CONTROL SYSTEM FOR A WORK MACHINE AND METHOD FOR CONTROLLING A HYDRAULIC CYLINDER IN A WORK MACHINE

Abstract

A control system for a work machine (101) including an electric machine (202), a hydraulic machine (204) and at least one hydraulic cylinder (108). The electric machine (202) is connected in a driving manner to the hydraulic machine (204). The hydraulic machine (204) is connected to a piston side (208) of the hydraulic cylinder (108) via a first line (210) and a piston-rod side (212) of the hydraulic cylinder (108) via a second line (214). The hydraulic machine (204) is adapted to be driven by the electric machine (202) and supply the hydraulic cylinder (108) with pressurized hydraulic fluid from a tank (216) in a first operating state and to be driven by a hydraulic fluid flow from the hydraulic cylinder (108) and drive the electric machine in a second operating state.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Application No. 60/759,996 filed 18 Jan. 2006 and claims priority to Swedish Application No. 0600087-1 filed 16 Jan. 2006. Said applications are expressly incorporated herein by reference in their entirety.

FIELD AND BACKGROUND

The present invention relates to a control system for a work machine and a method for controlling at least one hydraulic cylinder in a work machine. Herein, the work machine is described in terms of a wheel loader. This is a preferred but is in no way limiting to the invention as the he invention can also be used for other types of work machines (or work vehicles), such as an excavator loader (backhoe) and excavating machine.

The invention relates, for example, to controlling lifting and/or tilting cylinders for operating an implement.

More precisely, the invention relates to a control system which comprises a hydraulic machine that functions as both pump and motor. The hydraulic machine is connected in a driving manner to an electric machine which functions as both motor and generator.

The hydraulic machine therefore functions as a pump in a first operating state and supplies pressurized hydraulic fluid to the hydraulic cylinder. The hydraulic machine also functions as a hydraulic motor in a second operating state and is driven by a hydraulic fluid flow from the hydraulic cylinder. The electric machine therefore functions as an electric motor in the first operating state and as a generator in the second operating state.

The first operating state corresponds to a work operation, such as lifting or tilting, being carried out with the hydraulic cylinder. Hydraulic fluid is therefore directed to the hydraulic cylinder for movement of the piston of the cylinder. On the other hand, the second operating state is an energy recovery state.

SUMMARY

A first object of the invention is to provide a control system, preferably for a lifting and/or tilting function, which affords an opportunity for energy-efficient operation.

This object is achieved with a control system for a work machine, which system comprises an electric machine, a hydraulic machine and at least one hydraulic cylinder, the electric machine being connected in a driving manner to the hydraulic machine, the hydraulic machine being connected to a piston side of the hydraulic cylinder via a first line and a piston-rod side of the hydraulic cylinder via a second line, the hydraulic machine being adapted to be driven by the electric machine and supply the hydraulic cylinder with pressurized hydraulic fluid from a tank in a first operating state and to be driven by a hydraulic fluid flow from the hydraulic cylinder and drive the electric machine in a second operating state.

The hydraulic cylinder is preferably adapted to move an implement in order to perform a work function. According to a first example, the hydraulic cylinder comprises a lifting cylinder for moving a loading arm which is pivotably connected to a vehicle frame, the implement being arranged on the loading arm. According to a second example, the hydraulic cylinder comprises a tilting cylinder for moving the implement which is pivotably connected to the loading arm.

The speed of the cylinder is preferably controlled directly by the electric machine, that is to say no control valves are required between the hydraulic machine and the cylinder for regulating direction and speed of the movement. In some cases, on/off valves which open and respectively close a communication for the hydraulic fluid flow are required.

A second object of the invention is to provide a method for controlling a hydraulic cylinder, preferably for a lifting and/or tilting function, which provides smooth operation and reduces jerking/jolting of the driver.

This object is achieved by a method according to claim 31. It is therefore achieved with a method comprising the steps of detecting that a lowering movement of the implement is initiated, of before the lowering movement takes place pressurizing a first side of the piston of the hydraulic cylinder, which side is opposite a second side on which said load acts, and then reducing the pressurization so that the lowering movement can start.

A third object of the invention is to provide a method which takes account of the size of the load and provides energy-efficient operation.

This object is achieved by a method according to claim 36. It is therefore achieved with a method comprising the steps of detecting a load acting on the implement, of comparing the size of the detected load with a predetermined load level, and of, if the detected load lies below the predetermined load level, bringing the piston-rod side of the hydraulic cylinder into flow communication with the piston side so that hydraulic fluid coming from the piston-rod side is brought to the piston side without passing through the hydraulic machine.

A fourth object of the invention is to provide a method which provides energy-efficient operation during movement of the implement.

This object is achieved by a method according to claim 38. It is therefore achieved with a method comprising the steps of delivering such a pressure to the hydraulic cylinder that the implement is brought into a basic position, of, in the event of a disturbance which results in a downward movement of the implement, allowing driving of a hydraulic machine by a hydraulic fluid flow from the hydraulic cylinder, and of regenerating the energy from the hydraulic machine in an electric machine connected to it in a driving manner.

A fifth object of the invention is to provide a method which provides energy-efficient springing of the movement of the implement during transport.

This object is achieved by a method according to claim 44. It is therefore achieved with a method comprising the steps of delivering such a pressure to the hydraulic cylinder that the implement is brought into a basic position, of bringing a first port of the hydraulic machine into flow communication with a piston side of the hydraulic cylinder via a first line and a second port of the hydraulic machine into flow communication with a piston-rod side of the hydraulic cylinder via a second line, and of, in the event of a disturbance which results in an upward movement of the implement, supplying a corresponding quantity of hydraulic fluid to the hydraulic cylinder and, in the event of a downward movement of the implement, draining a corresponding quantity of hydraulic fluid from the hydraulic cylinder.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in greater detail below with reference to the embodiments shown in the accompanying drawings, in which:

FIG. 1 shows a side view of a wheel loader;

FIGS. 2-6 show different embodiments of a control system for controlling a work function of the wheel loader;

FIG. 7 shows an embodiment of a control system for controlling a number of functions of the wheel loader;

FIG. 8 shows a control system for controlling one or more of the functions of the wheel loader, and

FIG. 9 shows a further embodiment of the control system for controlling a work function of the wheel loader.

DETAILED DESCRIPTION

FIG. 1 shows a side view of a wheel loader 101. The wheel loader 101 comprises a front vehicle part 102 and a rear vehicle part 103, which parts each comprise a frame and a pair of drive axles 112, 113. The rear vehicle part 103 comprises a cab 114. The vehicle parts 102, 103 are coupled together with one another in such a way that they can be pivoted in relation to one another about a vertical axis by means of two hydraulic cylinders 104, 105 which are connected to the two parts. The hydraulic cylinders 104, 105 are thus arranged on different sides of a center line in the longitudinal direction of the vehicle for steering, or turning the wheel loader 101.

