Patent Publication Number: US-10787789-B2

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

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National stage application of International Application No. PCT/JP2017/026914, filed on Jul. 25, 2017. This U.S. National stage application claims priority under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2016-146382, filed in Japan on Jul. 26, 2016, 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 control method, and a work vehicle. 
     Background Information 
     An automatic control for automatically adjusting the position of a work implement has been conventionally proposed for work vehicles such as bulldozers or graders and the like. For example, Japanese Patent Publication No. 5247939 discloses excavation control and leveling control. 
     Under the excavation control, the position of the blade is automatically adjusted such that the load applied to the blade coincides with a target load. Under the leveling control, the position of the blade is automatically adjusted so that the tip of the blade moves along a design topography which describes a target shape of the excavation target. 
     SUMMARY 
     Work conducted by a work vehicle includes filling work as well as excavating work. During filling work, the work vehicle removes soil from a cutting with the work implement. The work vehicle then piles the removed soil in a predetermined position and compacts the piled soil by traveling over the piled soil. As a result for example, the depressed topography is filled in and a flat shape can be formed. 
     However, it is difficult to perform desirable filling work under the abovementioned automatic controls. For example as indicated in  FIG. 19 , under the leveling control, the position of the blade is automatically adjusted so that the blade tip  200  of the blade moves along the design topography  100 . As a result, when the filling work is performed using the leveling control, a large amount of soil is piled at one time in a position in front of the work vehicle  300  as illustrated in  FIG. 19  by the dashed line. In this case, it is difficult to compact the piled soil because the height of the piled soil is too large. As a result, there is a problem that the quality of the finished work is poor. 
     In addition, when the work vehicle travels over the actual topography having a steep slope, the attitude of the work vehicle may be suddenly changed. For example, the attitude of the work vehicle may change greatly when traveling over the top of the slope or the bottom of the slope. In this case, there is a problem that the quality of the work finish may deteriorate due to a delay in the tracking of the work implement with respect to the sudden change in the attitude. Sufficiently reducing the speed of the work vehicle could be thought of as a method for preventing a deterioration in the quality when traveling over the top or bottom of the slope. However in such a case, there is a problem that work efficiency is reduced. 
     An object of the present invention is to provide a control system for a work vehicle, a control method, and a work vehicle that enable filling work to be performed that is efficient and exhibits a quality finish using automatic controls. 
     A control system for a work vehicle according to a first aspect is provided with an actual topography acquisition device, a storage device, and a controller. The actual topography acquisition device acquires actual topography information which indicates an actual topography of a work target. The storage device stores design topography information which indicates a final design topography which is a target topography of the work target. The controller acquires the actual topography information from the actual topography acquisition device. The controller acquires the design topography information from the storage device. The controller generates a command signal to move the work implement along a locus that is more gently sloped than the actual topography when the actual topography positioned below the final design topography is sloped. 
     A control method of the work vehicle according to a second aspect includes the following steps. A first step is to acquire actual topography information which indicates an actual topography of a work target. A second step is to acquire design topography information which indicates a final design topography which is a target topography of the work target. A third step is to generate a command signal to move a work implement along a locus that is more gently sloped than the actual topography when the actual topography positioned below the final design topography is sloped. 
     A work vehicle according to a third aspect is provided with a work implement and a controller. The controller acquires actual topography information which indicates an actual topography of a work target. The controller acquires design topography information which indicates a final design topography which is a target shape of the work target. The controller moves the work implement along a locus that is more gently sloped than the actual topography when the actual topography positioned below the final design topography is sloped. 
     According to the present invention, the work implement is automatically controlled so that the work implement moves along a locus that is more gently sloped than the actual topography with regard to the actual topography positioned below the final design topography. As a result, soil can be piled thinly on the actual topography in comparison to a case of moving the work implement along the final design topography. Consequently, the piled up soil can be easily compacted by the work vehicle and the quality of the finish of the work can be improved. 
     In addition, the slope of the actual topography can be gradually made gentle by repeating the work. As a result, a sudden change in the attitude of the work vehicle can be suppressed. Consequently, unnecessary speed reduction of the work vehicle can be suppressed and work efficiency can be improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a work vehicle according to an embodiment. 
         FIG. 2  is a block diagram 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  illustrates an example of an actual topography, a final design topography, and an intermediate design topography during filling work. 
         FIG. 5  is a flow chart depicting automatic control processing of the work implement during soil piling. 
         FIG. 6  illustrates an example of actual topography information. 
         FIG. 7  is a flow chart depicting processing for determining the intermediate design topography. 
         FIG. 8  illustrates processing for determining a bottom height. 
         FIG. 9  illustrates a first upper limit height, a first lower limit height, a second upper limit height, and a second lower limit height. 
         FIG. 10  is a flow chart depicting processing for determining a pitch angle the intermediate design topography. 
         FIG. 11  illustrates processing for determining a first upper limit angle. 
         FIG. 12  illustrates processing for determining a first lower limit angle. 
         FIG. 13  illustrates processing for determining a shortest distance angle. 
         FIG. 14  illustrates processing for determining a shortest distance angle. 
         FIG. 15  illustrates processing for determining a shortest distance angle. 
         FIG. 16  illustrates an intermediate design topography according to a first modified example. 
         FIG. 17  is a block diagram of a control system according to another embodiment. 
         FIG. 18  is a block diagram of a control system according to another embodiment. 
         FIG. 19  illustrates conventional filling work. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
     A work vehicle according to an embodiment shall be explained in detail with reference to the drawings.  FIG. 1  is a side view of the work vehicle  1  according to an embodiment. The work vehicle  1  is a bulldozer according to the present embodiment. The work vehicle  1  is provided with 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 right crawler belt  16  is depicted 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 , a lift cylinder  19 , an angle cylinder  20 , and a tilt cylinder  21 . 
     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 motions 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  rotates up and down centered on the axis X due to the extension and contraction of the lift cylinder  19 . 
