Patent Publication Number: US-2020277750-A1

Title: Method and system for controlling wheel loader

Description:
PRIORITY STATEMENT 
     This application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2019-0023584, filed on Feb. 28, 2019 in the Korean Intellectual Property Office (KIPO), the contents of which are herein incorporated by reference in their entirety. 
     BACKGROUND 
     1. Field 
     Example embodiments relate to a method and a system for controlling a wheel loader. More particularly, example embodiments relate to a method of performing an autonomous excavation work for a wheel loader and a system for controlling a wheel loader. 
     2. Description of the Related Art 
     Wheel loaders are widely used at construction sites to excavate aggregate such as dirt, sand, gravel and the like and load it into dump truck. Since the aggregate may be inhomogeneous loose soil or a somewhat compact material, the excavation work for digging the aggregate may be easy or difficult depending on the type of the aggregate. Further, load applied to the wheel loader may vary depending on the aggregate, which may result in tire slip. In particular, when the autonomous wheel loader performs the autonomous excavation work, the tire slip may occur thereby reducing the tire life and deteriorating productivity. 
     SUMMARY 
     Example embodiments provide a method of controlling a wheel loader capable of performing an autonomous excavation function to improve fuel efficiency and productivity. 
     Example embodiments provide a control system of a wheel loader for performing the method. 
     According to example embodiments, in a method of controlling a wheel loader, the wheel loader is moved forwards such that a bucket penetrates into an aggregate to perform an excavation work. Signals able to be used to determine tire slip of the wheel loader are obtained during the excavation work. Prediction algorithms obtained through training are performed to determine whether or not the tire slip occurs. In case of the tire slip, an engine speed is decreased and the bucket is lifted to remove the tire slip. The bucket is moved along a predetermined autonomous excavation trajectory when the tire slip is removed. 
     In example embodiments, performing the prediction algorithms may include performing algorithms trained using data on a tire tractive force and a bucket breakout force as learning data for the tire slip determination. 
     In example embodiments, obtaining the signals able to be used to determine the tire slip of the wheel loader may include obtaining a first group of signals required for calculating a tractive force of the tire, and obtaining a second group of signals required for calculating a breakout force of the bucket. 
     In example embodiments, the first group of signals may include an engine rotational speed signal, a turbine rotational speed signal of a torque converter, a speed step signal of a transmission, a vehicle speed signal and a wheel rotational speed signal, and the second group of signals may include a stroke signal of a boom cylinder, a stroke signal of a bucket cylinder and a pressure signal of the boom cylinder. 
     In example embodiments, the wheel rotational speed signal may be obtained from an encoder installed in the tire. 
     In example embodiments, moving the wheel loader forwards to perform the excavation work may include increasing an engine speed without an operator stepping on an acceleration pedal. 
     In example embodiments, lifting the bucket when the tire slip occurs may include increasing a stroke of a boom cylinder. 
     In example embodiments, the method may further include determining a time when the bucket penetrates into the aggregate and a speed step of a transmission is shifted down from second step to first step as an entry time of the excavation work. 
     In example embodiments, the method may further include terminating the autonomous excavation work mode when an angle of the bucket is at the maximum crowd state. 
     According to example embodiments, a control system for a wheel loader includes a plurality of sensors installed respective in an engine and a work apparatus and a travel apparatus driven by the engine to detect signals able to be used to determine tire slip of the wheel loader, a control apparatus configured to output a control signal for performing an autonomous excavation work mode of the wheel loader, perform prediction algorithms obtained through training on the signals received from the sensors to determine whether or not the tire slip occurs and output first and second tire slip removal control signals so as to remove the tire slip within a desired value, an engine control device configured to decrease an engine rotational speed according to the first tire slip removal control signal, and a work control device configured to lift a bucket of the wheel loader according to the second tire slip removal control signal. 