The wheel loader 101 comprises an apparatus 111 for handling objects or material. The apparatus 111 comprises a lifting arm unit 106 and an implement 107 in the form of a bucket which is mounted on the lifting arm unit. Here, the bucket 107 is filled with material 116. A first end of the lifting arm unit 106 is coupled rotatably to the front vehicle part 102 for bringing about a lifting movement of the bucket. The bucket 107 is coupled rotatably to a second end of the lifting arm unit 106 for bringing about a tilting movement of the bucket.

The lifting arm unit 106 can be raised and lowered in relation to the front part 102 of the vehicle by means of two hydraulic cylinders 108, 109, which are each coupled at one end to the front vehicle part 102 and at the other end to the lifting arm unit 106. The bucket 107 can be tilted in relation to the lifting arm unit 106 by means of a third hydraulic cylinder 110, which is coupled at one end to the front vehicle part 102 and at the other end to the bucket 107 via a link arm system.

A number of embodiments of a control system for the hydraulic functions of the wheel loader 101 will be described in greater detail below. These embodiments relate to lifting and lowering of the lifting arm 106 via the lifting cylinders 108, 109, see FIG. 1. However, the various embodiments of the control system could also be used for tilting the bucket 107 via the tilting cylinder 110.

FIG. 2 shows a first embodiment of a control system 201 for performing lifting and lowering of the lifting arm 106, see FIG. 1. The hydraulic cylinder 108 in FIG. 2 therefore corresponds to the lifting cylinders 108, 109 (although only one cylinder is shown in FIG. 2).

The control system 201 comprises an electric machine 202, a hydraulic machine 204 and the lifting cylinder 108. The electric machine 202 is connected in a mechanically driving manner to the hydraulic machine 204 via an intermediate drive shaft 206. The hydraulic machine 204 is connected to a piston side 208 of the hydraulic cylinder 108 via a first line 210 and a piston-rod side 212 of the hydraulic cylinder 108 via a second line 214.

The hydraulic machine 204 is adapted to function as a pump, be driven by the electric machine 202 and supply the hydraulic cylinder 108 with pressurized hydraulic fluid from a tank 216 in a first operating state and to function as a motor, be driven by a hydraulic fluid flow from the hydraulic cylinder 108 and drive the electric machine 202 in a second operating state.

The hydraulic machine 204 is adapted to control the speed of the piston 218 of the hydraulic cylinder 108 in the first operating state. No control valves are therefore required between the hydraulic machine and the hydraulic cylinder for said control. More precisely, the control system 201 comprises a control unit 802, see FIG. 8, which is electrically connected to the electric machine 202 in order to control the speed of the piston of the hydraulic cylinder 108 in the first operating state by controlling the electric machine.

The hydraulic machine 204 has a first port 220 which is connected to the piston side 208 of the hydraulic cylinder via the first line 210 and a second port 222 which is connected to the piston-rod side 212 of the hydraulic cylinder via the second line 214. The second port 222 of the hydraulic machine 204 is moreover connected to the tank 216 in order to allow the hydraulic machine, in the first operating state, to draw oil from the tank 216 via the second port 222 and supply the oil to the hydraulic cylinder 108 via the first port 220.

In certain situations, such as when it is desired to press a material down or to flatten something, it is necessary to lower the bucket 107 with more force than is the case when only the load drives the movement of the piston 218. Such intensified lowering is usually referred to as “power down”. This power down function can also be used for lifting the vehicle. The control system 201 comprises a means 224 for controlling pressure, which pressure means 224 is arranged on a line 226 between the second port 222 of the hydraulic machine 204 and the tank 216 in order to allow pressure build-up on the piston-rod side 212. More precisely, the pressure control means 224 comprises an electrically controlled pressure-limiting valve.

The control system 201 also comprises a sensor 228 for sensing pressure on the piston side 208 of the hydraulic cylinder 108. When a low pressure value is detected on the piston side, the line 226 to the tank is blocked via the pressure-limiting valve 224, which results in the pressure in the line 214 to the piston-rod side being increased and said intensified downward movement (power down) being obtained. During lowering, the pressure sensor registers that the pressure is below a certain level (for example 20 bar) on the piston side. The pressure level on the electrically controlled pressure limiter is then increased to a suitable level so that pressure build-up takes place in the piston-rod side.

The first port 220 of the hydraulic machine 204 is connected to the tank 216 via a first suction line 230. A means 232, in the form of a non-return valve, is adapted to allow suction of hydraulic fluid from the tank and obstruction of a hydraulic fluid flow to the tank through the suction line 230.

The second port 222 of the hydraulic machine 204 is connected to the tank 216 via a second suction line 234. A means 236, in the form of a non-return valve, is adapted to allow suction of hydraulic fluid from the tank and obstruction of a hydraulic fluid flow to the tank through the suction line 234.

A means 237 for opening/closing is arranged on the second line 214 between the second port 222 of the hydraulic machine 204 and the piston-rod end 212 of the hydraulic cylinder 108. This means 237 comprises an electrically controlled valve with two positions. In a first position, the line 214 is open for flow in both directions. In a second position, the valve has a non-return valve function and allows flow in only the direction toward the hydraulic cylinder 108. During lifting movement, the electric valve 237 is opened and the rotational speed of the electric machine 202 determines the speed of the piston 218 of the hydraulic cylinder 108. Hydraulic fluid is drawn from the tank 216 via the second suction line 234 and is pumped to the piston side 208 of the hydraulic cylinder 108 via the first line 210.

An additional line 242 connects the second port 222 of the hydraulic machine 204 and the tank 216.

A means 243 for opening/closing is arranged on the first line 210 between the first port 220 of the hydraulic machine 204 and the piston end 208 of the hydraulic cylinder 108. This means 243 comprises an electrically controlled valve with two positions. In a first position, the line 210 is open for flow in both directions. In a second position, the valve has a non-return valve function and allows flow in only the direction toward the hydraulic cylinder 108.

According to an embodiment for lowering the implement, it is first detected that a lowering movement is initiated. The electric valve 243 is closed. Before the lowering movement takes place, a first side 208 of the piston 218 of the hydraulic cylinder is pressurized, which side is opposite a second side on which said load acts. In other words, the piston side 208 is pressurized. Before the lowering movement takes place, the hydraulic machine 204 is driven in a first rotation direction so that said first side 208 of the piston of the hydraulic cylinder is pressurized. The hydraulic machine 204 is therefore rotated by a certain angle in the “wrong direction”. A sensor 248 is adapted to sense the position of the piston rod. A detected upward movement of the piston rod indicates that the pressurization is complete. According to an alternative, the pump 204 is rotated by a predetermined angle in the “wrong direction”.

The valve 243 is then opened to the piston side 208, the rotation direction is changed for the hydraulic machine 204 and the lowering movement starts. The electrically controlled pressure limiter may need to be adjusted slightly in order to improve refilling to the piston-rod side.

The hydraulic machine is therefore allowed to rotate in a second rotation direction, opposite the first rotation direction, whereupon the lowering movement can start. The pressure applied is therefore reduced so that the lowering movement can start. A hydraulic flow from the hydraulic cylinder 108 drives the hydraulic machine 204 in the second rotation direction. More precisely, the pressurization of the first side 208 of the hydraulic cylinder is reduced gradually so that a smooth lowering movement is achieved.

