Patent Publication Number: US-9834905-B2

Title: Work machine control device, work machine, and work machine control method

Description:
FIELD 
     The present invention relates to a work machine control device, a work machine, and a work machine control method. 
     BACKGROUND 
     In a technical field related with a work machine such as an excavator, as disclosed in Patent Literature 1, there is known a work machine that controls a working implement so that a blade tip of a bucket moves along a target excavating topography (a design surface) indicating a target shape of an excavation target. 
     In the specification, a control for causing the blade tip of the bucket of the working implement to move along the target excavating topography will be referred to as a leveling assist control. In the leveling assist control, a target blade tip speed of the bucket is determined from a distance between the target excavating topography and the current blade tip position of the bucket, and the determined target blade tip speed is added to the blade tip speed counteracting the blade tip speed of the bucket in response to at least one of the arm operation amount and the bucket operation amount by the operator. Then, a target boom speed is calculated from the added value. Further, the target boom speed is corrected (compensated by integration) by using a correction amount obtained by the integration in time of the distance between the target excavating topography and the past blade tip position of the bucket, and the boom cylinder is controlled based on the target boom speed compensated by integration. In the leveling assist control using the compensation by integration, the boom cylinder is controlled so that the boom is raised when the blade tip of the bucket digs the target excavating topography. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: WO 2014/167718 A 
     SUMMARY 
     Technical Problem 
     In the excavator, a time delay exists in the responsiveness of a hydraulic cylinder with respect to a control signal for controlling the hydraulic cylinder due to a delay in the responsiveness of hydraulic pressure or hysteresis generated when driving a hydraulic driving unit. Particularly, a delay in the responsiveness of the hydraulic cylinder noticeably occurs when the hydraulic cylinder is operated from an acceleration state to a deceleration state. For that reason, when the ratio of the correction amount using the compensation by integration is large, overcompensation occurs. As a result, a phenomenon occurs in which the blade tip of the bucket is excessively separated from the target excavating topography. 
     For example, in a case where the boom is raised by the leveling assist control in which the blade tip of the bucket returns to the target excavating topography from the state where the blade tip of the bucket digs the target excavating topography, when the time in which the blade tip of the bucket exceeds the target excavating topography is long, the correction amount is excessively large when the blade tip of the bucket returns to the target excavating topography. Thus, when the boom is operated from an acceleration state to a deceleration state, the target boom speed is not decreased and the boom is excessively raised. Accordingly, a phenomenon occurs in which the blade tip of the bucket is excessively raised from the target excavating topography. As a result, a portion which is not excavated by the working implement is generated, and hence the leveling operation is performed in a state different from the target excavating topography. 
     An aspect of the invention is to provide a work machine control device, a work machine, and a work machine control method capable of suppressing degradation in excavating precision by preventing the blade tip from being raised until the blade tip of the bucket returns to the target excavating topography from the state where the blade tip digs the target excavating topography in the leveling assist control. 
     Solution to Problem 
     According to a first aspect of the present invention, a work machine control device for a work machine including a working implement with a boom, an arm, and a bucket, comprises: a distance acquiring unit which acquires distance data between the bucket and a target excavating topography; a target blade tip speed determining unit which determines a target blade tip speed of the bucket based on the distance data; an operation amount acquiring unit which acquires an operation amount for operating the working implement; a target boom speed calculating unit which calculates a target boom speed based on the target blade tip speed and at least one of an arm operation amount and a bucket operation amount acquired by the operation amount acquiring unit; a correction amount calculating unit which calculates a correction amount of the target boom speed based on an integration in time of a distance between the bucket and the target excavating topography; a correction amount limiting unit which limits the correction amount based on the distance between the bucket and the target excavating topography; and a working implement control unit which outputs an instruction for driving a boom cylinder driving the boom based on the target boom speed corrected by the correction amount. 
     According to a second aspect of the present invention, a work machine comprises: a working implement which includes a boom, an arm, and a bucket; a boom cylinder which drives the boom; an arm cylinder which drives the arm; a bucket cylinder which drives the bucket; an upper swing body which supports the working implement; a lower traveling body which supports the upper swing body; and a control device, wherein the control device includes a distance acquiring unit which acquires distance data between the bucket and a target excavating topography, a target blade tip speed determining unit which determines a target blade tip speed of the bucket based on the distance data, an operation amount acquiring unit which acquires an operation amount for operating the working implement, a target boom speed calculating unit which calculates a target boom speed based on the target blade tip speed and at least one of an arm operation amount and a bucket operation amount acquired by the operation amount acquiring unit, a correction amount calculating unit which calculates a correction amount of the target boom speed based on an integration in time of a distance between the bucket and the target excavating topography, a correction amount limiting unit which limits the correction amount based on the distance between the bucket and the target excavating topography, and a working implement control unit which outputs an instruction for driving the boom cylinder based on the target boom speed corrected by the correction amount. 
     According to a third aspect of the present invention, a method of controlling a work machine including a working implement with a boom, an arm, and a bucket, comprises: acquiring distance data between the bucket and a target excavating topography; determining a target blade tip speed of the bucket based on the distance data; calculating a target boom speed based on the target blade tip speed and at least one of an arm operation amount and a bucket operation amount; calculating a correction amount of the target boom speed based on an integration in time of a distance between the bucket and the target excavating topography; limiting the correction amount based on the distance between the bucket and the target excavating topography; and outputting an instruction for driving a boom cylinder driving the boom based on the target boom speed corrected by the correction amount. 
     Advantageous Effects of Invention 
     According to the aspect of the invention, it is possible to provide a work machine control device, a work machine, and a work machine control method capable of suppressing degradation in excavating precision by preventing the blade tip from being raised until the blade tip of the bucket returns to the target excavating topography from the state where the blade tip digs the target excavating topography in the leveling assist control. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view illustrating an example of an excavator according to an embodiment. 
         FIG. 2  is a schematic side view illustrating an example of the excavator according to the embodiment. 
         FIG. 3  is a schematic rear view illustrating an example of the excavator according to the embodiment. 
         FIG. 4  is a schematic view illustrating a leveling assist control according to the embodiment. 
         FIG. 5  is a schematic view illustrating an example of a hydraulic system according to the embodiment. 
         FIG. 6  is a schematic view illustrating an example of the hydraulic system according to the embodiment. 
         FIG. 7  is a functional block diagram illustrating an example of a control system according to the embodiment. 
         FIG. 8  is a schematic view illustrating a process of a target excavating topography data generating unit according to the embodiment. 
         FIG. 9  is a diagram illustrating a relation between a distance and a target blade tip speed according to the embodiment. 
         FIG. 10  is a flowchart illustrating an example of an excavator control method according to the embodiment. 
         FIG. 11  is a control block diagram illustrating an example of the control system according to the embodiment. 
         FIG. 12  is a diagram illustrating a state where a distance and a correction amount change in a comparative example. 
         FIG. 13  is a diagram illustrating a state where a distance and a correction amount change according to the embodiment. 
         FIG. 14  is a diagram illustrating a relation between an offset amount and a detection value of a pressure sensor according to the embodiment. 
         FIG. 15  is a diagram illustrating an example of an operation device according to the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments according to the invention will be described with reference to the drawings, but the invention is not limited thereto. The components of the embodiments to be described below can be appropriately combined with one another. Further, there is a case where a part of the components are not used. 
     [Work Machine] 
       FIG. 1  is a perspective view illustrating an example of a work machine  100  according to an embodiment. In the embodiment, an example will be described in which the work machine  100  is an excavator. In the description below, the work machine  100  will be appropriately referred to as the excavator  100 . 
     As illustrated in  FIG. 1 , the excavator  100  includes a working implement  1  which is operated by a hydraulic pressure, a vehicle body  2  which supports the working implement  1 , a traveling device  3  which supports the vehicle body  2 , an operation device  40  which is used to operate the working implement  1 , and a control device  50  which controls the working implement  1 . The vehicle body  2  is able to swing about a swing axis RX while being supported by the traveling device  3 . The vehicle body  2  is disposed on the traveling device  3 . In the description below, the vehicle body  2  will be appropriately referred to as the upper swing body  2 , and the traveling device  3  will be appropriately referred to as the lower traveling body  3 . 
     The upper swing body  2  includes a cab  4  which is occupied by an operator, a machine room  5  which accommodates an engine or a hydraulic pump, and a handrail  6 . The cab  4  includes a driver seat  4 S on which the operator sits. The machine room  5  is disposed in rear of the cab  4 . The handrail  6  is disposed in front of the machine room  5 . 
     The lower traveling body  3  includes a pair of crawlers  7 . By the rotation of the crawlers  7 , the excavator  100  travels. In addition, the lower traveling body  3  may be vehicle wheels (tires). 
     The working implement  1  is supported by the upper swing body  2 . The working implement  1  includes a bucket  11  having a blade tip  10 , an arm  12  connected to the bucket  11 , and a boom  13  connected to the arm  12 . The blade tip  10  of the bucket  11  may be a protruding blade tip provided in the bucket  11 . The blade tip  10  of the bucket  11  may be a straight blade tip provided in the bucket  11 . 
