Hydraulic control system for construction machine

Abstract

The present invention makes it possible to maintain a discharge pressure of a main pump at a pressure equal to or higher than a predetermined minimum pressure when an operation lever is not operated, and, when the operation lever is operated, to supply a sufficient volume of pressure oil to a hydraulic actuator of a direction control valve which is tandem-connected on the downstream side of a bypass cut valve disposed in the middle of a center bypass oil passage. Thus, the bypass cut valve is disposed between a first direction control valve and a second direction control valve of the center bypass oil passage. When all of the direction control valves are in neutral position, a controller maintains the opening area of the bypass cut valve in a predetermined minimum opening area, and controls the discharge volume of the main pump so that the discharge pressure of the main pump becomes a pressure equal to or higher than a predetermined minimum discharge pressure. When the second direction control valve positioned on the downstream side of the bypass cut valve is operated, the controller controls the opening area of the bypass cut valve to increase as the spool stroke amount of the second direction control valve increases.

Description

Hydraulic control systems for construction machinery

[0001] The present invention relates to a hydraulic control system for a construction machine such as a hydraulic excavator.

[0002] In a hydraulic control system for construction machinery in which pressurized oil discharged from a main pump is distributed to each hydraulic actuator via a directional control valve and excess oil is returned to a tank, it is known to maintain the discharge pressure of the main pump at or above a predetermined minimum pressure by throttling the bypass oil passage between the main pump and the hydraulic oil tank.

[0003] For example, in the hydraulic control system described in Patent Document 1, a bypass sequence valve is arranged at the most upstream of a center bypass oil passage connected to a main pump and in which a plurality of directional control valves are arranged, and when the control lever is not operated, the solenoid of the bypass sequence valve is de-energized to put the bypass sequence valve in an unloaded communicating state, and the pump discharge pressure is unloaded to the tank line to make it low pressure, while when the control lever is operated, the solenoid of the bypass sequence valve is excited to narrow the opening of the bypass sequence valve (throttle the center bypass oil passage), thereby maintaining the pressure of the pressurized oil discharged from the main pump (discharge pressure) at a pressure equal to or higher than the pilot primary pressure (predetermined minimum pressure) of the pilot hydraulic source, and by directing a portion of the pressurized oil to a pressure reducing valve of the pilot hydraulic source, it is possible to ensure a pilot hydraulic source without using a pilot pump.

[0004] Furthermore, in the hydraulic control system described in Patent Document 2, a bypass valve is arranged in a return oil line (bypass oil line) that branches off from the pump discharge oil line and connects the pump discharge oil line to a tank, and when the system is in a standby state where all operating levers are not operated, the opening area of ​​the bypass valve is controlled to a position that narrows it to a set value. This maintains the discharge pressure of the main pump at a pressure equal to or greater than a predetermined value when none of the operating levers are operated, and applies back pressure to the main pump, improving the responsiveness of the main pump when an operation input is made from the standby state and preventing damage to the sliding equipment of the main pump.

[0005] Patent No. 3692004 Patent No. 5622243

[0006] In the hydraulic control system described in Patent Document 1, when the operating lever is not operated, the solenoid of the bypass sequence valve is de-energized, putting the bypass sequence valve into an unloaded communicating state, and the pump discharge pressure is unloaded into the tank line to keep it at a low pressure. Therefore, as in Patent Document 2, it is not possible to apply back pressure to the main pump when none of the operating levers are operated, thereby improving the responsiveness of the main pump when an operating input is received, or preventing damage to the sliding equipment of the main pump.

[0007] Furthermore, in the hydraulic control system described in Patent Document 1, a bypass sequence valve (hereinafter referred to as a bypass cut valve) is disposed at the most upstream of the center bypass oil passage in order to maintain the discharge pressure of the main pump at or above a predetermined minimum pressure when the operating lever is operated. However, due to reasons such as constraints on the space within the valve block and the strength of the outer wall, it may be desirable to dispose the bypass cut valve not at the most upstream of the center bypass oil passage but midway along the center bypass oil passage (between adjacent directional control valves). When the bypass cut valve is disposed midway along the center bypass oil passage in this way, the inlet oil passage of the directional control valve connected in tandem downstream of the bypass cut valve is narrowed, reducing the flow rate (supply amount) of pressure oil supplied to the hydraulic actuator of that directional control valve, resulting in a problem of a decrease in speed.

[0008] An object of the present invention is to provide a hydraulic control system for construction machinery in which a bypass cut valve is arranged midway through a center bypass oil line, which maintains the discharge pressure of the main pump at a pressure equal to or higher than a predetermined minimum pressure when the operating lever is not operated, and which can supply a sufficient flow rate of pressurized oil to the hydraulic actuator of a directional control valve connected in tandem downstream of the bypass cut valve when the operating lever is operated.

[0009] The present invention includes a plurality of means for solving the above problems, examples of which are as follows.

[0010] That is, in order to solve the above-mentioned problems, the hydraulic control system for a construction machine of the present invention comprises a center bypass oil passage connected on the upstream side to a main pump and connected on the downstream side to a tank, a first directional control valve arranged in the center bypass oil passage, a plurality of directional control valves including a second directional control valve arranged on the center bypass oil passage downstream of the first directional control valve and connected in tandem to the first directional control valve so that pressure oil discharged from the main pump is preferentially supplied to the first directional control valve, and a controller, The system further includes a bypass cut valve whose opening area can be proportionally changed from a predetermined minimum opening area to a maximum opening area, and when all of the plurality of directional control valves arranged in the center bypass oil passage are in neutral positions, the controller maintains the opening area of ​​the bypass cut valve at the predetermined minimum opening area and controls the discharge flow rate of the main pump so that the discharge pressure of the main pump becomes a pressure equal to or higher than a predetermined minimum discharge pressure, and when the second directional control valve located downstream of the bypass cut valve is operated, controls the opening area of ​​the bypass cut valve so that it increases as the spool stroke amount of the second directional control valve increases.

[0011] According to the present invention configured as described above, in a hydraulic control system in which a bypass cut valve is arranged midway through a center bypass oil passage, when all of the multiple directional control valves arranged in the center bypass oil passage are in neutral positions, the opening area of ​​the bypass cut valve is maintained at a predetermined minimum opening area, and the discharge flow rate of the main pump is controlled so that the discharge pressure of the main pump is equal to or higher than a predetermined minimum discharge pressure, thereby maintaining the discharge pressure of the main pump at a pressure equal to or higher than the predetermined minimum pressure when the operating lever is not being operated, and when the second directional control valve located downstream of the bypass cut valve is operated, the opening area of ​​the bypass cut valve is controlled so that it increases as the spool stroke amount of the second directional control valve increases, so that a sufficient flow rate of pressurized oil can be supplied to the hydraulic actuator of the directional control valve connected in tandem downstream of the bypass cut valve when the operating lever is operated.

[0012] According to the present invention, in a hydraulic control system in which a bypass cut valve is arranged midway through a center bypass oil line, when the operating lever is operated, a sufficient flow rate of pressurized oil can be supplied to the hydraulic actuator of the directional control valve connected in tandem downstream of the bypass cut valve.

