Patent Description:
As a travel drive system of a wheel loader, a torque converter type travel drive system that transmits the drive force of an engine to wheels via a torque converter, has been known. In the wheel loader equipped with the torque converter type travel drive system, the rotational speed of an engine is changed based on the ratio of the rotational speed of an input shaft of a torque converter and the rotational speed of an output shaft (= output rotational speed/input rotational speed), and the rotation with the changed rate is transmitted to wheels.

For example, Patent Literature <NUM> discloses a wheel loader that includes: a travel drive device that transmits the rotation of an engine to tires via a torque converter and a transmission; a front working device that includes a lift arm rotatable in the vertical direction; a variable displacement hydraulic pump that is driven by the engine to supply pressurized oil to an actuator for driving the front working device; and a controller that controls each element of a vehicle body.

This wheel loader limits the maximum absorption torque of the hydraulic pump with respect to the actual rotational speed of the engine in a low speed range when the stepping amount of an accelerator pedal is smaller than a predetermined value, and limits the maximum absorption torque in the low speed range and a medium speed range when the stepping amount of the accelerator pedal is larger than the predetermined value, which increases the rate of increase in the actual rotational speed of the engine, and improves the blow-up performance of the engine.

<CIT> discloses a method for controlling a working machine comprising a bucket, a bucket height sensor and means for assessing traction force or engine speed. It can set engine speed in accordance with a bucket state.

Unfortunately, in the wheel loader described in Patent Literature <NUM>, the rate of increase in the actual rotational speed of the engine is high even in what is called a raise and run operation of moving a lift arm in the upper direction during forward travel of the vehicle body. Consequently, increase in the travel speed of the vehicle body is enhanced, and the lift arm lifting speed becomes relatively low with respect to the traveling speed. Accordingly, complete rise of the lift arm in the upper direction requires a certain time period. Consequently, it is required to set a long traveling distance required for the raise and run operation. Furthermore, increase in the traveling distance, in turn, increases the fuel consumption of the wheel loader.

'It is the object of the invention to provide a wheel loader that can reduce the traveling distance required for the raise and run operation, and reduce fuel consumption.

The above object is accomplished by the features of claim <NUM>.

A wheel loader has the features of claim <NUM>, amongst them a front working device including a lift arm rotatable in a vertical direction, the front working device being provided at a front of a vehicle body, the wheel loader traveling by transmitting a drive force of an engine to wheels via a torque converter, and further including: a traveling state sensor that detects a traveling state of the vehicle body; a motion sensor that detects that the lift arm is in a lifting motion; and a controller that controls the engine, wherein the controller determines whether a specific condition for specifying an operation of the lift arm in an upper direction during forward travel of the vehicle body is satisfied or not, based on the traveling state detected by the traveling state sensor, and on a state of a lifting motion of the lift arm detected by the motion sensor, and limits a vehicle speed by reducing a maximum rotational speed of the engine in a case of satisfying the specific condition. Claim <NUM> recites further features.

The present invention can reduce the traveling distance required for a raise and run operation, and reduce fuel consumption. Problems, configurations and advantageous effects other than those described above are clarified by the following description of embodiments.

The entire configuration and operations of a wheel loader according to each embodiment of the present invention are described with reference to <FIG>.

<FIG> is a side view showing an appearance of the wheel loader <NUM> according to each embodiment of the present invention.

The wheel loader <NUM> includes: a vehicle body that includes a front frame 1A and a rear frame 1B; and a front working device <NUM> provided at the front of the vehicle body. The wheel loader <NUM> is an articulate working machine that is steered by bending the vehicle body around the center. The front frame 1A and the rear frame 1B are joined by a center joint <NUM> so as to be freely rotatable in the lateral direction. The front frame 1A is bent in the lateral direction with respect to the rear frame 1B.

The front frame 1A is provided with a pair of left and right front wheels 11A, and the front working device <NUM>. The rear frame 1B includes a pair of left and right rear wheels 11B, an operating room <NUM> where an operator boards, a machine room <NUM> that accommodates various devices, such as an engine, a controller and a cooler, and a counter weight <NUM> for keeping the balance so as to prevent the vehicle body from inclining or rolling over. Note that <FIG> shows only the left front wheel 11A and rear wheel 11B among the pairs of left and right front wheels 11A and rear wheels 11B.

The front working device <NUM> includes: a lift arm <NUM> rotatable in the vertical direction; a pair of lift arm cylinders <NUM> that are extended and retracted to thereby drive the lift arm <NUM>; a bucket <NUM> attached to the distal end of the lift arm <NUM>; a bucket cylinder <NUM> that is extended and retracted to thereby rotate the bucket <NUM> in the vertical direction with respect to the lift arm <NUM>; a bellcrank <NUM> rotatably joined to the lift arm <NUM> to constitute a link mechanism between the bucket <NUM> and the bucket cylinder <NUM>; and a plurality of pipes (not shown) that guide pressurized oil to the pair of lift arm cylinders <NUM> and the bucket cylinder <NUM>. Note that <FIG> shows, with broken lines, only the lift arm cylinder <NUM> arranged to the left between the pair of the lift arm cylinders <NUM>.

The lift arm <NUM> is rotated in the upper direction by extending rods <NUM> of the respective lift arm cylinders <NUM>, and is rotated in the lower direction by retracting the rods <NUM>. The bucket <NUM> is rotated (tilted) in the upper direction with respect to the lift arm <NUM> by extending a rod <NUM> of the bucket cylinder <NUM>, and is rotated (dumped) in the lower direction with respect to the lift arm <NUM> by retracting the rod <NUM>.

