Work vehicle, and control device and control method for power machine

A control device for a power machine includes a target circuit pressure specifying unit configured to specify a target circuit pressure of the hydrostatic continuously variable transmission, a measurement value acquisition unit configured to acquire an actual circuit pressure of the hydrostatic continuously variable transmission, a brake torque determination unit configured to determine a brake torque based on the target circuit pressure and the actual circuit pressure, and a pump control unit configured to control the hydraulic pump based on the brake torque. The brake torque is a torque consumed by the hydraulic pump.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a U.S. National stage application of International Application No. PCT/JP2019/043760, filed on Nov. 7, 2019. This U.S. National stage application claims priority under 35 U.S.C. § 119(a) to Japanese Patent Application No. 2018-216736, filed in Japan on Nov. 19, 2018, the entire contents of which are hereby incorporated herein by reference.

BACKGROUND

Field of the Invention

The present invention relates to a work vehicle, and a control device and a control method for a power machine.

Background Information

A work vehicle such as a wheel loader equipped with a continuously variable transmission is known. Examples of the continuously variable transmission include a hydraulic static transmission (HST) and a hydraulic mechanical transmission (HMT). Japanese Unexamined Patent Application, First Publication No. 2015-096401 discloses the following technique. When the work vehicle equipped with the continuously variable transmission is braked, a braking force is obtained by rotation of a hydraulic pump connected to a power take off (PTO). In this manner, an excessive rotation speed of an engine is prevented.

SUMMARY

According to the technique disclosed in Japanese Unexamined Patent Application, First Publication No. 2015-096401, a control device accurately controls a braking force in an electric motor. On the other hand, regenerative power is absorbed by driving the hydraulic pump connected to the engine to prevent overspeed when engine rotation increases due to regeneration to the engine. However, in order to properly brake the work vehicle by using the technique disclosed in Japanese Unexamined Patent Application, First Publication No. 2015-096401, it is necessary to provide electrical hardware such as an electric motor capable of properly controlling torques and an inverter for driving the electric motor. Therefore, when a power system includes a hydraulic component, it is difficult to accurately control braking of the work vehicle.

An object of the present invention is to provide a work vehicle, and a control device and a control method for a power machine, which are capable of accurately controlling a braking force regardless of the presence or absence of electrical hardware.

According to a first aspect of the present invention, there is provided a control device for a power machine including a drive source, a hydraulic pump driven by a driving force of the drive source, a power take off device that transmits a portion of the driving force of the drive source to the hydraulic pump, a power transmission device including a hydrostatic continuously variable transmission, in which an input shaft is connected to the power take off device, an output shaft is connected to a load, and the driving force input to the input shaft is transmitted to the output shaft, and an operation device that provides an instruction regarding a magnitude of the driving force of the drive source. The control device includes a target circuit pressure specifying unit configured to specify a target circuit pressure which is a target pressure of the hydrostatic continuously variable transmission, a measurement value acquisition unit configured to acquire an actual circuit pressure which is a measurement value of a pressure of the hydrostatic continuously variable transmission, a brake torque determination unit configured to determine a brake torque based on the target circuit pressure and the actual circuit pressure, the brake torque which is a torque consumed by the hydraulic pump, and a pump control unit configured to control the hydraulic pump based on the brake torque.

According to at least one of the above-described aspects, the control device can accurately control a braking force of a power device regardless of whether or not the power device includes electrical hardware.

DETAILED DESCRIPTION OF EMBODIMENT(S)

First Embodiment

FIG.1is a side view of a work vehicle according to a first embodiment.

A work vehicle100according to the first embodiment is a wheel loader. The work vehicle100includes a vehicle body110, a work machine120, a front wheel part130, a rear wheel part140, and an operator cab150. The work vehicle100is an example of a power machine.

The vehicle body110includes a front vehicle body111, a rear vehicle body112, and a steering cylinder113. The front vehicle body111and the rear vehicle body112are attached to be pivotable around a steering shaft extending in an upward-downward direction of the vehicle body110. The front wheel part130is provided in a lower part of the front vehicle body111, and the rear wheel part140is provided in a lower part of the rear vehicle body112.

The steering cylinder113is a hydraulic cylinder. A base end portion of the steering cylinder113is attached to the rear vehicle body112, and a tip portion is attached to the front vehicle body111. The steering cylinder113is expanded and contracted by hydraulic oil, thereby defining an angle between the front vehicle body111and the rear vehicle body112. That is, a steering angle of the front wheel part130is defined by expansion and contraction of the steering cylinder113.