     The bucket  11  and the arm  12  are connected to each other through a bucket pin. The bucket  11  is supported by the arm  12  so as to be rotatable about the rotation axis AX 1 . The arm  12  and the boom  13  are connected to each other through an arm pin. The arm  12  is supported by the boom  13  so as to be rotatable about the rotation axis AX 2 . The boom  13  and the upper swing body  2  are connected to each other through a boom pin. The boom  13  is supported by the vehicle body  2  so as to be rotatable about the rotation axis AX 3 . 
     The rotation axis AX 1 , the rotation axis AX 2 , and the rotation axis AX 3  are parallel to one another. The rotation axes AX 1 , AX 2 , and AX 3  are orthogonal to an axis parallel to the swing axis RX. In the description below, the axial direction of each of the rotation axes AX 1 , AX 2 , and AX 3  will be appropriately referred to as the vehicle width direction of the upper swing body  2 , and the direction orthogonal to the rotation axes AX 1 , AX 2 , and AX 3  and the swing axis RX will be appropriately referred to as the front to back direction of the upper swing body  2 . A direction in which the working implement  1  exists based on the operator sitting on the driver seat  4 S will be set as the front direction. 
     In addition, the bucket  11  may be a tilt bucket. The tilt bucket is a bucket which is able to be tilted in the vehicle width direction by the operation of the bucket tilt cylinder. When the excavator  100  is operated in a slope, it is possible to freely mold and level a slope or an even ground by tilting the bucket  11  in the vehicle width direction. 
     The operation device  40  is disposed in the cab  4 . The operation device  40  includes an operation member that is operated by the operator of the excavator  100 . The operation member includes an operation lever or a joystick. By the operation of the operation member, the working implement  1  is operated. 
     The control device  50  includes a computer system. The control device  50  includes a processor such as a CPU (Central Processing Unit), a storage device such as a ROM (Read Only Memory) or a RAM (Random Access Memory), and an input/output interface device. 
       FIG. 2  is a schematic side view illustrating the excavator  100  according to the embodiment.  FIG. 3  is a schematic rear view illustrating the excavator  100  according to the embodiment. 
     As illustrated in  FIGS. 1 and 2 , the excavator  100  includes a hydraulic cylinder  20  which drives the working implement  1 . The hydraulic cylinder  20  is driven by hydraulic oil. The hydraulic cylinder  20  includes a bucket cylinder  21  which drives the bucket  11 , an arm cylinder  22  which drives the arm  12 , and a boom cylinder  23  which drives the boom  13 . 
     As illustrated in  FIG. 2 , the excavator  100  includes a bucket cylinder stroke sensor  14  disposed in the bucket cylinder  21 , an arm cylinder stroke sensor  15  disposed in the arm cylinder  22 , and a boom cylinder stroke sensor  16  disposed in the boom cylinder  23 . The bucket cylinder stroke sensor  14  detects the bucket cylinder length as the stroke length of the bucket cylinder  21 . The arm cylinder stroke sensor  15  detects the arm cylinder length as the stroke length of the arm cylinder  22 . The boom cylinder stroke sensor  16  detects the boom cylinder length as the stroke length of the boom cylinder  23 . 
     As illustrated in  FIGS. 2 and 3 , the excavator  100  includes a position detector  30  which detects the position of the upper swing body  2 . The position detector  30  includes a vehicle body position detector  31  which detects the position of the upper swing body  2  defined by a global coordinate system, a posture detector  32  which detects the posture of the upper swing body  2 , and a direction detector  33  which detects the direction of the upper swing body  2 . 
     The global coordinate system (the XgYgZg coordinate system) is a coordinate system that indicates an absolute position defined by a GPS (Global Positioning System). The local coordinate system (the XYZ coordinate system) is a coordinate system that indicates a relative position based on the reference position Ps of the upper swing body  2  of the excavator  100 . The reference position Ps of the upper swing body  2  is set in, for example, the swing axis RX of the upper swing body  2 . In addition, the reference position Ps of the upper swing body  2  may be set in the rotation axis AX 3 . By the position detector  30 , the three-dimensional position of the upper swing body  2  defined by the global coordinate system, the inclination angle of the upper swing body  2  with respect to the horizontal plane, and the direction of the upper swing body  2  with respect to the reference direction are detected. 
     The vehicle body position detector  31  includes a GPS receiver. The vehicle body position detector  31  detects the three-dimensional position of the upper swing body  2  defined by the global coordinate system. The vehicle body position detector  31  detects the Xg-direction position, the Yg-direction position, and the Zg-direction position of the upper swing body  2 . 
     The upper swing body  2  is provided with a plurality of GPS antennas  31 A. The GPS antennas  31 A are provided in the handrail  6  of the upper swing body  2 . In addition, the GPS antenna  31 A may be disposed on the counter weight disposed in rear of the machine room  5 . The GPS antennas  31 A receive radio waves from a GPS satellite and output signals based on the received radio waves to the vehicle body position detector  31 . The vehicle body position detector  31  detects the installation positions P 1  of the GPS antennas  31 A defined by the global coordinate system based on the signals supplied from the GPS antennas  31 A. The vehicle body position detector  31  detects the absolute position Pg of the upper swing body  2  based on the installation positions P 1  of the GPS antennas  31 A. 
     Two GPS antennas  31 A are provided in the vehicle width direction. The vehicle body position detector  31  detects the installation position Pla of one GPS antenna  31 A and the installation position Plb of the other GPS antenna  31 A. The vehicle body position detector  31 A detects the absolute position Pg and the direction of the upper swing body  2  by performing a calculation process based on the installation position P 1   a  and the installation position P 1   b . In the embodiment, the absolute position Pg of the upper swing body  2  is the installation position P 1   a . In addition, the absolute position Pg of the upper swing body  2  may be also the installation position P 1   b . 
     The posture detector  32  includes an IMU (Inertial Measurement Unit). The posture detector  32  is provided in the upper swing body  2 . The posture detector  32  is disposed at the lower portion of the cab  4 . The posture detector  32  detects the inclination angle of the upper swing body  2  with respect to the horizontal plane (the XgYg plane). The inclination angle of the upper swing body  2  with respect to the horizontal plane includes the inclination angle θa of the upper swing body  2  in the vehicle width direction and the inclination angle θb of the upper swing body  2  in the front to back direction. 
     The direction detector  33  has a function of detecting the direction of the upper swing body  2  in the reference direction defined by the global coordinate system based on the installation position P 1   a  of one GPS antenna  31 A and the installation position P 1   b  of the other GPS antenna  31 A. The reference direction indicates, for example, north. The direction detector  33  detects the direction of the upper swing body  2  with respect to the reference direction by performing a calculation process based on the installation position P 1   a  and the installation position P 1   b . The direction detector  33  calculates a line connecting the installation position P 1   a  and the installation position P 1   b , and detects the direction of the upper swing body  2  with respect to the reference direction based on the angle formed between the calculate line and the reference direction. 
     In addition, the direction detector  33  may be separated from the position detector  30 . The direction detector  33  may detect the direction of the upper swing body  2  by using a magnetic sensor. 
     The excavator  100  includes a blade tip position detector  34  which detects the relative position of the blade tip  10  with respect to the reference position Ps of the upper swing body  2 . 
     In the embodiment, the blade tip position detector  34  calculates the relative position of the blade tip  10  with respect to the reference position Ps of the upper swing body  2  based on the detection result of the bucket cylinder stroke sensor  14 , the detection result of the arm cylinder stroke sensor  15 , the detection result of the boom cylinder stroke sensor  16 , the length L 11  of the bucket  11 , the length L 12  of the arm  12 , and the length L 13  of the boom  13 . 
     The blade tip position detector  34  calculates the inclination angle θ 11  of the blade tip  10  of the bucket  11  with respect to the arm  12  based on the bucket cylinder length detected by the bucket cylinder stroke sensor  14 . The blade tip position detector  34  detects the inclination angle θ 12  of the arm  12  with respect to the boom  13  based on the arm cylinder length detected by the arm cylinder stroke sensor  15 . The blade tip position detector  34  calculates the inclination angle θ 13  of the boom  13  with respect to the Z axis of the upper swing body  2  based on the boom cylinder length detected by the boom cylinder stroke sensor  16 . 
     The length L 11  of the bucket  11  is a distance between the blade tip  10  of the bucket  11  and the rotation axis AX 1  (the bucket pin). The length L 12  of the arm  12  is a distance between the rotation axis AX 1  (the bucket pin) and the rotation axis AX 2  (the arm pin). The length L 13  of the boom  13  is a distance between the rotation axis AX 2  (the arm pin) and the rotation axis AX 3  (the boom pin). 
     The blade tip position detector  34  calculates the relative position of the blade tip  10  with respect to the reference position Ps of the upper swing body  2  based on the inclination angle θ 11 , the inclination angle θ 12 , the inclination angle θ 13 , the length L 11 , the length L 12 , and the length L 13 . 
     Further, the blade tip position detector  34  calculates the absolute position Pb of the blade tip  10  based on the absolute position Pg of the upper swing body  2  detected by the position detector  30  and the relative position between the reference position Ps of the upper swing body  2  and the blade tip  10 . The relative position between the absolute position Pg and the reference position Ps is given data derived from the specification data of the excavator  100 . Thus, the blade tip position detector  34  can calculate the absolute position Pb of the blade tip  10  based on the absolute position Pg of the upper swing body  2 , the relative position between the reference position Ps of the upper swing body  2  and the blade tip  10 , and the specification data of the excavator  100 . 
