Patent Publication Number: US-2022220704-A1

Title: Control device of work vehicle, work vehicle, and control method for work vehicle

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. National stage application of International Application No. PCT/JP2020/022585, filed on Jun. 8, 2020. This U.S. National stage application claims priority under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2019-108554, filed in Japan on Jun. 11, 2019, the entire contents of which are hereby incorporated herein by reference. 
    
    
     BACKGROUND 
     Filed of the Invention 
     The present invention relates to a control device of a work vehicle, a work vehicle, and a control method for a work vehicle. 
     BACKGROUND INFORMATION 
     A work vehicle such as a wheel loader provided with a continuously variable transmission is known. Examples of a continuously variable transmission include a hydraulic static transmission (HST) and a hydraulic mechanical transmission (HMT). Patent Document 1 discloses a technique of controlling a pump capacity of the HST in order to reduce a sudden feeling of acceleration that occurs when a clutch of the one-pump two-motor type HST is operated. 
     SUMMARY 
     As described in Japanese Unexamined Patent Application, First Publication No. 2012-231331, the pressure of an HST circuit of a continuously variable transmission mounted on the work vehicle may increase more than necessary due to various factors. For example, the pressure of the HST circuit rapidly changes when load fluctuation occurs due to work such as excavation and dozing or when load fluctuation occurs due to braking. When the pressure of the HST circuit rapidly increases, output torque increases more than necessary and riding comfort of the work vehicle deteriorates. 
     The object of the present invention is to provide a control device of a work vehicle, the work vehicle, and a control method for the work vehicle which can prevent a rapid change of output torque due to load fluctuation of a continuously variable transmission. 
     According to one aspect of the present invention, in a control device of a work vehicle that includes a power source; a travel device; and a power transmission device that includes a hydrostatic continuously variable transmission having a relief valve capable of setting a relief pressure, and is configured to transmit power of the power source to the travel device, the control device of the work vehicle comprises: a relief pressure setting unit that is configured to set the relief pressure of the relief valve in accordance with a target output value of the travel device. 
     According to the above aspect, the control device of the work vehicle can prevent a rapid change of the output torque due to load fluctuation of the power transmission device. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view of a work vehicle according to a first embodiment. 
         FIG. 2  is a view illustrating an internal configuration of a cab according to the first embodiment. 
         FIG. 3  is a schematic diagram illustrating a power system of the work vehicle according to the first embodiment. 
         FIG. 4  is a diagram illustrating a configuration of an HST included in a transmission according to the first embodiment. 
         FIG. 5  is a schematic block diagram illustrating a configuration of a control device of the work vehicle according to the first embodiment. 
         FIG. 6  is a flowchart illustrating a control method for the work vehicle according to the first embodiment. 
         FIG. 7  is a diagram illustrating an example of setting a relief pressure related to the first embodiment. 
         FIG. 8  is a diagram illustrating an effect of setting a relief pressure by a control device according to the first embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT(S) 
     First Embodiment 
     Embodiments will be described below in detail with reference to the drawings. 
       FIG. 1  is a side view of a work vehicle according to a first embodiment. 
     A work vehicle  100  according to the first embodiment is a wheel loader. 
     The work vehicle  100  includes a vehicle body  110 , work equipment  120 , a front wheel portion  130 , a rear wheel portion  140 , and a cab  150 . The work vehicle  100  is an example of a power machine. 
     The vehicle body  110  includes a front vehicle body  111 , a rear vehicle body  112 , and a steering cylinder  113 . The front vehicle body  111  and the rear vehicle body  112  are rotatably attached to each other around a steering axis extending in a vertical direction of the vehicle body  110 . The front wheel portion  130  is provided in a lower portion of the front vehicle body  111 , and the rear wheel portion  140  is provided in a lower portion of the rear vehicle body  112 . 
     The steering cylinder  113  is a hydraulic cylinder. A base end portion of the steering cylinder  113  is attached to the rear vehicle body  112 , and a distal end portion thereof is attached to the front vehicle body  111 . The steering cylinder  113  expands and contracts by hydraulic oil to define an angle between the front vehicle body  111  and the rear vehicle body  112 . That is, a steering angle of the front wheel portion  130  is defined by the expansion and contraction of the steering cylinder  113 . 
     The work equipment  120  is used for excavating and transporting a work object such as earth. The work equipment  120  is provided in a front portion of the vehicle body  110 . The work equipment  120  includes a boom  121 , a bucket  122 , a bell crank  123 , a lift cylinder  124 , and a bucket cylinder  125 . 
     A base end portion of the boom  121  is attached to the front portion of the front vehicle body  111  via a pin. 
     The bucket  122  includes a blade for excavating the work object and a container which transports the excavated work object. A base end portion of the bucket  122  is attached to a distal end portion of the boom  121  via a pin. 
     The bell crank  123  transmits power of the bucket cylinder  125  to the bucket  122 . A first end of the bell crank  123  is attached to a bottom portion of the bucket  122  via a link mechanism. A second end of the bell crank  123  is attached to a distal end portion of the bucket cylinder  125  via a pin. 
     The lift cylinder  124  is a hydraulic cylinder. A base end portion of the lift cylinder  124  is attached to the front portion of the front vehicle body  111 . A distal end portion of the lift cylinder  124  is attached to the boom  121 . The lift cylinder  124  expands and contracts by the hydraulic oil, so that the boom  121  is driven in an upward direction or a downward direction. 
     The bucket cylinder  125  is a hydraulic cylinder. A base end portion of the bucket cylinder  125  is attached to the front portion of the front vehicle body  111 . A distal end portion of the bucket cylinder  125  is attached to the bucket  122  via the bell crank  123 . The bucket cylinder  125  expands and contracts by the hydraulic oil, so that the bucket  122  swings in a tilt direction or a dump direction. 
     The cab  150  is a space in which an operator gets on and performs an operation of the work vehicle  100 . The cab  150  is provided in an upper portion of the rear vehicle body  112 . 
       FIG. 2  is a view illustrating an internal configuration of the cab according to the first embodiment. A seat  151 , an accelerator pedal  152 , a brake pedal  153 , a steering wheel  154 , a forward-rearward selector switch  155 , a shift switch  156 , a boom lever  157 , and a bucket lever  158  are provided inside the cab  150 . 