     The angle cylinder  20  is coupled to the lift frame  17  and the blade  18 . Due to the extension and contraction of the angle cylinder  20 , the blade  18  rotates around an axis Y that extends in roughly the up-down direction. 
     The tilt cylinder  21  is coupled to the lift frame  17  and the blade  18 . Due to the extension and contraction of the tilt cylinder  21 , the blade  18  rotates around an axis Z that extends in roughly the front-back direction of the vehicle. 
       FIG. 2  is a block diagram illustrating 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  is provided with 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 operating fluid. The operating fluid discharged from the hydraulic pump  23  is supplied to the lift cylinder  19 , the angle cylinder  20 , and the tilt cylinder  21 . 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 , for example, may be a hydrostatic transmission (HST). Alternatively, the power transmission device  24 , for example, may be a transmission having a torque converter or a plurality of speed change gears. 
     The control system  3  is provided with 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 travel device  12 . The operating device  25  is disposed in the operating cabin  4 . The operating device  25  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  includes, for example, an operating lever, a pedal, and a switch and the like. 
     The controller  26  is programmed to control the work vehicle  1  on the basis of acquired information. The controller  26  includes, for example, a processor such as a CPU. The controller  26  acquires operation signals from the operating device  25 . The controller  26  controls the control valve  27  on the basis of the operation signals. The controller  26  is not limited to one component and may be divided into a plurality of controllers. 
     The control valve  27  is a proportional control valve and is controlled by 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 angle cylinder  20 , and the tilt cylinder  21 . The amount of the operating fluid supplied from the hydraulic pump  23  to the lift cylinder  19 , the angle cylinder  20 , and the tilt cylinder  21  is controlled by the control valve  27 . The controller  26  generates a command signal to the control valve  27  so that the work implement  13  acts in accordance with the abovementioned operations of the operating device  25 . As a result, the lift cylinder  19 , the angle cylinder  20 , and the tilt cylinder  21  and the like are controlled in response to the operation amount of the operating device  25 . 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  is provided with a lift cylinder sensor  29 . The lift cylinder sensor  29  detects the stroke length (referred to below as “lift cylinder length L”) of the lift cylinder  19 . As depicted in  FIG. 3 , the controller  26  calculates a lift angle θlift of the blade  18  on the basis of the lift cylinder length L.  FIG. 3  is a schematic view of a configuration of the work vehicle  1 . 
     The origin position of the work implement  13  is depicted as a chain double-dashed line in  FIG. 3 . The origin position of the work implement  13  is the position of the blade  18  while the 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 origin position of the work implement  13 . 
     As illustrated in  FIG. 2 , the control system  3  is provided with a position detection device  31 . The position detection device  31  detects the position of the work vehicle  1 . The position detection device  31  is provided with a GNSS receiver  32  and an IMU  33 . The GNSS receiver  32  is disposed on the operating cabin  14 . The GNSS receiver  32  is, for example, an antenna for a global positioning system (GPS). The GNSS receiver  32  receives vehicle position information which indicates 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 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 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  computes a blade tip position P 1  from the lift cylinder length L, the vehicle position information, and the vehicle inclination angle information. As illustrated in  FIG. 3 , the controller  26  calculates global coordinates of the GNSS receiver  32  on the basis of the vehicle position information. The controller  26  calculates the lift angle θlift on the basis of the lift cylinder length L. The controller  26  calculates local coordinates of the blade tip position P 1  with respect to the GNSS receiver  32  on the basis of 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 the global coordinates of the blade tip position P 1  on the basis of the global coordinates of the GNSS receiver  32 , the local coordinates of the blade tip position P 1 , and the vehicle inclination angle information. The controller  26  acquires the global coordinates of the blade tip position P 1  as blade tip position information. 
     As illustrated in  FIG. 2 , the control system  3  is provided with a soil amount acquisition device  34 . The soil amount acquisition device  34  acquires soil amount information which indicates the amount of soil held by the work implement  13 . The soil amount acquisition device  34  generates a soil amount signal which indicates the soil amount information and sends the soil amount signal to the controller  26 . In the present embodiment, the soil amount information indicates the tractive force of the work vehicle  1 . The controller  26  calculates the held soil amount from the tractive force of the work vehicle  1 . For example, in the work vehicle  1  provided with the HST, the soil amount acquisition device  34  is a sensor for detecting the hydraulic pressure (driving hydraulic pressure) supplied to the hydraulic motor of the HST. In this case, the controller  26  calculates the tractive force from the driving hydraulic pressure and calculates the held soil amount from the calculated tractive force. 
     Alternatively, the soil amount acquisition device  34  may be a survey device that detects changes in the actual topography. In this case, the controller  26  may calculate the held soil amount from a change in the actual topography. Alternatively, the soil amount acquisition device  34  may be a camera that acquires image information of the soil carried by the work implement  13 . In this case, the controller  26  may calculate the held soil amount from the image information. 
     The control system  3  is provided with a storage device  28 . 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  stores design topography information. The design topography information indicates the position and the shape of a final design topography. The final design topography indicates a target topography of a work target at the work site. The controller  26  acquires actual topography information. The actual topography information indicates the position and shape of the actual topography of the work target at the work site. The controller  26  automatically controls the work implement  13  on the basis of the actual topography information, the design topography information, and the blade tip position information. 
     Automatic control of the work implement  13  during filling work and executed by the controller  26  will be explained below.  FIG. 4  depicts an example of a final design topography  60  and an actual topography  50  positioned below the final design topography  60 . During filling work, the work vehicle  1  piles up and compacts the soil on top of the actual topography  50  positioned below the final design topography  60 , whereby the work target is formed so as to become the final design topography  60 . 
     The controller  26  acquires actual topography information which indicates the actual topography  50 . For example, the controller  26  acquires position information which indicates the locus of the blade tip position P 1  as the actual topography information. Therefore, the position detection device  31  functions as an actual topography acquisition device for acquiring the actual topography information. 
     Alternatively, the controller  26  may calculate the position of the bottom surface of the crawler belt  16  from the vehicle position information and the vehicle dimension information, and may acquire the position information which indicates the locus of the bottom surface of the crawler belt  16  as the actual topography information. Alternatively, the actual topography information may be generated from survey data measured by a survey device outside of the work vehicle  1 . Alternatively, the actual topography  50  may be imaged by a camera and the actual topography information may be generated from image data acquired by the camera. 
