Hydraulic control system for construction machine

Abstract

When the control lever is not operated, the discharge pressure of the main pump can be maintained at a pressure above a predetermined minimum pressure. Additionally, when the control lever is operated, it ensures that sufficient flow rate of pressure oil is supplied to the hydraulic actuator of the directional control valve, which is tandem-connected downstream of the bypass cut valve placed in the middle of the center bypass oil passage. Therefore, a bypass cut valve is placed between the first directional control valve and the second directional control valve in the center bypass oil passage. The controller maintains the opening area of the bypass cut valve at a predetermined minimum opening area when all the directional control valves are in the neutral position, and controls the discharge flow rate of the main pump such that the discharge pressure of the main pump is at a pressure equal to or greater than a predetermined minimum discharge pressure. Then, when the second directional control valve, located downstream of the bypass cut-off valve, is operated, the controller controls the increase in the opening area of the bypass cut-off valve as the spool stroke amount of the second directional control valve increases.

Description

Technical Field

[0001] The present invention relates to a hydraulic control system for construction machinery such as hydraulic excavators.Background Art

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

[0003] For example, in the hydraulic control system described in patent document 1, a bypass sequence valve is placed at the most upstream part of the center bypass oil passage, which is connected to the main pump and has plurality directional control valves placed. When the control lever is not operated, the solenoid of the bypass sequence valve is de-energized to set the bypass sequence valve to a no-load communication state, thereby releasing the pump discharge pressure to the tank line and reducing it to a low pressure. On the other hand, when the control lever is operated, the solenoid of the bypass sequence valve is energized to narrow the opening of the bypass sequence valve (throttle the center bypass oil passage), thereby maintaining the pressure (discharge pressure) of the pressure oil discharged from the main pump at a pressure above the pilot primary pressure (predetermined minimum pressure) of the pilot hydraulic source. Then, by supplying a part of the pressure oil to the reducing valve of the pilot hydraulic source, the pilot hydraulic source can be secured without using a pilot pump.

[0004] In the hydraulic control system described in patent document 2, a bypass valve is placed in a return oil passage (bypass oil passage) that branches from the pump discharge oil passage and communicates the pump discharge oil passage with the tank. Then, when all the control levers are in a non-operating standby state, the opening area of the bypass valve is controlled to a position throttled to a set value. This maintains the discharge pressure of the main pump at a pressure above a predetermined value when all control levers are not operated, and by applying back pressure to the main pump, it improves the responsiveness of the main pump when there is an operation input from the standby state and prevents damage to the sliding equipment of the main pump.Prior Art DocumentsPatent Documents

[0005] Patent document 1: Japanese Patent No. 3692004, Patent document 2: Japanese Patent No. 5622243 Summary of the InventionProblems to be Solved by the Invention

[0006] The hydraulic control system described in patent document 1 releases the pump discharge pressure to the tank line to lower the pressure by de-energizing the solenoid of the bypass sequence valve and setting the bypass sequence valve to a no-load communication state when the control lever is not operated. Therefore, when all control levers are not operated, back pressure cannot be applied to the main pump (as in patent document 2), and it is not possible to improve the responsiveness of the main pump when there is an operation input or to prevent damage to the sliding equipment of the main pump.

[0007] Also, in the hydraulic control system described in Patent Document 1, to maintain the discharge pressure of the main pump at a pressure above a predetermined minimum pressure when the control lever is operated, the bypass sequence valve (hereinafter referred to as the bypass cut valve) is placed at the most upstream part of the center bypass oil passage. However, due to constraints such as space within the valve block and ensuring the strength of the outer wall, there may be cases where it is desired to place the bypass cut valve not at the most upstream part of the center bypass oil passage, but in the middle of the center bypass oil passage (between adjacent directional control valves). In such cases, the inflow oil passage of the directional control valve tandem-connected downstream of the bypass cut valve is throttled, reducing the flow rate (supply amount) of pressure oil supplied from that directional control valve to the hydraulic actuator. As a result, there is a problem of the speed reduction of the hydraulic actuator.

[0008] The purpose of the present invention is to provide a hydraulic control system for construction machinery, in which a bypass cut valve is placed in the middle of the center bypass oil passage, maintaining the discharge pressure of the main pump at a pressure above a predetermined minimum pressure when the control lever is not operated, and supplying a sufficient flow rate of pressure oil to the hydraulic actuator from the directional control valve tandem-connected downstream of the bypass cut valve when the control lever is operated.Means for Solving the Problems

[0009] The present invention includes plurality means for solving the above problems, and an example is as follows.

[0010] That is, the hydraulic control system for construction machinery of the present invention, in order to solve the above problems, includes: a center bypass oil passage having an upstream side connected to a main pump and a downstream side connected to a tank; a plurality of directional control valves including a first directional control valve placed in the center bypass oil passage and a second directional control valve placed in a downstream of the first directional control valve in the center bypass oil passage and tandem-connected to the first directional control valve such that pressure oil discharged from the main pump is preferentially supplied to the first directional control valve; a controller. The hydraulic control system further comprises a bypass cut valve placed between the first directional control valve and the second directional control valve in the center bypass oil passage, the bypass cut valve configured to proportionally change an opening area from a predetermined minimum opening area to a maximum opening area. The controller is configured to maintains the opening area of the bypass cut valve at the predetermined minimum opening area and control the discharge flow rate of the main pump such that the discharge pressure of the main pump is equal to or greater than a predetermined minimum discharge pressure when all of the plurality of directional control valves placed in the center bypass oil passage are in a neutral position. The controller is configured to control the opening area of the bypass cut valve to increase as the spool stroke amount of the second directional control valve located downstream of the bypass cut valve increases when the second directional control valve is operated.

