An actuator according to the present invention includes a cylinder, a piston inserted into the cylinder to be free to slide, a rod that is inserted into the cylinder and connected to the piston, a rod side chamber and a piston side chamber defined by the piston within the cylinder, a tank, a first opening/closing valve provided in a first passage that connects the rod side chamber to the piston side chamber, a second opening/closing valve provided in a second passage that connects the piston side chamber to the tank, a pump that supplies a working fluid to the rod side chamber, a motor that drives the pump, an exhaust passage that connects the rod side chamber to the tank, and a passive valve that is provided in the exhaust passage and has a predetermined pressure/flow rate characteristic.
1. An actuator unit, comprising:
a cylinder;
a piston inserted into the cylinder to be free to slide;
a rod that is inserted into the cylinder and connected to the piston;
a rod side chamber and a piston side chamber defined by the piston within the cylinder;
a tank;
a first opening/closing valve provided in a first passage that connects the rod side chamber to the piston side chamber;
a second opening/closing valve provided in a second passage that connects the piston side chamber to the tank;
a pump configured to supply a working fluid to the rod side chamber;
a motor configured to drive the pump;
an exhaust passage that connects the rod side chamber to the tank;
a passive valve that is provided in the exhaust passage and has a predetermined pressure/flow rate characteristic to cause a flow rate of the working fluid discharged from the rod side chamber to the tank to be increased with an increase in an opening degree of the passive valve caused by an increase in a pressure of the rod side chamber; and
a controller configured to control a rotation speed of the motor based on the pressure/flow rate characteristic of the passive valve to control an output thrust of the actuator unit.
2. The actuator unit as defined in claim 1 , wherein, in the pressure/flow rate characteristic, the flow rate of the working fluid is increased proportionally with the increase in the pressure of the rod side chamber.
3. The actuator unit as defined in claim 1 , wherein, in the pressure/flow rate characteristic, the degree of the increase in the pressure of the rod side chamber when the flow rate of the working fluid is more than a predetermined flow rate is smaller than that when the flow rate of the working fluid is less than the predetermined flow rate.
4. The actuator unit as defined in claim 1 , wherein the actuator unit is configured as a damper that is configured to generate a damping force corresponding to the pressure/flow rate characteristic of the passive valve when the pump is stopped.
5. The actuator unit as defined in claim 1 , further comprising:
a suction passage configured to cause the working fluid to flow only from the tank toward the piston side chamber; and
a rectifying passage configured to cause the working fluid to flow only from the piston side chamber toward the rod side chamber.
6. The actuator unit as defined in claim 1 , further comprising a check valve between the pump and the rod side chamber to prevent the working fluid from flowing from the rod side chamber toward the pump.
7. The actuator unit according to claim 1 , wherein the first opening/closing valve and the second opening/closing valve are solenoid opening/closing valves, which, when not energized, are biased to a cutoff position by a spring.
8. An actuator unit, comprising:
a cylinder;
a piston inserted into the cylinder to be free to slide;
a rod that is inserted into the cylinder and connected to the piston;
a rod side chamber and a piston side chamber defined by the piston within the cylinder;
a tank;
a first opening/closing valve provided in a first passage that connects the rod side chamber to the piston side chamber;
a second opening/closing valve provided in a second passage that connects the piston side chamber to the tank;
a pump configured to supply a working fluid to the rod side chamber;
a motor configured to drive the pump;
an exhaust passage that connects the rod side chamber to the tank;
a passive valve that is provided in the exhaust passage and has a predetermined pressure/flow rate characteristic to cause a flow rate of the working fluid discharged from the rod side chamber to the tank to be increased with an increase in an opening degree of the passive valve caused by an increase in a pressure of the rod side chamber; and
a controller configured to control a torque of the motor based on a target pressure in the cylinder to control an output thrust of the actuator unit.
9. The actuator unit according to claim 8 , wherein, in the pressure/flow rate characteristic, the flow rate of the working fluid is increased proportionally with the increase in the pressure of the rod side chamber.
10. The actuator unit according to claim 8 , wherein, in the pressure/flow rate characteristic, the degree of the increase in the pressure of the rod side chamber when the flow rate of the working fluid is more than a predetermined flow rate is smaller compared with that when the flow rate of the working fluid is less than the predetermined flow rate.
11. The actuator unit according to claim 8 , wherein the actuator unit is configured as a damper that is configured to generate a damping force corresponding to the pressure/flow rate characteristic of the passive valve when the pump is stopped.
12. The actuator unit according to claim 8 , further comprising a current loop configured to control the motor,
wherein the controller is configured to control the motor by determining a torque command to be applied to the current loop from the target pressure in the cylinder.
13. The actuator unit according to claim 8 , further comprising
a suction passage configured to cause the working fluid to flow only from the tank toward the piston side chamber; and
a rectifying passage configured to cause the working fluid to flow only from the piston side chamber toward the rod side chamber.
14. The actuator unit according to claim 8 , further comprising a check valve between the pump and the rod side chamber to prevent the working fluid from flowing from the rod side chamber toward the pump.
15. The actuator unit according to claim 8 , wherein the first opening/closing valve and the second opening/closing valve are solenoid opening/closing valves, which, when not energized, are biased to a cutoff position by a spring.