JP2020076474A - Construction machine - Google Patents

Abstract

To provide a construction machine capable of suppressing an increase in speed of traveling motors each using a variable displacement pump as a hydraulic source while increasing speed of work machine actuators each using a fixed displacement pump as a hydraulic source when both of the variable displacement pump and the fixed displacement pump are simultaneously driven by an engine.SOLUTION: A construction machine includes: traveling motors 22a, 22b each using a variable displacement pump 51 as a hydraulic source; work machine actuators each using a fixed displacement pump 52 as a hydraulic source; a pump regulator 51a controlling the variable displacement pump 51; a solenoid proportional valve 51c generating a pressure signal to be input to the pump regulator 51a; a controller 70 issuing a control command to be input to the solenoid proportional valve 51c; and a pressure sensor 51h generating an electric signal corresponding to request flow rates of the traveling motors 22a, 22b. When a traveling detection signal and a work machine detection signal are input, the controller 70 increases target speed of an engine 42 and reduces a discharge flow rate of the variable displacement pump 51.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a construction machine.

  Patent Document 1 below is a working machine for controlling a pump flow rate by a negative control, in which the discharge pressure of a variable displacement hydraulic pump is a pump discharge pressure at a light load and a variable displacement hydraulic pump. A technique is disclosed in which the displacement of the hydraulic pump is increased by performing an increase control of the engine speed when the maximum displacement and the negative control signal pressure are equal to or lower than a preset set signal pressure.

  In Patent Document 2 below, in a hydraulic pump that performs pump flow rate control by negative control, when the deceleration of the actual engine speed falls below a predetermined control start threshold value, the pump output is controlled by negative control. Techniques for lowering the target output for controlling the pump flow rate are disclosed.

JP, 2013-124752, A JP, 2016-17594, A

  In Patent Document 1, since the negative control signal pressure limits the discharge flow rate of the variable displacement pump irrespective of the increase in the engine speed, both the variable displacement pump and the fixed displacement pump by negative control are controlled. When the engine is driven simultaneously, if the engine speed is increased in order to increase the discharge flow rate of the fixed displacement pump, the discharge flow rate of the variable displacement pump also increases at the same time. Not only the actuator that uses the variable displacement pump, but also the actuator that uses the variable displacement pump that does not need to increase the speed as the hydraulic power source increases the speed, which causes a problem of impairing operability.

  Since Patent Document 2 is a control that feeds back the deceleration of the actual engine speed, it is not possible to minimize the effect of disturbance that occurs when another actuator is operated with respect to the actuator that is currently being operated. There is a problem.

  Therefore, in view of the above problems, the present invention provides a variable displacement type pump while increasing the speed of a working machine actuator using a fixed displacement type pump as a hydraulic power source when driving both a variable displacement type pump and a fixed displacement type pump simultaneously by an engine. An object of the present invention is to provide a construction machine capable of suppressing an increase in speed of a traveling motor that uses a pump as a hydraulic power source.

A construction machine according to the present invention includes a prime mover, a variable displacement pump and a fixed displacement pump driven by the prime mover, a first traveling motor having a first discharge port of the variable displacement pump as a hydraulic source, and the variable displacement pump. A second traveling motor that uses the second discharge port of the displacement pump as a hydraulic source, a work machine actuator that uses the fixed displacement pump as a hydraulic source, and an operation of the first traveling motor and the second traveling motor are detected. A travel detection device that sends a travel detection signal, a work machine detection device that detects the operation of the work machine actuator and sends a work machine detection signal, a pump regulator that controls the discharge flow rate of the variable displacement pump, and the pump A pressure signal generation device that generates a pressure signal that is input to the regulator, a controller that issues a control command that is input to the pressure signal generation device, and a required flow rate of the first traveling motor and the second traveling motor. An electric signal generation device for generating an electric signal and outputting the electric signal to the controller,
The controller has a target rotation speed setting unit that sets a target rotation speed of the prime mover, and a control command generation unit that multiplies the electric signal by a gain to generate the control command,
When the traveling detection signal and the working machine detection signal are input to the controller, the target rotation speed setting unit sets the target rotation speed from a first target rotation speed to a second target rotation speed higher than the first target rotation speed. The control command generator changes the gain so that the control command becomes a command to reduce the discharge flow rate of the variable displacement pump.

