Patent Publication Number: US-11391020-B2

Title: Work machine

Description:
TECHNICAL FIELD 
     The present invention relates to a work machine that carries out front device control such as area limiting excavation control, for example. 
     BACKGROUND ART 
     There is machine control (Machine Control: referred to as MC hereinafter) as a technique to improve the work efficiency of a work machine (for example, hydraulic excavator) including a work device (for example, front work implement) driven by a hydraulic actuator. The MC is a technique to carry out operation assist of an operator by carrying out semiautomatic control by which the work device is caused to operate according to a condition defined in advance when an operation device is operated by the operator. 
     When the MC works, operation of a work device (for example, front work implement) is limited in such a manner that the lower side of an excavation target surface is not excavated. 
     In patent document 1, a proportional solenoid valve is disposed on an operation signal line of an operation device and operation of a work device is limited by reducing an operation pilot pressure output from the operation device by the proportional solenoid valve such that the velocity of the work device may be kept from exceeding a limit value. 
     In patent document 2, when the MC is not carried out, a selector valve is switched to a first position to interrupt connection between an operation signal line of an operation device and a pressure reducing line including a proportional solenoid valve and connect the operation signal line directly to a signal input line of a corresponding flow control valve. Thereby, an operation pilot pressure output from the operation device is kept from passing through the proportional solenoid valve. When the MC is carried out, the selector valve is switched to a second position to connect the operation signal line to the signal input line of the flow control valve through the pressure reducing line and reduce the operation pilot pressure output from the operation device by the proportional solenoid valve. Thereby, operation of a work device is limited. 
     Furthermore, in patent document 1 and patent document 2, an operation signal line of boom raising of the operation device and a control signal line that introduces a control pilot pressure generated by the proportional solenoid valve are connected to each other through a shuttle valve, and the higher pressure of an operation pilot pressure of boom raising output from the operation device and the control pilot pressure output from the proportional solenoid valve is introduced to a signal input line of the boom raising side in the flow control valve. This allows execution of automatic boom raising and boom raising through operation of the operation device by an operator. 
     PRIOR ART DOCUMENT 
     Patent Documents 
     
         
         Patent Document 1: Japanese Patent No. 3091667 
         Patent Document 2: JP-2018-080762-A 
       
    
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     In the technique described in patent document 1, operation limitation of the work device by the MC and automatic boom raising by the MC can be carried out. However, the proportional solenoid valve exists on the operation signal line. Therefore, when the MC is not carried out, pressure loss occurs due to passing of the operation pilot pressure output from the operation device through the proportional solenoid valve. For this reason, there is a problem that the responsiveness of a hydraulic actuator to operation of the operation device by an operator lowers and it is impossible to obtain operability equivalent to that of a work machine that does not have MC functions. 
     Furthermore, in patent document 1, a proportional solenoid valve is not disposed in the operation pilot pressure circuit of the boom lowering side and therefore it is impossible to carry out automatic boom lowering by the MC. 
     In the technique described in patent document 2, when the MC is not carried out, the selector valve is switched to the first position to connect the operation signal line directly to the signal input line of the corresponding flow control valve, and the operation pilot pressure output from the operation device does not pass through the proportional solenoid valve. Thus, pressure loss does not occur and the responsiveness of a hydraulic actuator to operation of the operation device by an operator is improved, thus operability equivalent to that of a work machine that does not have MC functions is obtained. 
     However, also in patent document 2, a proportional solenoid valve is not disposed in the operation pilot pressure circuit of the boom lowering side and therefore it is impossible to carry out automatic boom lowering by the MC. 
     Here, boom lowering operation will be described by taking horizontal excavation by the MC as an example. 
     In the horizontal excavation by the MC, an arm is operated to the crowding side by operating an operation device of the arm. At this time, boom raising operation is automatically carried out in such a manner that the bucket claw tip is along an excavation target surface set in advance in line with the operation of the arm. After the arm becomes a posture perpendicular to the excavation target surface, the bucket claw tip operates in such a direction as to get further away from the excavation target surface due to arm crowding operation. Therefore, the boom raising operation becomes unnecessary. However, boom lowering operation needs to be carried out in order to cause the bucket claw tip to operate along the target surface. 
     In patent documents 1 and 2, an operator operates the operation device in the boom lowering direction and the output operation pilot pressure is reduced by the proportional solenoid valve. Thereby, boom lowering operation is limited in such a manner that the bucket claw tip does not enter the lower side of the excavation target surface, thus the horizontal excavation is implemented. 
     However, it is desired to automate the boom lowering operation such that the horizontal excavation in the MC can be carried out with only the operation device of the arm in the future. In this case, it is necessary that the boom lowering operation can be automatically carried out in the state in which the operation device of the boom is not being operated. In patent documents 1 and 2, the operation pilot pressure generated by operating the operation device of the boom in the lowering direction is employed as input to the proportional solenoid valve. Therefore, it is impossible to allow the boom lowering operation in the state in which the operation device of the boom is not being operated in the lowering direction. 
     Furthermore, if the circuit configuration of boom raising that allows execution of operation without operating the operation device is applied also to the boom lowering side, it becomes possible to allow the boom lowering operation in the state in which the operation device of the boom is not being operated in the lowering direction. However, the higher pressure of the control pilot pressure output from the proportional solenoid valve and the operation pilot pressure of boom lowering of the operation device is introduced to a signal input line of boom lowering in the flow control valve. Therefore, there is a problem that, although a signal for limiting operation of the work device is output to the proportional solenoid valve, the operation pilot pressure of boom lowering of the operation device is introduced to the signal input line of the flow control valve as it is without being reduced by the proportional solenoid valve and it becomes impossible to limit the operation of the work device. 
     An object of the present invention is to provide a work machine that can limit operation of a work device by the MC, and improves the responsiveness of a hydraulic actuator to operation of an operation device by an operator, and ensures operability equivalent to that of a work machine that does not have MC functions, and allows the hydraulic actuator for which the operation device is not being operated to automatically operate in either direction of the operation directions thereof. 
     Means for Solving the Problem 
     In order to solve such a problem, the present invention provides a work machine comprising: a work device; a plurality of hydraulic actuators that drive the work device; a plurality of operation devices that generate a plurality of operation pilot pressures to instruct operations of the plurality of hydraulic actuators; a plurality of flow control valves that are driven by the plurality of operation pilot pressures and control flow rates of hydraulic fluids supplied to the plurality of hydraulic actuators; a plurality of proportional solenoid valves that generate a plurality of control pilot pressures independently of the plurality of operation devices; a plurality of operation pressure sensors that sense the plurality of operation pilot pressures generated by the plurality of operation devices; a work device posture sensor that senses posture of the work device; and a controller that controls the plurality of proportional solenoid valves on a basis of signals from the plurality of operation pressure sensors and the work device posture sensor, the plurality of operation devices including a first operation device that instructs operation of a first hydraulic actuator in the plurality of hydraulic actuators, the plurality of flow control valves including a first flow control valve that is driven by an operation pilot pressure generated by the first operation device and controls a flow rate of a hydraulic fluid supplied to the first hydraulic actuator, the first operation device having a first output port that outputs a first operation pilot pressure to instruct operation of the first hydraulic actuator in a first direction and a second output port that outputs a second operation pilot pressure to instruct operation of the first hydraulic actuator in a second direction, the plurality of operation pressure sensors having a first operation pressure sensor that senses the first operation pilot pressure and a second operation pressure sensor that senses the second operation pilot pressure, wherein the plurality of proportional solenoid valves have a first proportional solenoid valve that generates a first control pilot pressure to instruct operation of the first hydraulic actuator in the first direction and a second proportional solenoid valve that generates a second control pilot pressure to instruct operation of the first hydraulic actuator in the second direction, the work machine further comprises a plurality of control pressure sensors that sense the plurality of control pilot pressures generated by the plurality of proportional solenoid valves and include a first control pressure sensor that senses the first control pilot pressure generated by the first proportional solenoid valve and a second control pressure sensor that senses the second control pilot pressure generated by the second proportional solenoid valve, a first selector valve disposed between the first output port of the first operation device and the first flow control valve and between the first proportional solenoid valve and the first flow control valve, and a second selector valve disposed between the second output port of the first operation device and the first flow control valve and between the second proportional solenoid valve and the first flow control valve, the first selector valve has a first position to interrupt connection between the first proportional solenoid valve and the first flow control valve and connect the first output port of the first operation device to the first flow control valve and a second position to interrupt connection between the first output port of the first operation device and the first flow control valve and connect the first proportional solenoid valve to the first flow control valve, the second selector valve has a first position to interrupt connection between the second proportional solenoid valve and the first flow control valve and connect the second output port of the first operation device to the first flow control valve and a second position to interrupt connection between the second output port of the first operation device and the first flow control valve and connect the second proportional solenoid valve to the first flow control valve, and the controller is configured to switch the first and second selector valves to either one of the first position and the second position on a basis of signals from the first and second operation pressure sensors and the first and second control pressure sensors and a target operation set in advance regarding the first and second selector valves. 
     By such configuration in which the first selector valve and the second selector valve are disposed and the first and second selector valves are switched to either one of the first position and the second position as above, operation of the work device can be limited by the MC and the responsiveness of the hydraulic actuator to operation of the operation device by the operator is improved. In addition, operability equivalent to that of a work machine that does not have MC functions is ensured and it becomes possible to automatically operate the hydraulic actuator for which the operation device is not being operated in either of the operation directions thereof. 
     Specifically, for example, by switching the first selector valve to the second position and controlling the first proportional solenoid valve to generate the first control pilot pressure obtained by reducing the first operation pilot pressure sensed by the first operation pressure sensor, operation of the first hydraulic actuator in the first direction can be limited and it becomes possible to limit operation of the work device by the MC. This is the same also in the case in which the second selector valve is switched to the second position. 
     Furthermore, for example, by causing the first selector valve to be switched to the first position when the operator operates the first operation device in the MC or when the MC is not carried out, the operation pilot pressure output from the first output port of the first operation device is introduced to the first flow control valve without passing through the first proportional solenoid valve. Due to this, pressure loss as in the conventional case in which the operation pilot pressure passes through the proportional solenoid valve does not occur, thus the responsiveness of the first hydraulic actuator to operation of the first operation device by the operator can be improved and operability equivalent to that of a work machine that does not have MC functions can be ensured. This is the same also in the case in which the second selector valve is caused to be switched to the first position. 