[0013] FIG. 1 is a diagram showing a hydraulic excavator (construction machine) according to a first embodiment of the present invention. FIG. 2 is a diagram showing a work implement portion of a front working machine of a hydraulic excavator equipped with a hydraulic grapple (attachment) instead of a bucket. FIG. 3 is a diagram showing a hydraulic circuit of a hydraulic control system for a construction machine according to a first embodiment of the present invention. FIG. 4 is a diagram showing valve functions of a swing directional control valve, a boom three-way control valve, and an attachment two-way control valve using hydraulic symbols. FIG. 5 is a diagram showing a control circuit portion of the hydraulic control system of the first embodiment. FIG. 6 is a diagram showing a control algorithm for the directional control valve of the controller. FIG. 7 is a diagram showing a control algorithm for the bypass cut valve of the controller. FIG. 8 is a diagram showing a control algorithm for the main pump of the controller. FIG. 9 is a diagram showing changes over time in the target opening area of ​​the bypass cut valve, the target flow rate of the main pump, and the discharge pressure of the main pump when the operation lever device for attachment 2 is operated alone. FIG. 10 is a diagram showing changes over time in the target opening area of ​​the bypass cut valve, the target flow rate of the main pump, and the discharge pressure of the main pump when the operation lever device for the boom is operated simultaneously in the boom-raising direction after the operation lever device for attachment 2 is operated alone. FIG. 11 is a diagram showing a hydraulic circuit of a hydraulic control system for a construction machine according to a second embodiment of the present invention. FIG. 12 is a diagram showing an actuator load estimation means (load estimation algorithm) based on detection values ​​from a pressure sensor. FIG. 10 is a diagram showing a control algorithm of the bypass cut valve of the controller.

[0014] Hereinafter, a plurality of embodiments of the present invention will be described with reference to the drawings.

[0015] First Embodiment - Construction Machine - FIG. 1A is a diagram showing a hydraulic excavator (construction machine) according to a first embodiment of the present invention.

[0016] The hydraulic excavator comprises a lower traveling structure 100, an upper rotating structure 101, and a front work implement 102. The lower traveling structure 100 has right and left crawler-type traveling devices 100a and 100b, which are driven by right and left traveling motors 100c and 100b. The upper rotating structure 101 is rotatably mounted on the lower traveling structure 100, and is driven to rotate by a swing motor 101a. The front work implement 102 is attached to the front of the upper rotating structure 101 so that it can be raised and lowered. The upper rotating body 101 is provided with a cabin (operator's compartment) 101b, and inside the cabin 101b are arranged a driver's seat (not shown), left and right operating lever devices 41a, 41b for front and rotation provided in front of the driver's seat (see Figure 3), left and right operating lever / pedal devices for travel (not shown) provided on the floor in front of the driver's seat, and operating lever devices 41c, 41d for attachment 1 and attachment 2 (see Figure 3) located in appropriate positions near the driver's seat.

[0017] The front work machine 102 has a multi-joint structure having a boom 102a, an arm 102b, and a bucket 102c. The boom 102a rotates up and down by the extension and contraction of a boom cylinder 112a, the arm 102b rotates up and down and in the front and rear directions by the extension and contraction of an arm cylinder 112b, and the bucket 102c rotates up and down and in the front and rear directions by the extension and contraction of a bucket cylinder 112c.

[0018] The front work implement 102 may be equipped with attachments such as a grapple, breaker, or crusher instead of the bucket 102c, and these attachments are also driven by hydraulic actuators (hereinafter sometimes simply referred to as actuators) such as hydraulic cylinders or hydraulic motors.

[0019] 1B is a diagram showing the implement portion of the front work implement 102 of the hydraulic excavator equipped with a hydraulic grapple 102d instead of the bucket 102c. The hydraulic grapple 102d incorporates a hydraulic cylinder 112d and a hydraulic motor (not shown) as actuators, and the hydraulic cylinder 112d opens and closes the claws 102d1 in directions A1 and A2, and the hydraulic motor rotates the claws 102d1 in directions B1 and B2.

[0020] -Hydraulic Circuit- Fig. 2A is a diagram showing a hydraulic circuit of the hydraulic control system for the construction machine according to the first embodiment of the present invention.

[0021] In FIG. 2A, the hydraulic control system of this embodiment includes three hydraulic circuits HC1, HC2, and HC3: a hydraulic circuit HC1 including a main pump (hydraulic pump) 20a, a hydraulic circuit HC2 including a main pump (hydraulic pump) 20b, and a hydraulic circuit HC3 including a main pump (hydraulic pump) 20c.

[0022] The hydraulic circuit HC1 includes a right-travel directional control valve 21, a bucket directional control valve 22, an arm 2 directional control valve 23, and a boom 1 directional control valve 24. These directional control valves 21-24 are connected in order from the main pump 20a side (upstream side) to a center bypass oil line 51a, whose upstream side is connected to the pump oil line of the main pump 20a and whose downstream side is connected to the tank T via a return oil line L1. The right-travel directional control valve 21, the bucket directional control valve 22, the arm 2 directional control valve 23, and the boom 1 directional control valve 24 control the flow direction and amount of pressure oil supplied from the main pump 20a to the right travel motor 100c, the bucket cylinder 112c, the arm cylinder 112b, and the boom cylinder 112a, which are hydraulic actuators shown in FIG. 1, respectively, to drive these hydraulic actuators. A throttle 33a for generating a minimum pump discharge pressure is provided in the return oil line L1.

[0023] Hydraulic circuit HC2 has a boom 2 directional control valve 25, an arm 1 directional control valve 26, an attachment 1 directional control valve 27, and a travel left directional control valve 28, and these directional control valves 25 to 28 are connected in order from the main pump 20b side (upstream side) to a center bypass oil line 51b whose upstream side is connected to the pump oil line of the main pump 20b and whose downstream side is connected to the tank T via a return oil line L2. The boom 2 directional control valve 25, the arm 1 directional control valve 26, and the travel left directional control valve 28 respectively control the flow direction and amount of pressure oil supplied from the main pump 20b to the boom cylinder 112a, the arm cylinder 112b, and the left traveling motor 100d, which are hydraulic actuators shown in FIG. 1, and drive these hydraulic actuators. When the bucket 102c is replaced with an attachment other than a bucket, the attachment 1 directional control valve 27 controls the flow direction and flow rate of pressure oil supplied from the main pump 20b to the first hydraulic actuator of that attachment (for example, a hydraulic motor, not shown, if the attachment is the hydraulic grapple 102d described above), and drives that first hydraulic actuator. A throttle 33b for generating the minimum pump discharge pressure is provided in the return oil line L2.

