Patent Publication Number: US-11047108-B2

Title: Work machine and control method for work machine

Description:
TECHNICAL FIELD 
     The present invention relates to a work machine including a work implement, and a control method for a work machine. 
     BACKGROUND ART 
     For a work machine that includes a front device provided with a bucket, there has been proposed such control that shifts the bucket along a boundary surface defining a target shape of an object of execution (for example, see PTDs 1 and 2). This control is referred to as intervention control. 
     In some situations, this intervention control for the target shape of the object of execution is difficult to perform depending on the attitude of the work implement of the work machine. 
     More specifically, during land grading by dumping with a dipper stick, a rapid change of a cylinder speed may be produced when the dipper stick arrives at a position close to a stroke end of a dipper stick cylinder. A change of the cylinder speed may affect accuracy of land grading. Accordingly, the intervention control may be brought to a stop when the dipper stick arrives at the position close to the stroke end of the dipper stick cylinder. 
     CITATION LIST 
     Patent Document 
     
         
         PTD 1: WO 2012/127912 
         PTD 2: WO 2016/056678 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, a rapid speed change of the work implement at the time of a stop of the intervention control applies a shock to the work machine. 
     The present disclosure has been developed to solve the aforementioned problems. An object of the present disclosure is to provide a work machine and a control method for a work machine capable of reducing a shock applied to a work implement at the time of a stop of intervention control. 
     Solution to Problem 
     A work machine according an aspect includes a work implement, an operation apparatus for operating the work implement, and a controller for controlling the work implement. The controller performs intervention control for lowering the work implement based on an operation command from the operation apparatus, and reduces a speed of the work implement during the intervention control to stop the work implement before completion of the intervention control. 
     Advantageous Effects of Invention 
     The work machine and the control method for the work machine are capable of reducing a shock applied to a work implement at the time of a stop of intervention control. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a work machine according to an embodiment. 
         FIG. 2  is a block diagram illustrating configurations of a control system  200  and a hydraulic system  300  included in a hydraulic excavator  100  according to the embodiment. 
         FIG. 3  is a diagram illustrating an example of a hydraulic circuit  301  included in a boom cylinder  10  according to the embodiment. 
         FIG. 4  is a block diagram of a work implement controller  26  according to the embodiment. 
         FIG. 5  is a chart illustrating target excavation topography data U and a bucket  8  according to the embodiment. 
         FIG. 6  is a diagram illustrating a boom speed limit Vcy_bm according to the embodiment. 
         FIG. 7  is a chart illustrating a speed limit Vc_lmt according to the embodiment. 
         FIG. 8  is a view illustrating an example of a relationship between bucket  8  and target excavation topography  43 I according to the embodiment. 
         FIG. 9  is another view illustrating the relationship between bucket  8  and target excavation topography  43 I according to the embodiment. 
         FIG. 10  is a chart illustrating a boom speed during boom intervention control for land grading according to the embodiment. 
         FIG. 11  is a chart illustrating a limiting table for a boom speed according to the embodiment. 
         FIG. 12  is a chart illustrating a flow of a control method for the work machine according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     An embodiment of the present invention is hereinafter described with reference to the drawings. In the following description, identical parts are given identical reference numbers. These identical parts have identical names and functions, wherefore details of these parts are not repeatedly described herein. Note that “upper”, “lower”, “fore”, “after”, “left”, and “right” in the following description are terms defined as viewed from a reference corresponding to an operator sitting on an operator&#39;s seat. 
     &lt;General Configuration of Work Machine&gt; 
       FIG. 1  is a perspective view of a work machine according to the embodiment. 
       FIG. 2  is a block diagram illustrating configurations of a control system  200  and a hydraulic system  300  included in a hydraulic excavator  100  according to the embodiment. 
     Referring to  FIG. 1 , hydraulic excavator  100  provided as a work machine includes a vehicular body  1  and a work implement  2 . 
     Vehicular body  1  includes an upper revolving unit  3  provided as a revolving unit, and a traveling apparatus  5  provided as a traveling unit. Upper revolving unit  3  accommodates an internal combustion engine provided as a power generator, hydraulic pumps, and other devices within an engine room  3 EG. Engine room  3 EG is disposed at an end of upper revolving unit  3 . 
     According to the embodiment, the internal combustion engine provided as a power generator of hydraulic excavator  100  is constituted by a diesel engine, for example. However, the power generator may be constituted by other types of power generator. 
     For example, the power generator of hydraulic excavator  100  may be a hybrid type device constituted by a combination of an internal combustion engine, a generator motor, and an electrical storage device. 
     The power generator of hydraulic excavator  100  may be constituted by a combination of an electrical storage device and a generator motor, excluding an internal combustion engine. 
     Upper revolving unit  3  includes an operator&#39;s cab  4 . Operator&#39;s cab  4  is disposed at the other end of upper revolving unit  3 . Operator&#39;s cab  4  is positioned on the side opposite to the side of engine room  3 EG. A display unit  29  and an operation apparatus  25  illustrated in  FIG. 2  are disposed within operator&#39;s cab  4 . 
     Traveling apparatus  5  supports upper revolving unit  3 . Traveling apparatus  5  includes crawler belts  5   a  and  5   b . One or both of travel motors  5   c  provided on the left and right of traveling apparatus  5  drive and rotate crawler belts  5   a  and  5   b  to allow traveling of hydraulic excavator  100 . Work implement  2  is attached to a side of operator&#39;s cab  4  of upper revolving unit  3 . 
     Hydraulic excavator  100  may include a traveling apparatus provided with tires instead of crawler belts  5   a  and  5   b , and transmit driving force of an engine to the tires via a transmission to allow traveling. Examples of hydraulic excavator  100  of this type include a wheel hydraulic excavator. 
     Hydraulic excavator  100  may be a backhoe loader, for example. 
     The front of upper revolving unit  3  corresponds to the side where work implement  2  and operator&#39;s cab  4  are disposed, while the rear of upper revolving unit  3  corresponds to the side where engine room  3 EG is disposed. The left side in the forward direction corresponds to the left of upper revolving unit  3 , while the right side in the forward direction corresponds to the right of upper revolving unit  3 . The left/right direction of upper revolving unit  3  is also referred to as a width direction. Traveling apparatus  5  side of hydraulic excavator  100  or vehicular body  1  with respect to upper revolving body  3  corresponds to the lower side, while upper revolving unit  3  side with respect to traveling apparatus  5  corresponds to the upper side. The fore/aft direction, the width direction, and the up/down direction of hydraulic excavator  100  correspond to an x direction, a y direction, and a z direction, respectively. When hydraulic excavator  100  is disposed on a horizontal plane, the lower side corresponds to the gravitating side in the direction of gravity identical to the perpendicular direction, while the upper side corresponds to the side opposite to the gravitating side in the perpendicular direction. 
