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BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a hydraulic control system for controlling a hydraulic actuator for a hydraulic excavator.  
         [0003]     2. Description of the Related Art  
         [0004]     According to a conventional method for performing a leveling work with use of a hydraulic excavator, a front end of a bucket is brought into contact with the ground while a boom and an arm are extended to a maximum and then boom raising operation and arm pulling operation are performed simultaneously to pull the bucket toward the excavator.  
         [0005]     In the leveling work, when the arm assumes a nearly vertical attitude, a peripheral speed of the bucket front end becomes low relative to a cylinder speed of an arm cylinder, so there sometimes occurs a case where the cylinder speed of the arm cylinder is insufficient when the leveling efficiency is to be improved.  
         [0006]     There is known a hydraulic excavator capable of solving the problem of delay in the boom speed which occurs upon simultaneous execution of the boom raising operation and the arm pulling operation (see, for example, Japanese Patent Laid-Open No. Hei 10-102547).  
         [0007]     This known hydraulic excavator is configured in such a manner that the cylinder speed of a boom cylinder which is set for a fine operation taking the lifting work into account can be increased in the leveling work so as to permit operation at a speed matching a high arm speed.  
         [0008]     In this hydraulic excavator, the boom whose speed is set for the fine operation can be actuated while allowing its speed to match the arm speed in the leveling work, but the excavator is not so configured as to permit an increase of the arm cylinder speed in the case where only the arm pulling operation is performed in the nearly vertical attitude of the arm.  
       SUMMARY OF THE INVENTION  
       [0009]     It is an object of the present invention to provide a hydraulic control system in an hydraulic excavator wherein a boom cylinder and an arm cylinder can be operated each independently when boom raising operation and arm pulling operation are performed simultaneously in a leveling work, while when the arm pulling operation alone is performed, the speed of the arm cylinder can be increased to improve the leveling work efficiency.  
         [0010]     The hydraulic control system for a hydraulic excavator according to the present invention comprises, as a basic configuration thereof, a boom control valve adapted to provide pressure oil from a first hydraulic pump with a boom cylinder in accordance with operation of a boom operation means, the boom control valve being disposed in a first oil path, an boom control valve adapted to provide pressure oil from a second hydraulic pump with an arm cylinder in accordance with operation of an arm operation means, the arm control valve being disposed in a second oil path, a confluence switching valve adapted to switch between a confluence position for joining the pressure oil from the first and second oil paths and a confluence stop position for stopping the joining, a flow control valve disposed in a return oil path for returning the pressure oil present in the first oil path to a tank, and a control means for controlling the confluence switching valve and the flow control valve. The control means is configured so as to switch the confluence switching valve to the confluence position and to make the flow control valve in a closed position when the arm operating means is operated independently and so as to switch the confluence switching valve to the confluence stop position and to make the flow control valve in an opened position when arm operating means and boom operating means are operated substantially simultaneously.  
         [0011]     According to this configuration, by an automatic switching of the confluence switching valve and the flow control valve, when boom raising and arm pulling operations are performed simultaneously, the respective cylinders of boom and arm can be operated each independently, and in case of performing the arm pulling operation alone, it is possible to increase the speed of the arm cylinder. Consequently, for example when the arm assumes a nearly vertical attitude and the arm speed becomes insufficient in a leveling work, it is possible to increase the flow rate of the pressure oil fed to the arm cylinder. As a result, the problem that the peripheral speed at a front end of a front attachment becomes low is solved and it is possible to improve the leveling work efficiency. 
     
    
     BREIF DESCRIPTION OF THE DRAWINGS  
       [0012]      FIG. 1  is a hydraulic circuit diagram according to a first embodiment of the present invention;  
         [0013]      FIG. 2  is a hydraulic circuit diagram according to a second embodiment of the present invention; and  
         [0014]      FIG. 3  is a flow chart showing contents of control made by a controller shown in  FIG. 2 . 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0015]     The present invention will be described in detail hereinunder by way of embodiments thereof illustrated in the drawings.  
         [0016]      FIG. 1  illustrates a hydraulic control circuit (hydraulic control system) in a hydraulic excavator according to a first embodiment of the present invention.  