[0024] The hydraulic circuit HC3 has a swing direction control valve 29, a boom three-way control valve 30, and an attachment two-way control valve 31, and these direction control valves 29 to 31 are connected in order from the main pump 20c side (upstream side) to a center bypass oil line 51c whose upstream side is connected to the pump oil line of the main pump 20c and whose downstream side is connected to the tank T via a return oil line L3. The swing direction control valve 29 and the boom three-way control valve 30 respectively control the flow direction and flow rate of pressure oil supplied from the main pump 20c to the swing motor 101a and the boom cylinder 112a, which are hydraulic actuators shown in Figure 1, and drive these hydraulic actuators. When the bucket 102c is replaced with an attachment and that attachment is equipped with a second hydraulic actuator (hydraulic cylinder 112d if the attachment is the hydraulic grapple 102d described above), the attachment two-way control valve 31 controls the flow direction and flow rate of pressure oil supplied from the main pump 20c to the second hydraulic actuator of that attachment, and drives that second hydraulic actuator.

[0025] Hydraulic Circuit HC3 The present invention is characterized by the hydraulic circuit HC3, which will be described in detail below with reference to Fig. 2B, which is a diagram showing the valve functions of the swing direction control valve 29, boom three-way control valve 30, and attachment two-way control valve 31 using hydraulic symbols.

[0026] In the hydraulic circuit HC3, the main pump 20c distributes its discharge oil to the directional control valves 29 to 31 through a center bypass oil line 51c and a parallel oil line 52c. A center bypass oil line 51c passes through the directional control valves 29 to 31, and when the directional control valves 29 to 31 are in neutral, the center bypass oil line 51c is connected to the return oil line L3 to the tank T. When the directional control valves 29 to 31 are operated, the center bypass throttle gradually throttles the center bypass oil line 51c in accordance with the operation amount (spool stroke amount), and when the directional control valves 29 to 31 are fully operated, the communication between the center bypass oil line 51c and the return oil line L3 is blocked. Furthermore, the directional control valves 29 to 31 supply the discharge oil from the main pump 20c to the actuator and discharge the discharge oil from the actuator to the tank via the return oil line L3 in accordance with the operation amount (spool stroke amount).

[0027] The swing direction control valve 29 is located at the most upstream side of the center bypass oil passage 51c, and the boom three-way control valve 30 is arranged downstream of the swing direction control valve 29, and the boom three-way control valve 30 is connected in tandem to the swing direction control valve 29 so that the pressure oil discharged from the main pump 20c is preferentially supplied to the upstream swing direction control valve 29. The attachment two-way control valve 31 is arranged downstream of the boom three-way control valve 30 on the center bypass oil passage 51c, and is connected in tandem to the boom three-way control valve 30 so that the pressure oil discharged from the main pump 20c is preferentially supplied to the upstream boom three-way control valve 30.

[0028] Furthermore, the swing direction control valve 29, boom 3-way control valve 30, and attachment 2-way control valve 31 are connected in parallel to each other to the main pump 20c via a parallel oil line 52c that is connected to the pump oil line of the main pump 20c. The input port of the boom 3-way control valve 30 is connected to the parallel oil line 52c via a feeder oil line 54a that is equipped with a check valve 54b for preventing backflow, and a parallel throttle (fixed throttle) 54c with a predetermined opening area is provided in the feeder oil line 54a between the input port of the boom 3-way control valve 30 and the check valve 54b. The input port of the attachment 2-way control valve 31 is connected to the parallel oil line 52c via a feeder oil line 55a that is equipped with a check valve 55b for preventing backflow, and a parallel throttle (fixed throttle) 55c with a predetermined opening area is also provided in the feeder oil line 55a between the input port of the attachment 2-way control valve 31 and the check valve 55b.

[0029] Furthermore, as a feature of this embodiment, the hydraulic circuit HC3 further includes a bypass cut valve 32 that is arranged between the boom three-way control valve 30 (first direction control valve) and the attachment two-way control valve 31 (second direction control valve) in the center bypass oil passage 51c and is capable of proportionally changing the opening area from a predetermined minimum opening area to a maximum opening area.

[0030] As described above, in the hydraulic circuit HC3 of this embodiment, the bypass cut valve 32 is disposed between the directional control valve 30 and the directional control valve 31, so that the center bypass oil passage 51c can be variably throttled. The directional control valve 31, located downstream of the bypass cut valve 32, is connected to both the center bypass oil passage 51c and the parallel oil passage 52c, and a parallel throttle 55c is disposed in the parallel oil passage 52c. Therefore, when the directional control valves 29 and 30 are in the neutral position, pressure oil is supplied to the directional control valve 31 with low pressure loss through the center bypass oil passage 51c. Meanwhile, when either of the directional control valves 29 and 30 is operated, the center bypass oil passage 51c is throttled by the directional control valves 29 and 30, and the amount of pressure oil supplied to the directional control valve 31 is suppressed by the parallel throttle 55c. Thus, in a tandem-connected circuit in which pressure oil is preferentially supplied to the upstream directional control valve, the speed balance (operation balance) between the actuators can be adjusted by the action of the parallel throttle 55c.

[0031] Here, "tandem connection" refers to a connection configuration in which, for example, the input port of the directional control valve 31 is connected to a center bypass oil passage 51c downstream of the directional control valve 30 via a feeder passage 31b equipped with a check valve 31a for preventing backflow, and the oil discharged from the main pump 20c is supplied from the center bypass oil passage 51c to the directional control valve 31. In this tandem connection, when the directional control valve 31 and the upstream directional control valve 29 or directional control valve 30 are operated simultaneously, the oil discharged from the main pump 20c is preferentially supplied to the upstream directional control valve 29 or directional control valve 30. If the upstream boom 3-way control valve 29 or swing direction control valve 30 is operated at a full stroke, the flow rate of pressure oil supplied from the main pump 20c to the directional control valve 31 becomes zero.

[0032] The term "parallel connection" refers to a connection configuration in which the feeder ports of the directional control valves 29, 30, and 31 are connected to the pump oil passage of the main pump 20c via the parallel oil passage 52c, and the oil discharged from the main pump 20c is directly supplied from the pump oil passage. In this parallel connection, if the feeder oil passages 54a and 55a are not provided with the parallel throttles 54c and 55c, when the directional control valves 29 and 31 are operated simultaneously, for example, the oil discharged from the main pump 20c is preferentially supplied to the directional control valve of the actuator on the low-load pressure side of the directional control valve 29 or the directional control valve 31, and most of the pressure oil discharged from the main pump 20c is supplied to the directional control valve of the actuator on the low-load pressure side.

[0033] In the case where the parallel throttles 54c, 55c are provided in the feeder oil passages 54a, 55a as in this embodiment, when the directional control valve 29 and the directional control valve 31 are operated simultaneously, pressurized oil can be supplied to the downstream directional control valve 31 via the parallel throttle 55c, and even if the directional control valve 31 is on the low load side, the parallel throttle 55c increases the discharge pressure of the main pump 20c, and the discharge oil of the main pump 20c can also be supplied to the directional control valve 29 of the actuator on the high load side, thereby driving the actuator on the high load side.

[0034] Pilot Hydraulic Source Circuit and Solenoid Valve Unit In FIG. 2A, the hydraulic control system of this embodiment includes a pilot hydraulic source circuit 34 and a solenoid valve unit 38.