The wheel loader <NUM> is a working machine for performing a loading operation that excavates earth, sand, minerals and the like in an opencast mine, for example, and loads them into a dump truck or the like. Next, V-shaped loading that is one of methods during the wheel loader <NUM> performing a digging operation and a loading operation, is described with reference to <FIG> and <FIG>.

<FIG> illustrates the V-shaped loading by the wheel loader <NUM>. <FIG> illustrates a raise and run operation of the wheel loader <NUM>.

First, as indicated by an arrow X1, the wheel loader <NUM> advances toward a ground 100A that is to be dug, and digs the bucket <NUM> into the ground 100A and performs the digging operation. After completion of the digging operation, the wheel loader <NUM> once goes back to the original position as indicated by an arrow X2.

Next, as indicated by an arrow Y1, the wheel loader <NUM> advances toward a dump truck 100B, and stops in front of the dump truck 100B. In <FIG>, the wheel loader <NUM> in the state of stopping in front of the dump truck 100B is indicated by broken lines.

Specifically, as shown in <FIG>, the operator presses the accelerator pedal to the floor (fully accelerating), while performing a lifting operation for the lift arm <NUM> (a state shown at the right in <FIG>). Next, with the fully accelerating state being kept, the lift arm <NUM> is further lifted in the upper direction (a state shown at the middle in <FIG>). The operator then brakes to stop in front of the dump truck 100B, and dumps a load (earth, sand, minerals and the like) from the bucket <NUM> to load it into the dump truck 100B. Note that this series of operations shown in <FIG> is called a "raise and run operation.

After completion of the loading operation, the wheel loader <NUM> goes back to the original position as indicated by an arrow Y2 in <FIG>. As described above, the wheel loader <NUM> travels to and fro between the ground 100A and the dump truck 100B in a V-shaped manner to perform the digging operation and the loading operation.

Next, a drive system of the wheel loader <NUM> is described with respect to each embodiment.

The drive system of a wheel loader <NUM> according to a first embodiment of the present invention is described with reference to <FIG>.

First, the travel drive system of the wheel loader <NUM> is described with reference to <FIG>.

<FIG> shows a hydraulic circuit and an electric circuit of the wheel loader <NUM> according to this embodiment. <FIG> is a graph showing the relationship between an accelerator pedal stepping amount and a target engine rotational speed. <FIG> is a graph showing the relationship between the vehicle speed and the drive force for each speed stage.

As for the wheel loader <NUM> according to this embodiment, the travel of the vehicle body is controlled by the torque converter type travel drive system. As shown in <FIG>, the wheel loader <NUM> includes: an engine <NUM>; a torque converter <NUM> that includes an input shaft joined to the output shaft of the engine <NUM>; a transmission <NUM> joined to an output shaft of the torque converter <NUM>; and a controller <NUM> that controls each device, such as the engine <NUM>.

The torque converter <NUM> is a fluid clutch that includes an impeller, a turbine and a stator, and has a function of increasing the output torque with respect to the input torque, that is, a function of causing the torque ratio (= output torque/input torque) to be one or more. The torque ratio decreases with increase in the torque converter speed ratio (= output shaft rotational speed/input shaft rotational speed) that is the ratio of the rotational speed of the input shaft and the rotational speed of the output shaft of the torque converter <NUM>. Thus, the rotational speed of the engine <NUM> is changed and is transmitted to the transmission <NUM>.

The transmission <NUM> is a variable speed gearbox that includes a plurality of solenoid valves for respective maximum vehicle speeds corresponding to first to fourth speed stages as shown in <FIG>, and changes the rotational speed of the output shaft of the torque converter <NUM>. Selection from among the first to fourth speed stages is performed through a speed stage switch <NUM> (see <FIG>) provided in the operating room <NUM>. The speed stage switch <NUM> is mainly used for forward travel of the wheel loader <NUM>.

When the operator selects a desired speed stage through the speed stage switch <NUM>, a speed stage signal pertaining to the selected speed stage is output from the speed stage switch <NUM> to the controller <NUM>. According to the speed stage signal output from the controller <NUM> to the transmission control unit <NUM>, the solenoid valves of the transmission <NUM> are driven.

As shown in <FIG>, the maximum vehicle speed is set to S1 at the first speed stage, the maximum vehicle speed is set to S2 at the second speed stage, the maximum vehicle speed is set to S3 at the third speed stage, and the maximum vehicle speed is set to S4 at the fourth speed stage. Note that the magnitude relationship among S1, S2, S3 and S4 is S1 < S2 < S3 < S4. In <FIG>, the first speed stage is indicated by a solid line, the second speed stage is indicated by a broken line, the third speed stage is indicated by a chain line, and the fourth speed stage is indicated by a chain double-dashed line.

Among the first to fourth speed stages, the first speed stage and the second speed stage correspond to "low speed stage" and the third speed stage and the fourth speed stage correspond to "medium to high speed stages. " The "low speed stage" is selected when the wheel loader <NUM> travels toward the dump truck 100B in the loading operation (in the case of indication by the arrow Y1 in <FIG>), that is, at the time of raise and run operation, and the maximum vehicle speed is set to range from <NUM> to <NUM>/hour, for example.