         [0017]     The hydraulic excavator, though not shown, comprises a lower traveling body and an upper rotating body mounted rotatably on the lower traveling body, with a front attachment being attached to a front portion of the upper rotating body. The front attachment comprises a boom, an arm and a bucket.  
         [0018]     In the same figure, a first hydraulic pump  2 , a second hydraulic pump  3  and a pilot pump  4  for producing a pilot pressure Pa are actuated by operation of an engine  1 .  
         [0019]     The first and second hydraulic pumps  2 ,  3  are variable capacity pumps. A discharge flow rate in each of the pumps varies in accordance with an inclination angle of a swash plate.  
         [0020]     Pressure oil discharged from the first and second hydraulic pumps  2 ,  3  is fed to a right traveling motor control valve  5 , a bucket cylinder control valve  6  and a boom cylinder control valve (boom control valve)  7  which are disposed in a left center bypass line (first oil path) LCB and is also fed to a left traveling motor control valve  8 , a swing motor control valve  9  and an arm cylinder control valve (arm control valve)  10  which are disposed in a right center bypass line (second oil path) RCB.  
         [0021]     A right traveling motor  11 , a bucket cylinder  12 , and a boom cylinder  13 , are connected to the right traveling motor control valve  5 , the bucket cylinder control valve  6 , and the boom cylinder control valve  7 , respectively, and pressure oil is fed to the respective actuators through the control valves.  
         [0022]     Likewise, a left traveling motor  14 , a swing motor  15 , and an arm cylinder  16 , are connected to the left traveling motor control valve  8 , the swing motor control valve  9 , and the arm cylinder control valve  10 , respectively.  
         [0023]     A straight traveling valve (confluence switching valve)  17  is disposed in an upstream oil path L 1  formed on the upstream of the right traveling motor control valve  5 . When a leveling work to be described later is not performed, the straight traveling valve  17  is switched so as to ensure straight traveling stability as a conventional function.  
         [0024]     More specifically, the straight traveling valve  17  has a flow dividing position a as a confluence stop position and a confluence position b and is normally held at the flow dividing position a. At the flow dividing position a, pressure oil discharged from the first hydraulic pump  2  is fed to the left center bypass line LCB through the oil path L 1 , while pressure oil discharged from the second hydraulic pump  3  is fed to the right center bypass line RCB through an oil path L 2 .  
         [0025]     In this case, the pressure oil from the first hydraulic pump  2  and the pressure oil from the second hydraulic pump  3  are fed each independently to the right traveling motor control valve  5  and the left traveling motor control valve  8 .  
         [0026]     Next, when the straight traveling valve  17  is switched from the flow dividing position a to the confluence position b, the pressure oil from the first hydraulic pump  2  is fed through an oil path L 3 . At a connecting point P, this oil path L 3  is connected with an oil path L 4  divided from a downstream side of the left traveling motor control valve  8  in the RCB. And the pressure oil is provided with the swing motor control valve  9  and the arm control valve  10  through an oil path L 5  extended from the connecting point P. Apart of the pressure oil flowing in the path L 3  is also able to be provided with the bucket cylinder control valve  6  and the boom cylinder control valve  7 .  
         [0027]     The pressure oil from the second hydraulic pump  3  is divided to flows in parallel through the oil paths L 1  and L 2  and is fed and distributed to the left and right traveling motor control valves  8 ,  5 . As a result, even when a composite operation is performed such as, for example, a boom hoisting operation under operation of the right and left traveling motors  11 ,  14 , the pressure oil from the second hydraulic pump  3  is fed equally to right and left traveling motors  11 ,  14 , whereby the straight traveling stability can be ensured.  
         [0028]     A left cut-off valve (flow control valve)  18  is disposed on the downstream (return oil path) side of the boom cylinder control valve  7  in the left center bypass line LCB and a right cut-off valve  19  is disposed on the downstream side of the arm cylinder control valve  10  in the right center bypass line RCB.  
         [0029]     Next, switching control system of the straight traveling valve  17  and the cut-off valve  18  will be explained hereafter. To the circuit in  FIGS. 1 and 2 , a bucket operating remote control valve  20  with a bucket operating lever  20   a , a boom operating remote control valve (boom operating means)  22  with a boom operating lever  22   a , and an arm operating lever  25   a  are connected. Those remote control valves  20 ,  22 ,  25  output a pilot pressure according to an operating direction and an operating amount of each of those operating levers  20   a ,  22   a ,  25   a.    