Pressurization can also be effected by the electric machine 202 first being driven with a certain torque in the “wrong direction”, where the torque level is based on the value of the pressure sensor 228 immediately before.

If the bucket 107 should stop suddenly during a lowering movement (which can happen if the bucket strikes the ground), the hydraulic machine 204 does not have time to stop. In this state, hydraulic fluid can be drawn from the tank 216 via the suction line 230 and on through the additional line 242.

The electrically controlled valves 237, 243 function as load-holding valves. They are closed in order that electricity is not consumed when there is a hanging load and also in order to prevent dropping when the drive source is switched off According to an alternative, the valve 237 on the piston-rod side 212 is omitted. However, it is advantageous to retain the valve 237 because external forces can lift the lifting arm 106.

A filtering unit 238 and a heat exchanger 240 are arranged on the additional line 242 between the second port 222 of the hydraulic machine 204 and the tank 216. An additional filtering and heating flow can be obtained by virtue of the hydraulic machine 204 driving a circulation flow from the tank 216 first via the first suction line 230 and then via the additional line 242 when the lifting function is in a neutral position. Before the tank, the hydraulic fluid thus passes through the heat exchanger 240 and the filter unit 238.

There is another possibility for additional heating of the hydraulic fluid by pressurizing the electrically controlled pressure limiter 224 at the same time as pumping-round takes place to the tank in the way mentioned above. This can of course take place when the lifting function is used.

The electrically controlled pressure limiter 224 can also be used as a back-up valve for refilling to the piston-rod side 212 when lowering takes place. The counter-pressure can be varied as required and kept as low as possible, which saves energy. The counter-pressure can be lower the hotter the oil is and lower the lower the lowering speed is. When the filtering flow is run, the counter-pressure can be zero.

A first pressure-limiting valve 245 is arranged on a line which connects the first port 220 of the hydraulic machine 204 to the tank 216. A second pressure-limiting valve 247 is arranged on a line which connects the piston side 208 of the hydraulic cylinder 108 to the tank 216. The two pressure-limiting valves 245, 247 are connected to the first line 210 between the hydraulic machine 204 and the piston side 208 of the hydraulic cylinder 108 on different sides of the valve 243. The two pressure-limiting valves 245, 247, which are also referred to as shock valves, are spring-loaded and adjusted to be opened at different pressures. According to an example, the first pressure-limiting valve 245 is adjusted to be opened at 270 bar, and the second pressure-limiting valve 247 is adjusted to be opened at 380 bar.

When the work machine 101 is driven toward a heap of gravel or stones and/or when the implement is lifted/lowered/tilted, the movement of the bucket may be counteracted by an obstacle. The pressure-limiting valves 245, 247 then ensure that the pressure is not built up to levels which are harmful for the system.

According to a first example, the bucket 107 is in a neutral position, that is to say stationary in relation to the frame of the front vehicle part 102. When the wheel loader 101 is driven toward a heap of stones, the second pressure limiter 247 is opened at a pressure of 380 bar.

During ongoing lowering, the valve 243 on the first line 210 between the hydraulic machine 204 and the piston side 208 of the hydraulic cylinder 108 is open. When the lifting arm 106 is lowered, the first pressure limiter 245 is opened at a pressure of 270 bar. If an external force should force the loading arm 106 upward during a lowering operation with power down, the pressure limiter 224 on the line 226 between the second port 222 of the hydraulic machine 204 and the tank 216 is opened.

According to an alternative to the pressure-limiting valves 245, 247 being adjusted to be opened at a predetermined pressure, the pressure-limiting valves can be designed with variable opening pressure. According to a variant, the pressure-limiting valves 245, 247 are electrically controlled. If electric control is used, only one valve 247 is sufficient for the shock function. This valve 247 is controlled depending on whether the valve 243 is open or closed. The opening pressure can be adjusted depending on activated or non-activated lifting/lowering function and also depending on the cylinder position.

A method for regenerating energy when the implement 107 is moved during movement of the work machine 101 is described below with reference to FIG. 2. The method can be said to constitute an active springing system for the lifting function. The method can either be selected by an operator via a control element or a control, such as a knob or lever, in the cab or be initiated automatically.

A sensor 248 is adapted for sensing the position of the lifting arm 106 in relation to the frame of the front vehicle part 102. Here, the sensor 248 is adapted to detect the position of the piston rod. The sensor 248 could alternatively detect the angular position of the loading arm 106 relative to the frame. The sensor 248 detects the position of the implement repeatedly, essentially continuously, and produces corresponding signals.

A control unit 802 (see FIG. 8) receives the position signals from the sensor 248. The control unit 802 is usually referred to as a CPU (central processing unit) and comprises a microprocessor and a memory.

The position of the loading arm 106 is stored in the memory before the energy regeneration function is activated. When the function is activated, the two valves 237 and 243 on both sides of the lifting cylinder 108 are opened. The hydraulic machine 204 is controlled so that such a pressure is delivered to the hydraulic cylinder 108 that the implement 107 is brought into a basic position. The loading arm 106 is therefore held in position with a certain torque.

During movement of the wheel loader 101, that is to say transport, the loading arm 106 will be acted on by vertical forces owing to the weight of the load and irregularities of the ground and move up and down. The sensor 248 registers such disturbances which result in the loading arm 106 being moved from the basic position.

In the event of a disturbance which results in a downward movement of the implement 107, the control unit 802 produces a signal for the electric machine 202 which allows the hydraulic machine 204 to be driven by a hydraulic fluid flow from the hydraulic cylinder 108 and the energy from the hydraulic machine 204 is regenerated in the electric machine 202. More precisely, the first port 220 of the hydraulic machine 204 is brought into flow communication with the piston side 208 of the hydraulic cylinder 108. The control unit 802 therefore sends a signal to the valve 243 on the first line 210, which is thus opened. When the lifting arm 106 moves downward, the basic position is passed through, the counter-torque of the electric machine 204 increasing so that the movement of the lifting arm is braked and in the end stops. Oil is then pumped into the cylinder 108 so the lifting arm 106 moves upward again.

If a disturbance means that the lifting arm 106 moves upward, the control unit 802 registers this. The control unit controls the hydraulic machine 204 (via the electric machine 202) so that the hydraulic machine follows with a certain torque and fills hydraulic fluid to the piston side 208. The torque applied decreases depending on how far from the basic position the lifting arm 106 is. A springing function is thus obtained.

More precisely, a second port 222 of the hydraulic machine 204 is brought into flow communication with the piston-rod side 212 of the hydraulic cylinder 108.

The hydraulic cylinder 108 is controlled continuously so that the implement 107 is kept within a predetermined range around the basic position. Adjustment is also carried out continuously between the disturbances so that the loading arm 106 does not move too far from the basic position.

If there are few disturbances, the valve 243 on the piston side 208 can be closed temporarily in order to save the energy which is consumed for holding the load.

The function also damps shocks which occur as a result of external forces such as, for example, collision with the bucket 107.