Selection of the traveling direction of the wheel loader <NUM>, that is, selection between the forward travel and reverse travel is performed by a forward and reverse switch <NUM> provided in the operating room <NUM> (see <FIG>). Specifically, when the operator selects the advance position by the forward and reverse switch <NUM>, a forward and reverse switching signal indicating the forward travel is output to the controller <NUM>, and the controller <NUM> outputs, to the transmission control unit <NUM>, a command signal for bringing a forward clutch of the transmission <NUM> into an engaged state. When the transmission control unit <NUM> receives a command signal pertaining to forward travel, a clutch control valve provided for the transmission control unit <NUM> operates to bring a forward clutch into an engaged state, and the vehicle body is switched to forward travel. Reverse travel of the vehicle body is selected also by a similar mechanism.

As for the torque converter type travel drive system, first, the operator presses the accelerator pedal <NUM> provided in the operating room <NUM> to rotate the engine <NUM>, and the input shaft of the torque converter <NUM> is rotated according to the rotation of the engine <NUM>. The output shaft of the torque converter <NUM> rotates according to the set torque converter speed ratio, and the output torque of the torque converter <NUM> is transmitted to the front wheels 11A and the rear wheels 11B via the transmission <NUM>, a propeller shaft <NUM> and an axle <NUM>, thereby allowing the wheel loader <NUM> to travel.

Specifically, as shown in <FIG>, the stepping amount on the accelerator pedal <NUM> detected by a stepping amount sensor <NUM> is input into the controller <NUM>. A target engine rotational speed is input as the command signal from the controller <NUM> to the engine <NUM>. The number of revolutions of the engine <NUM> is controlled in conformity with the target engine rotational speed. The rotational speed of the engine <NUM> is detected by a first engine rotational speed sensor <NUM> provided at an output shaft of the engine <NUM>.

As shown in <FIG>, the stepping amount on the accelerator pedal <NUM> and the target engine rotational speed have a proportional relationship. The more the stepping amount on the accelerator pedal <NUM> is, the higher the target engine rotational speed is. Accordingly, the rotational speed of the output shaft of the torque converter <NUM> increases, which in turn increases the vehicle speed. As shown in <FIG>, the vehicle speed is detected as the rotational speed of the propeller shaft <NUM> by a second rotational speed sensor <NUM>.

Note that in <FIG>, in a range of the stepping amount on the accelerator pedal <NUM> from <NUM>% to <NUM>% or <NUM>%, the target engine rotational speed is constant at the minimum target engine rotational speed Vmin irrespective of the stepping amount on the accelerator pedal <NUM>. In a range of the stepping amount on the accelerator pedal <NUM> from <NUM>% or <NUM>% to <NUM>%, the target engine rotational speed is constant at the maximum target engine rotational speed Vmax irrespective of the stepping amount on the accelerator pedal <NUM>.

As described above, with respect to the relationship between the stepping amount on the accelerator pedal <NUM> and the target engine rotational speed, setting is configured so as to maintain the target engine rotational speed at the minimum target engine rotational speed Vmin in a predetermined range with a small stepping amount on the accelerator pedal <NUM>, and to maintain the target engine rotational speed at the maximum target engine rotational speed Vmax in a predetermined range with a large stepping amount on the accelerator pedal <NUM>. Note that such setting can be freely changed.

Next, the drive system of the front working device <NUM> is described with reference to <FIG> and <FIG>.

<FIG> is a graph showing the relationship between the lifting operation amount for the lift arm <NUM> and the opening area of the spool.

As shown in <FIG>, the wheel loader <NUM> is driven by the engine <NUM>, and includes: an hydraulic pump <NUM> that supplies the front working device <NUM> with hydraulic oil; a hydraulic oil tank <NUM> that stores the hydraulic oil; a lift arm operating lever <NUM> for operating the lift arm <NUM>; a bucket operating lever <NUM> for operating the bucket <NUM>; and control valves <NUM> that control the flow of pressurized oil supplied from the hydraulic pump <NUM> to the lift arm cylinders <NUM> and the bucket cylinder <NUM>.

In this embodiment, the hydraulic pump <NUM> is a swash-plate or bent-axis type variable displacement hydraulic pump whose displacement volume is controlled in response to a tilt angle. The tilt angle is adjusted by a regulator <NUM> according to a command signal output from the controller <NUM>. Note that the hydraulic pump <NUM> is not necessarily a variable displacement hydraulic pump. A fixed displacement hydraulic pump may be adopted, instead.

When the operator operates the lift arm operating lever <NUM> in the direction of lifting the lift arm <NUM>, for example, the pilot pressure in response to the operation amount is generated. The pilot pressure corresponds to the lifting operation amount for the lift arm <NUM> through the lift arm operating lever <NUM>, and is detected by the operation amount sensor <NUM>.

The generated pilot pressure is applied to the control valve <NUM>, and the spool in the control valve <NUM> moves in a stroke according to the pilot pressure. The hydraulic oil discharged from the hydraulic pump <NUM> flows into the lift arm cylinders <NUM> via the control valve <NUM>, thereby extending the rods <NUM> of the lift arm cylinders <NUM>.

Consequently, as shown in <FIG>, the lifting operation amount [%] for the lift arm <NUM> and the spool opening area [%] of the control valve <NUM> have a proportional relationship. As the lifting operation amount for the lift arm <NUM> increases, the spool opening area increases accordingly. Consequently, when the lift arm operating lever <NUM> is operated largely in the direction of lifting the lift arm <NUM>, the hydraulic oil flow rate into the lift arm cylinders <NUM> increases to extend rapidly the rods <NUM> accordingly.

Note that in <FIG>, in a range of the lifting operation amount for the lift arm <NUM> from <NUM>% to <NUM>%, the spool is not opened, and the opening area is <NUM>% (dead zone). In a range of the lifting operation amount for the lift arm <NUM> from <NUM>% to <NUM>%, the spool opening area is constant at <NUM>%, and a full lever operation state is maintained.

