Patent Publication Number: US-10316866-B2

Title: Construction machine

Description:
TECHNICAL FIELD 
     The present invention relates to a construction machine equipped with a hydraulic actuator. 
     BACKGROUND ART 
     Generally speaking, a construction machine such as a hydraulic excavator is equipped with a hydraulic pump driven by a prime mover, a hydraulic actuator, and flow control valves controlling the supply and discharge of the hydraulic working fluid with respect to the hydraulic actuator. Each flow control valve has a meter-in restrictor and a meter-out restrictor. The meter-in restrictor controls the flow rate of the hydraulic working fluid flowing into the hydraulic actuator from a pump, and the meter-out restrictor controls the flow rate of the hydraulic working fluid discharged from the hydraulic actuator to a hydraulic working fluid tank. Examples of the hydraulic actuator in a hydraulic excavator include a boom cylinder driving a boom and an arm cylinder driving an arm. 
     In a construction machine equipped with such a hydraulic actuator, it can occur that the weight of the support object of the hydraulic actuator (which, in the case, for example, of an arm cylinder, includes an arm and a bucket (attachment) acts as a load in the same direction as the operational direction of the hydraulic actuator (hereinafter also referred to as the “negative load”). In this case, the operational speed of the hydraulic actuator increases, and there is a shortage of hydraulic working fluid flow rate on the meter-in side, so that there is a fear of generation of a breathing phenomenon (cavitation). The breathing phenomenon may lead to deterioration in the operability of the construction machine and to damage of the hydraulic apparatus. 
     To solve the above problem, there is known a construction in which a meter-out control valve is provided in a meter-out passage leading to a hydraulic working fluid tank from a hydraulic actuator and in which the opening area of the meter-out control valve is adjusted in accordance with a cylinder pressure, whereby the cylinder speed is suppressed and a breathing phenomenon is prevented (See, for example, Patent Document 1). 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: JP-2010-14244-A 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In a state in which warming up has not been sufficiently effected due to the low external air temperature as in the case of winter or a cold district, the viscosity of the hydraulic working fluid becomes high and it takes time to raise a pilot pressure for valve switching and to transmit it. As a result, in the case where the opening area of the meter-out control valve is controlled with the pilot pressure, the controllability of the meter-out control valve markedly deteriorates when the temperature of the hydraulic working fluid is low, so that it is advisable to refrain from performing opening area control on the meter-out control valve. 
     In the case where opening area control is not performed on the meter-out control valve, the meter-out control valve is fixed at a normal position (a position determined by a spring force pushing a spool/poppet valve in a non-control state). At this time, when, as in the case of the above-mentioned document, the meter-out control valve is of a structure exhibiting a normal open characteristic (a characteristic in which a maximum opening is assumed at a normal position), the meter-out side hydraulic working fluid restrictor is widened. Thus, in the case where the hydraulic cylinder is operated in the direction of fall, the direction being due to its own weight, it is impossible to raise a sufficient meter-out pressure, and the cylinder speed increases, so that there is a fear of generation of a breathing phenomenon. 
     The present invention has been made in view of the above problem. It is an object of the present invention to provide a construction machine which can prevent a breathing phenomenon of the hydraulic actuator even in the case where performing of the opening area control of the meter-out control valve is refrained from because of a low hydraulic working fluid temperature. 
     Means for Solving the Problem 
     To achieve the above object, there is provided, in accordance with the present invention, a construction machine including: a hydraulic pump pumping up a hydraulic working fluid in a tank and delivering it; a hydraulic actuator driven by the hydraulic working fluid delivered from the hydraulic pump; a meter-out passage through which the hydraulic working fluid discharged from the hydraulic actuator flows; a meter-out control valve provided in the meter-out passage and controlling the hydraulic working fluid flow rate in the meter-out passage by varying an opening area; a load sensor detecting a load acting on the hydraulic actuator; an operation device operating the hydraulic actuator; and an operation amount sensor detecting the operation amount of the operation device, the construction machine further including a control device configured to select one of a normal operation mode in which the opening area of the meter-out control valve is controlled based on the load and the operation amount and a substitution operation mode in which the opening area of the meter-out control valve is controlled based on the operation amount. The control device is configured to further increase the delivery flow rate of the hydraulic pump when the substitution operation mode is selected than when the normal operation mode is selected. 
     Advantages of the Invention 
     According to the present invention, even in the case where the opening area control of the meter-out control valve is not conducted, the pump flow rate is further increased than at the time of normal operation, whereby it is possible to prevent a breathing phenomenon of the hydraulic actuator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an overall view of a construction machine according to the present invention. 
         FIG. 2  is a conceptual diagram illustrating the construction of a hydraulic circuit and an apparatus according to a first embodiment of the present invention. 
         FIG. 3  is a flowchart illustrating an operational mode switching control according to the first embodiment of the present invention. 
         FIG. 4  is a control block diagram of a hydraulic pump and a meter-out opening limitation computation according to the first embodiment of the present invention. 
         FIG. 5  is a control block diagram illustrating a solenoid proportional valve electric current instruction value computation according to the first embodiment of the present invention. 
         FIG. 6  is a diagram illustrating a meter-out opening limitation value computation table according to the first embodiment of the present invention. 
         FIG. 7  is a diagram illustrating a pump flow rate correction value determination method according to the first embodiment of the present invention. 
         FIG. 8  is a conceptual diagram illustrating the construction of a hydraulic circuit and an apparatus according to a second embodiment of the present invention. 
         FIG. 9  is a flowchart illustrating an operational mode switching control according to the second embodiment of the present invention. 
         FIG. 10  is a flowchart illustrating an operational mode switching control according to a third embodiment of the present invention. 
         FIG. 11  is a control block diagram of a hydraulic pump and a meter-out opening limitation computation according to the third embodiment of the present invention. 
         FIG. 12  is a construction diagram of controller hardware according to the present invention. 
         FIG. 13  is a conceptual diagram illustrating the construction of a hydraulic circuit and an apparatus according to the second embodiment of the present invention. 
         FIG. 14  is a flowchart illustrating an operational mode switching control according to a fourth embodiment of the present invention. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     In the following, embodiments of the present invention will be described with reference to the drawings. In the description, the construction machine consists of a hydraulic excavator by way of example. 
     First Embodiment 
     In the present embodiment, there will be described how to prevent a breathing phenomenon in the case where the hydraulic working fluid is at low temperature and where the responsiveness of the mechanism adjusting the meter-out opening area in accordance with the actuator load deteriorates. 
