Patent Publication Number: US-11047112-B2

Title: Control system, work machine, and control method

Description:
FIELD 
     The present invention relates to a control system, a work machine, and a control method. 
     BACKGROUND 
     An excavator is known as a kind of work machine having a work unit. The work unit of the excavator is driven by a hydraulic cylinder. The hydraulic cylinder is actuated by hydraulic fluid discharged from a hydraulic pump. Patent Literature 1 discloses a hydraulic control device having a merging-separating valve that performs switching between a merged state in which hydraulic fluid discharged from a first hydraulic pump and hydraulic fluid discharged from a second hydraulic pump are merged and a separated state in which these two kinds of hydraulic fluid are not merged. In the separated state, a first hydraulic actuator is actuated by the hydraulic fluid discharged from the first hydraulic pump, and a second hydraulic actuator is actuated by the hydraulic fluid discharged from the second hydraulic pump. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: WO 2005/047709 A1 
     SUMMARY 
     Technical Problem 
     Each of a first hydraulic pump and a second hydraulic pump is driven by an engine. When output of an engine is decreased, a flow rate of hydraulic fluid discharged from each of the first hydraulic pump and the second hydraulic pump is decreased. In the case where a separated state is kept when the output of the engine is decreased, the flow rate of the hydraulic fluid supplied to each of a first hydraulic actuator and a second hydraulic actuator is decreased. As a result, an actuation speed of the work unit may be decreased, and workability of the work machine may be degraded. 
     An aspect of the present invention is directed to providing a technique in which an actuation speed of a work unit can be prevented from being decreased when output of an engine is decreased. Solution to Problem 
     According to an aspect of the present invention, a control system comprises: an engine; a first hydraulic pump and a second hydraulic pump driven by the engine; a switching device provided in a flow path that connects the first hydraulic pump to the second hydraulic pump, and configured to perform switching between a merged state in which the flow path is opened and a separated state in which the flow path is closed; a first hydraulic actuator to which hydraulic fluid discharged from the first hydraulic pump is supplied in the separated state; a second hydraulic actuator to which hydraulic fluid discharged from the second hydraulic pump is supplied in the separated state; a determining unit configured to determine whether output of the engine is limited; and a merging-separating control unit configured to control the switching device so as to perform switching to the merged state when the determining unit determines that output of the engine is limited. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     According to the aspect of the present invention, provided is the technique in which the actuation speed of the work unit can be prevented from being decreased when output of the engine is decreased. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view illustrating an exemplary work machine according to the present embodiment. 
         FIG. 2  is a diagram schematically illustrating an exemplary control system according to the present embodiment. 
         FIG. 3  is a diagram schematically illustrating an exemplary engine and an exemplary exhaust gas treatment device according to the present embodiment. 
         FIG. 4  is a diagram illustrating an exemplary hydraulic system according to the present embodiment. 
         FIG. 5  is a functional block diagram illustrating an exemplary control device according to the present embodiment. 
         FIG. 6  is a diagram illustrating an exemplary torque chart of an engine according to the present embodiment. 
         FIG. 7  is a flowchart illustrating an exemplary control method for the work machine according to the present embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following, an embodiment of the present invention will be described with reference to the drawings, but the present invention is not limited thereto. Note that components of each embodiment described hereafter can be suitably combined. Additionally, there may be a case where some of the components are not used. 
     [Work Machine] 
       FIG. 1  is a perspective view illustrating an exemplary work machine  1  according to the present embodiment. In the present embodiment, it is assumed that the work machine  1  is an excavator of a hybrid system. In the following description, the work machine  1  will be referred to as an excavator  1  as appropriate. 
     As illustrated in  FIG. 1 , the excavator  1  includes a work unit  10 , an upper swing body  2  that supports the work unit  10 , a lower traveling body  3  that supports the upper swing body  2 , an engine  4 , a generator motor  27  driven by the engine  4 , a hydraulic pump  30  driven by the engine  4 , a hydraulic cylinder  20  that actuates the work unit  10 , an electric motor  25  that swings the upper swing body  2 , a hydraulic motor  24  that causes the lower traveling body  3  to travel, an operation device  5  configured to operate the work unit  10 , a control device  100 , and an exhaust gas treatment device  200  that treats an exhaust gas of the engine  4 . 
     The engine  4  is an internal combustion engine that is a power source of the excavator  1 . The engine  4  has an output shaft  4 S connected to the generator motor  27  and the hydraulic pump  30 . The engine  4  is, for example, a diesel engine. The engine  4  is housed in a machine room  7  of the upper swing body  2 . 
     The generator motor  27  is connected to the output shaft  4 S of the engine  4 , and generates power by actuation of the engine  4 . The generator motor  27  is, for example, a switched reluctance motor. Note that the generator motor  27  may also be a permanent magnet (PM) motor. 
     The hydraulic pump  30  is connected to the output shaft  4 S of the engine  4 , and discharges hydraulic fluid by actuation of the engine  4 . In the present embodiment, the hydraulic pump  30  is connected to the output shaft  4 S, and includes: a first hydraulic pump  31  driven by the engine  4 ; and a second hydraulic pump  32  connected to the output shaft  4 S and driven by the engine  4 . The hydraulic pump  30  is housed in the machine room  7  of the upper swing body  2 . 
     The work unit  10  is supported by the upper swing body  2 . The work unit  10  includes a plurality of work unit elements which are movable relative to each other. The work unit elements of the work unit  1  includes a bucket  11 , an arm  12  connected to the bucket  11 , and a boom  13  connected to the arm  12 . The bucket  11  is rotatably connected to a distal end portion of the arm  12 . The arm  12  is rotatably connected to a distal end portion of the boom  13 . The boom  13  is rotatably connected to the upper swing body  2 . 
     The hydraulic cylinder  20  is actuated by hydraulic fluid supplied from the hydraulic pump  30 . The hydraulic cylinder  20  is a hydraulic actuator that generates power to actuate the work unit  10 . The work unit  10  can be actuated by the power generated by the hydraulic cylinder  20 . The hydraulic cylinder  20  includes a bucket cylinder  21  to actuate a bucket  11 , an arm cylinder  22  to actuate an arm  12 , and a boom cylinder  23  to actuate a boom  13 . 
     The electric motor  25  is actuated by power supplied from the generator motor  27 . The electric motor  25  is an electric actuator that generates power to swing the upper swing body  2 . The upper swing body  2  can swing about a swing shaft RX by the power generated by the electric motor  25 . 
     The hydraulic motor  24  is actuated by hydraulic fluid supplied from the hydraulic pump  30 . The hydraulic motor  24  is a hydraulic actuator that generates power to cause the lower traveling body  3  to travel. A crawler belt  3 C of the lower traveling body  3  can be rotated by the power generated by the hydraulic motor  24 . 
     The upper swing body  2  has a fuel tank  8  to store fuel and a hydraulic fluid tank  9  to store hydraulic fluid. The fuel stored in the fuel tank  8  is supplied to the engine  4 . The hydraulic fluid stored in the hydraulic fluid tank  9  is supplied to the hydraulic cylinder  20  and the hydraulic motor  24  via the hydraulic pump  30 . 
     The operation device  5  is arranged in an operating room  6 . The operation device  5  is operated in order to drive each of the hydraulic cylinder  20  and the hydraulic motor  24 . The operation device  5  includes an operating member to be operated by an operator of the excavator  1 . The operating member includes an operating lever or a joystick. When the operation device  5  is operated, the work unit  10  is actuated. 
     [Control System] 
       FIG. 2  is a diagram schematically illustrating an exemplary control system  1000  according to the present embodiment. The control system  1000  is mounted on the excavator  1  and controls the excavator  1 . The control system  1000  includes a control device  100 , a hydraulic system  1000 A, and an electric system  1000 B. 
     The hydraulic system  1000 A has the hydraulic pump  30 , a hydraulic circuit  40  where hydraulic fluid discharged from the hydraulic pump  30  flows, the hydraulic cylinder  20  actuated by hydraulic fluid supplied from the hydraulic pump  30  via the hydraulic circuit  40 , and the hydraulic motor  24  actuated by hydraulic fluid supplied from the hydraulic pump  30  via the hydraulic circuit  40 . 
     The output shaft  4 S of the engine  4  is connected to the hydraulic pump  30 . When the engine  4  is driven, the hydraulic pump  30  is actuated. The hydraulic cylinder  20  and the hydraulic motor  24  are actuated on the basis of the hydraulic fluid discharged from the hydraulic pump  30 . An engine speed sensor  4 R that detects an engine speed [rpm] of the engine  4  is provided in the engine  4 . 
     The hydraulic pump  30  is a variable displacement hydraulic pump. In the present embodiment, the hydraulic pump  30  is a swash plate hydraulic pump. A swash plate  30 A of the hydraulic pump  30  is driven by a servo mechanism  30 B. A capacity [cc/rev] of the hydraulic pump  30  is adjusted by adjusting an angle of the swash plate  30 A by the servo mechanism  30 B. The capacity of the hydraulic pump  30  represents a discharge amount [cc/rev] of the hydraulic fluid discharged from the hydraulic pump  30  when the output shaft  4 S of the engine  4  connected to the hydraulic pump  30  is rotated once. 
     In the present embodiment, the swash plate  30 A of the hydraulic pump  30  includes a swash plate  31 A of the first hydraulic pump  31  and a swash plate  32 A of the second hydraulic pump  32 . The servo mechanism  30 B includes: a servo mechanism  31 B to adjust an angle of the swash plate  31 A of the first hydraulic pump  31 ; and a servo mechanism  32 B to adjust an angle of the swash plate  32 A of the second hydraulic pump  32 . 
     The electric system  1000 B has the generator motor  27 , a storage battery  14 , a transformer  14 C, a first inverter  15 G, a second inverter  15 R, and the electric motor  25  actuated by the power supplied from the generator motor  27 . 
     The output shaft  4 S of the engine  4  is connected to the generator motor  27 . When the engine  4  is driven, the generator motor  27  is actuated. When the engine  4  is driven, a rotor of the generator motor  27  is rotated. The generator motor  27  generates power by rotation of the rotor of the generator motor  27 . Meanwhile, the generator motor  27  may also be connected to the output shaft  4 S of the engine  4  via a power transmission mechanism such as a power take off (PTO). 
     The electric motor  25  is actuated on the basis of power output from the generator motor  27 . The electric motor  25  generates power to swing the upper swing body  2 . A rotation sensor  16  is provided at the electric motor  25 . The rotation sensor  16  includes, for example, a resolver or a rotary encoder. The rotation sensor  16  detects a rotation angle or a rotation speed of the electric motor  25 . 
     The operating room  6  is provided with the operation device  5 , a throttle dial  33 , and a work mode selector  34  which are operated by an operator. 
     The operation device  5  includes an operating member to operate the lower traveling body  3 , an operating member to operate the upper swing body  2 , and an operating member to operate the work unit  10 . The hydraulic motor  24  that causes the lower traveling body  3  to travel is actuated on the basis of operation of the operation device  5 . The electric motor  25  that swings the upper swing body  2  is actuated on the basis of operation of the operation device  5 . The hydraulic cylinder  20  that actuates the work unit  10  is actuated on the basis of operation of the operation device  5 . 
     In the present embodiment, the operation device  5  includes: a right operating lever  5 R arranged on a right side of an operator seated on an operator&#39;s seat  6 S; and a left operating lever  5 L arranged on a left side thereof. 
     Further, the operation device  5  has a travel lever (not illustrated). A travel motor  24  is driven by operating the travel lever. 
     The control system  1000  has an operation amount sensor  90  that detects an operation amount of the operation device  5 . The operation amount sensor  90  includes: a bucket operation amount sensor  91  that detects an operation amount of the operation device  5  operated in order to drive the bucket cylinder  21  that actuates the bucket  11 ; an arm operation amount sensor  92  that detects an operation amount of the operation device  5  operated in order to drive the arm cylinder  22  that actuates the arm  12 ; and a boom operation amount sensor  93  that detects an operation amount of the operation device  5  operated in order to drive the boom cylinder  23  that actuates the boom  13 . 