According to a development of the energy regeneration function, pressure sensors are used for registering the course of the pressure variations which occur in the event of a disturbance. If pressure sensors are used, the valve 243 on the piston side 208 can if appropriate be closed as long as no lowering movement takes place (depending on how quickly it is possible to open in the event of a disturbance).

The hydraulic machine 204 is controlled so that a springing function is achieved. In other words, if a disturbance presses the lifting arm 106 down, the hydraulic machine 204 regenerates electricity and at the same time the torque is increased so that braking of the movement takes place (like a spring). This spring characteristic can be dependent on a number of different parameters and have a different appearance.

According to a preferred embodiment, the spring characteristic is dependent on the following parameters:

1) Level of Disturbance Force

The same spring travel is obtained for the same disturbance force (irrespective of the weight of the load). The spring travel is longer the greater the disturbance force is. The disturbance force can be registered via pressure sensors or the derivative on the position sensor.

2) How Heavy the Load is

It is possible, for example, to measure the pressure in the lifting cylinder and if appropriate in the tilting cylinder. According to a first variant, the springing is controlled so that the heavier the detected load is the shorter the spring travel. According to a second variant, the springing is controlled so that the lighter the detected load is the shorter the spring travel.

3) Type of Handling

The computer registers the type of handling (bucket, pallet fork, timber fork etc.) in a manner known per se.

4) Type of Handling Mode

Different characteristics if the machine is in transport mode or if work is in progress with the function. This could be indicated, for example, via machine speed and/or whether lever movement takes place.

The damping in the system is determined by the size of the torque which the pump applies when the unit is to be raised again after being pressed down. This torque application (spring characteristic) can also be a function of the parameters above.

FIG. 3 shows a second embodiment of the control system 301. Here, the first port 220 of the hydraulic machine 204 is connected to the piston-rod side 212 of the hydraulic cylinder 108 via a line 302 which connects the piston-rod side 212 and the piston side 208 of the hydraulic cylinder 108 in parallel to the hydraulic machine 204. A means 304 for flow control, in the form of an electrically controlled on/off valve, is arranged on said parallel line 302 in order to control the flow communication between the piston-rod side 212 and the piston side 208. By virtue of the valve 304, the maximum flow via the hydraulic machine 204 can be lowered, that is to say the pump displacement can be reduced or a lower maximum speed can be used.

The pressure sensor 228 indicates whether the weight of the load is below or above a predetermined value, which indicates whether the load is considered to be light or heavy. In the case of a lifting movement of a light load, the additional valve 304 is opened, which results in more rapid lifting being possible by virtue of hydraulic fluid for the piston side 208 being obtained both from the hydraulic machine 204 and from the piston-rod side 212. The electric valve 237 on the second line 214 on the piston-rod side 212 is therefore closed.

In the case of a lifting movement of a heavy load, the electric valve 237 on the second line 214 on the piston-rod side 212 is opened. The electric valve 304 on the parallel line 302 is closed. The lifting takes place somewhat more slowly due to the fact that the whole piston side 208 has to be filled by the hydraulic machine 204.

In the case of a light load, lowering can take place more rapidly, due to the fact that only the volume of the piston rod passes via the hydraulic machine 204. First, the additional valve 304 on the parallel line 302 is opened. Before the lowering movement, pressurization takes place, for example by the electric machine 202 first being driven with a certain torque in the “wrong direction”, where the torque level is based on the value of the pressure sensor 228 immediately before. Alternatively, the hydraulic machine 204 rotates by a certain angle in the “wrong direction”. The valve 243 on the first line 210 is then opened to the piston side 208, the rotation direction of the hydraulic machine 204 is changed, and the lowering movement starts.

The lowering movement of a heavy load can be performed as follows: the pressure sensor 228 indicates heavy load. The additional valve 304 on the parallel line 302 is closed. In this state, all the flow from the piston side 208 passes via the hydraulic machine 204. The electrically controlled pressure limiter may need to be adjusted slightly in order to improve refilling to the piston-rod side 212.

According to a preferred embodiment, the pressure sensor 228 therefore detects a load acting on the implement and generates a corresponding signal. The control unit 802, see FIG. 8, compares the size of the detected load with a predetermined load level. If the detected load is below the predetermined load level, a corresponding signal is sent to the valve 304, which is opened, the piston-rod side 212 of the hydraulic cylinder 108 being brought into flow communication with the piston side 208 so that hydraulic fluid coming from the piston-rod side is brought to the piston side without passing through the hydraulic machine 204. If on the other hand the detected load is above the predetermined load level, a corresponding signal is sent to the valve 237, which is opened, the piston-rod side of the hydraulic cylinder being brought into flow communication with the second port 222 of the hydraulic machine 204 so that hydraulic fluid coming from the piston-rod side 212 is brought to the second port of the hydraulic machine.

FIG. 4 shows a third embodiment of the control system 401. A flow control means 402, in the form of an electrically controlled proportional valve, is connected on a line 404 which extends between the first line 210 and the tank 216 in order to allow a certain leakage flow from the hydraulic machine 204 to the tank at the start of a lifting movement. The hydraulic machine 204 thus has a certain basic revolution before lifting takes place. This reduces starting friction. The valve 402 can then be closed gradually the greater the lifting speed becomes. The valve 402 is a small valve which only produces an adequate drainage flow so that the hydraulic machine 204 starts working before the cylinder movement starts.

A flow control means 406, in the form of an electrically controlled proportional valve, is connected on the first line 210 between the hydraulic machine 204 and the piston side 208 of the hydraulic cylinder in order to control the size of the hydraulic fluid flow from the hydraulic cylinder 108 to the hydraulic machine 204 at the start of the lowering movement. At the start of the lowering movement, the electric machine 202 has a low counter-torque in order to prevent starting friction and a jerky start. The valve 406 is opened proportionally and the piston speed is controlled. In parallel with the valve 406 being opened, the counter-torque in the electric machine 202 is increased and the hydraulic machine 204 gradually takes over the speed control of the lowering movement. In the end, the valve 406 is fully open and the lowering speed is controlled completely by the electric machine 202.

FIG. 5 shows a fourth embodiment of the control system 501. The hydraulic machine 204 can be connected via a connection means 502 to an additional hydraulic actuator 504 which is adapted to perform a work function which is separate from a work function performed by said hydraulic cylinder 108. Here, the connection means 502 consists of an electrically controlled directional valve. The additional work function can be, for example, implement locking or an emergency pump for the steering function.

FIG. 6 shows a fifth embodiment of the control system 601, which is a development of the first embodiment, see FIG. 2. Here, said means for allowing suction of hydraulic fluid from the tank 216 through the suction lines 230, 234 consist of electrically controlled on/off valves 632, 636 instead of non-return valves. This reduces problems of cavitation on the suction side.

The valve 636 which connects the second port 222 of the hydraulic machine 204 to the tank 216 can be open when the hydraulic machine rotates in the direction so that hydraulic fluid passes to the cylinder 108. The valve 636 is closed when the rotation is changed.

