JPH07190004A - Oil pressure controller for construction machine - Google Patents

Abstract

PURPOSE:To operate a boom in a desired action mode by changing the opening and closing characteristic of a pressure compensatory valve used in a load sensing control circuit in accordance with the operation mode of the boom. CONSTITUTION:An oil pressure controller is provided with a mode instructing means 3 for instructing the operation mode of a boom cylinder 24 and changing means 60 and 70 for changing the reference value of a pressure compensatory valve 34 in accordance with the instructed operation mode. When the operation mode is instructed, the opening and closing characteristic of the pressure compensatory valve 34 is changed and the pressure compensatory valve 34 is closed by a difference pressure smaller than a target pressure for load sensing at the time of pressure changing work.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydraulic control device for a construction machine such as a hydraulic excavator having a boom cylinder.

[0002]

2. Description of the Related Art FIG. 9 shows a conventionally known boom cylinder 99 of a hydraulic shovel and a hydraulic pilot type control valve 98 for controlling the amount and direction of oil supplied to the boom cylinder 99. . The control valve 98 is switched by the pilot pressure generated according to the operation amount of the operation lever (not shown). When the control valve 98 is switched to the L position, the boom cylinder 99 contracts and the boom descends, and when the control valve 98 is switched to the U position, the boom cylinder 99 extends and the boom rises. When the boom is lowered, the pressure oil of the hydraulic power source 97 is supplied from the P → A port to the boom cylinder 99.
The oil that is supplied to the rod chamber 99a and discharged from the bottom chamber 99b flows into the tank 96 from the B → T port.

FIG. 10 shows the spool opening area between the four ports of the control valve 98 according to the stroke thereof. The horizontal axis represents the stroke and the vertical axis represents the spool opening area. FIG. 10 shows a case where the flow rate between the P and A ports is smaller than the flow rate between the B and T ports. In this case, when the boom lowering operation is performed, the boom falls freely.

When a control valve having such a metering characteristic is used, the behavior of the pressure in the bottom chamber and the rod chamber of the boom cylinder becomes as shown in FIG. 11 when the operating lever is returned to the neutral position after the boom lowering operation. That is, the pressure oil on the rod chamber side cannot catch up with the pressure on the bottom chamber side, and the pressure in the rod chamber becomes a negative pressure for a while. Therefore, the boom is not settled and the vehicle body vibrates, resulting in poor stability.

On the other hand, FIG. 12 shows a case where the opening area between the P → A ports is larger than the opening area between the B → T ports. In this case, when the boom lowering operation is performed, the boom lowers at a speed that depends on the required amount of pressure oil.

When a control valve having such a metering characteristic is used, when the operation lever is returned to the neutral position after the boom lowering operation, the behavior of the pressure in the bottom chamber and the rod chamber of the boom cylinder becomes as shown in FIG. That is, since the amount of oil discharged from the bottom chamber side is less than the amount of oil supplied from the rod chamber side, pushing is self-contained, and the pressure in the rod chamber immediately rises. Therefore, the boom is settled immediately, the vibration of the vehicle body is suppressed, and the stability is good.

[0007]

However, the metering characteristic of the control valve is shown in FIG.
If the push-down operation is performed when the boom is lowered as described above, the vehicle body is jacked up during rolling compaction work, resulting in poor rolling compaction performance. On the other hand, if the boom is allowed to fall freely as shown in FIG. 10 so as to improve the compaction performance, the stability will deteriorate and the operability during excavation work will deteriorate.

It is an object of the present invention to change the opening / closing characteristics of a pressure compensation valve used in a load sensing control circuit according to the operation mode of a boom so that the boom can be operated in a desired operation mode. To provide.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 showing an embodiment, the present invention will be described. According to the present invention, a variable displacement hydraulic pump 1 driven by a prime mover 2 and a discharge oil from the hydraulic pump 1 are used for driving. The boom cylinder 24 for moving the boom up and down, the vertical movement command means 14a for instructing the vertical movement of the boom, and the direction and flow rate of the pressure oil to the boom cylinder 24 according to the command of the vertical movement command means 14a. A control valve 14 to be controlled, and a displacement volume adjusting means 50 for adjusting the displacement volume of the variable displacement hydraulic pump 1 so that the differential pressure across the control valve 14 becomes a predetermined value regardless of the load pressure of the boom cylinder 24. , Which is connected to the input port of the control valve 14 and is suitable for a hydraulic control device of a construction machine having a pressure compensating valve 34 which is opened until the differential pressure across the control valve 14 reaches a reference value and is closed when the reference value reaches the reference value. It is. And the boom cylinder 2
The above-described object is achieved by including the mode command means 3 for commanding the operation mode of No. 4 and the change means 60, 70 for changing the reference value of the pressure compensation valve 34 according to the commanded operation mode.

