Patent Publication Number: US-2019186100-A1

Title: Work vehicle and method for controlling work vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to a work vehicle. 
     BACKGROUND ART 
     A work vehicle such as a hydraulic excavator comprises a work implement having a boom, a dipper stick, and a bucket. When the hydraulic excavator starts an excavation work, the dipper stick is manipulated to cause the bucket to penetrate soil. As the bucket continues to operate, the bucket penetrates soil deeply and the soil&#39;s resistance increases, and accordingly, the boom is manipulated to add an operation to raise the bucket upward to make the bucket&#39;s excavation depth appropriate. Furthermore, the dipper stick and the bucket are manipulated and once sufficient soil is introduced into the bucket, the bucket is manipulated to lift up the soil, and furthermore, the boom is manipulated to raise the bucket upward. 
     For a hydraulic excavator&#39;s excavation work, it is necessary to move manipulation levers of three-axes for the boom, the dipper stick, and the bucket, respectively, to manipulate the movement of the bucket, and it is thus not easy to perform the excavation work efficiently and requires skill. 
     In this respect, for example, Japanese Patent Laying-Open No. 61-225429 discloses a method of correcting a bucket in posture by detecting a collision of a back surface of the bucket against an excavation surface in order to reduce an excavation load. 
     Furthermore, Japanese Patent Laying-Open No. 62-189222 discloses a method of adjusting a bucket&#39;s excavation depth by measuring the weight of the soil in the bucket. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Laying-Open No. 61-225429 
     PTL 2: Japanese Patent Laying-Open No. 62-189222 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, the excavation work in the above-mentioned publication requires various calculations and has a possibility of complicated control. 
     The present invention has been made in view of the above issue, and an object of the present invention is to provide a work vehicle which can perform an excavation work efficiently in a simple manner, and a method for controlling the work vehicle. 
     Solution to Problem 
     A work vehicle according to one aspect of the present invention comprises a vehicular body, a work implement, a manipulation unit and a controller. The work implement has a boom pivotable with respect to the vehicular body, a dipper stick pivotable with respect to the boom, and a bucket pivotable with respect to the dipper stick. The manipulation unit is operated to output a manipulation command for the work implement. The controller controls the work implement in accordance with the manipulation command received from the manipulation unit. When in accordance with the manipulation command via the manipulation unit the bucket travels in a direction on the side of the back surface of the bucket with respect to the direction of the cutting edge of the bucket, the controller limits movement of the bucket in the direction in accordance with the manipulation command. 
     Preferably, when in accordance with the manipulation command via the manipulation unit the bucket travels in the direction of the cutting edge of the bucket or a direction on the side of an open side of the bucket with respect to the direction of the cutting edge of the bucket, the controller moves the bucket in the direction in accordance with the manipulation command. 
     Preferably, when in accordance with the manipulation command via the manipulation unit the bucket travels in a direction within a range of a predetermined angle on the side of the open side of the bucket with respect to the direction of the cutting edge of the bucket, the controller limits movement of the bucket in the direction in accordance with the manipulation command. 
     Preferably, when in accordance with the manipulation command via the manipulation unit the bucket travels in a direction outside the range of the predetermined angle on the side of the open side of the bucket with respect to the direction of the cutting edge of the bucket, the controller moves the bucket in the direction in accordance with the manipulation command. 
     Preferably, limiting movement of the bucket in the direction in accordance with the manipulation command prohibits the bucket from moving in the direction. 
     Preferably, when in accordance with the manipulation command via the manipulation unit the bucket travels in the direction on the side of the back surface of the bucket with respect to the direction of the cutting edge of the bucket, the controller changes the direction in which the bucket travels to a predetermined direction on the side of the open side of the bucket, and the controller moves the bucket in the predetermined direction. 
     Preferably, the manipulation unit outputs at least one of a first manipulation command for the boom, a second manipulation command for the dipper stick, and a third manipulation command for the bucket. The controller prohibits movement of the boom when the bucket travels in the direction on the side of the back surface of the bucket with respect to the direction of the cutting edge of the bucket as the boom moves in response to the first manipulation command output from the manipulation unit. The controller prohibits movement of the dipper stick when the bucket travels in the direction on the side of the back surface of the bucket with respect to the direction of the cutting edge of the bucket as the dipper stick moves in response to the second manipulation command output from the manipulation unit. The controller prohibits movement of the bucket when the bucket travels in the direction on the side of the back surface of the bucket with respect to the direction of the cutting edge of the bucket in response to the third manipulation command output from the manipulation unit. 
     Preferably, the controller determines whether the work implement performs an excavation work, and when the controller determines that the work implement performs the excavation work, and the bucket travels in the direction on the side of the back surface of the bucket with respect to the direction of the cutting edge of the bucket in accordance with the manipulation command output from the manipulation unit, the controller limits movement of the bucket in the direction in accordance with the manipulation command. 
     Preferably, the controller determines whether the work implement performs an excavation work in response to a manipulation instruction of an operator. 
     According to one aspect, a method for controlling a work vehicle including a work implement having a boom pivotable with respect to a vehicular body, a dipper stick pivotable with respect to the boom, and a bucket pivotable with respect to the dipper stick, comprises: outputting a manipulation command for the work implement; and controlling the work implement in accordance with the manipulation command, the step of controlling the work implement including, when in accordance with the manipulation command the bucket travels in a direction on a side of a back surface of the bucket with respect to a direction of a cutting edge of the bucket, limiting movement of the bucket in the direction in accordance with the manipulation command. 
     Advantageous Effects of Invention 
     The present work vehicle and method for controlling the same allows an excavation work to be performed efficiently in a simple manner. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of one example of a work vehicle according to a first embodiment. 
         FIG. 2  schematically illustrates a work vehicle CM according to the first embodiment. 
         FIG. 3  is a functional block diagram representing a configuration of a control system  200  to control work vehicle CM according to the first embodiment. 
         FIG. 4  represents a relationship between an excavation angle of a bucket  8  and resistance of soil according to the first embodiment. 
         FIG. 5  is a flowchart of an excavation process by work vehicle CM according to the first embodiment. 
         FIG. 6  is a conceptual diagram illustrating a case where target cutting edge data is projected in a predetermined direction according to the first embodiment. 
         FIG. 7  is a diagram for illustrating a direction in which a cutting edge  8   a  of bucket  8  moves according to a second embodiment. 
         FIG. 8  is a flowchart of an excavation process by work vehicle CM according to the second embodiment. 
         FIG. 9  illustrates a specific example of a state of an excavation process by work vehicle CM according to the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings, although the present invention is not limited thereto. Any constituent element of each embodiment described below can be combined as appropriate. Some constituent element may not be used. 
     First Embodiment 
     [General Configuration of Work Vehicle] 
       FIG. 1  is a perspective view of one example of a work vehicle according to a first embodiment. 
     As shown in  FIG. 1 , in the present example, a work vehicle will be described by referring as an example to a hydraulic excavator CM including a hydraulically actuated work implement  2  as a work implement. 
     Hydraulic excavator CM includes a vehicular body  1  and work implement  2 . 
     Vehicular body  1  has a revolving unit  3 , an operator&#39;s cab  4 , and a travelling unit  5 . 
     Revolving unit  3  is disposed on a travelling unit  5 . Travelling unit  5  supports revolving unit  3 . Revolving unit  3  can revolve about an axis AX. An operator&#39;s seat  4 S on which an operator is seated is provided in operator&#39;s cab  4 . The operator manipulates hydraulic excavator CM in operator&#39;s cab  4 . Travelling unit  5  has a pair of crawler belts  5 Cr. Hydraulic excavator CM travels as crawler belts  5 Cr rotate. Note that travelling unit  5  may be composed of vehicular wheels (or tires). 
     In the first embodiment, a positional relationship of each part will be described with reference to an operator seated on operator&#39;s seat  4 S in the cab. 
