Patent Description:
Patent Literature <NUM> discloses a work vehicle (tractor) including a hydrostatic, continuously variable transmission device (continuously variable transmission section), a planetary transmission device (compound planetary power transmission section, speed change and output section), a forward/backward travel switching device, a speed change operation tool (shift lever), and a forward/backward travel switching tool (forward-reverse lever). The continuously variable transmission device receives and varies motive power from the engine and outputs the varied motive power. The planetary transmission device composites motive power from the engine and motive power from the continuously variable transmission device and outputs the composite motive power. The planetary transmission device also varies the composite motive power in response to the continuously variable transmission device being varied. The forward/backward travel switching device is switchable between a forward-travel power transmission state and a backward-travel power transmission state. In the forward-travel power transmission state, the forward/backward travel switching device switches the composite motive power from the planetary transmission device into forward-travel motive power and outputs the forward-travel motive power to travel devices (front wheels, rear wheels). In the backward-travel power transmission state, the forward/backward travel switching device switches the composite motive power from the planetary transmission device into backward-travel motive power and outputs the backward-travel motive power to the travel devices. The speed change operation tool is for use to vary the continuously variable transmission device to change the vehicle speed. The forward/backward travel switching tool is for use to switch the forward/backward travel switching device. Patent Literature <NUM> discloses a working vehicle including a vehicle body provided with a traveling device having a transmission device which comprises a hydraulic pump including a swashplate configured to change an output of the hydraulic pump according to a swashplate angle, the swashplate being controlled by a controller.

The work vehicle (tractor) configured as above has a vehicle speed determined on the basis of the rotation speed (number of revolutions) of the composite motive power. The rotation speed determined the ratio (gear ratio) between the number of revolutions of the engine and the number of revolutions of the output shaft. The rotation speed of the composite motive power is determined on the basis of the speed range of the planetary transmission device and motive power (swash plate angle) outputted from the continuously variable transmission device. The speed range is determined through an operation of clutches of the planetary transmission device. The continuously variable transmission device has a swash plate that is tilted across a neutral position to and from a state (-MAX) in which the swash plate is maximally tilted to one side and a state (+MAX) in which the swash plate is maximally tilted to the other side. The continuously variable transmission device, in other words, alternates between normal rotation and reverse rotation. Each change in the speed range switches the direction in which the swash plate is tilted and thereby switches between normal rotation and reverse rotation. This gradually changes the rotation speed of the composite motive power.

The continuously variable transmission device has a swash plate angle controlled with use of operating oil supplied from a hydraulic cylinder controlled with use of two electromagnetic valves, which are in turn controlled on the basis of the electric current value of a control signal controlled as the duty ratio of a pulse-width modulated (PWM) signal.

An electromagnetic valve has production variation in, for example, the internal resistance of its coil, and may receive electric current with a value (current electric current value) that varies relative to a target electric current value for the control signal, with the result of variation in operation between different electromagnetic valves. The internal resistance, for example, of the coil may be changed during the operation by an environmental factor such as the ambient temperature or electric current application.

In view of the above issue, the work vehicle is configured to control the current electric current value relative to a target electric current value through PWM-based proportional-integral (PI) control involving a feed-forward (FF) term (or environmental coefficient) and optimize and update the FF term with use of a learning function while the work vehicle is traveling.

The work vehicle is configured to, once it stops traveling (that is, the engine is turned off), initialize the FF term, as the environment may differ when the work vehicle restarts to travel. The FF term may thus be unsuitable for the environment when the work vehicle starts to travel. This may prevent appropriate PI control, and consequently prevent appropriate control of the electromagnetic valves.

The present invention has an object of accurately controlling electromagnetic valves even immediately after the start of the engine.

In order to attain the above object, an embodiment of the present invention relates to an electromagnetic valve control device configured to control an angle of a swash plate of a hydrostatic, continuously variable transmission device with use of at least one electromagnetic valve, when the hydrostatic, continuously variable transmission device varies motive power from a drive section, and a gear transmission composites motive power from the drive section and the varied motive power from the hydrostatic, continuously variable transmission device, varies the composite motive power, and outputs the varied composite motive power, the electromagnetic valve control device comprising a control signal generator configured to, through feedback control involving an environmental coefficient, generate a control signal intended for the at least one electromagnetic valve and corresponding to a target electric current value; a coefficient determiner configured to determine the environmental coefficient during a learning period extending from a start of the drive section over a predetermined time length; and an operation limiter configured to limit respective operations of the at least one electromagnetic valve and the gear transmission during the learning period.

A work vehicle according to an embodiment of the present invention includes: a drive section; a hydrostatic, continuously variable transmission device configured to vary motive power from the drive section and output the varied motive power; a gear transmission configured to composite motive power from the drive section and motive power from the hydrostatic, continuously variable transmission device, vary the composite motive power, and output the varied composite motive power; an electromagnetic valve configured to control a swash plate of the hydrostatic, continuously variable transmission device; a control signal generator configured to control the electromagnetic valve through feedback control involving an environmental coefficient; a coefficient determiner configured to determine the environmental coefficient during a learning period extending from a start of the drive section over a predetermined time length; and an operation limiter configured to limit respective operations of the at least one electromagnetic valve and the gear transmission during the learning period, wherein the work vehicle is configured to travel on motive power from the gear transmission.

An electromagnetic valve control device configured to generate a control signal for an electromagnetic valve through feedback control involving an environmental coefficient updates the environmental coefficient while the work vehicle is traveling to prevent the control signal from becoming less accurate due to a change in the state of the environment. The environmental coefficient is, however, not optimal immediately after the drive section has started. An electromagnetic valve control device is thus unable to easily generate a control signal accurately and thereby control the electromagnetic valve accurately at an initial stage at which the work vehicle has just started to travel (or travel and perform work), that is, until the electromagnetic valve control device updates the environmental coefficient to an appropriate value.

Each embodiment above is capable of learning an initial coefficient during a learning period after the drive section has started and determining an environmental coefficient corresponding to the current state of the environment. This in turn makes it possible to generate a control signal with use of an environmental coefficient corresponding to the current state of the environment even when the work vehicle starts to travel (or travel and perform work) after the learning period. Each embodiment above is therefore capable of generating a control signal accurately and thereby controlling the electromagnetic valve accurately when the work vehicle starts to travel (or travel and perform work) after the learning period.