     In addition, the work vehicle  100  may include one brake pedal  153  or a plurality of brake pedals  153 . For example, when the work vehicle  100  includes two brake pedals  153  as shown in  FIG. 2 , a left brake pedal  153  viewed from the rear may be assigned the same function as a right brake pedal  153 . Further, different functions may be assigned to the left brake pedal  153  and the right brake pedal  153 . In this case, for example, the degree of engagement of the clutch may be changed in accordance with the amount of operation of the left brake pedal  153  so as to release engagement of the clutch by operation of the left brake pedal  153 . 
     The accelerator pedal  152  is operated to set a traveling driving force (traction force) generated to the work vehicle  100 . As an operation amount of the accelerator pedal  152  is larger, target driving force (target traction force) is set higher. 
     The brake pedal  153  is operated to set a traveling braking force generated to the work vehicle  100 . As an operation amount of the brake pedal  153  is larger, the braking force is set higher. 
     The steering wheel  154  is operated to set the steering angle of the work vehicle  100 . 
     The forward-rearward selector switch  155  is operated to set a traveling direction of the work vehicle  100 . The traveling direction of the work vehicle is any of forward (F), rearward (R), or neutral (N). 
     The shift switch  156  is operated to set a speed range of a power transmission device. By operating the shift switch  156 , for example, one speed range is selected from a first gear, a second gear, a third gear, and a fourth gear. 
     The boom lever  157  is operated to set a movement amount of a raising operation or a lowering operation of the boom  121 . The boom lever  157  accepts the lowering operation by being tilted forward and accepts the raising operation by being tilted rearward. 
     The bucket lever  158  is operated to set a movement amount of a dump operation or a tilt operation of the bucket  122 . The bucket lever  158  accepts the dump operation by being tilted forward and accepts the tilt operation by being tilted rearward. 
     &lt;&lt;Power System&gt;&gt; 
       FIG. 3  is a schematic diagram illustrating a power system of the work vehicle according to the first embodiment. 
     The work vehicle  100  includes an engine  210 , a power take off (PTO)  220 , a transmission  230 , a front axle  240 , a rear axle  250 , a variable capacity pump  260 , and a fixed capacity pump  270 . 
     The engine  210  is a diesel engine, for example. The engine  210  is provided with a fuel injection device  211  and an engine tachometer  2101 . The fuel injection device  211  controls the driving force of the engine  210  by adjusting an injection amount of fuel into a cylinder of the engine  210 . The engine tachometer  2101  measures a rotation speed of the engine  210 . 
     The PTO  220  transmits part of the driving force of the engine  210  to the variable capacity pump  260  and the fixed capacity pump  270 . That is, the PTO  220  distributes the driving force of the engine  210  to the transmission  230 , the variable capacity pump  260 , and the fixed capacity pump  270 . 
     The transmission  230  is a continuously variable transmission including an HST  231  (hydrostatic continuously variable transmission). The transmission  230  may perform gear shifting control only by the HST  231 , or may be an HMT (hydraulic mechanical continuously variable transmission) that performs gear shifting control by a combination of the HST  231  and a planetary gear mechanism. The transmission  230  shifts the driving force input to an input shaft and outputs the shifted driving force from an output shaft. The input shaft of the transmission  230  is connected to the PTO  220 , and the output shaft thereof is connected to the front axle  240  and the rear axle  250 . 
     That is, the transmission  230  transmits the driving force of the engine  210  distributed by the PTO  220  to the front axle  240  and the rear axle  250 . The transmission  230  is provided with an input shaft tachometer  2301  and an output shaft tachometer  2302 . The input shaft tachometer  2301  measures a rotation speed of the input shaft of the transmission  230 . The output shaft tachometer  2302  measures a rotation speed of the output shaft of the transmission  230 . The HST  231  of the transmission  230  is provided with an HST pressure gauge  2303 . The HST pressure gauge  2303  measures the pressure of the HST  231 . 
     The front axle  240  transmits the driving force output by the transmission  230  to the front wheel portion  130 . As a result, the front wheel portion  130  is rotated. 
     The rear axle  250  transmits the driving force output by the transmission  230  to the rear wheel portion  140 . As a result, the rear wheel portion  140  is rotated. 
     The front axle  240  and the rear axle  250  are an example of a travel device. 
     The variable capacity pump  260  is driven by the driving force from the engine  210 . A discharge capacity of the variable capacity pump  260  is changed, for example, by controlling a tilt angle of a swash plate provided in the variable capacity pump  260 . The hydraulic oil discharged from the variable capacity pump  260  is supplied to the lift cylinder  124  and the bucket cylinder  125  via a control valve  261  and is supplied to the steering cylinder  113  via a steering valve  262 . 
     The control valve  261  controls a flow rate of the hydraulic oil discharged from the variable capacity pump  260  and distributes the hydraulic oil to the lift cylinder  124  and the bucket cylinder  125 . The steering valve  262  controls a flow rate of the hydraulic oil supplied to the steering cylinder  113 . 
     The variable capacity pump  260  is provided with a first pump pressure gauge  2601  and a pump capacity meter  2602 . The first pump pressure gauge  2601  measures a discharge pressure of the hydraulic oil from the variable capacity pump  260 . The pump capacity meter  2602  measures a capacity of the variable capacity pump  260  based on an angle of the swash plate of the variable capacity pump  260  and the like. 
     The variable capacity pump  260  is an example of a device to which the power is distributed from the PTO  220 . In another embodiment, the variable capacity pump  260  may be configured by a plurality of pumps or may have another supply destination such as a hydraulically driven fan (not shown) in place of or in addition to the variable capacity pump  260 . 
     The lift cylinder  124  is provided with a lift pressure sensor  2603 . The lift pressure sensor  2603  measures a bottom pressure of the lift cylinder  124 . 
     The fixed capacity pump  270  is driven by the driving force from the engine  210 . The hydraulic oil discharged from the fixed capacity pump  270  is supplied to a brake valve  271  in the transmission  230 . The brake valve  271  controls pressure of the hydraulic oil supplied to brake cylinders (not shown) built in each of axles. By supplying the hydraulic oil to the brake cylinders, each brake disc rotating together with the rotating shafts of the front wheel portion  130  and the rear wheel portion  140  is pressed against a non-rotating plate, and thus a braking force is generated. The fixed capacity pump  270  is provided with a second pump pressure gauge  2701 . The second pump pressure gauge  2701  measures a discharge pressure of the hydraulic oil from the fixed capacity pump  270 . The fixed capacity pump  270  is an example of the device to which the power is distributed from the PTO  220 . The fixed capacity pump  270  may include a plurality of pumps, or may have a supply destination such as a lubrication circuit (not shown). 