     Moreover, the first hydraulic actuator can be automatically operated in the first direction by switching the first selector valve to the second position and controlling the first proportional solenoid valve to generate the first control pilot pressure based on the MC. Similarly, the first hydraulic actuator can be automatically operated in the second direction by switching the second selector valve to the second position and controlling the second proportional solenoid valve to generate the second control pilot pressure based on the MC. Due to this, it becomes possible to automatically operate the hydraulic actuator for which the operation device is not being operated in either direction of the operation directions thereof. 
     Advantages of the Invention 
     According to the present invention, operation of the work device can be limited by the MC and the responsiveness of the hydraulic actuator to operation of the operation device by the operator is improved. In addition, operability equivalent to that of a work machine that does not have MC functions is ensured and it becomes possible to automatically operate the hydraulic actuator for which the operation device is not being operated in either direction of the operation directions thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a configuration diagram of a hydraulic excavator that is a work machine in a first embodiment of the present invention. 
         FIG. 2  is a diagram illustrating a front device control part of a drive system included in the work machine (hydraulic excavator) of the first embodiment of the present invention. 
         FIG. 3  is a diagram illustrating the arrangement and the operation form of an operation device for a boom, an operation device for an arm, and an operation device for a bucket. 
         FIG. 4  is a functional block diagram of a controller. 
         FIG. 5  is a functional block diagram of an MC control section illustrated in  FIG. 4 . 
         FIG. 6  is a diagram illustrating a control flow of selector valves in a selector valve operation calculating section illustrated in  FIG. 5 . 
         FIG. 7  is a diagram illustrating a control flow of proportional solenoid valves in an actuator control section (boom control section, arm control section, and bucket control section) illustrated in  FIG. 5 . 
         FIG. 8  is a diagram illustrating operation of horizontal excavation at the time of the MC and an image of synthesis of velocity vectors based on operation of the boom and the arm in the hydraulic excavator. 
         FIG. 9  is a diagram illustrating operation of position adjustment of the claw tip of the bucket to a target surface at the time of the MC in the hydraulic excavator. 
         FIG. 10  is a functional block diagram of the MC control section similar to  FIG. 5  in a second embodiment of the present invention. 
         FIG. 11  is a diagram that illustrates a control flow of the selector valves in the selector valve operation calculating section in the second embodiment of the present invention and is similar to  FIG. 6 . 
         FIG. 12  is a functional block diagram of the controller in a third embodiment of the present invention. 
         FIG. 13  is a functional block diagram of the MC control section in  FIG. 12 . 
         FIG. 14  is a diagram illustrating a control flow of the selector valves in the selector valve operation calculating section in the third embodiment of the present invention. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     Embodiments of the present invention will be described below according to the drawings. In the following description, a hydraulic excavator including a bucket  10  as work equipment (attachment) at the tip of a work device will be exemplified. However, the present invention may be applied to a work machine including an attachment other than the bucket. Moreover, application to a work machine other than the hydraulic excavator is also possible as long as it is what has an articulated work device configured by joining plural link members (attachment, arm, boom, and so forth). 
     First Embodiment 
     &lt;Work Machine&gt; 
       FIG. 1  is a configuration diagram of a hydraulic excavator that is a work machine in a first embodiment of the present invention. 
     In  FIG. 1 , a hydraulic excavator  1  is composed of an articulated front work device (hereinafter, often referred to simply as work device)  1 A and a machine body  1 B. The machine body  1 B has a lower track structure  11  that travels by left and right travelling hydraulic motors  3   a  and  3   b  and an upper swing structure  12  that is attached onto the lower track structure  11  and is swung by a swing hydraulic motor  4 . The front work device  1 A is configured by joining plural driven members (boom  8 , arm  9 , and bucket  10 ) that are each pivoted in the perpendicular direction. The base end of the boom  8  is pivotally supported at the front part of the upper swing structure  12  with the interposition of a boom pin. The arm  9  is pivotally joined to the tip of the boom  8  with the interposition of an arm pin and the bucket  10  is pivotally joined to the tip of the arm  9  with the interposition of a bucket pin. The boom  8  is driven by a hydraulic cylinder  5  (hereinafter, referred to as boom cylinder). The arm  9  is driven by a hydraulic cylinder  6  (hereinafter, referred to as arm cylinder). The bucket  10  is driven by a hydraulic cylinder  7  (hereinafter, referred to as bucket cylinder). 
     A boom angle sensor  30  is attached to the boom pin and an arm angle sensor  31  is attached to the arm pin and a bucket angle sensor  32  is attached to a bucket link  13  such that the pivot angles of the boom  8 , the arm  9 , and the bucket  10  can be measured. A machine body inclination angle sensor  33  that senses the inclination angle of the upper swing structure  12  (machine body  1 B) with respect to a reference plane (for example, horizontal plane) is attached to the upper swing structure  12 . The angle sensors  30 ,  31 , and  32  can be each replaced by an angle sensor with respect to a reference plane (for example, horizontal plane). 
     &lt;Drive System&gt; 
       FIG. 2  is a diagram illustrating a front device control part of a drive system included in the work machine (hydraulic excavator) of the first embodiment of the present invention. 
     In  FIG. 2 , the drive system includes an operation device  45   a  for the boom, an operation device  46   a  for the arm, and an operation device  45   b  for the bucket. The operation device  45   a  for the boom and the operation device  45   b  for the bucket are operation devices operated by one operation lever  1   a  disposed on the right side of an operation seat  24  illustrated in  FIG. 1 . The operation device  46   a  for the arm is an operation device operated together with an operation device  46   b  for swing (see  FIG. 3 ) by one operation lever  1   b  disposed on the left side of the operation seat  24  illustrated in  FIG. 1 . 
       FIG. 3  is a diagram illustrating the arrangement and the operation form of the operation device  45   a  for the boom, the operation device  46   a  for the arm, and the operation device  45   b  for the bucket. 
     The operation devices  45   a  and  45   b  are set on the right side of the front part of the operation seat  24  in an operation room (cabin)  23  of the hydraulic excavator illustrated in  FIG. 1  and the operation device  46   a  is set on the left side of the front part of the operation seat  24 . The operation devices  45   a  and  45   b  are configured as one operation lever unit  45  including the operation lever  1   a . The operation device  46   a  is configured as one operation lever unit  46  including the operation lever  1   b  together with the operation device  46   b  for swing. An operator operates the right operation lever  1   a  with the right hand and operates the left operation lever  1   b  with the left hand. 
     The operation lever units  45  and  46  can each instruct operation of two hydraulic actuators by one operation lever  1   a  or  1   b . The operation levers  1   a  and  1   b  can be each operated in an optional direction on the basis of the four directions of a cross. Operation of the operation lever  1   a  in the upward-downward direction in the diagrammatic representation corresponds to an operation instruction of the boom cylinder  5 . Operation of the operation lever  1   a  in the left-right direction in the diagrammatic representation corresponds to an operation instruction of the bucket cylinder  7 . Operation of the operation lever  1   b  in the left-right direction in the diagrammatic representation corresponds to an operation instruction of the arm cylinder  6 . Operation of the operation lever  1   b  in the upward-downward direction in the diagrammatic representation corresponds to an operation instruction of the swing hydraulic motor  4  (see  FIG. 1 ). Furthermore, operation of the operation lever  1   a  in the downward direction in the diagrammatic representation corresponds to an instruction of operation of the boom cylinder  5  in the extension direction (boom raising). Operation of the operation lever  1   a  in the upward direction in the diagrammatic representation corresponds to an instruction of operation of the boom cylinder  5  in the contraction direction (boom lowering). Operation of the operation lever  1   a  in the left direction in the diagrammatic representation corresponds to an instruction of operation of the bucket cylinder  7  in the extension direction (bucket crowding). Operation of the operation lever  1   a  in the right direction in the diagrammatic representation corresponds to an instruction of operation of the bucket cylinder  7  in the contraction direction (bucket dumping). Operation of the operation lever  1   b  in the right direction in the diagrammatic representation corresponds to an instruction of operation of the arm cylinder  6  in the extension direction (arm crowding). Operation of the operation lever  1   b  in the left direction in the diagrammatic representation corresponds to an instruction of operation of the arm cylinder  6  in the contraction direction (arm dumping). 
     Referring back to  FIG. 2 , the drive system includes a flow control valve  15   a  for the boom, a flow control valve  15   b  for the arm, and a flow control valve  15   c  for the bucket. The flow rate and the supply direction of a hydraulic fluid supplied from a main pump that is not illustrated in the diagram to the boom cylinder  5 , the arm cylinder  6 , and the bucket cylinder  7  are controlled by the flow control valve  15   a , the flow control valve  15   b , and the flow control valve  15   c.    
     For the operation device  45   a  for the boom, the operation device  46   a  for the arm, and the operation device  45   b  for the bucket, primary ports (input ports)  124 ,  125 , and  126  are connected to a pump line  48   a  of a pilot pump  48 . The operation device  45   a  for the boom, the operation device  46   a  for the arm, and the operation device  45   b  for the bucket use the pressure of the pump line  48   a  as a primary pressure to generate an operation pilot pressure (secondary pressure) according to the operation amount of the operation lever  1   a  or  1   b , and output the generated operation pilot pressure from secondary ports (output ports)  134   a ,  134   b ,  135   a ,  135   b ,  136   a , and  136   b  to operation pilot lines  144   a ,  144   b ,  145   a ,  145   b ,  146   a , and  146   b.    
     The operation device  45   a  for the boom, when the operation lever  1   a  is operated in the right direction in  FIG. 2  (downward direction in  FIG. 3 ), generates the operation pilot pressure to drive the boom  8  in the raising direction and outputs the operation pilot pressure to the operation pilot line  144   a . Furthermore, the operation device  45   a  for the boom, when the operation lever  1   a  is operated in the left direction in  FIG. 2  (upward direction in  FIG. 3 ), generates the operation pilot pressure to drive the boom  8  in the lowering direction and outputs the operation pilot pressure to the operation pilot line  144   b . The operation device  46   a  for the arm, when the operation lever  1   b  is operated in the right direction in  FIG. 2  (right direction in  FIG. 3 ), generates the operation pilot pressure to drive the arm  9  in the crowding direction and outputs the operation pilot pressure to the operation pilot line  145   a . Furthermore, the operation device  46   a  for the arm, when the operation lever  1   b  is operated in the left direction in  FIG. 2  (left direction in  FIG. 3 ), generates the operation pilot pressure to drive the arm  9  in the dumping direction and outputs the operation pilot pressure to the operation pilot line  145   b . The operation device  45   b  for the bucket, when the operation lever  1   a  is operated in the right direction in  FIG. 2  (left direction in  FIG. 3 ), generates the operation pilot pressure to drive the bucket  10  in the crowding direction and outputs the operation pilot pressure to the operation pilot line  146   a . Furthermore, the operation device  45   b  for the bucket, when the operation lever  1   a  is operated in the left direction in  FIG. 2  (right direction in  FIG. 3 ), generates the operation pilot pressure to drive the bucket  10  in the dumping direction and outputs the operation pilot pressure to the operation pilot line  146   b.    
