Patent Publication Number: US-11041289-B2

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

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National stage application of International Application No. PCT/JP2017/027129, filed on Jul. 26, 2017. This U.S. National stage application claims priority under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2016-154817, filed in Japan on Aug. 5, 2016, the entire contents of which are hereby incorporated herein by reference. 
     BACKGROUND 
     The present invention relates to a system for controlling a work vehicle, a method for controlling a work vehicle, and a work vehicle. 
     Traditionally, for a work vehicle such as a bulldozer or a grader, controlling of automatically adjusting the position of a work implement has been proposed. For example, Japanese Patent No. 5,247,939 discloses an excavation control and a ground leveling control. 
     In the excavation control, the position of a blade is automatically adjusted so that a load applied to the blade matches a target load. In the ground leveling control, the position of the blade is automatically adjusted so that an edge of the blade moves along a design landscape indicating the shape of a target to be subjected to excavation. 
     SUMMARY 
     According to the aforementioned control, the occurrence of a shoe slip can be reduced by lifting a work implement upon an excessive increase in a load applied to the work implement. This makes it possible to efficiently perform a work operation. 
     In the conventional control, however, as shown in  FIG. 18 , when a load applied to a work implement  100  increases after the start of excavation of a current landscape  300 , the work implement  100  is lifted by load controlling (refer to a trajectory  200  of the work implement  100 ). Then, when the load applied to the work implement  100  increases after the restart of the excavation, the work implement  100  is lifted again. When this operation is repeated, a landscape with a large irregularity is formed and it is difficult to perform a smooth excavation operation. In addition, there is a concern that the excavated landscape easily gets rough and the quality of a finish may be degraded. 
     In addition, as shown in  FIG. 18 , when a downward slope is excavated, a flat scaffold on a top portion of the current landscape  300  is narrowed due to the repetition of the excavation. In this case, when a work vehicle goes over the top portion, the orientation of the work vehicle may rapidly change and cause the landscape to get rough. In addition, there is a concern that it may become difficult to perform the work operation due to the narrowing of the scaffold and that the efficiency of the work operation may be reduced. 
     An object of the invention is to provide a system for controlling a work vehicle, a method for controlling a work vehicle, and a work vehicle, which enable an excavation operation to be efficiently performed with a high-quality finish. 
     A control system according to a first aspect is a system that controls a work vehicle including a work implement and includes a storage device and a controller. The storage device stores current landscape information indicating a current landscape to be subjected to a work operation. The controller communicates with the storage device. 
     The controller acquires an excavation start position at which the work implement starts excavation. When the current landscape includes an upward slope and a downward slope existing ahead of the upward slope, and the excavation start position is on the upward slope, the controller determines a first virtual design surface including a first design surface that is located below the current landscape and inclined at a smaller angle than the upward slope. The controller generates a command signal that causes the work implement to move along the first virtual design surface. 
     A computer-implemented method for controlling a work vehicle including a work implement according to a second aspect includes the following steps. The first step is to acquire current landscape information indicating a current landscape to be subjected to a work operation. The second step is to acquire an excavation start position at which the work implement starts excavation. The third step is to determine a first virtual design surface including a first design surface that is located below the current landscape and inclined at a smaller angle than an upward slope when the current landscape includes the upward slope and a downward slope existing ahead of the upward slope and the excavation start position is on the upward slope. The fourth step is to generate a command signal that causes the work implement to move along the first virtual design surface. 
     A work vehicle according to a third aspect includes a work implement and a controller. The controller is programmed to control the work implement. The controller acquires current landscape information indicating a current landscape to be subjected to a work operation. The controller acquires an excavation start position at which the work implement starts excavation. When the current landscape includes an upward slope and a downward slope existing ahead of the upward slope, and the excavation start position is on the upward slope, the controller determines a first virtual design surface including a first design surface that is located below the current landscape and inclined at a smaller angle than the upward slope. The controller generates a command signal that causes the work implement to move along the first virtual design surface. 
     According to the invention, excavation is performed along a first virtual design surface that is determined based on a current landscape. Thus, the excavation can be smoothly performed without forming a large irregularity. In addition, when the current landscape includes an upward slope and a downward scape, a first virtual design surface including a first design surface inclined at a smaller angle than the upward slope is determined. Thus, it is possible to secure a scaffold for a work vehicle and perform an efficient excavation operation with a high-quality finish. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view showing a work vehicle according to an embodiment. 
         FIG. 2  is a block diagram showing a configuration of a driving system and control system of the work vehicle. 
