Patent Publication Number: US-11643789-B2

Title: Control system for work vehicle, method, and work vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National stage application of International Application No. PCT/JP2019/005833, filed on Feb. 18, 2019. This U.S. National stage application claims priority under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2018-062238, filed in Japan on Mar. 28, 2018, the entire contents of which are hereby incorporated herein by reference. 
     BACKGROUND 
     Field of the Invention 
     The present invention relates to a control system for a work vehicle, a method, and a work vehicle. 
     Background Information 
     A control for automatically adjusting the position of a work implement such as a blade has been conventionally proposed for work vehicles such as bulldozers or graders and the like. For example, Japanese Patent Publication No. 5247939 describes automatically adjusting a blade by controlling the load so that the load applied to the blade matches a target load during excavating work. 
     SUMMARY 
     According to the abovementioned conventional control, the occurrence of shoe slip can be suppressed by raising the blade when the load on the blade becomes excessive. As a result, work can be performed with good efficiency. 
     However, as illustrated in  FIG.  20   , in the conventional control, first the blade is controlled so as to follow a design topography  100 . Thereafter, when the load on the blade becomes large, the blade is raised due to the load control (see the locus  200  of the blade in  FIG.  20   ). Therefore, when the blade is in a position deep in the design topography  100  with respect to the actual topography  300 , the load applied to the blade increases very quickly whereby the blade may be raised very quickly. In this case, because the terrain is formed with large undulations, it may be difficult to carry out excavating work smoothly. Moreover, there is a concern that the excavated terrain may easily become rough and the quality of the finish may decrease. 
     An object of the present invention is to cause a work vehicle to perform work efficiently and with a good finish quality with automatic control. 
     A first aspect is a control system for a work vehicle including a work implement, the control system including an operating device and a controller. The operating device outputs an operation signal indicating an operation by an operator. The controller communicates with the operating device and controls the work implement. The controller is programmed so as to execute the following processes. The controller determines a first target design topography. The controller generates a command signal for operating the work implement in accordance with the first target design topography. The controller obtains a displacement amount of the work implement from the first target design topography upon receiving the operation signal which indicates an operation of the work implement by the operator during work in accordance with the first target design topography. The controller determines a second target design topography based on the displacement amount. The controller generates a command signal for operating the work implement in accordance with the second target design topography. 
     A second aspect is a method executed by the controller in order to control a work vehicle including a work implement, the method including the following processes. A first process includes determining a first target design topography. A second process includes generating a command signal for operating the work implement in accordance with the first target design topography. A third process includes receiving an operation signal which indicates an operation by an operator, from an operating device. A fourth process includes obtaining a displacement amount of the work implement from the first target design topography upon receiving the operation signal which indicates an operation of the work implement by the operator during work in accordance with the first target design topography. A fifth process includes determining a second target design topography based on the displacement amount. A sixth process includes generating a command signal for operating the work implement in accordance with the second target design topography. 
     A third aspect is a work vehicle, the work vehicle including a work implement, an operating device, and a controller. The operating device outputs an operation signal indicating an operation by an operator. The controller receives the operation signal and controls the work implement. The controller is programmed so as to execute the following processes. The controller determines a first target design topography. The controller generates a command signal for operating the work implement in accordance with the first target design topography. The controller obtains a displacement amount of the work implement from the first target design topography upon receiving the operation signal which indicates an operation of the work implement by the operator during work in accordance with the first target design topography. The controller determines a second target design topography based on the displacement amount. The controller generates a command signal for operating the work implement in accordance with the second target design topography. 
     According to the present invention, a work vehicle can be made to perform work efficiently and with a good finish quality with automatic control. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a side view of a work vehicle according to an embodiment. 
         FIG.  2    is a block diagram of a configuration of a drive system and a control system of the work vehicle. 
         FIG.  3    is a schematic view of a configuration of the work vehicle. 
         FIG.  4    is a flow chart of an automatic control process of the work vehicle. 