     Moreover, the drive system includes pressure sensors (operation pressure sensors)  70   a  and  70   b  that are disposed on the operation pilot lines  144   a  and  144   b  of the operation device  45   a  for the boom and sense the operation pilot pressure generated by the operation device  45   a  and proportional solenoid valves  54   a  and  54   b  that have the primary port connected to the pump line  48   a  through control pilot lines  154   a  and  154   b  and reduce the pilot pressure from the pump line  48   a  to generate a control pilot pressure. The drive system includes also pressure sensors (control pressure sensors)  200   a  and  200   b  that are connected to control pilot lines  154   c  and  154   d  on the secondary port side of the proportional solenoid valves  54   a  and  54   b  and sense the control pilot pressure generated by the proportional solenoid valves  54   a  and  54   b  and selector valves  203   a  and  203   b  connected to the operation pilot lines  144   a  and  144   b  on the secondary port side of the operation device  45   a  for the boom and the control pilot lines  154   c  and  154   d  on the secondary port side of the proportional solenoid valves  54   a  and  54   b.    
     Drive pilot pressure input lines  164   a  and  164   b  are connected to hydraulic drive parts  150   a  and  150   b  of the flow control valve  15   a  for the boom. The selector valves  203   a  and  203   b  carry out switching about to which of the operation pilot line  144   a  or  144   b  and the control pilot line  154   c  or  154   d  the drive pilot pressure input line  164   a  or  164   b  is connected, on the basis of a control signal from a controller  40 . 
     Furthermore, the drive system, also for the operation device  46   a  for the arm, similarly includes pressure sensors  71   a  and  71   b , control pilot lines  155   a  and  155   b , proportional solenoid valves  55   a  and  55   b , control pilot lines  155   c  and  155   d , pressure sensors  201   a  and  201   b , drive pilot pressure input lines  165   a  and  165   b , and selector valves  204   a  and  204   b . Also for the operation device  45   b  for the bucket, similarly, the drive system includes pressure sensors  72   a  and  72   b , control pilot lines  156   a  and  156   b , proportional solenoid valves  56   a  and  56   b , control pilot lines  156   c  and  156   d , pressure sensors  202   a  and  202   b , drive pilot pressure input lines  166   a  and  166   b , and selector valves  205   a  and  205   b.    
     In  FIG. 2 , connection lines between the pressure sensors  70   a  to  72   b  and the pressure sensors  200   a  to  202   b  and the controller  40  are omitted for simplification of the diagrammatic representation. 
     In the proportional solenoid valves  54   a  to  56   b , the degree of opening is zero at the time of non-energization. The proportional solenoid valves  54   a  to  56   b  have a predetermined degree of opening at the time of energization and the degree of opening becomes higher as a current (control signal) from the controller  40  is increased. As above, the degree of opening of the proportional solenoid valves  54   a  to  56   b  becomes what depends on the control signal from the controller  40  and the proportional solenoid valves  54   a  to  56   b  reduce the pilot pressure from the pump line  48   a  according to the degree of opening to generate the control pilot pressure. 
     The selector valves  203   a  to  205   b  have a first position to form a circuit that connects the secondary port side of the operation device  45   a ,  45   b , or  46   b  to the hydraulic drive section  150   a  to  152   b  of the flow control valve  15   a ,  15   b , or  15   c  and a second position to form a circuit that connects the secondary port side of the proportional solenoid valve  54   a  to  56   b  to the hydraulic drive section  150   a  to  152   b  of the flow control valve  15   a ,  15   b , or  15   c . The selector valves  203   a  to  205   b  are switched to either position of the first position and the second position according to the control signal from the controller  40  to carry out switching of the circuit. The selector valves  203   a  to  205  are switched to the first position at the time of non-energization when the MC is not carried out, and are switched to the second position at the time of energization when the MC is carried out. 
     In the drive system configured as above, when the control signal is output from the controller  40  and the proportional solenoid valve  54   a  to  56   b  and the selector valve  203   a  to  205   b  are driven, the control pilot pressure is generated by the proportional solenoid valve  54   a  to  56   b  also in the case in which operator operation to the operation device  45   a ,  45   b , or  46   a  is not made, and boom raising operation, boom lowering operation, arm crowding operation, arm dumping operation, bucket crowding operation, or bucket dumping operation can be forcibly caused by introducing the control pilot pressure to the hydraulic drive section  150   a  to  152   b  of the flow control valve  15   a ,  15   b , or  15   c . Furthermore, similarly to this, when an operator is operating the operation device  45   a ,  45   b , or  46   a , the velocity of boom raising operation, boom lowering operation, arm crowding operation, arm dumping operation, bucket crowding operation, or bucket dumping operation can be forcibly reduced from the value of the operator operation by generating the control pilot pressure by the proportional solenoid valve  54   a  to  56   b  and introducing the control pilot pressure to the hydraulic drive section  150   a  to  152   b  of the flow control valve  15   a ,  15   b , or  15   c . Moreover, when the selector valve  203   a  to  205   b  exists at the first position, the operation pilot pressure generated by the operation device  45   a ,  45   b , or  46   a  is introduced to the hydraulic drive section  150   a  to  152   b  of the flow control valve  15   a ,  15   b , or  15   c  without passing through the proportional solenoid valve  54   a  to  56   b . Therefore, pressure loss as in the conventional case in which the operation pilot pressure passes through the proportional solenoid valve does not occur. Thus, the responsiveness of the hydraulic actuators  5 ,  6 , and  7  to operation of the operation devices  45   a ,  46   a , and  45   b  can be improved and operability equivalent to that of a work machine that does not have MC functions can be ensured. 
     Here, there is application to horizontal excavation as an MC function of the work machine. In this case, when an excavation operation signal (specifically, instruction of at least one of arm crowding, bucket crowding, and bucket dumping) is input through the operation devices  45   b  and  46   a , on the basis of the positional relation between a target surface  60  (see  FIG. 8 ) and a control point of the work device  1 A, for example, the tip of the bucket  10  (in the present embodiment, claw tip of the bucket  10 ), a control signal that causes at least one of the hydraulic actuators  5 ,  6 , and  7  to be forcibly operated (for example, causes the boom cylinder  5  to extend to forcibly carry out boom raising operation) in such a manner that the position of the control point of the work device  1 A is kept on the target surface  60  and in a region on the upper side thereof is output to the corresponding flow control valve  15   a ,  15   b , or  15   c . The claw tip of the bucket  10  is prevented from entering the lower side of the target surface  60  by this MC function. Therefore, excavation along the target surface  60  is enabled irrespective of the degree of skill of the operator. In the present embodiment, the control point of the front work device  1 A at the time of the MC is set to the claw tip of the bucket  10  of the hydraulic excavator (tip of the work device  1 A). However, the control point can be changed also to a point other than the bucket claw tip as long as it is a point on the tip part of the work device  1 A. For example, the bottom surface of the bucket  10  and the outermost part of the bucket link  13  can also be selected. 
     &lt;Controller  40 &gt; 
       FIG. 4  is a functional block diagram of the controller  40 . 
     The controller  40  has an MC control section  43 , a proportional solenoid valve control section  44 , a selector valve control section  213 , and a display control section  374 . 
     The MC control section  43  inputs signals from a work device posture sensor  50 , a target surface setting device  51 , an operation device secondary pressure sensor  52   a , and a proportional solenoid valve secondary pressure sensor  210  and carries out predetermined calculation on the basis of these signals to send calculation information to the proportional solenoid valve control section  44 , the selector valve control section  213 , and the display control section  374 . The proportional solenoid valve control section  44 , the selector valve control section  213 , and the display control section  374  output a control signal and display information to the proportional solenoid valves  54   a  to  56   b , the selector valves  203   a  to  205   b , and a display device  53  on the basis of the calculation information. 
     The work device posture sensor  50  is composed of the boom angle sensor  30 , the arm angle sensor  31 , the bucket angle sensor  32 , and the machine body inclination angle sensor  33 . These sensors  30 ,  31 ,  32 , and  33  function as a posture sensor of the work device  1 A. 
     The target surface setting device  51  is an interface with which information relating to the target surface  60  (see  FIG. 8 ) (including position information and inclination angle information of each target surface) can be input. The target surface setting device  51  is connected to an external terminal (not illustrated) in which three-dimensional data of target surfaces defined on the global coordinate system (absolute coordinate system) is stored. The input of the target surface through the target surface setting device  51  may be manually carried out by the operator. 
     The operation device secondary pressure sensor  52   a  is composed of the pressure sensors  70   a  to  72   b  that sense the operation pilot pressure generated in the operation pilot lines  144   a ,  144   b ,  145   a ,  145   b ,  146   a , and  146   b  through operation of the operation levers  1   a  and  1   b  (operation devices  45   a ,  45   b , and  46   a ). 
     The proportional solenoid valve secondary pressure sensor  210  is composed of the pressure sensors  200   a  to  202   b  that sense the control pilot pressure generated in the control pilot lines  154   c ,  154   d ,  155   c ,  155   d ,  156   c , and  156   d  on the secondary port side of the proportional solenoid valves  54   a  to  56   b.    
       FIG. 5  is a functional block diagram of the MC control section  43  illustrated in  FIG. 4 . 
     The MC control section  43  has an operation device secondary pressure calculating section  43   a , a posture calculating section  43   b , a target surface calculating section  43   c , an actuator control section  81  including a boom control section  81   a , an arm control section  81   b , and a bucket control section  81   c , a proportional solenoid valve secondary pressure calculating section  211 , and a selector valve operation calculating section  212 . 
     The operation device secondary pressure calculating section  43   a  computes the operation pilot pressures that are the pressures of the secondary port of the operation devices  45   a ,  45   b , and  46   a  from sensed values of the operation device secondary pressure sensor  52   a  (pressure sensors  70   a  to  72   b ). 
     The posture calculating section  43   b  calculates the posture of the front work device  1 A and the position of the claw tip of the bucket  10  in a local coordinate system (for example, machine body coordinate system set on the machine body  1 B in  FIG. 1 ) on the basis of sensed values from the work device posture sensor  50  (boom angle sensor  30 , arm angle sensor  31 , bucket angle sensor  32 , and machine body inclination angle sensor  33 ). 