  According to the present invention, while increasing the target rotational speed of the prime mover to increase the discharge flow rate of the fixed displacement pump, the discharge flow rate of the variable displacement pump is adjusted according to the required flow rates of the first traveling motor and the second traveling motor. By suppressing the increase in the discharge flow rate of the variable displacement pump by changing the gain for electrical control, the speed of the work machine actuator is increased and the increase in the speed of the first travel motor and the second travel motor is suppressed. it can.

It is a side view which shows the hydraulic excavator which concerns on this embodiment. It is a figure which shows the hydraulic circuit of the hydraulic shovel which concerns on 1st Embodiment. It is a figure which shows the hydraulic circuit of the hydraulic shovel which concerns on 2nd Embodiment.

  Embodiments of the present invention will be described below with reference to the drawings.

[Structure of hydraulic excavator]
First, a schematic structure of a hydraulic excavator 1 as an example of a construction machine will be described with reference to FIG.

  The lower traveling body 2 is driven by receiving power from the engine 42 to drive the hydraulic excavator 1. The lower traveling body 2 includes a pair of left and right crawlers 21 and 21 and a pair of left and right traveling motors 22 and 22 (the right traveling motor 22 is not shown in FIG. 1). The left and right traveling motors 22, 22, which are hydraulic motors, respectively drive the left and right crawlers 21, 21 to enable the hydraulic excavator 1 to move forward and backward. Further, the lower traveling body 2 is provided with a blade 23 and a blade cylinder 24 which is a hydraulic actuator for rotating the blade 23 in the vertical direction.

  The work machine 3 receives power from the engine 42 and is driven to perform an excavation work such as earth and sand. The work machine 3 includes a boom 31, an arm 32, and a bucket 33, and drives them independently to enable excavation work. The boom 31, the arm 32, and the bucket 33 each correspond to a working unit, and the hydraulic excavator 1 has a plurality of working units.

  The boom 31 has its base end supported by the front of the upper swing body 4 and is rotated by a boom cylinder 31a that is movable in an extendable and contractible manner. The arm 32 has a base end supported by the tip of the boom 31 and is rotated by an arm cylinder 32a that is movable in a telescopic manner. The bucket 33 has its base end supported by the tip of the arm 32, and is rotated by a bucket cylinder 33a that is movable in an expandable and contractible manner. The boom cylinder 31a, the arm cylinder 32a, and the bucket cylinder 33a correspond to hydraulic actuators that drive the working unit.

  The bucket 33 is a container-like member that is provided at the tip of the work machine 3 and has a claw for performing excavation work. The bucket 33 is rotatably attached to the tip of the arm 32 via a pin 34. Further, the bucket 33 is connected to the bucket cylinder 33a via a link mechanism 35.

  The upper revolving structure 4 is configured to be revolvable with respect to the lower traveling structure 2 via a revolving bearing (not shown). The upper revolving unit 4 is provided with a control unit 41, an engine 42, a revolving base 43, a revolving motor 44, and the like. The upper revolving structure 4 revolves through a revolving bearing by the driving force of a revolving motor 44 which is a hydraulic motor. The upper swing body 4 is provided with a plurality of hydraulic pumps (not shown in FIG. 1) driven by the engine 42. These hydraulic pumps supply pressure oil to the traveling motors 22, 22, the swing motor 44, the boom cylinder 31a, the arm cylinder 32a, the bucket cylinder 33a, and the like.

  A pilot seat 411 is arranged in the pilot section 41. A pair of work operation levers 412 and 412 are arranged on the left and right of the cockpit 411, and a pair of travel levers 413 and 413 are arranged on the front side. The operator sits on the cockpit 411 and operates the work operation levers 412, 412, the travel levers 413, 413, etc. to control the engine 42, each hydraulic motor, each hydraulic actuator, etc. to travel, turn, Can perform work, etc.

[Structure of hydraulic circuit]
[First Embodiment]
The hydraulic circuit 5 of the hydraulic excavator 1 will be described with reference to FIG. The hydraulic circuit 5 includes a first traveling motor 22a, a second traveling motor 22b (either the left traveling motor 22 or the right traveling motor 22), a first working machine actuator 30a, a second working machine actuator 30b, and a third working machine. The machine actuator 30 c (any one of the boom cylinder 31 a, the arm cylinder 32 a, and the bucket cylinder 33 a), the swing motor 44, the variable displacement pump 51, the fixed displacement pump 52, the pilot pump 53, and the controller 70.

