Patent Publication Number: US-11639593-B2

Title: Work machine

Description:
TECHNICAL FIELD 
     The present invention relates to a work machine such as a hydraulic excavator. 
     BACKGROUND ART 
     A hydraulic excavator includes a machine body including a lower track structure and an upper swing structure, and an articulated-type front work implement. The front work implement includes a boom rotatably mounted to a front portion of the upper swing structure, an arm mounted to a tip portion of the boom in a vertically rotatable manner, and a work tool (for example, a bucket) mounted to a tip portion of the arm in a vertically or front-rear directionally rotatable manner. The boom, the arm, and the bucket are driven by supplying a hydraulic fluid, delivered from a hydraulic pump driven by an engine, to a boom cylinder, an arm cylinder, and a bucket cylinder. With the boom cylinder, the arm cylinder, and the bucket cylinder driven according to lever operations by an operator, a desired operation of the front work implement is realized. 
     In addition, the hydraulic excavator includes one in which a function for automatically or semi-automatically operating the front work implement (the function will hereinafter be referred to machine control) is mounted. According to the machine control, it is easy, for example, to operate the front work implement in such a manner that the tip of the bucket is stopped on a target surface at the time of starting an operation such as excavation, or to operate the front work implement in such a manner that the tip of the bucket is moved along the target surface at the time of an arm crowding operation. Documents disclosing a prior art concerning machine control include, for example, Patent Document 1. 
     Patent Document 1 discloses a region limiting excavation controller for a construction machine including: a plurality of driven members inclusive of a plurality of front members which constitute an articulated-type front device (front work implement) and which are vertically rotatable; a plurality of hydraulic actuators that respectively drive the plurality of driven members; a plurality of operating means for instructing operations of the plurality of driven members; and a plurality of hydraulic control valves which are driven according to operation signals of the plurality of operating means and which control flow rates of a hydraulic fluid supplied to the plurality of hydraulic actuators. The region limiting excavation controller for the construction machine includes: region setting means for setting a region in which the front device can be moved; first detection means for detecting status quantities concerning position and posture of the front device; first calculation means for calculating the position and posture of the front device based on a signal from the first detection means; first signal correction means for performing a processing of reducing an operation signal of at least the operating means concerning a first specific front member of the plurality of operating means, when the front device is located in the set area and in a vicinity of a boundary of the region, based on a calculated value given by the first calculation means; mode selection means for selecting whether a processing of reducing the operation signal of the operating means by the first signal correction means is to be conducted; and second signal correction means for correcting the operation signal of at least the operating means concerning a second specific front member of the plurality of operating means, in such a manner that the front device is moved in a direction along the boundary of the set area and the moving speed in a direction for approaching the boundary of the set area is reduced, when the front device is located in the set area and in the vicinity of the boundary of the set area, based on the operation signal having undergone the processing of reducing by the first signal correction means and the calculated value given by the first calculation means in the case where it is selected by the mode selection means that the processing by the first signal correction means is to be conducted, and based on the operation signal of the operating means and the calculated value given by the first calculation means in the case where it is selected by the mode selection means that the processing by the first signal correction means is not to be conducted. 
     PRIOR ART DOCUMENT 
     Patent Document 
     Patent Document 1: JP-Hei-9-53259-A 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     According to the construction machine described in Patent Document 1, at the time of performing excavation with region limitation, it is possible to perform the work by selecting either of a work mode in which priority is given to accuracy such that the amount of penetration of the bucket tip into the outside of the set area is small (this mode will hereinafter referred to as accuracy priority mode) and a work mode in which priority is given to speed such that the front work implement can be moved fast (this mode will hereinafter referred to as speed priority mode) according to the operator&#39;s will. However, when the accuracy priority mode is selected, the amount of penetration of the bucket tip into the outside of the set area is suppressed, but the moving speed of the front work implement is reduced and, hence, the front work implement cannot be operated at a speed according to the operator&#39;s lever operation. On the other hand, when the speed priority mode is selected, the front work implement can be operated at a speed according to the operator&#39;s lever operation, but the amount of penetration of the bucket tip into the outside of the set area may be enlarged. 
     The present invention has been made in consideration of the above-mentioned problems. It is an object of the present invention to provide a work machine that can operate a front work implement at a speed according to an operator&#39;s lever operation, while securing the accuracy of work by machine control. 
     Means for Solving the Problems 
     In order to achieve the above object, according to the present invention, there is provided a work machine including: a machine body; an articulated-type work implement including a boom rotatably mounted to the machine body, an arm rotatably mounted to a tip portion of the boom, and a work tool rotatably mounted to the arm; a boom cylinder that drives the boom; an arm cylinder that drives the arm; a work tool cylinder that drives the work tool; an operation device for operating the work tool; and a controller that sets a target surface for the work tool, and controls an operation of the work implement in such a manner that the work tool does not penetrate to below the target surface, in which the controller sets a speed correction region on an upper side of the target surface, varies a width of the speed correction region in accordance with an operation amount of the operation device, and controls the operation of the work implement in such a manner that the work tool does not penetrate into the speed correction region. 
     According to the present invention configured as above, the speed correction region is set on the upper side of the target surface for the work tool, the width of the speed correction region is varied according to the operation amount of the operation device, and the operation of the front work implement is controlled in such a manner that the work tool does not penetrate into the speed correction region. As a result, it becomes possible to operate the front work implement at a speed according to the operator&#39;s lever operation, while securing the accuracy of work by machine control. 
     Advantages of the Invention 
     According to the present invention, a front work implement can be operated at a speed according to an operator&#39;s lever operation, while securing the accuracy of work by machine control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a hydraulic excavator according to an embodiment of the present invention. 
         FIG.  2    is a schematic configuration diagram of a hydraulic drive system mounted on the hydraulic excavator depicted in  FIG.  1   . 
         FIG.  3    is a configuration diagram of a hydraulic control unit depicted in  FIG.  2   . 
         FIG.  4    is a functional block diagram of a controller depicted in  FIG.  2   . 
         FIG.  5    is a figure depicting an example of a horizontal excavating operation by a machine control. 
         FIG.  6    is a functional block diagram of a target operation calculation section depicted in  FIG.  4   . 
         FIG.  7    is a flow chart depicting a processing of the target operation calculation section depicted in  FIG.  6   . 
         FIG.  8    is a flow chart depicting details of a speed correction region processing depicted in  FIG.  7   . 
         FIG.  9 A  is a diagram depicting the relation between arm lever operation amount and speed correction region width. 
         FIG.  9 B  is a diagram depicting the relation between boom lowering lever operation amount and speed correction region width. 
         FIG.  10    is a figure depicting the relation between target surface distance and corrected target surface distance. 
         FIG.  11    is a diagram depicting the relation between target surface distance and operation amount limit value. 
         FIG.  12    is a figure depicting a bucket positioning operation of the hydraulic excavator depicted in  FIG.  1   . 
         FIG.  13    illustrates figures depicting movements of a bucket with respect to a boom lowering operation. 
         FIG.  14    is a figure depicting a horizontal excavating operation of the hydraulic excavator depicted in  FIG.  1   . 
         FIG.  15    illustrates figures depicting movements of the bucket with respect to an arm crowding operation. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     A hydraulic excavator taken as an example of a work machine according to an embodiment of the present invention will be described below, referring to the drawings. Note that in the drawings, the same or equivalent members are denoted by the same reference characters, and repeated descriptions of them will be omitted. 
