Patent Publication Number: US-2022235533-A1

Title: Work vehicle and control method for work vehicle

Description:
FIELD 
     The present invention relates to a work vehicle and a control method for the work vehicle. 
     BACKGROUND 
     In a technical field related to a work vehicle, a hydraulic static transmission (HST) in which a hydraulic pump and a hydraulic motor are combined is known. In addition, a hydraulic mechanical transmission (HMT) in which the HST and a planetary gear mechanism are combined is known. 
     The HST is a purely hydraulic transmission that converts all power generated by an engine into a hydraulic pressure and transmits the hydraulic pressure. The HMT is a power split transmission that mechanically transmits a part of power generated by the engine, and converts a part of the power generated by the engine into a hydraulic pressure and then transmits the hydraulic pressure. In the HMT, a part of power is converted into a hydraulic pressure, and thus, power transmission efficiency is high. Therefore, the HMT is used in a work vehicle having a large load variation, such as a wheel loader or a bulldozer. 
     A continuously variable transmission function of the HMT is achieved by the planetary gear mechanism. Among three elements of the planetary gear mechanism including a sun gear, a carrier supporting a planetary gear, and a ring gear, a first element is connected to an input shaft, a second element is connected to an output shaft, and a third element is connected to the hydraulic pump or the hydraulic motor. 
     A power transmission device including the HMT capable of performing switching between a plurality of modes for a power transmission path is known. The power transmission device transmits power input from the engine to the input shaft to the output shaft connected to a traveling device of the work vehicle via a clutch mechanism. The power transmission device capable of performing switching between the plurality of modes for the power transmission path can achieve a wide range of speed ratio with a small-capacity hydraulic pump or hydraulic motor. Examples of the plurality of modes include a high-speed mode in which a traveling speed of the work vehicle is high and a low-speed mode in which the traveling speed of the work vehicle is low. For example, the mode switching is performed based on the speed ratio of the power transmission device. The speed ratio of the power transmission device refers to a ratio between a rotation speed of the input shaft and a rotation speed of the output shaft. The high-speed mode is selected when the speed ratio is greater than a predetermined threshold. The low-speed mode is selected when the speed ratio is equal to or less than the predetermined threshold. 
     In the power transmission device capable of performing switching between the plurality of modes for the power transmission path, the mode switching is performed by performing switching between a plurality of clutches provided in a clutch mechanism. Examples of the plurality of clutches include a high-speed clutch engaged in the high-speed mode and a low-speed clutch engaged in the low-speed mode. In the high-speed mode, the high-speed clutch is engaged and the low-speed clutch is released. In the low-speed mode, the low-speed clutch is engaged and the high-speed clutch is released. 
     In a case of changing the state of the traveling device from one of a forward state and a reverse state to the other, a forward-reverse operation device including a forward-reverse operation member is operated. When operated by a driver, the forward-reverse operation member is moved to a forward position, a neutral position, and a reverse position. When the forward-reverse operation member is moved to the forward position, the state of the traveling device is changed to the forward state. When the forward-reverse operation member is moved to the neutral position, the state of the traveling device is changed to a neutral state. When the forward-reverse operation member is moved to the reverse position, the state of the traveling device is changed to the reverse state. The forward-reverse operation member is moved from one of the forward position and the reverse position to the other via the neutral position. When the forward-reverse operation member is moved to the neutral position, the clutch is released to interrupt power transmission. When the forward-reverse operation member is moved from the neutral position to the forward position or the reverse position, the clutch is engaged to start the power transmission. When the forward-reverse operation member is moved to the neutral position, it is necessary to interrupt power transmitted to the output shaft and the subsequent portions. When the forward-reverse operation member is moved from the neutral position to the forward position or the reverse position, it is necessary to transmit power to the output shaft and the subsequent portions. As a method for switching between interruption and transmission of power, the clutch is released and engaged. However, there is a possibility that a shock occurs when the clutch is engaged and the driver of the work vehicle feels uncomfortable. Frequent clutch engagement also leads to wear of the clutch. For an electric continuously variable transmission described in Patent Literature 1, there is disclosed a technology related to a pseudo neutral control for generating a pseudo neutral state by reducing a torque of the output shaft by a highly accurate torque control using an electric motor and interrupting power transmission as much as possible in a state where the engagement of the clutch is maintained even when the forward-reverse operation member is moved to the neutral position. The pseudo neutral control prevents unintended power transmission at the time of a neutral operation performed by the driver, and prevents the occurrence of the shock by avoiding the release and reengagement of the clutch. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: WO 2015/093162 A 
     SUMMARY 
     Technical Problem 
     However, even in a case where the torque of the output shaft can be reduced by the electric motor, unintended power transmission may occur in a case where a rotational change of the output shaft is large or the like, due to an influence of inertia including the engine in a state where the clutch is engaged. Furthermore, in a case of a configuration of a hydraulic mechanical transmission using a hydraulic pump motor with low torque controllability instead of the electric motor, it is difficult to accurately interrupt power transmission while the clutch is engaged. Therefore, in a case where the pseudo neutral state is maintained for a long time, unintended power transmission may occur, which leads to sensory deterioration. For example, in a case where the driver quickly moves the forward-reverse operation member from one of the forward position and the reverse position to the other, a time for which the forward-reverse operation member is disposed at the neutral position is short, and thus the risk of sensory deterioration due to the pseudo neutral state is small. On the other hand, in a case where the driver slowly moves the forward-reverse operation member from one of the forward position and the reverse position to the other or causes the traveling device to travel by inertia in a state where the forward-reverse operation member is disposed at the neutral position, the time for which the forward-reverse operation member is disposed at the neutral position becomes long. In a case where the time for which the forward-reverse operation member is disposed at the neutral position is long, the pseudo neutral state is maintained for a long time, and the risk of sensory deterioration increases. In order to avoid the sensory deterioration due to the pseudo neutral state, it is necessary to release the clutch to completely interrupt the power, but once the clutch is released, a shock may occur when the clutch is engaged again. 
     An object of an aspect of the present invention is to reduce unintended power transmission and a shock caused by engagement of a clutch in a neutral or forward-reverse operation. 
     Solution to Problem 
     According to an aspect of the present invention, a work vehicle comprises: an input shaft that is connected to an engine; an output shaft that is connected to a traveling device; a power transmission device that transmits power input to the input shaft to the output shaft via a clutch mechanism; a forward-reverse operation device operated to change a state of the traveling device between a forward state, a neutral state, and a reverse state; and a control device, wherein the control device includes: an operation signal acquisition unit that acquires an operation signal generated by an operation of the forward-reverse operation device; a torque reduction command unit that outputs a torque reduction command for reducing a torque of the output shaft in a state where a first clutch among a plurality of clutches of the clutch mechanism is engaged, before a specified time elapses from a time point at which the operation signal for changing to the neutral state is acquired in a state where the first clutch is engaged; a clutch control unit that outputs a release command for releasing the first clutch after the specified time elapses; and a power control unit that outputs a control command for controlling the power transmission device so that a rotation speed of an input-side element of a second clutch to be engaged next coincides with a rotation speed of an output-side element of the second clutch in a state where the first clutch is released. 
     Advantageous Effects of Invention 
     According to an aspect of the present invention, it is possible to reduce unintended power transmission and a shock caused by engagement of the clutch in the neutral or forward-reverse operation. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side view schematically illustrating an example of a work vehicle according to the present embodiment. 
         FIG. 2  is a block diagram illustrating an example of the work vehicle according to the present embodiment. 
         FIG. 3  is a diagram schematically illustrating an example of a power transmission device according to the present embodiment. 
         FIG. 4  is a diagram schematically illustrating a relationship among a mode, a state of a clutch mechanism, and a state of each of a first hydraulic pump motor and a second hydraulic pump motor according to the present embodiment. 
         FIG. 5  is a functional block diagram illustrating an example of a control device according to the present embodiment. 
         FIG. 6  is a diagram schematically illustrating an example of correlation data according to the present embodiment. 
         FIG. 7  is a flowchart illustrating an example of a control method for the work vehicle according to the present embodiment. 
         FIG. 8  is a block diagram illustrating an example of a computer system according to the present embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments according to the present invention will be described with reference to the drawings, but the present invention is not limited thereto. Components of the embodiments to be described below can be combined as appropriate. In addition, some components are not used in some cases. 
     Work Vehicle 
       FIG. 1  is a side view schematically illustrating an example of a work vehicle  1  according to the present embodiment. In the present embodiment, the work vehicle  1  is assumed to be a wheel loader. In the following description, the work vehicle  1  is appropriately referred to as a wheel loader  1 . 
     As illustrated in  FIG. 1 , the wheel loader  1  includes a vehicle body frame  2 , working equipment  3 , a traveling device  4 , and a cab  5 . The wheel loader  1  performs excavation work and transportation work by using the working equipment  3 . 
     The vehicle body frame  2  functions as a body of the wheel loader  1 . The vehicle body frame  2  includes a front frame  2 F and a rear frame  2 R. The front frame  2 F and the rear frame  2 R are connected so as to be bendable in a left-right direction. 
     The working equipment  3  performs predetermined work such as the excavation work and the transportation work. The working equipment  3  includes a boom  3 A and a bucket  3 B. The boom  3 A is supported by the front frame  2 F. The working equipment  3  is driven by a lift cylinder  3 C and a bucket cylinder  3 D. Each of the lift cylinder  3 C and the bucket cylinder  3 D is a hydraulic cylinder. One end portion of the lift cylinder  3 C is connected to the front frame  2 F. The other end portion of the lift cylinder  3 C is connected to the boom  3 A. As the lift cylinder  3 C extends and retracts, the boom  3 A makes upward movement and downward movement. The bucket  3 B is connected to a distal end portion of the boom  3 A. One end portion of the bucket cylinder  3 D is connected to the front frame  2 F. The other end portion of the bucket cylinder  3 D is connected to the bucket  3 B via a bell crank  3 E. As the bucket cylinder  3 D extends and retracts, the bucket  3 B makes dumping movement and excavation movement. 