     The target surface calculating section  43   c  calculates position information of the target surface  60  (see  FIG. 8 ) on the basis of information from the target surface setting device  51 . 
     The proportional solenoid valve secondary pressure calculating section  211  computes the control pilot pressures that are the pressures of the secondary port side of the proportional solenoid valves  54   a  to  56   b  on the basis of sensed values from the proportional solenoid valve secondary pressure sensor  210  (pressure sensors  200   a  to  202   b ). 
     The actuator control section  81  (boom control section  81   a , arm control section  81   b , and bucket control section  81   c ), on the basis of the output of each the operation device secondary pressure calculating section  43   a , the posture calculating section  43   b , the target surface calculating section  43   c , the proportional solenoid valve secondary pressure calculating section  211 , and the selector valve operation calculating section  212 , calculates the target pilot pressure of the flow control valve  15   a ,  15   b , or  15   c  for the hydraulic actuator  5 ,  6 , or  7 , according to a condition defined in advance (for example, work mode of front device operation input by the operator) at the time of operation of the operation device  45   a ,  45   b , or  46   a  and outputs the calculated target pilot pressure to the proportional solenoid valve control section  44 . 
     Here, the boom control section  81   a  is a section for carrying out operation control of the boom  8  by the MC at the time of operation of the operation device  45   a ,  45   b , or  46   a . For example, when horizontal excavation and position adjustment of the claw tip of the bucket  10  (to be described later) are set in the controller  40  as the work mode, the boom control section  81   a , at the time of operation of the operation device  45   a ,  45   b , or  46   a , carries out MC to control operation of the boom cylinder  5  (boom  8 ) in such a manner that the claw tip (control point) of the bucket  10  is located on the target surface  60  or on the upper side thereof, on the basis of the position of the target surface  60  (see  FIG. 8 ), the posture of the front work device  1 A and the position of the claw tip of the bucket  10 , the operation amount of the operation device  45   a ,  45   b , or  46   a , the pressure of the secondary port side of the proportional solenoid valve  54   a  or  54   b , and the switching position of the selector valve  203   a  or  203   b . The boom control section  81   a  calculates the target pilot pressure (target value of the control pilot pressure) of the flow control valve  15   a  relating to the boom cylinder  5  for carrying out the MC. 
     The arm control section  81   b  is a section for carrying out operation control of the arm  9  by the MC at the time of operation of the operation device  45   a ,  45   b , or  46   a . The arm control section  81   b  calculates the target pilot pressure (target value of the control pilot pressure) of the flow control valve  15   b  relating to the arm cylinder  6  for carrying out the MC. 
     The bucket control section  81   c  is a section for carrying out bucket angle control by the MC at the time of operation of the operation device  45   a ,  45   b , or  46   a . The bucket control section  81   c  calculates the target pilot pressure (target value of the control pilot pressure) of the flow control valve  15   c  relating to the bucket cylinder  7  for carrying out the MC. 
     The proportional solenoid valve control section  44  calculates command values to the proportional solenoid valves  54   a  to  56   b  on the basis of the target pilot pressures of the respective flow control valves  15   a ,  15   b , and  15   c  output from the actuator control section  81 . 
     The selector valve operation calculating section  212  calculates the target switching position of the selector valves  203   a  to  205   b  according to a condition defined in advance (for example, work mode of front device operation) at the time of operation of the operation device  45   a ,  45   b , or  46   a  on the basis of the output of the operation device secondary pressure calculating section  43   a  and the output of the proportional solenoid valve secondary pressure calculating section  211 . 
     The selector valve control section  213  calculates command values to the selector valves  203   a  to  205   b  on the basis of the target switching position of the selector valves  203   a  to  205   b  output from the selector valve operation calculating section  212 . 
     The display control section  374  controls the display device  53  on the basis of the work device posture and the target surface output from the posture calculating section  43   b  and the target surface calculating section  43   c . In the display control section  374 , a display ROM in which a large number of pieces of display-related data including image and icon of the work device  1 A are stored is included. The display control section  374  reads out a predetermined program on the basis of a flag included in input information and carries out display control in the display device  53 . 
     &lt;Selector Valve Control Flow of Selector Valve Operation Calculating Section  212 &gt; 
       FIG. 6  is a diagram illustrating a control flow of the selector valves  203   a  to  205   b  in the selector valve operation calculating section  212  illustrated in  FIG. 5 . In the controller  40 , with respect to the selector valves  203   a  to  205   b , target operation for setting the target position according to a condition defined in advance (for example, work mode of front device operation) is set in advance. 
     In a step S 110  in  FIG. 6 , the selector valve operation calculating section  212  acquires the operation pilot pressures that are the pressures of the secondary port side of the operation devices  45   a ,  45   b , and  46   a  calculated in the operation device secondary pressure calculating section  43   a.    
     In a step S 120 , the selector valve operation calculating section  212  acquires the control pilot pressures that are the pressures of the secondary port side of the proportional solenoid valves  54   a  to  56   b  calculated in the proportional solenoid valve secondary pressure calculating section  211 . 
     In a step S 130 , the selector valve operation calculating section  212  determines whether or not the target operation set in advance regarding the selector valve  203   a  to  205   b  is keeping at the first position. When it is determined in the step S 130  that the target operation is keeping at the first position, progress to a step S 140  is made. When the target operation is other than keeping at the first position, progress to a step S 150  is made. 
     In the step S 140 , the selector valve operation calculating section  212  sets the target position of the selector valve  203   a  to  205   b  to the first position. 
     In the step S 150 , the selector valve operation calculating section  212  determines whether or not the target operation set in advance regarding the selector valve  203   a  to  205   b  is keeping at the second position. When it is determined in the step S 150  that the target operation is keeping at the second position, progress to a step S 160  is made. When the target operation is other than keeping at the second position, progress to a step S 170  is made. 
     In the step S 160 , the selector valve operation calculating section  212  sets the target position of the selector valve  203   a  to  205   b  to the second position. 
     In the step S 170 , the selector valve operation calculating section  212  compares the pressure of the secondary port side of the operation device  45   a ,  45   b , or  46   a  with the pressure of the secondary port side of the corresponding proportional solenoid valve  54   a  to  56   b  acquired in the step S 110  and the step S 120 , and determines whether or not the pressure of the secondary port side of the operation device  45   a ,  45   b , or  46   a  is higher. When it is determined in the step S 170  that the pressure of the secondary port side of the operation device  45   a ,  45   b , or  46   a  is higher than the pressure of the secondary port side of the proportional solenoid valve  54   a  to  56   b , progress to a step S 180  is made. When it is determined that the pressure of the secondary port side of the operation device  45   a ,  45   b , or  46   a  is equal to or lower than the pressure of the secondary port side of the proportional solenoid valve  54   a  to  56   b , progress to a step S 190  is made. 
     In the step S 180 , the selector valve operation calculating section  212  sets the target position of the selector valve  203   a  to  205   b  to the first position. 
     In the step S 190 , the selector valve operation calculating section  212  sets the target position of the selector valve  203   a  to  205   b  to the second position. 
     In a step S 270 , the selector valve operation calculating section  212  outputs the target position of the selector valve  203   a  to  205   b  to the selector valve control section  213 . 
     The selector valve control section  213  calculates a command value to the selector valve  203   a  to  205   b  on the basis of the target position of the selector valve  203   a  to  205   b  and outputs a control signal to cause the position of the selector valve  203   a  to  205   b  to become the target position. 
     &lt;Proportional Solenoid Valve Control Flow of Actuator Control Section  81 &gt; 
       FIG. 7  is a diagram illustrating a control flow of the proportional solenoid valves  54   a  to  56   b  in the actuator control section  81  (boom control section  81   a , arm control section  81   b , and bucket control section  81   c ) illustrated in  FIG. 5 . In the controller  40 , with respect to the proportional solenoid valves  54   a  to  56   b , target operation for setting the target pilot pressure according to a condition defined in advance (for example, work mode of front device operation) is set in advance. 
     In a step S 410 , the actuator control section  81  acquires the operation pilot pressures that are the pressures of the secondary port side of the operation devices  45   a ,  45   b , and  46   a  calculated in the operation device secondary pressure calculating section  43   a.    
     In a step S 420 , the actuator control section  81  acquires the control pilot pressures that are the pressures of the secondary port side of the proportional solenoid valves  54   a  to  56   b  calculated in the proportional solenoid valve secondary pressure calculating section  211 . 
     In a step S 430 , the actuator control section  81  acquires the target position of the selector valve  203   a  to  205   b  calculated in the selector valve operation calculating section  212 . 
     In a step S 440 , the actuator control section  81  determines whether or not the position of the selector valve  203   a  to  205   b  is the second position. When it is determined in the step S 440  that the position of the selector valve  203   a  to  205   b  is the second position, progress to a step S 450  is made. When it is determined that the position of the selector valve  203   a  to  205   b  is other than the second position, i.e. the first position, progress to a step S 470  is made. 
     In the step S 450 , the actuator control section  81  acquires the posture of the boom  8 , the arm  9 , and the bucket  10  calculated in the posture calculating section  43   b.    
     In a step S 460 , the actuator control section  81 , on the basis of the target operation set in advance, calculates and sets the target pilot pressure of the flow control valve  15   a ,  15   b , or  15   c  that should be generated by the proportional solenoid valve  54   a  to  56   b  and is based on the MC. 
     In the step S 470 , the actuator control section  81 , on the basis of the pressures of the secondary port side of the operation devices  45   a ,  45   b , and  46   a  (operation pilot pressures) acquired in the step S 410 , sets the target pilot pressure equal to these operation pilot pressures. 
     In the step S 480 , the actuator control section  81  outputs the target pilot pressure for the flow control valve  15   a ,  15   b , or  15   c  of the hydraulic actuator  5 ,  6 , or  7  to the proportional solenoid valve control section  44 . 
     The proportional solenoid valve control section  44  controls the proportional solenoid valves  54   a  to  56   b  in such a manner that the control pilot pressure equal to the target pilot pressure acts on the flow control valves  15   a ,  15   b , and  15   c  relating to the hydraulic actuators  5 ,  6 , and  7 . Due to this, for example, even when an operator is carrying out boom lowering operation through operating the operation device  45   a , operation of the boom  8  can be limited by generating the control pilot pressure in such a manner that the claw tip of the bucket  10  does not enter the target surface  60 . Furthermore, in the case in which boom lowering operation needs to be carried out in order to cause the claw tip of the bucket  10  to operate along the target surface  60  in horizontal excavation or the like, generating the control pilot pressure allows the boom lowering operation to be automatically carried out without operation of the operation device  45   a  by the operator. 