     (Configuration of HST) 
       FIG. 4  is a diagram illustrating a configuration of the HST included in the transmission according to the first embodiment. 
     The HST  231  includes an HST pump  2311 , an HST motor  2312 , a first relief valve  2313 , a first check valve  2314 , a second relief valve  2315 , and a second check valve  2316 . The HST pump  2311  and the HST motor  2312  are connected to each other via a first hydraulic path P 1  and a second hydraulic path P 2 . 
     The HST pump  2311  supplies the hydraulic oil to the HST motor  2312  via the first hydraulic path P 1  or the second hydraulic path P 2 . The HST motor  2312  is driven by the hydraulic oil supplied from the HST pump  2311 . 
     The first relief valve  2313  connects the first hydraulic path P 1  and a relief bypass P 3 . In a case where the pressure of the first hydraulic path P 1  is higher than that of the second hydraulic path P 2  and a differential pressure between the first hydraulic path P 1  and the second hydraulic path P 2  exceeds a predetermined relief pressure, the hydraulic oil flowing through the first hydraulic path P 1  passes through the relief bypass P 3 . 
     The first check valve  2314  is provided in parallel with the first relief valve  2313 , blocks the hydraulic oil flowing from the first hydraulic path P 1 , and passes the hydraulic oil flowing from the relief bypass P 3  to the first hydraulic path P 1 . 
     The second relief valve  2315  connects the second hydraulic path P 2  and the relief bypass P 3 . In a case where the pressure of the second hydraulic path P 2  is higher than that of the first hydraulic path P 1  and a differential pressure between the second hydraulic path P 2  and the first hydraulic path P 1  exceeds a predetermined relief pressure, the hydraulic oil flowing through the second hydraulic path P 2  passes through the relief bypass P 3 . 
     The second check valve  2316  is provided in parallel with the second relief valve  2315 , blocks the hydraulic oil flowing from the second hydraulic path P 2 , and passes the hydraulic oil flowing from the relief bypass P 3  to the second hydraulic path P 2 . 
     That is, when the pressure of the first hydraulic path P 1  is higher than that of the second hydraulic path P 2  and the differential pressure between the first hydraulic path P 1  and the second hydraulic path P 2  exceeds a predetermined relief pressure, the hydraulic oil flowing through the first hydraulic path P 1  flows to the second hydraulic path P 2  via the first relief valve  2313 , the relief bypass P 3  and the second check valve  2316 . On the other hand, when the pressure of the second hydraulic path P 2  is higher than that of the first hydraulic path P 1  and the differential pressure between the first hydraulic path P 1  and the second hydraulic path P 2  exceeds a predetermined relief pressure, the hydraulic oil flowing through the second hydraulic path P 2  flows to the first hydraulic path P 1  via the second relief valve  2315 , the relief bypass P 3 , and the first check valve  2314 . In addition, a hydraulic pump (not shown) for supplying the hydraulic oil to the relief bypass P 3  may be provided, for example, in order to compensate for a decrease in the oil amount due to leakage of the hydraulic oil from the HST  231 . 
     The relief pressures of the first relief valve  2313  and the second relief valve  2315  are capable of being changed by a control signal. For example, the first relief valve  2313  and the second relief valve  2315  may each include a solenoid for pressing a spring that closes each valve, and the relief pressure may be set by changing a position of the solenoid with an electric current. 
     In addition, the HST pressure gauge  2303  measures the differential pressure between the first hydraulic path P 1  and the second hydraulic path P 2 . The differential pressure between the first hydraulic path P 1  and the second hydraulic path P 2  may be measured by the difference between measured values of two pressure sensors by providing the pressure sensors in each of the first hydraulic path P 1  and the second hydraulic path P 2 . 
     (Control Device) 
     The work vehicle  100  includes a control device  300  for controlling the work vehicle  100 . The control device  300  is provided on the vehicle body  110 , preferably in the cab  150 . 
     The control device  300  outputs control signals to the fuel injection device  211 , the transmission  230 , the variable capacity pump  260 , and the control valve  261  in accordance with the operation amounts of each of the operating devices (the accelerator pedal  152 , the brake pedal  153 , the steering wheel  154 , the forward-rearward selector switch  155 , the shift switch  156 , the boom lever  157 , and the bucket lever  158 ) in the cab  150 . 
       FIG. 5  is a schematic block diagram illustrating a configuration of the control device of the work vehicle according to the first embodiment. The control device  300  is a computer which includes a processor  310 , a main memory  330 , a storage  350 , and an interface  370 . 
     The storage  350  is a non-transitory tangible storage medium. A hard disk drive (HDD), a solid-state drive (SSD), a magnetic disk, an optical magnetic disk, a compact disc read-only memory (CD-ROM), a digital versatile disc read-only memory (DVD-ROM), a semiconductor memory, and the like are examples of the storage  350 . The storage  350  may be an internal medium directly connected to a bus of the control device  300 , or an external medium connected to the control device  300  via the interface  370  or a communication line. The storage  350  stores a program for controlling the work vehicle  100 . 
     The program may be a program for realizing some of the functions exerted by the control device  300 . For example, the program may exert the functions in combination with another program already stored in the storage  350 , or in combination with another program mounted on another device. In addition, in another embodiment, the control device  300  may include a custom large-scale integrated circuit (LSI) such as a programmable logic device (PLD) in addition to or in place of the configuration described above. A programmable array logic (PAL), a generic array logic (GAL), a complex programmable logic device (CPLD), and a field-programmable gate array (FPGA) are examples of the PLD. In this case, some or all of the functions realized by the processor may be realized by an integrated circuit. 
     In a case where the program is distributed to the control device  300  via a communication line, the control device  300  that has received the distribution may load the program into the main memory  330  and execute the processing described above. 
     In addition, the program may be a program for realizing some of the functions described above. Further, the program may be a so-called difference file (difference program) which realizes the functions described above in combination with another program already stored in the storage  350 . 
     By executing the program, the processor  310  includes an operation amount acquisition unit  311 , a measurement value acquisition unit  312 , a vehicle state calculation unit  313 , a required PTO torque decision unit  314 , a required output torque decision unit  315 , a traveling load estimation unit  316 , a work state specification unit  317 , a target rotation speed decision unit  318 , an acceleration torque specification unit  319 , a target engine torque decision unit  320 , an engine control unit  321 , a target HST pressure specification unit  322  (target circuit pressure specification unit), a margin decision unit  323 , a relief pressure setting unit  324 , a target speed ratio decision unit  325 , and a transmission control unit  326 , and a pump control unit  327 . 
