US20070283688A1

US20070283688A1 - Arrangement And A Method For Controlling A Work Vehicle

Info

Publication number: US20070283688A1
Application number: US11/572,327
Authority: US
Inventors: Bo Vigholm; Patrik Fransson; Kjell Sjogren; Stefan Salomonsson
Original assignee: Volvo Construction Equipment AB
Current assignee: Volvo Construction Equipment AB
Priority date: 2004-07-26
Filing date: 2005-06-20
Publication date: 2007-12-13
Also published as: US7506507B2; WO2006011835A1; EP1781863A1; US20060018765A1; KR101239107B1; JP4769801B2; SE0402233D0; KR20070038520A; CN1989303A; SE527405C2; JP2008507677A; SE0402233L; CN1989303B

Abstract

An arrangement for controlling a work vehicle including a hydraulic system that includes at least one pump and at least one actuator operatively driven by hydraulic fluid delivered from the pump is provided. At least a first pump is a variable displacement pump. The system is of load-sensing type in that the pump displacement is controlled by a pilot pressure representing a load exerted on the system. The arrangement includes an arrangement for reducing the pilot pressure delivered to the first pump so that the first pump displacement is regulated down when there is a need for limiting hydraulic power consumption.

Description

BACKGROUND AND SUMMARY

The present invention relates to an arrangement for controlling a work vehicle comprising a hydraulic system. The hydraulic system comprises at least one pump and at least one actuator operatively driven by hydraulic fluid delivered from said pump, wherein at least a first pump is a variable displacement pump. The system is of load-sensing type in that the pump displacement is controlled by a pilot pressure representing a load exerted on the system.
The pump is normally operatively driven by an internal combustion engine arranged for propelling the work vehicle.
The term work vehicle comprises different types of material handling vehicles like construction machines, such as a wheel loader, a backhoe loader, a motor grader and an excavator. The invention will be described below in a case in which it is applied in a wheel loader. This is to be regarded only as an example of a preferred application.
Said actuator may be a linear actuator in the form of a hydraulic cylinder. A wheel loader comprises several such hydraulic cylinders in order to perform certain functions. A first pair of hydraulic cylinders are arranged for turning the wheel loader. Further, there are hydraulic cylinders provided for lifting a load arm unit and tilting an implement, for example a bucket, arranged on the load arm unit.
A load sensing hydraulic system is characterized by that the operating condition of the load is sensed and that the output pressure of the pump is controlled so that it exceeds the load pressure existing in the hydraulic actuator by a predetermined differential. More specifically, the pressure (an LS signal) from the hydraulic cylinder for the load is sensed via a shuttle valve and via an activated control valve unit associated with the hydraulic cylinder for the load. The pump then delivers a hydraulic fluid flow to the hydraulic cylinder, the level of which depends on the extent to which the activated control valve unit is operated.
In order for the work vehicle to function well, the engine, transmission and hydraulic system must be balanced with regard to available power and output power. It is difficult to find an engine that exactly manages the desired power outputs at different engine speeds. The problem with different output power demand is particularly pronounced at low engine speeds. If the driver utilizes the power from the engine at low engine speeds to drive the vehicle's half shafts at the same time as the hydraulic system is activated, then there is a risk that the engine will cut out or that the engine will “stick”, that is it will not be able to increase the engine speed when the driver depresses the accelerator pedal. The driver can, of course, adjust the power consumption via various controls, when he senses a loss of engine speed, but this can be problematical, particularly when the engine suddenly cuts out. Further, even skilled drivers overcompensate and therefore unnecessarily reduce the amount of hydraulic work the hydraulic system is truly capable of performing. As a result, machine productivity is reduced.