       FIG.  1    is a perspective view of a hydraulic excavator according to the present embodiment. 
     In  FIG.  1   , a hydraulic excavator  1  includes a machine body  1 A, and an articulated-type front work implement  1 B. The machine body  1 A includes a lower track structure  11 , and an upper swing structure  12  swingably mounted onto the lower track structure  11 . The lower track structure  11  is driven to travel by a track right motor (not illustrated) and a track left motor  3   b . The upper swing structure  12  is driven to swing by a swing hydraulic motor  4 . 
     The front work implement  1 B includes a boom  8  mounted to a front portion of the upper swing structure  12  in a vertically rotatable manner, an arm  9  mounted to a tip portion of the boom  8  rotatably vertically or in a front-rear direction, and a bucket (work tool)  10  mounted to a tip portion of the arm  9  rotatably vertically or in a front-rear direction. The boom  8  is rotated vertically by contracting/extending motions of a boom cylinder  5 . The arm  9  is rotated vertically or in a front-rear direction by contracting/extending motions of an arm cylinder  6 . The bucket  10  is rotated vertically or in a front-rear direction by contracting/extending motions of a bucket cylinder (work tool cylinder)  7 . 
     An operation room  1 C in which an operator rides is provided on a left side of a front portion of the upper swing structure  12 . In the operation room  1 C, there are disposed a track right lever  13   a  and a track left lever  13   b  for giving operation instructions to the lower track structure  11 , and an operation right lever  14   a  and an operation left lever  14   b  for giving operation instructions to the boom  8 , the arm  9 , the bucket  10 , and the upper swing structure  12 . 
     A boom angle sensor  21  for detecting a rotation angle of the boom  8  is attached to a boom pin that links the boom  8  to the upper swing structure  12 . An arm angle sensor  22  for detecting a rotation angle of the arm  9  is attached to an arm pin that links the arm  9  to the boom  8 . A bucket angle sensor  23  for detecting a rotation angle of the bucket  10  is attached to a bucket pin that links the bucket  10  to the arm  9 . A machine body inclination angle sensor  24  for detecting an inclination angle in the front-rear direction of the upper swing structure  12  (machine body  1 A) relative to a reference plane (for example, a horizontal plane) is attached to the upper swing structure  12 . Angle signals outputted from the angle sensors  21  to  23  and the machine body inclination angle sensor  24  are inputted to a controller  20  (depicted in  FIG.  2   ) which will be described later. 
       FIG.  2    is a schematic configuration diagram of a hydraulic drive system mounted on the hydraulic excavator  1  depicted in  FIG.  1   . Note that for simplification of explanation, in  FIG.  2   , only portions concerning the driving of the boom cylinder  5 , the arm cylinder  6 , the bucket cylinder  7 , and the swing hydraulic motor  4  are depicted, and portions concerning the driving of other hydraulic actuators are omitted. 
     In  FIG.  2   , the hydraulic drive system  100  includes the hydraulic actuators  4  to  7 , a prime mover  49 , the hydraulic pump  2  and a pilot pump  48  driven by the prime mover  49 , flow control valves  16   a  to  16   d  for controlling the directions and flow rates of a hydraulic fluid supplied from the hydraulic pump  2  to the hydraulic actuators  4  to  7 , hydraulic pilot type operation devices  15 A to  15 D for operating the flow control valves  16   a  to  16   d , a hydraulic control unit  60 , a shuttle block  46 , and the controller  20  as a control system. 
     The hydraulic pump  2  includes a tilting swash plate mechanism (not illustrated) that has a pair of input/output ports, and a regulator  47  for regulating the tilting angle of a swash plate to regulate the pump displacement volume. The regulator  47  is operated by a pilot pressure supplied from the shuttle block  46  described later. 
     The pilot pump  48  is connected to pilot pressure control valves  52  to  59  and the hydraulic control unit  60 , which will be described later, through a lock valve  51 . The lock valve  51  is opened and closed in accordance with an operation of a gate lock lever (not illustrated) provided in the vicinity of an entrance to the operation room  1 C. When the gate lock lever is operated to a position (push-down position) for restricting the entrance to the operation room  1 C, the lock valve  51  is opened by an instruction from the controller  20 . As a result, a delivery pressure of the pilot pump  48  (hereinafter referred to as pilot primary pressure) is supplied to the pilot pressure control valves  52  to  59  and the hydraulic control unit  60 , resulting in that operations of the flow control valves  16   a  to  16   d  by the operation devices  15 A to  15 D are possible. On the other hand, when the gate lock lever is operated to a position (push-up position) for opening the entrance to the operation room  1 C, the lock valve  51  is closed by an instruction from the controller  20 . As a result, the supply of the pilot primary pressure from the pilot pump  48  to the pilot pressure control valves  52  to  59  and the hydraulic control unit  60  is stopped, resulting in that the operations of the flow control valves  16   a  to  16   d  by the operation devices  15 A to  15 D are impossible. 
     The operation device  15 A includes a boom operation lever  15   a , the boom raising pilot pressure control valve  52 , and the boom lowering pilot pressure control valve  53 . Here, the boom operation lever  15   a  corresponds, for example, to the operation right lever  14   a  (depicted in  FIG.  1   ) when it is operated in the front-rear direction. 
     The boom raising pilot pressure control valve  52  decompresses the pilot primary pressure supplied through the lock valve  51 , to produce a pilot pressure according to a lever stroke (hereinafter referred to as operation amount) in the boom raising direction of the boom operation lever  15   a  (this pilot pressure will hereinafter be referred to as boom raising pilot pressure). The boom raising pilot pressure outputted from the boom raising pilot pressure control valve  52  is led to an operation section on one side (the left side in the figure) of the boom flow control valve  16   a  through the hydraulic control unit  60 , the shuttle block  46 , and a pilot line  529 , to drive the boom flow control valve  16   a  in the rightward direction in the figure. As a result, the hydraulic fluid delivered from the hydraulic pump  2  is supplied to the bottom side of the boom cylinder  5 , whereas the hydraulic fluid on the rod side is discharged into a tank  50 , and the boom cylinder  5  is extended. 
     The boom lowering pilot pressure control valve  53  decompresses the pilot primary pressure supplied through the lock valve  51 , to produce a pilot pressure according to an operation amount in the boom lowering direction of the boom operation lever  15   a  (this pilot pressure will hereinafter be referred to as boom lowering pilot pressure). The boom lowering pilot pressure outputted from the boom lowering pilot pressure control valve  53  is led to an operation section on the other side (the right side in the figure) of the boom flow control valve  16   a  through the hydraulic control unit  60 , the shuttle block  46 , and a pilot line  539 , to drive the boom flow control valve  16   a  in the leftward direction in the figure. As a result, the hydraulic fluid delivered from the hydraulic pump  2  is supplied to the rod side of the boom cylinder  5 , whereas the hydraulic fluid on the bottom side is discharged into the tank  50 , and the boom cylinder  5  is contracted. 
     The operation device  15 B includes the bucket operation lever (work tool operation lever)  15   b , the bucket crowding pilot pressure control valve  54 , and the bucket dumping pilot pressure control valve  55 . Here, the bucket operation lever  15   b  corresponds, for example, to the operation right lever  14   a  (depicted in  FIG.  1   ) when it is operated in the left-right direction. 