  The variable displacement pump 51 and the fixed displacement pump 52 are driven by the engine 42, and are hydraulic actuators (first working machine actuator 30a, second working machine actuator 30b, third working machine actuator 30c, first traveling motor 22a, The pressure oil supplied to the second traveling motor 22b and the turning motor 44) is discharged. The variable displacement pump 51 supplies pressure oil to the first traveling motor 22a and the second traveling motor 22b to drive it. The fixed displacement pump 52 supplies pressure oil to the first working machine actuator 30a, the second working machine actuator 30b, the third working machine actuator 30c, and the swing motor 44 to drive them.

  The variable displacement pump 51 can control the discharge flow rate of the pressure oil by changing the inclination angle of the movable swash plate 51b by driving the pump regulator 51a. The pump regulator 51a is driven by the pressure (pilot pressure) of the pilot oil discharged from the pilot pump 53.

  An electromagnetic proportional valve 51c (corresponding to a pressure signal generation device) is provided in an oil passage 53a between the pump regulator 51a and the pilot pump 53. The solenoid proportional valve 51c generates a pressure signal (pilot signal pressure) input to the pump regulator 51a. The solenoid proportional valve 51c can adjust the pilot signal pressure according to a control command from the controller 70.

  The variable displacement pump 51 is a so-called split flow type hydraulic pump including a first discharge port P1 and a second discharge port P2. The pressure oil discharged from the first discharge port P1 is supplied to the first traveling direction switching valve 55d described later via the first center bypass oil passage 51d, and the pressure oil discharged from the second discharge port P2 is It is supplied to a second traveling directional control valve 55e described later via the second center bypass oil passage 51e.

  A first negative control throttle 51f is provided on the most downstream side of the first center bypass oil passage 51d. The first negative control throttle 51f restricts the flow of the pressure oil flowing through the first center bypass oil passage 51d to generate the first negative control pressure upstream of the first negative control throttle 51f. Similarly, a second negative control throttle 51g is provided on the most downstream side of the second center bypass oil passage 51e. The second negative control throttle 51g restricts the flow of the pressure oil flowing through the second center bypass oil passage 51e to generate the second negative control pressure upstream of the second negative control throttle 51g.

  The pressure sensor 51h (corresponding to an electric signal generation device) detects the low-pressure side negative control pressure of the first negative control pressure and the second negative control pressure, converts the detected value into an electrical signal, and controls it as an electrical negative control pressure. Output to 70.

  Here, the first negative control pressure and the second negative control pressure detected by the pressure sensor 51h depend on the flow rate of the pressure oil flowing through the first center bypass oil passage 51d and the second center bypass oil passage 51e, in other words, the first It depends on the flow rate of the pressure oil supplied to the first traveling motor 22a and the second traveling motor 22b via the traveling direction switching valve 55d and the second traveling direction switching valve 55e. Therefore, the pressure sensor 51h can generate an electric signal according to the required flow rates of the first traveling motor 22a and the second traveling motor 22b.

  The pressure oil discharged from the fixed displacement pump 52 is passed through the third center bypass oil passage 52a to a later-described first working machine directional switching valve 55a, second working machine directional switching valve 55b, and third working machine directional switching valve 55a. It is supplied to the direction switching valve 55c and the turning direction switching valve 55f.

  The hydraulic actuators (the first working machine actuator 30a, the second working machine actuator 30b, the third working machine actuator 30c, the first traveling motor 22a, the second traveling motor 22b, and the turning motor 44) respectively have corresponding direction switching. A valve 55 is provided, and the directional switching valve 55 is a pilot type directional switching valve capable of switching the direction and volume of the pressure oil pumped from the variable displacement pump 51 and the fixed displacement pump 52 to the hydraulic actuator. The direction switching valve 55 can be switched to a plurality of positions by sliding the spool. When the pilot signal pressure is not applied to any of the two pilot ports of the direction switching valve 55, the direction switching valve 55 is held at the neutral position by the biasing force of the spring. When the direction switching valve 55 is in the neutral position, the pressure oil is not supplied to the corresponding hydraulic actuator and passes through the first center bypass oil passage 51d, the second center bypass oil passage 51e, and the third center bypass oil passage 52a. Flows to the oil tank. On the other hand, when the pilot signal pressure is applied to any pilot port of the direction switching valve 55, the direction switching valve 55 is switched from the neutral position to another position, and the pressure oil is supplied to the corresponding hydraulic actuator. ..