         FIG.  5    illustrates examples of a final design topography, an actual topography, and a target design topography. 
         FIG.  6    is a flow chart of a process for determining the target design topography. 
         FIG.  7    illustrates a process for determining the target design topography. 
         FIG.  8    illustrates a process for determining the target design topography. 
         FIG.  9    illustrates a process for determining the target design topography. 
         FIG.  10    illustrates a process for determining the target design topography. 
         FIG.  11    illustrates a process for determining the target design topography. 
         FIG.  12    illustrates a process for determining the target design topography. 
         FIG.  13    is a flow chart of a process when a manual operation is introduced. 
         FIG.  14    illustrates a process when a manual operation is introduced. 
         FIG.  15    illustrates a process when a manual operation is introduced. 
         FIG.  16    illustrates a process when a manual operation is introduced. 
         FIG.  17    is a block diagram of a configuration of a drive system and a control system of the work vehicle according to a first modified example. 
         FIG.  18    is a block diagram of a configuration of a drive system and a control system of the work vehicle according to a second modified example. 
         FIG.  19    illustrates a process when a manual operation is introduced according to another embodiment. 
         FIG.  20    illustrates excavation work according to the prior art. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
     A work vehicle according to an embodiment is discussed hereinbelow with reference to the drawings.  FIG.  1    is a side view of a work vehicle  1  according to an embodiment. The work vehicle  1  according to the present embodiment is a bulldozer. The work vehicle  1  includes a vehicle body  11 , a travel device  12 , and a work implement  13 . 
     The vehicle body  11  has an operating cabin  14  and an engine compartment  15 . An operator&#39;s seat that is not illustrated is disposed inside 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 bottom part of the vehicle body  11 . The travel device  12  has a pair of left and right crawler belts  16 . Only the crawler belt  16  on the left side is illustrated in  FIG.  1   . The work vehicle  1  travels due to the rotation of the crawler belts  16 . 
     The work implement  13  is attached to the vehicle body  11 . The work implement  13  has a lift frame  17 , a blade  18 , and a lift cylinder  19 . The lift frame  17  is attached to the vehicle body  11  in a manner that allows movement up and down centered on an axis X that extends 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 motion of the lift frame  17 . The lift frame  17  may be attached to the travel device  12 . 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 centered on the axis X. 
       FIG.  2    is a block diagram of a configuration of a drive system  2  and a control system  3  of the work vehicle  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 . While only one hydraulic pump  23  is illustrated in  FIG.  2   , a plurality of hydraulic pumps may be provided. 
     The power transmission device  24  transmits driving power from the engine  22  to the travel device  12 . The power transmission device  24  may be a hydrostatic transmission (HST), for example. Alternatively, the power transmission device  24 , for example, may be a transmission including a torque converter or a plurality of speed change 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  and the input device  25   b  are disposed in the operating cabin  14 . 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 operations from an operator for driving the work implement  13  and the travel device  12 , and outputs operation signals in accordance with the operations. The operating device  25   a  includes, for example, an operating lever, a pedal, and a switch and the like. 
     The input device  25   b  is a device for performing belowmentioned automatic control settings of the work vehicle  1 . The input device  25   b  receives an operation by an operator and outputs an operation signal corresponding to the operation. The operation signals of the input device  25   b  are output to the controller  26 . The input device  25   b  is, for example, a touch screen display. However, the input device  25   b  is not limited to a touch screen and may include hardware keys. 
     The controller  26  is programmed so as to control the work vehicle  1  based on obtained data. The controller  26  includes, for example, a processing device (processor) such as a CPU. The controller  26  obtains operation signals from the operating device  25   a  and the input device  25   b . The controller  26  is not limited to one component and may be divided into a plurality of controllers. The controller  26  controls the travel device  12  or the power transmission device  24  thereby causing the work vehicle  1  to travel. The controller  26  controls the control valve  27  thereby causing the blade  18  to move up and down. 
