Steering control device and method for controlling steering system

1. A steering control device for a steering system that changes steering torque required to steer a steering wheel according to motor torque that is applied by an actuator using a motor as a driving source of the actuator, the steering control device comprising

an electronic control unit configured to:

calculate target torque that is a target value of a motor torque;

control operation of a motor such that the motor torque is generated according to the target torque;

calculate a vehicle speed basic axial force based on a detected vehicle speed;

calculate another state quantity basic axial force based on a state quantity other than the detected vehicle speed;

calculate a distributed axial force by adding the vehicle speed basic axial force and the other state quantity basic axial force at individually set distribution ratios;

calculate the target torque based on the distributed axial force; and

reduce a distribution ratio of the vehicle speed basic axial force when the detected vehicle speed is abnormal as compared to when the detected vehicle speed is normal.

2. The steering control device according to claim 1 , wherein the electronic control unit is configured to set the distribution ratio of the vehicle speed basic axial force to zero when the detected vehicle speed is abnormal.

3. The steering control device according to claim 1 , wherein:

the vehicle speed basic axial force is at least one of an angle axial force that does not include road surface information and a vehicle state quantity axial force including information that is transmittable through a change in lateral behavior of a vehicle out of the road surface information; and

the other state quantity basic axial force is a road surface axial force including the road surface information.

4. The steering control device according to claim 1 , wherein:

the steering system has a structure in which power transmission to and from a steering unit is separated from power transmission to and from a steered unit that steers a steered wheel according to steering that is input to the steering unit;

the motor is a steering-side motor that applies the motor torque as a steering reaction force that is a force against the steering that is input to the steering unit; and

the electronic control unit is configured to calculate, as the target torque, target reaction torque that is a target value of the steering reaction force.

5. A method for controlling a steering system that changes steering torque required to steer a steering wheel according to motor torque that is applied by an actuator using a motor as a driving source of the actuator, the method comprising:

calculating, by an electronic control unit, target torque that is a target value of a motor torque;

controlling, by the electronic control unit, operation of a motor such that the motor torque is generated according to the target torque;

calculating, by the electronic control unit, a vehicle speed basic axial force based on a detected vehicle speed;

calculating, by the electronic control unit, another state quantity basic axial force based on a state quantity other than the detected vehicle speed;

calculating, by the electronic control unit, a distributed axial force by adding the vehicle speed basic axial force and the other state quantity basic axial force at individually set distribution ratios;

calculating, by the electronic control unit, the target torque based on the distributed axial force; and

reducing, by the electronic control unit, a distribution ratio of the vehicle speed basic axial force when the detected vehicle speed is abnormal as compared to when the detected vehicle speed is normal.