     In  FIG. 1 , the hydraulic excavator is equipped with a track structure  10 , a swing structure  20  swingably provided on the track structure  10 , and a front work device  30  attached to the swing structure  20 . 
     The track structure  10  is composed of a pair of crawlers  11   a  and  11   b , crawler frames  12   a  and  12   b  (solely one of which is shown in  FIG. 1 ), a pair of traveling hydraulic motors  13   a  and  13   b  independently drive-controlling the crawlers  11   a  and  11   b , a speed reduction mechanism thereof, etc. 
     The swing unit  20  is equipped with a swing frame  21 , an engine  22  as a prime mover provided on the swing frame  21 , a hydraulic pump  23  rotary driven by an engine  22  and pumping up a hydraulic working fluid in a hydraulic working fluid tank  40  (See  FIG. 2 ) and delivering it, hydraulic actuators (e.g., hydraulic cylinders  32 ,  34 , and  36 ) driven by the hydraulic working fluid delivered from the hydraulic pump  23 , and a control valve unit  24  equipped with a plurality of flow control valves (e.g., a flow control valve  41  in  FIG. 2 ) distributing the hydraulic working fluid delivered from the hydraulic pump  23  to the hydraulic actuators. Further, the swing structure  20  is equipped with a swing hydraulic motor  25  and a speed reduction mechanism thereof, and the swing hydraulic motor  25  swingably drives an upper swing structure  20  (swing frame  21 ) with respect to the lower track structure  10 . 
     Further, the front work device  30  is mounted on the swing structure  20 . The front work device  30  is composed of a boom  31  the proximal end portion of which is pivotably supported in a freely rotating manner by the swing structure  20 , a boom cylinder  32  for driving the boom  31 , an arm  33  pivotably supported in a freely rotating manner by a portion in the vicinity of the distal end portion of the boom  31 , an arm cylinder  34  for driving the arm  33 , a bucket  35  pivotably supported in a rotatable manner by the distal end of the arm  33 , a bucket cylinder  36  for driving the bucket  35 , etc. 
       FIG. 2  is a conceptual diagram illustrating the construction of a hydraulic circuit and an apparatus related to the arm cylinder  34  in the hydraulic control apparatus of a construction machine according to the first embodiment of the present invention. While in the following description the hydraulic actuator consists of the arm cylinder  34 , the present embodiment is also applicable to some other hydraulic actuator such as the bucket cylinder  36  so long as the hydraulic actuator is one in which the operational direction of the driving object of the hydraulic actuator, the direction being due to the weight of the driving object, can coincide with the operational direction of the driving object driven by the hydraulic actuator. 
     In  FIG. 2 , the hydraulic control apparatus according to the present invention is equipped with an engine  22 , a hydraulic pump  23  rotary driven by the engine  22 , a hydraulic working fluid tank  40  which is the hydraulic working fluid supply source to the hydraulic pump  23 , and a pilot valve  42  which is connected to a delivery line L 1  of the hydraulic pump  23  and which is an arm operation device controlling the flow rate and direction of the hydraulic working fluid supplied to the arm cylinder  34 . 
     The revolution speed of the engine  22  is detected by a pickup sensor SE 1  and input to a controller  44 . 
     The hydraulic pump  23  is of a variable displacement type and is equipped with a regulator (pump delivery flow rate control device)  23   a  varying the displacement volume (delivery flow rate) of the hydraulic pump  23  based on a command from the controller  44 . The delivery pressure of the hydraulic pump  23  is detected by a pump delivery pressure sensor SE 2 , and is input to the controller  44 . 
     The control valve  41  is of a center bypass type, and a center bypass portion  41   a  is connected to a center bypass line L 2  at a neutral position A. The downstream side of the center bypass line L 2  is connected to a hydraulic working fluid tank  40 . Further, the control valve  41  has a pump port  41   b , a tank port  41   c , and actuator ports  41   d  and  41   e . The pump port  41   b  is connected a delivery line L 1 . The tank port  41   c  is connected to the tank  40 . The actuator ports  41   d  and  41   e  are connected to a bottom side hydraulic fluid chamber or a rod side hydraulic fluid chamber of the arm cylinder  34  via an actuator line L 3  or L 4 . 
     The pilot valve  42  has an operation lever  42   a , and a pilot pressure generation portion  42   b  containing a pair of pressure reducing valves (not shown), and the pilot pressure generation portion  42   b  is connected to pilot pressure receiving portions  41   f  and  41   g  of the control valve  41  via pilot lines L 5  and L 6 . When the operation lever  42   a  is operated, the operation pilot pressure generation portion  42   b  operates one of the pair of pressure reducing valves in accordance with the operational direction thereof, and outputs a pilot pressure in accordance with the operation amount to one of the pilot lines L 5  and L 6 . The operation pilot pressure generated in L 5  and L 6  is detected by pilot pressure sensors SE 3  and SE 4 , and output to the controller  44 . 
     As its switching positions, the control valve  41  has a neutral position A, a switching position B, and a switching position C. When a pilot pressure is imparted to the pressure receiving portion  41   f  by the pilot line L 5 , switching is effected to the switching position B on the left-hand side as seen in the drawing. At this time, the actuator line L 3  is on the meter-in side, and L 4  is on the meter-out side. The hydraulic working fluid is supplied to the bottom side hydraulic fluid chamber of the arm cylinder  34 , and the piston rod of the arm cylinder  34  extends. On the other hand, when a pilot pressure is imparted to the pressure receiving portion  41   g  by the pilot line L 6 , switching is effected to the position C on the right-hand side as seen in the drawing. At this time, the actuator line L 4  is on the meter-in side, and L 3  is on the meter-out side. The hydraulic working fluid is supplied to the rod side hydraulic fluid chamber of the arm cylinder  34 , and the piston rod of the arm cylinder  34  contracts. The expansion of the piston rod of the arm cylinder  34  corresponds to the operation of drawing in the arm, that is, the crowding operation, and the contraction of the piston rod of the arm cylinder  34  corresponds to the operation of pushing out the arm, that is, the damping operation. 
     The pressure of the bottom side hydraulic fluid chamber (hereinafter referred to as the bottom pressure) can be detected by a pressure sensor SE 5 , and the pressure of the rod side hydraulic fluid chamber (hereinafter referred to as the rod pressure) can be detected by a pressure sensor SE 6 . The detection pressures of the pressure sensors SE 5  and SE 6  are input to the controller  44 . In the present embodiment, the pressure sensor SE 5  is utilized as a load sensor detecting the load acting on the arm cylinder  34 . 
