JP2024072154A - Work machine, work machine controller, and work machine control method - Google Patents

Abstract

To reduce shock occurring when switching from whole wheel driving to rear wheel driving.SOLUTION: A work machine has front wheels, a front wheel driving device to drive the front wheels to rotate, rear wheels, a rear wheel driving device to drive the rear wheels to rotate, a clutch that selectively couples the front wheels and the front feel driving device, and a controller to control the front wheel driving device and the clutch. When receiving input of command to maintain rotation driving of the rear wheels by the rear wheel driving device and stop rotation driving of the front wheels by the front wheel driving device, the controller starts reducing a rotation driving force transmitted from the front wheel driving device to the front wheels. When determining that the rotation driving force becomes smaller than a determination value, or that a prescribed time passed after starting reducing the rotation driving force, the controller releases the clutch.SELECTED DRAWING: Figure 5

Description

The present disclosure relates to a work machine, a work machine controller, and a work machine control method.

Working machines such as motor graders are sometimes equipped with all-wheel drive devices that drive both the front and rear wheels. U.S. Patent No. 6,644,429 (Patent Document 1) discloses a motor grader in which a clutch is placed between the front wheels and a hydraulic motor that drives the front wheels, and the clutch is released to put the machine in rear-wheel drive mode and engaged to put the machine in all-wheel drive mode.

U.S. Pat. No. 6,644,429

When switching from all-wheel drive to rear-wheel drive driving mode on all-wheel drive work machines, a shock may occur.

This disclosure proposes a work machine, a work machine controller, and a work machine control method that can reduce the shock when switching from all-wheel drive to rear-wheel drive.

According to one aspect of the present disclosure, a work machine is proposed that includes front wheels, a front-wheel drive unit that rotationally drives the front wheels, rear wheels, a rear-wheel drive unit that rotationally drives the rear wheels, a clutch that selectively connects the front wheels and the front-wheel drive unit, and a controller that controls the front-wheel drive unit and the clutch. When the controller receives an input of a command to maintain the rotational drive of the rear wheels by the rear-wheel drive unit and to stop the rotational drive of the front wheels by the front-wheel drive unit, the controller starts to reduce the rotational drive force transmitted from the front-wheel drive unit to the front wheels. When the controller determines that the rotational drive force has become smaller than a judgment value or that a certain time has elapsed since the controller started to reduce the rotational drive force, the controller releases the clutch.

According to one aspect of the present disclosure, a controller for a work machine is proposed. The controller starts reducing the rotational drive force transmitted from the front wheel drive unit to the front wheel upon receiving an input of a command to maintain the rotational drive of the rear wheels of the work machine by the rear wheel drive unit and to stop the rotational drive of the front wheels of the work machine by the front wheel drive unit. The controller judges whether the rotational drive force is smaller than a judgment value. The controller judges whether a certain time has passed since the reduction in the rotational drive force started. When the controller judges that the rotational drive force is smaller than the judgment value or that a certain time has passed since the reduction in the rotational drive force started, the controller releases the clutch between the front wheel and the front wheel drive unit.

According to one aspect of the present disclosure, a control method for a work machine is proposed. The control method includes the following steps. The first step is to receive an input of a command to maintain the rotational drive of the rear wheels of the work machine by the rear wheel drive unit and to stop the rotational drive of the front wheels of the work machine by the front wheel drive unit. The second step is to start reducing the rotational drive force transmitted from the front wheel drive unit to the front wheels. The third step is to determine whether the rotational drive force is smaller than a judgment value. The fourth step is to determine whether a certain time has passed since the reduction of the rotational drive force started. The fifth step is to release the clutch between the front wheels and the front wheel drive unit when it is determined that the rotational drive force has become smaller than the judgment value or that a certain time has passed since the reduction of the rotational drive force started.

The work machine, work machine controller, and work machine control method disclosed herein can reduce the shock that occurs when switching from all-wheel drive to rear-wheel drive.

1 is a side view showing a schematic configuration of a motor grader according to an embodiment. FIG. 2 is a diagram showing a schematic configuration of the motor grader shown in FIG. 1 . FIG. 4 is a diagram showing the configuration of a circuit for supplying clutch actuation pressure. FIG. 2 is a block diagram illustrating a functional configuration of a controller. 4 is a flowchart showing a process flow of travel control of a motor grader in an embodiment.

The following describes the embodiments with reference to the drawings. In the following description, identical parts and components are given the same reference numerals. Their names and functions are also the same. Therefore, detailed descriptions of these will not be repeated. It is also intended from the outset that any configuration may be extracted from the embodiments and that they may be combined in any manner.

In this embodiment, a motor grader 1 will be described as an example of a work machine. Figure 1 is a side view showing a schematic configuration of the motor grader 1 in this embodiment.

