JPH09165793A - Hydraulic driving device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a structure which can improve starting characteristic of a prime mover for a machine equipped with a pump controlling device executing negative control. SOLUTION: A controller decides, at the step 3, whether or not Ne inputted at the step 1 is below a specified threshold value N (for instance N=400rpm). In the case where Ne<=N, the decision made at the step 3 is satisfied and moved to the step 4, and a desired pump tilt and turn angle θ0 is decided at a function generator, regardless of a negative control Pn detected by the pressure sensor and inputted at the step 2, to be constantly a mechanical minimum value θmin for a swash plate, and a current value (command value) Imin corresponding to the objective pump tilt and turn angle θmin is calculated at the function generator. Then, the current value Imin is outputted to a solenoid operated proportional pressure reducing valve, and the tile and turn angle for the swash plate is finally controlled, through a regulator, to become, for instance, θmin =5 deg..

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hydraulic drive system provided in a construction machine such as a hydraulic excavator, and more particularly to a hydraulic drive system provided with means for controlling a variable displacement hydraulic pump.

[0002]

2. Description of the Related Art Conventionally, as an example of a publicly known technique for controlling a variable displacement hydraulic pump, for example, an actual Kaihei 5-64506.
There is one described in the publication. This known technique performs so-called negative control, and a variable displacement hydraulic pump, a tank, a cylinder driven by hydraulic pressure discharged from the variable displacement hydraulic pump, and a variable displacement hydraulic pump. A center bypass type directional control valve for controlling hydraulic pressure, a center bypass line connected to a variable displacement hydraulic pump and a tank, and a control pressure provided between the tank and the directional control valve on the center bypass line. A throttle valve to be generated, a sensor that detects the magnitude of control pressure, and a signal from the sensor are input to calculate the target pump displacement of the variable displacement hydraulic pump and drive according to the target pump displacement. A controller that outputs a current and a proportional electromagnetic decompression that is driven by the drive current from the controller and generates a command pressure according to the drive current And,
A regulator for controlling the displacement volume of the variable displacement hydraulic pump based on the command pressure generated by the proportional electromagnetic pressure reducing valve.

When the operation amount of the directional control valve is small and the passing flow rate of the center bypass line is large, the controller
That is, when the pressure on the upstream side of the throttle valve is high, the target pump displacement is calculated so as to reduce the pump discharge flow rate of the variable displacement hydraulic pump. Further, when the operation amount of the direction switching valve is large and the flow rate through the center bypass line is small, that is, when the pressure on the upstream side of the throttle valve is low, the target pump tilt should be increased to increase the discharge flow rate of the variable displacement hydraulic pump. Calculate

[0004]

However, the above-mentioned known techniques have the following problems. That is, since the variable displacement hydraulic pump obtains power from the prime mover, the discharge flow rate of the variable displacement hydraulic pump is 0 when the prime mover is stopped, and the variable displacement hydraulic pump increases as the number of revolutions after the start of the prime mover increases. The discharge flow rate of the pump increases.

Therefore, in the above-mentioned known technique, when the prime mover is started, the flow rate of passage through the center bypass line is small, that is, the pressure on the upstream side of the throttle valve is low, so the pump tilt is controlled by the controller so as to increase the pump discharge flow rate. Will be done. As a result, a relatively large load is transmitted from the variable displacement hydraulic pump to the prime mover, which hinders improvement of the startability of the prime mover.

Particularly when used in a region where the temperature is low, when used in a highland, or when poor fuel is used, the startability of the prime mover is good even when the load on the prime mover is small. Therefore, if the load transmitted from the variable displacement hydraulic pump to the prime mover increases, the startability of the prime mover deteriorates significantly.

In addition to this, when the prime mover is stopped,
As the number of revolutions of the prime mover decreases, the discharge flow rate of the variable displacement hydraulic pump decreases, the flow rate passing through the center bypass line gradually decreases, and the upstream pressure of the throttle valve decreases, so increase the pump discharge flow rate. The controller controls the tilting of the pump. As a result, the tilting of the variable displacement hydraulic pump is stopped in the state where a relatively large load is applied to the prime mover at the next start, and this is also a factor that hinders improvement in startability of the prime mover.

An object of the present invention is to provide a hydraulic drive system having a pump control means for performing negative control, which can improve the starting performance of a prime mover.