     In example embodiments, the control apparatus may include a data receiver configured to receive the signals from the sensors, a determiner configured to perform neural network algorithms on the signals to determine whether or not the tire slip occurs, and an output portion configured to output the first and second tire slip removal control signals to the engine control device and the work control device respectively. 
     In example embodiments, the sensors may include a first group of sensors for detecting signals required for calculating a tractive force of a tire and a second group of sensors for detecting signals required for calculating a breakout force of the bucket. 
     In example embodiments, the first group of sensors may include at least one of an engine speed sensor, a turbine rotational speed sensor of a torque converter, a sensor for detecting speed step of a transmission, a vehicle speed sensor and a wheel speed detection sensor, and a second group of sensors may include at least one of a boom angle sensor, a bucket angle sensor and a boom cylinder pressure sensor. 
     In example embodiments, the wheel speed detection sensor may include an encoder installed in the tire. 
     In example embodiments, the control apparatus may output an acceleration pedal output signal having a predetermined increase ratio value to the engine control device when the autonomous excavation work mode is entered, to increase the engine rotational speed. 
     In example embodiments, the first tire slip removal control signal may include an acceleration pedal output signal having a predetermined decrease ratio value. 
     In example embodiments, the second tire slip removal control signal may include a pilot pressure signal for increasing a stroke of a boom cylinder. 
     In example embodiments, the control apparatus may determine a time when the bucket penetrates into an aggregate and speed step of a transmission is shifted down from second step to first step as an entry time of the autonomous excavation work mode. 
     In example embodiments, the control apparatus may determine a time when an angle of the bucket is at the maximum crowd state as an end point of the autonomous excavation work mode. 
     According to example embodiments, a wheel loader may be controlled to perform an autonomous excavation work without an operator pressing an acceleration pedal when entering an autonomous excavation work mode. In addition, tire slip of the wheel loader may be determined by using prediction algorithm obtained through training such as neural network algorithms on signals received from sensors installed on the wheel loader, and when it is determined that the tire slip occurs, an engine speed may be decreased and the bucket may be lifted to remove the tire slip within a desired range. 
     Artificial neural network algorithms for a digging force and a tractive force that change according to the type and state of the aggregate may be used to control real-time equipment to thereby implement full autonomous excavation function. Thus, tire product life may be prevented from shortening due to excessive slippage of tires and optimized excavation trajectory control may be performed regardless of the operator&#39;s skill to thereby improve productivity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings. 
         FIG. 1  is a side view illustrating a wheel loader in accordance with example embodiments. 
         FIG. 2  is a block diagram illustrating a system for controlling the wheel loader in  FIG. 1 . 
         FIG. 3  is a block diagram illustrating a control system for a wheel loader in accordance with example embodiments. 
         FIG. 4  is a block diagram illustrating a control apparatus in  FIG. 3 . 
         FIG. 5  is a view illustrating a neural network circuit in a tire slip determiner in  FIG. 4 . 
         FIG. 6  is a view illustrating a signal transfer in each layer of the neural network in  FIG. 5 . 
         FIG. 7  is a graph illustrating a tractive force of a tire according to an acceleration pedal output signal inputted to an engine control unit from the control apparatus in  FIG. 3 . 
         FIG. 8  is a graph illustrating a height of a buck according to a pilot pressure signal inputted to a work control apparatus from the control apparatus in  FIG. 3 . 
         FIG. 9  is a flow chart illustrating a method of controlling a wheel loader in accordance with example embodiments. 
         FIG. 10  is views illustrating an entry time of an auto-excavation work mode in accordance with example embodiments. 
         FIG. 11  is graphs illustrating a tractive force of tire and a breakout force of a bucket in accordance with example embodiments. 
     
    
    
     DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Various example embodiments will be described more fully hereinafter with reference to the accompanying drawings, in which example embodiments are shown. Example embodiments may, however, be embodied in many different forms and should not be construed as limited to example embodiments set forth herein. Rather, these example embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of example embodiments to those skilled in the art. In the drawings, the sizes and relative sizes of components or elements may be exaggerated for clarity. 