     A frontward/rearward direction is a frontward/rearward direction with reference to the operator seated on operator&#39;s seat  4 S. A rightward/leftward direction is a rightward/leftward direction with respect to the operator seated on operator&#39;s seat  4 S. The rightward/leftward direction matches the vehicle&#39;s widthwise direction (a vehicular widthwise direction). When the operator is seated on operator&#39;s seat  4 S and faces frontward, the operator faces in the frontward direction, and a direction opposite to the frontward direction is the rearward direction. When the operator is seated on operator&#39;s seat  4 S and faces frontward, a direction on a right side of the operator is referred to as the rightward direction, and a direction on a left side of the operator is referred to as the leftward direction. The frontward/rearward direction is a direction along the x axis and the rightward/leftward direction is a direction along the y axis. When the operator is seated on operator&#39;s seat  4 S and faces frontward, the operator faces in the frontward direction (or a+x direction), and a direction opposite to the frontward direction is the rearward direction (or a−x direction). When the operator is seated on operator&#39;s seat  4 S and faces frontward, a direction on one side of the operator in the vehicular widthwise direction is the right direction (or a+z direction), and a direction on the other side of the operator in the vehicular widthwise direction is the left direction (or a−z direction). 
     Revolving unit  3  has an engine compartment  9  in which an engine is housed, and a counter weight provided at a rear portion of revolving unit  3 . Revolving unit  3  is provided with a handrail  19  in front of engine compartment  9 . The engine, a hydraulic pump, etc. are disposed in engine compartment  9 . 
     Work implement  2  is connected to revolving unit  3 . 
     Work implement  2  has a boom  6 , a dipper stick  7 , a bucket  8 , a boom cylinder  10 , a dipper stick cylinder  11 , and a bucket cylinder  12 . 
     Boom  6  is connected to revolving unit  3  via a boom pin  13 . Dipper stick  7  is connected to boom  6  via a dipper stick pin  14 . Bucket  8  is connected to dipper stick  7  via a bucket pin  15 . Boom cylinder  10  drives boom  6 . Dipper stick cylinder  11  drives dipper stick  7 . Bucket cylinder  12  drives bucket  8 . Boom  6  has a proximal end (or a boom foot) connected to revolving unit  3 . Boom  6  has a distal end (or a boom top) connected to a proximal end of dipper stick  7 . Dipper stick  7  has a distal end (or a dipper stick top) connected to a proximal end of bucket  8 . Boom cylinder  10 , dipper stick cylinder  11 , and bucket cylinder  12  are all a hydraulic cylinder driven with hydraulic oil. 
     Boom  6  is pivotable with respect to revolving unit  3  about boom pin  13  serving as a pivot. Dipper stick  7  is pivotable with respect to boom  6  about dipper stick pin  14  serving as a pivot parallel to boom pin  13 . Bucket  8  is pivotable with respect to dipper stick  7  about bucket pin  15  serving as a pivot parallel to boom pin  13  and dipper stick pin  14 . 
     Boom pin  13 , dipper stick pin  14 , and bucket pin  15  are all parallel to the z axis. Boom  6 , dipper stick  7 , and bucket  8  are all pivotable about an axis parallel to the z axis. 
       FIG. 2  schematically illustrates work vehicle CM according to the first embodiment. 
     As shown in  FIG. 2 , work vehicle CM is provided with a boom cylinder stroke sensor  16 , a dipper stick cylinder stroke sensor  17 , and a bucket cylinder stroke sensor  18 . 
     Boom cylinder stroke sensor  16  is disposed on boom cylinder  10  and senses a length of a stroke of boom cylinder  10  (a boom cylinder length). Dipper stick cylinder stroke sensor  17  is disposed on dipper stick cylinder  11  and senses a length of a stroke of dipper stick cylinder  11  (a dipper stick cylinder length). Bucket cylinder stroke sensor  18  is disposed on bucket cylinder  12  and senses a length of a stroke of bucket cylinder  12  (bucket cylinder length). 
     In the following description, a length of a stroke of boom cylinder  10  will also be referred to as a boom cylinder length or a boom stroke. A length of a stroke of a dipper stick cylinder  11  will also be referred to as a dipper stick cylinder length or a dipper stick stroke. A length of a stroke of bucket cylinder  12  will also be referred to as a bucket cylinder length or a bucket stroke. 
     Furthermore, the boom cylinder length, the dipper stick cylinder length and the bucket cylinder length will collectively be also referred to as cylinder length data. 
     Boom  6  has a length L 1 , which is a distance between boom pin  13  and dipper stick pin  14 . Dipper stick  7  has a length L 2 , which is a distance between dipper stick pin  14  and bucket pin  15 . Bucket  8  has a length L 3 , which is a distance between bucket pin  15  and a cutting edge  8   a  of bucket  8 . Bucket  8  has a plurality of blades, and in the present example, a tip portion of bucket  8  will be referred to as cutting edge  8   a.  Note that bucket  8  may have no blade. The tip portion of bucket  8  may be formed of a steel plate having a straight shape. 
     In the present example, an x- and y-axis vehicular body coordinate system with boom pin  13  as a reference point (or a reference position) is shown. 
     An inclination angle θ 1  of boom  6  with respect to a horizontal direction in the vehicular body coordinate system is calculated from cylinder length data sensed by boom cylinder stroke sensor  16 . 
     An inclination angle θ 2  of dipper stick  7  with respect to boom  6  is calculated from cylinder length data sensed by dipper stick cylinder stroke sensor  17 . 
     An inclination angle θ 3  of cutting edge  8   a  of bucket  8  with respect to dipper stick  7  is calculated from cylinder length data sensed by bucket cylinder stroke sensor  18 . 
     Based on lengths L 1  to L 3  and inclination angles θ 1  to θ 3  of boom  6 , dipper stick  7  and bucket  8 , a position of cutting edge  8   a  of bucket  8  and an angle of cutting edge  8   a  of bucket  8  (the cutting edge&#39;s direction) in the x- and y-axis vehicular body coordinate system can be calculated. 
     In the present example, positional coordinates [x0, y0] of cutting edge  8   a  of bucket  8  and a cutting edge angle [α0] of cutting edge  8   a  of bucket  8  with respect to the horizontal direction are shown. 
     While in the present example a method of sensing a stroke length by using a stroke sensor, and calculating inclination angle θ will be described, the inclination angle may be calculated by using an angle detector such as a rotary encoder. 
     [Configuration of Hydraulic System] 
       FIG. 3  is a functional block diagram representing a configuration of a control system  200  to control work vehicle CM according to the first embodiment. 
     As shown in  FIG. 3 , control system  200  according to the first embodiment controls an excavation process using work implement  2 . 
     Control system  200  includes boom cylinder stroke sensor  16 , dipper stick cylinder stroke sensor  17 , bucket cylinder stroke sensor  18 , a manipulation device  25 , a work implement controller  26 , a hydraulic cylinder  60 , a directional control valve  64 , and a pressure sensor  66 . 
     Manipulation device  25  is disposed in operator&#39;s cab  4 . Manipulation device  25  is manipulated by the operator. Manipulation device  25  receives a manipulation command of the operator to drive work implement  2 . Manipulation device  25  is a manipulation device of a pilot hydraulic system as an example. 
     Directional control valve  64  adjusts an amount of hydraulic oil supplied to hydraulic cylinder  60 . Directional control valve  64  is actuated by oil supplied. In the present example, oil supplied to a hydraulic cylinder (boom cylinder  10 , dipper stick cylinder  11 , and bucket cylinder  12 ) in order to actuate the hydraulic cylinder is also referred to as hydraulic oil. Furthermore, oil supplied to directional control valve  64  to actuate directional control valve  64  is referred to as pilot oil. Furthermore, the pilot oil&#39;s pressure is also referred to as pilot oil pressure. 
