Patent Publication Number: US-11035099-B2

Title: Work vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to a work vehicle. 
     BACKGROUND ART 
     A wheel loader, which is a type of the work vehicle, has a working device for excavation driven by a hydraulic actuator or the like in front of a frame. An operator of the wheel loader advances the vehicle and inserts a leading end of the working device into an excavation target such as crushed stone or earth and sand, then lifts the working device and scoops the excavation target into the working device, to thereby perform excavation. 
     In the excavation by the wheel loader, in order to prevent slipping of a tire, there is a need to lift the working device at an appropriate timing at the start of excavation. In other words, when the working device has been inserted into the excavation target, a resistance force exerted on the working device from the excavation target acts in a direction in which the hydraulic actuator connecting the working device and the frame contracts. At that time, if the working device is fixed in a vertical direction by the excavation target, the frame is lifted by the hydraulic actuator, a frictional force between a ground and the tire is reduced so that the tire may slip. When the tire slips, not only wear of the tire is accelerated, but also the tire scrapes off a road surface to thereby deteriorate a road surface condition, which leads to a decrease in working efficiency. Therefore, the operator of the wheel loader usually lifts the working device at the time of starting the excavation, and a load is applied to front wheels by a reaction force against the lifted working device, thereby preventing the tires from slipping. However, if the timing at which to lift the working device is early, the working device starts to be lifted before the working device is sufficiently inserted into the excavation target, and the amount of excavation target scooped by the working device decreases. Meanwhile, if the timing at which to lift the working device is delayed, the tire slips as described above. Therefore, the operator of the wheel loader needs to determine the start of excavation at an appropriate timing and perform the lifting operation of the working device. 
     In the conventional wheel loader, as described above, the operator needs to determine the appropriate excavation start timing. However, it may be difficult for an inexperienced operator to determine the appropriate excavation start timing, for example, when the leading end of the working device cannot be visually observed. To cope with the above case, Patent Literature 1 discloses a technique for determining a state of work in the work vehicle based on a hydraulic pressure of a hydraulic cylinder, an operating state of the working device by the operator, an accelerator opening degree of the work vehicle, and the like. 
     CITATION LIST 
     Patent Literature 
     
         
         PATENT LITERATURE 1: WO 2005/024208 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In the conventional technique disclosed in Patent Literature 1, the hydraulic pressure of the hydraulic cylinder is compared with a predetermined reference value, and whether the work vehicle is excavating, or not, is determined based on the comparison result. However, the above determination method makes it difficult to quickly and accurately determine the excavation start timing. 
     Solution to Problem 
     According to the present invention, there is provided a work vehicle including: a working device; a hydraulic actuator that drives the working device; a hydraulic pump that supplies hydraulic oil to the hydraulic actuator; a hydraulic actuator pressure detector that detects a pressure of the hydraulic actuator; a control valve that controls the amount of hydraulic oil to be supplied from the hydraulic pump to the hydraulic actuator; a vehicle acceleration detector that detects a vehicle acceleration in a longitudinal direction; and a control device that determines whether excavation of the working device has started, or not, based on the pressure of the hydraulic actuator detected by the hydraulic actuator pressure detector and the vehicle acceleration detected by the vehicle acceleration detector. 
     Advantageous Effects of Invention 
     According to the present invention, the excavation start timing can be quickly and accurately determined. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view of a wheel loader which is a work vehicle according to an embodiment of the present invention. 
         FIG. 2  is a system configuration diagram of a wheel loader according to the embodiment of the present invention. 
         FIG. 3  is a control block diagram of a control device. 
         FIG. 4  is a control block diagram of an excavation start determination section. 
         FIG. 5  is a diagram showing an example of changing a threshold value of a lift cylinder bottom pressure incremental acceleration and a threshold value of a vehicle acceleration according to a hardness of an excavation target. 
         FIG. 6  is a control block diagram of an excavation operation prediction section. 
         FIG. 7  is a diagram showing an example of the operation of a wheel loader according to the embodiment of the present invention. 
         FIG. 8  is a diagram showing a hardware configuration of the control device. 
         FIG. 9  is a diagram showing an example of a system configuration of the wheel loader when a torque converter power transmission mechanism is employed. 
         FIG. 10  is a diagram showing an example of a system configuration of the wheel loader employing an HST power transmission mechanism. 
         FIG. 11  is a diagram showing an example of a system configuration of the wheel loader employing an HMT power transmission mechanism. 
         FIG. 12  is a diagram showing an example of a system configuration of the wheel loader employing a hybrid power transmission mechanism. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. 
     [Configuration of Wheel Loader  100 ] 
       FIG. 1  is a side view of a wheel loader  100  which is a work vehicle according to an embodiment of the present invention. The wheel loader  100  shown in  FIG. 1  includes a frame  110  and an articulated working device  150  that is attached to a front of the frame  110 . 
     The working device  150  is a work device driven by at least one actuator. The working device  150  shown in  FIG. 1  includes lift arms  155  and a bucket  151 . Lift cylinders  152  and a bucket cylinder  153  are attached between the working device  150  and the frame  110  as hydraulic actuators (hydraulic cylinders) that drive the lift arms  155  and the bucket  151 , respectively. Incidentally, the lift arms  155  and the lift cylinders  152  are provided on the left and right sides of the frame  110  one by one, but in  FIG. 1 , the lift arm  155  and the lift cylinder  152  on the right side of the frame  110  are hidden. 
     The lift arms  155  rotate (elevate) in a vertical direction as the lift cylinders  152  expand and contract. The bucket  151  rotates (performs dump operation or cloud operation) in association with expansion and contraction of the bucket cylinder  153 . Meanwhile, a link mechanism for actuating the bucket  151  of the wheel loader  100  shown in  FIG. 1  is of a Z link type (bell crank type) using a bell crank  154 . 