There are known arrangements, which are adapted to relieve the engine load by reducing pump displacement when there is a risk for stalling the engine.
In U.S. Pat. No. 6,644,429, engine speed or engine torque is sensed and the pump displacement is reduced to zero for certain engine speed or engine torque limit values. A left and right solenoid is controlled by a controller and adapted to position the swash plates on the pumps.
In U.S. Pat. No. 4,335,577, engine speed is sensed and the pump displacement is reduced to zero for a certain engine speed limit value. The pump is short-circuited so that it regulates down, ie the pump output pressure is conveyed directly to the load-sensing conduit and the pressure is increased until the pump maximum pressure limitation is reached. A disadvantage is that when a higher flow is used for a function, the pump may stick on maximum displacement and cannot increase the pressure to the maximum level. The pump will then continue to pump with full flow and not maximum pressure although it is short-circuited.
It is desirable to achieve an alternative control arrangement for a work vehicle with a load-sensing hydraulic system that creates conditions for limiting the hydraulic power to relieve engine load, especially when there is a risk for stalling the engine. Further, the arrangement should be cost-efficient in operation and/or to install in the work vehicle.
According to an aspect of the present invention, an arrangement comprises means for reducing the pilot pressure delivered to the first pump so that the first pump displacement is regulated down when there is a need for limiting hydraulic power consumption. More specifically, the load signal delivered to the first pump is reduced to a pressure level below the load pressure resulting in that the displacement of the first pump is regulated down to zero output. Thus, the load sensing signal from the hydraulic cylinder is manipulated before reaching the first pump.
More specifically, the pump displacement is reduced by pressure control. This arrangement may easily be introduced in a conventional load-sensing system.
Further, the arrangement is adapted to sense a reduction of a value of an operation state of the vehicle's driveline, for example a reduction in the engine speed, resulting from excessive hydraulic loads and control the pressure reducing means based on the detected operation state value.
According to a preferred embodiment said means for reducing the pilot pressure to the first pump is adapted to reduce the pilot pressure to the first pump to such an extent that the first pump does not deliver any hydraulic fluid flow. Thus, the first pump is completely destroked. This is particularly advantageous when there are more than one pump delivering hydraulic fluid to the actuator.
In the case that the hydraulic system comprises at least two variable displacement pumps for delivering hydraulic fluid to said actuator the displacement of a second pump is preferably controlled by a pilot pressure representing the load exerted on the system in a non-manipulated form. Thus, only the input pilot pressure signal to the first pump is manipulated. Existing load-sensing hydraulic systems may easily and cost-efficiently be amended for achieving the limitation of the hydraulic power to relieve engine load in this way.
According to a preferred embodiment the arrangement comprises means for detecting an operation state of a driveline of the work vehicle and means connected to the detection means for evaluating the detected operation state and generating an operation state signal. Further, said evaluating means is connected to said means for reducing the pilot pressure to the first pump for controlling the displacement reduction depending on the operation state of the driveline.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained below, with reference to the embodiments shown on the appended drawings, wherein
FIG. 1 shows a wheel loader in a side view,
FIG. 2 schematically shows an exemplary embodiment of a driveline for the wheel loader, and
FIG. 3 illustrates diagrammatically a preferred embodiment of an arrangement for controlling the wheel loader.