     The throttle dial  33  is an operating member to set a fuel injection amount to be injected to the engine  4 . An upper limit engine speed Nmax [rpm] of the engine  4  is set by the throttle dial  33 . 
     The work mode selector  34  is an operating member to set an output characteristic of the engine  4 . Maximum output [kW] of the engine  4  is set by the work mode selector  34 . 
     The control device  100  includes a computer system. The control device  100  has an arithmetic processing device including a processor such as a central processing unit (CPU), a storage device including a memory such as a read only memory (ROM) or a random access memory (RAM), and an input/output interface device. The control device  100  outputs command signals to control the hydraulic system  1000 A and the electric system  1000 B. In the present embodiment, the control device  100  includes a pump controller  100 A to control the hydraulic system  1000 A, a hybrid controller  100 B to control the electric system  1000 B, and an engine controller  100 C to control the engine  4 . 
     The pump controller  100 A outputs a command signal to control the first hydraulic pump  31  and the second hydraulic pump  32  on the basis of at least one of a command signal transmitted from the hybrid controller  100 B, a command signal transmitted from the engine controller  100 C, and a detection signal transmitted from the operation amount sensor  90 . 
     In the present embodiment, the pump controller  100 A outputs a command signal to adjust the capacity [cc/rev] of the hydraulic pump  30 . The pump controller  100 A adjusts the capacity [cc/rev] of the hydraulic pump  30  by outputting a command signal to the servo mechanism  30 B and controlling the angle of the swash plate  30 A of the hydraulic pump  30 . The hydraulic pump  30  has a swash plate angle sensor  30 S that detects the angle of the swash plate  30 A. The inclination angle sensor  30 S includes an inclination angle sensor  31 S to detect the angle of the swash plate  31 A and an inclination angle sensor  32 S to detect the angle of the swash plate  32 A. A detection signal of the swash plate angle sensor  30 S is output to the pump controller  100 A. The pump controller  100 A controls the angle of the swash plate  30 A by outputting a command signal to the servo mechanism  30 B on the basis of the detection signal of the swash plate angle sensor  30 S. 
     The hydraulic pump  30  is driven by the engine  4 . When the engine speed [rpm] of the engine  4  is increased and the engine speed per unit time of the output shaft  4 S of the engine  4  connected to the hydraulic pump  30  is increased, a discharge flow rate Q [1/min] of hydraulic fluid discharged from the hydraulic pump  30  per unit time is increased. When the engine speed [rpm] of the engine  4  is decreased and the engine speed per unit time of the output shaft  4 S of the engine  4  connected to the hydraulic pump  30  is decreased, a discharge flow rate Q [1/min] of hydraulic fluid discharged from the hydraulic pump  30  per unit time is decreased. 
     When the engine  4  is driven at a maximum engine speed [rpm] in a state in which the hydraulic pump  30  is adjusted to a maximum capacity [cc/rev], the hydraulic pump  30  discharges hydraulic fluid at a maximum discharge flow rate Qmax [1/min]. 
     In the present embodiment, the pump controller  100 A outputs a command signal to adjust each of a capacity [cc/rev] of the first hydraulic pump  31  and a capacity [cc/rev] of the second hydraulic pump  32 . 
     The pump controller  100 A outputs a command signal to the servo mechanism  31 B on the basis of a detection signal of the swash plate angle sensor  31 S and controls the angle of the swash plate  31 A of the first hydraulic pump  31 , thereby adjusting the capacity [cc/rev] of the first hydraulic pump  31 . The pump controller  100 A outputs a command signal to the servo mechanism  32 B on the basis of a detection signal of the swash plate angle sensor  32 S and controls the angle of the swash plate  32 A of the second hydraulic pump  32 , thereby adjusting the capacity [cc/rev] of the second hydraulic pump  32 . 
     The discharge flow rate Q [1/min] of the hydraulic fluid discharged from the hydraulic pump  30  includes: a discharge flow rate Q 1  [1/min] of the hydraulic fluid discharged from the first hydraulic pump  31 ; and a discharge flow rate Q 2  [1/min] of the hydraulic fluid discharged from the second hydraulic pump  32 . When the engine speed of the engine  4  is increased and the engine speed per unit time of the output shaft  4 S of the engine  4  connected to the first hydraulic pump  31  and the second hydraulic pump  32  is increased, the discharge flow rate Q 1  [1/min] of the first hydraulic pump  31  and the discharge flow rate Q 2  [1/min] of the second hydraulic pump  32  are increased. When the engine speed of the engine  4  is decreased and the engine speed per unit time of the output shaft  4 S of the engine  4  connected to the first hydraulic pump  31  and the second hydraulic pump  32  is decreased, the discharge flow rate Q 1  [1/min] of the first hydraulic pump  31  and the discharge flow rate Q 2  [1/min] of the second hydraulic pump  32  are decreased. 
     The maximum discharge flow rate Qmax [1/min] of the hydraulic pump  30  includes: a maximum discharge flow rate Q 1 max [1/min] of the first hydraulic pump  31 ; and a maximum discharge flow rate Q 2 max [1/min] of the second hydraulic pump  32 . When the engine  4  is driven at the maximum engine speed with the first hydraulic pump  31  adjusted to the maximum capacity [cc/rev], the first hydraulic pump  31  discharges hydraulic fluid with the maximum discharge flow rate Q 1 max. Similarly, when the engine  4  is driven at the maximum engine speed with the second hydraulic pump  32  adjusted to the maximum capacity [cc/rev], the second hydraulic pump  32  discharges the hydraulic fluid at the maximum discharge flow rate Q 2 max. In the present embodiment, the maximum discharge flow rate Q 1 max and the maximum discharge flow rate Q 2 max are equal. 
     The hybrid controller  100 B controls the electric motor  25  on the basis of a detection signal of the rotation sensor  16 . The electric motor  25  is actuated on the basis of power supplied from the generator motor  27  or the storage battery  14 . In the present embodiment, the hybrid controller  100 B performs: control for power transfer among the transformer  14 C, the first inverter  15 G, and the second inverter  15 R; and control for power transfer between the transformer  14 C and the storage battery  14 . 
     Furthermore, the hybrid controller  100 B controls a temperature in each of the generator motor  27 , electric motor  25 , storage battery  14 , first inverter  15 G, and second inverter  15 R on the basis of a detection signal of a temperature sensor provided in each of the generator motor  27 , electric motor  25 , storage battery  14 , first inverter  15 G, and second inverter  15 R. Additionally, the hybrid controller  100 B performs: control for charge/discharge of the storage battery  14 ; control for the generator motor  27 ; 
     and assist control for the engine  4  by the generator motor  27 . 
     The engine controller  100 C generates a command signal on the basis of a setting value of the throttle dial  33  and outputs the same to a common rail control unit  29  provided in the engine  4 . The common rail control unit  29  adjusts a fuel injection amount to the engine  4  on the basis of a command signal transmitted from the engine controller  100 C. 
     [Engine and Exhaust Gas Treatment Device] 
       FIG. 3  is a diagram schematically illustrating an exemplary engine  4  and an exemplary exhaust gas treatment device  200  according to the present embodiment. The exhaust gas treatment device  200  treats an exhaust gas of the engine  4 . In the present embodiment, the exhaust gas treatment device  200  includes a urea selective catalytic reduction (SCR) system to reduce and purify nitrogen oxides (NOx) contained in the exhaust gas by utilizing a selective catalyst and a reducing agent. 
     The engine  4  has a fuel injection device  17 . The fuel injection device  17  injects fuel to a combustion chamber of the engine  4 . In the present embodiment, the fuel injection device  17  is a common rail system including an accumulator  17 A and an injector  17 B. The fuel injection device  17  is controlled by a control device  50  via the common rail control unit  29 . 
     The engine  4  is connected to each of an intake pipe  18  and an exhaust pipe  19 . An inlet of the intake pipe  18  is connected to an air cleaner  35  that collects a foreign matter in the air. An outlet of the intake pipe  18  is connected to an intake port of the engine  4 . The exhaust gas treatment device  200  is connected to an exhaust port of the engine  4  via the exhaust pipe  19 . 
     The exhaust gas treatment device  200  purifies the exhaust gas discharged from the engine  4 . The exhaust gas treatment device  200  decreases nitrogen oxides (NOx) contained in the exhaust gas. The exhaust gas treatment device  20  includes: a filter unit  201  connected to the exhaust pipe  19  and configured to collect particulates contained in the exhaust gas; a reducing catalyst  203  connected to the filter unit  201  via a pipe line  202  and configured to reduce NOx contained in the exhaust gas; and a reducing agent supply device  204  to supply a reducing agent R. 
     The filter unit  201  includes a diesel particulate filter (DPF) and collects the particulates contained in the exhaust gas. 
     The reducing catalyst  203  reduces NOx contained in the exhaust gas by the reducing agent R supplied from the reducing agent supply device  204 . The reducing catalyst  203  converts NOx into nitrogen and water by the reducing agent R. For example, a vanadium catalyst or a zeolite catalyst is used as the reducing catalyst  203 . 
     The reducing agent supply device  204  supplies the reducing agent R to the pipe line  202 . The reducing agent R is urea (aqueous urea). The reducing agent supply device  204  includes: a reducing agent tank  205  to store the reducing agent R; a supply pipe  206  connected to the reducing agent tank  205 ; a supply pump  207  provided in the supply pipe  206 ; and an injection nozzle  208  connected to the supply pipe  207 . The supply pump  207  pumps the reducing agent R stored in the reducing agent tank  205  to the injection nozzle  208 . The injection nozzle  208  injects the reducing agent R supplied from the reducing agent tank  205  to the inside of the pipe line  202 . 
     A supply amount (injection amount) of the reducing agent R by the reducing agent supply device  204  is controlled by the control device  100 . The reducing agent R supplied to the inside of the pipe line  202  is decomposed by heat of the exhaust gas, and changed into ammonia. In the paraphrase catalyst  203 , NOx and ammonia cause catalytic reaction and are converted into nitrogen and water. 
     In the present embodiment, a reducing agent sensor  209  that detects an amount (liquid level) of the reducing agent R is provided in the reducing agent tank  205  of the reducing agent supply device  204 . 
     Furthermore, in the present embodiment, the control system  1000  includes an exhaust gas sensor  300  in order to detect a state of the engine  4 . The exhaust gas sensor  300  detects the state of the engine  4  by detecting a state of the exhaust gas from the engine  4 . The state of the exhaust gas includes at least one of a concentration of NOx contained in the exhaust gas, a pressure of the exhaust gas, a temperature of the exhaust gas, and a flow rate of the exhaust gas. The reducing agent supply device  204  adjusts a supply amount of the reducing agent R to be supplied to the reducing catalyst  203  on the basis of a detection signal of the exhaust gas sensor  300 . 
     In the present embodiment, the exhaust gas sensor  300  includes an NOx sensor  301  that detects a concentration of NOx contained in an exhaust gas, a pressure sensor  302  and a pressure sensor  304  each of which detects a pressure of the exhaust gas, and a temperature sensor  303  that detects a temperature of the exhaust gas. 
     The NOx sensor  301  detects the concentration of NOx in an exhaust gas in the exhaust pipe  19 . The pressure sensor  302  detects a pressure of an exhaust gas in the pipe line  202 . The temperature sensor  303  detects a temperature of the exhaust gas in the pipe line  202 . The pressure sensor  304  detects a pressure of an exhaust gas having passed through the reducing catalyst  203 . 
