12 according to the deviation Δα. With an increasing deviation Δα, the first current value A1 supplied to the steering actuator 12 is increased, the output of the steering actuator 12 is increased, and the amount of change in the steered angle α is increased.

The control unit 15 computes the target reaction torque Tt to be generated in the reaction force actuator 13 according to the steered state of the front wheels 3 in particular according to the deviation Δα. The target reaction torque Tt may be computed by multiplying Δα by a predetermined coefficient. Then, the control unit 15 computes a second current value A2 to be supplied to the reaction force actuator 13 according to the computed target reaction torque Tt. The second current value A2 to be supplied to the reaction force actuator 13 may be determined in relation with the target reaction torque Tt by referring to a predetermined map. Alternatively, the control unit 15 may determine the second current value A2 according to the deviation Δα by referring to a predetermined map. The target reaction torque Tt and the second current value A2 are each greater in value as the deviation Δα of the steered angle α increases.

The control unit 15 supplies the second current value A2 to the reaction force actuator 13, and generates a corresponding driving force in the reaction force actuator 13. The driving force generated by the reaction force actuator 13 is applied to the steering shaft 18 as a reaction torque T that opposes the operation input of the vehicle operator. As a result, the vehicle operator can receive a reaction force (resistance force) against the steering operation applied to the steering wheel 19.

The vehicle 2 is provided with a control steering center setting switch 37 as a manual input switch for transmitting a steering center setting signal S to the control unit 15. The control steering center setting switch 37 is configured to accept a manual input, and to generate a steering center setting signal S when the manual input is received. As shown by the imaginary line in FIG. 1, alternatively, the control steering center setting switch 37 may be provided in a vehicle diagnosis device 39 which is provided separately from the vehicle 2, and can be selectively connected to the control unit 15.

FIG. 2 is a schematic cross-sectional view of the steering input mechanism 10. As described above, the steering shaft 18 is rotatably supported by the steering column 20. As shown in FIG. 2, a protrusion 41 is formed on the outer surface of the steering shaft 18, and a receiving groove 42 for receiving the protrusion 41 is formed on the steering column 20. The receiving groove 42 is formed on the inner periphery of the steering column 20 over an angular range smaller than 360 degrees, and the end walls defining the respective ends of the receiving groove 42 form physical stoppers 43 that limit the movement of the protrusion 41.

When the steering wheel 19 is turned clockwise, the protrusion 41 eventually comes into contact with the physical stopper 43 on the right side. The angular position where the protrusion 41 abuts on the physical stopper 43 on the right side is the physical end PE on the right side of the steering shaft 18. When the steering wheel 19 is turned counterclockwise, the protrusion 41 eventually comes into contact with the physical stopper 43 on the left side. The angular position where the protrusion 41 abuts on the physical stopper 43 on the left side is the physical end PE on the left side of the steering shaft 18. The center between the physical end PE on the right side and the physical end PE on the left side is the mechanical steering center of the steering shaft 18. The angular range from the mechanical steering center of the steering shaft 18 to the physical end PE on the right side is the physical maximum steering angle βmP on the right side, and the angular range from the mechanical steering center of the steering shaft 18 to the physical end PE on the left side is the physical maximum steering angle βmP on the left side. Thus, the left and right physical stoppers 43 each define the left or the right physical maximum steering angle βmP of the steering shaft 18.

The geometric angular center of the steering wheel 19, or the neutral position of the steering input mechanism 10 as perceived by the driver, usually coincides with the mechanical steering center of the steering shaft 18. When the steering shaft 18 is turned to the physical end PE, an impact noise is generated, and the impact may impair the mechanical durability of the steering input mechanism 10. It is therefore desirable to avoid such noise and the impairment of durability.

Similar physical limits are also provided on the front wheels 3, especially in the steering mechanism 11. The front wheels 3 can be steered only up to the physical end PE in each direction. The physical end PE can be defined in various ways, but is usually defined by the stroke end of the rack 26, commonly referred to as the “rack end”. The center between the left and right stroke ends of the rack 26 is the mechanical steering center of the steering mechanism 11 which usually coincides with the position of the front wheels 3 when the vehicle is traveling straight, or when the steered angle α of the steering mechanism 11 is 0°. The ratio K (=B/α) of the steering angle β to the steered angle α is set so that the physical end PE of the steering shaft 18 corresponds to the physical end PE of the rack 26. When the front wheels 3 are steered to the rack end, an impact noise is generated and the mechanical durability of the steering mechanism 11 may be impaired due to the impact. It is desirable to avoid such noise and impairment of durability.

In the present embodiment, a control end CE, which is the control rotation limit of the steering shaft 18, is set for each rotational direction at an angle smaller than the angle of the physical end PE by an angle β0. The control steering center (angular position of steering angle β=0) of the steering input mechanism 10 corresponds to the rotational angle of the steering shaft that causes the steered angle α of the front wheels 3 to be substantially zero, and the vehicle 2 travels straight. Ideally, the control steering center should correspond to the geometric steering center of the steering wheel 19. The angular range from the control steering center of the steering shaft 18 to the control end CE on the right side is defined as the control maximum steering angle βmC on the right side, and the angular range from the control steering center of the steering shaft 18 to the control end CE on the left side is defined as the control maximum steering angle βmC on the left side. The target steered angle αt is related to the steering angle β in such a manner that the steered angle α of the front wheels 3 increases as the steering angle β increases. However, when the steering angle β increases beyond the control end CE, the command to the steering actuator 12 is limited to the control maximum steering angle corresponding to the control maximum steering angle βmC. In other words, when the steering input mechanism 10 is turned beyond the maximum control steering angle βmC, the front wheels 3 are steered only up to the maximum control steered angle. The maximum control steered angle is smaller than the maximum physical steered angle.

