US 5,129,230 AGranted

Control system for load sensing hydraulic drive circuit

AssigneeHitachi Construction Machinery Co., Ltd.

InventorsEiki Izumi, Hiroshi Watanabe

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Abstract

A load sensing hydraulic drive circuit comprises a variable displacement pump delivering fluid to a flow control valve for controlling flow to an actuator. A pump controller controls a delivery rate of the pump such that a differential pressure between a delivery pressure of the pump and a load pressure of the actuator is equal to a first predetermined value. An unloading valve is connected between the pump and the flow control valve for holding the differential pressure between the delivery pressure of the pump and the load pressure of the actuator less than a second predetermined value. A demanded flow rate of the flow control valve is detected and the unloader valve is controlled based on the demanded flow rate such that when the demanded flow rate is small, the second predetermined value is smaller than the first predetermined value, and when the demanded flow rate increases, the second predetermined value becomes larger than the first predetermined value. This allows stable control of the differential pressure for both large and small changes in operation of the flow control valve.

Claims8

Dependent claims

A control system for a load sensing hydraulic drive circuit comprising at least one hydraulic pump provided with displacement volume varying means, at least one hydraulic actuator driven by a hydraulic fluid delivered from said hydraulic pump, a flow control valve connected between said hydraulic pump and said actuator for controlling a flow rate of the hydraulic fluid supplied to said actuator, pump control means for controlling a delivery rate of said hydraulic pump such that a delivery pressure of said hydraulic pump is higher by a first predetermined value than a load pressure of said actuator, and an unloading valve connected between said hydraulic pump and said actuator for holding a differential pressure between the delivery pressure of said hydraulic pump and the load pressure of said actuator less than a second predetermined value, said control system further comprising:

first means for detecting a value associated with a demanded flow rate of said flow control valve, and
second means for controlling said unloading valve based on said value associated with the demanded flow rate detected by said first means such that said second predetermined value is smaller than said first predetermined value when said demanded flow rate is small, and said second predetermined value becomes larger than said first predetermined value as said demanded flow rate increases.

Dependencies are inferred from the claim text.