     As illustrated in  FIG. 4 , the final design topography  60  is horizontal and flat in the present embodiment. However, a portion or all of the final design topography  60  may be inclined. In  FIG. 4 , the height of the final design topography in the range from −d2 to 0 is the same as the height of the actual topography  50 . 
     The controller  26  determines an intermediate design topography  70  that is positioned between the actual topography  50  and the final design topography  60 . In  FIG. 4 , a plurality of the intermediate design topographies  70  are indicated by dashed lines; however, only a portion, thereof is given the reference numeral “70.” As illustrated in  FIG. 4 , the intermediate design topography  70  is positioned above the actual topography  50  and below the final design topography  60 . The controller  26  determines the intermediate design topography  70  on the basis of the actual topography information, the design topography information, and the soil amount information. 
     The intermediate design topography  70  is set to the position of a predetermined distance D1 above the actual topography  50 . The controller  26  determines the next intermediate design topography  70  at the position of the predetermined distance D1 above the updated actual topography  50  each time the actual topography  50  is updated. As a result, the plurality of intermediate design topographies  70  which are stacked on the actual topography  50  are generated as illustrated in  FIG. 4 . The processing for determining the intermediate design topography  70  is explained in detail below. 
     The controller  26  controls the work implement  13  on the basis of intermediate topography information which indicates the intermediate design topography  70  and blade tip position information which indicates the blade tip position P 1 . Specifically, the controller  26  generates command signals for the work implement  13  so as to move the blade tip position P 1  of the work implement  13  along the intermediate design topography  70 . 
       FIG. 5  is a flow chart depicting automatic control processing of the work implement  13  during filling work. As illustrated in  FIG. 5 , the controller  26  acquires the current position information in step S 101 . As illustrated in  FIG. 6 , the controller  26  acquires the height Hm_- 1  of an intermediate design surface  70 _- 1  that is one position before the previously determined reference position P 0 , and a pitch angle θm_- 1  of the intermediate design surface  70 _- 1 . 
     However, during the initial state of the filling work, the controller  26  acquires the height of the actual surface  50 _- 1  which is one surface before the reference position P 0  in place of the height Hm_- 1  of the intermediate design topography  70 _- 1  that is one position before the previously determined reference position P 0 . During the initial state of the filling work, the controller  26  acquires the pitch angle of the actual surface  50 _- 1  which is one surface before the reference position P 0  in place of the pitch angle θm_- 1  of the intermediate design topography  70 _- 1  that is one position before the previously determined reference position P 0 . The initial state of the filling work can be a state when the work vehicle is switched, for example, from reverse travel to forward travel. 
     In step S 102 , the controller  26  acquires the actual topography information. As illustrated in  FIG. 6 , the actual topography  50  includes a plurality of actual surfaces  50 _ 1  to  50 _ 10  which are divided by a predetermined interval d1 from the predetermined reference position P 0  in the traveling direction of the work vehicle  1 . The reference position P 0  is the position where the actual topography  50  starts to slope downward from the final design topography  60  in the traveling direction of the work vehicle  1 . In other words, the reference position P 0  is the position where the height of the actual topography  50  starts to become smaller than the height of the final design topography  60  in the traveling direction of the work vehicle  1 . Alternatively, the reference position P 0  is a position in front of the work vehicle  1  by a predetermined distance. Alternatively, the reference position P 0  is the current position of the blade tip position P 1  of the work vehicle  1 . Alternatively, the reference position P 0  may be a position at the top of the slope of the actual topography  50 . In  FIG. 6 , the vertical axis indicates the height of the topography and the horizontal axis indicates the distance from the reference position P 0 . 
     The actual topography information includes the position information of the actual surfaces  50 _ 1  to  50 _ 10  for each predetermined interval d1 from the reference position P 0  in the traveling direction of the work vehicle  1 . That is, the actual topography information includes the position information of the actual surfaces  50 _ 1  to  50 _ 10  from the reference position P 0  as far forward as the predetermined distance d10. 
     As illustrated in  FIG. 6 , the controller  26  acquires the heights Ha_ 1  to Ha_ 10  of the actual surfaces  50 _ 1  to  50 _ 10  as the actual topography information. In the present embodiment, the actual surfaces acquired as the actual topography information include up to ten actual surfaces; however, the number of actual surfaces may be more than ten or less than ten. 
     In step S 103 , the controller  26  acquires the design topography information. As illustrated in  FIG. 6 , the final design topography  60  includes a plurality of final design surfaces  60 _ 1  to  60 _ 10 . Therefore, the design topography information includes the position information of the final design surfaces  60 _ 1  to  60 _ 10  at each predetermined interval d1 in the traveling direction of the work vehicle  1 . That is, the design topography information includes the position information of the final design surfaces  60 _ 1  to  60 _ 10  from the reference position P 0  as far forward as the predetermined distance d10. 
     As illustrated in  FIG. 6 , the controller  26  acquires the heights Hf_ 1  to Hf_ 10  of the final design surfaces  60 _ 1  to  60 _ 10  as the design topography information. In the present embodiment, the number of final design surfaces acquired as the design topography information includes up to ten final design surfaces; however, the number of final design surfaces may be more than ten or less than ten. 
     In step S 104 , the controller  26  acquires the soil amount information. In this case, the controller  26  acquires the current held soil amount Vs_ 0 . The held soil amount Vs_ 0  is represented, for example, as a ratio with respect to the capacity of the blade  18 . 
     In step S 105 , the controller  26  determines the intermediate design topography  70 . The controller  26  determines the intermediate design topography  70  from the actual topography information, the design topography information, the soil amount information, and the current position information. The processing for determining the intermediate design topography  70  is explained in detail below. 
       FIG. 7  is a flow chart depicting processing for determining the intermediate design topography  70 . In step S 201 , the controller  26  determines a bottom height Hbottom. In this case, the controller  26  determines the bottom height Hbottom so that the bottom soil amount coincides with the held soil amount. 
     As illustrated in  FIG. 8 , the bottom soil amount represents the amount of soil piled below the bottom height Hbottom and above the actual surface  50 . For example, the controller  26  calculates the bottom height Hbottom from the product of the total of bottom lengths Lb_ 4  to Lb_ 10  and the predetermined distance d1, and from the held soil amount. The bottom lengths Lb_ 4  to Lb_ 10  represent the distance from the actual topography  50  upwards to the bottom height Hbottom. 
     In step S 202 , the controller  26  determines a first upper limit height Hup 1 . The first upper limit height Hup 1  defines an upper limit of the height of the intermediate design topography  70 . However, the intermediate design topography  70  may be determined to be positioned above the first upper limit height Hup 1  in response to other conditions. The first upper limit height Hup 1  is defined using the following equation 1.
 