     Additionally, the exhaust gas sensor  300  includes an intake air flow rate sensor  305  that detects a flow rate of the air taken into the engine  4  via the intake pipe  18 . The flow rate of the exhaust gas is determined on the basis of the flow rate of the air taken into the engine  4 . The intake air flow rate sensor  305  functions as an exhaust gas flow rate sensor. 
     A detection signal of the NOx sensor  301 , a detection signal of the pressure sensor  302 , a detection signal of the temperature sensor  303 , a detection signal of the pressure sensor  304 , and a detection signal of the intake air flow rate sensor  305  are output to the control device  100 . 
     The control device  100  controls the supply amount of the reducing agent R to be supplied to the reducing catalyst  203  on the basis of at least the detection signal of the NOx sensor  301  and the detection signal of the pressure sensor  302 . For example, the control device  100  calculates a flow rate of the exhaust gas supplied from the pipe line  202  to the reducing catalyst  203  on the basis of the detection signal of the pressure sensor  302 . The control device  100  calculates a flow rate of NOx in the pipe line  202  on the basis of the flow rate of the exhaust gas in the pipe line  202  and the concentration of NOx in the exhaust gas detected by the NOx sensor  301 . The control device  100  determines the supply amount of the reducing agent R to be supplied to the reducing catalyst  203  on the basis of the flow rate of NOx in the pipe line  202 . 
     Meanwhile, the control device  100  may calculate the flow rate of the exhaust gas in the pipe line  202  on the basis of the detection signal of the intake air flow rate sensor  305  and a fuel injection amount supplied from the fuel injection device  17  to the engine  4 . 
     Meanwhile, the control device  100  may also control the supply amount of the reducing agent R to be supplied to the reducing catalyst  203  on the basis of the detection signal of the NOx sensor  301 , detection signal of the pressure sensor  302 , detection signal of the temperature sensor  303 , and detection signal of the pressure sensor  304 . 
     Furthermore, the exhaust gas sensor  300  includes an atmospheric pressure sensor  306 , an outside air temperature sensor  307 , and a coolant temperature sensor  308 . The atmospheric pressure sensor  306  detects an atmospheric pressure which is an environmental pressure at which the engine  4  and the exhaust gas treatment device  200  are used. Detects an outside air temperature which is an environmental temperature at which the engine  4  and the exhaust gas treatment device  200  are used. The coolant temperature sensor  308  detects a temperature of coolant that cools the engine  4 . 
     The NOx sensor  301  requires a certain period to be able to detect NOx after the engine  4  is started and the NOx sensor  301  is started. The NOx sensor  301  is required to keep a sensing portion at a high temperature due to a structure thereof. That is why the certain period is required for the NOx sensor  301  to be able to detect a concentration of NOx after the engine  4  is started. During a period in which the concentration of NOx cannot be detected by using the NOx sensor  301 , the control device  100  estimates the concentration of NOx on the basis of a detection signal of the engine speed sensor  4 R, a detection signal of the atmospheric pressure sensor  306 , a detection signal of the outside air temperature sensor  307 , and a detection signal of the coolant temperature sensor  308 , and controls the supply amount of the reducing agent R to be supplied from the reducing agent supply device  204  to the reducing catalyst  203  on the basis of the estimated NOx concentration. 
     [Hydraulic System] 
       FIG. 4  is a diagram illustrating an example of the hydraulic system  1000 A according to the present embodiment. 
     The hydraulic system  1000 A includes: the hydraulic pump  30  that discharges hydraulic fluid; the hydraulic circuit  40  where hydraulic fluid discharged from the hydraulic pump  30  flows; the hydraulic cylinder  20  to which the hydraulic fluid discharged from the hydraulic pump  30  is supplied via the hydraulic circuit  40 ; a main operation valve  60  that adjusts a direction of hydraulic fluid supplied to the hydraulic cylinder  20  and a distribution flow rate Qa of the hydraulic fluid; and a pressure compensating valve  70 . 
     The hydraulic pump  30  includes the first hydraulic pump  31  and the second hydraulic pump  32 . The hydraulic cylinder  20  includes the bucket cylinder  21 , arm cylinder  22 , and boom cylinder  23 . 
     The main operation valve  60  includes: a first main operation valve  61  that adjusts a direction of hydraulic fluid supplied from the hydraulic pump  30  to the bucket cylinder  21  and a distribution flow rate Qabk of the hydraulic fluid; a second main operation valve  62  that adjusts a direction of hydraulic fluid supplied from the hydraulic pump  30  to the arm cylinder  22  and a distribution flow rate Qaar of the hydraulic fluid; and a third main operation valve  63  that adjusts a direction of hydraulic fluid supplied from the hydraulic pump  30  to the boom cylinder  23  and a distribution flow rate Qabm of the hydraulic fluid. The main operation valve  60  is a direction control valve of a slide spool system. 
     The pressure compensating valve  70  includes a pressure compensating valve  71 , a pressure compensating valve  72 , a pressure compensating valve  73 , a pressure compensating valve  74 , a pressure compensating valve  75 , and a pressure compensating valve  76 . 
     Additionally, the hydraulic system  1000 A includes a first merging-separating valve  67  that is a switching device provided in a merging flow path  55  that connects the first hydraulic pump  31  to the second hydraulic pump  32 , and capable of performing switching between a merged state in which the merging flow path  55  is opened and a separated state in which the merging flow path  55  is closed. 
     The hydraulic circuit  40  has: a first hydraulic pump flow path  41  connected to the first hydraulic pump  31 ; and a second hydraulic pump flow path  42  connected to the second hydraulic pump  32 . 
     The hydraulic circuit  40  has: a first supply flow path  43  and a second supply flow path  44  which are connected to the first hydraulic pump flow path  41 ; and a third supply flow path  45  and a fourth supply flow path  46  which are connected to the second hydraulic pump flow path  42 . 
     The first hydraulic pump flow path  41  is branched into the first supply flow path  43  and the second supply flow path  44  at a first branch portion Br 1 . The second hydraulic pump flow path  42  is branched into the third supply flow path  45  and the fourth supply flow path  46  at a fourth branch portion Br 4 . 
     The hydraulic circuit  40  has: a first branch flow path  47  and a second branch flow path  48  which are connected to the first supply flow path  43 ; and a third branch flow path  49  and a fourth branch flow path  50  which are connected to the second supply flow path  44 . The first supply flow path  43  is branched into the first branch flow path  47  and the second branch flow path  48  at a second branch portion Br 2 . The second supply flow path  44  is branched into the third branch flow path  49  and the fourth branch flow path  50  at a third branch portion Br 3 . 
     The hydraulic circuit  40  has: a fifth branch flow path  51  connected to the third supply flow path  45 ; and a sixth branch flow path  52  connected to the fourth supply flow path  46 . 
     The first main operation valve  61  is connected to the first branch flow path  47  and the third branch flow path  49 . The second main operation valve  62  is connected to the second branch flow path  48  and the fourth branch flow path  50 . The third main operation valve  63  is connected to the fifth branch flow path  51  and the sixth branch flow path  52 . 
     The hydraulic circuit  40  has: a first bucket flow path  21 A that connects the first main operation valve  61  to a cap-side space  210  of the bucket cylinder  21 ; and a second bucket flow path  21 B that connects the first main operation valve  61  to a rod-side space  21 L of the bucket cylinder  21 . 
     The hydraulic circuit  40  has: a first arm flow path  22 A that connects the second main operation valve  62  to a rod-side space  22 L of the arm cylinder  22 ; and a second arm flow path  22 B that connects the second main operation valve  62  to a cap-side space  22 C of the arm cylinder  22 . 
     The hydraulic circuit  40  has: a first boom flow path  23 A that connects the third main operation valve  63  to a cap-side space  23 C of the boom cylinder  23 ; and a second boom flow path  23 B that connects the third main operation valve  63  to a rod-side space  23 L of the boom cylinder  23 . 
     The cap-side space of the hydraulic cylinder  20  is a space between a cylinder head cover and a piston. The rod-side space of the hydraulic cylinder  20  is a space in which a piston rod is arranged. 
     When hydraulic fluid is supplied to the cap-side space  21 C of the bucket cylinder  21  and the bucket cylinder  21  is extended, the bucket  11  performs excavating operation. 
     When hydraulic fluid is supplied to the rod-side space  21 L of the bucket cylinder  21  and the bucket cylinder  21  is retracted, the bucket  11  performs dumping operation. 
     When hydraulic fluid is supplied to the cap-side space  22 C of the arm cylinder  22  and the arm cylinder  22  is extended, the arm  12  performs excavating operation. When hydraulic fluid is supplied to the rod-side space  22 L of the arm cylinder  22  and the arm cylinder  22  is retracted, the arm  12  performs dumping operation. 
     When hydraulic fluid is supplied to the cap-side space  23 C of the boom cylinder  23  and the boom cylinder  23  is extended, the boom  13  performs lifting operation. When hydraulic fluid is supplied to the rod-side space  23 L of the boom cylinder  23  and the boom cylinder  23  is retracted, the boom  13  performs lowering operation. 
     The first main operation valve  61  supplies hydraulic fluid to the bucket cylinder  21  and recovers hydraulic fluid discharged from the bucket cylinder  21 . A spool of the first main operation valve  61  is movable to: a stop position PTO whereby supply of hydraulic fluid to the bucket cylinder  21  is stopped to stop the bucket cylinder  21 ; a first position PT 1  whereby the first branch flow path  47  and the first bucket flow path  21 A are connected such that hydraulic fluid is supplied to the cap-side space  21 C and the bucket cylinder  21  is extended; and a second position PT 2  whereby the third branch flow path  49  and the second bucket flow path  21 B are connected such that hydraulic fluid is supplied to the rod-side space  21 L and the bucket cylinder  21  is retracted. The first main operation valve  61  is operated such that the bucket cylinder  21  becomes at least one of a stopped state, an extended state, and a retracted state. 
     The second main operation valve  62  supplies hydraulic fluid to the arm cylinder  22  and recovers hydraulic fluid discharged from the arm cylinder  22 . The second main operation valve  62  has a structure similar to that of the first main operation valve  61 . A spool of the second main operation valve  62  is movable to: a stop position whereby supply of hydraulic fluid to the arm cylinder  22  is stopped to stop the arm cylinder  22 ; a second position whereby the fourth branch flow path  50  and the second arm flow path  22 B are connected such that hydraulic fluid is supplied to the cap-side space  22 C and the arm cylinder  22  is extended; and a first position whereby the second branch flow path  48  and the first arm flow path  22 A are connected such that hydraulic fluid is supplied to the rod-side space  22 L and the arm cylinder  22  is retracted. The second main operation valve  62  is operated such that the arm cylinder  22  becomes at least one of a stopped state, an extended state, and a retracted state. 
     The third main operation valve  63  supplies hydraulic fluid to the boom cylinder  23  and recovers hydraulic fluid discharged from the boom cylinder  23 . The third main operation valve  63  has a structure similar to that of the first main operation valve  61 . A spool of the third main operation valve  63  is movable to: a stop position whereby supply of hydraulic fluid to the boom cylinder  23  is stopped to stop the boom cylinder  23 ; a first position whereby the fifth branch flow path  51  and the first boom flow path  23 A are connected such that hydraulic fluid is supplied to the cap-side space  23 C and the boom cylinder  23  is extended; and a second position whereby the sixth branch flow path  52  and the second boom flow path  23 B are connected such that hydraulic fluid is supplied to the rod-side space  23 L and the boom cylinder  23  is retracted. The third main operation valve  63  is operated such that the boom cylinder  23  becomes at least one of a stopped state, an extended state, and a retracted state. 