         [0030]     Pilot pressures P 1  and P 2  outputted from the bucket operating remote control valve  20  are provided to respective pilot ports in the bucket cylinder control valve  6 . Either the pilot pressure P 1  or P 2  is selected by a high-order selection which is made by a shuttle valve  21 . The pilot pressure P 1  (or P 2 ) thus selected by the high-order selection and a boom raising pilot pressure P 3  outputted from the boom operating remote control valve  22  are further subjected to a high-order selection by a shuttle valve (detecting means for detecting a boom raising operation pressure)  23 . That is, the shuttle valves  21  and  23  are adapted to detect a composite operation.  
         [0031]     The pilot pressure selected by the shuttle valve  23  is provided to a pilot port of a flow control valve (control means)  24 . The flow control valve  24  is adapted to be switched between a cut-off position c and a communicating position d and is normally held at the communicating position d.  
         [0032]     A portion of an arm pulling pilot pressure P 4 , which is outputted from the arm operating remote control valve  25  by operation of an arm operating lever (arm operating means)  25   a , is provided to the flow control valve  24 . The arm pulling pilot pressure P 4  outgoing from the flow control valve  24  is provided to a pilot port of the straight traveling valve  17  through an oil path L 6  and a shuttle valve  26 . The shuttle valve  26  makes a high-order selection out of a pilot pressure based on simultaneous operation of traveling operation and attachment operation and the arm pulling pilot pressure P 4 .  
         [0033]     An oil path L 7  which branches from the oil path L 6  is connected to a pilot port of the left cut-off valve  18  through a shuttle valve  27 . The shuttle valve  27  makes a high-order selection out of an operating pressure other than the arm pulling pilot pressure (e.g., arm pushing pilot pressure) and the arm pulling pilot pressure P 4 .  
         [0034]     Numeral  28  in the figure denotes a return oil tank.  
         [0035]     In this embodiment, the straight traveling valve  17  is used as a confluence switching valve. The straight traveling valve  17  makes switching between the flow dividing position a (first switching position) in which the pressure oil from the first hydraulic pump  2  and the pressure oil from the second hydraulic pump  3  are fed each independently to the LCB (first oil path) and RCB (second oil path) and thence to the traveling motor  11  disposed in the LCB as the first oil path and the traveling motor  14  disposed in the RCB as the second oil path, and the confluence position b (second switching position) in which the pressure oil from either the first hydraulic pump  2  or the second hydraulic pump  3  is distributed to the traveling motors  11  and  14 . In this case, the effect of the present invention can be exhibited without any great design alteration of the existing circuit.  
         [0036]     The following description is now provided about the operation of the hydraulic control system described above.  
         [0037]     When the arm pulling operation alone is performed in the leveling work or the like or when the arm pulling operation alone is performed as a result of the boom raising operation being stopped during simultaneously simultaneous operation of boom raising and arm pulling operations, a pilot pressure is not provided from the bucket operating lever  20   a  or the boom operating lever (boom operating means)  22   a  to the pilot port of the flow control valve  24 . Consequently, a portion of the pilot pressure P 4  flows in the oil paths L 6  and L 7  through the communicating position d of the flow control valve  24 .  
         [0038]     In this case, the pilot pressure P 4  is provided to the pilot port of the straight traveling valve  17  through the oil path L 6  and is also provided to the pilot port of the left cut-off valve  18  through the oil path L 7 , whereby the straight traveling valve  17  switches from the flow dividing position a to the confluence position b and the left cut-off valve  18  switches from a communicating position e to a cut-off position f.  
         [0039]     When the straight traveling valve  17  switches to the confluence position b, the pressure oil from the first hydraulic pump  2  is fed to the arm cylinder control valve  10  through the oil paths L 3  to L 5 . On the other hand, the pressure oil from the second hydraulic pump  3  flows through the oil paths L 2  to L 4  because the left cut-off valve  18  in LCB is closed, and in the oil path L 5  it joins the pressure oil from the first hydraulic pump  2 .  
         [0040]     Consequently, when the arm pulling operation alone is performed, the flow rate of pressure oil fed to the arm cylinder control valve  10  increases and it is possible to increase the cylinder speed of the arm cylinder  16 .  