     The lift cylinders  152  are connected to the lift arms  155  and the frame  110 . Hereinafter, one side of the lift cylinders  152  connected to the lift arms  155  will be referred to as a rod side and the other side connected to the frame  110  will be referred to as a bottom side. With the supply of hydraulic oil from a hydraulic pump to be described later to the bottom side of the lift cylinders  152 , cylinder rods of the lift cylinders  152  are extended to lift the lift arms  155 . In addition, with the supply of hydraulic oil from a hydraulic pump to the rod side of the lift cylinders  152 , cylinder rods of the lift cylinders  152  are retracted to lower the lift arms  155 . 
     The bucket cylinder  153  is connected to the bell crank  154  and the frame  110 . Hereinafter, one side of the bucket cylinder  153  connected to the bell crank  154  will be referred to as the rod side and the other side connected to the frame  110  will be referred to as the bottom side. With the supply of hydraulic oil from the hydraulic pump to the bottom side of the bucket cylinder  153 , the cylinder rod of the bucket cylinder  153  is extended, and the bucket  151  rotates so that an opening of the bucket  151  faces upward. In addition, with the supply of hydraulic oil from the hydraulic pump to the rod side of the bucket cylinder  153 , the cylinder rod of the bucket cylinder  153  is retracted, and the bucket  151  rotates so that the opening faces downward. 
     A bucket cylinder stroke detector  250  for detecting a bucket cylinder stroke, that is, the stroke amount of the bucket cylinder  153  is attached to the bucket cylinder  153  in order to determine whether a bottom surface of the bucket  151  is horizontal with respect to the ground, or not. A lift arm angle detector  251  for detecting a lift arm angle, that is, an angle of the lift arm  155  is attached in the vicinity of a connecting portion of the lift arm  155  to the frame  110  in order to determine a height of the lift arm  155 . 
     At the time of starting excavation, an operator sets the bottom surface of the bucket  151  to be horizontal with respect to the ground, and advances the wheel loader  100  toward crushed stone or earth and sand which are the excavation target in an attitude in which the lift arms  155  are lowered to such an extent that the bucket  151  comes in contact with the ground. When a leading end of the working device  150 , that is, a leading end of the bucket  151  abuts against the excavation target, the resistance force from the excavation target acts so as to contract the lift cylinders  152 , and the pressure of the lift cylinders  152  on the bottom side increases. For that reason, a lift cylinder bottom pressure detector  252  for detecting a lift cylinder bottom pressure, that is, the bottom pressure of the lift cylinders  152  is attached to the lift cylinders  152  in order to detect the resistance force from the excavation target exerted on the working device  150 . A resistance force from an excavation target also acts on the bucket cylinder  153 , but a magnitude of a pressure change in the bucket cylinder  153  at that time largely changes depending on an angle of the bottom surface of the bucket  151  with respect to the ground. In addition, a pressure change in the lift cylinder  152  on the rod side due to the resistance force from the excavation target is smaller than the pressure change on the bottom side. Therefore, in order to detect the resistance force from the excavation target, it is suitable to detect the lift cylinder bottom pressure. 
     The frame  110  is provided with four wheels  1   a ,  1   b ,  1   c , and  1   d . In  FIG. 1 , wheels  1   a  and  1   b  on the right side of the frame  110  are hidden. Hereinafter, the wheels  1   a ,  1   b ,  1   c , and  1   d  may be collectively referred to as “wheels  1 ”. Each wheel  1  is driven by a power transmission device  210  (to be described later) with an engine  201  (to be described later) as a power source. A driving force is transmitted to the ground through each wheel  1  so that the wheel loader  100  travels forward or backward. 
       FIG. 2  is a system configuration diagram of the wheel loader according to the embodiment of the present invention shown in  FIG. 1 . 
     The engine  201  supplies a power to the power transmission device  210  and a hydraulic pump  220 . The engine  201  has an electronic control governor  202  that controls a fuel injection amount. The electronic control governor  202  controls a fuel injection amount of the engine  201  based on a manipulated variable of an accelerator pedal  264  detected by an accelerator manipulated variable detector  256 . 
     The power transmission device  210  is a power transmission mechanism that transmits a part of the power output from the engine  201  to the wheels  1 . For example, a torque converter type, an HST (hydro static transmission) type, an HMT (hydro mechanical transmission) type, a hybrid type, or the like can be adopted as a system of the power transmission device  210 . A specific example of the power transmission device  210  will be described later with reference to  FIGS. 9 to 12 . 
     The hydraulic pump  220  supplies the hydraulic oil through the control valve  221  to multiple hydraulic actuators associated with the working device  150  including the lift cylinders  152  and the bucket cylinder  153  described above, to thereby appropriately drive each hydraulic actuator. A power source of the hydraulic pump  220  is the engine  201 . For that reason, similarly, in each hydraulic actuator having the hydraulic pump  220  as a drive source, the engine  201  serves as a power source as with the wheels  1 . 
     The control valve  221  controls the amount of hydraulic oil to be supplied from the hydraulic pump  220  to the hydraulic actuators (lift cylinders  152 , bucket cylinder  153 ) according to pilot pressures described below. The pilot pressures are output from a working device control lever  261  for controlling the working device  150  and a control valve control section  262 . A higher one of the pilot pressures is selected by a high pressure selection valve  263  and acts on the control valve  221 . Incidentally, as will be described later, the control valve control section  262  is driven according to an excavation start determination command output from the control device  240 . 
     A vehicle traveling direction detector  253  detects whether a traveling direction of the vehicle, that is, a traveling direction of the wheel loader  100  is forward or backward, based on a rotational direction of a propeller shaft  230 , and outputs the detection result to the control device  240 . Alternatively, the vehicle traveling direction detector  253  can detect a rotational speed or the like of the propeller shaft  230  and calculate an acceleration and a traveling speed of the wheel loader  100  based on the detection result. For example, the acceleration of the wheel loader  100  can be obtained by differentiating the rotational speed of the propeller shaft  230  detected by the vehicle traveling direction detector  253 . 