     &lt;Setting of Target Operation of Selector Valves and Proportional Solenoid Valves&gt; 
     In the following, a setting example of the target operation of the selector valves and the proportional solenoid valves will be described by taking as an example the case in which horizontal excavation and position adjustment of the bucket claw tip are set as the work mode. 
       FIG. 8  is a diagram illustrating operation of the horizontal excavation at the time of the MC and an image of synthesis of velocity vectors based on operation of the boom  8  and the arm  9  in the hydraulic excavator configured as above. 
     In the horizontal excavation, the front work device  1 A makes transitions from a state S 1  ( FIG. 8 : excavation start posture) to a state S 2  ( FIG. 8 : arm vertical posture) and to a state S 3  ( FIG. 8 : excavation end posture). 
       FIG. 9  is a diagram illustrating operation of position adjustment of the claw tip of the bucket  10  to the target surface  60  at the time of the MC. 
     In the position adjustment of the claw tip of the bucket  10 , the front work device  1 A makes transitions from a state S 4  ( FIG. 9 : height of the claw tip of the bucket  10  is high) to a state S 5  ( FIG. 9 : height of the claw tip of the bucket  10  is middle) and to a state S 6  ( FIG. 9 : height of the claw tip of the bucket  10  is 0). 
     The controller  40 , in the horizontal excavation illustrated in  FIG. 8 , carries out boom raising control and boom lowering control as the MC by combining control of the proportional solenoid valves  54   a  and  54   b  by the boom control section  81   a  and control of the selector valves  203   a  and  203   b  by the selector valve operation calculating section  212 . 
     Furthermore, the controller  40 , in the operation of the position adjustment of the claw tip of the bucket  10  illustrated in  FIG. 9 , carries out boom lowering control as the MC by combining control of the proportional solenoid valve  54   b  by the boom control section  81   a  and control of the selector valve  203   b  by the selector valve operation calculating section  212 . 
     Here, when the horizontal excavation and the position adjustment of the bucket claw tip based on the MC are carried out, the work mode of the horizontal excavation and the position adjustment of the bucket claw tip is set in the controller  40  through operation by the operator and the target operation of the selector valves  203   a  to  205   b  and the proportional solenoid valves  54   a  to  56   b  is set in the controller  40  in advance on the basis of the work mode. 
     The target operation set in advance regarding the selector valves  203   a  to  205   b  includes first target operation of keeping each selector valve at the first position, second target operation of keeping each selector valve at the second position, and third target operation of switching each selector valve to either the first position or the second position to introduce, to the corresponding flow control valve, the higher pressure of the operation pilot pressure sensed by the pressure sensor  70   a  to  72   b  and the control pilot pressure sensed by the pressure sensor  200   a  to  202   b  (hereinafter, referred to as “switching to the higher-pressure selection position”). 
     The target operation set in advance regarding the proportional solenoid valves  54   a  to  56   b  includes first target operation of generating the target pilot pressure to equalize the control pilot pressure sensed by the pressure sensor  200   a  to  202   b  to the operation pilot pressure sensed by the pressure sensor  70   a  to  72   b  when the selector valve  203   a  to  205   b  exists at the first position, and second target operation of generating the target pilot pressure based on the MC when the selector valve  203   a  to  205   b  exists at the second position. 
     The selector valve operation calculating section  212  of the controller  40  sets the target position of the selector valves  203   a  to  205   b  to either the first position or the second position on the basis of the above-described target operation set in advance. 
     The actuator control section  81  of the controller  40  calculates and sets the target pilot pressures of the proportional solenoid valves  54   a  to  56   b  on the basis of the above-described target operation set in advance. 
     When the work mode input and set to the controller  40  by the operator is the horizontal excavation illustrated in  FIG. 8  and the position adjustment of the claw tip of the bucket  10  illustrated in  FIG. 9 , the target operation set for the selector valves  203   a  to  205   b  is as follows.
         1. Selector valves  204   a ,  204   b ,  205   a ,  205   b      Keeping at the first position (first target operation)   2. Selector valve  203   b      Keeping at the second position (second target operation)   3. Selector valve  203   a      Switching to the higher-pressure selection position (third target operation)   The controller  40  allows setting of a desired work mode through operation by the operator besides the horizontal excavation illustrated in  FIG. 8  and the position adjustment of the claw tip of the bucket  10  illustrated in  FIG. 9 . Furthermore, any of the above-described first target operation, second target operation, and third target operation is set in the selector valves  203   a  to  205   b  according to the work mode.       

     Summarization of Characteristics of Present Embodiment 
     As above, in the work machine of the present embodiment, the drive system includes the selector valve  203   a  (first selector valve) disposed between the secondary port  134   a  (first output port) of the operation device  45   a  (first operation device) and the flow control valve  15   a  (first flow control valve) and between the proportional solenoid valve  54   a  (first proportional solenoid valve) and the flow control valve  15   a  and the selector valve  203   b  (second selector valve) disposed between the secondary port  134   b  (second output port) of the operation device  45   a  and the flow control valve  15   a  and between the proportional solenoid valve  54   b  (second proportional solenoid valve) and the flow control valve  15   a.    
     Furthermore, the selector valve  203   a  (first selector valve) has the first position to interrupt the connection between the proportional solenoid valve  54   a  (first proportional solenoid valve) and the flow control valve  15   a  and connect the secondary port  134   a  (first output port) of the operation device  45   a  (first operation device) to the flow control valve  15   a  and the second position to interrupt the connection between the secondary port  134   a  of the operation device  45   a  and the flow control valve  15   a  and connect the proportional solenoid valve  54   a  to the flow control valve  15   a . The selector valve  203   b  (second selector valve) has the first position to interrupt the connection between the proportional solenoid valve  54   b  (second proportional solenoid valve) and the flow control valve  15   a  and connect the secondary port  134   b  (second output port) of the operation device  45   a  to the flow control valve  15   a  and the second position to interrupt the connection between the secondary port  134   b  of the operation device  45   a  and the flow control valve  15   a  and connect the proportional solenoid valve  54   b  to the flow control valve  15   a.    
     The controller  40  is configured to switch the selector valves  203   a  and  203   b  to either one of the first position and the second position on the basis of signals from the pressure sensors  70   a  and  70   b  (first and second operation pressure sensors) and the pressure sensors  200   a  and  200   b  (first and second control pressure sensors) and the target operation set in advance regarding the selector valves  203   a  and  203   b  (first and second selector valves). 
     Furthermore, the controller  40  is configured to, as the target operation set in advance regarding the selector valves  203   a  and  203   b  (first and second selector valves), set one of the first target operation of keeping at the first position, the second target operation of keeping at the second position, and the third target operation of switching to one of the first position and the second position to introduce, to the flow control valve  15   a , the higher pressure of the operation pilot pressure (first operation pilot pressure) output from the secondary port  134   a  (first output port) of the operation device  45   a  (first operation device) and the control pilot pressure (first control pilot pressure) generated by the proportional solenoid valve  54   a  (first proportional solenoid valve) and the higher pressure of the operation pilot pressure (second operation pilot pressure) output from the secondary port  134   b  (second output port) of the operation device  45   a  and the control pilot pressure (second control pilot pressure) generated by the proportional solenoid valve  54   b  (second proportional solenoid valve). In addition, the controller  40  sets the target position of the selector valves  203   a  and  203   b  on the basis of this set target operation to switch the selector valves  203   a  and  203   b  to either one of the first position and the second position. 
     Moreover, the controller  40  is configured to, as the target operation of the proportional solenoid valves  54   a  and  54   b  (first and second proportional solenoid valves), set the first target operation of equalizing the control pilot pressures (first and second control pilot pressures) sensed by the pressure sensors  200   a  and  200   b  (first and second control pressure sensors) to the operation pilot pressures (first and second operation pilot pressures) sensed by the pressure sensors  70   a  and  70   b  (first and second operation pressure sensors), respectively, when the selector valves  203   a  and  203   b  (first and second selector valves) exist at the first position, and set the second target operation on the basis of automatic control in advance when the selector valves  203   a  and  203   b  exist at the second position. In addition, the controller  40  sets the target pilot pressure of the proportional solenoid valves  54   a  and  54   b  (first and second proportional solenoid valves) on the basis of the set target operation and controls the proportional solenoid valves  54   a  and  54   b.    
     Furthermore, in the present embodiment, for each of the operation devices  45   a ,  46   a , and  45   b  (plural operation devices), the pressure sensors  70   a  and  70   b  (first and second operation pressure sensors), the pressure sensors  71   a  and  71   b  (first and second operation pressure sensors), the pressure sensors  72   a  and  72   b  (first and second operation pressure sensors), the proportional solenoid valves  54   a  and  54   b  (first and second proportional solenoid valves), the proportional solenoid valves  55   a  and  55   b  (first and second proportional solenoid valves), the proportional solenoid valves  56   a  and  56   b  (first and second proportional solenoid valves), the pressure sensors  200   a  and  200   b  (first and second control pressure sensors), the pressure sensors  201   a  and  201   b  (first and second control pressure sensors), the pressure sensors  202   a  and  202   b  (first and second control pressure sensors), the selector valves  203   a  and  203   b  (first and second selector valves), the selector valves  204   a  and  204   b  (first and second selector valves), and the selector valves  205   a  and  205   b  (first and second selector valves) are disposed, and the controller  40  is configured to switch the selector valves  203   a  and  203   b , the selector valves  204   a  and  204   b , and the selector valves  205   a  and  205   b  to either one of the first position and the second position on the basis of signals from the pressure sensors  70   a  and  70   b , the pressure sensors  71   a  and  71   b , the pressure sensors  72   a  and  72   b , the pressure sensors  200   a  and  200   b , the pressure sensors  201   a  and  201   b , and the pressure sensors  202   a  and  202   b  and the target operation set in advance regarding the selector valves  203   a  and  203   b , the selector valves  204   a  and  204   b , and the selector valves  205   a  and  205   b.    
