Actuator unit

1. An actuator unit comprising:

a cylinder;

a piston slidably inserted into the cylinder, the piston defining a rod side chamber and a piston side chamber in the cylinder;

a rod inserted into the cylinder and coupled to the piston;

a tank;

a first opening/closing valve provided on a first passage that communicates the rod side chamber with the piston side chamber;

a second opening/closing valve provided on a second passage that communicates the piston side chamber with the tank;

an suction passage configured to allow a working fluid to flow only from the tank toward the piston side chamber;

a rectifying passage configured to allow the working fluid to flow only from the piston side chamber toward the rod side chamber;

a pump configured to supply the working fluid to the rod side chamber;

a motor configured to drive the pump;

a first discharge passage and a second discharge passage that respectively communicate the rod side chamber to the tank;

a first passive valve provided on the first discharge passage, the first passive valve having a first pressure-flow rate characteristic, wherein a first pressure loss according to the first pressure-flow rate characteristic is determined unambiguously in relation to a first flow rate through the first passive valve;

a second passive valve provided on the second discharge passage, the second passive valve having a second pressure-flow rate characteristic, wherein a second pressure loss according to the second pressure-flow rate characteristic is determined unambiguously in relation to a second flow rate through the second passive valve; and

a switch valve provided on the first discharge passage in series with the first passive valve, the switch valve being configured to communicate and block at least one of the first discharge passage and the second discharge passage,

wherein the second pressure-flow rate characteristic of the second passive valve is set such that the second pressure loss is larger than the first pressure loss.

2. The actuator unit according to claim 1 , wherein, when the pump is driven with one of the first opening/closing valve and the second opening/closing valve at a communicating position, the switch valve blocks one of the first discharge passage and the second discharge passage, and when driving of the pump is stopped with both the first opening/closing valve and the second opening/closing valve at a blocking position, the switch valve communicates one of the first discharge passage and the second discharge passage.

3. The actuator unit according to claim 1 , wherein the second passive valve is constituted by an orifice.

4. The actuator unit according to claim 1 , further comprising a third passive valve provided on the second discharge passage in parallel with the second passive valve, the third passive valve having a third pressure-flow rate characteristic,

wherein the switch valve communicates and blocks the first discharge passage and the second discharge passage,

wherein the third passive valve is constituted by a relief valve, and

wherein the third pressure-flow rate characteristic is set such that a third pressure loss relative to a third flow rate passing through the third passive valve is larger than the first pressure loss relative to the first flow rate of the first pressure-flow rate characteristic of the first passive valve, and such that a pressure in the rod side chamber when the first discharge passage is blocked is less than the pressure in the rod side chamber when a torque of the motor is at a maximum.

5. The actuator unit according to claim 1 , wherein the switch valve is a solenoid valve which, when not energized, communicates at least one of the first discharge passage and the second discharge passage.