As shown in FIG. 1, the motor grader 1 of the embodiment is a vehicle with a total of six wheels. The motor grader 1 is equipped with running wheels consisting of a pair of left and right front wheels and two rear wheels on each side. The front wheels include a left front wheel 2 and a right front wheel not shown in FIG. 1. The rear wheels include a left rear front wheel 4, a left rear rear wheel 5, and a right rear front wheel and a right rear rear wheel not shown. The number and arrangement of the front and rear wheels are not limited to the example shown in FIG. 1.

The motor grader 1 is equipped with a work machine that includes a blade 50. The blade 50 is provided between the front and rear wheels. The motor grader 1 can use the blade 50 to perform tasks such as ground leveling, snow removal, and light cutting.

The motor grader 1 has a vehicle body frame. The vehicle body frame has a front frame 51 and a rear frame 52. The front frame 51 is rotatably connected to the rear frame 52.

The front wheels are mounted on the front frame 51 together with the blades 50. The front wheels are rotatably attached to the front end of the front frame 51. The rear wheels are mounted on the rear frame 52. The rear wheels are rotatably attached to the rear frame 52 by the driving force from the engine, as described below.

Figure 2 is a diagram showing the schematic configuration of the motor grader 1 shown in Figure 1. The pair of left and right front wheels described above includes a left front wheel 2 and a right front wheel 3. The motor grader 1 is equipped with an engine 6. The engine 6 is supported by a rear frame 52 shown in Figure 1. The engine 6 is a drive source that generates a driving force to rotate the front and rear wheels, and is, for example, a diesel engine.

The output side of the engine 6 is connected to the left rear wheels 4, 5, and the left rear wheels 4, 5 and a pair of right rear wheels (not shown) via a torque converter 8, a transmission 9, a final reduction gear 10, and a tandem device 11. The torque converter 8 is a fluid clutch that transmits driving force from the engine 6 using oil as a medium. The transmission 9 is a mechanical transmission. The transmission 9 has multiple clutches corresponding to multiple speed stages. The transmission 9 switches between multiple speed stages by switching between the connected and unconnected states of each clutch. The engine 6 drives the left rear wheels 4, 5 and the right rear wheel via the torque converter 8, the transmission 9, a final reduction gear 10, and a tandem device 11.

The torque converter 8, the transmission 9, the final reduction gear 10, and the tandem gear 11 constitute a rear wheel power transmission device that transmits the driving force generated by the engine 6 to the rear wheels. The final reduction gear 10 corresponds to an example of a rear wheel drive device that drives the rear wheels to rotate.

A pair of left and right hydraulic systems 7L, 7R are connected to the transmission 9. The hydraulic system 7L drives the left front wheel 2. The hydraulic system 7R drives the right front wheel 3. The engine 6 drives the left front wheel 2 and the right front wheel 3 via a torque converter 8, the transmission 9, and the hydraulic systems 7L, 7R. The hydraulic systems 7L, 7R may be connected to the other output side of the engine 6 without going through the mechanical transmission 9. The left and right hydraulic systems 7L, 7R each constitute an HST (hydraulic static transmission).

The motor grader 1 is an all-wheel drive vehicle in which the front wheels 2, 3, the left rear wheels 4, 5, and the right rear wheel can all be driven by power generation and transmission devices 6-11. These devices 6-11 make up the all-wheel drive device 12. Of the all-wheel drive device 12, the engine 6, parts of the hydraulic systems 7L, 7R, the torque converter 8, the transmission 9, and the final reduction gear 10 are supported by the rear frame 52.

The hydraulic system 7L includes a left hydraulic pump 15 and a left hydraulic motor 16. The hydraulic system 7R includes a right hydraulic pump 17 and a right hydraulic motor 18. The output of the engine 6 is transmitted to the left hydraulic pump 15 and the right hydraulic pump 17 via the PTO (Power Take-Off) 14 to drive the left hydraulic pump 15 and the right hydraulic pump 17. The left hydraulic motor 16 is rotated by hydraulic oil discharged from the left hydraulic pump 15 to drive the left front wheel 2. The right hydraulic motor 18 is rotated by hydraulic oil discharged from the right hydraulic pump 17 to drive the right front wheel 3.

Hydraulic pumps 15, 17 are variable displacement hydraulic pumps. Hydraulic pumps 15, 17 may be swash plate type hydraulic pumps having a variable swash plate. The angle of the variable swash plate of left hydraulic pump 15 is continuously and steplessly controlled by swash plate driver 15A in accordance with a control command value output from a controller described later. The angle of the variable swash plate of right hydraulic pump 17 is continuously and steplessly controlled by swash plate driver 17A in accordance with a control command value output from a controller described later, independently of the variable swash plate of left hydraulic pump 15. Swash plate drivers 15A, 17A are, for example, solenoids.

The hydraulic motors 16, 18 may be variable displacement motors. The hydraulic motors 16, 18 may be bent-axis axial motors. The displacement of the hydraulic motors 16, 18 is constant depending on the speed stage selected by the operator. The hydraulic motors 16, 18 may be fixed displacement motors.