 H up1=MIN(final design topography, actual topography+ D 1)  (Equation 1)
 
Therefore as illustrated in  FIG. 9 , the first upper limit height Hup 1  is positioned below the final design topography  60  and above the actual topography  50  by a predetermined distance D1. The predetermined distance D1 is the thickness of the piled soil to a degree that the piled soil can be appropriately compacted by the work vehicle  1  traveling one time over the piled soil.
 
     In step S 203 , the controller  26  determines a first lower limit height Hlow 1 . The first lower limit height Hlow 1  defines a lower limit of the height of the intermediate design topography  70 . However, the intermediate design topography  70  may be determined to be positioned below the first lower limit height Hlow 1  in response to other conditions. The first lower limit height Hlow 1  is defined using the following equation 2.
 
 H low1=MIN(final design topography, MAX(actual topography, bottom))  (Equation 2)
 
Therefore as illustrated in  FIG. 9 , when the actual topography  50  is positioned below the final design topography  60  and above the abovementioned bottom height Hbottom, the first lower limit height Hlow 1  coincides with the actual topography  50 . Additionally, when the bottom height Hbottom is positioned below the final design topography  60  and above the actual topography  50 , the first lower limit height Hlow 1  coincides with the bottom height Hbottom.
 
     In step S 204 , the controller  26  determines a second upper limit height Hup 2 . The second upper limit height Hup 2  defines an upper limit of the height of the intermediate design topography  70 . The second upper limit height Hup 2  is defined using the following equation 3.
 