     The first main operation valve  61  is operated by the operation device  5 . When the operation device  5  is operated, a pilot pressure determined on the basis of an operation amount of the operation device  5  acts on the first main operation valve  61 . When the pilot pressure acts on the first main operation valve  61 , a direction of hydraulic fluid supplied from the first main operation valve  61  to the bucket cylinder  21  and a distribution flow rate Qabk of the hydraulic fluid are determined. A rod of the bucket cylinder  21  is moved in a moving direction corresponding to the direction of the supplied hydraulic fluid, and actuated at a cylinder speed corresponding to the distribution flow rate Qabk of the supplied hydraulic fluid. When the bucket cylinder  21  is actuated, the bucket  11  is actuated on the basis of the moving direction and the cylinder speed of the bucket cylinder  21 . 
     Similarly, the second main operation valve  62  is operated by the operation device  5 . When the operation device  5  is operated, a pilot pressure determined on the basis of an operation amount of the operation device  5  acts on the second main operation valve  62 . When the pilot pressure acts on the second main operation valve  62 , a direction of hydraulic fluid supplied from the second main operation valve  62  to the arm cylinder  22  and a distribution flow rate Qaar of the hydraulic fluid are determined. A rod of the arm cylinder  22  is moved in a moving direction corresponding to the direction of the supplied hydraulic fluid, and actuated at a cylinder speed corresponding to the distribution flow rate Qaar of the supplied hydraulic fluid. When the arm cylinder  22  is actuated, the arm  12  is actuated on the basis of the moving direction and the cylinder speed of the arm cylinder  22 . 
     Similarly, the third main operation valve  63  is operated by the operation device  5 . When the operation device  5  is operated, a pilot pressure determined on the basis of an operation amount of the operation device  5  acts on the third main operation valve  63 . When the pilot pressure acts on the third main operation valve  63 , a direction of hydraulic fluid supplied from the third main operation valve  63  to the boom cylinder  23  and a distribution flow rate Qabm of the hydraulic fluid are determined. A rod of the boom cylinder  23  is moved in a moving direction corresponding to the direction of the supplied hydraulic fluid, and actuated at a cylinder speed corresponding to the distribution flow rate Qabm of the supplied hydraulic fluid. When the boom cylinder  23  is actuated, the boom  13  is actuated on the basis of the moving direction and the cylinder speed of the boom cylinder  23 . 
     The hydraulic fluid discharged from each of the bucket cylinder  21 , arm cylinder  22 , and boom cylinder  23  is recovered in a hydraulic fluid tank  9  via a discharge flow path  53 . 
     The first hydraulic pump flow path  41  and the second hydraulic pump flow path  42  are connected by the merging flow path  55 . The merging flow path  55  is a flow path that connects the first hydraulic pump  31  to the second hydraulic pump  32 . The merging flow path  55  connects the first hydraulic pump  31  to the second hydraulic pump  32  via the first hydraulic pump flow path  41  and the second hydraulic pump flow path  42 . 
     The first merging-separating valve  67  is a switching device to open and close the merging flow path  55 . The first merging-separating valve  67  performs switching between a merged state in which the merging flow path  55  is opened and a separated state in which the merging flow path  55  is closed by opening and closing the merging flow path  55 . In the present embodiment, the first merging-separating valve  67  is a switching valve. Note that as far as the merging flow path  55  can be opened and closed, the switching device that opens and closes the merging flow path  55  may not necessarily be the switching valve. 
     A spool of the first merging-separating valve  67  is movable to: a merging position whereby the first hydraulic pump flow path  41  and the second hydraulic pump flow path  42  are connected by opening the merging flow path  55 ; and a separating position whereby the first hydraulic pump flow path  41  and the second hydraulic pump flow path  42  are separated by closing the merging flow path  55 . The control device  100  controls the first merging-separating valve  67  such that the first hydraulic pump flow path  41  and the second hydraulic pump flow path  42  to become any one of the merged state and the separated state. 
     The merged state represents a state in which: the first hydraulic pump flow path  41  and the second hydraulic pump flow path  42  are connected via the merging flow path  55  when the merging flow path  55  that connects the first hydraulic pump flow path  41  to the second hydraulic pump flow path  42  is opened by the first merging-separating valve  67 ; and hydraulic fluid discharged from the first hydraulic pump flow path  41  and hydraulic fluid discharged from the second hydraulic pump flow path  42  are merged at the first merging-separating valve  67 . In the merged state, the hydraulic fluid discharged from both of the first hydraulic pump  31  and the second hydraulic pump  32  is supplied to each of the bucket cylinder  21 , the arm cylinder  22 , and the boom cylinder  23 . 
     The separated state represents a state in which: the first hydraulic pump flow path  41  and the second hydraulic pump flow path  42  are separated from each other when the merging flow path  55  that connects the first hydraulic pump flow path  41  to the second hydraulic pump flow path  42  is closed by the first merging-separating valve  67 ; and the hydraulic fluid discharged from the first hydraulic pump flow path  41  and the hydraulic fluid discharged from the second hydraulic pump flow path  42  are separated. In the separated state, the hydraulic fluid discharged from the first hydraulic pump  31  is supplied to the bucket cylinder  21  and the arm cylinder  22 , and the hydraulic fluid discharged from the second hydraulic pump  32  is supplied to the boom cylinder  23 . 
     In other words, in the present embodiment, the first hydraulic actuator to which the hydraulic fluid discharged from the first hydraulic pump  31  is supplied in the separated state corresponds to the bucket cylinder  21  that drives the bucket  11  and the arm cylinder  22  that drives the arm  12 . The second hydraulic actuator to which the hydraulic fluid discharged from the second hydraulic pump  32  is supplied in the separated state corresponds to the boom cylinder  23  that drives the boom  13 . In the separated state, the hydraulic fluid discharged from the first hydraulic pump  31  is not supplied to the boom cylinder  23 . In the separated state, the hydraulic fluid discharged from the second hydraulic pump  32  is not supplied to the bucket cylinder  21  and the arm cylinder  22 . 
     In the merged state, the hydraulic fluid discharged from each of the first hydraulic pump  31  and the second hydraulic pump  32  passes through each of the first hydraulic pump flow path  41 , second hydraulic pump flow path  42 , first main operation valve  61 , second main operation valve  62 , and third main operation valve  63  and then is supplied to each of the bucket cylinder  21 , arm cylinder  22 , and boom cylinder  23 . 
     In the separated state, the hydraulic fluid discharged from the first hydraulic pump  31  passes through the first hydraulic pump flow path  41 , first main operation valve  61 , and second main operation valve  62  and then is supplied to the bucket cylinder  21  and arm cylinder  22 . Additionally, in the separated state, the hydraulic fluid discharged from the second hydraulic pump  32  passes through the second hydraulic pump flow path  42  and the third main operation valve  63  and then is supplied to the boom cylinder  23 . 
     The hydraulic system  1000 A has: a shuttle valve  701  provided between the first main operation valve  61  and the second main operation valve  62 ; and a shuttle valve  702  provided between a second merging-separating valve  68  and the third main operation valve  63 . Additionally, the hydraulic system  1000 A has the second merging-separating valve  68  connected to the shuttle valve  701  and the shuttle valve  702 . 
     The second merging-separating valve  68  selects a maximum pressure of a load sensing pressure (LS pressure) obtained by depressurizing the hydraulic fluid supplied to each of the bucket cylinder  21 , arm cylinder  22 , and boom cylinder  23  by the shuttle valve  701  and the shuttle valve  702 . The load sensing pressure is a pilot pressure used for pressure compensation. 
     When the second merging-separating valve  68  is in the merged state, the maximum LS pressure among pressures in the bucket cylinder  21  to the boom cylinder  23  is selected and supplied to the pressure compensating valve  70  in each of the bucket cylinder  21  to the boom cylinder  23  and also supplied to the servo mechanism  31 B of the first hydraulic pump  31  and the servo mechanism  32 B of the second hydraulic pump  32 . 
     When the second merging-separating valve  68  is in the separated state, the maximum LS pressure in each of the bucket cylinder  21  and the arm cylinder  22  is supplied to the pressure compensating valve  70  in each of the bucket cylinder  21  and the arm cylinder  22  and the servo mechanism  31 B of the first hydraulic pump  31 , and the LS pressure of the boom cylinder  23  is supplied to the pressure compensating valve  70  of the boom cylinder  23  and the servo mechanism  32 B of the second hydraulic pump  32 . 
     The shuttle valve  701  and the shuttle valve  702  select a pilot pressure indicating a maximum value from among pilot pressures output from the first main operation valve  61 , second main operation valve  62 , and third main operation valve  63 . The selected pilot pressure is supplied to the pressure compensating valve  70  and the servo mechanism ( 31 B,  32 B) of the hydraulic pump  30  ( 31 ,  32 ). 
     &lt;Pressure Sensor&gt; 
     The hydraulic system  1000 A has a load pressure sensor  80  that detects a pressure PL of hydraulic fluid in the hydraulic cylinder  20 . The pressure PL of the hydraulic fluid in the hydraulic cylinder  20  is a load pressure of hydraulic fluid supplied to the hydraulic cylinder  20 . A detection signal of the load pressure sensor  80  is output to the control device  100 . 
     In the present embodiment, the load pressure sensor  80  includes: a bucket load pressure sensor  81  that detects a pressure PLbk of hydraulic fluid in the bucket cylinder  21 , an arm load pressure sensor  82  that detects a pressure PLar of hydraulic fluid in the arm cylinder  22 , and a boom load pressure sensor  83  that detects a pressure PLbm of the hydraulic fluid in the boom cylinder  23 . 
     The bucket load pressure sensor  81  includes: a bucket load pressure sensor  81 C provided in the first bucket flow path  21 A and detecting a pressure PLbkc of hydraulic fluid in the cap-side space  21 C of the bucket cylinder  21 ; and a bucket load pressure sensor  81 L provided in the second bucket flow path  21 B and detecting a pressure PLbkl of hydraulic fluid in the rod-side space  21 L of the bucket cylinder  21 . 
     The arm load pressure sensor  82  includes: an arm load pressure sensor  82 C provided in the second arm flow path  22 B and detecting a pressure PLarc of hydraulic fluid in the cap-side space  22 C of the arm cylinder  22 ; and an arm load pressure sensor  82 L provided in the first arm flow path  22 A and detecting a pressure PLarl of hydraulic fluid in the rod-side space  22 L of the arm cylinder  22 . 
     The boom load pressure sensor  83  includes: a boom load pressure sensor  83 C provided in the first boom flow path  23 A and detecting a pressure PLbmc of hydraulic fluid in the cap-side space  23 C of the boom cylinder  23 ; and a boom load pressure sensor  83 L provided in the second boom flow path  23 B and detecting a pressure PLbml of hydraulic fluid in the rod-side space  23 L of the boom cylinder  23 . 
     Furthermore, the hydraulic system  1000 A has a discharge pressure sensor  800  that detects a discharge pressure P of hydraulic fluid discharged from the hydraulic pump  30 . A detection signal of the discharge pressure sensor  800  is output to the control device  100 . 
     The discharge pressure sensor  800  includes: a discharge pressure sensor  801  provided between the first hydraulic pump  31  and the first hydraulic pump flow path  41  and detecting a discharge pressure P 1  of hydraulic fluid discharged from the first hydraulic pump  31 ; and a discharge pressure sensor  802  provided between the second hydraulic pump  32  and the second hydraulic pump flow path  42  and detecting a discharge pressure P 2  of hydraulic fluid discharged from the second hydraulic pump  32 . 
     &lt;Pressure Compensating Valve&gt; 
     The pressure compensating valve  70  has a selection port to make a selection from among communicating, throttling, and blocking. The pressure compensating valve  70  includes a throttle valve that enables switching between blocking, throttling, and communicating by self-pressure. The pressure compensating valve  70  is directed to compensating flow rate distribution in accordance with a ratio of a metering opening area of each main operation valve  60  even when a load pressure of each hydraulic cylinder  20  is different. In the case of having no pressure compensating valve  70 , most of hydraulic fluid flows into the hydraulic cylinder  20  on a low load side. The pressure compensating valve  70  implements a function of flow rate distribution because an outlet pressure of each main operation valve  60  is made uniform by making a pressure loss act on the hydraulic cylinder  20  having a low load pressure such that an outlet pressure of the main operation valve  60  of the hydraulic cylinder  20  having the low load pressure becomes equivalent to an outlet pressure of the main operation valve  60  of the hydraulic cylinder  20  having a maximum load pressure. 