The electromagnetic valve control device may be further configured such that the control signal is a pulse signal, and the coefficient determiner may determine the environmental coefficient for which a difference between the target electric current value and a current electric current value for the control signal is not larger than a predetermined value when the target electric current value and a duty ratio of the pulse signal are varied.

The above configuration makes it possible to accurately determine an environmental coefficient for generating a control signal corresponding to a target electric current value.

The electromagnetic valve control device may be further configured such that the control signal is a pulse signal, and the coefficient determiner determines the environmental coefficient by inputting the target electric current value, a duty ratio of the pulse signal, and a current electric current value for the control signal into a learned model machine-learned to output the environmental coefficient in response to receiving the target electric current value, the duty ratio, and the current electric current value.

The above configuration allows an environmental coefficient to be determined more easily and accurately.

The electromagnetic valve control device may be further configured such that the gear transmission varies motive power with use of a plurality of clutches, and the coefficient determiner determines the environmental coefficient when the drive section is in operation and the clutches are all in a power transmission disconnected state.

The above configuration makes it possible to determine an environmental coefficient with the gear transmission in a stable state, thereby making it possible to determine an environmental coefficient early and accurately.

The electromagnetic valve control device may be further configured such that the hydrostatic, continuously variable transmission device and the gear transmission are each operable in response to an external operation, and the operation limiter disables the hydrostatic, continuously variable transmission device and the gear transmission from accepting the external operation during the learning period.

The above configuration makes it possible to determine an environmental coefficient accurately while preventing the target electric current value from being changed.

The electromagnetic valve control device may be further configured such that the coefficient determiner continues to determine and update the environmental coefficient after the learning period has elapsed.

The above configuration makes it possible to generate a control signal with use of an environmental coefficient corresponding to the current state of the environment even while the work vehicle is traveling (or traveling and performing work), making it possible to generate a control signal accurately and thereby control the electromagnetic valve accurately.

The electromagnetic valve control device may be further configured such that the at least one electromagnetic valve includes: a first electromagnetic valve configured to rotationally move the swash plate in a normal rotation direction; and a second electromagnetic valve configured to rotationally move the swash plate in a reverse rotation direction.

The above configuration makes it possible to generate a control signal accurately and thereby control the electromagnetic valves accurately regardless of configuration of the electromagnetic valves.

The electromagnetic valve control device may be further configured such that the feedback control is proportional-integral control based on pulse width modulation and involving a feed-forward term as the environmental coefficient.

The above configuration makes it possible to generate a control signal accurately.

The description below deals with a tractor as an example of the work vehicle of the present invention with reference to drawings. The embodiment described below is of a tractor including a body. <FIG> shows arrow F to indicate the forward side of the body, arrow B to indicate the backward side of the body, arrow U to indicate the upward side of the body, and arrow D to indicate the downward side of the body. The front side of <FIG> corresponds to the left side of the body, whereas the back side of <FIG> corresponds to the right side of the body.

As illustrated in <FIG>, the tractor includes a pair of left and right turnable and drivable front wheels <NUM> ("travel device"), a pair of left and right drivable rear wheels <NUM> ("travel device"), and a body <NUM> supported by the front and rear wheels <NUM> and <NUM>. The tractor includes a motive section <NUM> including an engine <NUM> ("drive section") at a front portion of the body <NUM>. The tractor includes a driver section <NUM> and a link mechanism <NUM> at a back portion of the body <NUM>. The driver section <NUM> is configured to accommodate an operator for driving the tractor. The link mechanism <NUM> is configured to couple an implement such as a rotary tiller device to the tractor in such a manner that the implement is capable of being lifted and lowered. The driver section <NUM> includes a driver's seat <NUM>, a steering wheel <NUM> for use to turn the front wheels <NUM>, and a cabin <NUM> defining a driver space. The body <NUM> includes a body frame <NUM> including an engine <NUM>, a transmission case <NUM>, and front-wheel support frame members <NUM>. The transmission case <NUM> has a front portion coupled to a back portion of the engine <NUM>. The front-wheel support frame members <NUM> are coupled to a lower portion of the engine <NUM>. The tractor includes a power takeoff shaft <NUM> at a back portion of the transmission case <NUM>. The power takeoff shaft <NUM> is configured to take off motive power from the engine <NUM> and transmits the motive power to the implement coupled with use of the link mechanism <NUM>.

As illustrated in <FIG>, the tractor includes a power transmission device <NUM> for travel, a rear-wheel differential mechanism <NUM>, and a front-wheel differential mechanism <NUM>. The power transmission device <NUM> is configured to transmit motive power from the engine <NUM> to the front and rear wheels <NUM> and <NUM>. The power transmission device <NUM> includes a transmission <NUM> contained in the transmission case <NUM> and configured to vary motive power from the engine <NUM> and transmit the varied motive power to the rear-wheel differential mechanism <NUM> and the front-wheel differential mechanism <NUM>.

As illustrated in <FIG>, the transmission <NUM> includes an input shaft <NUM>, a main transmission section <NUM>, a forward/backward travel switching device <NUM>, a gear mechanism <NUM>, and a front-wheel power transmission section <NUM>. The input shaft <NUM> is disposed at a front portion of the transmission case <NUM> and configured to receive motive power from the output shaft 4a of the engine <NUM>. The main transmission section <NUM> is configured to receive motive power from the input shaft <NUM>, vary the motive power, and output the varied motive power. The forward/backward travel switching device <NUM> is configured to receive the motive power outputted from the main transmission section <NUM> and switch the rotation direction of the motive power between a forward-travel direction and a backward-travel direction. The gear mechanism <NUM> is configured to transmit the output from the forward/backward travel switching device <NUM> to the input shaft 16a of the rear-wheel differential mechanism <NUM>. The front-wheel power transmission section <NUM> is configured to receive motive power outputted from the forward/backward travel switching device <NUM>, vary the motive power, and output the varied motive power to the front-wheel differential mechanism <NUM>.

As illustrated in <FIG>, the main transmission section <NUM> includes a continuously variable transmission device <NUM> and a planetary transmission device <NUM> ("gear transmission"). The continuously variable transmission device <NUM> is configured to receive motive power from the input shaft <NUM>. The planetary transmission device <NUM> is configured to receive motive power from the input shaft <NUM> and the output from the continuously variable transmission device <NUM>.