[0035] The pilot hydraulic source circuit 34 is provided with a pilot pressure reducing valve 35 that receives a portion of the pressure oil discharged from the main pump 20c and reduces the pressure of the pressure oil to generate pilot primary pressure, a check valve 36 for maintaining the pilot primary pressure, and an accumulator 37 for smoothing the pilot primary pressure. By providing the check valve 36 and the accumulator 37, the pilot hydraulic source circuit 34 can maintain the pilot primary pressure even when the discharge pressure of the main pump 20c temporarily becomes lower than the set pressure of the pilot pressure reducing valve 35.

[0036] The solenoid valve unit 38 includes solenoid proportional valves 38a, 38b, 38c, 38d, 38e, 38f, and 38g that generate operating pilot pressures C1a, C1b, C2a, C2b, C3a, C3b, and D1 using the pilot primary pressure generated by the pilot pressure reducing valve 35 of the pilot hydraulic power source circuit 34 as a source pressure, and a plurality of solenoid proportional valves (not shown) that use the pilot primary pressure as a source pressure to generate operating pilot pressures A1a to A4a, A1b to A4b and operating pilot pressures B1a to B4a, B1b to B4b.

[0037] Control System FIG. 3 is a diagram showing a control circuit portion of the hydraulic control system of the first embodiment.

[0038] 3 , the hydraulic control system of this embodiment includes a controller 40; an operating device 41 including left and right operating lever devices 41a and 41b for front and swing operation, an operating lever device 41c for a first hydraulic actuator of the attachment, an operating lever device 41d for a second hydraulic actuator of the attachment, and left and right operating lever / pedal devices for travel (not shown); pressure sensors 42a and 42b for detecting the discharge pressures of the main pumps 20a and 20b, respectively; and a pressure sensor 42c (first pressure sensor) for detecting the discharge pressure of the main pump 20c. The operating lever devices 41a, 41b, 41c, and 41d of the operating device 41 and the pedal device are electric, each outputting an electrical operation signal. The operating lever device 41a functions as an operating device (first operating device) that drives the swing direction control valve 29 when operated in one direction of the cross, and the operating lever device 41b functions as an operating device that drives the boom three-way control valve 30 when operated in one direction of the cross. The operating lever device 41c functions as an operating device (second operating device) for the attachment two-way control valve 31, for example.

[0039] The controller 40 receives electrical operation signals output from the operation device 41, performs predetermined arithmetic processing, and outputs control signals IC1a, IC1b, IC2a, IC2b, IC3a, IC3b, ID1... to the solenoid proportional valves 38a to 38g... of the solenoid valve unit 38. The solenoid proportional valves 38a to 38g... of the solenoid valve unit 38 are driven by these control signals IC1a, IC1b, IC2a, IC2b, IC3a, IC3b, ID1... and output the above-mentioned operation pilot pressures A1a to A4a, A1b to A4b, operation pilot pressures B1a to B4a, B1b to B4b, operation pilot pressures C1a, C1b, C2a, C2b, C3a, C3b, and operation pilot pressure D1. These operation pilot pressures are sent to the directional control valves 21 to 24, 25 to 27, 29 to 31 and the bypass cut valve 32, and drive the corresponding directional control valves and bypass cut valve 32.

[0040] In addition, the controller 40 inputs the electrical operation signal output from the operating device 41 and the detection signals from the pressure sensors 42a, 42b, and 42c, performs predetermined calculations, and generates and outputs control signals (control currents) PC1, PC2, and PC3 that drive the regulators 50a, 50b, and 50c of the main pumps 20a, 20b, and 20c, thereby controlling the discharge flow rates of the main pumps 20a, 20b, and 20c.

[0041] Control Algorithm of Controller 40 The control algorithm of the controller 40 for the hydraulic circuit HC3, which is a characteristic feature of the present invention, will be described below.

[0042] Basic Concept of Control Algorithm First, we will explain the basic concept of the control algorithm of the controller 40. In the following explanation, the operating lever devices 41a and 41b of the directional control valves 29 and 30 will be referred to as a first operating device, the operating lever device 41c of the directional control valve 31 will be referred to as a second operating device, and the operating lever devices 41a, 41b, and 41c will be referred to as a plurality of operating devices.

[0043] (1) When all of the directional control valves 29, 30, 31 arranged in the center bypass oil passage 51c are in neutral positions (when not operated), the controller 40 maintains the opening area of ​​the bypass cut valve 32 at a predetermined minimum opening area A0 and controls the discharge flow rate of the main pump 20c so that the discharge pressure of the main pump 20c becomes equal to or higher than a predetermined minimum discharge pressure P0. When the directional control valve (second directional control valve) 31 located downstream of the bypass cut valve 32 is operated, the controller 40 controls the opening area of ​​the bypass cut valve 32 so that it increases as the operation amount (spool stroke amount) of the directional control valve 31 increases.

[0044] (2) The controller 40 sets in advance a predetermined minimum opening area A of the bypass cut valve 32 as a standby opening area when the multiple operating devices 41 a, 41 b, 41 c are not being operated, calculates an operating target opening area A of the bypass cut valve 32 based on the operation signal of the second operating lever device 41 c, selects the larger of the predetermined minimum opening area A of the bypass cut valve or the operating target opening area A as the target opening area for control At, and controls the opening area of ​​the bypass cut valve 32 to become the selected target opening area for control At.

[0045] (3) The controller 40 sets in advance a predetermined minimum discharge pressure P of the main pump 20c as a standby pressure when the multiple operating devices 41a, 41b, 41c are not being operated, calculates a required flow rate Q of the main pump 20c based on operation signals of the multiple operating lever devices 41a, 41b, 41c, calculates a standby flow rate Q of the main pump 20c required to maintain the discharge pressure of the main pump 20c at the predetermined minimum discharge pressure P when the multiple operating devices 41a, 41b, 41c are not being operated based on the discharge pressure of the main pump 20c detected by the pressure sensor 42c (first pressure sensor) and the predetermined minimum discharge pressure P of the main pump 20c, selects the larger of the required flow rate Q of the main pump 20c or the standby flow rate Q of the main pump 20c as a pump target flow rate Qt, and controls the discharge flow rate of the main pump 20c to achieve the selected pump target flow rate Qt.

[0046] The control algorithm of the controller 40 will now be described in more detail.

[0047] <Directional Control Valve Control Algorithm> FIG. 4A is a diagram showing the control algorithm of the controller 40 for the directional control valves 29, 30, and 31.

[0048] The controller 40 has target operating pressure calculation tables T41 and T42, a determination section O41, a conversion section O42, a zero setting section O40, selection sections O43 and O44, and control current calculation tables T43 and T44.

[0049] The control algorithm shown in FIG. 4A is for the case where the directional control valve is an attachment two-way control valve 31.