Also as for the operation of the bucket <NUM>, similar to the operation of the lift arm <NUM>, the pilot pressure generated in response to the operation amount for the bucket operating lever <NUM> acts on the control valve <NUM>, which controls the spool opening area of the control valve <NUM>, and adjusts the hydraulic oil flow rate into and from the bucket cylinder <NUM>.

Although illustration is omitted in <FIG>, operation amount (pilot pressure) sensors for detecting the lowering operation amount for the lift arm <NUM>, and tilt and dump operation amounts for the bucket <NUM> are provided on the respective pipe lines of the hydraulic circuit.

Next, the configuration and functions of the controller <NUM> are described with reference to <FIG>.

<FIG> is a functional block diagram showing functions that the controller <NUM> has. <FIG> is a flowchart showing the flow of processes executed by the controller <NUM>. <FIG> is a graph showing the relationship between the lifting operation amount of the lift arm <NUM> and the maximum rotational speed of the engine. <FIG> is a graph showing the relationship between the stepping amount on the accelerator pedal <NUM> and the target engine rotational speed in a case where the maximum rotational speed of the engine <NUM> is limited. <FIG> is a graph showing the relationship between the traveling distance of the wheel loader <NUM> and the lifting time of the lift arm <NUM>.

The controller <NUM> is configured such that a CPU, a RAM, a ROM, an HDD, an input I/F, and an output I/F are connected to each other via a bus. The various operation devices, such as the forward and reverse switch <NUM> and the speed stage switch <NUM>, the various sensors, such as the stepping amount sensor <NUM> and the operation amount sensor <NUM> (see <FIG>), and the like are connected to the input I/F. The engine <NUM>, the transmission control unit <NUM> of the transmission <NUM>, the regulator <NUM> of the hydraulic pump <NUM> and the like are connected to the output I/F.

In such a hardware configuration, the CPU reads a calculation program (software) stored in a recording medium, such as the ROM, the HDD or an optical disk, deploys the program on the RAM, and executes the deployed calculation program, which allows the calculation program and the hardware to cooperate with each other, and achieves the functions of the controller <NUM>.

In this embodiment, the configuration of the controller <NUM> is described with reference to the combination of the software and the hardware. Without limitation thereto, the configuration may be achieved using an integrated circuit that achieves the functions of the calculation program to be executed on the wheel loader <NUM>.

As shown in <FIG>, the controller <NUM> includes a data acquisition section <NUM>, a storage section <NUM>, a determination section <NUM>, a calculation section <NUM>, and a command signal output section <NUM>.

The data acquisition section <NUM> acquires data items pertaining to the forward and reverse switching signal that has been output from the forward and reverse switch <NUM> and indicates forward or reverse travel, the stepping amount on the accelerator pedal <NUM> detected by the stepping amount sensor <NUM>, the pilot pressure Ti as the lifting operation amount for the lift arm <NUM> detected by the operation amount sensor <NUM> (hereinafter, simply called "pilot pressure Ti"), and a speed stage signal output from the speed stage switch <NUM>.

The storage section <NUM> stores a first pilot threshold T1, a second pilot threshold T2 and a third pilot threshold T3 that pertain to the pilot pressure for the lifting operation for the lift arm <NUM>. The first pilot threshold T1 and the second pilot threshold T2 are pilot pressures in a state where the lift arm <NUM> is lifted in the upper direction higher than the lift arm <NUM> in a horizontal attitude. The second pilot threshold T2 is configured to have a larger value than the first pilot threshold T1 has (T1 < T2). For example, in this embodiment, the first pilot threshold T1 is <NUM>% (T1 = <NUM>%), and the second pilot threshold T2 is <NUM>% (T2 = <NUM>%). Note that the first pilot threshold T1 may be a pilot pressure at least when the lift arm <NUM> is in the horizontal attitude in situations where the lift arm <NUM> is performing the lifting operation. The third pilot threshold T3 is a pilot pressure with the lift arm <NUM> having been completely lifted in the upper direction, that is, <NUM>% (T3 = <NUM>%).

The determination section <NUM> determines whether the wheel loader <NUM> is traveling forward or not on the basis of the forward and reverse switching signal acquired by the data acquisition section <NUM> and of the stepping amount on the accelerator pedal <NUM>, and determines whether the lift arm <NUM> is in the lifting motion or not on the basis of the pilot pressure Ti acquired by the data acquisition section <NUM>, for example, of whether the pilot pressure Ti of the lift arm <NUM> in the lifting direction is equal to or more than the minimum value Ti_min of the pilot pressure or not. Hereinafter, a condition for specifying the operation of the lift arm <NUM> in the upper direction during forward travel of the wheel loader <NUM> is regarded as a "specific condition. " A case of satisfying the "specific condition" is a case of performing the raise and run operation described above.

Here, the forward and reverse switch <NUM> and the stepping amount sensor <NUM> are modes of a traveling state sensor that detects the traveling state of the vehicle body of the wheel loader <NUM>. The operation amount sensor <NUM> is a mode of a motion sensor that detects the lifting motion for the lift arm <NUM>.

Note that in this embodiment, advance travel of the vehicle body is determined on the basis of the forward and reverse switching signal that indicates forward travel and has been output from the forward and reverse switch <NUM> and of the stepping amount on the accelerator pedal <NUM> detected by the stepping amount sensor <NUM>. Without limitation thereto, the forward travel of the vehicle body may be integrally determined in consideration of traveling states detected by other traveling state sensors mounted on the vehicle body.