The continuously variable transmission device <NUM> is of a hydraulic type (that is, a hydrostatic transmission or HST), and includes a hydraulic pump P with a variable capacity and a hydraulic motor M. The continuously variable transmission device <NUM> is configured to vary motive power from the input shaft <NUM> into normal-direction motive power or reverse-direction motive power in accordance with the swash plate angle of the hydraulic pump P. The continuously variable transmission device <NUM> is also configured to continuously vary the rotation speed (that is, the number of revolutions) of the normal-direction motive power or reverse-direction motive power in accordance with the swash plate angle and output the resulting motive power from its motor shaft 28b. As illustrated in <FIG>, the continuously variable transmission device <NUM> includes a pump shaft 28a coupled. to the hydraulic pump P as well as to the input shaft <NUM> with a rotary shaft <NUM> and a first gear mechanism <NUM> in-between. The input shaft <NUM> is coupled to a front end portion of the rotary shaft <NUM>, which has a back end portion coupled to the first gear mechanism <NUM>, which is then coupled to the pump shaft 28a. The hydraulic motor M is configured to output to the motor shaft 28b motive power corresponding to pressure oil supplied from the hydraulic pump P.

As illustrated in <FIG>, the planetary transmission device <NUM> includes a planetary transmission unit 31A and an output section 31B. The planetary transmission unit 31A is configured to receive motive power from the input shaft <NUM> and the output from the continuously variable transmission device <NUM>. The output section 31B is configured to receive motive power from the planetary transmission unit 31A and output the motive power in one of four speed ranges. As illustrated in <FIG> and <FIG>, the planetary transmission unit 31A includes a first planetary transmission section <NUM> and a second planetary transmission section <NUM> backward of the first planetary transmission section <NUM>. The first planetary transmission section <NUM> includes (i) a first sun gear 32a, (ii) a first planetary gear 32b meshing with the first sun gear 32a, and (iii) a first ring gear 32c including inner teeth meshing with the first planetary gear 32b. The second planetary transmission section <NUM> includes (i) a second sun gear 33a, (ii) a second planetary gear 33b meshing with the second sun gear 33a, (iii) a second ring gear 33c including inner teeth meshing with the second planetary gear 33b, and (iv) a second carrier 33d holding the second planetary gear 33b.

As illustrated in <FIG>, the main transmission section <NUM> includes a second gear mechanism <NUM> extending from the first sun gear 32a to the motor shaft 28b of the continuously variable transmission device <NUM> and configured to transmit the output from the continuously variable transmission device <NUM> to the first sun gear 32a. The main transmission section <NUM> includes a third gear mechanism <NUM> extending from the first ring gear 32c to the input shaft <NUM> and configured to transmit motive power from the input shaft <NUM> to the first ring gear 32c. As illustrated in <FIG> and <FIG>, the first planetary transmission section <NUM> includes an interlocking gear 32d meshing with the first planetary gear 32b and coupled in an interlocked manner to the second planetary gear 33b with use of a coupler 33e. The first and second planetary transmission sections <NUM> and <NUM> constitute a so-called compound planetary transmission section.

As illustrated in <FIG> and <FIG>, the output section 31B includes a first input shaft 34a, a second input shaft 34b, and a third input shaft 34c in a triple-shaft structure as well as an output shaft <NUM> parallel to, for example, the first input shaft 34a. The first input shaft 34a is coupled to the second ring gear 33c. The second input shaft 34b is coupled to the second carrier 33d. The third input shaft 34c is coupled to the second sun gear 33a. The first input shaft 34a is coupled to a first range gear mechanism 36a. The main transmission section <NUM> includes a first clutch CL1 extending from the first range gear mechanism 36a to the output shaft <NUM>. The third input shaft 34c is coupled to a second range gear mechanism 36b. The main transmission section <NUM> includes a second clutch CL2 extending from the second range gear mechanism 36b to the output shaft <NUM>. The second input shaft 34b is coupled to a third range gear mechanism 36c. The main transmission section <NUM> includes a third clutch CL3 extending from the third range gear mechanism 36c to the output shaft <NUM>. The third input shaft 34c is coupled to a fourth range gear mechanism 36d. The main transmission section <NUM> includes a fourth clutch CL4 extending from the fourth range gear mechanism 36d to the output shaft <NUM>.

The main transmission section <NUM> is configured as follows: The motive power output from the engine <NUM> transmits through the input shaft <NUM>, the rotary shaft <NUM>, and the first gear mechanism <NUM> to the hydraulic pump P of the continuously variable transmission device <NUM>. The continuously variable transmission device <NUM> outputs the motive power from its motor shaft 28b as normal-direction motive power or reverse-direction motive power. The continuously variable transmission device <NUM> continuously varies the rotation speed (that is, the number of revolutions) of the normal-direction motive power or reverse-direction motive power. The continuously variable transmission device <NUM> transmits the motive power through the second gear mechanism <NUM> to the first sun gear 32a of the first planetary transmission section <NUM>. The engine <NUM> transmits motive power through the input shaft <NUM> and the third gear mechanism <NUM> to the first ring gear 32c of the first planetary transmission section <NUM>. The first and second planetary transmission sections <NUM> and <NUM> composite (i) the motive power outputted from the continuously variable transmission device <NUM> to the first ring gear 32c and (ii) the motive power outputted from the engine <NUM> to the first ring gear 32c. The second planetary transmission section <NUM> transmits the composite motive power to the output section 31B, which then outputs the composite motive power from its output shaft <NUM>.

The main transmission section <NUM> is configured as follows: With the first clutch CL1 engaged, the composite motive power as composited by the planetary transmission unit 31A is varied by the first range gear mechanism 36a and first clutch CL1 of the output section 31B into motive power in the first-gear range. The motive power in the first-gear range is transmitted from the second ring gear 33c to the first input shaft 34a of the output section 31B. During this operation, the motive power in the first-gear range is continuously varied through the variation of the continuously variable transmission device <NUM>, and is outputted from the output shaft <NUM>.

With the second clutch CL2 engaged, the composite motive power as composited by the planetary transmission unit 31A is varied by the second range gear mechanism 36b and second of the output section 31B into motive power in the second-gear range. The motive power in the second-gear range is transmitted from the second sun gear 33a to the third input shaft 34c of the output section 31B. During this operation, the motive power in the second-gear range is continuously varied through the variation of the continuously variable transmission device <NUM>, and is outputted from the output shaft <NUM>.