     The traveling device  4  travels while supporting the vehicle body frame  2 . The traveling device  4  includes traveling wheels  4 F and traveling wheels  4 R. The wheel loader  1  travels as the traveling wheels  4 F and  4 R rotate. A traveling direction of the wheel loader  1  is changed by a steering cylinder  4 S. The steering cylinder  4 S is a hydraulic cylinder. One end portion of the steering cylinder  4 S is connected to the front frame  2 F. The other end portion of the steering cylinder  4 S is connected to the rear frame  2 R. As the steering cylinder  4 S extends and retracts, the front frame  2 F and the rear frame  2 R are bent, and the traveling direction of the wheel loader  1  is changed to the left and right. 
     The cab  5  is a space on which a driver of the wheel loader  1  boards. The cab  5  is supported by the vehicle body frame  2 . A seat on which the driver of the wheel loader  1  sits and an operation device operated by the driver are disposed in the cab  5 . 
       FIG. 2  is a block diagram illustrating an example of the wheel loader  1  according to the present embodiment. As illustrated in  FIG. 2 , the wheel loader  1  includes an engine  6 , a power take-off (PTO)  7 , a working equipment pump  8 , a steering pump  9 , an input shaft  11 , an output shaft  12 , a power transmission device  10 , a clutch mechanism  30 , the traveling device  4 , an operation device  50 , and a control device  100 . 
     The engine  6  is a power source of the wheel loader  1 . The engine  6  generates power. The engine  6  is, for example, a diesel engine. The engine  6  is provided with a fuel injection device  6 A. The fuel injection device  6 A adjusts the amount of fuel injected into a cylinder of the engine  6  to adjust an output of the engine  6 . 
     The power take-off  7  distributes the power generated by the engine  6  to each of the working equipment pump  8 , the steering pump  9 , and the power transmission device  10 . 
     The working equipment pump  8  supplies hydraulic oil to each of the lift cylinder  3 C and the bucket cylinder  3 D. The working equipment pump  8  is driven by the engine  6 . The working equipment pump  8  is a variable displacement hydraulic pump. The hydraulic oil discharged from the working equipment pump  8  is supplied to each of the lift cylinder  3 C and the bucket cylinder  3 D via a working equipment control valve. 
     The steering pump  9  supplies hydraulic oil to the steering cylinder  4 S. The steering pump  9  is driven by the engine  6 . The steering pump  9  is a variable displacement hydraulic pump. The hydraulic oil discharged from the steering pump  9  is supplied to the steering cylinder  4 S via a steering control valve. Note that a pump (not illustrated) for driving auxiliary equipment such as a cooling fan or driving a clutch may be connected to the power take-off  7 . 
     The input shaft  11  is connected to the engine  6 . The output shaft  12  is connected to the traveling device  4 . The input shaft  11  receives power from the engine  6 . The power transmission device  10  transmits the power input to the input shaft  11  to the output shaft  12 . The output shaft  12  outputs power to the traveling device  4 . The power generated by the engine  6  is transmitted to the traveling device  4  via the power transmission device  10 . 
     The power transmission device  10  includes a hydraulic mechanical transmission (HMT). The power transmission device  10  includes a mechanical transmission mechanism  10 A that includes a planetary gear mechanism and mechanically transmits a part of the power input to the input shaft  11  to the output shaft  12 , and a hydraulic transmission mechanism  10 B that includes a first hydraulic pump motor P 1  and a second hydraulic pump motor P 2 , converts a part of the power input to the input shaft  11  into a hydraulic pressure, and transmits the hydraulic pressure to the output shaft  12 . The power transmission device  10  is a power split transmission that mechanically transmits a part of the power generated by the engine  6  to the output shaft  12  via the mechanical transmission mechanism  10 A, and converts a part of the power generated by the engine  6  into a hydraulic pressure in the hydraulic transmission mechanism  10 B to transmit the hydraulic pressure to the output shaft  12 . 
     The first hydraulic pump motor P 1  and the second hydraulic pump motor P 2  are connected via a hydraulic oil pipe  13 . The first hydraulic pump motor P 1  functions as one of a hydraulic pump and a hydraulic motor. The second hydraulic pump motor P 2  functions as the other of the hydraulic pump and the hydraulic motor. 
     In a case where the first hydraulic pump motor P 1  functions as the hydraulic pump, the second hydraulic pump motor P 2  functions as the hydraulic motor. Hydraulic oil discharged from the first hydraulic pump motor P 1  is supplied to the second hydraulic pump motor P 2  via the hydraulic oil pipe  13 . The second hydraulic pump motor P 2  is driven based on the hydraulic oil supplied from the first hydraulic pump motor P 1 . 
     In a case where the second hydraulic pump motor P 2  functions as the hydraulic pump, the first hydraulic pump motor P 1  functions as the hydraulic motor. Hydraulic oil discharged from the second hydraulic pump motor P 2  is supplied to the first hydraulic pump motor P 1  via the hydraulic oil pipe  13 . The first hydraulic pump motor P 1  is driven based on the hydraulic oil supplied from the second hydraulic pump motor P 2 . 
     Each of the first hydraulic pump motor P 1  and the second hydraulic pump motor P 2  is a variable displacement hydraulic pump motor. The power transmission device  10  includes a first capacity adjustment device Q 1  that adjusts a capacity Pc 1  of the first hydraulic pump motor P 1  and a second capacity adjustment device Q 2  that adjusts a capacity Pc 2  of the second hydraulic pump motor P 2 . The first capacity adjustment device Q 1  includes an actuator that drives an inclined shaft of the first hydraulic pump motor P 1 , and adjusts the capacity Pc 1  of the first hydraulic pump motor P 1  by driving the inclined shaft of the first hydraulic pump motor P 1 . The second capacity adjustment device Q 2  includes an actuator that drives an inclined shaft of the second hydraulic pump motor P 2 , and adjusts the capacity Pc 2  of the second hydraulic pump motor P 2  by driving the inclined shaft of the second hydraulic pump motor P 2 . 
     Note that, in the present embodiment, each of the first hydraulic pump motor P 1  and the second hydraulic pump motor P 2  is a bent-axis hydraulic pump motor whose capacity is adjusted as the inclined shaft is driven, but at least one of the first hydraulic pump motor P 1  or the second hydraulic pump motor P 2  may be a swash plate hydraulic pump motor whose capacity is adjusted as a swash plate is driven. 
     The clutch mechanism  30  transmits and interrupts power from the power transmission device  10 . The clutch mechanism  30  selects output rotation. The clutch mechanism  30  includes a plurality of clutches. The clutch mechanism  30  may include a gear. The clutch mechanism  30  may switch a reduction gear ratio and a rotation direction. Although the clutch mechanism  30  selects the output rotation in  FIG. 2 , but the clutch mechanism  30  may select input rotation of the power transmission device  10 . 
     The traveling device  4  includes an axle  4 A, the traveling wheels  4 F, and the traveling wheels  4 R. The axle  4 A transmits power from the power transmission device  10  to each of the traveling wheels  4 F and the traveling wheels  4 R. The traveling wheels  4 F and the traveling wheels  4 R rotate by the power transmitted from the axle  4 A. 
     The operation device  50  is operated by the driver in the cab  5 . The operation device  50  includes an accelerator/brake operation device  51  operated for driving and braking the traveling device  4 , and a forward-reverse operation device  52  operated to change the state of the traveling device  4  between a forward state, a neutral state, and a reverse state. 
     The forward-reverse operation device  52  includes a forward-reverse operation member. The forward-reverse operation member is movable to each of a forward position (F position) for bringing the traveling device  4  into the forward state, a neutral position (N position) for bringing the traveling device  4  into the neutral state, and a reverse position (R position) for bringing the traveling device  4  into the reverse state. The N position is disposed between the F position and the R position. An operation for switching between the F position and the R position is performed via the N position. 
     When the forward-reverse operation member is moved to the F position, the traveling device  4  can move forward. When the forward-reverse operation member is moved to the N position, the traveling device  4  is brought into the neutral state. When the forward-reverse operation member is moved to the R position, the traveling device  4  can be reversed. 
     In a case of changing the state of the traveling device  4  from the forward state to the reverse state, the driver moves the forward-reverse operation member from the F position to the R position. In a case of changing the state of the traveling device  4  from the reverse state to the forward state, the driver moves the forward-reverse operation member from the R position to the F position. The forward-reverse operation member is moved from the F position to the R position via the N position. The forward-reverse operation member is moved from the R position to the F position via the N position. That is, in a case of changing the state of the traveling device  4  between the forward state and the reverse state, the forward-reverse operation member is moved from one of the F position and the R position to the other via the N position. 
     Note that, although not illustrated, the operation device  50  further includes a working equipment operation device operated for operating the working equipment  3  and a steering operation device operated for changing the traveling direction of the wheel loader  1 . 
     The control device  100  includes a computer system that controls the wheel loader  1 . The control device  100  controls the fuel injection device  6 A to adjust the output of the engine  6 . The control device  100  controls the first capacity adjustment device Q 1  to adjust the capacity of the first hydraulic pump motor P 1 . The control device  100  controls the second capacity adjustment device Q 2  to adjust the capacity of the second hydraulic pump motor P 2 . 
     Further, the wheel loader  1  also includes an input shaft rotation speed sensor  41  and an output shaft rotation speed sensor  42 . 
     The input shaft rotation speed sensor  41  detects the rotation speed of the input shaft  11  per unit time. The rotating speed of the input shaft  11  is detected by detecting the rotation speed of the input shaft  11 . A detection signal of the input shaft rotation speed sensor  41  is output to the control device  100 . The rotation speed of the input shaft  11  corresponds to an engine speed of the engine  6  on a one-to-one basis. The control device  100  can calculate the engine speed of the engine  6  based on the rotation speed of the input shaft  11 . 
     The output shaft rotation speed sensor  42  detects the rotation speed of the output shaft  12  per unit time. The rotating speed of the output shaft  12  is detected by detecting the rotation speed of the output shaft  12 . A detection signal of the output shaft rotation speed sensor  42  is output to the control device  100 . The rotation speed of the output shaft  12  and a traveling speed of the wheel loader  1  correspond to each other on a one-to-one basis. The control device  100  can calculate the traveling speed of the wheel loader  1  based on the rotation speed of the output shaft  12 . 