     The vehicle acceleration detector  254  detects the vehicle acceleration in the longitudinal direction, that is, the acceleration of the wheel loader  100 , and outputs the detected acceleration to the control device  240 . When the vehicle acceleration is calculated based on the detection result of the vehicle traveling direction detector  253  as described above, the vehicle acceleration detector  254  may not be provided. 
     The excavation determination notification section  265  gives a notification to the operator according to an excavation start determination command output from the control device  240 . The excavation determination notification section  265  is configured by, for example, a monitor capable of displaying a predetermined screen. 
     The control device  240  is a computer for executing various types of information processing relating to the operation of the wheel loader  100 , and is configured using, for example, a microcomputer. The control device  240  determines whether the working device  150  has started excavation, or not, based on the lift cylinder bottom pressure detected by the lift cylinder bottom pressure detector  252  and the acceleration of the wheel loader  100  in the longitudinal direction detected by the vehicle acceleration detector  254 . When the control device  240  has determined that excavation has started, the control device  240  outputs the excavation start determination command. Details of a control process to be performed by the control device  240  will be described later. 
       FIG. 8  is a diagram showing a hardware configuration of the control device  240 . The control device  240  includes an input section  91 , a central processing unit (CPU)  92  which is a processor, a read only memory (ROM)  93  and a random access memory (RAM)  94  which are storage devices, and an output section  95 . 
     The input section  91  receives information and signals output from the lift cylinder bottom pressure detector  252 , the vehicle traveling direction detector  253 , the vehicle acceleration detector  254 , the lift arm angle detector  251 , the bucket cylinder stroke detector  250 , and the like described above, and outputs the received information and signals to the CPU  92 . At that time, the input section  91  performs A/D conversion as occasion demands. The ROM  93  is a recording medium in which programs and the like are stored. The CPU  92  performs predetermined arithmetic processing on the information and signals taken from the input section  91 , the ROM  93 , and the RAM  94  according to the programs stored in the ROM  93 . The output section  95  creates a signal for output according to the calculation result of the CPU  92 , and outputs the created signal to the control valve control section  262  and the excavation determination notification section  265 . Incidentally, although the control device  240  of  FIG. 8  includes the ROM  93  and the RAM  94  which are semiconductor memories as storage devices, the control device  240  may include a magnetic storage device such as a hard disk drive instead of those semiconductor memories and store the programs or the like in the magnetic storage device. 
     [Control Process of Control Device  240 ] 
     Next, details of the control process to be executed by the control device  240  will be described.  FIG. 3  is a control block diagram of the control device  240 . As shown in  FIG. 3 , the control device  240  includes a working device to ground angle acquisition section  321 , an excavation work prediction section  320 , and an excavation start determination section  310  as functions of the control process. 
     The working device to ground angle acquisition section  321  receives the bucket cylinder stroke detected by the bucket cylinder stroke detector  250  and the lift arm angle detected by the lift arm angle detector  251  as information for calculating the angle of the working device  150  to the ground. The work machine to ground angle acquisition section  321  calculates the working device to ground angle based on those pieces of input information, to thereby acquire the angle of the working device  150  to the ground and output the acquired angle to the excavation work prediction section  320 . For example, the working device to ground angle acquisition section  321  may calculate the working device to ground angle corresponding to the input bucket cylinder stroke and lift arm angle geometrically with the use of a mathematical formula based on dimensional parameters of the lift arm  155 , the bucket  151 , the bell crank  154 , and so on configuring the working device  150 . Alternatively, the working device to ground angle acquisition section  321  may tabulate a relationship between the stroke amount of the bucket cylinder  153  as well as the angle of the lift arms  155  and the working device to ground angle and store a resultant table in the control device  240  in advance, and obtain the working device to ground angle corresponding to the input bucket cylinder stroke and lift arm angle with the use of the table. Alternatively, the working device to ground angle acquisition section  321  may directly detect the angle of the working device  150  to the ground with the use of a ground angle sensor or the like without using the bucket cylinder stroke detected by the bucket cylinder stroke detector  250  or the lift arm angle detected by the lift arm angle detector  251 , to thereby acquire the working device to ground angle. 
     The excavation work prediction section  320  predicts whether the working device  150  is to perform excavation from now, or not, based on the working device to ground angle acquired by the working device to ground angle acquisition section  321  and the vehicle traveling direction detected by the vehicle traveling direction detector  253 . More specifically, when the vehicle traveling direction is forward and the working device to ground angle falls within a predetermined range, the excavation work prediction section  320  determines that the wheel loader  100  is in an excavation start attitude and predicts that the working device  150  will perform excavation. In this situation, the excavation work prediction section  320  outputs an excavation work prediction command to the excavation start determination section  310 . Meanwhile, if at least one of those conditions is not satisfied, the excavation work prediction section  320  predicts that the working device  150  will not perform excavation and does not output the excavation work prediction command. Details of the processing of the excavation work prediction section  320  will be described later. 
     The excavation start determination section  310  receives the excavation work prediction command from the excavation work prediction section  320 , the lift cylinder bottom pressure detected by the lift cylinder bottom pressure detector  252 , and the vehicle acceleration detected by the vehicle acceleration detector  254  as the information for determining a timing of the appropriate lifting operation of the working device  150 . The excavation start determination section  310  determines that the working device  150  has started excavation based on the lift cylinder bottom pressure and the vehicle acceleration when the excavation work prediction command is output from the excavation work prediction section  320 . More specifically, when the excavation work prediction command is output and an increasing speed of the lift cylinder bottom pressure is equal to or more than a predetermined threshold value and the vehicle acceleration is equal to or less than the predetermined threshold value, the excavation start determination section  310  detects that a leading end of the bucket  151  has abutted against the excavation target and determines that the working device  150  has started excavation. At that time, the excavation start determination section  310  outputs the excavation start determination command to the control valve control section  262  and the excavation determination notification section  265 . Meanwhile, when the excavation work prediction command is not output or when at least one of the increasing speed of the lift cylinder bottom pressure and the vehicle acceleration does not satisfy the above-described conditions, the excavation start determination section  310  determines that the working device  150  has not started excavation and does not output the excavation start determination command. The details of the processing of the excavation start determination section  310  will be described later. 
