JP7808530B2 - Work vehicles - Google Patents

Description

The present invention relates to a work vehicle.

A well-known example of a work vehicle is a wheel loader equipped with a travel device for moving the vehicle body and a work device with a bucket and arm for digging up soil and sand. Excavation, one type of work performed by such a work vehicle, involves the operator operating, for example, the accelerator pedal to increase the driving force for moving the vehicle forward, while penetrating the bucket into the work object, such as natural ground, and then lifting the bucket to scoop up the work object.

When a work vehicle is performing excavation work, if the driving force required to move the vehicle forward exceeds the maximum static friction force between the tires and the ground, the tires will slip. If slippage occurs during excavation work, the bucket cannot penetrate sufficiently into the work object, reducing work efficiency. The operator must adjust the driving force by feel to a level that prevents slippage during excavation work, which places a heavy burden on the operator.

In response, technology has been proposed to automatically suppress slippage during excavation work by work vehicles (see Patent Document 1). Patent Document 1 discloses "a driving force control device for a work vehicle equipped with a work implement, in which engine power is transmitted as driving force to tires via a driving force transmission path, the device comprising: driving force variable means provided in the driving force transmission path for freely varying the driving force transmitted to the tires; tire slip detection means for detecting the occurrence of tire slip; driving force measurement means for measuring the driving force; and driving force control means for controlling the driving force variable means so that the driving force is less than the driving force at the time tire slip is detected."

WO2008/146846A1

However, with technology that detects slip based on wheel speed (tire speed), such as the technology described in Patent Document 1, if slip occurs in which the wheel speed increases gradually, it takes time for the wheel speed to reach the threshold value for slip detection, which can result in a delay in slip detection. If there is a delay in slip detection, the time that slip occurs will be longer, resulting in reduced work efficiency.

The present invention aims to provide a work vehicle that can reduce the burden on the operator by automatically limiting the driving force when the wheels slip, and that can improve work efficiency by detecting wheel slip more quickly and shortening the duration of the slip.

A work vehicle according to one aspect of the present invention comprises a vehicle body having wheels, a hydraulic pump mounted on the vehicle body, a work device having a hydraulic actuator driven by hydraulic fluid discharged from the hydraulic pump, a pressure sensor that detects the pressure of the hydraulic actuator, a traveling drive device that drives the wheels, a driving force sensor that detects the driving force of the traveling drive device, and a control device that controls the driving force of the traveling drive device. The control device calculates the thrust of the hydraulic actuator in accordance with the reaction force that the working device receives from the work object based on the detection result of the pressure sensor, and limits the driving force when a slip determination condition is met, including the thrust of the hydraulic actuator not increasing even though the driving force of the traveling drive device is increasing.

The present invention provides a work vehicle that can reduce the burden on the operator by automatically limiting the driving force when wheels slip, and that can improve work efficiency by detecting wheel slip more quickly and shortening the duration of slippage.

FIG. 1 is a side view of a wheel loader. FIG. 2 is a configuration diagram of a control system for a wheel loader. FIG. 3 is a functional block diagram of the main controller. FIG. 4 is a diagram showing an example of a correlation map that defines the relationship between the thrust force during slip and the estimated friction coefficient value. FIG. 5 is a diagram showing an example of a correlation map that defines the relationship between the estimated friction coefficient value and the driving force upper limit value. FIG. 6 is a flowchart showing an example of a main flow of the driving force control executed by the main controller. FIG. 7 is a flowchart showing an example of the process of setting the excavation determination flag in FIG. FIG. 8 is a flowchart showing an example of the slip determination flag setting process of FIG. FIG. 9 is a diagram showing time series changes in the parameters (bucket operation amount, arm operation amount, travel driving force, arm cylinder thrust, wheel speed, and slip determination flag) of the wheel loaders according to this embodiment and the comparative example.

Embodiments of a work vehicle according to the present invention will be described below with reference to the drawings. In the description of the drawings, identical elements will be assigned the same reference numerals, and duplicate descriptions will be omitted. In this embodiment, an example will be described in which the work vehicle is an electrically driven wheel loader; however, the work vehicle of the present invention is not limited to wheel loaders and may be a bulldozer, etc. Furthermore, in this embodiment, an example will be described in which a hybrid system using an engine and a generator motor as a drive source is employed; however, a system using an engine alone as a drive source may also be employed. Furthermore, in the following description, the directions and positions of up/down, left/right, and front/rear will be based on the normal use state of the work vehicle, i.e., a state in which all four wheels are in contact with the traveling surface.

- Wheel loader configuration -
Figure 1 is a side view of a wheel loader 1. As shown in Figure 1, the wheel loader 1 comprises a vehicle body 8 on which an electric travel drive unit 45 is mounted, and an articulated working device 6 attached to the front of the vehicle body 8. The vehicle body 8 is of an articulated steering type (vehicle body articulation type) and has a front vehicle body 8A, a rear vehicle body 8B, and a center joint 10 that connects the front vehicle body 8A and the rear vehicle body 8B.

A working device 6 is attached to the front body 8A. A cab 12 and an engine compartment 16 are located in the rear body 8B. The cab 12 is equipped with a seat for the operator and operating devices operated by the operator. The engine compartment 16 is equipped with an engine 20 (see Figure 2), hydraulic pumps 30A, 30B, and 30C (see Figure 2) driven by the engine 20, and hydraulic equipment such as valves.

The working device 6 includes a lift arm (hereinafter simply referred to as the arm) 2 attached to the front vehicle body 8A so as to be freely rotatable in the vertical direction, a hydraulic cylinder (hereinafter also referred to as the arm cylinder) 4 that drives the arm 2, a bucket 3 attached to the tip of the arm 2 so as to be freely rotatable in the longitudinal direction, and a hydraulic cylinder (hereinafter also referred to as the bucket cylinder) 5 that drives the bucket 3. The arm 2, which is the driven member, is moved in response to the extension and retraction movement of the arm cylinder 4. The bucket 3, which is also the driven member, is moved in response to the extension and retraction movement of the bucket cylinder 5. One arm 2 and one arm cylinder 4 are provided on each side of the front vehicle body 8A. In this embodiment, a Z-link (bell crank) type link mechanism is used as the link mechanism for operating the bucket 3.

The wheel loader 1 is equipped with a travel drive device 45 that drives the wheels 7 attached to the vehicle body 8. The travel drive device 45 includes a travel motor 43 and a travel device 11 that receives driving force (hereinafter also referred to as travel drive force) from the travel motor 43. The travel device 11 has front wheels 7A that are wheels 7 attached to the front vehicle body 8A, rear wheels 7B that are wheels 7 attached to the rear vehicle body 8B, and a power transmission device that transmits the travel drive force (power) from the travel motor 43 to the wheels 7. The power transmission device is composed of an axle, a differential device, a propeller shaft, etc. Power from the travel motor 43 is transmitted to at least one of the front wheels 7A and the rear wheels 7B via the power transmission device.

The traveling motor 43 is an electric motor that drives the wheels 7 of the traveling device 11. The traveling motor 43 is driven and rotated by electricity generated by the generator motor 40 (see Figure 2), which is rotated by the power of the engine 20.

The wheel loader 1 is steered by a steering device 22 (see Figure 2) that has a pair of left and right hydraulic cylinders (hereinafter also referred to as steering cylinders) 15 that are provided to connect the front vehicle body 8A and the rear vehicle body 8B.

- Wheel loader control system -
Figure 2 is a configuration diagram of the control system of the wheel loader 1. As shown in Figure 2, the wheel loader 1 includes an engine 20, a fuel injection device 23 that supplies fuel to the engine 20, a generator motor 40 mechanically connected to the engine 20, hydraulic pumps 30A, 30B, 30C mechanically connected to the engine 20 and the generator motor 40, a working device 6 driven by hydraulic oil discharged from the hydraulic pump 30A, a front control unit 31 that controls the operation of the working device 6, a braking device 21 driven by hydraulic oil discharged from the hydraulic pump 30B, a brake control unit 32 that controls the operation of the braking device 21, a steering device 22 driven by hydraulic oil discharged from the hydraulic pump 30C, a steering control unit 33 that controls the steering device 22, and a traveling drive device 45 driven by electric power generated by the generator motor 40.

The working device 6 and the traveling drive device 45 are driven independently of each other by the power of the engine 20. The engine 20, which serves as the prime mover, is configured as an internal combustion engine such as a diesel engine. The generator motor 40 rotates due to the torque output from the engine 20 and functions as a generator that generates electricity.

Hydraulic pumps 30A, 30B, and 30C are driven by the torque output by engine 20 and discharge hydraulic oil. When generator motor 40 functions as an electric motor, hydraulic pumps 30A, 30B, and 30C are driven by the torque output by engine 20 and generator motor 40.

Hydraulic cylinders (hydraulic actuators) 4, 5, 15, 17, and 18 are driven to expand and contract by hydraulic oil (pressurized oil) discharged from hydraulic pumps 30A, 30B, and 30C, which are rotated by the torque output by the engine 20 (see Figure 2).