The valve 632 which connects the first port 220 of the hydraulic machine 204 to the tank 216 is opened when the filtering and heating flow is run. The valve 636 may also need to be opened if the unit stops dead during ongoing lowering, which results in cavitation occurring on account of the fact that the hydraulic machine 202 does not have time to stop. Such a course of events can be registered by, for example, registering the state of the hydraulic machine 202 and the state of the cylinder 108.

FIG. 7 shows a control system 701 comprising a subsystem 707 for the lifting function, a subsystem 709 for the tilting function, a subsystem 711 for the steering function and a subsystem 731 for an additional function. A number of different system embodiments for the lifting function have been described above.

The subsystem 709 shown in FIG. 7 for the tilting function has a construction corresponding to the system for the lifting function. FIG. 7 illustrates the electric machine with reference sign 703 and the hydraulic machine with reference sign 705. For the tilting function, a pressure-limiting valve 702, or shock valve, is added, which connects the piston-rod side of the tilting cylinder 110 to the tank.

The subsystem 711 shown in FIG. 7 for the steering function comprises said first and second steering cylinders 104, 105, which are adapted for frame-steering the wheel loader 101. The system also comprises a first drive unit 704 and a second drive unit 706, which each comprise an electric machine 708, 710 and a hydraulic machine 712, 714. Each electric machine 708, 710 is connected in a driving manner to its associated hydraulic machine 712, 714.

A first 712 of the two hydraulic machines is connected to a piston side 716 of the first hydraulic cylinder 104 and a piston-rod side 718 of the second hydraulic cylinder 105. A second 714 of the two hydraulic machines is connected to a piston side 720 of the second hydraulic cylinder 105 and a piston-rod side 722 of the first hydraulic cylinder 104.

For steering the wheel loader 101 in a direction (for example to the right), a first of the hydraulic machines 712 is adapted to be driven by its associated electric machine 708 and to supply the hydraulic cylinders 104, 105 with pressurized hydraulic fluid from the tank 216, and the second hydraulic machine 714 is adapted to be driven by a hydraulic fluid flow from the hydraulic cylinders 104, 105 and to drive its associated electric machine 710, and vice versa.

The hydraulic machines are therefore driven in opposite directions during operation.

A first electrically controlled control means (control valve) 724 is arranged between the hydraulic machine 712 of the first drive unit 704 and the steering cylinders 104, 105, and a second electrically controlled control means (control valve) 726 is arranged between the hydraulic machine 714 of the second drive unit 706 and the steering cylinders 104, 105.

The subsystem 731 shown in FIG. 7 for the additional function preferably comprises only one drive unit 734 for providing all the additional functions. This means that it is easier to add an additional function, see arrow 766, as only a valve unit has to be added. The drive unit 734 comprises a pump 736 which is driven mechanically by an electric motor 738. This additional function can consist of, for example, the implement 107 comprising parts which are movable relative to one another, the movement of which is controlled. Such functions can consist of a sweeping roller, clamping arms etc.

A hydraulic actuator in the form of a hydraulic cylinder 732 is adapted for carrying out the movement in the control system 731 shown. The pump 736 is connected to a piston side 740 and a piston-rod side 742 via a first and a second line 744, 746. An inlet valve in the form of an electrically controlled proportional valve 748, 750 is arranged on each of the first and second lines 744, 746. The piston side 740 and the piston-rod side 742 are connected to the tank 216 via a third and fourth line 752, 754. An outlet valve in the form of an electrically controlled proportional valve 756, 758 is arranged on each of the third and fourth lines 752, 754. A pressure sensor 760, 762 is arranged on each of the third and fourth lines 752, 754. An additional pressure sensor 764 is arranged on the line downstream of the pump 736 and upstream of the inlet valves 748, 750.

According to an alternative, more pumps and if appropriate electric motors can be added for the purpose of increasing the maximum flow. The pump for the lifting or the tilting function can moreover be connected in parallel for any topping of the flow. Functions with another type of valve can also be added.

The additional function can be controlled via inlet control: on activation of a function, the load pressure in the cylinder 732 is registered. The pump 736 is set with a torque which gives a certain level of higher pressure before the inlet valve 748, 750, which is registered via the pressure sensor 764 before the valve. This means that the inlet valve 748, 750 has a known pressure drop. By virtue of the fact that the pressure drop can be read off, the flow can now be adjusted via control of the inlet valve (regulating the opening area). If a number of functions are running at the same time, the pump 736 builds up a torque which is a certain level higher than the highest registered load pressure. The outlet valve 756, 758 opens to a level which gives a specific counter-pressure, which can be read off via the pressure sensor 760, 762 on the outlet side of the cylinder 732. If the counter-pressure is higher on account of a hanging load, the outlet valve 756, 758 is regulated so that the pressure on the inlet side does not fall below a certain level. Functions which have a motor instead of a cylinder can be regulated in the same way.

The additional function can alternatively be controlled via outlet control: the pump 736 is set with a torque which gives a certain pressure level before the outlet valve 756, 758, which is registered via the pressure sensor 760, 762 before the outlet valve. This means that the outlet valve 756, 758 has a pressure drop which is known (the tank side is in principle pressureless). According to an alternative/supplement, a pressure sensor is arranged on the tank side. It is then possible to have control of the pressure drop across the valve (in some cases the system is not pressureless).

By virtue of the fact that the pressure drop can be read off, the flow can now be adjusted via control of the outlet valve 756, 758 (regulating the opening area). If a number of functions are running at the same time, the pump builds up a torque which gives a certain level of pressure at the pressure sensor (on the outlet side) which has the lowest pressure.

The inlet valve 748, 750 can be opened fully so that no pressure drop occurs (lower losses). If it is hanging load, the cylinder 732 drives, or if a pump flow deficiency occurs, the outlet valve 756, 758 is also regulated so that the pressure on the inlet side of the cylinder 732 does not fall below a certain level. Prioritizing/weighting can take place between the functions is the pump flow is not sufficient.

Functions which have a motor instead of a cylinder can be regulated in the same way.

If use is made of a function which has a hydraulic motor (for example a sweeping roller), both the inlet valve 748, 750 and the outlet valve 756, 758 can be opened fully so that no pressure drop is generated. The speed of the sweeping roller is then controlled directly via the speed of the pump 736. If another function is temporarily controlled simultaneously, it is possible to change over temporarily to inlet control or outlet control.

The control system 731 affords opportunities for a maximum feed pressure limitation. The pressure can be read off via the pressure sensor, and the inlet valve can be throttled when the pressure becomes too high.

The control system 731 also affords opportunities for dealing with a shock pressure. The pressure can be read off via pressure sensor, and the outlet valve can drain to the tank when the pressure level becomes too high.

According to a development, a back-up valve can be added after the valve 756, 758 on the outlet side (toward the tank 216), together with refilling valves for the cylinder 732. This provides more available pump flow when a number of functions are running simultaneously and then if a function has a load which drives the flow.

