Patent ID: 12188208

DESCRIPTION OF EMBODIMENTS

Hereinafter, a work machine according to an embodiment will be described with reference to the drawings.FIG.1is a side view of a work machine1according to the embodiment. The work machine1according to the present embodiment is a bulldozer. The work machine1includes a vehicle body11, a travel device12, and a work implement13.

The vehicle body11includes an operating cabin14and an engine compartment15. An operator's seat that is not illustrated is disposed in the operating cabin14. The engine compartment15is disposed in front of the operating cabin14. The travel device12is attached to a lower portion of the vehicle body11. The travel device12includes a pair of left and right crawler belts16. Only the left crawler belt16is illustrated inFIG.1. The work machine1travels due to the rotation of the crawler belts16. The travel of the work machine1may be either autonomous travel, semi-autonomous travel, or travel under operation by an operator.

The work implement13is attached to the vehicle body11. The work implement13includes a lift frame17, a blade18, and a lift cylinder19. The lift frame17is attached to the vehicle body11so as to be movable up and down around an axis X extending in the vehicle width direction. The lift frame17supports the blade18. The blade18is disposed in front of the vehicle body11. The blade18moves up and down accompanying the up and down movements of the lift frame17. The lift cylinder19is coupled to the vehicle body11and the lift frame17. Due to the extension and contraction of the lift cylinder19, the lift frame17rotates up and down around the axis X.

FIG.2is a block diagram illustrating a configuration of a drive system2and a control system3of the work machine1. As illustrated inFIG.2, the drive system2includes an engine22, a hydraulic pump23, and a power transmission device24. The hydraulic pump23is driven by the engine22to discharge hydraulic fluid. The hydraulic fluid discharged from the hydraulic pump23is supplied to the lift cylinder19. Although one hydraulic pump23is illustrated inFIG.2, a plurality of hydraulic pumps may be provided.

The power transmission device24transmits driving force of the engine22to the travel device12. The power transmission device24may be, for example, a hydro static transmission (HST). Alternatively, the power transmission device24may be, for example, a transmission having a torque converter or a plurality of transmission gears.

The control system3includes an operating device25a, an input device25b, a controller26, a storage device28, and a control valve27. The operating device25ais a device for operating the work implement13and the travel device12. The operating device25ais disposed in the operating cabin14. The operating device25areceives an operation by an operator for driving the work implement13and the travel device12, and outputs an operation signal corresponding to the operation. The operating device25aincludes, for example, an operating lever, a pedal, a switch, and the like.

For example, the operating device25afor the travel device12is 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 device25ais output to the controller26. When the operating position of the operating device25ais the forward position, the controller26controls the travel device12or the power transmission device24so that the work machine1travels forward. When the operating position of the operating device25ais the reverse position, the controller26controls the travel device12or the power transmission device24so that the work machine1travels in reverse.

The input device25bis, for example, a touch screen type input device. The input device25bmay 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 device25b.

The controller26is programmed to control the work machine1based on acquired data. The controller26includes the storage device28and a processor30. The processor30includes, for example, a CPU. The storage device28includes, for example, a memory and an auxiliary storage device. The storage device28may be, for example, a RAM or a ROM. The storage device28may be a semiconductor memory, a hard disk, or the like. The storage device28is an example of a non-transitory computer-readable recording medium. The storage device28stores computer commands that are executable by the processor30and for controlling the work machine1.

The controller26acquires an operation signal from the operating device25a. The controller26controls the control valve27based on the operation signal. The controller26is not limited to one unit and may be divided into a plurality of controllers.

The control valve27is a proportional control valve and controlled by a command signal from the controller26. The control valve27is disposed between a hydraulic actuator, such as the lift cylinder19, and the hydraulic pump23. The control valve27controls the flow rate of the hydraulic fluid supplied from the hydraulic pump23to the lift cylinder19. The controller26generates a command signal to the control valve27so that the blade18operates according to the operation of the operating device25adescribed above. As a result, the lift cylinder19is controlled according to an amount of operation of the operating device25a. The control valve27may be a pressure proportional control valve. Alternatively, the control valve27may be an electromagnetic proportional control valve.