The work machine120is used for excavating and transporting a work object such as earth and sand. The work machine120is provided in a front part of the vehicle body110. The work machine120includes a boom121, a bucket122, a bell crank123, a lift cylinder124, and a bucket cylinder125.

A base end portion of the boom121is attached to a front part of the front vehicle body111via a pin.

The bucket122includes a blade for excavating the work object and a container for carrying the excavated work object. A base end portion of the bucket122is attached to a tip portion of the boom121via a pin.

The bell crank123transmits power of the bucket cylinder125to the bucket122. A first end of the bell crank123is attached to a bottom portion of the bucket122via a link mechanism. A second end of the bell crank123is attached to a tip portion of the bucket cylinder125via a pin.

The lift cylinder124is a hydraulic cylinder. A base end portion of the lift cylinder124is attached to a front part of the front vehicle body111. A tip portion of the lift cylinder124is attached to the boom121. As the lift cylinder124is expanded and contracted by hydraulic oil, the boom121is driven in an upward direction or a downward direction.

The bucket cylinder125is a hydraulic cylinder. A base end portion of the bucket cylinder125is attached to a front part of the front vehicle body111. A tip portion of the bucket cylinder125is attached to the bucket122via the bell crank123. As the bucket cylinder125is expanded and contracted by the hydraulic oil, the bucket122swings in a tilt direction or a dump direction.

The operator cab150is a space for an operator who rides in the space to operate the work vehicle100. The operator cab150is provided in an upper part of the rear vehicle body112.

FIG.2is a diagram showing an internal configuration of the operator cab according to the first embodiment. The operator cab150is internally provided with a seat151, an accelerator pedal152, a brake pedal153, a steering wheel154, a front/rear selection switch155, a shift switch156, a boom lever157, and a bucket lever158.

The accelerator pedal152is operated to set a driving force (traction force) for traveling which is generated by the work vehicle100. As an operation amount of the bucket lever158increases, a target driving force (target traction force) is set to be stronger.

The brake pedal153is operated to set a braking force for traveling which is generated by the work vehicle100. As an operation amount of the brake pedal153increases, the braking force is set to be stronger.

The steering wheel154is operated to set a steering angle of the work vehicle100.

The front/rear selection switch155is operated to set a traveling direction of the work vehicle100. The traveling direction of the work vehicle is either forward (F), rearward (R), or neutral (N).

The shift switch156is operated to set a speed range of the power transmission device. For example, the shift switch156is operated to select one speed range from a first speed, a second speed, a third speed, and a fourth speed.

The boom lever157is operated to set a movement amount of a raising operation or a lowering operation of the boom121. The boom lever157receives the lowering operation when tilted forward, and receives the raising operation when tilted rearward.

The bucket lever158is operated to set a movement amount of a dump operation or a tilt operation of the bucket122. The bucket lever158receives the dump operation when tilted forward, and receives the tilt operation when tilted rearward.

FIG.3is a schematic diagram showing a power system of the work vehicle according to the first embodiment.

The work vehicle100includes an engine210, a power take off220(PTO: power take off device), a transmission230, a front axle240, a rear axle250, a variable capacity pump260, and a fixed capacity pump270.

For example, the engine210is a diesel engine. The engine210is provided with a fuel injection device211and an engine tachometer212. The fuel injection device211controls a driving force of the engine210by adjusting the amount of fuel injected into a cylinder of the engine210. The engine tachometer212measures a rotation speed of the engine210.

The PTO220transmits a portion of the driving force of the engine210to the variable capacity pump260and the fixed capacity pump270. That is, the PTO220distributes the driving force of the engine210to the transmission230, the variable capacity pump260, and the fixed capacity pump270.

The transmission230is a continuously variable transmission including a hydrostatic continuously variable transmission (HST)231. The transmission230may perform shift control by using only the HST231, or may be a hydraulic mechanical continuously variable transmission (HMT) that performs shift control by using a combination of the HST231and a planetary gear mechanism. The transmission230shifts the driving force input to an input shaft, and outputs the driving force from an output shaft. The input shaft of the transmission230is connected to the PTO220, and the output shaft is connected to the front axle240and the rear axle250. That is, the transmission230transmits the driving force of the engine210which is distributed by the PTO220to the front axle240and the rear axle250. The transmission230is provided with an input shaft tachometer232and an output shaft tachometer233. The input shaft tachometer232measures a rotation speed of the input shaft of the transmission230. The output shaft tachometer233measures a rotation speed of the output shaft of the transmission230. The HST231of the transmission230is provided with an HST pressure gauge234. The HST pressure gauge234measures a pressure of the HST231.