     It will be understood that when an element or layer is referred to as being “on,” “connected to” or “coupled to” another element or layer, it can be directly on, connected or coupled to the other element or layer or intervening elements or layers may be present. In contrast, when an element or layer is referred to as being “directly on,” “directly connected to” or “directly coupled to” another element or layer, there are no intervening elements or layers present. Like numerals refer to like elements throughout. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of example embodiments. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of example embodiments. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which example embodiments belong. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
       FIG. 1  is a side view illustrating a wheel loader in accordance with example embodiments.  FIG. 2  is a block diagram illustrating a system for controlling the wheel loader in  FIG. 1 . 
     Referring to  FIGS. 1 and 2 , a wheel loader  10  may include a front body  12  and a rear body  14  connected to each other. The front body  12  may include a work apparatus and a front wheel  160 . The rear body  14  may include a driver cabin  40 , an engine bay  50  and a rear wheel  162 . 
     The work apparatus may include a boom  20  and a bucket  30 . The boom  20  may be freely pivotally attached to the front body  12 , and the bucket  30  may be freely pivotally attached to an end portion of the boom  20 . The boom  20  may be coupled to the front body  12  by a pair of boom cylinders  22 , and the boom  20  may be pivoted upwardly and downwardly by expansion and contraction of the boom cylinders  22 . A tilt arm  34  may be freely rotatably supported on the boom  20 , almost at its central portion. One end portion of the tilt arm  34  may be coupled to the front body  12  by a pair of bucket cylinders  32  and another end portion of the tilt arm  34  may be coupled to the bucket  30  by a tilt rod, so that the bucket  30  may pivot (crowd and dump) as the bucket cylinder  32  expands and contracts. 
     The front body  12  and the rear body  14  may be rotatably connected to each other through a center pin  16  so that the front body  12  may swing side to side with respect to the rear body  14  by expansion and contraction of a steering cylinder (not illustrated). 
     A travel apparatus for propelling the wheel loader  10  may be mounted at the rear body  14 . An engine  100  may be provided in the engine bay  50  to supply an output power to the travel apparatus. The travel apparatus may include a torque converter  120 , a transmission  130 , a propeller shaft  150 , axles  152 ,  154 , etc. The output power of the engine  100  may be transmitted to the front wheel  160  and the rear wheel  162  through the torque converter  120 , the transmission  130 , the propeller shaft  150  and the axles  152  and  154 , and thus the wheel loader  10  may travels. 
     In particular, the output power of the engine  100  may be transmitted to the transmission  130  through the torque converter  120 . An input shaft of the torque converter  120  may be connected to an output shaft of the engine  100 , and an output shaft of the torque converter  120  may be connected to the transmission  130 . The torque converter  120  may be a fluid clutch device including an impeller, a turbine and a stator. The transmission  130  may include hydraulic clutches that shift speed steps between first to fourth speeds, and rotation of the output shaft of the torque converter  120  may be shifted by the transmission  130 . The shifted rotation may be transmitted to the front wheel  160  and the rear wheel  162  through the propeller shaft  150  and the axles  152  and  154  and thus the wheel loader may travel. 
     The torque converter  120  may have a function to increase an output torque with respect to an input torque, i.e., a function to make the torque ratio 1 or greater. The torque ratio may decrease with an increase in the torque converter speed ratio e (=Nt/Ni), which is a ratio of the number of rotations Nt of the output shaft of the torque converter  120  to the number of rotations Ni of the input shaft of the torque converter  120 . For example, if travel load is increased while the vehicle is in motion in a state where the engine speed is constant, the number of rotations of the output shaft of the torque converter  120 , i.e., the vehicle speed may be decreased. At this time, the torque ratio may be increased and thus the vehicle may be allowed to travel with a greater travel driving force (traction force). 