     The bucket crowding pilot pressure control valve  54  decompresses the pilot primary pressure supplied through the lock valve  51 , to produce a pilot pressure according to an operation amount in the bucket crowding direction of the bucket operation lever  15   b  (this pilot pressure will hereinafter be referred to as bucket crowding pilot pressure). The bucket crowding pilot pressure outputted from the bucket crowding pilot pressure control valve  54  is led to an operation section on one side (left side in the figure) of the bucket flow control valve  16   b  through the hydraulic control unit  60 , the shuttle block  46 , and a pilot line  549 , to drive the bucket flow control valve  16   b  in the rightward direction in the figure. As a result, the hydraulic fluid delivered from the hydraulic pump  2  is supplied to the bottom side of the bucket cylinder  7 , whereas the hydraulic fluid on the rod side is discharged into the tank  50 , and the bucket cylinder  7  is extended. 
     The bucket dumping pilot pressure control valve  55  decompresses the pilot primary pressure supplied through the lock valve  51 , to produce a pilot pressure according to an operation amount in the bucket dumping direction of the bucket operation lever  15   b  (this pilot pressure will hereinafter be referred to as bucket dumping pilot pressure). The bucket dumping pilot pressure outputted from the bucket dumping pilot pressure control valve  55  is led to an operation section on the other side (the right side in the figure) of the bucket flow control valve  16   b  through the hydraulic control unit  60 , the shuttle block  46 , and a pilot line  559 , to drive the bucket flow control valve  16   b  in the leftward direction in the figure. As a result, the hydraulic fluid delivered from the hydraulic pump  2  is supplied to the rod side of the arm cylinder  6 , whereas the hydraulic fluid on the bottom side is discharged into the tank  50 , and the bucket cylinder  7  is contracted. 
     The operation device  15 C includes an arm operation lever  15   c , the arm crowding pilot pressure control valve  56 , and the arm dumping pilot pressure control valve  57 . Here, the arm operation lever  15   c  corresponds, for example, to the operation left lever  14   b  (depicted in  FIG.  1   ) when it is operated in the left-right direction. 
     The arm crowding pilot pressure control valve  56  decompresses the pilot primary pressure supplied through the lock valve  51 , to produce s pilot pressure according to an operation amount in the arm crowding direction of the arm operation lever  15   c  (this pilot pressure will hereinafter be referred to as arm crowding pilot pressure). The arm crowding pilot pressure outputted from the arm crowding pilot pressure control valve  56  is led to an operation section on one side (the left side in the figure) of the arm flow control valve  16   c  through the hydraulic control unit  60 , the shuttle block  46 , and a pilot line  569 , to drive the arm flow control valve  16   c  in the rightward direction in the figure. As a result, the hydraulic fluid delivered from the hydraulic pump  2  is supplied to the bottom side of the arm cylinder  6 , whereas the hydraulic fluid on the rod side is discharged into the tank  50 , and the arm cylinder  6  is extended. 
     The arm dumping pilot pressure control valve  57  decompresses the pilot primary pressure supplied through the lock valve  51 , to produce a pilot pressure according to an operation amount in the arm dumping direction of the arm operation lever  15   c  (this pilot pressure will hereinafter be referred to as arm dumping pilot pressure). The arm dumping pilot pressure outputted from the arm dumping pilot pressure control valve  57  is led to the operation section of the other side (the right side in the figure) of the arm flow control valve  16   c  through the hydraulic control unit  60 , the shuttle block  46 , and a pilot line  579 , to drive the arm flow control valve  16   c  in the leftward direction in the figure. As a result, the hydraulic fluid delivered from the hydraulic pump  2  is supplied to the rod side of the arm cylinder  6 , whereas the hydraulic fluid on the bottom side is discharged into the tank  50 , and the arm cylinder  6  is contracted. 
     The operation device  15 D includes a swing operation lever  15   d , the right swing pilot pressure control valve  58 , and the left swing pilot pressure control valve  59 . Here, the swing operation lever  15   d  corresponds, for example, to the operation left lever  14   b  (depicted in  FIG.  1   ) when it is operated in the front-rear direction. 
     The right swing pilot pressure control valve  58  decompresses the pilot primary pressure supplied through the lock valve  51 , to produce a pilot pressure according to an operation amount in the right swing direction of the swing operation lever  15   d  (this pilot pressure will hereinafter be referred to as right swing pilot pressure). The right swing pilot pressure outputted from the right swing pilot pressure control valve  58  is led to the operation section of one side (the right side in the figure) of the swing flow control valve  16   d  through the hydraulic control unit  60 , the shuttle block  46 , and a pilot line  589 , to drive the swing flow control valve  16   d  in the leftward direction in the figure. As a result, the hydraulic fluid delivered from the hydraulic pump  2  flows into the inlet/outlet port on one side (the right side in the figure) of the swing hydraulic motor  4 , whereas the hydraulic fluid flowing out from the inlet/outlet port on the other side (the left side in the figure) is discharged into the tank  50 , and the swing hydraulic motor  4  is rotated in one direction (a direction for putting the upper swing structure  12  into right swing). 
     The left swing pilot pressure control valve  59  decompresses the pilot primary pressure supplied through the lock valve  51 , to produce a pilot pressure according to an operation amount in the left swing direction of the swing operation lever  15   d  (this pilot pressure will hereinafter be referred to as left swing pilot pressure). The left swing pilot pressure outputted from the left swing pilot pressure control valve  59  is led to an operation section on the other side (the left side in the figure) of the swing flow control valve  16   d  through the hydraulic control unit  60 , the shuttle block  46 , and a pilot line  599 , to drive the swing flow control valve  16   d  in the rightward direction in the figure. As a result, the hydraulic fluid delivered from the hydraulic pump  2  flows into the inlet/outlet port on the other side (the left side in the figure) of the swing hydraulic motor  4 , whereas the hydraulic fluid flowing out from the inlet/outlet port on one side (the right side in the figure) is discharged into the tank  50 , and the swing hydraulic motor  4  is rotated in the other direction (a direction for putting the upper swing structure  12  into left swing). 
     The hydraulic control unit  60  is a device for executing a machine control, corrects the pilot pressures inputted from the pilot pressure control valves  52  to  59  according to instructions from the controller  20 , and outputs the corrected pilot pressures to the shuttle block  46 . As a result, it is possible to cause the front work implement  1 B to perform a desired operation, irrespectively of the operator&#39;s lever operation. 
     The shuttle block  46  outputs the pilot pressures inputted from the hydraulic control block to the pilot lines  529 ,  539 ,  549 ,  559 ,  569 ,  579 ,  589 , and  599 , selects, for example, a maximum pilot pressure of the inputted pilot pressures, and outputs the maximum pilot pressure to the regulator  47  of the hydraulic pump  2 . As a result, the delivery flow rate of the hydraulic pump  2  can be controlled according to the operation amounts of the operation levers  15   a  to  15   d.    
       FIG.  3    is a configuration diagram of the hydraulic control unit  60  depicted in  FIG.  2   . 
     In  FIG.  3   , the hydraulic control unit  60  includes a solenoid shut-off valve  61 , shuttle valves  522 ,  564 , and  574 , and solenoid proportional valves  525 ,  532 ,  542 ,  552 ,  562 ,  567 ,  572 , and  577 . 