  In the present embodiment, the first working machine directional switching valve 55a corresponding to the first working machine actuator 30a, the second working machine directional switching valve 55b corresponding to the second working machine actuator 30b, and the third working machine actuator 30c. Corresponding to the third working machine direction switching valve 55c, the first traveling motor 22a corresponding to the first traveling direction switching valve 55d, the second traveling motor 22b corresponding to the second traveling direction switching valve 55e, turning. A turning direction switching valve 55f corresponding to the motor 44 is provided. These directional control valves are collectively called control valves.

  The pilot pump 53 mainly discharges pilot oil as a command input to the direction switching valve 55. However, in FIG. 2, the oil passage from the pilot pump 53 to the direction switching valve 55 is not shown. The pilot pump 53 is driven by the engine 42 and discharges pressure oil to generate pilot signal pressure in the oil passage.

  An oil passage 53a between the pilot pump 53 and the pump regulator 51a is branched into a travel detection oil passage 53b, and the travel detection oil passage 53b is further branched into a work implement detection oil passage 53c. The travel detection oil passage 53b includes a first travel detection direction switching valve 56d that operates in conjunction with the first travel direction switching valve 55d, and a second travel detection direction that operates in association with the second travel direction switching valve 55e. It reaches the oil tank through the switching valve 56e. The working machine detection oil passage 53c moves in conjunction with the first working machine direction switching valve 55a, and moves in conjunction with the first working machine detecting direction switching valve 56a and the second working machine direction switching valve 55b. Machine detection direction switching valve 56b, third working machine direction switching valve 55c that operates in conjunction with the third work machine detection direction switching valve 56c, swing direction switching valve 55f that operates in conjunction with swing detection direction switching valve Return to the oil tank through 56f.

  The first traveling direction switching valve 56d is integrated with the first traveling direction switching valve 55d, and moves in conjunction with the first traveling direction switching valve 55d. The first traveling detection direction switching valve 56d can be switched to a plurality of positions by sliding the spool. When the first traveling direction switching valve 55d is held at the neutral position, the first traveling detection direction switching valve 56d is also held at the neutral position. When the first traveling direction switching valve 55d is switched from the neutral position to another position, the first traveling direction switching valve 56d is also switched from the neutral position to another position in conjunction with this.

  When the first traveling detection direction switching valve 56d is in the neutral position, the first traveling detection direction switching valve 56d does not block the traveling detection oil passage 53b. Therefore, the pressure oil can flow through the travel detection oil passage 53b. On the other hand, when the first traveling detection direction switching valve 56d is at a position other than the neutral position, the first traveling detection direction switching valve 56d closes the traveling detection oil passage 53b.

  Similarly, the second travel detection direction switching valve 56e can be switched to a communication position for communicating the travel detection oil passage 53b or a shut-off position for shutting off the travel detection oil passage 53b. Similarly, the first working machine detecting directional switching valve 56a, the second working machine detecting directional switching valve 56b, the third working machine detecting directional switching valve 56c, and the turning detection directional switching valve 56f are the working machine detecting It is possible to switch to a communication position for communicating the oil passage 53c or a shut-off position for shutting off the work implement detection oil passage 53c.

  The travel detection oil passage 53b is branched to the signal oil passage 53d on the upstream side of the first travel detection direction switching valve 56d. The signal oil passage 53d is connected to the pressure sensor 53e. The traveling levers 413 and 413 are operated to operate the first traveling direction switching valve 55d and the first traveling direction switching valve 56d or the second traveling direction switching valve 55e. Is shifted from the neutral position to a position other than the neutral position, the travel detection oil passage 53b is closed and a pressure rises downstream of the throttle 53f, and this pressure is input to the pressure sensor 53e via the signal oil passage 53d. .. The pressure sensor 53e converts the input signal pressure into an electric signal and outputs it as a travel detection signal to the controller 70.

  Similarly, the work implement detection oil passage 53c is branched to the signal oil passage 53g on the upstream side of the first work implement detection direction switching valve 56a. The signal oil passage 53g is connected to the pressure sensor 53h. The work operation levers 412 and 412 are operated to detect the first working machine detection direction switching valve 56a that interlocks with the first working machine direction switching valve 55a and the second working machine detection that interlocks with the second working machine direction switching valve 55b. Direction switching valve 56b, the third working machine detection direction switching valve 56c that interlocks with the third working machine direction switching valve 55c, or the swivel detection direction switching valve 56f that interlocks with the swiveling direction switching valve 55f from the neutral position. When the position is set to a position other than the neutral position, the working machine detection oil passage 53c is closed and a pressure rises downstream of the throttle 53i, and this pressure is input to the pressure sensor 53h via the signal oil passage 53g. The pressure sensor 53h converts the input signal pressure into an electric signal and outputs it as a work machine detection signal to the controller 70.