[0010]

The mode command means 3 commands the operation mode of the boom cylinder 24. The changing means 60, 70 changes the reference value of the pressure compensation valve 34 according to the commanded operation mode. As a result, the opening / closing characteristic of the pressure compensation valve 34 can be selectively used depending on whether it is closed at the target differential pressure set as the load sensing control or closed at a value lower than the target differential pressure. In the former, the boom is pushed freely, and in the latter, the boom falls freely.

Incidentally, in the section of means and action for solving the above problems for explaining the constitution of the present invention, the drawings of the embodiments are used for the purpose of making the present invention easy to understand. It is not limited to.

[0012]

【Example】

-First Embodiment- An embodiment in which the present invention is applied to a hydraulic excavator will be described with reference to Figs. FIG. 1 is a diagram showing the overall configuration of a drive control device for a hydraulic excavator, and 1 is a variable displacement hydraulic pump driven by an engine (motor) 2. The oil discharged from the variable displacement hydraulic pump 1 according to the engine speed is supplied to the swing hydraulic motor 21, the traveling hydraulic motors 22 and 23, the boom cylinder 24, the arm cylinder 25, and the bucket cylinder 26 via the control valves 11 to 16. To be done. In the present embodiment, the metering characteristic of the boom control valve 14 at the lower position is such that the opening area between the P → A ports is larger than the opening area between the B → T ports as shown in FIG. There is.

Reference numerals 31 to 36 are pressure compensating valves, which are provided for independently compensating the operation of the hydraulic actuators 21 to 26. When the difference between the inlet and outlet pressures of the respective control valves is below a predetermined reference value, the hydraulic pressure is reduced. The pressure oil from the pump 1 is supplied to each actuator. The turning pressure compensating valve 31 has a turning load pressure introduced via a pilot port 31a,
The swirl pump pressure introduced via the pilot port 31b acts in opposition to each other, resulting in a differential pressure Δ between the swirl pump pressure and the swirl load pressure.
P1 provides the valve closing force F1. On the other hand, the spring force of the spring 31c and the force corresponding to the swing control pressure Pc1 introduced from the control pressure generation circuit 60 described later via the pilot port 31d act in opposition to each other. The driving force by the spring force and the turning control pressure gives a valve opening force F2. When the sum of the swing load pressure and the spring force is larger than the sum of the swing pump pressure and the swing control pressure (in this case, the relationship between the valve opening force F2 and the valve closing force F1 is
F2-F1> 0) The discharge oil of the hydraulic pump 1 is guided to the swing hydraulic motor 21, and when the sum of the swing load pressure and the spring force is less than or equal to the sum of the swing pump pressure and the swing control pressure (in this case, the valve opening force). The relationship between F2 and the valve closing force F1 is F2-F1≤0) The discharge oil of the hydraulic pump 1 is shut off.

The other pressure compensating valves 32 to 36 are similarly constructed. Pilot ports 32a to 36a to which the load pressure of each actuator is guided, pilot ports 32b to 36b to which the pump pressure is guided, and springs 32c to 36c.
And pilot ports 32d to 3 on which control pressures Pc2 to Pc6 introduced from a control pressure generation circuit 60 described later act.
6d.

The load pressure on the downstream side of each control valve is the line 41
The maximum load pressure is taken out to the line 47 from a to 46a via the check valves 41b to 46b. The maximum load pressure and the discharge pressure of the hydraulic pump 1 are input to the differential pressure detection circuit 48 and the difference between the two is detected. Further, the maximum load pressure and the discharge pressure of the hydraulic pump 1 are also input to the tilt angle control device 50 and used for load sensing control described later.

The tilt angle of the variable displacement hydraulic pump 1, that is, the displacement volume, is controlled by a tilt angle control device 50.
The load sensing is the pressure across the control valves 11 to 16, that is, the differential pressure between the inlet pressure (pump pressure) and the outlet pressure (load sensing pressure) of the control valve of the variable displacement hydraulic pump 1 so as to be a constant value. The displacement volume (hereinafter, also referred to as tilt angle) is controlled to keep the pump pressure higher than the load sensing pressure by a predetermined target value.