The front control unit 31 controls the pressure, flow rate, and direction of hydraulic oil supplied from the hydraulic pump 30A to the arm cylinder 4 and bucket cylinder 5. This controls the extension and retraction of the arm cylinder 4 and bucket cylinder 5. The brake control unit 32 controls the pressure, flow rate, and direction of hydraulic oil supplied from the hydraulic pump 30B to the brake cylinder 17 and parking brake cylinder 18. This controls the extension and retraction of the brake cylinder 17 and parking brake cylinder 18. The steering control unit 33 controls the pressure, flow rate, and direction of hydraulic oil supplied from the hydraulic pump 30C to the steering cylinder 15. This controls the extension and retraction of the steering cylinder 15.

The wheel loader 1 is equipped with a main controller 100, which is a control device that controls the entire vehicle; an engine controller 25 that controls the fuel injector 23 based on an engine rotation speed command from the main controller 100; the fuel injector 23 that controls the fuel injection amount based on a fuel injection amount command from the engine controller 25; a generator-motor inverter (hereinafter referred to as the generator inverter) 41 that controls the generator-motor 40 based on a power generation voltage command input from the main controller 100; a travel motor inverter (hereinafter referred to as the travel inverter) 42 that controls the torque of the travel motor 43 (i.e., the travel driving force generated by the travel drive device 45) based on a travel drive torque command input from the main controller 100; and various operating devices (51-57) provided in the driver's cab 12.

The cab 12 is equipped with a forward/reverse switch 51, which is a forward/reverse switching device that switches the direction of travel of the vehicle body 8; an arm operating device 52 that operates the arm cylinder 4 (arm 2) of the work implement 6; a bucket operating device 53 that operates the bucket cylinder 5 (bucket 3) of the work implement 6; an accelerator operating device 56 that operates the travel drive unit 45; a brake operating device 57 that operates the brake cylinder 17; a parking brake operating device 54 that operates the parking brake cylinder 18; and a steering operating device 55 that operates the pair of left and right steering cylinders 15. The forward/reverse switch 51 has three operating positions: a forward position (F), a standby position (N), and a reverse position (R). For ease of explanation, the operating devices 52 and 53 of the work implement 6 and the operating device 56 of the travel drive unit 45 are collectively referred to as the operating device 50.

The arm operating device 52 includes an arm operating lever and an arm operation amount sensor 52a that detects the operation amount of the arm operating lever (hereinafter also referred to as the arm operation amount). The bucket operating device 53 includes a bucket operating lever and a bucket operation amount sensor 53a that detects the operation amount of the bucket operating lever (hereinafter also referred to as the bucket operation amount). The accelerator operating device 56 includes an accelerator pedal and an accelerator operation amount sensor 56a that detects the operation amount of the accelerator pedal (hereinafter also referred to as the accelerator operation amount). The brake operating device 57 includes a brake pedal and a brake operation amount sensor 57a that detects the operation amount of the brake pedal (hereinafter also referred to as the brake operation amount). The steering operating device 55 includes a steering wheel and a steering operation amount sensor 55a that detects the operation amount of the steering wheel (hereinafter also referred to as the steering operation amount). The arm operation amount sensor 52a, bucket operation amount sensor 53a, accelerator operation amount sensor 56a, brake operation amount sensor 57a, and steering operation amount sensor 55a are, for example, potentiometers that output a voltage to the main controller 100 according to the operation position of an operating member (operation lever or pedal).

The main controller 100 is composed of a microcomputer equipped with a CPU (Central Processing Unit) 101 as a processing device (operating circuit), ROM (Read Only Memory) 102 and RAM (Random Access Memory) 103 as storage devices, an input interface 104, an output interface 105, and other peripheral circuits. Like the main controller 100, the engine controller 25 is also composed of a microcomputer equipped with a processing device (operating circuit), storage device, input/output interface, etc. The main controller 100 and engine controller 25 may each be composed of a single microcomputer, or multiple microcomputers.

The ROM 102 of the main controller 100 is a non-volatile memory such as an EEPROM, and stores programs capable of executing various calculations. In other words, the ROM 102 of the main controller 100 is a storage medium from which the programs that realize the functions of this embodiment can be read. The RAM 103 is a volatile memory, and serves as work memory for directly inputting and outputting data to and from the CPU 101. The RAM 103 temporarily stores necessary data while the CPU 101 is executing the programs. The main controller 100 may also include a storage device such as a flash memory or a hard disk drive.

The CPU 101 is a processing device that loads programs stored in the ROM 102 into the RAM 103 and executes them, and performs predetermined calculations on signals received from the input interface 104, ROM 102, and RAM 103 in accordance with the programs.

Operation signals from various operating devices (51-57) and sensor signals from various sensors are input to the input interface 104. The input interface 104 converts the input signals into data that can be calculated by the CPU 101. The output interface 105 generates output signals based on the results of calculations by the CPU 101, and outputs these signals to the front control unit 31, brake control unit 32, steering control unit 33, generator inverter 41, driving inverter 42, engine controller 25, etc.

The main controller 100 comprehensively controls the front control unit 31, brake control unit 32, steering control unit 33, power generation inverter 41, driving inverter 42, and engine controller 25 based on operation signals input from various operating devices and sensor signals input from various other sensors.

Operation signals input to the main controller 100 include the accelerator operation amount detected by the accelerator operation amount sensor 56a, the brake operation amount detected by the brake operation amount sensor 57a, the arm operation amount detected by the arm operation amount sensor 52a, the bucket operation amount detected by the bucket operation amount sensor 53a, the steering operation amount detected by the steering operation amount sensor 55a, and a signal output from the forward/reverse switch 51 indicating the operation position of the forward/reverse switch 51.

The sensor signals input to the main controller 100 include a signal representing the angle detected by an arm relative angle sensor 62 attached to the connecting shaft connecting the vehicle body 8 and the arm 2, and a signal representing the angle detected by a bucket relative angle sensor 63 attached to the connecting shaft connecting the arm 2 and the bucket 3. The arm relative angle sensor 62 is a potentiometer that detects the relative angle (tilt angle) of the arm 2 with respect to the vehicle body 8 and outputs a signal representing the detected angle to the main controller 100. The bucket relative angle sensor 63 is a potentiometer that detects the relative angle (tilt angle) of the bucket 3 with respect to the arm 2 and outputs a signal representing the detected angle to the main controller 100. Because the angle of the vehicle body 8 with respect to the ground (driving surface) is constant, the angle detected by the arm relative angle sensor 62 can be said to correspond to the relative angle (tilt angle) of the arm 2 with respect to the ground.

Sensor signals input to the main controller 100 include a signal representing the wheel speed (rotational speed of the wheels (drive wheels) 7) detected by the wheel speed sensor 61. The wheel speed detected by the wheel speed sensor 61 can be converted into the vehicle's traveling speed (vehicle speed). The wheel speed sensor 61 detects the wheel speed and outputs a signal representing the detected wheel speed to the main controller 100. Further sensor signals input to the main controller 100 include signals representing the rotational speeds of the engine 20, generator motor 40, hydraulic pumps 30A, 30B, 30C, and traveling motor 43 detected by multiple rotational speed sensors, and a signal representing the pressure (load pressure) of the arm cylinder 4 detected by the arm cylinder pressure sensor 75.

The multiple rotational speed sensors include an engine rotational speed sensor 64 that detects the actual rotational speed of the engine 20, and a motor speed sensor 58 such as a resolver that detects the rotational speed of the traction motor 43 (hereinafter also referred to as motor speed). The engine rotational speed sensor 64 is, for example, a rotary encoder provided on the output shaft of the engine 20, and outputs a signal representing the detected actual engine rotational speed to the main controller 100. Note that the engine rotational speed sensor 64 is not limited to detecting the rotational speed of the output shaft of the engine 20, and may also detect the rotational speed of any shaft constituting the power transmission device. In this case, the main controller 100 calculates the actual engine rotational speed based on the detection result of the engine rotational speed sensor 64.

The motor speed sensor 58 detects the motor speed and outputs a signal representing the detection result to the main controller 100 and the driving force sensor 65 (described later). The motor speed detected by the motor speed sensor 58 is correlated with the wheel speed. In other words, the motor speed detected by the motor speed sensor 58 can be converted into vehicle speed.

In the illustrated example, the engine rotation speed sensor 64 is connected to the main controller 100, but it may also be connected to the engine controller 25. In this case, the main controller 100 obtains the actual engine rotation speed detected by the engine rotation speed sensor 64 via the engine controller 25.

Furthermore, the sensor signals input to the main controller 100 include a signal representing the driving force of the traveling drive device 45 detected by the driving force sensor 65. The driving force sensor 65 calculates the driving force of the traveling drive device 45 based on the driving torque command value calculated by the main controller 100, the motor speed input from the motor speed sensor 58, the radius of the wheel 7, and the inertia constant of the wheel 7, and outputs a signal representing the calculation result to the main controller 100. The radius and inertia constant of the wheel 7 are stored, for example, in memory provided in the driving force sensor 65. Note that the data stored in the memory of the driving force sensor 65 (such as the radius and inertia constant of the wheel 7) can be updated using data output from the main controller 100.