[0011] According to the present invention configured as above, in a hydraulic control system with a bypass cut valve placed in the middle of the center bypass oil passage, when all of the plurality directional control valves placed in the center bypass oil passage are in the neutral position, the opening area of the bypass cut valve is maintained at the predetermined minimum opening area, and the discharge flow rate of the main pump is controlled such that the discharge pressure of the main pump becomes a pressure above the predetermined minimum discharge pressure. This maintains the discharge pressure of the main pump at a pressure above a predetermined minimum pressure when the control lever is not 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 to increase as the spool stroke amount of the second directional control valve increases. As a result, when the control lever is operated, a sufficient flow rate of pressure oil can be supplied to the hydraulic actuator from the directional control valve tandem-connected downstream of the bypass cut valve.Advantages of the Invention

[0012] According to the present invention, in a hydraulic control system with a bypass cut valve placed in the middle of the center bypass oil passage, when the control lever is operated, a sufficient flow rate of pressure oil can be supplied to the hydraulic actuator from the directional control valve tandem-connected downstream of the bypass cut valve.Brief Description of the Drawings

[0013] [FIG. 1A] is a diagram showing a hydraulic excavator (construction machinery) according to the first embodiment of the present invention. [FIG. 1B] is a diagram showing the work tool portion of the front working machine of a hydraulic excavator equipped with a hydraulic grapple (attachment) instead of a bucket. [FIG. 2A] is a diagram showing the hydraulic circuit of the hydraulic control system for construction machinery according to the first embodiment of the present invention. [FIG. 2B] is a diagram showing the valve functions of the swing directional control valve, boom 3-directional control valve, and attachment 2-directional control valve with hydraulic symbols. [FIG. 3] is a diagram showing the control circuit portion of the hydraulic control system of the first embodiment. [FIG. 4A] is a diagram showing the control algorithm of the directional control valve of the controller. [FIG. 4B] is a diagram showing the control algorithm of the bypass cut valve of the controller. [FIG. 5] is a diagram showing the control algorithm of the main pump of the controller. [FIG. 6A] is a diagram showing the temporal changes of 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 control lever device for attachment 2 is operated alone. [FIG. 6B] is a diagram showing the temporal changes of 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 control lever device for attachment 2 is operated alone and then the control lever device for the boom is simultaneously operated in the boom raising direction. [FIG. 7] is a diagram showing the hydraulic circuit of the hydraulic control system for construction machinery according to the second embodiment of the present invention. [FIG. 8A] is a diagram showing the actuator load estimation means (load estimation algorithm) based on the detected value of the pressure sensor. [FIG. 8B] is a diagram showing the control algorithm of the bypass cut valve of the controller. Modes for Carrying out the Invention

[0014] Below, plurality embodiments of the present invention will be described with reference to the drawings.<First Embodiment>- Construction Machinery -

[0015] FIG. 1A illustrates a hydraulic excavator (construction machinery) according to the first embodiment of the present invention.

[0016] The hydraulic excavator comprises a lower travel body 100, an upper swing body 101, and a front work machine 102. The lower travel body 100 has right and left crawler travel devices 100a, 100b, and is driven by right and left travel motors 100c, 100b. The upper swing body 101 is mounted on the lower travel body 100 in a rotatable manner and is driven to swing by a swing motor 101a. The front work machine 102 is mounted on the front part of the upper swing body 101 in a manner that allows tilting. The upper swing body 101 is equipped with a cabin (operating room) 101b, which contains an operator's seat (not shown), left and right control lever devices 41a, 41b for front and swing operations installed at the front of the operator's seat (see FIG. 3), left and right control lever / pedal devices for travel installed on the front floor of the operator's seat (not shown), and control lever devices 41c, 41d for attachments 1 and 2 placed at appropriate locations near the operator's seat (see FIG. 3).

[0017] The front work machine 102 has a multi-joint structure comprising a boom 102a, an arm 102b, and a bucket 102c. The boom 102a rotates in the vertical direction by the extension and compression of the boom cylinder 112a, the arm 102b rotates in the vertical and longitudinal directions by the extension and compression of the arm cylinder 112b, and the bucket 102c rotates in the vertical and longitudinal directions by the extension and compression of the bucket cylinder 112c.

[0018] The front work machine 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 respective hydraulic actuators such as hydraulic cylinders or hydraulic motors (hereinafter simply referred to as actuators).

[0019] FIG. 1B illustrates the work tool portion of the front work machine 102 of a 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 opens and closes the claw 102d1 in the A1 and A2 directions by the hydraulic cylinder 112d, and rotates the claw 102d1 in the B1 and B2 directions by the hydraulic motor.- Hydraulic Circuit -

[0020] FIG. 2A illustrates the hydraulic circuit of the hydraulic control system for construction machinery 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, comprising a main pump (hydraulic pump) 20a, a main pump (hydraulic pump) 20b, and a main pump (hydraulic pump) 20c, respectively.

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

[0023] The hydraulic circuit HC2 includes a boom 2 direction control valve 25, an arm 1 direction control valve 26, an attachment 1 direction control valve 27, and a travel left direction control valve 28. These direction control valves 25 to 28 are connected in sequence from the upstream side of the main pump 20b to the center bypass oil passage 51b, which is connected to the pump oil passage of the main pump 20b on the upstream side and to the tank T via the return oil passage L2 on the downstream side. The boom 2 direction control valve 25, arm 1 direction control valve 26, and travel left direction control valve 28 each control the flow direction and flow rate of pressure oil supplied from the main pump 20b to the boom cylinder 112a, arm cylinder 112b, and left travel motor 100d, which are hydraulic actuators shown in FIG. 1, thereby driving these hydraulic actuators. The attachment 1 direction control valve 27 controls the flow direction and flow rate of the pressure oil supplied from the main pump 20b to the first hydraulic actuator of the attachment (when the attachment is the aforementioned hydraulic grapple 102d, for example, a hydraulic motor not shown) when the bucket 102c is replaced with an attachment other than the bucket, and drives the first hydraulic actuator. A throttle 33b for generating the minimum pump discharge pressure is formed in the return oil passage L2.

[0024] The hydraulic circuit HC3 includes a swing direction control valve 29, a boom 3 direction control valve 30, and an attachment 2 direction control valve 31. These direction control valves 29 to 31 are connected in sequence from the upstream side of the main pump 20c to the center bypass oil passage 51c, which is connected to the pump oil passage of the main pump 20c on the upstream side and to the tank T via the return oil passage L3 on the downstream side. The swing direction control valve 29 and boom 3 direction control valve 30 each control the flow direction and flow rate of pressure oil supplied from the main pump 20c to the swing motor 101a and boom cylinder 112a, which are hydraulic actuators shown in FIG. 1, thereby driving these hydraulic actuators. The attachment 2 direction control valve 31 controls the flow direction and flow rate of the pressure oil supplied from the main pump 20c to the second hydraulic actuator of the attachment (when the attachment is the aforementioned hydraulic grapple 102d, the hydraulic cylinder 112d) when the bucket 102c is replaced with an attachment, and drives the second hydraulic actuator.- Hydraulic Circuit HC3 -

[0025] The feature of the present invention is the hydraulic circuit HC3, and the hydraulic circuit HC3 will be described in detail below with reference to FIG. 2B. FIG. 2B illustrates the valve functions of the swing direction control valve 29, boom 3 direction control valve 30, and attachment 2 direction control valve 31 with hydraulic symbols.