In this embodiment, upon determination that the specific condition is satisfied (in the raise and run operation), the determination section <NUM> determines the magnitude relationship between the pilot pressure Ti and the first to third pilot thresholds T1, T2 and T3 on the basis of the pilot pressure Ti acquired by the data acquisition section <NUM> and of the first to third pilot thresholds T1, T2 and T3 read from the storage section <NUM>. Furthermore, the determination section <NUM> determines whether the low speed stage is selected or not on the basis of the speed stage signal acquired by the data acquisition section <NUM>.

When the determination section <NUM> determines that the specific condition is satisfied (in the raise and run operation), the calculation section <NUM> calculates the maximum rotational speed Vi of the engine <NUM>. The command signal output section <NUM> outputs, to the engine <NUM>, a command signal pertaining to the maximum rotational speed Vi of the engine <NUM> calculated by the calculation section <NUM>.

Next, a flow of specific processes executed in the controller <NUM> is described.

As shown in <FIG>, first, the data acquisition section <NUM> acquires the forward and reverse switching signal from the forward and reverse switch <NUM>, the stepping amount on the accelerator pedal <NUM> from the stepping amount sensor <NUM>, and the pilot pressure Ti from the operation amount sensor <NUM> (step S501).

Next, the determination section <NUM> determines whether the forward and reverse switching signal indicates forward travel or not (the wheel loader <NUM> is traveling forward or not) on the basis of the data items acquired in step S501, and determines whether the pilot pressure Ti of the lift arm <NUM> in the lifting direction is equal to or higher than the minimum value Ti_min of the pilot pressure or not (the lift arm <NUM> is performing the lifting motion or not) (step S502). That is, in step <NUM>, it is determined whether the specific condition is satisfied or not.

If it is determined that the forward and reverse switching signal indicates forward travel and the pilot pressure Ti of the lift arm <NUM> in the lifting direction is equal to or higher than the minimum value Ti_min of the pilot pressure (Ti ≥ Ti_min) in step S502, that is, it is determined that the specific condition is satisfied (step S502/YES), the data acquisition section <NUM> acquires the speed stage signal from the speed stage switch <NUM> (step S503). On the contrary, if it is determined that the specific condition is not satisfied in step S502 (step S502/NO), the processes in the controller <NUM> are finished.

The determination section <NUM> determines whether the speed stage is the low speed stage or not on the basis of the speed stage signal acquired in step S503 (step S504). If it is determined that the speed stage is the low speed stage in step S504 (step S504/YES), the magnitude relationship between the pilot pressure Ti acquired in step S501 and the first pilot threshold T1 and second pilot threshold T2 read from the storage section <NUM> is determined. Specifically, the determination section <NUM> determines whether or not the pilot pressure Ti is equal to or higher than the first pilot threshold T1 and lower than the second pilot threshold T2 (step S506).

In step S506, when it is determined that the pilot pressure Ti is equal to or higher than the first pilot threshold T1 and is lower than the second pilot threshold T2 (T1 ≤ Ti < T2) (step S506/YES), the calculation section <NUM> calculates the maximum rotational speed Vi of the engine <NUM> according to Vi = k1×Ti (k1 < <NUM>: proportional constant) (step S507). The command signal output section <NUM> outputs, to the engine <NUM>, the command signal pertaining to the maximum rotational speed Vi of the engine <NUM> calculated in step S507 (step S510).

That is, as shown in <FIG>, when the detected pilot pressure Ti is a value ranging from the first pilot threshold T1 to the second pilot threshold T2 (T1 ≤ Ti < T2), the controller <NUM> gradually reduces the maximum rotational speed Vi of the engine <NUM> to a predetermined value Vth such that the pilot pressure Ti and the maximum rotational speed Vi of the engine <NUM> satisfy an inversely proportional relationship, and limits (reduces) the vehicle speed. Accordingly, in this embodiment, only after the detected pilot pressure Ti reaches the first pilot threshold T1, the controller <NUM> executes a process for limiting the vehicle speed.

In <FIG>, when the pilot pressure Ti is <NUM>% (first pilot threshold T1), the maximum rotational speed Vi of the engine <NUM> is <NUM>,<NUM> [rpm], which is the rated value (= <NUM>%). When the pilot pressure Ti is <NUM>% (second pilot threshold T2), the maximum rotational speed Vi of the engine <NUM> is <NUM>,<NUM> [rpm], which is <NUM>% of the rated value. Thus, as the pilot pressure Ti increases from <NUM>% to <NUM>%, the maximum rotational speed Vi of the engine <NUM> is gradually limited from the <NUM>% (rated value) to <NUM>% (predetermined value Vth).

On the contrary, if it is not determined that the pilot pressure Ti is equal to or higher than the first pilot threshold T1 and is lower than the second pilot threshold T2 (T1 ≤ Ti < T2) in step S506 (step S506/NO), the determination section <NUM> further determines whether or not the pilot pressure Ti is equal to or higher than the second pilot threshold T2 and lower than the third pilot threshold T3 (step S508).

In step S508, when it is determined that the pilot pressure Ti is equal to or higher than the second pilot threshold T2 and is lower than the third pilot threshold T3 (T2 ≤ Ti < T3) (step S508/YES), the calculation section <NUM> calculates the maximum rotational speed Vi of the engine <NUM> as the predetermined value Vth (Vi = Vth) irrespective of increase in pilot pressure Ti (step S509). The command signal output section <NUM> outputs, to the engine <NUM>, the command signal pertaining to the maximum rotational speed Vi (= Vth) of the engine <NUM> calculated in step S509 (step S510).