     Power Transmission Device 
       FIG. 3  is a diagram schematically illustrating an example of the power transmission device  10  according to the present embodiment. The power transmission device  10  transmits the power input to the input shaft  11  to the output shaft  12  via the clutch mechanism  30 . 
     The power transmission device  10  includes a mechanical transmission mechanism  10 A that includes a planetary gear mechanism  15 , a planetary gear mechanism  16 , and a planetary gear mechanism  17  and transmits a part of the power input to the input shaft  11  to the output shaft  12  via the clutch mechanism  30 , and the hydraulic transmission mechanism  10 B that includes the first hydraulic pump motor P 1  and the second hydraulic pump motor P 2  and transmits a part of the power input to the input shaft  11  to the output shaft  12  via a part of the mechanical transmission mechanism  10 A and the clutch mechanism  30 . 
     The input shaft  11  is connected to the engine  6  via the power take-off  7 . The power generated by the engine  6  is transmitted to the input shaft  11  via the power take-off  7 . The input shaft  11  rotates based on the power transmitted from the engine  6 . An input gear  14  is fixed to the input shaft  11 . 
     The planetary gear mechanism  15  includes a sun gear  15 S, a plurality of planetary gears  15 P disposed around the sun gear  15 S, and a carrier  15 C that rotatably supports the plurality of planetary gears  15 P. The plurality of planetary gears  15 P mesh with each of the sun gear  15 S and a ring gear  18 . The planetary gear  15 P can revolve around the sun gear  15 S. The ring gear  18  is disposed around the plurality of planetary gears  15 P. The sun gear  15 S is connected to the first hydraulic pump motor P 1  via a transmission shaft  19 . A carrier gear  20  is provided on an outer circumference of the carrier  15 C. The input gear  14  meshes with the carrier gear  20 . 
     The planetary gear mechanism  16  includes a sun gear  16 S, a plurality of planetary gears  16 P disposed around the sun gear  16 S, and a carrier  16 C that rotatably supports the plurality of planetary gears  16 P. The plurality of planetary gears  16 P mesh with each of the sun gear  16 S and the ring gear  18 . The planetary gear  16 P can revolve around the sun gear  16 S. The ring gear  18  is disposed around the plurality of planetary gears  16 P. The sun gear  16 S is connected to the planetary gear  15 P via a sleeve  15 R. The sleeve  15 R is disposed around the transmission shaft  19 . 
     The planetary gear mechanism  17  includes a sun gear  17 S, a plurality of planetary gears  17 P disposed around the sun gear  17 S, and a ring gear  17 R disposed around the planetary gears  17 P. The plurality of planetary gears  17 P mesh with each of the sun gear  17 S and the ring gear  17 R. The planetary gear  17 P and the planetary gear  16 P are connected via the carrier  16 C. As the planetary gear  16 P revolves, the planetary gear  17 P revolves. 
     In the present embodiment, a rotation axis of the sun gear  15 S of the planetary gear mechanism  15 , a rotation axis of the sun gear  16 S of the planetary gear mechanism  16 , a rotation axis of the sun gear  17 S of the planetary gear mechanism  17 , and a rotation axis of the transmission shaft  19  coincide with each other. 
     The hydraulic transmission mechanism  10 B includes a transmission shaft  22  connected to the second hydraulic pump motor P 2 . A gear  23  is fixed to the transmission shaft  22 . The gear  23  meshes with an outer circumference gear  24  provided on an outer circumference of the ring gear  18 . 
     Clutch Mechanism 
     The clutch mechanism  30  includes a low-speed gear  31 , a medium-speed gear  32 , a high-speed gear  33 , a forward-low-speed clutch FL, a forward-medium-speed clutch FM, and a forward-high-speed clutch FH. A rotation axis of the low-speed gear  31 , a rotation axis of the medium-speed gear  32 , a rotation axis of the high-speed gear  33 , and a rotation axis of the transmission shaft  19  coincide with each other. 
     The low-speed gear  31  is connected to the transmission shaft  19 . The low-speed gear  31  is rotatable together with the transmission shaft  19 . The low-speed gear  31  is connected to the output shaft  12  via the forward-low-speed clutch FL. 
     The medium-speed gear  32  is connected to the sun gear  17 S. The medium-speed gear  32  is formed integrally with the sun gear  17 S. Note that the medium-speed gear  32  may be provided separately from the sun gear  17 S. The medium-speed gear  32  is rotatable together with the sun gear  17 S. The medium-speed gear  32  is connected to the output shaft  12  via the forward-medium-speed clutch FM. 
     The high-speed gear  33  is connected to the transmission shaft  19  via the forward-high-speed clutch FH. In a state where the forward-high-speed clutch FH is engaged, the high-speed gear  33  is rotatable together with the transmission shaft  19 . The high-speed gear  33  is connected to the output shaft  12 . 
     The forward-low-speed clutch FL, the forward-medium-speed clutch FM, and the forward-high-speed clutch FL are, for example, hydraulic clutches. The forward-low-speed clutch FL, the forward-medium-speed clutch FM, and the forward-high-speed clutch FL are controlled by the control device  100 . 
     The forward-low-speed clutch FL performs switching between connection and disconnection between the output shaft  12  and the transmission shaft  19 . In a state where the forward-low-speed clutch FL is engaged, the rotation of the transmission shaft  19  is transmitted to the output shaft  12  via the low-speed gear  31 . 
     The forward-medium-speed clutch FM performs switching between connection and disconnection between the output shaft  12  and the sun gear  17 S. In a state where the forward-medium-speed clutch FM is engaged, the rotation of the sun gear  17 S is transmitted to the output shaft  12  via the medium-speed gear  32 . 
     The forward-high-speed clutch FH performs switching between connection and disconnection between the output shaft  12  and the transmission shaft  19 . In a state where the forward-high-speed clutch FH is engaged, the rotation of the transmission shaft  19  is transmitted to the output shaft  12 . 
     Note that, in  FIG. 3 , some of components between the forward-low-speed clutch FL, the forward-medium-speed clutch FM, and the forward-high-speed clutch FH, and the output shaft  12  are omitted. Another clutch or gear may be disposed between the output shaft  12  and at least one of the forward-low-speed clutch FL, the forward-medium-speed clutch FM, or the forward-high-speed clutch FH. For example, a forward gear and a reverse gear may be disposed between the output shaft  12  and at least one of the forward-low-speed clutch FL, the forward-medium-speed clutch FM, or the forward-high-speed clutch FH. In the present embodiment, a reverse-low-speed clutch RL and a reverse-medium-speed clutch RM are provided. 
     In the following description, the forward-low-speed clutch FL, the forward-medium-speed clutch FM, the forward-high-speed clutch FH, the reverse-low-speed clutch RL, and the reverse-medium-speed clutch RM are collectively referred to as clutches as appropriate. The clutch includes an input-side element and an output-side element. The input-side element and the output-side element are connectable and disconnectable. The input-side element refers to an element that rotates in synchronization with the input shaft  11  by the rotation of the input shaft  11  even when the input-side element and the output-side element are disconnected. The output-side element refers to an element that rotates in synchronization with the output shaft  12  by the rotation of the output shaft  12  even when the output-side element and the input-side element are disconnected. Even when the output shaft  12  rotates in a state where the input-side element and the output-side element are disconnected, the input-side element is not affected by the rotation of the output shaft  12 . Even when the input shaft  11  rotates in a state where the output-side element and the input-side element are disconnected, the output-side element is not affected by the rotation of the input shaft  11 . Generally, the clutch includes a disc and a plate. One of the disc and the plate functions as the input-side element, and the other of the disc and the plate functions as the output-side element. 
     As the clutch is engaged, the input-side element and the output-side element of the clutch are connected. As the clutch is released, the input-side element and the output-side element of the clutch are disconnected. 
     The clutch mechanism  30  performs switching between a plurality of modes for a power transmission path. In the present embodiment, the modes include a forward-low-speed mode, a forward-medium-speed mode, a forward-high-speed mode, a reverse-low-speed mode, and a reverse-medium-speed mode. The forward-low-speed mode is a mode in which a speed ratio of the wheel loader  1  that moves forward is low. The forward-medium-speed mode is a mode in which the speed ratio of the wheel loader  1  that moves forward is medium. The forward-high-speed mode is a mode in which the speed ratio of the wheel loader  1  that moves forward is high. The reverse-low-speed mode is a mode in which the speed ratio of the wheel loader  1  that reverses is low. The reverse-medium-speed mode is a mode in which the speed ratio of the wheel loader  1  that reverses is medium. 
     Mode Switching 
       FIG. 4  is a diagram schematically illustrating a relationship among the mode, a state of the clutch mechanism  30 , and a state of each of the first hydraulic pump motor P 1  and the second hydraulic pump motor P 2  according to the present embodiment. 
     The forward-low-speed clutch FL is engaged in the forward-low-speed mode. The input-side element of the forward-low-speed clutch FL is connected to the low-speed gear  31 . The output-side element of the forward-low-speed clutch FL is connected to the output shaft  12  via a gear (not illustrated) or another clutch. The forward-low-speed clutch FL performs switching between connection and disconnection between the low-speed gear  31  and the output shaft  12 . 
     The forward-medium-speed clutch FM is engaged in the forward-medium-speed mode. The input-side element of the forward-medium-speed clutch FM is connected to the medium-speed gear  32 . The output-side element of the forward-medium-speed clutch FM is connected to the output shaft  12  via a gear (not illustrated) or another clutch. The forward-medium-speed clutch FM performs switching between connection and disconnection between the medium-speed gear  32  and the output shaft  12 . 
     The forward-high-speed clutch FH is engaged in the forward-high-speed mode. The input-side element of the forward-high-speed clutch FH is connected to the high-speed gear  33 . The output-side element of the forward-high-speed clutch FH is connected to the output shaft  12  via a gear (not illustrated) or another clutch. The forward-high-speed clutch FH performs switching between connection and disconnection between the high-speed gear  33  and the output shaft  12 . 