     The pressure compensating valve  70  includes a pressure compensating valve  71  and a pressure compensating valve  72  which are connected to the first main operation valve  61 , a pressure compensating valve  73  and a pressure compensating valve  74  which are connected to the second main operation valve  62 , a pressure compensating valve  75  and a pressure compensating valve  76  which are connected to the third main operation valve  63 . 
     The pressure compensating valve  71  compensates a differential pressure (metering differential pressure) between before and after the first main operation valve  61  in a state in which the first branch flow path  47  and the first bucket flow path  21 A are connected such that hydraulic fluid is supplied to the cap-side space  21 C. The pressure compensating valve  72  compensates a differential pressure (metering differential pressure) between before and after the first main operation valve  61  in a state in which the third branch flow path  49  and the second bucket flow path  21 B are connected such that hydraulic fluid is supplied to the rod-side space  21 L. 
     The pressure compensating valve  73  compensates a differential pressure (metering differential pressure) between before and after the second main operation valve  62  in a state in which the second branch flow path  48  and the first arm flow path  22 A are connected such that hydraulic fluid is supplied to the rod-side space  22 L. The pressure compensating valve  74  compensates a differential pressure (metering differential pressure) between before and after the second main operation valve  62  in a state in which the fourth branch flow path  50  and the second arm flow path  22 B are connected such that hydraulic fluid is supplied to the cap-side space  22 C. 
     Meanwhile, the differential pressure (metering differential pressure) between before and after the main operation valve  60  represents a difference between a pressure at an inlet port corresponding to the hydraulic pump  30  side of the main operation valve  60  and a pressure at an outlet port corresponding to the hydraulic cylinder  20  side, and corresponds to a differential pressure to measure a flow rate (metering). 
     Using the pressure compensating valve  70 , hydraulic fluid can be distributed to each of the bucket cylinder  21  and the arm cylinder  22  at a flow rate according to an operation amount of the operation device  5  even in the case where a light load acts on the hydraulic cylinder  20  corresponding to one of the bucket cylinder  21  and the arm cylinder  22  and a heavy load acts on the hydraulic cylinder  20  corresponding to the other thereof. 
     The pressure compensating valve  70  enables supply at a flow rate based on operation regardless of loads acting on the plurality of hydraulic cylinders  20 . For example, in the case where a heavy load acts on the bucket cylinder  21  while a light load acts on the arm cylinder  22 , the pressure compensating valve  70  ( 73 ,  74 ) arranged on the light load side compensates a metering differential pressure ΔP 2  on the arm cylinder  22  side, namely, the light load side so as to become a pressure substantially equal to a metering differential pressure ΔP 1  on the bucket cylinder  21  side such that supply is performed at a flow rate based on an operation amount of the second main operation valve  62  when hydraulic fluid is supplied from the second main operation valve  62  to the arm cylinder  22 , regardless of the metering differential pressure ΔP 1  generated by hydraulic fluid is supplied from the first main operation valve  61  to the bucket cylinder  21 . 
     In the case where a heavy load acts on the arm cylinder  22  while a light load acts on the bucket cylinder  21 , the pressure compensating valve  70  ( 71 ,  72 ) arranged on the light load side compensates the metering differential pressure ΔP 1  on the light load side such that supply is performed at a flow rate based on an operation amount of the first main operation valve  61  when hydraulic fluid is supplied from the first main operation valve  61  to the bucket cylinder  21 , regardless of the metering differential pressure ΔP 2  generated by hydraulic fluid being supplied from the second main operation valve  62  to the arm cylinder  22 . 
     &lt;Unload Valve&gt; 
     The hydraulic circuit  40  has an unloading valve  69 . In the hydraulic circuit  40 , even when the hydraulic cylinder  20  is not driven, hydraulic fluid at a flow rate corresponding to a minimum capacity is discharged from the hydraulic pump  30 . When the hydraulic cylinder  20  is not driven, the hydraulic fluid discharged from the hydraulic pump  30  is discharged (unloaded) via the unloading valve  69 . 
     [Control Device] 
       FIG. 5  is a functional block diagram illustrating an exemplary control device  100  according to the present embodiment. The control device  100  includes a computer system. The control device  100  has an arithmetic processing device  101 , a storage device  102 , and an input/output interface device  103 . 
     The control device  100  is connected to the first merging-separating valve  67  and the second merging-separating valve  68 , and outputs command signals to the first merging-separating valve  67  and the second merging-separating valve  68 . 
     Furthermore, the control device  100  is connected to the fuel injection device  17  (common rail control unit  29 ) and outputs a command signal to the fuel injection device  17 . 
     Additionally, the control device  100  is connected to each of the load pressure sensor  80  that detects a pressure PL of the hydraulic cylinder  20 , the discharge pressure sensor  800  that detects a discharge pressure P of hydraulic fluid discharged from the hydraulic pump  30 , the operation amount sensor  90  that detects an operation amount S of the operation device  5 , the engine speed sensor  4 R, the reducing agent sensor  209 , and the exhaust gas sensor  300 . 
     In the present embodiment, the operation amount sensor  90  ( 91 ,  92 ,  93 ) is a pressure sensor. When the operation device  5  is operated in order to drive the bucket cylinder  21 , a pilot pressure acting on the first main operation valve  61  is changed on the basis of an operation amount Sbk of the operation device  5 . Furthermore, when the operation device  5  is operated in order to drive the arm cylinder  22 , a pilot pressure acting on the second main operation valve  62  is changed on the basis of an operation amount Sar of the operation device  5 . Additionally, when the operation device  5  is operated in order to drive the boom cylinder  23 , a pilot pressure acting on the third main operation valve  63  is changed on the basis of an operation amount Sbm of the operation device  5 . The bucket operation amount sensor  91  detects the pilot pressure acting on the first main operation valve  61  when the operation device  5  is operated in order to drive the bucket cylinder  21 . The arm operation amount sensor  92  detects the pilot pressure acting on the second main operation valve  62  when the operation device  5  is operated in order to drive the arm cylinder  22 . The boom operation amount sensor  93  detects the pilot pressure acting on the third main operation valve  63  when the operation device  5  is operated in order to drive the boom cylinder  23 . 
     The arithmetic processing device  101  includes a distribution flow rate calculation unit  112 , a determination unit  114 , a determining unit  116 , a merging-separating control unit  118 , an exhaust gas treatment control unit  120 , and an engine control unit  122 . 
     &lt;Distribution Flow Rate Calculation Unit&gt; 
     The distribution flow rate calculation unit  112  calculates a distribution flow rate Qa of hydraulic fluid supplied to each of the plurality of hydraulic cylinders  20  on the basis of a pressure PL of hydraulic fluid in each of the plurality of hydraulic cylinders  20  and an operation amount S of the operation device  5  operated in order to drive each of the plurality of hydraulic cylinders  20 . In the present embodiment, the distribution flow rate calculation unit  112  calculates the distribution flow rate Qa on the basis of the pressure PL of hydraulic fluid in the hydraulic cylinder  20 , the operation amount S of the operation device  5 , and the discharge pressure P of hydraulic fluid discharged from the hydraulic pump  30 . 
     The pressure PL of the hydraulic fluid of the hydraulic cylinder  20  is detected by the load pressure sensor  80 . The distribution flow rate calculation unit  112  acquires the pressure PLbk of the hydraulic fluid in the bucket cylinder  21  from the bucket load pressure sensor  81 , acquires the pressure PLar of the hydraulic fluid in the arm cylinder  22  from the arm load pressure sensor  82 , and acquires the pressure PLbm of the hydraulic fluid in the boom cylinder  23  from the boom load pressure sensor  83 . 
     The operation amount S of the operation device  5  is detected by the operation amount sensor  90 . The distribution flow rate calculation unit  112  acquires the operation amount Sbk of the operation device  5  operated in order to drive the bucket cylinder  21  from the bucket operation amount sensor  91 , acquires the operation amount Sar of the operation device  5  operated in order to drive the arm cylinder  22  from the arm operation amount sensor  92 , and acquires the operation amount Sbm of the operation device  5  operated in order to drive the boom cylinder  23  from the boom operation amount sensor  93 . 
     The discharge pressure P of the hydraulic fluid in the hydraulic pump  30  is detected by the discharge pressure sensor  800 . The distribution flow rate calculation unit  112  acquires the discharge pressure P 1  of the hydraulic fluid in the first hydraulic pump  31  from the discharge pressure sensor  801 , and acquires the discharge pressure P 2  of the hydraulic fluid in the second hydraulic pump  32  from the discharge pressure sensor  802 . 
     The distribution flow rate calculation unit  112  calculates the distribution flow rate Qa (Qabk, Qaar, Qabm) of hydraulic fluid supplied to each of the plurality of hydraulic cylinder  20  ( 21 ,  22 ,  23 ) on the basis of the pressure PL (PLbk, PLar, PLbm) of the hydraulic fluid in each of the plurality of hydraulic cylinders  20  ( 21 ,  22 ,  23 ) and the operation amount S (Sbk, Sar, Sbm) of the operation device  5  operated in order to drive each of the plurality of hydraulic cylinders  20  ( 21 ,  22 ,  23 ). 
     The distribution flow rate calculation unit  112  calculates the distribution flow rate Qa on the basis of Expression (1).
 
 Qa=Qd ×√{( P−PL )/ ΔPC}   (1)
 
     In Expression (1), Qd represents a required flow rate of the hydraulic fluid in the hydraulic cylinder  20 . P represents a discharge pressure of the hydraulic fluid discharged from the hydraulic pump  30 . PL represents a load pressure of the hydraulic fluid in the hydraulic cylinder  20 . ΔPC represents a setting differential pressure between an inlet side and an outlet side of the main operation valve  60 . In the present embodiment, the differential pressure between the inlet side and the outlet side of the main operation valve  60  is set as the setting differential pressure ΔPC. The setting differential pressure ΔPC is preset for each of the first main operation valve  61 , second main operation valve  62 , and third main operation valve  63 , and stored in the storage device  102 . 
     The distribution flow rate Qabk of the bucket cylinder  21 , the distribution flow rate Qaar of the arm cylinder  22 , and the distribution flow rate Qabm of the boom cylinder  23  are respectively calculated on the basis of Expressions (2), (3), and (4).
 
 Qabk=Qdbk ×√{( P−PLbk )/Δ PC}   (2)
 
 Qaar=Qdar ×√{( P−PLar )/Δ PC}   (3)
 
 Qabm=Qdbm ×√{( P−PLbm )/Δ PC}   (4)
 
     In Expression (2), Qdbk represents a required flow rate of the hydraulic fluid in the bucket cylinder  21 . PLbk represents a pressure of the hydraulic fluid in the bucket cylinder  21 . In Expression (3), Qdar represents a required flow rate of the hydraulic fluid in the arm cylinder  22 . PLar represents a pressure of the hydraulic fluid in the arm cylinder  22 . In Expression (4), Qdbm represents a required flow rate of the hydraulic fluid in the boom cylinder  23 . PLbm is a load pressure of the hydraulic fluid in the boom cylinder  23 . In the present embodiment, a setting differential pressure ΔPC between an inlet side and an outlet side of the first main operation valve  61 , a setting differential pressure ΔPC between an inlet side and an outlet side of the second main operation valve  62 , and a setting differential pressure ΔPC between an inlet side and an outlet side of the third main operation valve  63  are the same values. 