     The control valve control section  262  performs a control for lifting the working device  150  at the appropriate timing on the control valve  221  in response to the excavation start determination command output from the excavation start determination section  310 . Specifically, when the excavation start determination command is output from the excavation start determination section  310 , the control valve control section  262  outputs a predetermined pilot pressure to the control valve  221  and controls the control valve  221  so that the supply of hydraulic oil to the bottom side of the lift cylinder  152  starts. Whereas, when the excavation start determination command is not input, the control valve control section  262  does not supply hydraulic oil to the bottom side of the lift cylinder  152  without outputting the pilot pressure. As a result, since hydraulic oil is supplied from the hydraulic pump  220  to the bottom side of the lift cylinder  152  at an appropriate timing determined by the excavation start determination section  310 , and the working device  150  can be lifted without delay, the wheels  1  can be prevented from slipping. When the excavation start determination command is output, the control valve control section  262  may control the control valve  221  so as to supply a maximum amount of hydraulic oil that can be supplied by the hydraulic pump  220  to the bottom side of the lift cylinder  152 . Alternatively, the control valve control section  262  may control the control valve  221  so as to have a predetermined supply amount that is less than the maximum amount. 
     The excavation determination notification section  265  gives a notification to the effect that the operator is urged to lift the working device  150  at an appropriate timing in response to the excavation start determination command output from the excavation start determination section  310 . More specifically, when the excavation start determination command is output from the control device  240 , the excavation determination notification section  265  displays the determination that the excavation has started on a monitor for the operator. Meanwhile, when the excavation start determination command is not output from the control device  240 , the excavation determination notification section  265  does not perform display on the monitor. In this manner, the excavation determination notification section  265  notifies the operator of appropriate excavation start timing, as a result of which the operator can know that the operation of lifting the working device  150  has been performed at an appropriate timing through the control of the control valve control section  262 . Further, when the working device  150  is not automatically lifted according to the excavation start timing, for example, when the wheel loader  100  is not provided with the control valve control section  262 , the operator can perform a lifting work of the working device  150  without any delay according to notification of the excavation start timing by the excavation determination notification section  265 . As a result, the wheels  1  can be prevented from slipping. Incidentally, in addition to the monitor display as described above or in place of the monitor display, the excavation determination notification section  265  may give a notification to the operator by other methods. For example, the excavation determination notification section  265  notifies the operator of the determination that the excavation has started with a change in illuminance of an illumination device in a cabin not shown, occurrence of a sound, or vibration of the working device control lever  261 . 
     [Control Process of Excavation Work Prediction Section  320 ] 
     Next, the details of the control process to be executed by the excavation work prediction section  320  will be described.  FIG. 6  is a control block diagram of the excavation work prediction section  320 . As shown in  FIG. 6 , the excavation work prediction section  320  includes a vehicle traveling direction determination section  610 , a working device to ground angle determination section  620 , and an excavation work prediction command section  630  as functions of the control process. 
     The vehicle traveling direction determination section  610  receives the vehicle traveling direction detected by the vehicle traveling direction detector  253 . The vehicle traveling direction determination section  610  determines whether the input vehicle traveling direction is forward, or not, and outputs a Boolean value indicating the determination result to the excavation work prediction command section  630 . In other words, the vehicle traveling direction determination section  610  outputs “TRUE” when the traveling direction of the vehicle is forward, and outputs “FALSE” when the traveling direction of the vehicle is other than forward (in the case of backward). 
     The working device to ground angle determination section  620  receives the working device to ground angle acquired by the working device to ground angle acquisition section  321 . The working device to ground angle determination section  620  determines whether the input working device to ground angle falls within a predetermined range, or not, and outputs the Boolean value indicating the determination result to the excavation work prediction command section  630 . In other words, the working device to ground angle determination section  620  outputs “TRUE” when the working device to ground angle falls within the predetermined range, and outputs “FALSE” when the working device to ground angle falls outside the predetermined range. In general, in order to make it easier to insert the bucket  151  into the excavation target at the time of starting excavation, the angle of the working device  150  to the ground is set substantially horizontal. For that reason, in the working machine to ground angle determination section  620 , it is preferable that a range of the working device to ground angle described above is set corresponding to the angle to the ground at which the working device  150  is substantially horizontal. Further, the range may be a preset value, or may be set by the operator as an arbitrary value from an input device such as a button, a dial, or a touch panel. 
     The excavation work prediction command section  630  receives the Boolean value output from the vehicle traveling direction determination section  610  and the Boolean value output from the working device to ground angle determination section  620 . The excavation work prediction command section  630  predicts whether the working device  150  will perform excavation, or not, based on those input Boolean values and outputs the excavation work prediction command to the excavation start determination section  310  according to the prediction result. In other words, when both of the two Boolean values are “TRUE”, the wheel loader  100  is in the excavation start attitude, and the excavation work prediction command section  630  predicts that the working device  150  will perform excavation from now and outputs the excavation work prediction command. Whereas, when one or both of the two Boolean values are “FALSE”, the wheel loader  100  is not in the excavation start attitude, and the excavation work prediction command section  630  predicts that the working device  150  will not perform excavation, and does not output the excavation work prediction command. 