[0050] Table T41 receives the lever stroke amount (e.g., -100% to 100%) calculated from the operation signal of the operating lever device 41c and calculates the target operating pressure (target operating pressure of operating pilot pressure C3a) P3ta when the lever stroke amount is a positive value. Conversion unit O42 converts the lever stroke amount when it is a negative value into a positive value, and table T42 calculates the target operating pressure (target operating pressure of operating pilot pressure C3b) P3tb when the lever stroke amount is a negative value from the lever stroke amount converted to a positive value.

[0051] The determination unit O41 determines whether the lever stroke amount is positive or negative, and the selection units O43, O44 select the target operating pressure P3ta or P3tb according to the operating direction of the operating lever based on the determination result. That is, the selection units O43, O44 select the target operating pressure P3ta or P3tb if the target operating pressure P3ta or P3tb input from the conversion units O41, O42 is a positive value (true), and select the zero target operating pressure set in the zero setting unit O40 if the target operating pressure P3ta or P3tb is a negative value (false).

[0052] The control current calculation tables T43, T44 convert the target operating pressure P3ta or P3tb selected by the selection units O43, O44 into a target current for the electromagnetic proportional valves 38e, 38f, and output control currents IC3a, IC3b corresponding to the electromagnetic proportional valves 38e, 38f, thereby generating operating pilot pressures C3a, C3b by the electromagnetic proportional valves 38e, 38fg and controlling the spool of the attachment two-way control valve 31.

[0053] When the direction control valve is the swing direction control valve 29 or the boom three-way control valve 30, the target operating pressure P1ta or P1tb, P2ta or P2tb is similarly calculated from the lever stroke amount, and the control current IC1a or IC1b and the control current IC2a ​​or IC2b are output to the solenoid proportional valves 38a, 38b; 38c, 38d, thereby controlling the spools of the swing direction control valve 29 and the boom three-way control valve 30.

[0054] <Bypass Cut Valve Control> FIG. 4B is a diagram showing a control algorithm of the bypass cut valve 32 by the controller 40. As shown in FIG.

[0055] The controller 40 has a minimum opening area setting section O47 for the bypass cut valve 32, opening area calculation tables T45 and T46 for the bypass cut valve 32, a maximum selection section O45, a maximum selection section O46, and a control current calculation table T47.

[0056] In the minimum opening area setting section O47, a predetermined minimum opening area A0 of the bypass cut valve 32 is set in advance as a standby opening area when the plurality of operating devices 41a, 41b, 41c are not operated.

[0057] Table T45 converts the target operating pressure P3ta of the operating pilot pressure C3a of the attachment two-way control valve 31 selected in selection section O43 in Fig. 4A into the operational opening area of ​​the bypass cut valve 32. Table T46 converts the target operating pressure P3tb of the operating pilot pressure C3b of the attachment two-way control valve 31 selected in selection section O44 in Fig. 4A into the operational opening area of ​​the bypass cut valve 32.

[0058] The maximum selection unit O45 selects the larger of the operational opening areas obtained from the tables T45, T46 as the operational target opening area A1 of the bypass cut valve 32, and the maximum selection unit O46 selects the larger of the operational target opening area A1 and the predetermined minimum opening area A0 (standby opening area when not in operation) of the bypass cut valve 32 set in the minimum opening area setting unit O47 as the target opening area At for control of the bypass cut valve 32.

[0059] The control current calculation table T47 converts the target opening area At for control selected in the selection section O46 into a target current for the electromagnetic proportional valve 38d, and outputs a control current ID1 corresponding to the electromagnetic proportional valve 38g, causing the electromagnetic proportional valve 38g to generate an operating pilot pressure D1 and control the spool of the bypass cut valve 32.

[0060] The predetermined minimum opening area A of the bypass cut valve 32 (standby opening area when not operated) is set to, for example, a size that can generate a pressure loss that results in an appropriate set pressure from the standby flow rate when the main pump 20C is not operated, and can be calculated, for example, using the following Bernoulli equation:

[0061] In the following equation, A is the predetermined minimum opening area of ​​the bypass cut valve 32, Q is the standby flow rate of the main pump 20c, that is, the discharge flow rate when the main pump 20c is rotating at the minimum volume (for example, when the minimum volume is 20 cc / rev and the prime mover rotation speed is 1800 rev / min, the discharge flow rate is 20 × 1800 / 1000, or 36 L / min), P is a predetermined minimum discharge pressure of the main pump 20c that is set in advance as a standby pressure (for example, 4 MPa if a supply pressure of 4 MPa or more is required as a pilot hydraulic source), P is the hydraulic oil pressure of the tank T (tank pressure), ρ is the hydraulic oil density, and c is a flow coefficient.

[0062]

[0063] Since the motor rotation speed changes depending on the control state and the operator's command input, the standby flow rate Q0 also changes (for example, if the minimum volume is 20 cc / rev and the rotation speed range is 800 to 2000 rev / min, the standby flow rate Q0 is 16 to 40 l / min).

[0064] <Main Pump Control Algorithm> FIG. 5 is a diagram showing the control algorithm of the controller 40 for the main pump 20c.

[0065] The controller 40 has required flow rate calculation tables T51a, T51b; T52a, T52b; T53a, T53b for the main pump 20c, a maximum selection unit O51, a minimum pump discharge pressure setting unit O55, a subtraction unit O52, a PID calculation unit O53, a maximum selection unit O54, and a control current calculation table T48.

[0066] The required flow rate calculation tables T51a, T51b; T52a, T52b; T53a, T53b of the main pump 20c input the target operating pressures P1ta, P1tb, P2ta, P2tb, P3ta, and P3tb calculated from the lever stroke amount using the control algorithm of Figure 4A, and convert them into actuator required flow rates.

[0067] The maximum selection unit O51 selects the maximum value of the actuator required flow rates converted from the target operating pressures P1ta, P1tb, P2ta, P2tb, P3ta, and P3tb as the pump required flow rate Qreq (positive control flow rate).

[0068] A minimum pump discharge pressure setting unit O55 presets a predetermined minimum discharge pressure Po of the main pump 20c as a standby pressure when the operating devices 41a, 41b, 41c are not operated, a subtraction unit O52 subtracts the discharge pressure of the main pump 20c detected by the pressure sensor 42c from the predetermined minimum discharge pressure Po to calculate a discharge pressure deviation, and a PID calculation unit O53 performs a feedback control calculation on the discharge pressure deviation to calculate a standby flow rate Qo (pressure feedback flow rate) of the main pump 20c necessary to maintain the discharge pressure of the main pump 20c at the predetermined minimum discharge pressure Po. A maximum selection unit O54 selects the larger of the required pump flow rate Qreq selected by the maximum selection unit O51 or the standby flow rate Qo calculated by the PID calculation unit O53 as the pump target flow rate Qt. The control current calculation table T48 converts the pump target flow rate Qt selected by the selection unit O53 into a control current PC3 for the regulator 50c, and by outputting the control current PC3 to the regulator 50c, the volume of the main pump 20c is controlled so that the discharge flow rate of the main pump 20c becomes the pump target flow rate Qt.