     The hydraulic oil and the pilot oil may be pumped from the same hydraulic pump. For example, the hydraulic oil pumped from the hydraulic pump may have a portion reduced in pressure by a reducing valve, and the hydraulic oil reduced in pressure may be used as the pilot oil. Further, a hydraulic pump (a main hydraulic pump) for pumping the hydraulic oil and a hydraulic pump (a pilot hydraulic pump) for pumping the pilot oil may be different hydraulic pumps. 
     Further, in the present example, the pilot oil pumped from the main hydraulic pump and reduced in pressure by the reducing valve is supplied to manipulation device  25 . 
     The pilot oil pressure is adjusted based on the amount of manipulating manipulation device  25 . Pressure sensor  66  is connected to manipulation device  25 . Pressure sensor  66  senses a pilot oil pressure generated in response to manipulation of a lever of manipulation device  25  and outputs it to work implement controller  26 . 
     In response to the pilot oil pressure sensed by pressure sensor  66 , work implement controller  26  drives directional control valve  64  passing the hydraulic oil supplied to hydraulic cylinder  60  (boom cylinder  10 , dipper stick cylinder  11 , and bucket cylinder  12 ). 
     Manipulation device  25  includes a first manipulation lever  25 R, a second manipulation lever  25 L, and an excavation mode setting button  25 P. First manipulation lever  25 R is disposed, for example, on the right side of driver&#39;s seat  4 S. 
     Second manipulation lever  25 L is disposed, for example, on the left side of operator&#39;s seat  4 S. For first manipulation lever  25 R and second manipulation lever  25 L, forward, backward, rightward and leftward operations correspond to operations along two axes. 
     First manipulation lever  25 R is manipulated to manipulate boom  6  and bucket  8 . A forward/backward manipulation of first manipulation lever  25 R corresponds to a manipulation of boom  6 , and in response to the forward/backward manipulation, boom  6  is raised/lowered. The lever is manipulated to manipulate boom  6 . 
     A rightward/leftward manipulation of first manipulation lever  25 R corresponds to a manipulation of bucket  8 , and in response to the rightward/leftward manipulation, bucket  8  is operated to excavate soil and be released. The lever is manipulated to manipulate bucket  8 . 
     Second manipulation lever  25 L is manipulated to manipulate dipper stick  7  and revolving unit  3 . 
     A forward/backward manipulation of second manipulation lever  25 L corresponds to a manipulation of dipper stick  7 , and in response to the forward/backward manipulation, dipper stick  7  is raised/lowered. The lever is manipulated to manipulate dipper stick  7 . 
     A rightward/leftward manipulation of second manipulation lever  25 L corresponds to revolution of revolving unit  3 , and in response to the rightward/leftward manipulation, revolving unit  3  revolves rightward and leftward. 
     In response to an amount of manipulating first manipulation lever  25 R in the forward/backward direction (an amount of manipulating the boom), as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to boom cylinder  10  for driving boom  6 . 
     In response to an amount of manipulating first manipulation lever  25 R in the rightward and leftward direction (an amount of manipulating the bucket), as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to bucket cylinder  12  for driving bucket  8 . 
     In response to an amount of manipulating second manipulation lever  25 L in the forward/backward direction (an amount of manipulating the dipper stick), as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to dipper stick cylinder  11  for driving dipper stick  7 . 
     In response to an amount of manipulating second manipulation lever  25 L in the rightward and leftward direction, as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to a hydraulic actuator for driving revolving unit  3 . 
     Note that a rightward/leftward manipulation of first manipulation lever  25 R may correspond to a manipulation of boom  6  and a forward/backward manipulation thereof may correspond to that of bucket  8 . Note that a rightward/leftward manipulation of second manipulation lever  25 L may correspond to a manipulation of dipper stick  7  and a forward/backward manipulation thereof may correspond to that of revolving unit  3 . 
     Excavation mode setting button  25 P is a setting button for setting an excavation mode. Work implement controller  26  shifts from a normal mode to the excavation mode in response to an instruction issued as the operator presses excavation mode setting button  25 P. Furthermore, work implement controller  26  shifts from the excavation mode to the normal mode in response to an instruction issued as the operator again presses excavation mode setting button  25 P. 
     Work implement controller  26  limits movement of bucket  8  in a predetermined direction in response to the normal mode being shifted to the excavation mode. 
     [Resistance of Soil] 
       FIG. 4  represents a relationship between an excavation angle of bucket  8  and resistance of soil according to the first embodiment. 
     In the present example, an excavation angle represents an angle between a direction of cutting edge  8   a  of bucket  8  and a direction in which cutting edge  8   a  travels when bucket  8  moves. With reference to the direction of cutting edge  8   a  of bucket  8 , when bucket  8  moves and cutting edge  8   a  travels in a direction to a side on the open side of bucket  8 , the angle has a positive value, whereas when cutting edge  8   a  travels in the opposite direction, i.e., to a side on the side of the back surface of bucket  8 , the angle has a negative value. 
     As shown in  FIG. 4 , an excavation angle of bucket  8  around 0° is indicated as a limit angle. 
     When bucket  8  has an excavation angle smaller than the limit angle, bucket  8  has its exterior or back surface pressed against soil, which rapidly increases a value of resistance of soil against bucket  8 . When bucket  8  travels in a direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 , resistance of soil has an increased value. 
     On the other hand, the figure shows that when bucket  8  has an excavation angle larger than the limit angle, bucket  8  experiences resistance of soil of a small value against it. When bucket  8  travels in the direction of cutting edge  8   a  of bucket  8  or a direction on the open side of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 , resistance of soil has a decreased value. 
     Furthermore, the figure shows that when bucket  8  has an excavation angle of a predetermined angle Q, bucket  8  experiences resistance of soil of a minimum value against it. 
     It should be noted that the limit angle and the predetermined angle Q are merely examples and can be set to different values depending on the form of bucket  8 . 
     Work vehicle CM according to the first embodiment performs an excavation work efficiently in a simple manner. Specifically, the operation of work implement  2  is controlled so as to avoid performing an excavation process at an excavation angle for which resistance of soil has a large value. Specifically, when bucket  8  travels in a direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 , work vehicle CM limits movement of bucket  8  in the direction on the side of the back surface of bucket  8 . This limits an excavation process performed at an excavation angle for which resistance of soil has a large value, and allows an excavation work to be performed efficiently in a simple manner. 
     [Process of Operation] 
       FIG. 5  is a flowchart of a process of an operation of an excavation work by work vehicle CM according to the first embodiment. 
     As shown in  FIG. 5 , work implement controller  26  determines whether the excavation mode is set (step S 2 ). Specifically, work implement controller  26  determines whether a setting instruction via the excavation mode setting button to set the excavation mode in response to a manipulation command of the operator is received. 
     In step S 2 , if work implement controller  26  determines that the excavation mode is set, work implement controller  26  calculates cutting edge data (step S 4 ). 
     Specifically, work implement controller  26  calculates a boom cylinder length, a dipper stick cylinder length, and a bucket cylinder length based on detection results obtained from boom cylinder stroke sensor  16 , dipper stick cylinder stroke sensor  17 , and bucket cylinder stroke sensor  18 . Work implement controller  26  calculates inclination angle θ 1  of boom  6  with respect to the horizontal direction from the boom cylinder length. Work implement controller  26  calculates inclination angle θ 2  of dipper stick  7  with respect to boom  6  from the dipper stick cylinder length. Work implement controller  26  calculates inclination angle θ 3  of cutting edge  8   a  of bucket  8  with respect to dipper stick  7  from the bucket cylinder length. Thus, work implement controller  26  calculates cutting edge data [x0, y0, α0] indicating a position of bucket  8  and a direction of cutting edge  8   a  of bucket  8  (the cutting edge&#39;s direction) in the x- and y-axis vehicular body coordinate system. 
     Subsequently, work implement controller  26  accepts an input via a manipulation lever (step S 6 ). 