         FIG. 3  is a schematic diagram showing a configuration of the work vehicle. 
         FIG. 4  is a flowchart showing a process of automatic control of the work implement in an excavation operation. 
         FIG. 5  is a diagram showing an example with a final design landscape, a current landscape, and a virtual design surface. 
         FIG. 6  is a flowchart showing a process of automatic control of the work implement. 
         FIG. 7  is a diagram showing an example with a final design landscape, a current landscape, and a virtual design surface. 
         FIG. 8  is a diagram showing an example with a final design landscape, a current landscape, and a virtual design surface. 
         FIGS. 9A and 9B  are diagrams showing an example of an inclination angle of a virtual design surface. 
         FIG. 10  is a diagram showing an example with a final design landscape, a current landscape, and a virtual design surface. 
         FIG. 11  is a diagram showing an example with a final design landscape, a current landscape, and a virtual design surface. 
         FIG. 12  is a diagram showing an example with a final design landscape, a current landscape, and a virtual design surface. 
         FIG. 13  is a flowchart showing a process of automatic control of the work implement. 
         FIG. 14  is a diagram showing an example with a final design landscape, a current landscape, and a virtual design surface. 
         FIG. 15  is a diagram showing an example with a final design landscape, a current landscape, and a virtual design surface. 
         FIG. 16  is a block diagram showing a configuration of a control system according to a modified example. 
         FIG. 17  is a block diagram showing a configuration of a control system according to another modified example. 
         FIG. 18  is a diagram showing excavation according to a conventional technique. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Hereinafter, a work vehicle according to an embodiment is described with reference to the accompanying drawings.  FIG. 1  is a side view showing the work vehicle  1  according to the embodiment. The work vehicle  1  according to the embodiment is a bulldozer. The work vehicle  1  includes a vehicle body  11 , a traveling device  12 , and a work implement  13 . 
     The vehicle body  11  includes an operator cab  14  and an engine compartment  15 . In the operator cab  14 , an operator seat, which is not shown, is disposed. The engine compartment  15  is disposed in front of the operator cab  14 . The traveling device  12  is attached to a lower portion of the vehicle body  11 . The traveling device  12  includes a pair of left and right crawlers  16 . Note that  FIG. 1  shows only the left crawler  16 . The rotation of the crawlers  16  allows the work vehicle  1  to travel. 
     The work implement  13  is attached to the vehicle body  11 . The work implement  13  includes a lift frame  17 , a blade  18 , a lift cylinder  19 , an angle cylinder  20 , and a tilt cylinder  21 . 
     The lift frame  17  is attached to the vehicle body  11  and capable of pivoting up and down about an axial line X extending in a vehicle width direction. The lift frame  17  holds the blade  18 . The blade  18  is disposed in front of the vehicle body  11 . The blade  18  moves up and down together with upward and downward movements of the lift frame  17 . 
     The lift cylinder  19  is coupled to the vehicle body  11  and the lift frame  17 . The lift frame  17  pivots up and down about the axial line X in accordance with the expansion and contraction of the lift cylinder  19 . 
     The angle cylinder  20  is coupled to the lift frame  17  and the blade  18 . The blade  18  pivots about an axial line Y extending in a substantially up-down direction in accordance with the expansion and contraction of the angle cylinder  20 . 
     The tilt cylinder  21  is coupled to the lift frame  17  and the blade  18 . The blade  18  pivots about an axial line Z extending in a substantially vehicle front-back direction in accordance with the expansion and contraction of the tilt cylinder  21 . 
       FIG. 2  is a block diagram showing a configuration of a driving system  2  and control system  3  of the work vehicle  1 . As shown in  FIG. 2 , the driving 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  and discharges a hydraulic fluid. The hydraulic fluid discharged from the hydraulic pump  23  is supplied to the lift cylinder  19 , the angle cylinder  20 , and the tilt cylinder  21 . Note that although  FIG. 2  shows the single hydraulic pump  23 , multiple hydraulic pumps  23  may be disposed. 
     The power transmission device  24  transmits driving force of the engine  22  to the traveling device  12 . The power transmission device  24  may be a hydro static transmission (HST), for example. Alternatively, the power transmission device  24  may be a torque converter or a transmission having multiple transmission gears, for example. 
     The control system  3  includes an operating device  25 , a controller  26 , and a control valve  27 . The operating device  25  is a device for operating the work implement  13  and the traveling device  12 . The operating device  25  is disposed in the operator cab  14 . The operating device  25  includes an operation lever, a pedal, a switch, and the like, for example. 
