Hybrid excavator boom actuating system and method for controlling same

1. A boom actuating system for use with a hybrid excavator, comprising:

an electric motor operated as a motor or a generator;

an electricity storage device configured to store electricity generated in the electric motor;

a hydraulic pump motor driven by the electric motor to supply hydraulic fluid to a boom;

a boom control valve configured to selectively connect or disconnect an outlet line and an inlet line of the hydraulic pump motor to or from a head or a rod of a boom cylinder operating the boom;

a motor bypass valve connected to the outlet line and the inlet line to interconnect the outlet line and the inlet line or disconnect the outlet line from the inlet line; and

a controller configured to control the electric motor, the hydraulic pump motor, the boom control valve, and the motor bypass valve;

wherein in a case where a flow rate discharged from the head of the boom after the descending of the boom is initiated is equal to or larger than a predetermined value, the controller operates the hydraulic pump motor, and switches the motor bypass valve so as to disconnect the outlet line and the inlet line.

2. The boom actuating system of claim 1 , wherein when descending of the boom is initiated, the controller switches the motor bypass valve so that the outlet line and the inlet line are interconnected.

3. The boom actuating system of claim 2 , further comprising:

a second control valve configured to selectively connect the outlet line and a tank,

wherein the controller switches the second control valve to interconnect the outlet line and the tank when the boom descends, so that the flow rate discharged from one side of the boom is returned to the tank through the second control valve or supplied to the other side of the boom after passing through the boom control valve, the inlet line, and the hydraulic pump motor, and

the electric motor is operated as the generator by the hydraulic pump motor driven by the flow rate discharged from the one side of the boom and then supplied to the other side of the boom or the tank when the boom descends, and the generated electric power is stored in an electricity storage device.

4. The boom actuating system of claim 1 , further comprising:

main pumps configured to supply hydraulic fluid to other working devices, except for the boom, and receive power from a power source separately installed from the electric motor;

a boom assistant line configured to guide a flow rate supplied from the main pumps to the outlet line; and

a boom assistant valve configured to selectively open or close the boom assistant line,

wherein in a case where the flow rate discharged from the hydraulic pump motor and supplied to the boom through the outlet line is smaller than a necessary flow rate, the controller opens the boom assistant valve so that the flow rate supplied from the main pumps is additionally supplied to the boom.

5. The boom actuating system of claim 4 , further comprising:

a second control valve configured to selectively connect the outlet line and a tank,

wherein the controller switches the second control valve to interconnect the outlet line and the tank when the boom descends, so that the flow rate discharged from one side of the boom is returned to the tank through the second control valve or supplied to the other side of the boom after passing through the boom control valve, the inlet line, and the hydraulic pump motor, and

the electric motor is operated as the generator by the hydraulic pump motor driven by the flow rate discharged from the one side of the boom and then supplied to the other side of the boom or the tank when the boom descends, and the generated electric power is stored in an electricity storage device.

6. The boom actuating system of claim 1 , further comprising:

a second control valve configured to selectively connect the outlet line and a tank,

wherein the controller switches the second control valve to interconnect the outlet line and the tank when the boom descends, so that the flow rate discharged from one side of the boom is returned to the tank through the second control valve or supplied to the other side of the boom after passing through the boom control valve, the inlet line, and the hydraulic pump motor, and

the electric motor is operated as the generator by the hydraulic pump motor driven by the flow rate discharged from the one side of the boom and then supplied to the other side of the boom or the tank when the boom descends, and the generated electric power is stored in an electricity storage device.

7. A method of controlling a boom actuating system for use with a hybrid excavator, comprising:

detecting an amount of operation of a boom joystick;

determining whether a boom descends according to the operation of the boom joystick;

when descending of the boom is initiated, switching a motor bypass valve so as to interconnect an outlet line and an inlet line;

determining whether the flow rate discharged from a head of the boom is equal to or larger than a predetermine value; and

when the flow rate is equal to or larger than the predetermine value, operating a hydraulic pump motor, and switching the motor bypass valve so as to disconnect the outlet line and the inlet line.