     The control valve  27  is a proportional control valve and is controlled with command signals from the controller  26 . The control valve  27  is disposed between the hydraulic pump  23  and hydraulic actuators such as the lift cylinder  19 . 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  moves. As a result, the lift cylinder  19  is controlled. The control valve  27  may also be a pressure proportional control valve. Alternatively, the control valve  27  may be an electromagnetic proportional control valve. 
     The control system  3  includes a work implement sensor  29 . The work implement sensor  29  detects the position of the work implement  13  and outputs a work implement position signal which indicates the position of the work implement  13 . The work implement sensor  29  may be a displacement sensor that detects displacement of the work implement  13 . Specifically, the work implement sensor  29  detects the stroke length (referred to below as “lift cylinder length L”) of the lift cylinder  19 . As illustrated in  FIG.  3   , the controller  26  calculates a lift angle θlift of the blade  18  based on the lift cylinder length L. The work implement sensor  29  may be a rotation sensor that directly detects the rotation angle of the work implement  13 . 
       FIG.  3    is a schematic view of a configuration of the work vehicle  1 . The reference position of the work implement  13  is depicted as a chain double-dashed line in  FIG.  3   . The reference position of the work implement  13  is the position of the blade  18  while the blade tip of the blade  18  is in contact with the ground surface on a horizontal ground surface. The lift angle θlift is the angle from the reference position of the work implement  13 . 
     As illustrated in  FIG.  2   , the control system  3  includes a positional sensor  31 . The positional sensor  31  measures the position of the work vehicle  1 . The positional sensor  31  includes a global navigation satellite system (GNSS) receiver  32  and an IMU  33 . The GNSS receiver  32  is, for example, a receiving apparatus for a global positioning system (GPS). For example, an antenna of the GNSS receiver  32  is disposed on the operating cabin  14 . The GNSS receiver  32  receives a positioning signal from a satellite, computes the position of the antenna from the positioning signal, and generates vehicle body position data. The controller  26  obtains the vehicle body position data from the GNSS receiver  32 . The controller  26  derives the traveling direction and the vehicle speed of the work vehicle  1  from the vehicle body position data. 
     The vehicle body position data may not be data of the antenna position. The vehicle body position data may be data that indicates a position of an arbitrary position having a fixed positional relationship with an antenna inside the work vehicle  1  or in the surroundings of the work vehicle  1 . 
     The IMU  33  is an inertial measurement device. The IMU  33  obtains vehicle body inclination angle data. The vehicle body inclination angle data includes the angle (pitch angle) relative to horizontal in the vehicle front-back direction and the angle (roll angle) relative to horizontal in the vehicle lateral direction. The controller  26  obtains the vehicle body inclination angle data from the IMU  33 . 
     The controller  26  computes a blade tip position Pb 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 θlift based on the lift cylinder length L. The controller  26  calculates local coordinates of the blade tip position Pb with respect to the GNSS receiver  32  based on the lift angle θlift and vehicle body dimension data. The vehicle body dimension data 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 the global coordinates of the blade tip position Pb based on the global coordinates of the GNSS receiver  32 , the local coordinates of the blade tip position Pb, and the vehicle body inclination angle data. The controller  26  obtains the global coordinates of the blade tip position Pb as blade tip position data. 
     The storage device  28  includes, for example, a memory and an auxiliary storage device. The storage device  28  may be a RAM or a ROM, for example. The storage device  28  may be a semiconductor memory or a hard disk and the like. The storage device  28  is an example of a non-transitory computer-readable recording medium. The storage device  28  records computer commands for controlling the work vehicle  1  and that are executable by the processor. 
     The storage device  28  stores design topography data and work site topography data. The design topography data indicates the final design topography. The final design topography is the final target shape of the surface of a work site. The work site topography data is, for example, a civil engineering diagram map in a three-dimensional data format. The work site topography data indicates the topography of a wide area of the work site. The work site topography data is, for example, an actual topographical survey map in a three-dimensional data format. The work site topography data can be derived, for example, from an aerial laser survey. 
