Patent Publication Number: US-2023160184-A1

Title: System and method for controlling work machine, and work machine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National stage application of International Application No. PCT/JP2021/018271, filed on May 13, 2021. This U.S. National stage application claims priority under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2020-105941, filed in Japan on Jun. 19, 2020, the entire contents of which are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a system and a method for controlling a work machine, and a work machine. 
     BACKGROUND INFORMATION 
     A control for automatically adjusting a position of a work implement, such as a blade, has been conventionally proposed for work vehicles, such as bulldozers or graders. For example, in Japanese Patent Publication No. 5247939, the position of the blade is automatically adjusted by a load control that makes the load on the blade match a target load in digging work. 
     SUMMARY 
     With the conventional control described above, the occurrence of shoe slip can be suppressed by raising the blade when the load on the blade becomes excessively large. This allows the work to be performed efficiently. 
     However, with the conventional control, as illustrated in  FIG.  15   , the blade is first controlled to conform to a design topography  100 . If the load on the blade subsequently increases, the blade is raised by the load control (see a trajectory  200  of the blade in  FIG.  15   ). Therefore, when a topography  300  with large undulations is dug, the load applied to the blade may increase rapidly, causing the blade to rise suddenly. If that happens, a topography with large unevenness will be formed. Once the topography with large unevenness is formed, it becomes difficult to perform subsequent digging work smoothly. Therefore, it is preferable to perform digging work in such a way that a topography with large unevenness is not formed. 
     An object of the present disclosure is to perform work efficiently under automatic control and to prevent a topography with large unevenness from being formed due to the work. 
     A system according to a first aspect of the present disclosure is a system for controlling a work machine including a work implement. The system includes a sensor and a controller. The sensor detects a current position of the work machine. The controller communicates with the sensor. The controller is programmed to execute the following processes. The controller acquires current position data indicative of the current position of the work machine. The controller acquires actual topography data indicative of an actual topography to be worked by the work machine. The controller acquires a target soil amount in one work path with respect to the actual topography. The controller determines a target profile in the one work path based on the target soil amount. The controller performs work in the one work path by operating the work implement according to the target profile. The controller acquires a maximum traction force of the work machine during the one work path. The controller determines whether the maximum traction force is smaller than a reference traction force. The controller increases the target soil amount in a next work path when the maximum traction force is smaller than the reference traction force. The controller determines the target profile in the next work path based on the increased target soil amount. 
     A method according to a second aspect of the present disclosure is a method for controlling a work machine including a work implement. The method includes the following processes. A first process is to acquire current position data indicative of a current position of the work machine. A second process is to acquire actual topography data indicative of an actual topography to be worked by the work machine. A third process is to acquire a target soil amount in one work path with respect to the actual topography. A fourth process is to determine a target profile in the one work path based on the target soil amount. A fifth process is to perform work in the one work path by operating the work implement according to the target profile. A sixth process is to acquire a maximum traction force of the work machine during the one work path. A seventh process is to determine whether the maximum traction force is smaller than a reference traction force. An eighth process is to increase the target soil amount in a next work path when the maximum traction force is smaller than the reference traction force. A ninth process is to determine the target profile in the next work path based on the increased target soil amount. The execution order of the processes is not limited to the above order and may be changed. 
     A work machine according to a third aspect of the present disclosure is a work machine including a work implement, a sensor, and a controller. The sensor detects a current position of the work machine. The controller communicates with the sensor. The controller is programmed to execute the following processes. The controller acquires current position data indicative of the current position of the work machine. The controller acquires actual topography data indicative of an actual topography to be worked by the work machine. The controller acquires a target soil amount in one work path with respect to the actual topography. The controller determines a target profile in the one work path based on the target soil amount. The controller performs work in the one work path by operating the work implement according to the target profile. The controller acquires a maximum traction force of the work machine during the one work path. The controller determines whether the maximum traction force is smaller than a reference traction force. The controller increases the target soil amount in a next work path when the maximum traction force is smaller than the reference traction force. The controller determines the target profile in the next work path based on the increased target soil amount. 
     According to the present disclosure, it is possible to perform work efficiently under automatic control and to prevent a topography with large unevenness from being formed due to the work. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a side view of a work machine according to an embodiment. 
         FIG.  2    is a block diagram illustrating a configuration of a drive system and a control system of the work machine. 
         FIG.  3    is a schematic view illustrating a configuration of the work machine. 
