Lane deviation prevention device and method

1. A lane departure prevention controller for a vehicle, comprising:

a lane departure tendency determining device configured to determine a departure tendency of the vehicle to depart from a travel lane;

a yaw moment computing device configured to compute a reference yaw moment based on the departure tendency;

a yaw moment correction device configured to compute a target yaw moment based on at least the reference yaw moment; and

a controller configured to apply the target yaw moment to the vehicle;

wherein when the reference yaw moment computed by the yaw moment computing device is lower than a prescribed threshold, the yaw moment correction device is further configured to gradually increase the target yaw moment over time until the target yaw moment is greater than the prescribed threshold.

2. The lane departure prevention controller of claim 1 wherein the yaw moment correction device is further configured to set a minimum control yaw moment used to compute the target yaw moment.

3. The lane departure prevention controller of claim 2 wherein, when the reference yaw moment computed by the yaw moment computing device exceeds the prescribed threshold, the yaw moment correction device is further configured to set the target yaw moment based on the larger of the reference yaw moment computed with the yaw moment computing device and the minimum control yaw moment set by the yaw moment correction device.

4. The lane departure prevention controller of claim 2 wherein when the reference yaw moment set by the yaw moment correction device exceeds the prescribed threshold, the yaw moment correction device is further configured to set the target yaw moment based on the larger of the reference yaw moment computed with the yaw moment computing device and the minimum control yaw moment set by the yaw moment correction device.

5. The lane departure prevention controller of claim 2 wherein the yaw moment correction device is configured to gradually increase the target yaw moment over time by calculating a gain in the minimum control yaw moment over time.

6. The lane departure prevention controller of claim 5 wherein the minimum control yaw moment increases when a road surface friction coefficient increases.

7. The lane departure prevention controller of claim 5 , wherein a rate of increase of the minimum control yaw moment increases over time.

8. The lane departure prevention controller of claim 5 wherein the minimum control yaw moment increases as a road condition index value increases.

9. The lane departure prevention controller of claim 5 wherein the minimum control yaw moment increases one of linearly and step-wise over time.

10. A method of preventing departure of a vehicle from a travel lane comprising:

determining the departure tendency of the vehicle to depart from the travel lane with a lane departure tendency determining device;

calculating a reference yaw moment based on the degree of the departure tendency with a yaw moment computing device;

calculating a target yaw moment based on at least the reference yaw moment with a yaw moment correction device, wherein the target yaw moment is gradually increased over time by the yaw moment correction device when the reference yaw moment is lower than a prescribed threshold, until the target yaw moment exceeds the prescribed threshold irrespective of the departure tendency; and

applying the target yaw moment to the vehicle.

11. The lane departure prevention control method of claim 10 , further comprising:

setting the target yaw moment with the yaw moment correction device based on the larger of the reference yaw moment calculated based on the departure tendency and the target yaw moment that was gradually increased when the reference yaw moment exceeds the prescribed threshold.

12. The lane departure prevention control method of claim 10 , further comprising:

setting a minimum control yaw moment with the yaw moment correction device; and wherein when the reference yaw moment exceeds the prescribed threshold, calculating the target yaw moment comprises setting the target yaw moment based on the larger of the reference yaw moment and the minimum control yaw moment.

13. The lane departure prevention control method of claim 12 wherein the minimum control yaw moment is increased as a road surface friction coefficient increases.

14. The lane departure prevention control method of claim 12 wherein the minimum control yaw moment is increased with time.

15. The lane departure prevention control method of claim 14 wherein gradually increasing the target yaw moment comprises calculating a gain in a minimum control yaw moment over time.

16. A lane departure prevention controller for a vehicle, comprising:

means for determining a departure tendency of the vehicle;

means for computing a reference yaw moment based on the departure tendency of the vehicle;

means for computing a target yaw moment based on at least the reference yaw moment; and

means for applying the target yaw moment to the vehicle, wherein when the reference yaw moment is lower than a prescribed threshold, the means for computing the target yaw moment is configured to gradually increase the target yaw moment over time until the target yaw moment is greater than the prescribed threshold.

17. The lane departure prevention controller of claim 16 wherein the means for computing the target yaw moment further comprises means for setting a minimum control yaw moment, wherein the minimum control yaw moment is also used to compute the target yaw moment.

18. The lane departure prevention controller of claim 17 wherein when the reference yaw moment exceeds the prescribed threshold, the means for computing the target yaw moment further comprises means for setting the target yaw moment based on the larger of the reference yaw moment and the minimum control yaw moment.