[0026] In the hydraulic circuit HC3, the main pump 20c distributes its discharge oil to the direction control valves 29 to 31 through the center bypass oil passage 51c and the parallel oil passage 52c. The direction control valves 29 to 31 have the center bypass oil passage 51c passing through them, and when the direction control valves 29 to 31 are in neutral, the center bypass oil passage 51c is communicated with the return oil passage L3 to the tank T. When operating the direction control valves 29 to 31, the center bypass oil passage 51c is gradually throttled by the center bypass throttle according to the operation amount (spool stroke amount). When the direction control valves 29 to 31 are fully operated, the communication between the center bypass oil passage 51c and the return oil passage L3 is cut off. Additionally, the direction control valves 29 to 31 supply the discharge oil from the main pump 20c to the actuators according to the operation amount (spool stroke amount), and the discharge oil from the actuators is discharged to the tank via the return oil passage L3.

[0027] The swing direction control valve 29 is located at the most upstream position of the center bypass oil passage 51c, and the boom 3 direction control valve 30 is placed downstream of the swing direction control valve 29. The boom 3 direction control valve 30 is tandem-connected to the swing direction control valve 29 such that the pressure oil discharged from the main pump 20c is preferentially supplied to the upstream swing direction control valve 29. The attachment 2 direction control valve 31 is placed downstream of the boom 3 direction control valve 30 in the center bypass oil passage 51c, and is tandem-connected to the boom 3 direction control valve 30 such that the pressure oil discharged from the main pump 20c is preferentially supplied to the upstream boom 3 direction control valve 30.

[0028] Additionally, the swing directional control valve 29, boom 3 directional control valve 30, and attachment 2 directional control valve 31 are connected in parallel to the main pump 20c via a parallel oil passage 52c connected to the pump oil passage of the main pump 20c. The input port of the boom 3 directional control valve 30 is connected to the parallel oil passage 52c via a feeder oil passage 54a equipped with a check valve 54b for backflow prevention. Furthermore, a parallel throttle (fixed throttle) 54c with a predetermined opening area is formed between the input port of the boom 3 directional control valve 30 and the check valve 54b in the feeder oil passage 54a. The input port of the attachment 2 directional control valve 31 is connected to the parallel oil passage 52c via a feeder oil passage 55a equipped with a check valve 55b for backflow prevention. Furthermore, a parallel throttle (fixed throttle) 55c with a predetermined opening area is also formed between the input port of the attachment 2 directional control valve 31 and the check valve 55b in the feeder oil passage 55a.

[0029] Furthermore, as a feature of this embodiment, the hydraulic circuit HC3 is placed between the boom 3 directional control valve 30 (first directional control valve) and the attachment 2 directional control valve 31 (second directional control valve) of the center bypass oil passage 51c. The hydraulic circuit HC3 further includes a bypass cut valve 32, which can proportionally change the opening area from a predetermined minimum to a maximum opening area.

[0030] In this way, in the hydraulic circuit HC3 of this embodiment, a bypass cut valve 32 is placed between the directional control valves 30 and 31, allowing the center bypass oil passage 51c to be variably throttled. Additionally, 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 placed 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 through the center bypass oil passage 51c with low pressure loss. On the other hand, when either of the directional control valves 29 or 30 is operated, the center bypass oil passage 51c is throttled by these directional control valves 29 and 30, and the supply amount of pressure oil to the directional control valve 31 is suppressed by the parallel throttle 55c. As a result, in a tandem connection circuit where pressure oil is preferentially supplied to the so-called upstream directional control valve, the action of the parallel throttle 55c allows for the adjustment of the speed balance (operation balance) between each actuator.

[0031] Here, "tandem connection" refers to a configuration where, for example, the input port of the directional control valve 31 is connected to the center bypass oil passage 51c downstream of the directional control valve 30 via a feeder passage 31b equipped with a check valve 31a for backflow prevention. This refers to a connection form where the discharge oil from the main pump 20c is supplied to the directional control valve 31 from the center bypass oil passage 51c. 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 discharge oil from the main pump 20c is preferentially supplied to the upstream directional control valve 29 or directional control valve 30. And if the upstream boom 3 directional control valve 29 or the swing directional control valve 30 is operated at full stroke, the flow rate of pressure oil supplied to the directional control valve 31 from the main pump 20c becomes zero.

[0032] "Parallel connection" refers to a configuration where, like the directional control valves 29, 30, and 31, each feeder port is connected to the pump oil passage of the main pump 20c via the parallel oil passage 52c. This refers to a connection form where the discharge oil 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 do not have parallel throttles 54c and 55c formed, for example, when the directional control valves 29 and 31 are operated simultaneously, the discharge oil from the main pump 20c is preferentially supplied to the directional control valve of the actuator on the low load pressure side among the directional control valves 29 and 31. As a result, the majority of the pressure oil discharged from the main pump 20c is supplied to the directional control valve of the actuator on the low load side.

[0033] In the case where parallel throttles 54c and 55c are formed in the feeder oil passages 54a and 55a as in this embodiment, when the directional control valves 29 and 31 are operated simultaneously, pressure oil can be supplied to the downstream directional control valve 31 via the parallel throttle 55c. At the same time, even if the directional control valve 31 is on the low load side, the discharge pressure of the main pump 20c is increased by the parallel throttle 55c, allowing the discharge oil 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.- Pilot Hydraulic Source Circuit and Solenoid Valve Unit -

[0034] 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 guides a portion of the pressure oil discharged from the main pump 20c and reduces its pressure. Additionally, the pilot hydraulic source circuit 34 includes a pilot reducing valve 35 for generating a pilot primary pressure, a check valve 36 for maintaining the pilot primary pressure, and an accumulator 37 for smoothing the pilot primary pressure. By including the check valve 36 and the accumulator 37, the pilot hydraulic source circuit 34 can maintain the pilot primary pressure even if the discharge pressure of the main pump 20c temporarily falls below the setting pressure of the pilot reducing valve 35.