     The controller  40  is configured for each of the operation devices  45   a ,  46   a , and  45   b  (plural operation devices) to set, as the target operation set in advance regarding the selector valves  203   a  and  203   b  (first and second selector valves), the selector valves  204   a  and  204   b  (first and second selector valves), and the selector valves  205   a  and  205   b  (first and second selector valves), one of the first target operation of keeping at the first position, the second target operation of keeping at the second position, and the third target operation of switching to one of the first position and the second position to introduce, to the flow control valves  15   a ,  15   b , and  15   c  (plural flow control valves), the higher pressure of the operation pilot pressure (first operation pilot pressure) sensed by the pressure sensors  70   a ,  71   a , 72   a  and the control pilot pressure (first control pilot pressure) sensed by the pressure sensors  200   a ,  201   a ,  202   a  and the higher pressure of the operation pilot pressure (second operation pilot pressure) sensed by the pressure sensors  70   b ,  71   b ,  72   b  and the control pilot pressure (second control pilot pressure) sensed by the pressure sensors  200   b ,  201   b ,  202   b . In addition, the controller  40  decides the target position of the selector valves  203   a  and  203   b , the selector valves  204   a  and  204   b , and the selector valves  205   a  and  205   b  on the basis of the set target operation to switch the selector valves  203   a  and  203   b , the selector valves  204   a  and  204   b , and the selector valves  205   a  and  205   b  to either one of the first position and the second position. 
     &lt;Operation&gt; 
     Next, description will be made about operator operation and operation of the controller  40  (actuator control section  81  and selector valve operation calculating section  212 ) in the case in which, in the horizontal excavation illustrated in  FIG. 8 , the front work device  1 A makes transitions from the state S 1  ( FIG. 8 : excavation start posture) to the state S 2  ( FIG. 8 : arm vertical posture) and to the state S 3  ( FIG. 8 : excavation end posture). 
     During the state from the state S 1  to the state S 3  in  FIG. 8 , the operator operates only the operation lever  1   b  and inputs arm crowding operation. 
     In the state S 1  in  FIG. 8 , on the basis of the above-described third target operation (switching to the higher-pressure selection position) set in advance regarding the selector valve  203   a , NO is determined in the step S 130  in FIG.  6  regarding the selector valve  203   a  and NO is determined also in the step S 150 . Furthermore, NO is determined in the step S 170  because the operator is not operating the operation device  45   a  and therefore the pressure of the secondary port side of the operation device  45   a  (operation pilot pressure) is 0. As a result, the target position of the selector valve  203   a  is set to the second position in the step S 190  and control is carried out to set the selector valve  203   a  to the second position in the selector valve control section  213 . 
     Moreover, since the position of the selector valve  203   a  is the second position, YES is determined in the step S 440  in  FIG. 7 . Then, in the step S 460 , the target pilot pressure of raising operation of the boom  8  by the MC is calculated on the basis of the second target operation (generation of the target pilot pressure based on the MC) set in advance regarding the proportional solenoid valve  54   a . Then, a command value to the proportional solenoid valve  54   a  is calculated in the proportional solenoid valve control section  44  on the basis of the target pilot pressure for the flow control valve  15   a , and the proportional solenoid valve  54   a  is controlled. Due to this, raising operation of the boom  8  is automatically carried out by the MC in such a manner that the claw tip of the bucket  10  does not enter the target surface  60 . 
     The above operation is carried out until a transition to the state S 2  in  FIG. 8  is made. 
     In the state S 2  in  FIG. 8 , on the basis of the above-described third target operation (switching to the higher-pressure selection position) set in advance regarding the selector valve  203   a , NO is determined in the step S 130  in  FIG. 6  regarding the selector valve  203   a  and NO is determined in the step S 150 . Then, NO is determined in the step S 170  because the operator is not operating the operation device  45   a  and therefore the pressure of the secondary port side of the operation device  45   a  is 0. As a result, the target position of the selector valve  203   a  is set to the second position in the step S 190  and control is carried out to set the selector valve  203   a  to the second position in the selector valve control section  213 . 
     Moreover, since the position of the selector valve  203   a  is the second position, YES is determined in the step S 440  in  FIG. 7 . Then, in the step S 460 , the target pilot pressure of boom raising operation by the MC is calculated on the basis of the second target operation set in advance regarding the proportional solenoid valve  54   a . Then, a command value to the proportional solenoid valve  54   a  is calculated in the proportional solenoid valve control section  44  on the basis of the target pilot pressure for the flow control valve  15   a , and the proportional solenoid valve  54   a  is controlled. However, in the state S 2 , the arm  9  operates almost horizontally and therefore the target pilot pressure of the boom raising operation calculated by the MC is almost 0. 
     After the state S 2  in  FIG. 8  and until the state S 3 , on the basis of the above-described second target operation (keeping at the second position) set in advance regarding the selector valve  203   b , NO is determined in the step S 130  in  FIG. 6  regarding the selector valve  203   b  and YES is determined in the step S 150 . Then, the target position of the selector valve  203   b  is set to the second position in the step S 160  and control is carried out to cause the selector valve  203   b  to be kept at the second position in the selector valve control section  213 . Furthermore, since the position of the selector valve  203   b  is the second position, YES is determined in the step S 440  in  FIG. 7 . Then, in the step S 460 , the target pilot pressure of boom lowering operation by the MC is calculated on the basis of the second target operation set in advance regarding the proportional solenoid valve  54   b . Then, a command value to the proportional solenoid valve  54   b  is calculated in the proportional solenoid valve control section  44  on the basis of the target pilot pressure for the flow control valve  15   a , and the proportional solenoid valve  54   b  is controlled. Due to this, lowering operation of the boom  8  is automatically carried out by the MC in such a manner that the claw tip of the bucket  10  does not get separated from the target surface  60 . 
     Furthermore, during the state from the state S 1  to the state S 3  in  FIG. 8 , on the basis of the above-described third target operation (switching to the higher-pressure selection position) set in advance regarding the selector valve  203   a , the selector valve  203   a  is set to introduce the higher pressure of the operation pilot pressure and the control pilot pressure to the hydraulic drive section  150   a  of the flow control valve  15   a . Thus, when the operation lever  1   a  is operated and boom raising operation is input, YES is determined in the step S 170  in  FIG. 6 . Then, the target position of the selector valve  203   a  is set to the first position in the step S 180  and control is carried out to set the selector valve  203   a  to the first position in the selector valve control section  213 . Due to the setting of the selector valve  203   a  to the first position, the operation pilot line  144   a  of the operation device  45   a  and the hydraulic drive section  150   a  of the flow control valve  15   a  are connected to each other and normal operation by the operator becomes valid for the boom raising operation. Due to this, even in MC operation, it is also possible to raise the boom  8  on the basis of operator&#39;s intention to separate the claw tip of the bucket  10  from the target surface  60  in the case in which the bucket  10  is filled up with earth and sand in the middle of excavation, or the like. 
     Furthermore, at this time, the pressure of the secondary port side of the operation device  45   a  (operation pilot pressure) is introduced to the hydraulic drive section  150   a  of the flow control valve  15   a  without passing through the proportional solenoid valve  54   a . Thus, pressure loss as in the conventional case in which the operation pilot pressure passes through the proportional solenoid valve does not occur, thus the responsiveness of the hydraulic actuator  5  to operation of the operation device  45   a  can be improved and operability equivalent to that of a work machine that does not have MC functions can be ensured. 
     Moreover, during the state from the state S 1  to the state S 3  in  FIG. 8 , the selector valves  204   a ,  204   b ,  205   a , and  205   b  are always controlled to the first position on the basis of the first target operation (keeping at the first potential) set in advance. Therefore, also when the operator operates the operation device  46   a  or  45   b , the operation pilot pressure is introduced to the hydraulic drive section  151   a ,  151   b ,  152   a , or  152   b  of the flow control valve  15   b  or  15   c  without passing through the proportional solenoid valve. Thus, also in this case, pressure loss as in the conventional case in which the operation pilot pressure passes through the proportional solenoid valve does not occur and operability equivalent to that of a machine that is not equipped with MC functions can be ensured regarding arm crowding operation, arm dumping operation, bucket crowding operation, and bucket dumping operation. 
     Next, description will be made about operator operation and operation of the controller  40  (actuator control section  81  and selector valve operation calculating section  212 ) in the case in which, in the operation of the position adjustment of the claw tip of the bucket  10  to the target surface  60  illustrated in  FIG. 9 , the front work device  1 A makes transitions from the state S 4  ( FIG. 9 : height of the claw tip of the bucket  10  is high) to the state S 5  ( FIG. 9 : height of the claw tip of the bucket  10  is middle) and to the state S 6  ( FIG. 9 : height of the claw tip of the bucket  10  is 0). 
     During the state from the state S 4  to the state S 6  in  FIG. 9 , the operator operates only the operation lever  1   a  and inputs boom lowering operation. 
     In the state S 4  to the state S 6  in  FIG. 9 , on the basis of the above-described second target operation (keeping at the second position) set in advance regarding the selector valve  203   b , NO is determined in the step S 130  in  FIG. 6  regarding the selector valve  203   b  and YES is determined in the step S 150 . Then, the target position of the selector valve  203   b  is set to the second position in the step S 160 . Thus, control is carried out to set the selector valve  203   b  to the second position in the selector valve control section  213 . Furthermore, since the position of the selector valve  203   b  is the second position, YES is determined in the step S 440  in  FIG. 7 . Then, in the step S 460 , the target pilot pressure of lowering operation of the boom  8  by the MC is calculated on the basis of the second target operation set in advance regarding the proportional solenoid valve  54   b . Then, a command value to the proportional solenoid valve  54   b  is calculated in the proportional solenoid valve control section  44  on the basis of the target pilot pressure for the flow control valve  15   a , and the proportional solenoid valve  54   b  is controlled. 
     Here, in the state S 4 , the distance between the target surface  60  and the claw tip of the bucket  10  is long. Therefore, limitation of the boom lowering operation by the MC is not carried out, and the control pilot pressure equal to the operation pilot pressure of the boom lowering operation calculated in the operation device secondary pressure calculating section  43   a  is calculated as the target pilot pressure and the target pilot pressure is output from the boom control section  81   a.    
     The above operation is carried out until a transition to the state S 5  is made. 
     In the state S 5 , the distance between the target surface  60  and the claw tip of the bucket  10  is short and therefore limitation (velocity reduction) of the boom lowering operation is started in the MC in order to prevent entry into the target surface  60 . In the boom control section  81   a , a value obtained by reducing the operation pilot pressure of the boom lowering operation calculated in the operation device secondary pressure calculating section  43   a  is output as the target pilot pressure according to the distance between the target surface  60  and the claw tip of the bucket  10 . 
     In the state S 6 , the claw tip of the bucket  10  has reached the target surface  60  and therefore limitation (stop) of the boom lowering operation is carried out in the MC in order to prevent entry into the target surface  60 . In the boom control section  81   a , 0 is output as the target pilot pressure. 