     Further, the reverse-low-speed clutch RL (not illustrated) is engaged in the reverse-low-speed mode, and the reverse-medium-speed clutch RM (not illustrated) is engaged in the reverse-medium-speed mode. 
     As illustrated in  FIG. 4 , each of the capacity Pc 1  of the first hydraulic pump motor P 1  and the capacity Pc 2  of the second hydraulic pump motor P 2  is adjusted based on the speed ratio. The speed ratio refers to a ratio between the rotation speed of the input shaft  11  and the rotation speed of the output shaft  12 . That is, a relationship of [speed ratio]=[rotation speed of output shaft  12 ]/[rotation speed of input shaft  11 ] is established. In a case where the rotation speed of the input shaft  11  is constant, the speed ratio corresponds to the traveling speed of the wheel loader  1 . 
     A power transmission path in the forward-low-speed mode will be described. In the forward-low-speed mode, the forward-low-speed clutch FL is engaged, and other clutches are released. 
     When power is input from the engine  6  to the input shaft  11 , the input gear  14  rotates, and the carrier  15 C rotates. As the carrier  15 C rotates, the planetary gear  15 P revolves, and the sun gear  15 S rotates. As the sun gear  15 S rotates, the transmission shaft  19  rotates. As the transmission shaft  19  rotates, the low-speed gear  31  rotates. Since the forward-low-speed clutch FL is engaged, the output shaft  12  rotates by the rotation of the low-speed gear  31 . 
     Next, a power transmission path in the forward-medium-speed mode will be described. In the forward-medium-speed mode, the forward-medium-speed clutch FM is engaged, and other clutches are released. 
     When power is input from the engine  6  to the input shaft  11 , the input gear  14  rotates, and the carrier  15 C rotates. As the carrier  15 C rotates, the planetary gear  15 P revolves, and the planetary gear  16 P connected to the planetary gear  15 P via the carrier  16 C revolves. As the planetary gear  16 P revolves, the medium-speed gear  32  rotates. Since the forward-medium-speed clutch FM is engaged, the output shaft  12  rotates by the rotation of the medium-speed gear  32 . 
     Next, a power transmission path in the forward-high-speed mode will be described. In the forward-high-speed mode, the forward-high-speed clutch FH is engaged, and other clutches are released. 
     When power is input from the engine  6  to the input shaft  11 , the input gear  14  rotates, and the carrier  15 C rotates. As the carrier  15 C rotates, the planetary gear  15 P revolves, and the sun gear  15 S rotates. As the sun gear  15 S rotates, the transmission shaft  19  rotates. As the transmission shaft  19  rotates, the high-speed gear  33  rotates. Since the high-speed clutch FM is engaged, the output shaft  12  is engaged by the rotation of the high-speed gear  33 . 
     The power transmission paths in the forward-low-speed mode, the forward-medium-speed mode, and the forward-high-speed mode have been described above. A description of power transmission paths in the reverse-low-speed mode and the reverse-medium-speed mode will be omitted. 
     The clutch to be engaged is switched at a predetermined switching speed ratio Cv. As illustrated in  FIG. 4 , the switching speed ratio Cv includes a reference switching speed ratio Cv 0 , a first switching speed ratio Cv 1 , a second switching speed ratio Cv 2 , and a third switching speed ratio Cv 3 . A value of the reference switching speed ratio Cv 0  is zero. Note that the reference switching speed ratio Cv 0  may have a value approximate to zero. The first switching speed ratio Cv 1  is a speed ratio when the wheel loader  1  moves forward, and is a value higher than the reference switching speed ratio Cv 0 . The second switching speed ratio Cv 2  is a speed ratio when the wheel loader  1  moves forward, and is a value higher than the first switching speed ratio Cv 1 . The third switching speed ratio Cv 3  is a speed ratio when the wheel loader  1  reverses, and is a value lower than the reference switching speed ratio Cv 0 . 
     A clutch engaged at a speed ratio between the reference switching speed ratio Cv 0  and the first switching speed ratio Cv 1  is the forward-low-speed clutch FL. A clutch engaged at a speed ratio between the first switching speed ratio Cv 1  and the second switching speed ratio Cv 2  is the forward-medium-speed clutch FM. A clutch engaged at a speed ratio higher than the second switching speed ratio Cv 2  is the forward-high-speed clutch FH. A clutch engaged at a speed ratio between the reference switching speed ratio Cv 0  and the third switching speed ratio Cv 3  is the reverse-low-speed clutch RL. A clutch engaged at a speed ratio lower than the third switching speed ratio Cv 3  is the reverse-medium-speed clutch RM. 
     At the first switching speed ratio Cv 1 , the clutch to be engaged is switched from one of the forward-low-speed clutch FL and the forward-medium-speed clutch FM to the other. At the second switching speed ratio Cv 2 , the clutch to be engaged is switched from one of the forward-medium-speed clutch FM and the forward-high-speed clutch FH to the other. At the third switching speed ratio Cv 3 , the clutch to be engaged is switched from one of the reverse-low-speed clutch RL and the reverse-medium-speed clutch RM to the other. At the reference switching speed ratio Cv 0 , the clutch to be engaged is switched from one of the forward-low-speed clutch FL and the reverse-low-speed clutch RL to the other. 
     Control Device 
       FIG. 5  is a functional block diagram illustrating an example of the control device  100  according to the present embodiment. As illustrated in  FIG. 5 , the control device  100  is connected to the operation device  50  including the accelerator/brake operation device  51  and the forward-reverse operation device  52 . The control device  100  is connected to each of the input shaft rotation speed sensor  41  and the output shaft rotation speed sensor  42 . The control device  100  is connected to each of the first hydraulic pump motor P 1  including the first capacity adjustment device Q 1  and the second hydraulic pump motor P 2  including the second capacity adjustment device Q 2 . The control device  100  is connected to the clutch mechanism  30  including the forward-low-speed clutch FL, the forward-medium-speed clutch FM, the forward-high-speed clutch FH, the reverse-low-speed clutch RL, and the reverse-medium-speed clutch RM. 
     The control device  100  includes a computer system. The control device  100  includes an arithmetic processing device including a processor such as a central processing unit (CPU), a storage device including a memory such as a read only memory (ROM) or a random access memory (RAM), and an input/output interface. 
     The control device  100  includes an operation signal acquisition unit  101 , a detection signal acquisition unit  102 , a target output shaft torque determination unit  103 , an engine acceleration/deceleration torque determination unit  104 , an output shaft rotation speed prediction unit  105 , an input shaft rotation speed prediction unit  106 , a speed ratio calculation unit  107 , a power control unit  108 , a clutch control unit  109 , a storage unit  110 , a timer unit  111 , and a torque reduction command unit  112 . 
     The operation signal acquisition unit  101  acquires an operation signal generated by the operation of the operation device  50 . 
     When the accelerator/brake operation device  51  is operated by the driver, the accelerator/brake operation device  51  generates at least one of an operation signal for driving the traveling device  4  or an operation signal for braking the traveling device  4 . When the accelerator/brake operation device  51  is operated by the driver, the operation signal acquisition unit  101  acquires at least one of the operation signal for driving the traveling device  4  or the operation signal for braking the traveling device  4 . 
     When the forward-reverse operation device  52  is operated by the driver, the forward-reverse operation device  52  outputs at least one of an operation signal for changing the state of the traveling device  4  to the forward state, an operation signal for changing the state of the traveling device  4  to the neutral state, or an operation signal for changing the state of the traveling device  4  to the reverse state. When the forward-reverse operation device  52  is operated by the driver, the forward-reverse operation device  52  acquires at least one of the operation signal for changing the state of the traveling device  4  to the forward state, the operation signal for changing the traveling device  4  to the neutral state, or the operation signal for changing the state of the traveling device  4  to the reverse state. 
     As described above, the forward-reverse operation device  52  includes the forward-reverse operation member. In a case of changing the state of the traveling device  4  between the forward state and the reverse state, the forward-reverse operation member is moved from one of the F position and the R position to the other via the N position. In a case where the forward-reverse operation member is moved from the N position to the F position, the forward-reverse operation device  52  outputs the operation signal for changing the state of the traveling device  4  to the forward state. In a case where the forward-reverse operation member is moved from the N position to the R position, the forward-reverse operation device  52  outputs the operation signal for changing the state of the traveling device  4  to the reverse state. In a case where the forward-reverse operation member is moved from one of the F position and the R position to the N position, the forward-reverse operation device  52  outputs the operation signal for changing the state of the traveling device  4  to the neutral state. 
     The detection signal acquisition unit  102  acquires a detection signal of the input shaft rotation speed sensor  41  and a detection signal of the output shaft rotation speed sensor  42 . The detection signal of the input shaft rotation speed sensor  41  indicates the rotation speed of the input shaft  11 . The detection signal of the output shaft rotation speed sensor  42  indicates the rotation speed of the output shaft  12 . 
     The target output shaft torque determination unit  103  calculates a target output shaft torque indicating a target torque of the output shaft  12  based on the traveling speed of the wheel loader  1  and the operation signal of the accelerator/brake operation device  51 . The target output shaft torque determination unit  103  calculates the traveling speed of the wheel loader  1  based on the detection signal of the output shaft rotation speed sensor  42  acquired by the detection signal acquisition unit  102 . The target output shaft torque determination unit  103  acquires the operation signal of the accelerator/brake operation device  51  from the operation signal acquisition unit  101 . The target output shaft torque determination unit  103  determines the target output shaft torque based on the traveling speed of the wheel loader  1  calculated from the detection signal of the output shaft rotation speed sensor  42  acquired by the detection signal acquisition unit  102  and the operation signal of the accelerator/brake operation device  51  acquired by the operation signal acquisition unit  101 . For example, when the wheel loader  1  moves forward, in a case where the operation signal for driving the traveling device  4  is acquired, the target output shaft torque increases. In a case where the operation signal for braking the traveling device  4  is acquired, the target output shaft torque decreases. In a case where the traveling speed of the wheel loader  1  is low, the target output shaft torque increases. In a case where the traveling speed of the wheel loader  1  is high, the target output shaft torque decreases. 