[0009]

In order to achieve the above object, according to the present invention, a variable displacement hydraulic pump driven by a prime mover and at least one hydraulic pump driven by oil discharged from the hydraulic pump. A hydraulic actuator, a center bypass type flow control valve that guides the discharge oil from the hydraulic pump to the hydraulic actuator, and a downstream side of a center bypass line that passes through the flow control valve, and a downstream side of the center bypass line. Throttle means for generating a control pressure according to the flow rate of the pressure oil flowing through, a displacement volume calculating means for calculating a target displacement volume of the hydraulic pump according to the control pressure, and the hydraulic pump based on the target displacement volume. In the hydraulic drive device having a pump control means for controlling the displacement of the When the number of revolutions detecting means for outputting a corresponding signal and the signal from the number of revolutions detecting means is inputted and the number of revolutions detected by the number of revolutions detecting means is lower than a predetermined threshold value, A hydraulic drive system is provided, which has a target displacement limiting means for limiting a target displacement of the hydraulic pump to a minimum value.

That is, during normal operation, general negative control is performed. For example, if the operator intends to quickly operate the hydraulic actuator and increases the operation amount of the flow control valve, the pressure oil flow rate on the downstream side of the center bypass line is increased. The control pressure generated by the throttle means also decreases, and the displacement volume calculation means calculates the target displacement volume of the hydraulic pump of a relatively large value corresponding to this, and the pump control means increases the displacement volume of the hydraulic pump. Control to be. In addition, if the operator decreases the operation amount of the flow control valve with the intention of gently moving the hydraulic actuator,
The pressure oil flow rate on the downstream side of the center bypass line increases,
The control pressure generated by the throttle means also becomes large, and the displacement volume calculation means calculates the target displacement volume of the hydraulic pump of a relatively small value corresponding thereto, and the pump control means reduces the displacement volume of the hydraulic pump. Control.

Here, when the prime mover is started, the rotation speed detected by the rotation speed detection means is small and becomes lower than the threshold value, and the target displacement capacity of the hydraulic pump is minimized by the target displacement capacity limiting means, for example, control. It is limited to the minimum value in the possible range or the minimum value in the mechanical movable range. Therefore, as in the conventional case, the target displacement of the hydraulic pump having a relatively large value may be calculated based on the small control pressure corresponding to the small pressure oil flow rate on the downstream side of the center bypass line as in the case where the normal operation amount is small. Since it does not exist, the load transmitted from the hydraulic pump to the prime mover can be reduced, and the startability when the prime mover starts can be improved. Then, when the rotation of the prime mover increases after the start and exceeds the threshold value and the rotation of the prime mover becomes stable, the limitation by the target displacement volume limiting means is released, and the general negative control during normal operation is performed by the pump control means.

Also, when the prime mover is stopped, as in the case of starting the prime mover, the number of revolutions detected by the number of revolutions detecting means is small, so that it becomes lower than the threshold value, and the target displacement of the hydraulic pump is minimized by the target displacement limiting means. Limited to. Therefore, unlike the conventional case, the hydraulic pump does not stop in a state where a large load is applied to the prime mover, so that the startability at the next start of the prime mover can be improved.

Preferably, in the hydraulic drive system,
A hydraulic drive system is provided, wherein the target displacement volume limiting unit limits the target displacement volume of the hydraulic pump to a minimum value within a controllable range by the pump control unit.

Further preferably, in the hydraulic drive system, the target displacement volume limiting means limits the target displacement volume of the hydraulic pump to a minimum value within a mechanical movable range of the hydraulic pump. A hydraulic drive is provided.

Further, preferably, in the hydraulic drive system, an actuator detecting means for detecting whether the hydraulic actuator is in an operating state or a non-operating state and outputting a corresponding signal, and a signal from the actuator detecting means. Is input and the operating state of the hydraulic actuator is detected by the actuator detecting means, the hydraulic drive device further includes limit releasing means for releasing the limitation by the target displacement volume limiting means. Will be provided.

As a result, if the rotation speed detection means fails and a signal indicating a rotation speed lower than the threshold value is output even when the actuator is in the operating state, the target displacement capacity limiting means limits the target displacement capacity of the hydraulic pump. Even if is limited to the minimum value, the actuator detecting means detects the operating state of the actuator, and at the same time, the limitation releasing means releases the limitation by the target displacement volume limiting means. Therefore, when the actuator is operating, the discharge flow rate from the hydraulic pump is not insufficient and the required oil amount of the actuator cannot be secured.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a first embodiment of the present invention.
This will be described with reference to FIG. A hydraulic circuit diagram showing the hydraulic drive system according to the present embodiment is shown in FIG.