         [0041]     On the other hand, when the boom raising and arm pulling operations are performed substantially simultaneously, the boom raising pilot pressure P 3  is provided to the pilot port of the flow control valve  24  through the shuttle valve  23  and the flow control valve  24  is switched from the communicating position d to the cut-off position c.  
         [0042]     When bucket excavation and bucket release are operated in the leveling work, the flow control valve  24  also switches from the communicating portion d to the cut-off position c upon receipt of the pilot pressure P 1  (or P 2 ).  
         [0043]     When the arm operating remote control valve  25  and the oil path L 6  are cut off, a pilot pressure is not developed in the oil paths L 6  and L 7 , the straight traveling valve  17  switches from the confluence position b to the flow dividing position a and the left cut-off valve  18  switches from the cut-off position f to the communicating position e.  
         [0044]     In this case, therefore, the pressure oil from the first hydraulic pump  2  and the pressure oil from the second hydraulic pump  3  do not join, but as the pressure oil from the first hydraulic pump  2  is fed to the boom cylinder control valve  7  and the pressure oil from the second hydraulic pump  3  is fed to the arm cylinder control valve  10 , whereby operating pressures for valves can be ensured.  
         [0045]     Of course, the straight traveling valve  17  and the left cut-off valve  18  are configured so as to be switched by ON-OFF operation (opening and closing operation). The straight traveling valve  17  and the left cut-off valve  18  are also configured so as to switch gradually in proportion to the operation amount of the boom operating lever  22   a , instead of by ON-OFF operation. Therefore, the confluence and flow division can be switched from one to the other without causing any shock.  
         [0046]     Thus, it is preferable that the straight traveling valve  17  and the left cut-off valve  18  be configured so as to switch in proportion to the operation amount of the boom operating lever  22   a . According to this configuration, in the arm pulling operation, the straight traveling valve  17  and the left cut-off valve  18  are switched gradually without any shock.  
         [0047]     When the boom raising and arm pulling operations are performed simultaneously, the straight traveling valve  17  is switched to the flow dividing side and the left cut-off valve  18  is opened as the operation amount of the boom operating lever  22   a  increases, and thus a shock-free operation is ensured.  
         [0048]     As to the pump flow rate on the confluence side, in the case of a negative control device, a negative control pressure drops and the flow rate increases upon closure of the left cut-off valve  18 , while in the case of a positive control valve and if control is to be made hydraulically, the pump flow rate of the first hydraulic pump  2  can be increased with the pilot pressure which is for switching the straight traveling valve  17  and the left cut-off valve  18 .  
         [0049]     The afore-mentioned flow control valve  24  and the shuttle valve  26  are constituted as a control means which is adapted to switch the position of the straight traveling valve  17  according to the condition of an arm operation and a boom operation. This straight traveling valve  17  serves as well for a confluence switching valve for switching supplying condition of the pressure oil from the pumps  2 , 3  to the boom cylinder control valve  7  and the arm cylinder control valve  10 .  
         [0050]      FIG. 2  illustrates a second embodiment of the present invention, in which the above control for confluence and flow division is performed electrically using a controller.  
         [0051]     In  FIG. 2 , the same constituent elements as in  FIG. 1  are identified by the same reference numerals as in  FIG. 1 , and explanations thereof will be omitted.  
         [0052]     In  FIG. 2 , bucket pilot pressure sensors  30  and  31  are provided on the secondary side of a bucket operating remote control valve  20 . A bucket excavation pilot pressure S 1  and a bucket release pilot pressure S 2  are outputted as electric signals from the sensors  30  and  31  respectively.  
         [0053]     A boom pilot pressure sensor (detecting means for detecting a boom raising operation pressure)  32  is provided on the secondary side (boom raising side) of a boom operating remote control valve  22  and a boom raising pilot pressure S 3  is outputted from the sensor  32 .  
         [0054]     The sensors  30  to  32  are adapted to detect a composite operation of actuators.  
         [0055]     Further, an arm pilot pressure sensor  33  is provided on the secondary side (arm pulling side) of an arm operating remote control valve  25  and an arm pulling pilot pressure S 4  is outputted from the sensor  33 .  