The left hydraulic pump 15 and the left hydraulic motor 16 are connected by the left hydraulic circuit 21. The hydraulic oil discharged from the left hydraulic pump 15 is supplied to the left hydraulic motor 16 via the left hydraulic circuit 21. The rotation speed of the left front wheel 2 when the left front wheel 2 is driven is controlled by the hydraulic oil discharged from the left hydraulic pump 15. The left hydraulic circuit 21 is provided with pressure sensors 27L, 28L that detect the pressure of the hydraulic oil in the left hydraulic circuit 21. The pressure sensors 27L, 28L output signals that indicate the hydraulic pressure in the left hydraulic circuit 21.

The right hydraulic pump 17 and the right hydraulic motor 18 are connected by a right hydraulic circuit 22. The hydraulic oil discharged from the right hydraulic pump 17 is supplied to the right hydraulic motor 18 via the right hydraulic circuit 22. The rotation speed of the right front wheel 3 when the right front wheel 3 is driven is controlled by the hydraulic oil discharged from the right hydraulic pump 17. The right hydraulic circuit 22 is provided with pressure sensors 27R, 28R that detect the pressure of the hydraulic oil in the right hydraulic circuit 22. The pressure sensors 27R, 28R output signals indicative of the hydraulic pressure in the right hydraulic circuit 22.

The power transmission device for transmitting the driving force from the engine 6 to the front wheels 2, 3 generates pressure in the hydraulic oil by driving the hydraulic pumps 15, 17 with the engine 6, and the hydraulic motors 16, 18 are driven by the pressurized oil discharged from the hydraulic pumps 15, 17, thereby generating rotational force again. In other words, as described above, the left and right hydraulic systems 7L, 7R each constitute an HST.

The pressure sensors 27L and 27R are provided in an oil passage through which hydraulic oil flows from the hydraulic pump to the hydraulic motor when the motor grader 1 is traveling forward. The pressure sensors 27L and 27R detect the pressure of the high-pressure hydraulic oil discharged from the hydraulic pump when the motor grader 1 is traveling forward. The pressure sensors 28L and 28R are provided in an oil passage through which hydraulic oil flows from the hydraulic pump to the hydraulic motor when the motor grader 1 is traveling backward. The pressure sensors 28L and 28R detect the pressure of the high-pressure hydraulic oil discharged from the hydraulic pump when the motor grader 1 is traveling backward.

The left hydraulic clutch mechanism 23 and the left reducer 25 are provided between the left front wheel 2 and the left hydraulic motor 16. The right hydraulic clutch mechanism 24 and the right reducer 26 are provided between the right front wheel 3 and the right hydraulic motor 18. When hydraulic pressure is supplied to the left hydraulic clutch mechanism 23 and the right hydraulic clutch mechanism 24, power is transmitted to the left front wheel 2 and the right front wheel 3, and the motor grader 1 becomes all-wheel drive. When the supply of hydraulic pressure to the left hydraulic clutch mechanism 23 and the right hydraulic clutch mechanism 24 is cut off, the motor grader 1 is released from all-wheel drive and becomes rear-wheel drive.

The torque converter 8, the transmission 9, the PTO 14, the hydraulic systems 7L, 7R, the clutch mechanisms 23, 24, and the reducers 25, 26 constitute a front wheel power transmission device that transmits the driving force generated by the engine 6 to the front wheels. The hydraulic motors 16, 18 correspond to an example of a front wheel drive device that drives the front wheels to rotate. The rotation speed of the front wheels can be increased by increasing the amount of hydraulic oil supplied from the hydraulic pumps 15, 17 to the hydraulic motors 16, 18 (the hydraulic pump discharge amount). The rotation speed of the front wheels can be decreased by decreasing the amount of hydraulic oil supplied from the hydraulic pumps 15, 17 to the hydraulic motors 16, 18.

The speed sensor 31 is provided on the output shaft of the transmission 9. The speed sensor 31 detects the rotational speed of the rear wheels when the motor grader 1 is moving (driving) by measuring the rotational speed of the output shaft of the transmission 9. The speed sensor 31 outputs a signal indicating the rotational speed of the rear wheels.

Figure 3 is a diagram showing the configuration of a circuit that supplies clutch actuation pressure. The motor grader 1 of the embodiment has an external pressure clutch that has a clutch control circuit that supplies clutch actuation pressure, separate from the HST that transmits rotational driving force to the front wheels 2, 3. Of the left and right hydraulic circuits and clutches, Figure 3 representatively illustrates the left hydraulic circuit 21 and the left hydraulic clutch mechanism 23. Figure 3 also representatively illustrates the charge circuit 40 that switches the left hydraulic clutch mechanism 23 between an engaged state and a released state. The hydraulic clutch mechanisms 23, 24 are hereinafter also simply referred to as clutches 23, 24.