     The required flow rate Qd (Qdbk, Qdar, Qdbm) is calculated on the basis of the operation amount S (Sbk, Sar, Sbm) of the operation device  5 . In the present embodiment, the required flow rate Qd (Qdbk, Qdar, Qdbm) is calculated on the basis of a pilot pressure detected by the operation amount sensor  90  ( 91 ,  92 ,  93 ). The operation amount S (Sbk, Sar, Sbm) of the operation device  5  corresponds one-to-one with the pilot pressure detected by the operation amount sensor  90  ( 91 ,  92 ,  93 ). The distribution flow rate calculation unit  112  converts the pilot pressure detected by the operation amount sensor  90  into a spool stroke of the main operation valve  60 , and calculates the required flow rate Qd on the basis of the spool stroke. The first correlation data indicating a relation between the pilot pressure and the spool stroke of the main operation valve  60  and the second correlation data indicating a relation between the spool stroke of the main operation valve  60  and the required flow rate Qd are known data and stored in the storage device  102 , respectively. The first correlation data indicating the relation between the pilot pressure and the spool stroke of the main operation valve  60  and the second correlation data indicating the relation between the spool stroke of the main operation valve  60  and the required flow rate Qd each include conversion table data. 
     The distribution flow rate calculation unit  112  acquires a detection signal of the bucket operation amount sensor  91  that has detected the pilot pressure acting on the first main operation valve  61 . The distribution flow rate calculation unit  112  converts the pilot pressure acting on the first main operation valve  61  into a spool stroke of the first main operation valve  61  by using the first correlation data stored in the storage device  102 . Consequently, the spool stroke of the first main operation valve  61  is calculated on the basis of the detection signal of the bucket operation amount sensor  91  and the first correlation data stored in the storage device  102 . Furthermore, the distribution flow rate calculation unit  112  converts the calculated spool stroke of the first main operation valve  61  into a required flow rate Qdbk of the bucket cylinder  21  by using the second correlation data stored in the storage device  102 . Consequently, the distribution flow rate calculation unit  112  can calculate the required flow rate Qdbk of the bucket cylinder  21 . 
     The distribution flow rate calculation unit  112  acquires a detection signal of the arm operation amount sensor  92  that has detected the pilot pressure acting on the second main operation valve  62 . The distribution flow rate calculation unit  112  converts the pilot pressure acting on the second main operation valve  62  into a spool stroke of the second main operation valve  62  by using the first correlation data stored in the storage device  102 . Consequently, the spool stroke of the second main operation valve  62  is calculated on the basis of the detection signal of the arm operation amount sensor  92  and the first correlation data stored in the storage device  102 . Furthermore, the distribution flow rate calculation unit  112  converts the calculated spool stroke of the second main operation valve  62  into a required flow rate Qdar of the arm cylinder  22  by using the second correlation data stored in the storage device  102 . Consequently, the distribution flow rate calculation unit  112  can calculate the required flow rate Qdar of the arm cylinder  22 . 
     The distribution flow rate calculation unit  112  acquires a detection signal of the boom operation amount sensor  93  that has detected the pilot pressure acting on the third main operation valve  63 . The distribution flow rate calculation unit  112  converts the pilot pressure acting on the third main operation valve  63  into a spool stroke of the third main operation valve  63  by using the first correlation data stored in the storage device  102 . Consequently, the spool stroke of the third main operation valve  63  is calculated on the basis of the detection signal of the boom operation amount sensor  93  and the first correlation data stored in the storage device  102 . Furthermore, the distribution flow rate calculation unit  112  converts the calculated spool stroke of the third main operation valve  63  into a required flow rate Qdbm of the boom cylinder  23  by using the second correlation data stored in the storage device  102 . Consequently, the distribution flow rate calculation unit  112  can calculate the required flow rate Qdbm of the boom cylinder  23 . 
     Meanwhile, as described above, the bucket load pressure sensor  81  includes the bucket load pressure sensor  81 C and the bucket load pressure sensor  81 L, and the pressure PLbk of the hydraulic fluid in the bucket cylinder  21  includes the pressure PLbkc of the hydraulic fluid in the cap-side space  21 C of the bucket cylinder  21  and the pressure PLbkl of the hydraulic fluid in the rod-side space  21 L of the bucket cylinder  21 . In the case of calculating the distribution flow rate Qabk by using Expression (2), the distribution flow rate calculation unit  112  selects any one of the pressure PLbkc and the pressure PLbkl on the basis of a moving direction of the spool of the first main operation valve  61 . For example, in the case where the spool of the first main operation valve  61  is moved in a first direction, the distribution flow rate calculation unit  112  calculates, on the basis of Expression (2), the distribution flow rate Qabk by using the pressure PLbkc detected by the bucket load pressure sensor  81 C. In the case where the spool of the first main operation valve  61  is moved in a second direction that is an opposite direction of the first direction, the distribution flow rate calculation unit  112  calculates, on the basis of Expression (2), the distribution flow rate Qabk by using the pressure PLbkl detected by the bucket load pressure sensor  81 L. 
     Similarly, the arm load pressure sensor  82  includes the arm load pressure sensor  82 C and the arm load pressure sensor  82 L, and the pressure PLar of hydraulic fluid in the arm cylinder  22  includes the pressure PLarc of the hydraulic fluid in the cap-side space  22 C of the arm cylinder  22  and the pressure PLarl of the hydraulic fluid in the rod-side space  22 L of the arm cylinder  22 . In the case of calculating the distribution flow rate Qaar by using Expression (3), the distribution flow rate calculation unit  112  selects any one of the pressure PLarc and the pressure PLarl on the basis of a moving direction of the spool of the second main operation valve  62 . For example, in the case where the spool of the second main operation valve  62  is moved in a first direction, the distribution flow rate calculation unit  112  calculates, on the basis of Expression (3), the distribution flow rate Qaar by using the pressure PLarc detected by the arm load pressure sensor  82 C. In the case where the spool of the second main operation valve  62  is moved in a second direction that is an opposite direction of the first direction, the distribution flow rate calculation unit  112  calculates, on the basis of Expression (3), the distribution flow rate Qaar by using the pressure PLarl detected by the arm load pressure sensor  82 L. 
     Similarly, the boom load pressure sensor  83  includes the boom load pressure sensor  83 C and the boom load pressure sensor  83 L, and the pressure PLbm of hydraulic fluid in the boom cylinder  23  includes the pressure PLbmc of the hydraulic fluid in the cap-side space  23 C of the boom cylinder  23  and the pressure PLbml of the hydraulic fluid in the rod-side space  23 L of the boom cylinder  23 . In the case of calculating the distribution flow rate Qabm by using Expression (4), the distribution flow rate calculation unit  112  selects any one of the pressure PLbmc and the pressure PLbml on the basis of a moving direction of the spool of the third main operation valve  63 . For example, in the case where the spool of the third main operation valve  63  is moved in a first direction, the distribution flow rate calculation unit  112  calculates, on the basis of Expression (4), the distribution flow rate Qabm by using the pressure PLbmc detected by the boom load pressure sensor  83 C. In the case where the spool of the third main operation valve  63  is moved in a second direction that is an opposite direction of the first direction, the distribution flow rate calculation unit  112  calculates, on the basis of Expression (4), the distribution flow rate Qabm by using the pressure PLbml detected by the boom load pressure sensor  83 L. 
     In the present embodiment, the discharge pressure P of the hydraulic fluid discharged from the hydraulic pump  30  is detected by the discharge pressure sensor  800 . Meanwhile, when the discharge pressure P of the hydraulic fluid discharged from the hydraulic pump  30  is unknown in Expressions (1) to (4), the distribution flow rate calculation unit  112  may calculate the distribution flow rates Qabk, Qaar, and Qabm by repeating numerical calculation such that Expression (5) become convergent.
 
 Qlp=Qabk+Qaar+Qabm   (5)
 
     In Expression (5), Qlp represents a pump limit flow rate. The pump limit flow rate Qlp is set to a minimum value among the maximum discharge flow rate Qmax of the hydraulic pump  30 , a target discharge flow rate Qt 1  of the first hydraulic pump  31  determined on the basis of target output of the first hydraulic pump  31 , and a target discharge flow rate Qt 2  of the second hydraulic pump  32  determined on the basis of target output of the second hydraulic pump  32 . 
     Meanwhile, in the present embodiment, the operation device  5  includes an operating lever of a pilot pressure system, and a pressure sensor is used as the operation amount sensor  90  ( 91 ,  92 ,  93 ). The operation device  5  may also include an operating lever of an electric system. In the case where the operation device  5  includes the operating lever of the electric system, a stroke sensor that can detect a lever stroke indicating a stroke of the operating lever is used as the operation amount sensor ( 91 ,  92 ,  93 ). The distribution flow rate calculation unit  112  converts a lever stroke detected by the operation amount sensor  90  into a spool stroke of the main operation valve  60 , and can calculate the required flow rate Qd on the basis of the spool stroke. The distribution flow rate calculation unit  112  can convert the lever stroke into the spool stroke by using a predetermined conversion table. 
     &lt;Determination Unit&gt; 
     The determination unit  114  determines to perform switching to the merged state or switching to the separated state on the basis of the distribution flow rate Qa calculated in the distribution flow rate calculation unit  201 . In the present embodiment, the determination unit  114  determines to perform switching to the merged state or switching the separated state on the basis of a comparison result between the distribution flow rate Qa calculated in the distribution flow rate calculation unit  112  and a threshold value Qs. 
     The threshold value Qs is a threshold value for the distribution flow rate Qa of the hydraulic cylinder  20 . When the distribution flow rate Qa calculated in the distribution flow rate calculation unit  112  is the threshold value Qs or less, the determination unit  114  determines to perform switching to the separated state. When the distribution flow rate Qa calculated in the distribution flow rate calculation unit  112  is larger than the threshold value Qs, the determination unit  112  determines to perform switching to the merged state. 
     In the present embodiment, the threshold value Qs is the maximum discharge flow rate Qmax of the hydraulic fluid that can be discharged by each of the first hydraulic pump  31  and the second hydraulic pump  32 . In other words, in the present embodiment, the determination unit  114  determines to perform switching to the merged state or switching the separated state on the basis of a comparison result between the distribution flow rate Qa and the maximum discharge flow rate Qmax. When the distribution flow rate Qa is the most discharge flow rate Qmax or less, the determination unit  114  determines to perform switching to the separated state. When the distribution flow rate Qa is larger than the maximum discharge flow rate Qmax, the determination unit  114  determines to perform switching to the merged state. 
     In the present embodiment, when the sum of the distribution flow rate Qabk of the hydraulic fluid supplied to the bucket cylinder  21  and the distribution flow rate Qaar of the hydraulic fluid supplied to the arm cylinder  22  is equal to or less than the maximum discharge flow rate Q 1 max of the first hydraulic pump  31  and also when the distribution flow rate Qabm of the hydraulic fluid supplied to the boom cylinder  23  is equal to or less than the maximum discharge flow rate Q 2 max of the second hydraulic pump  32 , the determination unit  114  determines to perform switching to the separated state. When the sum of the distribution flow rate Qabk of the hydraulic fluid supplied to the bucket cylinder  21  and the distribution flow rate Qaar of the hydraulic fluid supplied to the arm cylinder  22  is larger than the maximum discharge flow rate Q 1 max of the first hydraulic pump  31  or when the distribution flow rate Qabm of the hydraulic fluid supplied to the boom cylinder  23  is larger than the maximum discharge flow rate Q 2 max of the second hydraulic pump  32 , the determination unit  114  determines to perform switching to the merged state. 
     In the following description, a state in which following conditions are satisfied will be referred to as satisfying separating conditions: the distribution flow rate Qa calculated in the distribution flow rate calculation unit  112  is the threshold value Qs or less; and the determination unit  114  can determine to perform switching to the separated state. 