In the hydraulic circuit shown in FIG. 1, the hydraulic drive system of the present embodiment is mounted on a construction machine such as a hydraulic excavator, for example, and includes a variable displacement hydraulic pump 1.
From the pilot pump 5, the relief valve 6 that limits the maximum discharge pressure of the pilot pump 5, the tank 15, the prime mover 4 that starts the variable displacement hydraulic pump 1 by the start switch 4a, and the variable displacement hydraulic pump 1. An actuator driven by the discharged pressure oil, for example, a hydraulic cylinder 8, a center bypass type directional control valve (flow rate control valve) 7 for controlling the pressure oil discharged from the variable displacement hydraulic pump 1, and a pipe (not shown). An operation lever device 12 for switching the position of the directional control valve 7 using the pilot pressure from the pilot pump 5 guided by the passage, a center bypass passage 13 connected to the variable displacement hydraulic pump 1 and the tank 15, and a center It is provided between the tank 15 of the bypass passage 13 and the direction switching valve 7, and is for controlling according to the operation amount of the operation lever device 12. A pressure generator 9 for generating a force,
The center bypass pressure transmission passage 14 that guides the control pressure generated by the pressure generator 9 and the control pressure (hereinafter, referred to as negative control pressure) Pn that is guided to the center bypass pressure transmission passage 14 are detected, and the detected value is electrically detected. The pressure sensor 10 for converting into a signal, the rotation speed Ne of the prime mover 4, and the rotation speed detector 19 for converting the detected value into an electric signal, the negative control pressure Pn detected by the pressure sensor 10 and the rotation speed detector. The controller 11 outputs the drive current I according to the calculation result by inputting the prime mover rotation speed Ne detected in step 3 and performing a predetermined calculation process (described later).

Further, the hydraulic drive system of this embodiment is driven by the drive current I from the controller 11 to generate the command pressure P using the pressure oil from the pilot pump 5, and the proportional electromagnetic pressure reduction valve 3. It further has a regulator 2 that controls the discharge flow rate of the variable displacement hydraulic pump 1 based on the command pressure P generated by the valve 3.

The regulator 2 includes a servo piston 2a for operating the swash plate 1a of the variable displacement hydraulic pump 1, a small diameter side chamber 2b for storing the small diameter side of the servo piston 2a, and a large diameter for storing the large diameter side of the servo piston 2a. Radial chamber 2c,
The control spool 2d is configured to control the drive of the servo piston 2a according to the command pressure P from the proportional electromagnetic pressure reducing valve 3.

The control spool 2d has a force urged by a spring 2e provided at one end and a command pressure P from the proportional electromagnetic pressure reducing valve 3.
The balance determines the position. That is, when the command pressure P from the proportional electromagnetic pressure reducing valve 3 becomes higher than the set value of the spring 2e, the control spool 2d is moved to the right position in the drawing, and the hydraulic pressure from the pilot pump 5 is applied to both the small diameter side chamber 2b and the large diameter side chamber 2c. The servo piston 2a is moved in the direction in which the tilting amount of the swash plate 1a on the left side of the drawing is increased due to the pressure receiving area difference between the small diameter chamber side 2b and the large diameter side chamber 2c. On the contrary, the command pressure P from the proportional electromagnetic pressure reducing valve 3 is the spring 2e.
When it becomes lower than the set value of, the control spool 2d is moved to the position on the left side in the drawing, the hydraulic pressure from the pilot pump 5 is supplied only to the small diameter side chamber 2b, the large diameter side chamber 2c communicates with the tank 15, and the servo piston 2a is moved to the right side in the drawing. The swash plate 1a is moved in a direction that reduces the tilt amount.

The controller 1 which is the main part of this embodiment
FIG. 2 shows a flow chart showing the control contents performed in 1. In FIG. 2, first, in step 1, the rotational speed N _e of the prime mover 4 is input from the rotational speed detector 19, and step 2
Then, the negative control pressure P _n is input from the pressure sensor 10.

Then, the process proceeds to step 3, where N _e input in step 1 is stored in the controller 11 in advance, or a predetermined threshold value N input by an input means (not shown) (for example, idling of the prime mover 4). When the rotation speed is 900 rpm, it is determined whether N = 400 rpm or less.