     The operating device  25  includes an operating device  251  for the traveling device  12  and an operating device  252  for the work implement  13 . The operating device  251  for the traveling device  12  is disposed so that the operating device  251  can be operated in a forward position, a reverse position, and a neutral position. When an operational position of the operating device  251  for the traveling device  12  is the forward position, the traveling device  12  or the power transmission device  24  is controlled so that the work vehicle  1  moves forward. When the operational position of the operating device  251  for the traveling device  12  is the reverse position, the traveling device  12  or the power transmission device  24  is controlled so that the work vehicle  1  moves backward. 
     The operating device  252  for the work implement  13  is mounted in a manner capable of operating the lift cylinder  19 , the angle cylinder  20 , and the tilt cylinder  21 . By operating the operating device  252  for the work implement  13 , a lift operation, angle operation, and tilt operation of the blade  18  can be performed. 
     The operating device  25  includes sensors  25   a  and  25   b  that detect an operation of the operating device  25  performed by an operator. The operating device  25  receives an operation performed by the operator to drive the work implement  13  and the traveling device  12 , and the sensors  25   a  and  25   b  output operation signals based on the operation. The sensor  25   a  outputs an operation signal based on an operation of the operating device  251  for the traveling device  12 . The sensor  25   b  outputs an operation signal based on an operation of the operating device  252  for the work implement  13 . 
     The controller  26  is programmed to control the work vehicle  1  based on acquired information. The controller  26  includes a processor such as a CPU, for example. The controller  26  acquires the operation signals from the sensors  25   a  and  25   b  of the operating device  25 . The controller  26  controls the control valve  27  based on the operation signals. The controller  26  is not limited to a single unit and may be separated in multiple controllers. 
     The control valve  27  is a proportional control valve and is controlled by a command signal from the controller  26 . The control valve  27  is disposed between the hydraulic pump  23  and hydraulic actuators for the lift cylinder  19 , the angle cylinder  20 , and the tilt cylinder  21 . The control valve  27  controls a flow rate of the hydraulic fluid supplied from the hydraulic pump  23  toward the lift cylinder  19 , the angle cylinder  20 , and the tilt cylinder  21 . The controller  26  generates the command signal to the control valve  27  so that the work implement  13  operates based on operations of the aforementioned operating device  252 . Thus, the lift cylinder  19 , the angle cylinder  20 , and the tilt cylinder  21  are controlled based on the amounts of the operations of the operating device  252 . 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 a stroke length (hereinafter referred to as “lift cylinder length L”) of the lift cylinder  19 . As shown in  FIG. 3 , the controller  26  calculates a lift angle θlift of the blade  18  based on the lift cylinder length L.  FIG. 3  is a schematic diagram showing a configuration of the work vehicle  1 . 
     In  FIG. 3 , the original position of the work implement  13  is indicated by an alternate long and two short dashes line. The original position of the work implement  13  is the position of the blade  18  in a state in which an edge of the blade  18  is in contact with a horizontal ground surface. The lift angle θlift is an angle with respect to the original position of the work implement  13 . 
     As shown in  FIG. 2 , the control system  3  includes a position detecting device  31 . The position detecting device  31  detects the position of the work vehicle  1 . The position detecting device  31  includes a GNSS receiver  32  and an IMU  33 . The GNSS receiver  32  is disposed on the operator cab  14 . The GNSS receiver  32  is, for example, an antenna for the Global Positioning System (GPS). The GNSS receiver  32  receives vehicle position information indicating the position of the work vehicle  1 . The controller  26  acquires the vehicle position information from the GNSS receiver  32 . 
     The IMU  33  is an inertial measurement unit. The IMU  33  acquires vehicle inclination angle information. The vehicle inclination angle information indicates an angle (pitch angle) with respect to a horizontal direction in a vehicle front-back direction and an angle (roll angle) with respect to the horizontal direction in the vehicle width direction. The IMU  33  transmits the vehicle inclination angle information to the controller  26 . The controller  26  acquires the vehicle inclination angle information from the IMU  33 . 
     The controller  26  calculates an edge position P 0  based on the lift cylinder length L, the vehicle position information, and the vehicle inclination angle information. As shown in  FIG. 3 , the controller  26  calculates global coordinates of the GNSS receiver  32  based on the vehicle position information. The controller  26  calculates the lift angle θlift based on the lift cylinder length L. The controller  26  calculates local coordinates of the edge position P 0  with respect to the GNSS receiver  32  based on the lift angle θlift and vehicle dimension information. The vehicle dimension information is stored in the storage device  28  and indicates the position of the work implement  13  with respect to the GNSS receiver  32 . The controller  26  calculates global coordinates of the edge position P 0  based on the global coordinates of the GNSS receiver  32 , the local coordinates of the edge position P 0 , and the vehicle inclination angle information. The controller  26  acquires the global coordinates of the edge position P 0  as edge position information. 
