EP3530962B1 - Method for controlling a lowering pressure of an electrohydraulic power lift and electro-hydraulic power lift - Google Patents

Description

The invention relates to a method for controlling a lowering pressure of an electrohydraulic power lift, which has at least one working cylinder with a piston, with a lifting side, which the piston delimits with a lifting-side effective area, to which pressure medium is supplied for lifting a load and in which a lifting pressure is present, and with a lowering side, which the piston delimits with a lowering-side effective area, to which pressure medium is supplied for lowering a load and in which the lowering pressure is present. The electrohydraulic power lift also comprises a first pressure sensor for detecting the lifting pressure and a second pressure sensor for detecting the lowering pressure, an adjustable pressure valve for setting a lowering pressure and an electronic control unit, to which the pressures detected by the pressure sensors are fed and by which the pressure valve can be controlled.

From the EP 2 884 118 A1 An electro-hydraulic power lift with a working cylinder that has a piston and a piston rod on one side of the piston is already known. The piston thus separates a cylinder chamber on the rod side or on the bottom and a ring-shaped cylinder chamber on the rod side. In this case, pressure medium is supplied to the bottom cylinder chamber for lifting and to the rod side cylinder chamber for lowering. The bottom cylinder chamber therefore represents the lifting side and the rod side cylinder chamber represents the lowering side of the working cylinder. The effective area on the lifting side, with which the piston limits the lifting side, is larger than the effective area on the lowering side, with which the piston limits the lowering side. The lifting side and the lowering side can be compared to the EP2 884 118 The arrangement shown may also be reversed. For example, in the case of rear-mounted tractor lifting gear, the ground-side Cylinder chamber is the lifting side, while in front loaders the rod-side cylinder chamber usually represents the lifting side.

The one from the EP 2 884 118 A1 Known electro-hydraulic power lifts have an adjustable pressure control valve assigned to the lowering side, with which a certain pressure can be regulated on the lowering side. There is also a pressure sensor with which the pressure on the lifting side can be recorded. The pressure sensor is intended to detect certain operating situations, for example excessive pressure on the lifting side or a pressure drop on the lifting side, and then to change the control of the pressure control valve.

From the DE 101 38 389 A1 An electro-hydraulic power lift is already known in which a first pressure sensor measures the pressure on the lifting side and a second pressure sensor measures the pressure on the lowering side of a working cylinder. The signals from the two pressure sensors are fed to an electronic control unit.

The invention is based on the object of specifying a method with which the functioning of an electro-hydraulic power lift with the features specified at the outset is improved.

The following procedural steps are planned to solve this task:
Continuous determination of an equivalent lowering pressure by forming the difference between the lifting pressure multiplied by k as the minuend and the lowering pressure as the subtrahend in the electronic control unit, where k is the ratio of the size of the lifting-side effective area to the size of the lowering-side effective area of the piston, forming a pressure value as the difference between a target pressure set on the control unit on the lowering side as the minuend and the equivalent lowering pressure as the subtrahend and controlling the pressure valve with a control signal corresponding to the pressure value.

In an advantageous embodiment of the method according to the invention, the equivalent sink pressure is set equal to zero for the formation of the pressure value if the ongoing determination of the equivalent sink pressure results in a negative value.

If a negative pressure value is obtained, the pressure valve is advantageously controlled according to the lowest pressure that can be set on it. Usually, the pressure valve is not controlled in this case. Unless it is a pressure valve with a negative characteristic, such as one in the EP 2 884 118 A1 is shown.

It is advantageous if the equivalent sink pressure is dampened by an electronic filter.

The pressure setting on the sink side can be further improved by increasing or decreasing the control signal for the pressure valve depending on the current volume flow and its sign in order to compensate for a volume flow-related pressure drop between the pressure valve and the sink side. The control signal for the pressure valve can be increased or decreased according to a characteristic curve. It is also possible to increase or decrease it according to a factor.

If the electro-hydraulic power lift includes a load-compensated LS directional valve that can be connected to the lifting side of the working cylinder, a volume flow flowing between the pressure valve and the lowering side of the working cylinder, in particular a volume flow flowing away from the lowering side, can be determined via the control signal with which the LS (load sensing) directional valve is controlled. The lifting mode is particularly suitable for determining the volume flow in this way, since lifting is usually load-compensated via the LS directional valve and the target volume flow given as a control signal to the LS directional valve corresponds to the actual volume flow, as long as no stop is reached or the hydraulic energy is limited and a pump does not deliver as much pressure medium as required.