     In the present example, work implement controller  26  accepts manipulation inputs via first manipulation lever  25 R and second manipulation lever  25 L. 
     As has been previously discussed, first manipulation lever  25 R is manipulated to manipulate boom  6  and bucket  8 . Furthermore, when second manipulation lever  25 L is manipulated, dipper stick  7  and revolving unit  3  are manipulated. In the present example, to simply description, a case where second manipulation lever  25 L is manipulated to manipulate dipper stick  7  will be described, and a case where the manipulation lever is manipulated to manipulate revolving unit  3  will not be described. This is because even when revolving unit  3  is manipulated, bucket  8  does not vary in posture in the x- and y-axis vehicular body coordinate system with boom pin  13  serving as a reference point (or a reference position). 
     Subsequently, work implement controller  26  calculates an amount of rotating the boom, an amount of rotating the dipper stick, and an amount of rotating the bucket in accordance with manipulation inputs received via the manipulation levers (step S 7 ). 
     Specifically, work implement controller  26  calculates the amount of rotating the bucket and the amount of rotating the boom based on a pressure generated in response to a manipulation input via first manipulation lever  25 R and sensed by and output from pressure sensor  66  (i.e., a manipulation command). Furthermore, work implement controller  26  calculates the amount of rotating the dipper stick based on a pressure generated in response to a manipulation input via second manipulation lever  25 L and sensed by and output from pressure sensor  66  (i.e., a manipulation command). 
     In the present example, work implement controller  26  calculates an amount  401  of rotating the boom, an amount  402  of rotating the dipper stick, and an amount  403  of rotating the bucket. 
     Subsequently, work implement controller  26  calculates target cutting edge data for cutting edge  8   a  of bucket  8  moving in response to an input via a manipulation lever (step S 8 ). 
     Specifically, based on the boom  6  inclination angle θ 1 +Δθ 1 , the dipper stick  7  inclination angle θ 2 +Δθ 2 , and the bucket  8  inclination angle θ 3 +Δθ 3 , target cutting edge data [x1, y1, α1] indicating a position of bucket  8  and a direction of cutting edge  8   a  of bucket  8  (the cutting edge&#39;s direction) in the x- and y-axis vehicular body coordinate system is calculated. 
     Subsequently, work implement controller  26  calculates an excavating direction vector V which represents a direction in which cutting edge  8   a  of bucket  8  travels (step S 9 ). 
     Excavating direction vector V in the vehicular body coordinate system of the present example has components dx and dy along the x and y axes, respectively, which are represented by the following expressions: 
         Vx=x 1− x 0
 
         Vy=y 1− y 0
 
     Subsequently, work implement controller  26  calculates an excavation angle φ (step S 10 ). 
     Excavation angle φ is calculated by the following equation: 
       φ=tan(− Vy/Vx ) −1 −α0.
 
     Subsequently, work implement controller  26  determines whether excavation angle φ is equal to or larger than the limit angle (step S 12 ). In this example, the limit angle is set to 0° as an example. 
     In step S 12 , when work implement controller  26  determines that excavation angle φ is not equal to or larger than the limit angle (NO in step S 12 ), work implement controller  26  projects the target cutting edge data in a predetermined direction to calculate corrected target cutting edge data (step S 14 ). 
       FIG. 6  is a conceptual diagram illustrating a case where target cutting edge data is projected in a predetermined direction according to the first embodiment. 
       FIG. 6  represents cutting edge data [x0, y0, α0] indicating a position of bucket  8  and a direction of cutting edge  8   a  (or the cutting edge&#39;s direction). 
     The figure also represents target cutting edge data [x1, y1, α1] for cutting edge  8   a  of bucket  8  moving in response to an input via a manipulation lever, as described above. 
     The figure shows a side closer to the back surface of bucket  8  with respect to the direction of cutting edge  8   a  and a side closer to the open side of bucket  8  with respect to the direction of cutting edge  8   a,  and, as shown in the figure, excavating direction vector V is on the side closer to the back surface of bucket  8  than the direction of the cutting edge. 
     Therefore, excavation angle φ will be negative (&lt;0). 
     Therefore, as described above, the target cutting edge data [x1, y1, α1] is changed to the corrected target cutting edge data [x1′, y1′, α1′]. 
     Specifically, the target cutting edge data is projected in a predetermined direction to calculate the corrected target cutting edge data. 
     The predetermined direction is, for example, an excavation angle β. The target cutting edge data is projected in the direction of cutting edge  8   a  of bucket  8  as the predetermined direction. 
     In this respect, an amount of movement D when the target cutting edge data is projected in the predetermined direction is represented by the following equation: 
         D=Vx ×cos(α0+β)+ Vy ×(−sin(α0+β)),
 
     where β represents an excavation angle representing an angle between a direction of cutting edge  8   a  of bucket  8  and a direction in which cutting edge  8   a  travels when bucket  8  moves. Note that β is set to 0° or more. 
     When the target cutting edge data is projected in the predetermined direction and the corrected target cutting edge data is obtained, the corrected target cutting edge data has positional coordinates represented by the following equation: 
       Δ x=D ×cos(α0+β)
 
       Δ y=−D ×sin(α0+β)
 
         x 1′= x 0+Δ x  
 
         y 1′= y 0+Δ y  
 
       α1′=α0+β
 
     Thus, new corrected target cutting edge data [x1′, y1′, α1′] is calculated. 
     Referring again to  FIG. 5 , subsequently, work implement controller  26  controls the work implement based on the calculated corrected target cutting edge data (step S 16 ). 
     Specifically, work implement controller  26  calculates an inclination angle θ 1 ′ of boom  6 , an inclination angle θ 2 ′ of dipper stick  7 , and an inclination angle θ 3 ′ of bucket  8  in accordance with the corrected target cutting edge data [x1′, y1′, α1′] of cutting edge  8   a  of bucket  8  in the x- and y-axis vehicular body coordinate system. Work implement controller  26  calculates a boom cylinder length, a dipper stick cylinder length and a bucket cylinder length based on inclination angles θ 1 ′ to θ 3 ′ of boom  6 , dipper stick  7  and bucket  8 . 
     Then, work implement controller  26  drives directional control valve  64  so as to adjust hydraulic oil supplied to hydraulic cylinder  60  so as to achieve the calculated boom, dipper stick and bucket cylinder lengths. 
     Thus, boom  6 , dipper stick  7  and bucket  8  are automatically controlled so that cutting edge  8   a  of bucket  8  has a position and a direction as indicated by the corrected target cutting edge data. 
     In step S 12  when work implement controller  26  determines that excavation angle φ is equal to or larger than the limit angle (YES in step S 12 ), work implement controller  26  controls the work implement based on the calculated target cutting edge data (step S 16 ). 
     Specifically, work implement controller  26  calculates an inclination angle θ 1 ′ of boom  6 , an inclination angle θ 2 ′ of dipper stick  7 , and an inclination angle θ 3 ′ of bucket  8  in accordance with the target cutting edge data [x1, y1, α1] of cutting edge  8   a  of bucket  8  in the x- and y-axis vehicular body coordinate system. Work implement controller  26  calculates a boom cylinder length, a dipper stick cylinder length and a bucket cylinder length based on inclination angles θ 1 ′ to θ 3 ′ of boom  6 , dipper stick  7  and bucket  8 . 
     Then, work implement controller  26  drives directional control valve  64  so as to adjust hydraulic oil supplied to hydraulic cylinder  60  so as to achieve the calculated boom, dipper stick and bucket cylinder lengths. 
     Thus, boom  6 , dipper stick  7  and bucket  8  are automatically controlled so that cutting edge  8   a  of bucket  8  has a position and a direction as indicated by the target cutting edge data. 
     Subsequently, work implement controller  26  determines whether a work has ended (step S 18 ). When work implement controller  26  determines that the work has ended is for example when the engine is stopped. 
     In step S 18 , if work implement controller  26  determines that the work has ended (YES in step S 18 ), work implement controller  26  ends the process (END). 