     Further, the control valve  41  has meter-in restrictors  41   h  and  41   i  and meter-out restrictors  41   j  and  41   k . These restrictors  41   h ,  41   i ,  41   j , and  41   k  function as variable restrictors varying in opening area in accordance with the switching position of the control valve  41 . The meter-out restrictors  41   j  and  41   k  cause the control valve  41  to function as a meter-out control valve controlling the flow rate of the hydraulic working fluid in the meter-out passage (actuator line L 4  or L 3 ). When the control valve  41  is at the switching position B, the hydraulic working fluid supplied to the arm cylinder  34  is controlled by the meter-in restrictor  41   h , and the return flow rate from the arm cylinder  34  is controlled by the meter-out restrictor  41   j . On the other hand, when the control valve  41  is at the switching position C, the hydraulic working fluid supplied to the arm cylinder  34  is controlled by the meter-in restrictor  41   i , and the return flow rate from the arm cylinder  34  is controlled by the meter-out restrictor  41   k.    
     Further, the hydraulic control apparatus of the construction machine according to the present embodiment is equipped with a solenoid proportional valve  43  installed in the pilot line L 5 . The solenoid proportional valve  43  is driven based on a solenoid valve electric current (control signal) input from the controller  44 , and functions as a control device (meter-out control valve control device) controlling the opening area of the meter-out restrictor  41   j  of the control valve  41 . The solenoid valve electric current value input to the solenoid proportional valve  43  assumes a value somewhere between a solenoid proportional valve minimum electric current IMIN (e.g., 100 mA) which is zero or more and a solenoid proportional valve maximum electric current IMAX (e.g., 600 mA). When the solenoid valve electric current value IMIN, a solenoid valve spool  43   a  is at a switching position D, and the opening of a hydraulic line  43   b  is maximum. At this time, the pilot pressure generated at the operation pilot pressure generation portion  42   b  is directly guided to the pressure receiving portion  41   f . When the solenoid valve electric current value IMAX, a solenoid valve spool a is at a switching position F, and interrupts the hydraulic line  43   b , thereby preventing the pilot pressure generated in the pilot line L 5  from being guided to the pressure receiving portion  41   f . At the same time, the opening of the hydraulic line  43   c  is maximum, and the hydraulic working fluid at the pressure receiving portion  41   f  is discharged to a drain circuit L 7 . When the solenoid valve electric current value is in a control region between IMIN and IMAX, the solenoid proportional valve  43  controls the spool  43   a  between the switching position D and the switching position E, whereby the hydraulic line  43   b  from the operation pilot pressure generation portion  42   b  to the pressure receiving portion  41   f  is restricted. At the same time, the hydraulic working fluid of the pressure receiving portion  41   f  is partially discharged to the drain circuit L 7  through the hydraulic line  43   c . Through this operation, an arbitrary pressure not higher than the pilot pressure generated in the operation pilot pressure generation portion  42   b  can be guided to the pressure receiving portion  41   f  as the pilot pressure. 
     The hydraulic working fluid tank  40  is equipped with a hydraulic working fluid temperature sensor (temperature sensor) SE 7 , and the temperature of the hydraulic working fluid in the hydraulic working fluid tank  40  is detected and output to the controller  44 . 
     Further, the hydraulic control apparatus of the construction machine according to the present embodiment is equipped with the controller  44 . The controller  44  is formed by a computer, which acquires the values of the sensors SE 1  through SE 7  and controls a pump regulator  23   a  and a solenoid proportional valve  43 . 
       FIG. 12  shows the hardware construction of the controller  44 . The controller  44  has an input unit  91 , a central processing unit (CPU)  92  that is a processor, read-only memory (ROM)  93  and random access memory (RAM)  94  that are storage devices, and an output unit  95 . The input unit  91  inputs signals from the sensors SE 1  through SE 7 , and performs A/D conversion. The ROM  93  is a storage medium storing a control program for executing the processing illustrated in the flowcharts of  FIG. 3 , etc. described below, and various items of information, etc. necessary for executing the processing of the flowcharts. In accordance with the control program stored in the ROM  93 , the CPU  92  performs a predetermined computation processing with respect to signals acquired from the input unit  91 , the memory  93 , and the memory  94 . The output unit  95  prepares an output signal in accordance with the computation result at the CPU  92 , and outputs the signal to the solenoid proportional valve  43  and the pump regulator  23   a , whereby it is possible to control the opening area of the meter-out restrictor  41   j  of the control valve  41  and to control the delivery flow rate of the hydraulic pump  23 . While the controller  44  of  FIG. 12  is equipped with semiconductor memories, i.e., the ROM  93  and the RAM  94 , as the storage devices, this allows replacement by some other device so long as it is a storage device. For example, a magnetic storage device such as a hard disk drive may be provided. 
       FIG. 3  shows a flowchart for the operational mode switching control in the first embodiment. It is to be assumed that, at the start of the flowchart, a key switch is at an OFF position, and that a normal operation mode is selected as the machine body operation mode. 
     In step S 1 , it is determined whether or not the key switch is switched to the ON position (key ON) by the operator. When it is determined that the system is in the key ON mode, the controller  44  is activated, and the procedure advances to step S 2 . In step S 2 , it is determined whether or not the key switch is switched to the start position from the ON position. When it is determined that the key is at the start position, the engine  22  is started, and the procedure advances to step S 20 . Next, in step S 20 , the controller  44  acquires the hydraulic working fluid temperature T 0  detected by the hydraulic working fluid temperature sensor SE 7 , and the procedure advances to step S 21 . 
     In step S 21 , the controller  44  compares with each other the hydraulic working fluid temperature T 0 , the meter-out opening limitation non-effective temperature threshold value T 1 , and the meter-out opening limitation effective temperature threshold value T 2 . Between the meter-out opening limitation non-effective temperature threshold value T 1  and the meter-out opening limitation effective temperature threshold value T 2 , the following relationship holds good: T 1 &lt;T 2 . For example, the maximum value of the temperature range where the viscosity of the hydraulic working fluid is high and where the meter-out opening limitation control is difficult can be set as the meter-out opening limitation non-effective temperature threshold value T 1 , and a value higher than the temperature range concerned can be set as the meter-out opening limitation non-effective temperature threshold value T 2 . Further, the difference between T 1  and T 2  is set to a value that is sufficiently large with respect to a short-period change amount of the hydraulic working fluid temperature (for example, T 1 =0° C. and T 2 =5° C.) 
