Vehicle driver assistance system

1. A vehicle driver assistance system, comprising:

a steering assist torque generation device configured to generate steering assist torque;

a control device configured to control the steering assist torque generation device; and

a switch configured to switch a steering mode between a manual steering mode and an automatic steering mode, the manual steering mode being a mode in which a steered wheel is steered as a driver operates a steering wheel, and the automatic steering mode being a mode in which the steered wheel is steered by the steering assist torque generated by the steering assist torque generation device, wherein:

the control device is configured to acquire information on steering torque;

the control device is configured to calculate desired steering assist torque based on the steering torque, the desired steering assist torque being torque that reduces a burden of operation of the steering wheel on the driver;

the control device is configured to control the steering assist torque generation device so as to control the steering assist torque to the desired steering assist torque when the steering mode is the manual steering mode;

the control device is configured to calculate a desired index value of a steering angle of the steered wheel and desired automatic steering torque and control the steering assist torque generation device so as to control the steering assist torque to a sum of the desired automatic steering torque and the desired steering assist torque when the steering mode is the automatic steering mode, the desired index value of the steering angle of the steered wheel being a value for controlling a traveling state of a vehicle to a desired traveling state, and the desired automatic steering torque being torque for controlling an index value of the steering angle of the steered wheel to the desired index value;

the control device is configured to determine whether the driver is holding the steering wheel; and

the control device is configured to calculate the desired automatic steering torque so as to make a rate of change in the desired automatic steering torque lower when the steering mode switches from the manual steering mode to the automatic steering mode in a situation where determination is made that the driver is holding the steering wheel than when the steering mode switches from the manual steering mode to the automatic steering mode in a situation where determination is made that the driver is not holding the steering wheel.

2. The vehicle driver assistance system according to claim 1 , further comprising a touch sensor configured to output a hold signal when the driver is holding the steering wheel, wherein the control device is configured to determine that the driver is holding the steering wheel when the hold signal has been output from the touch sensor.

3. The vehicle driver assistance system according to claim 1 , wherein the control device is configured to determine that the driver is holding the steering wheel when a magnitude of the steering torque is equal to or larger than a reference value for determining whether the driver is holding the steering wheel.

4. The vehicle driver assistance system according to claim 1 , further comprising a touch sensor configured to output a hold signal when the driver is holding the steering wheel, wherein the control device is configured to determine that the driver is holding the steering wheel when the hold signal has been output from the touch sensor and a magnitude of the steering torque is equal to or larger than a reference value for determining whether the driver is holding the steering wheel.

5. The vehicle driver assistance system according to claim 1 , wherein the control device is configured to make the rate of change in the desired automatic steering torque lower when a magnitude of the steering torque is equal to or larger than a reference value for determining whether the driver is steering the steering wheel than when the magnitude of the steering torque is smaller than the reference value for determining whether the driver is steering the steering wheel, in the case where the steering mode switches from the manual steering mode to the automatic steering mode in the situation where determination is made that the driver is holding the steering wheel.

6. The vehicle driver assistance system according to claim 1 , wherein the control device is configured to variably set the rate of change in the desired automatic steering torque according to a magnitude of the steering torque in such a manner that the rate of change in the desired automatic steering torque decreases as the magnitude of the steering torque increases.

7. The vehicle driver assistance system according to claim 1 , wherein the control device is configured not to reduce the rate of change in the desired automatic steering torque when the desired index value of the steering angle of the steered wheel is a desired index value of the steering angle for moving the vehicle in a lateral direction in order to further improve driving safety of the vehicle.

8. The vehicle driver assistance system according to claim 1 , wherein:

the desired automatic steering torque is a sum of desired automatic steering torque of feedforward control and desired automatic steering torque of feedback control that are torques for controlling the index value of the steering angle of the steered wheel to the desired index value; and

the control device is configured to reduce a rate of change in the desired automatic steering torque of the feedforward control.

9. The vehicle driver assistance system according to claim 1 , wherein:

the desired automatic steering torque is a sum of desired automatic steering torque of feedforward control and desired automatic steering torque of feedback control that are torques for controlling the index value of the steering angle of the steered wheel to the desired index value; and

the control device is configured to reduce a rate of change in the sum.