 H up2=MIN(final design topography, MAX(actual topography+ D 2, bottom))  (Equation 3)
 
Therefore as illustrated in  FIG. 9 , the second upper limit height Hup 2  is positioned below the final design topography  60  and above the actual topography  50  by a predetermined distance D2. The predetermined distance D2 is larger than the predetermined distance D1.
 
     In step S 205 , the controller  26  determines a second lower limit height Hlow 2 . The second lower limit height Hlow 2  defines a lower limit of the height of the intermediate design topography  70 . The second lower limit height Hlow 2  is defined using the following equation 4.
 
 H low2=MIN(final design topography− D 3, MAX(actual topography− D 3, bottom))  (Equation 4)
 
Therefore as illustrated in  FIG. 9 , the second lower limit height Hlow 2  is positioned below the final design topography  60  by a predetermined distance D3. The second lower limit height Hlow 2  is positioned below the first lower limit height Hlow 1  by the predetermined distance D3.
 
     In step S 206 , the controller  26  determines the pitch angle of intermediate design topography. As illustrated in  FIG. 4 , the intermediate design topography includes the plurality of intermediate design surfaces  70 _ 1  to  70 _ 10  separated from each other by the predetermined distance d1. The controller  26  determines the pitch angle for each of the plurality of intermediate design surfaces  70 _ 1  to  70 _ 10 . The intermediate design topography  70  depicted in  FIG. 4  has different pitch angles for the respective intermediate design surfaces  70 _ 1  to  70 _ 4 . In this case, the intermediate design topography  70  has a shape that is bent at a plurality of locations as illustrated in  FIG. 4 . 
       FIG. 10  is a flow chart illustrating processing for determining the pitch angles of the intermediate design topography  70 . The controller  26  determines the pitch angle of the intermediate design surface  70 _ 1  that is one position ahead the reference position P 0  by using the processing illustrated in  FIG. 10 . 
     In step S 301 , the controller  26  determines a first upper limit angle θup 1  as illustrated in  FIG. 10 . The first upper limit angle θup 1  defines an upper limit of the pitch angle of the intermediate design topography  70 . However, the pitch angle of the intermediate design topography  70  may be larger than the first upper limit angle θup 1  in response to other conditions. 
     As illustrated in  FIG. 11 , the first upper limit angle θup 1  is the pitch angle of the intermediate design surface  70 _ 1  so that the intermediate design surface  70 _ 1  does not exceed the first upper limit height Hup 1  up to the distance d10 when the pitch angle of the intermediate design topography  70  is set to the degree (previous degree −A1) for each interval d1. The first upper limit angle θup 1  is determined as indicated below. 
     When the pitch angle of the intermediate design topography  70  is set as the degree (previous degree −A1) at each interval d1, the pitch angle θn of the intermediate design surface  70 _ 1  is determined using the following equation 5 such that the nth ahead intermediate design surface  70 _ n  is equal to or less than the first upper limit height Hup 1 .
 
θ n =( H up1_ n−Hm _-1+ A 1*( n *( n− 1)/2))/ n   (Equation 5)
 
Hup 1 _ n  is the first upper limit height Hup 1  at the nth ahead intermediate design surface  70 _ n . Hm_- 1  is the height of the intermediate design surface  70 _- 1  which is one position behind the reference position P 0 . A1 is a predetermined constant. θn values are determined from n=1 to 10 using equation 5, and the minimum θn value is selected as the first upper limit angle θup 1 . In  FIG. 11 , the minimum θn value from n=1 to 10 becomes the pitch angle θ 2  that does not exceed the first upper limit height Hup 1  at the distance d2 in front of the reference position P 0 . In this case, δ 2  is selected as the first upper limit angle θup 1 .
 
     However, when the selected first upper limit angle θup 1  is larger than a predetermined change upper limit θlimit 1 , the change upper limit θlimit 1  is selected as the first upper limit angle θup 1 . The change upper limit θlimit 1  is a threshold for limiting the change in the pitch angle from the previous pitch angle to +A1 or less. 
     In the present embodiment, while the pitch angle is determined on the basis of the intermediate design surfaces  70 _ 1  to  70 _ 10  as far as ten positions in front of the reference position P 0 , the number of intermediate design surfaces used in the computation of the pitch angle is not limited to ten and may be more than ten or less than ten. 
     In step S 302 , the controller  26  determines a first lower limit angle θlow 1 . The first lower limit angle θlow 1  defines a lower limit of the pitch angle of the intermediate design topography  70 . However, the pitch angle of the intermediate design topography  70  may be less than the first lower limit angle θlow 1  in response to other conditions. As illustrated in  FIG. 12 , the first lower limit angle θlow 1  is the pitch angle of the intermediate design surface  70 _ 1  so that the intermediate design surface  70 _ 1  does not fall below the first lower limit height Hlow 1  as far forward as the distance d10 when the pitch angle of the intermediate design topography  70  is set to the degree (previous degree −A1) for each interval d1. The first lower limit angle θlow 1  is determined as indicated below. 
     When the pitch angle of the intermediate design topography  70  is set as the degree (previous degree +A1) at each interval d1, one pitch angle θn in front is determined using the following equation 6 such that the nth ahead intermediate design surface  70 _ n  is equal to or greater than the first lower limit height Hlow 1 .
 