A volume flow between the pressure valve and the lowering side of the working cylinder can also be determined by deriving the signal from a position sensor assigned to the working cylinder. This is particularly advantageous when lowering, as it prevents overpressure. The pressure value is only increased when a volume flow is also flowing.

Depending on the sign and/or amount of the determined equivalent lowering pressure, different target pressures can be specified by the control unit.

A shut-off valve can be arranged between the pressure valve and the lowering side of the working cylinder, and pressure medium can be discharged from the lowering side of the working cylinder via a relief valve, bypassing the pressure valve. It can be advantageous if the shut-off valve is closed and the relief valve is opened when the target volume flow to the lifting side and thus the target speed for lifting exceeds a certain value and the pressure value falls below a certain value.

The pressure valve is preferably a pressure control valve. A pressure control valve usually has a consumer connection connected to a hydraulic consumer - here to the lower side of the working cylinder - a pressure connection and a tank connection and is able to allow pressure medium to flow to the hydraulic consumer as well as to flow away from the hydraulic consumer for pressure control.

The invention is also implemented in an electro-hydraulic power lift, which has at least one working cylinder with a piston, with a lifting side, which the piston delimits with a lifting-side effective surface, to which pressure medium is supplied for lifting a load and in which a lifting pressure is present, and with a lowering side, which the piston delimits with a lowering-side effective surface, to which pressure medium is supplied for lowering a load and in which the lowering pressure is present, a first pressure sensor for detecting the lifting pressure and a second pressure sensor for detecting the lowering pressure, an adjustable pressure valve for setting a lowering pressure and an electronic control unit, to which the pressures detected by the pressure sensors are supplied and by which the pressure valve can be controlled, wherein the electronic control unit is designed to carry out a method as described above.

An electrohydraulic power lift to which the method according to the invention can be applied as well as various process sequences are shown in the drawings. The invention will now be explained in more detail with reference to these drawings.

Figur 1

ein Schaltbild des elektrohydraulischen Krafthebers,

Figur 2

ein Verfahrensablauf ohne Kompensation von durch einen Volumenstrom verursachten Druckabfall zwischen dem Arbeitszylinder und dem Druckventil,

Figur 3

eine Erweiterung des Verfahrens gemäß Figur 2 um eine Volumenstromkompensation, wobei der Volumenstrom mit Hilfe eines Wegsensors ermittelt wird, und

Figur 4

eine Erweiterung des Verfahrens gemäß Figur 2 um eine Volumenstromkompensation, wobei der Volumenstrom mit Hilfe eines des Ansteuersignals eines LS-Wegeventils ermittelt wird.

Show it

Figure 1

a circuit diagram of the electro-hydraulic power lift,

Figure 2

a process sequence without compensation of pressure drop caused by a volume flow between the working cylinder and the pressure valve,

Figure 3

an extension of the procedure according to Figure 2 a volume flow compensation, whereby the volume flow is determined using a displacement sensor, and

Figure 4

an extension of the procedure according to Figure 2 It is a volume flow compensation, whereby the volume flow is determined using the control signal of an LS directional control valve.

The Figure 1 The electro-hydraulic power lift shown is in this case the rear lifting mechanism on a tractor and has a working cylinder 10. Inside a housing of the working cylinder, a piston 11 is movable, to which a piston rod 12 is attached on one side, which emerges from the housing on one end, the so-called head of the working cylinder. The piston and the piston rod form and separate a cylinder chamber 13 on the rod side or base side and a ring-shaped cylinder chamber 14 on the rod side. In the lifting mechanism shown, pressure medium is supplied to the base side cylinder chamber 13 for lifting and to the rod side cylinder chamber 14 for lowering. The cylinder chamber on the bottom side therefore represents the lifting side and the cylinder chamber on the rod side represents the lowering side of the working cylinder. The lifting-side effective surface 15, with which the piston 11 limits the lifting side, is larger than the lowering-side, annular effective surface 16, with which the piston limits the lowering side.

The free end of the piston rod 12 is connected to an attachment, shown here as a plow, via lifting arms 17 hinged to the tractor. The connection is drawn very schematically. The position assumed by the lifting arms 17 is detected by a position sensor 18 and converted into an electrical signal.

The pressure in the cylinder chamber 13 can be detected by a first pressure sensor 19. The pressure in the cylinder chamber 14 can be detected by a second pressure sensor 20, which, like the first pressure sensor, converts the pressure into an electrical signal.