     Work implement  2  includes a boom  6 , a dipper stick  7 , a bucket  8  provided as a work tool, a boom cylinder  10 , a dipper stick cylinder  11 , and a bucket cylinder  12 . A proximal end of boom  6  is attached to a front portion of vehicular body  1  via a boom pin  13 . A proximal end of dipper stick  7  is attached to a distal end of boom  6  via a dipper stick pin  14 . Bucket  8  is attached to a distal end of dipper stick  7  via a bucket pin  15 . Bucket  8  is movable around bucket pin  15 . A plurality of cutters  8 B are attached to bucket  8  on the side opposite to bucket pin  15 . Cutting edges  8 T correspond to distal ends of cutters  8 B. 
     According to the embodiment, rising of work implement  2  refers to a movement of work implement  2  in the direction from a ground engaging surface of hydraulic excavator  100  toward upper revolving unit  3 . Lowering of work implement  2  refers to a movement of work implement  2  in the direction from upper revolving unit  3  of hydraulic excavator  100  toward the ground engaging surface. The ground engaging surface of hydraulic excavator  100  is a flat surface defined by at least three points of engaging portions between crawler belts  5   a  and  5   b  and the ground. 
     In case of a work machine not provided with upper revolving unit  3 , rising of implement  2  refers to a movement of work implement  2  in the direction away from a ground engaging surface of the work machine. Lowering of work implement  2  refers to a movement of work implement  2  in the direction of approach toward the ground engaging surface of the work machine. When the work machine has wheels instead of crawler belts, the ground engaging surface is a flat surface defined by ground engaging portions of at least three wheels. 
     Bucket  8  is not required to have the plurality of cutters  8 B. Such a bucket is adoptable which does not have cutters  8 B illustrated in  FIG. 1 , but has a cutting edge constituted by a steel plate in a straight shape. Work implement  2  may include a tilt bucket having a single cutter, for example. The tilt bucket herein is a bucket that includes a bucket tilt cylinder, and tilts toward the left and right to form or grade a slope or a flat land into a desired shape, and also perform rolling compaction by using a bottom plate even when the hydraulic excavator is on a slope area. Alternatively, work implement  2  may include a drilling attachment provided with a slope bucket or a drilling chip as a work tool, for example, in place of bucket  8 . 
     Each of boom cylinder  10 , dipper stick cylinder  11 , and bucket cylinder  12  illustrated in  FIG. 1  is a hydraulic cylinder driven by a pressure of hydraulic oil (hereinafter referred to as oil pressure where appropriate). Boom cylinder  10  drives boom  6  to raise and lower boom  6 . Dipper stick cylinder  11  drives dipper stick  7  to move dipper stick  7  around dipper stick pin  14 . Bucket cylinder  12  drives bucket  8  to move bucket  8  around bucket pin  15 . 
     A direction control valve  64  illustrated in  FIG. 2  is provided between the hydraulic cylinders such as boom cylinder  10 , dipper stick cylinder  11 , and bucket cylinder  12 , and hydraulic pumps  36  and  37  illustrated in  FIG. 2 . Direction control valve  64  controls flow rates of hydraulic oil supplied from hydraulic pumps  36  and  37  to boom cylinder  10 , dipper stick cylinder  11 , bucket cylinder  12  and others, and switches flow directions of hydraulic oil. Direction control valve  64  includes a travel direction control valve for driving travel motors  5   c , and a work implement direction control valve for controlling revolving motors that revolve boom cylinder  10 , dipper stick cylinder  11 , bucket cylinder  12 , and upper revolving unit  3 . 
     Work implement controller  26  illustrated in  FIG. 2  controls a control valve  27  illustrated in  FIG. 2  to control a pilot pressure of hydraulic oil supplied from operation apparatus  25  to direction control valve  64 . Control valve  27  is included in a hydraulic system of boom cylinder  10 , dipper stick cylinder  11 , and bucket cylinder  12 . Work implement controller  26  controls control valve  27  included in a pilot oil path  450  to control movements of boom cylinder  10 , dipper stick cylinder  11 , and bucket cylinder  12 . 
     Work implement controller  26  according to the embodiment closes control valve  27  to reduce respective speeds of boom cylinder  10 , dipper stick cylinder  11 , and bucket cylinder  12 . 
     Antennas  21  and  22  are attached to an upper part of upper revolving unit  3 . Antennas  21  and  22  are used to detect a current position of hydraulic excavator  100 . Antennas  21  and  22  are electrically connected with a position detection device  19  illustrated in  FIG. 2  and provided as a position detector for detecting a current position of hydraulic excavator  100 . 
     Position detection device  19  detects a current position of hydraulic excavator  100  by utilizing real time kinematic-global navigation satellite systems (Real Time Kinematic-Global Navigation Satellite Systems). In the following description, antennas  21  and  22  are referred to as GNSS antennas  21  and  22  where appropriate. When GNSS antennas  21  and  22  receive a GNSS radio wave, a signal in the GNSS radio wave is input to position detection device  19 . Position detection device  19  detects installation positions of GNSS antennas  21  and  22 . Position detection device  19  includes a three-dimensional position sensor, for example. 
     &lt;Hydraulic System  300 &gt; 
     Referring to  FIG. 2 , hydraulic system  300  of hydraulic excavator  100  includes an internal combustion engine  35  provided as a power generation source, and hydraulic pumps  36  and  37 . Hydraulic pumps  36  and  37  driven by internal combustion engine  35  discharge hydraulic oil. The hydraulic oil discharged from hydraulic pumps  36  and  37  is supplied to boom cylinder  10 , dipper stick cylinder  11 , and bucket cylinder  12 . 
     Hydraulic excavator  100  includes a revolving motor  38 . Revolving motor  38  is a hydraulic motor driven by hydraulic oil discharged from hydraulic pumps  36  and  37 . Revolving motor  38  revolves upper revolving unit  3 . Note that only a single hydraulic pump may be provided instead of two hydraulic pumps  36  and  37  illustrated in  FIG. 2 . Revolving motor  38  may be a motor other than a hydraulic motor, such as an electric motor. 
     &lt;Control System  200 &gt; 
     Referring to  FIG. 2 , control system  200  provided as a control system for the work machine includes position detection device  19 , a global coordinate calculating unit  23 , operation apparatus  25 , work implement controller  26  provided as a controller of the work machine according to the embodiment, a sensor controller  39 , a display controller  28 , and display unit  29 . 
     Operation apparatus  25  is a device for operating work implement  2  and upper revolving unit  3  illustrated in  FIG. 1 . Operation apparatus  25  is a device for operating work implement  2 . Operation apparatus  25  receives an operation for driving work implement  2  from the operator, and outputs a pilot oil pressure corresponding to a manipulated variable. 
     The pilot oil pressure corresponding to a manipulated variable is equivalent to an operation command. This operation command is a command for moving work implement  2 . 
     The operation command is generated by operation apparatus  25 . Operation apparatus  25  is operated by the operator, wherefore the operation command is a command for moving work implement  2  based on an operation input by the operator as a manual operation. 