The control system3includes a lift cylinder sensor29. The lift cylinder sensor29detects the stroke length of the lift cylinder19(hereinafter referred to as “lift cylinder length L”). As illustrated inFIG.3, the controller26calculates a lift angle θ lift of the blade18based on the lift cylinder length L.FIG.3is a schematic view illustrating a configuration of the work machine1.

InFIG.3, the origin position of the work implement13is indicated by a chain double-dashed line. The origin position of the work implement13is the position of the blade18in a state where the tip of the blade18is 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 implement13.

As illustrated inFIG.2, the control system3includes a position sensor31. The position sensor31measures a position of the work machine1. The position sensor31includes a global navigation satellite system (GNSS) receiver32, an IMU33, and an antenna35. The GNSS receiver32is, for example, a receiver for global positioning system (GPS). The GNSS receiver32receives a positioning signal from a satellite and calculates the position of the antenna35from the positioning signal to generate vehicle body position data. The controller26acquires the vehicle body position data from the GNSS receiver32.

The IMU33is an inertial measurement unit. The IMU33acquires 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 machine1. The controller26acquires the traveling direction and vehicle speed of the work machine1from the vehicle body acceleration data. The controller26acquires the vehicle body inclination angle data and the vehicle body acceleration data from the IMU33.

The controller26calculates a blade tip position P0from the lift cylinder length L, the vehicle body position data, and the vehicle body inclination angle data. As illustrated inFIG.3, the controller26calculates global coordinates of the GNSS receiver32based on the vehicle body position data. The controller26calculates the lift angle θ lift based on the lift cylinder length L. The controller26calculates local coordinates of the blade tip position P0with respect to the GNSS receiver32based on the lift angle θ lift and vehicle body dimension data. The controller26calculates the traveling direction and vehicle speed of the work machine1from the vehicle body acceleration data. The vehicle body dimension data is stored in the storage device28and indicates a position of the work implement13with respect to the GNSS receiver32. The controller26calculates global coordinates of the blade tip position P0based on the global coordinates of the GNSS receiver32, the local coordinates of the blade tip position P0, and the vehicle body inclination angle data. The controller26acquires the global coordinates of the blade tip position P0as blade tip position data.

The control system3includes an output sensor34that measures an output of the power transmission device24. When the power transmission device24is an HST including a hydraulic motor, the output sensor34may be a pressure sensor that detects driving hydraulic pressure of the hydraulic motor. The output sensor34may be a rotation sensor that detects an output rotation speed of the hydraulic motor. When the power transmission device24includes a torque converter, the output sensor34may 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 sensor34is output to the controller26.

The controller26calculates a traction force of the work machine1from the detection value of the output sensor34. When the power transmission device24of the work machine1is an HST, the controller26can 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 machine1.

When the power transmission device24includes a torque converter and a transmission, the controller26can calculate the traction force from the output rotation speed of the torque converter. Specifically, the controller26calculates 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 device28stores 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 controller26acquires actual topography data. The actual topography data indicates an actual topography50of the work site.FIG.4indicates a cross section of the actual topography50. InFIG.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 machine1.

The actual topography data is information indicative of a topography positioned in the traveling direction of the work machine1. The actual topography data is acquired by calculation in the controller26from the work site data, the position of the work machine1acquired from the afore-mentioned position sensor31, and the traveling direction of the work machine1.

Specifically, the actual topography data includes heights Z0to Zn of the actual topography50at a plurality of reference points from the current position to a predetermined topography recognition distance do in the traveling direction of the work machine1. In the present embodiment, the current position is a position determined based on the current blade tip position P0of the work machine1. The current position may be determined based on a current position of another portion of the work machine1. The plurality of reference points are arranged at a predetermined interval, for example, every one meter.

The design topography data indicates a final design topography60. The final design topography60is 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 inFIG.4, the design topography data includes a height Z design of the final design topography60at a plurality of reference points in the traveling direction of the work machine1. The plurality of reference points indicate a plurality of points at a predetermined interval along the traveling direction of the work machine1. InFIG.4, the final design topography60has a flat shape parallel to the horizontal direction, but may have a different shape.

The controller26automatically controls the work implement13based on the actual topography data, the design topography data, and the blade tip position data. The automatic control of the work implement13may be a semi-automatic control that is performed in combination with manual operations by an operator. Alternatively, the automatic control of the work implement13may be a fully automatic control that is performed without manual operations by an operator.