The electro-hydraulic power lift shown also has a load-sensing controlled variable displacement pump 25, which sucks pressure medium from a tank 26 and discharges it into a pump line 27. Load-sensing controlled means that the variable displacement pump delivers so much pressure medium that the pressure in the pump line 27 is a certain pressure difference above a control pressure reported to the load-sensing regulator 28 of the variable displacement pump, which is usually the highest load pressure of all hydraulic consumers simultaneously supplied with pressure medium.

A pressure control valve 35, which is proportionally adjustable with the aid of a proportional electromagnet 36, is connected with a pressure connection 37 to the pump line 27 and with a relief connection 38 to a tank line 39, via which pressure medium flows back from the hydraulic consumers of the tractor, in particular from the working cylinder 10, to the tank 26. The pressure at a control connection 40 of the pressure control valve, which can be connected to the rod-side cylinder chamber 14, is fed back to a measuring surface on the pressure control valve via a control line 41 and generates a force on the measuring surface which, together with a force exerted by a control spring 42, acts against the force of the proportional electromagnet 36. At the control connection 40 of the pressure control valve, such a pressure is therefore established that there is an equilibrium between the opposing forces. Since the pressure force and the magnetic force act against each other, the pressure control valve 35 has a positive characteristic curve. The control pressure therefore increases with the current flowing through the electromagnet 36. According to Figure 1 The pressure control valve 35 is shown as a directly controlled pressure control valve. However, a pilot-operated pressure control valve can also be used.

A shut-off valve 45 is arranged between the control connection 40 of the pressure control valve 35 and the rod-side cylinder chamber 14, which is designed as a 2/2-way seat valve, assumes its blocking position under the action of a return spring 46, in which the connection between the control connection 40 and the cylinder chamber 14 is interrupted, and can be brought by a switching magnet 47 against the force of the return spring 46 into a through position in which the connection between the control connection 40 and the cylinder chamber 14 is open.

A relief valve 50 is arranged between the rod-side cylinder chamber 14 of the working cylinder 10 and the tank line 39. Like the valve 45, this relief valve is designed as a 2/2-way seat valve. Under the action of a return spring 51, it assumes its blocking position in which the connection between the tank line 39 and the cylinder chamber 14 is interrupted. It can be brought into a through position by a switching magnet 52 against the force of the return spring 51, in which the connection between the cylinder chamber 14 and the tank line 39 is open. The relief valve 50 is used in certain operating states to complete pressure relief of the rod-side cylinder chamber 14. When the relief valve 50 is open, the shut-off valve 45 is closed. When the shut-off valve 45 is open, the relief valve 50 is closed.

The inflow of pressure medium to the cylinder chamber 13 on the bottom of the working cylinder 10 and the outflow of pressure medium from this cylinder chamber can be controlled with an electrohydraulic lifting gear control valve arrangement 60. For this purpose, the lifting gear control valve arrangement 60 has a consumer connection 61 that is fluidically connected to the cylinder chamber 13. In addition, the lifting gear control valve arrangement has a pump connection 62 connected to the pump line 27 and a tank connection 63 connected to the tank line 39. In order to be able to control the inflow of pressure medium to the cylinder chamber 13 independently of the load pressure, the lifting gear control valve arrangement 60 comprises an individual pressure compensator (not shown in detail) that keeps the pressure drop constant via an adjustable metering orifice so that the amount of pressure medium flowing through the metering orifice only depends on the current flow cross-section of the metering orifice. The outflow of pressure medium from the cylinder chamber 13 is not controlled independently of the load pressure.

The lifting gear valve arrangement 60 also has a load sensing connection 64, from which a control line 65 leads to a first input of a shuttle valve 66. The second input of the shuttle valve 66 is connected to the control connection 40 of the pressure control valve 35 via a control line 67. The output of the shuttle valve 66 is connected to the load sensing regulator 28 of the variable displacement pump 25. The load sensing regulator 28 is therefore informed of the higher of the pressures present in the cylinder chambers 13 and 14.

There is an electronic control unit 70 to which the electrical signals of the position sensor 18, the two pressure sensors 19 and 20 and an electrical target signal, which is generated with the aid of an operating element 71, are fed via electrical lines 72, 73, 74 and 75. The electronic control unit calculates control signals for the electromagnets of the pressure control valve 35, the shut-off valve 45, the relief valve 50 and the lifting gear valve arrangement 60 from the electrical signals supplied and controls the electromagnets accordingly via electrical lines 76, 77, 78 and 79.