     An inlet port of the solenoid shut-off valve  61  is connected to an outlet port of the lock valve  51  (depicted in  FIG.  2   ). An outlet port of the solenoid shut-off valve  61  is connected to inlet ports of the solenoid proportional valves  525 ,  567 , and  577 . Of the solenoid shut-off valve  61 , the opening is zero when no current is passed, and the opening is maximized by the supply of current from the controller  20 . In the case of making the machine control valid, the opening of the solenoid shut-off valve  61  is maximized, and the supply of the pilot primary pressure to the solenoid proportional valves  525 ,  567 , and  577  is started. On the other hand, in the case of making the machine control invalid, the opening of the solenoid shut-off valve  61  is set to zero, and the supply of the pilot primary pressure to the solenoid proportional valves  525 ,  567 , and  577  is stopped. 
     The shuttle valve  522  has two inlet ports and one outlet port, and the higher one of pressures inputted from the two inlet ports is outputted from the outlet port. The inlet port on one side of the shuttle valve  522  is connected to the boom raising pilot pressure control valve  52  through a pilot line  521 . The inlet port on the other side of the shuttle valve  522  is connected to an outlet port of the solenoid proportional valve  525  through a pilot line  524 . The outlet port of the shuttle valve  522  is connected to the shuttle block  46  through a pilot line  523 . 
     An inlet port of the solenoid proportional valve  525  is connected to the outlet port of the solenoid shut-off valve  61 . The outlet port of the solenoid proportional valve  525  is connected to the inlet port on the other side of the shuttle valve  522  through a pilot line  524 . Of the solenoid proportional valve  525 , the opening is set to zero when no current is passed, and the opening is increased according to a current supplied from the controller  20 . The solenoid proportional valve  525  decompresses the pilot primary pressure supplied through the solenoid shut-off valve  61  in accordance with the opening thereof, and outputs the decompressed pilot pressure to the pilot line  524 . As a result, a boom raising pilot pressure can be supplied to the pilot line  523  even in the case where the boom raising pilot pressure is not supplied from the boom raising pilot pressure control valve  52  to the pilot line  521 . Note that in the case where the machine control with respect to a boom raising operation is not conducted, the solenoid proportional valve  525  is set into a non-current-passed state, and the opening of the solenoid proportional valve  525  is set to zero. In this instance, the boom raising pilot pressure supplied from the boom raising pilot pressure control valve  52  is led to an operation section on one side of the boom flow control valve  16   a , and, therefore, a boom raising operation according to an operator&#39;s lever operation can be performed. 
     An inlet port of the solenoid proportional valve  532  is connected to the boom lowering pilot pressure control valve  53  through a pilot line  531 . An outlet port of the solenoid proportional valve  532  is connected to the shuttle block  46  through a pilot line  533 . Of the solenoid proportional valve  532 , the opening is maximized when no current is passed, and the opening is reduced from the maximum to zero according to a current supplied from the controller  20 . The solenoid proportional valve  532  decompresses the boom lowering pilot pressure supplied through the pilot line  531  in accordance with the opening thereof, and outputs the decompressed pilot pressure to the pilot line  533 . As a result, it is possible to decompress, or reduce to zero, the boom lowering pilot pressure due to an operator&#39;s lever operation. Note that in the case where the machine control with respect to a boom lowering operation is not conducted, the solenoid proportional valve  532  is set into a non-current-passed state, and the opening of the solenoid proportional valve  532  is full open. In this instance, the boom lowering pilot pressure supplied from the boom lowering pilot pressure control valve  53  is led to an operation section on the other side of the boom flow control valve  16   a , and, therefore, a boom lowering operation according to an operator&#39;s lever operation can be performed. 
     An inlet port of the solenoid proportional valve  542  is connected to the bucket crowding pilot pressure control valve  54  through a pilot line  541 . An outlet port of the solenoid proportional valve  542  is connected to the shuttle block  46  through a pilot line  543 . Of the solenoid proportional valve  542 , the opening is maximized when no current is passed, and the opening is reduced from the maximum to zero in accordance with a current supplied from the controller  20 . The solenoid proportional valve  542  decompresses the bucket crowding pilot pressure inputted through the pilot line  541  in accordance with the opening thereof, and outputs the decompressed pilot pressure to the pilot line  543 . As a result, it is possible to decompress, or to reduce to zero, the bucket crowding pilot pressure due to an operator&#39;s lever operation. Note that in the case where the machine control with respect to a bucket crowding operation is not conducted, the solenoid proportional valve  542  is set into a non-current-passed state, and the opening of the solenoid proportional valve  542  is full open. In this instance, the bucket crowding pilot pressure supplied from the bucket crowding pilot pressure control valve  54  is led to an operation section on one side of the bucket flow control valve  16   b , and, therefore, a bucket dumping operation according an operator&#39;s lever operation can be performed. 
     An inlet port of the solenoid proportional valve  552  is connected to the bucket dumping pilot pressure control valve  55  through a pilot line  551 . An outlet port of the solenoid proportional valve  552  is connected to the shuttle block  46  (depicted in  FIG.  2   ) through a pilot line  553 . Of the solenoid proportional valve  552 , the opening is maximized when no current is passed, and the opening is reduced from the maximum to zero according to a current supplied from the controller  20 . The solenoid proportional valve  552  decompresses the bucket dumping pilot pressure inputted through the pilot line  551  in accordance with the opening thereof, and outputs the decompressed pilot pressure to the pilot line  553 . As a result, it is possible to decompress, or to reduce to zero, the bucket dumping pilot pressure due to an operator&#39;s lever operation. Note that in the case where the machine control with respect to a bucket dumping operation is not conducted, the solenoid proportional valve  552  is set into a non-current-passed state, and the opening of the solenoid proportional valve  552  is full open. In this instance, the bucket dumping pilot pressure supplied from the bucket dumping pilot pressure control valve  55  is led to an operation section on the other side of the bucket flow control valve  16   b , and, therefore, a bucket dumping operation according to an operator&#39;s lever operation can be performed. 
     The shuttle valve  564  has two inlet ports and one outlet port, and a higher one of pressures inputted from the two inlet ports is outputted from the output port. The inlet port on one side of the shuttle valve  564  is connected to an outlet port of the solenoid proportional valve  562  through a pilot line  563 . The inlet port on the other side of the shuttle valve  564  is connected to an outlet port of the solenoid proportional valve  567  through a pilot line  566 . The outlet port of the shuttle valve  522  is connected to the shuttle block  46  through a pilot line  565 . 
     An inlet port of the solenoid proportional valve  562  is connected to the arm crowding pilot pressure control valve  56  through a pilot line  561 . An outlet port of the solenoid proportional valve  562  is connected to the inlet port on one side of the shuttle valve  564  through the pilot line  563 . Of the solenoid proportional valve  562 , the opening is maximized when no current is passed, and the opening is reduced from the maximum to zero according to a current supplied from the controller  20 . The solenoid proportional valve  562  decompresses the arm crowding pilot pressure inputted through the pilot line  561  in accordance with the opening thereof, and outputs the decompressed pilot pressure to the pilot line  563 . As a result, it is possible to decompress, or to reduce to zero, the arm crowding pilot pressure due to an operator&#39;s lever operation. 