[0036] The solenoid valve unit 38 includes a solenoid proportional valve 38a, 38b, 38c, 38d, 38e, 38f, 38g that generates the control pilot pressure C1a, C1b, C2 a, C2b, C3a, C3b, D1 using the pilot primary pressure (main pressure) generated by the pilot reducing valve 35 of the pilot hydraulic source circuit 34, and plurality solenoid proportional valves (not shown) that generate the control pilot pressure A1a ~< 4a, A1b ~< A4b and control pilot pressure B1a ~< B4a, B1b ~< B4b using the same pilot primary pressure (main pressure).- Control System -

[0037] FIG. 3 illustrates the control circuit portion of the hydraulic control system of the first embodiment.

[0038] In FIG. 3, the hydraulic control system of this embodiment includes a controller 40, left and right control lever devices 41a, 41b for front and swing, a control lever device 41c for the first hydraulic actuator of the attachment, a control lever device 41d for the second hydraulic actuator of the attachment, and operation devices 41 including left and right control lever / pedal devices for travel (not shown), pressure sensors 42a, 42b for detecting the discharge pressures of the main pumps 20a, 20b respectively, and a pressure sensor 42c (first pressure sensor) for detecting the discharge pressure of the main pump 20c. The control lever devices 41a, 41b, 41c, 41d, and pedal devices of the operation device 41 are electric and output electrical operation signals respectively. Additionally, the control lever device 41a functions as an operation device (first operation device) that drives the swing directional control valve 29 when operated in one direction of the cross, and the control lever device 41b functions as an operation device that drives the boom 3 directional control valve 30 when operated in one direction of the cross. The control lever device 41c functions, for example, as an operation device (second operation device) for the attachment 2 directional control valve 31.

[0039] The controller 40 inputs the electrical operation signals output from the operation device 41, performs predetermined calculation processing, and outputs control signals IC1a, IC1b, IC2a, IC2b, IC3a, IC3b, ID1... to the solenoid proportional valves 38a ~< 38g... of the solenoid valve unit 38. The solenoid proportional valves 38a ~< 38g... of the solenoid valve unit 38 are driven by their control signals IC1a, IC1b, IC2a, IC2b, IC3a, IC3b, ID1..., and output the aforementioned operation pilot pressures A1a ~< 4a, A1b ~< A4b, operation pilot pressures B1a ~< B4a, B1b ~< 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 ~< 24, 25 ~< 27, 29 ~< 31 and the bypass cut valve 32, driving the corresponding directional control valves and the bypass cut valve 32.

[0040] Additionally, the controller 40 inputs the electrical operation signals output from the operation device 41 and the detection signals from the pressure sensors 42a, 42b, 42c. Then, the controller 40 performs the predetermined calculation processing and generates and outputs control signals (control currents) PC1, PC2, PC3 to drive the regulators 50a, 50b, 50c of the main pumps 20a, 20b, 20c, thereby controlling the discharge flow rate of the main pumps 20a, 20b, 20c.- Control Algorithm of Controller 40 -

[0041] Below, the control algorithm of the controller 40 related to the hydraulic circuit HC3, which is a feature of the present invention, will be explained.- Basic Concept of the Control Algorithm -

[0042] First, the basic concept of the control algorithm of the controller 40 will be explained. In the following description, the control lever devices 41a, 41b of the directional control valves 29, 30 are referred to as the first operation device, and the control lever device 41c of the directional control valve 31 is referred to as the second operation device, and the control lever devices 41a, 41b, 41c may be collectively referred to as plurality operation devices. (1) The controller 40 maintains the opening area of the bypass cut valve 32 at a predetermined minimum opening area A0 when all the directional control valves 29, 30, 31 placed in the center bypass oil passage 51c are in the neutral position (non-operation), and controls the discharge flow rate of the main pump 20c such that the discharge pressure of the main pump 20c becomes a pressure equal to or greater than the predetermined minimum discharge pressure P0. Then, when the directional control valve (second directional control valve) 31 located downstream of the bypass cut valve 32 is operated, the opening area of the bypass cut valve 32 is controlled to increase as the operation amount (spool stroke amount) of the directional control valve 31 increases. (2) The controller 40 pre-sets the predetermined minimum opening area A0 of the bypass cut valve 32 as the standby opening area during non-operation of the plurality operation devices 41a, 41b, 41c, and calculates the operation target opening area A1 of the bypass cut valve 32 based on the operation signal of the second control lever device 41c. Then, the controller 40 selects the larger of the predetermined minimum opening area A0 and the operation target opening area A1 of the bypass cut valve as the control target opening area At, and controls the opening area of the bypass cut valve 32 to become the selected control target opening area At. (3) The controller 40 pre-sets the predetermined minimum discharge pressure P0 of the main pump 20c as the standby pressure during non-operation of the plurality operation devices 41a, 41b, 41c, and calculates the demand flow rate Qreq of the main pump 20c based on the operation signals of the plurality control lever devices 41a, 41b, 41c. Additionally, the controller 40 calculates the standby flow rate Q0 of the main pump 20c required to maintain the discharge pressure of the main pump 20c at the predetermined minimum discharge pressure P0 during non-operation of the plurality operation devices 41a, 41b, 41c, 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 P0 of the main pump 20c. Then, the controller 40 selects the larger of the demand flow rate Qreq and the standby flow rate Q0 of the main pump 20c as the pump target flow rate Qt, and controls the discharge flow rate of the main pump 20c to become the selected pump target flow rate Qt.

[0043] The control algorithm of the controller 40 will be explained in further detail.<Directional Control Valve Control Algorithm>

[0044] FIG. 4A illustrates the control algorithm of the directional control valves 29, 30, 31 of the controller 40.

[0045] The controller 40 includes target operation pressure calculation tables T41, T42, a determination section 041, a converting section O42, a zero setting section O40, selection sections O43, O44, and control current calculation tables T43, T44.

[0046] The control algorithm shown in FIG. 4A is for the case where the directional control valve is the attachment 2 directional control valve 31.

[0047] Table T41 inputs the lever stroke amount (e.g., -100% to 100%) calculated from the operation signal of the control lever device 41c, and calculates the target operation pressure (target operation pressure of operation pilot pressure C3a) P3ta when the lever stroke amount is a positive value. The converting section O42 converts the lever stroke amount to a positive value when the lever stroke amount is negative, and table T42 calculates the target operation pressure (target operation pressure of operation pilot pressure C3b) P3tb from the lever stroke amount converted to a positive value when the lever stroke amount is negative.