If N _e > N, the determination is not satisfied and the routine proceeds to step 4, where the displacement of the variable displacement hydraulic pump 1 by the negative control pressure P _n, that is, the tilt angle control of the swash plate 1a is performed. Details of step 3 are shown in FIG.

FIG. 3 is a functional block diagram showing the control procedure performed in step 3 of the controller 11. FIG.
The function indicated by is obtained by the function generator 11a that calculates the target pump displacement angle θ ₀ according to the negative control pressure P _n detected by the pressure sensor 10 and input in step 2, and this function generator 11a. The function generator 11b calculates a current value (command value) I ₀ according to the target pump displacement angle θ ₀ . The command value obtained by the function generator 11b is given to a power supply device (not shown), and this power supply device outputs the commanded current value I ₀ to the proportional electromagnetic pressure reducing valve 3.

With the functions of the two function generators 11a and 11b, for example, when the hydraulic cylinder 8 is not driven and the directional control valve 7 is in the neutral position, the negative control pressure P _n detected by the pressure sensor 10 is It becomes the maximum value, and as is clear from the characteristic shown in the function generator 11a, the target pump tilt angle θ ₀ becomes the minimum value. Here, when the operator operates the operation lever device 12, the direction switching valve 7 is switched, and the detected value of the negative control pressure P _n by the pressure sensor 10 decreases as the switching amount increases. Therefore, the target pump tilt angle θ ₀ increases in accordance with the characteristic shown in the function generator 11a, and the target drive current I ₀ also increases in accordance with the characteristic shown in the function generator 11b. As a result, the tilt angle of the swash plate 1a increases, so that the variable displacement hydraulic pump 1 discharges sufficient pressure oil to drive the hydraulic cylinder 8.

As a result of such negative control, the tilt angle of the swash plate 1a of the variable displacement hydraulic pump 1 is, for example, 10 ° to
It will be controlled in the range of 30 °.

On the other hand, when N _e ≦ N, the determination at step 3 is satisfied and the routine proceeds to step 4 where control is performed so that the tilt angle of the swash plate 1a of the variable displacement hydraulic pump 1 is minimized. Details of step 4 are shown in FIG.

FIG. 4 is a functional block diagram showing the control procedure performed in step 4 of the controller 11. FIG.
The function shown in (1) is for generating a function that always sets the target pump tilt angle θ ₀ to the mechanical minimum value θ _{min of the} swash plate 1a regardless of the negative control pressure P _n detected by the pressure sensor 10 and input in step 2. An electric current value (command value) I ₀ (= I _min ) corresponding to the target pump displacement angle θ ₀ (= θ _min ) obtained by the function generator 11c and the function generator 11c is calculated, which has been described with reference to FIG. It is composed of the same function generator 11b as the above. As in FIG.
The command value obtained by the function generator 11b is given to a power supply device (not shown), and the current value I _min commanded by this power supply device.
Is output to the proportional electromagnetic pressure reducing valve 3. Finally, the tilt angle of the swash plate 1a is controlled to, for example, θ _min = 5 ° via the proportional electromagnetic pressure reducing valve 3 and the regulator 2.

According to the present embodiment configured as described above, when the prime mover 4 is started, the rotation speed N _e detected by the rotation speed detector 19 is small, and therefore becomes lower than the threshold value N.
When the determination in step 3 is satisfied, in step 5, the target tilt angle θ ₀ of the swash plate 1a of the variable displacement hydraulic pump 1 is limited to the minimum value θ _min within the mechanical movable range. Therefore, unlike the conventional case, the target tilt angle of the comparatively large value similar to the case where the operation amount in the normal operation is small does not occur, so that the load transmitted from the variable displacement hydraulic pump 1 to the prime mover 4 can be reduced. The startability when the prime mover 4 is started can be improved. And, after starting, the prime mover 4
The rotation speed N _e of the motor increases and exceeds the threshold value N, so that the prime mover 4
When the rotation is stable, the determination at step 3 is not satisfied, and the routine proceeds to step 4, where general negative control during normal operation is performed at step 4.

Further, even when the prime mover 4 is stopped, the rotation speed N _e detected by the rotation speed detector 19 is small and becomes lower than the threshold value N, as in the start-up, and at step 5, the variable displacement hydraulic pump 1 is operated. The target tilt angle θ is limited to the minimum value θ _min . Therefore, unlike the conventional case, the variable displacement hydraulic pump 1 does not stop in a state where a large load is applied to the prime mover 4, so that the startability at the next start of the prime mover 4 can be improved. Therefore, it is particularly effective for improving the startability when the starting condition of the prime mover 4 is bad, such as when it is used in a region where the temperature is low, when it is used at high altitudes, or when poor fuel is used.