     An inlet port of the solenoid proportional valve  567  is connected to the output port of the solenoid shut-off valve  61 , and an outlet port of the solenoid proportional valve  567  is connected to the inlet port on the other side of the shuttle valve  564  through a pilot line  566 . Of the solenoid proportional valve  567 , the opening is set to zero when no current is passed, and the opening is increased according to a current supplied from the controller  20 . The solenoid proportional valve  567  decompresses the pilot primary pressure supplied through the solenoid shut-off valve  61  in accordance with the opening thereof, and outputs the decompressed pilot pressure to the pilot line  566 . As a result, even in the case where the arm crowding pilot pressure is not supplied from the arm crowding pilot pressure control valve  56  to the pilot line  563 , the arm crowding pilot pressure can be supplied to the pilot line  565 . Note that in the case where the machine control with respect to an arm crowding operation is not conducted, the solenoid proportional valves  562  and  567  are set into a non-current-passed state, the opening of the solenoid proportional valve  562  is full open, and the opening of the solenoid proportional valve  567  is zero. In this instance, the arm crowding pilot pressure supplied from the arm crowding pilot pressure control valve  56  is led to an operation section on one side of the arm flow control valve  16   c , and, therefore, an arm crowding operation according to an operator&#39;s lever operation can be performed. 
     The shuttle valve  574  has two inlet ports and one outlet port, and the higher one of pressures inputted from the two inlet ports is outputted from the outlet port. The inlet port on one side of the shuttle valve  574  is connected to an outlet port of the solenoid proportional valve  572  through a pilot line  573 . The inlet port on the other side of the shuttle valve  574  is connected to an outlet port of the solenoid proportional valve  577  through a pilot line  576 . The outlet port of the shuttle valve  574  is connected to the shuttle block  46  through a pilot line  575 . 
     An inlet port of the solenoid proportional valve  572  is connected to the arm dumping pilot pressure control valve  57  through a pilot line  571 . The outlet port of the solenoid proportional valve  572  is connected to the inlet port on one side of the shuttle valve  574  through the pilot line  573 . Of the solenoid proportional valve  572 , the opening is maximized when no current is passed, and the opening is reduced from the maximum to zero according to a current supplied from the controller  20 . The solenoid proportional valve  572  decompresses the arm dumping pilot pressure inputted through the pilot line  571  in accordance with the opening thereof, and supplies the decompressed pilot pressure to the pilot line  573 . As a result, it is possible to decompress, or to reduce to zero, the arm dumping pilot pressure due to an operator&#39;s lever operation. 
     An inlet port of the solenoid proportional valve  577  is connected to the outlet port of the solenoid shut-off valve  61 . An outlet port of the solenoid proportional valve  577  is connected to the inlet port on the other side of the shuttle valve  574  through a pilot line  576 . Of the solenoid proportional valve  577 , the opening is set to zero when no current is passed, and the opening is increased according to a current supplied from the controller  20 . The solenoid proportional valve  577  decompresses the pilot primary pressure supplied through the solenoid shut-off valve  61  in accordance with the opening thereof, and supplies the decompressed pilot pressure to the pilot line  576 . As a result, even in the case where the arm dumping pilot pressure is not supplied from the arm dumping pilot pressure control valve  57  to the pilot line  573 , the arm dumping pilot pressure can be supplied to the pilot line  575 . Note that in the case where the machine control with respect to an arm dumping operation is not conducted, the solenoid proportional valves  572  and  577  are set into a non-current-passed state, the opening of the solenoid proportional valve  572  is full open, and the opening of the solenoid proportional valve  577  is zero. In this instance, the arm dumping pilot pressure supplied from the arm dumping pilot pressure control valve  57  is led to an operation section on the other side of the arm flow control valve  16   c , and, therefore, an arm dumping operation according to an operator&#39;s lever operation can be performed. 
     The pilot line  521  is provided with a pressure sensor  526  for detecting the boom raising pilot pressure supplied from the boom raising pilot pressure control valve  52 . The pilot line  531  is provide with a pressure sensor  534  for detecting the boom lowering pilot pressure supplied from the boom lowering pilot pressure control valve  53 . The pilot line  541  is provide with a pressure sensor  544  for detecting the bucket crowding pilot pressure supplied from the bucket crowding pilot pressure control valve  54 . The pilot line  551  is provided with a pressure sensor  554  for detecting the bucket dumping pilot pressure supplied from the bucket dumping pilot pressure control valve  55 . The pilot line  561  is provided with a pressure sensor  568  for detecting the arm crowding pilot pressure supplied from the arm crowding pilot pressure control valve  56 . The pilot line  571  is provided with a pressure sensor  578  for detecting the arm dumping pilot pressure supplied from the arm dumping pilot pressure control valve  57 . The pilot pressures detected by the pressure sensors  526 ,  534 ,  544 ,  554 ,  568 , and  578  are inputted to the controller  20  as operation signals. 
       FIG.  4    is a functional block diagram of the controller depicted in  FIG.  2   . 
     In  FIG.  4   , the controller  20  includes a work implement posture calculation section  30 , a target surface calculation section  31 , a target operation calculation section  32 , and a solenoid valve control section  33 . 
     The work implement posture calculation section  30  calculates the posture of the front work implement  1 B based on information from a work implement posture sensor  34 . Here, the work implement posture sensor  34  includes the boom angle sensor  21 , the arm angle sensor  22 , the bucket angle sensor  23 , and the machine body inclination angle sensor  24 . 
     The target surface calculation section  31  calculates a target surface based on information from a target surface setting device  35 . Here, the target surface setting device  35  is an interface through which information regarding the target surface can be inputted. The input to the target surface setting device  35  may be manually made by the operator, or may be taken in from the exterior via a network or the like. In addition, a satellite communication antenna may be connected to the target surface setting device  35 , and the position of the hydraulic excavator  1  and a target surface position in a global coordinate system may be calculated. 
     The target operation calculation section  32  calculates a target operation of the front work implement  1 B in such a manner that the bucket  10  is moved without penetrating into the target surface, based on information from the work implement posture calculation section  30 , the target surface calculation section  31 , and an operator&#39;s operation sensor  36 . Here, the operator&#39;s operation sensor  36  includes the pressure sensors  526 ,  534 ,  544 ,  554 ,  568 , and  578  (depicted in  FIG.  3   ). 
     The solenoid valve control section  33  outputs instructions to the solenoid shut-off valve  61  and a solenoid proportional valve  500 , based on information from the target operation calculation section  32 . Here, the solenoid proportional valve  500  is representative of the solenoid proportional valves  525 ,  532 ,  542 ,  552 ,  562 ,  567 ,  572 , and  577  (depicted in  FIG.  3   ). 
     An example of a horizontal excavating operation by machine control is depicted in  FIG.  5   . For example, in the case where the operator operates the operation device  15  to perform horizontal excavation by a pulling operation of the arm  9  in the direction of arrow A, the solenoid proportional valve  525  is controlled to automatically perform a raising operation of the boom  8  in such a manner that the tip of the bucket  10  does not penetrate to below a target surface. In addition, in the case where the bucket  10  has penetrated to below the target surface at the time of performing horizontal excavation by the pulling operation of the arm  9  in the direction of arrow A, the solenoid proportional valve  525  is controlled to automatically perform the raising operation of the boom  8  in such a manner that the bucket  10  returns to above the target surface. In addition, in the case where the bucket  10  is brought close to the target surface by a lowering operation of the boom  8 , the solenoid proportional valve  532  is controlled such as to reduce the speed of the boom  8  in such a manner that the bucket  10  does not penetrate to below the target surface, and to reduce the speed of the boom  8  to zero in a state in which the bucket  10  reaches the target surface. In addition, the solenoid proportional valve  542  is controlled and a pulling operation of the arm  9  is performed, in such a manner as to realize an excavation speed, or excavation accuracy, required by the operator. In this instance, for enhancing the accuracy of excavation, the speed of the arm  9  may be reduced as required. In addition, in order that angle B of the bucket  10  relative to the target surface becomes a fixed value and leveling work is facilitated, the solenoid proportional valve  577  may be controlled such that the bucket is automatically rotated in the direction of arrow C. 