The driving force sensor 65 calculates the traveling driving force F _M according to the following equation (1).
F _M =(T _{M_COM} -M×ΔS _M )/R...(1)
Here, T _{M_COM} is a command value of the traveling drive torque calculated by the main controller 100, M is the inertia constant of the wheel 7, and R is the radius of the wheel 7. ΔS _M is the angular acceleration of the traveling motor 43, and is calculated based on the motor speed detected by the motor speed sensor 58.

The driving force sensor 65 is not limited to the above configuration. For example, the driving force sensor 65 may calculate the driving force based on the output torque (motor torque) of the traveling motor 43 detected by a torque sensor, or may calculate the driving force based on the current flowing through the traveling motor 43 detected by a current sensor. The functions of the driving force sensor 65 may also be performed by the main controller 100. In this case, the driving force sensor 65 is omitted, and the motor speed sensor 58 functions as a driving force sensor that detects a physical quantity related to driving force (motor speed), and the main controller 100 calculates the driving force F _M based on the detection result of the motor speed sensor 58.

The main controller 100 calculates the target rotation speed of the engine 20 (hereinafter also referred to as the target engine rotation speed) based on the accelerator operation amount, arm operation amount, bucket operation amount, etc. The main controller 100 calculates a rotation speed command value based on the target engine rotation speed and outputs it to the engine controller 25. The main controller 100 also outputs the actual engine rotation speed detected by the engine rotation speed sensor 64 to the engine controller 25.

The engine controller 25 compares the rotation speed command value obtained from the main controller 100 with the actual engine rotation speed detected by the engine rotation speed sensor 64, and controls the fuel injector 23 so that the actual engine rotation speed matches the rotation speed command value. The fuel injector 23 controls the fuel injection amount based on the fuel injection command output from the engine controller 25, and operates the engine 20.

In this way, the main controller 100, engine controller 25, and fuel injector 23 cooperate to form an engine control device that controls the operation of the engine 20.

The main controller 100 outputs a front control command based on the operation direction and operation amount of the arm operation device 52 and the bucket operation device 53. The front control unit 31 adjusts the pressure, flow rate, and direction of the hydraulic oil discharged from the hydraulic pump 30A based on the front control command from the main controller 100, and operates the arm cylinder 4 and the bucket cylinder 5. The front control unit 31 has a directional control valve (flow rate control valve) that controls the flow of hydraulic oil discharged from the hydraulic pump 30A, and a solenoid valve that generates pilot pressure that is input to the pilot chamber of this directional control valve, etc.

The main controller 100 outputs a brake control command based on the amount of operation of the brake operating device 57 and the operating position of the operating switch of the parking brake operating device 54. The brake control unit 32 adjusts the pressure, flow rate, and direction of the hydraulic oil discharged from the hydraulic pump 30B based on the brake control command from the main controller 100, and operates the brake cylinder 17 and the parking brake cylinder 18. The brake control unit 32 includes a directional control valve (flow rate control valve) that controls the flow of hydraulic oil discharged from the hydraulic pump 30B, and a solenoid valve that generates pilot pressure that is input to the pilot chamber of this directional control valve.

The main controller 100 outputs steering control commands based on the direction and amount of operation of the steering wheel of the steering operation device 55. The steering control unit 33 adjusts the pressure, flow rate, and direction of the hydraulic oil discharged from the hydraulic pump 30C based on the steering control commands from the main controller 100, and operates the steering cylinder 15. The steering control unit 33 includes a directional control valve (flow rate control valve) that controls the flow of hydraulic oil discharged from the hydraulic pump 30C, and a solenoid valve that generates pilot pressure that is input to the pilot chamber of this directional control valve.

The power generation inverter 41 and the traction inverter 42 are connected by a DC section (DC bus) 44. The power generation inverter 41 controls the bus voltage of the DC section 44 using the power supplied from the generator motor 40 based on a power generation voltage command from the main controller 100. The traction inverter 42 drives the traction motor 43 using the power from the DC section 44 based on a traction drive torque command from the main controller 100.

In this embodiment, hydraulic pumps 30A, 30B, and 30C are driven by the torque output by the engine 20, and the working device 6, braking device 21, and steering device 22 are driven by the hydraulic oil discharged from the hydraulic pumps 30A, 30B, and 30C. Also, in this embodiment, the generator motor 40 is driven by the torque output by the engine 20, and the traveling motor 43 is driven by the electricity generated by the generator motor 40.

When the arm operation lever of the arm operation device 52 is operated, the arm cylinder 4 extends and retracts, causing the arm 2 to rotate vertically (elevate and lower). When the bucket operation lever of the bucket operation device 53 is operated, the bucket cylinder 5 extends and retracts, causing the bucket 3 to rotate forward and backward (dump or crowd).

When the steering wheel of the steering operation device 55 is operated, the front body 8A is turned (steered) left and right around the center joint 10 relative to the rear body 8B in response to the extension and retraction of the steering cylinder 15. When the accelerator pedal of the accelerator operation device 56 is operated, the wheels 7 are rotated by the drive of the traveling motor 43, causing the wheel loader 1 to travel.

When the forward/reverse switch 51 is set to the forward position (F) and the accelerator pedal of the accelerator operating device 56 is depressed, the wheels 7 rotate in the forward direction, causing the vehicle body 8 to travel forward. When the forward/reverse switch 51 is set to the reverse position (R) and the accelerator pedal of the accelerator operating device 56 is depressed, the wheels 7 rotate in the reverse direction, causing the vehicle body 8 to travel backward. When the forward/reverse switch 51 is set to the standby position (N), the wheels 7 do not rotate and the vehicle body 8 does not travel, even if the accelerator pedal of the accelerator operating device 56 is depressed. Note that when the forward/reverse switch 51 is set to the forward position (F) or reverse position (R), the vehicle body 8 travels forward or backward (creep) at extremely low speeds, even if the accelerator pedal is not depressed.

- Main controller functions -
3 is a functional block diagram of the main controller 100. The main controller 100 generates a traveling drive torque command based on detection signals from each sensor and outputs the command to the traveling inverter 42, thereby controlling the traveling drive force generated by the traveling drive device 45. Below, with reference to FIG. 3, details of each function when the main controller 100 generates the traveling drive torque command will be described.

As shown in FIG. 3, the main controller 100 executes the programs stored in the ROM 102 to function as a motor speed change amount calculation unit 112, a thrust force calculation unit 110, a thrust force change amount calculation unit 111, a driving force change amount calculation unit 113, an excavation operation determination unit 114, a slip determination unit 115, a thrust force retention unit during slip 116, a friction coefficient estimation unit 117, a driving force upper limit value calculation unit 118, a traveling required driving force calculation unit 119, a minimum value selection unit 120, and a torque command generation unit 121.

The motor speed change amount calculation unit 112 calculates the time rate of change (hereinafter also referred to as the speed change rate) ΔS _M of the motor speed S _M detected by the motor speed sensor 58. The motor speed S _M is a positive value when the vehicle body 8 moves forward, and a negative value when the vehicle body 8 moves backward. The motor speed change amount calculation unit 112 calculates the speed change rate ΔS _M (= (S M b - S M _a ) / (tb - ta)) by dividing the difference (S _M b - S _M a) between the previous value S _{M a} and the current value S _M b of the motor speed S M, which are repeatedly detected at a predetermined control period, by the time _Δt (= tb - ta) from the time ta when the previous value S _M a was detected to the time tb when the current value S _M b was detected. Therefore, the speed change rate ΔS _M is a positive value when the forward moving vehicle body 8 is accelerating, and a negative value when the forward moving vehicle body 8 is decelerating. Moreover, the speed change rate ΔS _M is 0 when the vehicle body 8 is traveling at a constant speed.

The excavation operation determination unit 114 determines whether the current state is an excavation operation state in which the work device 6 is excavating a work object such as natural ground, or a non-excavation operation state in which the work device 6 is not excavating a work object. If the excavation operation determination unit 114 determines that the current state is an excavation operation state, it sets the excavation determination flag FLdig on (FLdig = 1), and if the current state is a non-excavation operation state, it sets the excavation determination flag FLdig off (FLdig = 0).

The digging determination flag FLdig indicates whether the wheel loader 1 is in an digging operation state or a non-digging operation state. Therefore, the main controller 100 can determine whether the wheel loader 1 is in an digging operation state based on the setting state of the digging determination flag FLdig.

The excavation operation determination unit 114 determines that the excavation operation is in progress when the excavation operation conditions are met. When the wheel loader 1 moves forward toward a work object such as a mound of natural ground and then the working implement 6 enters the work object, the vehicle body 8 decelerates and the reaction force that the working implement 6 receives from the work object increases. As the reaction force increases, the thrust of the arm cylinder 4 also increases. For this reason, in this embodiment, the excavation operation conditions include the following (Condition I) and (Condition II).
(Condition I) The forward moving vehicle body 8 is decelerating.
(Condition II) The thrust of the arm cylinder 4 is equal to or greater than a predetermined value.