     With the control configuration described above, the excavation work prediction section  320  can predict whether the working device  150  will perform excavation, or not, based on the working device to ground angle acquired by the working device to ground angle acquisition section  321  and the vehicle travel direction detected by the vehicle traveling direction detector  253 . 
     [Excavation Start Determination Section  310 ] 
     Next, the details of the control process to be executed by the excavation start determination section  310  will be described.  FIG. 4  is a control block diagram of the excavation start determination section  310 . As shown in  FIG. 4 , the excavation start determination section  310  includes, as functions of the control process, an excavation work prediction determination section  410 , a lift cylinder bottom pressure increasing speed determination section  420 , a lift cylinder bottom pressure increasing speed calculation section  421 , a vehicle acceleration determination section  430 , and an excavation start determination command section  440 . 
     The excavation work prediction determination section  410  receives the excavation work prediction command output from the excavation work prediction section  320 . The excavation work prediction determination section  410  determines whether to have received the excavation work prediction command, or not, and outputs a Boolean value indicating the determination result to the excavation start determination command section  440 . In other words, the excavation work prediction determination section  410  outputs “TRUE” when having received the excavation work prediction command, and outputs “FALSE” when having not received the excavation work prediction command. 
     The lift cylinder bottom pressure increasing speed calculation section  421  receives the lift cylinder bottom pressure detected by the lift cylinder bottom pressure detector  252 . The lift cylinder bottom pressure increasing speed calculation section  421  obtains an increment of the input lift cylinder bottom pressure per unit time. In this example, since the increment of the lift cylinder bottom pressure per unit time (hydraulic actuator pressure) is synonymous with a speed at which the lift cylinder bottom pressure increases, in the following description, the increment of the lift cylinder bottom pressure per unit time will be referred to as “lift cylinder bottom pressure increasing speed”. Then, the lift cylinder bottom pressure increasing speed calculation section  421  outputs the calculated lift cylinder bottom pressure increasing speed to the lift cylinder bottom pressure increasing speed determination section  420 . 
     The lift cylinder bottom pressure increasing speed determination section  420  receives the lift cylinder bottom pressure increasing speed calculated by the lift cylinder bottom pressure increasing speed calculation section  421 . The lift cylinder bottom pressure increasing speed determination section  420  determines whether the input lift cylinder bottom pressure increasing speed is equal to or more than a predetermined threshold value, or not, and outputs a Boolean value indicating the determination result to the excavation start determination command section  440 . In other words, the lift cylinder bottom pressure increasing speed determination section  420  outputs “TRUE” when the lift cylinder bottom pressure increasing speed is equal to or more than the threshold value, and outputs “FALSE” when the lift cylinder bottom pressure increasing speed is less than the threshold value. 
     The vehicle acceleration determination section  430  receives the vehicle acceleration detected by the vehicle acceleration detector  254 . The vehicle acceleration determination section  430  determines whether the input vehicle acceleration is equal to or less than a predetermined threshold value, or not, that is, whether the deceleration of the wheel loader  100  is equal to or more than a predetermined value, or not, and outputs a Boolean value indicating the determination result to the excavation start determination command section  440 . In other words, the vehicle acceleration determination section  430  outputs “TRUE” when the vehicle acceleration is equal to or less than the threshold value (when the deceleration is equal to or more than the predetermined value), and outputs “FALSE” when the vehicle acceleration exceeds the threshold value (when the deceleration is less than the predetermined value). 
     The excavation start determination command section  440  receives a Boolean value output from the excavation work prediction determination section  410 , a Boolean value output from the lift cylinder bottom pressure increasing speed determination section  420 , and a Boolean value output from the vehicle acceleration determination section  430 . The excavation start determination command section  440  performs the excavation start determination of the working device  150  based on those input Boolean values and outputs the excavation start determination command to the control valve control section  262  and the excavation determination notification section  265  according to the determination result. In other words, when all of three Boolean values are “TRUE”, the excavation start determination command section  440  determines that the excavation has started and outputs the excavation start determination command. Whereas, when at least one of the three Boolean values is “FALSE”, the excavation start determination command section  440  determines that the excavation has not started, and does not output the excavation start determination command. 
     Meanwhile, in the lift cylinder bottom pressure increasing speed determination section  420 , it is preferable to set a different threshold value for the lift cylinder bottom pressure increasing acceleration according to the hardness of the excavation target. For example, when the excavation target is relatively soft, the increasing speed of the resistance force received from the excavation target when the bucket  151  is abutted against the excavation target is smaller than that when the excavation target is hard. For that reason, if the same threshold value as that when the excavation target is hard is used, a timing at which the Boolean value output from the lift cylinder bottom pressure increasing speed determination section  420  changes from “FALSE” to “TRUE” is delayed. As a result, the output of the excavation start determination command from the excavation start determination command section  440  is delayed, resulting in a possibility that the wheels  1  may slip. Therefore, it is preferable that the threshold value of the lift cylinder bottom pressure increasing speed is set to be larger as the excavation target is harder. 
     Also, it is preferable to set a different threshold value for the vehicle acceleration in the vehicle acceleration determination section  430  according to the hardness of the excavation target. However, it is preferable that the threshold value of the vehicle acceleration is set to be smaller as the excavation target is harder. 
       FIG. 5  is a diagram showing an example of changing the threshold value of the lift cylinder bottom pressure increasing speed and the threshold value of the vehicle acceleration according to the hardness of the excavation target. In  FIG. 5 , a graph  510  shows an example of a relationship between the hardness of the excavation target and the threshold value of the lift cylinder bottom pressure increasing speed. In the example of the graph  510 , the threshold value of the lift cylinder bottom pressure increasing speed is set so as to linearly increase as the excavation target becomes hard. The present invention is not limited to the example of the graph  510 , if the threshold value of the lift cylinder bottom pressure increasing speed increases monotonically as the hardness of the excavation target increases, the threshold value of the lift cylinder bottom pressure increasing speed is available in the determination of the lift cylinder bottom pressure incremental acceleration determination section  420 . This includes a monotonous increase in a broad sense, for example, such as a form including a section where the threshold value of the lift cylinder bottom pressure increasing speed is kept constant even if the hardness of the excavation target changes. 