Hereinafter, the automatic control of the work implement13in digging executed by the controller26will be described.FIG.5is a flowchart illustrating processes of the automatic control of the work implement13in digging work.FIG.5illustrates the processes in one work path in digging work. The one work path indicates steps from when the work machine1starts traveling forward from a digging start position and then performs a series of digging work until the work machine1starts traveling in reverse in order to move to a next digging start position.

As illustrated inFIG.5, in step S101, the controller26acquires current position data. At this time, the controller26acquires the current blade tip position P0of the blade18as described above. In step S102, the controller26acquires the afore-mentioned design topography data. In step S103, the controller26acquires the afore-mentioned actual topography data.

In step S104, the controller26acquires a digging start position (work start position). For example, the controller26acquires, as the digging start position, the position when the blade tip position P0first drops below the height Z0of the actual topography50. As a result, the position where the tip of the blade18is lowered and digging of the actual topography50is started is acquired as the digging start position. However, the controller26may acquire the digging start position by another method. For example, the controller26may acquire the digging start position based on the operation of the operating device25a. For example, the controller26may acquire the digging start position based on an operation of a button, a screen operation using a touch screen, or the like.

In step S105, the controller26acquires a movement amount of the work machine1. The controller26acquires, as the movement amount, the distance that the work machine1travels from the digging start position to the current position. The movement amount of the work machine1may be the movement amount of the vehicle body11. Alternatively, the movement amount of the work machine1may be the movement amount of the blade tip position P0of the blade18.

In step S106, the controller26determines a target profile70. As illustrated inFIG.4, the target profile70indicates a desired trajectory of the tip of the blade18in work. The target profile70is a target shape of the topography to be worked and indicates a desired shape as a result of digging work.

The controller26determines the target profile70so that the target profile70does not go below the final design topography60. Therefore, at the time of digging work, the controller26determines the target profile70positioned at or above the final design topography60and below the actual topography50.

As illustrated inFIG.4, the controller26determines the target profile70that is displaced downward from the actual topography50by 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 blade18. For example, the controller26may calculate the target displacement dZ from the target soil amount S_target and the width of the blade18.

The controller26refers to target soil amount data C to determine the target soil amount S_target according to the movement amount of the work machine1.FIG.6is 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 machine1in the horizontal direction. The controller26refers to the target soil amount data C illustrated inFIG.6to determine the target soil amount S_target from the movement amount n of the work machine1.

As illustrated inFIG.6, the target soil amount data C defines the relationship between the movement amount n of the work machine1and the target soil amount S_target. The target soil amount data C is stored in the storage device28. The target soil amount data C includes data at start c1, data during digging c2, data during transition c3, and data during soil transportation c4.

The data at start c1defines 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 c1, 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 c1defines the target soil amount S_target that increases linearly with respect to the movement amount n.

The data during digging c2defines 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 c2, the target soil amount S_target is defined as a constant value in the digging region. The data during digging c2defines the target soil amount S_target that is constant with respect to the movement amount n.

The data during transition c3defines 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 c3, 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 c3defines the target soil amount S_target that decreases linearly with respect to the movement amount n.

The data during soil transportation c4defines 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 c4, the target soil amount S_target is defined as a constant value in the soil transportation region. The data during soil transportation c4defines 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 b1and ends at a first end value b2. The soil transportation region starts at a second start value b3. The first end value b2is smaller than the second start value b3. 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 a1. The target soil amount S_target in the soil transportation region is constant at a second target value a2. The first target value a1is larger than the second target value a2. Therefore, as illustrated inFIG.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 a0. The start value a0is smaller than the first target value a1. The start target value a0is smaller than the second target value a2.

FIG.7is a flowchart illustrating processes for determining the target soil amount S_target. The determination processes start when the operating device25amoves to the forward position. In step S201, the controller26determines whether the movement amount n is equal to or greater than zero and less than the first start value b1. When the movement amount n is equal to or greater than zero and less than the first start value b1, the controller26gradually increases the target soil amount S_target from the start value a0as the movement amount n increases in step S202.

The start value a0is a constant and stored in the storage device28. The start value a0is preferably a small value at which the load on the blade18at the digging start will not be excessively large. The first start value b1is acquired by calculation from an inclination c1in the digging start region as illustrated inFIG.6, the start value a0, and the first target value a1. The inclination c1is a constant and stored in the storage device28. The inclination c1is preferably a value at which a quick transition from the digging start to the digging work can be performed and the load on the blade18will not be excessively large.