     In addition, the blade tip position detector  34  may include an angle sensor such as a potentiometer and an angle meter. The angle sensor may be used to detect the inclination angle θ 11  of the bucket  11 , the inclination angle θ 12  of the arm  12 , and the inclination angle θ 13  of the boom  13 . 
     [Leveling Assist Control] 
       FIG. 4  is a schematic view illustrating the operation of the excavator  100  according to the embodiment. In the embodiment, the control device  50  performs a leveling assist control on the working implement  1  so that the blade tip  10  of the bucket  11  moves along the target excavating topography (the design surface) indicating the target shape of the excavation target. The control device  50  performs a leveling assist control on the working implement  1  by, for example, a PI control (proportional-integral control). 
     By the operation of the operation device  40 , the dumping operation of the bucket  11 , the excavating operation of the bucket  11 , the dumping operation of the arm  12 , the excavating operation of the arm  12 , the raising operation of the boom  13 , and the lowering operation of the boom  13  are performed. 
     In the embodiment, the operation device  40  includes a right operation lever disposed at the right side of the operator sitting on the driver seat  4 S and a left operation lever disposed at the left side thereof. When the right operation lever is operated in the front to back direction, the lowering operation and the raising operation of the boom  13  are performed. When the right operation lever is operated in the left and right direction (the vehicle width direction), the excavating operation and the dumping operation of the bucket  11  are performed. When the left operation lever is operated in the front to back direction, the dumping operation and the excavating operation of the arm  12  are performed. When the left operation lever is operated in the left and right direction, the upper swing body  2  swings left and right. In addition, when the left operation lever is operated in the front to back direction, the upper swing body  2  may swing right and left. Then when the left operation lever is operated in the left and right direction, the arm  12  may perform the dumping operation and the excavating operation. 
     In the leveling assist control, the bucket  11  and the arm  12  are driven based on the operation of the operation device  40  by the operator. The boom  13  is driven based on at least one of the operator&#39;s operation of the operation device  40  and the control of the control device  50 . 
     As illustrated in  FIG. 4 , when the excavation target is excavated, the bucket  11  and the arm  12  are used to perform the excavating operation. The control device  50  performs a control related with the movement of the boom  10  so that the blade tip  10  of the bucket  11  moves along the target excavating topography while the bucket  11  and the arm  12  are used for the excavating operation by the operation of the operation device  40 . In the example illustrated in  FIG. 4 , the control device  50  controls the boom cylinder  23  so that the boom  13  is raised while the bucket  11  and the arm  12  are used for the excavating operation. 
     [Hydraulic System] 
     Next, an example of a hydraulic system  300  according to the embodiment will be described. The hydraulic cylinder  20  including the bucket cylinder  21 , the arm cylinder  22 , and the boom cylinder  23  is operated by the hydraulic system  300 . The hydraulic cylinder  20  is operated by the operation device  40 . 
     In the embodiment, the operation device  40  is a pilot hydraulic operation device. In the description below, the oil supplied to the hydraulic cylinder  20  in order to operate the hydraulic cylinder  20  (the bucket cylinder  21 , the arm cylinder  22 , and the boom cylinder  23 ) will be appropriately referred to as the hydraulic oil. The hydraulic oil supply amount with respect to the hydraulic cylinder  20  is adjusted by a direction control valve  41 . The direction control valve  41  is operated by the supplied oil. In the description below, the oil supplied to the direction control valve  41  in order to operate the direction control valve  41  will be appropriately referred to as the pilot oil. Further, the pressure of the pilot oil will be appropriately referred to as the pilot hydraulic pressure. 
       FIG. 5  is a schematic view illustrating an example of the hydraulic system  300  operated by the arm cylinder  22 . By the operation of the operation device  40 , the arm  12  performs two kinds of operations, the excavating operation and the dumping operation. When the arm cylinder  22  is lengthened, the arm  12  performs the excavating operation. Then, when the arm cylinder  22  is shortened, the arm  12  performs the dumping operation. 
     The hydraulic system  300  includes a variable displacement main hydraulic pump  42  which supplies the hydraulic oil to the arm cylinder  22  through the direction control valve  41 , a pilot hydraulic pump  43  which supplies the pilot oil, the operation device  40  which adjusts the pilot hydraulic pressure with respect to the direction control valve  41 , oil passages  44 A and  44 B through which the pilot oil flows, pressure sensors  46 A and  46 B respectively disposed in the oil passages  44 A and  44 B, and the control device  50 . The main hydraulic pump  42  is driven by a motor such as an engine (not illustrated). 
     The direction control valve  41  controls the hydraulic oil flow direction. The hydraulic oil supplied from the main hydraulic pump  42  is supplied to the arm cylinder  22  through the direction control valve  41 . The direction control valve  41  is of a spool type that changes the hydraulic oil flow direction by moving a rod-shaped spool. When the spool moves in the axial direction, the supply of the hydraulic oil with respect to a cap side oil chamber  20 A (an oil passage  47 A) of the arm cylinder  22  and the supply of the hydraulic oil with respect to a rod side oil chamber  20 B (an oil passage  47 B) thereof are switched. In addition, the cap side oil chamber  20 A is a space which is formed between a cylinder head cover and a piston. The rod side oil chamber  20 B is a space in which a piston rod is disposed. Further, when the spool moves in the axial direction, the hydraulic oil supply amount (the supply amount per unit time) with respect to the arm cylinder  22  is adjusted. When the hydraulic oil supply amount with respect to the arm cylinder  22  is adjusted, the cylinder speed is adjusted. 
     The direction control valve  41  is operated by the operation device  40 . The pilot oil fed from the pilot hydraulic pump  43  is supplied to the operation device  40 . In addition, the pilot oil which is fed from the main hydraulic pump  42  and is decreased in pressure by a pressure reduction valve may be supplied to the operation device  40 . The operation device  40  includes a pilot hydraulic pressure adjusting valve. Based on the operation amount of the operation device  40 , the pilot hydraulic pressure is adjusted. By the pilot hydraulic pressure, the direction control valve  41  is driven. When the pilot hydraulic pressure is adjusted by the operation device  40 , the movement amount and the movement speed of the spool in the axial direction are adjusted. 
     The direction control valve  41  includes a first pressure receiving chamber and a second pressure receiving chamber. When the spool is driven by the pilot hydraulic pressure of the oil passage  44 A, the first pressure receiving chamber is connected to the main hydraulic pump  42  so that the hydraulic oil is supplied to the first pressure receiving chamber. When the spool is driven by the pilot hydraulic pressure of the oil passage  44 B, the second pressure receiving chamber is connected to the main hydraulic pump  42  so that the hydraulic oil is supplied to the second pressure receiving chamber. 
     The pressure sensor  46 A detects the pilot hydraulic pressure of the oil passage  44 A. The pressure sensor  46 B detects the pilot hydraulic pressure of the oil passage  44 B. The detection signals of the pressure sensors  46 A and  46 B are output to the control device  50 . 
     When the operation lever of the operation device  40  is operated toward one side in relation to the neutral position, the pilot hydraulic pressure set in response to the operation amount of the operation lever acts on the first pressure receiving chamber of the spool of the direction control valve  41 . When the operation lever of the operation device  40  is operated toward the other side in relation to the neutral position, the pilot hydraulic pressure set in response to the operation amount of the operation lever acts on the second pressure receiving chamber of the spool of the direction control valve  41 . 
     The spool of the direction control valve  41  moves by the distance set in response to the pilot hydraulic pressure adjusted by the operation device  40 . For example, when the pilot hydraulic pressure acts on the first pressure receiving chamber, the hydraulic oil is supplied from the main hydraulic pump  42  to the cap side oil chamber  20 A of the arm cylinder  22  so that the arm cylinder  22  is lengthened. When the arm cylinder  22  is lengthened, the arm  12  performs the excavating operation. When the pilot hydraulic pressure acts on the second pressure receiving chamber, the hydraulic oil is supplied from the main hydraulic pump  42  into the rod side oil chamber  20 B of the arm cylinder  22  so that the arm cylinder  22  is shortened. When the arm cylinder  22  is shortened, the arm  12  performs the dumping operation. Based on the movement amount of the spool of the direction control valve  41 , the hydraulic oil supply amount per unit time supplied from the main hydraulic pump  42  to the arm cylinder  22  through the direction control valve  41  is adjusted. When the hydraulic oil supply amount per unit time is adjusted, the cylinder speed is adjusted. 
     The hydraulic system  300  that operates the bucket cylinder  21  has the same configuration as the hydraulic system  300  that operates the arm cylinder  22 . By the operation of the operation device  40 , the bucket  11  performs two kinds of operations, the excavating operation and the dumping operation. When the bucket cylinder  21  is lengthened, the bucket  11  performs the excavating operation. When the bucket cylinder  21  is shortened, the bucket  11  performs the dumping operation. The detailed description of the hydraulic system  300  operating the bucket cylinder  21  will be omitted. 