FIG. 8 shows a control system for controlling the control system 701 shown in FIG. 7 for the lifting function, the tilting function, the steering function and the additional function. A number of elements, or controls, 804, 806, 808, 810, 812, 814 are arranged in the cab 114 for manual operation by the driver and are electrically connected to the control unit 802 for controlling the various functions. A wheel 804 and a control lever 806 are adapted for controlling the steering function. A lifting lever 808 is adapted for the lifting function and a tilting lever 810 is adapted for the tilting function. A lever 812 is adapted for controlling the third function, and an additional control 814 is adapted for pump control (adjustable flow) for the third function. A number of additional functions with associated controls can be added.

The electric machines 202, 703, 708, 710, 738 are electrically connected to the control unit 802 in such a way that they are controlled by the control unit and that they can provide operating state signals to the control unit.

The control system comprises one or more energy storage means 820 connected to one or more of said electric machines 202, 703, 708, 710, 738. The energy storage means 820 can consist of a battery or a supercapacitor, for example. The energy storage means 820 is adapted to provide the electric machine with energy when the electric machine 202 is to function as a motor and drive its associated pump 204. The electric machine 202 is adapted to charge the energy storage means 820 with energy when the electric machine 202 is driven by its associated pump 204 and functions as a generator.

The wheel loader 101 also comprises a power source 822 in the form of an internal combustion engine, which usually consists of a diesel engine, for propulsion of the vehicle. The diesel engine 822 is connected in a driving manner to the wheels of the vehicle via a drive line (not shown). The diesel engine 822 is moreover connected to the energy storage means 820 via a generator (not shown) for energy transmission.

It is possible to imagine alternative machines/units adapted for generating electric power. According to a first alternative, use is made of a fuel cell which provides the electric machine with energy. According to a second alternative, use is made of a gas turbine with an electric generator for providing the electric machine with energy.

FIG. 8 also shows the other components which are connected to the control unit 802 according to the first embodiment of the control system for the lifting function, see FIG. 2, such as the electrically controlled valves 224, 237, 243, the position sensor 248 and the pressure sensor 228. It will be understood that corresponding components for the tilting function and the steering function and the additional function are connected to the control unit 802.

FIG. 9 shows a further embodiment of the control system 901. The control system 901 comprises a hydraulic cylinder 902 which is reversed, which means that a load 904 draws the cylinder out via its weight. This control system 901 can be said to be a variant of the control system 201 according to the first embodiment, see FIG. 2.

In order to bring about necessary refilling to the piston side 906 of the cylinder 902 during a lowering movement, the system comprises an additional, smaller pump 908. The smaller pump has a driving connection to the hydraulic machine 204.

During lowering, the hydraulic fluid passes from the piston-rod side 910 of the cylinder 902 to the piston side 906 via the larger hydraulic machine 204. The small pump 908 contributes to pumping hydraulic fluid from the tank 216 to the piston side 906 via a suction line 912. During a lifting movement, the small pump 908 performs no useful work. The small pump 908 only pumps hydraulic fluid round through itself via a small non-return valve 914. The non-return valve 914 is therefore connected between an inlet side 916 and an outlet side 918 of the additional pump 908, so that, during a lifting movement, the pump 908 only pumps hydraulic fluid in a circuit 920 comprising the non-return valve 914. The non-return valve 914 is therefore arranged in parallel with the small pump 908.

Otherwise, this system 901 functions similarly to the basic system (see FIG. 2), apart from the filtering and heating flow being a little greater.

According to a previously known pump, there is a regulator in the pump, which provides a pressure-limiting function so that the displacement of the pump is adjusted down in the event of pressure being too high. According to one embodiment of the control method, the built-in pressure-limiting function of the pump can be omitted, and a simpler/cheaper pump can therefore be used as a hydraulic machine.

A first embodiment of the regulating method comprises the steps of detecting an operating parameter and of generating a corresponding parameter signal, of determining a level of said pressure based on the level of the detected operating parameter, of comparing the determined pressure level with a predetermined maximum level and of controlling the hydraulic machine so that a delivered pressure lies below the predetermined maximum level. More precisely, the parameter signal generated is received by the control unit (computer) and is processed, after which a control signal is sent to the electric machine which has a driving connection to the hydraulic machine to reduce the delivered torque if the determined pressure level exceeds the predetermined maximum level.

The preferred embodiment comprises the step of detecting a torque delivered by the electric machine and of determining the level of said pressure based on the detected torque. Furthermore, a level of said pressure based on at least the detected torque and the displacement of the hydraulic machine is calculated.

According to an alternative to detecting the delivered torque of the electric machine, it is possible to detect the pressure of the hydraulic fluid in a line downstream of the hydraulic machine and to compare the detected pressure level with the predetermined maximum level.

The invention is not to be regarded as being limited to the illustrative embodiments described above, but a number of further variants and modifications are conceivable within the scope of the following patent claims.

Claims

1. A control system for a work machine (101) comprising:

an electric machine (202);

an hydraulic machine (204); and

at least one hydraulic cylinder (108);

the electric machine (202) being connected in a driving manner to the hydraulic machine (204);

the hydraulic machine (204) being connected to a piston side (208) of the hydraulic cylinder (108) via a first line (210) and a piston-rod side (212) of the hydraulic cylinder (108) via a second line (214); and

the hydraulic machine (204) being adapted to be driven by the electric machine (202) and supply the hydraulic cylinder (108) with pressurized hydraulic fluid from a tank (216) in a first operating state and to be driven by a hydraulic fluid flow from the hydraulic cylinder (108) and drive the electric machine in a second operating state.

2. The control system as recited in claim 1, wherein the hydraulic machine (204) is adapted to control the speed of the piston (218) of the hydraulic cylinder (108) in the first operating state.

3. The control system as recited in claim 1, wherein the control system comprises a control unit (802) which is electrically connected to the electric machine (202) in order to control the speed of the piston (218) of the hydraulic cylinder (108) in the first operating state by controlling the electric machine.

4. The control system as recited in claim 1, wherein the hydraulic machine (204) has a first port (220) which is connected to the piston side (208) of the hydraulic cylinder (108) via the first line (210) and a second port (222) which is connected to the piston-rod side (212) of the hydraulic cylinder (108) via the second line (214).

5. The control system as recited in claim 1, wherein a second port (222) of the hydraulic machine (204) is connected to the tank (216) in order to allow the hydraulic machine (204), in the first operating state, to draw oil from the tank via the second port (222) and supply the oil to the hydraulic cylinder via a first port (220).

6. The control system as recited in claim 4, wherein the system comprises a means (224) for controlling pressure, which pressure means (224) is arranged on a line (226) between the second port (222) of the hydraulic machine and the tank in order to allow pressure build-up on the piston-rod side (212).

7. The control system as claimed in claim 6, wherein the pressure control means (224) comprises an electrically controlled pressure-limiting valve.

8. The control system as recited in claim 1, wherein the system comprises a sensor (228) for sensing pressure on the piston side (208) of the hydraulic cylinder.

9. The control system as recited in claim 1, wherein a first port (220) of the hydraulic machine is connected to the tank (216) via a suction line (230).