The front axle240transmits the driving force output by the transmission230to the front wheel part130. In this manner, the front wheel part130is rotated.

The rear axle250transmits the driving force output by the transmission230to the rear wheel part140. In this manner, the rear wheel part140is rotated.

The front axle240and the rear axle250are examples of a traveling device.

The variable capacity pump260is driven by a driving force transmitted from the engine210. For example, discharge capacity of the variable capacity pump260is changed by controlling a tilt angle of a swash plate provided inside the variable capacity pump260. Hydraulic oil discharged from the variable capacity pump260is supplied to the steering cylinder113, the lift cylinder124, and the bucket cylinder125via a control valve261. In addition, the hydraulic oil discharged from the variable capacity pump260is discharged via a brake valve265and a relief valve266.

The control valve261controls a flow rate of the hydraulic oil discharged from the variable capacity pump260, and distributes the hydraulic oil to the steering cylinder113, the lift cylinder124, and the bucket cylinder125. The brake valve265controls the flow rate of the hydraulic oil to be supplied to the relief valve266. The relief valve266releases the pressure when the pressure of the hydraulic oil exceeds a predetermined relief pressure, and discharges the hydraulic oil.

The variable capacity pump260is provided with a first pump pressure gauge262and a pump capacity meter263. The first pump pressure gauge262measures a discharge pressure of the hydraulic oil discharged from the variable capacity pump260. The pump capacity meter263measures capacity of the variable capacity pump260based on a swash plate angle of the variable capacity pump260.

The lift cylinder124is provided with a cylinder pressure gauge264. The cylinder pressure gauge264measures the pressure of the lift cylinder124.

The variable capacity pump260is an example of a device to which the power is distributed from the PTO220. In another embodiment, the variable capacity pump260may be configured to include a plurality of pumps, or may include other destinations such as hydraulically driven fans (not shown), instead of or in addition to the variable capacity pump260.

The fixed capacity pump270is driven by a driving force transmitted from the engine210. The hydraulic oil discharged from the fixed capacity pump270is supplied to a clutch (not shown) inside the transmission230. The fixed capacity pump270is provided with a second pump pressure gauge271. The second pump pressure gauge271measures the discharge pressure of the hydraulic oil discharged from the fixed capacity pump270. The fixed capacity pump270is an example of a device to which the power is distributed from the PTO220. The fixed capacity pump270may be configured to include a plurality of pumps, or may have a supply destination such as a lubrication circuit (not shown).

The work vehicle100includes a control device300for controlling the work vehicle100.

The control device300outputs a control signal to the fuel injection device211, the transmission230, the variable capacity pump260, the control valve261and the brake valve265in response to an operation amount of each operation device (accelerator pedal152, brake pedal153, steering wheel154, front/rear selection switch155, shift switch156, boom lever157, and bucket lever158) inside the operator cab150.

FIG.4is a schematic block diagram showing a configuration of the control device of the work vehicle according to the first embodiment. The control device300is a computer including a processor310, a main memory330, a storage350, and an interface370.

The storage350is a non-temporary tangible storage medium. Examples of the storage350include a hard disk drive (HDD), a solid state drive (SSD), a magnetic disk, a magneto-optical disc, a compact disc read only memory (CD-ROM), and a digital versatile disc read only memory (DVD-ROM), and a semiconductor memory. The storage350may be an internal medium directly connected to a bus of the control device300, or may be an external medium connected to the control device300via the interface370or a communication line. The storage350stores a program for controlling the work vehicle100.

The program may partially realize functions of the control device300. For example, the program may fulfill a function in combination with another program previously stored in the storage or in combination with another program installed in another device. In another embodiment, the computer may include a custom large scale integrated circuit (LSI) such as a programmable logic device (PLD) in addition to the above-described configuration or instead of the above-described configuration. Examples of the PLD include a programmable array logic (PAL), a generic array logic (GAL), a complex programmable logic device (CPLD), and a field programmable gate array (FPGA). In this case, functions realized by the processor may be partially or entirely realized by the integrated circuit.

In a case where the program is distributed to the control device300via a communication line, the control device300receiving the distribution may deploy the program in the main memory330, and may execute the above-described process.

In addition, the program may partially realize the above-described function. Furthermore, the program may be a so-called difference file (difference program) that realizes the above-described function in combination with another program previously stored in the storage350.

