Construction Machine

A construction machine provided with a hydraulic drive system comprising:

• a variable displacement hydraulic pump;
• a plurality of actuators driven by a hydraulic fluid delivered from the hydraulic pump;
• a control valve device that distributes and supplies the hydraulic fluid delivered from the hydraulic pump to the plurality of actuators;
• a plurality of operation lever devices that instructs drive directions and speeds of the plurality of actuators, respectively;
• a pump regulating device that controls a delivery flow rate of the hydraulic fluid from the hydraulic pump in such a manner that the hydraulic fluid is delivered at a flow rate to match with input amounts of operation levers of the plurality of operation lever devices;
• an unloading valve that discharges the hydraulic fluid in a hydraulic fluid supply line of the hydraulic pump to a tank when a pressure in the hydraulic fluid supply line of the hydraulic pump exceeds a set pressure determined by adding at least a target differential pressure to a highest load pressure of the plurality of actuators;
• a plurality of first pressure sensors that detect load pressures of the plurality of actuators, respectively;
• and
• a controller that controls the control valve device,
• wherein the control valve device includes
• a plurality of directional control valves that are changed over by the plurality of operation lever devices and are associated with the plurality of actuators so as to control the drive directions and the speeds of the actuators, respectively, and
• a plurality of flow control valves disposed between the hydraulic fluid supply line of the hydraulic pump and the plurality of directional control valves to control flow rates of the hydraulic fluid supplied to the plurality of directional control valves by changing opening areas of the flow control valves, respectively, and
• the controller is configured to:
• compute demanded flow rates of the plurality of actuators on the basis of input amounts of the operation levers of the plurality of operation lever devices and compute differential pressures between a highest load pressure among load pressures of the plurality of actuators and the load pressures of the plurality of actuators, compute target opening areas of the plurality of flow control valves on the basis of the demanded flow rates of the plurality of actuators and the differential pressures and control opening areas of the plurality of flow control valves in such a manner that the opening areas are equal to the target opening areas.