     The operation amount acquisition unit  311  acquires the operation amount from each of the accelerator pedal  152 , the brake pedal  153 , the steering wheel  154 , the forward-rearward selector switch  155 , the shift switch  156 , the boom lever  157 , and the bucket lever  158 . Hereinafter, the operation amount of the accelerator pedal  152  is referred to as an accelerator operation amount, the operation amount of the brake pedal  153  is referred to as a brake operation amount, the operation amount of the steering wheel  154  is referred to as a steering operation amount, a value in response to an operation position of the forward-rearward selector switch  155  is referred to as an FNR operation amount, a value in response to an operation position of the shift switch  156  is referred to as a shift operation amount, the operation amount of the boom lever  157  is referred to as a boom operation amount, and the operation amount of the bucket lever  158  is referred to as a bucket operation amount. 
     The measurement value acquisition unit  312  acquires the measurement values from the fuel injection device  211 , the engine tachometer  2101 , the input shaft tachometer  2301 , the output shaft tachometer  2302 , the HST pressure gauge  2303 , the first pump pressure gauge  2601 , the pump capacity meter  2602 , the lift pressure sensor  2603 , and the second pump pressure gauge  2701 . That is, the measurement value acquisition unit  312  acquires the measurement value of each of a fuel injection amount of the engine  210 , the rotation speed of the engine  210 , the rotation speed of the input shaft of the transmission  230 , the rotation speed of the output shaft of the transmission  230 , the pressure of the HST  231 , a pump pressure of the variable capacity pump  260 , the capacity of the variable capacity pump  260 , the bottom pressure of the lift cylinder  124 , and a pump pressure of the fixed capacity pump  270 . 
     The vehicle state calculation unit  313  calculates the output torque of the engine  210 , upper limit torque of the engine  210 , angular acceleration of the engine  210 , torque (PTO torque) distributed by the PTO  220  to the variable capacity pump  260  and the fixed capacity pump  270 , an input-output speed ratio of the transmission  230 , angular acceleration of the output shaft of the transmission  230 , and a traveling speed of the work vehicle  100 , based on the measurement value acquired by the measurement value acquisition unit  312 . The output torque of the engine  210  is the torque that the engine  210  can actually exert, which is calculated based on the fuel injection amount. The upper limit torque of the engine  210  is the maximum torque that the engine  210  is capable of exerting. 
     The required PTO torque decision unit  314  decides a required value of the torque (required PTO torque) distributed from the PTO  220  to the variable capacity pump  260  and the fixed capacity pump  270 , based on the steering operation amount, the boom operation amount, and the bucket operation amount acquired by the operation amount acquisition unit  311  and the measurement values of the pump pressure of the variable capacity pump  260 , the capacity of the variable capacity pump  260 , the pump pressure of the fixed capacity pump  270  acquired by the measurement value acquisition unit  312 . For example, the required PTO torque decision unit  314  obtains the required flow rate of the variable capacity pump  260  from the steering operation amount based on a PTO conversion function that defines the relationship between the operation amount and the required flow rate. In addition, for example, the required PTO torque decision unit  314  obtains a required flow rate of the variable capacity pump  260  from the boom operation amount and the bucket operation amount based on the PTO conversion function. Then, the required PTO torque decision unit  314  decides the required PTO torque based on the measurement values of the pump pressure of the variable capacity pump  260 , the capacity of the variable capacity pump  260 , the pump pressure of the fixed capacity pump  270 , and the specified required flow rate of the variable capacity pump  260 . 
     The required output torque decision unit  315  decides a required value of the torque (required output torque) of the output shaft of the transmission  230  based on the accelerator operation amount, the brake operation amount, the shift operation amount, and the FNR operation amount acquired by the operation amount acquisition unit  311  and the traveling speed calculated by the vehicle state calculation unit  313 . For example, the required output torque decision unit  315  decides the required output torque from the traveling speed calculated by the vehicle state calculation unit  313  based on a traveling conversion function that defines a relationship between the traveling speed and the required output torque. At this time, the required output torque decision unit  315  decides a characteristic of a traveling conversion function based on the accelerator operation amount, the brake operation amount, the shift operation amount, and the FNR operation amount. 
     Specifically, the required output torque decision unit  315  specifies a traveling conversion function corresponding to the speed range specified by the shift operation amount among a plurality of the traveling conversion functions corresponding to a plurality of the speed ranges. In a case in which there is an accelerator operation, the required output torque decision unit  315  transforms the specified traveling conversion function based on a magnification related to the accelerator operation amount. In a case in which there is a brake operation, the required output torque decision unit  315  transforms the specified traveling conversion function based on a magnification related to the brake operation amount. The required output torque decision unit  315  decides a sign of the required output torque based on the FNR operation amount. In addition, in a case where the signs of the required output torque and the traveling speed do not match (a case in which a sign of the product of the required output torque and the traveling speed is negative), the torque on a braking side is exerted by the transmission  230 . 
     With the traveling conversion function, when the traveling speed exceeds a predetermined speed, the required output torque is a value on the braking side. Therefore, in the required output torque decision unit  315 , in a case in which the traveling speed calculated by the vehicle state calculation unit  313  exceeds an upper limit of the speed range specified by the shift operation amount, the accelerator operation amount, and the brake operation amount, the required output torque becomes a value on the braking side (sign opposite to the traveling speed). 
     The traveling load estimation unit  316  estimates a traveling load torque T load  related to traveling based on an output torque T eng  of the engine  210 , an angular acceleration α eng  of the engine  210 , a PTO torque T PTO , an input-output speed ratio i of the transmission  230 , and an angular acceleration α out  of the output shaft of the transmission  230  calculated by the vehicle state calculation unit  313 . 
     The traveling load torque T load  can be calculated based on the following expression (1). 
     