With the third clutch CL3 engaged, the composite motive power as composited by the planetary transmission unit 31A is varied by the third range gear mechanism 36c and third clutch CL3 of the output section 31B into motive power in the third-gear range. The motive power in the third-gear range is transmitted from the second carrier 33d to the second input shaft 34b of the output section 31B. During this operation, the motive power in the third-gear range is continuously varied through the variation of the continuously variable transmission device <NUM>, and is outputted from the output shaft <NUM>.

With the fourth clutch CL4 engaged, the composite motive power as composited by the planetary transmission unit 31A is varied by the fourth range gear mechanism 36d and fourth clutch CL4 of the output section 31B into motive power in the fourth-gear range. The motive power in the fourth-gear range is transmitted from the second sun gear 33a to the third input shaft 34c of the output section 31B. During this operation, the motive power in the fourth-gear range is continuously varied through the variation of the continuously variable transmission device <NUM>, and is outputted from the output shaft <NUM>.

As illustrated in <FIG>, the forward/backward travel switching device <NUM> includes an input shaft 23a, an output shaft 23b, a forward-travel gear interlocking mechanism 23c, and a backward-travel gear interlocking mechanism 23d. The input shaft 23a is coupled to the output shaft <NUM> of the planetary transmission device <NUM>. The output shaft 23b is parallel to the input shaft 23a. The input shaft 23a is provided with a forward clutch CLF and a reverse clutch CLR. The forward-travel gear interlocking mechanism 23c extends from the forward clutch CLF to the output shaft 23b. The backward-travel gear interlocking mechanism 23d extends from the reverse clutch CLR to the output shaft 23b.

Engaging the forward clutch CLF couples the input shaft 23a to the forward-travel gear interlocking mechanism 23c. This achieves a forward-travel power transmission state, in which motive power from the input shaft 23a is transmitted through the forward-travel gear interlocking mechanism 23c to the output shaft 23b. Engaging the reverse clutch CLR couples the input shaft 23a to the backward-travel gear interlocking mechanism 23d. This achieves a backward-travel power transmission state, in which motive power from the input shaft 23a is transmitted through the backward-travel gear interlocking mechanism 23d to the output shaft 23b.

The forward/backward travel switching device <NUM> receives the output from the planetary transmission device <NUM> at the input shaft 23a. Engaging the forward clutch CLF causes motive power from the input shaft 23a to be converted by the forward clutch CLF and the forward-travel gear interlocking mechanism 23c into forward-travel motive power to be transmitted to the output shaft 23b. Engaging the reverse clutch CLR causes motive power from the input shaft 23a to be converted by the reverse clutch CLR and the backward-travel gear interlocking mechanism 23d into backward-travel motive power to be transmitted to the output shaft 23b. The output shaft 23b transmits the forward-travel motive power and backward-travel motive power through the gear mechanism <NUM> to the rear-wheel differential mechanism <NUM> and the front-wheel power transmission section <NUM>.

The rear-wheel differential mechanism <NUM> receives the forward-travel motive power or backward-travel motive power from the forward/backward travel switching device <NUM>, and transmits the motive power from a pair of left and right output shafts 16b to the respective rear wheels <NUM>. The left output shaft 16b transmits its motive power through a planetary deceleration mechanism <NUM> to the left rear wheel <NUM>. The left output shaft 16b is provided with a steering brake <NUM>. The right output shaft 16b transmits its motive power to the right rear wheel <NUM> in a system including a planetary deceleration mechanism <NUM> and a steering brake <NUM> (not illustrated in the drawing) similarly to the power transmission system for the left rear wheel <NUM>. The body <NUM> (see <FIG>) is easily turnable in accordance with how each steering brake <NUM> is operated.

As illustrated in <FIG>, the front-wheel power transmission section <NUM> includes an input shaft 25a and an output shaft 25b. The input shaft 25a is coupled to the output shaft 24a of the gear mechanism <NUM>. The output shaft 25b is parallel to the input shaft 25a. The input shaft 25a is provided with a constant-rate clutch CLT and a rate-increasing clutch CLH backward of the constant-rate clutch CLT. The front-wheel power transmission section <NUM> includes a constant-rate gear mechanism <NUM> extending from the constant-rate clutch CLT to the output shaft 25b and a rate-increasing gear mechanism <NUM> extending from the rate-increasing clutch CLH to the output shaft 25b. The output shaft 24a of the gear mechanism <NUM> is provided with a parking brake <NUM>.

The front-wheel power transmission section <NUM> is configured as follows: Engaging the constant-rate clutch CLT causes motive power from the input shaft 25a to be transmitted through the constant-rate clutch CLT and the constant-rate gear mechanism <NUM> to the output shaft 25b. The constant-rate gear mechanism <NUM> achieves a constant-rate power transmission state, in which the output shaft 25b outputs motive power for driving the front wheels <NUM> such that the front wheels <NUM> have a circumferential speed equal to that of the rear wheels <NUM>. Engaging the rate-increasing clutch CLH causes motive power from the input shaft 25a to be transmitted through the rate-increasing clutch CLH and the rate-increasing gear mechanism <NUM> to the output shaft 25b. The rate-increasing gear mechanism <NUM> achieves a front-wheel rate-increasing power transmission state, in which the output shaft 25b outputs motive power for driving the front wheels <NUM> such that the front wheels <NUM> have a circumferential speed higher than that of the rear wheels <NUM>. The output from the output shaft 25b is received by the front-wheel differential mechanism <NUM> through a rotary shaft <NUM> coupling the output shaft 25b to the input shaft 17a of the front-wheel differential mechanism <NUM>.

The body <NUM> (see <FIG>) is configured as follows: Engaging the constant-rate clutch CLT leads to a four-wheel drive mode in which the front and rear wheels <NUM> and <NUM> are driven such that the front wheels <NUM> have an average circumferential speed equal to that of the rear wheels <NUM>. Engaging the rate-increasing clutch CLH leads to a four-wheel drive mode in which the front and rear wheels <NUM> and <NUM> are driven such that the front wheels <NUM> have an average circumferential speed higher than that of the rear wheels <NUM>. Engaging the rate-increasing clutch CLH allows the body <NUM> to turn with a radius smaller than when the constant-rate clutch CLT is engaged.

