Steering apparatus

1. A steering apparatus comprising:

a first shaft which is coupled to a steering wheel, the steering wheel being a steering wheel by which a vehicle is steered;

a torsion bar;

a second shaft which is coupled to the first shaft through the torsion bar;

a torsion bar torque detection unit, the torsion bar torque detection unit detecting a torsion bar torque which is applied to the torsion bar; and

an electronic control unit configured to estimate a driver torque which is applied to the steering wheel by a driver, wherein the electronic control unit is configured to:

i) compute a steering wheel rotation angle which is a rotation angle of the steering wheel;

ii) compute a value including an added value, as the driver torque, the added value being a value resulting from adding the torsion bar torque, a steering wheel inertia torque compensation value, and a gravity torque compensation value, the steering wheel inertia torque compensation value being a product of a second-order differential value of the steering wheel rotation angle and an inertia moment of the steering wheel, the gravity torque compensation value being a compensation value for a gravity torque to be applied to the first shaft by gravity that acts on a gravity center of the steering wheel, wherein:

the electronic control unit is configured to compute the driver torque by adding a coulomb friction torque compensation value to the added value, the coulomb friction torque compensation value being a compensation value for a coulomb friction torque that acts on the first shaft and the steering wheel, and

the coulomb friction torque compensation value is a product of a hyperbolic tangent and a predetermined coulomb friction torque coefficient, the hyperbolic tangent being a hyperbolic tangent of a product of a predetermined coulomb friction torque gradient and a first-order differential value of the steering wheel rotation angle.

2. The steering apparatus according to claim 1 , further comprising a rotational angle detection unit which directly or indirectly detects a rotation angle of the second shaft, wherein the electronic control unit is configured to compute the steering wheel rotation angle using the rotation angle of the second shaft and the torsion bar torque.

3. The steering apparatus according to claim 1 , wherein:

the electronic control unit is configured to compute the driver torque by adding a viscous friction torque compensation value to the added value, the viscous friction torque compensation value being a compensation value for a viscous friction torque that acts on the first shaft and the steering wheel; and

the viscous friction torque compensation value is a product of a first-order differential value of the steering wheel rotation angle and a predetermined viscous friction torque coefficient.

4. The steering apparatus according to claim 1 , wherein:

the gravity torque compensation value is a product of a distance between a gravity center position of the steering wheel and a rotation center position, a mass of the steering wheel, a sine value of the steering wheel rotation angle, and a cosine value of a steering wheel tilt angle;

the steering wheel tilt angle is an angle between a vertical line and a rotation plane of the steering wheel in a state where the steering wheel is mounted on the vehicle, the vertical line being a vertical line that passes through the rotation center position of the steering wheel; and

the steering wheel rotation angle is an angle corresponding to a rotation amount from a neutral position and a rotation direction, the neutral position being a steering wheel position at which an orientation of the vehicle is a straight direction.

5. The steering apparatus according to claim 1 , wherein the electronic control unit is configured to:

i) compute an acceleration in a vehicle-lateral direction that acts on the gravity center of the steering wheel; and

ii) compute the driver torque by adding a lateral acceleration torque compensation value to the added value, the lateral acceleration torque compensation value being a compensation value for a lateral acceleration torque which is applied to the first shaft by the acceleration in the vehicle-lateral direction that acts on the gravity center of the steering wheel.

6. The steering apparatus according to claim 5 , wherein the lateral acceleration torque compensation value is a product of a distance between a gravity center position of the steering wheel and a rotation center position, a mass of the steering wheel, the acceleration in the vehicle-lateral direction that acts on the gravity center of the steering wheel, and a cosine value of the steering wheel rotation angle.

7. The steering apparatus according to claim 1 , wherein:

the electronic control unit is configured to determine whether a hands-on state is formed, based on the driver torque; and

the hands-on state is a state where the driver is holding the steering wheel.

8. The steering apparatus according to claim 7 , further comprising an electric motor configured to assist steering, wherein the electronic control unit is configured to:

i) generate a manual steering command value using the torsion bar torque, only when it is determined that the hands-on state is formed;

ii) compute a summed angle command value by adding the manual steering command value to an automatic steering command value; and

iii) control the electric motor based on the summed angle command value.

9. The steering apparatus according to claim 8 , wherein the electronic control unit is configured to:

i) set an assist torque command value based on the torsion bar torque; and

ii) generate the manual steering command value using the torsion bar torque and the assist torque command value, only when it is determined that the hands-on state is formed.

10. The steering apparatus according to claim 1 , further comprising an electric motor configured to assist steering, wherein the electronic control unit is configured to:

i) generate a manual steering command value using the driver torque;

ii) compute a summed angle command value by adding the manual steering command value to an automatic steering command value; and

iii) control the electric motor based on the summed angle command value.

11. The steering apparatus according to claim 10 , wherein the electronic control unit is configured to:

i) set an assist torque command value based on the driver torque; and

ii) generate the manual steering command value using the driver torque and the assist torque command value.

12. The steering apparatus according to claim 1 , further comprising an electric motor configured to assist steering, wherein the electronic control unit is configured to:

i) generate a manual steering command value using the torsion bar torque, only when an absolute value of the driver torque is equal to or larger than a predetermined value;

ii) compute a summed angle command value by adding the manual steering command value to an automatic steering command value; and

iii) control the electric motor based on the summed angle command value.

13. The steering apparatus according to claim 12 , wherein the electronic control unit is configured to:

i) set an assist torque command value based on the torsion bar torque; and

ii) generate the manual steering command value using the torsion bar torque and the assist torque command value, only when the absolute value of the driver torque is equal to or larger than the predetermined value.