Vehicle motion control device

1. A vehicle motion control device, comprising:

a motion state obtaining means for obtaining, once per a predetermined calculation period, a physical quantity and an actual slip ratio being actually generated at a wheel of a vehicle, the physical quantity representing a state of motion of the vehicle in a direction of lateral overturn;

a mode setting means for being in an anti-lateral overturn mode when the physical quantity obtained by the motion state obtaining means is larger than or equal to a motion state threshold, the anti-lateral overturn mode being for suppressing the lateral overturn of the vehicle;

an increase gradient obtaining means for directly obtaining an increase gradient which is a ratio of an increase amount of the physical quantity to a decrease amount of the actual slip ratio, by calculating the decrease amount of the actual slip ratio, calculating the increase amount of the physical quantity, and dividing the increase amount of the physical quantity by the decrease amount of the actual slip ratio;

an increase gradient determination means for determining whether or not the increase gradient obtained by the increase gradient obtaining means is larger than or equal to a predetermined increase gradient threshold; and

a depressurization speed suppression means for decreasing, when the vehicle motion control device is in the anti-lateral overturn mode and the increase gradient determination means determines that the increase gradient is larger than or equal to the increase gradient threshold, a speed of depressurization of a wheel cylinder pressure at a wheel cylinder for generating a brake force at the wheel, compared to when it is neither determined that the increase gradient is larger than nor equal to the increase gradient threshold, and thereby for suppressing decrease of the actual slip ratio and suppressing increase of the physical quantity and a roll angle of the vehicle which are caused by the decrease of the actual slip ratio.

2. The vehicle motion control device according to claim 1 , wherein the depressurization speed suppression means changes a way of decreasing the speed of the depressurization based on a total weight of the vehicle so that the speed of the depressurization becomes smaller as the total weight of the vehicle increases.

3. The vehicle motion control device according to claim 1 , wherein the depressurization speed suppression means changes the way of decreasing the speed of the depressurization so that the speed of the depressurization becomes smaller as the increase gradient directly obtained by the increase gradient obtaining means increases.

4. The vehicle motion control device according to claim 1 , further comprising a boosting speed enhancement means for increasing, when the vehicle motion control device is in the anti-lateral overturn mode and the increase gradient determination means determines that the increase gradient is larger than or equal to the increase gradient threshold, a speed of pressure boosting of the wheel cylinder pressure at the wheel cylinder for generating the brake force at the wheel, compared to when it is neither determined that the increase gradient is larger than nor equal to the increase gradient threshold.

5. The vehicle motion control device according to claim 2 , wherein the depressurization speed suppression means changes the way of decreasing the speed of the depressurization so that the speed of the depressurization becomes smaller as the increase gradient directly obtained by the increase gradient obtaining means increases.

6. The vehicle motion control device according to claim 2 , further comprising a boosting speed enhancement means for increasing, when the vehicle motion control device is in the anti-lateral overturn mode and the increase gradient determination means determines that the increase gradient is larger than or equal to the increase gradient threshold, a speed of pressure boosting of the wheel cylinder pressure at the wheel cylinder for generating the brake force at the wheel, compared to when it is neither determined that the increase gradient is larger than nor equal to the increase gradient threshold.