work machine

Abstract

Work machine, comprising: a hydrostatic pump designed as a variable displacement swash plate pump driven by an internal combustion engine; a hydrostatic motor connected in a closed circuit to the hydrostatic pump via a pair of speed-changing oil lines for driving a traveling device by means of oil supplied from the hydrostatic pump; a control unit for controlling an engine speed of the internal combustion engine and for controlling a swash plate of the hydrostatic pump; and an accelerator pedal for changing the engine speed to control the vehicle speed; a throttle adjustment element for adjusting the engine speed independently of a degree of depression of the accelerator pedal; a servo cylinder for changing an inclination angle of the swash plate of the hydrostatic pump to tilt the swash plate to rotate the hydrostatic motor in a direction that causes normal rotation of the hydrostatic motor or in a direction that causes reverse rotation of the hydrostatic motor; a swash plate control valve constructed as an electromagnetic proportional valve for supplying a control pressure to the servo cylinder, the control pressure changing the inclination angle of the swash plate with the aid of a command signal from the control unit [0040]; and an accelerator pedal position sensor for detecting a degree of operation of the accelerator pedal and inputting the detected degree to the control unit, wherein the control unit has a normal mode for performing vehicle speed control by controlling the engine speed based on a degree of depression of the accelerator pedal and by controlling a swash plate of the hydrostatic pump based on the engine speed, and a device mode for performing vehicle speed control by controlling the swash plate of the hydrostatic pump based on the degree of depression of the accelerator pedal regardless of the engine speed, characterized in that in the normal mode, the control unit outputs a first command signal to the internal combustion engine in such a manner that the engine speed is controlled based on the degree of depression of the accelerator pedal and also outputs a second command signal to the swash plate control valve in such a manner that the inclination angle of the swash plate is controlled based on the engine speed controlled by the first command signal, and in the device mode, the control unit does not output a command signal to the internal combustion engine for controlling the engine speed based on the degree of depression of the accelerator pedal detected by the accelerator position sensor, and outputs a third command signal to the internal combustion engine based on the engine speed adjusted by the throttle adjusting element, thereby maintaining the engine speed, and outputs a fourth command signal to the swash plate control valve based on the degree of depression of the accelerator pedal detected by the accelerator position sensor, thereby changing the inclination angle of the swash plate in accordance with the degree of depression of the accelerator pedal detected by the accelerator position sensor.

Description

[0001] The present invention relates to a working machine, such as a wheel loader.

[0002] Previously known work machines include, for example, a wheel loader as described in JP H08 - 40 223 A (1996).

[0003] This wheel loader consists of a front carriage with front wheels and a rear carriage with rear wheels. The front carriage has a working device, and the rear carriage is equipped with an internal combustion engine and a hydrostatic drive (HST).

[0004] The working device has a lifting frame that can be freely pivoted up and down, and a bucket located at the front end of the lifting frame for free pivoting movement. Instead of the bucket, another attachment can also be attached to the front end of the lifting frame.

[0005] The hydrostatic drive system includes a hydrostatic pump (HST pump), a variable displacement swashplate pump driven by an internal combustion engine, and a hydrostatic motor connected to the hydrostatic pump in a closed circuit via a pair of speed-variable oil lines. The hydrostatic motor is driven by oil supplied by the hydrostatic pump, and the rear wheel is driven by rotational force generated by the hydrostatic motor. By adjusting the angle of the hydrostatic pump's swashplate, it is possible to vary the direction and rate of hydraulic oil supplied by the hydrostatic pump, allowing the wheel loader to move forward or backward while continuously varying the vehicle speed.

[0006] The wheel loader also has an accelerator pedal. Pressing the accelerator pedal controls the engine speed, and changing the engine speed varies the control pressure for the swash plate of the hydrostatic pump. The vehicle speed is controlled based on the engine speed and the control pressure for the swash plate.

[0007] Additionally, the wheel loader features a creep pedal for slowing and stopping the wheel loader. The creep pedal is designed to reduce the angle of the hydrostatic pump's swashplate when depressed, allowing the wheel loader to slow down and stop.

[0008] In the wheel loader described above, normal driving control is achieved by operating the accelerator pedal. This means that as the accelerator pedal is depressed more, the vehicle speed increases, and as the accelerator pedal is depressed more, the vehicle speed decreases.

[0009] Furthermore, in a case where an attachment requiring a drive section to rotate at a fixed speed, such as a sweeper (cleaning device) or a mower (lawn mower), is mounted on the wheel loader instead of the bucket, work is performed while maintaining a constant engine speed. In this case, travel control cannot be performed using the accelerator pedal, so the creep pedal is used for travel control.

[0010] In the case where travel control is performed by the creep pedal, the wheel loader starts traveling when the creep pedal is returned (released) in a pressed state, and when it is desired to stop the wheel loader, the creep pedal is stepped on.

[0011] This means that driving control via the creeper pedal is the opposite of driving control via the accelerator pedal. This poses a problem for an inexperienced user due to the difficulty of adjusting the vehicle speed. Furthermore, US 5 335 750 A describes a control system for a hydrostatic transmission control of vehicles. US 2009 / 0 320 462 A1 describes a hydraulic drive device and a hydraulically driven vehicle with a motor-driven, adjustable hydraulic pump. US 7 962 768 B2 describes a machine control system having a power-adapted operating mode.