θ n =( H low1_ n−Hm _-1− A 1*( n *( n− 1)/2))/ n   (Equation 6)
 
Hlow 1 _ n  is the first lower limit height Hlow 1  with respect to the nth ahead intermediate design surface  70 _ n . θn values are determined from n=1 to 10 using equation 6, and the maximum of the θn values is selected as the first lower limit angle θlow 1 . In  FIG. 12 , the maximum of the θn values from n=1 to 10 becomes the pitch angle θ 3  that does not exceed the first upper limit height Hup 1  at the distance d3 in front of the reference position P 0 . In this case, θ 3  is selected as the first lower limit angle θlow 1 .
 
     However, when the selected first lower limit angle θlow 1  is smaller than a predetermined change lower limit θlimit 2 , the change lower limit θlimit 2  is selected as the first lower limit angle θlow 1 . The change lower limit θlimit 2  is a threshold for limiting a change in the pitch angle from the previous pitch angle to −A1 or greater. 
     In step S 303 , the controller  26  determines a second upper limit angle θup 2 . The second upper limit angle θup 2  defines an upper limit of the pitch angle of the intermediate design topography  70 . The second upper limit angle θup 2  is the pitch angle of the intermediate design surface  70 _ 1  so that the intermediate design surface  70 _ 1  does not exceed the second upper limit height Hup 2  as far forward as the distance d10 when the pitch angle of the intermediate design topography  70  is set to the degree (previous degree −A1) for each interval d1. The second upper limit angle θup 2  is determined in the same way as the first upper limit angle θup 1  with the following equation 7.
 
θ n =( H up2_ n−Hm _-1 +A 1*( n *( n −1)/2))/ n   (Equation 7)
 
     Hup 2 _ n  is the second upper limit height Hup 2  with respect to the nth ahead intermediate design surface  70 _ n . θn values are determined from n=1 to 10 using equation 7, and the minimum θn value is selected as the second upper limit angle θup 2 . 
     In step S 304 , the controller  26  determines a second lower limit angle θlow 2 . The second lower limit angle θlow 2  defines a lower limit of the pitch angle of the intermediate design topography  70 . The second lower limit angle θlow 2  is the pitch angle of the intermediate design surface one position in front of the reference position P 0  so as not to fall below the second lower limit height Hlow 2  second lower limit height Hlow 2  as far forward as the distance d10 when the pitch angle of the intermediate design topography  70  is set to the degree (previous degree +A2) for each interval d1. The angle A2 is larger than the abovementioned angle A1. The second lower limit angle θlow 2  is defined using the following equation 8 in the same way as the first lower limit angle θlow 1 .
 
θ n =( H low2_ n−Hm _-1 −A 2*( n *( n −1)/2))/ n   (Equation 8)
 
     Hlow 2 _ n  is the second lower limit height Hlow 2  with respect to the nth ahead intermediate design surface  70 _ n . A2 is a predetermined constant. θn values are determined from n=1 to 10 using equation 8, and the maximum θn value is selected as the second lower limit angle θlow 2 . 
     However, when the selected second lower limit angle θlow 2  is smaller than a predetermined change lower limit θlimit 3 , the change lower limit θlimit 3  is selected as the first lower limit angle θlow 1 . The change lower limit θlimit 3  is a threshold for limiting the change in the pitch angle from the previous pitch angle to −A2 or greater. 
     In step S 305 , the controller  26  determines a shortest distance angle θs. As illustrated in  FIG. 13 , the shortest distance angle θs is the pitch angle of the intermediate design topography  70  that has the shortest intermediate design topography  70  length between the first upper limit height Hup 1  and the first lower limit height Hlow 1 . For example, the shortest distance angle θs is determined using the following equation 9.
 
θ s =MAX(θlow1_1, MIN(θup1_1, MAX(θlow1_2, MIN(θup1_2, . . . MAX(θlow1_ n , MIN(θup1_ n , . . . MAX(θlow1_10, MIN(θup1_10,θ m _-1))) . . . )))  (Equation 9)
 
As illustrated in  FIG. 14 , θlow 1 _ n  is the pitch angle of a straight line that connects the reference position P 0  and the nth ahead first lower limit height Hlow 1  (four in front in  FIG. 14 ). θup 1 _ n  is the pitch angle of a straight line that connects the reference position P 0  and the nth ahead first upper limit height Hup 1 . θm_- 1  is the pitch angle of the intermediate design surface  70 _- 1  which is one position in front of the reference position P 0 . Equation 9 can be represented as indicated in  FIG. 15 .
 
     In step S 306 , the controller  26  determines whether predetermined pitch angle change conditions are satisfied. The pitch angle change conditions are conditions which indicate that an intermediate design topography  70  is formed so as to be inclined by an angle −A1 or greater. That is, the pitch angle change conditions indicate that a gradually sloped intermediate design topography  70  has been generated. 
     Specifically, the pitch angle change condition includes the following first to third conditions. The first change condition is that the shortest distance angle θs is an angle −A1 or greater. The second change condition is that the shortest distance angle θs is greater than θlow 1 _ 1 . The third change condition is that θlow 1 _ 1  is an angle −A1 or greater. When all of the first to third conditions are satisfied, the controller  26  determines that the pitch angle change conditions are satisfied. 
     The routine advances to step S 307  if the pitch angle change conditions are not satisfied. In step S 307 , the controller  26  determines the shortest distance angle θs derived in step S 306  as a target pitch angle θt. 
     The routine advances to step S 308  if the pitch angle change conditions are satisfied. In step S 308 , the controller  26  determines θlow 1 _ 1  as the target pitch angle θt. θlow 1 _ 1  is the pitch angle that follows the first lower limit height Hlow 1 . 
     In step S 309 , the controller  26  determines a command pitch angle. The controller  26  determines a command pitch angle θc using the following equation 10.
 