     The controller  26  obtains actual topography data. The actual topography data indicates the actual topography of the work site. The actual topography of the work site is the topography of an area in the traveling direction of the work vehicle  1 . The actual topography data is obtained by computations by the controller  26  from the work site topography data, the position of the work vehicle  1  obtained by the abovementioned positional sensor  31 , and from the traveling direction. The actual topography data may be obtained by performing distance surveying on the actual topography with an on-board laser imaging detection and ranging device (LIDAR). 
     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 accompaniment with manual operations by the 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. The traveling of the work vehicle  1  may be controlled automatically by the controller  26 . For example, the travel control of the work vehicle  1  may be a fully automatic control that is performed without manual operations by an operator. Alternatively, the travel control may be a semi-automatic control that is performed in accompaniment with manual operations by an operator. Alternatively, the travel of the work vehicle  1  may be performed with manual operations by the operator. 
     Automatic control of the work vehicle  1  during excavation and executed by the controller  26  will be explained below. The controller  26  starts the automatic control when a predetermined starting condition is met. The predetermined starting condition may be, for example, that an operation signal which indicates a lowering operation of the work implement  13  is received from the operating device  25   a . Alternatively, the predetermined starting condition may be that an operation signal indicating an automatic control starting command is received by the controller  26  from the input device  25   b.    
       FIG.  4    is a flow chart of automatic control processes of the work vehicle  1 . As illustrated in  FIG.  4   , the controller  26  obtains the current position data in step S 101 . The controller  26  obtains the current blade tip position Pb of the work implement  13  as indicated above. 
     In step S 102 , the controller  31  obtains the design topography data. As illustrated in  FIG.  5   , the design topography data includes a height Zdesign of a final design topography  60  at a plurality of reference points Pn (n=0, 1, 2, 3, . . . , A) in the traveling direction of the work vehicle  1 . The plurality of reference points Pn indicate a plurality of spots at predetermined intervals in the traveling direction of the work vehicle  1 . The plurality of reference points Pn are on the travel path of the blade  18 . In  FIG.  5   , while the final design topography  60  has a shape that is flat and parallel to the horizontal direction, the shape of the final design topography  60  may be different. 
     In step S 103 , the controller  26  obtains the actual topography data. The controller  26  obtains the actual topography data by computations of the work site topography data obtained from the storage device  28  and the vehicle body position data and the traveling direction data obtained by the positional sensor  31 . 
     The actual topography data is information which indicates the topography located in the traveling direction of the work vehicle  1 .  FIG.  5    illustrates a cross-section of actual topography  50 . In  FIG.  5   , the vertical axis indicates the height of the topography and the horizontal axis indicates the distance from the current position in the traveling direction of the work vehicle  1 . 
     Specifically, the actual topography data includes a height Zn of the actual topography  50  at each of the plurality of reference points Pn from the current position to a predetermined topography recognition distance dA in the traveling direction of the work vehicle  1 . In the present embodiment, the current position may be a position defined based on the current blade tip position Pb of the work vehicle  1 . However, the current position may also be defined based on the current position of another portion of the work vehicle  1 . The plurality of reference points are aligned with a predetermined interval, for example 1 m, between each point. 
     In step S 104 , the controller  26  determines the target design topography data. The target design topography data represents a target design topography  70  indicated by the dashed line in  FIG.  5   . The target design topography  70  represents a desired locus of the blade tip of the blade  18  during the work. The target design topography  70  is a target profile of the topography that is the work object and represents the desired shape as a result of the excavating work. As illustrated in  FIG.  5   , the controller  26  determines at least a portion of the target design topography  70  located below the actual topography  50 . 
     The controller  26  determines the target design topography  70  so as not to go below the final design topography  60 . Therefore, the controller  26  determines the target design topography  70  located above the final design topography  60  and below the actual topography  50  during the excavating work. 