DETAILED DESCRIPTION

FIG. 1 shows a wheel loader 1. The body of the wheel loader 1 comprises a front body section 2 and a rear body section 3, which sections each has a pair of half shafts 12, 120. The rear body section 3 comprises a cab 101. The body sections are connected to each other in such a way that they can pivot. The body sections 2, 3 can pivot in relation to each other around a vertical axis by means of two actuators in the form of hydraulic cylinders 4, 5 arranged between the two sections. The hydraulic cylinders 4, 5 are thus arranged to turn the wheel loader 1.
The wheel loader 1 comprises an equipment 11 for handling objects or material. The equipment 11 comprises a load-arm unit 6 and an implement 7 in the form of a bucket fitted on the load-arm unit. A first end of the load-arm unit 6 is pivotally connected to the front vehicle section 2. The implement 7 is connected to a second end of the load-arm unit 6.
The load-arm unit 6 can be raised and lowered relative to the front section 2 of the vehicle by means of two second actuators in the form of two hydraulic cylinders 8, 9, each of which is connected at one end to the front vehicle section 2 and at the other end to the load-arm unit 6. The bucket 7 can be tilted relative to the load-arm unit 6 by means of a third actuator in the form of a hydraulic cylinder 10, which is connected at one end to the front vehicle section 2 and at the other end to the bucket 7 via a link-arm system. FIG. 2 illustrates schematically an example of the wheel loader's 1 driveline 13. The driveline 13 comprises an internal combustion engine 14, in the form of a diesel engine, an automatic gearbox 15 and a hydrodynamic torque converter 16. Advantageously, the engine comprises a turbocharger (not shown) . The gearbox 15 consists of an electrically controlled automatic gearbox of the power-shift type. The gearbox 15 comprises a forward and reverse gear 17.
FIG. 2 also shows two pumps 18, 180 in the wheel loader's hydraulic system for supplying the hydraulic cylinders 4, 5, 8, 9, 10 with hydraulic fluid. The pumps 18, 180 (like the torque converter 16) is driven by an output shaft 19 from the engine 14. In the illustrated embodiment, the pumps 18, 180 are drivingly connected between the torque converter 16 and the gearbox 15. More specifically, the pumps 18, 180 are driven by a torque converter output shaft 19 via a transmission 24. An output shaft 20 from the gearbox 15 leads to a differential gear 21, which is drivingly connected to said half-shafts 12, on which the vehicle's driving wheels 23 are arranged.
FIG. 3 illustrates an embodiment of an arrangement 25 for controlling the wheel loader 1. The solid lines indicate main hydraulic conduits, the dashed lines indicate pilot hydraulic conduits and the lines with a longer dash followed by two shorter dashes indicate lines for electric signals.
It is schematically shown in FIG. 3 that the pumps 18, 180 are drivingly connected to the engine 14. The control arrangement 25 comprises a control unit 26, or computer. A number of electric operating levers 64 arranged in the cab are connected to the control unit 26, and this is adapted to handle the signals from the levers. A number of electrically controlled hydraulic valve units 27, 28 in a hydraulic system 22 are electrically connected to the control unit 26 and hydraulically connected to the hydraulic cylinders 4, 5, 8, 9, 10 for regulating the reciprocating work of these. The first and second pump 18 and 180, respectively are provided in order to supply the hydraulic cylinders 4, 5, 8, 9, 10 with hydraulic oil through the hydraulic valve units 27, 28. Each of said valve units 27, 28 comprises a directional control valve (not shown) . The directional control valve has a pair of service passages being connected to opposite ends of each of the double-acting hydraulic cylinders.
The hydraulic system 22 further comprises a valve unit 31 arranged between said pumps 18, 180 and said actuators 4, 5, 7, 8, 9 for controlling a flow of hydraulic fluid supplied to the actuators from said pumps. Said valve unit comprises a prioritizing valve connected between the pumps 18, 180 and the electric valves 27, 28. This prioritizing valve unit 31 is adapted for prioritizing steering hydraulics over lifting hydraulics.
An accumulator 32 with an associated valve unit 71 is connected to the loading cylinders 8, 9 in such a way that spring-action characteristics are obtained when the vehicle is driven with a loaded implement.
The signals from the electric operating levers 64 may be converted in a characteristic way in the control unit 26 and are then sent as output signals to the valve units 27, 28 in the form of electric pilot hydraulic valves, which in turn control the hydraulic cylinders 4, 5, 8, 9, 10.
Another valve unit 33 is indicated in FIG. 3. This valve unit 33 is intended to regulate the supply of hydraulic oil to an actuator of an implement and is coupled hydraulically to the pumps 18, 180 through the prioritizing valve unit 31 and electrically to the control unit 26. Said actuator of the implement can consist of, for example, a working cylinder of the gripping arms for moving these relative to one another or a working cylinder of the fork implement for relative movement of the two legs. The prioritizing valve unit 31 is also adapted to prioritize the steering hydraulics over the hydraulics for the implement concerned.