θ c =MAX(θlow2, MIN(θup2, MAX(θlow1, MIN(θup1,θ t ))))  (Equation 10)
 
The command pitch angle determined as indicated above is determined as the pitch angle of the intermediate design surface  70 _ 1  in step S 206  in  FIG. 7 . As a result, the intermediate design topography  70  is determined in step S 105  in  FIG. 5 . That is, the intermediate design surface  70 _ 1  that fulfills the abovementioned command pitch angle is determined for the intermediate design topography  70  at the reference position P 0 .
 
     As illustrated in  FIG. 5 , the controller  26  generates a command signal for the work implement  13  in step S 106 . In this case, the controller  26  generates a command signal for the work implement  13  so as to move the blade tip position P 1  of the work implement  13  along the determined intermediate design topography  70 . In addition, the controller  26  generates a command signal for the work implement  13  so that the blade tip position P 1  of the work implement  13  does not go above the final design topography  60 . The generated command signals are input to the control valve  27 . Consequently, the work implement  13  is controlled so that the blade tip position P 1  of the work implement  13  moves along the intermediate design topography  70 . 
     The processing illustrated in  FIG. 5 ,  FIG. 7  and  FIG. 10  is repeated and the controller  26  acquires new actual topography information and updates the actual topography information. For example, the controller  26  may acquire and update the actual topography information in real time. Alternatively, the controller may acquire and update the actual topography information when a predetermined action is carried out. 
     The controller  26  determines the next intermediate design topography  70  on the basis of the updated actual topography information. The work vehicle  1  then moves the work implement  13  along the intermediate design topography  70  while traveling forward again, and upon reaching a certain position, the work vehicle  1  travels backward and returns. The work vehicle  1  repeats the above actions whereby the soil is repeatedly stacked on the actual topography  50 . Consequently, the actual topography  50  is gradually piled up and as a result the final design topography  60  is formed. 
     The intermediate design topography  70  is determined as depicted in  FIG. 4  as a result of the above processing. Specifically, the intermediate design topography  70  is determined so as to conform to the following conditions. 
     (1) The first condition is that the intermediate design topography  70  is lower than the first upper limit height Hup 1 . According to the first condition, the intermediate design topography  70  can be determined that is stacked on the actual topography  50  with a thickness within the predetermined distance D1 as illustrated in  FIG. 4 . As a result, the stacked thickness of the piled soil can be held to within D1 so long as there are no constraints due to other conditions. As a result, the vehicle does not have to repeatedly travel over the piled soil to compact the piled soil. Consequently, work efficiency can be improved. 
     (2) The second condition is that the intermediate design topography  70  is higher than the first lower limit height Hlow 1 . According to the second condition, scraping away of the actual topography  50  can be suppressed so long as there are no constraints due to other conditions. 
     (3) The third condition is that the intermediate design topography  70  approaches the first lower limit height Hlow 1  while the pitch angle of the intermediate design topography  70  at each interval d1 is limited to be equal to or less than the angle (previous angle −A1). According to the third condition, the change de of the pitch angle in the downward direction can be limited to be equal to or less than the angle A1. As a result, a sudden change in the attitude of the vehicle body can be prevented and the work can be performed at a high speed. As a result, work efficiency can be improved. In particular, the inclination angle of the intermediate design topography  70  near the top of the slope is gentler and a change of the attitude of the work vehicle  1  at the top of the slope can be reduced. 
     (4) The fourth condition is that the pitch angle intermediate design topography  70  is greater than the first lower limit angle θlow 1 . According to the fourth condition, the change dθ of the pitch angle in the upward direction can be limited to be equal to or less than the angle A1. As a result, a sudden change in the attitude of the vehicle body  11  can be prevented and the work can be performed at a high speed. As a result, work efficiency can be improved. In particular, the inclination angle of the intermediate design topography  70  near the bottom of the slope can be gentler. Furthermore, scraping away of the actual topography  50  can be suppressed below the first lower limit height Hlow 1  when the intermediate design topography  70  is set below the first lower limit height Hlow 1  due to modification of the pitch angle. 
     (5) The fifth condition is that the shortest distance angle θs is selected as the pitch angle of the intermediate design topography  70  when the shortest distance angle θs is greater than the first lower limit angle θlow 1 . According to the fifth condition, the bending points of the intermediate design topography  70  can be reduced each time the stacking is repeated, and the maximum inclination angle of the intermediate design topography  70  can be gentler as illustrated in  FIG. 4 . As a result, a gradually smoother intermediate design topography can be generated each time stacking is repeated. 
     (6) The sixth condition is that θlow 1 _ 1  along the first lower limit height Hlow 1  is selected as the pitch angle of the intermediate design topography  70  when the pitch angle change conditions are satisfied. After a gentle inclined surface at the inclination angle A1 is formed in front of the work vehicle  1  on the actual topography  50 ′ as illustrated in  FIG. 4  as a result of the fifth condition, the soil piling of the actual topography  50 ′ at the back of the inclined surface can be prioritized. 