[0012] The present invention has been made in view of the above-mentioned problem, and accordingly, an object of the present invention is to provide a work machine in which vehicle speed control is possible through the accelerator pedal operation even during execution of work under a condition in which an engine speed is kept constant.

[0013] The problem described above is solved by a work machine which includes the features of claim 1.

[0014] The following are the features of technical measures taken by the present invention to solve the above-mentioned problem.

[0015] Preferably, the work machine further comprises: a servo cylinder for controlling the swash plate of the hydrostatic pump; a swash plate control valve constructed as an electromagnetic proportional valve for supplying hydraulic oil from a hydraulic pump driven by an internal combustion engine to the servo cylinder as a control pressure for the swash plate; and an accelerator pedal position sensor for detecting a degree of operation of the accelerator pedal and inputting the detected degree to the control unit, wherein in the normal mode, a command signal is output from the control unit to the internal combustion engine in such a manner that the engine speed is controlled based on the degree of depression of the accelerator pedal, and also a command signal is output from the control unit to the swash plate control valve in such a manner that the control pressure for the swash plate of the hydrostatic pump is controlled based on the engine speed, and wherein, in the device mode, a command signal is output from the control unit to the swash plate control valve in such a manner that the control pressure for the swash plate of the hydrostatic pump is controlled based on the degree of depression of the accelerator pedal.

[0016] In an advantageous embodiment, the work machine further comprises: a creep pedal; and a creeper pedal position sensor for detecting a degree of operation of the creeper pedal and for inputting the detected degree to the control unit, wherein in the normal mode and in the implement mode, a command signal is output from the control unit to the swash plate control valve in such a manner that the control pressure for the swash plate of the hydrostatic pump is reduced by a pressing operation of the creep pedal.

[0017] In an advantageous embodiment, the work machine further comprises: a throttle adjustment element for adjusting an engine speed to a predetermined speed level in the device mode.

[0018] In a special version, the work machine also includes: a work device for performing work, wherein the control unit is capable of controlling an engine speed based on first to third control characteristics defining a relationship between the degree of operation of the accelerator pedal and engine speed, and wherein the control unit comprises: a first engine speed control section for controlling the engine speed according to the work speed-oriented first control characteristic during execution of work with the work device; a second engine speed control section for controlling the engine speed according to the fuel-economy-oriented second control map when a driving load is less than a predetermined load during a trip; and a third engine speed control section for controlling the engine speed according to the third control characteristic which is between the first control characteristic and the second control characteristic when a traveling load is equal to or greater than the predetermined load during traveling.

[0019] In a further specific embodiment of the invention, the work machine further comprises: a servo cylinder for controlling the swash plate of the hydrostatic pump; a swash plate control valve constructed as an electromagnetic proportional valve for supplying hydraulic oil from a hydraulic pump driven by an internal combustion engine to the servo cylinder as a control pressure for the swash plate, a creep pedal for reducing vehicle speed; and a creeper pedal position sensor for detecting the degree of depression of the creeper pedal and for inputting the detected degree to the control unit, wherein the swash plate control valve is controlled by a command signal output from the control unit in such a manner that the control pressure for the swash plate is reduced by a pressing operation of the creep pedal, and wherein the first engine speed control section controls the engine speed during operation of the creeper pedal according to the first control characteristic.

[0020] In a preferred embodiment of the invention, the working machine is capable of making a selection between the normal mode and an economy mode, wherein in the normal mode, an engine speed is generally controlled according to the first control characteristic, and in the economy mode, the engine speed is controlled by the first to third engine speed control sections.

[0021] The present invention offers the following effects.

[0022] According to the invention of claim 1, in the normal mode, the engine speed is controlled based on the degree of depression of the accelerator pedal, and the swash plate of the hydrostatic pump is controlled based on the engine speed. This makes it possible to exercise vehicle speed control through the usual accelerator pedal operation; that is, it makes it possible to control the vehicle speed while adjusting the engine speed through the accelerator pedal depression.

[0023] Furthermore, in implement mode, the swash plate of the hydrostatic pump is controlled based on the degree of accelerator pedal depression, regardless of the engine speed. This makes it possible to control the engine speed by pressing the accelerator pedal even in a state where the engine speed is maintained at a constant level. Accordingly, even when using an implement that requires a drive section to rotate at a fixed speed, the vehicle speed can be controlled by the same operation as the usual accelerator pedal operation.

[0024] Furthermore, it is possible to enable realization of a system that can make a selection between a method of exercising vehicle speed control by controlling the engine speed through the depression of the accelerator pedal and a method of exercising vehicle speed control by controlling the swash plate of the hydrostatic pump through the depression of the accelerator pedal regardless of the engine speed.

[0025] In particular, the vehicle speed can be reduced satisfactorily in both normal mode and device mode.

[0026] Furthermore, in device mode, the engine speed can be set to a predetermined speed level using the throttle adjustment element.

[0027] In particular, during execution of work with the working device, the engine speed is controlled according to the work speed-oriented first control characteristic, whereby the work can be performed with high efficiency.

[0028] In addition, during a trip, the engine speed is controlled according to the fuel-economy-oriented second control map when a driving load is smaller than a predetermined load, whereby fuel consumption can be reduced.

