Power plants – Pressure fluid source and motor – Having condition responsive control in a system of distinct...
Reexamination Certificate
2000-10-24
2002-08-27
Lopez, F. Daniel (Department: 3745)
Power plants
Pressure fluid source and motor
Having condition responsive control in a system of distinct...
C060S452000, C091S447000
Reexamination Certificate
active
06438952
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a hydraulic circuit system which is mounted on a construction machine including a plurality of hydraulic actuators often simultaneously operated, such as a hydraulic excavator, and which can provide a smooth start-up characteristic regardless of the magnitude of an inertia body to be driven.
BACKGROUND ART
There are two types of hydraulic circuit systems mounted on a construction machine such as a hydraulic excavator; one employing a center bypass control valve and including a bleed-off circuit, and the other employing a closed center control valve and including no bleed-off circuit. The latter hydraulic circuit system employs a load sensing system for controlling a delivery rate of a hydraulic pump so that a hydraulic fluid can be basically supplied at a flow rate demanded by the control valve. In the case of intending simplification of hydraulic equipment, the latter hydraulic circuit system is more advantageous because of including no bleed-off circuit. The absence of a bleed-off circuit however gives rise to the problem that, when a hydraulic actuator having large inertia is driven, the actuator is abruptly accelerated in a transient state due to a sudden rise of pressure, or the actuator is free from a smooth start-up characteristic because vibration of pressure (pressure pulsation) does not attenuate early.
More specifically, in the load sensing system, the delivery rate of the hydraulic pump is controlled so that the hydraulic fluid can be supplied at the flow rate demanded by the control valve. Accordingly, where a load to be driven by the actuator is an inertia body such as a swing body and the actuator cannot fully consume the hydraulic fluid delivered from the hydraulic pump, the delivery pressure of the hydraulic pump abruptly rises and the energy delivered from the hydraulic pump is accumulated in a piping system. Then, when the actuator has passed an acceleration range and pressure for acceleration is no longer required, the energy accumulated in the piping system is released upon lowering of the driving pressure, causing the actuator to overshoot. This overshoot further lowers the driving pressure. After that, the actuator speed is reduced, whereupon the driving pressure rises again, thus repeating changes in the actuator speed and the driving pressure. Stated otherwise, the actuator is brought into such a transient state that a sudden rise of pressure occurs and pressure pulsation does not attenuate early.
In view of the above problem, JP,A 4-191501, JP,A 5-263804, and JP,A 10-89304 propose methods for reducing a supply flow rate to the actuator with an increase of the driving pressure and suppressing a sudden rise of pressure.
The methods disclosed in JP,A 4-191501 and JP,A 5-263804 have the same purport and are intended to propose a control valve for controlling a displacement of a proportional seat valve having a slit in accordance with a valve opening of a pilot valve, wherein a displacement of the pilot valve is controlled depending on a driving pressure of an actuator to thereby control the displacement of the proportional seat valve. More specifically, a pressure having been introduced from an inlet portion of a hydraulic motor through a throttle is introduced to the pilot valve against the force acting upon the pilot valve for operation. The pressure having been introduced from the inlet portion of the hydraulic motor through the throttle is a pressure that increases in proportion to a driving pressure of the hydraulic motor. Therefore, the valve opening of the pilot valve is reduced in proportion to the driving pressure of the hydraulic motor, whereupon the valve opening of the proportional valve is also reduced. A hydraulic fluid delivered from a hydraulic pump is further controlled so as to reduce correspondingly. This reduction of the delivered hydraulic fluid contributes to moderating a sudden rise of pressure and attenuating pressure pulsation.
According to JP,A 10-89304, a pressure compensation valve provided for enabling the combined operation to be performed in the load sensing system is given with a load dependent characteristic that reduces a compensation differential pressure as a load pressure increases. This results in such control that as the load pressure increases, a supply flow rate to an actuator is reduced and a delivery rate of a hydraulic pump is also reduced. The load dependent characteristic of the pressure compensation valve is provided by setting, of pressure bearing areas of the pressure compensation valve, a pressure bearing area against which a pressure on the inlet side of a meter-in variable throttle acts in the closing direction, to be larger than a pressure bearing area against which a pressure on the outlet side of the meter-in variable throttle acts in the opening direction. By so setting a difference between both the pressure bearing areas, there occurs a hydraulic force that acts in the closing direction corresponding to the difference between both the pressure bearing areas, and is increased as the load pressure rises. In proportion to the load pressure, therefore, the differential pressure across the meter-in variable throttle is controlled so as to decrease and the supply flow rate to the actuator is reduced. With a reduction of the supply flow rate to the actuator, the delivery rate of the hydraulic pump under load sensing control is reduced. As a result, a sudden rise of pressure is avoided and pressure pulsation attenuates more early.
Meanwhile, JP,A 2-296002 proposes a hydraulic circuit system including a load sensing system, wherein a driving speed of a particular hydraulic actuator only is slowed down to achieve fine-speed operation without changing a target differential pressure of load sensing control set on pump control means. According to this proposal, a spring force of a check valve for detecting a load pressure is set to a certain degree of strength so that the load pressure is modulated with a pressure loss produced by the check valve. A detected signal pressure is lowered from the load pressure by an amount corresponding to the pressure loss, and a differential pressure between a delivery pressure of a hydraulic pump under the load sensing control and the load pressure is also lowered from an originally set value by an amount corresponding to the pressure loss. Consequently, the flow rate delivered under the load sensing control is reduced.
Further, PCT Laid-Open Publication WO98/31940 discloses a control valve for use in a hydraulic circuit system including a load sensing system, the control valve being constructed as a valve assembly in combination of a flow distribution valve and a hold check valve for simplification. In the disclosed control valve, a valve body of the flow distribution valve is partly incorporated in a hollow valve body of the hold check valve, a load pressure detecting hydraulic line of the control valve is formed as an internal passage (hydraulic line slit) of the flow distribution valve, and the internal passage is utilized to provide a check valve function. As a result, a check valve as a separate valve element is no longer required and the control valve is simplified in its overall construction.
DISCLOSURE OF THE INVENTION
With the proposals disclosed in JP,A 4-191501, JP,A 5-263804 and JP,A 10-89304, in proportion to the load pressure, the supply flow rate to the hydraulic actuator is reduced and the delivery rate of the hydraulic pump is also reduced. Upon driving of the hydraulic actuator, therefore, a sudden rise of pressure is avoided and pressure pulsation attenuates more early. A smooth start-up characteristic is thus obtained regardless of the magnitude of an inertia body to be driven. However, those prior-art techniques have the following problems.
The proposals disclosed in JP,A 4-191501 and JP,A 5-263804 are difficult to implement using an ordinary spool-type control valve from the structural point of view because the control valve employed in those proposals is constructed so as to control the valve opening of th
Nishimura Yoshizumi
Nozawa Yusaku
Takahashi Kinya
Tougasaki Mitsuhisa
Hitachi Construction Machinery Co. Ltd.
Lazo Thomas E.
Lopez F. Daniel
Mattingly Stanger & Malur, P.C.
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