     Meanwhile, in  FIG. 5 , a graph  520  shows an example of a relationship between the hardness of the excavation target and the threshold value of the vehicle acceleration. In the example of the graph  520 , the threshold value of the vehicle acceleration is set so as to linearly decrease more as the excavation target becomes harder. It should be noted that the present invention is not limited to the example of the graph  520  and the threshold value of the vehicle acceleration is available in the determination of the vehicle acceleration determination section  430  as long as the threshold value of the vehicle acceleration decreases monotonically as the hardness of the excavation target increases. This includes a monotonic decrease in a broad sense, for example, such as a form including a section in which the threshold value of the vehicle acceleration is kept constant even if the hardness of the excavation target changes. 
     As described above, in the case where the threshold value of the lift cylinder bottom pressure increasing speed and the threshold value of the vehicle acceleration are changed according to a hardness of the excavation target, it is preferable that those threshold values are set taking a vehicle rank of the wheel loader  100  into consideration. For example, a table indicating a relationship between the hardness of the excavation target and the threshold value of the lift cylinder bottom pressure increasing speed and the threshold value of the vehicle acceleration according to the vehicle rank of the wheel loader  100  is stored in advance in the control device  240 . When the hardness of the excavation target is set when the wheel loader  100  performs the excavation work, the control device  240  obtains the threshold value of the lift cylinder bottom pressure increasing speed and the threshold value of the vehicle acceleration corresponding to the set hardness of the excavation target from the table, and uses those threshold values thus obtained in the determinations of the cylinder bottom pressure increasing speed determination section  420  and the vehicle acceleration determination section  430 . Incidentally, the hardness of the excavation target may be set to an arbitrary value by the operator through an input device such as a button, a dial, or a touch panel, or may be determined based on a previous excavation work. 
     In the present embodiment described above, as understood from a series of processes of the excavation start determination section  310 , when the increasing speed of the lift cylinder bottom pressure exceeds the threshold value and the vehicle acceleration falls below the threshold value, the control device  240  determines that the bucket  151  has abutted against the excavation target and determines that the excavation has started. As described above, the determination is performed with the use of the increasing speed of the lift cylinder bottom pressure, thereby making it possible to determine that the excavation has started more rapidly than the determination when using the lift cylinder bottom pressure as it is. In the case where the determination is performed with the use of the lift cylinder bottom pressure as it is, as the threshold value for the lift cylinder bottom pressure is set smaller, an excavation reaction force can be detected more quickly and the excavation start determination can be performed. However, the possibility of the erroneous determination is increased as much. For example, when excavating clay-based earth and sand, earth and sand may remain without being dropped from the bucket  151  after being loaded on a dump truck. In that case, since a weight of the earth and sand remaining in the bucket  151  is added to the lift cylinder bottom pressure, although the bucket  151  is not abutted against the excavation target, the lift cylinder bottom pressure exceeds the threshold value and the erroneous determination may be performed. Therefore, in order to prevent the erroneous determination, there is a need to set the threshold value of the lift cylinder bottom pressure to a certain high level. For that reason, as compared with the present embodiment, the determination of the excavation start is delayed, which may cause the wheels  1  to slip. 
     Further, according to the present embodiment, in addition to the lift cylinder bottom pressure increasing speed, the determination of the excavation start is performed with the use of the vehicle acceleration. This makes it possible to avoid the erroneous determination caused by a variation in the lift cylinder bottom pressure increasing speed occurring when the lift arms  155  are greatly shaken due to the bound of the frame  110  or the like during traveling on a rough road, and to determine the excavation start more accurately. 
     Furthermore, according to the present embodiment, in addition to the above determination, the excavation work is predicted by the excavation work prediction section  320 , and the excavation start determination is performed with the use of the prediction result. As a result, when traveling except for immediately before the excavation, for example, when performing a carrying work for transporting an excavated load or performing a “rise and run” function or the like for traveling forward while raising the working device  150  in order to load the excavation target onto the dump truck, or the like, the excavation start determination may not be performed. Therefore, the erroneous determination which can occur except for during the excavation work can be avoided and the excavation start can be determined more accurately. 
     [Actual Operation] 
       FIG. 7  is a diagram showing an example of the operation of the wheel loader  100  according to the embodiment of the present invention configured as described above. In  FIG. 7 , a graph  710  shows the transition of the traveling speed, a graph  720  shows the transition of the lift cylinder bottom pressure, a graph  730  shows the transition of the vehicle acceleration, a graph  740  shows the transition of the lift cylinder bottom pressure increasing speed, a graph  750  shows the transition of the working device to ground angle, a graph  760  shows the transition of the excavation start determination command, and a graph  770  shows the transition of the hydraulic oil supply amount to the lift cylinder bottom side. A threshold value  731  in the graph  730  indicates the threshold value of the vehicle acceleration in the vehicle acceleration determination section  430  described above and a threshold value  741  in the graph  740  indicates the threshold value of the lift cylinder bottom pressure increasing speed in the lift cylinder bottom pressure increasing speed determination section  420  described above. In addition, an upper limit threshold value  751  and a lower limit threshold value  752  of the graph  750  indicate a range of the working device to ground angle in the working device to ground angle determination section  620  described above. 
     At a time  0 , the wheel loader  100  is traveling in a state where the working device to ground angle is large, that is, in a state where the bucket opening faces upward, as shown by the graph  750 . In a period from the above time  0  to a time T 1 , since the process is transitioned to the excavation work, the wheel loader  100  advances toward the excavation target as shown in the graph  710  while the working device to ground angle is adjusted to be smaller as shown in the graph  750 . 