         FIG.  4    is a graph illustrating an example of a final design topography, an actual topography, and a target profile. 
         FIG.  5    is a flowchart illustrating processes of automatic control of a work implement. 
         FIG.  6    is a graph illustrating an example of target soil amount data. 
         FIG.  7    is a flowchart illustrating processes for determining a target soil amount. 
         FIG.  8    is a graph illustrating an example of the modified target soil amount data. 
         FIG.  9    is a graph illustrating an example of the target profile in a current work path and the target profile in a next work path. 
         FIG.  10    is a block diagram illustrating a configuration of the control system according to another embodiment. 
         FIG.  11    is a block diagram illustrating a configuration of the control system according to another embodiment. 
         FIG.  12    is a graph illustrating the target profile according to a first modified example. 
         FIG.  13    is a graph illustrating the target profile according to a second modified example. 
         FIG.  14    is a graph illustrating the target profile according to a third modified example. 
         FIG.  15    is a view illustrating digging work according to a prior art. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, a work machine according to an embodiment will be described with reference to the drawings.  FIG.  1    is a side view of a work machine  1  according to the embodiment. The work machine  1  according to the present embodiment is a bulldozer. The work machine  1  includes a vehicle body  11 , a travel device  12 , and a work implement  13 . 
     The vehicle body  11  includes an operating cabin  14  and an engine compartment  15 . An operator&#39;s seat that is not illustrated is disposed in the operating cabin  14 . The engine compartment  15  is disposed in front of the operating cabin  14 . The travel device  12  is attached to a lower portion of the vehicle body  11 . The travel device  12  includes a pair of left and right crawler belts  16 . Only the left crawler belt  16  is illustrated in  FIG.  1   . The work machine  1  travels due to the rotation of the crawler belts  16 . The travel of the work machine  1  may be either autonomous travel, semi-autonomous travel, or travel under operation by an operator. 
     The work implement  13  is attached to the vehicle body  11 . The work implement  13  includes a lift frame  17 , a blade  18 , and a lift cylinder  19 . The lift frame  17  is attached to the vehicle body  11  so as to be movable up and down around an axis X extending in the vehicle width direction. The lift frame  17  supports the blade  18 . The blade  18  is disposed in front of the vehicle body  11 . The blade  18  moves up and down accompanying the up and down movements of the lift frame  17 . The lift cylinder  19  is coupled to the vehicle body  11  and the lift frame  17 . Due to the extension and contraction of the lift cylinder  19 , the lift frame  17  rotates up and down around the axis X. 
       FIG.  2    is a block diagram illustrating a configuration of a drive system  2  and a control system  3  of the work machine  1 . As illustrated in  FIG.  2   , the drive system  2  includes an engine  22 , a hydraulic pump  23 , and a power transmission device  24 . The hydraulic pump  23  is driven by the engine  22  to discharge hydraulic fluid. The hydraulic fluid discharged from the hydraulic pump  23  is supplied to the lift cylinder  19 . Although one hydraulic pump  23  is illustrated in  FIG.  2   , a plurality of hydraulic pumps may be provided. 
     The power transmission device  24  transmits driving force of the engine  22  to the travel device  12 . The power transmission device  24  may be, for example, a hydro static transmission (HST). Alternatively, the power transmission device  24  may be, for example, a transmission having a torque converter or a plurality of transmission gears. 
     The control system  3  includes an operating device  25   a,  an input device  25   b,  a controller  26 , a storage device  28 , and a control valve  27 . The operating device  25   a  is a device for operating the work implement  13  and the travel device  12 . The operating device  25   a  is disposed in the operating cabin  14 . The operating device  25   a  receives an operation by an operator for driving the work implement  13  and the travel device  12 , and outputs an operation signal corresponding to the operation. The operating device  25   a  includes, for example, an operating lever, a pedal, a switch, and the like. 
     For example, the operating device  25   a  for the travel device  12  is configured to be operated at a forward position, a reverse position, and a neutral position. An operation signal indicative of a position of the operating device  25   a  is output to the controller  26 . When the operating position of the operating device  25   a  is the forward position, the controller  26  controls the travel device  12  or the power transmission device  24  so that the work machine  1  travels forward. When the operating position of the operating device  25   a  is the reverse position, the controller  26  controls the travel device  12  or the power transmission device  24  so that the work machine  1  travels in reverse. 
