Pumps – Condition responsive control of drive transmission or pump... – Having condition responsive pumped fluid control
Patent
1992-09-29
1994-03-22
Berisch, Richard A.
Pumps
Condition responsive control of drive transmission or pump...
Having condition responsive pumped fluid control
4172222, 417270, 60368, 60445, F04B 4900
Patent
active
052957950
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a hydraulic drive system for construction machines such as hydraulic excavators, and more particularly to a hydraulic drive system for construction machines for controlling a delivery rate of a hydraulic pump dependent upon a control pressure produced by a flow resistive element.
BACKGROUND OF THE INVENTION
A conventional hydraulic drive system for construction machines comprises, as disclosed in JP, A, 1-25921, a hydraulic pump of variable displacement type, a pump regulator for controlling a delivery rate of the hydraulic pump, a plurality of hydraulic actuators driven by a hydraulic fluid supplied from the hydraulic pump, a plurality of directional control valves of center bypass type for controlling respective flows of the hydraulic fluid supplied from the hydraulic pump to the plural hydraulic actuators, a center bypass line for connecting in series center bypasses of the plural directional control valves to a reservoir, a flow resistive element, e.g., a fixed restrictor, disposed in a downstream portion of the center bypass line for producing a control pressure, a pressure sensor for detecting the control pressure produced by the fixed restrictor and outputting a corresponding electric signal, and a function generator for storing preset one kind of pump flow rate characteristic that defines the relationship between a value of the electric signal outputted from the pressure sensor and a delivery rate of the hydraulic pump, determining the delivery rate corresponding to the value of the electric signal outputted from the pressure sensor based on the preset pump flow rate characteristic, and outputting, as a drive signal, a signal corresponding to the determined delivery rate. The pump regulator is driven with the drive signal.
In the above prior art, a variable restrictor for bleeding-off is disposed in the center bypass of each of the plural directional control valves. This variable restrictor is fully opened when the associated directional control valve is at its neutral position, and its opening is reduced as an input amount of the directional control valve increases. As a result, with the directional control valve being at its neutral position, the flow rate of the hydraulic fluid passing through the center bypass is maximized and, therefore, the control pressure produced by the fixed restrictor is also maximized. Then, as the input amount of the directional control valve increases, the flow rate through the center bypass is reduced and so is the control pressure. The pump flow rate characteristic preset in the function generator is set such that the delivery rate of the hydraulic pump is increased with the control pressure becoming smaller. Accordingly, the delivery rate of the hydraulic pump is controlled to increase dependent upon the input amount of the directional control valve.
Meanwhile, there are various kinds of operations to be performed by construction machines such as hydraulic excavators, and the directional control valve requires control characteristics different from each other dependent upon the kinds of operations. For example, in the work such as craning which requires fine operation, a control characteristic superior in the metering property is needed. On the other hand, in the work such as digging which requires powerful operation, a control characteristic superior in rising of the metering property and capable of easily supplying the hydraulic fluid at a large rate is needed.
In the conventional hydraulic drive system as stated above, however, the control characteristic of the delivery rate of the hydraulic pump is uniquely determined dependent upon the setting in the function generator and, correspondingly, the control characteristic of the directional control valve is also uniquely determined. This has raised the problem that good operating efficiency cannot be ensured in other kinds of work than particular one.
More specifically, in the above-explained prior hydraulic drive system, the control characteristic of the directi
REFERENCES:
patent: 4600364 (1986-07-01), Nakatani et al.
patent: 5073091 (1991-12-01), Burgess et al.
patent: 5077973 (1992-01-01), Suzuki et al.
Aoyagi Yukio
Yasuda Tomohiko
Berisch Richard A.
Hitachi Construction Machinery Co. Ltd.
Scheuermann David W.
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