Both pumps 18, 180 have variable displacement. The hydraulic output by the pumps can thereby be controlled. The pumps 18, 180 feed hydraulic fluid to. said actuators 4, 5, 8, 9, 10 and the associated loads coupled thereto. The pumps 18, 180 are provided so as to supply fluid to the control valves 27, 28 through fluid supply conduits.
Each of the pumps 18, 180 has a main inlet 34 and 35, respectively, connected to a sump 36 and a main outlet 37 and 38, respectively, connected to an inlet port 39, 40 of the control valve unit 31 through fluid supply conduits 41, 42.
A displacement control arrangement 43 and 44, respectively, is arranged at each pump 18, 180 for controlling the displacement thereof. The pump 18, 180 includes a swash plate being rotatable for varying pump displacement. The displacement control arrangement 43, 44 is hydraulically activated and comprises a mechanical element coupled to the swash plate for rotating the same and set it in a desired position. The displacement control arrangement 43, 44 works against spring force. The displacement control arrangement 43, 44 comprises a pilot inlet port 45 and 47, respectively. The displacement control arrangement 43, 44 is operative to increase the displacement of the pumps 18, 180 in response to receipt of respective increased pressure signals.
The hydraulic system 22 is load sensing and the pump displacement is therefore automatically controlled by a pilot pressure signal representing a load exerted on the system. In other words, the displacement of said first pump 18 is controlled by a load signal (pressure signal) representing an actual load. Thus, both pumps 18, 180 have their displacements controlled automatically in response to the requirement of various hydraulic functions.
The first pump 18 and the second pump 180 are hydraulically interconnected so that the pilot pressure is supplied to the input pilot port 45 of the second pump 180. An output pilot port 46 of the second pump 180 is connected to the input pilot port 47 of the first pump 18 through a hydraulic fluid conduit 48.
The displacement control arrangement 43 of the second pump 180 is connected to said inlet port 45 so that it controls the pump displacement depending on the input pilot pressure. The displacement control arrangement 44 of the first pump 18 is connected to said inlet port 47 so that it controls the pump displacement depending on the input pilot pressure.
The hydraulic system 22 further comprises shuttle valve means 49 operable for routing the larger fluid pressure existing at the output ports of the valve units 27, 28, 33 to the pump displacement control arrangements 43, 44 to thereby automatically control the pumps 18, 180 to satisfy the highest demand of the loads associated with the valve units 27, 28, 33. The pumps 18, 180 are hydraulically controlled to deliver an output pressure equal to such highest load pressure plus the load sensing pressure drop of the valve unit 27, 28, 33 associated with the load. For example, the pumps may be adapted to deliver a pressure of 25 bar higher than the load pressure.
More specifically, pilot conduits 50, 51, 52 connect the outlet ports of the control valve units 27, 28, 33 to first and second inlet ports 53, 54 of the shuttle valve 49. The shuttle valve 49 comprises a ball 55, which is arranged in a central space, and two opposite seats 56, 57 for the ball 55, each associated with one of said first and second inlet ports 53, 54. Each seat 56, 57 is arranged so that the ball 55 will block the respective inlet port 53, 54. The shuttle valve comprises an outlet port 58 connected to the inlet ports 53, 54 by said central space. In this way, the greater of the pressures in the pilot conduits connected to the inlet ports 53, 54 will act on the ball 55 and seat the ball in the opposite seat so that the larger of the pressures in the pilot conduits is communicated to the outlet port 58. The outlet port 58 of the shuttle valve 49 is connected, by a further pilot fluid conduit 59, to the displacement control arrangement 43 of the second pump 180.
For example, when the operator wants to lift the load-arm unit 11, he operates the specific control lever 64 associated with this function. The valve unit 28 associated with the lift cylinders 8, 9 will then start opening. The valve unit 28 is arranged to first open a port to the conduit 51 connected to the shuttle valve 49. The pressure of the lift cylinders 8, 9 will then be forwarded to the second pump 180 via the conduit 59 connected to the inlet port 45. The second pump 180 will now deliver a flow so that the output pressure from the second pump will be a specific differential higher, for example 25 bar, than the lift cylinder pressure.
The pressure from the lift cylinder 8, 9 will be forwarded from the second pump outlet port 46 to the inlet port 47 of the first pump 18. The first pump 18 will in the same way as the second pump 180 deliver a flow so that the output pressure from the first pump 18 will be a specific differential higher, for example 25 bar, than the lift cylinder pressure.