[0029] In addition, during a trip, the engine speed is controlled according to the third control map, which is located between the first control map and the second control map, when a driving load is greater than or equal to the predetermined load. Accordingly, even if a driving load frequently becomes greater than or equal to the predetermined load during engine speed control according to the second control map, a reduction in fuel consumption can be achieved.

[0030] As described above, it is possible to provide a working machine that is capable of both increasing fuel efficiency and performing work with the highest efficiency.

[0031] Specifically, the degree of depression of the creep pedal is detected by the creep position sensor, and the detected degree is input to the control unit. Then, a command signal can be output from the control unit to the swash plate control valve in such a way that the control pressure for the swash plate of the hydrostatic pump is reduced based on the degree of depression of the creep pedal. Furthermore, since the creep pedal is used to operate the work machine, it is possible to use the detection of the pedal depression by the creep pedal position sensor to detect the execution and work.

[0032] In this way, the first engine speed control section controls the engine speed during creep pedal operation according to the first control characteristic. This eliminates the need for additional detection means for detecting work execution, thereby making it possible to build the machine at a lower cost.

[0033] In particular, operators can choose between normal mode, which is geared to ground speed and working rpm, and economical mode, which allows for reduced fuel consumption. This is very convenient for operators. [ Fig. 1] Fig. 1 is a side view of a wheel loader. [ Fig. 2] Fig. Figure 2 is a block diagram showing part of the hydraulic circuit and electrical control system of the wheel loader. [ Fig. 3A] Fig. Figure 3A is a block diagram showing a vehicle speed control system in normal mode. [ Fig. 3B] Fig. Figure 3B is a block diagram showing a vehicle speed control system in device mode. [ Fig. 4A] Fig. 4A is an engine speed control map according to the present invention for defining the relationship between engine speed and the degree of accelerator pedal pressure. [ Fig. 4B] Fig. Figure 4B is an engine speed control map referring to a comparative example.

[0034] The following are explanations of embodiments of the present invention with reference to the drawings.

[0035] Fig. 1 illustrates a wheel loader 1 as a working machine.

[0036] The wheel loader 1 is an articulated work machine whose mobile machine body 1A consists of a front carriage 2 and a rear carriage 3. The front carriage 2 has a pair of right and left front wheels 5, and the rear carriage 3 has a pair of right and left rear wheels 6.

[0037] At the front of the rear carriage 3, a coupling element 4 is arranged for free rotation within a predetermined rotation range about an axis arranged in a front-to-back direction or the front-to-back axis, and the rear of the front carriage 2 is coupled to the coupling element 4 for free articulation movement from one side to the other about an axis arranged in a direction from top to bottom or the vertical axis.

[0038] A steering cylinder 7, designed as a hydraulic cylinder, is arranged transversely to the coupling element 4 and the front carriage 2. With the telescopic movement of the steering cylinder 7, the front carriage 2 is pivoted from side to side relative to the rear carriage 3, thereby imparting a side-to-side pivoting movement to the wheel loader 1.

[0039] Furthermore, the front end 2 is equipped with a work device 8 (front work device). The work device 8 includes a pair of right and left sides of the lifting frame 9 and a bucket 10. The right and left sides of the lifting frame 9 are supported at their base sides for free rotation around an axis extending in a right-left direction, or the horizontal axis, via a support frame 11 fixed in the front end 2 so that it can be pivoted up and down. The bucket 10 is coupled to the front end of the right and left sides of the lifting frame 9 for free pivoting movement around the horizontal axis. The right and left sides of the lifting frame 9 are driven by a lift cylinder 12, and the bucket 10 is driven by a bucket cylinder 13. The lift cylinder 12 and the bucket cylinder 13 are each constructed as a hydraulic cylinder.

[0040] However, the bucket 10 is installed detachably so that an attachment other than the bucket 10, such as a sweeper, a mower or a crusher, can be attached to the front end of the lifting frame 9.

[0041] The rear carriage 3 is equipped with a driver's seat 14, a roof with four pillars 15, which serves as a flat protective device for the driver's seat, a steering wheel 16 for actuating the steering cylinder 7, a working device actuating lever 17 for actuating the working device 8 and an internal combustion engine 18 (diesel engine).

[0042] Fig. Figure 2 shows part of the hydraulic circuit and the electrical control system of the wheel loader 1.

[0043] In Fig. 2, reference numeral 18 represents an internal combustion engine, reference numeral 19, a hydrostatic drive (HST) for a traveling device, reference numeral 20, a control unit for controlling the internal combustion engine 18 and the hydrostatic drive 19, reference numeral 21, an accelerator pedal for increasing and decreasing the speed of the wheel loader 1, reference numeral 22, a creep pedal for decelerating and stopping the wheel loader 1, reference numeral 23, a main pump, and reference numeral 24, a sub-pump (charging pump).

[0044] The hydrostatic drive 19 includes a hydrostatic pump 26 driven by the internal combustion engine 18 and a hydrostatic motor 29 connected to the hydrostatic pump 26 via a pair of speed-variable oil lines 27 and 28 in a closed circuit. The hydrostatic motor 29 is driven by oil delivered by the hydrostatic pump 26, and the front wheel 5 and the rear wheel 6, in this embodiment, are driven by a rotational force generated by the hydrostatic motor 29.