       
         
           
             
               
                 
                   
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     I eng  is the moment of inertia of the engine  210 . I v  is the moment of inertia of the work vehicle  100 . η t  is the torque efficiency of the transmission  230 . N is an axle reduction ratio between the output shaft of the transmission  230  and the front wheel portion  130  and the rear wheel portion  140 . The moment of inertia I eng , the moment of inertia I v , the torque efficiency η t , and the axle reduction ratio N are constant. 
     In addition, the expression (1) can be derived from an expression (2) indicating a relationship between the output torque T eng  of the engine  210  and the output torque T out  of the transmission  230  and an expression (3) indicating a relationship between the output torque T out  of the transmission  230  and the acceleration αout of the work vehicle  100 . In addition, in another embodiment, the traveling load torque T load  may be calculated based on an expression other than the expression (1). For example, in place of the expression (2), an expression specifying the traveling load torque T load  may be derived by using an expression indicating a relationship between the pressure of the HST  231  measured by the HST  231 , a capacity command of the variable capacity pump of the HST  231  or the pump capacity measured by the pump capacity meter provided in the variable capacity pump, and the output torque T out . In addition, in another embodiment, in a case where an electric motor is provided in the transmission  230 , an expression specifying the traveling load torque T load  may be derived by using a torque command of the electric motor or electric motor output torque estimated from the voltage and current. 
     
       
         
           
             
               
                 
                   
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                   ) 
                 
               
             
           
         
       
     