The reaction torque T that is applied to the steering input mechanism 10 is normally controlled so as to increase independence on the deviation Δα in the steered angle α from the target steered angle αt. However, when the steering angle β is equal to or greater than the control maximum steering angle βmC, the control unit 15 applies a steering limit reaction torque Ts1 to the steering input mechanism 10 to prevent the steering input mechanism 10 from being turned beyond the control maximum steering angle βmC. The steering limit reaction torque Ts1 is a value substantially greater than the normal value of the reaction torque T that could be generated in other situations. The control unit 15 computes a second current value A2 that is required to be supplied to the reaction force actuator 13 to generate the steering limit reaction torque Ts1.

When the steering angle β of the front wheels 3 reaches the control maximum steering angle βmC, the steering limit reaction force torque Ts1 is applied to the steering input mechanism 10. As a result, the driver recognizes that the steered angle α of the front wheels 3 has reached the maximum angle due to the reaction force received from the steering input mechanism 10, and is thereby urged not to turn the steering input mechanism 10 excessively.

As described above, the steering system 1 is provided with a physical stopper 43 that defines the physical maximum steering angle βmP on the left and right sides of the steering shaft 18. Therefore, problems such as damaging wire harness by excessive rotation of the steering shaft 18 can be avoided.

Further, the control unit 15 sets a control maximum steering angle βmC smaller than the physical maximum steering angle βmP, and drives the reaction force actuator 13 so as to generate a steering limit reaction torque Ts1 that defines the control maximum steering angle βmC. When the control unit 15 generates the steering limit reaction torque Ts1, the steering shaft 18 is prevented from colliding with the physical stopper 43. As a result, the generation of collision noise and impact even when the steering wheel 19 is turned excessively. Since the maximum control steering angle βmC of the steering wheel 19 is set on each side of the geometric steering center of the steering wheel 19 (clockwise rotation and counter-clockwise rotation), the maximum steering angle of the steering wheel 19 is evenly distributed to the left and right.

FIG. 3 is a diagram showing the relationship between the physical maximum steering angle βmP and the control maximum steering angle βmC as a result of a proper assembly work. As shown in FIG. 3, the steering wheel 19 is assembled to the steering shaft 18 properly so that the geometric steering center of the steering wheel 19 coincides with the mechanical steering center (which is the same as the control steering center in this case) of the steering shaft 18. The steering angle β (angular displacement from the steering center β=0) of the steering shaft 18 detected by the steering angle sensor 21 and the angular displacement of the steering wheel 19 from the geometric steering center as perceived by the drive are in agreement with each other.

However, when assembling the vehicle 2 at the factory, the steering wheel 19 may be assembled to the steering shaft 18 in such a manner that the geometric steering center of the steering wheel 19 is offset from the mechanical steering center of the steering shaft 18. It is also possible that the geometric steering center of the steering wheel 19 may be offset from the mechanical steering center of the steering shaft 18 when the steering wheel 19 is removed from the steering shaft 18 and reassembled during post-factory maintenance.

FIG. 4 is a diagram illustrating the process of correcting the output of the steering angle sensor 21 when a small assembly error was made when installing the steering wheel 19. In this example, as shown in FIG. 4(A), the steering wheel 19 is assembled to the steering shaft 18 such that the steering wheel 19 is turned slightly to the left from the geometric steering center thereof when the steering shaft 18 is at the mechanical steering center. Alternatively, as shown in FIG. 4(B), the steering wheel 19 is assembled to the steering shaft 18 such that the steering wheel 19 is at the geometrical steering center while the steering shaft 18 is slightly turned to the right from the mechanical steering center thereof.

In either case, the person who has performed the assembly work can readily correct the output of the steering angle sensor 21 by operating the control steering center setting switch 37 and inputting the steering center setting signal S to the control unit 15. More specifically, the worker manually operates the control steering center setting switch 37 in a state where the steering wheel 19 is placed at the geometric steering center as shown in FIG. 4(B). When the control steering center setting switch 37 is operated, the steering center setting signal S is generated, and the steering center setting signal S is forwarded to the control unit 15. When the steering center setting signal S is received, the control unit 15 corrects the output of the steering angle sensor 21 so that the current steering angle β is brought to or brought closer to the value corresponding to the control steering center (β=0).

As a result, the worker can correct the output of the steering angle sensor 21 such that the steering angle β at that time is the value corresponding to the control steering center. In other words, even if a setting error should occur when assembling the steering wheel 19, the output of the steering angle sensor 21 can be reset to the value corresponding to the mechanical steering center while the steering wheel 19 is held at the geometrical steering center thereof without requiring the steering wheel 19 to be reassembled. Therefore, the work efficiency in assembling the steering system 1 can be improved. Further, since no physical positioning mechanism is required for the steering system 1, an increase in size and cost of the steering system 1 can be avoided.

The control steering center setting switch 37 may be a switch provided on the vehicle 2 as described above. By providing the control steering center setting switch 37 in the vehicle 2, the worker can easily correct the output of the steering angle sensor 21 by operating the control steering center setting switch 37.

Alternatively, the control steering center setting switch 37 may be a switch provided in the vehicle diagnosis device 39 that is separate from the vehicle 2 as mentioned above. In this case, when the worker operates the control steering center setting switch