[0048] The determination section 041 determines the sign of the lever stroke amount, and the selection sections O43, O44 select the target operation pressure P3ta or P3tb based on the determination result according to the operation direction of the control lever. That is, the selection sections O43, O44 select the target operation pressure P3ta or P3tb when the target operation pressure P3ta or P3tb input from the converting section 041, O42 is a positive value (TRUE), and select the zero target operation pressure set in the zero setting section O40 when it is a negative value (FALSE).

[0049] The control current calculation tables T43, T44 convert the target operation pressure P3ta or P3tb selected by the selection sections O43, O44 into the target current of the electromagnetic proportional valves 38e, 38f, and output the control currents IC3a, IC3b corresponding to the electromagnetic proportional valves 38e, 38f. Then, the operation pilot pressures C3a, C3b are generated by the electromagnetic proportional valves 38e, 38f, and the spool of the attachment 2 directional control valve 31 is controlled.

[0050] In the case where the directional control valve is the swing directional control valve 29, the boom 3 directional control valve 30, similarly, the target operation pressures P1ta or P1tb, P2ta or P2tb are calculated from the lever stroke amount, and the control currents ICla or IC1b and control currents IC2a or IC2b are output to the electromagnetic proportional valves 38a, 38b; 38c, 38d, thereby controlling the spools of the swing directional control valve 29, the boom 3 directional control valve 30.<Bypass Cut Valve Control>

[0051] FIG. 4B illustrates the control algorithm of the bypass cut valve 32 of the controller 40.

[0052] The controller 40 includes a minimum opening area setting section O47 for the bypass cut valve 32, opening area calculation tables T45, T46 for the bypass cut valve 32, maximum selection sections O45, O46, and a control current calculation table T47.

[0053] In the minimum opening area setting section O47, the predetermined minimum opening area A0 of the bypass cut valve 32 is pre-set as the standby opening area during non-operation of the plurality operation devices 41a, 41b, and 41c.

[0054] Table T45 converts the target operation pressure P3ta of the operation pilot pressure C3a of the attachment 2 directional control valve 31 selected by the selection section O43 in FIG. 4A into the operation opening area of the bypass cut valve 32. Table T46 converts the target operation pressure P3tb of the operation pilot pressure C3b of the attachment 2 directional control valve 31 selected by the selection section O44 in FIG. 4A into the operation opening area of the bypass cut valve 32.

[0055] The maximum selection section O45 selects the larger of the operation opening areas obtained from tables T45 and T46 as the operation target opening area A1 of the bypass cut valve 32. Then, the maximum selection section O46 selects the larger of the operation target opening area A1 and the predetermined minimum opening area A0 (standby opening area during non-operation) of the bypass cut valve 32 set in the minimum opening area setting section O47 as the control target opening area At of the bypass cut valve 32.

[0056] The control current calculation table T47 converts the control target opening area At selected by the maximum selection section O46 into the target current of the electromagnetic proportional valve 38d. Then, by outputting the control current ID1 corresponding to the electromagnetic proportional valve 38g, the electromagnetic proportional valve 38g generates the operation pilot pressure D1, and the spool of the bypass cut valve 32 is controlled.

[0057] The predetermined minimum opening area A0 (standby opening area during non-operation) of the bypass cut valve 32 is set to a size that can cause a pressure loss resulting in an appropriate setting pressure from the standby flow rate during non-operation of the main pump 20C, and can be calculated from the following Bernoulli's equation, for example.

[0058] In the following equation, A0 is the predetermined minimum opening area of the bypass cut valve 32, Q0 is the standby flow rate of the main pump 20c, i.e., the discharge flow rate when the main pump 20c is rotating at minimum volume (for example, when the minimum volume is 20cc / rev and prime mover speed is 1800rev / min, the discharge flow rate is 36L / min (= 20×1800 / 1000)), P0 is the predetermined minimum discharge pressure of the main pump 20c pre-set as the standby pressure (for example, if a supply pressure of 4MPa or more is required as a pilot hydraulic source, it is 4MPa), Pt is the hydraulic fluid pressure of the tank T (tank pressure), ρ is the hydraulic fluid density, and c is the flow coefficient. A 0 = Q 0 c 2 p 0 − p t / ρ

[0059] Since the prime mover speed changes according to the control state and operator command input, the standby flow rate Q0 also changes (for example, if the minimum volume is 20cc / rev and the speed change range is 800 to 2000rev / min, the standby flow rate Q0 is 16 to 40l / min).<Control Algorithm of the Main Pump>

[0060] FIG. 5 illustrates the control algorithm of the main pump 20c of the controller 40.

[0061] The controller 40 includes demand flow rate calculation tables T51a, T51b, T52a, T52b, T53a, T53b for the main pump 20c, a maximum selection section 051, a minimum pump discharge pressure setting section O55, a subtraction section O52, a PID calculation section O53, a maximum selection section O54, and a control current calculation table T48.

[0062] The demand flow rate calculation tables T51a, T51b, T52a, T52b, T53a, T53b of the main pump 20c input the target operation pressures P1ta, P1tb, P2ta, P2tb, P3ta, P3tb calculated from the lever stroke amount by the control algorithm of FIG. 4A and convert them into actuator demand flow rates.

[0063] The maximum selection section O51 selects the maximum value of the actuator demand flow rates converted from the target operation pressures P1ta, P1tb, P2ta, P2tb, P3ta, P3tb as the pump demand flow rate Qreq (positive control flow rate).

[0064] On the other hand, the minimum pump discharge pressure setting section O55 is pre-set with the predetermined minimum discharge pressure P0 of the main pump 20c as the standby pressure when plurality operation devices 41a, 41b, 41c are not operated. The subtraction section O52 calculates the discharge pressure deviation by subtracting the discharge pressure of the main pump 20c detected by the pressure sensor 42c from the predetermined minimum discharge pressure P0. The PID calculation section O53 performs feedback control calculation on the discharge pressure deviation and calculates the standby flow rate Q0 (pressure feedback flow) of the main pump 20c necessary to maintain the discharge pressure of the main pump 20c at the predetermined minimum discharge pressure P0. The maximum selection section O54 selects the larger of the pump demand flow rate Qreq selected by the maximum selection section 051 and the standby flow rate Q0 calculated by the PID calculation section 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 section O53 into the control current PC3 of the regulator 50c. Then, by outputting the control current PC3 to the regulator 50c, the volume of the main pump 20c is controlled such that the discharge flow rate of the main pump 20c becomes the pump target flow rate Qt.