In order to execute the program, the processor310includes an operation amount acquisition unit311, a measurement value acquisition unit312, a vehicle state calculation unit313, a required PTO torque determination unit314, a required output torque determination unit315, a traveling load estimation unit316, a target rotation speed determination unit317, an acceleration torque specifying unit318, a target engine torque determination unit319, an engine control unit320, a brake control determination unit321, a target HST pressure specifying unit322(target circuit pressure specifying unit), an offset torque determination unit323, a compensation torque determination unit324, an assist torque determination unit325, a brake torque determination unit326, a target speed ratio determination unit327, a transmission control unit328, and a pump control unit329.

The operation amount acquisition unit311acquires the operation amount from each of the accelerator pedal152, the brake pedal153, the steering wheel154, the front/rear selection switch155, the shift switch156, the boom lever157, and the bucket lever158. Hereinafter, the operation amount of the accelerator pedal152will be referred to as an accelerator operation amount, the operation amount of the brake pedal153will be referred to as a brake operation amount, the operation amount of the steering wheel154will be referred to as a steering operation amount, a value corresponding to an operation position of the front/rear selection switch155will be referred to as an FNR operation amount, a value corresponding to an operation position of the shift switch156will be referred to as a shift operation amount, the operation amount of the boom lever157will be referred to as a boom operation amount, and the operation amount of the bucket lever158will be referred to as a bucket operation amount.

The measurement value acquisition unit312acquires measurement values from the fuel injection device211, the engine tachometer212, the input shaft tachometer232, the output shaft tachometer233, the HST pressure gauge234, the first pump pressure gauge262, the pump capacity meter263, the cylinder pressure gauge264, and the second pump pressure gauge271. That is, the measurement value acquisition unit312acquires each measurement value of a fuel injection amount of the engine210, a rotation speed of the engine210, a rotation speed of the input shaft of the transmission230, a rotation speed of the output shaft of the transmission230, a pressure of the HST231, a pump pressure of the variable capacity pump260, capacity of the variable capacity pump260, a pressure of the lift cylinder124, and a pump pressure of the fixed capacity pump270.

Based on the measurement values acquired by the measurement value acquisition unit312, the vehicle state calculation unit313calculates an output torque of the engine210, an upper limit torque of the engine210, angular acceleration of the engine210, torques (PTO torques) distributed to the variable capacity pump260and the fixed capacity pump270by the PTO220, an input/output speed ratio of the transmission230, angular acceleration of the output shaft of the transmission230, and a traveling speed of the work vehicle100. The output torque of the engine210is a torque actually achieved by the engine210which is calculated based on the fuel injection amount. The upper limit torque of the engine210is a maximum torque which can be achieved by the engine210.

The required PTO torque determination unit314determines a required value (required PTO torque) of the torques distributed from the PTO220to the variable capacity pump260and the fixed capacity pump270, based on the steering operation amount, the boom operation amount, and the bucket operation amount which are acquired by the operation amount acquisition unit311, and the measurement values of the pump pressure of the variable capacity pump260, the capacity of the variable capacity pump260, and the pump pressure of the fixed capacity pump270which are acquired by the measurement value acquisition unit312. For example, the required PTO torque determination unit314obtains a required flow rate of the variable capacity pump260from the steering operation amount, based on a PTO conversion function that defines a relationship between the operation amount and the required flow rate. Further, for example, the required PTO torque determination unit314obtains the required flow rate of the variable capacity pump260from the boom operation amount and the bucket operation amount based on the PTO conversion function. Then, the required PTO torque determination unit314determines the required PTO torque, based on the measurement values of the pump pressure of the variable capacity pump260, the capacity of the variable capacity pump260, the pump pressure of the fixed capacity pump270, and the required flow rate of the specified variable capacity pump260.

The required output torque determination unit315determines the required value (required output torque) of the torque of the output shaft of the transmission230, based on the accelerator operation amount, the brake operation amount, the shift operation amount, and the FNR operation amount which are acquired by the operation amount acquisition unit311, and the traveling speed calculated by the vehicle state calculation unit313. For example, the required output torque determination unit315determines the required output torque from the traveling speed calculated by the vehicle state calculation unit313, based on a traveling conversion function that defines a relationship between the traveling speed and the required output torque. In this case, the required output torque determination unit315determines characteristics of the traveling conversion function, based on the accelerator operation amount, the brake operation amount, the shift operation amount, and the FNR operation amount.