The continuously variable transmission device <NUM> is controlled by the hydraulic circuit illustrated in <FIG> as an example. The hydraulic circuit includes a hydraulic cylinder <NUM>, a speed change valve unit <NUM> including electromagnetically operated valves, and a hydraulic pump <NUM>. The continuously variable transmission device <NUM> is controlled on the basis of a change (tilt) in the angle (swash plate angle) of a swash plate <NUM> ("pump swash plate") of the hydraulic pump P, and outputs from the hydraulic motor M motive power corresponding to the swash plate angle of the hydraulic pump P. The swash plate <NUM> is controlled on the basis of the amount of operating oil supplied from the hydraulic pump <NUM> through the hydraulic cylinder <NUM> and the pressure of the operating oil (operating oil pressure). The operating oil supplied (or discharged) from the hydraulic cylinder <NUM> is controlled by the speed change valve unit <NUM>.

As illustrated in <FIG>, the hydraulic cylinder <NUM> is coupled to the swash plate <NUM>. The hydraulic cylinder <NUM> includes two oil chambers 50A and 50B. The speed change valve unit <NUM> is connected to the hydraulic cylinder <NUM> through an operating oil path <NUM>, and controls the hydraulic cylinder <NUM> to cause the hydraulic cylinder <NUM> to discharge operating oil to the hydraulic pump P. The hydraulic pump <NUM> is connected to the speed change valve unit <NUM> through a supply oil path <NUM>. The speed change valve unit <NUM> includes electromagnetically operated valves and an electromagnetic operation section 52a, and is controlled on the basis of the duty ratio <NUM> (that is, the electric current value; see <FIG>) of a control signal inputted to the electromagnetic operation section 52a. The hydraulic pump P is connected to the hydraulic motor M through a drive oil path <NUM> connected to an emergency relief valve <NUM>.

The hydraulic circuit is configured such that the speed change valve unit <NUM> is switchable to cause operating oil from the hydraulic pump <NUM> to be supplied from either of the two oil chambers of the hydraulic cylinder <NUM>. The speed change valve unit <NUM> includes a first electromagnetic valve 52A and a second electromagnetic valve 52B. The first electromagnetic valve 52A is configured to cause operating oil to be discharged from the oil chamber 50A to the normal rotation side of the neutral position. The second electromagnetic valve 52B is configured to cause operating oil to be discharged from the oil chamber 50B to the reverse rotation side of the neutral position. As described above, the first and second electromagnetic valves 52A and 52B are each switched across the neutral position of the swash plate <NUM> so that the swash plate <NUM> is tilted to an inclination angle corresponding to the respective positions of the first and second electromagnetic valves 52A and 52B as operated, thereby operating the continuously variable transmission device <NUM>.

With reference to <FIG> and <FIG>, the description below deals with how the main transmission section <NUM> is configured to change the vehicle speed. <FIG> shows a vertical axis indicative of the calculated gear ratio G ("current gear ratio") and the rotation speed V of the input shaft 16a (which corresponds to the vehicle speed). The calculated gear ratio G refers to the ratio of the number of revolutions of the input shaft 16a to the number of revolutions of the input shaft <NUM>. <FIG> shows a horizontal axis indicative of how the continuously variable transmission device <NUM> is varied. The symbol "N" indicates the neutral state. The symbol "-MAX" indicates that the continuously variable transmission device <NUM> has been varied to output reverse-direction motive power for the maximum speed (that is, the largest swash plate angle for the reverse rotation). The symbol "+MAX" indicates that the continuously variable transmission device <NUM> has been varied to output normal-direction motive power for the maximum speed (that is, the largest swash plate angle for the normal rotation). The symbols "G1", "G2", "G3", and "G4" refer to preset gear ratios.

Varying the continuously variable transmission device <NUM> from -MAX toward +MAX with the first clutch CL1 engaged increases the rotation speed V in the first-gear range continuously from zero speed. In response to the calculated gear ratio G reaching G1, speed change control means <NUM> disengages the first clutch CL1 and engages the second clutch CL2. Varying the continuously variable transmission device <NUM> from +MAX toward -MAX with the second clutch CL2 engaged increases the rotation speed V in the second-gear range continuously. In response to the calculated gear ratio G reaching G2, the speed change control means <NUM> disengages the second clutch CL2 and engages the third clutch CL3. Varying the continuously variable transmission device <NUM> from -MAX toward +MAX with the third clutch CL3 engaged increases the rotation speed V in the third-gear range continuously. In response to the calculated gear ratio G reaching G3, the speed change control means <NUM> disengages the third clutch CL3 and engages the fourth clutch CL4. Varying the continuously variable transmission device <NUM> from +MAX toward -MAX with the fourth clutch CL4 engaged increases the rotation speed V in the fourth-gear range continuously.

With reference to <FIG>, <FIG>, and <FIG>, the description below deals with how speed change is controlled.

The driver section <NUM> includes, for example, a shift pedal <NUM> as a speed change operation tool for varying the continuously variable transmission device <NUM>. The driver section <NUM> includes a potentiometer <NUM> configured to detect the position of the shift pedal <NUM> as operated. The present embodiment, which includes a potentiometer <NUM>, may alternatively include any of various position detecting mechanisms such as a mechanism including a detection switch.

The tractor includes a controller <NUM> ("electromagnetic valve control device") configured to control the speed change operation on the basis of the shift pedal <NUM> as operated. The controller <NUM> includes a processor such as a central processing unit (CPU) or electronic control unit (ECU). The controller <NUM> is linked to the continuously variable transmission device <NUM> with the first electromagnetic valve 52A or second electromagnetic valve 52B in-between. The controller <NUM> is also linked to the first to fourth clutches CL1 to CL4 of the planetary transmission device <NUM> ("gear transmission"). The controller <NUM> is configured to detect on the basis of information detected by the potentiometer <NUM> that the shift pedal <NUM> has been operated and vary the continuously variable transmission device <NUM>. The controller <NUM> is also configured to control how the first to fourth clutches CL1 to CL4 are switched.

With reference to <FIG>, <FIG>, and <FIG>, the description below deals with a control signal for controlling the first and second electromagnetic valves 52A and 52B.