     The engine acceleration/deceleration torque determination unit  104  calculates an engine acceleration/deceleration torque indicating a target torque of the engine  6  based on the traveling speed of the wheel loader  1  and the operation signal of the accelerator/brake operation device  51 . The engine acceleration/deceleration torque corresponds to a target input shaft torque indicating a target torque of the input shaft  11 . The engine acceleration/deceleration torque determination unit  104  calculates the traveling speed of the wheel loader  1  based on the detection signal of the output shaft rotation speed sensor  42  acquired by the detection signal acquisition unit  102 . The engine acceleration/deceleration torque determination unit  104  acquires the operation signal of the accelerator/brake operation device  51  from the operation signal acquisition unit  101 . The engine acceleration/deceleration torque determination unit  104  determines the engine acceleration/deceleration torque based on the traveling speed of the wheel loader  1  calculated from the detection signal of the output shaft rotation speed sensor  42  acquired by the detection signal acquisition unit  102  and the operation signal of the accelerator/brake operation device  51  acquired by the operation signal acquisition unit  101 . For example, in a case where the operation signal for driving the traveling device  4  is acquired, the engine acceleration/deceleration torque increases. 
     Based on the target output shaft torque and the detection signal of the output shaft rotation speed sensor  42 , the output shaft rotation speed prediction unit  105  calculates an estimated output shaft rotation speed indicating a predictive value of the rotation speed of the output shaft  12  at a prediction time point after a predetermined time from a current time point. Note that the output shaft rotation speed prediction unit  105  may estimate the predictive value of the rotation speed of the output shaft by using an output shaft load torque estimated by a certain method. The current time point includes a time point at which the target output shaft torque is calculated and a time point at which the detection signal of the output shaft rotation speed sensor  42  is acquired by the detection signal acquisition unit  102 . 
     Based on the engine acceleration/deceleration torque and the detection signal of the input shaft rotation speed sensor  41 , the input shaft rotation speed prediction unit  106  calculates an estimated input shaft rotation speed indicating a predictive value of the rotation speed of the input shaft  11  at a prediction time point after a predetermined time from a current time point. The current time point includes a time point at which the engine acceleration/deceleration torque is calculated and a time point at which the detection signal of the input shaft rotation speed sensor  41  is acquired by the detection signal acquisition unit  102 . 
     Based on the estimated output shaft rotation speed and the estimated input shaft rotation speed, the speed ratio calculation unit  107  calculates a target speed ratio indicating a target value of the speed ratio at a prediction time point after a predetermined time from a current time point. 
     The power control unit  108  outputs a control command for controlling the power transmission device  10 . In the present embodiment, the power control unit  108  outputs a control command for controlling at least one of the first hydraulic pump motor P 1  or the second hydraulic pump motor P 2 . The control command output from the power control unit  108  includes a capacity command for changing at least one of the capacity Pc 1  of the first hydraulic pump motor P 1  or the capacity Pc 2  of the second hydraulic pump motor P 2 . The power control unit  108  can output a capacity command for changing the capacity Pc 1  of the first hydraulic pump motor P 1  to the first capacity adjustment device Q 1 . The power control unit  108  can output a capacity command for changing the capacity Pc 2  of the second hydraulic pump motor P 2  to the second capacity adjustment device Q 2 . 
     The clutch control unit  109  outputs a control command for controlling the clutch mechanism  30 . The control command output from the clutch control unit  109  includes a clutch command for engaging a specified clutch among the plurality of clutches of the clutch mechanism  30  and a release command for releasing the engaged clutch. 
     The storage unit  110  stores data used for a control of at least one of the first hydraulic pump motor P 1 , the second hydraulic pump motor P 2 , or the clutch mechanism  30 . In the present embodiment, the storage unit  110  stores correlation data indicating a relationship between the speed ratio and the capacity Pc 1  of the first hydraulic pump motor P 1  and the capacity Pc 2  of the second hydraulic pump motor P 2 . 
     The power control unit  108  outputs the capacity command for changing at least one of the capacity Pc 1  of the first hydraulic pump motor P 1  or the capacity Pc 2  of the second hydraulic pump motor P 2  based on the target speed ratio calculated by the speed ratio calculation unit  107  and the correlation data stored in the storage unit  110 . In addition, the power control unit  108  calculates an actual speed ratio based on the detection signal of the input shaft rotation speed sensor  41  and the detection signal of the output shaft rotation speed sensor  42 , and outputs the capacity command for changing at least one of the capacity Pc 1  of the first hydraulic pump motor P 1  or the capacity Pc 2  of the second hydraulic pump motor P 2  based on the actual speed ratio and the correlation data stored in the storage unit  110 . 
     The clutch control unit  109  outputs the clutch command for engaging a specified clutch among the plurality of clutches of the clutch mechanism  30  based on the target speed ratio calculated by the speed ratio calculation unit  107 . In addition, the clutch control unit  109  calculates an actual speed ratio based on the detection signal of the input shaft rotation speed sensor  41  and the detection signal of the output shaft rotation speed sensor  42 , and outputs the clutch command for engaging a specified clutch among the plurality of clutches of the clutch mechanism  30  based on the actual speed ratio. 
     The timer unit  111  measures an elapsed time from a time point t 0  at which an operation signal for changing the state of the forward-reverse operation device  52  from the forward state or the reverse state to the neutral state is acquired by the operation signal acquisition unit  101 , the operation signal being generated by the operation of the forward-reverse operation device  52  in a state where a first clutch among the plurality of clutches of the clutch mechanism  30  is engaged. That is, the timer unit  111  measures the elapsed time from the time point t 0  at which the forward-reverse operation member is moved from one of the F position and the R position to the N position. 
     The torque reduction command unit  112  outputs a torque reduction command for reducing a torque of the output shaft  12  in a state where the first clutch is engaged before a specified time elapses from the time point t 0 . In the present embodiment, the torque reduction command unit  112  outputs the torque reduction command to the target output shaft torque determination unit  103 . When the torque reduction command is output, the target output shaft torque determination unit  103  determines the target output shaft torque to be zero. 
     The clutch control unit  109  outputs a release command for releasing the first clutch after a specified time elapses from the time point t 0 . 
     In a state where the first clutch is released, the power control unit  108  outputs the control command for controlling at least one of the first hydraulic pump motor P 1  or the second hydraulic pump motor P 2  so that a rotation speed of an input-side element of a second clutch to be engaged next coincides with a rotation speed of an output-side element of the second clutch. 
     The second clutch to be engaged next is determined based on the speed ratio. The clutch control unit  109  determines the second clutch to be engaged next based on the speed ratio indicating the ratio between the rotation speed of the input shaft  11  and the rotation speed of the output shaft  12 . 
     The clutch control unit  109  determines the second clutch to be engaged next based on the actual speed ratio calculated from the detection signal of the input shaft rotation speed sensor  41  and the detection signal of the output shaft rotation speed sensor  42 . 
     After the second clutch is determined, the power control unit  108  outputs a control command based on the actual speed ratio and the correlation data stored in the storage unit  110  so that the rotation speed of the input-side element of the second clutch to be engaged next coincides with the rotation speed of the output-side element. In the present embodiment, the power control unit  108  outputs the capacity command for changing at least one of the capacity Pc 1  of the first hydraulic pump motor P 1  or the capacity Pc 2  of the second hydraulic pump motor P 2  based on the actual speed ratio and the correlation data stored in the storage unit  110  so that the rotation speed of the input-side element of the second clutch to be engaged next coincides with the rotation speed of the output-side element. 
     A function as the hydraulic pump and a function as the hydraulic motor of the first hydraulic pump motor P 1  and the second hydraulic pump motor P 2  are switched based on the speed ratio. The power control unit  108  outputs a capacity command so that the capacity (Pc 1  or Pc 2 ) of at least one of the first hydraulic pump motor P 1  or the second hydraulic pump motor P 2  that functions the hydraulic motor is changed at the speed ratio at which the second clutch is engaged. 
     The clutch control unit  109  outputs a clutch command for engaging the second clutch when the second clutch to be engaged next is determined, the neutral state of the forward-reverse operation device  52  is released, and the rotation speed of the input-side element of the second clutch coincides with the rotation speed of the output-side element. The clutch control unit  109  may output the clutch command for engaging the second clutch after a difference between the rotation speed of the input-side element and the rotation speed of the output-side element of the second clutch becomes equal to or less than a predetermined allowable value. 
     Correlation Data 
       FIG. 6  is a diagram schematically illustrating an example of the correlation data according to the present embodiment. As illustrated in  FIG. 6 , the correlation data indicates the relationship between the speed ratio and the capacity Pc 1  of the first hydraulic pump motor P 1  and the capacity Pc 2  of the second hydraulic pump motor P 2 . In  FIG. 6 , a horizontal axis represents the speed ratio, and a vertical axis represents the capacity [cc/rev].  FIG. 6  illustrates the correlation data when the wheel loader  1  moves forward. 
     As illustrated in  FIG. 6 , in the present embodiment, the correlation data is set so that both the capacity Pc 1  of the first hydraulic pump motor P 1  and the capacity Pc 2  of the second hydraulic pump motor P 2  are changed with a change in the speed ratio in a predetermined speed ratio range CM between a first speed ratio Ca and a second speed ratio Cb higher than the first speed ratio Ca. The first speed ratio Ca is a value higher than the reference switching speed ratio Cv 0 . The second speed ratio Cb is a value higher than the first speed ratio Ca. 
     The correlation data is set so that, in the predetermined speed ratio range CM, when one of the capacity Pc 1  of the first hydraulic pump motor P 1  and the capacity Pc 2  of the second hydraulic pump motor P 2  increases with a change in the speed ratio, the other of the capacity Pc 1  of the first hydraulic pump motor P 1  and the capacity Pc 2  of the second hydraulic pump motor P 2  decreases. 