Whether the advancing vehicle body 8 is decelerating can be determined based on the speed change rate ΔS _M calculated by the motor speed change amount calculation unit 112. The excavation operation determination unit 114 determines that (Condition I) is met when the speed change rate ΔS _M is equal to or less than a predetermined speed change rate threshold S1. The excavation operation determination unit 114 determines that (Condition I) is not met when the speed change rate ΔS _M is greater than the speed change rate threshold S1. The speed change rate threshold S1 is stored in advance in the ROM 102. The speed change rate threshold S1 is a threshold for determining whether the advancing vehicle body 8 is decelerating due to the wheel loader 1 entering a work object, and is a value obtained by subtracting a margin value (>0) from 0. In other words, the speed change rate threshold S1 is less than 0 (S1<0). This margin value is determined in advance by calculation or experiment in order to prevent erroneous determinations due to changes in the speed of the traveling motor 43 caused by vibrations of the vehicle body 8 or detection errors of the motor speed sensor 58 .

The thrust of the arm cylinder 4, which increases when the working device 6 receives a reaction force from the work object, is proportional to the pressure of the hydraulic oil in the bottom-side oil chamber (not shown) of the arm cylinder 4. For this reason, in this embodiment, it is determined whether or not (Condition II) is satisfied based on the arm cylinder pressure _PA detected by the arm cylinder pressure sensor 75.

As shown in FIG. 2 , the arm cylinder pressure sensor 75 is a pressure sensor provided in an oil passage connecting a bottom-side oil chamber (not shown) of the arm cylinder 4 and the front control unit 31, and detects the pressure of hydraulic oil (arm cylinder pressure) _PA in the bottom-side oil chamber of the arm cylinder 4 and outputs a signal representing the detection result to the main controller 100. The excavation operation determination unit 114 shown in FIG. 3 determines that (Condition II) is satisfied when the arm cylinder pressure _PA is equal to or greater than the cylinder pressure threshold P1. The excavation operation determination unit 114 determines that (Condition II) is not satisfied when the arm cylinder pressure _PA is less than the cylinder pressure threshold P1. The cylinder pressure threshold P1 corresponds to the arm cylinder pressure detected when excavation operation is started by calculation or experiment, and is stored in advance in the ROM 102.

When the digging determination flag FLdig is set to off and both (Condition I) and (Condition II) are satisfied, the digging operation determination unit 114 determines that the digging operation conditions are met. That is, the digging operation determination unit 114 determines that the state of the wheel loader 1 has transitioned from a non-digging operation state to an digging operation state, and switches the digging determination flag FLdig from off to on (FLdig = 1). Note that when the digging determination flag FLdig is set to off and either (Condition I) or (Condition II) is not satisfied, the digging operation determination unit 114 determines that the digging operation conditions are not met. That is, the digging operation determination unit 114 determines that the non-digging operation state is continuing. That is, the digging determination flag FLdig remains off.

When the wheel loader 1 finishes excavation work, it travels in reverse and moves away from the work object. Therefore, the excavation work determination unit 114 in this embodiment determines whether the excavation work end condition has been met, i.e., whether the wheel loader 1 has transitioned from a working state to a non-working state, based on the operating position of the forward/reverse switch 51. The excavation work end condition includes the forward/reverse switch 51 being operated to the reverse position (R).

When the digging determination flag FLdig is set to on and the forward/reverse switch 51 is operated to the reverse position (R), the digging operation determination unit 114 determines that the digging operation end condition is met. That is, the digging operation determination unit 114 determines that the wheel loader 1 has transitioned from a working state to a non-working state, and switches the digging determination flag FLdig from on to off (FLdig = 0). When the digging determination flag FLdig is set to on and the forward/reverse switch 51 is not operated to the reverse position (R), the digging operation determination unit 114 determines that the digging operation end condition is not met. That is, the digging operation determination unit 114 determines that the digging operation state is continuing. That is, the digging determination flag FLdig remains on.

The thrust calculation unit 110 calculates the arm cylinder thrust F _A by the following equation (2).
F _A = P _A × S _A ... (2)
Here, P _A is the arm cylinder pressure detected by the arm cylinder pressure sensor 75, and S _A is the pressure-receiving area (cross-sectional area) of the arm cylinder 4 stored in advance in ROM 102. In an excavation work state, the arm cylinder thrust F _A increases in accordance with an increase in the reaction force that the work implement 6 receives from the work object.

The thrust change amount calculation unit 111 calculates the time rate of change ΔF _A (hereinafter also referred to as thrust change rate) of the arm cylinder thrust F _A calculated by the thrust calculation unit 110. The thrust change amount calculation unit 111 calculates the thrust change rate ΔF _A (= (F _{A b} - F A a) / (tb - ta)) by dividing the difference (F _{A b} - F _A a) between the previous value F _A a and the current value F _A b of the arm cylinder thrust F A, which are repeatedly calculated at a predetermined control cycle, by the time _Δt (= tb - ta) from the time ta when the previous value F _A a is detected to the time tb when the current value F _A b is detected. Therefore, the thrust change rate ΔF _A is a positive value when the arm cylinder thrust F _A is increasing, and is a negative value when the arm cylinder thrust F _A is decreasing. Furthermore, the thrust change rate ΔF _A is 0 when the arm cylinder thrust F _A is constant.

The driving force change amount calculation unit 113 calculates the time rate of change ΔF _M of the driving force F _M detected by the driving force sensor 65 (hereinafter also referred to as the driving force change rate). The driving force change amount calculation unit 113 calculates the driving force change rate ΔF _M (= (F M b - F _M a) / (tb - ta)) by dividing the difference ( _{F M b} - F _M a) between the previous value F _M a and the current value F _M b of the driving force F _M , which are repeatedly detected at a predetermined control cycle, by the time Δt (= tb - ta) from the time ta when the previous value F _{M a} was detected to the time tb when the current value F _{M b} was detected. Therefore, the driving force change rate ΔF _M is positive when the driving force F _M is increasing and negative when the driving force F M is decreasing. Furthermore, the driving force change rate ΔF _M is zero when the driving force F _M is constant.

The slip determination unit 115 determines that the wheel 7 is slipping if the slip determination conditions are met, and determines that the wheel 7 is not slipping if the slip determination conditions are not met. The slip determination conditions include the following (Condition 1) to (Condition 5). If all of (Condition 1) to (Condition 5) are met, the slip determination unit 115 determines that the slip determination conditions are met and sets the slip determination flag FLslip to on (FLslip = 1). Note that if any of (Condition 1) to (Condition 5) are not met, the slip determination unit 115 determines that the slip determination conditions are not met and sets the slip determination flag FLslip to off (FLslip = 0).
(Condition 1) Excavation work must be in progress.
(Condition 2) The arm cylinder thrust is not increasing.
(Condition 3) The driving force is increasing.
(Condition 4) The arm operating device is not being operated.
(Condition 5) The bucket operating device is not being operated.

The slip determination unit 115 determines whether (Condition 1) is satisfied based on the setting state of the digging determination flag FLdig. If the digging determination flag FLdig is set to on (FLdig = 1), the slip determination unit 115 determines that (Condition 1) is satisfied, and if the digging determination flag FLdig is set to off (FLdig = 0), the slip determination unit 115 determines that (Condition 1) is not satisfied.

The slip determination unit 115 determines whether or not (Condition 2) is satisfied based on the thrust change rate ΔF _A. If the thrust change rate ΔF _A is equal to or less than the thrust change rate threshold F _A1 , the slip determination unit 115 determines that (Condition 2) is satisfied, and if the thrust change rate ΔF _A is greater than the thrust change rate threshold F _A1 , the slip determination unit 115 determines that (Condition 2) is not satisfied. The thrust change rate threshold F _A1 is a threshold for determining whether or not a sudden decrease (loss) in arm cylinder thrust has occurred at the start of slippage, and is determined by calculation or experiment. The thrust change rate threshold F _A1 is a value equal to or less than 0, and is stored in ROM 102.

The slip determination unit 115 determines whether or not (Condition 3) is satisfied based on the driving force change rate ΔF _M. If the driving force change rate ΔF _M is greater than 0, the slip determination unit 115 determines that (Condition 3) is satisfied, and if the driving force change rate ΔF _M is 0 or less, the slip determination unit 115 determines that (Condition 3) is not satisfied.

The slip determination unit 115 determines whether or not (Condition 4) is satisfied based on the arm operation amount R _A detected by the arm operation amount sensor 52 a. The slip determination unit 115 determines that (Condition 4) is satisfied when the absolute value |R _{A |} of the arm operation amount R _{A is equal to or smaller than the arm operation amount threshold R A1 ,} and determines that (Condition 4) is not satisfied when the absolute value |R _A | of the arm operation amount R _A is greater than the arm operation amount threshold R A1. The arm operation amount threshold R _A1 is a threshold for determining whether or not the arm operating device 52 is being operated, and is stored in the ROM 102. For example, when the maximum operation amount for arm-raising is 100% and the maximum operation amount for arm-lowering is −100%, the arm operation amount threshold R _A1 corresponds to an operation amount of about 5%.

