Variable displacement pump

1. A variable displacement pump for supplying working fluid to a hydraulic device mounted on a vehicle, the variable displacement pump comprising:

a body;

a drive shaft rotatably supported by the body;

a rotor mounted in the body, and arranged to be rotated by the drive shaft;

a cam ring mounted radially outside of the rotor in the body, and arranged to move with a change in an eccentricity of the cam ring with respect to the rotor, wherein change of the eccentricity causes a change in a specific discharge rate as a quantity of discharge of working fluid per one rotation of the rotor;

an electromagnetic actuator arranged to actuate the cam ring for regulating the eccentricity; and

a controller configured to: receive an input signal outputted from a sensor arranged to measure a state of operation of the vehicle; and output a drive signal to the electromagnetic actuator, wherein the controller is programmed to:

control operation of the electromagnetic actuator with reference to the input signal by outputting the drive signal; and

set a first response slower than a second response, during control of operation of the electromagnetic actuator, wherein the first response is a response of movement of the cam ring to a change of the input signal in a first direction to request a decrease in the specific discharge rate, and the second response is a response of movement of the cam ring to a change of the input signal in a second direction to request an increase in the specific discharge rate,

wherein the variable displacement pump further comprises:

a discharge passage formed in the body for guiding working fluid to outside of the body after the working fluid is pressurized by a pumping effect resulting from rotation of the rotor;

a metering orifice provided in the discharge passage;

a first fluid pressure chamber defined radially outside of the came ring in the body, and arranged to contract with an increase in the eccentricity;

a second fluid pressure chamber defined radially outside of the cam ring in the body, and arranged to contract with a decrease in the eccentricity; and

a control valve arranged to regulate at least one of an internal pressure of the first fluid pressure chamber and an internal pressure of second fluid pressure chamber with a valve element arranged to be operated by a differential pressure between an upstream side and a downstream side of the metering orifice,

wherein the electromagnetic actuator is arranged to regulate a cross-sectional flow area of the metering orifice for actuating the cam ring with the control valve;

wherein the control valve is arranged so that the specific discharge rate increases with an increase in the cross-sectional flow area of the metering orifice;

wherein the controller is programmed to set the cross-sectional flow area of the metering orifice to be larger in response to a change of the input signal in the second direction when the cam ring is moving in a direction to reduce the specific discharge rate than when the cam ring is stationary, and

wherein the controller is programmed to determine with reference to an actual supply current flowing through the electromagnetic actuator whether the cam ring is moving in the direction to reduce the specific discharge rate or in a direction to increase the specific discharge rate.

2. The variable displacement pump as claimed in claim 1 , wherein the controller is programmed to determine with reference to a change of the actual supply current whether the cam ring is moving in the direction to reduce the specific discharge rate or in the direction to increase the specific discharge rate.

3. The variable displacement pump as claimed in claim 1 , wherein the controller is programmed to use a longer time constant for control of operation of the electromagnetic actuator in response to a change of the input signal in the first direction than in response to a change of the input signal in the second direction.

4. The variable displacement pump as claimed in claim 1 , wherein the metering orifice includes:

a variable orifice having a cross-sectional flow area that is regulated by the electromagnetic actuator; and

a constant orifice arranged in parallel to the variable orifice.

5. The variable displacement pump as claimed in claim 1 , wherein the electromagnetic actuator is arranged to press the valve element of the control valve with the differential pressure in a direction to change a state of flow of the control valve.

6. The variable displacement pump as claimed in claim 1 , wherein:

the hydraulic device is a hydraulic power steering device; and

the controller is programmed to change the specific discharge rate with reference to a travel speed of the vehicle.

7. The variable displacement pump as claimed in claim 6 , wherein the controller is programmed to increase the specific discharge rate with a decrease in the travel speed of the vehicle.

8. The variable displacement pump as claimed in claim 1 , wherein the controller is programmed to prevent the cam ring from overshooting a target position, by reducing the specific discharge rate, immediately before the target position, while the cam ring is moving in the direction to increase the specific discharge rate.

9. The variable displacement pump as claimed in claim 1 , wherein the controller is programmed to reduce a speed of the cam ring as the cam ring approaches a target position, while the cam ring is moving in the direction to increase the specific discharge rate.

10. The variable displacement pump as claimed in claim 1 , wherein

the controller is configured to

wait a predetermined delay period before allowing the cam ring to move in the direction to reduce the specific discharge rate, in response to a change of the input signal in the first direction during control of operation of the electromagnetic actuator.