     The input device  25   b  is, for example, a touch screen type input device. The input device  25   b  may be another type of input device, such as a switch. The operator can input a setting for automatic control described later by using the input device  25   b.    
     The controller  26  is programmed to control the work machine  1  based on acquired data. The controller  26  includes the storage device  28  and a processor  30 . The processor  30  includes, for example, a CPU. The storage device  28  includes, for example, a memory and an auxiliary storage device. The storage device  28  may be, for example, a RAM or a ROM. The storage device  28  may be a semiconductor memory, a hard disk, or the like. The storage device  28  is an example of a non-transitory computer-readable recording medium. The storage device  28  stores computer commands that are executable by the processor  30  and for controlling the work machine  1 . 
     The controller  26  acquires an operation signal from the operating device  25   a.  The controller  26  controls the control valve  27  based on the operation signal. The controller  26  is not limited to one unit and may be divided into a plurality of controllers. 
     The control valve  27  is a proportional control valve and controlled by a command signal from the controller  26 . The control valve  27  is disposed between a hydraulic actuator, such as the lift cylinder  19 , and the hydraulic pump  23 . The control valve  27  controls the flow rate of the hydraulic fluid supplied from the hydraulic pump  23  to the lift cylinder  19 . The controller  26  generates a command signal to the control valve  27  so that the blade  18  operates according to the operation of the operating device  25   a  described above. As a result, the lift cylinder  19  is controlled according to an amount of operation of the operating device  25   a.  The control valve  27  may be a pressure proportional control valve. Alternatively, the control valve  27  may be an electromagnetic proportional control valve. 
     The control system  3  includes a lift cylinder sensor  29 . The lift cylinder sensor  29  detects the stroke length of the lift cylinder  19  (hereinafter referred to as “lift cylinder length L”). As illustrated in  FIG.  3   , the controller  26  calculates a lift angle  0  lift of the blade  18  based on the lift cylinder length L.  FIG.  3    is a schematic view illustrating a configuration of the work machine  1 . 
     In  FIG.  3   , the origin position of the work implement  13  is indicated by a chain double-dashed line. The origin position of the work implement  13  is the position of the blade  18  in a state where the tip of the blade  18  is in contact with the ground surface on a horizontal ground surface. The lift angle θ lift is the angle from the origin position of the work implement  13 . 
     As illustrated in  FIG.  2   , the control system  3  includes a position sensor  31 . The position sensor  31  measures a position of the work machine  1 . The position sensor  31  includes a global navigation satellite system (GNSS) receiver  32 , an IMU  33 , and an antenna  35 . The GNSS receiver  32  is, for example, a receiver for global positioning system (GPS). The GNSS receiver  32  receives a positioning signal from a satellite and calculates the position of the antenna  35  from the positioning signal to generate vehicle body position data. The controller  26  acquires the vehicle body position data from the GNSS receiver  32 . 
     The IMU  33  is an inertial measurement unit. The IMU  33  acquires vehicle body inclination angle data and vehicle body acceleration data. The vehicle body inclination angle data includes an angle with respect to the horizontal in the vehicle longitudinal direction (pitch angle) and an angle with respect to the horizontal in the vehicle lateral direction (roll angle). The vehicle body acceleration data includes the acceleration of the work machine  1 . The controller  26  acquires the traveling direction and vehicle speed of the work machine  1  from the vehicle body acceleration data. The controller  26  acquires the vehicle body inclination angle data and the vehicle body acceleration data from the IMU  33 . 
     The controller  26  calculates a blade tip position PO from the lift cylinder length L, the vehicle body position data, and the vehicle body inclination angle data. As illustrated in  FIG.  3   , the controller  26  calculates global coordinates of the GNSS receiver  32  based on the vehicle body position data. The controller  26  calculates the lift angle θ lift based on the lift cylinder length L. The controller  26  calculates local coordinates of the blade tip position PO with respect to the GNSS receiver  32  based on the lift angle θ lift and vehicle body dimension data. The controller  26  calculates the traveling direction and vehicle speed of the work machine  1  from the vehicle body acceleration data. The vehicle body dimension data is stored in the storage device  28  and indicates a position of the work implement  13  with respect to the GNSS receiver  32 . The controller  26  calculates global coordinates of the blade tip position PO based on the global coordinates of the GNSS receiver  32 , the local coordinates of the blade tip position P 0 , and the vehicle body inclination angle data. The controller  26  acquires the global coordinates of the blade tip position P 0  as blade tip position data. 