     According to the embodiment, operation apparatus  25  includes a left control lever  25 L provided on the left side of the operator, and a right control lever  25 R provided on the right side of the operator. 
     For example, an operation of right control lever  25 R in the fore/aft direction is associated with an operation of boom  6 . When right control lever  25 R is operated forward, boom  6  lowers. When right control lever  25 R is operated rearward, boom  6  rises. The lowering and rising movements of boom  6  are performed in accordance with operations in the fore/aft direction. 
     An operation of right control lever  25 R in the left/right direction is associated with an operation of bucket  8 . When right control lever  25 R is operated leftward, bucket  8  performs excavation. When right control lever  25 R is operated rightward, bucket  8  performs dumping. The excavation or dumping movement of bucket  8  is performed in accordance with an operation in the left/right direction. 
     An operation of left control lever  25 L in the fore/aft direction is associated with an operation of dipper stick  7 . When left control lever  25 L is operated forward, dipper stick  7  performs dumping. When left control lever  25 L is operated rearward, dipper stick  7  performs excavation. 
     An operation of left control lever  25 L in the left/right direction is associated with a revolution of upper revolving unit  3 . When left control lever  25 L is operated leftward, upper revolving unit  3  revolves leftward. When left control lever  25 L is operated rightward, upper revolving unit  3  revolves rightward. 
     According to the embodiment, operation apparatus  25  is a device of pilot hydraulic type. Hydraulic oil having a pressure reduced to a predetermined pilot pressure by pressure reducing valve  25 V is supplied from hydraulic pump  36  to operation apparatus  25  in accordance with a boom operation, a bucket operation, a dipper stick operation, and a revolving operation. 
     An operation of right control lever  25 R in the fore/aft direction allows supply of a pilot oil pressure to pilot oil path  450 . In this state, the operation of boom  6  is received from the operator. Hydraulic oil is supplied to pilot oil path  450  by opening of the valve device of right control lever  25 R in accordance with a manipulated variable of right control lever  25 R. 
     Pressure sensor  66  detects a pressure of hydraulic oil within pilot oil path  450  at the time of the supply of hydraulic oil as a pilot pressure. 
     Pressure sensor  66  designates the detected pilot pressure as a boom manipulated variable MB, and transmits boom manipulated variable MB to work implement controller  26 . A manipulated variable of right control lever  25 R in the fore/aft direction is hereinafter referred to as boom manipulated variable MB where appropriate. A control valve (hereinafter referred to as intervention valve where appropriate)  27 C, and a shuttle valve  51  are included in pilot oil path  50 . Intervention valve  27 C and shuttle valve  51  will be detailed below. 
     An operation of right control lever  25 R in the left/right direction allows supply of a pilot oil pressure to pilot oil path  450 . In this state, the operation of bucket  8  is received from the operator. Hydraulic oil is supplied to pilot oil path  450  by opening of the valve device of right control lever  25 R in accordance with a manipulated variable of right control lever  25 R. 
     Pressure sensor  66  detects a pressure of hydraulic oil within pilot oil path  450  at the time of the supply of hydraulic oil as a pilot pressure. Pressure sensor  66  designates the detected pilot pressure as a bucket manipulated variable MT, and transmits bucket manipulated variable MT to work implement controller  26 . A manipulated variable of right control lever  25 R in the left/right direction is hereinafter referred to as bucket manipulated variable MT where appropriate. 
     An operation of left control lever  25 L in the fore/aft direction allows supply of a pilot oil pressure to pilot oil path  450 . In this state, the operation of dipper stick  7  is received from the operator. Hydraulic oil is supplied to pilot oil path  450  by opening of a valve device of left control lever  25 L in accordance with a manipulated variable of left control lever  25 L. 
     Pressure sensor  66  detects a pressure of hydraulic oil within pilot oil path  450  at the time of the supply of hydraulic oil as a pilot pressure. Pressure sensor  66  designates the detected pilot pressure as an dipper stick manipulated variable MA, and transmits dipper stick manipulated variable MA to work implement controller  26 . A manipulated variable of left control lever  25 L in the fore/aft direction is hereinafter referred to as dipper stick manipulated variable MA where appropriate. 
     When right control lever  25 R is operated, operation apparatus  25  supplies to direction control valve  64  a pilot oil pressure at a level corresponding to a manipulated variable of right control lever  25 R. 
     When left control lever  25 L is operated, operation apparatus  25  supplies to direction control valve  64  a pilot oil pressure at a level corresponding to a manipulated variable of left control lever  25 L. Direction control valve  64  moves in accordance with a pilot oil pressure supplied from operation apparatus  25  to direction control valve  64 . 
     Control system  200  includes a first stroke sensor  16 , a second stroke sensor  17 , and a third stroke sensor  18 . For example, first stroke sensor  16  is included in boom cylinder  10 , second stroke sensor  17  is included in dipper stick cylinder  11 , and third stroke sensor  18  is included in bucket cylinder  12 . 
     Sensor controller  39  includes a storage unit such as a random access memory (RAM) and a read only memory (ROM), and a processing unit such as a central processing unit (CPU). 
     Sensor controller  39  calculates an inclination angle θ 1  of boom  6  with respect to a direction (z-axis direction) perpendicular to a horizontal plane (x-y plane) in a local coordinate system of hydraulic excavator  100 , more specifically, a local coordinate system of vehicular body  1 , based on a boom cylinder length LS 1  detected by first stroke sensor  16 , and outputs calculated inclination angle θ 1  to work implement controller  26  and display controller  28 . 
     Sensor controller  39  calculates an inclination angle θ 2  of dipper stick  7  with respect to boom  6  based on a dipper stick cylinder length LS 2  detected by second stroke sensor  17 , and outputs calculated inclination angle θ 2  to work implement controller  26  and display controller  28 . 
     Sensor controller  39  calculates an inclination angle θ 3  of cutting edges  8 T of bucket  8  with respect to dipper stick  7  based on a bucket cylinder length LS 3  detected by third stroke sensor  18 , and outputs calculated inclination angle θ 3  to work implement controller  26  and display controller  28 . 
     Inclination angles θ 1 , θ 2 , and θ 3  may be detected by methods other than the use of first stroke sensor  16 , second stroke sensor  17 , and third stroke sensor  18 . For example, an angle sensor such as a potentiometer may be used to detect inclination angles θ 1 , θ 2 , and θ 3 . 
     An inertial measurement unit (IMU)  24  is connected to sensor controller  39 . IMU  24  acquires information about inclination of the vehicular body such as a pitch around the y axis and a roll around the x axis of hydraulic excavator  100  illustrated in  FIG. 1 , and outputs the acquired information to sensor controller  39 . 
     Work implement controller  26  includes a storage unit  26 Q such as a RAM and a read only memory (ROM), and a processing unit  26 P such as a CPU. Work implement controller  26  controls intervention valve  27 C and control valve  27  based on boom manipulated variable MB, bucket manipulated variable MT, and dipper stick manipulated variable MA illustrated in  FIG. 2 . 