Specifically, the required output torque determination unit315specifies the traveling conversion function corresponding to a speed range specified by the shift operation amount, out of a plurality of the traveling conversion functions corresponding to a plurality of the speed ranges. When there is an accelerator operation, the required output torque determination unit315transforms the specified traveling conversion function, based on a magnification relating to the accelerator operation amount. When there is a brake operation, the required output torque determination unit315transforms the specified traveling conversion function, based on the magnification relating to the brake operation amount. The required output torque determination unit315determines a code of the required output torque, based on the FNR operation amount. When codes of the required output torque and the traveling speed do not coincide with each other (when the code of a product of the required output torque and the traveling speed is negative), the torque on the braking side is achieved by the transmission230.

According to the traveling conversion function, when the traveling speed exceeds a predetermined speed, the required output torque has a value on the braking side. Therefore, in the required output torque determination unit315, when the traveling speed calculated by the vehicle state calculation unit313exceeds an upper limit of the speed range specified by the shift operation amount, the accelerator operation amount, and the brake operation amount, the required output torque has a value on the braking side (code opposite to the traveling speed).

The traveling load estimation unit316estimates a traveling load torque Tloadrelating to the traveling based on the output torque Tengof the engine210, angular acceleration αengof the engine210, a PTO torque TPTO, an input/output speed ratio i of the transmission230, and angular acceleration αoutof the output shaft of the transmission230which are calculated by the vehicle state calculation unit313.

The traveling load torque Tloadcan be calculated based on Equation (1) below.

Iengis a moment of inertia of the engine210. Ivis a moment of inertia of the work vehicle100. ηtis torque efficiency of the transmission230. N is an axle reduction ratio from the output shaft of the transmission230to the front wheel part130and the rear wheel part140. The moment of inertia Ieng, the moment of inertia Iv, the torque efficiency ηt, and the axle reduction ratio N are constants.

Equation (1) can be derived from Equation (2) showing a relationship between the output torque Tengof the engine210and the output torque Toutof the transmission230, and Equation (3) showing a relationship between the output torque Toutof the transmission230, and the acceleration αoutof the work vehicle100. In another embodiment, the traveling load torque Tloadmay be calculated based on an equation other than Equation (1). For example, instead of Equation (2), an equation for specifying the traveling load torque Tloadmay be derived by using an equation showing a relationship among the pressure of the HST231which is measured by the HST231, a capacity command of the variable capacity pump of the HST231or the pump capacity measured by the pump capacity meter provided in the variable capacity pump, and the output torque Tout. In addition, in another embodiment, when the transmission230includes an electric motor, an equation for specifying the traveling load torque Tloadmay be derived by using a torque command of the electric motor or an electric motor output torque estimated from a voltage/current.

The target rotation speed determination unit317determines a target engine rotation speed used for controlling the engine210, based on a required engine output which is a sum of required traveling power calculated from the required output torque and the traveling speed, and the required PTO output calculated from the required PTO torque and the measurement value of the rotation speed of the engine210. The target rotation speed determination unit317determines a target engine rotation speed based on a rotation speed conversion function that defines a relationship between the required engine output and the engine rotation speed which are determined in advance by design. For example, the rotation speed conversion function may be designed to suppress the rotation of the engine210to the low rotation speed side as much as possible within a range in which the required engine output can be achieved and the engine acceleration is not hindered.

In addition, the target rotation speed determination unit317determines the engine rotation speed (required PTO rotation speed) required for realizing the required flow rate of the variable capacity pump260which is calculated by the required PTO torque determination unit314. The target rotation speed determination unit317determines the required PTO rotation speed based on a rotation speed conversion function that defines a relationship between the required flow rate of the variable capacity pump260and the engine rotation speed which are determined in advance by design. When the target engine rotation speed falls below the required PTO rotation speed, the target rotation speed determination unit317determines the target engine rotation speed as the required PTO rotation speed.

The acceleration torque specifying unit318calculates a target acceleration torque required for rotating the engine210at a target engine rotation speed based on the measurement value of the rotation speed of the engine210which is acquired by the measurement value acquisition unit312and the target engine rotation speed determined by the target rotation speed determination unit317. That is, the acceleration torque specifying unit318determines target engine acceleration from the rotation speed of a difference between the measurement value of the rotation speed of the engine210and the target engine rotation speed, and multiplies the target engine acceleration by the moment of inertia of the engine210, thereby calculating the target acceleration torque.