As shown in FIG. 2, the tilt angle control device 50.
Has a load sensing regulator 51 that switches according to the differential pressure between the pump pressure and the load sensing pressure, and when the differential pressure between the pump pressure and the load sensing pressure exceeds the pressure set by the spring 51a, the load sensing regulator 51 switches in the direction of b position according to the pressure. At the position b, the pump pressure is introduced to both the rod chamber and the bottom chamber of the servo cylinder 52, the servo cylinder 52 expands due to the difference in area of the pistons, the displacement volume of the hydraulic pump 1 becomes small, and the pump discharge flow rate decreases. To do. On the contrary, when the differential pressure becomes less than the pressure set by the spring 51a, the load sensing regulator 51 switches to the a position and the servo cylinder 52 is connected to the tank. As a result, the servo cylinder 52 contracts, the displacement volume increases, and the pump discharge flow rate increases.

According to the load sensing control, the displacement volume of the variable displacement hydraulic pump 1 is controlled so that the differential pressure across each control valve becomes a constant value, and the pump pressure is higher than the load sensing pressure. Since the pump is kept high by a predetermined target value, the pump tilt angle is controlled so that the pump discharge flow rate is the flow rate required by the control valve of the actuator with the highest load pressure, and there is no discharge of excess flow rate It is possible to improve fuel efficiency and operability by eliminating waste caused by.

On the other hand, a torque limit control regulator 53 composed of a servo valve is also provided, and the pump pressure is controlled by the spring 5.
When it exceeds the pressure set in 3a, it switches to b position,
Similarly to the above, the servo cylinder 52 expands to reduce the displacement volume of the hydraulic pump 1, and when the pump pressure becomes less than the pressure set by the spring 53a, the position is switched to the a position, and the servo cylinder 52 contracts similarly to the above. As a result, the displacement volume of the hydraulic pump 1 increases.

According to the torque limiting control, the displacement volume of the pump 1 can be controlled in accordance with the PQ curve determined in advance from the horsepower characteristic of the engine 2, and the torque of the hydraulic pump 1 is within the range of the output torque of the engine 2. Therefore, the engine 2 is prevented from being overloaded.

Referring again to FIG. 1, the controller 70 includes a differential pressure signal detected by the differential pressure detection circuit 48, a boom operation mode signal instructed by the mode switch 3, and the hydraulic pump 1 detected by the temperature sensor 4. The temperature signal of the discharged oil and the boom lowering detection signal detected by the boom lowering detection switch 5 are input. The switch 5 is a switch that is turned on when the boom lever 14a is operated downward. The controller 70 responds to these input signals by pressure compensating valves 31-36 of the actuators 21-26.
The control signals a to f for changing the shunting characteristics are generated and output to the control pressure generation circuit 60.

The controller 70 includes function generators 71 to 76, as shown in FIG. Function generators 71-76
Is the control pressure Pc1 corresponding to each hydraulic actuator according to the differential pressure signal ΔPLS detected by the differential pressure detection circuit 48.
To Pc6 are output. The boom cylinder function generators 74a and 74b output control signals Pc4a and Pc4b with different functions. The control signal Pc4a is a control signal suitable for excavation work, and the control signal Pc4b is a control signal suitable for rolling compaction work. These control signals Pc
Either 4a or Pc4b is selected by the switch 77 in the subsequent stage.

The switch 77 is switched by the output signal from the AND gate 78. The AND gate 78 outputs an ON signal when both the mode signal from the mode switch 3 and the detection signal from the boom lowering detection switch 5 are ON.
If the mode switch 3 is turned off at the time of excavation work and turned on at the time of rolling work, the AND gate 78 switches the switch 77 to the b contact when the boom lowering detection signal is turned on during rolling work. As a result, the function generator 74b is selected and the large control signal Pc4b is output with a small pressure difference. On the other hand, when the mode switch 3 is turned off during excavation, the AND gate 78 is turned off, so that the switch 77 is switched to the a contact, the function generator 74a is selected, and the control signal Pc4a smaller than that during the rolling operation is output. It

The controller 70 further includes a temperature sensor 4
It has a function generator 79 that outputs a correction coefficient K according to the temperature signal TH detected in. The output signal of the function generator 79 is sent to the multipliers 81 to 83 for the boom cylinder control signal P.
c4, the arm cylinder control signal Pc5, and the bucket cylinder control signal Pc6 are multiplied to correct the oil temperature. The controller 70 further includes delay circuits 84 to 89, respectively, and controls signals Pc1 to Pc6.
Is subjected to first-order lag filter processing, and is input to the subsequent control pressure generating circuit 60.