Slip determination unit 115 determines whether or not (Condition 5) is satisfied based on bucket manipulation amount R _B detected by bucket manipulation amount sensor 53 a. Slip determination unit 115 determines that (Condition 5) is satisfied if the absolute value |R _B | of the bucket manipulation amount R _B is equal to or smaller than bucket manipulation amount threshold R _B1 , and determines that (Condition 5 ₎ is not satisfied if the absolute value |R _B | of the bucket manipulation amount R _B is greater than bucket manipulation amount threshold R B1. Bucket manipulation amount threshold R _B1 is a threshold for determining whether or not bucket manipulation device 53 is being operated, and is stored in ROM 102. Bucket manipulation amount threshold R _B1 corresponds to an operation amount of about 5%, for example, when the maximum operation amount on the bucket dump side is 100% and the maximum operation amount on the bucket crowding side is −100%.

The slip thrust holding unit 116 holds the arm cylinder thrust F _A when the slip determination flag FLslip is switched from off to on, i.e., the arm cylinder thrust F _A when the slip determination condition is met, as the slip thrust F _{A_slip} (F _{A_slip} = F _A ). The slip thrust F _{A_slip} continues to be held at least until the slip determination flag FLslip is switched from on to off. Furthermore, the slip thrust F _{A_slip} may continue to be held until the slip determination flag FLslip is switched from on to off and then turned on again.

The friction coefficient estimating unit 117 refers to a correlation map M _μ (see FIG. 4 ) stored in advance in the ROM 102, and calculates an estimated friction coefficient μ based on the slip thrust F _{A_slip} stored by the slip thrust storing unit 116. The correlation map M _μ is prescribed data that defines the relationship between the slip thrust F _{A_slip} and the estimated friction coefficient μ. The correlation map M _μ may be stored, for example, in the form of a look-up table or in the form of a function (approximate formula).

As shown in the correlation map M _μ in FIG. 4 , the estimated coefficient of friction μ relative to the slip thrust F _{A_slip} has a nonlinear characteristic. The correlation map M _μ defines a characteristic in _which the estimated coefficient of friction μ is 0 when the slip thrust F A_slip is 0, and the estimated coefficient of friction μ increases as the slip thrust F _{A_slip} increases from 0. The characteristic of this correlation map M _μ is determined in advance by calculation or experiment. The larger the slope of the correlation map M _μ (increase in estimated coefficient of friction/increase in slip thrust), the larger the estimated coefficient of friction μ at a certain slip thrust F _{A_slip} .

Driving force upper limit calculation unit 118 shown in FIG. 3 references correlation map M _L (see FIG. 5 ) stored in advance in ROM 102 and calculates driving force upper limit value F _{M_LIM} based on the estimated friction coefficient value μ calculated by friction coefficient estimation unit 117. Correlation map M _L is prescribed data that defines the relationship between the estimated friction coefficient value μ and driving force upper limit value F _{M_LIM} . Correlation map M _L may be stored in the form of a look-up table, for example, or in the form of a function (approximate equation).

As shown in Figure 5, the correlation map _{M_L} defines characteristics such that when the estimated coefficient of friction μ is 0, the driving force upper limit value F _{M_LIM} is at a minimum value F _{M_LIM_MIN} , and as the estimated coefficient of friction μ increases from 0 to a predetermined value μp, the driving force upper limit value F _{M_LIM} increases, and when the estimated coefficient of friction μ is equal to or greater than the predetermined value μp, the driving force upper limit value F _{M_LIM} is at a maximum value F _{M_LIM_MAX} . The maximum value F _{M_LIM_MAX} of the driving force upper limit value F _{M_LIM} corresponds to the maximum driving force that can be output by the traveling motor 43. The minimum value F _{M_LIM_MIN} of the driving force upper limit value F _{M_LIM} corresponds to the minimum driving force that can move the wheel loader 1 forward. The characteristics of this correlation map _{M_L} are determined in advance by calculation or experiment. The greater the gradient of the correlation map _{M_L} (increase in driving force upper limit value/increase in estimated friction coefficient value), the greater the driving force upper limit value _{F_M_LIM} at a given estimated friction coefficient value μ.

Note that the driving force upper limit value _{FM_LIM} uses the result calculated using the characteristics of the correlation map _ML only when the slip determination flag FLslip is set to ON. When the slip determination flag FLslip is set to OFF, driving force upper limit value calculation unit 118 sets the driving force upper limit value _{FM_LIM} to the maximum value _{FM_LIM_MAX} . Therefore, when the slip determination condition is met, the traveling driving force _FM is limited by the driving force upper limit value _{FM_LIM} set using the characteristics of the correlation map _ML . On the other hand, when the slip determination condition is not met, the driving force upper limit value _{FM_LIM} is set to the maximum value _{FM_LIM_MAX} , so the traveling driving force is not substantially limited.

The traveling demand driving force calculation unit 119 shown in Figure 3 calculates the traveling demand driving force based on the motor speed S _M detected by the motor speed sensor 58 and the accelerator operation amount detected by the accelerator operation amount sensor 56a. A traveling demand driving force table used to calculate the traveling demand driving force is stored in ROM 102. The traveling demand driving force table stores a plurality of driving force curves corresponding to the accelerator operation amount so that the traveling demand driving force increases or decreases according to an increase or decrease in the accelerator operation amount. The traveling demand driving force table is set so that the traveling demand driving force increases as the accelerator operation amount increases, and the traveling demand driving force decreases as the rotation speed (motor speed) of the traveling motor 43 increases.

The traveling required driving force calculation unit 119 selects a driving force curve that corresponds to the magnitude of the accelerator operation amount, and calculates the traveling required driving force F _{M_REQ} based on the rotation speed (motor speed) S _M of the traveling electric motor 43. For example, when the accelerator operation device 56 is fully operated, the traveling required driving force calculation unit 119 selects the driving force curve indicated by the solid line, and calculates the traveling required driving force F _{M_REQ} based on the rotation speed of the traveling electric motor 43 by referring to the selected driving force curve.

The minimum value selection unit 120 selects the smallest one of the driving force upper limit value F _{M_LIM} calculated by the driving force upper limit value calculation unit 118 and the traveling required driving force F _{M_REQ} calculated by the traveling required driving force calculation unit 119, and determines the selected value as the traveling target driving force F _{M_TGT} .

The torque command generation unit 121 generates a traveling drive torque command T _{M_COM} based on the traveling target drive force F _{M_TGT} selected by the minimum value selection unit 120. The traveling drive torque command T _{M_COM} generated by the torque command generation unit 121 is output to the traveling inverter 42 and the drive force sensor 65, and the traveling electric motor 43 is controlled so that the traveling drive force detected by the drive force sensor 65 becomes the traveling target drive force.

- Flow of driving force control -
The contents of the control executed by the main controller 100 to suppress slipping will be described with reference to Figures 6 to 8. Figure 6 is a flowchart showing an example of a main flow of the travel driving force control executed by the main controller 100, Figure 7 is a flowchart showing an example of the process of setting the excavation determination flag in Figure 6, and Figure 8 is a flowchart showing an example of the process of setting the slip determination flag in Figure 6.

The processing shown in the flowchart of Fig. 6 is started, for example, when an ignition switch (engine key switch) is turned on, and after initial setting (not shown) is performed, is repeatedly executed at a predetermined control period. Note that in the initial setting, the slip determination flag FLslip and the excavation determination flag FLdig are set to off. Furthermore, the processing shown in the flowcharts of Figs. 6 to 8 is executed based on the detection results of each sensor (e.g., arm cylinder pressure _PA , arm operation amount _RA , bucket operation amount _RB ) repeatedly acquired at a predetermined control period, the calculation results of each calculation unit (e.g., arm cylinder thrust F _A , thrust force change rate ΔF _A , speed change rate ΔS _M , driving force change rate ΔF _M ), and a signal indicating the operation position from the forward/reverse switch 51, etc.

As shown in FIG. 6, in step S100, the excavation operation determination unit 114 executes the process of setting the excavation determination flag FLdig. As shown in FIG. 7, when the process of setting the excavation determination flag begins, in step S110, the excavation operation determination unit 114 determines whether the currently set excavation determination flag FLdig is on. If it is determined that the currently set excavation determination flag FLdig is off, the process proceeds to step S120. If it is determined that the currently set excavation determination flag FLdig is on, the process proceeds to step S150.

In step S120, the excavation operation determination unit 114 determines whether the speed change rate ΔS _M , which is the time change rate of the motor speed calculated by the motor speed change amount calculation unit 112, is equal to or less than the speed change rate threshold S1. If it is determined that the speed change rate ΔS _M is equal to or less than the speed change rate threshold S1, the process proceeds to step S130. If it is determined that the speed change rate ΔS _M is greater than the speed change rate threshold S1, the excavation determination flag setting process shown in FIG. 7 ends.