That is, as shown in <FIG>, when the detected pilot pressure Ti is a value ranging from the second pilot threshold T2 (= <NUM>%) to the third pilot threshold T3 (= <NUM>%) (T2 ≤ Ti < T3), the controller <NUM> limits (reduces) the vehicle speed so as to maintain the maximum rotational speed Vi of the engine <NUM> to be the predetermined value Vth (= <NUM>,<NUM> rpm) irrespective of increase in pilot pressure Ti.

As described above, when it is determined that the forward and reverse switching signal is forward travel and the pilot pressure Ti of the lift arm <NUM> in the lifting direction is equal to or higher than the minimum value Ti_min of the pilot pressure (Ti ≥ Ti_min) in step S502, that is, the specific condition is satisfied (in the raise and run operation) (step S502/YES), the maximum rotational speed Vi of the engine <NUM> is limited, thereby limiting the target engine maximum rotational speed with respect to the stepping amount on the accelerator pedal <NUM> from Vmax1 to Vmax2 (Vmax1 → Vmax2 and Vmax2 < Vmax1) as shown in <FIG>.

Accordingly, as shown in <FIG>, during the raise and run operation, the discharge rate of the hydraulic pump <NUM> driven by the engine <NUM> decreases, and the time to the complete rise of the lift arm <NUM> in the upper direction (lifting time) extends from t1 to t2 (t1 → t2 and t2 > t1), which is longer than that in a case without limitation on the vehicle speed.

Meanwhile, the traveling distance from the wheel loader <NUM> to the dump truck 100B (the distance from the wheel loader <NUM> indicated by the solid line to the wheel loader <NUM> indicated by the broken line in <FIG>), that is, the traveling distance required for the raise and run operation is reduced from L1 to L2 (L1 → L2 and L2 < L1), which is shorter than that in the case without limitation on the vehicle speed.

Without any limitation on the vehicle speed with respect to the lifting motion rate of the lift arm <NUM>, the wheel loader <NUM> possibly reaches the front of the dump truck 100B before the lift arm <NUM> has been completely lifted in the upper direction. In this case, the traveling distance is required to be long. However, by the controller <NUM> limiting the vehicle speed in consideration of the lifting motion rate of the lift arm <NUM>, the lift arm <NUM> can be completely lifted even with a small traveling distance. Accordingly, the cycle time of the operation of V-shaped loading is reduced, which improves the operation efficiency and reduces the fuel consumption of the wheel loader <NUM>.

To determine whether the specific condition is satisfied or not, presence or absence of the lifting motion for the lift arm <NUM> is determined using the pilot pressure Ti detected by the operation amount sensor <NUM>. Consequently, for example, in comparison with the case of detecting the bottom pressure of the lift arm cylinders <NUM>, erroneous determinations of the lifting motion for the lift arm <NUM> can be reduced, and abrupt change in vehicle speed is suppressed. This is because of the following reasons. Unlike the case of using the bottom pressure of the lift arm cylinders <NUM>, use of the pilot pressure generated by operating the lift arm operating lever <NUM> can directly detect the lifting motion for the lift arm <NUM>. Accordingly, adverse effects of variation in pressure due to a load in the bucket <NUM> and vibrations of the vehicle body are small.

Furthermore, in this embodiment, only in the latter half of the raise and run operation, at least until the lift arm <NUM> is completely lifted from the horizontal attitude in the upper direction (with the pilot pressure ranging from <NUM>% to <NUM>% in <FIG>), the maximum rotational speed (vehicle speed) of the engine <NUM> is limited by the controller <NUM>. When the lifting motion for the lift arm <NUM> is not largely performed, the maximum rotational speed of the engine <NUM> is not limited. Accordingly, when the lifting motion for the lift arm <NUM> is not largely performed, the blow-up of the engine <NUM> can be improved to enhance the acceleration performance.

If it is not determined that the pilot pressure Ti is equal to or higher than the second pilot threshold T2 and is lower than the third pilot threshold T3 (T2 ≤ Ti < T3) in step S508 (step S508/NO), that is, if the lift arm <NUM> is not subjected to a large lifting motion (Ti < T1), or if the raise and run operation has been completely finished (Ti = T3), the processes in the controller <NUM> are finished.

After the command signal output section <NUM> outputs the command signal to the engine <NUM> in step S510, the processing returns to step S501, and the processes are repeated.

This embodiment is configured such that if the speed stage is not the low speed stage in step S504 (step S504/NO), the processing returns to step S503, and does not proceed to the process of controlling the maximum rotational speed of the engine <NUM> to limit the vehicle speed (the processes in step S506 and thereafter) until the speed stage becomes the low speed stage. The low speed stage (in particular, the second speed stage in <FIG>) is suitable for the raise and run operation. It is desirable to limit the vehicle speed only when the low speed stage is selected.

Note that the controller <NUM> may omit steps S503 and S504, and control the maximum rotational speed of the engine <NUM> irrespective of the type of the selected speed stage.

In this embodiment, the wheel loader <NUM> includes an adjustment device <NUM> as shown in <FIG>. The adjustment device <NUM> allows the operator to adjust freely the change rate (proportional constant k1) of the maximum rotational speed of the engine <NUM> with respect to the pilot pressure Ti. The controller <NUM> stores the change rate preset in the storage section <NUM> by the adjustment device <NUM>, and the calculation section <NUM> calculates the maximum rotational speed of the engine <NUM> in conformity with the stored change rate.