The target engine torque determination unit319determines a target engine torque which is a torque to be output by the engine210, based on the PTO torque calculated by the vehicle state calculation unit313, the upper limit torque of the engine210, the input/output speed ratio of the transmission230, the required output torque determined by the required output torque determination unit315, and the measurement value of the rotation speed of the engine210. The target engine torque determination unit319calculates a required input torque which is a torque of the engine210required for obtaining the required output torque by multiplying the required output torque by the input/output speed ratio of the transmission230. The target engine torque determination unit319determines the smaller one of the sum of the PTO torque and the required input torque and the maximum value of the engine torque as the target engine torque.

The engine control unit320outputs an engine torque command to the fuel injection device211. Specifically, the engine control unit320outputs the engine torque command indicating the target engine torque determined by the target engine torque determination unit319. The engine control unit320is an example of a drive source control unit.

The brake control determination unit321determines whether or not to perform PTO brake control for obtaining a braking force by driving the variable capacity pump260. Specifically, the brake control determination unit321determines to perform the PTO brake control when the required output torque determined by the required output torque determination unit315has a value on the braking side.

In another embodiment, the brake control determination unit321may determine to perform the PTO brake control when the code of the required output torque on the braking side and an absolute value exceed a certain threshold value.

The target HST pressure specifying unit322determines the pressure of the HST231corresponding to the required output torque determined by the required output torque determination unit315as a target HST pressure (target circuit pressure) which is a control target of the HST231. A relationship between the output torque of the transmission230and the pressure of the HST231is determined by a relationship of a gear ratio between the output shaft and the HST motor which are determined by the design of the transmission230, and the capacity of the HST motor at that time.

The offset torque determination unit323determines the offset torque used for the PTO brake control by multiplying the required input torque by a predetermined coefficient. The required input torque is calculated by multiplying the required output torque determined by the required output torque determination unit315by the input/output speed ratio of the transmission230. The offset torque determination unit323may determine the offset torque by using the required input torque calculated by the target engine torque determination unit319. The coefficient used for determining the offset torque is a value greater than −1 and equal to or smaller than 0. The PTO brake control is performed when the required output torque is a negative value. Accordingly, the offset torque is a positive value. The offset torque is a feedforward correction factor for estimating the PTO brake torque that is expected to be required empirically or experimentally.

The compensation torque determination unit324determines a compensation torque used for the PTO brake control by performing proportional integral (PI) control based on a difference between an actual HST pressure (actual circuit pressure) which is the measurement value of the pressure of the HST231acquired by the measurement value acquisition unit312and a target HST pressure specified by the target HST pressure specifying unit. The compensation torque determination unit324according to another embodiment may determine the compensation torque by performing other feedback control based on the actual HST pressure and the target HST pressure. The compensation torque is a feedback correction factor for accurately controlling the braking force in accordance with the target.

The assist torque determination unit325determines an assist torque for assisting the rotation acceleration of the engine210, based on the measurement value of the rotation speed of the engine210which is acquired by the measurement value acquisition unit312. For example, the assist torque determination unit325performs P control based on a difference between the smaller of the target rotation speed of the engine and an assist guaranteed rotation speed and the measurement value of the rotation speed of the engine210. In this manner, the assist torque determination unit325determines a low rotation assist torque for assisting the rotation acceleration of the engine210when the rotation speed of the engine210is lower than the assist guaranteed rotation speed. In order to secure the power for the traveling or the work machine, the assist guaranteed rotation speed is set so that the engine rotation speed does not fall below a prescribed rotation speed.

In addition, the assist torque determination unit325performs the P control based on a difference between the rotation speed of the engine required for realizing the required flow rate of the variable capacity pump260calculated by the required PTO torque determination unit314and the measurement value of the rotation speed of the engine210. In this manner, the assist torque determination unit325determines a work machine assist torque for assisting the rotation acceleration of the engine210when the hydraulic oil is required for driving the work machine120. The assist torque determined by the assist torque determination unit325is a value equal to or greater than 0. The assist torque is a correction factor for assisting the engine rotation acceleration when it is necessary to quickly increase the engine rotation.

The brake torque determination unit326determines the brake torque consumed by the variable capacity pump260in the PTO brake control, based on the offset torque, the compensation torque, and the assist torque. Specifically, the brake torque determination unit326determines the brake torque by adding the offset torque and the compensation torque and subtracting the assist torque.