       FIG. 6  is a schematic view illustrating an example of the hydraulic system  300  operating the boom cylinder  23 . By the operation of the operation device  40 , the boom  13  performs two kinds of operations, the raising operation and the lowering operation. The direction control valve  41  includes a first pressure receiving chamber and a second pressure receiving chamber. When the spool is driven by the pilot hydraulic pressure of the oil passage  44 A, the first pressure receiving chamber is connected to the main hydraulic pump  42  so that the hydraulic oil is supplied to the first pressure receiving chamber. When the spool is driven by the pilot hydraulic pressure of the oil passage  44 B, the second pressure receiving chamber is connected to the main hydraulic pump  42  so that the hydraulic oil is supplied to the second pressure receiving chamber. The hydraulic oil supplied from the main hydraulic pump  42  is supplied to the boom cylinder  23  through the direction control valve  41 . When the spool of the direction control valve  41  moves in the axial direction, the supply of the hydraulic oil with respect to the cap side oil chamber  20 A (the oil passage  47 B) of the boom cylinder  23  and the supply of the hydraulic oil with respect to the rod side oil chamber  20 B (the oil passage  47 A) thereof are switched. When the hydraulic oil is supplied to the first pressure receiving chamber, the hydraulic oil is supplied to the rod side oil chamber  20 B through the oil passage  47 A so that the boom cylinder  13  is shortened and the boom  13  is lowered. When the hydraulic oil is supplied to the second pressure receiving chamber, the hydraulic oil is supplied to the cap side oil chamber  20 A through the oil passage  47 B so that the boom cylinder  13  is lengthened and the boom  13  is raised. 
     As illustrated in  FIG. 6 , the hydraulic system  300  operating the boom cylinder  23  includes the main hydraulic pump  42 , the pilot hydraulic pump  43 , the direction control valve  41 , the operation device  40  adjusting the pilot hydraulic pressure for the direction control valve  41 , the oil passages  44 A,  44 B, and  44 C causing the pilot oil to flow therethrough, control valves  45 A,  45 B, and  45 C disposed in the oil passages  44 A,  44 B, and  44 C, the pressure sensors  46 A and  46 B disposed in the oil passages  44 A,  44 B, and  44 C, and the control device  50  controlling the control valves  45 A,  45 B, and  45 C. 
     The control valves  45 A,  45 B, and  45 C are electromagnetic proportional control valves. The control valves  45 A,  45 B, and  45 C adjust the pilot hydraulic pressure based on the instruction signal from the control device  50 . The control valve  45 A adjusts the pilot hydraulic pressure of the oil passage  44 A. The control valve  45 B adjusts the pilot hydraulic pressure of the oil passage  44 B. The control valve  45 C adjusts the pilot hydraulic pressure of the oil passage  44 C. 
     As described above by referring to  FIG. 5 , the pilot hydraulic pressure set in response to the operation amount of the operation device  40  acts on the direction control valve  41  by the operation of the operation device  40 . The spool of the direction control valve  41  moves in response to the pilot hydraulic pressure. Based on the movement amount of the spool, the hydraulic oil supply amount per unit time supplied from the main hydraulic pump  42  to the boom cylinder  23  through the direction control valve  41  is adjusted. 
     The control device  50  can decrease the pilot hydraulic pressure acting on the first pressure receiving chamber by controlling the control valve  45 A. The control device  50  can decrease the pilot hydraulic pressure acting on the second pressure receiving chamber by controlling the control valve  45 B. In the example illustrated in  FIG. 6 , when the pilot hydraulic pressure adjusted by the operation of the operation device  40  is decreased by the control valve  45 A, the pilot oil supplied to the direction control valve  41  is limited. When the pilot hydraulic pressure acting on the direction control valve  41  is decreased by the control valve  45 A, the lowering operation of the boom  13  is limited. Similarly, when the pilot hydraulic pressure adjusted by the operation of the operation device  40  is decreased by the control valve  45 B, the pilot oil supplied to the direction control valve  41  is limited. When the pilot hydraulic pressure acting on the direction control valve  41  is decreased by the control valve  45 B, the raising operation of the boom  13  is limited. The control device  50  controls the control valve  45 A based on the detection signal of the pressure sensor  46 A. The control device  50  controls the control valve  45 B based on the detection signal of the pressure sensor  46 B. 
     In the embodiment, the oil passage  44 C is provided with the control valve  45 C which is operated based on the instruction signal related with the leveling assist control and output from the control device  50  for the leveling assist control. The pilot oil fed from the pilot hydraulic pump  43  flows in the oil passage  44 C. The oil passage  44 C and the oil passage  44 B are connected to a shuttle valve  48 . The shuttle valve  48  supplies the pilot oil of the oil passage having a higher pilot hydraulic pressure among the oil passage  44 B and the oil passage  44 C to the direction control valve  41 . 
     The control valve  45 C is controlled based on the instruction signal output from the control device  50  for the leveling assist control. 
     The control device  50  does not output the instruction signal to the control valve  45 C so that the direction control valve  41  is driven based on the pilot hydraulic pressure adjusted by the operation of the operation device  40  when the leveling assist control is not performed. For example, the control device  50  fully opens the control valve  45 B and closes the oil passage  44 C by the control valve  45 C so that the direction control valve  41  is driven based on the pilot hydraulic pressure adjusted by the operation of the operation device  40 . 
     When the leveling assist control is performed, the control device  50  controls the control valves  45 B and  45 C so that the direction control valve  41  is driven based on the pilot hydraulic pressure adjusted by the control valve  45 C. For example, when the leveling assist control of limiting the movement of the boom  13  is performed, the control device  50  controls the control valve  45 C so as to realize the pilot hydraulic pressure in response to the target boom speed. For example, the control device  50  controls the control valve  45 C so that the pilot hydraulic pressure adjusted by the control valve  45 C becomes higher than the pilot hydraulic pressure adjusted by the operation device  40 . When the pilot hydraulic pressure of the oil passage  44 C becomes higher than the pilot hydraulic pressure of the oil passage  44 B, the pilot oil is supplied from the control valve  45 C to the direction control valve  41  through the shuttle valve  48 . 
     When the pilot oil is supplied to the direction control valve  41  through at least one of the oil passage  44 B and the oil passage  44 C, the hydraulic oil is supplied to the cap side oil chamber  20 A through the oil passage  47 B. Accordingly, the boom cylinder  23  is lengthened so that the boom  13  is raised. 
     When the raising operation amount of the boom  13  caused by the operation device  40  is large so that the target excavating topography is not dug by the blade tip  10  of the bucket  11 , the leveling assist control is not performed. When the operation device  40  is operated so that the boom  13  is raised at a speed faster than the target boom speed and the pilot hydraulic pressure is adjusted based on the operation amount, the pilot hydraulic pressure adjusted by the operation of the operation device  40  becomes higher than the pilot hydraulic pressure adjusted by the control valve  45 C. Accordingly, the pilot oil of the pilot hydraulic pressure adjusted by the operation of the control valve  45 C of the control device  50  is selected by the shuttle valve  48  and is supplied to the direction control valve  41 . Further, when the pilot hydraulic pressure set based on the instruction from the control device  50  to be described later to the control valve  45 C is lower than the pilot hydraulic pressure based on the boom operation amount, the pilot oil adjusted by the operation of the operation device  40  is selected by the shuttle valve  48  and the boom  13  is operated. 
     [Control System] 
     Next, a control system  200  of the excavator  100  according to the embodiment will be described.  FIG. 7  is a functional block diagram illustrating an example of the control system  200  according to the embodiment. 
     As illustrated in  FIG. 7 , the control system  200  includes the control device  50  controlling the working implement  1 , the position detector  30 , the blade tip position detector  34 , the operation device  40 , the control valve  45  ( 45 A,  45 B, and  45 C), a pressure sensor  46  ( 46 A and  46 B), and a target construction data generating device  70 . 
     As described above, the position detector  30  including the vehicle body position detector  31 , the posture detector  32 , and the direction detector  33  detects the absolute position Pg of the upper swing body  2 . In the description below, the absolute position Pg of the upper swing body  2  will be appropriately referred to as the vehicle body position Pg. 
     The control valve  45  ( 45 A,  45 B, and  45 C) adjusts the hydraulic oil supply amount with respect to the hydraulic cylinder  20 . The control valve  45  is operated based on the instruction signal from the control device  50 . The pressure sensor  46  ( 46 A and  46 B) detects the pilot hydraulic pressure of an oil passage  44  ( 44 A and  44 B). The detection signal of the pressure sensor  46  is output to the control device  50 . 
     The target construction data generating device  70  includes a computer system. The target construction data generating device  70  generates target construction data indicating a three-dimensional design topography as the target shape of the construction area. The target construction data indicates the three-dimensional target shape obtained after the construction by the working implement  1 . The target construction data includes coordinate data and angle data necessary for generating the target excavating topography data. 
     The target construction data generating device  70  is provided in, for example, a remote place separated from the excavator  100 . The target construction data generating device  70  is provided in, for example, a construction management facility. A radio communication can be allowed between the target construction data generating device  70  and the control device  50 . The target construction data generated by the target construction data generating device  70  is wirelessly transmitted to the control device  50 . 
     In addition, the target construction data generating device  70  and the control device  50  may be connected via a wire so that the target construction data is transmitted from the target construction data generating device  70  to the control device  50 . In addition, the target construction data generating device  70  may include a storage medium storing the target construction data and the control device  50  may include a device capable of reading the target construction data from the storage medium. 