     The work state specification unit  317  specifies a work state of the work vehicle  100  based on the operation amount acquired by the operation amount acquisition unit  311  and the measurement values acquired by the measurement value acquisition unit  312 . The value of the work state includes, for example, a “low-speed traveling state”, a “high-speed traveling state”, an “excavation state”, or a “braking state”. 
     The “low-speed traveling state” is a state in which the work vehicle  100  is traveling at a low speed. The work state specification unit  317  can determine that the work state is the “low-speed traveling state”, when, for example, the absolute value of the traveling speed is less than a predetermined value. The work state specification unit  317  can determine that the work state is the “low-speed traveling state”, when, for example, the shift operation amount is the first gear or the second gear. 
     The “high-speed traveling state” is a state in which the work vehicle  100  is moving forward or rearward. The work state specification unit  317  can determine that the work state is the “high-speed traveling state”, when, for example, the absolute value of the traveling speed is equal to or greater than a predetermined value. The work state specification unit  317  can determine that the work state is the “high-speed traveling state”, when, for example, the shift operation amount is the third gear or the fourth gear. 
     The “excavation state” is a state in which the work vehicle  100  is performing excavation work using the work equipment  120 . The work state specification unit  317  can determine that the work state is the “excavation state”, when, for example, the measured value of the bottom pressure of the lift cylinder  124  is equal to or greater than a predetermined value. 
     The “braking state” is a state in which the work vehicle  100  is braking. The work state specification unit  317  can determine that the work state is the “braking state”, when, for example, the brake operation amount is larger than 0. 
     The target rotation speed decision unit  318  decides a target engine rotation speed used for controlling the engine  210  based on a required traveling power calculated from the required output torque and the traveling speed and a required engine output, which is a sum of the required PTO torque and a required PTO output calculated from the measurement value of the rotation speed of the engine  210 . The target rotation speed decision unit  318  decides the target engine rotation speed based on a rotation speed conversion function that defines a relationship between the required engine output and the engine rotation speed, which is determined in advance by design or the like. The rotation speed conversion function may be designed to suppress the rotation of the engine  210  to a low rotation speed side as much as possible within a range, for example, in which the required engine output is capable of being exerted and the engine acceleration is not inhibited. 
     In addition, the target rotation speed decision unit  318  decides the engine rotation speed (PTO required rotation speed) required to realize the required flow rate of the variable capacity pump  260 , which is calculated by the required PTO torque decision unit  314 . The target rotation speed decision unit  318  decides the PTO required rotation speed based on a rotation speed conversion function that defines a relationship between the required flow rate of the variable capacity pump  260  and the engine rotation speed, which is determined in advance by design or the like. In a case where the target engine rotation speed is less than the PTO required rotation speed, the target rotation speed decision unit  318  decides the target engine rotation speed to be the PTO required rotation speed. 
     The acceleration torque specification unit  319  calculates a target acceleration torque required to rotate the engine  210  at the target engine rotation speed based on the measurement value of the rotation speed of the engine  210  acquired by the measurement value acquisition unit  312  and the target engine rotation speed decided by the target rotation speed decision unit  318 . That is, the acceleration torque specification unit  319  decides a target engine acceleration from a difference between the measurement value of the rotation speed of the engine  210  and the target engine rotation speed and multiplies the target engine acceleration by the moment of inertia of the engine  210  to calculate the target acceleration torque. 
     The target engine torque decision unit  320  decides a target engine torque, which is the torque to be output by the engine  210 , based on the PTO torque, the upper limit torque of the engine  210 , and the input-output speed ratio of the transmission  230  calculated by the vehicle state calculation unit  313 , the required output torque decided by the required output torque decision unit  315 , and the measurement value of the rotation speed of the engine  210 . The target engine torque decision unit  320  calculates a required input torque, which is the torque of the engine  210  required to obtain the required output torque, by multiplying the required output torque by the input-output speed ratio of the transmission  230 . The target engine torque decision unit  320  decides a smaller value out of the sum of the PTO torque and the required input torque and the maximum value of the engine torque, as the target engine torque. 
     The engine control unit  321  outputs an engine torque command to the fuel injection device  211 . Specifically, the engine control unit  321  outputs the engine torque command indicating the target engine torque decided by the target engine torque decision unit  320 . 
     The target HST pressure specification unit  322  determines the pressure of the HST  231  corresponding to the required output torque decided by the required output torque decision unit  315  as a target HST pressure (target circuit pressure) which is a control target of the HST  231 . The relationship between the output torque of the transmission  230  and the pressure of the HST  231  is determined by the relationship of the gear ratio between the output shaft and the HST motor  2312  defined by the design of the transmission  230 , and the capacity of the HST motor  2312  at that time. That is, the target HST pressure decreases as the capacity of the HST motor  2312  increases, and the target HST pressure increases as the required output torque increases. 
     The margin decision unit  323  decides a margin pressure to be allowed for the difference between and the target HST pressure and the relief pressures of the first relief valve  2313  and the second relief valve  2315  based on the work state of the work vehicle  100  specified by the work state specification unit  317 . 
     The margin pressure is decided in advance for each work state. When arranging the order of the margin pressures related to the work states in ascending manner, it is the margin pressure related to the braking state, the margin pressure related to the excavation state, the margin pressure related to the low-speed traveling state, and the margin pressure related to the high-speed traveling state. The margin pressure related to the braking state may be set so as to monotonically decrease with respect to the pressing amount of the brake pedal  153 . When it is desired to apply a strong braking force, the margin pressure relating to the braking state may be set to a negative number. 
     The relief pressure setting unit  324  sets the relief pressures of the first relief valve  2313  and the second relief valve  2315  in the HST  231  to the sum of the target HST pressure specified by the target HST pressure specification unit  322  and the margin pressure decided by the margin decision unit  323 . 