[0065] The regulator 50c of the main pump 20c introduces the discharge pressure of the main pump 20c as the drive pressure into the pressure receiving chamber and adjusts the pump tilt angle (pump volume) with that drive pressure. This allows the regulator 50c of the main pump 20c to apply back pressure due to self-pressure, improving the responsiveness of the main pump when there is an operation input from the standby state and preventing damage to the sliding equipment of the main pump. A hydraulic pump equipped with a regulator that adjusts the pump capacity by applying back pressure due to self-pressure is described, for example, in JP 2019-65569 A.

[0066] If the pump 20c is a variable capacity pump and is driven to rotate by a prime mover (e.g., an electric motor), the target volume and target prime mover speed that satisfy the pump target flow rate may be calculated and output respectively.

[0067] The control algorithm for the main pumps 20a, 20b of the controller 40 is similar. That is, the throttles 33a, 33b formed in the return oil paths L1, L2 of the hydraulic circuits HC1, HC2 have the same opening area as the minimum opening area A0 of the bypass cut valve 32, and based on the operation pressure of the operation device, the pump target flow rate Qt is calculated by the algorithm shown in FIG. 5, and control currents PC1, PC2 are output to the regulators 50a, 50b. Thus, even in the hydraulic circuits HC1, HC2, when the operation device is not operated and all the directional control valves 21 to 24 or 25 to 28 are in the neutral position in the standby state, the discharge pressure of the main pumps 20a, 20b is controlled to be at a pressure near the predetermined minimum discharge pressure P0. Therefore, the responsiveness of the main pump when there is an operation input from the standby state is improved, and damage to the sliding equipment of the main pump is prevented.- Operation Example -

[0068] Next, an operation example according to this embodiment will be described with reference to FIGS. 6A and 6B.Operation Example 1

[0069] FIG. 6A is a diagram showing the temporal changes 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 control lever device 41c for the attachment 2 is operated alone.

[0070] In FIGS. 6A, (1) to (5) show the temporal changes of the following state quantities. (1) Operation signal of the control lever device 41c for the attachment (2) Target operation pressure P3ta of the attachment 2 directional 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 operation pressure P3ta of (2) (4) Target discharge flow rate Qt of the main pump 20c calculated from the operation signal of (1) and the detected value of the pressure sensor 42c (5) Discharge pressure of the main pump 20c that fluctuates due to the driving conditions of the main pump 20c and the bypass cut valve 32, and the actuator load In addition, the time periods (a), (b), and (c) on the horizontal axis indicate the following states. (a) Non-operation (b) Attachment 2 single operation (c) Non-operation

[0071] During non-operation in (a) and (c), the target opening area At of the bypass cut valve 32 is controlled to a size 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 the 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, 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 there is an operation input from the standby state, and preventing damage to the sliding equipment of the main pump 20c. Furthermore, the pressure of the pressure oil discharged from the main pump 20c (discharge pressure) is maintained at a pressure equal to or greater than the pilot primary pressure (predetermined minimum pressure) of the pilot hydraulic source circuit 34. This allows a part of the pressure oil to be applied to the reducing valve 35 of the pilot hydraulic source circuit 34, ensuring a pilot hydraulic source without using a pilot pump, even when all control levers are not operated and all directional control valves are in the neutral position.

[0072] During the single operation of the attachment in (b), the opening area of the bypass cut valve 32 increases due to the operation signal of the control lever device 41c for the attachment. This prevents a decrease in the flow rate of the pressure oil by the bypass cut valve 32, and sufficient flow rate of pressure oil is supplied to the hydraulic cylinder 112d of the attachment 102d with minimal pressure loss.Operation Example 2

[0073] FIG. 6B is a diagram showing the temporal changes 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 control lever device 41c for the attachment 2 is operated alone and then the control lever device 41b for the boom is simultaneously operated in the boom raising direction.

[0074] In (1) to (5) of FIG. 6B, the temporal changes of the state quantities related to the operation of the control lever device 41c for the attachment are shown by dotted lines.

[0075] Additionally, the time periods on the horizontal axis (a), (b), (c), (d), (d) indicate the following states. (a) Non-operation (b) Attachment 2 single operation (c) Attachment 2 and boom raising combined operation (d) Boom raising single operation (e) Non-operation

[0076] In this operational example, the load pressure of the boom cylinder is higher than the load pressure of the hydraulic cylinder 112d of attachment 102d, but the present invention is not limited to this.

[0077] In this operational example as well, during non-operation in (a) and (e), the target opening area At of the bypass cut valve 32 is controlled to a size 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 the predetermined minimum discharge pressure P0 (standby pressure).

[0078] During the single operation of attachment 2 in (b), the opening area of the bypass cut valve 32 increases due to the operation signal from the control lever device 41c for the attachment. This prevents a decrease in the flow rate of pressure oil by the bypass cut valve 32, and ensures that sufficient flow rate of pressure oil is supplied to the hydraulic cylinder 112d of attachment 102d with minimal pressure loss.

[0079] During the combined operation of attachment 2 and boom raising in (c), when the boom 3 directional control valve 30 is actuated by the boom raising operation, the center bypass oil passage 51c is cut off. At this time, pressure oil is supplied to the downstream directional control valve 31 through the parallel throttle 55c of the parallel oil passage 52c. Simultaneously, even if the directional control valve 31 is on the low load side, the discharge pressure of the main pump 20c increases due to the parallel throttle 55c, allowing the discharge oil of the main pump 20c to be supplied to the directional control valve 29 of the high load side actuator, thereby driving the high load side actuator. Thus, in a tandem connection circuit where pressure oil is preferentially supplied to the upstream directional control valve, the action of the parallel throttle 55c allows for the adjustment of the speed balance (operation balance) between each actuator.

[0080] During the single operation of boom raising in (d), pressure oil is supplied only to the boom 3 directional control valve 30, increasing the boom speed. Additionally, since the load pressure of the boom cylinder 112a is higher than the load pressure of the hydraulic cylinder 112d of attachment 102d, the discharge pressure of the main pump 20c also slightly increases.