     The control device  50  includes a vehicle body position data acquiring unit  51  which acquires vehicle body position data indicating the vehicle body position Pg of the upper swing body  2  supporting the working implement  1 , a blade tip position data acquiring unit  52  which acquires blade tip position data indicating the relative position of the blade tip  10  of the bucket  11  with respect to the reference position Ps of the upper swing body  2  in the local coordinate system, a target excavating topography data generating unit  53  which generates target excavating topography data indicating the target shape of the excavation target, a distance acquiring unit  54  which acquires distance data indicating the distance between the target excavating topography and the blade tip position of the bucket  11 , a target blade tip speed determining unit  55  which determines the target blade tip speed of the bucket  11  based on the distance data, an operation amount acquiring unit  56  which acquires the operation amount for operating the working implement  1 , a target boom speed calculating unit  57  which calculates a target boom speed based on the target blade tip speed and at least one of the arm operation amount and the bucket operation amount acquired by the operation amount acquiring unit  56 , a correction amount calculating unit  58  which calculates a correction amount of the target boom speed based on the integration in time of the distance between the blade tip position and the target excavating topography, a correction amount limiting unit  59  which limits the correction amount based on the distance between the blade tip position and the target excavating topography, a working implement control unit  60  which controls the boom cylinder  23  driving the boom  13  based on the target boom speed corrected by the correction amount, a storage unit  61  which stores the specification data of the excavator  100 , and an input/output unit  62 . 
     The processor of the control device  50  includes the vehicle body position data acquiring unit  51 , the blade tip position data acquiring unit  52 , the target excavating topography data generating unit  53 , the distance acquiring unit  54 , the target blade tip speed determining unit  55 , the operation amount acquiring unit  56 , the target boom speed calculating unit  57 , the correction amount calculating unit  58 , the correction amount limiting unit  59 , and the working implement control unit  60 . The storage device of the control device  50  includes the storage unit  61 . The input/output interface device of the control device  50  includes the input/output unit  62 . 
     The vehicle body position data acquiring unit  51  acquires the vehicle body position data indicating the vehicle body position Pg from the position detector  30  through the input/output unit  62 . The vehicle body position Pg is a current absolute position defined by the global coordinate system. The vehicle body position detector  31  detects the vehicle body position Pg based on at least one of the installation position Pla and the installation position Plb of the GPS antenna  31 . The vehicle body position data acquiring unit  51  acquires the vehicle body position data indicating the vehicle body position Pg from the vehicle body position detector  31 . 
     The blade tip position data acquiring unit  52  acquires the blade tip position data indicating the blade tip position from the blade tip position detector  34  through the input/output unit  56 . The blade tip position is a current relative position defined by the local coordinate system. The blade tip position data acquiring unit  52  acquires the blade tip position data indicating the blade tip position as the relative position of the blade tip  10  with respect to the reference position Ps of the upper swing body  2  from the blade tip position detector  34 . In addition, the blade tip position detector  34  can calculate the current absolute position Pb of the blade tip  10  based on the vehicle body position Pg of the upper swing body  2 , the relative position between the reference position Ps of the upper swing body  2  and the blade tip  10 , and the specification data of the excavator  100 . The blade tip position data acquired by the blade tip position data acquiring unit  52  from the blade tip position detector  32  may include the current absolute position Pb of the blade tip  10 . 
     The target excavating topography data generating unit  53  generates the target excavating topography data indicating the target shape of the excavation target corresponding to the blade tip position by using the target construction data and the blade tip position data supplied from the target construction data generating device  70 . The target excavating topography data generating unit  53  generates the target excavating topography data in the local coordinate system. 
       FIG. 8  is a diagram illustrating a relation between the target excavating topography data and the target construction data indicating the three-dimensional design topography. As illustrated in  FIG. 8 , the target excavating topography data generating unit  53  acquires the intersection line E between the three-dimensional design topography and the work machine operation plane MP of the working implement  1  defined in the front to back direction of the upper swing body  2  as the candidate line of the target excavating topography based on the target construction data and the blade tip position data. The target excavating topography data generating unit  53  sets the direct lower point of the blade tip  10  in the candidate line of the target excavating topography as the reference point AP of the target excavating topography. The control device  50  determines a single inflection point and a plurality of inflection points before and after the reference point AP of the target excavating topography and the front and rear lines thereof as the target excavating topography as the excavation target. The target excavating topography data generating unit  53  generates the target excavating topography data indicating the design topography as the target shape of the excavation target. 
     In  FIG. 7 , the distance acquiring unit  54  calculates the distance d between the blade tip position Pb and the target excavating topography based on the blade tip position acquired by the blade tip position data acquiring unit  52  and the target excavating topography generated by the target excavating topography data generating unit  53 . 
     In addition, in the embodiment, the blade tip position Pb is used as the control target. However, the distance between the arbitrary point of the bucket  11  including the outer periphery of the bucket  11  and the target excavating topography may be set as the distance d between the bucket  11  and the target excavating topography by the use of the outer shape dimension of the bucket  11 . 
     The target blade tip speed determining unit  55  determines the target blade tip speed of the bucket  11  based on the distance d between the blade tip position Pb and the target excavating topography. 
       FIG. 9  is a diagram illustrating an example of a relation between the distance d and the target blade tip speed. In the graph illustrated in  FIG. 9 , the horizontal axis indicates the distance d, and the vertical axis indicates the target blade tip speed. In  FIG. 9 , the distance d has a positive value when the surface of the target excavating topography is not invaded by the blade tip  10 . The distance d has a negative value when the surface of the target excavating topography is invaded by the blade tip  10 . The non-invasion state in which the surface of the target excavating topography is not invaded by the blade tip  10  indicates a state where the blade tip  10  exists outside (above) the surface of the target excavating topography. In other words, the blade tip exists at a position not exceeding the target excavating topography. The invasion state in which the surface of the target excavating topography is invaded by the blade tip  10  indicates a state where the blade tip  10  exists inside (below) the surface of the target excavating topography. In other words, the blade tip exists at a position exceeding the target excavating topography. In the non-invasion state, the blade tip  10  is raised from the target excavating topography. In the invasion state, the target excavating topography is dug by the blade tip  10 . The distance d is zero when the blade tip  10  matches the surface of the target excavating topography. 
     In the embodiment, the speed at which the blade tip  10  is directed from the inside of the target excavating topography toward the outside thereof is set to a positive value, and the speed at which the blade tip  10  is directed from the outside of the target excavating topography toward the inside thereof is set to a negative value. That is, the speed at which the blade tip  10  is directed toward the upside of the target excavating topography is set to a positive value, and the speed at which the blade tip  10  is directed toward the downside of the target excavating topography is set to a negative value. 
     As illustrated in  FIG. 9 , the target blade tip speed determining unit  55  determines whether the target blade tip speed is positive or negative so that the blade tip  10  matches the target excavating topography. Further, the target blade tip speed determining unit  55  determines the target blade tip speed so that the absolute value of the target blade tip speed increases as the distance d increases and the absolute value of the target blade tip speed decreases as the distance d decreases. 
     In  FIG. 7 , the operation amount acquiring unit  56  acquires the operation amount of the operation device  40 . The operation amount of the operation device  40  is correlated with the pilot hydraulic pressure of the oil passages  44 A and  44 B. The pilot hydraulic pressure of the oil passages  44 A and  44 B is detected by the pressure sensors  46 A and  46 B. The correlation data indicating the correlation between the operation amount of the operation device  40  and the pilot hydraulic pressure of the oil passages  44 A and  44 B is obtained in advance by a preliminary test or a simulation and is stored in the storage unit  61 . The operation amount acquiring unit  56  acquires the operation amount data indicating the operation amount of the operation device  40  from the detection signals (PPC pressure) from the pressure sensors  46 A and  46 B based on the detection signals of the pressure sensors  46 A and  46 B and the correlation data stored in the storage unit  61 . The operation amount acquiring unit  56  acquires the bucket operation amount of the operation device  40  for operating the bucket  11 , the arm operation amount of the operation device  40  for operating the arm  12 , and the boom operation amount of the operation device  40  for operating the boom  13 . 
     The target boom speed calculating unit  57  calculates the target boom speed based on the target blade tip speed determined by the target blade tip speed determining unit  55  and at least one of the arm operation amount and the bucket operation amount acquired by the operation amount acquiring unit  56 . In the leveling assist control, the movement of the bucket  11  and the movement of the arm  12  are set based on the operation of the operation device  40  by the operator. In the leveling assist control, the movement of the boom  10  is controlled by the control device  50  so that the blade tip  10  of the bucket  11  moves along the target excavating topography while the bucket  11  and the arm  12  are operated through the operation device  40 . The target boom speed calculating unit  55  calculates the blade tip speed when the bucket  11  is operated from the bucket operation amount for operating the bucket  11  by the operation device  40  and calculates the target boom speed counteracting the blade tip speed based on the movement of the bucket  11  so as to offset a deviation between the blade tip  10  and the target excavating topography during the operation of the bucket  11 . Similarly, the target boom speed calculating unit  55  calculates the blade tip speed when the arm  12  is operated from the arm operation amount for operating the arm  12  by the operation device  40  and calculates the target boom speed counteracting the blade tip speed based on the movement of the arm  12  so as to offset a deviation between the blade tip  10  and the target excavating topography during the operation of the arm  12 . Since the target boom speed is calculated based on the target blade tip speed and at least one of the arm operation amount and the bucket operation amount of the operation device  40  and the movement of the boom  13  is controlled at the target boom speed, the blade tip  10  and the target excavating topography can be close to each other. 