     On the other hand, if work implement controller  26  determines in step S 18  that the work has not ended (NO in step S 18 ), work implement controller  26  returns to step S 2  and repeats the above process. 
     On the other hand, if work implement controller  26  determines in step S 2  that the excavation mode is not set, work implement controller  26  accepts an input via a manipulation lever (step S 20 ). 
     In the present example, manipulation inputs via first manipulation lever  25 R and second manipulation lever  25 L are accepted. 
     As has been previously discussed, in the normal mode, first manipulation lever  25 R is manipulated to manipulate boom  6  and bucket  8 . Furthermore, second manipulation lever  25 L is manipulated to manipulate dipper stick  7  and revolving unit  3 . 
     And work implement controller  26  controls the work implement (step S 22 ). 
     In response to an amount of manipulating first manipulation lever  25 R in the forward/backward direction (an amount of manipulating the boom), as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to boom cylinder  10  for driving boom  6 . 
     In response to an amount of manipulating first manipulation lever  25 R in the rightward and leftward direction (an amount of manipulating the bucket), as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to bucket cylinder  12  for driving bucket  8 . 
     In response to an amount of manipulating second manipulation lever  25 L in the forward/backward direction (an amount of manipulating the dipper stick), as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to dipper stick cylinder  11  for driving dipper stick  7 . 
     In response to an amount of manipulating second manipulation lever  25 L in the rightward and leftward direction, as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to the hydraulic actuator for driving revolving unit  3 . 
     Subsequently, the control proceeds to step S 18 . 
     The process subsequent to the step is similar to that described above, and accordingly, will not be described repeatedly in detail. 
     In the above description, when an excavation angle of bucket  8  controlled according to first manipulation lever  25 R and second manipulation lever  25 L is less than the limit angle (φ&lt;0 in the present example), projection is done so that the excavating direction vector is in the predetermined direction and bucket  8  is moved in the predetermined direction as projected. In the present example, bucket  8  is moved so that as the predetermined direction, excavation angle β is 0° or more. 
     Thus, when bucket  8  travels in a direction to press the back surface of bucket  8 , that is, to a side on the side of the back surface of bucket  8  (i.e., when bucket  8  has an excavation angle of less than the limit angle (φ&lt;0 in the present example)), movement of bucket  8  in that direction is limited. 
     Then, the direction in which bucket  8  travels is compulsorily changed to a predetermined direction on the side of the open side of bucket  8 , and bucket  8  is moved in that direction. 
     This can restrain bucket  8  from traveling in a direction on the side of the back surface of bucket  8  with respect to the direction of the cutting edge of bucket  8  and change the direction in which bucket  8  travels to a predetermined direction on the side of the open side of bucket  8  accompanied by low resistance of soil to suppress increase of resistance of soil against bucket  8  to perform an efficient excavation work. 
     More specifically, when bucket  8  travels in a direction with excavation angle φ&lt;0, excavation angle β is set to the predetermined angle Q. This can minimize a value of resistance of soil against bucket  8  to allow an efficient excavation work to be performed, as has been described with reference to  FIG. 4 . 
     Further, in the above description, a configuration has been described in which whether bucket  8  travels in a direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8  is determined with reference to a limit angle of 0°. 
     However, a limit angle for which resistance of soil has a sharply increasing value may have a value larger than 0° in some cases based on the shape of bucket  8 , friction or the like. 
     Specifically, resistance of soil may also have an increasing value when bucket  8  travels in a direction within a range up to a predetermined angle P on the side of the open side of bucket  8  with respect to the direction of cutting edge  8   a  (a range up to the predetermined angle P shown in  FIG. 4 ). 
     Accordingly, the predetermined angle P may be set to the limit angle. 
     In that case, when bucket  8  travels in a direction with excavation angle φ&lt;P, excavation angle β is set to the predetermined angle Q. Note that it is assumed that angle P&lt;Q. This can minimize a value of resistance of soil against bucket  8  to allow an efficient excavation work to be performed, as has been described with reference to  FIG. 4 . 
     Note that when bucket  8  travels in a direction with excavation angle φ≥P, excavation angle φ is set to the predetermined angle Q and, without changing the direction to the predetermined direction, the work implement is controlled based on the calculated target cutting edge data. 
     Furthermore, while in the above embodiment a method to provide projection so that bucket  8  travels in a predetermined direction on the side of the open side of bucket  8  has been described, bucket  8  may be stopped from moving when bucket  8  travels in a direction with excavation angle φ&lt;P. 
     This can restrain bucket  8  from traveling in a direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8  and within the range up to the predetermined angle P on the side of the open side of bucket  8 , and suppress increase of resistance of soil against bucket  8  to allow an efficient excavation work to be performed. 
     Further, in the present example, while an excavation mode is set in accordance with an instruction issued as an operator presses excavation mode setting button  25 P, an excavation work is performed efficiently with a small load as cutting edge  8   a  of bucket  8  moves in accordance with a predetermined excavating direction vector, and improved fuel economy can be achieved. 
     Further, in the present example, the excavation mode can be set in response to an instruction issued as an operator presses excavation mode setting button  25 P, and an excavation work can be performed efficiently with the operator&#39;s intention reflected. 
     Second Embodiment 
     In the first embodiment has been described a method in which target cutting edge data according to which cutting edge  8   a  of bucket  8  moves is calculated according to a manipulation input via a manipulation lever to limit movement accompanied by increased resistance of soil. 
     In the second embodiment, another method for limiting such movement of cutting edge  8   a  of bucket  8  as accompanied by increased resistance of soil will be described. 
       FIG. 7  is a diagram for illustrating a direction in which cutting edge  8   a  of bucket  8  moves according to the second embodiment. 
     As shown in  FIG. 7 , in the present example, directions in which cutting edge  8   a  of bucket  8  moves and hence travels as work implement  2  moves are shown. 
     Specifically, directions in which cutting edge  8   a  of bucket  8  moves and hence travels as boom  6 , dipper stick  7 , and bucket  8  move are shown. 
     In the present example, a direction in which cutting edge  8   a  of bucket  8  moves and hence travels as boom  6  moves is indicated as directions Vb− and Vb+ (also collectively referred to as a direction Vb). 
     Direction Vb in which cutting edge  8   a  of bucket  8  moves and hence travels as boom  6  moves is a direction perpendicular to a line connecting boom pin  13  and cutting edge  8   a  of bucket  8 . 
     Direction Vb− indicates a direction in which cutting edge  8   a  of bucket  8  moves and hence travels as boom  6  is lowered (or pivots counterclockwise) about boom pin  13 . 
     Direction Vb+ indicates a direction in which cutting edge  8   a  of bucket  8  moves and hence travels as boom  6  is raised (or pivots clockwise) about boom pin  13 . 
     In the present example, a direction in which cutting edge  8   a  of bucket  8  moves and hence travels as dipper stick  7  moves is indicated as directions Va− and Va+ (also collectively referred to as a direction Va). 
     Direction Va in which cutting edge  8   a  of bucket  8  moves and hence travels as dipper stick  7  moves is a direction perpendicular to a line connecting dipper stick pin  14  and cutting edge  8   a  of bucket  8 . 
     Direction Va− indicates a direction in which cutting edge  8   a  of bucket  8  moves and hence travels as dipper stick  7  is lowered (or pivots counterclockwise) about dipper stick pin  14 . 
     Direction Va+ indicates a direction in which cutting edge  8   a  of bucket  8  moves and hence travels as dipper stick  7  is raised (or pivots clockwise) about dipper stick pin  14 . 
     In the present example, a direction in which cutting edge  8   a  of bucket  8  moves and hence travels as bucket  8  moves is indicated as directions Vk− and Vk+ (also collectively referred to as a direction Vk). 