In step S130, the excavation operation determination unit 114 determines whether the arm cylinder pressure _PA detected by the arm cylinder pressure sensor 75 is equal to or greater than the cylinder pressure threshold value P1. If it is determined that the arm cylinder pressure _PA is equal to or greater than the cylinder pressure threshold value P1, the process proceeds to step S140. If it is determined that the arm cylinder pressure _PA is less than the cylinder pressure threshold value P1, the excavation determination flag setting process shown in Figure 7 ends.

In step S140, the excavation operation determination unit 114 sets the excavation determination flag to on (FLdig = 1) and terminates the excavation determination flag setting process shown in Figure 7.

In step S150, the excavation operation determination unit 114 determines whether the forward/reverse switch 51 is operated to the reverse position (R) based on the operation position signal from the forward/reverse switch 51. If it is determined that the forward/reverse switch 51 is operated to the reverse position (R), processing proceeds to step S160. If it is determined that the forward/reverse switch 51 is not operated to the reverse position (R), the excavation determination flag setting process shown in Figure 7 ends.

In step S160, the excavation operation determination unit 114 sets the excavation determination flag to off (FLdig = 0) and terminates the excavation determination flag setting process shown in Figure 7.

When the excavation determination flag setting process is completed, processing proceeds to step S200 in Figure 6. As shown in Figure 6, in step S200, the slip determination unit 115 executes the process of setting the slip determination flag FLslip. As shown in Figure 8, when the slip determination flag setting process is started, in step S210, the slip determination unit 115 determines whether the currently set excavation determination flag FLdig is on, similar to step S110 above. If it is determined that the currently set excavation determination flag FLdig is on, processing proceeds to step S220. If it is determined that the currently set excavation determination flag FLdig is off, processing proceeds to step S270.

In step S220, slip determination unit 115 determines whether the driving force change rate ΔF _M , which is the time change rate of traveling driving force F _M calculated by driving force change amount calculation unit 113, is greater than 0. If it is determined that the driving force change rate ΔF _M is greater than 0, processing proceeds to step S230. If it is determined that the driving force change rate ΔF _M is equal to or less than 0, processing proceeds to step S270.

In step S230, the slip determination unit 115 determines whether the thrust change rate ΔF _A , which is the time change rate of the arm cylinder thrust F _A calculated by the thrust change amount calculation unit 111, is equal to or less than the thrust change rate threshold F _{A 1.} If it is determined that the thrust change rate ΔF _A is equal to or less than the thrust change rate threshold F A 1, the process proceeds to step S240. If it is determined that the thrust change rate ΔF _A is greater than the thrust change rate threshold F _A 1, the process proceeds to step S270.

In step S240, the slip determination unit 115 determines whether the arm operation amount R _A detected by the arm operation amount sensor 52 a is equal to or less than the arm operation amount threshold R _A 1. If it is determined that the arm operation amount R _A is equal to or less than the arm operation amount threshold R _A 1, the process proceeds to step S250. If it is determined that the arm operation amount R _A is greater than the arm operation amount threshold R _A 1, the process proceeds to step S270.

In step S250, slip determination unit 115 determines whether bucket operation amount R _B detected by bucket operation amount sensor 53 a is equal to or less than bucket operation amount threshold R _B 1. If it is determined that bucket operation amount R _B is equal to or less than bucket operation amount threshold R _B 1, the process proceeds to step S260. If it is determined that bucket operation amount R _B is greater than bucket operation amount threshold R _B 1, the process proceeds to step S270.

In step S260, the slip determination unit 115 sets the slip determination flag to on (FLslip = 1) and ends the slip determination flag setting process shown in Figure 8. In step S270, the slip determination unit 115 sets the slip determination flag to off (FLslip = 0) and ends the slip determination flag setting process shown in Figure 8. Once the slip determination flag setting process is completed, the process proceeds to step S310 in Figure 6.

As shown in FIG. 6, in step S310, the slip thrust holding unit 116 and the driving force upper limit calculation unit 118 determine whether the currently set slip determination flag FLslip is on. If it is determined that the currently set slip determination flag FLslip is on, processing proceeds to step S320. If the currently set slip determination flag FLslip is off, processing proceeds to step S350.

In step S320, the slip-time thrust holding unit 116 holds the arm cylinder thrust F _A calculated by the thrust calculation unit 110 as the slip-time thrust F _{A_slip} . In other words, the slip-time thrust holding unit 116 holds the arm cylinder thrust F _A when the slip determination flag FLslip switches from off to on as the slip-time thrust F _{A_slip} . When the processing of step S320 ends, the processing proceeds to step S330.

In step S330, the friction coefficient estimating unit 117 calculates the estimated friction coefficient μ using the characteristics of the correlation map M _μ (see FIG. 4) based on the slip thrust F _{A — slip} held in step S320, and then the process proceeds to step S340.

In step S340, driving force upper limit calculation unit 118 calculates driving force upper limit F _{M_LIM} based on the estimated friction coefficient μ calculated in step S330 and the characteristics of correlation map M _L (see FIG. 5).

In step S350, driving force upper limit calculation unit 118 sets driving force upper limit value _{FM_LIM} to maximum value _{FM_LIM_MAX} . When the processing of step S340 or step S350 is completed, the processing of the flowchart shown in Figure 6 for this control cycle ends, and in the next control cycle, the processing of step S100 to step S340 or step S350 is executed again.

In this way, when slip is detected by switching the slip determination flag from off to on, the driving force upper limit value _{FM_LIM} is set based on the thrust force _{FA_slip} at the time the slip was detected. Therefore, the traveling driving force _FM is limited so as not to exceed the driving force upper limit value _{FM_LIM,} which is set within the range from _{FM_LIM_MIN} to _{FM_LIM_MAX} . On the other hand, when slip is not detected, the driving force upper limit value is always set to the maximum value _{FM_LIM_MAX} , so the traveling driving force is not limited. In other words, the traveling driving force _FM is controlled to become the required traveling driving force _{FM_REQ} , which is calculated based on the detection results of the accelerator depression amount sensor 56a and the motor speed sensor 58.

-Operation-
An example of the operation and effects of the wheel loader 1 according to this embodiment during excavation work will now be described with reference to Figure 9. Figure 9 is a diagram showing time series changes in the parameters of the wheel loader 1 according to this embodiment (bucket operation amount R _B , arm operation amount R _A , travel drive force F _M , arm cylinder thrust F _A , wheel speed V _W , and slip determination flag FLslip).

Furthermore, in order to clarify the effects of this embodiment, the description will be made in comparison with a comparative example in which processing for setting a slip determination flag is executed based on the wheel speed _VW detected by the wheel speed sensor 61. It should be noted that the wheel loader 1 according to this embodiment and the wheel loader according to the comparative example of this embodiment have the same operation procedures and operation amounts by the operator.

The main controller of the wheel loader according to the comparative example determines whether or not slipping is occurring using the following (Condition 2') instead of the above (Condition 2). In other words, when all of (Condition 1), (Condition 2'), and (Condition 3) to (Condition 5) are met, the main controller of the wheel loader according to the comparative example determines that the slip determination condition is met and switches the slip determination flag FLslip from off to on.
(Condition 2') The wheel speed _VW is equal to or greater than the wheel speed threshold _VW0 ( _VW ≥ _VW0 ).
The wheel speed threshold _Vw0 is stored in advance in the ROM 102. The wheel speed threshold _Vw0 is determined based on the wheel speed when slippage occurs through experiments or the like. Note that the wheel speed threshold _Vw0 is a value obtained by adding a margin value (>0) to the wheel speed when slippage occurs in experiments or the like in order to prevent erroneous detection of slippage due to changes in wheel speed caused by vibrations of the vehicle body 8 or detection errors of the wheel speed sensor 61.

In Figure 9, the time series changes of each parameter in this embodiment are shown by solid lines, and the time series changes of each parameter in the comparative example are shown by dashed lines. The horizontal axes of Figures 9(a) to 9(g) represent time (elapsed time). The vertical axes of Figure 9(a) represent bucket operation amount R _B detected by bucket operation amount sensor 53a, and the vertical axis of Figure 9(b) represents arm operation amount R _A detected by arm operation amount sensor 52a. The vertical axis of Figure 9(c) represents traveling drive force F _M ' detected by drive force sensor 65 in the comparative example, and the vertical axis of Figure 9(d) represents traveling drive force F _M detected by drive force sensor 65 in this embodiment. The vertical axis of Figure 9(e) represents arm cylinder thrust F _A calculated by thrust calculation unit 110, the vertical axis of Figure 9(f) represents wheel speed V _W detected by wheel speed sensor 61, and the vertical axis of Figure 9(g) represents slip determination flag FLslip.

FIG. 9 shows time series changes after the wheel loader 1 enters the work object and the excavation determination flag FLdig is switched from off to on. In FIG. 9, time _t0 is the time when, with the bucket 3 having penetrated the work object, such as a mound of natural ground, the operator begins to further depress the accelerator pedal in order to cause the bucket 3 to further penetrate into the work object. In other words, time _t0 is the time when the travel driving forces F _M and F _M ' in the direction moving the vehicle body 8 forward begin to increase. The accelerator pedal is gently depressed. Time _t1 is the time when the slip determination flag FLslip is switched from off to on in this embodiment, which detects slip based on the arm cylinder thrust F _A. Time _t2 is the time when the slip determination flag FLslip is switched from off to on in a comparative example, which detects slip based on the wheel speed V _W.