     The correction amount calculating unit  58  calculates the correction amount of the target boom speed based on the integration in time of the distance d between the blade tip position Pb and the target excavating topography. The correction amount calculating unit  58  calculates the correction amount based on the integration in time of the distance d from a predetermined past time point to a current time point and compensates the target boom speed by integration. 
     The correction amount is calculated based on the integration in time of the distance d when the blade tip  10  is separated from the target excavating topography. Since the target boom speed is compensated by integration based on the distance d when the target excavating topography is dug by the blade tip  10 , the boom  13  can be driven so that the distance d becomes zero from the state where the target design topography is dug. 
     The correction amount limiting unit  59  limits the correction amount calculated by the correction amount calculating unit  58  so that the speed is not overcompensated based on the distance d between the blade tip position Pb and the target excavating topography. The correction amount limiting unit  59  calculates the upper limit of the correction amount based on the distance d. In the embodiment, the correction amount limiting unit  59  calculates the upper limit of the correction amount based on the target blade tip speed determined from the distance d. 
     The working implement control unit  60  controls the boom cylinder  23  so that the boom  13  is driven based on the target boom speed corrected by the correction amount. The working implement control unit  60  compares the correction amount calculated by the correction amount calculating unit  58  with the upper limit calculated by the correction amount limiting unit  59  and determines the instruction signal output to the control valve  45 C based on the upper limit when the correction amount calculated by the correction amount calculating unit  58  is larger than the upper limit calculated by the correction amount limiting unit  59 . The working implement control unit  60  controls the boom cylinder  23  by outputting the instruction signal to the control valve  45 C and controls the boom cylinder  23  based on the correction amount when the correction amount is equal to or smaller than the upper limit. 
     [Excavator Control Method] 
     Next, a method of controlling the excavator  100  according to the embodiment will be described with reference to  FIGS. 10 and 11 .  FIG. 10  is a flowchart illustrating a method of controlling the excavator  100  according to the embodiment.  FIG. 11  is a control block diagram of the excavator  100  according to the embodiment. 
     The target construction data is supplied from the target construction data generating device  70  to the control device  50 . The target excavating topography data generating unit  53  generates the target excavating topography data by using the target construction data supplied from the target construction data generating device  70  (step SP 1 ). 
     The blade tip position data is supplied from the blade tip position detector  34  to the control device  50 . The blade tip position data acquiring unit  52  acquires the blade tip position data from the blade tip position detector  34  (step SP 2 ). 
     The distance acquiring unit  54  calculates the distance d between the blade tip position and the target excavating topography based on the target excavating topography generated by the target excavating topography data generating unit  53  and the blade tip position data acquired by the blade tip position data acquiring unit  52  (step SP 3 ). Accordingly, the distance data between the blade tip position of the bucket  11  and the target excavating topography is acquired. 
     The target blade tip speed determining unit  55  determines the target blade tip speed Vr of the bucket  11  based on the distance data (step SP 4 ). As described above by referring to  FIG. 9 , map data indicating a relation between the distance d and the target blade tip speed Vr is stored in the storage unit  61 . The target blade tip speed determining unit  55  determines the target blade tip speed Vr in response to the distance d based on the distance data acquired by the distance acquiring unit  54  and the map data stored in the storage unit  61 . 
     The target boom speed calculating unit  57  calculates the target boom speed Vb for the leveling assist control based on the target blade tip speed Vr determined by the target blade tip speed determining unit  55  and at least one of the arm operation amount and the bucket operation amount acquired by the operation amount acquiring unit  56  (step SP 5 ). 
     As illustrated in  FIG. 11 , the determined target blade tip speed Vr is added to the counter blade tip speed Va counteracting the blade tip speed Vs set in response to the arm operation amount and the bucket operation amount of the operation device  40 . Specifically, the target blade tip speed Vr is added to the first counter blade tip speed Va 1  counteracting the blade tip speed Vs 1  set in response to the bucket operation amount of the operation device  40  and the second counter blade tip speed Va 2  counteracting the blade tip speed Vs 2  set in response to the arm operation amount of the operation device  40 . The first counter blade tip speed Va 1  and the second counter blade tip speed Va 2  have negative values. From the added value of the target blade tip speed Vr, the first counter blade tip speed Va 1 , and the second counter blade tip speed Va 2 , the target boom speed Vb is calculated. 
     The target boom speed calculating unit  57  calculates the blade tip speed Vs 1  when the bucket  11  is operated by the bucket operation amount from the bucket operation amount for operating the bucket  11  by the operation device  40 . As described above, when the hydraulic oil supply amount per unit time supplied from the main hydraulic pump  42  to the bucket cylinder  21  through the direction control valve  41  is adjusted, the bucket cylinder speed is adjusted. The bucket cylinder speed is correlated with the movement amount of the spool of the direction control valve  41 . The movement amount of the spool of the direction control valve  41  is correlated with the pilot hydraulic pressure of the oil passages  44 A and  44 B. The pilot hydraulic pressure of the oil passages  44 A and  44 B is correlated with the bucket operation amount by the operation device  40 . Further, the pilot hydraulic pressure of the oil passages  44 A and  44 B is detected by the pressure sensors  46 A and  46 B. The correlation data indicating such a correlation is obtained in advance by a preliminary test or a simulation and is stored in the storage unit  61 . Thus, the target boom speed calculating unit  57  can calculate the bucket cylinder speed from the detection signals (PPC pressure) of the pressure sensors  46 A and  46 B based on the detection signals of the pressure sensors  46 A and  46 B of the bucket cylinder  21  and the correlation data stored in the storage unit  61 , and calculate the blade tip speed Vs 1  of the bucket  11  when the bucket cylinder  21  is driven at the bucket cylinder speed based on the bucket cylinder speed. Similarly, the target boom speed calculating unit  57  can calculate the arm cylinder speed based on the detection signals of the pressure sensors  46 A and  46 B of the arm cylinder  22  and the correlation data stored in the storage unit  61 , and calculate the blade tip speed Vs 2  of the bucket  11  when the arm cylinder  22  is driven at the arm cylinder speed based on the arm cylinder speed. 
     The target boom speed calculating unit  57  calculates the first counter blade tip speed Va 1  counteracting the blade tip speed Vs 1  of the bucket  11  when the bucket cylinder  21  is driven at a predetermined bucket cylinder speed and the second counter blade tip speed Va 2  counteracting the blade tip speed Vs 2  of the bucket  11  when the arm cylinder  22  is driven at a predetermined arm cylinder speed. The first counter blade tip speed Va 1  is a value used to offset the blade tip speed Vs 1  of the bucket  11  generated by the driving of the bucket cylinder  21  by the blade tip speed Vs 3  of the bucket  11  generated by the driving of the boom cylinder  23 . The second counter blade tip speed Va 2  is a value used to offset the blade tip speed Vs 2  of the bucket  11  generated by the driving of the arm cylinder  22  by the blade tip speed Vs 3  of the bucket  11  generated by the driving of the boom cylinder  23 . The target boom speed calculating unit  55  calculates the target boom speed Vb for the leveling assist control based on the target blade tip speed Vr, the first counter blade tip speed Va 1 , and the second counter blade tip speed Va 2 . 
     The correction amount calculating unit  58  calculates the correction amount R of the target boom speed Vb based on the integration in time of the distance d (step SP 6 ). 
     The correction amount calculating unit  58  calculates the correction amount R based on the integration in time of the distance d from the time point (the past time point) at which the leveling assist control is started to the current time point and compensates the target boom speed Vb by integration. 
     The time point at which the leveling assist control is started is a time point at which an instruction for selecting a control mode so that the operator starts the excavating operation is transmitted to the control device  50  through a mode selecting unit (not illustrated) and the control signal starts to be output from the control device  50  to the control valve  45 C. In the leveling assist control, the boom  13  is raised so that the blade tip  10  is disposed at the same position as the target excavating topography from the state where the blade tip  10  digs the target excavating topography. The correction amount calculating unit  58  calculates the correction amount R based on the integration in time of the distance d from the past time point at which the leveling assist control is started to the current time point at which the blade tip  10  is disposed on the target excavating topography. 
     The correction amount limiting unit  59  calculates the upper limit A of the correction amount R based on the distance d at the current time point (step SP 7 ). In the embodiment, the correction amount limiting unit  59  calculates the upper limit A of the correction amount R based on the target blade tip speed Vr determined from the distance d at the current time point. 
     In the embodiment, the upper limit A is determined based on Equation (1) below.
 
 A=a×Vr+S    (1)
 
     In Equation (1), A indicates the upper limit of the correction amount R, Vr indicates the target blade tip speed, a indicates the coefficient, and S indicates the offset amount. The offset amount S is determined arbitrarily. As indicated in Equation (1), the upper limit A and the target blade tip speed Vr are proportional to each other. As the target blade tip speed Vr decreases, the upper limit A decreases. Further, the upper limit A of the correction amount R is changed when the offset amount S is changed. As the offset amount S decreases, the upper limit A decreases, and hence the limitation for the correction amount R becomes strict. As the offset amount S increases, the upper limit A increases, and hence the limitation for the correction amount R becomes moderate. 
