turning actuator  44  on the basis of the vehicle speed range, it is possible to reduce occurrence of self-steer due to erroneous connection of the clutch  30  in the case where the vehicle speed is higher than the threshold even when the steering reaction actuator  18  includes a steering reaction motor having a low rating. 
         [0040]    When the vehicle speed is lower than or equal to the threshold, by setting the maximum value of supplied current to Ia such that the turning motor is able to generate the rated maximum torque, the turning actuator  44  is allowed to turn the steered wheels even at the time of, for example, stationary steering. If the clutch  30  is erroneously connected at this time, there is a possibility that self-steer occurs; however, it is possible to suppress the influence of self-steer on vehicle behavior to a significantly lesser degree because of a minute low speed. In the above description, when the vehicle speed falls within the minute low vehicle speed range (0 to 5 km/h) in S 10 , the rated maximum torque of the turning motor is not limited. Instead, the rated maximum torque of the turning motor may not be limited only during a vehicle stop by setting the threshold to 0 km/h in order to minimize the influence of self-steer on vehicle behavior. 
         [0041]    Next, turning torque required for turning in terms of the steering angle will be studied. For example, at the time of stationary steering, the maximum turning torque is required around the ends of the range in which the steered wheels are turnable, that is, around points at which the steered angle of the steered wheels is maximum. In other words, when the steered angle of the steered wheels is smaller than a predetermined value (a value just before the maximum value of the steered angle), the turning actuator  44  does not need to generate large turning torque, and turning is not impaired even when the maximum torque of the turning actuator  44  is limited. That is, when the steered angle is smaller than the predetermined value, the generatable maximum torque of the turning actuator  44  is allowed to be set so as to be smaller than the rating. Therefore, an example in which the turning ECU  64  limits the maximum value of turning torque, by using the steering angle of the steering wheel  10  at the time when the steered angle of the steered wheels becomes a predetermined value, will be described. 
         [0042]      FIG. 4  shows another example of a flowchart for limiting the maximum value of turning torque. The turning ECU  64  receives the detected steering angle from the steering angle sensor  14  via the steering reaction ECU  62 , and acquires the steering angle of the steering wheel  10 . When the steering angle is smaller than a threshold (Y in S 20 ), the turning ECU  64  limits the maximum value of turning torque generated by the turning actuator  44  (S 22 ). Specifically, the turning ECU  64  sets Ib (&lt;Ia) for the maximum current value that is supplied to the turning motor, thus setting the generatable maximum torque such that the generatable maximum torque is smaller than the rated maximum torque. On the other hand, when the steering angle is larger than or equal to the threshold (N in S 20 ), the turning ECU  64  sets the generatable maximum torque to the rated maximum torque, thus setting the supplied maximum current value to Ia. The threshold to be compared with the steering angle in S 20  is the steering angle at the time when the steered angle of the steered wheels becomes the predetermined value as described above. 
         [0043]    The maximum value of turning torque, which is limited in S 22 , may be the same as the maximum value of turning torque, which is limited in S 12 . When the steering angle is smaller than the threshold, the turning ECU  64  sets the generatable maximum torque of the turning actuator  44  such that the maximum torque that the turning actuator  44  is able to transmit via the backup mechanism  22  becomes smaller than or equal to the maximum torque that the steering reaction actuator  18  is able to transmit via the backup mechanism  22 . In this way, by changing the generatable maximum torque of the turning actuator  44  on the basis of the vehicle speed range, it is possible to reduce occurrence of self-steer due to erroneous connection of the clutch  30  in the case where the steering angle is smaller than the threshold even when the steering reaction actuator  18  includes a steering reaction motor having a low rating. 
         [0044]    When the steering angle is larger than or equal to the threshold, by setting the maximum value of supplied current to Ia such that the turning motor is able to generate the rated maximum torque, the turning actuator  44  is allowed to turn the steered wheels even, for example, around the ends at the time of stationary steering. At this time, when the clutch  30  is erroneously connected, there is a possibility that self-steer occurs; however, the steered wheels are turned to around the ends only when the vehicle speed is 0 or minute low speed, so it is possible to suppress the influence of self-steer on vehicle behavior to a significantly lesser degree. 
         [0045]    The invention is described on the basis of the embodiment. However, the embodiment is only illustrative, and it should be understood by persons skilled in the art that various modifications are applicable in combinations of components and processes and the invention also encompasses such modifications. 
         [0046]    In the flowchart shown in  FIG. 4 , the maximum value of turning torque is limited on the basis of the steering angle; instead, the maximum value of turning torque may be limited on the basis of not the steering angle but the steered angle. The two flowcharts for limiting the maximum value of turning torque are shown in  FIG. 3  and  FIG. 4 ; and these two flowcharts may be utilized in AND condition or OR condition. The case where the two flowcharts are utilized in AND condition means that step of S 12  (or S 22 ) is executed when both Y in S 10  and Y in S 20  are satisfied. The case where the two flowcharts are utilized in OR condition means that the control unit  60  executes the two flowcharts independently.