     When T 0 &lt;T 1  in step S 21 , the procedure advances to step S 22 . When T 1 ≤T 0 &lt;T 2 , the procedure advances to step S 23 , and when T 2 ≤T 0 , the procedure advances to step S 24 . In step S 22 , the operation mode of the machine body (the initial value of which is the normal operation mode) is switched to a substitution operation mode (described below), and the procedure returns to step S 20 . In step S 23 , the operation mode at that point in time is maintained, and the procedure returns to step S 21 . In step S 24 , the operation mode is switched to the normal operation mode (described below), and the procedure returns to step S 20 . 
     Next, referring to  FIGS. 4 and 5 , the control of the delivery flow rate of the hydraulic pump  23  and of the solenoid proportional valve  43  by the controller  44  in the normal operation mode and the substitution operation mode will be described. 
     In  FIG. 4 , by using table T 1 , a flow rate reference value Q 1  of the pump  23  is determined from an arm crowding operation pilot pressure (arm crowding operation amount) detected by the pilot pressure sensor SE 3 . Further, an arm crowding power demanded value POW 1  is computed from a pump output reference value set such that the engine speed does not undergo lug-down and from an arm crowding operation amount, and this is divided by the pump delivery pressure detected by the pump delivery pressure sensor SE 2 , whereby a pump flow rate limitation value Qlim in terms of horsepower is computed. The minimum value of the flow rate reference value Q 1  and the pump flow rate limitation value Qlim in terms of horsepower will be regarded as a pump flow rate demanded value Q 2 . 
     Further, from the arm crowding operation pilot pressure (arm crowding operation amount) and the arm bottom pressure (arm cylinder load) detected by the pressure sensor SE 5 , the opening area value of the meter-out restrictor  41   j  (hereinafter also referred to as the meter-out opening limitation value) is computed by using the table T 2 . The table T 2  has a characteristic in which the larger the arm crowding operation pilot pressure (the larger the arm speed), the large the meter-out opening limitation value. The arrow in the table T 2  indicates the magnitude of the arm bottom pressure, and the table T 2  is of a characteristic in which the lower the arm bottom pressure (i.e., when the liability of generation of breathing in the arm cylinder  34  is high), the smaller the meter-out opening limitation value. The graph when the arm bottom pressure is at the highest level coincides with the meter-out opening characteristic A 0  of the control valve  41  (See  FIG. 6  referred to below). 
     The switching position of the switch SW 1  is selectively switched in accordance with the operation mode determined in the flowchart of  FIG. 3 . In the normal operation mode, switch SW 1  is switched to a position Ps 1 , and the opening area value calculated by using the table T 2  is output to the table T 4  of  FIG. 5 . On the other hand, in the substitution operation mode, the switch SW 1  is switched to a position Ps 2 , and the maximum value Amax (See  FIG. 6  referred to below) when the control valve  41  assumes the meter-out opening characteristic A 0  is output to the table T 4  of  FIG. 5  without taking the arm bottom pressure into consideration. 
     In  FIG. 5 , to be described will be a computation method for determining the control signal (solenoid proportional valve electric current instruction value) to the solenoid proportional valve  43  based on the meter-out opening limitation value. First, from the meter-out opening limitation value of the table T 2 , a solenoid proportional valve secondary pressure target value (pilot pressure) is computed by using the table T 4 . Here, in the table T 4 , the vertical axis and the horizontal axis of the opening characteristic of the meter-out restrictor  41   j  with respect to the pressure of the pressure receiving portion  41   f  are interchanged with each other. When Amax is input to T 4  (when SW 1  is at Ps 2  in the substitution operation mode), the solenoid proportional valve secondary pressure target value assumes a maximum value. 
     Next, by using the table T 5 , the solenoid valve electric current instruction value is computed from the solenoid proportional valve secondary pressure target value of T 4 . Here, in the table T 5 , the vertical axis and the horizontal axis of the electric current-secondary pressure characteristic (I-P characteristic) of the solenoid proportional valve  43  are interchanged with each other. When the solenoid proportional valve secondary pressure target value assumes a maximum value (when SW 1  is at Ps 2  in the substation operation mode), the electric current value is zero, so that the control valve  41  is driven by the pilot pressure generated by the operation pilot pressure generation portion  42   b . Here, when the substation operation mode is selected, the electric current instruction value calculated by the table T 5  is zero. However, it may also be a value in excess of zero so long as it is within the electric current value range in which the solenoid proportional valve  43  is retained at the normal position. 
     As described above, through computation using the tables T 4  and T 5 , the controller  44  outputs the solenoid valve electric current instruction value of T 5  to the solenoid proportional valve  43 , and controls the solenoid proportional valve  43  such that the opening area of the meter-out restrictor  41   j  assumes the target value. 
     Next, referring back to  FIG. 4 , a method of computing a pump flow rate correction value ΔQ will be described. From the arm crowding operation pilot pressure and the arm bottom pressure, the pump flow rate correction value is calculated by using the table T 3 . The table T 3  is of a characteristic in which the higher the operation pilot pressure, the further the pump flow rate correction value ΔQ increases. The arrow in the table T 3  indicates the magnitude of the arm bottom pressure, and the table T 3  is of a characteristic in which the lower the bottom pressure (actuator load) (the higher the possibility of generation of breathing in the arm cylinder), the further the pump flow rate correction value ΔQ increases. Further, it is of a characteristic in which when the bottom pressure is high (when there is little possibility of generation of breathing in the arm cylinder), the pump flow rate correction value ΔQ decreases as compared with the case where the bottom pressure is low. The pump flow rate correction value ΔQ calculated by using the table T 3  is output to the switch SW 2 . 
     The switching position of the switch SW 2  is alternatively switched in accordance with the operation mode determined by the flowchart of  FIG. 3 . In the normal operation mode, the switch SW 2  is switched to the position Ps 1 , and zero is output as the pump flow rate correction value ΔQ. On the other hand, in the substitution operation mode, the switch SW 2  is switched to the position Ps 2 , and the value calculated by using the table T 3  is output as the pump flow rate correction value ΔQ. 
     The pump flow rate correction value ΔQ output from the switch SW 2  is added to the pump flow rate demanded value Q 2 , whereby the final pump flow rate target value Q 3  is determined. Based on the pump flow rate target value Q 3 , an electric current instruction value to the pump regulator  23   a  is generated. The controller  44  outputs the electric current instruction value to the pump regulator  23   a , and controls the pump regulator  23   a  such that the delivery flow rate of the hydraulic pump  23  attains the target value (Q 2  or Q 2 +ΔQ). Due to this operation, when the substitution operation mode is selected, the pump flow rate correction value ΔQ which is larger than zero is added to Q 2 , so that the delivery flow rate of the hydraulic pump  23  is increased as compared with the case where the normal operation mode is selected and where Q 2  is constantly maintained, and the shortage of flow rate on the meter-in side is mitigated/eliminated. 