     Due to this, even when the operator operates the operation lever  1   a  to continue to input the boom lowering operation, the claw tip of the bucket  10  can be automatically stopped at the target surface  60  and the position adjustment can be carried out. 
     &lt;Effects&gt; 
     According to the present embodiment, the following effects are obtained. 
     1. As in the above-described operation example of the position adjustment of the bucket claw tip illustrated in  FIG. 9 , while the work device  1 A is in the state S 5  to S 6 , by switching the selector valve  203   b  to the second position and controlling the proportional solenoid valve  54   b  to generate the control pilot pressure obtained by reducing the operation pilot pressure sensed by the pressure sensor  70   b , operation of the boom cylinder  5  in the boom lowering direction can be limited and it becomes possible to limit operation of the work device  1 A by the MC. Also in the cases in which the selector valves  203   a ,  204   a ,  204   b ,  205   a , and  205   b  are switched to the second position and the proportional solenoid valves  54   a ,  55   a ,  55   b ,  56   a , and  56   b  are similarly controlled in other work modes, similarly it becomes possible to limit operation of the work device  1 A by the MC. 
     2. When the work mode is not set and the MC is not carried out, all proportional solenoid valves  54   a  to  56   b  become non-excited and switching to the first position is carried out. Also in the case of carrying out normal work based on operator operation, the responsiveness of the hydraulic actuators  5 ,  6 , and  7  to the operator operation can be improved and operability equivalent to that of a work machine that does not have MC functions can be ensured. 
     Furthermore, as in the above-described operation example of the horizontal excavation illustrated in  FIG. 8 , when the operator operates the first operation device in MC operation while the work device  1 A is in the state S 1  to S 3 , the operation pilot pressure output from the secondary port  134   a  of the operation device  45   a  is introduced to the flow control valve  15   a  without passing through the proportional solenoid valve  54   a  by switching the selector valve  203   a  to the first position. Thus, pressure loss as in the conventional case in which the operation pilot pressure passes through the proportional solenoid valve does not occur, thus the responsiveness of the boom cylinder  5  to operation of the operation device  45   a  by the operator can be improved and operability equivalent to that of a work machine that does not have MC functions can be ensured. Also in the cases in which the selector valves  203   b ,  204   a ,  204   b ,  205   a , and  205   b  are switched to the first position when the operator operates the operation device in other work modes, similarly the responsiveness of the hydraulic actuators  5 ,  6 , and  7  to the operation of the operation devices  45   a ,  46   a , and  45   b  by the operator can be improved and operability equivalent to that of a work machine that does not have MC functions can be ensured. 
     Moreover, in the operation example of the horizontal excavation illustrated in  FIG. 8  by the MC, during the state from the state S 1  to the state S 3  in  FIG. 8 , the selector valves  204   a ,  204   b ,  205   a , and  205   b  are always controlled to the first position on the basis of the first target operation (keeping at the first position) set in advance. Thus, also when the operator operates the operation device  46   a ,  45   b , the operation pilot pressure is introduced to the hydraulic drive section  151   a ,  151   b ,  152   a , or  152   b  of the flow control valve  15   b  or  15   c  without passing through the proportional solenoid valve. Therefore, also in this case, pressure loss as in the conventional case in which the operation pilot pressure passes through the proportional solenoid valve does not occur and operability equivalent to that of a machine that is not equipped with MC functions can be ensured regarding arm crowding operation, arm dumping operation, bucket crowding operation, and bucket dumping operation. 
     3. As in the above-described operation example of the horizontal excavation illustrated in  FIG. 8 , the boom cylinder  5  can be automatically operated in the boom raising direction by switching the selector valve  203   a  to the second position and controlling the proportional solenoid valve  54   a  to generate the control pilot pressure based on the MC. In addition, the boom cylinder can be automatically operated in the boom lowering direction by switching the selector valve  203   b  to the second position and controlling the proportional solenoid valve  54   b  to generate the second control pilot pressure based on the MC. This makes it possible to cause the boom cylinder  5  that is the hydraulic actuator for which the operation device  45   a  is not being operated to automatically operate in either direction of the boom raising direction and the boom lowering direction. Also in the cases in which the selector valves  204   a ,  204   b ,  205   a , and  205   b  for which the operation device is not being operated to the second position in other work modes, similarly the hydraulic actuators  5 ,  6 , and  7  can be operated in either direction of the operation directions thereof. 
     Modification Example 
     In the first embodiment, for each of the operation devices  45   a ,  46   a , and  45   b , the pressure sensors  70   a  and  70   b ;  71   a  and  71   b ; and  72   a  and  72   b , the proportional solenoid valves  54   a  and  54   b ;  55   a  and  55   b ; and  56   a  and  56   b , the pressure sensors  200   a  and  200   b ;  201   a  and  201   b ; and  202   a  and  202   b , and the selector valves  203   a  and  203   b ;  204   a  and  204   b ; and  205   a  and  205   b  are disposed. The controller  40  switches the selector valves  203   a  and  203   b ;  204   a  and  204   b ; and  205   a  and  205   b  to either one of the first position and the second position on the basis of signals from the pressure sensors  70   a  to  72   b  and the pressure sensors  200   a  to  202   b  and the target operation set in advance regarding the selector valves  203   a  to  205   b.    
     Due to this, the drive system is allowed to have general-purpose versatility and front device operation by the MC can be carried out whatever kind of work mode is set in the controller  40 . 
     On the other hand, it is also possible to cause the drive system to have a configuration specialized for the horizontal excavation illustrated in  FIG. 8  and the position adjustment of the claw tip of the bucket  10  described above. In this case, it suffices that the pressure sensors  70   a  and  70   b , the proportional solenoid valves  54   a  and  54   b , the pressure sensors  200   a  and  200   b , and the selector valves  203   a  and  203   b  are disposed only for the operation device  45   a  and the controller  40  switches the selector valves  203   a  and  203   b  to either one of the first position and the second position on the basis of signals from the pressure sensors  70   a  and  70   b  and the pressure sensors  200   a  and  200   b  and the target operation set in advance regarding the selector valves  203   a  and  203   b.    
     This can also obtain the effects relating to the selector valves  203   a  and  203   b  in the above-described 1 to 3. 
     Second Embodiment 
     A second embodiment of the present invention will be described with reference to  FIG. 10  and  FIG. 11 . 
     The second embodiment is different from the first embodiment in the configuration of the selector valve operation calculating section  212  in  FIG. 5 . The configuration other than it is the same as the first embodiment. 
       FIG. 10  is a functional block diagram of the MC control section  43  similar to  FIG. 5  in the present embodiment. 
       FIG. 11  is a diagram that illustrates a control flow of the selector valves  203   a  to  205   b  in the selector valve operation calculating section  212  in the present embodiment and is similar to  FIG. 6 . 
     The difference between  FIG. 5  and  FIG. 6  will be described below. 
     &lt;Controller&gt; 
     In  FIG. 10 , to the selector valve operation calculating section  212  of the controller  40 , the outputs of the posture calculating section  43   b  and the target surface calculating section  43   c  are input in addition to the outputs of the operation device secondary pressure calculating section  43   a  and the proportional solenoid valve secondary pressure calculating section  211 . The selector valve operation calculating section  212  calculates the target switching position of the selector valve  203   a  to  205   b  as illustrated in  FIG. 11 , according to a condition defined in advance (for example, work mode of front device operation), at the time of operation of the operation device  45   a ,  45   b , or  46   a.    
     &lt;Selector Valve Control Flow of Selector Valve Operation Calculating Section  212 &gt; 
     In  FIG. 11 , the processing of the steps S 110  to S 190  is the same as the first embodiment illustrated in  FIG. 6 . In the present embodiment, the following processing is further executed after the target position of the selector valve  203   a  to  205   b  is set in the step S 140 , S 160 , S 180 , or S 190 . 
     First, in a step S 230 , the selector valve operation calculating section  212  acquires the posture of the boom  8 , the arm  9 , and the bucket  10  calculated in the posture calculating section  43   b.    
     In a step S 240 , the selector valve operation calculating section  212  acquires position information of a target surface calculated in the target surface calculating section  43   c.    
     In a step S 250 , the selector valve operation calculating section  212  determines whether or not the distance between the target surface  60  and the claw tip of the bucket  10  is shorter than a first distance set in advance from the output of the posture calculating section  43   b  and the output of the target surface calculating section  43   c . When it is determined in the step S 250  that the distance between the target surface  60  and the claw tip of the bucket  10  is equal to or shorter than the first distance set in advance, progress to a step S 270  is made. When it is determined in the step S 250  that the distance between the target surface  60  and the claw tip of the bucket  10  is longer than the first distance set in advance, progress to a step S 260  is made. 
     In the step S 260 , the selector valve operation calculating section  212  sets the target position of the selector valve  203   a  to  205   b  to the first position. That is, even in the state in which the MC is valid, the target position of the selector valve  203   a  to  205   b  is set to the first position when the claw tip of the bucket  10  is separate from the target surface  60  by the first distance set in advance or longer. 
     In the step S 270 , the selector valve operation calculating section  212  outputs the target position of the selector valve  203   a  to  205   b  to the selector valve control section  213 . 
     As above, in the present embodiment, the controller  40  calculates the distance between a control point of the work device  1 A (for example, claw tip of the bucket  10 ) and the excavation target surface on the basis of signals from the work device posture sensor  50  (boom angle sensor  30 , arm angle sensor  31 , bucket angle sensor  32 , and machine body inclination angle sensor  33 ). The controller  40  keeps the selector valve  203   b  (second selector valve) at the first position when the distance between the control point and the excavation target surface is longer than the first distance set in advance, and switches the selector valve  203   b  (second selector valve) to the second position when the distance between the control point and the excavation target surface becomes equal to or shorter than the first distance. 
     &lt;Operation&gt; 
     Similarly to the first embodiment, description will be made about operator operation and operation of the controller  40  (actuator control section  81  and selector valve operation calculating section  212 ) in the case in which, in the operation of the position adjustment of the claw tip of the bucket  10  to the target surface  60  by the MC in  FIG. 9 , the front work device  1 A makes transitions from the state S 4  ( FIG. 9 : distance between the claw tip of the bucket  10  and the target surface  60 &gt;first distance) to the state S 5  ( FIG. 9 : distance between the claw tip of the bucket  10  and the target surface  60 =first distance) and to the state S 6  ( FIG. 9 : distance between the claw tip of the bucket  10  and the target surface  60 &lt;first distance). 