     The control system  3  includes an output sensor  34  that measures an output of the power transmission device  24 . When the power transmission device  24  is an HST including a hydraulic motor, the output sensor  34  may be a pressure sensor that detects driving hydraulic pressure of the hydraulic motor. The output sensor  34  may be a rotation sensor that detects an output rotation speed of the hydraulic motor. When the power transmission device  24  includes a torque converter, the output sensor  34  may be a rotation sensor that detects an output rotation speed of the torque converter. A detection signal indicative of a detection value of the output sensor  34  is output to the controller  26 . 
     The controller  26  calculates a traction force of the work machine  1  from the detection value of the output sensor  34 . When the power transmission device  24  of the work machine  1  is an HST, the controller  26  can calculate the traction force from the driving hydraulic pressure of the hydraulic motor and the rotation speed of the hydraulic motor. The traction force is a load received by the work machine  1 . 
     When the power transmission device  24  includes a torque converter and a transmission, the controller  26  can calculate the traction force from the output rotation speed of the torque converter. Specifically, the controller  26  calculates the traction force from the following formula (1). 
         F=k×T×R /( L×Z )   (1)
 
     At this time, F is a traction force, k is a constant, T is a transmission input torque, R is a reduction ratio, L is a crawler belt link pitch, and Z is the number of sprocket teeth. The input torque T is calculated based on the output rotation speed of the torque converter. The method for detecting the traction force is not limited to the afore-mentioned method and may be another method. 
     The storage device  28  stores work site data and design topography data. The work site data indicates an actual topography of the work site. The work site data is, for example, an actual topography survey map in a three-dimensional data format. The work site data can be acquired, for example, by aerial laser survey. 
     The controller  26  acquires actual topography data. The actual topography data indicates an actual topography  50  of the work site.  FIG.  4    indicates a cross section of the actual topography  50 . In  FIG.  4   , the vertical axis indicates the height of the topography and the horizontal axis indicates the distance from a current position in the traveling direction of the work machine  1 . 
     The actual topography data is information indicative of a topography positioned in the traveling direction of the work machine  1 . The actual topography data is acquired by calculation in the controller  26  from the work site data, the position of the work machine  1  acquired from the afore-mentioned position sensor  31 , and the traveling direction of the work machine  1 . 
     Specifically, the actual topography data includes heights Z 0  to Zn of the actual topography  50  at a plurality of reference points from the current position to a predetermined topography recognition distance do in the traveling direction of the work machine  1 . In the present embodiment, the current position is a position determined based on the current blade tip position P 0  of the work machine  1 . The current position may be determined based on a current position of another portion of the work machine  1 . The plurality of reference points are arranged at a predetermined interval, for example, every one meter. 
     The design topography data indicates a final design topography  60 . The final design topography  60  is a final target shape of a surface of the work site. The design topography data is, for example, a construction drawing in a three-dimensional data format. As illustrated in  FIG.  4   , the design topography data includes a height Zdesign of the final design topography  60  at a plurality of reference points in the traveling direction of the work machine  1 . The plurality of reference points indicate a plurality of points at a predetermined interval along the traveling direction of the work machine  1 . In  FIG.  4   , the final design topography  60  has a flat shape parallel to the horizontal direction, but may have a different shape. 
     The controller  26  automatically controls the work implement  13  based on the actual topography data, the design topography data, and the blade tip position data. The automatic control of the work implement  13  may be a semi-automatic control that is performed in combination with manual operations by an operator. Alternatively, the automatic control of the work implement  13  may be a fully automatic control that is performed without manual operations by an operator. 
     Hereinafter, the automatic control of the work implement  13  in digging executed by the controller  26  will be described.  FIG.  5    is a flowchart illustrating processes of the automatic control of the work implement  13  in digging work.  FIG.  5    illustrates the processes in one work path in digging work. The one work path indicates steps from when the work machine  1  starts traveling forward from a digging start position and then performs a series of digging work until the work machine  1  starts traveling in reverse in order to move to a next digging start position. 
     As illustrated in  FIG.  5   , in step S 101 , the controller  26  acquires current position data. At this time, the controller  26  acquires the current blade tip position P 0  of the blade  18  as described above. In step S 102 , the controller  26  acquires the afore-mentioned design topography data. In step S 103 , the controller  26  acquires the afore-mentioned actual topography data. 