[0045] The hydrostatic pump 26 is designed as a variable-displacement swash plate pump. By adjusting the angle of the swash plate of the hydrostatic pump 26, it is possible to vary the delivery direction and also the delivery rate of the hydraulic oil delivered by the hydrostatic pump. This allows the present design to vary the rotational speed of the output shaft of the hydrostatic motor 29 (and the vehicle speed of the wheel loader 1) in a stepless manner in a direction for moving the wheel loader 1 forward (a direction for causing normal rotation of the hydrostatic motor 29) or in a direction for moving the wheel loader 1 backward (a direction for causing reverse rotation of the hydrostatic motor 29).

[0046] The hydrostatic motor 29 is constructed as a variable displacement swash plate motor, in which the angle of the swash plate of the hydrostatic motor 29 (the inclination angle of the swash plate) can be controlled with a command signal from the control unit 20, and the hydrostatic braking force can be exerted by increasing the angle of the swash plate of the hydrostatic motor 29. Power is transmitted from an output shaft 30 of the hydrostatic motor 29 to a transmission shaft 32 via a gear transmission mechanism 31, and is then transmitted from the rear side of the transmission shaft 32 to right and left rear axles 34 via a rear wheel differential device 33, and finally, the power is transmitted from the rear axles 34 to the rear wheels 6, thereby driving the rear wheels 6.In addition, the power from the front of the transmission shaft 32 is also transmitted to the front wheels 5 via a drive shaft, a front wheel differential device, front axles, and so on, thereby driving the front wheels 5.

[0047] The front and rear wheels 5 and 6, a bearing housing for supporting the front and rear wheels 5 and 6, a power transmission system for transmitting power from the hydrostatic motor 29 to the front and rear wheels 5 and 6, and so on together constitute the traveling device.

[0048] The rotational speed of the transmission shaft 32 can be detected by a rotation sensor 35. The rotation sensor 35 is connected to the control unit 20 so that a value detected by the rotation sensor 35 can be input to the control unit 20. In the control unit 20, a vehicle speed calculation is performed based on the value detected by the rotation sensor 35.

[0049] When an actual vehicle speed obtained based on the value detected by the rotation sensor 35 is lower than a target vehicle speed obtained from an engine speed command value output from the control unit 20 (that is, when there is a reduction in vehicle speed), it is detected that a traveling load is applied to the wheel loader 1. Furthermore, the degree of the traveling load is determined based on the difference between the target vehicle speed and the actual vehicle speed.

[0050] The term “driving load” as used here refers to a load applied to the wheel loader 1 when driving uphill, entering a curve or at other times.

[0051] The swash plate of the hydrostatic pump 26 is subjected to an angle-changing actuation by a servo cylinder 36. The servo cylinder 36 is connected to a forward-reverse selector valve 39 via a forward oil line 37 and a reverse oil line 38. The forward-reverse selector valve 39, which is designed as an electromagnetic 4 / 3-way valve, is held in a neutral position 39a by a spring 41 upon demagnetization of a coil 40. In response to an excitation signal from the control unit 20, the forward-reverse selector valve 39 is shifted from the neutral position 39a to a forward travel position 39b or a reverse travel position 39c.

[0052] After moving the forward-reverse selector valve 39 to the forward travel position 39b, the swash plate of the hydrostatic pump 26 becomes tiltable to allow normal rotation of the hydrostatic motor 29. On the other hand, after moving the forward-reverse selector valve 39 to the reverse travel position 39c, the swash plate of the hydrostatic pump 26 becomes tiltable to allow reverse rotation of the hydrostatic motor 29.

[0053] The forward-reverse selector valve 39 is connected to a swash plate control valve 43 via a pilot pressure supply line 42. The swash plate control valve 43 is designed to control the pressure of the pressurized oil supplied to the servo cylinder 36 (swash plate control pressure of the hydrostatic pump 26). The swash plate control valve 43, which is designed as an electromagnetic proportional valve, is capable of controlling the pressure of the pressurized oil supplied to the servo cylinder 36 using a command signal (a current command value) from the control unit 20.

[0054] The control unit 20 is connected to a forward-reverse selection switch 44, and by actuating the forward-reverse selection switch 44, the forward-reverse selection valve 39 can be energized so that it can be moved to the forward driving position 39b or the reverse driving position 39c.

[0055] The control unit 20 is also connected to a neutral switch 45, and by actuating the neutral switch 45, the forward-reverse selector valve 39 is demagnetized so that it can be moved from the forward travel position 39b or the reverse travel position 39c to the neutral position 39a.

[0056] The accelerator pedal 21 is designed to control the vehicle speed through its depression. The degree of depression of the accelerator pedal 21 is detected by an accelerator position sensor 46, which is designed as a potentiometer. The accelerator position sensor 46 is connected to the control unit 20 so that a detected value (the degree of depression of the accelerator pedal 21) from the accelerator position sensor 46 can be input to the control unit 20.

[0057] The creep pedal 22, which is capable of push-operation, is designed so that after the pedal is depressed to a predetermined degree of depression, a master cylinder 47 can be actuated for braking by a further push-operation. The operating range of the creep pedal, which extends from a state in which the creep pedal 22 is not depressed to a state immediately before operation of the master cylinder 44, is hereinafter referred to as an "increep operating range A."