     Direction control valve  64  illustrated in  FIG. 2  is a proportional control valve, for example, and is controlled by hydraulic oil supplied from operation apparatus  25 . 
     Direction control valve  64  is disposed between the section of boom cylinder  10 , dipper stick cylinder  11 , bucket cylinder  12 , and a hydraulic actuator such as revolving motor  38 , and the section of hydraulic pumps  36  and  37 . 
     Direction control valve  64  controls flow rates and directions of hydraulic oil supplied from hydraulic pumps  36  and  37  to boom cylinder  10 , dipper stick cylinder  11 , bucket cylinder  12 , and revolving motor  38 . 
     Position detection device  19  contained in control system  200  includes GNSS antennas  21  and  22  described above. When GNSS antennas  21  and  22  receive a GNSS radio wave, a signal in the GNSS radio wave is input to global coordinate calculating unit  23 . 
     GNSS antenna  21  receives reference position data P 1  indicating a self-position from a positioning satellite. GNSS antenna  22  receives reference position data P 2  indicating a self-position from the positioning satellite. 
     GNSS antennas  21  and  22  receive reference position data P 1  and P 2  in a predetermined cycle. Each of reference position data P 1  and P 2  is information indicating the installation position of the corresponding GNSS antenna. GNSS antennas  21  and  22  output reference position data P 1  and P 2  to global coordinate calculating unit  23  every time GNSS antennas  21  and  22  receive these data P 1  and P 2 . 
     Global coordinate calculating unit  23  includes a storage unit such as a RAM and a ROM, and a processing unit such as a CPU. Global coordinate calculating unit  23  generates revolving unit position data indicating a position of upper revolving unit  3  based on two reference position data P 1  and P 2 . 
     According to the embodiment, the revolving unit position data includes reference position data P corresponding to one of two reference position data P 1  and P 2 , and revolving unit direction data Q generated based on two reference position data P 1  and P 2 . Revolving unit direction data Q indicates a direction in which work implement  2 , i.e., upper revolving unit  3 , faces. 
     Global coordinate calculating unit  23  updates reference position data P and revolving unit direction data Q each indicating revolving unit position data, and outputs the updated data to display controller  28  every time two reference position data P 1  and P 2  are acquired from GNSS antennas  21  and  22  in a predetermined cycle. 
     Display controller  28  includes a storage unit such as a RAM and a ROM, and a processing unit such as a CPU. Display controller  28  acquires reference position data P and revolving unit direction data Q each indicating revolving unit position data from global coordinate calculating unit  23 . 
     According to the embodiment, display controller  28  generates, as work implement position data, bucket cutting edge position data S indicating a three-dimensional position of cutting edges  8 T of bucket  8 . Display controller  28  subsequently generates target excavation topography data U based on bucket cutting edge position data S and target execution information T. 
     Target execution information T is information indicating a service object by work implement  2  included in hydraulic excavator  100 , or a finishing target of an excavation object according to the embodiment. Examples of target execution information T include design information about an execution object by hydraulic excavator  100 . Examples of a service object by work implement  2  include land. Examples of a service performed by work implement  2  include an excavation service and a land grading service. However, the service by work implement  2  is not limited to these examples. 
     Display controller  28  derives target excavation landform data Ua for display based on target excavation landform data U, and displays a target shape of a service object by work implement  2 , such as a landform, on display unit  29  based on target excavation landform data Ua for display. 
     Display unit  29  is a liquid crystal display apparatus that receives input via a touch panel, for example. However, display unit  29  is not limited to this type. According to the embodiment, a switch  29 S is provided adjacent to display unit  29 . Switch  29 S is an input device operated to perform intervention control described below, or stop the intervention control being performed. 
     Work implement controller  26  acquires boom manipulated variable MB, bucket manipulated variable NIT, and dipper stick manipulated variable MA from pressure sensor  66 . Work implement controller  26  acquires inclination angle θ 1  of boom  6 , inclination angle θ 2  of dipper stick  7 , and inclination angle θ 3  of bucket  8  from sensor controller  39 . 
     Work implement controller  26  acquires target excavation topography data U from display controller  28 . Target excavation topography data U is information included in target execution information T and indicating a range of a service that will be performed by hydraulic excavator  100 . 
     Target excavation topography data U is a part of target execution information T. Target excavation topography data U indicates a shape of a finishing target of a service object of work implement  2  similarly to target execution information T. The shape of the finishing target is hereinafter referred to as target excavation topography where appropriate. 
     Work implement controller  26  calculates a position of cutting edges  8 T of bucket  8  (hereinafter referred to as cutting edge position where appropriate) based on an angle of work implement  2  acquired from sensor controller  39 . 
     Work implement controller  26  controls a movement of work implement  2  based on a distance between target excavation topography data U and cutting edges  8 T of bucket  8 , and on a speed of work implement  2  such that cutting edges  8 T of bucket  8  can shift in accordance with target excavation topography data U. 
     Work implement controller  26  performs such control as to maintain a speed of work implement  2  in a direction of approach toward an execution object at a speed less than or equal to a speed limit to prevent bucket  8  from invading a target shape of a service object of work implement  2  indicated by target excavation topography data U. This control is referred to as intervention control where appropriate. 
     For example, the intervention control is performed when the operator of hydraulic excavator  100  selects performance of the intervention control by using switch  29 S illustrated in  FIG. 2 . When a distance between target excavation topography described below and bucket  8  is calculated, a reference position of bucket  8  is not limited to the position of cutting edges  8 T but may be other appropriate positions. 
     During the intervention control, work implement controller  26  generates a boom command signal CBI, and outputs generated boom command signal CBI to intervention valve  27 C illustrated in  FIG. 2  to control work implement  2  such that cutting edges  8 T of bucket  8  can shift in accordance with target excavation topography data U. 
     Boom  6  moves based on boom command signal CBI. A speed of work implement  2 , more specifically a speed of bucket  8 , is controlled by a movement of boom  6  based on boom command signal CBI. An approaching speed of bucket  8  toward target excavation topography data U is regulated in accordance with a distance between bucket  8  and target excavation topography data U. 
     &lt;Configuration of Hydraulic Circuit  301 &gt; 
       FIG. 3  is a diagram illustrating an example of hydraulic circuit  301  of boom cylinder  10  according to the embodiment. 
     Referring to  FIG. 3 , hydraulic circuit  301  includes pilot oil path  450  between operation apparatus  25  and direction control valve  64 . Direction control valve  64  is a valve for controlling a flow direction of hydraulic oil supplied to boom cylinder  10 . 
     According to the embodiment, direction control valve  64  is a spool valve that shifts a rod-shaped spool  64 S to switch a flow direction of hydraulic oil. 