     At the time T 1 , when the vehicle traveling direction is forward as shown in the graph  710  and the working device to ground angle falls within a range between the upper limit threshold value  751  and the lower limit threshold value  752  as shown in the graph  750 , the excavation work prediction section  320  outputs the excavation work prediction command to the excavation start determination section  310 . Thereafter, when the leading end of the working device  150  abuts against the excavation target at a time T 2 , the lift cylinder bottom pressure increasing speed starts to increase as shown in the graph  720 , and the vehicle acceleration starts to decrease as shown in the graph  730 . 
     At a time T 3 , as described above, when the excavation work prediction command by the excavation work prediction section  320  is input, and the lift cylinder bottom pressure increasing speed exceeds the threshold value  741  as shown in the graph  740  and the vehicle acceleration falls below the threshold value  731  as shown in the graph  730 , the excavation start determination section  310  determines that the excavation is to be started and outputs the excavation start determination command to the control valve control section  262  and excavation determination notification section  265 . When the excavation start determination command is output as described above, the control valve control section  262  controls the control valve  221  so as to start supplying the hydraulic oil to the bottom side of the lift cylinder  152 , or the excavation determination notification section  265  notifies the operator of the excavation start, and the operator performs the lifting operation of the working device  150  according to the notification of the excavation start. As a result, the lift arms  155  are lifted. 
     As described above, the control device  240  according to the present embodiment determines the excavation start based on the increasing speed of the hydraulic actuator pressure, that is, the lift cylinder bottom pressure increasing speed. As a result, the excavation start can be determined without any delay as compared with the case where the hydraulic actuator pressure is used as it is. Further, when the vehicle acceleration exceeds a predetermined threshold value or when it is predicted that the working device  150  will not perform the excavation, it is not determined that the excavation is started. As a result, the erroneous determination can be avoided and the timing of the excavation start can be determined more accurately. Therefore, the lifting operation of the working device  150  can be performed at an appropriate timing. 
     [Power Transmission Device  210 ] 
     Finally, a specific example of the power transmission device  210  will be described below with reference to  FIGS. 9 to 12 . 
       FIG. 9  is an example of a system configuration diagram of the wheel loader  100  in the case where a torque converter power transmission mechanism that converts a power of the engine  201  into a flow of hydraulic oil and transmits the flow of hydraulic oil to the wheels  1  is adopted as the power transmission device  210 . In the example shown in  FIG. 9 , the wheel loader  100  includes a torque converter  211  that is connected to an output shaft of the engine  201 , and a stepped transmission  212  that changes the power output from the torque converter  211  by a gear mechanism. The stepped transmission  212  rotationally drives the respective wheels  1  through the propeller shaft  230 . 
       FIG. 10  is an example of a system configuration diagram of the wheel loader  100  employing an HST power transmission mechanism that converts the power of the engine  201  into a hydraulic pressure and transmits the hydraulic pressure to the wheels  1  as the power transmission device  210 . In the example shown in  FIG. 10 , the wheel loader  100  includes a hydraulic pump  213  that is connected to the output shaft of the engine  201  and a hydraulic motor  214  that is rotationally driven by the hydraulic oil discharged from the hydraulic pump  213 . The hydraulic motor  214  rotationally drives the respective wheels  1  through the propeller shaft  230 . 
       FIG. 11  is an example of a system configuration diagram of the wheel loader  100  that employs an HMT power transmission mechanism as the power transmission device  210 . In the example shown in  FIG. 11 , the wheel loader  100  further includes a power transmission mechanical section  215  in addition to the hydraulic pump  213  and the hydraulic motor  214 . In this example, the hydraulic pump  213  drives the propeller shaft  230  through the hydraulic motor  214  to drive the wheels  1  while the engine  201  drives the propeller shaft  230  through the power transmission mechanical section  215  to drive the wheels  1 . The power transmission mechanical section  215  is a mechanical mechanism for mechanically connecting the output shaft of the engine  201  and the propeller shaft  230 , and is configured by using, for example, a swash plate piston, a planetary gear, or the like. 
       FIG. 12  is an example of a system configuration diagram of the wheel loader  100  that adopts a hybrid power transmission mechanism that converts the power of the engine  201  into electricity and transmits the electricity to the wheels  1  as the power transmission device  210 . In the example of  FIG. 12 , the wheel loader  100  includes a motor generator (motor/generator)  216  that is mechanically connected to the engine  201  and driven by the engine  201 , an inverter  218  that controls the motor generator  216 , a traveling motor  217  that drives the four wheels  1  attached to the propeller shaft  230  through a differential gear Dif and a gear G, an inverter  219  that controls the traveling motor  217 , and an electrical storage device  290  that is electrically connected to the inverters  218  and  219  through a DC-DC converter  291 . The electrical storage device  290  is configured by, for example, a secondary battery or a capacitor, and exchanges a DC power between the inverter  218  and the inverter  219 . In the system configuration diagram of  FIG. 12 , a configuration example of a so-called series hybrid system is shown, but a parallel hybrid system is also available. 
     According to the embodiment of the present invention described above, the following operational effects are obtained. 
     (1) The wheel loader  100  which is a work vehicle includes the working device  150 , the lift cylinder  152  which is a hydraulic actuator for driving the working device  150 , the hydraulic pump  220  which supplies hydraulic oil to the lift cylinder  152 , the hydraulic actuator pressure detector that detects the pressure of the lift cylinder  152 , that is, the lift cylinder bottom pressure detector  252  that detects the lift cylinder bottom pressure, the control valve  221  that controls the amount of hydraulic oil to be supplied from the hydraulic pump  220  to the lift cylinder  152 , the vehicle acceleration detector  254  that detects the vehicle acceleration in the longitudinal direction, and the control device  240 . The control device  240  determines whether the working device  150  has started excavation, or not, based on the lift cylinder bottom pressure detected by the lift cylinder bottom pressure detector  252  and the vehicle acceleration detected by the vehicle acceleration detector  254 . With the above configuration, the excavation start timing can be determined quickly and accurately.