[0058] The master cylinder 47 is connected via a brake oil line 49 to a brake mechanism 48 for braking the rear axle 34, and by actuating the master cylinder 47 via the operation of the creeper pedal 22, the brake mechanism 48 is actuated to brake the right and left rear axles 34.

[0059] The degree of depression of the inching pedal 22 is detected by an inching pedal position sensor 51, which is constructed as a potentiometer. The inching pedal position sensor 51 is connected to the control unit 20, so that a detected value (the degree of depression of the inching pedal 22) from the inching pedal position sensor 51 can be input to the control unit 20.

[0060] In the creep operating range A, a command signal is output from the control unit 20 to a coil 52 of the swash plate control valve 43 in such a manner that the control pressure for the swash plate of the hydrostatic pump 26 is reduced while the creep pedal 22 is depressed.

[0061] The main pump 23 and the sub-pump 24 are both designed as a fixed-displacement hydraulic pump driven by the power of the internal combustion engine 18. The main pump 23 supplies hydraulic oil to the hydraulic actuators (such as the steering cylinder 7, the lifting cylinder 12, and the bucket cylinder 13) mounted on the wheel loader 1 and to the hydraulic cylinder on an attachment installed on the wheel loader 1 instead of the bucket 10.

[0062] The lower pump 24 is connected to the swash plate control valve 43 via a hydraulic oil supply line 53 to supply hydraulic oil to the servo cylinder 36. Furthermore, the lower pump 24 supplies pressurized oil to a pilot valve for operating a pilot control valve for controlling the hydraulic actuators, a charging circuit for refilling the speed change oil line 27, 28 on the low-pressure side of the hydrostatic drive with hydraulic oil, and so on.

[0063] The control unit 20 has a normal mode, an appliance mode, and an economy mode. At the start of operation of the internal combustion engine 18, the control unit 20 remains in normal mode. In this mode, the control unit 20 switches to the appliance mode by operating an appliance mode switch 54 or switches to the economy mode by operating an economy mode switch 55. The appliance mode switch 54 and the economy mode switch 55 are connected to the control unit 20.

[0064] In normal mode, as in Fig. 3A, the number of revolutions or the rotational speed of the engine 18 (the target engine speed) is controlled by the accelerator pedal 21. Specifically, when the accelerator pedal 21 is not depressed, the rotational speed of the engine 18 corresponds to an idling speed. Then, a command signal is output from the control unit 20 to the engine 18 in such a manner that, when the accelerator pedal 21 is depressed, the rotational speed of the engine 18 is correspondingly increased, and, conversely, when the accelerator pedal 21 is released, the rotational speed of the engine 18 is correspondingly decreased.

[0065] In addition, in normal mode, the control pressure for the swash plate of the hydrostatic pump 26 is controlled based on the speed of the internal combustion engine 18.Specifically, a map including characteristic curves indicating the relationship between the engine speed and the control pressure for the swash plate of the hydrostatic pump 26 is included in the control unit 20, and in accordance with the map, a command signal is output from the control unit 20 to the coil 52 of the swash plate control valve 43 in such a manner that the control pressure for the swash plate of the hydrostatic pump 26 is made to correspond to the speed of the internal combustion engine 18 (the swash plate control valve 43 is controlled by the control unit 20 in such a manner that the control pressure for the swash plate of the hydrostatic pump 26 is increased with an increase in the engine speed, while the control pressure for the swash plate of the hydrostatic pump 26 is decreased with a decrease in the engine speed).

[0066] In this way, in the normal mode, under a condition where the creeper pedal 22 is not depressed, the discharge rate of the hydrostatic pump 26 is determined according to the engine speed (revolutions of the hydrostatic pump 26) based on the depression of the accelerator pedal 21 and according to the control pressure for the swash plate of the hydrostatic pump 26 based on the engine speed to determine the vehicle speed.

[0067] In the normal mode, when the forward-reverse selector valve 39 assumes the neutral position, the servo cylinder 36 is balanced to minimize the swash plate angle of the hydrostatic pump 26 (the inclination angle of the swash plate) so that no hydraulic oil is discharged from the hydrostatic pump 26.

[0068] In the case of a shift of the forward-reverse selector valve 39 from the neutral position 39a to the forward travel position 39b or the reverse travel position 39c, when the engine speed coincides with an idling speed, the control pressure for the swash plate of the hydrostatic pump 26 is at a minimum (the swash plate angle of the hydrostatic pump 26 is at the smallest) and the wheel loader 1 stops.

[0069] In this state, when the accelerator pedal 21 is depressed, the engine speed increases and the control pressure for the swash plate of the hydrostatic pump 26 also increases, whereupon the swash plate angle increases, thereby performing a forward-backward movement. Furthermore, when the accelerator pedal 21 is fully depressed to maximize the engine speed, the control pressure for the swash plate of the hydrostatic pump 26 increases to a maximum.

[0070] Furthermore, when the creep pedal 22 is depressed while the accelerator pedal 21 is depressed, a command signal is output from the control unit 20 to the coil 52 of the swash plate control valve 43 in such a way as to reduce the control pressure for the swash plate of the hydrostatic pump 26. Consequently, the swash plate angle of the hydrostatic pump 26 is reduced, and the wheel loader 1 decelerates, while the rotational speed of the internal combustion engine 18 remains at a level determined by the operation of the accelerator pedal 21.