     In step S 104 , the controller  26  acquires a digging start position (work start position). For example, the controller  26  acquires, as the digging start position, the position when the blade tip position P 0  first drops below the height Z 0  of the actual topography  50 . As a result, the position where the tip of the blade  18  is lowered and digging of the actual topography  50  is started is acquired as the digging start position. However, the controller  26  may acquire the digging start position by another method. For example, the controller  26  may acquire the digging start position based on the operation of the operating device  25   a.  For example, the controller  26  may acquire the digging start position based on an operation of a button, a screen operation using a touch screen, or the like. 
     In step S 105 , the controller  26  acquires a movement amount of the work machine  1 . The controller  26  acquires, as the movement amount, the distance that the work machine  1  travels from the digging start position to the current position. The movement amount of the work machine  1  may be the movement amount of the vehicle body  11 . Alternatively, the movement amount of the work machine  1  may be the movement amount of the blade tip position P 0  of the blade  18 . 
     In step S 106 , the controller  26  determines a target profile  70 . As illustrated in  FIG.  4   , the target profile  70  indicates a desired trajectory of the tip of the blade  18  in work. The target profile  70  is a target shape of the topography to be worked and indicates a desired shape as a result of digging work. 
     The controller  26  determines the target profile  70  so that the target profile  70  does not go below the final design topography  60 . Therefore, at the time of digging work, the controller  26  determines the target profile  70  positioned at or above the final design topography  60  and below the actual topography  50 . 
     As illustrated in  FIG.  4   , the controller  26  determines the target profile  70  that is displaced downward from the actual topography  50  by a target displacement dZ. The target displacement dZ is the target depth at each reference point in the vertical direction. The target displacement dZ is determined from a target soil amount S_target per unit of movement amount to be dug by the blade  18 . For example, the controller  26  may calculate the target displacement dZ from the target soil amount S_target and the width of the blade  18 . 
     The controller  26  refers to target soil amount data C to determine the target soil amount S_target according to the movement amount of the work machine  1 .  FIG.  6    is a graph illustrating an example of the target soil amount data C. The target soil amount data C indicates the target soil amount S_target per unit of movement amount as a dependent variable of movement amount n of the work machine  1  in the horizontal direction. The controller  26  refers to the target soil amount data C illustrated in  FIG.  6    to determine the target soil amount S_target from the movement amount n of the work machine  1 . 
     As illustrated in  FIG.  6   , the target soil amount data C defines the relationship between the movement amount n of the work machine  1  and the target soil amount S_target. The target soil amount data C is stored in the storage device  28 . The target soil amount data C includes data at start c 1 , data during digging c 2 , data during transition c 3 , and data during soil transportation c 4 . 
     The data at start c 1  defines the relationship between the movement amount n and the target soil amount S_target in a digging start region. The digging start region is the region from a digging start point S to a steady digging start point D. As indicated by the data at start c 1 , the target soil amount S_target that gradually increases as the movement amount n increases is defined in the digging start region. The data at start c 1  defines the target soil amount S_target that increases linearly with respect to the movement amount n. 
     The data during digging c 2  defines the relationship between the movement amount n and the target amount of soil S_target in a digging region. The digging region is the region from the steady digging start point D to a transitional soil transportation start point T. As indicated by the data during digging c 2 , the target soil amount S_target is defined as a constant value in the digging region. The data during digging c 2  defines the target soil amount S_target that is constant with respect to the movement amount n. 
     The data during transition c 3  defines the relationship between the movement amount n and the target soil amount S_target in a transitional soil transportation region. The transitional soil transportation region is the region from a steady digging end point T to a soil transportation start point P. As indicated by the data during transition c 3 , the target soil amount S_target that gradually decreases as the movement amount n increases is defined in the transitional soil transportation region. The data during transition c 3  defines the target soil amount S_target that decreases linearly with respect to the movement amount n. 
     The data during soil transportation c 4  defines the relationship between the movement amount n and the target soil amount S_target in a soil transportation region. The soil transportation region is the region starting from the soil transportation start point P. As indicated by the data during soil transportation c 4 , the target soil amount S_target is defined as a constant value in the soil transportation region. The data during soil transportation c 4  defines the target soil amount S_target that is constant with respect to the movement amount n. 