     The control system  3  includes the storage device  28 . The storage device  28  for example includes a memory and an auxiliary storage device. The storage device  28  may be a RAM, a ROM, or the like, for example. The storage device  28  may be a semiconductor storage device, a hard disk, or the like, for example. The controller  26  communicates with the storage device  28  via a cable or wirelessly to acquire information stored in the storage device  28 . 
     The storage device  28  stores the edge position information, current landscape information, and design landscape information. The design landscape information indicates the position and shape of a final design landscape. The final design landscape is a target landscape to be subjected to a work operation at a work site. The controller  26  acquires the current landscape information. The current landscape information indicates the position and shape of a current landscape to be subjected to the work operation at the work site. The controller  26  automatically controls the work implement  13  based on the current landscape information, the design landscape information, and the edge position information. 
     Note that the automatic control of the work implement  13  may be semi-automatic control to be performed together with a manual operation by an operator. Alternatively, the automatic control of the work implement  13  may be complete automatic control to be performed without a manual operation by an operator. 
     An automatic control, to be performed by the controller  26 , of the work implement  13  in an excavation operation is described below.  FIG. 4  is a flowchart showing a process of the automatic control of the work implement  13  in the excavation operation. 
     As shown in  FIG. 4 , in step S 101 , the controller  26  acquires current position information. In this case, the controller  26  acquires the current edge position P 0  of the work implement  13 , as described above. 
     In step S 102 , the controller  26  acquires the design landscape information. As shown in  FIG. 5 , the design landscape information includes heights of multiple points (refer to “−d 5 ” to “d 7 ” shown in  FIG. 5 ) located on a final design landscape  60  and arranged at predetermined intervals in a traveling direction of the work vehicle  1 . Thus, the final design landscape  60  is recognized as multiple final design surfaces  60 _ 1 ,  60 _ 2 , and  60 _ 3  obtained by dividing the final design landscape  60  at the multiple points. 
     Note that, in the drawing, only some of the final design surfaces are indicated by the reference symbols, while reference symbols of the other final design surfaces are omitted. In  FIG. 5 , the final design landscape  60  is formed in a flat shape parallel to the horizontal direction but may be formed in a different shape. 
     In step S 103 , the controller  26  acquires the current landscape information. As shown in  FIG. 5 , the current landscape information indicates a cross-sectional surface of a current landscape  50  in the traveling direction of the work vehicle  1 . 
     Note that, in  FIG. 5 , the ordinate indicates the height of the landscape and an estimated amount, described later, of soil to be held, and the abscissa indicates a distance from a reference position d 0  in the traveling direction of the work vehicle  1 . The reference position may be the current edge position P 0  of the work vehicle  1 . Specifically, the current landscape information includes the heights of the multiple points of the current landscape  50  in the traveling direction of the work vehicle  1 . The multiple points are arranged at the predetermined intervals of, for example, 1 meter (refer to “−5d” to “7d” shown in  FIG. 5 ). 
     Thus, the current landscape  50  is recognized as multiple current surfaces  50 _ 1 ,  50 _ 2 , and  50 _ 3  obtained by dividing the current landscape  50  at the multiple points. Note that, in the drawing, only some of the current surfaces are indicated by the reference symbols, while reference symbols of the other current surfaces are omitted. 
     The controller  26  acquires, as the current landscape information, positional information indicating the latest trajectory of the edge position P 0 . Thus, the position detecting device  31  functions as a current landscape acquiring device that acquires the current landscape information. In response to a movement of the edge position P 0 , the controller  26  updates the current landscape information to the latest current landscape and causes the latest current landscape to be stored in the storage device  28 . 
     Alternatively, the controller  26  may calculate the positions of bottom surfaces of the crawlers  16  from the vehicle position information and the vehicle dimension information and acquire, as the current landscape information, position information indicating trajectories of the bottom surfaces of the crawlers  16 . Alternatively, the current landscape information may be generated from data measured by an external measuring device of the work vehicle  1 . Alternatively, the current landscape information may be generated from image data obtained by causing a camera to capture images of the current landscape  50 . 