     The correction amount limiting unit  59  performs a correction limiting process of limiting the correction amount R calculated by the correction amount calculating unit  58  using the calculated upper limit A (step SP 8 ). 
     The correction amount limiting unit  59  compares the correction amount R calculated by the correction amount calculating unit  58  with the upper limit A calculated by the correction amount limiting unit  59 , outputs the upper limit A calculated by the correction amount limiting unit  59  as the correction amount Rs for correcting the target boom speed Vb to the working implement control unit  60  when the correction amount R calculated by the correction amount calculating unit  58  is larger than the upper limit A calculated by the correction amount limiting unit  59 , and outputs the correction amount R calculated by the correction amount calculating unit  58  as the correction amount Rs for correcting the target boom speed Vb to the working implement control unit  60  when the correction amount R calculated by the correction amount calculating unit  58  is equal to or smaller than the upper limit A calculated by the correction amount limiting unit  59 . 
     The working implement control unit  60  performs a correction process of correcting (compensating by integration) the target boom speed Vr calculated in step SP 5  by using the correction amount Rs used in the correction amount limiting process of step SP 8  (step SP 9 ). 
     The working implement control unit  60  outputs the instruction signal for performing the leveling assist control on the boom cylinder  23  to the control valve  45 C based on the corrected target boom speed Vb (step SP 10 ). The working implement control unit  60  outputs the instruction signal for controlling the boom cylinder  23  based on the upper limit A calculated by the correction amount limiting unit  59  when the correction amount R calculated by the correction amount calculating unit  58  is larger than the upper limit A calculated by the correction amount limiting unit  59 . The working implement control unit  60  outputs the instruction signal for controlling the boom cylinder  23  based on the correction amount R calculated by the correction amount calculating unit  58  when the correction amount R calculated by the correction amount calculating unit  58  is equal to or smaller than the upper limit A calculated by the correction amount limiting unit  59 . 
     COMPARATIVE EXAMPLE 
     A comparative example will be described. In the control device according to the comparative example, the correction amount limiting process is not performed. In the comparative example, the correction amount R is directly output and is added to the target boom speed Vb. 
       FIG. 12  is a graph illustrating an operation when the excavator  100  is controlled by the control method according to the comparative example.  FIG. 12(A)  illustrates a relation between the distance d and the elapse time t from the time point at which the leveling assist control is started. In  FIG. 12(A) , the horizontal axis indicates the elapse time t, and the vertical axis indicates the distance d.  FIG. 12(B)  illustrates a relation of the target blade tip speed Vr and the correction amount R with respect to the elapse time t from the time point at which the leveling assist control is started. In  FIG. 12(B) , the horizontal axis indicates the elapse time t, and the vertical axis indicates the speed. 
     In  FIG. 12(A) , the blade tip position Pb matches the target excavating topography when the distance d is “0”. When the distance d has a positive value, the blade tip  10  is raised from the target excavating topography. When the distance d has a negative value, the blade tip  10  digs the target excavating topography. In the leveling assist control, the boom  13  is raised while the boom cylinder  23  is controlled so that the blade tip  10  of the bucket  11  returns to the target excavating topography from the state where the target excavating topography is dug by the blade tip  10  of the bucket  11 . 
     In the control system according to the comparative example, the target blade tip speed Vr of the bucket  11  is determined from the distance d between the current blade tip position of the bucket  11  and the target excavating topography, and the determined target blade tip speed Vr and the counter blade tip speed Va (the first counter blade tip speed Va 1  and the second counter blade tip speed Va 2 ) counteracting the blade tip speed of the bucket  11  in response to the arm operation amount and the bucket operation amount by the operator are subtracted, so that the target boom speed Vr is calculated. The correction amount R is calculated based on the integration in time of the distance d (corresponding to a portion indicated by the diagonal line M in  FIG. 12(A) ) from the time point at which the leveling assist control is started and the blade tip  10  digs the target excavating topography to the time point at which the blade tip returns to the target excavating topography. The target boom speed Vr is corrected (compensated by integration) by using the calculated correction amount R, and the control signal for controlling the boom cylinder  23  based on the target boom speed Vr compensated by integration is output. 
     As illustrated in  FIG. 12(A) , even in the leveling assist control using the compensation by integration according to the comparative example, the boom cylinder  23  is controlled so that the boom  13  is raised when the blade tip  10  of the bucket  11  digs the target excavating topography. 
     In the excavator  100 , a time delay exists in the responsiveness of the boom cylinder  23  with respect to the instruction signal for controlling the boom cylinder  23  due to an increase in the weight of the working implement  1 , a delay in the responsiveness of hydraulic pressure, or hysteresis generated when driving a hydraulic driving unit. For that reason, in a case where the boom  13  is raised by the leveling assist control so that the blade tip  10  of the bucket  11  returns to the target excavating topography from the state where the blade tip digs the target excavating topography, when the time T (see  FIG. 12(A) ) in which the blade tip  10  of the bucket  11  digs the target excavating topography is long, the correction amount R increases excessively (so as to be overcompensated) when the blade tip  10  of the bucket  11  returns to the target excavating topography as illustrated in  FIG. 12(B) , and hence the boom  13  is raised continuously even when the blade tip  10  is raised. As a result, as illustrated in  FIG. 12(A) , a phenomenon occurs in which the blade tip  10  of the bucket  11  is excessively separated (raised) from the target excavating topography. Consequently, a portion which is not excavated by the working implement  1  is generated, and hence the leveling operation is performed in a state different from the target excavating topography. 
     [Operation and Effect] 
       FIG. 13  is a graph illustrating an operation when the excavator  100  is controlled by the control method according to the embodiment.  FIG. 13(A)  illustrates a relation between the distance d and the elapse time t from the time point at which the leveling assist control is started. In  FIG. 13(A) , the horizontal axis indicates the elapse time t, and the vertical axis indicates the distance d.  FIG. 13(B)  illustrates a relation of the target blade tip speed Vr and the correction amount Rs with respect to the elapse time t from the time point at which the leveling assist control is started. In  FIG. 13(B) , the horizontal axis indicates the elapse time t, and the vertical axis indicates the speed. 
     In the leveling assist control, the working implement control unit  60  raises the boom  13  by controlling the boom cylinder  23  so that the blade tip  10  of the bucket  11  returns to the target excavating topography from the state where the blade tip  10  of the bucket  11  digs the target excavating topography. 
     The correction amount calculating unit  58  calculates the correction amount R based on the integration in time of the distance d (corresponding to a portion indicated by the diagonal line M in  FIG. 13(A) ) from the time point at which the leveling assist control is started and the blade tip  10  digs the target excavating topography to the time point at which the blade tip  10  returns to the target excavating topography by the raising operation of the boom  13 . The correction amount limiting unit  59  limits the correction amount R in the raising operation of the boom  13 . 
     Since the correction amount R is limited in the raising operation of the boom  13 , an increase in correction amount R is suppressed as illustrated in  FIG. 13(B)  even when a state in which the blade tip  10  of the bucket  11  digs the target topography changes to a state where the blade tip is disposed at the same position as the target excavating topography, and hence the overcompensation of the correction amount R is prevented. Since the target boom speed Vb is corrected by the correction amount Rs preventing the overcompensation thereof, it is possible to suppress the blade tip  10  of the bucket  11  from being excessively raised from the target excavating topography as illustrated in  FIG. 13(A) , and hence to decrease the raising amount. 
     In this way, according to the embodiment, since the correction amount R is limited, it is possible to suppress degradation in excavating precision by preventing the blade tip  10  from being raised until the blade tip  10  of the bucket  11  returns to the target excavating topography from the state where the blade tip digs the target excavating topography in the leveling assist control. 
     Further, in the embodiment, as indicated by Equation (1), the upper limit A of the correction amount R is calculated, the correction amount limiting process of the correction amount R is performed so as not to exceed the upper limit A, and hence the correction amount Rs is calculated. Thus, it is possible to smoothly perform a strict or moderate limitation for the correction amount R just by changing the upper limit A. 
     Further, as indicated by Equation (1), the upper limit A and the target blade tip speed Vr are proportional to each other. Further, as described above by referring to  FIG. 9 , the target blade tip speed Vr is proportional to the distance d. Thus, the upper limit A is proportional to the distance d. In the embodiment, the correction amount limiting unit  59  decreases the upper limit A of the correction amount R as the distance d (the target blade tip speed Vr) at the current time point decreases. Accordingly, since the overcompensation is suppressed, the correction amount R can be also zero when the distance d (the target blade tip speed Vr) at the current time point is zero. 
     Further, as indicated by Equation (1), it is possible to smoothly perform a strict or moderate limitation for the correction amount R just by changing the offset amount S for the upper limit A. 
     [Other Embodiments] 
     The correction amount limiting unit  59  can change the upper limit A of the correction amount R based on the arm operation amount or the arm speed (the arm cylinder speed). For example, the correction amount limiting unit  59  increases the upper limit A as the arm operation amount or the arm speed decreases (for a moderate limitation) and decreases the upper limit A as the arm operation amount or the arm speed increases (for a strict limitation). When the arm  12  moves at a low speed, the raising of the blade tip  10  in the leveling assist control is suppressed even when the correction amount R is not limited. When the arm  12  moves at a high speed, the raising of the blade tip  10  in the leveling assist control can be suppressed while the correction amount R is limited. 