     In step S 105 , the controller  26  controls the work implement  13  in accordance with the target design topography  70 . The controller  26  generates command signals for the work implement  13  so as to move the blade tip position of the blade  18  in accordance with the target design topography  70 . The generated command signal is inputted to the control valve  27 . Consequently, the blade tip position PB of the blade  18  moves toward the target design topography  70 . 
     In step S 106 , the controller  26  updates the work site topography data. The controller  26  updates the work site topography data with the position data that indicates the most recent locus of the blade tip position Pb. The update of the work site topography data may be performed at any time. Alternatively, the controller  26  may calculate the location of the bottom surface of the crawler belts  16  from the vehicle body position data and the vehicle body dimension data, and may update the work site topography data with the position data that indicates the locus of the bottom surface of the crawler belts  16 . In this case, the updating of the work site topography data can be performed promptly. 
     Alternatively, the work site topography data may be generated from survey data measured by a survey device outside of the work vehicle  1 . For example, aerial laser surveying may be used as the external measurement device. Alternatively, the actual topography  50  may be imaged by a camera and the work site topography data may be generated from image data captured by the camera. For example, aerial photography surveying performed with an unmanned aerial vehicle (UAV) may be used. When using the external surveying device or a camera, the updating of the work site topography data may be performed at predetermined periods or at any time. 
     By repeating the above processes, the excavating is performed so that the actual topography  50  approaches the final design topography  60 . 
     The processing for determining the target design topography  70  is explained in detail below.  FIG.  6    is a flow chart of a process for determining the target design topography  70 . As illustrated in  FIG.  6   , in step S 201 , the controller  26  determines a starting point S 0 . As illustrated in  FIG.  7   , the controller  26  determines, as the starting point S, a position that is a predetermined distance L 1  in front of the blade tip position Pb at the point in time that the automatic control starts. The predetermined distance L 1  is saved in the storage device  28 . The input device  25   b  may be used to allow setting of the predetermined distance L 1 . 
     In step S 202 , the controller  26  determines a plurality of division points An (n=1, 2, . . . ) based on the actual topography data. As illustrated in  FIG.  7   , the controller  26  demarcates the actual topography  50  into a plurality of divisions according to the division points An. The division points An are spots positioned away from each other by a predetermined interval L 2  on the actual topography  50 . The predetermined interval L 2  is, for example, 3 m. However, the predetermined interval L 2  may be less than 3 m or greater than 3 m. The predetermined interval L 2  is saved in the storage device  28 . The input device  25   b  may be used to allow setting of the predetermined interval L 2 . The controller  26  determines, as the division points An, a plurality of spots at each predetermined interval L 2  in the traveling direction of the work vehicle  1  from the starting point S 0 . 
     In step S 203 , the controller  26  smooths the actual topography data. The controller  26  smooths the actual topography data by linear interpolation. Specifically, as illustrated in  FIG.  8   , the controller  26  smooths the actual topography data by replacing the actual topography  50  with straight lines that link each of the division points An. 
     In step S 204 , the controller  26  determines a target depth L 3 . The controller  26  determines the target depth L 3  in accordance with a control mode set with the input device  25   b . For example, the operator is able to select any of a first mode, a second mode, and a third mode with the input device  25   b . The first mode is a control mode with the greatest load and the third mode is a control mode with the smallest load. The second mode is a control mode with a load between the first mode and the third mode. 
     The target depths L 3  corresponding to each mode are saved in the storage device  28 . The controller  26  selects, as the target depth L 3 , a first target depth of the first mode, a second target depth of the second mode, or a third target depth of the third mode. The first target depth is greater than the second target depth. The second target depth is greater than the third target depth. The input device  25   b  may be used to allow optional setting of the target depth L 3 . 
     In step S 205 , the controller  26  determines a plurality of reference points. As illustrated in  FIG.  9   , the controller  26  determines, as respective reference points B 1  and B 2 , spots displaced downward by the target depth L 3  from the first preceding division point A 1  and from the second preceding division point A 2 . 