[0069] The regulator 50c of the main pump 20c introduces the discharge pressure of the main pump 20c as a driving pressure into a pressure-receiving chamber and adjusts the pump displacement angle (pump volume) using that driving pressure, thereby applying back pressure due to the main pump's own pressure to the regulator 50c of the main pump 20c, improving the responsiveness of the main pump when an operation input is made from the standby state and preventing damage to the sliding equipment of the main pump. A hydraulic pump equipped with a regulator that adjusts pump volume by applying back pressure due to the main pump's own pressure is described, for example, in JP 2019-65569 A.

[0070] If the pump 20c is a variable displacement pump that is rotationally driven by a prime mover (for example, an electric motor), a target volume and a target prime mover rotation speed that satisfy the target pump flow rate may be calculated and output.

[0071] The control algorithm of the controller 40 for the main pumps 20a, 20b is also similar. That is, the throttles 33a, 33b provided in the return oil passages L1, L2 of the hydraulic circuits HC1, HC2 have an opening area equal to the minimum opening area A of the bypass cut valve 32, and the pump target flow rate Qt is calculated based on the operating pressure of the operating device using the algorithm shown in FIG. 5, and control currents PC1, PC2 are output to the regulators 50a, 50b. As a result, in the hydraulic circuits HC1, HC2, when the operating device is not operated and the hydraulic circuits are in a standby state where all of the directional control valves 21-24 or 25-28 are in the neutral position, the discharge pressure of the main pumps 20a, 20b is controlled to be near a predetermined minimum discharge pressure P0. This improves the responsiveness of the main pumps when an operating input is received from the standby state, and prevents damage to the sliding devices of the main pumps.

[0072] Operation Example Next, an operation example according to this embodiment will be described with reference to FIGS. 6A and 6B.

[0073] Operation Example 1 FIG. 6A is a diagram showing the changes over time in the target opening area of ​​the bypass cut valve 32, the target flow rate of the main pump 20c, and the discharge pressure of the main pump 20c when the operating lever device 41c for the attachment 2 is operated alone.

[0074] In FIG. 6A, (1) to (5) indicate the changes over time of the following state quantities.

[0075] (1) Operation signal of the operating lever device 41c for the attachment. (2) Target operating pressure P3ta of the attachment two-way control valve 31 converted from the operation signal of (1). (3) Target opening area At of the bypass cut valve 32 calculated from the target operating pressure P3ta of (2). (4) Target discharge flow rate Qt of the main pump 20c calculated from the operation signal of (1) and the detection value of the pressure sensor 42c. (5) Discharge pressure of the main pump 20c which varies depending on the driving status of the main pump 20c and the bypass cut valve 32 and the actuator load. Furthermore, the time periods (a), (b), and (c) on the horizontal axis indicate the following states.

[0076] (a) Not operated (b) Operation of attachment 2 alone (c) Not operated During the non-operation states (a) and (c), the target opening area At of the bypass cut valve 32 is controlled to a value corresponding to the standby flow rate Q0 of the main pump 20c, and the discharge pressure of the main pump 20c is maintained at a pressure equal to or greater than a predetermined minimum discharge pressure P0 (standby pressure). By maintaining the discharge pressure of the main pump 20c at a pressure equal to or greater than the predetermined minimum discharge pressure P0 in the standby state in this way, back pressure is applied to the regulator 50c of the main pump 20c even in the standby state, improving the responsiveness of the main pump 20c, including the regulator 50c, when an operation input is received from the standby state, and preventing damage to the sliding devices of the main pump 20c. Furthermore, by maintaining the pressure (discharge pressure) of the pressurized oil discharged from the main pump 20c at a pressure equal to or higher than the pilot primary pressure (predetermined minimum pressure) of the pilot hydraulic power source circuit 34 and guiding a portion of the pressurized oil to the pressure reducing valve 35 of the pilot hydraulic power source circuit 34, even when none of the operating levers are operated and all of the directional control valves are in the neutral position, it is possible to ensure a pilot hydraulic power source without using a pilot pump.

[0077] When the attachment is operated independently (b), the opening area of ​​the bypass cut valve 32 is increased by an operation signal from the attachment operating lever device 41c. This prevents a decrease in the flow rate of pressure oil due to the bypass cut valve 32, and a sufficient flow rate of pressure oil is supplied to the hydraulic cylinder 112d of the attachment 102d with little pressure loss.

[0078] Operation Example 2 FIG. 6B is a diagram showing the changes over time in the target opening area of ​​the bypass cut valve 32, the target flow rate of the main pump 20c, and the discharge pressure of the main pump 20c when the operating lever device 41c for the attachment 2 is operated alone and then the operating lever device 41b for the boom is simultaneously operated in the boom-raising direction.

[0079] In (1) to (5) of FIG. 6B, the change over time in the state quantity related to the operation of the attachment operating lever device 41c is indicated by a dotted line.

[0080] Additionally, the time periods (a), (b), (c), (d), and (d) on the horizontal axis indicate the following states: (a) No operation (b) Operation of attachment 2 alone (c) Combined operation of attachment 2 and boom raising (d) Operation of boom raising alone (e) No operation In this operation example, a case is shown in which the load pressure of the boom cylinder is higher than the load pressure of hydraulic cylinder 112d of attachment 102d, but the present invention is not limited to this.

[0081] In this operation example, when (a) and (e) are not operated, the target opening area At of the bypass cut valve 32 is controlled to a value corresponding to the standby flow rate Q of the main pump 20c, and the discharge pressure of the main pump 20c is maintained at a pressure equal to or higher than a predetermined minimum discharge pressure P (standby pressure).

[0082] When the attachment 2 is operated independently (b), the opening area of ​​the bypass cut valve 32 is increased by an operation signal from the attachment operating lever device 41c. This prevents a decrease in the flow rate of pressure oil due to the bypass cut valve 32, and a sufficient flow rate of pressure oil is supplied to the hydraulic cylinder 112d of the attachment 102d with little pressure loss.

[0083] During the combined operation of attachment 2 and boom raising (c), when the boom 3 directional control valve 30 is driven by the boom-raising operation, the center bypass oil line 51c is shut off. At this time, pressure oil can be supplied to the downstream directional control valve 31 through the restrictor 55c of the parallel oil line 52c, and even if the directional control valve 31 is on the low-load side, the parallel restrictor 55c increases the discharge pressure of the main pump 20c, allowing the oil discharged from the main pump 20c to be supplied to the directional control valve 29 of the actuator on the high-load side, thereby driving the actuator on the high-load side. In this way, in a tandem-connected circuit in which pressure oil is supplied preferentially to the upstream directional control valve, the speed balance (operation balance) between the actuators can be adjusted by the action of the parallel restrictor 55c.

[0084] During the boom-raising operation alone (d), pressure oil is supplied only to the boom three-way control valve 30, increasing the boom speed. In addition, since the load pressure of the boom cylinder 112a is higher than the load pressure of the hydraulic cylinder 112d of the attachment 102d, the discharge pressure of the main pump 20c also increases slightly.