     Specifically, the digging region starts at a first start value b 1  and ends at a first end value b 2 . The soil transportation region starts at a second start value b 3 . The first end value b 2  is smaller than the second start value b 3 . Therefore, the digging region starts when the movement amount n in the digging region is smaller than the movement amount n in the soil transportation region. The target soil amount S_target in the digging region is constant at a first target value a 1 . The target soil amount S_target in the soil transportation region is constant at a second target value a 2 . The first target value al is larger than the second target value a 2 . Therefore, as illustrated in  FIG.  4   , the target displacement dZ defined in the digging region is larger than the target displacement dZ in the soil transportation region. 
     The target soil amount S_target at the digging start position is a start value a 0 . The start value a 0  is smaller than the first target value a 1 . The start target value a 0  is smaller than the second target value a 2 . 
       FIG.  7    is a flowchart illustrating processes for determining the target soil amount S_target. The determination processes start when the operating device  25   a  moves to the forward position. In step S 201 , the controller  26  determines whether the movement amount n is equal to or greater than zero and less than the first start value b 1 . When the movement amount n is equal to or greater than zero and less than the first start value b 1 , the controller  26  gradually increases the target soil amount S_target from the start value a 0  as the movement amount n increases in step S 202 . 
     The start value a 0  is a constant and stored in the storage device  28 . The start value a 0  is preferably a small value at which the load on the blade  18  at the digging start will not be excessively large. The first start value b 1  is acquired by calculation from an inclination c 1  in the digging start region as illustrated in  FIG.  6   , the start value a 0 , and the first target value a 1 . The inclination c 1  is a constant and stored in the storage device  28 . The inclination c 1  is preferably a value at which a quick transition from the digging start to the digging work can be performed and the load on the blade  18  will not be excessively large. 
     In step S 203 , the controller  26  determines whether the movement amount n is equal to or greater than the first start value b 1  and less than the first end value b 2 . When the movement amount n is equal to or greater than the first start value b 1  and less than the first end value b 2 , the controller  26  sets the target soil amount S_target to the first target value al in step S 204 . The first target value a 1  is a constant and stored in the storage device  28 . The first target value a 1  is preferably a value at which the digging can be performed efficiently and the load on the blade  18  will not be excessively large. 
     In step S 205 , the controller  26  determines whether the movement amount n is equal to or greater than the first end value b 2  and less than the second start value b 3 . When the movement amount n is equal to or greater than the first end value b 2  and less than the second start value b 3 , the controller  26  gradually decreases the target soil amount S_target from the first target value al as the movement amount n increases in step S 206 . 
     The first end value b 2  is a movement amount at a time when a current amount of soil held by the blade  18  exceeds a predetermined threshold. Therefore, when the current amount of soil held by the blade  18  exceeds the predetermined threshold, the controller  26  decreases the target soil amount S_target from the first target value al. The predetermined threshold is determined based, for example, on the maximum capacity of the blade  18 . For example, the current amount of soil held by the blade  18  may be determined by measuring the load on the blade and calculating from this load. Alternatively, the current amount of soil held by the blade  18  may be calculated by capturing an image of the blade  18  with a camera and analyzing this image. 
     At the start of work, a predetermined initial value is set as the first end value b 2 . After the start of work, the movement amount when the amount of soil held by the blade  18  exceeds the predetermined threshold is stored as an updated value, and the first end value b 2  is updated based on the stored updated value. 
     In step S 207 , the controller  26  determines whether the movement amount n is equal to or greater than the second start value b 3 . When the movement amount n is equal to or greater than the second start value b 3 , the controller  26  sets the target soil amount S_target to the second target value a 2  in step S 208 . 
     The second target value a 2  is a constant and stored in the storage device  28 . The second target value a 2  is preferably set to a value suitable for the soil transportation work. The second start value b 3  is acquired by calculation from an inclination c 3  in the transitional soil transportation region as illustrated in  FIG.  6   , the first target value a 1 , and the second target value a 2 . The inclination c 3  is a constant and stored in the storage device  28 . The inclination c 3  is preferably a value at which a quick transition from the digging work to the soil transportation work can be performed and the load on the blade  18  will not be excessively large. 
     The start value a 0 , the first target value a 1 , and the second target value a 2  may be changed according to a condition of the work machine  1 , or the like. The first start value b 1 , the first end value b 2 , and the second start value b 3  may be stored in the storage device  28  as constants. 
     The target soil amount S_target is determined as described above. The controller  26  determines the target displacement dZ according to the movement amount n from the target soil amount S_target. Then, the height Z of the target profile  70  is determined from the height Z of the actual topography  50  and the target displacement dZ. 