     Direction Vk in which cutting edge  8   a  of bucket  8  moves and hence travels as bucket  8  moves is a direction perpendicular to a line connecting bucket pin  15  and cutting edge  8   a  of bucket  8 . 
     Direction Vk− indicates a direction in which cutting edge  8   a  of bucket  8  moves and hence travels as bucket  8  performs an excavating operation (or pivots counterclockwise) about bucket pin  15 . 
     Direction Vk+ indicates a direction in which cutting edge  8   a  of bucket  8  moves and hence travels as bucket  8  is released (or pivots clockwise) about bucket pin  15 . 
     Herein, the direction of cutting edge  8   a  of bucket  8  is indicated as the cutting edge&#39;s direction. 
     If the direction in which cutting edge  8   a  travels is on the side of the open side of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 , it is inferred that resistance of soil is small. In the present example, it is determined that a direction in which cutting edge  8   a  travels that is within a range of 180° from the direction of cutting edge  8   a  of bucket  8  toward the open side of the bucket is a direction in which cutting edge  8   a  travels which is accompanied by small resistance of soil. In contrast, it is determined that a direction in which cutting edge  8   a  travels that is outside the above range, or a range on the side of the back surface of bucket  8 , is a direction in which cutting edge  8   a  travels which is accompanied by large resistance of soil. 
     For example, in the present example, direction Vb− is not on the side of the open side of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8  but on the side of the back surface of bucket  8 , and accordingly, accompanied by increased resistance of soil. Accordingly, movement of bucket  8  by boom  6  in direction Vb− is prohibited. 
     On the other hand, direction Vb+ is on the side of the open side of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 , and accordingly, accompanied by decreased resistance of soil. Accordingly, movement of bucket  8  by boom  6  in direction Vb+ is permitted. When direction Vb+ and the direction of cutting edge  8   a  are identical, movement of bucket  8  is permitted. 
     Furthermore, direction Va− is on the side of the open side of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 , and accordingly, accompanied by decreased resistance of soil. Accordingly, movement of bucket  8  by dipper stick  7  in direction Va− is permitted. 
     On the other hand, direction Va+ is on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 , and accordingly, accompanied by increased resistance of soil. Accordingly, movement of bucket  8  by dipper stick  7  in direction Va+ is permitted. When direction Va+ and the direction of cutting edge  8   a  are identical, movement of bucket  8  is permitted. 
     Furthermore, direction Vk− is on the side of the open side of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 , and accordingly, accompanied by decreased resistance of soil. Accordingly, movement of bucket  8  in direction Vk− is permitted. 
     On the other hand, direction Vk+ is on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 , and accordingly, accompanied by increased resistance of soil. Accordingly, movement of bucket  8  in direction Vk+ is prohibited. 
     In the second embodiment, whether movement of bucket  8  in response to a command to manipulate boom  6 , movement of bucket  8  in response to a command to manipulate dipper stick  7 , and movement of bucket  8  in response to a command to manipulate bucket  8  are permitted or prohibited is determined and work implement  2  is controlled in accordance with a result of the determination. 
     Specifically, when movement of bucket  8  is permitted, the manipulation command is enabled, whereas when movement of bucket  8  is prohibited, the manipulation command is disabled. 
     Specifically, when the manipulation command is enabled, work implement controller  26  drives directional control valve  64 , whereas when the manipulation command is disabled, work implement controller  26  does not drive directional control valve  64 . 
     [Process of Operation] 
       FIG. 8  is a flowchart of a process of an operation of an excavation work by work vehicle CM according to the second embodiment. 
     As shown in  FIG. 8 , work implement controller  26  determines whether the excavation mode is set (step S 2 ). Specifically, work implement controller  26  determines whether a setting instruction via the excavation mode setting button to set the excavation mode in response to a manipulation command of the operator is received. 
     In step S 2 , if work implement controller  26  determines that the excavation mode is set, work implement controller  26  calculates cutting edge data (step S 4 ). 
     Specifically, work implement controller  26  calculates a boom cylinder length, a dipper stick cylinder length, and a bucket cylinder length based on detection results obtained from boom cylinder stroke sensor  16 , dipper stick cylinder stroke sensor  17 , and bucket cylinder stroke sensor  18 . Inclination angle θ 1  of boom  6  with respect to the horizontal direction is calculated from the boom cylinder length. Inclination angle θ 2  of dipper stick  7  with respect to boom  6  is calculated from the dipper stick cylinder length. Inclination angle θ 3  of cutting edge  8   a  of bucket  8  with respect to dipper stick  7  is calculated from the bucket cylinder length. Thus, cutting edge data [x1, y1, α1] indicating a position of bucket  8  and a direction of cutting edge  8   a  of bucket  8  (the cutting edge&#39;s direction) in the x- and y-axis vehicular body coordinate system is calculated. 
     Subsequently, work implement controller  26  accepts an input via a manipulation lever (step S 6 ). 
     In the present example, manipulation inputs via first manipulation lever  25 R and second manipulation lever  25 L are accepted. 
     As has been previously discussed, first manipulation lever  25 R is manipulated to manipulate boom  6  and bucket  8 . Furthermore, second manipulation lever  25 L is manipulated to manipulate dipper stick  7  and revolving unit  3 . In the present example, a case where second manipulation lever  25 L is manipulated to manipulate dipper stick  7  will be described, and a case where the manipulation lever is manipulated to manipulate revolving unit  3  will not be described. This is because even when revolving unit  3  is manipulated, bucket  8  does not vary in posture in the x- and y-axis vehicular body coordinate system with boom pin  13  serving as a reference point (or a reference position). 
     Subsequently, work implement controller  26  determines whether a boom command input via a manipulation lever has been received (step S 30 ). Work implement controller  26  determines whether an input through a manipulation of first manipulation lever  25 R in the forward/backward direction has been received. 
     In step S 30 , when the boom command input is received, work implement controller  26  performs a boom command determination process (step S 32 ). 
     More specifically, as has been described with reference to  FIG. 7 , a process is performed to determine whether direction Vb in which cutting edge  8   a  of bucket  8  moves and hence travels according to the boom command input is on the side of the open side of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 . 
     Subsequently, work implement controller  26  determines whether a dipper stick command input via a manipulation lever has been received (step S 34 ). Work implement controller  26  determines whether an input through a manipulation of second manipulation lever  25 L in the forward/backward direction has been received. 
     In step S 34 , when the dipper stick command input is received, work implement controller  26  performs a dipper stick command determination process (step S 36 ). 
     More specifically, as has been described with reference to  FIG. 7 , a process is performed to determine whether direction Va in which cutting edge  8   a  of bucket  8  moves and hence travels according to the dipper stick command input is on the side of the open side of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 . 
     Subsequently, work implement controller  26  determines whether a bucket command input via a manipulation lever has been received (step S 38 ). Work implement controller  26  determines whether an input through a manipulation of first manipulation lever  25 R in the rightward/leftward manipulation has been received. 
     In step S 38 , when the bucket command input is received, work implement controller  26  performs a bucket command determination process (step S 40 ). 
     More specifically, as has been described with reference to  FIG. 7 , a process is performed to determine whether direction Vk in which cutting edge  8   a  of bucket  8  moves and hence travels according to the bucket command input is on the side of the open side of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 . 
     Subsequently, work implement controller  26  controls the work implement according to the determination result (step S 42 ). 
     Specifically, work implement controller  26  drives directional control valve  64  according to the determination result. 
     Work implement controller  26  drives directional control valve  64  according to determination results respectively of the boom, dipper stick and bucket command determination processes. 
     As an example, when a manipulation command is enabled as a result of determination through the boom command determination process, then, in response to an amount of manipulating first manipulation lever  25 R in the forward/backward direction (an amount of manipulating the boom), as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to boom cylinder  10  for driving boom  6 . 
     In contrast, when the manipulation command is disabled as a result of determination through the boom command determination process, work implement controller  26  does not drive directional control valve  64 . 