     Next, the role of the table T 2  will be described with reference to  FIG. 6 .  FIG. 6  is a schematic view of the table T 2 . In table T 2 , when the level of the arm bottom pressure is highest, that is, when breathing phenomenon is not easily generated in the arm cylinder, the meter-out opening limitation value assumes the meter-out opening characteristic (A 0  in the drawing) of the control valve  41 . At this time, the arm crowding operation pilot pressure and the solenoid valve secondary pressure coincide with each other, so that a reduction in the pilot pressure is not effected. As indicated at A 1  in the drawing, in the case where the arm bottom pressure is low and where there is the possibility of generation of a breathing phenomenon, the characteristic in which the opening is reduced by a fixed degree from A 0  is regarded as the meter-out opening limitation value. At this time, the meter-out restrictor  41   j  is restricted, so that the arm cylinder rod pressure increases, and the cylinder speed decreases, thereby preventing breathing. In the case where the arm bottom pressure is further reduced, the characteristic in which the opening is further reduced from A 1  is regarded as the meter-out opening limitation value. The degree to which the opening is reduced with respect to the arm bottom pressure is derived from an experiment. 
     Next, by using the equations of  FIG. 7 , the method of deriving the table T 3  will be described. Assuming that the table T 2  is determined through experiment, the requisite meter-out pressure pMO (which coincides with the arm cylinder rod pressure here) for preventing the breathing phenomenon is derived from equation (1). Here, Q(PI) corresponds to the pump reference flow rate corresponding to the operation pilot pressure PI, c corresponds to the flow rate coefficient, and A 1  (PI) corresponds to the characteristic of A 1  of  FIG. 5 . In the substitution operation mode, the meter-out opening is not limited, so that the characteristic of the meter-out restrictor opening is the meter-out opening characteristic A 0  of the control valve  41 . To prevent the breathing phenomenon, it is necessary to maintain, also in the substitution operation mode, a meter-out pressure equivalent to that of the normal operation mode. Here, A 1  is smaller than A 0 , so that, as in the case of equation (2), a positive-value pump correction flow rate ΔQ is added to the pump reference flow rate Q. From equations (1) and (2), the pump correction flow rate ΔQ is determined uniquely as in equation (3). 
     While in the above description one operation mode is automatically selected based on the flowchart of  FIG. 3 , that is, the hydraulic working fluid temperature, it is also possible to provide an operation mode changeover switch (not shown), and, by this switch, the switching positions of the switch SW 1  and the switch SW 2  may be changed in accordance with the operation mode as desired by the operator. 
     As described above, in the present embodiment, there is provided a hydraulic excavator including: a hydraulic pump  23  pumping up and delivering the hydraulic working fluid in a hydraulic working fluid tank  40 ; an arm cylinder  34  driven by the hydraulic working fluid delivered from the hydraulic pump  23 ; a meter-out passage L 4  through which the hydraulic working fluid discharged from the arm cylinder  34  flows; a control valve  41  provided in the meter-out passage L 4  and configured to control the flow rate of the hydraulic working fluid in the meter-out passage L 4  by changing the opening area of a restrictor  41   j ; a pressure sensor SE 5  detecting the load (actuator load) acting on the arm cylinder  34 ; an operation device  42  operating the arm cylinder  34 ; and a pressure sensor SE 3  detecting the operation amount of the operation device  42 . In the hydraulic excavator, there is provided a controller  44  configured to control the opening area of the restrictor  41   j  by selecting one of a normal operation mode in which the opening area of the restrictor  41   j  is controlled based on the actuator load detected by the sensor SE 5  and the operation amount detected by the sensor SE 3 , and a substitution operation mode in which the actuator load is not taken into consideration and in which the opening are of the restrictor  41   j  is controlled based solely on the operation amount detected by the sensor SE 3 . Further, the controller  44  is configured to increase the delivery flow rate of the hydraulic pump  23  when the substitution operation mode is selected as compared with the case where the normal operation mode is selected and where the operation amount is the same. 
     In the hydraulic excavator constructed as described above, the opening area of the restrictor  41   j  of the control valve  41  is controlled, whereby, in the case where the flow rate of the hydraulic working fluid in the meter-out passage (L 4 ) is not controlled in accordance with the actuator load (that is, in the case where the substitution operation mode is selected), the delivery flow rate of the hydraulic pump  23  increases as compared with the case where the normal operation mode is selected, making it possible to avoid a shortage of hydraulic working fluid flow rate in the meter-in passage (L 3 ). Thus, it is possible to prevent generation of the breathing phenomenon in the arm cylinder (hydraulic actuator)  34 . As a result, it is possible to prevent deterioration in the operability of the hydraulic excavator and damage of the hydraulic apparatus. 
     Further, in the present embodiment, due to the provision of the table T 3 , the controller  44  performs control, when the substitution operation mode is selected, such that the smaller the actuator load, the higher the delivery flow rate of the hydraulic pump  23 , and that the larger the operation amount, the higher the delivery flow rate of the hydraulic pump  23 . 
     In the hydraulic excavator constructed as described above, the smaller the actuator load, and the higher the possibility of generation of the breathing phenomenon, the higher the delivery flow rate of the hydraulic pump  23 , so that it is possible to achieve an improvement in terms of the reliability in the prevention of the generation of the breathing phenomenon. 
     Further, in the present embodiment, there is further provided the temperature sensor SE 7  detecting the hydraulic working fluid temperature in the hydraulic working fluid tank  40 . When the hydraulic working fluid temperature T 0  acquired by the temperature sensor SE 7  is below the threshold value T 1 , the controller  44  selects the substitution operation mode, and when the hydraulic working fluid temperature attains a value (T 2 ) that is the threshold value T 1  or more, the controller selects the normal operation mode. 
     In the hydraulic excavator constructed as described above, when the hydraulic working fluid temperature is lowered due to the external air temperature, etc., and the viscosity of the hydraulic working fluid becomes so high as to make it difficult to perform the meter-out opening limitation control (i.e., to control the hydraulic working fluid flow rate in the meter-out passage (L 4 ) in accordance with the actuator load through the control of the opening area of the restrictor  41   j  of the control valve  41 ), the substitution operation mode is automatically selected, and the execution of the meter-out opening limitation control is avoided. Further, the delivery flow rate of the hydraulic pump  23  increases. As a result, the execution/non-execution of the meter-out flow rate control in accordance with the load is automatically selected in accordance with the hydraulic working fluid temperature, and, at the same time, even when the meter-out flow rate control is not executed, it is possible to prevent generation of the breathing phenomenon in the arm cylinder (hydraulic actuator)  34 , so that it is possible to prevent deterioration in the operability of the hydraulic excavator and damage of the hydraulic apparatus. 