The left hydraulic clutch mechanism 23 is a wet multi-plate clutch. The clutch plates 23A and 23B shown in FIG. 3 are representative of two adjacent plates among the multiple plates included in the left hydraulic clutch mechanism 23. The clutch plates 23A and 23B are arranged facing each other.

The left front wheel rotating shaft 29A is connected to the left hydraulic motor 16 so as to be rotatable integrally with the output shaft of the left hydraulic motor 16. The left front wheel rotating shaft 29B is connected to the left front wheel 2 so as to be rotatable integrally with the left front wheel 2. Clutch plates 23A, 23B are disposed between the left front wheel rotating shaft 29A and the left front wheel rotating shaft 29B.

The clutch 23 is in an engaged state when the clutch plates 23A and 23B are in contact and rotate together. The clutch 23 is in a released state when the clutch plates 23A and 23B are spaced apart, a gap is formed between the clutch plates 23A and 23B, and the rotation of the clutch plates 23A is not transmitted to the clutch plates 23B even when the clutch plates 23A rotate.

The rotation sensor 32 is provided on the left front wheel rotating shaft 29A and detects the rotation speed of the left front wheel rotating shaft 29A. The rotation sensor 32 outputs a signal indicating the rotation speed of the left front wheel rotating shaft 29A. The rotation sensor 33 is provided on the left front wheel rotating shaft 29B and detects the rotation speed of the left front wheel rotating shaft 29B. The rotation sensor 33 outputs a signal indicating the rotation speed of the left front wheel rotating shaft 29B.

The piston rod 45 presses the clutch plate 23A toward the clutch plate 23B to engage the clutch 23. The clutch cylinder 44 has a cylindrical shape and houses a piston inside. The piston rod 45 extends from the inside to the outside of the clutch cylinder 44. The base end of the piston rod 45 is disposed inside the clutch cylinder 44 and is attached to the piston. The tip of the piston rod 45 is disposed outside the clutch cylinder 44. The piston rod 45 is configured to be able to reciprocate relative to the clutch cylinder 44 in the axial direction of the cylindrical clutch cylinder 44 (in FIG. 3, the up-down direction in the figure), and the length by which the piston rod 45 protrudes from the clutch cylinder 44 can be changed. A return spring 46 is disposed inside the clutch cylinder 44.

When pressure oil is supplied to the oil chamber inside the clutch cylinder 44 via the charge oil passage 42, the pressure oil acts on the piston, and the piston rod 45 moves in a direction that pushes it out of the clutch cylinder 44. The piston rod 45 presses the clutch plate 23A in a direction that brings the clutch plate 23A closer to the clutch plate 23B. As a result, the clutch plate 23A comes into contact with the clutch plate 23B, and the clutch 23 enters an engaged state.

When the supply of pressurized oil to the oil chamber inside the clutch cylinder 44 is stopped, the return spring 46 applies a biasing force that draws the piston rod 45 into the clutch cylinder 44. The piston rod 45 and clutch plate 23A move in a direction in which the clutch plate 23A moves away from the clutch plate 23B. As a result, the clutch plate 23A and the clutch plate 23B are no longer in contact with each other, and the clutch 23 is released.

The charge pump 41 is provided as a separate component from the left hydraulic circuit 21. The charge pump 41 supplies pressurized oil to the charge oil passage 42. The charge pump 41 supplies pressurized oil to the clutch cylinder 44 via the charge oil passage 42. A clutch control valve 43 is provided in the charge oil passage 42. The clutch control valve 43 is, for example, a solenoid valve. When the clutch control valve 43 is in a non-energized state, the supply of pressurized oil to the oil chamber inside the clutch cylinder 44 is stopped, and the clutch 23 is released. When the clutch control valve 43 is switched to an energized state, pressurized oil is supplied to the oil chamber inside the clutch cylinder 44, and the clutch 23 is engaged.

The charge circuit 40 that switches the left hydraulic clutch mechanism 23 between the engaged and released states has been described with reference to Figure 3, but the charge circuit (not shown) that switches the right hydraulic clutch mechanism 24 (Figure 2) between the engaged and released states is also provided separately from the right hydraulic circuit 22 and has a similar configuration to the charge circuit 40 for the left hydraulic clutch mechanism 23. The charge pump 41 is common to the left and right charge circuits. There is one charge pump 41, and the left and right charge circuits each have a clutch control valve that supplies charge pressure to the clutch.

Figure 4 is a block diagram explaining the functional configuration of the controller 60. The motor grader 1 is equipped with the controller 60. The controller 60 reads and executes various programs using a CPU (central processing unit) (not shown) and performs various calculation processes.

The driving mode changeover switch 80 is operated by an operator. The amount of operation of the driving mode changeover switch 80 by the operator is converted into an electrical signal and input to the controller 60.