10. The control system as recited in claim 9, wherein a means (232, 632) is arranged on the suction line (230) in order to allow suction of hydraulic fluid from the tank and obstruction of a hydraulic fluid flow to the tank.

11. The control system as recited in claim 10, wherein the means comprises a non-return valve (232).

12. The control system as recited in claim 10, wherein the means comprises an electrically controlled on/off valve (632).

13. The control system as recited in claim 1, wherein a second port (222) of the hydraulic machine is connected to the tank (216) via a suction line (234).

14. The control system as recited in claim 13, wherein a means (236, 636) is arranged on the suction line (234) in order to allow suction of hydraulic fluid from the tank and obstruction of a hydraulic fluid flow to the tank.

15. The control system as recited in claim 14, wherein the means comprises a non-return valve (236).

16. The control system as recited in claim 14, wherein the means comprises an electrically controlled on/off valve (636).

17. The control system as recited in claim 1, wherein a second port (222) of the hydraulic machine (204) is connected to the tank (216) via a line (242).

18. The control system as recited in claim 17, wherein a filtering unit (238) is arranged on the line (242) between the second port (222) of the hydraulic machine (204) and the tank (216).

19. The control system as recited in claim 1, wherein the hydraulic machine (204) can be connected via a connection means (502) to a hydraulic actuator (504) which is adapted to perform a work function which is separate from a work function performed by said hydraulic cylinder (108).

20. The control system as recited in claim 4, wherein a first port (220) of the hydraulic machine (204) is connected to the piston-rod side (212) of the hydraulic cylinder (108).

21. The control system as recited in claim 1, wherein the system comprises a line (302) which connects the piston-rod side (212) and the piston side (208) of the hydraulic cylinder (108) in parallel to the hydraulic machine (204).

22. The control system as recited in claim 21, wherein the system comprises a means (304) for flow control, which is arranged on said parallel line (302) in order to control the flow communication between the piston-rod side (212) and the piston side (208).

23. The control system as recited in claim 1, wherein a first port (220) of the hydraulic machine (204) is connected to a piston side (208) of the hydraulic cylinder (108) via a first line (210), and wherein a flow control means (402) is connected between the first line (210) and the tank (216) in order to allow a certain leakage flow from the hydraulic machine (204) to the tank at the start of a lifting movement.

24. The control system as recited in claim 1, wherein a first port (220) of the hydraulic machine (204) is connected to a piston side (208) of the hydraulic cylinder (108) via a first line (210), and wherein a flow control means (406) is connected on the first line (210) in order to control the size of the hydraulic fluid flow from the hydraulic cylinder (108) to the hydraulic machine (204) at the start of a lowering movement.

25. The control system as recited in claim 1, wherein the hydraulic cylinder is adapted to move an implement (107) in order to perform a work function.

26. The control system as recited in claim 25, wherein the hydraulic cylinder comprises a lifting cylinder (108, 109) for moving a loading arm (106) which is pivotably connected to a vehicle frame, the implement (107) being arranged on the loading arm (106).

27. The control system as recited in claim 25, wherein the hydraulic cylinder comprises a tilting cylinder (1 10, 902) for moving the implement (107), which is pivotably connected to a loading arm (106), which is in turn pivotably connected to a vehicle frame.

28. The control system as recited in claim 27, wherein the tilting cylinder (110) is adapted so that a load (904) which acts on the tilting cylinder draws the piston rod of the tilting cylinder out via its weight.

29. The control system as recited in claim 28, wherein the control system comprises an additional, smaller pump (908), which has a driving connection to the hydraulic machine (204), and wherein this additional pump (908) is connected to the piston side (906) of the tilting cylinder and to the tank (216) in order to pump hydraulic fluid to the piston side during a lowering movement.

30. The control system as recited in claim 29, wherein the control system comprises a non-return valve (914) which is connected between an inlet side (916) and an outlet side (918) of the additional pump (908) so that the pump (908) only pumps hydraulic fluid in a circuit (920) comprising the non-return valve (914) during a lifting movement.

31. A method for controlling a hydraulic cylinder (108, 109, 110) of a work machine (101 ), which hydraulic cylinder is adapted to move an implement (107) which is subjected to a load (116), the hydraulic cylinder being controlled by a hydraulic machine (204), said method comprising:

detecting that a lowering movement of the implement is initiated, and of before the lowering movement takes place driving the hydraulic machine (204) in a first rotation direction so that a first line (220) which connects the hydraulic machine (204) to said first side (208) of the piston (218) of the hydraulic cylinder is pressurized, which side is opposite a second side (212) on which said load acts.

32. The method as recited in claim 31, further comprising, after the pressurization, allowing the hydraulic machine (204) to rotate in a second rotation direction, opposite the first rotation direction, whereupon the lowering movement can start and a hydraulic flow from the hydraulic cylinder drives the hydraulic machine (204) in the second rotation direction.

33. The method as recited in claim 31, further comprising a controllable means (243) for opening and closing a flow connection between the hydraulic machine (204) and the hydraulic cylinder (108) being arranged on the first line (210), comprising the steps of keeping the controllable means (243) closed for flow in the direction from the hydraulic cylinder to the hydraulic machine (204) and of pressurizing the line (210) between the hydraulic cylinder (204) and the controllable means (243).

34. The method as recited in claim 33, further comprising, after the pressurization, opening the controllable means (243) in order to allow the hydraulic machine (204) to rotate in a second rotation direction, opposite the first rotation direction, whereupon the lowering movement can start and a hydraulic flow from the hydraulic cylinder drives the hydraulic machine (204) in the second rotation direction.

35. The method as recited in claim 31, further comprising the step of gradually reducing the pressurization so that a smooth lowering movement is achieved.

36. A method for controlling a hydraulic cylinder (108, 109, 110) of a work machine (101) for the purpose of moving an implement (107) which is connected to the hydraulic cylinder, a hydraulic machine (204) providing the hydraulic cylinder with pressurized hydraulic fluid, said method comprising:

detecting a load (116) acting on the implement (107);

comparing the size of the detected load with a predetermined load level, and, if the detected load lies below the predetermined load level, bringing the piston-rod side (212) of the hydraulic cylinder into flow communication with the piston side (208) so that hydraulic fluid coming from the piston-rod side (212) is brought to the piston side (208) without passing through the hydraulic machine (204).

37. The method as recited in claim 36, the hydraulic machine (204) providing the hydraulic cylinder with pressurized hydraulic fluid from a first port (220), comprising the step of, if the detected load (116) exceeds the predetermined load level, bringing the piston-rod side (212) of the hydraulic cylinder into flow communication with a second port (222) of the hydraulic machine (204) so that hydraulic fluid coming from the piston-rod side (212) is brought to the second port (220) of the hydraulic machine (204).

38. A method for regenerating energy when an implement (107) of a work machine (101) is moved during movement of the work machine, at least one hydraulic cylinder (108, 109, 110) being connected to the implement for controlling its movements, said method comprising:

delivering such a pressure to the hydraulic cylinder that the implement is brought into a basic position, of, in the event of a disturbance which results in a downward movement of the implement;

allowing driving of a hydraulic machine (204) by a hydraulic fluid flow from the hydraulic cylinder; and

regenerating the energy from the hydraulic machine (204) in an electric machine (202) connected to it in a driving manner.