In step S203, the controller26determines whether the movement amount n is equal to or greater than the first start value b1and less than the first end value b2. When the movement amount n is equal to or greater than the first start value b1and less than the first end value b2, the controller26sets the target soil amount S_target to the first target value a1in step S204. The first target value a1is a constant and stored in the storage device28. The first target value a1is preferably a value at which the digging can be performed efficiently and the load on the blade18will not be excessively large.

In step S205, the controller26determines whether the movement amount n is equal to or greater than the first end value b2and less than the second start value b3. When the movement amount n is equal to or greater than the first end value b2and less than the second start value b3, the controller26gradually decreases the target soil amount S_target from the first target value a1as the movement amount n increases in step S206.

The first end value b2is a movement amount at a time when a current amount of soil held by the blade18exceeds a predetermined threshold. Therefore, when the current amount of soil held by the blade18exceeds the predetermined threshold, the controller26decreases the target soil amount S_target from the first target value a1. The predetermined threshold is determined based, for example, on the maximum capacity of the blade18. For example, the current amount of soil held by the blade18may be determined by measuring the load on the blade and calculating from this load. Alternatively, the current amount of soil held by the blade18may be calculated by capturing an image of the blade18with a camera and analyzing this image.

At the start of work, a predetermined initial value is set as the first end value b2. After the start of work, the movement amount when the amount of soil held by the blade18exceeds the predetermined threshold is stored as an updated value, and the first end value b2is updated based on the stored updated value.

In step S207, the controller26determines whether the movement amount n is equal to or greater than the second start value b3. When the movement amount n is equal to or greater than the second start value b3, the controller26sets the target soil amount S_target to the second target value a2in step S208.

The second target value a2is a constant and stored in the storage device28. The second target value a2is preferably set to a value suitable for the soil transportation work. The second start value b3is acquired by calculation from an inclination c3in the transitional soil transportation region as illustrated inFIG.6, the first target value a1, and the second target value a2. The inclination c3is a constant and stored in the storage device28. The inclination c3is 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 blade18will not be excessively large.

The start value a0, the first target value a1, and the second target value a2may be changed according to a condition of the work machine1, or the like. The first start value b1, the first end value b2, and the second start value b3may be stored in the storage device28as constants.

The target soil amount S_target is determined as described above. The controller26determines the target displacement dZ according to the movement amount n from the target soil amount S_target. Then, the height Z of the target profile70is determined from the height Z of the actual topography50and the target displacement dZ.

In step S107illustrated inFIG.5, the controller26controls the blade18toward the target profile70. At this time, the controller26generates a command signal to the work implement13so that the tip position of the blade18moves toward the target profile70determined in step S106. The generated command signal is input to the control valve27. As a result, the blade tip position P0of the work implement13moves along the target profile70.

In the afore-mentioned digging region, the target displacement dZ between the actual topography50and the target profile70is large in comparison with the other regions. Accordingly, the digging work of the actual topography50is performed in the digging region. In the soil transportation region, the target displacement dZ between the actual topography50and the target profile70is small in comparison with the other regions. Accordingly, the digging of the ground surface is suppressed and the soil held by the blade18is transported in the soil transportation region.

In step S108, the controller26acquires a traction force of the work machine1. The controller26acquires the traction force of the work machine1during the one work path at a predetermined sampling cycle and stores it in the storage device28.

In step S109, the controller26updates the work site data. The controller26acquires the position data indicative of the latest trajectory of the blade tip position P0as the actual topography data and updates the work site data according to the acquired actual topography data. Alternatively, the controller26may calculate a position of the bottom surface of the crawler belts16from 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 belts16as 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 machine1. For example, aerial laser survey may be used as an external survey device. Alternatively, the actual topography50may 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 S110, the controller26determines whether the current work path is completed. The controller26determines that the current work path is completed when the work machine1reaches a predetermined work end position. Alternatively, the controller26may determine that the current work path is completed when the work machine1is switched from the forward travel to the travel in reverse. When the current work path is completed, the process proceeds to step S111. When the current work path is not completed, the process returns to step S105.