For example, if it is intended to limit largely the vehicle speed, the adjustment device <NUM> configures setting such that the change rate of the maximum rotational speed of the engine <NUM> with respect to the pilot pressure Ti is increased, as indicated by chain double-dashed lines in <FIG>. As described above, the wheel loader <NUM> is provided with the adjustment device <NUM>, which can freely adjust the limit on the vehicle speed in conformity with the preferences of the operator, the environment of the field site, etc., and improve the user-friendliness.

Next, a wheel loader <NUM> according to a second embodiment of the present invention is described with reference to <FIG>. In <FIG>, configuration elements common to those described on the wheel loader <NUM> according to the first embodiment are assigned the same symbols. The description thereof is omitted.

<FIG> shows a hydraulic circuit and an electric circuit of the wheel loader <NUM> according to the second embodiment. <FIG> is a functional block diagram showing functions that a controller 5A according to the second embodiment has. <FIG> is a flowchart showing the flow of processes executed by the controller 5A according to the second embodiment. <FIG> is a graph showing the relationship between the discharge pressure Pa of the hydraulic pump <NUM> and the maximum rotational speed Vi of the engine <NUM>.

In the wheel loader <NUM> according to this embodiment, for determination of whether the specific condition is satisfied or not, the controller 5A determines whether the lift arm <NUM> is in the lifting motion or not, on the basis of the discharge pressure Pa of the hydraulic pump <NUM> in response to the lifting operation for the lift arm <NUM>, instead of the pilot pressure Ti pertaining to the lifting operation for the lift arm <NUM>.

Consequently, as shown in <FIG>, the wheel loader <NUM> according to this embodiment includes a pressure sensor <NUM> that detects the discharge pressure Pa of the hydraulic pump <NUM>, as a mode of a motion sensor that detects the lifting motion for the lift arm <NUM>. Other configuration elements are similar to those of the first embodiment. The travel drive system in this embodiment is also a torque converter type travel drive system.

As shown in <FIG> and <FIG>, a data acquisition section 51A acquires data pertaining to the forward and reverse switching signal output from the forward and reverse switch <NUM>, the stepping amount detected by the stepping amount sensor <NUM>, the discharge pressure Pa of the hydraulic pump <NUM> detected by the pressure sensor <NUM>, and the speed stage signal output from the speed stage switch <NUM> (step S501A).

Next, the determination section 53A determines whether the vehicle body is in forward travel or not on the basis of the forward and reverse switching signal and the stepping amount on the accelerator pedal <NUM> acquired in step S501A (step S511).

When it is determined to be in forward travel in step S511 (step S511/YES), the determination section 53A determines the magnitude relationship between the discharge pressure Pa of the hydraulic pump <NUM> acquired in step S501A and the first pump threshold P1 read from a storage section 52A (step S512). That is, in step S512, it is determined whether the lift arm <NUM> is performing the lifting motion or not.

As described above, unlike the case of using the bottom pressures of the lift arm cylinders <NUM>, also in a case of using the discharge pressure Pa detected by the pressure sensor <NUM> to determine presence or absence of the lifting motion for the lift arm <NUM>, adverse effects of variation in pressure due to a load in the bucket <NUM>, vibrations of the vehicle body and the like are small. Accordingly, erroneous determination of the lifting operation for the lift arm <NUM> can be reduced and therefore the increase rate of the lift arm <NUM> and abrupt change in vehicle speed are suppressed.

The storage section 52A stores the first pump threshold P1, a second pump threshold P2 and a third pump threshold P3 that pertain to the discharge pressure of the hydraulic pump <NUM> and are required when the lift arm <NUM> lifts the bucket <NUM> in a state of being loaded. The first pump threshold P1 is the discharge pressure of the hydraulic pump <NUM> when the lift arm <NUM> starts the operation of lifting upward the bucket <NUM> in the state of being loaded. The second pump threshold P2 is the discharge pressure of the hydraulic pump <NUM> when the lift arm <NUM> is in a horizontal attitude. The third pump threshold P3 is the discharge pressure of the hydraulic pump <NUM> when the lift arm <NUM> has been completely lifted in the upper direction, that is, a relief pressure.

If it is determined that the discharge pressure Pa is equal to or higher than the first pump threshold P1 in step S512 (Pa ≥ P1), that is, it is determined that the lift arm <NUM> is performing the lifting motion (step S512/YES), the processing proceeds to the process in step S503.

On the other hand, if it is determined that the vehicle is not in forward travel in step S511 (in a stop state or during reverse travel) (step S511/NO), and if it is determined that the discharge pressure Pa is lower than the first pump threshold P1 in step S512 (Pa < P1), that is, it is determined that the lift arm <NUM> is not performing the lifting motion (step S512/NO), the processing in the controller 5A is finished. This is because the specific condition is not satisfied in these cases. In other words, in this embodiment, "the case of satisfying the specific condition" is at least YES in step S511 and YES in step S512.

In step S506A, the determination section 53A determines the magnitude relationship between the discharge pressure Pa acquired in step S501A and the first pump threshold P1 and second pump threshold P2 read from the storage section 52A. Specifically, the determination section 53A determines whether or not the discharge pressure Pa is equal to or higher than the first pump threshold P1 and is lower than the second pump threshold P2.

In step S506A, when it is determined that the discharge pressure Pa is equal to or higher than the first pump threshold P1 and is lower than the second pump threshold P2 (P1 ≤ Pa < P2) (step S506A/YES), the calculation section 54A calculates the maximum rotational speed Vi of the engine <NUM> according to Vi = k2×Pa (k2 < <NUM>: proportional constant) (step S507A). The command signal output section 55A outputs, to the engine <NUM>, the command signal pertaining to the maximum rotational speed Vi of the engine <NUM> calculated in step S507A (step S510A).