In the first embodiment, the swash plate 1a of the variable displacement hydraulic pump 1 is used in step 5 of FIG.
The tilt angle of the negative control range (for example, 10 ° to 30
However, the invention is not limited to this, although the mechanical minimum value (for example, 5 °) is smaller than That is, in step 5, the minimum value (for example, 10 °) in the negative control range may be set, and in this case, the same effect is obtained.

Further, in the first embodiment, the hydraulic cylinder 8 has been described as an example of the actuator, but the present invention is not limited to this, and other actuators such as a hydraulic motor are also applicable. It goes without saying that it is possible. Further, the number of the actuators 8, the direction switching valves 7, and the operation lever devices 12 is not limited to one, and may be plural. Similar effects are obtained in these cases as well.

A second embodiment of the present invention will be described with reference to FIGS. The same reference numerals are given to members and procedures equivalent to those of the first embodiment. FIG. 5 is a hydraulic circuit diagram showing the hydraulic drive system according to the present embodiment.

In the hydraulic circuit diagram shown in FIG. 5, the difference from the first embodiment is that the pressure valve 207a that moves in conjunction with the operation of the direction switching valve 7 and the operating pressure passage 217 in which this pressure valve 207a is arranged. An operating pressure transmission passage 218 for guiding the pressure generated in the actuator, and a pressure switch 216 for detecting whether the actuator is in an operating state or a non-operating state by the pressure in the operating pressure transmission passage 218 and outputting a corresponding signal to the controller 11. Is further provided, and accordingly, the control contents in the controller 11 are different.

FIG. 6 is a flowchart showing the contents of control performed in the controller 11. In FIG. 6, the first
2 is different from the flow chart shown in FIG. 2 in that the step 5 does not immediately proceed to step 5 even when the determination of N _e ≦ N in step 3 is satisfied, and the signal from the pressure switch 216 is input in step 6. In step 7, it is determined based on the signal whether the actuator (hydraulic cylinder) 8 is in the non-operation state.

When the actuator is not operated, the determination is satisfied and the routine goes to Step 5, where the tilt angle θ ₀ of the swash plate 1a of the variable displacement hydraulic pump 1 is limited to the minimum value, as in FIG. Control is performed.

When the actuator is in the operating state,
The determination is not satisfied, and the routine goes to Step 4, where, similarly to FIG. 2, the tilt angle control by the negative control pressure P _n is performed. Other configurations and functions are almost the same as those in the first embodiment.

According to this embodiment, in addition to the same effects as the first embodiment, the following effects are obtained. That is, even if the rotation speed detector 19 fails and the actuator 8 is in the operating state and outputs a signal indicating a rotation speed lower than the threshold value N and the determination in step 3 is satisfied, the pressure switch is satisfied. When the operating state of the actuator 8 is detected in 216, the determination in step 7 is not satisfied, and therefore the target tilt angle θ ₀ is not limited to the minimum value θ _min , the process _proceeds to step 4, and the normal operation is performed. Negative control is performed. Therefore, when the actuator 8 is operating, the discharge flow rate from the variable displacement hydraulic pump 1 will not be insufficient and the required oil amount of the actuator 8 cannot be secured. That is, even in such a case, the work load of the construction machine can be secured.

In the second embodiment, as in the first embodiment, the actuator 8, the direction switching valve 7,
There may be a plurality of operation lever devices 12, respectively.
In this case, one pressure valve 207a is provided for each direction switching valve 7, and the pressure generated in the operation pressure passage in which each pressure valve 207a is arranged is adjusted by the operation pressure transmission passage 21.
Alternatively, each pressure switch 216 may be used to detect the pressure. Further, the maximum pressure in each operation pressure transmission passage 218 may be detected by one pressure switch 216 via the shuttle valve. Similar effects are obtained in these cases.

