Vehicle steering device

1. A vehicle steering device, which includes a right steering operation mechanism that steers a right steered wheel and a left steering operation mechanism that steers a left steered wheel, and in which a steering member that is operated to steer a vehicle is mechanically coupled to neither the right steering operation mechanism nor the left steering operation mechanism, the right steering operation mechanism being driven by a right steering motor, and the left steering operation mechanism being driven by a left steering motor, the vehicle steering device comprising:

a higher-level control device that sets a right target steered angle and a left target steered angle, the right target steered angle being a target value for a steered angle of the right steered wheel, and the left target steered angle being a target value for a steered angle of the left steered wheel;

a left steering control device that drivingly controls the left steering motor; and

a right steering control device that drivingly controls the right steering motor, wherein:

the left steering control device

switches a left control mode between a first control mode and a second control mode, the first control mode being a mode in which the left steering control device controls the left steering motor so that the steered angle of the left steered wheel becomes equal to the left target steered angle received from the higher-level control device, and the second control mode being a mode in which the left steering control device controls the left steering motor by torque feedback control so that the steered angle of the left steered wheel becomes equal to an angle corresponding to a neutral position of the left steered wheel independent of a steering wheel, and

normally sets the left control mode to the first control mode, and switches the left control mode to the second control mode when abnormal communication occurs between the higher-level control device and the left steering control device;

the right steering control device

switches a right control mode between a third control mode and a fourth control mode, the third control mode being a mode in which the right steering control device controls the right steering motor so that the steered angle of the right steered wheel becomes equal to the right target steered angle received from the higher-level control device, and the fourth control mode being a mode in which the right steering control device controls the right steering motor by torque feedback control so that the steered angle of the right steered wheel becomes equal to an angle corresponding to a neutral position of the right steered wheel independent of the steering wheel, and

normally sets the right control mode to the third control mode, and switches the right control mode to the fourth control mode when abnormal communication occurs between the higher-level control device and the right steering control device.

2. The vehicle steering device according to claim 1 , further comprising:

a left steered angle sensor that detects a left steered angle that is the steered angle of the left steered wheel; and

a right steered angle sensor that detects a right steered angle that is the steered angle of the right steered wheel, wherein

the left steering control device is further configured to:

if communication between the higher-level control device and the left steering control device is restored in the second control mode, switch the left control mode to the first control mode when an absolute value of a difference between the left target steered angle received from the higher-level control device and the left steered angle detected by the left steered angle sensor becomes smaller than a first predetermined value; and

the right steering control device is further configured to:

if communication between the higher-level control device and the right steering control device is restored in the fourth control mode, switch the right control mode to the third control mode when an absolute value of a difference between the right target steered angle received from the higher-level control device and the right steered angle detected by the right steered angle sensor becomes smaller than a second predetermined value.

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

the left steering control device increases an apparent overall gear ratio for the left steered wheel when the left control mode of the left steering control device is the first control mode and the right control mode of the right steering control device is the fourth control mode; and

the right steering control device increases an apparent overall gear ratio for the right steered wheel when the right control mode of the right steering control device is the third control mode and the left control mode of the left steering control device is the second control mode.

4. The vehicle steering device according to claim 2 , wherein:

the left steering control device increases an apparent overall gear ratio for the left steered wheel when the left control mode of the left steering control device is the first control mode and the right control mode of the right steering control device is the fourth control mode; and

the right steering control device increases an apparent overall gear ratio for the right steered wheel when the right control mode of the right steering control device is the third control mode and the left control mode of the left steering control device is the second control mode.