     Spool  64 S is shifted by hydraulic oil supplied from operation apparatus  25  illustrated in  FIG. 2  (hereinafter referred to as pilot oil where appropriate). Direction control valve  64  supplies hydraulic oil to boom cylinder  10  by a shift of spool  64 S to move boom cylinder  10 . 
     Pilot oil path  50  and pilot oil path  450 B are connected to shuttle valve  51 . 
     Shuttle valve  51  and one end of direction control valve  64  are connected with each other via an oil path  452 B. The other end of direction control valve  64  and operation apparatus  25  are connected with each other via a pilot oil path  450 A and a pilot oil path  452 A. Pilot oil path  50  includes intervention valve  27 C. Intervention valve  27 C adjusts a pilot pressure of pilot oil path  50 . 
     Pilot oil path  450 B includes a pressure sensor  66 B and a control valve  27 B. Pilot oil path  450 A includes a pressure sensor  66 A provided between a control valve  27 A and operation apparatus  25 . A detection value obtained by pressure sensor  66  is acquired by work implement controller  26  illustrated in  FIG. 2 , and used for control of boom cylinder  10 . 
     Each of pressure sensor  66  and pressure sensor  66 B corresponds to pressure sensor  66  illustrated in  FIG. 2 . Each of control valve  27 A and control valve  27 B corresponds to control valve  27  illustrated in  FIG. 2 . 
     Hydraulic oil supplied from hydraulic pumps  36  and  37  is further supplied to boom cylinder  10  via direction control valve  64 . Supply of hydraulic oil is switched between supply to a cap side oil chamber  48 R of boom cylinder  10  and supply to a rod side oil chamber  47 R of boom cylinder  10  by a shift of spool  64 S in the axial direction. 
     A flow rate of hydraulic oil, i.e., a supply rate of hydraulic oil to boom cylinder  10  per unit time is adjusted by a shift of spool  64 S in the axial direction. A moving speed of boom cylinder  10  is adjusted by adjustment of the flow rate of hydraulic oil to boom cylinder  10 . 
     When spool  64 S of direction control valve  64  shifts in a first direction, hydraulic oil is supplied from direction control valve  64  to cap side oil chamber  48 R. When hydraulic oil is returned from rod side oil chamber  47 R to direction control valve  64 , a piston  10 P of boom cylinder  10  shifts from cap side oil chamber  48 R toward rod side oil chamber  47 R. As a result, a rod  10 L connected to piston  10 P extends from boom cylinder  10 . 
     When spool  64 S of direction control valve  64  shifts in a second direction opposite to a first direction based on a command from operation apparatus  25 , hydraulic oil is returned from cap side oil chamber  48 R to direction control valve  64 . When hydraulic oil is supplied from direction control valve  64  to rod side oil chamber  47 R, a piston  10 P of boom cylinder  10  shifts from rod side oil chamber  47 R to cap side oil chamber  48 R. As a result, rod  10 L connected to piston  10 P contracts into boom cylinder  10 . In this manner, a moving direction of boom cylinder  10  changes in accordance with adjustment of the shift direction of spool  64 S of direction control valve  64 . 
     The flow rate of hydraulic oil supplied to boom cylinder  10  and returned from boom cylinder  10  to direction control valve  64  changes in accordance with the adjustment of the shift amount of spool  64 S of direction control valve  64 . In this case, each shift speed of piston  10 P and rod  10 L corresponding to a moving speed of boom cylinder  10  changes accordingly. 
     As described above, a movement of direction control valve  64  is controlled by operation apparatus  25 . Hydraulic oil discharged from hydraulic pump  36  illustrated in  FIG. 2  and subjected to pressure reduction by pressure reducing valve  25 V is supplied to operation apparatus  25  as pilot oil. 
     Operation apparatus  25  adjusts the pilot oil pressure based on operations of the respective control levers. Direction control valve  64  is driven by the adjusted pilot oil pressure. The shift amount and shift direction of spool  64 S in the axial direction are adjusted by adjustment of the level and direction of the pilot oil pressure by operation apparatus  25 . Accordingly, the moving speed and moving direction of boom cylinder  10  are allowed to change. 
     As described above, work implement controller  26  during the intervention control regulates a speed of boom  6  based on target excavation topography (target excavation topography data U) that indicates design topography corresponding to a target shape of an excavation object, and on inclination angles θ 1 , θ 2 , and θ 3  used for obtaining a position of bucket  8 , such that an approaching speed of bucket  8  toward target excavation topography  43 I decreases in accordance with a distance between target excavation topography  43 I and bucket  8 . 
     According to the embodiment, work implement controller  26  generates boom command signal CBI and controls a movement of boom  6  based on generated boom command signal CBI to prevent invasion of target excavation topography  43 I by cutting edges  8 T of bucket  8  when work implement  2  moves based on an operation from operation apparatus  25 . 
     More specifically, work implement controller  26  raises or lowers boom  6  to prevent invasion of target excavation topography  43 I by cutting edges  8 T during the intervention control. The control for raising or lowering boom  6  performed during the intervention control is referred to as boom intervention control where appropriate. 
     According to the embodiment, work implement controller  26  generates a boom command signal CBI indicating the boom intervention control, and outputs generated boom command signal CBI to intervention valve  27 C or a control valve  27 A to achieve the boom intervention control. 
     Intervention valve  27 C is capable of adjusting a pilot oil pressure of pilot oil path  50 . Shuttle valve  51  includes two inlet ports  51 Ia and  51 Ib, and one outlet port  51 E. Inlet port  51 Ia provided as one of the inlet ports is connected to intervention valve  27 C. Inlet port  51 Ib provided as the other inlet port is connected to control valve  27 B. Outlet port  51 E is connected to oil path  452 B connected to direction control valve  64 . 
     Shuttle valve  51  connects oil path  452 B and the inlet port having a higher pilot oil pressure in two inlet ports  51 Ia and  51 Ib. 
     When the pilot oil pressure of inlet port  51 Ia is higher than the pilot oil pressure of inlet port  51 Ib, for example, shuttle valve  51  connects intervention valve  27 C and oil path  452 B. As a result, the pilot oil having passed through intervention valve  27 C is supplied to oil path  452 B via shuttle valve  51 . When the pilot oil pressure of inlet port  51 Ib is higher than the pilot oil pressure of inlet port  51 Ia, shuttle valve  51  connects control valve  27 B with oil path  452 B. As a result, the pilot oil having passed through control valve  27 B is supplied to oil path  452 B via shuttle valve  51 . 
     During a stop of the boom intervention control, direction control valve  64  is driven based on a pilot oil pressure adjusted by an operation from operation apparatus  25 . For example, work implement controller  26  opens (full-opens) pilot oil path  450 B by controlling control valve  27 B, and closes pilot oil path  50  by controlling intervention valve  27 C to drive direction control valve  64  based on a pilot oil pressure adjusted by an operation from operation apparatus  25 . 