As shown in FIG. 4, the control pressure generating circuit 60 has proportional electromagnetic pressure reducing valves 61 to 66 corresponding to the respective hydraulic actuators 21 to 26. Each proportional electromagnetic pressure reducing valve 61 to
The control signals a to f from the controller 70 described above are applied to the proportional solenoid 66, and the electromagnetic pressure reducing valves 61 to 6 are applied.
6 outputs the discharge pressure of the hydraulic pump 6 to the pilot ports 31d to 36d of the pressure compensating valves 31 to 36 as the control pressure according to the applied control signal via the lines 41c to 46c. 67 is a relief valve.

The operation of the hydraulic control device thus configured will be described. When any one or more of the control valves 11 to 16 are operated, the pressure oil from the hydraulic pump 1 causes the pressure compensating valve 3 to operate.
1-36 and control valves 11-16 to the corresponding actuators. At this time, the tilt angle control device 5
With 0, the displacement volume of the hydraulic pump 1 is subjected to load sensing control within the area surrounded by the PQ diagram. The differential pressure ΔPLS detected by the differential pressure detection circuit 48 is input to the controller 70. On the other hand, the oil temperature signal detected by the temperature sensor 4 is also input to the controller 70. Further, on / off signals from the mode switch 3 and the boom lowering detection switch 5 are also input to the controller 70.

The controller 70 calculates control signals a to f for controlling the characteristics of the pressure compensating valve in accordance with these input signals and supplies them to the control pressure generating circuit 60. The control pressure generation circuit 60 supplies control pressures corresponding to the input control signals a to f to the pilot ports 31d to 36d of the respective pressure compensation valves. Therefore, the pressure compensating valves 31 to 33 of the swing hydraulic motor 21 and the traveling hydraulic motors 22 and 23 are controlled according to the characteristics of the function generators 71 to 73 of FIG. On the other hand, the pressure compensating valves 34 to 36 of the boom cylinder 24, the arm cylinder 25, and the arm cylinder 26 are controlled by the characteristics of the function generators 74a and 74b to 76 corrected by the oil temperature.

Here, by operating the mode switch 3,
When the boom is lowered, the boom can be freely dropped or pushed down freely. In this embodiment, the mode switch 3 is turned off during excavation work. When the boom lever 14a of the boom control valve 14 is lowered, the pressure oil of the main pump 1 is supplied to the rod chamber 24a of the boom cylinder 24. At this time, the AND gate 78 of the controller 70 is turned off, and the switch 77 is switched to the a contact. Therefore, the control pressure Pc4a obtained by the function generator 74a is output from the switch 77 to the multiplier 81 as the control signal Pc4. Since the function generator 79 outputs the correction coefficient K according to the oil temperature, the control signal Pc4 is multiplied by the correction coefficient K by the multiplier 81, and the control signal d corrected according to the oil temperature is supplied to the control pressure generation circuit 60. Is supplied to the proportional electromagnetic pressure reducing valve 64.
The proportional electromagnetic pressure reducing valve 64 reduces the discharge pressure of the hydraulic pump 6 according to the control signal d and supplies the pressure to the pilot port 34d of the pressure compensating valve 34.

On the other hand, the mode switch 3 is turned on during the rolling operation. In this case, the AND gate 78 is turned on and the switch 77 is switched to the b contact. Therefore, the control pressure Pc4b obtained by the function generator 74b is equal to the control signal Pc.
4 is output from the switch 77 to the multiplier 81, and the temperature is similarly corrected by the multiplier 81. The proportional electromagnetic pressure reducing valve 64 is driven by this control signal d, and the proportional electromagnetic pressure reducing valve 64 is
The discharge pressure of the hydraulic pump 6 is reduced according to the control signal d and supplied to the pilot port 34d of the pressure compensating valve 34.

Since the function generator 74b is set to have a larger value for the differential pressure ΔPLS than the function generator 74a, the difference between the front and rear of the control valve 14 is changed unless the boom stroke is fully operated during the rolling operation. The pressure compensating valve 34 is closed before the pressure reaches the target differential pressure for load sensing control (set by the spring force of the spring 51a of the load sensing regulator 51 of FIG. 2), and as a result, the flow rate of the control valve 14 passes through. The boom cylinder 24 is in a free fall operation because it is smaller than the same stroke during the load sensing control. Therefore, when the boom lowering operation is stopped, as shown in FIG. 11, the rod chamber pressure does not immediately become a positive pressure, so that the vehicle body is not jacked up during the rolling operation, and the rolling operation is efficiently performed.