  The controller 70 sends a control command to the solenoid proportional valve 51c. The operation of the solenoid proportional valve 51c is controlled by the controller 70, and the pilot signal pressure for the pump regulator 51a can be adjusted according to the magnitude of the applied control current value. That is, the control command is, for example, a control current value.

  The controller 70 includes a control command generator 71. The control command generation unit 71 multiplies the electric signal input from the pressure sensor 51h by a gain to generate a control command. By adjusting the magnitude of the gain and adjusting the control command, the pilot signal pressure applied to the pump regulator 51a can be adjusted. In the present embodiment, the electromagnetic proportional valve 51c increases the flow rate of pilot pressure oil supplied to the pump regulator 51a by decreasing the magnitude of the gain and the control command (the pilot applied to the pump regulator 51a. Increase the signal pressure). At this time, the pump regulator 51a reduces the discharge flow rate of the variable displacement pump 51.

  The controller 70 also includes a target rotation speed setting unit 72 that sets a target rotation speed of the engine 42. The controller 70 controls the rotation speed of the engine 42 so as to reach the target rotation speed set by the target rotation speed setting unit 72.

  When the traveling detection signal from the pressure sensor 53e and the working machine detection signal from the pressure sensor 53h are not input, or when only one of them is input, the controller 70 determines the target rotation speed of the engine 42 as a worker or the like. The gain that is set to the first target rotation speed instructed by and multiplied by the electric signal input from the pressure sensor 51h is set to the first gain defined in advance. On the other hand, when the traveling detection signal from the pressure sensor 53e and the working machine detection signal from the pressure sensor 53h are input to the controller 70, the target rotation speed setting unit 72 sets the target rotation speed from the first target rotation speed to the first target rotation speed. It is configured to change to a second target rotation speed higher than the target rotation speed, and the control command generation unit 71 changes the gain so that the control command becomes a command to reduce the discharge flow rate of the variable displacement pump 51. .. In the present embodiment, the control command generator 71 is configured to change the gain from the first gain to the second gain lower than the first gain.

  A traveling motor (first traveling motor 22a and second traveling motor 22b) having a variable displacement pump 51 as a hydraulic source, and a work machine actuator (first working machine actuator 30a, which has a fixed displacement pump 52 as a hydraulic source). When the second work machine actuator 30b, the third work machine actuator 30c, and the swing motor 44) are simultaneously driven, the target rotation speed of the engine 42 is increased to increase the discharge flow rate of the fixed displacement pump 52, thereby performing the work. The speed of the machine actuator can be increased. When the target rotation speed of the engine 42 is increased, the discharge flow rate of the variable displacement pump 51 also increases, and the speed of the traveling motor also tries to increase. However, the discharge flow rate of the variable displacement pump 51 is increased according to the required flow rate of the traveling motor. By suppressing the increase in the discharge flow rate of the variable displacement pump 51 by reducing the gain for electrical control, it is possible to suppress the increase in the speed of the travel motor (travel speed). Therefore, when both the variable displacement pump 51 and the fixed displacement pump 52 are driven by the engine 42 at the same time, the variable displacement pump 51 is hydraulically operated while increasing the speed of the working machine actuator that uses the fixed displacement pump 52 as a hydraulic source. It is possible to suppress an increase in the speed of the traveling motor serving as the source.

[Second Embodiment]
Although the discharge flow rate of the variable displacement pump 51 is controlled by the negative control method in the first embodiment, the discharge flow rate of the variable displacement pump 51 is controlled by the positive control method in the second embodiment. Here is an example:

  Here, regarding the hydraulic circuit 5 of the second embodiment, only the points different from the hydraulic circuit 5 of the first embodiment will be described. In the second embodiment, the left and right traveling remote control valves 413a and 413a are operated according to the operation of the left and right traveling levers 413 and 413 as electric signals corresponding to the required flow rates of the first traveling motor 22a and the second traveling motor 22b. The pressure sensor 413b detects the pilot signal pressure generated in step 1 and converts the detected value into an electric signal. The pressure sensor 413b is connected to the left and right traveling remote control valves 413a and 413a via a plurality of shuttle valves, and the pressure sensor 413b is connected to the left and right traveling remote control valves 413a and 413a. The pilot signal pressure of the higher one (the one of the left and right traveling levers 413, 413 having the larger operation amount) is detected.