[0081] Thus, in this operational example as well, in a hydraulic control system with a bypass cut valve 32 placed in the middle of the center bypass oil passage 51c, the discharge pressure of the main pump 20c is maintained at a pressure equal to or greater than the predetermined minimum discharge pressure P0 during non-operation of the control lever, and during operation of the control lever, a decrease in the flow rate of pressure oil by the bypass cut valve 32 is prevented. As a result, sufficient flow rate of pressure oil can be supplied to the hydraulic cylinder 112d of the directional control valve 31 tandem-connected downstream of the bypass cut valve 32.

[0082] Additionally, when the directional control valve 29 and the directional control valve 41 are operated simultaneously, in a tandem connection circuit where pressure oil is preferentially supplied to the upstream directional control valve, the action of the parallel throttle 55c allows for the adjustment of the speed balance (operation balance) between each actuator.- Effects -

[0083] 1. According to this embodiment, in a hydraulic control system with a bypass cut valve 32 placed in the middle of the center bypass oil passage 51c, the discharge pressure of the main pump 20c is maintained at a pressure equal to or greater than the predetermined minimum discharge pressure P0 during non-operation of all control levers. Thus, even in a standby state where all directional control valves are in a neutral position, back pressure is applied to the regulator 50c of the main pump 20c, improving the responsiveness of the main pump 20c, including the regulator 50c, and preventing damage to the sliding equipment of the main pump 20c, when there is an operation input from the standby state, . Additionally, even in a 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 by applying a portion of the pressure oil to the reducing valve 35 of the pilot hydraulic source circuit 34, a pilot hydraulic source can be secured without using a pilot pump. 2. Additionally, during operation of the control lever, a decrease in the flow rate of pressure oil by the bypass cut valve 32 is prevented, and sufficient flow rate of pressure oil can be supplied to the hydraulic cylinder 112d of the directional control valve 31 tandem-connected downstream of the bypass cut valve 32. 3. When the directional control valve 29 and the directional control valve 31 are operated simultaneously, in a tandem connection circuit where pressure oil is preferentially supplied to the upstream directional control valve, the action of the parallel throttle 55c allows for the adjustment of the speed balance (operation balance) between each actuator. <Second Embodiment>

[0084] FIG. 7 illustrates the hydraulic circuit of a hydraulic control system for construction machinery according to the second embodiment of the present invention.

[0085] In FIG. 7, the hydraulic control system of this embodiment further includes pressure sensors 61, 62 (second pressure sensor and third pressure sensor) for detecting the supply pressure, which is the load pressure of the hydraulic actuator (hydraulic cylinder of hydraulic grapple 102d) driven by the directional control valve 31 (second directional control valve) of the hydraulic circuit HC3.

[0086] As shown by the dotted line in FIG. 3, the controller 40 inputs the detection signals from the pressure sensors 61, 62 to estimate 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 to decrease as the estimated load pressure decreases.

[0087] 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), and corrects the operation target opening area A1 of the bypass cut valve 32 to decrease as the estimated load pressure decreases. Then, the controller 40 selects the larger of the predetermined minimum opening area A0 and the corrected operation target opening area A2 of the bypass cut valve 32 as the target opening area At for control, and controls the opening area of the bypass cut valve 32 to achieve the target opening area At for control.

[0088] FIG. 8A illustrates the actuator load estimation means (load estimation algorithm) based on the detection values of the pressure sensors 61, 62.

[0089] The controller 40 includes a zero setting section O60, a determination section 061, a selection section O62, a determination section O63, and a selection section O64.

[0090] The determination section 061 determines whether the load pressure of the hydraulic cylinder 112d detected by the pressure sensor 61 is greater than the threshold th. The selection section O62 selects the load pressure if the load pressure of the hydraulic cylinder 112d is greater than the threshold th (TRUE), and selects the zero load pressure set in the zero setting section O60 if it is less than or equal to the threshold th (FALSE).

[0091] Additionally, the determination section O63 determines whether the load pressure of the hydraulic cylinder 112d detected by the pressure sensor 62 is greater than the threshold th. The selection section O64 selects the load pressure as the load pressure estimation value PLs if the load pressure of the hydraulic cylinder 112d is greater than the threshold th (TRUE), and selects the load pressure selected by the selection section O62 as the load pressure estimation value PLs if it is less than or equal to the threshold th (FALSE).

[0092] The threshold th is a value near the minimum pressure of the meter-in when the discharge oil from the main pump 20c is supplied to the hydraulic cylinder 112d, and is pre-set in the controller 40.

[0093] FIG. 8B illustrates the control algorithm of the bypass cut valve 32 of the controller 40.

[0094] The controller 40, similar to the control algorithm in the first embodiment shown in FIG. 4B, includes a minimum opening area setting section O47 for the bypass cut valve 32, opening area calculation tables T45, T46 for the bypass cut valve 32, maximum selection sections O45, O46, and a control current calculation table T47.

[0095] Additionally, the controller 40 includes an opening correction gain calculation table T81 and a multiplication section 081.

[0096] Table T81 converts the estimated load pressure PLs of the hydraulic cylinder 112d, calculated by the control algorithm in FIG. 8A, into an opening correction gain Kc. In table T81, for example, when the estimated load pressure PLs of the hydraulic cylinder 112d is small (e.g., near the threshold th), Kc is approximately 0.5, and as the estimated load pressure PLs increases to near the intermediate pressure, the relationship between the estimated load pressure PLs and the opening correction gain Kc is set such that Kc increases to a maximum of 1.

[0097] The multiplication section 081 multiplies the operation target opening area A1 of the bypass cut valve 32, selected by the maximum selection section O45, by the opening correction gain Kc to calculate the corrected operation target opening area A2. The maximum selection section O46 selects the larger of the corrected operation target opening area A2 and the predetermined minimum opening area A0 of the bypass cut valve 32, set in the minimum opening area setting section O47, as the target opening area At for controlling the bypass cut valve 32.

[0098] The actuator load estimation means shown in FIG. 8A estimates the actuator load by detecting the load pressure of the hydraulic cylinder 112d, but it may also be estimated by calculating the actuator load from the self-weight and posture information of the attachment 102d, for example. Additionally, by determining the actuator load in advance according to the type of hydraulic actuator, it may be uniquely set.