In step S111, the controller26determines whether a maximum traction force Fmax during the current work path is smaller than a reference traction force Fref. The controller26acquires, 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 machine1can produce. The reference traction force Fref may be a fixed value. The reference traction force Fref may be set by the input device25b. When the maximum traction force Fmax is smaller than the reference traction force Fref, the process proceeds to step S112.

In step S112, the controller26modifies the target soil amount data C. As illustrated inFIG.8, the controller26increases the target soil amount S_target in the digging region from the first target value a1by an increment r1in the target soil amount data C. As a result, the controller26modifies the target soil amount data C indicated by the chain double-dashed line inFIG.8to the target soil amount data C′ indicated by the solid line.

Upon completing the one work path as described above, the work machine1travels in reverse in order to move to a next digging start position. Then, when the work machine1travels forward again, a next work path is started. The controller26executes the above processes for the next work path.

That is, the controller26updates the actual topography50based on the updated work site data. The controller26refers to the modified target soil amount data to determine the target soil amount S_target according to the movement amount of the work machine1. 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 inFIG.8. The controller26determines a target displacement dZ′ from the increased target soil amount S_target. Therefore, as illustrated inFIG.9, the target displacement dZ′ in the next work path is larger than the target displacement dZ in the previous work path. The controller26determines a target profile70′ in the next work path based on the increased target displacement dZ′. Then, the controller26controls the blade18according to the newly determined target profile70′. These processes are repeated to perform digging so that the actual topography50approaches the final design topography60.

With the control system3of the work machine1according 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 profile70′ 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 machine1is not limited to a bulldozer and may be another vehicle, such as a wheel loader, a motor grader, or the like.

The work machine1may be a vehicle that can be remotely operated. In this case, a portion of the control system3may be disposed outside of the work machine1. For example, the controller26may be disposed outside of the work machine1. The controller26may be disposed in a control center that is away from the work site.

The controller26may have a plurality of controllers that are separate from each other. For example, as illustrated inFIG.10, the controller26may include a remote controller261disposed outside of the work machine1and an onboard controller262mounted on the work machine1. The remote controller261and the onboard controller262may be able to wirelessly communicate with each other via communication devices38and39. A portion of the afore-mentioned functions of the controller26may be executed by the remote controller261and the remaining functions may be executed by the onboard controller262. For example, the processes for determining the target profile70may be executed by the remote controller261and the processes for outputting the command signal to the work implement13may be executed by the onboard controller262.

The operating device25aand the input device25bmay be disposed outside of the work machine1. In this case, the operating cabin may be omitted from the work machine1. Alternatively, the operating device25aand the input device25bmay be omitted from the work machine1. The work machine1may be operated with only the automatic control by the controller26without operations by the operating device25a.

The actual topography50may be acquired by another device, instead of the afore-mentioned position sensor31. For example, as illustrated inFIG.11, the actual topography50may be acquired by an interface device37that receives data from an external device. The interface device37may wirelessly receive the actual topography data measured by a measuring device41disposed outside. Alternatively, the interface device37may be a recording medium reading device and may receive the actual topography data measured by the external measuring device41via the recording medium.

The processes by the controller26are not limited to those of the above embodiment and may be changed. For example, the processes for determining the target profile70may be changed. The target soil amount may be determined regardless of the movement amount n of the work machine1. As illustrated inFIG.12, in a case where a start point Ps and an end point Pe of the target profile70are determined, the controller26may determine the target displacement dZ of the target profile70in one work path so that the total soil amount between the actual topography50and the target profile70is the target soil amount S. Also, when the maximum traction force in the one work path is smaller than the reference traction force, the controller26may determine the target displacement dZ′ of the target profile70′ in a next work path so that the total soil amount between the actual topography50and the target profile70′ is the increased target soil amount S′.

Alternatively, the controller26may determine a starting end or a terminating end of the target profile70based on the target soil amount. For example, as illustrated inFIG.13, the controller26may determine a starting end Ps1of the target profile70in 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 controller26may determine a starting end Ps2of the target profile70in a next work path based on the increased target soil amount S′.

The target profile70may be determined independently of the shape of the actual topography50. That is, the target profile70does not have to be parallel to the actual topography50. For example, the target profile70may 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 inFIG.14, the controller26may determine an inclination angle θ1of the target profile70in 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 controller26may determine an inclination angle θ2of the target profile70′ 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.