[0071] Meanwhile, in device mode, as in Fig. 3B, even when the accelerator pedal 21 is depressed, the rotational speed of the engine 18 remains unchanged. In this case, when the accelerator pedal 21 is depressed, in response to the degree of depression of the pedal, a command signal is output from the control unit 20 to the coil 52 of the swash plate control valve 43 in such a manner that the control pressure for the swash plate of the hydrostatic pump 26 is changed according to the degree of depression of the accelerator pedal 21. That is, in the device mode, the swash plate of the hydrostatic pump 26 is controlled based on the degree of depression of the accelerator pedal 21, regardless of the rotational speed of the engine 18. A command signal is output from the control unit 20 to the coil 52 of the swash plate control valve 43 in such a manner that when the accelerator pedal 21 is not depressed, the control pressure for the swash plate of the hydrostatic pump 26 is minimized.In addition, a command signal is output from the control unit 20 to the swash plate control valve 43 in such a manner that when the accelerator pedal 21 is depressed, the control pressure for the swash plate of the hydrostatic pump 26 is raised (the swash plate angle of the hydrostatic pump 26 is increased).

[0072] In device mode, the speed of the internal combustion engine 18 is adjusted by means of a throttle adjustment controller 56 (a throttle adjustment element). In particular, the throttle adjustment controller 56 is connected to the control unit 20, so that a command signal can be output from the control unit 20 to the internal combustion engine 18 in such a way that the speed of the internal combustion engine 18 matches the engine speed determined by the throttle adjustment controller 56. Furthermore, by rotating the throttle adjustment controller 56, the speed of the internal combustion engine 18 can be adjusted within a range of, for example, 1700 rpm. -1- 2400 min -1 be changed stepwise or continuously and the speed of the internal combustion engine 18 can also be maintained at a predetermined level.

[0073] In the implement mode, when the forward-reverse selector valve 39 assumes the neutral position 39a, as in the normal mode, the swash plate angle of the hydrostatic pump 26 is at a minimum, whereby no hydraulic oil is supplied from the hydrostatic pump 26.

[0074] Furthermore, even if the forward-reverse selector valve 39 is shifted from the neutral position 39a to the forward travel position 39b or the reverse travel position 39c, and even if the engine speed is adjusted to a speed higher than the idle speed by the throttle control knob 56, as long as the accelerator pedal 21 is not depressed, the control pressure for the swash plate of the hydrostatic pump 26 is at a minimum (the swash plate angle of the hydrostatic pump 26 is at its smallest). Consequently, neither forward movement nor reverse movement is performed.

[0075] In this state, pressing the accelerator pedal 21 sets the wheel loader 1 in motion, and pressing the accelerator pedal 21 further controls the control pressure for the swash plate in such a way that the swash plate angle of the hydrostatic pump 26 is increased. Consequently, the wheel loader 1 accelerates. At this time, the engine speed remains at the speed level determined by the throttle control 56.

[0076] Furthermore, while the creep pedal 22 is depressed during the depressing of the accelerator pedal 21, a command signal is output from the control unit 20 to the coil 52 of the swash plate control valve 43 in such a manner that the control pressure for the swash plate of the hydrostatic pump 26 is reduced and, consequently, the swash plate angle of the hydrostatic pump 26 is lowered. Consequently, the wheel loader 1 decelerates while the engine speed determined by the throttle control controller 56 remains the same.

[0077] In normal mode and also in implement mode, the final control pressure for the swash plate of the hydrostatic pump 26 (the current value given to the swash plate control valve 43 for controlling a control pressure for the swash plate of the hydrostatic pump 26) is determined by the expression Y x (Z / 100).

[0078] In the expression, Y denotes a control pressure for the swash plate of the hydrostatic pump 26 which depends on the operation of the accelerator pedal 21, and Z denotes the degree of depression of the creep pedal 22 within the creep operation range A. Z indicates 100 percent at the time of release of the creep pedal (in a state in which the creep pedal 22 is not depressed), while it indicates 0 percent at a time of full depression of the creep pedal within the creep operation range A.

[0079] Consequently, in the normal mode and also in the implement mode, regardless of the degree of depression of the accelerator pedal 21, by depressing the creep pedal 22 to the point immediately before the degree of depression reaches a level that allows actuation of the master cylinder 47, the control pressure for the swash plate of the hydrostatic pump 26 is reduced to a minimum (zero) and the wheel loader 1 stops.

[0080] As described above, in the normal mode, the engine speed is controlled based on the degree of depression of the accelerator pedal 21, and the swash plate of the hydrostatic pump 26 is controlled based on the engine speed. This makes it possible to control the vehicle speed through the usual operation of the accelerator pedal 21; that is, it becomes possible to control the vehicle speed while adjusting the engine speed through the depression operation of the accelerator pedal 21.

[0081] Furthermore, in implement mode, the swash plate of the hydrostatic pump 26 is controlled based on the degree of depression of the accelerator pedal 21, regardless of the engine speed. This makes it possible to control the vehicle speed by pressing the accelerator pedal 21 while maintaining the engine speed constant. Accordingly, even when using an implement where the drive section must rotate at a fixed speed, vehicle speed control can be achieved by the same operation as the usual operation of the accelerator pedal 21.

