Suspension system for vehicle

1. A suspension system for a vehicle, comprising:

(a) a shock absorber disposed between a sprung portion and an unsprung portion and having a damping-force changing mechanism for changing a magnitude of a damping force, the shock absorber being configured to generate the damping force with respect to a relative movement of the sprung portion and the unsprung portion such that the magnitude of the damping force is changeable; and

(b) a controller configured to control the damping-force changing mechanism so as to control the damping force to be generated by the shock absorber,

wherein the controller includes a target sprung-speed determining portion configured to determine, as a target sprung speed which is a target of a speed of the sprung portion, a speed of the sprung portion in a behavior of a body of the vehicle that matches an operation input to the vehicle by a driver, for permitting the behavior to match the operation input, the controller being configured to control the damping force such that the speed of the sprung portion becomes equal to the target sprung speed

wherein the controller further includes:

a basic damping-force determining portion configured to determine a basic damping force which has a magnitude in accordance with a predetermined control rule and which is a basis when determining a target of the damping force to be generated by the shock absorber; and

a target damping-force determining portion configured to determine a target damping force as the target of the damping force to be generated by the shock absorber by correcting the basic damping force based on the target sprung speed,

wherein the controller further includes an actual sprung-speed obtaining portion configured to obtain an actual sprung speed that is an actual speed of the sprung portion, and

wherein the target damping-force determining portion is configured to correct the basic damping force based on a sprung speed deviation that is a deviation of the actual sprung speed from the target sprung speed.

2. The suspension system for the vehicle according to claim 1 , wherein the target sprung-speed determining portion is configured to determine the target sprung speed based on a steering operation input by the driver.

3. The suspension system for the vehicle according to claim 2 , wherein the target sprung-speed determining portion is configured to determine, based on an operation speed of the steering operation and a speed of the vehicle, a target roll speed as a target of a speed at which a body of the vehicle is caused to be rolled and to determine the target sprung speed based on the target roll speed.

4. The suspension system for the vehicle according to claim 3 , wherein the target sprung-speed determining portion is configured to determine the target roll speed by a product of a gain determined based on the speed of the vehicle and the operation speed of the steering operation.

5. The suspension system for the vehicle according to claim 4 , wherein the target sprung-speed determining portion is configured to determine the target sprung speed based on the operation input to the vehicle by the driver and information relating to motion control being executed in the vehicle.

6. The suspension system for the vehicle according to claim 5 ,

wherein the vehicle on which the suspension system is installed is configured to execute, as the motion control being executed in the vehicle, vehicle-body turning control in which an orientation of a body of the vehicle is changed, independently of changing of the orientation of the body of the vehicle by a steering operation input by the driver, and

wherein the target sprung-speed determining portion is configured to determine the target sprung speed based on the steering operation input by the driver and the information relating to the vehicle-body turning control.

7. The suspension system for the vehicle according to claim 1 ,

wherein the target damping-force determining portion is configured to:

(i) decrease the damping force to be generated by the shock absorber with respect to the basic damping force when the sprung speed deviation is positive and the sprung portion and the unsprung portion are relatively moved in a direction away from each other or when the sprung speed deviation is negative and the sprung portion and the unsprung portion are relatively moved in a direction toward each other, and

(ii) increase the damping force to be generated by the shock absorber with respect to the basic damping force when the sprung speed deviation is negative and the sprung portion and the unsprung portion are relatively moved in the direction away from each other or when the sprung speed deviation is positive and the sprung portion and the unsprung portion are relatively moved in the direction toward each other.

8. The suspension system for the vehicle according to claim 1 , wherein the target damping-force determining portion is configured to increase an amount of correction of the damping force to be generated by the shock absorber with respect to the basic damping force with an increase in the sprung speed deviation.

9. The suspension system for the vehicle according to claim 1 , wherein the target damping-force determining portion is configured to cut a component of the sprung speed deviation having a frequency lower than a set frequency.