When the valve unit 28 opens further, a main inlet port 72 of the valve unit 28 will open so that hydraulic fluid flow from the pumps 18, 180 is delivered from the pumps to the lift cylinder.
The control arrangement 25 comprises means 60, 61, 70 for detecting an operation state of the driveline 13 of the work vehicle. Said detection means 60, 61, 70 (sensors) may be adapted to sense a reduction of a value of the operation state, for example a change in the engine speed, resulting from excessive hydraulic loads, and producing parameter signals in response to the detected operating state. The control unit 26 is connected to the detection means 60, 61, 70 and comprises means 62 for evaluating the detected operation state and generating an operation state signal. Said evaluating means comprises software code for performing the evaluation. Thus, it is programmed with certain algorithms.
The evaluating means 62 is connected to means 63 for reducing the pilot pressure signal to the first pump 18 for controlling the displacement reduction depending on the operation state of the driveline. More specifically, the pilot pressure signal is reduced to a pressure level below the load pressure resulting in that the displacement of the first pump is regulated down to zero. Said means 63 for reducing the pressure signal to the first pump 18 is connected to said evaluation means 62 for receiving the operation state signal and reduce said pressure signal depending on the operation state signal.
Said detection means comprises means 60 for detecting an engine parameter. The engine parameter detection means 60 include a boost pressure sensor located at the inlet manifold of the engine downstream of the turbocharger, an ambient pressure sensor and an engine speed sensor.
The engine speed sensor may be a magnetic pick-up device sensitive to the movement of a gear tooth in the engine, which is proportional to crankshaft speed. The boost pressure sensor and the ambient pressure sensor are preferably pulse-width modulated pressure sensors of a type well known in the art producing signals having duty-cycles proportional to sensed pressure levels.
According to a first embodiment, the engine speed is sensed. When the engine speed falls to a predetermined minimum, the control unit 26 will output a signal with a level as a function of the detected engine speed. Said detection means may also comprise means 70 for detecting the position of an accelerator pedal 71. Therefore, as an alternative, the control unit 26 will output a signal with a level as a function of both the detected engine speed and the detected position of an accelerator pedal 71.
According to a second embodiment, the turbocharger pressure is sensed. When the turbocharger pressure falls to a predetermined minimum, the control unit 26 will output a signal with a level as a function of the detected turbocharger speed. As an alternative, the control unit 26 will output a signal with a level as a function of both the detected turbocharger pressure and the detected position of an accelerator pedal 71.
According to a third embodiment, a driveline torque or output power is sensed. In this embodiment the engine torque is sensed. The pressure in a clutch in the gear box is used as a measure of the engine torque. Such clutch pressure signals are directly related to the torque being transmitted by the clutch to the wheels and by the wheels to the ground. When the torque falls to a predetermined minimum, the control unit 26 will output a signal with a level as a function of accessible engine torque. As an alternative, the control unit 26 will output a signal with a level as a function of both accessible engine torque and the detected position of an accelerator pedal 71.
Said detection means further comprises means 61 for detecting a gear state in the gear box 15. More specifically, the detection means 61 detects a neutral state in the gear box 15. If the driver uses the hydraulic system 22 for a certain work operation when the vehicle is standing still, the driver normally puts the gear in the neutral state. Thus, in such a case, the engine is not used for propelling the vehicle. Thus, when a neutral state is detected, there is no need for destroking/disconnecting the first pump 18. Therefor, according to a further embodiment, when a neutral state is detected, the control unit 26 will not initiate destroking/disconnection of the first pump 18 independent of the engine speed.
Further, in a certain work situation, the driver may want a higher engine speed and he therefore depresses the accelerator pedal 71. Said means 70 detects the position of the accelerator pedal 71 and at a certain predefined depression, suitably substantially corresponding to a desired full gas, the control unit 26 will produce a signal to the pilot pressure reducing means 63 in order to regulate the first pump displacement down (or disconnecting the pump). Thus, there is no requirement for the engine speed to be substantially decreased (close to engine cut out) for regulating the pump displacement down. Instead, the first pump displacement may be regulated down at a comparably low pilot pressure (system load) in order to increase the engine speed for a different reason.