On the other hand, the control signal d calculated during the excavation work
Is smaller than the signal calculated for the same detected differential pressure ΔPLS during the rolling operation, the pressure compensating valve 34 is opened until it comes closer to the target differential pressure compared to the rolling operation. As a result, the control valve 14 supplies the pump discharge oil to the hydraulic cylinder 24 according to the metering characteristic of FIG. 12, and the boom cylinder 24 is operated by pushing. Therefore, as shown in FIG. 13, the rod chamber pressure immediately rises when the boom lowering operation is stopped, and the stability of the boom cylinder during excavation work is good.

-Second Embodiment- FIGS. 5 to 7 show a second embodiment. In the first embodiment,
The excavation work mode and the compaction work mode are selected by turning the mode switch 3 on and off. However, in the second embodiment, the excavation work or the compaction work is discriminated from the operation state of the boom lever 14a. The mode is switched. Therefore, as shown in FIG. 5, a displacement sensor 7 that outputs a signal proportional to the operation amount of the boom lever 14a, and a signal that switches the switch 77 by determining the work content based on the signal from the sensor 7 are output. Work discriminator 7
8A is provided. Other than that, the configuration can be similar to that of the first embodiment.

In FIG. 7 showing the work content determination program, the state of boom raising and lowering operations is read from the displacement sensor 7 in step S11, and whether the boom raising and lowering switching operation is performed N times or more within a predetermined time in step S12. To determine. If this step S12 is positive,
In step S13, the switch 77 is determined to be the rolling operation.
Is switched to the contact b, and if the result is negative, the excavation work is determined in step S14 and the switch 77 is switched to the contact a. In this embodiment, the mode switch 3 is unnecessary, and even if the operator forgets to operate the switch 3, there is no inconvenience in each work. The tilt control is the same as that of the first embodiment, and the description is omitted.

Therefore, as in the case of the first embodiment, when it is determined that the excavation work is being performed, the boom cylinder 24 is lowered by pushing. Therefore, the behaviors of the rod chamber pressure and the bottom chamber pressure when the boom lever 14a is returned to the neutral position are as shown in FIG. 13, and when the boom lever 14a is returned to the neutral position, the rod chamber pressure immediately rises. Settles quickly.

On the other hand, when it is determined that the work is rolling, the boom cylinder 24 is freely dropped. Therefore, as in the first embodiment, the rod chamber pressure does not immediately become a positive pressure even when the boom lever 14a is returned to the neutral position, so that the jack-up phenomenon of the vehicle body is suppressed.

-Third Embodiment- FIG. 8 shows a third embodiment. In the first and second embodiments, the function generator 74 is operated according to the differential pressure ΔPLS during the rolling operation.
Although the control signal Pc4b output from b is used as it is, the dial 8 that is arbitrarily operated by the operator, the function generator 78C that outputs the correction coefficient α according to the operation amount of the dial 8, and the correction Coefficient α and control signal Pc
And a multiplier 78D for multiplying 4b, respectively, so that the control pressure Pc4b is set to an arbitrary value.

According to this embodiment, when it is desired to freely drop the boom cylinder when the boom lever 14a is slightly operated during rolling work, the correction coefficient α is increased by the dial 8 so that the boom lever can be operated even during rolling work. When the boom cylinder is to be operated in a free fall operation when 14a is operated considerably, the correction coefficient α is set to a small value by dialing. Therefore, it is possible to realize the driving performance according to the operator's preference during the rolling operation. Other tilt control is the same as that of the first embodiment.

In the configuration of the above embodiment, the mode switch 3 serves as the mode command means, the boom lever 14a serves as the up-and-down motion command means, the control pressure generating circuit 60 and the controller 7.
0 constitutes each changing means.

The present invention can be applied to a wheel type hydraulic excavator, as well as to other hydraulic construction machines having a boom. Also, temperature compensation is not essential.

[0040]

As described above, according to the present invention, the opening / closing characteristics of the pressure compensating valve used for load sensing control are set according to the work content, so that the boom can be pushed freely according to the work content. Can be lowered and free fall, especially when the boom lowering operation is stopped, it is possible to efficiently perform the compaction work without causing the jack-up phenomenon of the vehicle body during compaction work. During excavation work, the boom settling time is shortened and workability is improved.