     The transmission  130  may include a forward hydraulic clutch for forward movement, a reverse hydraulic clutch for reverse movement, and first to fourth hydraulic clutches for the first to the fourth speeds. The hydraulic clutches may be each engaged or released by pressure oil (clutch pressure) supplied via a transmission control unit (TCU)  140 . The hydraulic clutches may be engaged when the clutch pressure supplied to the hydraulic clutches is increased, while the hydraulic clutches may be released when the clutch pressure is decreased. 
     When travel load is decreased and the torque converter speed ratio e is increased to be equal to or greater than a predetermined value eu, a speed step may be shifted up by one step. On the other hand, when travel load is increased and the torque converter speed ratio e is decreased to be equal to or less than a predetermined value ed, the speed step may be shifted down by one step. 
     A variable capacity hydraulic pump  200  for supplying a pressurized hydraulic fluid to the boom cylinder  22  and the bucket cylinder  32  may be mounted at the rear body  14 . The variable capacity hydraulic pump  200  may be driven using at least a portion of the power outputted from the engine  100 . For example, the output power of the engine  100  may drive the hydraulic pump  200  for the work apparatus and a hydraulic pump (not illustrated) for the steering cylinder via a power take-off (PTO) such as a gear train  110 . 
     A pump control device (EPOS, Electronic Power Optimizing System) may be connected to the variable capacity hydraulic pump  200 , and the hydraulic fluid discharged from the variable capacity hydraulic pump  200  may be controlled by the pump control device. A main control valve (MCV) including a boom control valve  210  and a bucket control valve  212  may be installed on a hydraulic circuit of the hydraulic pump  200 . The hydraulic fluid discharged from the hydraulic pump  200  may be supplied to the boom cylinder  22  and the bucket cylinder  32  through the boom control valve  210  and the bucket control valve installed in a hydraulic line  202  respectively. The main control valve (MCV) may supply the hydraulic fluid discharged from the hydraulic pump  200  to the boom cylinder  22  and the bucket cylinder  32  according to a pilot pressure in proportion to an operation rate of an operating lever. Thus, the boom  20  and the bucket  30  may be driven by the pressure of the hydraulic fluid discharged from the hydraulic pump  200 . 
     A maneuvering device may be provided within the driver cabin  40 . The maneuvering device may include an acceleration pedal  142 , a brake pedal, an FNR travel lever, the operating levers for operating the cylinders such as the boom cylinder  22  and the bucket cylinder  32 , etc. 
     As mentioned above, the wheel loader  10  may include a traveling operating system for driving the travel apparatus via the PTO and a hydraulic operating system for driving the work apparatus such as the boom  20  and the bucket  30  using the output power of the engine  100 . 
     Further, a control apparatus  300  for the wheel loader  10  such as a portion of a vehicle control unit (VCU) or a separate control unit may be mounted in the rear body  14 . The control apparatus  300  may include an arithmetic processing unit having a CPU which executes a program, a storage device such as a memory, other peripheral circuit, and the like. 
     The control apparatus  300  may receive signals from various sensors (detectors) which are installed in the wheel loader  10 . For example, the control apparatus  300  may be connected to an engine speed sensor  104  for detecting a rotational speed of the engine, an acceleration pedal detection sensor  143  for detecting an operation amount of the acceleration pedal  142 , a brake pedal detection sensor for detecting an operation amount of the brake pedal, and an FNR travel lever position sensor for detecting a manipulation position of the FNR travel lever, for example, forward (F), neutral (N) and reverse (R). Additionally, the control apparatus  300  may receive an engine rotational speed signal and an acceleration pedal signal from an engine control unit (ECU) connected to the engine speed sensor  104  and the acceleration pedal detection sensor  143 . Further, the control apparatus  300  may receive a speed step signal of the transmission through the transmission control unit (TCU)  140 . 