39. The method as recited in claim 38, comprising the step of bringing a first port (220) of the hydraulic machine (204) into flow communication with a piston side (208) of the hydraulic cylinder via a first line (210) and a second port (222) of the hydraulic machine (204) into flow communication with a piston-rod side (212) of the hydraulic cylinder via a second line (214).

40. The method as recited in claim 38, comprising the step of controlling the hydraulic machine (204) so as to deliver such a pressure to the hydraulic cylinder that the implement is brought into said basic position.

41. The method as recited in claim 38, comprising the steps of repeatedly detecting the position of the implement (107) and of, in the event of a disturbance which results in an upward movement of the implement, supplying a corresponding quantity of hydraulic fluid to the hydraulic cylinder and, in the event of a downward movement of the implement, draining a corresponding quantity of hydraulic fluid from the hydraulic cylinder.

42. The method as recited in claim 38, comprising the step of the electric machine (202) driving the hydraulic machine (204) so that the hydraulic fluid is supplied to the hydraulic cylinder for upward movement of the implement.

43. The method as recited in claim 38, comprising the step of continuously controlling the hydraulic cylinder so that the implement is kept within a predetermined range around the basic position.

44. A method for springing a movement of an implement (107) of a work machine (101) during movement of the work machine, at least one hydraulic cylinder (108, 109, 110) being connected to the implement for controlling its movements, said method comprising:

delivering such a pressure to the hydraulic cylinder that the implement is brought into a basic position; and

bringing a first port (220) of the hydraulic machine (204) into flow communication with a piston side (208) of the hydraulic cylinder via a first line (210) and a second port (222) of the hydraulic machine (204) into flow communication with a piston-rod side (212) of the hydraulic cylinder via a second line (210), and of, in the event of a disturbance which results in an upward movement of the implement, supplying a corresponding quantity of hydraulic fluid to the hydraulic cylinder and, in the event of a downward movement of the implement, draining a corresponding quantity of hydraulic fluid from the hydraulic cylinder.

45. The method as recited in claim 44, further comprising the steps of, in the event of a disturbance which results in a downward movement of the implement (107), allowing driving of the hydraulic machine (204) by a hydraulic fluid flow from the hydraulic cylinder, and of regenerating the energy from the hydraulic machine (204) in the electric machine (202).

46. The method as recited in claim 44, further comprising the step of the electric machine (202) controlling the hydraulic machine (204) to pump hydraulic fluid to the hydraulic cylinder.

47. The method as recited in claim 44, further comprising the step of controlling the hydraulic machine so as to deliver such a pressure to the hydraulic cylinder that the implement is brought into said basic position.

48. The method as recited in claim 44, further comprising the steps of at least one operating parameter being detected and of the springing movement being controlled according to a spring characteristic depending on the detected operating parameter.

49. The method as recited in claim 44, further comprising the steps of at least one operating parameter being detected and of a damping movement being controlled depending on the detected operating parameter.

50. The method as recited in claim 48, further comprising the steps of detecting a level of the disturbance force and of controlling the springing and/or the damping depending on the disturbance force level.

51. The method as recited in claim 48, further comprising the steps of detecting the weight of the load and of controlling the springing and/or the damping depending on the weight.

52. The method as recited in claim 48, further comprising the steps of detecting the type of implement and of controlling the springing and/or the damping depending on the detected implement type.

53. The method as recited in claim 48, further comprising the steps of detecting the type of handling mode and of controlling the springing and/or the damping depending on the detected handling mode.

54. A control system for a work machine (101) comprising:

a first subsystem (707, 709) for performing a first work operation and a second subsystem (731) for performing at least one second work operation;

said first subsystem comprises an electric machine (202), a hydraulic machine (204) and at least one hydraulic cylinder (108, 109, 110), the electric machine (202) being connected in a driving manner to the hydraulic machine (204), the hydraulic machine (204) being connected to a piston side of the hydraulic cylinder via a first line and a piston-rod side of the hydraulic cylinder via a second line, the hydraulic machine being adapted to be driven by the electric machine and supply the hydraulic cylinder with pressurized hydraulic fluid from a tank in a first operating state and to be driven by a hydraulic fluid flow from the hydraulic cylinder and drive the electric machine in a second operating state;

said second subsystem (731) comprises a drive unit (734) and a hydraulic actuator (732), the drive unit (734) comprising an electric machine (738) and a hydraulic machine (736), the electric machine being connected in a driving manner to the hydraulic machine, the hydraulic machine (736) being adapted for flow communication with the hydraulic actuator (732), and a means (748, 750, 756, 758) being adapted for controlling the movement of the hydraulic actuator (732).

55. The control system as recited in claim 54, wherein said control means (748, 750) is arranged on an inlet side of the hydraulic actuator (732).

56. The control system as recited in claim 54, wherein said control means (756, 758) is arranged on an outlet side of the hydraulic actuator (732).

57. The control system as recited in claim 54, wherein said control means (748, 750, 756, 758) comprises at least one valve.

58. The control system as recited in claim 54, wherein the control system comprises a third subsystem (711) for frame-steering the vehicle (101) and said third subsystem (711) comprising a first steering cylinder (104) and a second steering cylinder (105), which steering cylinders are adapted for frame-steering the vehicle, a first drive unit (704) and a second drive unit (706), which each comprise an electric machine (708, 710) and a hydraulic machine (712, 714), each electric machine being connected in a driving manner to its associated hydraulic machine, a first (712) of the two hydraulic machines being adapted for flow communication with a piston side (716) of the first steering cylinder (104) and a piston-rod side (718) of the second steering cylinder (105), a second (714) of the two hydraulic machines being adapted for flow communication with a piston side (720) of the second steering cylinder and a piston-rod side (722) of the first steering cylinder.

59. A method for limiting a pressure which is delivered by a hydraulic machine (204, 705, 712, 714, 736) forming part of a control system, when the hydraulic machine is used as a pump, an electric machine (202, 703, 708, 710, 738) being connected in a driving manner to the hydraulic machine (204), said method comprising:

detecting an operating parameter and of generating a corresponding parameter signal;

determining a level of said pressure based on the level of the detected operating parameter; and

comparing the determined pressure level with a predetermined maximum level and of controlling the hydraulic machine so that a delivered pressure lies below the predetermined maximum level.

60. The method as recited in claim 59, further comprising detecting a torque delivered by the electric machine and of determining the level of said pressure based on the detected torque.

61. The method as recited in claim 60, further comprising calculating a level of said pressure based on at least the detected torque and the displacement of the hydraulic machine.

62. The method as recited in claim 59, further comprising detecting the pressure of the hydraulic fluid in a line downstream of the hydraulic machine and of comparing the detected pressure level with the predetermined maximum level.

63. The method as recited in claim 59, further comprising:

providing the hydraulic machine with an hydraulic actuator (104, 105, 108, 109, 110, 732, 902) with pressurized hydraulic fluid.