9(a) and 9(b), when the bucket 3 has penetrated the work object, the bucket operation amount _RB and the arm operation amount _RA do not change. In other words, the work implement 6 maintains the attitude of penetrating into the work object (the attitude shown in FIG. 1). In this attitude, the bottom surface of the bucket 3 is parallel to the ground so that the bucket 3 can easily penetrate into the work object.

As shown in Figures 9(c) and 9(d), the traveling drive forces F _M and F _M ' gradually increase as the accelerator operation amount (not shown) increases from time _t0 . Furthermore, as shown in Figure 9(e), the arm cylinder thrust force F _A gradually increases in response to the increase in the traveling drive force. When the traveling drive force F _M exceeds the maximum static friction force generated in the wheel 7, slippage occurs. When slippage occurs, the friction force generated in the wheel 7 switches from the maximum static friction force to kinetic friction force, and the arm cylinder thrust force F _A begins to decrease rapidly.

As shown in Figures 9(f) and 9(g), in the wheel loader according to the comparative example, the slip determination flag FLslip is switched from off to on when the wheel speed _VW detected by the wheel speed sensor 61 rises to a wheel speed threshold _VW0 at a level that does not result in erroneous detection of slip. As shown in Figure 9(c), in the wheel loader according to the comparative example, the slip determination flag FLslip is switched from off to on at time _t2 , and thereafter the traveling drive force F _M ' is limited so that it does not exceed the drive force upper limit value F _{M_LIM} '. By limiting the traveling drive force F _M ' to the drive force upper limit value F _{M_LIM} ', the occurrence of subsequent slippage is prevented.

In contrast to this, in this embodiment, one of the slip determination conditions is that the arm cylinder thrust F _A is not increasing, rather than the wheel speed V _W (condition 2). As shown in FIG. 9( e), the arm cylinder thrust F _A immediately and suddenly decreases when slip occurs. For this reason, in this embodiment, as shown in FIG. 9( g), the slip determination flag FLslip is switched from off to on at time _t1 , which is earlier than in the comparative example. As shown in FIG. 9( d), in the wheel loader 1 according to this embodiment, the slip determination flag FLslip is switched from off to on at time _t1 , and thereafter the traveling drive force F _M is limited to the drive force upper limit value F _{M_LIM} , thereby preventing subsequent slip from occurring.

As described above, in the comparative example, slip cannot be detected until the wheel speed _VW reaches or exceeds the wheel speed threshold _VW0 , which is low enough to prevent false detection of slip. In contrast, in the present embodiment, slip can be detected based on the behavior of the arm cylinder thrust immediately after slippage begins, without using the wheel speed _VW . This allows for smoother excavation work. Furthermore, in the present embodiment, the slippage time is shorter than in the comparative example, so the time during which the running surface at the work site is scraped by the wheels 7 can be shortened. As a result, the effort required for subsequent repair work on the running surface can be reduced compared to the comparative example. Furthermore, in the present embodiment, the slippage time is shorter than in the comparative example, so the amount of tire wear can be reduced compared to the comparative example. Note that the drive force is limited automatically upon detection of slippage, regardless of the operator's operation, thereby reducing the operational burden on the operator.

The above-described embodiment provides the following advantages:

(1) A wheel loader (work vehicle) 1 includes a vehicle body 8 having wheels 7, a hydraulic pump 30A mounted on the vehicle body 8, a working device 6 having an arm cylinder (hydraulic actuator) 4 driven by hydraulic oil discharged from the hydraulic pump 30A, an arm cylinder pressure sensor (pressure sensor) 75 that detects the pressure of the arm cylinder 4, a traveling drive device 45 that drives the wheels 7, a driving force sensor 65 that detects the driving force of the traveling drive device 45, and a main controller (control device) 100 that controls the driving force of the traveling drive device 45. The main controller 100 calculates the thrust of the arm cylinder 4 corresponding to the reaction force that the working device 6 receives from a work object based on the detection result of the arm cylinder pressure sensor 75. The main controller 100 limits the driving force when slip determination conditions are met, including the driving force of the traveling drive device 45 (traveling driving force F _M ) being increased but the thrust of the arm cylinder 4 (arm cylinder thrust force F _A ) not increasing (i.e., (condition 2) and (condition 3)).

According to this embodiment, if the wheels 7 slip, the drive force is automatically limited to eliminate the slip state, allowing work to continue while preventing further slippage, thereby reducing the burden on the operator. Furthermore, according to this embodiment, compared to technologies that detect slippage based on wheel speed (such as the comparative example described above), wheel 7 slippage can be detected earlier, shortening the duration of slippage and improving the efficiency of excavation work.

(2) The slip determination conditions include an excavation operation state in which the work implement 6 is performing work to excavate a work object (i.e., (Condition 1)). When the excavation operation state is in effect and (Condition 2) to (Condition 5) are met, the main controller 100 limits the traveling drive force F _M. On the other hand, when the excavation operation state is not in effect, the main controller 100 does not limit the traveling drive force F _M even when (Condition 2) to (Condition 5) are met.

This configuration allows for proper detection of wheel 7 slippage that occurs when excavation work is being performed, and prevents erroneous detection of slippage when excavation work is not being performed.

(3) The wheel loader 1 is equipped with a motor speed sensor 58 that detects the rotational speed (motor speed) of the traveling motor 43 provided in the traveling drive device 45. The main controller 100 determines whether the advancing vehicle body 8 is decelerating based on the detection result of the motor speed sensor 58. The main controller 100 determines that the vehicle is in an excavation operation state when the following conditions for determining excavation operation are met: (Condition I) that the advancing vehicle body 8 is decelerating; and (Condition II) that the thrust of the arm cylinder 4 is equal to or greater than a predetermined value. In this embodiment, the main controller 100 determines that (Condition I) is met when the speed change rate ΔS _M of the traveling motor 43 is equal to or less than the speed change rate threshold S1, and determines that (Condition II) is met when the pressure _PA of the arm cylinder 4 is equal to or greater than the cylinder pressure threshold P1. Note that the speed change rate threshold S1 is smaller than 0 (S1<0). Note that the predetermined value in (Condition II) above corresponds to the value obtained by multiplying the cylinder pressure threshold P1 by the pressure-receiving area of the arm cylinder 4.

With this configuration, when the wheel loader 1 enters the work object, it is immediately determined that the excavation work state is in progress. This makes it possible to appropriately determine that the wheel loader 1 is in an excavation work state at the start of excavation work.

(4) The main controller 100 calculates a driving force change rate ΔF _M , which is the time rate of change of the traveling driving force, and calculates a thrust change rate ΔF _A , which is the time rate of change of the thrust of the arm cylinder 4. The slip determination conditions also include the working implement 6 not being operated (i.e., (Condition 4) and (Condition 5)). The main controller 100 according to this embodiment determines that the wheels 7 are slipping when all of the following conditions are met: the excavation work is in progress; the driving force change rate ΔF _M is greater than 0; the thrust change rate ΔF _A is equal to or less than the thrust change rate threshold F _A 1; and the arm 2 and bucket 3 that constitute the working implement 6 are not being operated.

The thrust of the arm cylinder 4 may change when the working device 6 is operated. In this embodiment, the slip determination conditions include the working device 6 not being operated, so a sudden decrease (loss) in the thrust of the arm cylinder 4 can be limited to cases where it is caused by slip. Therefore, this embodiment can prevent erroneous detection of slip when the working device 6 is operated.

(5) The main controller 100 calculates an estimated friction coefficient μ based on the thrust of the arm cylinder 4 when the slip determination condition is met (thrust during slip), and calculates a driving force upper limit value F _{M_LIM} based on the calculated estimated friction coefficient μ. Furthermore, the main controller 100 controls the driving force F M of the traveling drive device 45 so that the driving force F _M does not exceed the driving force upper limit value F _{M_LIM} .

According to this configuration, by calculating the friction coefficient estimated value μ, which changes non-linearly in accordance with the thrust F _A of the arm cylinder 4, and then setting the driving force upper limit value F _{M_LIM} , it is possible to increase the driving force to the limit of the driving force within the range in which slippage does not occur, thereby further improving the efficiency of excavation work.

The following modifications are also within the scope of the present invention, and it is possible to combine the configurations shown in the modifications with the configurations described in the above embodiments, or to combine the configurations described in the different modifications below.

<Modification 1>
In the above embodiment, the main controller 100 calculates the estimated friction coefficient μ and calculates the driving force upper limit F _{M_LIM} using the calculated estimated friction coefficient μ. However, the present invention is not limited to this. For example, the main controller 100 may calculate the driving force upper limit using a correlation map that defines the relationship between the slip thrust F _{A_slip} and the driving force upper limit. Furthermore, the main controller 100 may set the driving force upper limit (fixed value) to approximately 95% of the traveling driving force F _M detected by the driving force sensor 65 when slippage is detected.