[0041]

According to the present invention, when the prime mover is started, the rotation speed detected by the rotation speed detection means is small and therefore becomes lower than the threshold value, and the target displacement capacity of the hydraulic pump is minimized by the target displacement capacity limiting means. Limited to. Therefore, the load transmitted from the hydraulic pump to the prime mover can be reduced, and the startability when the prime mover is started can be improved. Also, even when the prime mover is stopped, as in the case of starting the prime mover, the number of revolutions detected by the number of revolutions detecting means is small, so it becomes lower than the threshold value, and the target displacement of the hydraulic pump is limited to the minimum value by the target displacement volume limiting means. It Therefore, unlike the conventional case, it will not stop in the state where a large load is applied to the prime mover from the hydraulic pump,
The startability at the time of starting the prime mover next time can be improved.
Therefore, it is particularly effective for improving the startability when the prime mover starting condition is bad, such as when it is used in a region where the temperature is low, when it is used at high altitudes, or when poor fuel is used.

Further, even if the rotation speed detection means fails and a signal indicating a rotation speed lower than a threshold value is output even when the actuator is in an operating state, the target displacement capacity of the hydraulic pump is controlled by the target displacement capacity limiting means. Even if the limit is limited to the minimum value, the limit releasing means releases the limitation by the target displacement volume limiting means. Therefore, when the actuator is operating, the discharge flow rate from the hydraulic pump is not insufficient and the required oil amount of the actuator cannot be secured. Therefore, even in such a case, the work load of the construction machine can be secured.

[Brief description of the drawings]

FIG. 1 is a hydraulic circuit diagram showing a hydraulic drive system according to a first embodiment of the present invention.

FIG. 2 is a flowchart showing the contents of control performed in the controller shown in FIG.

FIG. 3 is a functional block diagram showing a control procedure performed in step 3 shown in FIG.

FIG. 4 is a functional block diagram showing a control procedure performed in step 4 shown in FIG.

FIG. 5 is a hydraulic circuit diagram showing a hydraulic drive system according to a second embodiment of the present invention.

FIG. 6 is a flowchart showing the contents of control performed in the controller 11 shown in FIG.

[Explanation of symbols]

 1 Variable displacement hydraulic pump 1a Swash plate 2 Regulator 2a Servo piston 2b Small diameter side chamber 2c Large diameter side chamber 2d Control spool 2e Spring 3 Proportional electromagnetic pressure reducing valve 4 Prime mover 4a Start switch 5 Pilot pump 6 Relief valve 7 Directional switching valve 8 Actuator 9 Pressure Generator 10 Pressure sensor 11 Controller 12 Operation lever device 13 Center bypass passage 14 Center bypass pressure transmission passage 15 Tank 19 Rotation speed detector 207a Pressure valve 216 Pressure switch 217 Operation pressure passage 218 Operation pressure transmission passage

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigehiro Yoshinaga 650 Jinrachicho, Tsuchiura City, Ibaraki Prefecture Hitachi Construction Machinery Co., Ltd. Tsuchiura Factory

Claims (4)

[Claims] 1. A variable displacement hydraulic pump driven by a prime mover, at least one hydraulic actuator driven by discharge oil from the hydraulic pump, and a center for guiding discharge oil from the hydraulic pump to the hydraulic actuator. A bypass type flow control valve, and a throttle means installed on the downstream side of a center bypass line that passes through the flow control valve and generating control pressure according to the flow rate of pressure oil flowing to the downstream side of the center bypass line, A hydraulic drive device comprising: a displacement volume calculating means for calculating a target displacement volume of the hydraulic pump according to the control pressure; and a pump control means for controlling a displacement volume of the hydraulic pump based on the target displacement volume, A rotation speed detecting means for detecting the rotation speed of the prime mover and outputting a corresponding signal, and this rotation speed detecting means. When the signal from the detecting means is input and the rotation speed detected by the rotation speed detecting means is lower than a predetermined threshold value, the target displacement volume limiting means for limiting the target displacement volume of the hydraulic pump to the minimum value. And a hydraulic drive device. 2. The hydraulic drive according to claim 1, wherein
The hydraulic drive system characterized in that the target displacement volume limiting unit limits the target displacement volume of the hydraulic pump to a minimum value within a controllable range by the pump control unit. 3. The hydraulic drive system according to claim 1, wherein
A hydraulic drive system characterized in that the target displacement volume limiting means limits the target displacement volume of the hydraulic pump to a minimum value within a mechanical movable range of the hydraulic pump. 4. The hydraulic drive system according to claim 1, wherein
An actuator detection unit that detects whether the hydraulic actuator is in an operating state or a non-operation state and outputs a corresponding signal, and a signal from the actuator detection unit is input, and the actuator detection unit receives the signal from the hydraulic actuator. A hydraulic drive system further comprising: a limit releasing means for releasing the limitation by the target displacement volume limiting means when an operating state is detected.