     In addition, the control apparatus  300  may be connected to a turbine rotational speed sensor  122  for detecting a rotational speed of the turbine of the torque converter  120 , a vehicle speed sensor  132  for detecting a rotational speed of an output shaft of the transmission  130 , i.e., and a wheel speed detection sensors  170 ,  172  for detecting a wheel speed. The wheel speed detection sensors  170 ,  172  may include an encoder installed in a tire. Alternatively, the control apparatus  300  may be connected to a GPS receiver installed in the wheel loader, to receive a current speed of the vehicle. 
     Further, the control apparatus  300  may be connected to a pressure sensor  204  installed in the hydraulic line  202  in front end of the main control valve (MCV) to detect a pressure of the hydraulic fluid discharged from the hydraulic pump  200 , and a boom cylinder pressure sensor  222  for detecting a cylinder head pressure at a head of the boom cylinder  22 . Furthermore, the control apparatus  300  may be connected to a boom angle sensor  224  for detecting a rotational angle of the boom  20  and a bucket angle sensor  234  for detecting a rotational angle of the bucket  30 . 
     As illustrated in  FIGS. 1 and 2 , the signals detected by the sensors may be inputted into the control apparatus  300 . As mentioned later, the control apparatus  300  may select one or more signals of the signals received from the sensors installed in the wheel loader  10 , perform prediction algorithms obtained through training such as neural network algorithms to determine whether or not tire slip occurs. Further, the control apparatus  300  may output a control signal to the engine control unit (ECU), the transmission control unit (TCU)  140 , and the pump control device (EPOS), etc, to selectively control the travel apparatus and the work apparatus of the wheel loader  10  based on the occurrence of the tire slip. 
     Hereinafter, the control apparatus for controlling the wheel loader will be explained. 
       FIG. 3  is a block diagram illustrating a control system for a wheel loader in accordance with example embodiments.  FIG. 4  is a block diagram illustrating a control apparatus in  FIG. 3 .  FIG. 5  is a view illustrating a neural network circuit in a tire slip determiner in  FIG. 4 .  FIG. 6  is a view illustrating a signal transfer in each layer of the neural network in  FIG. 5 .  FIG. 7  is a graph illustrating a tractive force of a tire according to an acceleration pedal output signal inputted to an engine control unit from the control apparatus in  FIG. 3 .  FIG. 8  is a graph illustrating a height of a buck according to a pilot pressure signal inputted to a work control apparatus from the control apparatus in  FIG. 3 . 
     Referring to  FIGS. 3 to 8 , a control system for a wheel loader may include a plurality of sensors, a control apparatus  300  for performing an autonomous excavation work mode, a travel apparatus control device and a work apparatus control device. 
     The sensors may be installed in the engine  100 , the work apparatus and the travel apparatus to detect signals representing state information of the wheel loader. In particular, the control system form a wheel loader may include a first group of sensors for detecting signals required for calculating a tractive force of a tire of the wheel loader  10  and a second group of sensors for detecting signals required for calculating a breakout (digging) force of a bucket. 
     For example, the first group of sensors may include the engine speed sensor  104 , the turbine rotational speed sensor  122 , the sensor for detecting the speed step of the transmission, the vehicle speed sensor  132 , the wheel speed detection sensor, etc. The second group of sensors may include the boom angle sensor  224 , the bucket angle sensor  234 , the boom cylinder pressure sensor  222 , etc. 
     The control apparatus  300  may include a data receiver  310 , a determiner  320  and an output portion  330 . 
     The data receiver  310  may receive signals from the sensors. Additionally, the data receiver  310  may receive an autonomous excavation work mode selection signal from a selection portion  302 . When the autonomous excavation work mode is selected by an operator, the selection portion  302  may output the autonomous excavation work mode selection signal to the control apparatus  300 . Further, the operator may select detail working conditions of the autonomous excavation work mode through the selection portion  302 . The detail working conditions may include an excavation workload, an excavation work speed, an allowable range of tire slip, and the like. 