The driving modes include an all-wheel drive driving mode and a rear-wheel drive driving mode. In the all-wheel drive driving mode, power is transmitted to the front wheels 2 and 3, and also to the left and right rear wheels, and the motor grader 1 drives with all six driving wheels as drive wheels. In the rear-wheel drive driving mode, the transmission of power to the front wheels 2 and 3 is cut off, power is transmitted to the left and right rear wheels, and the motor grader 1 drives with the four left and right rear wheels as drive wheels.

When the operator manually operates the driving mode changeover switch 80, the driving mode selected by the operator is input to the driving mode input unit 61 of the controller 60. The driving mode input unit 61 accepts the input of the driving mode by the operator's operation.

The pressure sensor 27L provided in the left hydraulic circuit 21 and the pressure sensor 27R provided in the right hydraulic circuit 22 shown in FIG. 2 are collectively referred to as pressure sensor 27. The pressure sensor 28L provided in the left hydraulic circuit 21 and the pressure sensor 28R provided in the right hydraulic circuit 22 are collectively referred to as pressure sensor 28. The detection results of the pressure of the hydraulic oil in the hydraulic circuits from the pressure sensors 27 and 28 are input to the pressure detection value input unit 63 of the controller 60.

The controller 60 has a memory 72. The memory 72 stores a program for controlling the operation of the motor grader 1, and various data required to execute the program. The memory 72 also temporarily stores working data that is generated as work is performed. The controller 60 has a timer 73. The timer 73 keeps track of time.

Figure 5 is a flowchart showing the flow of the process of driving control of the motor grader 1 in the embodiment. Below, the process executed when the controller 60 switches the driving mode of the motor grader 1 from the all-wheel drive driving mode to the rear-wheel drive driving mode will be explained with appropriate reference to Figures 4 and 5.

As shown in FIG. 5, in step S1, the controller 60 receives an input that the operator has switched the driving mode. More specifically, in step S1 of FIG. 5, the driving mode input unit 61 receives an input from the driving mode changeover switch 80 that the operator has operated the driving mode changeover switch 80 to switch the driving mode from the all-wheel drive (AWD) driving mode to the rear-wheel drive (RWD) driving mode. The driving mode input unit 61 receives an input of a command to maintain the rotational drive of the rear wheels by the rear-wheel drive unit, while stopping the rotational drive of the front wheels 2, 3 by the front-wheel drive unit.

In step S2, the controller 60 controls the capacity of the hydraulic pumps 15, 17. The hydraulic pumps 15, 17 are variable capacity hydraulic pumps, and the pump capacity command unit 71 reduces the flow rate of hydraulic oil discharged by the hydraulic pumps 15, 17. The pump capacity command unit 71 controls the flow rate of hydraulic oil discharged by the hydraulic pumps 15, 17 to be gradually reduced over time, rather than being reduced all at once to zero.

Specifically, the pump capacity command unit 71 sets the flow rate of hydraulic oil that is smaller than the current flow rate of hydraulic oil discharged by the hydraulic pumps 15, 17 and greater than zero. The pump capacity command unit 71 sets the flow rate of hydraulic oil so that it decreases over time. The pump capacity command unit 71 may decrease the flow rate of hydraulic oil in stages. The pump capacity command unit 71 may decrease the flow rate of hydraulic oil gradually, or may decrease the flow rate of hydraulic oil so that it is proportional to time.

The pump capacity command unit 71 sends a control signal to the swash plate drive units 15A, 17A of each hydraulic pump 15, 17 so that hydraulic oil is supplied at that flow rate from the hydraulic pumps 15, 17 to the hydraulic motors 16, 18. Even if the engine 6 rotation speed is the same, the flow rate of hydraulic oil discharged from the hydraulic pumps 15, 17 can be changed by controlling the swash plates of each hydraulic pump 15, 17. In this manner, the hydraulic pumps 15, 17 are controlled.

In the all-wheel drive mode, the clutches 23, 24 are engaged. When control to reduce the flow rate of hydraulic oil discharged by the hydraulic pumps 15, 17 is started, the clutches 23, 24 are maintained in the engaged state. When control to reduce the flow rate of hydraulic oil discharged by the hydraulic pumps 15, 17 is started, no command is output to the clutch control valve 43 to release the clutches 23, 24.

In step S3, the controller 60 determines whether the estimated front wheel drive force is smaller than the judgment value.

The estimated front wheel driving force is calculated by adding the estimated value of the rotational driving force applied to the left front wheel 2 (referred to herein as the "estimated front wheel driving force (left)") and the estimated value of the rotational driving force applied to the right front wheel 3 (referred to herein as the "estimated front wheel driving force (right)"). That is, the estimated front wheel driving force is calculated by the following formula (1).

The estimated front wheel driving force (left) and the estimated front wheel driving force (right) are collectively referred to as the estimated front wheel driving force (left/right). The estimated front wheel driving force (left/right) is calculated using the following formula (2).