     In step S 104 , the controller  26  acquires a target amount of soil St. The target amount of soil St may be a fixed value determined based on the capacity of the blade  18 , for example. Alternatively, the target amount of soil St may be arbitrarily set by an operation of the operator. 
     In step S 105 , the controller  26  acquires an excavation start position Ps. In this case, the controller  26  acquires the excavation start position Ps based on an operation signal from the operating device  25 . For example, the controller  26  may determine, as the excavation start position Ps, the edge position P 0  when the controller  26  receives, from the operating device  252 , a signal indicating an operation of lowering the blade  18 . Alternatively, the excavation start position Ps may be stored in the storage device  28  in advance so that the excavation start position Ps can be acquired from the storage device  28 . 
     In step S 106 , a virtual design surface  70  is determined. The controller  26  determines the virtual design surface  70  as shown in  FIG. 5 , for example. The virtual design surface  70  is recognized as multiple design surfaces (divided unit surfaces)  70 _ 1 ,  70 _ 2 , and  70 _ 3  obtained by dividing the virtual design surface  70  at multiple points, similarly to the current landscape  50 . Note that in the drawing, only some of the current surfaces are indicated by the reference symbols, while reference symbols of the other current surfaces are omitted. A method for determining the virtual design surface  70  is described later in detail. 
     In step S 107 , the work implement  13  is controlled based on the virtual design surface  70 . In this case, the controller  26  generates a command signal to the work implement  13  so that the edge position P 0  of the work implement  13  moves along the virtual design surface  70  generated in step S 106 . The generated command signal is input to the control valve  27 . Accordingly, an operation of excavating the current landscape  50  is performed in response to the movement of the edge position P 0  of the work implement  13  along the virtual design surface  70 . 
     Next, a method for determining the virtual design surface  70  is described.  FIG. 6  is a flowchart showing a process, to be performed by the controller  26 , of determining the virtual design surface  70 . 
     As shown in  FIG. 6 , in step S 201 , an estimated amount of soil S to be held by the work implement  13  is calculated. As shown in  FIG. 5 , the estimated amount of soil S to be held is an estimated value of the amount of soil that is held by the work implement  13  when the edge position P 0  of the work implement  13  moves along the virtual design surface  70 . The controller  26  calculates an amount of soil between the virtual design surface  70  and the current landscape  50  as the estimated amount of soil S to be held. The alternate long and two short dashes line shown in  FIG. 5  indicates changes in the estimated amount of soil S to be held. 
     The virtual design surface  70  is located above the final design landscape  60 , while at least a portion of the virtual design surface  70  is located below the current landscape  50 . The virtual design surface  70  linearly extends from the excavation start position Ps. 
     The amount of soil between the virtual design surface  70  and the current landscape  50  is calculated as an amount corresponding to a cross-sectional area (or the area of a portion hatched in  FIG. 5 ) between the virtual design surface  70  and the current landscape  50 , as shown in  FIG. 5 . Here in the present embodiment, the size of the current landscape  50  in the width direction of the work vehicle  1  is not taken in consideration. The amount of soil, however, may be calculated with the size of the current landscape  50  in the width direction of the work vehicle  1  taken in consideration. 
     Note that as shown in  FIG. 7 , when the current landscape  50  includes a recess, the virtual design surface  70  may include portions (hereinafter referred to as “portions to be excavated”)  70   a  and  70   c  located below the current landscape  50  and a portion (hereinafter referred to as “portion to be raised”)  70   b  located above the current landscape  50 . In this case, the controller  26  calculates, as the estimated amount of soil S to be held, the sum of amounts of soil between the virtual design surface  70  and the current landscape  50  by adding the amount of soil between the portions  70   a  and  70   c  to be excavated and the current landscape  50  and subtracting the amount of soil between the portion  70   b  to be raised and the current landscape  50 . 
     For example, in  FIG. 7 , an amount Si of soil between the portion  70   a  to be excavated and the current landscape  50  and an amount S 3  of soil between the portion  70   c  to be excavated and the current landscape  50  are added to the estimated amount of soil S to be held, and an amount S 2  of soil between the portion  70   b  to be raised and the current landscape  50  is subtracted from the estimated amount of soil S to be held. Thus, the controller  26  calculates the estimated amount of soil S to be held, by S=Si+(−S 2 )+S 3 . 
     In step S 202 , an inclination angle α of the virtual design surface  70  is calculated. In this case, the controller  26  determines the inclination angle α so that the estimated amount of soil S, calculated in step S 201 , of soil to be held matches the target amount of soil St acquired in step S 104 . 
