Vehicle steering device

1. A vehicle steering device comprising:

an electric motor that applies a steering force to a turning mechanism of a vehicle;

a microcomputer configured to:

set a target assist torque in accordance with a steering torque;

set an angle controlling target torque for reducing an angular deviation between a target steering angle and an actual steering angle;

estimate a compensation object load with respect to the angle controlling target torque;

calculate a target automatic steering torque based on the angle controlling target torque set by the microcomputer and the compensation object load estimated by the microcomputer; and

perform weighted addition of the target automatic steering torque and the target assist torque in accordance with a value that changes in accordance with a driver input to calculate a target motor torque that is a target value of a motor torque of the electric motor; and

a drive circuit configured to control the electric motor based on the target motor torque that is calculated by the microcomputer.

2. The vehicle steering device according to claim 1 , wherein the compensation object load includes a road surface load torque that a drive object of the electric motor receives from a road surface.

3. The vehicle steering device according to claim 1 , wherein a torsion bar is included in middle of a torque transmission path between a steering member and the turning mechanism and

the compensation object load includes a friction torque that is generated in a torque transmission path from the torsion bar to a turned wheel.

4. The motor controller according to claim 1 , wherein the value that changes in accordance with the driver input used by the microcomputer is the angular deviation or the steering torque.

5. The vehicle steering device according to claim 4 , wherein the value that changes in accordance with the driver input used by the microcomputer is the angular deviation,

the microcomputer is configured to calculate the target motor torque by adding a value resulting from multiplying the target automatic steering torque by a first weighting factor and a value resulting from multiplying the target assist torque by a second weighting factor,

the first weighting factor is set to become 0 when an absolute value of the angular deviation is more than a predetermined value and to decrease gradually from 1 to 0 as the absolute value of the angular deviation increases from 0 when the absolute value of the angular deviation is not more than the predetermined value, and

the second weighting factor is set to become 1 when the absolute value of the angular deviation is more than the predetermined value and to increase gradually from 0 to 1 as the absolute value of the angular deviation increases from 0 when the absolute value of the angular deviation is not more than the predetermined value.

6. The motor controller according to claim 1 , wherein the microcomputer is further configured to:

perform a predetermined feedback calculation on the angular deviation to calculate a feedback control torque,

multiply a second order derivative of the target steering angle by a moment of inertia of the vehicle steering device to calculate a feedforward control torque, and

add the feedforward control torque to the feedback control torque to calculate the angle controlling target torque.

7. The vehicle steering device according to claim 1 , wherein the microcomputer is further configured to:

restrict the angle controlling target torque set by the microcomputer to be between a predetermined upper limit value and a predetermined lower limit value and

calculate the target automatic steering torque based on the angle controlling target torque after a restriction process by the microcomputer and the compensation object load estimated by the microcomputer.