In equation (2), the HST circuit pressure is the pressure of the hydraulic oil in the hydraulic circuit detected by the pressure sensors 27, 28 and input to the pressure detection value input unit 63. The motor capacity is the capacity of the hydraulic motors 16, 18. The reduction ratio is the reduction ratio of the reducers 25, 26. The efficiency is a coefficient that takes into account the efficiency of the hydraulic motors 16, 18 and the efficiency of the reducers 25, 26. The tire load radius is the effective radius of the front wheels 2, 3 that is calculated from the distance that the front wheels 2, 3 actually travel. The reduction ratio, efficiency, and tire load radius are stored in memory 72.

The judgment value is calculated by multiplying the running resistance by a coefficient. In other words, the judgment value is calculated by the following formula (3).

The running resistance in equation (3) is calculated using the weight (machine weight) of the motor grader 1 acting on the front wheels 2 and 3 and the rolling resistance coefficient of the front wheels 2 and 3, as shown in the following equation (4).

The coefficient in equation (3) is set to a value that can sufficiently reduce the shock that occurs even if the clutches 23, 24 are released while the motor grader 1 is traveling. The coefficient is set to a value greater than 0 and less than 1. The coefficient may be set to a value of 0.5 or less. For example, the coefficient may be set to 0.3. The coefficient may be set to 0.2.

The front wheel driving force determination unit 64 shown in FIG. 4 applies the HST circuit pressure input to the pressure detection value input unit 63, the capacity of the hydraulic motors 16, 18, and the reduction ratio, efficiency, and tire load radius read from the memory 72 to equations (1) and (2). In this way, the front wheel driving force determination unit 64 calculates an estimated front wheel driving force. The front wheel driving force determination unit 64 also calculates a determination value based on equations (3) and (4).

The front wheel drive force determination unit 64 compares the calculated estimated front wheel drive force with the calculated determination value. If it is determined that the estimated front wheel drive force is equal to or greater than the determination value (NO in step S3), in step S4, the controller 60 determines whether a certain amount of time has elapsed since control to reduce the flow rate of hydraulic oil discharged by the hydraulic pumps 15, 17 was started. The time determination unit 66 reads the current time from the timer 73. The time determination unit 66 calculates the elapsed time from the time when the pump capacity command unit 71 started sending a control signal to the swash plate drive units 15A, 17A in the processing of step S2 to the current time.

The time determination unit 66 judges whether the elapsed time has reached a predetermined time threshold. The time determination unit 66 reads from the memory 72 the time threshold (the "fixed time" shown in FIG. 5) stored in the memory 72. The time determination unit 66 compares the calculated elapsed time with the integrated time read from the memory 72. If the current elapsed time is less than the fixed time as a result of comparing the elapsed time with the fixed time, the time determination unit 66 judges that the fixed time has not yet elapsed since control to reduce the flow rate of hydraulic oil was started.

If it is determined that a certain amount of time has not yet elapsed since control to reduce the flow rate of hydraulic oil was started (NO in step S4), the clutches 23, 24 are maintained in an engaged state in step S6. The clutch command unit 70 sends a control signal to the clutch control valve 43 to continue supplying pressure oil to the clutch cylinder 44 and maintain the state in which clutch plate 23A is pressed against clutch plate 23B. This maintains the clutches 23, 24 in an engaged state. Then, the process returns to the determination in step S3.

If it is determined in step S3 that the estimated front wheel drive force is less than the judgment value (YES in step S3), or if it is determined in step S4 that a certain amount of time has elapsed since control to reduce the flow rate of hydraulic oil was started (YES in step S4), a clutch release command is output.

In step S7, the clutches 23, 24 are released. The clutch command unit 70 sends a control signal to the clutch control valve 43 to stop the supply of pressurized oil to the clutch cylinder 44 and separate the clutch plate 23A from the clutch plate 23B. This releases the clutches 23, 24 and switches the motor grader 1 to a rear-wheel drive state. In this way, the switching of the drive mode of the motor grader 1 from the all-wheel drive drive mode to the rear-wheel drive drive mode is completed ("END" in Figure 5).

In addition to the flow shown in FIG. 5, when the clutch release condition is satisfied, the clutches 23 and 24 are released. The clutch release condition determination unit 67 shown in FIG. 4 determines whether the clutch release condition is satisfied.

The clutch release condition determination unit 67 may receive input of the rotation state of the engine 6 from, for example, an engine speed sensor (not shown). The clutch release condition determination unit 67 may determine that the clutch release condition is met when the engine 6 speed is lower than a threshold value.

The clutch release condition determination unit 67 may receive an input of the pressure of the hydraulic oil in the charge oil passage 42, for example, from a pressure sensor (not shown). The clutch release condition determination unit 67 may determine that the clutch release condition is met when the pressure of the hydraulic oil in the charge oil passage 42 is lower than a threshold value.

The clutch release condition determination unit 67 may receive an input of the traveling direction of the motor grader 1 from, for example, a travel operation detection unit (not shown) that detects the amount of operation of a travel operation device operated for traveling the motor grader 1. The clutch release condition determination unit 67 may determine that the clutch release condition is met when an operation of a travel operation device that switches between forward traveling and reverse traveling of the motor grader 1 is performed.