     In this instance, the work implement posture calculation section  30  calculates the posture of the front work implement  1 B, based on information from the work implement posture sensor  34 . The target surface calculation section  31  calculates the target surface, based on information from the target surface setting device  35 . The target operation calculation section  32  calculates a target operation of the front work implement  1 B such that the bucket  10  is moved without penetrating to below the target surface, based on information from the work implement posture calculation section  30  and the target surface calculation section  31 . The solenoid valve control section  33  calculates control inputs to the solenoid shut-off valve  61  and the solenoid proportional valve  500 , based on information from the target operation calculation section  32 . 
     In the case of making the machine control invalid, the solenoid valve control section  33  gives an instruction to the solenoid shut-off valve  61  and the solenoid proportional valve  500  not to perform a control intervention. Specifically, the opening of the solenoid shut-off valve  61  is set to zero, such as to prevent the hydraulic fluid coming from the pilot pump  48  through the lock valve  51  from flowing into the hydraulic control unit  60 . In addition, with respect to the solenoid proportional valves  532 ,  542 ,  552 ,  562 , and  572  of which the openings are to be full open when no current is passed, the openings are set full open, such as not to intervene in the pilot pressure due to an operator&#39;s operation. Besides, with respect to the solenoid proportional valves  525 ,  567 , and  577  of which the openings are to be zero when no current is passed, the openings are set to zero, such as to prevent the front work implement  1 B to be operated without an operator&#39;s operation. 
       FIG.  6    is a functional block diagram of the target operation calculation section depicted in  FIG.  5   . 
     In  FIG.  6   , the target operation calculation section  32  includes a target surface distance calculation section  70 , a speed correction region calculation section  71 , a target surface distance correction section  72 , and an operation signal correction section  73 . 
     The target surface distance calculation section  70  calculates the distance from the tip of the bucket to a target surface (hereinafter referred to as target surface distance), based on a bucket tip position inputted from the work implement posture calculation section  30  and a target surface inputted from the target surface calculation section  31 , and outputs the target surface distance to the target surface distance correction section  72 . 
     The speed correction region calculation section  71  calculates a speed correction region width, which will be described later, based on the lever operation amount inputted from the operator&#39;s operation sensor  36 , and outputs the speed correction region width to the target surface distance correction section  72 . 
     The target surface distance correction section  72  calculate a corrected target surface distance based on a target surface distance inputted from the target surface distance calculation section  70  and a speed correction region width inputted from the speed correction region calculation section  71 , and outputs the corrected target surface distance to the operation signal correction section  73 . 
     The operation signal correction section  73  corrects an operation signal, inputted from the operator&#39;s operation sensor  36 , based on the corrected target surface distance inputted from the target surface distance correction section  72 , and outputs the corrected operation signal to the solenoid valve control section  33 . 
       FIG.  7    is a flow chart depicting a processing of the target operation calculation section  32  depicted in  FIG.  6   . The steps will be sequentially described below. 
     First, in step S 100 , it is determined whether or not the boom operation lever  15   a  has been operated in a boom lowering direction, or whether or not the arm operation lever  15   c  or the bucket operation lever  15   b  has been operated. 
     When it is determined in step S 100  that the boom operation lever  15   a  has been operated in the boom lowering direction or that the arm operation lever  15   c  or the bucket operation lever  15   b  has been operated (YES), a processing of setting a speed correction region on an upper side of the target surface (speed correction region processing) is conducted in step S 101 . The details of the speed correction region processing will be described later. 
     Subsequently to step S 101 , calculation for correcting the operation signal (operation signal correction calculation) is performed in step S 102 . The details of the operation signal correction calculation will be described later. 
     Subsequently to step S 102 , a boom raising control according to the operation signal corrected in step S 102  is carried out in step S 103 . 
     Subsequently to step S 103 , or when the determination in step S 100  is NO, the control returns to step S 100 . 
       FIG.  8    is a flow chart depicting in detail the speed correction region processing (step S 101 ) depicted in  FIG.  7   . The steps will be sequentially described below. 
     First, an operation signal is inputted in step S 200 . 
     Subsequently to step S 200 , whether or not the target surface distance is smaller than a predetermined distance is determined in step S 201 . Here, the predetermined distance is set to a value greater than a maximum value Rmax of a speed correction region width R which will be described later. 
     When it is determined in step S 201  that the target surface distance is smaller than the predetermined distance (YES), the operation signals are subjected to a low-pass filter treatment with respect to the respective operation signals in step S 202 . As a result, high-frequency components of the operation signals are removed, and, therefore, sudden changes in the speed correction region width R, which will be described later, can be prevented. 
     Subsequently to step S 202 , whether or not the arm operation lever  15   c  has been operated is determined in step S 203 . 
     When it is determined in step S 203  that the arm operation lever  15   c  has been operated (YES), a speed correction region width R corresponding to the operation amount of the arm operation lever  15   c  is calculated in step S 204 . Specifically, referring to a conversion table depicted in  FIG.  9 A , the speed correction region width R corresponding to the operation amount of the arm operation lever  15   c  is calculated. When the arm lever operation amount is equal to or less than a lower limit PAmin, the speed correction region width R is constant at zero. When the arm lever operation amount is between the lower limit PAmin and a predetermined upper limit PAmax, the speed correction region width R increases from zero to a predetermined maximum value Rmax, in proportion to the arm lever operation amount. When the arm lever operation amount is equal to or more than the upper limit PAmax, the speed correction region width R is constant at the maximum value Rmax. 
     When it is determined in step S 203  that the arm operation lever  15   c  has not been operated (NO), whether or not the boom operation lever  15   a  has been operated in a boom lowering direction is determined in step S 207 . 
     When it is determined in step S 207  that the boom operation lever  15   a  has been operated in the boom lowering direction (YES), a speed correction region width R corresponding to the operation amount in the boom lowering direction is calculated in step S 208 . Specifically, referring to a conversion table depicted in  FIG.  9 B , the speed correction region width R corresponding to the operation amount of the boom operation lever  15   a  in the boom lowering direction is calculated. When the operation amount in the boom lowering direction is equal to or less than a predetermined lower limit PBDmin, the speed correction region width R is constant at zero. When the lever operation amount in the boom lowering direction is between the lower limit PBDmin and a predetermined upper limit PBDmax, the speed correction region width R increases from zero to a predetermined maximum value Rmax, in proportion to the lever operation amount in the boom lowering direction. When the boom lowering lever operation amount is equal to or more than the upper limit PBDmax, the speed correction region width R is constant at the maximum value Rmax. 
     When it is determined in step S 201  that the target surface distance is equal to or greater than the predetermined distance (NO), the maximum value Rmax is set as the speed correction region width R in step S 209 . This ensures that in the case where the bucket  10  is largely spaced from the target surface, an upper surface of the speed correction region is set higher than the target surface by the speed correction region width Rmax, irrespectively of the operator&#39;s lever operation. As a result, for example, even in the case where the bucket  10  is moved at high speed from a remote position toward the target surface and where setting of the speed correction region width R is too late due to a delay in calculation by the controller  20 , the tip of the bucket can be prevented from penetrating to below the target surface. 
     Subsequently to step S 204 , S 208 , or S 209 , or when it is determined in step S 207  that the boom operation lever  15   a  has not been operated in the boom lowering direction (NO), setting of a speed correction region is conducted in step S 205 . Specifically, a speed correction region having the speed correction region width calculated in step S 204 , S 208 , or S 209  is set on the upper side of the target surface. 