     During the state from the state S 4  to the state S 6  in  FIG. 9 , the operator operates only the operation lever  1   a  and inputs boom lowering operation. 
     In the state S 4  to the state S 6  in  FIG. 9 , on the basis of the second target operation (keeping at the second position) set in advance regarding the selector valve  203   b , NO is determined in the step S 130  in  FIG. 11  regarding the selector valve  203   b  and YES is determined in the step S 150 . Then, the target position of the selector valve  203   b  is set to the second position in the step S 160 . 
     In the state S 4 , the distance between the target surface  60  and the claw tip of the bucket  10  is longer than the first distance. Therefore, NO is determined in the step S 250  in  FIG. 11  and the target position of the selector valve  203   b  is rewritten to the first position in the step S 260 . Due to this, in the state in which the distance between the claw tip of the bucket  10  and the target surface  60 &gt;first distance is satisfied, in which there is no fear of entry of the claw tip of the bucket  10  into the target surface  60 , the selector valve  203   b  is controlled to the first position and therefore the pressure of the secondary port side of the operation device  45   a  (operation pilot pressure) is introduced to the hydraulic drive section  150   b  of the flow control valve  15   a  without passing through the proportional solenoid valve  54   b . Thus, pressure loss as in the conventional case in which the operation pilot pressure passes through the proportional solenoid valve does not occur, thus the responsiveness of the hydraulic actuator  5  to operation of the operation device  45   a  can be improved and operability equivalent to that of a work machine that does not have MC functions can be ensured. 
     Furthermore, in the state S 4 , since the position of the selector valve  203   b  is the first position, NO is determined in the step S 440  in  FIG. 7 . Then, in the step S 470 , the control pilot pressure equal to the operation pilot pressure of boom lowering operation calculated in the operation device secondary pressure calculating section  43   a  is calculated as the target pilot pressure on the basis of the first target operation of the proportional solenoid valve  54   b  set in advance, and the target pilot pressure is output from the boom control section  81   a . Thereby, the pressure of the secondary port side of the proportional solenoid valve  54   b  (control pilot pressure) is controlled to become equal to the operation pilot pressure of the operation pilot line  144   b  of the operation device  45   a.    
     In the state S 5 , the distance between the target surface  60  and the claw tip of the bucket  10  is the first distance. Therefore, YES is determined in the step S 250  in  FIG. 11  and the target position of the selector valve  203   b  remains at the second position set in the step S 160 . Thus, the selector valve  203   b  is switched from the first position to the second position in the state S 5 . At this time, the pressure of the secondary port side of the proportional solenoid valve  54   b  (control pilot pressure) is equal to the operation pilot pressure of the operation pilot line  144   b  of the operation device  45   a . Therefore, sudden variation in the pressure that acts on the hydraulic drive section  150   b  of the flow control valve  15   a  does not occur at the moment of the switching of the selector valve  203   b  and shock to the front work device  1 A can be suppressed. 
     &lt;Effects&gt; 
     According to the present embodiment, while operability equivalent to that of a machine that is not equipped with MC functions is ensured in the state in which there is no fear of entry of the claw tip of the bucket  10  into the target surface  60 , the MC can be carried out in the state in which there is a fear of entry of the claw tip of the bucket  10  into the target surface  60 . Moreover, the switching thereof can be automatically carried out without operation of a switch or the like by the operator. Furthermore, the occurrence of shock at the moment of switching of the selector valve  203   a  to  205   b  can be suppressed and it is possible to continue to smoothly operate the front work device  1 A. 
     Third Embodiment 
     A third embodiment of the present invention will be described with reference to  FIG. 12 ,  FIG. 13 , and  FIG. 14 .  FIG. 12 ,  FIG. 13 , and  FIG. 14  are diagrams obtained by changing part of  FIG. 4 ,  FIG. 5 , and  FIG. 6  and the difference will be described below. 
     &lt;Basic Configuration&gt; 
     A hydraulic excavator according to the third embodiment includes an MC validity-invalidity switching device  214  for alternatively selecting validity or invalidity (ON or OFF) of the MC. 
     &lt;Controller  40 &gt; 
       FIG. 12  is a functional block diagram of the controller  40 . Output from the MC validity-invalidity switching device  214  is input to the MC control section  43  of the controller  40 .  FIG. 13  is a functional block diagram of the MC control section  43  in  FIG. 12 . 
     The MC control section  43  includes an MC validity-invalidity determining section  215  in addition to the operation device secondary pressure calculating section  43   a , the posture calculating section  43   b , the target surface calculating section  43   c , the boom control section  81   a , the arm control section  81   b , the bucket control section  81   c , the proportional solenoid valve secondary pressure calculating section  211 , and the selector valve operation calculating section  212 . To the selector valve operation calculating section  212 , the output of the MC validity-invalidity determining section  215  is input in addition to the outputs of the operation device secondary pressure calculating section  43   a , the proportional solenoid valve secondary pressure calculating section  211 , the posture calculating section  43   b , and the target surface calculating section  43   c.    
     The MC validity-invalidity determining section  215  determines whether a signal of the MC validity-invalidity switching device  214  is valid (ON) or invalid (OFF) on the basis of the input from the MC validity-invalidity switching device  214 . 
     The selector valve operation calculating section  212  calculates the target position of the selector valves  203   a  to  205   b , according to a condition defined in advance (for example, work mode of front device operation) on the basis of the outputs of the operation device secondary pressure calculating section  43   a , the posture calculating section  43   b , the target surface calculating section  43   c , the proportional solenoid valve secondary pressure calculating section  211 , and the MC validity-invalidity determining section  215 . 
     &lt;Selector Valve Control Flow of Selector Valve Operation Calculating Section  212 &gt; 
       FIG. 14  is a diagram illustrating a control flow of the selector valves  203   a  to  205   b  in the selector valve operation calculating section  212  in the present embodiment. 
     In  FIG. 14 , the processing of the steps S 110  to S 190  is the same as the first embodiment illustrated in  FIG. 6  and the processing of the steps S 230  to S 270  is the same as the second embodiment illustrated in  FIG. 11 . In the present embodiment, after the target position of the selector valve  203   a  to  205   b  is set in the step S 140 , S 160 , S 180 , or S 190 , the following processing is executed before the processing of the step S 210  to the step S 270  is executed. 
     In a step S 200 , the selector valve operation calculating section  212  acquires the signal of the MC validity-invalidity switching device  214  determined in the MC validity-invalidity determining section  215 . 
     In the step S 210 , the selector valve operation calculating section  212  determines whether or not the signal of the MC validity-invalidity switching device  214  acquired in the step S 200  is valid. When it is determined that the signal is valid in the step S 210 , progress to a step S 230  is made. When it is determined that the signal is other than valid in the step S 210 , progress to a step S 220  is made. 
     In the step S 220 , the selector valve operation calculating section  212  sets the target position of the selector valves  203   a  to  205   b  to the first position. That is, when the signal of the MC validity-invalidity switching device  214  is other than valid, the target position of the selector valves  203   a  to  205   b  is set to the first position irrespective of the target operation set in advance. 
     As above, the work machine of the present embodiment further includes the MC validity-invalidity switching device  214  (switching device) that outputs the signal to carry out switching between validity and invalidity of control of the controller  40 . The controller  40  rewrites the target position of the selector valves  203   a  and  203   b  (first and second selector valves) to the first position when the signal to make the control of the controller  40  invalid is input from the MC validity-invalidity switching device  214 . 
     &lt;Operation and Effects&gt; 
     In the hydraulic excavator configured as above, even when the work mode of front device operation is set in the controller  40 , the position of the selector valves  203   a  to  205   b  becomes the first position through setting of the MC validity-invalidity switching device  214  to invalidity (OFF) by the operator, and the pressures of the secondary port side of the operation devices  45   a ,  45   b , and  46   a  (operation pilot pressures) are introduced to the hydraulic drive sections  150   a  to  152   b  of the flow control valves  15   a ,  15   b , and  15   c  without passing through the proportional solenoid valves  54   a  to  56   b . Thus, when the MC is not carried out, pressure loss as in the conventional case in which the operation pilot pressure passes through the proportional solenoid valve does not occur in all of boom raising operation, boom lowering operation, arm crowding operation, arm dumping operation, bucket crowding operation, and bucket dumping operation. Thus, the responsiveness of the hydraulic actuators  5 ,  6 , and  7  to operation of the operation devices  45   a ,  45   b , and  46   a  can be improved and operability equivalent to that of a work machine that does not have MC functions can be ensured. 
     In the present embodiment, the MC validity-invalidity switching device  214  for alternatively selecting validity or invalidity (ON or OFF) of the MC is disposed in the hydraulic excavator according to the second embodiment. However, the MC validity-invalidity switching device  214  may be disposed in the hydraulic excavator according to the first embodiment and the same effects are obtained also by this. 
     DESCRIPTION OF REFERENCE CHARACTERS 
     
         
           1 A: Front work device (work device) 
           5 : Boom cylinder (hydraulic actuator) 
           6 : Arm cylinder (hydraulic actuator) 
           7 : Bucket cylinder (hydraulic actuator) 
           8 : Boom 
           9 : Arm 
           10 : Bucket 
           15   a ,  15   b ,  15   c : Flow control valve 
           30 : Boom angle sensor (work device posture sensor  50 ) 
           31 : Arm angle sensor (work device posture sensor  50 ) 
           32 : Bucket angle sensor (work device posture sensor  50 ) 
           40 : Controller 
           43 : MC control section 
           43   a : Operation device secondary pressure calculating section 
           43   b : Posture calculating section 
           43   c : Target surface calculating section 
           44 : Proportional solenoid valve control section 
           45   a : Operation device for the boom 
           45   b : Operation device for the bucket 
           46   a : Operation device for the arm 
           50 : Work device posture sensor 
           51 : Target surface setting device 
           52   a : Operation device secondary pressure sensor 
           54   a  to  56   b : Proportional solenoid valve 
           70   a  to  72   b : Pressure sensor (operation pressure sensor) 
           200   a  to  202   b : Pressure sensor (control pressure sensor) 
           81 : Actuator control section 
           81   a : Boom control section 
           81   b : Arm control section 
           81   c : Bucket control section 
           134   a  to  136   b : Secondary port (output port) 
           203   a  to  205   b : Selector valve 
           210 : Proportional solenoid valve secondary pressure sensor 
           211 : Proportional solenoid valve secondary pressure calculating section 
           212 : Selector valve operation calculating section 
           213 : Selector valve control section 
           214 : MC validity-invalidity switching device (switching device) 
           215 : MC validity-invalidity determining section 
           374 : Display control section