     Second Embodiment 
     Next, the second embodiment of the present invention will be described.  FIG. 8  is a construction diagram of a hydraulic circuit and an apparatus according to the present embodiment. The construction of the hydraulic circuit and the apparatus differs from that of the first embodiment in that the hydraulic working fluid temperature sensor SE 7  is removed. Otherwise, it is the same as that of the first embodiment, so that a description thereof will be left out. 
       FIG. 13  is a control block diagram illustrating the hydraulic pump and the meter-out opening limitation computation. In  FIG. 13 , reference character T 2  indicates the table T 2  in  FIG. 4 , and reference characters T 4  and T 5  indicate the tables T 4  and T 5  in  FIG. 5 . The difference from the control block diagrams of  FIGS. 4 and 5  lies in the fact that there is provided a switch SW 3  instead of the switch SW 1 . The switching position for the switch SW 3  is alternatively switched in accordance with the operation mode determined in the flowchart of  FIG. 9  referred to below. In the normal operation mode, the switch SW 3  is switched to a position Ps 1 , and an electric current instruction value calculated by using the tables T 2 , T 4 , and T 5  is output to the solenoid proportional valve  43 . On the other hand, in the substitution operation mode, the switch SW 3  is switched to a position Ps 2 , and the electrical connection between the controller  44  and the solenoid proportional valve  43  is cut off. As a result, the electric current output to the solenoid proportional valve  43  is not effected (that is, the electric current instruction value is zero), and the solenoid proportional valve  43  assumes a maximum opening at the normal position. As a result, the control valve  41  is driven by the pilot pressure generated by the operation pilot pressure generation portion  42   b  independently of the actuator load. 
       FIG. 9  shows the operation mode switching control flowchart of the first embodiment. The same processing as that in the above flowchart is indicated by the same reference character, and a description thereof may be left out. 
     When it is confirmed that the key switch is at the ON position in step S 1 , the controller  44  is activated, and the procedure advances to step S 30 . 
     In step S 30 , the controller  44  determines whether or not the operation mode when the key was OFF last time was the substitution mode. The operation mode when the key was OFF last time is stored in the ROM  93  of the controller  44 , and the controller  44  makes the determination in step S 30  based on the information. When it is determined in step S 30  that the operation mode was the substitution mode, the operation mode is switched to the normal operation mode in step S 34 , and the procedure advances to step S 2 . On the other hand, when in step S 30  the mode was determined to be the normal operation mode, the procedure advances to step S 2 . 
     In step S 3 , the controller  44  outputs a solenoid proportional valve electric current instruction value I, which is determined by the control shown in  FIG. 13 . In step S 4 , a current sensor of the controller  44  detects an electric current (feedback electric current value) IFB output to the solenoid proportional valve  43 , and the procedure advances to step S 5 . There may be constructed such that, in step S 3 , the presence or absence of an output demand for the solenoid proportional valve electric current instruction value I is detected, and when there is the output demand, the procedure advances to step S 4 , and when there is no output demand, the procedure may return to step S 3  (See step S 40  of  FIG. 14  referred to below). 
     In step S 5 , it is determined whether or not either the solenoid proportional valve feedback electric current IFB of S 4  exceeds a feedback electric current upper limit threshold value Ith 1  (e.g., 900 mA) or it is below a feedback electric current lower limit threshold value Ith 2  (e.g., 50 mA). Here, Ith 1  is a value larger than the solenoid proportional valve maximum electric current IMAX, and is an electric current value making it possible to determine whether or not the solenoid or wire harness of the solenoid proportional valve  43  suffers short-circuiting. Ith 2  is a value smaller than the solenoid proportional valve minimum electric current IMIN and not less than zero, and is an electric current value making it possible to determine whether or not the solenoid or wire harness of the solenoid proportional valve  43  suffers disconnection. That is, in step S 5 , it is determined whether or not there is failure accompanying short-circuiting/disconnection of the solenoid proportional valve  43 . When, in step S 5 , either the solenoid proportional valve feedback electric current IFB exceeds a feedback electric current upper limit threshold value Ith 1  or it is below a feedback electric current lower limit threshold value Ith 2  (that is, when there is a fear of short-circuiting/disconnection), the procedure advances to step S 6 . 
     In step S 6 , the computation cycle (e.g., 0.01 sec) of the controller  44  is added to a timer Ta (the initial value of which is zero), and the procedure advances to step S 8 . 
     On the other hand, when, in step S 5 , the solenoid proportional valve feedback electric current IFB is equal to or less than the feedback electric current upper limit threshold value Ith 1  or it is equal to or more than the feedback electric current lower limit threshold value Ith 2 , the procedure advances to step S 7 . In step S 7 , the timer Ta is set to zero, and the procedure advances to step S 8 . 
     In step S 8 , the timer Ta and a timer threshold value Tth (e.g., 5 sec) are compared with each other. When the timer Ta is equal to or less than the timer threshold value Tth, the procedure advances to step S 9 . When the timer Ta is more than the timer threshold value Tth, it is determined that abnormality is generated in the solenoid proportional valve  43  (meter-out control valve control device), and the procedure advances to step S 10 . 
     In step S 9 , the operation mode of the machine body is set to the normal operation mode, and it is determined whether or not the key switch is at the OFF position (S 36 ). When, in step S 36 , the key is OFF, the engine  22  and the controller  44  are stopped to complete the processing. When the key is ON, the procedure returns to step S 3 . 
     In step S 10 , the controller  44  switches the operation mode of the machine body to the substitution operation mode, and the switch SW 3  is switched to the position Ps 2 . As a result, the solenoid proportional valve electric current instruction value I is set to zero in step S 11  (that is, the control valve  41  is driven by the pilot pressure generated by the operation pilot pressure generation portion  42   b ), and the processing is completed. As a result, in the case where switching is effected to the substitution operation mode, switching to the normal operation mode is not effected so long as the turning OFF/ON of the key is not performed next time. 
     While in the above-described case the operation mode when the key was OFF last time is stored and it is confirmed in step S 30 , the storage of the operation mode and steps S 30  and S 34  may be omitted, and it is possible to adopt a construction in which the operation mode at the start of the flow of  FIG. 9  is always the normal operation mode. 