[0085] In this manner, in this operation example as well, in the hydraulic control system in which the bypass cut valve 32 is arranged midway through the center bypass oil passage 51c, the discharge pressure of the main pump 20c is maintained at a pressure equal to or higher than the predetermined minimum discharge pressure P0 when the control lever is not operated, and a reduction in the flow rate of pressurized oil by the bypass cut valve 32 is prevented when the control lever is operated, making it possible to supply a sufficient flow rate of pressurized oil to the hydraulic cylinder 112d of the directional control valve 31 connected in tandem downstream of the bypass cut valve 32.

[0086] Furthermore, when the directional control valves 29 and 41 are operated simultaneously, in a tandem-connected circuit in which pressure oil is preferentially supplied to the upstream directional control valve, the speed balance (operation balance) between the actuators can be adjusted by the action of the parallel throttle 55c.

[0087] Effects 1. According to this embodiment, in a hydraulic control system in which a bypass cut valve 32 is disposed in the center bypass oil line 51c, the discharge pressure of the main pump 20c is maintained at a pressure equal to or greater than a predetermined minimum discharge pressure P0 when none of the operation levers are operated, and even in a standby state in which all of the directional control valves are in the neutral position, back pressure is applied to the regulator 50c of the main pump 20c, thereby improving the responsiveness of the main pump 20c, including the regulator 50c, when an operation input is received from the standby state and preventing damage to the sliding equipment of the main pump 20c. Furthermore, even in the standby state, the pressure (discharge pressure) of the pressure oil discharged from the main pump 20c is maintained at a pressure equal to or greater than the pilot primary pressure (predetermined minimum pressure) of the pilot hydraulic source circuit 34, and a portion of the pressure oil is guided to the pressure reducing valve 35 of the pilot hydraulic source circuit 34, thereby ensuring a pilot hydraulic source without using a pilot pump.

[0088] 2. In addition, when the operating lever is operated, a reduction in the flow rate of pressure oil due to the bypass cut valve 32 is prevented, and a sufficient flow rate of pressure oil can be supplied to the hydraulic cylinder 112d of the directional control valve 31 connected in tandem downstream of the bypass cut valve 32.

[0089] 3. When the directional control valves 29 and 31 are operated simultaneously, the speed balance (operation balance) between the actuators can be adjusted by the action of the parallel throttle 55c in a tandem-connected circuit in which pressure oil is preferentially supplied to the upstream directional control valve.

[0090] Second Embodiment FIG. 7 is a diagram showing a hydraulic circuit of a hydraulic control system for a construction machine according to a second embodiment of the present invention.

[0091] In FIG. 7, the hydraulic control system of this embodiment further includes pressure sensors 61, 62 (second pressure sensor and third pressure sensor) that detect the supply pressure, which is the load pressure of the hydraulic actuator (hydraulic cylinder of the hydraulic grapple 102d) 112d driven by the directional control valve 31 (second directional control valve) of the hydraulic circuit HC3.

[0092] The controller 40 receives detection signals from the pressure sensors 61 and 62 as indicated by the dotted lines in FIG. 3, estimates the load pressure of the hydraulic cylinder 112d driven by the directional control valve 31 (second directional control valve), and controls the opening area of ​​the bypass cut valve 32 so that it becomes smaller as the estimated load pressure decreases.

[0093] More specifically, the controller 40 estimates the load pressure of the hydraulic cylinder 112d driven by the directional control valve 31 (second directional control valve) based on the detection values ​​of the pressure sensors 61, 62 (second and third pressure sensors), corrects the operation target opening area A1 of the bypass cut valve 32 so that it becomes smaller as the estimated load pressure decreases, selects the larger of the above-mentioned predetermined minimum opening area A0 of the bypass cut valve 32 and the corrected operation target opening area A2 as the target opening area At for control, and controls the opening area of ​​the bypass cut valve 32 so that it becomes the target opening area At for control.

[0094] FIG. 8A is a diagram showing an actuator load estimation means (load estimation algorithm) based on the detected values ​​of the pressure sensors 61 and 62.

[0095] The controller 40 has a zero setting unit O60, a determination unit O61, a selection unit O62, a determination unit O63, and a selection unit O64.

[0096] The judgment unit O61 judges whether the load pressure of the hydraulic cylinder 112d detected by the pressure sensor 61 is greater than the threshold value th, and the selection unit O62 selects the load pressure if the load pressure of the hydraulic cylinder 112d is greater than the threshold value th (if TRUE), and selects the zero load pressure set in the zero setting unit O60 if the load pressure is equal to or less than the threshold value th (if FALSE).

[0097] In addition, the judgment unit O63 judges whether the load pressure of the hydraulic cylinder 112d detected by the pressure sensor 62 is greater than the threshold value th, and if the load pressure of the hydraulic cylinder 112d is greater than the threshold value th (if TRUE), the selection unit O64 selects that load pressure as the load pressure estimated value PLs, and if the load pressure is equal to or less than the threshold value th (if FALSE), the selection unit O64 selects the load pressure selected by the selection unit O62 as the load pressure estimated value PLs.

[0098] The threshold value th is a value near the minimum meter-in pressure when oil discharged from the main pump 20c is supplied to the hydraulic cylinder 112d, and is set in advance in the controller 40.

[0099] FIG. 8B is a diagram showing a control algorithm of the bypass cut valve 32 of the controller 40.

[0100] The controller 40 has, similar to the control algorithm in the first embodiment shown in FIG. 4B, a minimum opening area setting section O47 for the bypass cut valve 32, opening area calculation tables T45 and T46 for the bypass cut valve 32, a maximum selection section O45, a maximum selection section O46, and a control current calculation table T47.

[0101] The controller 40 also has an aperture correction gain calculation table T81 and a multiplication section O81.

[0102] Table T81 converts the estimated load pressure value PLs of the hydraulic cylinder 112d calculated by the control algorithm of Fig. 8A into an opening correction gain Kc. In table T81, the relationship between the estimated load pressure value PLs and the opening correction gain Kc is set so that, for example, when the estimated load pressure value PLs of the hydraulic cylinder 112d is small (for example, when it is near the threshold value th), Kc is about 0.5, and when the estimated load pressure value PLs increases to near the intermediate pressure, Kc increases to a maximum of 1.

[0103] A multiplication unit O81 multiplies the operation target opening area A1 of the bypass cut valve 32 selected by the maximum selection unit O45 by the opening correction gain Kc to calculate a corrected operation target opening area A2. A maximum selection unit O46 selects the larger of the corrected operation target opening area A2 or the predetermined minimum opening area A0 of the bypass cut valve 32 set in a minimum opening area setting unit O47 as the target opening area At for control of the bypass cut valve 32.

[0104] 8A detects the load pressure of the hydraulic cylinder 112d to estimate the actuator load, but the actuator load may also be estimated by calculating it from the weight and posture information of the attachment 102d, for example. Also, the actuator load may be determined in advance and uniquely set depending on the type of hydraulic actuator.

[0105] According to this embodiment, the following effects can be obtained.