     The correction amount limiting unit  59  can change the upper limit A by changing the offset amount S indicated by Equation (1) based on the arm operation amount or the arm speed (the arm cylinder speed). 
     As described above, the arm cylinder speed is correlated with the pilot hydraulic pressure of the oil passages  44 A and  44 B. The pilot hydraulic pressure of the oil passages  44 A and  44 B is detected by the pressure sensors  46 A and  46 B. The correlation data is stored in the storage unit  61 . The detection signals of the pressure sensors  46 A and  46 B are output to the control device  50 . The correction amount limiting unit  59  can acquire the arm operation amount or the arm speed (the arm cylinder speed) based on the detection signals of the pressure sensors  46 A and  46 B. The correction amount limiting unit  59  can change the offset amount S based on the detection values of the pressure sensors  46 A and  46 B. 
       FIG. 14  is a diagram illustrating a relation between each of the detection values of the pressure sensors  46 A and  46 B and the offset amount S. As illustrated in  FIG. 14 , a large offset amount S is set as the detection values of the pressure sensors  46 A and  46 B decrease (the arm cylinder speed decreases), and the limitation becomes moderate. A small offset amount S is set as the detection values of the pressure sensors  46 A and  46 B increase (the arm cylinder speed increases), and the limitation becomes strict. The map data illustrated in  FIG. 14  is stored in the storage unit  61 . The correction amount limiting unit  59  determines the offset amount S in response to the arm cylinder speed based on the detection values of the pressure sensors  46 A and  46 B and the map data of the storage unit  61 . 
     In addition, the correction amount limiting unit  59  may change the upper limit A of the correction amount R based on the weight of the bucket  11  when the bucket  11  connected to the arm  12  can be replaced. For example, the correction amount limiting unit  59  increases the upper limit A as the weight of the bucket  11  decreases (for a moderate limitation) and decreases the upper limit A as the weight of the bucket  11  increases (for a strict limitation). When the weight of the bucket  11  is small, the raising of the blade tip  10  in the leveling assist control is suppressed even when the correction amount R is not limited. When the weight of the bucket  11  is large, the raising of the blade tip  10  in the leveling assist control can be suppressed while the correction amount R is limited. 
     In addition, in the above-described embodiment, the operation device  40  is set as the pilot hydraulic operation device. The operation device  40  may be of an electric type.  FIG. 15  is a diagram illustrating an example of an electric operation device  40 B. As illustrated in  FIG. 15 , the operation device  40 B includes an operation member  400  that corresponds to an electric lever and an operation amount sensor  49  that electrically detects the operation amount of the operation member  400 . The operation amount sensor  49  includes a potentiometer and an angle meter and detects the inclination angle of the inclined operation member  400 . The detection signal of the operation amount sensor  49  is output to the control device  50 . The operation amount acquiring unit  56  of the control device  50  acquires the detection signal of the operation amount sensor  49  as the operation amount. The control device  50  outputs an instruction signal (electric signal) for driving the direction control valve  41  based on the detection signal of the operation amount sensor  49 . The direction control valve  41  is operated by an actuator such as a solenoid operated by electric power. The instruction signal is output from the control device  50  to the actuator of the direction control valve  41 . The actuator of the direction control valve  41  moves the spool of the direction control valve  41  based on the instruction signal output from the control device  50 . 
     In addition, similarly to the operation device  40  described in the above-described embodiment, the operation device  40 B also includes a right operation lever and a left operation lever. When the right operation lever is operated in the front to back direction, the boom  13  is lowered and raised. When the right operation lever is operated in the left and right direction (the vehicle width direction), the bucket  11  performs the excavating operation and the dumping operation. When the left operation lever is operated in the front to back direction, the arm  12  performs the dumping operation and the excavating operation. When the left operation lever is operated in the left and right direction, the upper swing body  2  swings left and right. In addition, when the left operation lever is operated in the front to back direction, the upper swing body  2  may swing right and left. Then, when the left operation lever is operated in the left and right direction, the arm  12  may perform the dumping operation and the excavating operation. 
     In addition,  FIG. 15  illustrates an example in which the arm cylinder  22  is operated by the operation device  40 B. The hydraulic oil is supplied to the cap side oil chamber  20 A of the arm cylinder  22  through the oil passage  47 A and the hydraulic oil is supplied to the rod side oil chamber  20 B through the oil passage  47 B. The bucket cylinder  21  has the same configuration as the arm cylinder  22 . In the boom cylinder  23 , the hydraulic oil is supplied to the cap side oil chamber  20 A of the boom cylinder  23  through the oil passage  47 B and the hydraulic oil is supplied to the rod side oil chamber  20 B through the oil passage  47 B. 
     In addition, in the above-described embodiment, the leveling assist control is performed based on the local coordinate system. The leveling assist control may be performed based on the global coordinate system. 
     In addition, in the above-described embodiment, the operation device  40  is provided in the excavator  100 . The operation device  40  may be provided in a remote place separated from the excavator  100  so as to remotely operate the excavator  100 . When the working implement  1  is operated remotely, the instruction signal indicating the operation amount of the working implement  1  is wirelessly transmitted from the operation device  40  provided in a remote plate to the excavator  100 . The operation amount acquiring unit  56  of the control device  50  acquires the wirelessly transmitted instruction signal indicating the operation amount. 
     In addition, in the above-described embodiment, the excavator  100  is operated by the operation of the operation device  40  by the operator. The control device  50  of the excavator  100  may autonomically control the working implement  1  based on the target excavating topography data regardless of the operation of the operator. When the working implement  1  is autonomically controlled, the operation amount data for autonomically controlling the working implement  1  is wirelessly transmitted from, for example, a computer system provided in a remote place. The operation amount acquiring unit  56  of the control device  50  acquires the wirelessly transmitted operation amount data. 
     In addition, in the above-described embodiment, the work machine  100  is set as the excavator  100 . The control device  50  and the control method described in the above-described embodiment can be applied to the entire work machine including a working implement other than the excavator  100 . 
     REFERENCE SIGNS LIST 
       1  WORKING IMPLEMENT 
       2  VEHICLE BODY (UPPER SWING BODY) 
       3  TRAVELING DEVICE (LOWER TRAVELING BODY) 
       4  CAB 
       4 S DRIVER SEAT 
       5  MACHINE ROOM 
       6  HANDRAIL 
       7  CRAWLER 
       10  BLADE TIP 
       11  BUCKET 
       12  ARM 
       13  BOOM 
       14  BUCKET CYLINDER STROKE SENSOR 
       15  ARM CYLINDER STROKE SENSOR 
       16  BOOM CYLINDER STROKE SENSOR 
       20  HYDRAULIC CYLINDER 
       20 A CAP SIDE OIL CHAMBER 
       20 B ROD SIDE OIL CHAMBER 
       21  BUCKET CYLINDER 
       22  ARM CYLINDER 
       23  BOOM CYLINDER 
       30  POSITION DETECTOR 
       31  VEHICLE BODY POSITION DETECTOR 
       31 A GPS ANTENNA 
       32  POSTURE DETECTOR 
       33  DIRECTION DETECTOR 
       34  BLADE TIP POSITION DETECTOR 
       40  OPERATION DEVICE 
       41  DIRECTION CONTROL VALVE 
       42  MAIN HYDRAULIC PUMP 
       43  PILOT HYDRAULIC PUMP 
       44 A,  44 B,  44 C OIL PASSAGE 
       45 A,  45 B,  45 C CONTROL VALVE 
       46 A,  46 B PRESSURE SENSOR 
       47 A,  47 B OIL PASSAGE 
       48  SHUTTLE VALVE 
       49  OPERATION AMOUNT SENSOR 
       50  CONTROL DEVICE 
       51  VEHICLE BODY POSITION DATA ACQUIRING UNIT 
       52  BLADE TIP POSITION DATA ACQUIRING UNIT 
       53  TARGET EXCAVATING TOPOGRAPHY DATA GENERATING UNIT 
       54  DISTANCE ACQUIRING UNIT 
       55  TARGET BLADE TIP SPEED DETERMINING UNIT 
       56  OPERATION AMOUNT ACQUIRING UNIT 
       57  TARGET BOOM SPEED CALCULATING UNIT 
       58  CORRECTION AMOUNT CALCULATING UNIT 
       59  CORRECTION AMOUNT LIMITING UNIT 
       60  WORKING IMPLEMENT CONTROL UNIT 
       61  STORAGE UNIT 
       62  INPUT/OUTPUT UNIT 
       70  TARGET CONSTRUCTION DATA GENERATING DEVICE 
       100  EXCAVATOR 
       200  CONTROL SYSTEM 
       300  HYDRAULIC SYSTEM 
     AX 1  ROTATION AXIS 
     AX 2  ROTATION AXIS 
     AX 3  ROTATION AXIS 
     L 11  LENGTH 
     L 12  LENGTH 
     L 13  LENGTH 
     Pb ABSOLUTE POSITION OF BLADE TIP 
     Pg ABSOLUTE POSITION OF VEHICLE BODY 
     RX SWING AXIS 
     θ 11  INCLINATION ANGLE 
     θ 12  INCLINATION ANGLE 
     θ 13  INCLINATION ANGLE