     When performing the boom intervention control, work implement controller  26  controls control valve  27  to drive direction control valve  64  based on a pilot oil pressure adjusted by intervention valve  27 C. For example, when performing control for regulating a shift of bucket  8  toward target excavation topography  43 I as the boom intervention control, work implement controller  26  controls intervention valve  27 C to raise a pilot oil pressure of pilot oil path  50  adjusted by intervention valve  27 C to a pressure higher than a pilot oil pressure of pilot oil path  450 B adjusted by operation apparatus  25 . In this manner, pilot oil from intervention valve  27 C is supplied to direction control valve  64  via shuttle valve  51 . 
     When performing the boom intervention control, work implement controller  26  generates boom command signal CBI as a speed command for raising or lowering boom  6  to control intervention valve  27 C or control valve  27 A, for example. 
     More specifically, hydraulic oil is supplied to boom cylinder  10  under control of intervention valve  27 C to raise boom  6  at a speed corresponding to boom command signal CBI. In addition, hydraulic oil is supplied to boom cylinder  10  under control of control valve  27 A to lower boom  6  at a speed corresponding to boom command signal CBI. In this manner, direction control valve  64  of boom cylinder  10  supplies sufficient hydraulic oil to boom cylinder  10  to raise or lower boom  6  at a speed corresponding to boom command signal CBI. Accordingly, boom cylinder  10  is allowed to raise or lower boom  6 . 
     Each of the hydraulic circuit of dipper stick cylinder  11  and the hydraulic circuit of bucket cylinder  12  has a configuration similar to the configuration of hydraulic circuit  301  of boom cylinder  10  described above, except that intervention valve  27 C, shuttle valve  51 , and pilot oil path  50  are eliminated. 
     According to the embodiment, the intervention control is defined as control performed by work implement controller  26  to move at least one of boom  6 , dipper stick  7 , and bucket  8  constituting work implement  2  when work implement  2  moves based on an operation from operation apparatus  25 . 
     The intervention control is control performed by work implement controller  26  to achieve movement of the work implement when work implement  2  moves based on a manual operation corresponding to an operation from operation apparatus  25 . The boom intervention control described above is a mode of the intervention control. 
       FIG. 4  is a block diagram illustrating work implement controller  26  according to the embodiment. 
       FIG. 5  is a chart illustrating target excavation topography data U and bucket  8  according to the embodiment. 
       FIG. 6  is a diagram illustrating a boom speed limit Vcy_bm according to the embodiment. 
       FIG. 7  is a chart illustrating a speed limit Vc_lmt according to the embodiment. 
     Work implement controller  26  includes a decision unit  26 J and a control unit  26 CNT. Control unit  26 CNT includes a relative position calculation unit  26 A, a distance calculation unit  26 B, a target speed calculation unit  26 C, an intervention speed calculation unit  26 D, an intervention command calculation unit  26 E, and an intervention speed correction unit  26 F. 
     Functions of decision unit  26 J, relative position calculation unit  26 A, distance calculation unit  26 B, target speed calculation unit  26 C, intervention speed calculation unit  26 D, intervention command calculation unit  26 E, and intervention speed correction unit  26 F are performed by processing unit  26 P of work implement controller  26  illustrated in  FIG. 2 . 
     For performing the intervention control, work implement controller  26  generates boom command signal CBI necessary for the intervention control based on boom manipulated variable MB, dipper stick manipulated variable MA, bucket manipulated variable MT, target excavation topography data U and bucket cutting edge position data S acquired from display controller  28 , and inclination angles θ 1 , θ 2 , and θ 3  acquired from sensor controller  39 , and generates a dipper stick command signal and a bucket command signal as necessary to control work implement  2  by driving control valve  27  and intervention valve  27 C based on the generated command signal. 
     Relative position calculation unit  26 A acquires bucket cutting edge position data S from display controller  28 , and acquires inclination angles θ 1 , θ 2 , and θ 3  from sensor controller  39 . Relative position calculation unit  26 A obtains a cutting edge position Pb indicating a position of cutting edges  8 T of bucket  8  based on acquired inclination angles θ 1 , θ 2 , and θ 3 . 
     Distance calculation unit  26 B calculates a distance d indicating a minimum distance between cutting edges  8 T of bucket  8  and target excavation topography  43 I expressed by target excavation topography data U as a part of target execution information T based on cutting edge position Pb obtained by relative position calculation unit  26 A and target excavation topography data U acquired from display controller  28 . Distance d is a distance between cutting edge position Pb, and a position Pu corresponding to an intersection of target excavation topography data U and a line crossing target excavation topography  43 I at right angles and passing through cutting edge position Pb. 
     Target excavation topography  43 I is obtained as a line of intersection formed by a plane of work implement  2  defined in the fore/aft direction of upper revolving unit  3  and passing through an excavation target position Pdg, and target execution information T expressed by a plurality of target execution surfaces. 
     More specifically, target excavation topography  43 I is the line of intersection described above, and formed by a single or a plurality of inflection points fore and after excavation target position Pdg of target execution information T, and lines fore and after the inflection points. 
     According to an example illustrated in  FIG. 5 , target excavation topography  43 I is formed by two inflection points Pv 1  and Pv 2 , and lines fore and after inflection points Pv 1  and Pv 2 . Excavation target position Pdg is a point located directly below cutting edge position Pb corresponding to the position of cutting edges  8 T of bucket  8 . Accordingly, target excavation topography  43 I is a part of target execution information T. Target excavation topography  43 I is generated by display controller  28  illustrated in  FIG. 2 . 
     Target speed calculation unit  26 C determines a boom target speed Vc_bm, a dipper stick target speed Vc_am, and a bucket target speed Vc_bkt. Boom target speed Vc_bm is a speed of cutting edges  8 T during driving of boom cylinder  10 . Dipper stick target speed Vc_am is a speed of cutting edges  8 T during driving of dipper stick cylinder  11 . Bucket target speed Vc_bkt is a speed of cutting edges  8 T during driving of bucket cylinder  12 . Boom target speed Vc_bm is calculated based on boom manipulated variable MB. Dipper stick target speed Vc_am is calculated based on dipper stick manipulated variable MA. Bucket target speed Vc_bkt is calculated based on bucket manipulated variable MT. 
     Intervention speed calculation unit  26 D obtains speed limit (boom speed limit) Vcy_bm of boom  6  based on distance d between cutting edges  8 T of bucket  8  and target excavation topography  43 I. 
     Referring to  FIG. 6 , intervention speed calculation unit  26 D calculates boom speed limit Vcy_bm by subtracting dipper stick target speed Vc_am and bucket target speed Vc_bkt from speed limit Vc_lmt indicating the overall speed limit of work implement  2  illustrated in  FIG. 1 . 
     Speed limit Vc_lmt is an allowable shift speed of cutting edges  8 T in the direction of approach of cutting edges  8 T of bucket  8  toward target excavation topography  43 I. 
