Furthermore, the identification unit 222 identifies whether the second set condition is satisfied.

The second set condition is a condition that is required for the load state of the engine E to correspond to a light load state.

Whether the second set condition is satisfied may be identified using, for example, the information from the engine torque sensor 320 and the vehicle speed difference computed by the computation unit 221.

More specifically, the second set condition may be the condition in which the vehicle speed difference is smaller than or equal to a set value or the torque of the engine is lower than or equal to a first specific value. In this case, the first specific value is set to a value smaller than the second specific value, and may be set based on, for example, a negligible load (theoretically, no load).

The determination unit 223 determines the current load state of the engine E based on whether the first or second set condition is satisfied.

According to an appropriate embodiment of the present disclosure, the current load state is set to one of three load states.

The three load states are light load, heavy load, and intermediate load states.

The light load state is a state having a relatively low load, and the heavy load state is a state having a relatively high load. Furthermore, the intermediate load state refers to a state between the light load and heavy load states.

That is, according to an embodiment, the determination unit 223 determines whether the current load state is a light load, heavy load, or intermediate load state based on whether the first or second set condition is satisfied.

The control means 230 controls the first servo-mechanism 120 while varying a control mode according to the load state of the engine E determined by the determination means 220. It is obvious that the angular position of the first swash plate 111 is varied in response to the control of the first servo-mechanism 120.

The control mode in the light load state is a light load mode.

In the light load mode, the control means 230 controls the first servo-mechanism 120 so that the angular position of the first swash plate 111 is at the minimum angle. Accordingly, in the light load mode, the rotational speed of the output shaft OS is controlled to have a minimum value theoretically.

The control mode in the heavy load state is a heavy load mode.

In the heavy load mode, the control means 230 controls the first servo-mechanism 120 so that the angular position of the first swash plate 111 is at the maximum angle. Accordingly, in the heavy load mode, the rotational speed of the output shaft OS is controlled to have a maximum value theoretically.

The control mode in the intermediate load mode is an intermediate load mode.

According to a preferred embodiment of the present disclosure, in the intermediate load mode, the first swash plate 111 is determined to be at any angular position between the minimum and maximum angles. Accordingly, in the intermediate load mode, the rotational speed of the output shaft OS is positioned between the maximum and minimum values theoretically.

In the intermediate load mode, when the current vehicle speed is higher than the set vehicle speed, the first swash plate 111 is moved toward the minimum angle. When the current vehicle speed is lower than the set vehicle speed, the first swash plate 111 is moved toward the maximum angle.

Furthermore, depending on the implementation, when the torque of the engine is equal to or higher than the second specific value in the intermediate load mode, the first swash plate 111 may be additionally moved toward the maximum angle.

FIG. 5 shows changes in the angle of the first swash plate 111 in the intervals of light load, heavy load, and intermediate load states.

Referring to FIG. 5, changes in the angular position of the first swash plate 111 will be described in more detail.

In the interval of the light load state, the first swash plate 111 maintains a minimum angle. In the interval of the heavy load state, the first swash plate 111 maintains a maximum angle. Furthermore, in the interval of the intermediate load state, the first swash plate 111 may be positioned at any angular position between the minimum and maximum angles. That is, according to a preferred embodiment of the present disclosure, in the interval of the intermediate load state, the first swash plate 111 may be selectively positioned at any consecutive angular positions between the minimum and maximum angles. Furthermore, the theoretical rotational speed of the output shaft OS is determined in response to these angular positions.

Next, a method of controlling the hydrostatic transmission 100 having the above-described configuration will be described with reference to the flowchart of FIG. 6.

1. Information Collection <S10>

The collection means 210 collects the information used to determine a load state according to driving and/or work. In this case, the collected information includes the amount the foot pedal is pressed, the torque of the engine, the RPM of the engine, the current vehicle speed, and the temperature of working fluid received from the foot pedal sensor 310, the engine RPM sensor 330, the vehicle speed sensor 340, and the fluid temperature sensor 350. It is obvious that any information that allows for the more accurate determination of the load state of the engine E may be added.

2. Load Determination <S20>

The determination means 220 identifies the current load state of the engine E by analyzing the information collected in step S10, and then determines the current load state of the engine E.

The load states of the engine E may be classified into, for example, light load, heavy load, and intermediate load states.

The light load state is a state in which a driving and/or work load is slight, so that the rotational speed of the output shaft OS may be theoretically made to become a minimum speed. For example, the light load state may be a state in which a driving and/or work load is low because a work vehicle drives downhill due to a slope on a driving surface or a work target surface is soft.

The heavy load state is a state in which the driving and/or work load is high, so that it is necessary that the rotational speed of the output shaft OS be maximized. For example, a heavy load state may be a state in which a driving and/or work load is high because a work vehicle drives uphill due to a slope on a driving surface or a work target surface is hard, so that the speed of the work vehicle is decreased.

The intermediate load state is a state in the interval between the light load and heavy load states.

It is obvious that depending on the implementation, it may be possible to further subdivide one or more of the light load, heavy load, and intermediate load states.

Step S20 may be further subdivided into more specific steps, as shown in the flowchart of FIG. 7.