That is, as shown in <FIG>, when the detected discharge pressure Pa is a value ranging from the first pump threshold P1 to the second pump threshold P2 (P1 ≤ Pa < P2), the controller 5A gradually reduces the maximum rotational speed Vi of the engine <NUM> to a predetermined value Vth (= <NUM>,<NUM> rpm) such that the discharge pressure Pa and the maximum rotational speed Vi of the engine <NUM> satisfy an inversely proportional relationship, and limits (reduces) the vehicle speed.

On the contrary, if it is not determined that the discharge pressure Pa is equal to or higher than the first pump threshold P1 and is lower than the second pump threshold P2 (P1 ≤ Pa < P2) in step S506A (step S506A/NO), the determination section 53A further determines whether or not the discharge pressure Pa is equal to or higher than the second pump threshold P2 and is lower than the third pump threshold P3 (step S508A).

In step S508A, when it is determined that the discharge pressure Pa is equal to or higher than the second pump threshold P2 and is lower than the third pump threshold P3 (P2 ≤ Pa < P3) (step S508/YES), the calculation section 54A calculates the maximum rotational speed Vi of the engine <NUM> as the predetermined value Vth (Vi = Vth) irrespective of increase in discharge pressure Pa (step S509A). The command signal output section 55A outputs, to the engine <NUM>, the command signal pertaining to the maximum rotational speed Vi (= Vth) of the engine <NUM> calculated in step S509A (step S510A).

That is, as shown in <FIG>, when the detected discharge pressure Pa pertaining to the lifting operation for the lift arm <NUM> is a value ranging from the second pump threshold P2 to the third pump threshold P3 (P2 ≤ Pa < P3), the controller 5A limits (reduces) the vehicle speed so as to maintain the maximum rotational speed Vi of the engine <NUM> to be the predetermined value Vth (= <NUM>,<NUM> rpm) irrespective of increase in discharge pressure Pa.

As described above, in the case of satisfying the specific condition, the controller 5A may limit the vehicle speed by reducing the maximum rotational speed of the engine <NUM> according to increase in the discharge pressure Pa of the hydraulic pump <NUM>. At this time, irrespective of the discharge pressure Pa of the hydraulic pump <NUM> pertaining to the lifting operation for the lift arm <NUM>, the vehicle speed may be limited in response to increase in the input torque of the hydraulic pump <NUM> pertaining to the lifting operation for the lift arm <NUM>.

The controller 5A thus limits the vehicle speed on the basis of the discharge pressure Pa of the hydraulic pump <NUM> detected by the pressure sensor <NUM> (the input torque). Without limitation thereto, the vehicle speed may be limited on the basis of the average discharge pressure Pav (average input torque) in a predetermined setting time period. In this case, even if the detected value varies due to occurrence of instantaneous large vibrations, collision or the like at the vehicle body, the vehicle speed can be stably limited using the average value.

As shown in <FIG>, in this embodiment, in the former half of the raise and run operation, that is, until the lift arm <NUM> is in the horizontal attitude after start of the lifting operation for the lift arm <NUM>, the maximum rotational speed Vi of the engine <NUM> is gradually reduced to the predetermined value Vth as the discharge pressure Pa of the hydraulic pump <NUM> increases. Accordingly, the vehicle speed is smoothly limited, and the vibrations and shocks on the vehicle body and the operator accompanied by abrupt reduction in speed can be suppressed.

As shown in <FIG>, similar to the first embodiment, the wheel loader <NUM> according to this embodiment may include an adjustment device 65A that can adjust the change rate (proportional constant k2) of the maximum rotational speed Vi of the engine <NUM> with respect to the discharge pressure Pa of the hydraulic pump <NUM> pertaining to the lifting operation for the lift arm <NUM>.

The embodiments of the present invention have thus been described above. Note that the present invention is not limited to the embodiments described above, but is defined by the appended claims. For example, the aforementioned embodiments are detailed description for illustrating the present invention in an understandable manner, and does not necessarily impose limitation to those including the entire configuration described above.

Claim 1:
A wheel loader (<NUM>) comprising a front working device (<NUM>) including a lift arm (<NUM>) rotatable in a vertical direction, the front working device (<NUM>) being provided at a front of a vehicle body, the wheel loader being configured for (<NUM>) traveling by transmitting a drive force of an engine (<NUM>) to wheels via a torque converter (<NUM>), and comprising:
a traveling state sensor (<NUM>) configured to detect a traveling state of the vehicle body;
a motion sensor (<NUM>) configured to detect that the lift arm (<NUM>) is in a lifting motion;
a speed stage switch (<NUM>) configured to select one of a plurality of speed stages (S1 - S4) to which different maximum vehicle speeds are set, and
a controller (<NUM>, 5A) configured to control the engine (<NUM>),
wherein the controller (<NUM>, 5A)
is configured to determine whether a specific condition for specifying an operation of the lift arm (<NUM>) in an upward direction during forward travel of the vehicle body is satisfied or not, and
is configured to limit a vehicle speed by reducing a maximum rotational speed of the engine (<NUM>) in a case of satisfying the specific condition,
wherein the specific condition is that the traveling state sensor (<NUM>) detects (S502) forward traveling of the vehicle body and the motion sensor (<NUM>) detects (S502) a lifting motion of the lift arm (<NUM>) and the speed stage switch (<NUM>) is switched to select a low speed stage (S504).