     In step S 206 , the controller  26  determines a plurality of reference topographies. As illustrated in  FIG.  9   , the controller  26  determines a first reference topography C 1  and a second reference topography C 2 . The first reference topography C 1  is represented by a straight line that links the starting point S 0  and the first preceding reference point B 1 . The second reference topography C 2  is represented by a straight line that links the starting point S 0  and the second preceding reference point B 2 . 
     In step S 207 , the controller  26  determines the target design topography  70 . The controller  26  determines the target design topography  70  for each division demarcated by the plurality of division points An. As illustrated in  FIG.  10   , the controller  26  determines a first target design topography  70 _ 1  that passes between the first reference topography C 1  and the second reference topography C 2 . The first target design topography  70 _ 1  is the target design topography  70  in the division (referred to below as “first division”) between the starting point S 0  and the first preceding division point A 1 . 
     Specifically, the controller  26  calculates the average angle of the first reference topography C 1  and the second reference topography C 2 . The average angle is the average value between the angle of the first reference topography C 1  with respect to the horizontal direction and the angle of the second reference topography C 2  with respect to the horizontal direction The controller  26  determines, as the first target design topography  70 _ 1 , a straight line that is inclined by the average angle with respect to the horizontal direction. 
     When the first target design topography  70 _ 1  is determined as indicated above, the controller  26  controls the work implement  13  in accordance with the first target design topography  70 _ 1  in accordance with the abovementioned process of step S 105  as illustrated in  FIG.  11   . 
     In step S 208 , the controller  26  determines the next starting point S 1 . The next starting point S 1  is the starting point of the next target design topography  70 , namely a second target design topography  70 _ 2 . The second target design topography  70 _ 2  is the target design topography  70  in the division (referred to below as the “second division”) between the next starting point S 1  and the first preceding division point A 2 . As illustrated in  FIG.  12   , the next starting point S 1  is the end position of the first target design topography  70 _ 1  and is positioned directly below the division point A 1 . 
     Upon determining the next starting point S 1 , the controller  26  determines the second target design topography  70 _ 2  by repeating the processes from step S 205  to step S 207 . The controller  26  determines the second target design topography  70 _ 2  while working in accordance with the first target design topography  70 _ 1 . 
     Specifically, as illustrated in  FIG.  12   , the controller  26  determines, as the next first reference topography C 1 , a straight line that links the next starting point S 1  and the first preceding reference point B 2 . The controller  26  also determines, as the next second reference topography C 2 , a straight line that links the next starting point S 1  and the second preceding reference point B 3 . The controller  26  determines the second target design topography  70 _ 2  from the average angle of the first reference topography C 1  and the second reference topography C 2 . 
     When the work vehicle  1  reaches the next starting point S 1 , the controller  26  controls the work implement  13  in accordance with the second target design topography  70 _ 2  in accordance with the abovementioned process of step S 105 . The controller  26  then continues the excavation of the actual topography  50  by repeating the above processes. 
     When a predetermined completion condition is satisfied, the controller  26  finishes the abovementioned processes for determining the target design topography  70 . The predetermined completion condition is, for example, that the amount of material held by the work implement  13  has reached a predetermined upper limit. When the predetermined completion condition is satisfied, the controller  26  controls the work implement  13  so as to follow the actual topography  50 . Consequently, the excavated material can be transported smoothly. 
     The process when a manual operation of the work implement  13  is introduced by the operator during the abovementioned automatic control is explained next.  FIG.  13    is a flow chart of a process when a manual operation is introduced. In the following explanation as illustrated in  FIG.  14   , a manual operation of the work implement  13  is introduced by the operator during the work in accordance with the first target design topography  70 _ 1 . 
     In step S 301 , the controller  26  determines that a manual operation has been performed. The controller  26  determines that a manual operation has been performed when an operation signal which indicates an operation for causing the work implement  13  to move up or down is received from the operating device  25   a . The process advances to S 302  when the manual operation is performed. 