     For example, as shown in  FIG. 5 , when a point indicated by a distance d 0  (hereinafter referred to as “point d 0 ”) is at the excavation start position Ps, the controller  26  calculates the inclination angle α that provides the sum (indicated by a portion hatched in  FIG. 5 ) of amounts of soil between the virtual design surface  70  extending from the excavation start position Ps and the current landscape  50  matches the target amount of soil St. As a result, the virtual design surface  70  linearly extending from the excavation start position Ps to a point d 3  at which the target amount of soil St is achieved is determined. Regarding points following the point d 3  at which the target amount of soil St is achieved, the virtual design surface  70  is determined so that the virtual design surface  70  extends along the current landscape  50 . 
     Note that, in order to easily calculate the amount of soil, in the embodiment, the amount of soil between a point at which the target amount of soil St is achieved and a point at which the virtual design surface  70  is determined to extend along the current landscape  50  is not takin into consideration for the calculation of the estimated amount of soil S to be held. For example, in  FIG. 7 , at a point d 2 , the estimated amount of soil S to be held matches the target amount of soil St. The controller  26  determines the height of the virtual design surface  70  at the point d 3  next to the point d 2  so that the height of the virtual design surface  70  matches the height of the current landscape  50  at the point d 3  next to the point d 2 . Thus, the amount of soil between the point d 2  at which the target amount of soil St is achieved and the point d 3  at which the virtual design surface  70  is determined to extend along the current landscape  50  is not included in the estimated amount of soil S to be held. The estimated amount of soil S to be held, however, may be calculated with the amount of soil in this portion taken into consideration. 
     The controller  26  determines the virtual design surface  70  so that the virtual design surface  70  does not fall below the final design landscape  60 . Thus, as shown in  FIG. 8 , the inclination angle α is determined so that the estimated amount of soil S to be held between the virtual design surface  70 , the final design landscape  60 , and the current landscape  50  matches the target amount of soil St. Hence, as shown in  FIG. 8 , when the excavation is started at the point d 2 , the controller  26  determines the virtual design surface  70  so that the virtual design surface  70  reaches the final design landscape  60  at a point d 4  and extends along the final design landscape  60  at points following the point d 4 . 
     In step S 203 , it is determined whether or not the inclination angle α is an angle indicating a downward slope. In this case, when the inclination angle α calculated in step S 202  indicates a downward slope in the traveling direction of the work vehicle with respect to the horizontal direction, the controller  26  determines that the inclination angle α is an angle indicating a downward slope. When the current landscape  50  includes an upward slope and a downward slope existing ahead of the upward slope, the inclination angle α may be an angle indicating an upward slope as shown in  FIG. 9A  in some cases, and in other cases may be an angle indicating a downward slope as shown in  FIG. 9B . 
     When it is determined that the inclination angle α is an angle indicating a downward slope in step S 203 , the process proceeds to step S 204 . In step S 204 , whether a current surface behind the excavation start position Ps is an upward slope or not is determined. In this case, when the current surface (refer to, for example, the current surface  50 _ 1  shown in  FIG. 5 ), which is located immediately behind the excavation start position Ps in the traveling direction of the work vehicle  1 , extends upwardly with respect to the horizontal direction and also forms an angle equal to or more than a predetermined angular threshold with respect to the horizontal direction, the controller  26  determines that the current surface behind the excavation start position Ps is an upward slope. To ignore a small undulation such as the current surface  50 _ 1  shown in  FIG. 5 , the angular threshold may be a small value in a range from 1 degree to 6 degrees, for example. Alternatively, the angular threshold may be 0. 
     When it is determined that the current surface behind the excavation start position Ps is not an upward slope in step S 204 , the process proceeds to step S 205 . Thus, when the current surface behind the excavation start position Ps is a downward slope or a horizontal surface, the process proceeds to step S 205 . In step S 205 , a virtual design surface  70  inclined at the inclination angle α is determined as the virtual design surface  70  (second virtual design surface) to be used to control the work implement  13 . For example, as shown in  FIG. 5 , the controller  26  determines the virtual design surface  70  extending from the excavation start position Ps in a direction inclined at the inclination angle α. 
     In step S 206 , whether or not an initial design surface (the initial design surface among multiple surfaces into which the virtual design surface  70  is divided) of the virtual design surface  70  is located above the current landscape  50  is determined. The initial design surface is a design surface located immediately ahead of the excavation start position Ps. For example, as shown in  FIG. 10 , when the design surface  70 _ 2  immediately ahead of the excavation start position Ps is located above the current landscape  50 , it is determined that the initial design surface  70 _ 2  is located above the current landscape  50 , and the process proceeds to step S 207 . 