[0106] If the bypass cut valve 32 is opened while the directional control valve 31 connected in tandem downstream of the bypass cut valve 32 in the center bypass oil passage 51c is being driven, the hydraulic cylinder 112d is driven by the weight of the load, and if the load pressure decreases, the discharge pressure of the main pump 20c drops suddenly, and it may become impossible to maintain the minimum required discharge pressure. On the other hand, if the opening of the bypass cut valve 32 is limited to maintain the discharge pressure of the main pump 20c, there is a risk that the amount of oil supplied to the hydraulic cylinder 112d will be insufficient.

[0107] In this embodiment, the actuator negative pressure is estimated and the opening degree of the bypass cut valve 32 is adjusted in accordance with the negative pressure, so that the opening degree of the bypass cut valve 32 can be adjusted to an appropriate value in accordance with the actuator load conditions. In other words, even when there is a possibility that the actuator negative pressure will become extremely low, the amount of pressure oil supplied to the hydraulic cylinder 112d by the directional control valve 31 connected in tandem downstream of the bypass cut valve 32 can be ensured while maintaining the discharge pressure of the main pump 20c.

[0108] <Others> In the above embodiment, the hydraulic circuit HC3 includes the swing directional control valve 29, the boom three-way control valve 30, and the attachment two-way control valve 31, but the types of directional control valves are not limited to these. Furthermore, while the attachment two-way control valve 31 is connected downstream of the bypass cut valve 32, other directional control valves may be connected, or multiple directional control valves may be connected. When multiple directional control valves are connected downstream of the bypass cut valve 32, by controlling the opening area of ​​the bypass cut valve 32 so that it increases as the operation amount (spool stroke amount) of each directional control valve increases when the operating device of each directional control valve is operated, a sufficient flow rate of pressurized oil can be supplied to the hydraulic actuator of each directional control valve with little pressure loss.

[0109] HC1, HC2, HC3 Hydraulic circuit T Tank 20a, 20b, 20c Main pump 29 Directional control valve 30 Directional control valve (first directional control valve) 31 Directional control valve (second directional control valve) 32 Bypass cut valve 34 Pilot hydraulic power source circuit 38 Solenoid valve unit 40 Controller 41 Operation device 41a, 41b, 41c, 41d Operation lever device 42a, 42b Pressure sensor 42c Pressure sensor (first pressure sensor) 50a, 50b, 50c Regulator 51c Center bypass oil passage 52c Parallel oil passage 55a Feeder oil passage 55b Check valve 55c Throttle (parallel throttle) 61, 62 Pressure sensor (second and third pressure sensors) 112d Hydraulic cylinder 112d (actuator or second actuator) A0 Prescribed minimum opening area A1 Operation target opening area A2 Corrected operation target opening area At Target opening area for control P0 Prescribed minimum discharge pressure Q0 Standby flow rate PLs Estimated load pressure

Claims

1. A hydraulic control system for a construction machine, comprising: a center bypass oil passage connected at its upstream side to a main pump and at its downstream side to a tank; a first directional control valve arranged in said center bypass oil passage; a plurality of directional control valves including a second directional control valve arranged in said center bypass oil passage downstream of the first directional control valve and connected in tandem to the first directional control valve so that pressure oil discharged from the main pump is preferentially supplied to the first directional control valve; and a controller, further comprising a bypass cut valve arranged between the first directional control valve and the second directional control valve in the center bypass oil passage and capable of proportionally changing an opening area from a predetermined minimum opening area to a maximum opening area, and the controller: a control valve for controlling a discharge flow rate of the main pump so that a discharge pressure of the main pump is equal to or higher than a predetermined minimum discharge pressure when all of the plurality of directional control valves arranged in the center bypass oil passage are in neutral positions, and a control valve for controlling an opening area of ​​the bypass cut valve so that the opening area of ​​the bypass cut valve increases as a spool stroke amount of the second directional control valve increases when the second directional control valve located downstream of the bypass cut valve is operated.

2. A hydraulic control system for construction machinery as described in claim 1, further comprising a plurality of operating devices including first and second operating devices which generate operating signals for operating the first and second directional control valves, respectively, wherein the controller presets the predetermined minimum opening area of ​​the bypass cut valve as a standby opening area when the plurality of operating devices are not being operated, calculates an operating target opening area of ​​the bypass cut valve based on the operating signal of the second operating device, selects the larger of the predetermined minimum opening area and the operating target opening area of ​​the bypass cut valve as a target opening area for control, and controls the opening area of ​​the bypass cut valve to become the target opening area for control.

3. A hydraulic control system for construction machinery as claimed in claim 1, further comprising: a plurality of operating devices including first and second operating devices which generate operating signals for operating the first and second directional control valves, respectively; and a first pressure sensor which detects the discharge pressure of the main pump, wherein the controller: presets the predetermined minimum discharge pressure of the main pump as a standby pressure when the plurality of operating devices are not being operated; calculates a required flow rate of the main pump based on the operating signals of the plurality of operating devices; calculates a standby flow rate of the main pump required to maintain the discharge pressure of the main pump at the predetermined minimum discharge pressure when the plurality of operating devices are not being operated, based on the discharge pressure of the main pump detected by the first pressure sensor and the predetermined minimum discharge pressure of the main pump; selects the larger of the required flow rate of the main pump and the standby flow rate of the main pump as a pump target flow rate, and controls the discharge flow rate of the main pump to reach the selected pump target flow rate.

4. A hydraulic control system for construction machinery as described in claim 1, wherein the controller estimates the load pressure of the hydraulic actuator driven by the second directional control valve, and controls the opening area of ​​the bypass cut valve so that it becomes smaller as the estimated load pressure decreases.

5. A hydraulic control system for construction machinery according to claim 4, further comprising a plurality of operating devices including first and second operating devices which generate operating signals for operating the first and second directional control valves, respectively, and a second pressure sensor and a third pressure sensor which detect a supply pressure and a discharge pressure, respectively, of a hydraulic actuator driven by the second directional control valve, wherein the controller: presets the predetermined minimum opening area of ​​the bypass cut valve as a standby opening area when the plurality of operating devices are not operated; calculates an operating target opening area of ​​the bypass cut valve based on the operating signal of the second operating device; estimates the load pressure of the hydraulic actuator driven by the second directional control valve based on the detection values ​​of the second and third pressure sensors; corrects the operating target opening area of ​​the bypass cut valve so as to become smaller as the estimated load pressure decreases; selects the larger of the predetermined minimum opening area and the corrected operating target opening area of ​​the bypass cut valve as a target opening area for control, and controls the opening area of ​​the bypass cut valve to become the target opening area for control.

6. A hydraulic control system for construction machinery as described in claim 1, wherein the second directional control valve, which is disposed downstream of the bypass cut valve and connected in tandem to the first directional control valve, is further connected in parallel to the first directional control valve to the main pump via a parallel oil line, the input port of the second directional control valve is connected to the parallel oil line via a feeder oil line equipped with a check valve for preventing backflow, and a fixed throttle is provided between the input port of the feeder oil line and the check valve.

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