As described above, in this embodiment, as shown in Figures 4 and 5, the controller 60 (travel mode input unit 61) receives an input of a command to switch the travel mode of the motor grader 1 from an all-wheel drive travel mode to a rear-wheel drive travel mode. The controller 60 (pump capacity command unit 71) reduces the flow rate of hydraulic oil supplied to the hydraulic motors 16 and 18. As the flow rate of hydraulic oil decreases, the pressure of the hydraulic oil in the hydraulic circuit gradually decreases. The hydraulic motors 16 and 18 are front-wheel drive devices that drive the front wheels 2 and 3 to rotate, and the pressure of the hydraulic oil supplied to the hydraulic motors 16 and 18 corresponds to an example of the rotational drive force transmitted to the front wheels 2 and 3.

As shown in Figures 4 and 5, when the controller 60 (front wheel drive force determination unit 64) determines that the estimated front wheel drive force is smaller than the determination value, or when the controller 60 (time determination unit 66) determines that a certain time has elapsed since the rotational drive force transmitted to the front wheels 2 and 3 started to decrease, the controller 60 (clutch command unit 70) releases the clutches 23 and 24.

When the estimated front wheel driving force is equal to or greater than the judgment value, it is considered that the driving force of the front wheels 2, 3 accounts for a large proportion of the running resistance of the motor grader 1. If the clutches 23, 24 are instantly released and the driving force is suddenly no longer applied to the front wheels 2, 3 when the front wheel driving force is greater than the running resistance, running using the driving force of the rear wheels begins from the point of release. If the running resistance of the motor grader 1 is greater than the rotational driving force of the rear wheels, a shock may occur. Shock is more likely to occur when the speed ratio, which is the ratio of the speed of the front wheels 2, 3 to the speed of the rear wheels, is large (for example, speed ratio > 1).

After the driving mode is switched, the flow rate of hydraulic oil discharged from the hydraulic pumps 15, 17 is reduced while the clutches 23, 24 are maintained in an engaged state for a certain period of time. The pressure of the hydraulic oil supplied to the hydraulic motors 16, 18 is reduced before the clutches 23, 24 are released, thereby reducing the rotational driving force transmitted to the front wheels 2, 3. The clutches 23, 24 are released only after it is determined that the proportion of the driving force borne by the rear wheels relative to the running resistance of the motor grader 1 has increased to such an extent that no shock occurs when the clutches 23, 24 are switched from an engaged state to a released state. By switching from the all-wheel drive driving mode to the rear-wheel drive driving mode in this way, the shock when switching the driving mode can be reduced.

When it is determined that a certain amount of time has passed since the rotational driving force transmitted to the front wheels 2, 3 started to decrease, the clutches 23, 24 are released even if the estimated front wheel driving force is still equal to or greater than the determination value. If the operator switches the driving mode from all-wheel drive driving mode to rear-wheel drive driving mode and then leaves the driving mode in all-wheel drive driving mode for a long time, the responsiveness to the operation is impaired, which may cause the operator discomfort. When a certain amount of time has passed since receiving a command from the operator, the clutches 23, 24 are released even if some shock occurs. By not leaving the clutches 23, 24 engaged for a long time and improving the responsiveness to the operator's operation, the operator's discomfort can be eliminated.

The estimated front wheel drive force (left/right) is proportional to the pressure of the hydraulic oil supplied to the hydraulic motors 16, 18. When the controller 60 (front wheel drive force determination unit 64) determines that the estimated front wheel drive force is smaller than the determination value, the pressure of the hydraulic oil supplied to the hydraulic motors 16, 18 is smaller than a predetermined value. By determining that the pressure of the hydraulic oil supplied to the hydraulic motors 16, 18 is smaller than the predetermined value and releasing the clutches 23, 24, the possibility of shock occurring when the clutches 23, 24 are released can be reduced.

As shown in Figures 4 and 5, the clutches 23, 24 are maintained engaged until a certain time has elapsed since the rotational drive force of the front wheels 2, 3 began to decrease, and as long as the rotational drive force of the front wheels 2, 3 is equal to or greater than the judgment value. Even if a command is input to switch the driving mode of the motor grader 1 from the all-wheel drive driving mode to the rear-wheel drive driving mode, the clutches 23, 24 are not suddenly released, but the rotational drive force transmitted to the front wheels 2, 3 is reduced before the clutches 23, 24 are released. This reliably reduces the possibility of shock occurring when the clutches 23, 24 are released.

In the above embodiment, power is transmitted to the front wheels 2, 3 by the HST, while mechanical power is transmitted to the rear wheels via the transmission 9. This example is not limited to this, and any power transmission device can be selected as long as the rear wheels are rotationally driven by a rear-wheel drive device, and the front wheels are rotationally driven by a front-wheel drive device independently of the rear wheels. For example, power may be transmitted to the rear wheels via an HST separate from the hydraulic systems 7L, 7R. The front-wheel drive device that rotates the front wheels 2, 3 is not limited to the hydraulic motors 16, 18, and may be, for example, an electric motor.