     When trouble or failure is generated in the solenoid proportional valve  43 , it is difficult to output a proper secondary pressure from the solenoid proportional valve  43 , so that it is impossible to perform a proper meter-out flow rate control in accordance with the actuator load. 
     In view of this, in the present embodiment, constructed as described above, the hydraulic excavator is constructed as follows. Driving is effected based on the solenoid proportional valve electric current instruction value I (control signal) input from the controller  44 . When the controller  44  detects abnormality in the solenoid proportional valve  43  functioning as the meter-out control valve control device controlling the opening area of the restrictor  41   j  of the control valve  41 , the output of the electric current to the solenoid proportional valve  43  is stopped, and the substitution operation mode is selected as the operation mode. 
     When the hydraulic excavator is thus constructed, in the case where the meter-out flow rate control cannot be performed due to failure of the solenoid proportional valve  43 , the operation mode is automatically switched to the substitution operation mode, and the pump flow rate increases, so that it is possible to prevent the breathing phenomenon. 
     In the above-described system, in order to prevent an erroneous electric current from being output to the solenoid proportional valve  43  due to failure in the solenoid proportional valve  43  and the peripheral equipment thereof, the connection between the solenoid proportional valve  43  and the controller  44  is interrupted by SW 3  in the substitution operation mode. Instead of the control of the solenoid proportional valve  43  of  FIG. 13 , however, the control may be performed based on  FIGS. 4 and 5  as in the first embodiment. 
     Third Embodiment 
     Next, the third embodiment of the present invention will be described. In the third embodiment, the breathing phenomenon is prevented also in the case where the sensor used for the meter-out opening limitation computation suffers failure. In the following, the arm cylinder bottom pressure sensor SE 5  will be taken as an example of the sensor used for the meter-out opening limitation computation. The construction of the hydraulic circuit and the apparatus is the same as that of the second embodiment of the present invention. 
       FIG. 11  shows a method of controlling the delivery flow rate of the hydraulic pump  23  and the solenoid proportional valve  43  in the normal operation mode and the substitution operation mode in the present embodiment. The method of controlling the delivery flow rate of the hydraulic pump  23  and the solenoid proportional valve  43  is substantially the same as that of the first embodiment. The only difference lies in the fact that the pump correction flow rate ΔQ is computed solely from the operation pilot pressure (table T 3   a ) without using the arm bottom pressure. In the table T 3   a  of this example, there is utilized the characteristic when the arm bottom pressure is minimum in the table T 3  of  FIG. 4 . 
       FIG. 10  shows the flowchart for the operation mode switching control in the present embodiment. Steps S 1  and S 2  are the same as those of the first embodiment. Next, in step S 12 , the output voltage V 0  of the arm bottom pressure sensor SE 5  is detected, and the procedure advances to step S 13 . In step S 13 , it is determined whether or not either the cylinder pressure sensor voltage V 0  is below the cylinder pressure sensor voltage minimum value VMIN or it exceeds the cylinder pressure sensor voltage maximum value VMAX. The cylinder pressure sensor voltage minimum value VMIN is of a value making it possible to detect short-circuiting of the cylinder pressure sensor. The cylinder pressure sensor voltage maximum value VMAX is of a value making it possible to detect disconnection of the cylinder pressure sensor. When either the cylinder pressure sensor voltage V 0  is below the cylinder pressure sensor voltage minimum value VMIN or it exceeds the cylinder pressure sensor voltage maximum value VMAX, the procedure advances to step S 14 . Otherwise, the procedure advances to step S 15 . 
     In step S 14 , the computation cycle of the controller  44  is added to the timer Ta (the initial value of which is zero), and the procedure advances to step S 16 . 
     In step S 15 , the timer Ta is set to zero, and the procedure advances to step S 16 . 
     In step S 16 , the timer Ta and the timer threshold value Tth (e.g., 5 sec) are compared with each other. When the timer Ta is equal to or less than the timer threshold value Tth, the procedure advances to step S 17 , and when the timer Ta is more than the timer threshold value Tth, the procedure advances to step S 18 . 
     In step S 17 , the operation mode of the machine body is set to the normal operation mode (the initial state is the normal mode), and the procedure advances to step S 36 . 
     On the other hand, in step S 18 , the operation mode of the machine body is switched to the substitution operation mode, and the procedure advances to step S 19 . In step S 19 , the electric current instruction value of the solenoid valve  43  is reduced to a minimum value (which is an electric current value at which the solenoid valve  43  is maintained at the normal position and which can, for example, be zero), and the processing is completed. 
     In the case where the sensor used for controlling the operation of the control valve  41  such as the cylinder pressure sensor SE 5  suffers failure, it is difficult to property adjust the meter-out restrictor opening which is necessary for preventing the breathing phenomenon. Thus, in this case, the meter-out flow rate control should not be performed at least by the conventional method. 
     In view of this, in the present embodiment, the hydraulic excavator is constructed such that the controller  44  selects the substitution operation mode when abnormality of the sensor SE 5  is detected. 
     Due to this construction of the hydraulic excavator, even in the case where the sensor used for the meter-out flow rate control suffers failure and where the control valve  41  cannot be controlled by the conventional method, it is possible to prevent the breathing phenomenon by increasing the pump flow rate. 
     In particular, in the table T 3   a  of  FIG. 11 , the characteristic when the arm bottom pressure is minimum in the table T 3  of  FIG. 4  (that is, the characteristic in the case where the possibility of generation of breathing is highest) is utilized. When the pump correction flow rate ΔQ is thus computed, the hydraulic working fluid on the meter-in side is secured to the maximum even in the case where abnormality is generated in the bottom pressure sensor SE 5 , so that it is possible to prevent generation of the breathing phenomenon. 
     Fourth Embodiment 
     Next, the fourth embodiment of the present invention will be described. In the fourth embodiment, when abnormality of the meter-out control valve control device is overcome, and a permission signal permitting the change from the substitution operation mode to the normal operation mode is input, switching is effected from the substitution operation mode to the normal operation mode. 
       FIG. 14  is a flowchart illustrating the operation mode switching control according to the fourth embodiment. Otherwise, the present embodiment is of the same construction as the second embodiment, and a redundant description thereof will be left out. 
     In step S 8 , the timer Ta and the timer threshold value Tth (e.g., 5 sec) are compared with each other. When the timer Ta is equal to or lower than the timer threshold value Tth, the procedure advances to step S 42 . 
     In step  42 , the controller  44  determines whether or not the current operation mode is the normal operation mode. In the case of the normal operation mode, the procedure advances to step S 9 , and, in the case of the substitution operation mode, the procedure advances to step S 44 . 