     The target speed ratio decision unit  325  decides a target input-output speed ratio of the transmission  230  based on the measurement value of the rotation speed of the input shaft of the transmission  230 , the measurement value of the rotation speed of the output shaft of the transmission  230 , the traveling load torque estimated by the traveling load estimation unit  316 , the target output torque decided by the required output torque decision unit  315 , and the target engine acceleration specified by the acceleration torque specification unit  319 . Specifically, the target speed ratio decision unit  325  estimates the rotation speed of the output shaft of the transmission  230  after a lapse of time related to a predetermined control cycle based on the rotation speed of the output shaft of the transmission  230 , the traveling load torque, and the target output torque, and sets the estimated rotation speed as the target rotation speed of the output shaft. The target speed ratio decision unit  325  estimates the rotation speed of the input shaft of the transmission  230  after a lapse of time related to a predetermined control cycle based on the rotation speed of the input shaft of the transmission  230  and the target engine acceleration, and sets the estimated rotation speed as the target rotation speed of the input shaft. The target speed ratio decision unit  325  decides the target input-output speed ratio by dividing the target rotation speed of the output shaft by the target rotation speed of the input shaft. 
     The transmission control unit  326  outputs a control command for the transmission  230  in order to achieve the target input-output speed ratio decided by the target speed ratio decision unit  325 . The transmission control unit  326  outputs, for example, a capacity command for the HST  231  included in the transmission  230 . 
     The pump control unit  327  outputs a control command for the variable capacity pump  260  in order to achieve the required PTO torque decided by the required PTO torque decision unit  314 . 
     (Control Method for Work Vehicle) 
       FIG. 6  is a flowchart showing a control method for the work vehicle according to the first embodiment. 
     First, the operation amount acquisition unit  311  acquires the operation amount from each of the accelerator pedal  152 , the brake pedal  153 , the steering wheel  154 , the forward-rearward selector switch  155 , the shift switch  156 , the boom lever  157 , and the bucket lever  158  (step S 1 ). In addition, the measurement value acquisition unit  312  acquires the measurement values from the fuel injection device  211 , the engine tachometer  2101 , the input shaft tachometer  2301 , the output shaft tachometer  2302 , the HST pressure gauge  2303 , the first pump pressure gauge  2601 , the pump capacity meter  2602 , and the second pump pressure gauge  2701  (step S 2 ). 
     Next, the vehicle state calculation unit  313  calculates the output torque of the engine  210 , the upper limit torque of the engine  210 , the angular acceleration of the engine  210 , the PTO torque, the input-output speed ratio of the transmission  230 , the angular acceleration of the output shaft of the transmission  230 , and the traveling speed of the work vehicle  100 , based on the measurement value acquired in step S 2  (step S 3 ). 
     The required PTO torque decision unit  314  decides the required PTO torque based on the steering operation amount, the boom operation amount, and the bucket operation amount acquired in step S 1  the pump pressure and the capacity of the variable capacity pump  260  and the measurement value of the pump pressure of the fixed capacity pump  270  acquired in step S 2  (step S 4 ). The required output torque decision unit  315  decides the required output torque based on the operation amount related to traveling acquired in step S 1  and the traveling speed calculated in step S 3  (step S 5 ). The traveling load estimation unit  316  estimates the traveling load torque based on a value of a vehicle state calculated in step S 3  (step S 6 ). 
     The work state specification unit  317  specifies the work state of the work vehicle  100  based on the operation amount acquired in step S 1  and the measurement values acquired in step S 2  (step S 7 ). That is, the work state specification unit  317  specifies which one of the “low-speed traveling state”, the “high-speed traveling state”, the “excavation state”, and the “braking state” the work state is. 
     The target rotation speed decision unit  318  decides a target engine rotation speed based on a required traveling power calculated from the required output torque and the traveling speed and a required engine output that is a sum of the required PTO torque and a required PTO output calculated from the measurement value of the rotation speed of the engine  210  (step S 8 ). The acceleration torque specification unit  319  calculates the target acceleration torque based on the measurement value of the rotation speed of the engine  210  and the target engine rotation speed decided in step S 8  (step S 9 ). The target engine torque decision unit  320  decides the target engine torque based on the required output torque, the PTO torque, the upper limit torque of the engine, the input-output speed ratio of the transmission  230  calculated in step S 3 , and the measurement value of the rotation speed of the engine  210  acquired in step S 2  (step S 10 ). The engine control unit  321  outputs the engine torque command indicating the target engine torque decided in step S 10  (step S 11 ). 
     The target HST pressure specification unit  322  decides the target HST pressure based on the required output torque decided in step S 5  and the latest capacity command of the HST motor  2312  (step S 12 ). The margin decision unit  323  decides the margin pressure based on the work state of the work vehicle  100  specified in step S 7  (step  13 ). Then, the relief pressure setting unit  324  sets the relief pressures of the first relief valve  2313  and the second relief valve  2315  of the HST  231  to the sum of the target HST pressure decided in step S 12  and the margin pressure decided in step S 13  (step S 14 ). 
     The target speed ratio decision unit  325  decides the target input-output speed ratio based on the measurement value of the rotation speed of the input shaft of the transmission  230 , the measurement value of the rotation speed of the output shaft of the transmission  230 , the load torque, the target output torque, and the target engine acceleration (step S 15 ). The transmission control unit  326  outputs a control command for the transmission  230  to achieve the target input-output speed ratio (step S 16 ). 
     The control device  300  executes the above-described control processing for each predetermined control cycle. 
     (Examples of Setting of Relief Pressure) 
     Here, setting of the relief pressure by the control device  300  will be described using specific examples.  FIG. 7  is a diagram illustrating an example of setting of the relief pressure according to the first embodiment. As described above, the relief pressure  HST_relief  is obtained by adding the margin pressure P HST_margin  to the target HST pressure P HST_target . 