     &lt;Determining Unit&gt; 
     The determining unit  116  determines whether output of the engine  4  is limited. When it is determined that the exhaust gas treatment device  200  is in an abnormal state, the determining unit  116  determines that the output of the engine  4  is limited. Furthermore, when it is determined that the exhaust gas sensor  300  is in an abnormal state, the determining unit  116  determines that the output of the engine  4  is limited. The determining unit  116  determines that the output of the engine  4  is limited when the engine  4  cannot be protected, for example, when it is determined that at least one of the outside air temperature sensor  307  and the coolant temperature sensor  308  which constitute the part of the exhaust gas sensor  300 , and an engine hydraulic sensor not illustrated is in an abnormal state. 
     The state in which the exhaust gas treatment device  200  is in an abnormal state means the state of occurrence of an event in which treatment performance (purification performance) for the exhaust gas by the exhaust gas treatment device  200  is degraded or may be degraded. For example, in occurrence of an event in which an amount of the reducing agent R stored in the reducing agent tank  205  is decreased to a value less than an allowable value due to consumption, leakage, or the like, the treatment performance (purification performance) for the exhaust gas by the exhaust gas treatment device  200  is degraded or may be degraded. The amount of the reducing agent R stored in the reducing agent tank  205  is detected by the reducing agent sensor  209 . The determining unit  116  determines that output of the engine  4  is limited when it is determined that the amount of the reducing agent R stored in the reducing agent tank  205  is decreased to an amount less than the allowable value on the basis of a detection signal of the reducing agent sensor  209 . 
     The state in which the exhaust gas sensor  300  is in an abnormal state means the state of occurrence of an event in which detection accuracy for the exhaust gas state by the exhaust gas sensor  300  is degraded or an event in which the exhaust gas state cannot be detected. For example, in the case of failure of the NOx sensor  301 , an abnormality signal indicating the failure of the NOx sensor  301  is transmitted to the determining unit  116 . The determining unit  116  determines that the output of the engine  4  is limited when it is determined that the NOx sensor  301  cannot detect the NOx concentration on the basis of the acquired abnormality signal. Additionally, even in the case of failure of the intake air flow rate sensor  305  or in the case of failure of the atmospheric pressure sensor  306 , an abnormality signal is transmitted to the determining unit  116 . The determining unit  116  determines that the output of the engine  4  is limited when it is determined on the basis of the acquired abnormality signal that the flow rate of NOx cannot be calculated on the basis of the detection signal of the intake air flow rate sensor  305  or when it is determined that the flow rate of NOx cannot be estimated on the basis of the detection signal of the atmospheric pressure sensor  306 . 
     &lt;Merging-Separating Control Unit&gt; 
     The merging-separating control unit  118  outputs a command signal to control the first merging-separating valve  67  on the basis of a determination result of the determination unit  114  and a determination result of the determining unit  116 . When the determining unit  116  determines that output of the engine  4  is limited, the merging-separating control unit  118  outputs, to the first merging-separating valve  67 , a command signal to control the first merging-separating valve  67  so as to perform switching to the merged state. 
     In the present embodiment, when the determining unit  116  determines that the output of the engine  4  is limited even though the determination unit  114  determines to perform switching to the separated state, the merging-separating control unit  118  outputs, to the first merging-separating valve  67 , a command signal to control the first merging-separating valve  67  so as to perform switching to the merged state. 
     When the determining unit  116  determines that the output of the engine  4  is not limited, the merging-separating control unit  118  outputs, on the basis of the determination result of the determination unit  114 , a command signal to control the first merging-separating valve  67  to the first merging-separating valve  67  so as to perform switching to any one of the merged state and the separated state. 
     &lt;Exhaust Gas Treatment Control Unit&gt; 
     The exhaust gas treatment control unit  120  outputs a command signal to control the exhaust gas treatment device  200 . The exhaust gas treatment control unit  120  acquires a detection signal of the exhaust gas sensor  300  and determines a supply amount of the reducing agent R to be supplied to the reducing catalyst  203  on the basis of the detection signal of the exhaust gas sensor  300 . The exhaust gas treatment control unit  120  outputs a command signal to control, for example, the supply pump  207  such that the determined supply amount of the reducing agent R is supplied. 
     &lt;Engine Control Unit&gt; 
     The engine control unit  122  controls output of the engine  4 . The engine control unit  122  controls the output of the engine  4  by outputting a command signal to the fuel injection device  17  to control a fuel injection amount to the engine  4 . 
     In the present embodiment, when the exhaust gas treatment device  200  is in an abnormal state, the engine control unit  122  limits output of the engine  4  by controlling the fuel injection amount to the engine  4 . Furthermore, when the exhaust gas sensor  300  is in an abnormal state, the engine control unit  122  limits output of the engine  4  by controlling the fuel injection amount to the engine  4 . The engine control unit  122  decreases the output of the engine  4  by decreasing the fuel injection amount injected from the fuel injection device  17 . Furthermore, when the exhaust gas is not normally controlled, the engine control unit  122  limits the output of the engine  4 . Additionally, the engine control unit  122  limits the output of the engine  4  when the engine  4  cannot be protected, for example, when at least one of the outside air temperature sensor  307  an the coolant temperature sensor  308  which constitute the part of the exhaust gas sensor  300 , and an engine hydraulic sensor not illustrated is in an abnormal state. 
     As described above, the state in which the exhaust gas treatment device  200  is in an abnormal state means the state of occurrence of an event in which the treatment performance (purification performance) for the exhaust gas by the exhaust gas treatment device  200  is degraded or may be degraded. When the engine  4  is actuated with high output although the exhaust gas treatment device  200  is in an abnormal state, a large amount of exhaust gas discharged from the engine  4  cannot be sufficiently purified. As a result, a large amount of exhaust gas not sufficiently purified is emitted to an atmospheric space. Therefore, when it is determined that the exhaust gas treatment device  200  is in an abnormal state, the engine control unit  122  limits the output of the engine  4  by decreasing the fuel injection amount to the engine  4 . For example, when it is determined that the amount of the reducing agent R stored in the reducing agent tank  205  is decreased to an amount smaller than the allowable value on the basis of a detection signal of the reducing agent sensor  209 , the engine control unit  122  decreases the output of the engine  4 . Consequently, an amount of the exhaust gas discharged from the engine  4  becomes a small amount, and it is possible to prevent a large amount of exhaust gas not sufficiently purified from being emitted to the atmospheric space. 
     As described above, the state in which the exhaust gas sensor  300  is in an abnormal state means the state of occurrence of an event in which detection accuracy for an exhaust gas state by the exhaust gas sensor  300  is degraded or an event in which the exhaust gas state cannot be detected. When the exhaust gas sensor  300  is in an abnormal state, it is difficult for the exhaust gas treatment control unit  120  to determine an appropriate supply amount of the reducing agent R to be supplied to the reducing catalyst  203  on the basis of the detection signal of the exhaust gas sensor  300 . For example, when the supplied reducing agent R is excessive, there is higher possibility that ammonia may be emitted to the atmospheric space together with the exhaust gas. On the other hand, when the supplied reducing agent R is too little, there is higher possibility that NOx is not sufficiently decreased and emitted to the atmospheric space. Therefore, when it is determined that the exhaust gas sensor  300  is in an abnormal state, the engine control unit  122  limits output of the engine  4  by decreasing the fuel injection amount to the engine  4 . For example, when an abnormality signal indicating failure of the NOx sensor  301  is acquired, the engine control unit  122  decreases the output of the engine  4 . The exhaust gas treatment control unit  120  estimates the flow rate of NOx contained in the exhaust gas from the engine  4  having the output decreased, and can determine the supply amount of the reducing agent R such that NOx contained in the exhaust gas is decreased. 
       FIG. 6  is a diagram illustrating an exemplary torque chart of the engine  4  according to the present embodiment. An upper limit torque characteristic of the engine  4  is defined by a maximum output torque line La illustrated in  FIG. 6 . A droop characteristic of the engine  4  is defined by an engine droop line Lb illustrated in  FIG. 6 . Engine target output is defined by an equal output line Lc illustrated in  FIG. 6 . 
     The engine control unit  122  controls the engine  4  on the basis of the upper limit torque characteristic, droop characteristic, and engine target output. The engine control unit  122  controls the engine  4  such that the engine speed and torque of the engine  4  do not exceed the maximum output torque line La, engine droop line Lb, and equal output line Lc. 
     In other words, the engine control unit  122  outputs a command signal to control the fuel injection amount to the engine  4  such that the engine speed and torque of the engine  4  do not exceed an engine output torque line Lt defined by the maximum output torque line La, engine droop line Lb, and equal output line Lc. 
     When output of the engine  4  is not limited, the engine control unit  122  sets output of the engine  4  to target output indicated by an equal output line Lc 1 . When the output of the engine  4  is not limited, the engine control unit  122  adjusts the fuel injection amount to the engine  4  such that the engine speed and torque of the engine  4  do not exceed the equal output line Lc 1 . 
     When at least one of the exhaust gas treatment device  200  and the exhaust gas sensor  300  is in an abnormal state and it is necessary to limit the output of the engine  4 , the engine control unit  122  sets the output of the engine  4  to target output indicated by an equal output line Lc 2 . The output of the engine  4  indicated by the equal output line Lc 2  is smaller than the output of the engine  4  indicated by the equal output line Lc 1 . When the output of the engine  4  is limited, the engine control unit  122  adjusts the fuel injection amount to the engine  4  such that the engine speed and torque of the engine  4  do not exceed the equal output line Lc 2 . 
     [Control Method] 
       FIG. 7  is a flowchart illustrating an exemplary control method for the excavator  1  according to the present embodiment. The distribution flow rate calculation unit  112  calculates the distribution flow rate Qa (Qabk, Qaar, Qabm) (step SP 10 ). 
     The determination unit  114  compares the distribution flow rate Qa calculated in the distribution flow rate calculation unit  112  with the threshold value Qs and determines whether the separating conditions by which switching to the separated state can be determined are satisfied (step SP 20 ). 
     In step SP 20 , in the case of determining that the separating conditions are not satisfied (step SP 20 : No), the determination unit  114  determines to perform switching to the merged state. The merging-separating control unit  118  outputs a command signal to the first merging-separating valve  67  so as to perform switching to the merged state. Consequently, the hydraulic system  1000 A is actuated in the merged state (step SP 40 ). 
     Meanwhile, when the hydraulic system  1000 A is actuated in the merged state at the time of determining whether the separating conditions are satisfied in step SP 20 , the merging-separating control unit  118  controls the first merging-separating valve  67  such that the merged state is kept. When the hydraulic system  1000 A is actuated in the separated state at the time of determining whether the separating conditions are satisfied, the merging-separating valve control unit  118  controls the first merging-separating valve  67  so as to perform switching from the merged state to the separated state. 
     In the case of determining in step SP 20  that the separating conditions are satisfied (step SP 20 : Yes), the determination unit  114  determines to perform switching to the separated state. The determining unit  116  determines whether output of the engine  4  is limited (step SP 30 ). 
     For example, in the case where the amount of the reducing agent R stored in the reducing agent tank  205  is less than the allowable value, an abnormality signal indicating that the exhaust gas treatment device  200  is in an abnormal state is transmitted to the determining unit  116 . Furthermore, when the exhaust gas sensor  300  is in an abnormal state, an abnormality signal indicating that the exhaust gas sensor  300  is in an abnormal state is transmitted to the determining unit  116 . These abnormality signals are limiting signals indicating that the output of the engine  4  is limited. When the limiting signal is acquired, the determining unit  116  determines that the output of the engine  4  is limited. 
     In the case of determining in step SP 30  that the output of the engine  4  is not limited (step SP 30 : No), the merging-separating control unit  118  outputs a command signal to the first merging-separating valve  67  so as to perform switching to the separated state. Consequently, the hydraulic system  1000 A is actuated in the separated state (step SP 50 ). 