The target speed ratio determination unit327determines a target input/output speed ratio of the transmission230based on the measurement value of the rotation speed of the input shaft of the transmission230, the measurement value of the rotation speed of the output shaft of the transmission230, the traveling load torque estimated by the traveling load estimation unit316, the target output torque determined by the required output torque determination unit315, and the target engine acceleration specified by the acceleration torque specifying unit318. Specifically, the target speed ratio determination unit327estimates the rotation speed of the output shaft of the transmission230after the lapse of time relating to a predetermined control cycle, and sets the rotation speed of the output shaft of the transmission230as the target rotation speed, based on the rotation speed of the output shaft of the transmission230, the traveling load torque, and the target output torque. The target speed ratio determination unit327estimates the rotation speed of the input shaft of the transmission230after the lapse of time relating to a predetermined control cycle, and sets the rotation speed of the input shaft of the transmission230as the target rotation speed of the input shaft, based on the rotation speed of the input shaft of the transmission230and the target engine acceleration. The target speed ratio determination unit327determines a target input/output speed ratio by dividing the target rotation speed of the output shaft by the target rotation speed of the input shaft.

The transmission control unit328outputs a control command of the transmission230in order to realize the target input/output speed ratio determined by the target speed ratio determination unit327. For example, the transmission control unit328outputs a capacity command of the HST231included in the transmission230.

The pump control unit329outputs a control command of the variable capacity pump260in order to realize an absorption torque which is a sum of the required PTO torque determined by the required PTO torque determination unit314and the brake torque determined by the brake torque determination unit326. In addition, when the PTO brake control is performed, the pump control unit329outputs a control command of the brake valve265in order to realize the brake torque.

(Control Method for Work Vehicle)

FIG.5is a flowchart showing a control method for the work vehicle according to the first embodiment.

First, the operation amount acquisition unit311acquires the operation amount from each of the accelerator pedal152, the brake pedal153, the steering wheel154, the front/rear selection switch155, the shift switch156, the boom lever157, and the bucket lever158(Step S1). In addition, the measurement value acquisition unit312acquires the measurement values from the fuel injection device211, the engine tachometer212, the input shaft tachometer232, the output shaft tachometer233, the HST pressure gauge234, the first pump pressure gauge262, the pump capacity meter263, the cylinder pressure gauge264, and the second pump pressure gauge271(Step S2).

Next, based on the measurement values acquired in Step S2, the vehicle state calculation unit313calculates the output torque of the engine210, the upper limit torque of the engine210, the angular acceleration of the engine210, the PTO torque, the input/output speed ratio of the transmission230, the angular acceleration of the output shaft of the transmission230, and the traveling speed of the work vehicle100(Step S3).

The required PTO torque determination unit314determines the required PTO torque based on the measurement values of the steering operation amount, the boom operation amount, and the bucket operation amount which are acquired in Step S1, the pump pressure and the capacity of the variable capacity pump260which are acquired in Step S2, and the pump pressure of the fixed capacity pump270(Step S4). The required output torque determination unit315determines the required output torque based on the operation amount relating to the traveling acquired in Step S1and the traveling speed calculated in Step S3(Step S5). The traveling load estimation unit316estimates the traveling load torque based on the value of the vehicle state calculated in Step S3(Step S6).

The target rotation speed determination unit317determines the target engine rotation speed based on the required engine output which is a sum of the required traveling power calculated from the required output torque and the traveling speed, and the required PTO output calculated from the required PTO torque and the measurement value of the rotation speed of the engine210(Step S7). The acceleration torque specifying unit318calculates the target acceleration torque based on the measurement value of the rotation speed of the engine210and the target engine rotation speed determined in Step S7(Step S8). The target engine torque determination unit319determines the target engine torque based on the measurement values of the required output torque, the PTO torque calculated in Step S3, the upper limit torque of the engine, the input/output speed ratio of the transmission230, and the rotation speed of the engine210acquired in Step S2(Step S9). The engine control unit320outputs an engine torque command indicating the target engine torque determined in Step S9(Step S10).

Next, the brake control determination unit321determines whether or not the required output torque determined in Step S5has a value on the braking side (Step S11). When the required output torque has a value on the drive side (Step S11: NO), the brake control determination unit321determines that it is not necessary to perform the PTO brake control.

When the required output torque has a value on the braking side (Step S11: YES), the brake control determination unit321determines that it is necessary to perform the PTO brake control. When the required output torque has a value on the braking side (Step S11: YES), the target HST pressure specifying unit322determines the target HST pressure which is the control target of the HST231, based on the required output torque determined in Step S5(Step S12). The offset torque determination unit323determines the offset torque by multiplying the required input torque by a predetermined coefficient (Step S13).