     In step S 302 , the controller  26  obtains the displacement amount of the work implement  13 . Specifically, as illustrated in  FIG.  14   , the controller  26  calculates a displacement amount L 4  in the vertical direction of the blade tip position Pb with respect to the first target design topography  70 _ 1 . 
     In step S 303 , the controller  26  corrects the target design topography  70  during the current work. That is, as illustrated in  FIG.  15   , the controller  26  corrects the first target design topography  70 _ 1  so as to match the modified height of the blade tip position Pb. In addition, the controller  26  controls the work implement  13  in accordance with the corrected first target design topography  70 _ 1 . 
     The controller  26  raises the first target design topography  70 _ 1  to match the height of the blade tip position Pb when a raising operation of the work implement  13  is performed. The controller  26  lowers the first target design topography  70 _ 1  to match the height of the blade tip position Pb when a lowering operation of the work implement  13  is performed. 
     In step S 304 , the controller  26  corrects the target depth L 3  based on the displacement amount L 4 . When a raising operation of the work implement  13  is performed, the controller  26  reduces the target depth L 3  by the displacement amount L 4 . When a lowering operation of the work implement  13  is performed, the controller  26  increases the target depth L 3  by the displacement amount L 4 . 
     In step S 305 , the controller  26  determines the target design topography  70  for the next division based on a corrected target depth L 3 ′. That is, as illustrated in  FIG.  16   , the controller  26  determines the second target design topography  70 _ 2  based on the corrected target depth L 3 ′. The second target design topography  70 _ 2  is determined in accordance with the abovementioned processes from step S 205  to step S 207 . 
     Specifically, the controller  26  determines the next starting point S 1  from the corrected first target design topography  70 _ 1 . The controller  26  determines respective reference points B 2  and B 3  by displacing the division points A 2  and A 3  in the vertical direction by the corrected target depth L 3 ′. The controller  26  determines, as the next first reference topography C 1 , a straight line that links the next starting point S 1  and the first preceding reference point B 2 . The controller  26  also determines, as the next second reference topography C 2 , a straight line that links the next starting point S 1  and the second preceding reference point B 3 . The controller  26  determines the second target design topography  70 _ 2  from the average angle pf the first reference topography C 1  and the second reference topography C 2 . 
     When the work vehicle  1  reaches the next starting point S 1 , the controller  26  controls the work implement  13  in accordance with the second target design topography  70 _ 2  in accordance with the abovementioned process of step S 105 . The controller  26  then continues the excavation of the actual topography  50  by repeating the above processes. 
     In the above examples, the first target design topography  70 _ 1  is the target design topography  70  of the first division where the automatic control is started. However, the first target design topography  70 _ 1  may be the target design topography of another division. That is, the first division signifies the division where work is being performed when the manual introduction occurs and is not limited to the target design topography  70  of the first division where the automatic control is started. 
     In the control system  3  of the work vehicle  1  according to the present embodiment explained above, the controller  26  operates the work implement  13  in accordance with the target design topography  70 . As a result, when the final design topography  60  is still in a deep position, excavating by the work implement  13  is performed in accordance with the target design topography  70  that is positioned above the final design topography  60 . As a result, a situation in which the load on the work implement  13  becomes excessive is suppressed. In addition, the sudden raising or lowering of the work implement  13  is suppressed. Accordingly, the work vehicle  1  can be made to perform work efficiently and with a good finish quality. 
     When a manual operation of the work implement  13  is introduced by the operator during the automatic control, the controller  26  corrects the first target design topography  70 _ 1  in response to the displacement amount L 4  of the work implement  13 . The controller  26  also corrects the target depth L 3  in response to the displacement amount L 4  of the work implement  13  and determines the second target design topography  70 _ 2  based on the corrected target depth L 3 ′. As a result, the intention of the operator can be reflected in the automatic control. 