     The predetermined speed ratio range CM includes the switching speed ratio Cv at which the clutch to be engaged is switched. As described above, the switching speed ratio Cv includes the first switching speed ratio Cv 1 , the second switching speed ratio Cv 2  higher than the first switching speed ratio Cv 1 , and the reference switching speed ratio Cv 0  lower than the first switching speed ratio Cv 1 . As described with reference to  FIG. 4 , the switching speed ratio Cv includes the third switching speed ratio Cv 3  lower than the reference switching speed ratio Cv 0 . 
     In the present embodiment, the predetermined speed ratio range CM includes the first switching speed ratio Cv 1  and the second switching speed ratio Cv 2 . 
     The correlation data is set so that the capacity increases or decreases as the speed ratio increases from the switching speed ratio Cv, and the capacity increases or decreases as the speed ratio decreases from the switching speed ratio Cv. That is, in the correlation data, an inflection point of the capacity is set to the switching speed ratio Cv. 
     For example, the capacity Pc 1  of the first hydraulic pump motor P 1  increases as the speed ratio increases from the first switching speed ratio Cv 1 , the capacity Pc 1  of the first hydraulic pump motor P 1  increases as the speed ratio decreases from the first switching speed ratio Cv 1 , the capacity Pc 1  of the first hydraulic pump motor P 1  decreases as the speed ratio increases from the second switching speed ratio Cv 2 , and the capacity Pc 1  of the first hydraulic pump motor P 1  decreases as the speed ratio decreases from the second switching speed ratio Cv 2 . The capacity Pc 2  of the second hydraulic pump motor P 2  decreases as the speed ratio increases from the first switching speed ratio Cv 1 , the capacity Pc 2  of the second hydraulic pump motor P 2  decreases as the speed ratio decreases from the first switching speed ratio Cv 1 , the capacity Pc 2  of the second hydraulic pump motor P 2  increases as the speed ratio increases from the second switching speed ratio Cv 2 , and the capacity Pc 2  of the second hydraulic pump motor P 2  increases as the speed ratio decreases from the second switching speed ratio Cv 2 . 
     The correlation data is set so that when one of the capacity Pc 1  of the first hydraulic pump motor P 1  and the capacity Pc 2  of the second hydraulic pump motor P 2  increases at a speed ratio between the first switching speed ratio Cv 1  and the second switching speed ratio Cv 2 , the other of the capacity Pc 1  of the first hydraulic pump motor P 1  and the capacity Pc 2  of the second hydraulic pump motor P 2  decreases. 
     As described above, the function as the hydraulic pump and the function as the hydraulic motor of the first hydraulic pump motor P 1  and the second hydraulic pump motor P 2  are switched at the switching speed ratio Cv. In the present embodiment, for example, when the wheel loader  1  is accelerated by power from the engine  6 , the first hydraulic pump motor P 1  functions as the hydraulic pump and the second hydraulic pump motor P 2  functions as the hydraulic motor at a speed ratio between the first switching speed ratio Cv 1  and the second switching speed ratio Cv 2 . Note that the function as the hydraulic pump and the function as the hydraulic motor are switched not only by the speed ratio but also by whether a torque transmitted to the output shaft  12  is for acceleration or for deceleration. The correlation data is set so that the capacity Pc 2  of the second hydraulic pump motor P 2  decreases when the capacity Pc 1  of the first hydraulic pump motor P 1  increases at a speed ratio between the first switching speed ratio Cv 1  and the second switching speed ratio Cv 2 . 
     In the predetermined speed ratio range CM, the capacity Pc 1  of the first hydraulic pump motor P 1  is equal to or less than a maximum capacity Pc 1   m  of the first hydraulic pump motor P 1 . The capacity Pc 2  of the second hydraulic pump motor P 2  is equal to or less than a maximum capacity Pc 2 m of the second hydraulic pump motor P 2 . 
     In the correlation data, the capacity Pc 1  of the first hydraulic pump motor P 1  at the switching speed ratio Cv is equal to or less than the maximum capacity Pc 1   m  of the first hydraulic pump motor P 1 , and the capacity Pc 2  of the second hydraulic pump motor P 2  at the switching speed ratio Cv is equal to or less than the maximum capacity Pc 2 m of the second hydraulic pump motor P 2 . 
     The maximum capacity Pc 1   m  is the capacity Pc 1  of the first hydraulic pump motor P 1  when the inclined shaft of the first hydraulic pump motor P 1  is driven to a maximum angle, and is a value uniquely determined based on specifications of the first hydraulic pump motor P 1 . The maximum capacity Pc 2 m is the capacity Pc 2  of the second hydraulic pump motor P 2  when the inclined shaft of the second hydraulic pump motor P 2  is driven to a maximum angle, and is a value uniquely determined based on specifications of the second hydraulic pump motor P 2 . 
     Neutral Control 
     Next, a control method for the wheel loader  1  will be described. As described above, in a case of changing the state of the traveling device  4  from one of the forward state and the reverse state to the other, the forward-reverse operation member of the forward-reverse operation device  52  is operated by the driver. In a case of changing the state of the traveling device  4  from the forward state to the reverse state, the forward-reverse operation member is moved from the F position to the R position via the N position. In a case of changing the state of the traveling device  4  from the reverse state to the forward state, the forward-reverse operation member is moved from the R position to the F position via the N position. In addition, the forward-reverse operation member may be moved from the F position to the N position and then moved to the F position again, or the forward-reverse operation member may be moved from the R position to the N position and then moved to the R position again. 
     For example, in a case where it is desired to change the state of the wheel loader  1  from the forward state in which the forward-medium-speed clutch FM is engaged to the reverse state, the driver moves the forward-reverse operation member from the F position to the R position. When the forward-reverse operation member is moved from the F position to the R position, the forward-reverse operation member passes through the N position. In addition, for example, in the forward state of the wheel loader  1  in which the forward-medium-speed clutch FM is engaged, the driver may move the forward-reverse operation member from the F position to the N position and then move the forward-reverse operation member to the F position again. Also in this case, the forward-reverse operation member passes through the N position. 
     In the present embodiment, even in a case where the forward-reverse operation member is disposed at the N position, when the elapsed time from the time point t 0  at which the forward-reverse operation member is moved from the F position to the N position has not reached the specified time, the forward-medium-speed clutch FM is not released, and a state where the forward-medium-speed clutch FM is engaged is maintained. In the state where the forward-medium-speed clutch FM is engaged, the torque reduction command for reducing the torque of the output shaft  12  is output from the torque reduction command unit  112 . That is, even in a case where the forward-reverse operation member is moved to the N position, when the elapsed time from the time point t 0  has not reached the specified time, the torque of the output shaft  12  is reduced in a state where the forward-medium-speed clutch FM is engaged, and a pseudo neutral state is established. 
     For example, in a case where the driver quickly moves the forward-reverse operation member from the F position to the R position, the time for which the forward-reverse operation member is disposed at the N position is short. That is, in a case where the driver quickly moves the forward-reverse operation member from the F position to the R position, the elapsed time from the time point t 0  does not reach the specified time even when the forward-reverse operation member is moved to the N position. When the elapsed time from the time point t 0  has not reached the specified time, the torque reduction command for reducing the torque of the output shaft  12  is output from the torque reduction command unit  112  in a state where the forward-medium-speed clutch FM is engaged, and the pseudo neutral state is established. As a result, the traveling device  4  is brought into the neutral state in a pseudo manner. 
     The elapsed time from the time point t 0  is measured by the timer unit  111 . The torque reduction command unit  112  outputs the torque reduction command for reducing the torque of the output shaft  12  in a state where the forward-medium-speed clutch FM is engaged before the specified time elapses from the time point t 0 . In the present embodiment, the torque reduction command unit  112  outputs the torque reduction command to the target output shaft torque determination unit  103 . When the torque reduction command is output, the target output shaft torque determination unit  103  determines the target output shaft torque to be zero. The engine acceleration/deceleration torque is adjusted so that the target output shaft torque becomes zero, at least one of the capacity Pc 1  of the first hydraulic pump motor P 1  or the capacity Pc 2  of the second hydraulic pump motor P 2  is adjusted, the torque of the output shaft decreases, and the pseudo neutral state is established. 
     On the other hand, when the elapsed time from the time point t 0  at which the forward-reverse operation member is moved from the F position to the N position has reached the specified time, the forward-medium-speed clutch FM is released. 
     For example, in a case where the driver slowly moves the forward-reverse operation member from the F position to the R position or causes the traveling device  4  to travel by inertia in a state where the forward-reverse operation member is disposed at the N position, the time for which the forward-reverse operation member is disposed at the N position becomes long. Since it is difficult to accurately control the output shaft torque in the hydraulic transmission, it is difficult to maintain the pseudo neutral state when the time for which the forward-reverse operation member is disposed at the N position is long. Therefore, when the time for which the forward-reverse operation member is disposed at the N position is long, that is, when the elapsed time from the time point t 0  has reached the specified time, the release command for releasing the forward-medium-speed clutch FM is output from the clutch control unit  109 . As a result, the traveling device  4  is brought into a substantially neutral state. 
     The elapsed time from the time point t 0  is measured by the timer unit  111 . The clutch control unit  109  outputs the release command for releasing the medium-speed clutch RM after the specified time elapses from the time point t 0 . As a result, the forward-medium-speed clutch FM is released, and the substantial neutral state is established. 
     After the forward-medium-speed clutch FM is released, a candidate for a clutch to be engaged next is any one of the reverse-medium-speed clutch RM, the reverse-low-speed clutch RL, the forward-low-speed clutch FL, the forward-medium-speed clutch FM, and the forward-high-speed clutch FH. The clutch to be engaged next is determined based on the speed ratio. 
     The clutch control unit  109  determines the clutch to be engaged next based on the speed ratio indicating the ratio between the rotation speed of the input shaft  11  and the rotation speed of the output shaft  12 . As described with reference to  FIG. 4 , the clutch to be engaged is determined in advance based on the speed ratio. The clutch control unit  109  determines the clutch to be engaged next based on the detection signal of the input shaft rotation speed sensor  41  and the detection signal of the output shaft rotation speed sensor  42 . That is, the clutch control unit  109  determines the clutch to be engaged next based on the actual speed ratio calculated from the detection signal of the input shaft rotation speed sensor  41  and the detection signal of the output shaft rotation speed sensor  42 . 
