Vehicle suspension system

1. A vehicle suspension system, comprising:

an electromagnetic damper provided with a sprung member and an unsprung member of a vehicle to apply a drive force and a damping force between the sprung member and the unsprung member; and

a control unit for controlling the electromagnetic damper;

the control unit comprising:

an unsprung member demand load computing unit for computing an unsprung member demand load that attenuates a vertical vibration of the unsprung member;

a sprung member demand load computing unit for computing a sprung member demand load that restrains a vertical displacement of the sprung member; and

a target load computing unit for computing a target load for the electromagnetic damper based on the unsprung member demand load and the sprung member demand load;

the sprung member demand load computing unit being configured to reduce an absolute value of the sprung member demand load to be forwarded to the target load computing unit when a sprung member frequency given as a frequency of a vertical vibration of the sprung member is in an unsprung member resonance frequency range where a resonance of the unsprung member can occur.

2. The vehicle suspension system according to claim 1 , further comprising a correction unit for reducing an absolute value of the sprung member demand load to be forwarded to the target load computing unit when the sprung member frequency is in the unsprung member resonance frequency range by reducing the absolute value of the sprung member demand load forwarded to the target load computing unit when the sprung member frequency is higher than a prescribed threshold frequency, the correction unit being configured to vary the threshold frequency based on a yaw rate of the vehicle.

3. The vehicle suspension system according to claim 2 , wherein the correction unit raises the threshold frequency with a decreasing yaw rate.

4. The vehicle suspension system according to claim 2 , wherein the correction unit is configured to vary the threshold frequency based on a yaw rate of the vehicle and a vehicle speed.

5. The vehicle suspension system according to claim 4 , wherein the correction unit is configured to raise the threshold frequency when the yaw rate is smaller than a prescribed yaw rate determination value or the vehicle speed is lower than a prescribed vehicle speed determination value to a value higher than that when the yaw rate is greater than the prescribed yaw rate determination value and the vehicle speed is higher than the prescribed vehicle speed determination value.

6. The vehicle suspension system according to claim 5 , wherein the correction unit is configured to set the threshold frequency in a range of 3 Hz and 6 Hz when the yaw rate is greater than the prescribed yaw rate determination value and the vehicle speed is higher than the prescribed vehicle speed determination value.

7. The vehicle suspension system according to claim 2 , wherein the correction unit is configured to gradually decrease the sprung member demand load with an increase in the sprung member frequency when the sprung member frequency is greater than the threshold frequency.

8. The vehicle suspension system according to claim 1 , wherein the unsprung member demand load computing unit is configured to compute the unsprung member demand load based on a relative speed between the sprung member and the unsprung member, and to compute the sprung member demand load based on a vertical velocity of the sprung member.

9. The vehicle suspension system according to claim 1 , further comprising a sprung member velocity computing unit for computing a vertical velocity of the sprung member by integrating the sprung member acceleration received from the corresponding sprung member acceleration sensor with time,

wherein the sprung member demand load computing unit is configured to compute the sprung member demand load based on the vertical velocity of the sprung member.