[0099] According to this embodiment, the following effects are obtained.

[0100] When the bypass cut valve 32 is opened during the operation of the directional control valve 31, which is tandem-connected downstream of the bypass cut valve 32 in the center bypass oil passage 51c, the hydraulic cylinder 112d is driven by the self-weight of the load. Then, when the load pressure decreases, there is a risk that the discharge pressure of the main pump 20c will drop sharply, making it impossible to maintain the minimum required discharge pressure. On the other hand, if the opening degree of the bypass cut valve 32 is restricted to maintain the discharge pressure of the main pump 20c, there may be a shortage in the supply of hydraulic oil to the hydraulic cylinder 112d.

[0101] In this embodiment, by estimating the actuator load pressure and adjusting the opening degree of the bypass cut valve 32 according to the load pressure, the opening degree of the bypass cut valve 32 can be adjusted to an appropriate value according to the actuator load conditions. That is, even when the actuator load pressure may become extremely low, it is possible to maintain the discharge pressure of the main pump 20c and to ensure the supply of pressure oil to the hydraulic cylinder 112d by the directional control valve 31, which is tandem-connected downstream of the bypass cut valve 32.<Others>

[0102] In the above embodiments, the hydraulic circuit HC3 is assumed to have a swing directional control valve 29, a boom 3 directional control valve 30, and an attachment 2 directional control valve 31, but the types of directional control valves are not limited to these. Additionally, although the attachment 2 directional control valve 31 is connected downstream of the bypass cut valve 32, other directional control valves may be connected, and plurality directional control valves may be connected. When plurality directional control valves are connected downstream of the bypass cut valve 32, and the operation device of each directional control valve is operated, the opening area of the bypass cut valve 32 is controlled to increase as the operation amount (spool stroke amount) of the directional control valve increases. This allows for the supply of pressure oil with sufficient flow rate to the hydraulic actuators of each directional control valve with minimal pressure loss.Description of Reference Characters

[0103] 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 source circuit, 38: Solenoid valve unit, 40: Controller, 41: Operation device, 41a,41b,41c,41d: Control 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, third pressure sensor), 112d: Hydraulic cylinder (actuator or second actuator), A0: Predetermined minimum opening area, A1: Operation target opening area, A2: Corrected operation target opening area, At: Control target opening area, P0: Predetermined minimum discharge pressure, Q0: Standby flow rate, PLs: Estimated load pressure

Claims

1. A hydraulic control system for construction machinery comprising: a center bypass oil passage having an upstream side connected to a main pump and a downstream side connected to a tank; a plurality of directional control valves including a first directional control valve placed in the center bypass oil passage and a second directional control valve placed in a downstream of the first directional control valve in the center bypass oil passage and tandem-connected to the first directional control valve such that pressure oil discharged from the main pump is preferentially supplied to the first directional control valve; a controller; and a bypass cut valve placed between the first directional control valve and the second directional control valve in the center bypass oil passage, the bypass cut valve configured to proportionally change an opening area from a predetermined minimum opening area to a maximum opening area, wherein the controller is configured to maintain the opening area of the bypass cut valve at the predetermined minimum opening area and control the discharge flow rate of the main pump such that the discharge pressure of the main pump is equal to or greater than a predetermined minimum discharge pressure when all of the plurality of directional control valves placed in the center bypass oil passage are in a neutral position, and the controller is configured to control the opening area of the bypass cut valve to increase as the spool stroke amount of the second directional control valve located downstream of the bypass cut valve increases when the second directional control valve is operated.

2. The hydraulic control system for construction machinery according to claim 1,further comprising a plurality of operation devices including first and second operation devices for generating operation signals to operate the first and second directional control valves, respectively, wherein the controller is configured to: pre-set the predetermined minimum opening area of the bypass cut valve as a standby opening area when the plurality of operation devices are not operated; calculate an operation target opening area of the bypass cut valve based on an operation signal of the second operation device; select the larger of the predetermined minimum opening area and the operation target opening area of the bypass cut valve as a control target opening area; and control the opening area of the bypass cut valve to become the control target opening area.

3. The hydraulic control system for construction machinery according to claim 1, further comprising: a plurality of operation devices including first and second operation devices for generating operation signals to operate the first and second directional control valves, respectively; and a first pressure sensor for detecting a discharge pressure of the main pump, wherein the controller is configured to: pre-set the predetermined minimum discharge pressure of the main pump as a standby pressure when the plurality of operation devices are not operated; calculate a demand flow rate of the main pump based on operation signals of the plurality of operation devices; calculate 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 operation devices are not 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; select the larger of the demand flow rate of the main pump and the standby flow rate of the main pump as a pump target flow rate; and control the discharge flow rate of the main pump to become the selected pump target flow rate.

4. The hydraulic control system for construction machinery according to claim 1, wherein the controller is configured to estimate a load pressure of a hydraulic actuator driven by the second directional control valve and control the opening area of the bypass cut valve to decrease as the estimated load pressure decreases.

5. The hydraulic control system for construction machinery according to claim 4, further comprising: a plurality of operation devices including first and second operation devices for generating operation signals to operate the first and second directional control valves, respectively; and second and third pressure sensors for detecting a supply pressure and a discharge pressure of a hydraulic actuator driven by the second directional control valve, respectively, wherein the controller is configured to: pre-set the predetermined minimum opening area of the bypass cut valve as a standby opening area when the plurality of operation devices are not operated; calculate an operation target opening area of the bypass cut valve based on an operation signal of the second operation device; estimate a load pressure of a hydraulic actuator driven by the second directional control valve based on detection values of the second and third pressure sensors; correct an operation target opening area of the bypass cut valve such that the operation target opening area of the bypass cut valve decreases as the estimated load pressure decreases; select the larger of the predetermined minimum opening area and the corrected operation target opening area of the bypass cut valve as a control target opening area; and control the opening area of the bypass cut valve to become the control target opening area.

6. The hydraulic control system for construction machinery according to claim 1, wherein the second directional control valve is placed downstream of the bypass cut valve and tandem-connected to the first directional control valve and connected in parallel to the main pump via a parallel oil passage, an input port of the second directional control valve is connected to the parallel oil passage via a feeder oil passage installed with a check valve for preventing backflow, and a fixed throttle is formed between the input port of the feeder oil passage and the check valve.

Images