     (7) The seventh condition is that the bottom height Hbottom is determined so that the bottom soil amount coincides with the held soil amount. According to the seventh condition, the controller  26  changes the predetermined distance D1 from the actual topography  50  to the intermediate design topography  70  in response to the held soil amount. The stacking thickness of the piled soil can thereby be modified in response to the held soil amount. As a result, the soil remaining on the blade  18  can be reduced without using the piled soil. 
     (8) The eighth condition is that the pitch angle intermediate design topography  70  is less than the second upper limit angle θup 2 . According to the eighth condition, the maximum stacked thickness can be suppressed to be equal to or less than D2 as illustrated in  FIG. 4 . 
     When the actual topography is steep due to the pitch angle of the intermediate design topography  70  being reduced more than the second upper limit angle θup 2 , the intermediate design surface  70  is determined so as to scrape away the top of the slope as illustrated in  FIG. 4 . 
     (9) The ninth condition is that the pitch angle intermediate design topography  70  is greater than the second lower limit angle θlow 2 . Even if the pitch angle is lowered according to the eighth condition, excessive scraping away of the actual topography  50  is suppressed due to the ninth condition. 
     As explained above, the work implement  13  is automatically controlled by the control system  3  of the work vehicle  1  according to the present embodiment, so that the work implement  13  moves along a locus that is more gently sloped than the actual topography  50 . As a result, soil can be piled thinly on the actual topography  50  in comparison to a case of moving the work implement  13  along the final design topography  60 . Consequently, the piled up soil can be easily compacted by the work vehicle  1  and the quality of the finish of the work can be improved. 
     In addition, the slope of the actual topography  50  can be gradually made gentler by repeating the work. As a result, a sudden change in the attitude of the work vehicle  1  can be suppressed. Consequently, unnecessary speed reduction of the work vehicle  1  can be suppressed and work efficiency can be improved. 
     Although the embodiment of the present invention has been described so far, the present invention is not limited to the above embodiment and various modifications may be made within the scope of the invention. 
     The work vehicle is not limited to a bulldozer, and may be another type of work vehicle such as a wheel loader or the like. 
     The processing for determining the intermediate design topography is not limited to the processing described above and may be modified. For example, a portion of the aforementioned first to ninth conditions may be modified or omitted. Alternatively, a different condition may be added to the first to ninth conditions. 
     In the above embodiment, the actual topography  50  is inclined so as to drop downward in the forward direction from the reference position P 0 . However, the actual topography  50  may be inclined so as to rise up in the forward direction from the reference position P 0 . For example,  FIG. 16  illustrates an intermediate design topography  70  according to a modified example. As illustrated in  FIG. 16 , the actual topography  50  may be inclined so as to rise up in the forward direction from the reference position P 0 . In this case as well, the controller may determine the intermediate design topography  70  that is more gently sloped than the actual topography  50  with respect to the actual topography  50  that is positioned below the final design topography  60  as illustrated in  FIG. 16 . Consequently, the work implement  13  is automatically controlled so that the blade tip of the work implement  13  moves along the locus that is more gently sloped than the actual topography  50 . 
     The controller may have a plurality of controllers separated from each other. For example as illustrated in  FIG. 17 , the controller may include a first controller (remote controller)  261  disposed outside of the work vehicle  1  and a second controller (on-board controller)  262  mounted on the work vehicle  1 . The first controller  261  and the second controller  262  may be able to communicate wirelessly via communication devices  38 ,  39 . A portion of the abovementioned functions of the controller  26  may be executed by the first controller  261 , and the remaining functions may be executed by the second controller  262 . For example, the processing for determining a virtual design surface  70  may be performed by the remote controller  261 . That is, the processing from steps S 101  to S 105  illustrated in  FIG. 5  may be performed by the first controller  261 . Additionally, the processing (step S 106 ) to output the command signals to the work implement  13  may be performed by the second controller  262 . 
     The work vehicle may be remotely operated. In this case, a portion of the control system may be disposed outside of the work vehicle. For example, the controller may be disposed outside the work vehicle  1 . The controller may be disposed inside a control center separated from the work site. The operating devices may also be disposed outside of the work vehicle. In this case, the operating cabin may be omitted from the work vehicle. Alternatively, the operating devices may be omitted. The work vehicle may be operated with only the automatic control by the controller without operations by the operating devices. 
     The actual topography acquisition device is not limited to the abovementioned position detection device  31  and may be another device. For example, as illustrated in  FIG. 18 , the actual topography acquisition device may be an interface device  37  that receives information from external devices. The interface device  37  may wirelessly receive actual topography information 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 information measured by the external measurement device  41  via a recording medium. 
     According to the present invention, there are provided a control system for a work vehicle, a control method, and a work vehicle that enable filling work that is efficient and exhibits a quality finish using automatic controls.