<Modification 2>
The method of limiting the driving force is not limited to controlling the driving force so as not to exceed the driving force upper limit. For example, when slippage is detected, the main controller 100 may calculate a correction coefficient c according to the slip thrust F _{A_slip} and calculate the target driving force F _{M_TGT} by multiplying the required driving force F _{M_REQ} by the correction coefficient c. The correction coefficient c is a value greater than 0 and less than 1 (0<c<1). The larger the slip thrust F _{A_slip} , the larger the correction coefficient c.

<Modification 3>
The method of determining whether a transition from a non-excavation work state to an excavation work state has occurred is not limited to the method described in the above embodiment. In other words, the excavation work conditions are not limited to the examples described in the above embodiment. For example, the excavation work determination unit 114 may determine that the excavation work conditions are met when the accelerator operation amount of the accelerator operation device 56 is equal to or greater than a predetermined value and the speed change rate ΔS _M is equal to or less than the speed change rate threshold S1. According to this modification, the excavation work state is determined when the wheel loader 1 enters a work object with the accelerator pedal depressed. While the example of determining whether (Condition I) is satisfied based on the speed change rate ΔS _M has been described, whether (Condition I) is satisfied may also be determined based on the time change rate of the wheel (drive wheel) speed detected by the wheel speed sensor 61.

<Modification 4>
Furthermore, the method of determining whether or not an excavation operation state has been transitioned to a non-excavation operation state is not limited to the method described in the above embodiment. In other words, the excavation operation end condition is not limited to the example described in the above embodiment. For example, it may be determined that the excavation operation end condition has been met when the attitude of the working implement 6 changes from a plunge attitude, in which the bottom surface of the bucket 3 is parallel to the ground, to a transport attitude, in which the angle between the extension of the bottom surface of the bucket 3 and the ground is equal to or greater than a predetermined value due to the crowding operation of the bucket 3. The attitude of the working implement 6 can be detected by the arm relative angle sensor 62 and the bucket relative angle sensor 63.

<Modification 5>
In the above embodiment, the main controller 100 may perform moving average processing or low-pass filtering on values used in various determinations and calculations to avoid the effects of disturbances and noise. By performing moving average processing or low-pass filtering, it is possible to suppress sudden fluctuations in the estimated friction coefficient μ and the driving force upper limit value F _{M_LIM} . As a result, it is possible to improve stability and operability when the driving force is limited during excavation work.

<Modification 6>
In the above embodiment, an example has been described in which the wheel loader 1 is equipped with a single travel motor 43 that supplies power to the traveling device 11, but the present invention is not limited to this. The present invention may also be applied to a wheel loader 1 that is equipped with a plurality of travel motors 43. For example, the present invention can be applied to a wheel loader 1 that is equipped with a travel motor 43 that drives the left front wheel 7A and a travel motor 43 that drives the right front wheel 7A. The present invention can also be applied to a wheel loader 1 that is equipped with four travel motors 43 that drive each of a pair of left and right front wheels 7A and a pair of left and right rear wheels 7B. The travel motor 43 may be connected to the wheels 7 via a transmission, or may be configured to be integrated into the wheels 7.

<Modification 7>
In the above embodiment, an example has been described in which the work vehicle is an electrically driven wheel loader 1, but the present invention is not limited to this. For example, the present invention may be applied to a torque converter driven wheel loader or a hydraulic static transmission (HST) driven wheel loader that converts the power of the engine 20 into hydraulic power and transmits it to the wheels 7. The present invention may also be applied to a wheel loader equipped with a differential limiting device that limits the speed difference between the left and right wheels 7.

<Modification 8>
Some or all of the functions of the main controller 100 described in the above embodiment may be realized by hardware (for example, by designing logic for executing each function as an integrated circuit).

<Modification 9>
In the above embodiment, an example has been described in which the work vehicle is a wheel loader 1, but the present invention is not limited to this. The present invention can be applied to various work vehicles equipped with work implements such as a bulldozer.

The above describes embodiments of the present invention, but the above embodiments merely illustrate some of the application examples of the present invention, and are not intended to limit the technical scope of the present invention to the specific configurations of the above embodiments. The above-mentioned embodiments and variations are provided as examples to explain the present invention in an easy-to-understand manner, and are not necessarily limited to those that include all of the configurations described. Note that the control lines and information lines shown in the figures indicate those that are considered necessary for explanation, and do not necessarily represent all control lines and information lines required in the product. In reality, it can be assumed that almost all configurations are interconnected.

1...wheel loader (work vehicle), 2...arm, 3...bucket, 4...arm cylinder (hydraulic cylinder, hydraulic actuator), 5...bucket cylinder (hydraulic cylinder, hydraulic actuator), 6...working device, 7...wheel, 8...vehicle body, 8A...front vehicle body, 8B...rear vehicle body, 10...center joint, 11...traveling device, 20...engine, 25...engine controller, 30A, 30B, 30C...hydraulic pump, 31...front control unit, 32...brake control unit, 33...steering control unit, 40...generator motor, 41...generator inverter, 42...traveling inverter, 43...traveling motor, 45...traveling drive device, 50...operation device, 51...forward/reverse switch, 52...arm operation device, 52a...arm operation amount sensor, 53 ...Bucket operation device, 53a...Bucket operation amount sensor, 56...Accelerator operation device, 56a...Accelerator operation amount sensor, 58...Motor speed sensor, 61...Wheel speed sensor, 62...Arm relative angle sensor, 63...Bucket relative angle sensor, 65...Driving force sensor, 75...Arm cylinder pressure sensor (pressure sensor), 100...Main controller (control device), 110...Thrust calculation unit, 111...Thrust force change amount calculation unit, 112...Motor speed change amount calculation unit, 113...Driving force change amount calculation unit, 114...Excavation operation determination unit, 115...Slip determination unit, 116...Thrust force holding unit during slip, 117...Friction coefficient estimating unit, 118...Driving force upper limit value calculation unit, 119...Traveling required driving force calculation unit, 120...Minimum value selecting unit, 121...Torque command generating unit, F _A ...arm cylinder thrust, F _{A_slip} ...thrust force during slip, F _A1 ...thrust force change rate threshold, FLdig...digging determination flag, F _M ...traveling drive force (drive force), F _{M_LIM} ...drive force upper limit value, F _{M_LIM_MAX} ...maximum drive force upper limit value, F _{M_LIM_MIN} ...minimum drive force upper limit value, F _{M_REQ} ...traveling required drive force, F _{M_TGT} ...traveling target drive force, M _L , M _μ ...correlation map, P1...cylinder pressure threshold, P _A ...arm cylinder pressure, R _A ...arm operation amount, R _A1 ...arm operation amount threshold, R _B ...bucket operation amount, R _B1 ...bucket operation amount threshold, S1...speed change rate threshold, S _M ...motor speed, T _{M_COM} ...traveling drive torque command, V _W ...wheel speed, V _W 0...wheel speed threshold, ΔF _A ...thrust force change rate (time rate of change of thrust of hydraulic actuator), ΔF _M ...driving force change rate (time rate of change of driving force), ΔS _M ...speed change rate (time rate of change of rotation speed of traveling motor), μ...estimated friction coefficient

Claims (5)

a vehicle body having wheels;
a hydraulic pump mounted on the vehicle body;
a working device having a hydraulic actuator driven by hydraulic oil discharged from the hydraulic pump;
a pressure sensor for detecting the pressure of the hydraulic actuator;
a travel drive device that drives the wheels;
a driving force sensor that detects the driving force of the traveling drive device;
a control device for controlling the driving force of the traveling drive device,
The control device
calculating a thrust of the hydraulic actuator corresponding to a reaction force that the working device receives from a work object based on the detection result of the pressure sensor;
a limiting unit for limiting the driving force when a slip determination condition is met, the limiting unit including a condition that the driving force of the travel drive device is increasing but the thrust of the hydraulic actuator is not increasing. The work vehicle according to claim 1,
The work vehicle, wherein the slip determination conditions include a state in which the work vehicle is in an excavation operation state in which the work implement is performing an excavation operation on the work object. The work vehicle according to claim 2,
a motor speed sensor for detecting a rotation speed of a traveling motor provided in the traveling drive device;
The control device
determining whether the forward moving vehicle body is decelerating based on the detection result of the motor speed sensor;
a work vehicle that determines that the work vehicle is in the excavation work state when excavation work determination conditions are met, including that the forward moving vehicle body is decelerating and that the thrust of the hydraulic actuator is equal to or greater than a predetermined value. The work vehicle according to claim 2,
The work vehicle, wherein the slip determination conditions include a condition in which the work implement is not being operated. The work vehicle according to claim 1,
The control device
calculating a friction coefficient estimate value based on the thrust of the hydraulic actuator when the slip determination condition is met;
calculating a driving force upper limit value based on the estimated friction coefficient value;
A work vehicle, characterized in that the driving force of the travel drive device is controlled so as not to exceed the driving force upper limit value.