[Other Embodiments]
In the above-described embodiment, the variable displacement pump is the split flow type variable displacement pump 51 including the first discharge port P1 and the second discharge port P2, but the variable displacement pump is not limited to this. For example, a tandem type variable displacement pump including a variable displacement pump including a first variable displacement pump having a first discharge port P1 and a second variable displacement pump having a second discharge port P2. May be In the tandem type variable displacement pump, the discharge flow rates of the two variable displacement pumps may be controlled by one pump regulator or may be controlled by different pump regulators.

  In the above-described embodiment, an example in which the travel detection oil passage 53b, the work equipment detection oil passage 53c, the pressure sensor 53e, the pressure sensor 53h, and the like are used as the travel detection device and the work implement detection device has been shown, but the present invention is not limited thereto. .. As the traveling detection device and the work implement detection device, hydraulic actuators (first work implement actuator 30a, second work implement actuator 30b, third work implement actuator 30c, first travel motor 22a, second travel motor 22b, turning) are used. A method of measuring and detecting the pressure of the motor 44) with a pressure sensor or the like may be used.

  Although the embodiments of the present invention have been described above with reference to the drawings, it should be considered that the specific configurations are not limited to these embodiments. The scope of the present invention is shown not only by the above description of the embodiments but also by the claims, and further includes meanings equivalent to the claims and all modifications within the scope.

1 Hydraulic Excavator 2 Lower Traveling Body 3 Working Machine 4 Upper Revolving Body 5 Hydraulic Circuit 22a First Traveling Motor 22b Second Traveling Motor 30a First Working Machine Actuator 30b Second Working Machine Actuator 30c Third Working Machine Actuator 42 Engine 44 Swing motor 51 Variable displacement pump 51a Pump regulator 51c Electromagnetic proportional valve 52 Fixed displacement pump 53 Pilot pump 55 Directional switching valve 70 Controller 71 Control command generation unit 72 Target rotation speed setting unit

Claims (2)

A prime mover, a variable displacement pump and a fixed displacement pump driven by the prime mover, a first traveling motor whose hydraulic source is a first discharge port of the variable displacement pump, and a second discharge of the variable displacement pump. A second traveling motor that uses a port as a hydraulic source, a work machine actuator that uses the fixed displacement pump as a hydraulic source, a traveling that detects an operation of the first traveling motor and the second traveling motor, and issues a traveling detection signal. A detection device, a work machine detection device that detects an operation of the work machine actuator and sends a work machine detection signal, a pump regulator that controls the discharge flow rate of the variable displacement pump, and a pressure signal that is input to the pump regulator. A pressure signal generating device, a controller for issuing a control command input to the pressure signal generating device, an electric signal corresponding to the required flow rate of the first traveling motor and the second traveling motor, and And an electric signal generation device for outputting to the controller,
The controller has a target rotation speed setting unit that sets a target rotation speed of the prime mover, and a control command generation unit that multiplies the electric signal by a gain to generate the control command,
When the traveling detection signal and the working machine detection signal are input to the controller, the target rotation speed setting unit sets the target rotation speed from a first target rotation speed to a second target rotation speed higher than the first target rotation speed. The construction machine, wherein the control command generation unit changes the gain so that the control command becomes a command to reduce the discharge flow rate of the variable displacement pump. A first communication port that communicates with the oil tank from the first discharge port through the first direction switching valve when the first direction switching valve that controls the flow rate and direction of the pressure oil from the first discharge port is neutral. A center bypass flow passage, a first negative control throttle arranged on the most downstream side of the first center bypass flow passage, and a second directional control valve for controlling the flow rate and direction of the pressure oil from the second discharge port are neutral. The second center bypass passage communicating with the oil tank from the second discharge port through the second directional control valve, and the second negative control throttle arranged at the most downstream of the second center bypass passage. And
The construction machine according to claim 1, wherein the electric signal generation device compares pressures generated from the first negative control throttle and the second negative control throttle, selects a low pressure side and converts the pressure into an electric signal.

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