     In step S 107  illustrated in  FIG.  5   , the controller  26  controls the blade  18  toward the target profile  70 . At this time, the controller  26  generates a command signal to the work implement  13  so that the tip position of the blade  18  moves toward the target profile  70  determined in step S 106 . The generated command signal is input to the control valve  27 . As a result, the blade tip position P 0  of the work implement  13  moves along the target profile  70 . 
     In the afore-mentioned digging region, the target displacement dZ between the actual topography  50  and the target profile  70  is large in comparison with the other regions. Accordingly, the digging work of the actual topography  50  is performed in the digging region. In the soil transportation region, the target displacement dZ between the actual topography  50  and the target profile  70  is small in comparison with the other regions. Accordingly, the digging of the ground surface is suppressed and the soil held by the blade  18  is transported in the soil transportation region. 
     In step S 108 , the controller  26  acquires a traction force of the work machine  1 . The controller  26  acquires the traction force of the work machine  1  during the one work path at a predetermined sampling cycle and stores it in the storage device  28 . 
     In step S 109 , the controller  26  updates the work site data. The controller  26  acquires the position data indicative of the latest trajectory of the blade tip position P 0  as the actual topography data and updates the work site data according to the acquired actual topography data. Alternatively, the controller  26  may calculate a position of the bottom surface of the crawler belts  16  from the vehicle body position data and the vehicle body dimension data and acquire the position data indicative of the trajectory of the bottom surface of the crawler belts  16  as the actual topography data. In this case, the update of the work site data can be performed instantly. 
     Alternatively, the actual topography data may be generated from survey data measured by a survey device outside of the work machine  1 . For example, aerial laser survey may be used as an external survey device. Alternatively, the actual topography  50  may be captured by a camera and the actual topography data may be generated from image data acquired by the camera. For example, aerial photographic survey using an unmanned aerial vehicle (UAV) may be used. In the case of using the external survey device or camera, the work site data may be updated at a predetermined interval, or as needed. 
     In step S 110 , the controller  26  determines whether the current work path is completed. The controller  26  determines that the current work path is completed when the work machine  1  reaches a predetermined work end position. Alternatively, the controller  26  may determine that the current work path is completed when the work machine  1  is switched from the forward travel to the travel in reverse. When the current work path is completed, the process proceeds to step S 111 . When the current work path is not completed, the process returns to step S 105 . 
     In step S 111 , the controller  26  determines whether a maximum traction force Fmax during the current work path is smaller than a reference traction force Fref The controller  26  acquires, as the maximum traction force Fmax, the largest of traction forces detected during the current work path. The reference traction force Fref may be determined from the maximum value of the traction force that the work machine  1  can produce. The reference traction force Fref may be a fixed value. The reference traction force Fref may be set by the input device  25   b.  When the maximum traction force Fmax is smaller than the reference traction force Fref, the process proceeds to step S 112 . 
     In step S 112 , the controller  26  modifies the target soil amount data C. As illustrated in  FIG.  8   , the controller  26  increases the target soil amount S_target in the digging region from the first target value a 1  by an increment r 1  in the target soil amount data C. As a result, the controller  26  modifies the target soil amount data C indicated by the chain double-dashed line in  FIG.  8    to the target soil amount data C′ indicated by the solid line. 
     Upon completing the one work path as described above, the work machine  1  travels in reverse in order to move to a next digging start position. Then, when the work machine  1  travels forward again, a next work path is started. The controller  26  executes the above processes for the next work path. 
     That is, the controller  26  updates the actual topography  50  based on the updated work site data. The controller  26  refers to the modified target soil amount data to determine the target soil amount S_target according to the movement amount of the work machine  1 . When the maximum traction force Fmax during the previous work path is smaller than the reference traction force Fref, the target soil amount S_target is increased in the next work path as illustrated in  FIG.  8   . The controller  26  determines a target displacement dZ′ from the increased target soil amount S_target. Therefore, as illustrated in  FIG.  9   , the target displacement dZ′ in the next work path is larger than the target displacement dZ in the previous work path. The controller  26  determines a target profile  70 ′ in the next work path based on the increased target displacement dZ′. Then, the controller  26  controls the blade  18  according to the newly determined target profile  70 ′. These processes are repeated to perform digging so that the actual topography  50  approaches the final design topography  60 . 
