A hydraulic system includes a hydraulic valve, a load-holding valve arrangement, a flow-control valve arrangement, a working pressure sensor, a differential pressure sensor, and a speed sensor. The load-holding valve arrangement includes first and second load-holding valves connected to opposite ends of the hydraulic cylinder. The flow-control valve arrangement includes a directional flow-control valve and a hydraulic pump. The differential pressure sensor measures pressure over the load-holding valve arrangement. A desired extension or retraction speed of the hydraulic cylinder is controlled by adjusting the position of the directional flow-control valve, the first load-holding valve and the second load-holding valve based on a measured working pressure in the hydraulic system and a measured speed of the hydraulic cylinder or a differential pressure over the load-holding valve arrangement.

Provided is a hydraulic drive system for a work machine configured with a single solenoid proportional valve for a regeneration circuit, wherein substantially the same actuator speed can be secured irrespective of whether or not hydraulic fluid discharged from a hydraulic actuator is regenerated for driving of another hydraulic actuator. The hydraulic drive system includes: a regeneration line that connects a bottom-side hydraulic chamber of a hydraulic cylinder 4 to a portion between a hydraulic pump device 50 and a second hydraulic actuator 8, and a regeneration flow rate adjustment device that supplies, at an adjusted flow rate, at least part of the discharged hydraulic fluid to a portion between the hydraulic pump device 50 and the second hydraulic actuator; a discharge flow rate adjustment device that discharges, at an adjusted flow rate, the discharged hydraulic fluid to a tank; one electric drive device 22 that simultaneously controls the regeneration flow rate adjustment device and the discharge flow rate adjustment device; and a control unit 27 that outputs a control command to the electric drive device in such a manner that falling speed of a first driven body does not vary significantly, irrespective of the magnitude of the regeneration flow rate caused by the regeneration flow rate adjustment device.

A hydraulic drive system for an actuator uses a pair of pressure compensated hydraulic machines to control flow to and from the drive chambers of the actuator. The capacity of one of the machines is limited in a motoring mode to determine a maximum rate of efflux from one of the chambers. The pressure of fluid supplied to the other of said chambers is maintained at a predetermined level to provide motive force. The machines are mechanically coupled to permit energy recovery and charge an accumulator to store surplus energy.

To provide a work machine that makes it possible to drive actuators faster and more accurately by supplying flows to the actuators accurately at target rates without depending on load variations in a case where the machine body is controlled automatically by command inputs of a controller, while high operability is ensured for manual operation by an operator. In a case where a machine control function is cancelled via a machine control switch, a controller cancels limitation of the flow rate of a hydraulic fluid supplied to a plurality of directional control valves, the limitation being performed by the auxiliary flow rate control devices, and in a case where the machine control function is selected via the machine control switch, the controller causes the auxiliary flow rate control devices to limit the flow rate of the hydraulic fluid supplied to the plurality of directional control valves.

A method is provided for compensating for the flow rate of a variable capacity-type hydraulic pump such that, when a manipulation lever is manipulated, a discontinuous section, which has no change in the discharge flow rate of the hydraulic pump, is eliminated.

A driving stage of a servo valve, including a hydraulic ejector and a hydraulic receiver able to be moved relative to each other, one of the two hydraulic units being integral with a mobile unit, movable relative to a body of the servo valve through actuation means, characterized in that the actuation means comprise two piezoelectric actuators connected in series. Control device comprising a servo valve comprising such a driving stage.

A driving stage of a servo valve, including a hydraulic ejector and a hydraulic receiver able to be moved relative to each other, one of the two hydraulic units being integral with a mobile unit, movable relative to a body of the servo valve through actuation means, characterized in that the actuation means comprise two piezoelectric actuators connected in series. Control device comprising a servo valve comprising such a driving stage.

The present invention relates to a hydraulic control system for construction machinery capable of variably adjusting a set pressure of a relief valve, which controls a set pressure of a hydraulic system, according to a value inputted by means of pressure in the hydraulic system or the manipulation of a joystick. The hydraulic control system comprises: a flow control valve installed in a passage between a hydraulic pump and a hydraulic actuator, for controlling the driving of the hydraulic actuator by being switched by a control signal according to the manipulation of the joystick; a main relief valve installed in a passage between an upstream-side discharge passage of the hydraulic pump and a hydraulic tank, for returning a working fluid to the hydraulic tank when a high load exceeding a set pressure occurs in the system; a pressure controlling means for controlling, either continuously or in stages, a set pressure of the main relief valve; a pressure sensing means for sensing a pressure at a discharge side of the hydraulic pump; and a controller for determining the value inputted by the manipulation of the joystick, and a required set pressure for the relief valve according to the system pressure sensed by the pressure sensing means, and then outputting a control signal to the pressure controlling means to enable a set pressure of the relief valve to be variably adjusted to a determined set pressure.

To provide a construction machine that can highly precisely control branch flows from a hydraulic pump to a plurality of hydraulic actuators without being affected by load conditions. A controller (100) has a meter-out valve control section (140) configured to calculate a target opening area of a second meter-out valve (65a) (65b) according to a pressure difference between a supply pressure and a second meter-in pressure, or calculate a target opening area of a first meter-out valve (55a) (55b) according to a pressure difference between the supply pressure and the first meter-in pressure.

To provide a construction machine that can highly precisely control branch flows from a hydraulic pump to a plurality of hydraulic actuators without being affected by load conditions. A controller (100) has a meter-out valve control section (140) configured to calculate a target opening area of a second meter-out valve (65a) (65b) according to a pressure difference between a supply pressure and a second meter-in pressure, or calculate a target opening area of a first meter-out valve (55a) (55b) according to a pressure difference between the supply pressure and the first meter-in pressure.