 
(2) The control device  240  includes the excavation start determination section  310 . The excavation start determination section  310  calculates the hydraulic actuator pressure increasing speed, that is, the lift cylinder bottom pressure increasing speed according to the lift cylinder bottom pressure by the lift cylinder bottom pressure increasing speed calculation section  421 . When the lift cylinder bottom pressure increasing speed determination section  420  determines that the lift cylinder bottom pressure increasing speed is equal to or more than a predetermined threshold value and the vehicle acceleration determination section  430  determines that the vehicle acceleration is equal to or less than a predetermined threshold value, the excavation start determination command section  440  determines that the working device  150  has started the excavation. With the above configuration, the excavation start can be determined quickly while the erroneous determination caused by an unexpected variation in the lift cylinder bottom pressure increasing speed or the like is avoided.
 
(3) The wheel loader  100  further includes the vehicle traveling direction detector  253  that detects whether the vehicle traveling direction is forward or backward. The control device  240  includes the working device to ground angle acquisition section  321  that acquires an angle of the working device  150  to the ground, and the excavation work prediction section  320  that predicts that the working device  150  will perform the excavation when the vehicle traveling direction is forward and the angle of the working device  150  to the ground falls within a predetermined range. When the excavation work prediction section  320  predicts that the working device  150  will perform the excavation, the excavation start determination section  310  determines whether the working device  150  has started the excavation, or not, based on the lift cylinder bottom pressure and the vehicle acceleration. With the above configuration, the erroneous determination that can occur except for the excavation work can be avoided and the excavation start can be determined more accurately.
 
(4) The wheel loader  100  further includes the excavation determination notification section  265  that gives a notification to the effect that the operator is urged to lift the working device  150  when it is determined by the control device  240  that the working device  150  has started the excavation. With the above configuration, the operator can be caused to lift the working device  150  without any delay at the time of starting the excavation.
 
(5) In addition, the wheel loader  100  further includes the control valve control section  262  that controls the control valve  221  and starts the supply of hydraulic oil from the hydraulic pump  220  to the lift cylinder  152  when the control device  240  determines that the working device  150  has started the excavation. With the above configuration, the lifting operation of the working device  150  can be carried out without any delay at the time of starting the excavation.
 
     In the embodiment described above, the example in which the wheel loader  100  includes both of the control valve control section  262  and the excavation determination notification section  265  has been described. Alternatively, only one of those sections may be provided. Furthermore, without the provision of both of the control valve control section  262  and the excavation determination notification section  265 , the excavation start determination command output from the control device  240  may be output to the outside through an output terminal or the like provided in the wheel loader  100 . 
     In the embodiment described above, as shown in  FIG. 3 , the control device  240  includes the working device to ground angle acquisition section  321 , the excavation work prediction section  320 , and the excavation start determination section  310 . However, there is no need to provide the working device to ground angle acquisition section  321  and the excavation work prediction section  320 . In that case, the excavation start determination section  310  does not need to include the excavation work prediction determination section  410 , and the excavation start determination command section  440  may perform the excavation start determination based on the Boolean value output from the lift cylinder bottom pressure increasing speed determination section  420  and the Boolean value output from the vehicle acceleration determination section  430 . In other words, when both of those two Boolean values are “TRUE”, the vehicle acceleration determination section  430  determines that the excavation is to be started and outputs the excavation start determination command. Whereas, when either one or both of the two Boolean values are “FALSE”, the vehicle acceleration determination section  430  determines that the excavation is not to be started, and does not output the excavation start determination command. Alternatively, this process may be carried out. 
     The present invention is not limited to the embodiments described above, but includes various modifications without departing from the spirit of the present invention. For example, the present invention is not limited to the provision of all the configurations described in the above embodiments but includes the deletion of a part of the configurations. Also, a part of the configuration of one embodiment can be added or replaced with the configuration of another embodiment. In addition, other modes conceivable to fall within a technical concept of the present invention also fall within the scope of the present invention. 
     REFERENCE SIGNS LIST 
     
         
           100  . . . wheel loader 
           110  . . . frame 
           150  . . . working device 
           151  . . . bucket 
           152  . . . lift cylinder 
           153  . . . bucket cylinder 
           154  . . . bell crank 
           155  . . . lift arm 
           201  . . . engine 
           202  . . . electrically controlled governor 
           210  . . . power transmission device 
           220  . . . hydraulic pump 
           221  . . . control valve 
           230  . . . propeller shaft 
           240  . . . control device 
           250  . . . bucket cylinder stroke detector 
           251  . . . lift arm angle detector 
           252  . . . lift cylinder bottom pressure detector 
           253  . . . vehicle traveling direction detector 
           254  . . . vehicle acceleration detector 
           256  . . . accelerator manipulated variable detector 
           261  . . . working device control lever 
           262  . . . control valve control section 
           263  . . . high pressure selection valve 
           264  . . . accelerator pedal 
           265  . . . excavation determination notification section 
           310  . . . excavation start determination section 
           320  . . . excavation work prediction section 
           321  . . . working device to ground angle acquisition section 
           410  . . . excavation work prediction determination section 
           420  . . . lift cylinder bottom pressure increasing speed determination section 
           421  . . . lift cylinder bottom pressure increasing speed calculation section 
           430  . . . vehicle acceleration determination section 
           440  . . . excavation start determination command section 
           610  . . . vehicle traveling direction determination section 
           620  . . . working device to ground angle determination section 
           630  . . . excavation work prediction command section