     Here, it is assumed that the forward-low-speed clutch FL is determined as the clutch to be engaged next. 
     The power control unit  108  outputs the control command for controlling the power transmission device  10  so that the rotation speed of the input-side element of the forward-low-speed clutch FL to be engaged next coincides with the rotation speed of the output-side element of the forward-low-speed clutch FL in a state where the forward-medium-speed clutch FM is released. As described with reference to  FIG. 3 , the input-side element of the forward-low-speed clutch FL is connected to the low-speed gear  31 . The output-side element of the forward-low-speed clutch FL is connected to the output shaft  12  via another clutch or gear (not illustrated). 
     As described above, even when the output shaft  12  rotates in a state where the input-side element and the output-side element are disconnected, the input-side element is not affected by the rotation of the output shaft  12 . Even when the input shaft  11  rotates in a state where the output-side element and the input-side element are disconnected, the output-side element is not affected by the rotation of the input shaft  11 . 
     Therefore, in a state where all the clutches of the clutch mechanism  30  including the forward-medium-speed clutch FM are released, the input-side element of the forward-low-speed clutch FL and the output-side element of the forward-low-speed clutch FL rotate separately. 
     When the input-side element of the forward-low-speed clutch FL and the output-side element of the forward-low-speed clutch FL are connected to each other in a state where the rotation speed of the input-side element of the forward-low-speed clutch FL does not coincide with the rotation speed of the output-side element of the forward-low-speed clutch FL, a shock occurs, which causes deterioration of ride comfort. This also causes wear of the clutch. 
     Therefore, in the present embodiment, the power control unit  108  outputs the control command for controlling the power transmission device  10  so that the rotation speed of the input-side element of the forward-low-speed clutch FL to be engaged next coincides with the rotation speed of the output-side element of the forward-low-speed clutch FL in the neutral state established by the release of the clutch. 
     The output-side element of the forward-low-speed clutch FL rotates in synchronization with the output shaft  12 . The power control unit  108  outputs a control command for controlling the rotation speed of the input-side element of the forward-low-speed clutch FL so that the rotation speed of the input-side element of the forward-low-speed clutch FL coincides with the rotation speed of the output-side element of the forward-low-speed clutch FL. 
     In the present embodiment, the power control unit  108  outputs the capacity command for changing at least one of the capacity Pc 1  of the first hydraulic pump motor P 1  or the capacity Pc 2  of the second hydraulic pump motor P 2  so that the rotation speed of the input-side element of the forward-low-speed clutch FL to be engaged next coincides with the rotation speed of the output-side element of the forward-low-speed clutch FL. 
     The power control unit  108  outputs a capacity command for making the rotation speed of the input-side element of the forward-low-speed clutch FL coincide with the rotation speed of the output-side element based on the actual speed ratio calculated based on the detection signal of the input shaft rotation speed sensor  41  and the detection signal of the output shaft rotation speed sensor  42  and the correlation data stored in the storage unit  110 . 
     The rotation speed of the input-side element of the forward-low-speed clutch FL is changed based on the rotation speed of the input shaft  11 , the capacity Pc 1  of the first hydraulic pump motor P 1 , and the capacity Pc 2  of the second hydraulic pump motor P 2 . First relevant data indicating a relationship between the rotation speed of the input-side element of the forward-low-speed clutch FL and the rotation speed of the input shaft  11  is known data that can be derived from design data of the power transmission device  10 , design data of the clutch mechanism  30 , the capacity of the first hydraulic pump motor, and the capacity of the second hydraulic pump motor, and is stored in the storage unit  110 . The rotation speed of the output-side element of the forward-low-speed clutch FL is changed based on the rotation speed of the output shaft  12 . Second relevant data indicating a relationship between the rotation speed of the output-side element of the forward-low-speed clutch FL and the rotation speed of the output shaft  12  is known data that can be derived from the design data of the power transmission device  10 , the design data of the clutch mechanism  30 , and the like, and is stored in the storage unit  110 . Therefore, the power control unit  108  can calculate the rotation speed of the input-side element of the forward-low-speed clutch FL based on the detection signal of the input shaft rotation speed sensor  41  and the first relevant data stored in the storage unit  110 . The power control unit  108  can calculate the rotation speed of the output-side element of the forward-low-speed clutch FL based on the detection signal of the output shaft rotation speed sensor  42  and the second relevant data stored in the storage unit  110 . The power control unit  108  can determine, based on the actual speed ratio and the correlation data stored in the storage unit  110 , a capacity Pc 1   s  of the first hydraulic pump motor P 1  and a capacity Pc 2   s  of the second hydraulic pump motor P 2  for making the rotation speed of the input-side element of the forward-low-speed clutch FL coincide with the rotation speed of the output-side element of the forward-low-speed clutch FL. The power control unit  108  can output a capacity command so that the capacity Pc 1  of the first hydraulic pump motor P 1  reaches the determined capacity Pc 1   s.  The power control unit  108  can output a capacity command so that the capacity Pc 2  of the second hydraulic pump motor P 2  reaches the determined capacity Pc 2   s.  As a result, the rotation speed of the input-side element of the forward-low-speed clutch FL coincides with the rotation speed of the output-side element of the forward-low-speed clutch FL. 
     After at least one of the capacity Pc 1  or the capacity Pc 2  is controlled so that the rotation speed of the input-side element of the forward-low-speed clutch FL coincides with the rotation speed of the output-side element of the forward-low-speed clutch FL, the clutch control unit  109  outputs a clutch command for engaging the forward-low-speed clutch FL. As a result, the forward-low-speed clutch FL is engaged in a state where the occurrence of the shock is suppressed. 
     Control Method 
     Next, a control method for the work vehicle  1  according to the present embodiment will be described.  FIG. 7  is a flowchart illustrating an example of a control method for the wheel loader  1  according to the present embodiment. 
     The operation signal acquisition unit  101  determines whether or not the operation signal for changing the state of the traveling device  4  to the neutral state has been acquired (Step S 1 ). 
     For example, in a case of changing the state of the wheel loader  1  from the reverse state to the forward state or in a case of changing the state of the wheel loader  1  from the forward state to the reverse state, the driver operates the forward-reverse operation member of the forward-reverse operation device  52 . When the forward-reverse operation member is moved from the R position to the F position via the N position, or when the forward-reverse operation member is moved from the F position to the R position via the N position, the operation signal for changing the state of the traveling device  4  to the neutral state is output from the forward-reverse operation device  52  while the forward-reverse operation member is at the N position. 
     In a case where it is determined in Step S 1  that the operation signal for changing the state of the traveling device  4  to the neutral state has not been acquired (Step S 1 : No), the target output shaft torque determination unit  103  calculates the target torque of the output shaft  12  based on the traveling speed of the wheel loader  1  and the operation signal of the accelerator/brake operation device  51 , and determines the calculated target torque as the target output shaft torque (Step S 2 ). 
     The engine acceleration/deceleration torque determination unit  104  calculates the target torque of the engine  6  based on the traveling speed of the wheel loader  1  and the operation signal of the accelerator/brake operation device  51 , and determines the calculated target torque as the engine acceleration/deceleration torque (Step S 3 ). 
     Based on the target output shaft torque determined in Step S 2  and the detection signal of the output shaft rotation speed sensor  42 , the output shaft rotation speed prediction unit  105  calculates the predictive value of the rotation speed of the output shaft  12  at the prediction time point after the predetermined time from the current time point, and determines the calculated predictive value as the estimated output shaft rotation speed (Step S 4 ). 
     Based on the engine acceleration/deceleration torque determined in Step S 3  and the detection signal of the input shaft rotation speed sensor  41 , the input shaft rotation speed prediction unit  106  calculates the predictive value of the rotation speed of the input shaft  11  at the prediction time point after the predetermined time from the current time point, and determines the calculated predictive value as the estimated input shaft rotation speed (Step S 5 ). 
     Based on the estimated output shaft rotation speed determined in Step S 4  and the estimated input shaft rotation speed determined in Step S 5 , the speed ratio calculation unit  107  calculates the target value of the speed ratio at the prediction time point after the predetermined time from the current time point, and determines the calculated target value as the target speed ratio (Step S 6 ). 
     The clutch control unit  109  determines a clutch to be engaged based on the target speed ratio determined in Step S 6  (Step S 7 ). 
     As described with reference to  FIG. 4 , the clutch to be engaged is determined in advance based on the speed ratio. The clutch control unit  109  determines the clutch to be engaged based on the target speed ratio. 
     The power control unit  108  determines the capacity Pc 1  of the first hydraulic pump motor P 1  and the capacity Pc 2  of the second hydraulic pump motor P 2  based on the target speed ratio determined in Step S 6  and the correlation data stored in the storage unit  110  (Step S 8 ). 
     The clutch control unit  109  outputs the clutch command for engaging the clutch determined in Step S 7  to the clutch mechanism  30  (Step S 9 ). 
     The power control unit  108  outputs the capacity command for changing at least one of the capacity Pc 1  or the capacity Pc 2  so as to become the capacity Pc 1  or the capacity Pc 2  determined in Step S 8  (Step S 10 ). 
     In a case where it is determined in Step S 1  that the operation signal for changing the state of the traveling device  4  to the neutral state has been acquired (Step S 1 : Yes), the timer unit  111  starts measurement of the elapsed time from the time point t 0  at which the state of the traveling device  4  is changed from the forward state or the reverse state to the neutral state based on the operation signal acquired by the operation signal acquisition unit  101 . 
     In a state where the elapsed time from the time point t 0  at which the state of the traveling device  4  is changed from the forward state or the reverse state to the neutral state has not reached the specified time, a specific first clutch among the plurality of clutches of the clutch mechanism  30  is in an engaged state. The timer unit  111  measures the elapsed time from the time point t 0  in a state where the first clutch is engaged. 
     The timer unit  111  determines whether or not the specified time has elapsed from the time point t 0  (Step S 11 ). 