     The determiner  320  may determine the entry time and end point of the autonomous excavation work mode. The determiner  320  may determine a time when the bucket  30  penetrates into the aggregate as the entry time of the autonomous excavation work mode. When the bucket  30  digs the aggregate and load is applied to the travel apparatus by the reaction force, and the speed step of the transmission  130  is shifted down to the first step, it may be determined as the entry time of the autonomous excavation work mode. When the angle of the bucket  30  is at the maximum crowd state, it may be determined as the end point of the autonomous excavation work mode. 
     Additionally, the determiner  320  may include neural network circuits that perform neural network algorithms to determine whether or not the tire slip occurs. 
     As illustrated in  FIGS. 5 and 6 , the neural network circuit may include multilayer perceptrons having a multi-input layer, a hidden layer and an output layer. Neurons may be arranged in each layer, and the neurons in each layer may be connected by connection weights. Input data may be inputted to the neurons in the input layer and transferred to the output layer though the hidden layer. 
     Training the neural network algorithm may be a process of tuning the interconnection weights between each nodes in order to minimize an error between an expectation value and an output value of the neural network algorithms for a specific input (actual detected data). For example, back propagation algorithm may be used for training the neural networks. Accordingly, the neural network circuits of the determiner  320  may vary the connection weights between the input layer, the hidden layer and the output layer using pre-collected data to provide neural network algorithms as prediction models. 
     In example embodiments, data obtained from the first group of sensors and the second group of sensors may be accumulated and may be used as learning data. For example, the tire slip moments may be recorded on the basis of the number of the tire revolutions obtained from the external encoder  170 ,  172  installed in the tire, and tire slip occurrence data may be accumulated and used as learning data. The GPS speed of the wheel loader  10 , the breakout force of the bucket  30 , the acceleration pedal value from the engine control unit  400 , etc. may be used as supervised leaning data for the tire slip determination. As an example, although the tire tractive force is greater than a predetermined value and the acceleration pedal signal value does not decrease (not have a negative rate of change), data when the tire tractive force decreases by a predetermined level or more may be used as supervised learning data for the tire slip determination. Additionally, data when the bucket breakout force increases and the number of the tire revolutions increases may be used as supervised learning data for the tire slip determination. 
     Through supervised learning, the sensor signal weight of the artificial neural network logic may be determined and the tire slip may be determined from the sensor signals. 
     The output portion  330  may output an autonomous excavation work mode control signal for the autonomous excavation work mode and first and second tire slip removal control signals for removing the tire slip within a desired value. 
     The autonomous excavation work mode control signal may include an acceleration pedal output signal having a predetermined increase ratio value. The output portion  330  may output the autonomous excavation work mode control signal to the engine control device  400  when the autonomous excavation work mode is entered. The engine control device  400  may increase the engine speed by controlling a fuel injector  102  according to the autonomous excavation work mode control signal without the operator pressing the acceleration pedal. 
     The first tire slip removal control signal may include an acceleration pedal output signal having a predetermined decrease ratio value. The output portion  330  may output the acceleration pedal output signal to the engine control device  400  when the tire slip occurs. The engine control device  400  may decrease the engine speed by controlling the fuel injector  102  according to the first tire slip removal control signal. 
     The second tire slip removal control signal may include a pilot pressure signal for increasing a stroke of the boom cylinder  22 . The output portion  330  may output the pilot pressure signal to the work control apparatus, that is, the boom control valve  210  of the main control valve MCV when the tire slip occurs. The boom control valve  210  may increase the stroke of the boom cylinder  210  according to the pilot pressure signal to increase a height of the bucket  30 . 
     The control apparatus  300  may further include a storage portion. The storage portion may store data required for learning in a predictive model and calculation in the neural network algorithm which are performed in the determiner  330 , a control map required for determination of the control signal which is performed in the output portion  330 , etc. 
     As illustrated in  FIG. 7 , in response to the acceleration pedal output signal having the predetermined decrease ratio value, the fuel injection amount may be decreased and thus the engine speed may be also decreased. In this case, the tractive force of the tire may be decreased according to the acceleration pedal decrease ratio (%) (point A-&gt;point B). As the tractive force of the tire is decreased the tire slip may be removed. 