     Subsequently to step S 205 , correction of a target surface distance D is conducted in step S 206 . Specifically, as depicted in  FIG.  10   , the speed correction region width R calculated in step S 204 , S 208 , or S 209  is subtracted from the target surface distance D, to calculate a corrected target surface distance Da. This ensures that when the speed correction region width R is zero, machine control is carried out with the target surface as a reference, whereas when the speed correction region width R is greater than zero, machine control is carried out with the speed correction region upper surface set higher than the target surface by the speed correction region width R as a reference. 
     Subsequently to step S 206 , an operation signal correction calculation is conducted in step S 102  depicted in  FIG.  7   . Specifically, the operation signal inputted in step S 200  is corrected, based on the corrected target surface distance Da calculated in step S 206 . Here, as an example, a case of correcting the boom lowering pilot pressure which is one of the operation signals will be described.  FIG.  11    is a diagram depicting the relation between target surface distance and operation amount limit value. The boom lowering pilot pressure is compared with an operation amount limit value set according to the target surface distance; when the boom lowering pilot pressure is greater than the operation amount limit value, it is corrected to coincide with the operation amount limit value. In  FIG.  11   , for a target surface distance equal to or smaller than a predetermined distance Dlim, an operation amount limit value proportional to the target surface distance is set, and, for a target surface distance greater than the predetermined distance Dlim, infinity is set as the operation amount limit value. Therefore, when the target surface distance Da is equal to or smaller than the predetermined distance Dlim, the operation signal is corrected such that the boom lowering pilot pressure is equal to or less than the operation amount limit value, and, when the target surface distance is greater than the predetermined distance Dlim, the operation signal is not corrected. As a result, when the target surface distance (or the corrected target surface distance) is less than the predetermined distance Dlim, the boom lowering operation is decelerated as the bucket tip approaches the target surface (or the upper surface of the speed correction region), and, therefore, the bucket tip can be prevented from penetrating to below the target surface (or into the speed correction region). 
     An operation of the hydraulic excavator  1  will be described below. 
     &lt;Bucket Aligning Operation&gt; 
     As depicted in  FIG.  12   , a bucket aligning operation is carried out by operating the boom  8  in a lowering direction (the direction of arrow D) until the tip of the bucket  10  is disposed on the target surface. 
     When an operation amount of the boom operation lever  15   a  in a boom lowering direction is equal to or less than PBDmin, zero is set as the speed correction region width R based on the conversion table depicted in  FIG.  9 B , and, therefore, the corrected target surface distance Da coincides with the target surface distance D. As a result, when the tip of the bucket  10  is largely spaced from the target surface, the boom lowering operation is conducted at a speed according to the operation amount of the boom operation lever  15   a  in the boom lowering direction. As the tip of the bucket  10  approaches the target surface, the boom lowering pilot pressure is reduced in such a manner that the distance from the tip of the bucket  10  to the target surface (target surface distance D) does not become less than zero. In this instance, the operation amount of the boom operation lever  15   a  is equal to or less than the lower limit PBDmin, and the boom lowering speed is low; therefore, the accuracy of machine control is maintained, and the bucket  10  can be stopped when the tip of the bucket  10  comes to be located on the target surface, as depicted in  FIG.  13 ( a ) . 
     When the operation amount of the boom operation lever  15   a  in the boom lowering direction is between the lower limit PBDmin and the upper limit PBDmax, a value in the range of zero to the maximum value Rmax is set as the speed correction region width R in accordance with the operation amount, and the corrected target surface distance Da is smaller than the target surface distance D by the speed correction region width R. As a result, when the tip of the bucket  10  is largely spaced from the speed correction region upper surface (indicated by broken line in the figure), the boom lowering operation is performed at a speed according to the operation amount of the boom operation lever  15   a  in the boom lowering direction. When the tip of the bucket  10  approaches the speed correction region upper surface, the boom lowering pilot pressure is reduced in such a manner that the distance from the tip of the bucket  10  to the speed correction region upper surface (corrected target surface distance Da) does not become less than zero. As a result, the boom lowering operation is stopped in a state in which the bucket tip is disposed on the speed correction region upper surface, as depicted in  FIG.  13 ( b ) . In this instance, since the operation amount of the boom operation lever  15   a  is larger than the lower limit PBDmin and the boom lowering speed is not small, the accuracy of machine control may not be maintained, and the bucket tip may penetrate into the speed correction region. However, since the speed correction region upper surface is set higher than the target surface by the speed correction region width R according to the operation amount of the boom operation lever  15   a  in the boom lowering direction (that is, the boom lowering speed), the bucket tip can be prevented from penetrating to below the target surface. 
     When the operation amount of the boom operation lever  15   a  in the boom lowering direction is equal to or more than PBDmax, the maximum value Rmax is set as the speed correction region width R, and, therefore, the corrected target surface distance Da is smaller than the target surface distance D by the speed correction region width Rmax. As a result, when the tip of the bucket  10  is largely spaced from the speed correction region upper surface, the boom lowering operation is conducted at a speed according to the operation amount of the boom operation lever  15   a  in the boom lowering direction. When the tip of the bucket  10  approaches the speed correction region upper surface, the boom lowering pilot pressure is reduced in such a manner that the distance from the tip of the bucket  10  to the speed correction region upper surface (corrected target surface distance Da) does not become less than zero. As a result, as depicted in  FIG.  12 ( c ) , the boom lowering operation is stopped in a state in which the bucket tip is disposed on the speed correction region upper surface. In this instance, since the operation amount of the boom operation lever  15   a  is equal to or more than the upper limit PBDmax and the boom lowering speed is high, the accuracy of machine control may not be maintained, and the bucket tip may penetrate into the speed correction region. However, since the speed correction region upper surface is set higher than the target surface by the speed correction region width Rmax according to the operation amount of the boom operation lever  15   a  in the boom lowering direction (that is, the boom lowering speed), the bucket tip can be prevented from penetrating to below the target surface. Note that the bucket tip cannot be moved into the speed correction region during when the operation amount in the boom lowering direction is larger than the lower limit PBDmin, but, by reducing the operation amount in the boom lowering direction to the lower limit PBDmin, the bucket tip can be made to reach the target surface. 
     &lt;Horizontal Excavating Operation&gt; 
     A horizontal excavating operation is performed by operating the arm  9  in a crowding direction (the direction of arrow B) in a state in which the tip of the bucket  10  is disposed on the target surface, as depicted in  FIG.  14   . 
     When the operation amount of the arm operation lever  15   c  in an arm crowding direction is equal to or less than a lower limit PAmin, zero is set as the speed correction region width R based on the conversion table depicted in  FIG.  9 A , and, therefore, the corrected target surface distance Da coincides with the target surface distance D. As a result, a boom raising operation is automatically conducted in such a manner that the bucket  10  is moved at a speed according to the operation amount of the arm operation lever  15   c , and the bucket tip is moved along the target surface, as depicted in  FIG.  15 ( a ) . In this instance, since the operation amount of the arm operation lever  15   c  is equal to or less than the lower limit PAmin and the arm crowding speed is low, the accuracy of machine control is maintained, and the bucket tip can be prevented from penetrating to below the target surface. 