[Brief description of drawings]

FIG. 1 is a diagram showing an overall configuration of a first embodiment of a hydraulic control device for a construction machine according to the present invention.

FIG. 2 is a circuit diagram showing details of the tilt angle control device of FIG.

FIG. 3 is a circuit diagram showing details of the controller of FIG.

FIG. 4 is a circuit diagram showing details of a control pressure generation circuit of FIG.

FIG. 5 is a diagram showing an overall configuration of a second embodiment of a hydraulic control device for a construction machine according to the present invention.

FIG. 6 is a diagram showing details of a controller in the second embodiment.

FIG. 7 is a diagram illustrating an example of a work determination program according to a second embodiment.

FIG. 8 is a diagram showing details of a controller of a third embodiment of the hydraulic control device for the construction machine according to the present invention.

FIG. 9 is a view showing a conventional boom cylinder and its control valve.

FIG. 10 is a diagram showing an example of metering characteristics of a boom cylinder control valve.

11 is a graph showing the behavior of rod chamber pressure and bottom chamber pressure when the control valve having the metering characteristic of FIG. 10 is used.

FIG. 12 is a diagram showing another example of the metering characteristic of the boom cylinder control valve.

13 is a graph showing the behavior of the rod chamber pressure and the bottom chamber pressure when the control valve with the metering characteristic of FIG. 12 is used.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Variable displacement hydraulic pump 2 Engine 3 Mode switch 5 Boom lowering detection switch 7 Boom operation amount detection sensor 8 Dial 11-16 Control valve 21-26 Hydraulic actuator 31-36 Pressure compensation valve 50 Tilt angle control device 60 Control pressure generation circuit 70 Controller 74a Boom Function Generator for Excavation Work 74b Boom Function Generator for Rolling Work 77 Switcher 78 Andgate 78A Work Discriminator

Claims (6)

[Claims] 1. A variable displacement hydraulic pump driven by a prime mover, a boom cylinder driven by discharge oil from the hydraulic pump to move a boom up and down, and a vertical movement command means for commanding vertical movement of the boom. A control valve that controls the direction and flow rate of pressure oil to the boom cylinder according to the command of the up-and-down movement command means, and the differential pressure across the control valve reaches a predetermined value regardless of the load pressure of the boom cylinder. A displacement volume adjusting means for adjusting the displacement volume of the variable displacement hydraulic pump so that it is connected to the input port of the control valve, and is opened until the differential pressure across the control valve reaches a reference value, In a hydraulic control device for a construction machine equipped with a pressure compensation valve that closes when a value is reached, a mode command means for commanding an operation mode of a boom cylinder, and the commanded operation mode Flip, the hydraulic control system for a construction machine characterized in that it comprises a changing means for changing the reference value of the pressure compensating valve. 2. The hydraulic control device for a construction machine according to claim 1, wherein the operation mode is a first mode in which the boom is allowed to freely fall when the lowering of the boom is commanded by the vertical movement command means, and Secondly, when the vertical movement command means instructs the lowering of the boom, the boom is pushed in and lowered freely.
The hydraulic control device for a construction machine, wherein the reference value is made smaller when the first mode is selected than when the second mode is selected. 3. The hydraulic control device for a construction machine according to claim 1, wherein the pressure compensating valve drives the pilot port in the valve closing direction by the pressure on the upstream side of the control valve, and the pressure on the downstream side of the control valve. A pilot port for driving in the valve opening direction, a biasing means for driving in the valve opening direction by a biasing force, and a pilot port for driving in the valve closing direction by an input variable control pressure, and the commanded operation mode A hydraulic control device for a construction machine, comprising: a pressure generating unit that generates the variable control pressure adjusted according to the above. 4. The hydraulic control device for a construction machine according to claim 3, further comprising an external setting operation member for arbitrarily setting a value of the control pressure generated by the pressure generating means. apparatus. 5. The hydraulic control device for a construction machine according to any one of claims 1 to 4, wherein the mode commanding means determines the work content based on a command from the up / down motion commanding means to set the operation mode. A hydraulic control device for a construction machine, which is characterized by instructing. 6. The hydraulic control device for a construction machine according to claim 5, wherein the mode command means lowers the reference value when the determined work content is a compaction work, and sets the reference value when the work is an excavation work. A hydraulic control device for a construction machine, wherein the changing means is controlled so as to increase a reference value.