The compensation torque determination unit324determines the compensation torque by performing the PI control based on a difference between the actual HST pressure which is the measurement value of the pressure of the HST231acquired by the measurement value acquisition unit312, and the target HST pressure specified by the target HST pressure specifying unit (Step S14). When the actual HST pressure is lower than the target HST pressure, the compensation torque has a positive value. When the actual HST pressure is higher than the target HST pressure, the compensation torque has a negative value.

The assist torque determination unit325determines the low rotation assist torque by performing the P control based on a difference between the smaller of the target rotation speed of the engine and the assist guaranteed rotation speed and the measurement value of the rotation speed of the engine210(Step S15). In addition, the assist torque determination unit325determines the work machine assist torque by performing the P control based on a difference between the rotation speed of the engine required for an operator to quickly realize the work machine speed, and the measurement value of the rotation speed of the engine210(Step S16).

The brake torque determination unit326determines the brake torque by subtracting the low rotation assist torque and the work machine assist torque from the torque value obtained by adding the offset torque and the compensation torque (Step S17).

The target speed ratio determination unit327determines the target input/output speed ratio based on the measurement value of the rotation speed of the input shaft of the transmission230, the measurement value of the rotation speed of the output shaft of the transmission230, the load torque, the target output torque, and the target engine acceleration (Step S18). The transmission control unit328outputs a control command of the transmission230for achieving the target input/output speed ratio (Step S19).

The pump control unit329specifies the capacity of the variable capacity pump260required for realizing the absorption torque which is a sum of the required PTO torque determined in Step S4and the brake torque determined in Step S17, and outputs a control command indicating the capacity to the variable capacity pump260(Step S20).

In addition, the pump control unit329determines whether or not to perform the PTO brake control based on a determination result in Step S11(Step S21). In Step S11, the brake control determination unit321determines to perform the PTO brake control when the required output torque has a value on the braking side.

When the PTO brake control is performed (Step S21: YES), the pump control unit329outputs a control command instructing the brake valve265to open in accordance with the brake torque (Step S22). In this manner, the hydraulic oil is discharged from the relief valve266when the work vehicle100is braked. Therefore, out of the torques distributed from the PTO220to the variable capacity pump260, the torque which is not used for controlling the work machine120and controlling the steering is consumed as heat.

The control device300performs the above-described control processes at every predetermined control cycle.

As described above, according to the control device300of the first embodiment, the brake torque which is the torque consumed by the variable capacity pump260is determined based on the target HST pressure and the actual HST pressure of the HST231, and the variable capacity pump260is controlled based on the brake torque. In this manner, the control device300feeds back the torque of the output shaft of the transmission230by the HST pressure. Accordingly, the control device300can accurately control the braking force of the work vehicle100even when the power system of the work vehicle100does not include the electrical hardware.

In addition, according to the control device300of the first embodiment, the required output torque is specified based on the operation amount of the operation device and the traveling speed of the work vehicle100, and the target HST pressure is determined based on the required output torque. In this manner, the control device300enables the work vehicle100to achieve the braking force in accordance with the operation amount of the operator.

In addition, according to the control device300of the first embodiment, the brake torque is determined based on the sum of the offset torque obtained by multiplying the required output torque by the speed ratio and the predetermined coefficient, and the compensation torque obtained by performing the feedback control based on the difference between the target HST pressure and the actual HST pressure. The control device300can improve followability of the braking immediately after the braking starts by using the offset torque, or can achieve both high responsiveness and stability of the braking force. Therefore, the control device300can improve accuracy in a final braking force by using the compensation torque. Furthermore, the control device300according to the first embodiment determines the assist torque based on the rotation speed of the drive source, and determines the brake torque by subtracting the assist torque from the sum of the offset torque and the compensation torque. In this manner, the control device300can further use a portion of the braking force to assist the rotation acceleration of the engine210.

In another embodiment, the control device300may determine the brake torque without using at least any one of the offset torque and the assist torque.

The embodiment has been described above in detail with reference to the drawings. However, the specific configuration is not limited to the above-described embodiment, and various design changes can be made.

In addition, the work vehicle100according to the first embodiment is the wheel loader. However, the configuration is not limited thereto. For example, the work vehicle100according to another embodiment may be another work vehicle100such as a bulldozer or a tractor. In addition, in another embodiment, the control device300may be applied to a power machine other than a work vehicle.

According to at least one of the above-described aspects, the control device can accurately control a braking force of a power device regardless of whether or not the power device includes electrical hardware.