     Referring to  FIG. 7 , work implement  2  has a negative value and lowers when distance d is a positive value. In this case, speed limit Vc_lmt is a lowering speed of work implement  2 . On the other hand, work implement  2  has a positive value and rises when distance d is a negative value. In this case, limiting speed Vc_lmt is a rising speed of work implement  2 . 
     A negative value of distance d indicates an invaded state of target excavation topography  43 I by bucket  8 . The absolute value of speed limit Vc_lmt decreases as distance d decreases. After a change of distance d to a negative value, the absolute value of the speed increases as the absolute value of distance d increases. 
     Decision unit  26 J decides whether to correct boom speed limit Vcy_bm. 
     When decision unit  26 J decides to correct boom speed limit Vcy_bm, intervention speed correction unit  26 F corrects boom speed limit Vcy_bm, and outputs corrected boom speed limit Vcy_bm. The corrected boom speed limit is expressed as Vcy_bm′. 
     When decision unit  26 J decides not to correct boom speed limit Vcy_bm, intervention speed correction unit  26 F outputs boom speed limit Vcy_bm without correction. Intervention command calculation unit  26 E generates boom command signal CBI based on boom speed limit Vcy_bm obtained by intervention speed correction unit  26 F. 
     Boom command signal CBI is a command for setting an opening of intervention valve  27 C to a degree sufficient for providing a pilot pressure for shuttle valve  51  to raise boom  6  at boom speed limit Vcy_bm. According to the embodiment, boom command signal CBI is a current value corresponding to the boom command speed. 
     &lt;Mode of Boom Intervention Control&gt; 
       FIG. 8  is a view illustrating an example of a relationship between bucket  8  and target excavation topography  43 I according to the embodiment. 
     Referring to  FIG. 8 , the intervention control is control for shifting bucket  8  to prevent invasion of target excavation topography  43 I by bucket  8 . 
     According to the present embodiment, land grading is achieved by a shift of bucket  8  along target excavation topography  43 I in a direction indicated by an arrow Y. 
     More specifically, dipper stick  7  performs dumping in accordance with an operation command input from the operator to operation apparatus  25 . 
     Work implement controller  26  calculates a damping shift amount of dipper stick  7  based on dipper stick manipulated variable MA, and controls lowering of boom  6  such that bucket  8  can shift along target excavation topography  43 I in accordance with the calculated dumping shift amount. 
       FIG. 9  is another view illustrating the relationship between bucket  8  and target excavation topography  43 I according to the embodiment. 
     Referring to  FIG. 9 , dipper stick  7  dumps by a shift of bucket  8  in the direction of arrow Y from the state illustrated in  FIG. 8 . With continuation of dumping by dipper stick  7 , dipper stick cylinder  11  may arrive at a position close to the stroke end of dipper stick cylinder  11 . 
     In general, dipper stick cylinder  11  has such a characteristic that a cylinder speed may change at a position close to the stroke end. 
     The change of the cylinder speed may affect accuracy of land grading. Accordingly, the intervention control is canceled, and switched to control for stopping the work implement when dipper stick cylinder  11  arrives at the position close to the stroke end. 
     As a result, boom  6  stops in response to switching to the control for stopping the work implement, whereby the speed of boom  6  comes to zero. 
     When a large speed change is produced at the time of the stop of boom  6 , a greater shock may be applied to boom  6 . In this case, the operator may have a sense of discomfort that lowers service efficiency of land grading. 
       FIG. 10  is a chart illustrating a boom speed during boom intervention control for land, grading according to the embodiment. 
       FIG. 10  shows a boom speed Vbm of a movement of boom  6  with respect to time t. 
     A positive value of boom speed Vbm indicates a rising speed of boom  6  in a rising state, while a negative value of boom speed Vbm indicates a lowering speed of boom  6  in a lowering state. 
     Boom  6  is provided as a part of work implement  2 , wherefore boom speed Vbm is equivalent to a speed of work implement  2 . The rising speed of boom  6  corresponds to a rising speed of work implement  2 , while the lowering speed of boom  6  corresponds to a lowering speed of work implement  2 . 
     According to the embodiment, each of the rising speed and the lowering speed of work implement  2  is referred to as a shift speed of work implement  2 . The shift speed of work implement  2  during rising has a positive value, while the shift speed of work implement  2  during lowering has a negative value. 
     According to the present embodiment presented by way of example, boom speed Vbm is set to a predetermined boom speed limit Vcy_bm during lowering of boom  6 . 
     In this case, dipper stick cylinder  11  arrives at a position close to the stroke end at a time t 0 . The intervention control is canceled at this time. The intervention control thereby switches to control for stopping the work implement. 
     According to the present embodiment, the intervention control is canceled in response to an arrival of dipper stick cylinder  11  at the position close to the stroke end, and boom speed Vbm is set to zero. 
     In this case, a large speed change of the boom speed is produced at the time of canceling of the intervention control. Accordingly, a shock may be caused along with the speed change of the boom speed. 
     According to the embodiment, reduction of the boom speed is initiated to stop boom  6  before cancellation (stop) of the intervention control. 
     More specifically, there is provided a limiting table that specifies a speed limit of the boom speed in accordance with a cylinder length of the dipper stick cylinder. 
       FIG. 11  is a chart illustrating the limiting table for the boom speed according to the embodiment. 
       FIG. 11  illustrates increase in reduction of the boom speed with nearness to the stroke end of dipper stick cylinder  11 . 
     According to the present embodiment, the speed limit of the boom speed is set to a lower limit β when dipper stick cylinder  11  enters a range of a predetermined distance α from the stroke end. Thereafter, the boom speed changes from boom speed lower limit β at a rate of predetermined deceleration with nearness to the stroke end of dipper stick cylinder  11 . 
     With reference to the limiting table, the boom speed is allowed to change at the rate of predetermined deceleration before cancellation (stop) of the intervention control. 
     Accordingly, reduction of a rapid speed change of the boom speed, and therefore reduction of a shock applied to boom  6  along with the speed change are achievable. 
     Note that the predetermined deceleration defined by the limiting table may be varied to any values depending on the characteristics of hydraulic excavator  100 . 
     Work implement controller  26  starts limiting the boom speed with reference to the limiting table before cancellation (stop) of the intervention control. 
     The intervention control is canceled (stopped) when dipper stick cylinder  11  arrives at the position close to the stroke end. The position close to the stroke end corresponds to an area around the stroke end. Whether or not dipper stick cylinder  11  has arrived at the position close to the stroke end may be calculated based on dipper stick cylinder length LS 2  detected by second stroke sensor  17 . Similarly, whether or not dipper stick cylinder  11  has entered the range of predetermined distance α from the stroke end may be calculated based on dipper stick cylinder length LS 2  detected by second stroke sensor  17 . 
     According to the embodiment, work implement controller  26  limits the boom speed with reference to the limiting table when determining that dipper stick cylinder  11  has entered the range of predetermined distance a from the stroke end based on dipper stick cylinder length LS 2  detected by second stroke sensor  17 . 