2-1. First Computation <S21>

The computation unit 221 computes a set vehicle speed using the information collected from the foot pedal sensor 310, the engine RPM sensor 330, and the fluid temperature sensor 350 out of the information collected in step S10.

For example, the information from the foot pedal sensor 310 may be the amount the foot pedal is pressed by a driver for the purpose of gear shifting.

For example, the information from the engine RPM sensor 330 may be the RPM of the engine. An implementation may be made such that the RPM of the engine is set by the driver using the RPM pedal or a separate hand lever.

For example, the information from the fluid temperature sensor 350 may be the temperature of working fluid. The viscosity of the working fluid may be varied depending on the temperature, which may in turn affect the vehicle speed. That is, since the set vehicle speed may be affected by the temperature of the working fluid, the information collected from the fluid temperature sensor 350 is required to accurately compute the set vehicle speed.

2-2. First Identification <S22>

The identification unit 222 identifies whether a first set condition regarding the torque of the engine and the RPM of the engine is satisfied.

The first set condition is a condition that is required for the torque of the engine and the RPM of the engine to correspond to a heavy load state.

According to a preferred embodiment, the first set condition may be a condition in which the RPM of the engine decreases and the torque of the engine is equal to or higher than the second specific value.

2-3. Second Computation <S23>

When the first set condition is not satisfied in step S22, the computation unit 221 computes the vehicle speed difference between the set vehicle speed and the current vehicle speed.

For example, when the RPM of the engine increases or the torque of the engine is lower than the second specific value, the computation unit 221 computes the vehicle speed difference using the set vehicle speed computed in step S21 and the current vehicle speed collected from the vehicle speed sensor 340.

2-4. Second Identification <S24>

The identification unit 222 identifies whether a second set condition regarding the torque of the engine and the vehicle speed difference computed in step S23 is satisfied.

The second set condition is a condition that is required for the torque of the engine and vehicle speed difference to correspond to a light load state.

According to a preferred embodiment, the second set condition may be a condition in which the vehicle speed difference is equal to or smaller than a set value or the torque of the engine is equal to or lower than the first specific value. In this case, the set value for the vehicle speed difference is preferably an optimal value selected through numerous repeated tests.

For reference, in the case where the set value is 0.1 km/h, the second set condition may be satisfied when the vehicle speed difference is smaller than 0.1 km/h. Furthermore, even in the case where the vehicle speed difference is larger than 0.1 km/h, the second set condition may be satisfied when the torque of the engine is lower than or equal to the first specific value.

In the present embodiment, the vehicle speed difference is an absolute value that can only define the magnitude thereof. That is, for the first set condition to be satisfied when the set value is 0.1 km/h, the vehicle speed difference needs to fall within the range of (−) 0.1 km/h to (+) 0.1 km/h.

2-5. Determination <S25>

The determination unit 223 determines whether a current state is a light load, heavy load, or intermediate load state based on the identifications made in steps S22 and S24.

Step S25 will be discussed in more detail with reference to the reference diagram of FIG. 8.

According to the present example, when it is identified in step S22 that the first set condition is currently satisfied, the determination unit 223 determines that the current state is a heavy load state.

According to the present example, when it is identified in step S24 that the second set condition is currently satisfied, the determination unit 223 determines that the current state is a light load state.

According to the present example, when it is identified in step S22 that the first set condition is not satisfied and it is also identified in step S24 that the second set condition is not satisfied, the determination unit 23 determines that the current state is an intermediate load state between the light load and heavy load states.

3. Angular Position Adjustment <S30>

When the current load state is determined in step S20, the control means 230 varies the angular position of the first swash plate 111 to a control mode based on the determined load state.

In the light load state, the angular position of the first swash plate 111 is varied to a light load mode, which is the control mode in the light load state.

According to the present example, in the light load mode, the angular position of the first swash plate 111 is adjusted to a minimum angle. Accordingly, the rotational speed of the output shaft OS is controlled to have a minimum value. In the light load mode, driving and/or work is performed primarily based on inertia.

According to the present example, in the heavy load mode, the angular position of the first swash plate 111 is adjusted to a maximum angle. Accordingly, the rotational speed of the output shaft OS is controlled to have a maximum value.

According to the present example, in the intermediate load mode, the angular position of the first swash plate 111 is adjusted to any angular position between the minimum and maximum angles. The angular position of the first swash plate 111 in the intermediate load mode will be described in more detail below.

In the intermediate load mode, when the current vehicle speed is higher than a set vehicle speed, the angular position of the first swash plate 111 is moved toward the minimum angle. In this case, the rotational speed of the output shaft OS decreases, so that as the current vehicle speed decreases, the current vehicle speed converges to the set vehicle speed.

In the intermediate load mode, when the current vehicle speed is lower than the set vehicle speed, the angular position of the first swash plate 111 is moved toward the maximum angle. In this case, the rotational speed of the output shaft OS increases, so that as the current vehicle speed increases, the current vehicle speed converges to the set vehicle speed.

Furthermore, even in a work situation in which the driving load is low and thus the RPM of the engine is maintained, the torque of the engine may be equal to or higher than the second specific value due to the workload. Accordingly, an implementation may be made such that when the RPM of the engine is maintained and the torque of the engine is equal to or higher than the second specific value, the angular position of the first swash plate