     In step S 207 , the initial design surface is changed. In this case, the controller  26  changes the position of a design surface next to the excavation start position Ps to a position below the current landscape  50  by a predetermined distance. The predetermined distance may be a small value in a range from 0 cm to 10 cm, for example. As a result, the initial design surface  70 _ 2  is changed to be located below the current landscape  50 , as shown in  FIG. 11 . When the predetermined distance is 0 cm, the initial design surface  70 _ 2  is changed to extend along the current landscape  50 . 
     In addition, in step S 208 , the inclination angle α of the virtual design surface  70  is recalculated. In this case, the controller  26  recalculates the inclination angle α so that the estimated amount of soil S to be held, which is calculated for at a point (for example, a point −d 2  shown in  FIG. 11 ) next to the excavation start position Ps as a temporary excavation start position Ps′, matches the target amount of soil St. Then, in the aforementioned step S 107 , the work implement  13  is controlled so that the work implement  13  moves along the virtual design surface  70  inclined at the recalculated inclination angle α. 
     Normally, the amount of soil held by the work implement  13  at the excavation start position Ps is 0 or an extremely small value. Thus, as shown in  FIG. 10 , even when the current landscape  50  includes a recess located immediately ahead of the excavation start position Ps, the recess cannot be filled with soil. Therefore, changing the initial design surface  70 _ 2  in the aforementioned manner makes it possible to prevent the work implement  13  from swinging without touching soil. 
     On the other hand, when it is determined that the initial design surface of the virtual design surface  70  is not located above the current landscape  50  in step S 206 , the initial design surface is not changed. Thus, for example, as shown in  FIG. 7 , when the current landscape  50  includes a recess somewhere in the virtual design surface  70 , the work implement  13  is controlled to pass over the recess. In this case, the work implement  13  holds soil that has been excavated before the work implement  13  reaches the recess from the excavation start position Ps. Thus, the work implement  13  can fill the recess with the soil by moving along the virtual design surface  70  that passes over the recess. 
     As shown in the aforementioned  FIG. 9A , when the current landscape  50  includes an upward slope and a downward slope located ahead of the upward slope, the inclination angle α calculated in step S 202  may be an angle indicating a horizontal surface or an upward slope. In this case, the process proceeds from step S 203  to step S 209 . 
     In step S 209 , the virtual design surface  70  (first virtual design surface) including a scaffold surface  701  (first design surface) is determined. As shown in  FIG. 12 , the scaffold surface  701  is located below the current landscape  50  and extends in the horizontal direction. The scaffold surface  701  reaches the downward slope. A length of the scaffold surface  701  is larger than the length of the work vehicle  1 . The controller  26  determines a virtual design surface  70  including the scaffold surface  701  extending in the horizontal direction from a point (refer to a point −d 1  shown in  FIG. 12 ) next to the excavation start position Ps and an initial design surface (refer to a design surface  70 _ 1  shown in  FIG. 12 ) connecting the excavation start position Ps to the scaffold surface  701 . 
     Note that the scaffold surface  701  may not be completely parallel to the horizontal direction. The scaffold surface  701  may extend in a direction forming a small angle with the horizontal direction. For example, the scaffold surface  701  may be inclined at a smaller angle than an inclination angle of an upward slope at the excavation start position Ps. 
     In step S 210 , the controller  26  determines the height of the scaffold surface  701  so that an estimated amount of soil S to be held between the virtual design surface  70  and the current landscape  50  matches the target amount of soil St. The controller  26  determines the virtual design surface  70  so that the virtual design surface  70  extends along the current landscape  50  at points following the point (point dl shown in  FIG. 12 ) at which the amount of soil between the virtual design surface  70  and the current landscape  50  reaches the target amount of soil St. 
     In this way, when the inclination angle α is an angle indicating an upward slope, the controller  26  controls the work implement  13  so that the work implement  13  moves along the virtual design surface  70  including the scaffold surface  701 . As a result, a flat landscape serving as a scaffold for the work vehicle  1  is formed, and thereby the work operation can be efficiently performed thereafter. 
     When the inclination angle α is an angle indicating a downward slope in step S 203 , the process proceeds to step S 204 . As shown in  FIG. 9B , when the current surface located behind the excavation start position Ps is an upward slope, the process proceeds to step S 211  shown in  FIG. 13 . 