     In the case of determining in step SP 30  that the output of the engine  4  is limited (step SP 30 : Yes), the merging-separating control unit  118  outputs a command signal to the first merging-separating valve  67  so as to perform switching to the merged state. Consequently, the hydraulic system  1000 A is actuated in the merged state (step SP 40 ). 
     When the hydraulic system  1000 A is actuated in the merged state and it is determined that the output of the engine  4  is limited, the merging-separating control unit  118  controls the first merging-separating valve  67  such that the merged state is kept. In the case of determining in step SP 30  that the output of the engine  4  is limited while the hydraulic system  1000 A is actuated in the separated state, the merging-separating control unit  118  controls the first merging-separating valve  67  so as to perform switching from the separated state to the merged state. 
     When the hydraulic system  1000 A is actuated in the merged state (step SP 40 ), the hydraulic fluid discharged from the first hydraulic pump  31  and the hydraulic fluid discharged from the second hydraulic pump  32  are supplied to each of the bucket cylinder  21 , arm cylinder  22 , and boom cylinder  23 . 
     When the hydraulic system  1000 A is actuated in the separated state (step SP 50 ), the hydraulic fluid discharged from the first hydraulic pump  31  is supplied to the bucket cylinder  21  and the arm cylinder  22 , and the hydraulic fluid discharged from the second hydraulic pump  32  is supplied to the boom cylinder  23 . 
     [Effects] 
     As described above, according to the present embodiment, when output (engine speed) of the engine  4  is limited in the control system  1000  where the state can be switched between the merged state and the separated state, the state in the hydraulic system  1000 A is switched to the merged state. In the case where the state is switched to the separated state in the hydraulic system  1000 A when output of the engine  4  is decreased, the flow rate of the hydraulic fluid supplied to each of the bucket cylinder  21  and the arm cylinder  22  is decreased. As a result, an actuation speed of the bucket  21  or an actuation speed of the arm  22  may be decreased and workability of the excavator  1  may be degraded. In the present embodiment, when the output of the engine  4  is limited, the state of the hydraulic system  1000 A is restricted from being switched to the separated state, and is switched to the merged state, and therefore, the flow rate of the hydraulic fluid supplied to each of the bucket cylinder  21  and the arm cylinder  22  is prevented from being decreased. Therefore, workability of the excavator  1  is prevented from being degraded. 
     Furthermore, the separating conditions are not satisfied even when the hydraulic system  1000 A is switched to the separated state even in the case where the output (engine speed) of the engine  4  is decreased, and the state can be easily switched back to the merged state from the separated state. In the case where a difference between the pressure of the discharge hydraulic fluid from the first hydraulic pump  31  and the pressure of the discharge hydraulic fluid from the second hydraulic pump  32  is large when the state is switched back to the merged state from the separated state, there may be possibility of occurrence of shock. In the present embodiment, when output of the engine  4  is decreased, the state of the hydraulic system  1000 A is switched to the merged state, and therefore, occurrence of such shock is suppressed. 
     Furthermore, in the present embodiment, when the exhaust gas treatment device  200  is in an abnormal state, it is determined that the output of the engine  4  is limited. Since the output of the engine  4  is limited when the exhaust gas treatment device  200  is in an abnormal state, a large amount of NOx is prevented from being emitted to the atmospheric space. 
     Moreover, in the present embodiment, when the exhaust gas sensor  300  is in an abnormal state, output of the engine  4  is limited. Since the output of the engine  4  is limited when the exhaust gas sensor  300  is in an abnormal state, ammonia or NOx is prevented from being emitted to a standby space. 
     Additionally, in the present embodiment, when it is determined that output of the engine  4  is limited even in the case where the separating conditions are satisfied, the state in the hydraulic system  1000 A is switched to the merged state. Therefore, the flow rate of the hydraulic fluid supplied to each of the bucket cylinder  21  and the arm cylinder  22  is prevented from being decreased, and workability of the excavator  1  is prevented from being degraded. 
     Moreover, in the present embodiment, output of the engine  4  is limited by decreasing the fuel injection amount to the engine  4 . Consequently, the amount of generated NOx is decreased. 
     Meanwhile, in the above embodiment, it is assumed that the threshold value Qs used to determine whether to actuate the first merging-separating valve  67  is the maximum discharge flow rate Qmax. The threshold value Qs may also be a value smaller than the maximum discharge flow rate Qmax. 
     Meanwhile, in the above embodiment, it is assumed that the work machine  1  is the excavator  1  of the hybrid system. The work machine  1  may not necessarily be the excavator  1  of the hybrid system. In the above-described embodiment, it is assumed that the upper swing body  2  is swung by the electric motor  25 , but may also be swung by a hydraulic motor. The hydraulic motor may calculate a distribution flow rate and pump output by including a swing motor in either the first hydraulic actuator or the second hydraulic actuator. 
     Meanwhile, in the above embodiment, it is assumed that the control system  1000  is applied to the excavator  1 . The work machine to which the control system  1000  is applied is not limited to the excavator  1 , and the control system can be widely applied to hydraulically driven work machines other than the excavator. 
     REFERENCE SIGNS LIST 
       1  EXCAVATOR (WORK MACHINE) 
       2  UPPER SWING BODY 
       3  LOWER TRAVELING BODY 
       3 C CRAWLER 
       4  ENGINE 
       4 R ENGINE SPEED SENSOR 
       4 S OUTPUT SHAFT 
       5  OPERATION DEVICE 
       5 L LEFT OPERATING LEVER 
       5 R RIGHT OPERATING LEVER 
       6  OPERATING ROOM 
       6 S OPERATOR&#39;S SEAT 
       7  MACHINE ROOM 
       8  FUEL TANK 
       9  HYDRAULIC FLUID TANK 
       10  WORK UNIT 
       11  BUCKET 
       12  ARM 
       13  BOOM 
       14  STORAGE BATTERY 
       14 C TRANSFORMER 
       15 G FIRST INVERTER 
       15 R SECOND INVERTER 
       16  ROTATION SENSOR 
       17  FUEL INJECTION DEVICE 
       17 A ACCUMULATOR 
       17 B INJECTOR 
       18  INTAKE PIPE 
       19  EXHAUST PIPE 
       20  HYDRAULIC CYLINDER 
       21  BUCKET CYLINDER 
       21 A FIRST BUCKET FLOW PATH 
       21 B SECOND BUCKET FLOW PATH 
       21 C CAP-SIDE SPACE 
       21 L ROD-SIDE SPACE 
       22  ARM CYLINDER 
       22 A FIRST ARM FLOW PATH 
       22 B SECOND ARM FLOW PATH 
       22 C CAP-SIDE SPACE 
       22 L ROD-SIDE SPACE 
       23  BOOM CYLINDER 
       23 A FIRST BOOM FLOW PATH 
       23 B SECOND BOOM FLOW PATH 
       23 C CAP-SIDE SPACE 
       23 L ROD-SIDE SPACE 
       24  HYDRAULIC MOTOR 
       25  ELECTRIC MOTOR 
       27  GENERATOR MOTOR 
       29  COMMON RAIL CONTROL UNIT 
       30  HYDRAULIC PUMP 
       30 A SWASH PLATE 
       30 S SWASH PLATE ANGLE SENSOR 
       31  FIRST HYDRAULIC PUMP 
       31 A SWASH PLATE 
       31 B SERVO MECHANISM 
       31 S INCLINATION ANGLE SENSOR 
       32  SECOND HYDRAULIC PUMP 
       32 A SWASH PLATE 
       32 B SERVO MECHANISM 
       32 S INCLINATION ANGLE SENSOR 
       33  THROTTLE DIAL 
       34  WORK MODE SELECTOR 
       35  AIR CLEANER 
       40  HYDRAULIC CIRCUIT 
       41  FIRST HYDRAULIC PUMP FLOW PATH 
       42  SECOND HYDRAULIC PUMP FLOW PATH 
       43  FIRST SUPPLY FLOW PATH 
       44  SECOND SUPPLY FLOW PATH 
       45  THIRD SUPPLY FLOW PATH 
       46  FOURTH SUPPLY FLOW PATH 
       47  FIRST BRANCH FLOW PATH 
       48  SECOND BRANCH FLOW PATH 
       49  THIRD BRANCH FLOW PATH 
       50  FOURTH BRANCH FLOW PATH 
       51  FIFTH BRANCH FLOW PATH 
       52  SIXTH BRANCH FLOW PATH 
       53  DISCHARGE FLOW PATH 
       55  MERGING FLOW PATH 
       60  MAIN OPERATION VALVE 
       61  FIRST MAIN OPERATION VALVE 
       62  SECOND MAIN OPERATION VALVE 
       63  THIRD MAIN OPERATION VALVE 
       67  FIRST MERGING-SEPARATING VALVE (SWITCHING DEVICE) 
       68  SECOND MERGING-SEPARATING VALVE 
       69  UNLOAD VALVE 
       70  PRESSURE COMPENSATING VALVE 
       71 ,  72 ,  73 ,  74 ,  75 ,  76  PRESSURE COMPENSATING VALVE 
       80  LOAD PRESSURE SENSOR 
       81  BUCKET LOAD PRESSURE SENSOR 
       81 C,  81 L BUCKET LOAD PRESSURE SENSOR 
       82  ARM LOAD PRESSURE SENSOR 
       82 C,  82 L ARM LOAD PRESSURE SENSOR 
       83  BOOM LOAD PRESSURE SENSOR 
       83 C,  83 L BOOM PRESSURE SENSOR 
       90  OPERATION AMOUNT SENSOR 
       91  BUCKET OPERATION AMOUNT SENSOR 
       92  ARM OPERATION AMOUNT SENSOR 
       93  BOOM OPERATION AMOUNT SENSOR 
       100  CONTROL DEVICE 
       100 A PUMP CONTROLLER 
       100 B HYBRID CONTROLLER 
       100 C ENGINE CONTROLLER 
       101  ARITHMETIC PROCESSING DEVICE 
       102  STORAGE DEVICE 
       103  INPUT/OUTPUT INTERFACE DEVICE 
       112  DISTRIBUTION FLOW RATE CALCULATION UNIT 
       114  DETERMINATION UNIT 
       116  DETERMINING UNIT 
       118  MERGING-SEPARATING CONTROL UNIT 
       120  EXHAUST GAS TREATMENT CONTROL UNIT 
       122  ENGINE CONTROL UNIT 
       200  EXHAUST GAS TREATMENT DEVICE 
       201  FILTER UNIT 
       202  PIPE LINE 
       203  REDUCING CATALYST 
       204  REDUCING AGENT SUPPLY DEVICE 
       205  REDUCING AGENT TANK 
       206  SUPPLY PIPE 
       207  SUPPLY PUMP 
       208  INJECTION NOZZLE 
       209  REDUCING AGENT SENSOR 
       300  EXHAUST GAS SENSOR 
       301  NOx SENSOR 
       302  PRESSURE SENSOR 
       303  TEMPERATURE SENSOR 
       304  PRESSURE SENSOR 
       305  INTAKE AIR FLOW RATE SENSOR 
       306  ATMOSPHERIC PRESSURE SENSOR 
       307  OUTSIDE AIR TEMPERATURE SENSOR 
       308  COOLANT TEMPERATURE SENSOR 
       701  SHUTTLE VALVE 
       702  SHUTTLE VALVE 
       800  DISCHARGE PRESSURE SENSOR 
       801  DISCHARGE PRESSURE SENSOR 
       802  DISCHARGE PRESSURE SENSOR 
       1000  CONTROL SYSTEM 
       1000 A HYDRAULIC SYSTEM 
       1000 B ELECTRIC SYSTEM 
     Br 1  FIRST BRANCH PORTION 
     Br 2  SECOND BRANCH PORTION 
     Br 3  THIRD BRANCH PORTION 
     Br 4  FOURTH BRANCH PORTION 
     R REDUCING AGENT 
     RX SWING SHAFT