[0082] In Economy mode, the speed of the internal combustion engine 18 (the target engine speed) is controlled in accordance with an engine speed control map 57 as shown in Fig. 4A. The engine speed control map 57 defines the relationship between the degree of depression of the accelerator pedal 21 and the engine speed. In the map, the degree of depression of the accelerator pedal 21 is plotted along the abscissa and the engine speed along the ordinate.

[0083] The engine speed control map 57 includes a first control characteristic 58, a second control characteristic 59, and a third control characteristic 60. The control unit 20 includes a first engine speed control section 61 for controlling the engine speed according to the first control characteristic 58; a second engine speed control section 62 for controlling the engine speed according to the second control characteristic 59; and a third engine speed control section 63 for controlling the engine speed according to the third control characteristic 60.

[0084] The first control characteristic 58 is intended to control the speed of the internal combustion engine 18 to a high speed in a manner oriented to the speed of a working device 8, and in particular the first control characteristic 58 is intended, for example, to control the engine speed to a rated speed under a condition in which the accelerator pedal 21 is fully depressed.

[0085] The second control characteristic 59 is intended to control the speed of the internal combustion engine 18 to a low speed (which is lower than the speed defined by the first control characteristic 58) in a fuel-saving manner.

[0086] The third control characteristic curve 60, which is arranged between the first control characteristic curve 58 and the second control characteristic curve 59, is intended to control the speed of the internal combustion engine 18 to a speed level that falls between the speed defined by the first control characteristic curve 58 and the speed defined by the second control characteristic curve 59.

[0087] In this embodiment, the maximum engine speed corresponding to the first control characteristic 58 (a speed obtained at a time of full depression of the accelerator pedal 2) is set to 2400 min -1 set; the maximum engine speed, which corresponds to the second control characteristic 59, is 1800 min -1 set; and the maximum engine speed, which corresponds to the third control characteristic 60, is 2200 min -1 set.

[0088] During the execution of work with the work machine 8, the first engine speed control section 61 of the control unit 20 controls the speed of the internal combustion engine 18 according to the first control characteristic 58. This makes it possible to perform work with high efficiency.

[0089] During the execution of work with the work device 8, the creep pedal 22 is operated to repeatedly start and stop the wheel loader 1 in small movements (move the wheel loader 1 forward or backward at a creeping speed), which is the so-called creep operation (creep travel). Furthermore, during the execution of work with the work device 8, in order to speed up the operation of the work device 8, the starting and stopping of the wheel loader and vehicle acceleration and deceleration are performed by the creep pedal 22, with the engine speed being kept high by the accelerator pedal 21. Therefore, since the creep pedal 22 is used to execute work with the work device 8 in this embodiment, execution of work with the work device 8 is identified by the presence of the operation of the creep pedal 22.That is, during the operation of the creeper pedal 22, the first engine speed control section 61 controls the engine speed according to the first control characteristic 58.

[0090] Execution of work with the work device 8 can also be detected by detecting an operation of the lifting frame 9 and the bucket 10 or an operation of the work device operating lever 17. In this case, however, an additional detection means, such as a sensor, must be provided. In contrast, in the wheel loader 1 according to the invention, to control the control pressure of the swash plate of the hydrostatic pump 26, the degree of operation of the creep pedal 22 is detected by the creep pedal position sensor 51, and the detection result is input to the control unit 20. Accordingly, the detection of the pedal pressure operation by the creep pedal position sensor 51 can be used to detect execution of work with the work device 8. This offers the advantage that no additional detection means need to be provided to detect execution of work with the work device 8.

[0091] Meanwhile, during travel (during travel of the wheel loader without performing work), when a travel load is less than a predetermined load, the second engine speed control section 62 of the control unit 20 controls the speed of the internal combustion engine 18 according to the second control map 59, and on the other hand, when a travel load is equal to or greater than the predetermined load, the third engine speed control section 63 of the control unit 20 controls the speed of the internal combustion engine 18 according to the third control map 60.

[0092] For example, during flat road travel, the rotational speed of the engine 18 is controlled according to the second control characteristic 59, and when a travel load greater than or equal to the predetermined load is applied to the wheel loader 1 during uphill travel, the rotational speed of the engine 18 is controlled according to the third control characteristic 60. In this way, a sufficient reduction in fuel consumption can be achieved.

[0093] That is, in a case where the speed of the internal combustion engine 18 is controlled according to an engine speed control map 56 as shown in Fig. 4B (a map including the first control map 58 and the second control map 59 but excluding the third control map 60), when a traveling load frequently becomes equal to or greater than the predetermined load during traveling, the engine is controlled to operate at a high speed, making it impossible to achieve a sufficient reduction in fuel consumption. Moreover, when the engine speed corresponding to the case where a load becomes equal to or greater than the predetermined load is set lower than the engine speed corresponding to the case where control according to the first control map 58 is performed, the speed of the working device 8 is reduced during execution of work with the working device 8.Therefore, taking into account the efficiency of the work performed by the working device 8 (a reduction in the working speed), the engine speed corresponding to the case where the load becomes equal to or greater than the predetermined load cannot be significantly reduced.