     As illustrated in  FIG. 8 , in response to the pilot pressure signal, the stroke of the boom cylinder  22  may be increased, thereby raising the height of the bucket  30 . In this case, the height of the bucket  30  may be increased according to the stroke increase rate of the boom cylinder  22  (point C-&gt;point D). The bucket  30  may lift the aggregate upwards and thus the load on the tire may be increased to thereby remove the tire slip. 
     As described above, the control apparatus  300  of the wheel loader may control the wheel loader  10  to perform the autonomous excavation work without the operator pressing the acceleration pedal when entering the autonomous excavation work mode. In addition, the control apparatus  300  of the wheel loader may determine the tire slip of the wheel loader  10  by using prediction algorithm obtained through training such as neural network algorithms on the signals received from the sensors installed on the wheel loader  10 , and when it is determined that the tire slip occurs, may decrease the engine speed and lift the bucket  30  to remove the tire slip. 
     The control apparatus  300  of the wheel loader may learn data of the tire slip by using the artificial neural network algorithms for the digging force and tractive force that change according to the type and state of the aggregate to adjust the determination weight of the equipment sensor signal and to control the real-time equipment to thereby implement full autonomous excavation function. Thus, tire product life may be prevented from shortening due to excessive slippage of tires and optimized excavation trajectory control may be performed regardless of the operator&#39;s skill to thereby improve productivity. 
     Hereinafter, a method of controlling a wheel loader using the control apparatus in  FIG. 3  will be explained. 
       FIG. 9  is a flow chart illustrating a method of controlling a wheel loader in accordance with example embodiments.  FIG. 10  is views illustrating an entry time of an auto-excavation work mode in accordance with example embodiments.  FIG. 11  is graphs illustrating a tractive force of tire and a breakout force of a bucket in accordance with example embodiments. 
     Referring to  FIGS. 1, 2, 3 and 9 to 11 , first, an entry time of an autonomous excavation work mode may be determined (S 100 ), and when the autonomous excavation work mode is entered, a wheel loader  10  may be accelerated to perform an excavation work (S 110 ). 
     In example embodiments, in case that an operator selects the autonomous excavation work mode through an selection portion  302 , a time when a bucket  30  penetrates into an aggregate may be determined as the entry time of the autonomous excavation work mode. 
     As illustrated in  FIG. 10( a ) , the wheel loader  10  may move forwards and start to penetrate into the aggregate M. An angle of a bottom face of the bucket  30  may be kept parallel with the ground, and the boom  20  may be lowered so that the bottom face of the bucket  30  approaches closely to the ground. Then, as illustrated in  FIG. 10( b ) , the bucket  30  may dig the aggregate, and then, load is applied to a travel apparatus by the reaction force and the speed step of a transmission  130  is shifted down to the first step, it may be determined as the entry time of the autonomous excavation work mode. 
     Then, when the autonomous excavation work mode is entered, the wheel loader  10  may be accelerated to perform an autonomous excavation work. 
     For example, a control apparatus  300  may output an autonomous excavation work mode control signal to an engine control device  400  when the autonomous excavation work mode is entered. The autonomous excavation work mode control signal may include an acceleration pedal output signal having a predetermined increase ratio value. The engine control device  400  may increase an engine speed by controlling a fuel injector  102  according to the autonomous excavation work mode control signal without the operator pressing the acceleration pedal. 
     Then, during the autonomous excavation work mode, prediction algorithms obtained through training may be performed to determine whether or not tire slip occurs. 
     In example embodiments, during autonomous excavation work mode, the signals able to be used to determine the tire slip of the wheel loader  10  may be obtained. A first group of signals required for calculating a tractive force of a tire of the wheel loader  10  and a second group of signals required for calculating a breakout (digging) force of a bucket  30 . The first group of signals may include an engine rotational speed signal, a turbine rotational speed signal of a torque converter, a speed step signal of a transmission, a vehicle speed signal and a wheel rotational speed signal. The second group of signals may include a stroke signal of a boom cylinder, a stroke signal of a bucket cylinder and a pressure signal of the boom cylinder. 