     When a large speed change of boom  6  is produced at the time of canceling (stopping) of the intervention control, the operator has a sense of discomfort caused by a rapid speed reduction of boom  6 . 
     According to the present embodiment, work implement controller  26  limits the boom speed with reference to the limiting table such that the boom speed gradually approaches zero when determining that dipper stick cylinder  11  has entered the range of predetermined distance α from the stroke end based on dipper stick cylinder length LS 2  detected by second stroke sensor  17 . 
     This change reduces a rapid speed decrease of boom  6 , wherefore discomfort given to the operator decreases. Moreover, a shock caused by rapid speed reduction of boom  6  decreases. 
     More specifically, intervention speed calculation unit  26 D of the work implement controller illustrated in  FIG. 4  obtains boom speed limit Vcy_bm. 
     Subsequently, decision unit  26 J of work implement controller  26  illustrated in  FIG. 4  performs decision. 
     Decision unit  26 J determines whether or not dipper stick cylinder  11  has entered the range of predetermined distance a from the stroke end based on dipper stick cylinder length LS 2  detected by second stroke sensor  17 . 
     When determining that dipper stick cylinder  11  has entered the range of predetermined distance α from the stroke end, decision unit  26 J decides to correct boom speed limit Vcy_bm, and instructs intervention speed correction unit  26 F to correct boom speed limit Vcy_bm. 
     Intervention speed correction unit  26 F of control unit  26 CNT obtains corrected boom speed limit Vcy_bm′, and outputs boom speed limit Vcy_bm′ to intervention command calculation unit  26 E of control unit  26 CNT. More specifically, intervention speed correction unit  26 F obtains corrected boom speed limit Vcy_bm′ based on the limiting table. 
     Intervention command calculation unit  26 E of control unit  26 CNT generates boom command signal CBI based on corrected boom speed limit Vcy_bm′ to control intervention valve  27 C. Work implement controller  26  changes the lowering speed of boom  6  by performing these processes. 
     More specifically, intervention speed correction unit  26 F performs control such that boom speed limit Vcy_bm finally becomes zero at the rate of predetermined deceleration. 
     On the other hand, when determining that dipper stick cylinder  11  is out of the range of predetermined distance a from the stroke end, decision unit  26 J decides not to correct boom speed limit Vcy_bm. Intervention speed correction unit  26 F outputs boom speed limit Vcy_bm to intervention command calculation unit  26 E without correction. In this case, boom command signal CBI is generated based on boom speed limit Vcy_bm to control intervention valve  27 C. 
     According to the present embodiment described herein, intervention speed correction unit  26 F obtains boom speed limit Vcy_bm′ corrected with reference to the limiting table to limit the speed of boom  6 . Alternatively, boom command signal CBI output from intervention command calculation unit  26 E may be corrected instead of correction into boom speed limit Vcy_bm′. More specifically, the speed of boom  6  may be reduced by limiting a current value corresponding to a boom command speed output from intervention command calculation unit  26 E 
     Control Method for Work Machine of Embodiment 
       FIG. 12  is a chart illustrating a flow of a control method for the work machine according to the embodiment. 
     Referring to  FIG. 12 , the control method for the work machine according to the embodiment is performed by work implement controller  26 . 
     In step S 2 , decision unit  26 J of work implement controller  26  illustrated in  FIG. 4  determines whether or not dipper stick cylinder  11  has entered the range of predetermined distance α from the stroke end. More specifically, decision unit  26 J determines whether or not dipper stick cylinder  11  has entered the range of predetermined distance a from the stroke end based on dipper stick cylinder length LS 2  detected by second stroke sensor  17 . 
     When decision unit  26 J determines in step S 2  that dipper stick cylinder  11  is out of the range of predetermined distance α from the stroke end (NO in step S 2 ), intervention command calculation unit  26 E of work implement controller  26  in step S 16  generates boom command signal CBI based on boom speed limit Vcy_bm not subjected to correction to control intervention valve  27 C or control valve  27 A. 
     On the other hand, when decision unit  26 J determines in step S 2  that dipper stick cylinder  11  has entered the range of predetermined distance a from the stroke end (YES in step S 2 ), boom command signal CBI is generated based on the corrected boom speed limit to control intervention valve  27 C or control valve  27 A (step S 8 ). More specifically, intervention speed correction unit  26 F obtains corrected boom speed limit Vcy_bin′ based on the limiting table. Intervention command calculation unit  26 E generates boom command signal CBI based on corrected boom speed limit Vcy_bm′, and controls intervention valve  27 C or control valve  27 A. Work implement controller  26  changes the lowering speed of boom  6  by performing these processes. Thereafter, the process ends (END). 
     &lt;Electric Control Lever&gt; 
     According to the embodiment, operation apparatus  25  includes pilot hydraulic control levers. However, operation apparatus  25  may include an electric left control lever  25 La and an electric right control lever  25 Ra. 
     When each of left control lever  25 La and right control lever  25 Ra is constituted by an electric lever, a manipulated variable input by each control lever is detected by a potentiometer. The manipulated variable input by each of left control lever  25 La and right control lever  25 Ra and detected by the potentiometer is acquired by work implement controller  26 . 
     Work implement controller  26  having detected an operation signal of the electric control lever performs control similar to the corresponding control performed by using the pilot hydraulic control lever. 
     According to the embodiment described above, work implement controller  26  limits the boom speed based on the limiting table when determining that dipper stick cylinder  11  has entered the range of predetermined distance α from the stroke end based on dipper stick cylinder length LS 2  detected by second stroke sensor  17 . 
     Work implement  2  includes boom  6 , dipper stick  7 , and bucket  8 . However, the attachment of work implement  2  is not limited to them, and other types of attachment than bucket  8  may be employed. The work machine is only required to include a certain work implement. The work implement included in the work machine is not limited to hydraulic excavator  100 . 
     The embodiment disclosed herein is presented by way of example, and therefore is not limited to the specific details described herein. It is intended that the scope of the present invention is defined only by the appended claims, and therefore includes all changes made within meanings and ranges equivalent to the scope of the appended claims. 
     REFERENCE SIGNS LIST 
       1 : vehicular body,  2 : work implement,  3 : upper revolving unit,  4 : operator&#39;s cab,  5 : traveling apparatus,  6 : boom,  7 : dipper stick,  8 : bucket,  10 : boom cylinder,  11 : dipper stick cylinder,  12 : bucket cylinder,  13  boom pin,  14 : dipper stick pin,  15 : bucket pin,  16 : first stroke sensor,  17 : second stroke sensor,  18 : third stroke sensor,  19 : position detection device,  26 : work implement controller,  26 A: relative position calculation unit,  26 B: distance calculation unit,  26 C: target speed calculation unit,  26 CNT: control unit,  26 D: intervention speed calculation unit,  26 E: intervention command calculation unit,  26 F: intervention speed correction unit,  26 J: decision unit,  26 P: processing unit,  26 Q: storage unit.