[0094] In view of this problem, as in Fig.4A, in addition to the first control map 58 and the second control map 59, the third control map 60 is provided, by which different control maps for engine speed control can be used for both of two cases, namely, a case where, in traveling under a traveling load less than a predetermined load, the traveling load becomes greater than or equal to the predetermined load, and a case where work is performed with the work device 8. This makes it possible to achieve both sufficient reduction in fuel consumption and highly efficient work execution.

[0095] It should be noted that when traveling without performing work, pressing the creeper pedal changes the engine speed to a high speed level defined by the first control characteristic 58. However, when traveling without performing work, a speed reduction is generally achieved by releasing the accelerator pedal, so no problem arises here.

[0096] Furthermore, in normal mode and equipment mode, the speed of the internal combustion engine 18 is controlled according to the first control characteristic 58, regardless of the level of driving load. This makes it possible to select between speed-oriented driving and fuel-economy-oriented driving.

Claims

[1] Work machine, comprising: a hydrostatic pump designed as a variable displacement swash plate pump driven by an internal combustion engine; a hydrostatic motor connected in a closed circuit to the hydrostatic pump via a pair of speed-changing oil lines for driving a traveling device by means of oil supplied from the hydrostatic pump; a control unit for controlling an engine speed of the internal combustion engine and for controlling a swash plate of the hydrostatic pump; and an accelerator pedal for changing the engine speed to control the vehicle speed; a throttle adjustment element for adjusting the engine speed independently of a degree of depression of the accelerator pedal; a servo cylinder for changing an inclination angle of the swash plate of the hydrostatic pump to tilt the swash plate to rotate the hydrostatic motor in a direction that causes normal rotation of the hydrostatic motor or in a direction that causes reverse rotation of the hydrostatic motor; a swash plate control valve constructed as an electromagnetic proportional valve for supplying a control pressure to the servo cylinder, the control pressure changing the inclination angle of the swash plate with the aid of a command signal from the control unit [0040]; and an accelerator pedal position sensor for detecting a degree of operation of the accelerator pedal and inputting the detected degree to the control unit, wherein the control unit has a normal mode for performing vehicle speed control by controlling the engine speed based on a degree of depression of the accelerator pedal and by controlling a swash plate of the hydrostatic pump based on the engine speed, and a device mode for performing vehicle speed control by controlling the swash plate of the hydrostatic pump based on the degree of depression of the accelerator pedal regardless of the engine speed, characterized by , that in the normal mode, the control unit outputs a first command signal to the internal combustion engine in such a manner that the engine speed is controlled based on the degree of depression of the accelerator pedal and also outputs a second command signal to the swash plate control valve in such a manner that the inclination angle of the swash plate is controlled based on the engine speed controlled by the first command signal, and in the device mode, the control unit does not output a command signal to the internal combustion engine for controlling the engine speed based on the degree of depression of the accelerator pedal detected by the accelerator position sensor, and outputs a third command signal to the internal combustion engine based on the engine speed adjusted by the throttle adjusting element, thereby maintaining the engine speed, and outputs a fourth command signal to the swash plate control valve based on the degree of depression of the accelerator pedal detected by the accelerator position sensor, thereby changing the inclination angle of the swash plate in accordance with the degree of depression of the accelerator pedal detected by the accelerator position sensor. [2] Work machine according to claim 1, further comprising: a creep pedal; and a creeper pedal position sensor for detecting the degree of operation of the creeper pedal and for inputting the detected degree to the control unit, wherein in the normal mode and in the implement mode, a command signal is output from the control unit to the swash plate control valve in such a manner that the control pressure for the swash plate of the hydrostatic pump is reduced by a pressing operation of the creep pedal. [3] Working machine according to claim 1 or 2, further comprising: a work device for performing work, wherein the control unit is capable of controlling an engine speed based on first to third control characteristics defining a relationship between the degree of operation of the accelerator pedal and engine speed, and wherein the control unit comprises: a first engine speed control section for controlling the engine speed according to the work speed-oriented first control characteristic during execution of work with the work device; a second engine speed control section for controlling the engine speed according to the fuel-economy-oriented second control map when a driving load is less than a predetermined load during a trip; and a third engine speed control section for controlling the engine speed according to the third control characteristic which is between the first control characteristic and the second control characteristic when a traveling load during traveling is equal to or greater than the predetermined load. [4] Work machine according to claim 3, further comprising: a servo cylinder for controlling the swash plate of the hydrostatic pump; a swash plate control valve constructed as an electromagnetic proportional valve for supplying hydraulic oil from a hydraulic pump driven by an internal combustion engine to the servo cylinder as a control pressure for the swash plate, a creep pedal for reducing vehicle speed; and a creeper pedal position sensor for detecting the degree of depression of the creeper pedal and for inputting the detected degree to the control unit, wherein the swash plate control valve is controlled by a command signal output from the control unit in such a manner that the control pressure for the swash plate is reduced by a pressing operation of the creep pedal, and wherein the first engine speed control section controls the engine speed during operation of the creeper pedal according to the first control characteristic. [5] Working machine according to claim 3 or 4, where a choice can be made between normal mode and economy mode, and wherein in the normal mode, an engine speed is controlled solely according to the first control characteristic, and in the economy mode, the engine speed is controlled by the first to third engine speed control sections.

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