     With the control system  3  of the work machine  1  according to the present embodiment described above, it is determined whether the maximum traction force Fmax during the one work path is smaller than the reference traction force Fref When the maximum traction force Fmax is smaller than the reference traction force Fref, the target soil amount S_target in the next work path is increased. Then, the target profile  70 ′ in the next work path is determined based on the increased target soil amount S_target. As a result, it is possible to perform work efficiently under automatic control and to prevent a topography with large unevenness from being formed due to the work. 
     Although an embodiment of the present invention has been described so far, the present invention is not limited to the above embodiment and various modifications can be made without departing from the gist of the invention. 
     The work machine  1  is not limited to a bulldozer and may be another vehicle, such as a wheel loader, a motor grader, or the like. 
     The work machine  1  may be a vehicle that can be remotely operated. In this case, a portion of the control system  3  may be disposed outside of the work machine  1 . For example, the controller  26  may be disposed outside of the work machine  1 . The controller  26  may be disposed in a control center that is away from the work site. 
     The controller  26  may have a plurality of controllers that are separate from each other. For example, as illustrated in  FIG.  10   , the controller  26  may include a remote controller  261  disposed outside of the work machine  1  and an onboard controller  262  mounted on the work machine  1 . The remote controller  261  and the onboard controller  262  may be able to wirelessly communicate with each other via communication devices  38  and  39 . A portion of the afore-mentioned functions of the controller  26  may be executed by the remote controller  261  and the remaining functions may be executed by the onboard controller  262 . For example, the processes for determining the target profile  70  may be executed by the remote controller  261  and the processes for outputting the command signal to the work implement  13  may be executed by the onboard controller  262 . 
     The operating device  25   a  and the input device  25   b  may be disposed outside of the work machine  1 . In this case, the operating cabin may be omitted from the work machine  1 . Alternatively, the operating device  25   a  and the input device  25   b  may be omitted from the work machine  1 . The work machine  1  may be operated with only the automatic control by the controller  26  without operations by the operating device  25   a.    
     The actual topography  50  may be acquired by another device, instead of the afore-mentioned position sensor  31 . For example, as illustrated in  FIG.  11   , the actual topography  50  may be acquired by an interface device  37  that receives data from an external device. The interface device  37  may wirelessly receive the actual topography data measured by a measuring device  41  disposed outside. Alternatively, the interface device  37  may be a recording medium reading device and may receive the actual topography data measured by the external measuring device  41  via the recording medium. 
     The processes by the controller  26  are not limited to those of the above embodiment and may be changed. For example, the processes for determining the target profile  70  may be changed. The target soil amount may be determined regardless of the movement amount n of the work machine  1 . As illustrated in  FIG.  12   , in a case where a start point Ps and an end point Pe of the target profile  70  are determined, the controller  26  may determine the target displacement dZ of the target profile  70  in one work path so that the total soil amount between the actual topography  50  and the target profile  70  is the target soil amount S. Also, when the maximum traction force in the one work path is smaller than the reference traction force, the controller  26  may determine the target displacement dZ′ of the target profile  70 ′ in a next work path so that the total soil amount between the actual topography  50  and the target profile  70 ′ is the increased target soil amount S′. 
     Alternatively, the controller  26  may determine a starting end or a terminating end of the target profile  70  based on the target soil amount. For example, as illustrated in  FIG.  13   , the controller  26  may determine a starting end Psi of the target profile  70  in one work path based on the target soil amount S. When the maximum traction force in the one work path is smaller than the reference traction force, the controller  26  may determine a starting end Ps 2  of the target profile  70  in a next work path based on the increased target soil amount S′. 
     The target profile  70  may be determined independently of the shape of the actual topography  50 . That is, the target profile  70  does not have to be parallel to the actual topography  50 . For example, the target profile  70  may be a horizontal surface. Alternatively, the target profile may be an inclined surface inclined at a predetermined angle with respect to the horizontal surface. As illustrated in  FIG.  14   , the controller  26  may determine an inclination angle θ 1  of the target profile  70  in one work path based on the target soil amount S. When the maximum traction force in the one work path is smaller than the reference traction force, the controller  26  may determine an inclination angle θ 2  of the target profile  70 ′ in a next work path based on the increased target soil amount S′. 
     According to the present disclosure, it is possible to perform work efficiently under automatic control and to prevent a topography with large unevenness from being formed due to the work.