The controller that executes the process of switching the driving mode of the motor grader 1 described in the above embodiment does not necessarily have to be mounted on the motor grader 1. A system may be configured in which the controller 60 mounted on the motor grader 1 performs the process of transmitting the detection values of the pressure sensors 27, 28 to an external controller, and the external controller that receives the signal switches the driving mode. The external controller may be located at the work site of the motor grader 1, or may be located in a remote location away from the work site of the motor grader 1.

In the above embodiment, a motor grader 1 is given as an example of a work machine, but the present invention is not limited to a motor grader 1 and can be applied to other types of work machines.

The embodiments disclosed herein are illustrative in all respects and should not be considered limiting. The scope of the present invention is indicated by the claims, not by the above description, and is intended to include all modifications within the meaning and scope of the claims.

1 Motor grader, 2 Left front wheel, 3 Right front wheel, 4 Left rear front wheel, 5 Left rear rear wheel, 6 Engine, 7L, 7R Hydraulic system, 8 Torque converter, 9 Transmission, 10 Final reduction gear, 11 Tandem gear, 12 All-wheel drive gear, 15 Left hydraulic pump, 15A, 17A Swash plate drive unit, 16 Left hydraulic motor, 17 Right hydraulic pump, 18 Right hydraulic motor, 21 Left hydraulic circuit, 22 Right hydraulic circuit, 23 Left hydraulic clutch mechanism, 23A, 23B Clutch plate, 24 Right hydraulic clutch mechanism, 25 Left reduction gear, 26 Right reduction gear, 27, 28 Pressure sensor, 29A, 29B Left front wheel rotating shaft, 31 Speed sensor, 32, 33 Rotation sensor, 40 Charge circuit, 41 Charge pump, 42 Charge oil passage, 43 Clutch control valve, 44 Clutch cylinder, 45 Piston rod, 46 return spring, 50 blade, 51 front frame, 52 rear frame, 60 controller, 61 driving mode input section, 63 pressure detection value input section, 64 front wheel drive force determination section, 66 time determination section, 67 clutch release condition determination section, 70 clutch command section, 71 pump capacity command section, 72 memory, 73 timer, 80 driving mode changeover switch.

Claims (6)

The front wheel and
a front wheel drive device that drives and rotates the front wheels;
The rear wheel and
a rear wheel drive device that drives and rotates the rear wheels;
a clutch selectively connecting the front wheels to the front wheel drive system;
a controller for controlling the front wheel drive device and the clutch,
When the controller receives an input of a command to maintain the rotational driving of the rear wheels by the rear wheel drive device and to stop the rotational driving of the front wheels by the front wheel drive device, it begins to reduce the rotational driving force transmitted from the front wheel drive device to the front wheels, and when it is determined that the rotational driving force has become smaller than a judgment value or when it is determined that a certain time has elapsed since the reduction in the rotational driving force began, the controller releases the clutch. The front wheel drive device includes a hydraulic motor that is rotated by receiving a supply of hydraulic oil,
The work machine according to claim 1 , wherein the controller starts reducing the pressure of the hydraulic oil supplied to the hydraulic motor when a command to stop driving the rotation of the front wheels is received. The work machine according to claim 2, wherein the controller releases the clutch when it is determined that the pressure of the hydraulic oil is less than a predetermined value. The working machine according to any one of claims 1 to 3, wherein the controller maintains the clutch engaged until a certain time has elapsed since the rotational drive force started to decrease and the rotational drive force is equal to or greater than the determination value. upon receiving an input of a command to maintain the rotational driving of the rear wheels of the work machine by the rear wheel drive device and to stop the rotational driving of the front wheels of the work machine by the front wheel drive device, starting a reduction in the rotational driving force transmitted from the front wheel drive device to the front wheels;
determining whether the rotational driving force is smaller than a threshold value;
determining whether a certain time has elapsed since the reduction in the rotational drive force was started;
A controller for a work machine that releases a clutch between the front wheels and the front wheel drive device when it is determined that the rotational driving force has become smaller than the judgment value, or when it is determined that the certain time has elapsed since the rotational driving force began to decrease. receiving an input of a command to maintain rotational driving of the rear wheels of the work machine by the rear wheel drive device and to stop rotational driving of the front wheels of the work machine by the front wheel drive device;
commencing a reduction in rotational drive force transmitted from the front wheel drive system to the front wheels;
determining whether the rotational driving force is smaller than a threshold value;
determining whether a certain time has elapsed since the rotational drive force started to decrease;
and releasing a clutch between the front wheels and the front wheel drive device when it is determined that the rotational driving force has become smaller than the judgment value or when it is determined that the certain time has elapsed since the rotational driving force started to decrease.

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