     The work vehicle  100  starts traveling at the time T 0 . The traveling speed of the work vehicle  100  reaches a threshold speed at which the low-speed traveling state and the high-speed traveling state are distinguished from each other at the time T 1 . At this time, the work state specification unit  317  specifies that the work state of the work vehicle  100  is the low-speed traveling state from the time T 0  to the time T 1 . For this reason, the margin decision unit  323  decides the margin pressure P HST_margin  to be added to the target HST pressure P HST_target  as the margin pressure related to the low-speed traveling state. When the work vehicle  100  is traveling at a low speed, fine accelerator work is required, and there is a high possibility of shifting to excavation work. Therefore, by setting the margin pressure P HST_margin  to a value smaller than that in the high-speed traveling state, the control device  300  can prevent the pressure of the HST  231  from being largely away from the target HST pressure P HST_target . 
     From the time T 1  to the time T 2 , the traveling speed of the work vehicle  100  further increases. At this time, the work state specification unit  317  specifies that the work state of the work vehicle  100  is the high-speed traveling state from the time T 1  to the time T 2 . Therefore, the margin decision unit  323  decides the margin pressure P HST_margin  to be a value larger than that in the low-speed traveling state. When the work vehicle  100  is traveling at a high speed, there is a low possibility of occurrence of a sudden load. Therefore, the control device  300  can reduce a release loss of the HST  231  by setting the margin pressure P HST_margin  to a value larger than that in the low-speed traveling state. 
     Next, during the period from the time T 2  to the time T 3 , the operator of the work vehicle  100  presses the brake pedal  153 . As a result, the traveling speed of the work vehicle  100  becomes less than the threshold speed at the time T 3 . At this time, the work state specification unit  317  specifies that the work state of the work vehicle  100  is the braking state from the time T 2  to the time T 3 . Therefore, the margin decision unit  323  decides the margin pressure P HST_margin  to be a value that monotonically decreases with the pressing amount of the brake pedal  153 . In addition, in the example shown in  FIG. 7 , the margin pressure P HST_margin  is a negative value. Thus, the control device  300  can positively promote the relief of the hydraulic oil and suppress the traction force to improve the braking performance. 
     Thereafter, during the period from time T 3  to time T 4 , the work vehicle continues to travel at a speed less than the threshold speed. At this time, the work state specification unit  317  specifies that the work state of the work vehicle  100  is the low-speed traveling state from the time T 3  to the time T 4 . Therefore, the margin decision unit  323  decides the margin pressure P HST_margin  to be a margin pressure related to low-speed traveling. In addition, the margin pressure P HST_margin  from the time T 3  to the time T 4  is the same value as the margin pressure P HST_margin  from the time T 0  to the time T 1 . 
     When the bucket  122  of the work vehicle  100  pushes earth at the time T 4 , the pressure of the lift cylinder  124  increases. At this time, the work state specification unit  317  specifies that the work state of the work vehicle  100  is the excavation state on or after the time T 4 . Therefore, the margin decision unit  323  decides the margin pressure P HST_margin  to be a value smaller than that in the low-speed traveling state. When the work vehicle  100  performs excavation, there is a high possibility of occurrence of a rapid load fluctuation such as a load loss due to a tilt operation during excavation or a load loss due to the completion of excavation. Therefore, by setting the margin pressure P HST_margin  to a value smaller than that in the low-speed traveling state, the control device  300  can prevent the engine rotation speed from decreasing and prevent the vehicle from being jerked. 
     (Operation and Effects) 
       FIG. 8  is a diagram illustrating an effect of setting the relief pressure by the control device according to the first embodiment. 
     The control device  300  according to the first embodiment sets the relief pressure P HST_relief  of the first relief valve  2313  and the second relief valve  2315  in accordance with the target HST pressure P HST_target . In addition, the set relief pressure P HST_relief  of the first relief valve  2313  and the second relief valve  2315  may be the same value. As a result, as shown in  FIG. 8 , when a load fluctuation such as an external force being applied to the work vehicle  100  occurs, the internal pressure P HST  of the HST  231  can be suppressed to a pressure corresponding to the target HST pressure P HST_target  or less. Therefore, according to the first embodiment, it is possible to prevent a rapid change in output torque due to a load fluctuation of the transmission  230 . 
     As a comparative example,  FIG. 8  shows the behavior in the case where the relief pressure P HST_relief  is not set; however, it can be seen that the internal pressure P HST_cp  of the HST  231  greatly increases according to the load fluctuation in the case where the relief pressure P HST_relief  is not set. Thereafter, the control device  300  controls the internal pressure of the HST  231  so as to be close to the target HST pressure P HST_target , thereby causing a sway back of the internal pressure. This sway back causes shaking of the pitch of the vehicle body, and thus the ride comfort is reduced. On the other hand, according to the first embodiment, since the internal pressure P HST  of the HST  231  is suppressed to be equal to or less than the pressure corresponding to the target HST pressure P HST_target , it is possible to suppress the sway back and bring the internal pressure close to the target HST pressure P HST_target  at an early stage. 
     In addition, referring to  FIG. 8 , in the control according to the comparative example, the engine rotation speed remarkably decreases immediately after the occurrence of the load fluctuation. This is because a rapid load is transmitted from the HST  231  to the engine  210 . On the other hand, according to the first embodiment, it is possible to prevent the load from being transmitted to the engine  210  by the relief of the HST  231  and to suppress a decrease in the engine rotation speed. 
     Although one embodiment has been described in detail with reference to the drawings, the specific configuration is not limited to that described above, and various design changes and the like can be made. 
     In addition, the work vehicle  100  according to the first embodiment is a wheel loader, but the present invention is not limited thereto. For example, the work vehicle  100  according to another embodiment may be another work vehicle  100 , such as a bulldozer and a tractor. Also, in another embodiment, the control device  300  may be applied to a power machine other than the work vehicle. 
     Further, for example, according to the embodiment described above, the control device  300  sets the relief pressure based on the target HST pressure regardless of the work state; however, it is not limited to this in another embodiment. For example, another embodiment may be configured in a manner such that the control device  300  does not set the relief pressure in a case where the work state is in the high-speed traveling state, however sets the relief pressure in a case where the work state is in the other state. 
     In the above-described embodiment, the control device  300  varies the margin pressure according to the work state, however the present invention is not limited to this. For example, in another embodiment, the control device  300  may set the relief pressure by always using the same margin pressure. 
     According to the above disclosure of the present invention, the control device for the work vehicle can prevent a rapid change in output torque due to load fluctuation of the power transmission device.