     When the operation amount of the arm operation lever  15   c  is between the lower limit PAmin to an upper limit PAmax, a value in the range of zero to a maximum value Rmax is set as the speed correction region width R in accordance with the operation amount, and, therefore, the corrected target surface distance Da is smaller than the target surface distance D by the speed correction region width R. This ensures that a boom raising control is automatically conducted until the bucket tip is disposed on the speed correction region upper surface (indicated by broken line in the figure), and the boom raising operation is automatically performed in such a manner that the bucket  10  is moved at a speed according to the operation amount of the arm operation lever  15   c  and that the bucket tip is moved along the speed correction region upper surface located to be higher than the target surface by the speed correction region width R, as depicted in  FIG.  15 ( b ) . In this instance, since the operation amount of the arm operation lever  15   c  is larger than the lower limit PAmin and the arm crowding speed is not low, the accuracy of machine control may not be maintained, and the bucket tip may penetrate into the speed correction region. However, since the speed correction region upper surface is set higher than the target surface by the speed correction region width R according to the operation amount of the arm operation lever  15   c  in the arm crowding direction (that is, the arm crowding speed), the bucket tip can be prevented from penetrating to below the target surface. 
     When the operation amount of the arm operation lever  15   c  in the arm crowding direction is equal to or more than the upper limit PAmax, the maximum value Rmax is set as the speed correction region width R, and, therefore, the corrected target surface distance Da is smaller than the target surface distance D by the speed correction region width Rmax. As a result, a boom raising control is automatically conducted until the bucket tip is disposed on the speed correction region upper surface, and the boom raising operation is automatically performed in such a manner that the bucket  10  is moved at a speed according to the operation amount of the arm operation lever  15   c  and that the bucket tip is moved along the speed correction region upper surface located to be higher than the target surface by the maximum correction amount Rmax, as depicted in  FIG.  15 ( c ) . In this instance, since the operation amount of the arm operation lever  15   c  is equal to or more than the upper limit PAmax and the arm crowding speed is high, the accuracy of machine control may not be maintained, and the bucket tip may penetrate into the speed correction region. However, since the speed correction region upper surface is set higher than the target surface by the speed correction region width Rmax according to the operation amount of the arm operation lever  15   c  in the arm crowding direction (that is, the arm crowding speed), the bucket tip can be prevented from penetrating to below the target surface. 
     According to the hydraulic excavator  1  configured as above, when the operation amount of the operation device  15 A or  15 C is equal to or less than the predetermined operation amount PBDmin or PAmin, the operation of the front work implement  1 B is controlled in such a manner that the distance from the bucket tip to the target surface (target surface distance D) does not become less than zero. On the other hand, when the operation amount of the operation device  15 A or  15 C is larger than the predetermined operation amount PBDmin or PAmin, the speed correction region upper surface is set higher than the target surface by the speed correction region width R according to the operation amount, and the operation of the front work implement  1 B is controlled in such a manner that the distance from the bucket tip to the speed correction region upper surface (corrected target surface distance Da) does not become less than zero. As a result, it becomes possible to operate the front work implement  1 B at a speed according to the operator&#39;s lever operation, while securing the accuracy of work by machine control. 
     While the embodiment of the present invention has been described in detail above, the present invention is not limited to the above embodiment, but include various modifications. For instance, while the hydraulic excavator  1  having the bucket  10  has been described as an example of the work tool in the above embodiment, the present invention is applicable to hydraulic excavators having other work tool than the bucket, and to other work machines than the hydraulic excavator. In addition, while a case of performing machine control with respect to the position of the tip of the bucket  10  has been described in the above embodiment, the present invention is applicable also to a case of performing machine control with respect other position of the bucket  10 . Besides, while cases of correcting the target surface distance D according to the operation amount of the boom operation lever  15   a  in the boom lowering direction and the operation amount of the arm operation lever  15   c  have been described in the above embodiment, the target surface distance D may be corrected according to the operation amount of the bucket operation lever  15   b . In addition, the above embodiment has been described in detail for easily understandably explaining the present invention, and the present invention is not limited to an embodiment that has all the above-described configurations. 
     Description of Reference Characters 
     
         
           1 : Hydraulic excavator 
           1 A: Machine body 
           1 B: Front work implement 
           1 C: Operation room 
           2 : Hydraulic pump 
           4 : Swing hydraulic motor 
           5 : Boom cylinder 
           6 : Arm cylinder 
           7 : Bucket cylinder 
           8 : Boom 
           9 : Arm 
           10 : Bucket 
           11 : Lower track structure 
           12 : Upper swing structure 
           13   a : Track right lever 
           13   b : Track left lever 
           14   a : Operation right lever 
           14   b : Operation left lever 
           15 A to  15 D: Operation device 
           15   a : Boom operation lever 
           15   b : Bucket operation lever 
           15   c : Arm operation lever 
           15   d : Swing operation lever 
           16   a : Boom flow control valve 
           16   b : Bucket flow control valve 
           16   c : Arm flow control valve 
           16   d : Swing flow control valve 
           20 : Controller 
           21 : Boom angle sensor 
           22 : Arm angle sensor 
           23 : Bucket angle sensor 
           24 : Machine body inclination angle sensor 
           30 : Work implement posture calculation section 
           31 : Target surface calculation section 
           32 : Target operation calculation section 
           33 : Solenoid valve control section 
           34 : Work implement posture sensor 
           35 : Target surface setting device 
           36 : Operator&#39;s operation sensor 
           46 : Shuttle block 
           47 : Regulator 
           48 : Pilot pump 
           49 : Prime mover 
           50 : Tank 
           51 : Lock valve 
           52 : Boom raising pilot pressure control valve 
           53 : Boom lowering pilot pressure control valve 
           54 : Bucket crowding pilot pressure control valve 
           55 : Bucket dumping pilot pressure control valve 
           56 : Arm crowding pilot pressure control valve 
           57 : Arm dumping pilot pressure control valve 
           58 : Right swing pilot pressure control valve 
           59 : Left swing pilot pressure control valve 
           60 : Hydraulic control unit 
           61 : Solenoid shut-off valve 
           70 : Target surface distance calculation section 
           71 : Speed correction region calculation section 
           72 : Target surface distance correction section 
           73 : Operation signal correction section 
           100 : Hydraulic drive system 
           500 : Solenoid proportional valve 
           521 : Pilot line 
           522 : Shuttle valve 
           523 : Pilot line 
           524 : Pilot line 
           525 : Solenoid proportional valve 
           526 : Pressure sensor 
           529 : Pilot line 
           531 : Pilot line 
           532 : Solenoid proportional valve 
           533 : Pilot line 
           534 : Pressure sensor 
           539 : Pilot line 
           541 : Pilot line 
           542 : Solenoid proportional valve 
           543 : Pilot line 
           544 : Pressure sensor 
           549 : Pilot line 
           551 : Pilot line 
           552 : Solenoid proportional valve 
           553 : Pilot line 
           554 : Pressure sensor 
           559 : Pilot line 
           561 : Pilot line 
           562 : Solenoid proportional valve 
           563 : Pilot line 
           564 : Shuttle valve 
           565 : Pilot line 
           566 : Pilot line 
           567 : Solenoid proportional valve 
           568 : Pressure sensor 
           569 : Pilot line 
           571 : Pilot line 
           572 : Solenoid proportional valve 
           573 : Pilot line 
           574 : Shuttle valve 
           575 : Pilot line 
           576 : Pilot line 
           577 : Solenoid proportional valve 
           578 : Pressure sensor 
           579 : Pilot line 
           589 : Pilot line 
           599 : Pilot line.