The continuously variable transmission device <NUM> includes a hydraulic pump P with a swash plate <NUM> controlled with use of operating oil supplied from the hydraulic cylinder <NUM>, which is controlled by the first and second electromagnetic valves 52A and 52B, which are then each controlled on the basis of a control signal inputted to the electromagnetic operation section 52a. The control signal is represented as the duty ratio <NUM> of a pulse-width modulated (PWM) pulse signal.

The controller <NUM> generates a control signal on the basis of the position of the shift pedal <NUM> as operated. Specifically, the controller <NUM> generates a control signal in correspondence with a target electric current value <NUM> set in correspondence with the position of the shift pedal <NUM> as operated, the position being detected by the potentiometer <NUM>.

The controller <NUM> is linked to an electric current detector 52C and an electric current detector 52D. The electric current detector 52C is configured to measure the value of electric current ("control signal") inputted to the first electromagnetic valve 52A and transmit information on the electric current value to the controller <NUM>. The electric current detector 52D is configured to measure the value of electric current ("control signal") inputted to the second electromagnetic valve 52B and transmit information on the electric current value to the controller <NUM>.

The controller <NUM> functions as an electromagnetic valve control device configured to, as part of the operation of varying the continuously variable transmission device <NUM>, select an electric current value ("control signal") to be transmitted to the first electromagnetic valve 52A or second electromagnetic valve 52B and output information on the electric current value to the first electromagnetic valve 52A or second electromagnetic valve 52B.

The controller <NUM> includes a data communicator <NUM>, a current electric current value obtainer <NUM>, a duty ratio obtainer <NUM>, a coefficient determiner <NUM>, a control signal generator <NUM>, and a storage <NUM>.

The storage <NUM> is configured to store various items of information.

The data communicator <NUM> is connected to elements such as the potentiometer <NUM> and the electric current detectors 52C and 52D in such a manner as to be capable of data communication, and is configured to receive necessary information from those elements. The data communicator <NUM> is also configured to transmit a control signal to elements such as the first and second electromagnetic valves 52A and 52B and the planetary transmission device <NUM> ("gear transmission").

The controller <NUM> obtains information on the position of the shift pedal <NUM> as operated from the potentiometer <NUM> through the data communicator <NUM>. The controller <NUM> calculates a target electric current value <NUM> corresponding to the position to be given to the first electromagnetic valve 52A or second electromagnetic valve 52B, and stores information on the target electric current value <NUM> in the storage <NUM>.

The current electric current value obtainer <NUM> is configured to obtain through the data communicator <NUM> information on a current electric current value <NUM> as the value of electric current being inputted to each of the first and second electromagnetic valves 52A and 52B, and stores the information in the storage <NUM>.

The control signal generator <NUM> is configured to generate a control signal corresponding to the target electric current value <NUM> over time and transmit the control signal to the first electromagnetic valve 52A or second electromagnetic valve 52B. During this operation, the controller <NUM> performs feedback control on the control signal with use of a feed-forward (FF) term. This is to reliably input an appropriate control signal (or electric current value) to the first electromagnetic valve 52A or second electromagnetic valve 52B (hereinafter the two may collectively be referred to simply as "electromagnetic valves") and cause the electromagnetic valve to operate in correspondence with how the speed change operation tool has been operated.

The feedback control is, for example, PWM-based proportional-integral (PI) control. The controller <NUM> performs feed-forward control with use of a FF term and feedback control both on the target electric current value <NUM> to input an appropriate electric current value to the electromagnetic valve. The FF term is an environmental coefficient <NUM> based comprehensively on environmental states such as the ambient temperature. This prevents the electric current value to be inputted to the electromagnetic valve from being unintentionally varied due to a change in, for example, the internal resistance of a coil caused by the environment, and thereby prevents variation in how the electromagnetic valve operates.

Specifically, as illustrated in <FIG>, while the tractor is traveling (that is, the engine is running), a current electric current value <NUM> as the value of electric current being inputted to an electromagnetic valve, that is, a control signal being inputted to the first electromagnetic valve 52A or second electromagnetic valve 52B is detected.

In response to a target electric current value <NUM> being generated, the controller <NUM> proportions the target electric current value <NUM> to the current electric current value <NUM> and integrates the target electric current value <NUM> with the current electric current value <NUM>. The controller <NUM> also performs a FF process on the target electric current value <NUM>. The controller <NUM> then performs pulse-width modulation on the resulting target electric current value <NUM> to generate a control signal with a duty ratio <NUM> corresponding to the target electric current value <NUM>.

The FF term is an environmental coefficient <NUM> that changes constantly due to the environment. The FF term is thus constantly learned and updated while the tractor is traveling (that is, the engine is running). The duty ratio obtainer <NUM>, the coefficient determiner <NUM>, and the control signal generator <NUM> operate to learn the FF term.

The duty ratio obtainer <NUM> is configured to obtain information on the duty ratio <NUM> of a control signal inputted to the first electromagnetic valve 52A or second electromagnetic valve 52B. The duty ratio <NUM> refers to the proportion of a high or low period during a single cycle of a control signal as a pulse signal.

The coefficient determiner <NUM> is configured to, while the tractor is traveling (that is, the engine is running), compare with the target electric current value <NUM> and the current electric current value <NUM> of a control signal that the control signal generator <NUM> has generated, and learn and update the environmental coefficient <NUM> to bring the control signal closer to a signal corresponding to the target electric current value <NUM>.

Specifically, while the tractor is traveling (that is, the engine is running), the control signal generator <NUM> first generates with use of an environmental coefficient <NUM> a control signal corresponding to the target electric current value <NUM>. The coefficient determiner <NUM> determines an electric current deviation <NUM> as the difference between the current electric current value <NUM> and target electric current value <NUM> of the control signal, and stores information on the electric current deviation <NUM> in the storage <NUM>. The coefficient determiner <NUM> stores in the storage <NUM> information on the environmental coefficient <NUM> used.