The control unit 26 is programmed to activate the means 63 for reducing the pilot pressure signal to the first pump 18. More specifically, the pilot pressure signal reducing means 63 is formed by a solenoid-operated valve arranged in a branch conduit 65 connected to the pilot conduit 48 connecting the second pump outlet port 46 and the first pump inlet port 47. Thus, the pilot pressure signal reducing means 63 will control the hydraulic load signal to a pressure level corresponding to the electric signal from the control unit 26. The branch conduit 65 is connected to the sump 36. The hydraulic pilot signal will thus be drained to the sump 36 if the load pressure level is higher than the regulated pressure. The first pump 18 will then be destroked (the displacement of the first pump 18 will be reduced to zero and will not deliver any hydraulic fluid flow).
A flow restrictor 66, or orifice, is arranged on the conduit 48 connecting the output pilot port 46 of the second pump 180 and the input pilot port 47 of the first pump 18. The flow restrictor 66 is arranged upstream of said means 63 for reducing the pressure signal to said first pump 18. Thanks to the flow restrictor 66, the second pump 180 can maintain its pressure level and thereby continue to deliver fluid flow. This is due to that a flow will continuously be guided from the hydraulic cylinder to the pilot pressure inlet port 45 for the second pump 180. The flow is large enough for saturating the orifice 66.
When there is no need for reducing the power of the hydraulic system, the control unit will send a high electric signal to the solenoid valve 63 so that its opening pressure is higher than the maximum pump pressure.
When the control unit 26 regulates the solenoid valve 63 to a certain extent, the first pump 18 will be disconnected (stroked down) if the load pressure is larger than the regulated pressure level. The first pump 18 will be connected (and delivering fluid) as long as the load pressure is smaller than the regulated pressure level.
When the driveline is in a neutral state, an upper engine speed limit boundary may be used for activating the first pump. Thus, when the detected engine speed reaches said upper engine speed limit boundary, the control unit 26 will send a signal to the solenoid valve 63 for deactivating the same so that it does not influence the load pressure from the hydraulic cylinder independently of whether the above described methods of controlling the solenoid valve 63 based on the operation parameter of the driveline is in operation or not.
Further, in case the engine speed is reduced to a predefined lower critical level, the control unit 26 sends a signal to the solenoid valve 63 to reduce the pilot input pressure to the first pump and thereby disconnect the pump.
The ambient atmospheric pressure may further be sensed and used as an input for determining the level of manipulation of the input pressure signal to the first pump.
The invention is also directed to a computer program comprising code means for performing the method steps described above when said program is run on a computer. Said computer program is loaded in a memory in the control unit 26. Said computer program may be sent to the control unit by wireless technique, for example via the internet.
The invention is further directed to a computer program product comprising program code means stored on a computer readable medium for performing the method described above when said program product is run on a computer. Said computer readable medium may be in the form of a floppy disk or a CD-ROM.
The abovementioned control unit (ECU) 26 is also often called a CPU (Control Power Unit) or plainly vehicle computer.
The invention has above been described for solving the problem of limiting hydraulic power output at low engine speeds. The invention may of course also be used for limiting hydraulic power also at high engine speeds, which may be necessary when an engine with “too little” power is used for an arrangement where “too high” power outputs are demanded.
The invention is not in any way limited to the above described embodiments, instead a number of alternatives and modifications are possible without departing from the scope of the following claims. For example, the driveline described above in connection with FIG. 2 should only be regarded as an example. Any type of transmission may be used.
As an alternative or complement to the control methods described above, said detection means comprises means for detecting an engine fan torque or output power and the detected value is used to set the input pressure level to the first pump.
Further, output power may be sensed instead of torque in the methods described above.
The specific hydraulic system for controlling the first pump by manipulating its input pilot pressure may be solved in a number of different ways. One conduit for guiding the pilot pressure should be connected to an input pilot port of the first pump, one conduit for guiding the pilot pressure should be connected to an input pilot port of the second pump and the conduit leading to the first pump (to be regulated down) should have an orifice and a downstream bypass conduit to tank. Thus, the embodiment described and showed in FIG. 3 should only be regarded as an example.