     In step S 44 , a flag for determining whether or not failure of the solenoid proportional valve  43  is overcome (which is referred to as the normal flag) is set to 1, and the procedure advances to step S 36 . When the normal flag is 0, it indicates that abnormality is generated in the solenoid proportional valve  43 , and when the normal flag is 1, it indicates that the abnormality of the solenoid proportional valve  43  is overcome. 
     In the case where it is determined in step S 36  that the key switch is at the OFF position and where the non-operation of the front work device  30  is secured, it is determined in step S 48  whether or not the normal flag is 1. When the normal flag is 1, the operation mode is changed from the substitution operation mode to the normal operation mode to complete the processing. When the normal flag is 0, the processing is completed, with the operation mode remaining the normal operation mode. In step S 36 , it is determined whether or not the key switch is at the OFF position based on a signal (referred to as the permission signal) which is input to the controller  44  when the key switch is switched to the OFF position. The permission signal is a signal permitting the change from the substitution operation mode to the normal operation mode. 
     When the operation mode is restored from the substitution operation mode to the normal operation mode using as a trigger solely the fact that the abnormality of the solenoid proportional valve  43  is overcome, there is the possibility of the operation mode being changed during the operation of the front work device  30  to thereby impair the operational sensation for the operator. 
     However, in the hydraulic excavator constructed as described above, the operation mode is restored to the normal operation mode using as a trigger the fact that the abnormality generated in the solenoid proportional valve  43  is overcome and that the key switch is switched to the OFF position to guarantee the non-operation of the front work device  30 . Thus, it is possible to avoid a change in operation mode during the operation of the front work device  30 , making it possible to maintain a satisfactory operational sensation for the operator. Further, in the case where the abnormality of the solenoid proportional valve  43  is overcome, it is possible for the operation mode to be quickly restored to the normal operation mode. 
     While in the above description the permission signal is output to the controller  44  when the key switch is switched to the OFF position, the permission signal may also be output in other cases so long as the non-operation of the front work device  30  is guaranteed. For example, the permission signal can be output in the following cases: a case where the key switch is switched to the ON position or the start position; a case where there is erected a gate lock lever (not shown) controlling as to whether or not the pilot pressure is output from the pilot valve  42  to the control valve  41  (a case where switching is effected to the pilot pressure interrupting position); a case where an automatic idling control of the engine  22  is started; and a case where the operation lever  42   a  is not operated for a predetermined period of time. Further, a dedicated switch for the output of the permission signal may be installed in the cab, making it possible to output the permission signal with a timing as desired by the operator. In this case, the control of the present embodiment is also applicable to the first embodiment. 
     The present embodiment is also applicable to the case where abnormality of a sensor according to the third embodiment (e.g., the sensor SE 5 ) is overcome. 
     Additional Remark 
     While in the above description the pressure sensor SE 5  detecting the bottom pressure of the arm cylinder  34  is utilized as the load sensor of the arm cylinder  34 , the pressure sensor SE 6  may be utilized as the load sensor in addition to the pressure sensor SE 5 . In this case, it is possible to detect the load of the arm cylinder  34  from the differential pressure between the pressure sensors SE 5  and SE 6 . Further, instead of the pressure sensor SE 5 , the pressure sensor SE 2  detecting the pump delivery pressure may be utilized as the load sensor. 
     The first embodiment is constructed such that, from the viewpoint of preventing a frequent change in the operation mode as a result of frequent fluctuation in a short period of time of the hydraulic working fluid temperature around the threshold value T 1 , the substitution mode is selected when the hydraulic working fluid temperature T 0  is below the threshold value T 1 , and the normal operation mode is selected when the hydraulic working fluid temperature T 0  attains a value (T 2 ) that is equal to or more than the threshold value TO. That is, the two threshold values of T 1  and T 2  are used. However, in the case of use etc. in an environment in which the hydraulic working fluid temperature increases or decreases monotonously, only one threshold value may be used. Further, while in the above example the maximum value of the temperature range where the meter-out opening limitation control is difficult is T 1 , this should not be construed restrictively. A desired value can be set as T 1  in accordance with the viscosity of the hydraulic working fluid. 
     While in the flowchart of each embodiment described above the point in time when the key switch is switched to the start position (S 1  and S 2 ) is the actual point in time when the processing is started, steps S 1  and S 2  may be omitted, starting the processing at an appropriate point in time after the activation of the controller and after the start of the engine. Further, the order in which the processing of each flowchart is conducted may be changed as appropriate so long as the result attained is the same. 
     While in the above description the flow rate control of the meter-out passage (actuator line) L 4  is performed by the restrictor  41   j  in the control valve  41 , the meter-out flow rate control system is not restricted thereto but allows various modifications. For example, it is possible to connect some other passage to the actuator line L 4 , and to control the opening area of a variable restrictor provided in that passage. Further, the meter-out flow rate may be controlled by the sum total of the opening area of that variable restrictor and that of the restrictor  41   j.    
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
           10 : Track structure 
           11 : Crawler 
           12 : Crawler frame 
           13 : Traveling hydraulic motor 
           20 : Swing structure 
           21 : Swing frame 
           22 : Engine 
           23 : Hydraulic pump 
           23   a : Pump regulator 
           24 : Control valve unit 
           25 : Swing hydraulic motor 
           30 : Front work device 
           31 : Boom 
           32 : Boom cylinder 
           33 : Arm 
           34 : Arm cylinder (hydraulic actuator) 
           35 : Bucket 
           36 : Bucket cylinder 
           40 : Hydraulic working fluid tank 
           41 : Control valve (meter-out control valve) 
           42 : Pilot valve (operation device) 
           43 : Solenoid proportional valve 
           44 : Controller (control device) 
         SE 1 : Engine speed pickup sensor 
         SE 2 : Pump delivery pressure sensor 
         SE 3 : Operation pilot pressure sensor (arm crowding operation) 
         SE 4 : Operation pilot pressure sensor (arm damping operation) 
         SE 5 : Arm bottom pressure sensor 
         SE 6 : Arm rod pressure sensor 
         SE 7 : Hydraulic working fluid temperature sensor 
         SW 1 : Switch 
         SW 2 : Switch 
         SW 3 : Switch 
         L 1 : Delivery line 
         L 2 : Center bypass line 
         L 3 : Actuator line (arm bottom side) 
         L 4 : Actuator line (arm rod side meter-out passage) 
         L 5 : Pilot line (arm crowding) 
         L 6 : Pilot line (arm damping) 
         L 7 : Drain hydraulic fluid line