     Furthermore, when a manipulation command is enabled as a result of determination through the dipper stick command determination process, then, in response to an amount of manipulating second manipulation lever  25 L in the forward/backward direction (an amount of manipulating the dipper stick), as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to dipper stick cylinder  11  for driving dipper stick  7 . 
     In contrast, when the manipulation command is disabled as a result of determination through the dipper stick command determination process, work implement controller  26  does not drive directional control valve  64 . 
     Furthermore, when a manipulation command is enabled as a result of determination through the bucket command determination process, then, in response to an amount of manipulating first manipulation lever  25 R in the rightward and leftward direction (an amount of manipulating the bucket), as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to bucket cylinder  12  for driving bucket  8 . 
     In contrast, when the manipulation command is disabled as a result of determination through the bucket command determination process, work implement controller  26  does not drive directional control valve  64 . 
     Subsequently, work implement controller  26  determines whether a work has ended (step S 18 ). When work implement controller  26  determines that the work has ended is for example when the engine is stopped. 
     In step S 18 , if work implement controller  26  determines that the work has ended (YES in step S 18 ), work implement controller  26  ends the process (END). 
     On the other hand, if work implement controller  26  determines in step S 18  that the work has not ended (NO in step S 18 ), work implement controller  26  returns to step S 2  and repeats the above process. 
     On the other hand, if work implement controller  26  determines in step S 2  that the excavation mode is not set, work implement controller  26  accepts an input via a manipulation lever (step S 20 ). 
     In the present example, manipulation inputs via first manipulation lever  25 R and second manipulation lever  25 L are accepted. 
     As has been previously discussed, in the normal mode, first manipulation lever  25 R is manipulated to manipulate boom  6  and bucket  8 . Furthermore, second manipulation lever  25 L is manipulated to manipulate dipper stick  7  and revolving unit  3 . 
     And work implement controller  26  controls the work implement (step S 22 ). 
     In response to an amount of manipulating first manipulation lever  25 R in the forward/backward direction (an amount of manipulating the boom), as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to boom cylinder  10  for driving boom  6 . 
     In response to an amount of manipulating first manipulation lever  25 R in the rightward and leftward direction (an amount of manipulating the bucket), as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to bucket cylinder  12  for driving bucket  8 . 
     In response to an amount of manipulating second manipulation lever  25 L in the forward/backward direction (an amount of manipulating the dipper stick), as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to dipper stick cylinder  11  for driving dipper stick  7 . 
     In response to an amount of manipulating second manipulation lever  25 L in the rightward and leftward direction, as based on a result of detection by pressure sensor  66 , work implement controller  26  drives directional control valve  64  to supply hydraulic oil to the hydraulic actuator for driving revolving unit  3 . 
     Subsequently, the control proceeds to step S 18 . 
     The process subsequent to the step is similar to that described above, and accordingly, will not be described repeatedly in detail. 
       FIG. 9  illustrates a specific example of a state of an excavation process by work vehicle CM according to the second embodiment. 
     As shown in  FIG. 9 , a case is shown in which a boom command for boom  6  and a dipper stick command for dipper stick  7  are input as manipulation inputs via the manipulation levers. 
     In a state (1), a boom command for which first manipulation lever  25 R is manipulated in the forward direction is disabled, and a boom command for which the lever is manipulated in the backward direction is enabled. 
     Furthermore, a dipper stick command for which second manipulation lever  25 L is manipulated in the forward direction is disabled, and a dipper stick command for which the lever is manipulated in the backward direction is enabled. 
     In a state (2), a boom command for which first manipulation lever  25 R is manipulated in the forward direction is disabled, and a boom command for which the lever is manipulated in the backward direction is enabled. 
     Furthermore, a dipper stick command for which second manipulation lever  25 L is manipulated in the forward direction is enabled, and a dipper stick command for which the lever is manipulated in the backward direction is disabled. 
     In the second embodiment, when a movement of bucket  8  by a manipulation command moves cutting edge  8   a  of bucket  8  with increased resistance of soil accompanied, the movement can be limited. Specifically, whether the manipulation command is enabled/disabled is determined and only when it is enabled the work implement is controlled based on the accepted manipulation command input. 
     This can restrain bucket  8  from traveling in a direction on the side of the back surface of bucket  8  with respect to the direction of the cutting edge of bucket  8  and suppress increase of resistance of soil against bucket  8  to allow an efficient excavation work to be performed. 
     Other Embodiment 
       FIG. 9  is a diagram for illustrating an idea of a work vehicle system based on another embodiment. 
     As shown in  FIG. 9 , the work vehicle system according to the other embodiment configures a control system to control work vehicle CM from an external base station  300 . More specifically, it is a configuration in which a function of work implement controller  26  and manipulation device  25  described in  FIG. 3  is provided in external base station  300  or the like. 
     Base station  300  includes a work implement controller  26 # similar in function to work implement controller  26  and a manipulation device  25 # similar in function to manipulation device  25 . 
     Work implement controller  26 # receives a manipulation command via manipulation device  25 # and outputs an operation command for controlling work vehicle CM. Work vehicle CM operates in response to the operation command issued from work implement controller  26 #. More specifically, work implement controller  26 # outputs an operation command for driving directional control valve  64  described in  FIG. 3 . Further, work implement controller  26 # receives information from boom cylinder stroke sensor  16 , dipper stick cylinder stroke sensor  17  and bucket cylinder stroke sensor  18 . 
     This configuration also allows the process for the operation of the excavation work described in the first and second embodiments with reference to  FIGS. 5 and 8  to be performed by work implement controller  26 #. 
     Thus, even when the work vehicle is controlled from the remote base station  300 , the configuration in accordance with the present embodiment can be applied to perform an efficient excavation work. 
     While in the present embodiment a configuration is described in which an operator controls work vehicle CM in accordance with a manipulation input via a manipulation lever which is a manipulation device, the present invention is also applicable to a configuration in which the manipulation device is not provided and work vehicle CM is autonomously controlled. For example, the present invention can also be applied to a case where a manipulation command to perform an excavation work is preprogrammed and the work implement controller operates in response to the programmed manipulation command. Specifically, it suffices to include a process in which when an autonomous control program for autonomously controlling work vehicle CM is started in accordance with a user&#39;s instruction and the work implement controller operates in response to the programmed manipulation command, and bucket  8  travels in a direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 , then, work vehicle CM limits movement of bucket  8  in the direction on the side of the back surface of bucket  8 . 
     &lt;Function and Effect&gt; 
     A function and effect of the present embodiment will be described. 
     According to the present embodiment, work vehicle CM includes vehicular body  1  and work implement  2 , as shown in  FIG. 1 . Work implement  2  has boom  6  pivotable with respect to vehicular body  1 , dipper stick  7  pivotable with respect to boom  6 , and bucket  8  pivotable with respect to dipper stick  7 . As shown in  FIG. 3 , work vehicle CM is provided with manipulation device  25  and work implement controller  26 . Pressure sensor  66  senses pressure generated in response to manipulation of a lever of manipulation device  25  and outputs it to work implement controller  26 . Work implement controller  26  drives directional control valve  64  according to the sensed pressure received from pressure sensor  66  to control work implement  2 . When bucket  8  travels in a direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8  in accordance with manipulation of a lever of manipulation device  25 , work implement controller  26  limits movement of bucket  8  in the direction in accordance with manipulation of the lever of manipulation device  25 . 
     As shown in  FIG. 4 , when bucket  8  travels in a direction to a side on the side of the back surface of bucket  8 , resistance of soil has an increased value. Accordingly, when bucket  8  travels in the direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 , movement of bucket  8  in the direction on the side of the back surface of bucket  8  is limited to suppress an excavation work with an increased value of resistance of soil to thus perform an excavation work efficiently in a simple manner. 