Claims

1. An arrangement for controlling a work vehicle (1) comprising a hydraulic system (22), which comprises at least one pump (18,180) and at least one actuator (4,5,8,9,10) operatively driven by hydraulic fluid delivered from said pump, wherein at least a first pump (18) is a variable displacement pump, the system (22) being of load-sensing type in that the pump displacement is controlled by a pilot pressure representing a load exerted on the system, characterized in that the arrangement comprises means (63) for reducing the pilot pressure delivered to the first pump (18) so that the first pump displacement is regulated down when there is a need for limiting hydraulic power consumption.

2. An arrangement according to claim 1, characterized in that said means (63) for reducing the pilot pressure to the first pump (18) is adapted to reduce the pilot pressure to the first pump to such an extent that the first pump does not deliver any hydraulic fluid flow.

3. An arrangement according to claim 1 or 2, characterized in that the hydraulic system (22) comprises at least two variable displacement pumps (18,180) for delivering hydraulic fluid to said actuator (4,5,8,9,10).

4. An arrangement to claim 3 characterized in that the displacement of a second pump (180) is controlled by a pilot pressure representing the load exerted on the system in a non-manipulated form.

5. An arrangement to claim 3 or 4 characterized in that conduits (48,59) for the pilot pressure are connected both to an input pilot port (47) of the first pump (18) and to an input pilot port (45) of a second pump (180), that a flow restrictor (66) is arranged on the conduit (48) to the input pilot port (47) of the first pump (18) and that the flow restrictor is arranged upstream of said means (63) for reducing the pilot pressure to said first pump.

6. An arrangement according to any of the preceding claims, characterized in that said means (63) for reducing the pressure to said first pump (18) is formed by a valve, and that the valve in an activated state is adapted to at least partly route the pilot pressure in a conduit (65) bypassing the first pump.

7. An arrangement according to any of the preceding claims characterized in that each pump (18,180) has an associated hydraulically controlled arrangement (43,44) for controlling the displacement of the pump and that the displacement control arrangement controls the pump displacement depending on the input pilot pressure.

8. An arrangement according to any of the preceding claims characterized in that said pump (18,180) is operatively driven by an engine (14) arranged for propelling the vehicle.

9. An arrangement according to claim 8 characterized in that the arrangement comprises means (60,61,70) for detecting an operation state of a driveline (13) of the work vehicle and means (62) connected to the detection means for evaluating the detected operation state and generating an operation state signal.

10. An arrangement according to claim 9 characterized in that said evaluating means (62) is connected to said means (63) for reducing the pilot pressure to the first pump (18) for controlling the displacement reduction depending on the operation state of the driveline.

11. An arrangement according to claim 9 or 10 characterized in that said detection means comprises means (60) for detecting an engine speed.

12. An arrangement according to any of claims 9-11 characterized in that said detection means comprises means for detecting a turbocharger pressure.

13. An arrangement according to any of claims 9-12 characterized in that said detection means comprises means for detecting an engine torque or output power.

14. An arrangement according to any of claims 9-13 characterized in that said detection means comprises means (61) for detecting a gear state in a vehicle gear box (15).

15. An arrangement according to any of claims 9-14 characterized in that said detection means comprises means (70) for detecting a position of an accelerator pedal (71).

16. An arrangement according to claim any of claims, characterized in that at least one of said actuators is arranged to move an implement of the work vehicle.

17. A method of controlling a work vehicle with a hydraulic system for operating at least one actuator, comprising the steps of sensing a load exerted on the system and controlling a displacement of a first pump with variable displacement by means of a pilot pressure representing the load, the pump operatively driving the actuator by delivering hydraulic fluid to it, and reducing the pilot pressure to the first pump in case the hydraulic power needs to be limited.

18. A method according to claim 17, wherein the pilot pressure to the first pump is reduced to such an extent that the first pump does not deliver any hydraulic fluid flow.

19. A method according to claim 17 or 18, wherein said pump is operatively driven by an engine arranged for propelling the vehicle.

20. A method according to any of claims 17-19, wherein an operation state of a driveline of the work vehicle is detected, the detected operation state is evaluated and an operation state signal generated for controlling the pump displacement reduction depending on the operation state of the driveline.

21. A method according to claim 20, wherein an engine speed is detected.

22. A method according to claim 20 or 21, wherein a turbocharger pressure is detected.

23. A method according to any of claims 20-22, wherein an engine torque or output power is detected.

24. A method according to any of claims 20-23, wherein a position of an accelerator pedal (71) is detected.

25. A method according to any of claims 20-24, wherein a gear state in a vehicle gear box (15) is detected.

26. A computer program comprising code means for performing all the method steps described in any of claims 17-25 when said program is run on a computer.

27. A computer program product comprising program code means stored on a computer readable medium for performing the method described in any of claims 17-25 when said program product is run on a computer.