Power plants – Pressure fluid source and motor – Condition responsive control of pump or motor displacement
Reexamination Certificate
1998-06-01
2001-02-27
Lopez, F. Daniel (Department: 3745)
Power plants
Pressure fluid source and motor
Condition responsive control of pump or motor displacement
C060S452000, C060S468000
Reexamination Certificate
active
06192681
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a hydraulic drive system, and more particularly to a hydraulic drive system operating under load sensing control to control the displacement of a hydraulic pump so that a differential pressure between a delivery pressure of the hydraulic pump and a maximum load pressure among a plurality of actuators is maintained at a setting value.
BACKGROUND ART
As to the load sensing control technique for controlling the displacement of a hydraulic pump so that a differential pressure between a delivery pressure of the hydraulic pump and a maximum load pressure among a plurality of actuators is maintained at a setting value, there are known a pump displacement control system disclosed in JP, A, 5-99126 and a control system for a variable displacement hydraulic pump disclosed in GB Patent 1599233.
The pump displacement control system disclosed in JP, A, 5-99126 comprises a servo piston for tilting a swash plate of a variable displacement hydraulic pump, and a tilting control unit for supplying a pump delivery pressure to the servo piston in accordance with a differential pressure &Dgr;PLS between a delivery pressure Ps of the hydraulic pump and a load pressure PLS of an actuator driven by the hydraulic pump so as to maintain the differential pressure &Dgr;PLS at a setting value &Dgr;PLSref, thereby controlling the pump displacement. The disclosed pump displacement control system further comprises a fixed displacement hydraulic pump driven by an engine along with the variable displacement hydraulic pump, a throttle disposed in a delivery line of the fixed displacement hydraulic pump, and setting modifying means for modifying the setting value &Dgr;PLSref of the tilting control unit in accordance with a differential pressure &Dgr;Pp across the throttle. The setting value &Dgr;PLSref of the tilting control unit is modified by detecting an engine rotational speed based on change in the differential pressure across the throttle disposed in the delivery line of the fixed displacement hydraulic pump.
The control system disclosed in GB Patent 1599233 also has a similar construction. More specifically, a throttle is provided in a delivery line of a fixed pump and a differential pressure &Dgr;Pp across the throttle is introduced to control pressure chambers at opposite ends of a setting adjust valve. When the rotational speed of a prime mover is sufficiently high and the differential pressure &Dgr;Pp is larger than the pressure set by a spring, a valve apparatus
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establishes communication with the II side and a target load-sensing differential pressure &Dgr;PLSref of a tilting control valve involved in load sensing control is set to a relatively high value. When the prime mover comes into an overload condition and its rotational speed lowers upon change in loads of actuators connected respectively to a plurality of flow control valves, a delivery rate of the fixed pump connected to the prime mover is reduced. If the setting value of the spring becomes higher than the differential pressure &Dgr;Pp across the throttle upon reduction in the pump delivery rate, the setting adjust valve is shifted to establish communication with the I side and the target load-sensing differential pressure &Dgr;PLSref of the tilting control valve involved in load sensing control is set to a relatively low value, thereby relieving a load imposed on the prime mover.
DISCLOSURE OF THE INVENTION
In the pump displacement control system disclosed in JP, A, 5-99126, when flow control valves are operated, the load sensing differential pressure &Dgr;PLSref corresponding to the engine rotational speed is set in the tilting control unit by the setting modifying means, and the pressure Ps in a pump delivery line of the variable displacement hydraulic pump is held at a pressure higher than a maximum load pressure PLS among the actuators operated by the flow control valves by the load sensing differential pressure &Dgr;PLSref, i.e., Ps=PLS+&Dgr;PLSref.
On the other hand, when no flow control valves are operated, the maximum load pressure PLS is given by a reservoir pressure and hence the tilting control unit minimizes a tilting angle of the variable displacement hydraulic pump for lowering the pressure in the pump delivery line. In this condition, there produces a small pump delivery rate, or even if the setting is made to null out the pump delivery rate, a small flow rate still produces due to a delay in operation of the swash plate of the hydraulic pump. This brings a hydraulic fluid into an enclosed state because of all the flow control valves being in neutral positions, thus developing a pressure in the pump delivery line.
In a general hydraulic circuit, therefore, a safety valve (relief valve) is connected to the pump delivery line for limiting the pressure in the pump delivery line to a maximum pressure value allowable in the entire circuit.
Further, in a hydraulic system operating under load sensing control, an unloading valve is generally connected to a pump delivery line for the purpose of improving energy efficiency of a hydraulic pump in its non-load condition. The unloading valve controls the pressure in the pump delivery line to be held higher than a maximum load pressure PLS by a differential pressure &Dgr;Pun set by a spring when no flow control valves are operated.
The setting differential pressure &Dgr;Pun of the unloading valve is set to a higher value than the load sensing differential pressure &Dgr;PLSref set in the tilting control unit. Accordingly, when flow control valves are operated, the pressure Ps in the pump delivery line is controlled by the tilting control unit to meet Ps=PLS+&Dgr;PLSref under a condition where the system is normally operating. Thus the unloading valve does not operate to avoid interference with the load sensing control effected by the tilting control unit.
When the maximum load pressure PLS varies upon a variation in working load, the pressure Ps in the delivery line of the hydraulic pump is also adjusted by the tilting control unit following such a variation. Due to a delay in pump tilting under the load sensing control, however, there may produce a flow rate more than demanded by actuators. A resulting flow rate difference deviates the pressure in the delivery line from the target pressure in the load sensing control, causing an oscillation in the entire system.
The unloading valve operates to stabilize the system against such an oscillation phenomenon by releasing the hydraulic fluid in the pump delivery line when the pressure in the pump delivery line exceeds the setting differential pressure &Dgr;Pun. This is equivalent to that the hydraulic fluid corresponding to a flow rate produced due to a delay in tilting of the hydraulic pump is released. As a result, the entire system is stabilized.
By setting both values of the setting differential pressure &Dgr;Pun of the unloading valve and the setting differential pressure &Dgr;PLSref for load sensing control close to each other, stability of the entire system is improved.
Moreover, in the pump displacement control system disclosed in JP, A, 5-99126, the setting modifying means detects the engine rotational speed based on the delivery rate of the fixed displacement pump and variably adjusts the setting differential pressure &Dgr;PLSref for load sensing control, thereby realizing an improvement of operability depending on the engine rotational speed. Supposing a system that an unloading valve is provided in a hydraulic circuit including the disclosed pump displacement control system and the setting differential pressure &Dgr;Pun of the unloading valve is set slightly higher than the load-sensing setting differential pressure &Dgr;PLSref at the rated rotational speed of an engine, such a system can improve stability of the entire system at the rated rotational speed of the engine. However, when the engine rotational speed is lowered, the load-sensing setting differential pressure &Dgr;PLSref is reduced, whereas the setting differential pressure of the unloading valv
Kanai Takashi
Kawamoto Junya
Tsuruga Yasutaka
Hitachi Construction Machinery Co. Ltd.
Lopez F. Daniel
Mattingly, Stanger & Malur
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