     In step S 211 , a virtual design surface  70  including the scaffold surface  701  and a surface  702  inclined with respect to the scaffold surface  701  is determined. As shown in  FIG. 14 , the scaffold surface  701  is located below the current landscape  50  and extends from the excavation start position Ps in the horizontal direction. Note that the scaffold surface  701  may not be completely parallel to the horizontal direction. The scaffold surface  701  may extend in a direction forming a small angle with respect to the horizontal direction. For example, the scaffold surface  701  may be inclined at a smaller angle than an inclination angle of the upward slope behind or ahead of the excavation start position Ps. 
     The scaffold surface  701  extends to a point located immediately behind a current restoration point Q. The current restoration point Q is a point at which the extension of the scaffold surface  701  overlaps the current landscape  50 . The inclined surface  702  extends from a point located immediately behind the current restoration point Q. In  FIG. 14 , the inclined surface  702  extends from a point d 1  located immediately behind the current restoration point Q. 
     In step S 212 , an inclination angle α of the inclined surface  702  is calculated. In this case, the controller  26  calculates the inclination angle a of the inclined surface  702  so that the amount of soil between the current landscape  50  and the virtual design surface  70  including the scaffold  701  and the inclined surface  702  matches the target amount of soil St. 
     As described above, when the excavation start position Ps is located on the upward slope, and the inclination angle α calculated in step S 202  is an angle indicating a downward slope, the controller  26  determines the virtual design surface  70  including the scaffold surface  701  extending from the excavation start position Ps and the inclined surface  702  with respect to the scaffold surface  701 . Then, the controller  26  controls the work implement  13  so that the work implement  13  moves along the virtual design surface  70  including the scaffold surface  701  and the inclined surface  702 . As a result, a flat landscape serving as a scaffold for the work vehicle  1  is formed, and thereby the work operation can be efficiently performed thereafter. 
     In addition, in this case, when only the scaffold surface  701  is formed, the work implement  13  has an available space to hold soil. Thus, by moving the work implement  13  along the inclined surface  702 , the excavation can be performed along the inclined surface  702  on the side of the downward slope without wasting the space available to hold soil. This therefore makes it possible to improve the efficiency of the work operation. 
     Note that even when the current landscape  50  includes an upward slope and a downward slope, the excavation start position Ps is located on a downward slope as shown in  FIG. 15 , and the inclination angle α calculated in step S 202  is an angle indicating the downward slope, the controller  26  controls the work implement  13  so that the work implement  13  moves along the virtual design surface  70  inclined at the inclination angle α. 
     Although the embodiment of the invention has been described above, the invention is not limited to the aforementioned embodiment and may be variously changed without departing from the gist of the invention. 
     The work vehicle is not limited to the bulldozer and may be another vehicle such as a wheel loader. 
     The work vehicle  1  may be a remotely controllable vehicle. In this case, a portion of the control system  3  may be disposed outside the work vehicle  1 . For example, the controller  26  may be disposed outside the work vehicle  1 . The controller  26  may be disposed in a control center separated from the work site. 
     The controller may be separated in multiple controllers. For example, as shown in  FIG. 16 , the controller may include a remote controller  261  disposed outside the work vehicle  1  and an in-vehicle controller  262  disposed in the work vehicle  1 . The remote controller  261  and the in-vehicle controller  262  may wirelessly communicate with each other via communication devices  38  and  39 . Then, one or more of the aforementioned functions of the controller  26  may be performed by the remote controller  261 , while the other functions may be performed by the in-vehicle controller  262 . For example, the process of determining the virtual design surface  70  may be performed by the remote controller  261 , while the process of outputting the command signal to the work implement  13  may be performed by the in-vehicle controller  262 . 
     The operating device  25  may be disposed outside the work vehicle  1 . In this case, the operator cab may be omitted from the work vehicle  1 . Alternatively, the operating device  25  may be omitted from the work vehicle  1 . The work vehicle  1  may be operated by only the automatic control via the controller  26  without an operation via the operating device  25 . 
     The current landscape acquiring device is not limited to the aforementioned position detecting device  31  and may be another device. For example, as shown in  FIG. 17 , the current landscape acquiring device may be the interface device  37  that receives information from an external device. The interface device  37  may wirelessly receive current landscape information measured by an external measuring device  41 . Alternatively, the interface device  37  may be a device for reading a storage medium and may receive the current landscape information measured by the external measuring device  41  via the storage medium. 
     According to the invention, a system for controlling a work vehicle, a method for controlling a work vehicle, and a work vehicle can be provided which enable an efficient excavation operation with a high-quality finish.