The control signal generator <NUM> changes the environmental coefficient <NUM> (that is, a FF term) before generating a subsequent control signal, and determines the electric current deviation <NUM>. The coefficient determiner <NUM> compares the current electric current deviation <NUM> with the electric current deviation <NUM> stored in the storage <NUM>. If the current electric current deviation <NUM> is smaller than the electric current deviation <NUM> stored in the storage <NUM>, the coefficient determiner <NUM> replaces the environmental coefficient <NUM> and electric current deviation <NUM> stored in the storage <NUM> with the environmental coefficient <NUM> as changed and the electric current deviation <NUM> as determined. If the current electric current deviation <NUM> is not smaller than the electric current deviation <NUM> stored in the storage <NUM>, the coefficient determiner <NUM> inverts the first environmental coefficient <NUM> (that is, a FF term) in generating a subsequent control signal.

The coefficient determiner <NUM> repeats the above process to learn and update the environmental coefficient <NUM> for generation of a control signal corresponding to the target electric current value <NUM>.

The above configuration keeps optimizing the environmental coefficient <NUM> corresponding to the state of the environment while the tractor is traveling (that is, the engine is running). Thus, even with a change in, for example, the internal resistance caused by production variation or the environment, the above configuration allows accurate generation of a control signal (electric current value) and accurate control of the electromagnetic valves.

Constantly optimizing the environmental coefficient <NUM> while the tractor is traveling (that is, the engine is running) as described above allows accurate generation of a control signal (electric current value) and accurate control of the electromagnetic valves. At the start of the engine <NUM>, however, the current state of the environment may differ from the state in which the tractor was traveling previously, meaning that the environmental coefficient <NUM> is not necessarily appropriate.

It is thus appropriate to learn an initial coefficient at the start of the engine <NUM> to determine an environmental coefficient <NUM> suitable for the current state of the environment before starting to travel or perform work. The controller <NUM> learns an initial coefficient during a learning period that extends from the start of the engine <NUM> over a predetermined time length. The learning period is, for example, <NUM> seconds.

With reference to <FIG>, <FIG>, and <FIG>, the description below deals with how the controller <NUM> learns an initial coefficient.

To learn an initial coefficient, the controller <NUM> further includes a state obtainer <NUM> and an operation limiter <NUM>.

The state obtainer <NUM> is configured to obtain information on the respective states of various components of the tractor such as the engine <NUM> and the planetary transmission device <NUM> over time through the data communicator <NUM>.

The operation limiter <NUM> is configured to limit the respective operations of components such as the first and second electromagnetic valves 52A and 52B and the planetary transmission device <NUM> while the controller <NUM> is learning an initial coefficient.

The controller <NUM> detects on the basis of information that the state obtainer <NUM> has obtained from the engine <NUM> that the engine <NUM> has started (step #<NUM> in <FIG>).

If the engine <NUM> has started (yes in step #<NUM> in <FIG>), the controller <NUM> initializes the functional elements of the tractor such as the engine <NUM> and the planetary transmission device <NUM>. The controller <NUM> determines on the basis of information that the state obtainer <NUM> has obtained whether the initialization has ended successfully, until the initialization ends successfully (step #<NUM> in <FIG>).

If the initialization has ended successfully (yes in step #<NUM> in <FIG>), the controller <NUM> determines on the basis of information that the state obtainer <NUM> has obtained whether the transmission <NUM> is off in a normal state. Specifically, the controller <NUM> determines whether the various clutches of the transmission <NUM> are disengaged for disconnection of power transmission. The controller <NUM> determines, for example, whether the first to fourth clutches CL1 to CL4 of the planetary transmission device <NUM> are disengaged for disconnection of power transmission ("power transmission disconnected state") (step #<NUM> in <FIG>). If the clutches are not disengaged (no in step #<NUM> in <FIG>), the controller <NUM> avoids learning an initial coefficient and prompts, for example, a restart of the engine <NUM>. The controller <NUM> may alternatively determine for this step whether the forward clutch CLF and the reverse clutch CLR are disengaged for disconnection of power transmission and further determine whether the power transmission device <NUM> is capable of its normal operation.

If the clutches are disengaged for disconnection of power transmission (yes in step #<NUM> in <FIG>), the controller <NUM> starts learning an initial coefficient (step #<NUM> in <FIG>).

For the controller <NUM> to learn an initial coefficient, the operation limiter <NUM> first limits the respective operations of components such as the first and second electromagnetic valves 52A and 52B and the planetary transmission device <NUM> while the controller <NUM> is learning an initial coefficient to prevent the power transmission device <NUM> from being operated externally (step #<NUM> in <FIG>). The operation limiter <NUM>, for instance, disables operation tools for accepting an external operation such as the shift pedal <NUM>.

Next, to determine an accurate environmental coefficient <NUM> for learning an initial coefficient, the coefficient determiner <NUM> applies separately to the first and second electromagnetic valves 52A and 52B a control signal corresponding to predetermined learning electric current <NUM> as a target electric current value <NUM> (step #<NUM> in <FIG>). The learning electric current <NUM> may have a single predetermined value or two or more predetermined values. The coefficient determiner <NUM> may alternatively apply separately to the first and second electromagnetic valves 52A and 52B a control signal corresponding to learning electric current <NUM> while continuously changing the electric current value of the learning electric current <NUM> within a predetermined range. The control signal is generated by the control signal generator <NUM> performing PI control with use of a predetermined initial environmental coefficient <NUM> and a PWM process.

Next, the current electric current value obtainer <NUM> obtains a current electric current value <NUM> of electric current flowing through the first electromagnetic valve 52A or second electromagnetic valve 52B in response to the application of the control signal corresponding to the learning electric current <NUM> as the target electric current value <NUM>.

The coefficient determiner <NUM>, to learn an environmental coefficient <NUM>, changes the environmental coefficient <NUM> within a predetermined range during the application of the control signal corresponding to the learning electric current <NUM>. The coefficient determiner <NUM> then determines an electric current deviation <NUM> as the difference between (i) the current electric current value <NUM> obtained during the application of a control signal generated with use of each environmental coefficient <NUM> and (ii) the electric current value of the learning electric current <NUM> as the target electric current value <NUM> (step #<NUM> in <FIG>).

The coefficient determiner <NUM> continues to learn an initial coefficient until the learning period elapses (step #<NUM> in <FIG>). When the learning period has elapsed (yes in step #<NUM> in <FIG>), the coefficient determiner <NUM> stops the application of the control signal corresponding to the learning electric current <NUM> (step #<NUM> in <FIG>).