         [0056]     The pilot pressures S 1  to S 4  are provided to a controller  34 . In accordance with the pilot pressures S 1  to S 4  the controller  34  controls a proportional valve  35  connected to a pilot port of a straight traveling valve  17  and also controls a left cut-off valve  18  through a proportional valve  36 . The controller  34  and the proportional valve  36  function as control means.  
         [0057]     Reference will be made below to the contents of control performed by the controller  34  in accordance with the flow chart of  FIG. 3 .  
         [0058]     In the same figure, the controller  34  makes a high-order selection out of the boom raising pilot pressure S 3 , the bucket excavation pilot pressure S 1  and the bucket release pilot pressure S 2  (step S 1 ). As a result, a pilot pressure as an operating pressure is selected and is made a pilot pressure f 0 .  
         [0059]     As described in  FIG. 3 , within the controller  34  is pre-stored a map having a characteristic C 1  such that a pressure f 1  is kept to a minimum pressure in a region where the pilot pressure f 0  selected by the high-order selection is less than a predetermined value and that the pressure f 1  becomes higher as the pilot pressure f 0  selected by the high-order selection rises. The pressure f 1  corresponding to the selected pilot pressure f 0  is determined on the basis of the map (step S 2 ).  
         [0060]     Next, a command f 2  for the straight traveling valve  17  and the left cut-off valve  18  is determined in accordance with the following equation (step S 3 ): 
 
 f   2 =(arm pulling pilot pressure  S   4 )− f   1  
 
         [0061]     This calculation aims at suppressing f 2  and canceling a confluence command for the straight traveling valve  17  in case of arm pulling and boom raising (or bucket excavation/bucket release) operations being performed simultaneously.  
         [0062]     Then, a confluence command I 1  for the proportional valve  35  is calculated on the basis of a map of the confluence command I 1  which has characteristic C 2  increasing with an increase of the f 2  (step S 4 ).  
         [0063]     Next, a closing command I 2  for the proportional valve  36  is calculated on the basis of the closing command I 2  which has characteristic C 3  increasing with an increase of the f 2  (step S 5 ).  
         [0064]     Subsequently, the thus-determined confluence command I 1  and closing command I 2  are outputted to the proportional valves  35  and  36 , respectively (step S 6 ), whereby the straight traveling valve  17  and the left cut-off valve  18  are controlled.  
         [0065]     That is, with an increase of f 2 , in other word, with a decrease of the f 1  corresponding to the boom raising operation amount, the values of confluence command I 1  and closing command I 2  become higher, whereby the straight traveling valve  17  is gradually switched from the flow dividing position a to the confluence position b and the left cut-off valve  18  is also switched gradually from the communicating position e to the cut-off position f, whereby the pressure oil from the first hydraulic pump  2  and the pressure oil from the second hydraulic pump  3  are joined and fed to the arm cylinder control valve  10 .  
         [0066]     On the other hand, when the value of f 2  is small under the boom raising operation or under the bucket excavation/bucket release, the confluence command I 1  does not increase in the characteristic C 2  and the closing command I 2  does not increase, either, in the characteristic C 3 . Consequently, the proportional valves  35  and  36  do not operate and the pressure oil from the first hydraulic pump  2  and the pressure oil from the second hydraulic pump  3  do not join.  
         [0067]     In this case, it is possible to ensure operating pressures of boom cylinder  13  and arm cylinder  10 .  
         [0068]     Although the invention has been described with reference to the preferred embodiments in the attached figures, it is noted that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims.

Summary:
A hydraulic control system for a hydraulic excavator of the present invention comprises a boom control valve in a first oil path for providing pressure oil from a first hydraulic pump with a boom cylinder according to operation of a boom operating means, an arm control valve in a second oil path for providing pressure oil from a second hydraulic pump with an arm cylinder according to operation of an arm operating means, a confluence switching valve having a confluence position for joining the paths and a confluence stop position for stopping the confluence, a flow control valve for returning pressure oil in the first oil path to a tank, and a control means for controlling the valves, so as to switch the confluence switching valve to the confluence position and close the flow control valve when the arm operating means is operated independently and so as to switch the confluence switching valve to the confluence stop position and open the flow control valve when both of the operating means are operated simultaneously, thereby, the arm pulling operation is performed independently, an arm cylinder speed increases and a leveling efficiency is improved.