     In a case where it is determined in Step S 11  that the specified time has not elapsed from the time point t 0  (Step S 11 : No), the torque reduction command unit  112  outputs the torque reduction command for reducing the torque of the output shaft  12  to the target output shaft torque determination unit  103  in a state where the first clutch is engaged before the specified time elapses from the time point t 0  (Step S 12 ). 
     The target output shaft torque determination unit  103  determines the target output shaft torque to be zero based on the torque reduction command output from the torque reduction command unit  112  (Step S 2 ). The processes from Step S 3  to Step S 10  are performed based on the target output shaft torque determined to be zero. By adjusting at least one of the speed ratio, the capacity Pc 1 , or the capacity Pc 2  so that the torque of the output shaft  12  becomes zero, the torque of the output shaft  12  becomes zero in a state where the first clutch is engaged, and the traveling device  4  becomes the neutral state in a pseudo manner. Note that the target output shaft torque does not have to be determined to be zero, and may be determined to be a value approximate to zero. 
     In a case where it is determined in Step S 11  that the specified time has elapsed from the time point t 0  (Step S 11 : Yes), the clutch control unit  109  outputs the release command to all the clutches of the clutch mechanism  30  including the engaged first clutch (Step S 13 ). 
     When the release command is output, all the clutches of the clutch mechanism  30  are released. 
     The clutch control unit  109  calculates the actual speed ratio based on the detection signal of the input shaft rotation speed sensor  41  and the detection signal of the output shaft rotation speed sensor  42  acquired by the detection signal acquisition unit  102  (Step S 14 ). 
     The clutch control unit  109  determines the clutch to be engaged next based on the actual speed ratio calculated in Step S 14  (Step S 15 ). 
     As described with reference to  FIG. 4 , the clutch to be engaged is determined in advance based on the speed ratio. The clutch control unit  109  determines the clutch to be engaged based on the actual speed ratio. 
     Based on the actual speed ratio calculated in Step S 14  and the correlation data stored in the storage unit  110 , the power control unit  108  determines at least one of the capacity Pc 1  of the first hydraulic pump motor P 1  or the capacity Pc 2  of the second hydraulic pump motor P 2  so that the rotation speed of the input-side element of the clutch to be engaged next coincides with the rotation speed of the output-side element (Step S 16 ). 
     The power control unit  108  outputs the capacity command for changing at least one of the capacity Pc 1  or the capacity Pc 2  so as to become the capacity Pc 1  or the capacity Pc 2  determined in Step S 16  (Step S 10 ). 
     Computer System 
       FIG. 8  is a block diagram illustrating an example of a computer system  1000  according to the present embodiment. The control device  100  described above includes the computer system  1000 . The computer system  1000  includes a processor  1001  such as a central processing unit (CPU), a main memory  1002  including a non-volatile memory such as a read only memory (ROM) and a volatile memory such as a random access memory (RAM), a storage  1003 , and an interface  1004  including an input/output circuit. A function of the control device  100  described above is stored in the storage  1003  as a computer program. The processor  1001  reads the computer program from the storage  1003 , loads the computer program to the main memory  1002 , and performs the above-described processing according to the computer program. Note that the computer program may be distributed to the computer system  1000  via a network. 
     According to the above-described embodiment, the computer program can cause the computer system  1000  to perform: transmitting power input to the input shaft  11  connected to the engine  6  to the output shaft  12  connected to the traveling device  4  via the power transmission device  10  and the clutch mechanism  30 ; acquiring the operation signal of the forward-reverse operation device  52  operated to change the state of the traveling device  4  between the forward state, the neutral state, and the reverse state; measuring the elapsed time from the time point t 0  at which the operation signal for changing to the neutral state is acquired in a state where the first clutch among the plurality of clutches of the clutch mechanism  30  is engaged; outputting the torque reduction command for reducing the torque of the output shaft  12  in a state where the first clutch is engaged before the specified time elapses from the time point t 0 ; outputting the release command for releasing the first clutch after the specified time elapses; and outputting the control command for controlling the power transmission device  10  so that the rotation speed of the input-side element of the second clutch to be engaged next coincides with the rotation speed of the output-side element of the second clutch in a state where the first clutch is released. 
     Effects 
     As described above, according to the present embodiment, when the forward-reverse operation device  52  is operated so that the traveling device  4  is in the neutral state, the torque reduction command for reducing the torque of the output shaft  12  is output in a state where the first clutch is engaged, in a state where the elapsed time from the time point t 0  at which the forward-reverse operation member is moved to the N position has not reached the specified time. As a result, the pseudo neutral state is established. Since the first clutch is engaged, the occurrence of the shock is suppressed. Further, according to the present embodiment, it is possible to reduce a time loss until power transmission starts when the neutral state is canceled. 
     In a state where the elapsed time from the time point t 0  at which the forward-reverse operation member is moved to the N position has reached the specified time, the first clutch is released, and the substantially neutral state is established. After the substantial neutral state is established, at least one of the capacity Pc 1  of the first hydraulic pump motor P 1  or the capacity Pc 2  of the second hydraulic pump motor P 2  is controlled so that the rotation speed of the input-side element of the second clutch to be engaged next coincides with the rotation speed of the output-side element. As a result, the second clutch is engaged in a state where the rotation speed of the input-side element of the second clutch coincides with the rotation speed of the output-side element. Therefore, the occurrence of the shock is suppressed. 
     Other Embodiments 
     Note that, in the above-described embodiment, the power transmission device  10  is an HMT including both the mechanical transmission mechanism  10 A and the hydraulic transmission mechanism  10 B. The power transmission device  10  may be an HST that includes the hydraulic transmission mechanism  10 B and does not include the mechanical transmission mechanism  10 A. The power transmission device  10  may include the mechanical transmission mechanism  10 A and does not have to include the hydraulic transmission mechanism  10 B. 
     Note that, in the above-described embodiment, the work vehicle  1  is a wheel loader. The work vehicle  1  to which the components described in the above-described embodiment are applied is not limited to the wheel loader, and can be widely applied to the work vehicle  1  including the clutch mechanism. For example, the work vehicle  1  may be a bulldozer. 
     REFERENCE SIGNS LIST 
       1  WHEEL LOADER (WORK VEHICLE)
       2  VEHICLE BODY FRAME     2 F FRONT FRAME     2 R REAR FRAME     3  WORKING EQUIPMENT     3 A BOOM     3 B BUCKET     3 C LIFT CYLINDER     3 D BUCKET CYLINDER     3 E BELL CRANK     4  TRAVELING DEVICE     4 A AXLE     4 F TRAVELING WHEEL     4 R TRAVELING WHEEL     4 S STEERING CYLINDER     5  CAB     6  ENGINE     6 A FUEL INJECTION DEVICE     7  POWER TAKE-OFF     8  WORKING EQUIPMENT PUMP     9  STEERING PUMP     10  POWER TRANSMISSION DEVICE     10 A MECHANICAL TRANSMISSION MECHANISM     10 B HYDRAULIC TRANSMISSION MECHANISM     11  INPUT SHAFT     12  OUTPUT SHAFT     13  HYDRAULIC OIL PIPE     14  INPUT GEAR     15  PLANETARY GEAR MECHANISM     15 C CARRIER     15 P PLANETARY GEAR     15 R SLEEVE     15 S SUN GEAR     16  PLANETARY GEAR MECHANISM     16 C CARRIER     16 P PLANETARY GEAR     16 S SUN GEAR     17  PLANETARY GEAR MECHANISM     17 P PLANETARY GEAR     17 R RING GEAR     17 S SUN GEAR     18  RING GEAR     19  TRANSMISSION SHAFT     20  CARRIER GEAR     22  TRANSMISSION SHAFT     23  GEAR     24  OUTER CIRCUMFERENCE GEAR     30  CLUTCH MECHANISM     31  LOW-SPEED GEAR     32  MEDIUM-SPEED GEAR     33  HIGH-SPEED GEAR     41  INPUT SHAFT ROTATION SPEED SENSOR     42  OUTPUT SHAFT ROTATION SPEED SENSOR     50  OPERATION DEVICE     51  ACCELERATOR/BRAKE OPERATION DEVICE     52  FORWARD-REVERSE OPERATION DEVICE     100  CONTROL DEVICE     101  OPERATION SIGNAL ACQUISITION UNIT     102  DETECTION SIGNAL ACQUISITION UNIT     103  TARGET OUTPUT SHAFT TORQUE DETERMINATION UNIT     104  ENGINE ACCELERATION/DECELERATION TORQUE DETERMINATION UNIT     105  OUTPUT SHAFT ROTATION SPEED PREDICTION UNIT     106  INPUT SHAFT ROTATION SPEED PREDICTION UNIT     107  SPEED RATIO CALCULATION UNIT     108  POWER CONTROL UNIT     109  CLUTCH CONTROL UNIT     110  STORAGE UNIT     111  TIMER UNIT     112  TORQUE REDUCTION COMMAND UNIT   Ca FIRST SPEED RATIO   Cb SECOND SPEED RATIO   CM PREDETERMINED SPEED RATIO RANGE   Cv SWITCHING SPEED RATIO   Cv 0  REFERENCE SWITCHING SPEED RATIO   

     Cv 1  FIRST SWITCHING SPEED RATIO
     Cv 2  SECOND SWITCHING SPEED RATIO   Cv 3  THIRD SWITCHING SPEED RATIO   FH FORWARD-HIGH-SPEED CLUTCH   FL FORWARD-LOW-SPEED CLUTCH   FM FORWARD-MEDIUM-SPEED CLUTCH   P 1  FIRST HYDRAULIC PUMP MOTOR   P 2  SECOND HYDRAULIC PUMP MOTOR   Pc 1  CAPACITY   Pc 1   m  MAXIMUM CAPACITY   Pc 2  CAPACITY   Pc 2   m  MAXIMUM CAPACITY   Q 1  FIRST CAPACITY ADJUSTMENT DEVICE   Q 2  SECOND CAPACITY ADJUSTMENT DEVICE   RL REVERSE-LOW-SPEED CLUTCH   RM REVERSE-MEDIUM-SPEED CLUTCH