The coefficient determiner <NUM> selects, as an environmental coefficient <NUM> for use to generate a control signal while the tractor travels after the learning period has elapsed, (i) an environmental coefficient <NUM> with which the electric current deviation <NUM> is not larger than a predetermined value or (ii) an environmental coefficient <NUM> with which the electric current deviation <NUM> was at its minimum during the learning period, and stores information on the selected environmental coefficient <NUM> in the storage <NUM> (step #<NUM> in <FIG>). The coefficient determiner <NUM> should preferably select the above environmental coefficient <NUM> on the basis of the duty ratio <NUM> of the control signal (or pulse signal). The coefficient determiner <NUM>, in other words, selects an environmental coefficient <NUM> on the basis of the difference (that is, the electric current deviation <NUM>) between the current electric current value <NUM> and the target electric current value <NUM> while generating a control signal (or pulse signal) with a duty ratio <NUM> within an appropriate range. The coefficient determiner <NUM> may also change not only the learning electric current <NUM> as the target electric current value <NUM> but also the duty ratio <NUM> in generating a control signal and determine an environmental coefficient <NUM> with use of the current electric current value <NUM> obtained during the change.

Then, the operation limiter <NUM> stops limiting the respective operations of components such as the first and second electromagnetic valves 52A and 52B and the planetary transmission device <NUM>. This causes the tractor to start traveling (or traveling and performing work). When the tractor starts traveling (or traveling and performing work), the control signal generator <NUM> starts generating a control signal with use of the environmental coefficient <NUM> that the coefficient determiner <NUM> has selected during the process of learning an initial coefficient. The coefficient determiner <NUM> learns and updates the environmental coefficient <NUM> as described above while the tractor is traveling.

Learning an initial coefficient at the start of the engine <NUM> before starting to travel (or travel and perform work) as above allows determination of an environmental coefficient <NUM> corresponding to the state of the environment in which the tractor will travel (or travel and perform work). This allows the control signal generator <NUM> to generate a control signal with use of the environmental coefficient <NUM> when the tractor starts to travel (or travel and perform work). This in turn allows accurate generation of a control signal (or electric current value) and accurate control of the electromagnetic valves at the start of the engine <NUM>.

For instance, the coefficient determiner <NUM> may be configured to determine an environmental coefficient <NUM> with use of a learned model <NUM> machine-learned to output an environmental coefficient <NUM> in response to receiving a target electric current value <NUM>, the duty ratio <NUM> of a control signal (or pulse signal), and the current electric current value <NUM>.

The coefficient determiner <NUM>, in this case, inputs into the learned model <NUM> input data <NUM> in the form of a target electric current value <NUM>, the duty ratio <NUM> of a control signal (or pulse signal) that the control signal generator <NUM> has generated, and the current electric current value <NUM> at least either while the controller <NUM> is learning an initial coefficient or while the tractor is traveling to obtain an environmental coefficient <NUM> from the learned model <NUM>.

The above configuration allows an environmental coefficient <NUM> to be determined more easily and accurately.

(<NUM>) For each of the embodiments described above, the control signal is not necessarily a pulse signal, and may be in any form.

(<NUM>) The embodiments described above are each configured such that the oil chambers 50A and 50B of the hydraulic cylinder <NUM> are an oil chamber configured to discharge operating oil for tilting the swash plate <NUM> of the hydraulic pump P on the normal rotation side of the neutral position and an oil chamber configured to discharge operating oil for tilting the swash plate <NUM> on the reverse rotation side of the neutral position. The oil chambers 50A and 50B may alternatively be an oil chamber configured to discharge operating oil for tilting the swash plate <NUM> to the normal rotation side ("normal rotation direction") and an oil chamber configured to discharge operating oil for tilting the swash plate <NUM> to the reverse rotation side ("reverse rotation direction"). In other words, the embodiments described above may each be configured such that the first electromagnetic valve 52A is configured to tilt the swash plate <NUM> to the normal rotation side and that the second electromagnetic valve 52B is configured to tilt the swash plate <NUM> to the reverse rotation side.

(<NUM>) For each of the embodiments described above, the controller <NUM> does not necessarily include functional blocks such as the above, and may include any functional blocks. The functional blocks of the controller <NUM> may, for instance, each be divided further, or a portion of or the entire functional block may be combined with another functional block. The functions of the controller <NUM> are not necessarily performed by the above functional blocks, and may each be performed by any functional block. Further, one or more or all of the functions of the controller <NUM> may be performed by software. Programs as such software are stored in a storage device such as the storage <NUM> and executed by a processor included in the controller <NUM> such as a CPU or a separate processor.

(<NUM>) The embodiments described above are each configured such that the planetary transmission device <NUM> outputs composite motive power in one of four speed ranges. The planetary transmission device <NUM> may alternatively output composite motive power in one of three or less or five or more speed ranges.

(<NUM>) The embodiments described above are each an example including front and rear wheels <NUM> and <NUM>. The tractor may alternatively include as its travel device a crawler travel device or a combination of a mini crawler and wheels.

(<NUM>) The embodiments described above are each an example including a shift pedal <NUM>. The present invention is, however, not limited to such a configuration. The tractor may alternatively include a shift lever as its speed change operation tool.

(<NUM>) The electromagnetic valve control device for each of the embodiments described above is not necessarily mounted in a tractor, and may be mounted in any of various work vehicles such as an agricultural work vehicle.

Claim 1:
An electromagnetic valve control device configured to control an angle of a swash plate (<NUM>) of a hydrostatic, continuously variable transmission device with use of at least one electromagnetic valve (52A,52B), when the hydrostatic, continuously variable transmission device varies motive power from a drive section (<NUM>), and a gear transmission (<NUM>) composites motive power from the drive section and the varied motive power from the hydrostatic, continuously variable transmission device, varies the composite motive power, and outputs the varied composite motive power,
the electromagnetic valve control device being characterized in that the electromagnetic valve control device comprises:
a control signal generator (<NUM>) configured to, through feedback control involving an environmental coefficient (<NUM>), generate a control signal intended for the at least one electromagnetic valve and corresponding to a target electric current value (<NUM>);
a coefficient determiner (<NUM>) configured to determine the environmental coefficient during a learning period extending from a start of the drive section (<NUM>) over a predetermined time length; and
an operation limiter (<NUM>) configured to limit respective operations of the at least one electromagnetic valve (52A,52B) and the gear transmission (<NUM>) during the learning period.