     Data obtained from the first group of signals and the second group of signals may be accumulated to be used as learning data. For example, the tire slip moments obtained from an external encoder  170 ,  172  installed in the tire may be recorded, and tire slip occurrence data may be accumulated to be used as learning data. The GPS speed of the wheel loader  10 , the tractive force of the tire, the breakout force of the bucket  30 , the acceleration pedal value from the engine control unit  400 , etc. may be used as supervised leaning data for the tire slip determination. Through supervised learning, the sensor signal weight of the artificial neural network logic may be determined and the tire slip may be determined from the sensor signals. 
     Then, when the tire slip occurs, the engine speed of the wheel loader  10  may be decreased and the bucket  30  may be lifted until the tire slip is removed within a desired value ( 130 ). 
     For example, the control apparatus  300  may output a first tire slip removal control signal to the engine control device  400  when the tire slip occurs. The engine control device  400  may decrease the engine speed by controlling the fuel injector  102  according to the first tire slip removal control signal. 
     A fuel injection amount may be decreased in response to an acceleration pedal output signal having a predetermined decrease ratio value, and thus, the engine speed may be also decreased. The engine control device  400  may increase the engine speed by controlling a fuel injector  102  according to the autonomous excavation work mode control signal without the operator pressing the acceleration pedal. In this case, the tractive force of the tire may be decreased according the acceleration pedal decrease ratio and thus the tire slip may be removed. 
     Additionally, the control apparatus  300  may output a second tire slip removal control signal to a work control apparatus, that is, a boom control valve  210  of a main control valve MCV when the tire slip occurs. The second tire slip removal control signal may include a pilot pressure signal for increasing a stroke of the boom cylinder  22 . The boom control valve  210  may increase the stroke of the boom cylinder  210  according to the pilot pressure signal to increase a height of the bucket  30 . 
     The stroke of the boom cylinder  22  may be increased in response to the pilot pressure signal, thereby increasing the height of the bucket  30 . In this case, the height of the bucket  30  may be increased according to the stroke increase rate of the boom cylinder  22 . The bucket  30  may lift the aggregate upwards and thus the load on the tire may be increased to thereby remove the tire slip. 
     As illustrated in  FIG. 11 , graph G 1  represents the tire tractive force and graph G 2  represents the bucket digging force, and graphs G 3  and G 4  represent speeds of left and right wheels. In the tire slip section, the tire tractive force decreases and the GPS speed of the wheel loader  10  is constant or decreases, while the tire rotational speed (wheel speed) increases while vibrating rapidly. At this time, if the engine speed is decreased and the buck  20  is lifted to increase the bucket digging force, after the point at which the tire tractive force and the bucket dogging force are equal to each other, the tire slippage may disappear as the friction force with the ground increases, and thus, the tractive force may be increased again and the digging operation may be done smoothly. 
     Then, when the tire slip is removed, the bucket  30  may be moved along a predetermined autonomous digging trajectory, and the autonomous excavation work mode may be terminated. 
     For example, the control apparatus  300  may output the autonomous excavation control signal to the engine control device  400  and the work control device when the tire slip disappears. Thus, the strokes of the boom cylinder  22  and the bucket cylinder  32  may be controlled such that the end portion of the bucket  30  moves along the predetermined digging trajectory. 
     Then, when the wheel loader  10  moves forward while digging the aggregate and the angle of the bucket  30  is at the maximum crowd state, the autonomous excavation work mode may be terminated. 
     The foregoing is illustrative of example embodiments and is not to be construed as limiting thereof. Although a few example embodiments have been described, those skilled in the art will readily appreciate that many modifications are possible in example embodiments without materially departing from the novel teachings and advantages of the present invention. Accordingly, all such modifications are intended to be included within the scope of example embodiments as defined in the claims.