     When bucket  8  travels in the direction of cutting edge  8   a  of bucket  8  or a direction opposite to the side of the back surface of bucket  8 , that is, a direction on the side of the open side of bucket  8 , with respect to the direction of cutting edge  8   a  of bucket  8  in accordance with manipulation of a lever of manipulation device  25 , work implement controller  26  moves bucket  8  in the direction in accordance with manipulation of the lever of manipulation device  25 . 
     As shown in  FIG. 4 , when bucket  8  travels in the direction to the side on the side of the open side of bucket  8 , resistance of soil has a decreased value. Accordingly, when bucket  8  travels in the direction of cutting edge  8   a  of bucket  8  or in the direction on the side of the open side of bucket  8 , bucket  8  can be moved in that direction to perform an excavation work with a small value of resistance of soil and hence efficiently in a simple manner. 
     When bucket  8  travels in a direction within the range of the predetermined angle P on the side of the open side of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8  in accordance with manipulation of a lever of manipulation device  25 , work implement controller  26  limits movement of bucket  8  in the direction in accordance with manipulation of the lever of manipulation device  25 . 
     As shown in  FIG. 4 , depending on the shape of bucket  8 , friction or the like, resistance of soil may also have an increasing value when bucket  8  travels in a direction within the range of the predetermined angle P on the side of the open side of bucket  8  with respect to the direction of cutting edge  8   a,  and limiting the movement of bucket  8  traveling in a direction within the range of the predetermined angle P on the side of the open side of bucket  8  can also suppress an excavation work with an increased value of resistance of soil to thus perform an excavation work efficiently in a simple manner. 
     When bucket  8  travels in a direction outside the range of the predetermined angle P on the side of the open side of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8  in accordance with manipulation of a lever of manipulation device  25 , work implement controller  26  moves bucket  8  in the direction in accordance with manipulation of the lever of manipulation device  25 . 
     As shown in  FIG. 4 , when bucket  8  travels in the direction outside the range of the predetermined angle P on the side of the open side of the bucket  8 , resistance of soil has a decreased value. Accordingly, when bucket  8  travels in the direction of cutting edge  8   a  of bucket  8  or in the direction outside the range of the predetermined angle P on the side of the open side of the bucket  8 , bucket  8  can be moved in that direction to perform an excavation work with a small value of resistance of soil and hence efficiently in a simple manner. 
     When work implement controller  26  limits movement of bucket  8  in a direction in accordance with manipulation of a lever of manipulation device  25 , work implement controller  26  prohibits bucket  8  from moving in that direction. 
     Prohibiting a movement in a direction accompanied by a large value of resistance of soil can stop an excavation work with an increased value of resistance of soil to thus perform an excavation work efficiently in a simple manner. 
     When bucket  8  travels in a direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8  in accordance with manipulation of a lever of manipulation device  25 , then, as shown in  FIG. 6 , the direction in which bucket  8  travels is projected in a predetermined direction on the side of the open side of bucket  8 , and bucket  8  is moved in that predetermined direction. 
     Work implement controller  26  changes target cutting edge data [x1, y1, α1] for bucket  8  moving in response to manipulation of a lever of manipulation device  25  to target cutting edge data [x1′, y1′, α1′] in a predetermined direction. Bucket  8  moving in the predetermined direction on the side of the open side of bucket  8  allows an excavation work to be performed with a small value of resistance of soil and hence efficiently in a simple manner. Furthermore, even when an unskilled operator manipulates a lever, bucket  8  moves in the predetermined direction, and an efficient excavation work can be performed. 
     At least one of a boom command for boom  6 , a dipper stick command for dipper stick  7 , and a bucket command for bucket  8  is output to work implement controller  26  in response to manipulation of a lever of manipulation device  25 . When bucket  8  travels in a direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8  as boom  6  moves in response to a boom command received via manipulation device  25 , work implement controller  26  prohibits boom  6  from moving. When bucket  8  travels in a direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8  as dipper stick  7  moves in response to a dipper stick command received via manipulation device  25 , work implement controller  26  prohibits dipper stick  7  from moving. When bucket  8  travels in a direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8  in response to a bucket command received via manipulation device  25 , work implement controller  26  prohibits bucket  8  from moving. 
     When bucket  8  travels in a direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8  in response to at least one of a boom command, a dipper stick command, and a bucket command received through manipulation of a lever of manipulation device  25 , bucket  8  can be prohibited from moving and thus restrained from traveling in the direction on the side of the back surface of bucket  8  with respect to the direction of the cutting edge of bucket  8 , to suppress increase of resistance of soil against bucket  8  to perform an efficient excavation work. 
     Work implement controller  26  determines whether work implement  2  executes an excavation mode, and when work implement controller  26  determines that work implement  2  executes the excavation mode, and bucket  8  travels in a direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8  in accordance with manipulation of a lever of manipulation device  25 , work implement controller  26  limits movement of bucket  8  in the direction in accordance with manipulation of the lever of manipulation device  25 . 
     When work implement controller  26  determines that the excavation mode is executed, and bucket  8  travels in a direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8  in accordance with manipulation of a lever of manipulation device  25 , work implement controller  26  limits movement of bucket  8  in the direction in accordance with manipulation of the lever of manipulation device  25 . When the excavation mode is not executed, the movement is not limited to allow an excavation work to be performed efficiently in a simple manner. 
     Work implement controller  26  determines, according to an instruction issued as the operator presses excavation mode setting button  25 P, whether the current mode is an excavation mode in which work implement  2  performs an excavation work. 
     Whether the current mode is the excavation mode can be determined according to an instruction issued as the operator presses excavation mode setting button  25 P, and an efficient excavation work can be performed efficiently with the operator&#39;s intention reflected. 
     According to the present embodiment, work vehicle CM includes vehicular body  1  and work implement  2 , as shown in  FIG. 1 . Work implement  2  has boom  6  pivotable with respect to vehicular body  1 , dipper stick  7  pivotable with respect to boom  6 , and bucket  8  pivotable with respect to dipper stick  7 . A method for controlling work vehicle CM comprises: receiving a command for work implement  2 ; and controlling work implement  2  in response to the command. The step of controlling work implement  2  includes, when bucket  8  travels in a direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8  in accordance with a command, limiting movement of bucket  8  in the direction in accordance with the command. 
     As shown in  FIG. 4 , when bucket  8  travels in a direction to a side on the side of the back surface of bucket  8 , resistance of soil has an increased value. Accordingly, when bucket  8  travels in the direction on the side of the back surface of bucket  8  with respect to the direction of cutting edge  8   a  of bucket  8 , movement of bucket  8  in the direction on the side of the back surface of bucket  8  can be limited to restrain an excavation work with an increased value of resistance of soil to thus perform an excavation work efficiently in a simple manner. 
     While a hydraulic excavator has been described as a work vehicle in the present example, the work vehicle is also applicable to a crawler dozer, a wheel loader and other similar work vehicles. 
     While the present invention has been described in embodiments, it should be understood that the embodiments disclosed herein are illustrative and non-restrictive in any respect. The scope of the present invention is defined by the terms of the claims, and is intended to include any modifications within the meaning and scope equivalent to the terms of the claims. 
     REFERENCE SIGNS LIST 
     
         
           1  vehicular body,  2  work implement,  3  revolving unit,  4  operator&#39;s cab,  4   s  operator&#39;s seat,  5  traveling unit,  5 Cr crawler,  6  boom,  7  dipper stick,  8  bucket,  8   a  cutting edge,  9  engine room,  10  boom cylinder,  11  dipper stick cylinder,  12  bucket cylinder,  13  boom pin,  14  dipper stick pin,  15  bucket pin,  16  boom cylinder stroke sensor,  17  dipper stick cylinder stroke sensor,  18  bucket cylinder stroke sensor,  19  handrail,  25  manipulation device,  25 L second manipulation lever,  25 P excavation mode setting button,  25 R first manipulation lever,  26  work implement controller,  60  hydraulic cylinder,  64  directional control valve,  66  pressure sensor.