Control apparatus for steering system

1. A control apparatus for a steering system, the steering system being configured to turn turning wheels of a vehicle and including an electric motor, the control apparatus being configured to control the steering system, and the control apparatus comprising:

control circuitry configured to perform:

an angle control process for controlling, to an angle command value, a convertible angle that is convertible into a rotational angle of the electric motor,

a predetermined component calculation process for calculating a predetermined component containing at least one of two components that are a viscosity component and a friction component of the steering system, while using a value of a variable regarding a control quantity of the electric motor as an input to the predetermined component calculation process, the predetermined component calculation process including a viscosity component calculation process for calculating the viscosity component as the predetermined component, the viscosity component calculation process including:

a disturbance torque calculation process for calculating, as a disturbance torque, a torque component influencing the convertible angle other than a torque of the electric motor, while using a value of a variable regarding the torque of the electric motor, a detection value of the convertible angle, and the angle command value as values of variables regarding the control quantity, and

a filtering process for allowing selective transmission of a specific frequency component of the disturbance torque having a center frequency of 7 to 9 Hz, while using the disturbance torque as an input to the viscosity component calculation process, the viscosity component calculation process using an output of the filtering process as the viscosity component, and

a relationship change process for changing a relationship of an output from a control unit for the steering system to an input to the control unit, based on the predetermined component calculated through the predetermined component calculation process, the relationship change process including a process for changing the relationship of the output from the control unit for the steering system to the input to the control unit, based on the viscosity component calculated through the viscosity component calculation process.

2. The control apparatus for the steering system according to claim 1 , wherein the filtering process includes an intensity change process for changing an intensity of the output in accordance with a temperature of the steering system, even when the input to the filtering process remains unchanged.

3. The control apparatus for the steering system according to claim 1 , wherein the convertible angle is convertible into a turning angle of the turning wheels.

4. The control apparatus for the steering system according to claim 3 , wherein the control unit is configured to control the convertible angle to the angle command value.

5. The control apparatus for the steering system according to claim 4 , wherein the steering system includes a steering wheel that is displaceable without transmitting power to the turning wheels.

6. The control apparatus for the steering system according to claim 3 , wherein:

the steering system includes a steering wheel that is displaceable without transmitting power to the turning wheels;

the electric motor is a turning-side electric motor;

the steering system includes a steering-side electric motor configured to apply a torque against displacement of the steering wheel; and

the control unit is configured to control the torque of the steering-side electric motor.

7. The control apparatus for the steering system according to claim 1 , wherein:

the steering system includes a steering wheel that is displaceable without transmitting power to the turning wheels; and

the electric motor is a steering-side electric motor configured to apply a torque against displacement of the steering wheel.

8. The control apparatus for the steering system according to claim 7 , wherein the control unit is configured to control the torque of the steering-side electric motor.

9. The control apparatus for the steering system according to claim 7 , wherein:

the electric motor is a turning-side electric motor configured to turn the turning wheels; and

the control unit is configured to control a torque of the turning-side electric motor.