     Although an embodiment of the present invention has been described so far, the present invention is not limited to the above embodiments and various modifications may be made within the scope of the invention. 
     The work vehicle  1  is not limited to a bulldozer, and may be another type of work vehicle such as a wheel loader, a motor grader, a hydraulic excavator, or the like. 
     The work vehicle  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 vehicle  1 . For example, the controller  26  may be disposed outside the work vehicle  1 . The controller may be disposed inside a control center spaced away from the work site. In this case, the work vehicle  1  may not be provided with the operating cabin  14 . 
     The work vehicle  1  may be driven by an electric motor. In this case, the power source may be disposed outside of the work vehicle  1 . The internal combustion engine or the engine compartment may not be provided in the work vehicle  1  in which the power source is supplied from the outside. 
     The controller  26  may have a plurality of controllers  26  separate from each other. For example, as illustrated in  FIG.  17   , the controller  26  may include a remote controller  261  disposed outside of the work vehicle  1  and an on-board controller  262  mounted in the work vehicle  1 . The remote controller  261  and the on-board controller  262  may be able to communicate wirelessly via communication devices  38  and  39 . A portion of the abovementioned functions of the controller  26  may be executed by the remote controller  261 , and the remaining functions may be executed by the on-board controller  262 . For example, the processing for determining the target design topography  70  may be performed by the remote controller  261 , and the process for outputting the command signals to the work implement  13  may be performed by the on-board controller  262 . 
     The operating device  25   a  and the input device  25   b  may also be disposed outside of the work vehicle  1 . In this case, the operating cabin may be omitted from the work vehicle  1 . Alternatively, the operating device  25   a  and the input device  25   b  may be omitted from the work vehicle  1 . 
     The actual topography  50  may be obtained with another device and is not limited to being obtained with the abovementioned positional sensor  31 . For example, as illustrated in  FIG.  18   , the actual topography  50  may be obtained with an interface device  37  that receives data from an external device. The interface device  37  may wirelessly receive the actual topography data measured by an external measurement device  41 . Alternatively, the interface device  37  may be a recording medium reading device and may receive the actual topography data measured by the external measurement device  41  via a recording medium. 
     The method for setting the target design topography  70  is not limited to the method of the above embodiment and may be changed. For example, the target design topography  70  is determined based on two preceding reference points from the starting point in the above embodiment. However, the target design topography  70  may be determined based on three or more preceding reference points from the starting point. The controller  26  may determine the first target design topography  70 _ 1  based on another parameter without being limited to the target depth. For example, the controller  26  may determine the first target design topography  70 _ 1  based on the load on the work implement  13 , a target angle, a target position, or another parameter. Alternatively, the first target design topography  70 _ 1  may be determined ahead of time. 
     The controller  26  determines the target design topography  70  based on the average angle between the first reference topography C 1  and the second reference topography C 2  in the above embodiment. However, the determination is not limited to the average angle and the controller  26  may determine the target design topography  70  by implementing a process such as weighting the angle of the first reference topography C 1  and the angle of the second reference topography C 2 . 
     The controller  26  determines the second target design topography  70 _ 2  during the work on the first target design topography  70 _ 1  and before reaching the next starting position S 1  in the above embodiment. However, the controller  26  may determine the second target design topography  70 _ 2  upon reaching the next starting point S 1 . 
     The controller  26  may determine the target design topography  70  above the actual topography  50 . For example, as illustrated in  FIG.  19   , when the operator raises the blade tip position Pb to a position above the actual topography  50 , the controller  26  may raise the first target design topography  70 _ 1  to a position above the actual topography  50  to match the height of the blade tip position Pb. The controller  26  may determine the second target design topography  70 _ 2  so as to be positioned above the actual topography  50 . Consequently, for example, the material held by the work implement  13  can be leveled on the actual topography  50 . 
     According to the present invention, a work vehicle can be made to perform work efficiently and with a good finish quality with automatic control.