The present disclosure relates to a hydraulic control system. More particularly, the present disclosure relates to a positive hydraulic control system for preventing a working device from abnormally shaking, for example, hunting oscillations, wherein a lowest recognition pressure value is pre-stored in a database, a pump discharge pressure value is measured, when the measured pump discharge pressure value is lower than the lowest recognition pressure value, a control pressure is recognized as having the pre-stored lowest recognition pressure value, and when the measured pump discharge pressure value is greater than the lowest recognition pressure value, the control pressure is recognized as having the measured pump discharge pressure value.
Apparatuses using hydraulic pressure, such as construction machinery, are designed to obtain optimal output characteristics by matching the input horsepower of a pump to the output horsepower of an engine.
Generally, the hydraulic control systems of construction machinery are designed to perform constant-horsepower control to prevent an engine, a pump, a pipeline, a cylinder, or the like from being subjected to an excessive amount of pressure during high-load operations, so that the pipeline or an actuator is not fractured or the engine or the pump is not damaged.
Technological features related to constant-horsepower control will be described with reference to FIGS. 1 to 3 hereinafter. FIG. 1 is a block diagram illustrating a general constant-horsepower control system, FIG. 2 is a graph illustrating a relationship between pilot pressure and pump displacement preset in a hydraulic controller illustrated in FIG. 1, and FIG. 3 shows a pump constant torque curve preset in the hydraulic controller illustrated in FIG. 1, i.e. a graph depicting constant horsepower control through adjustment of the pump displacement and pump torque depending on discharge pressures of a variable displacement hydraulic pump.
As illustrated in FIG. 1, the hydraulic control system performing constant-horsepower control includes a control lever 1, a pressure sensor 2, a flow control valve 3, pump discharge pressure detectors 4, a hydraulic controller 5, variable displacement hydraulic pumps 6, electro-proportional pressure reducing valves 7, and an engine 8.
Describing an operation of the system for constant-horsepower control, the pressure sensor 2 detects a pilot pressure output by the control lever 1 and delivers a detected pressure value to the hydraulic controller 5.
Then, as illustrated in FIG. 2, the hydraulic controller 5 performs an operation of opening or closing an electro-proportional pressure reducing valve 7 by sending an electronic signal to the electro-proportional pressure reducing valve 7 to change displacement of the variable displacement hydraulic pump 6 in accordance with the curve of preset pump displacements with respect to pilot pressures.
The hydraulic controller 5 adjusts pump displacement depending on pilot pressure as illustrated in the pump volume curve of FIG. 2, as long as the variable displacement hydraulic pump operates within a preset maximum allowable torque value in the pump constant torque curve illustrated in FIG. 3. This prevents the engine 8 and the system from being damaged during high load operations, thereby protecting the engine 8 and the system.
FIG. 4 is a graph illustrating the pilot pressure of the control lever 1, the discharge pressure of a variable displacement hydraulic pump 6, and a control signal by which the hydraulic controller 5 instructs the variable displacement hydraulic pumps 6, plotted with time.
In a low temperature environment, for example, in the middle of the winter, since the viscosity of hydraulic fluid is increased, the pressure may abruptly change in a specific operation during hydraulic control. If such an abrupt change occurs when a pump discharge pressure P1 or P2 of the variable displacement hydraulic pump 6 is passing a pressure value at which the constant horsepower control, as illustrated in FIG. 3, is started, the hydraulic controller 5 transmits a control signal for increasing or reducing the displacement of the variable displacement hydraulic pump 6 accordingly, to the electro-proportional pressure reducing valves 7, so that constant horsepower is maintained.
However, there is a minute time difference between a point in time at which the pressure of the pump is abruptly changed and a point in time at which the displacement of the variable displacement hydraulic pump 6 is actually changed in response to the control signal transmitted to the electro-proportional pressure reducing valve 7 by the hydraulic controller 5. Thus, when an abrupt pressure change occurs at a low temperature, such a method of controlling the displacement of the variable displacement hydraulic pump 6 may lead to resonance, such that the pump discharge pressure vibrates, as illustrated in FIG. 4. Consequently, the hydraulic working devices are subjected to hunting oscillations, abrupt shaking, which is problematic.
Accordingly, an aspect of the present disclosure has been made in consideration of the above-described problems occurring in the related art, and the present disclosure proposes a hydraulic control system for preventing a hydraulic working device from experiencing hunting oscillations, or abruptly shaking, in a low-temperature environment. When discharge pressure abruptly surges in such a low-temperature environment, a hydraulic controller performing constant horsepower control by changing displacement of the variable displacement hydraulic pump issues a control signal for controlling the variable displacement hydraulic pump in which a control pressure may be recognized as having a lowest recognition pressure value. This can consequently prevent resonance of the discharge pressure of the variable displacement hydraulic pump, thereby preventing the hydraulic working device from experiencing hunting oscillations, or abruptly shaking.
According to an aspect of the present disclosure, a hydraulic control system may include: a flow control valve; a variable displacement hydraulic pump connected to the flow control valve to discharge pressurized fluid toward the flow control valve; a pump discharge pressure detector disposed on a passage between the flow control valve and the variable displacement hydraulic pump, the pump discharge pressure detector detecting a discharge pressure of the pressurized fluid discharged toward the flow control valve by the variable displacement hydraulic pump; and a hydraulic controller. The hydraulic controller includes: a detector connected to the pump discharge pressure detector to convert the detected discharge pressure to a pump discharge pressure value; a comparator receiving the pump discharge pressure value from the detector, comparing the pump discharge pressure value with a pre-stored lowest recognition pressure value, and determining whether the pump discharge pressure value is higher or lower than the pre-stored lowest recognition pressure value; and a calculator cooperating with the comparator to calculate a control pressure which the variable displacement hydraulic pump is controlled based on, wherein, when the pump discharge pressure value is lower than the lowest recognition pressure value, the calculator recognizes the control pressure as having the lowest recognition pressure value.
The hydraulic control system may further include a control lever connected to the flow control valve to control opening and closing the flow control valve.
The hydraulic control system may further include a pressure sensor detecting a pilot pressure applied to the flow control valve by the control lever.
The hydraulic control system may further include an engine working in concert with the variable displacement hydraulic pump to drive the variable displacement hydraulic pump.
The hydraulic control system may further include an electro-proportional pressure reducing valve connected to the variable displacement hydraulic pump to change pump displacement of the variable displacement hydraulic pump according to opening and closing operations of the electro-proportional pressure reducing valve.
The calculator may receive a value of pilot pressure, calculate a pump displacement from the value of pilot pressure with reference to a pre-stored pilot pressure-pump displacement relationship, calculate a torque of the variable displacement hydraulic pump from the calculated pump displacement and the calculated control pressure and transmit a control signal to the electro-proportional pressure reducing valve such that the variable displacement hydraulic pump operates within a maximum allowable torque value.
The lowest recognition value may be set to be higher than a value at which the control signal otherwise vibrates when an abrupt change of the discharge pressure occurs.
When the pump discharge pressure value is greater than the lowest recognition pressure value, the calculator may recognize the control pressure as having the pump discharge pressure value.
According to an aspect of the present disclosure, when discharge pressure abruptly surges in a low-temperature environment, a hydraulic controller performing constant horsepower control by changing displacement of a variable displacement hydraulic pump issues a control signal for controlling the variable displacement hydraulic pump in which a control pressure may be recognized as having a lowest recognition pressure value. This can consequently prevent resonance of the discharge pressure of the variable displacement hydraulic pump, thereby preventing a hydraulic working device from experiencing hunting oscillations, or abruptly shaking.