Now consider a situation in which the lifting gear is raised, i.e. has no contact with the ground, and is to be lowered and pressed into the ground. A lowering pressure of, for example, 50 bar is specified on the control element 71 for the rod-side cylinder chamber 14. The ratio k between the lifting-side effective area and the lowering-side effective area on the piston 11 of the working cylinder 10 is 1.25. The raised plough is supposed to generate a lifting pressure of, for example, 200 bar in the bottom-side cylinder chamber 13. This lifting pressure therefore prevails in the bottom-side cylinder chamber 13 when there is no pressure in the rod-side cylinder chamber.

According to the procedure according to Figure 2 the electronic control unit 70 continuously determines an equivalent lowering pressure by forming the difference between the lifting pressure (DS carrying) multiplied by k and recorded by the pressure sensor 19 as the minuend and the lowering pressure (DS pressing) recorded by the pressure sensor 20 as the subtrahend. In this case, the equivalent lowering pressure is therefore 200 bar times 1.25 - 0, equal to 250 bar. The calculated equivalent lowering pressures (differential pressure (carrying _* k - pressing)) are filtered using a low-pass filter (filter (low-pass)). If the equivalent lowering pressure is positive (limit positive), a pressure value is determined as the difference between the target pressure on the lowering side specified using the control element 71 and transmitted to the control unit 70 and stored there as the minuend and the equivalent lowering pressure as the subtrahend. If the equivalent lowering pressure is negative, a value of 0 is assumed for the equivalent lowering pressure to determine the pressure value. In this case, the equivalent lowering pressure is 250 bar. The pressure value is therefore 50 bar minus 250 equals minus 200 bar. However, the fact that the pressure value is negative means that the lifting gear is still supporting the plow. This information is processed in the electronic control unit 70. Accordingly, the electronic control unit 70 does not control the pressure control valve 35, so that it is set to the smallest setting value.

Nothing changes until the plow hits the ground. The bearing pressure now decreases. In the example considered, the electronic control unit begins to control the pressure control valve 35 with a control signal greater than zero when the bearing pressure has dropped to 40 bar. From this point on, the control signal for the pressure control valve 35 (static DRV setpoint) increases to the setpoint of 50 bar.

The procedure according to Figure 2 can be supplemented by a volume flow compensation, as shown in two alternatives in the Figures 3 and 4 is shown. According to Figure 3 the volume flow is multiplied by a proportionality factor (P-Gain), especially during lifting, whereby a correction value for a pressure is determined. The correction value is subtracted from the static setpoint for the pressure control valve (static DRV setpoint). If the difference is positive, it is equal to a dynamic setpoint for the pressure control valve 35 during lifting. The volume flow that is displaced from the rod-side cylinder chamber 14 during lifting can be determined from the control signal for the electrohydraulic lifting valve arrangement, which is a measure of the amount of pressure medium flowing into the cylinder chamber 13, and the area ratio k of the working cylinder 10.

To determine the volume flow during lowering, according to Figure 4 the signal of the position sensor 18 is derived over time and multiplied by a proportionality factor (P-Gain). The difference is formed between the static setpoint for the pressure control valve and a correction pressure corresponding to the volume flow, whereby only negative correction values are taken into account due to the opposite flow direction of the pressure medium compared to the lifting. The difference is equal to the dynamic setpoint of the control signal for the pressure control valve 35 when lowering. However, a maximum pressure (limit pMax) should not be exceeded.

List of reference symbols

10

Working cylinder

11

Piston of 10

12

Piston rod of 10

13

bottom cylinder space of 10

14

rod-side cylinder space of 10

15

lifting side effective area at 11

16

sink-side effective area at 11

17

Lifting arms

18

Position sensor

19

first pressure sensor

20

second pressure sensor

25

Variable displacement pump

26

tank

27

Pump line

28

Load-sensing controller

35

Pressure control valve

36

Proportional electromagnet

37

Pressure connection of 35

38

Relief connection of 35

39

Tank line

40

Standard connection of 35

41

Control line to 35

42

Control spring of 35

45

Shut-off valve

46

Return spring of 45

47

Switching magnets from 45

50

Relief valve

51

Return spring of 50

52

Switching magnet of 50

60

Hoist control valve arrangement

61

Consumer connection of 60

62

Pump connection of 60

63

Tank connection of 60

64

Load signaling connection of 60

65

Control line

66

Shuttle valve

67

Control line

70

electronic control unit

71

Control element

72

electrical line

73

electrical line

74

electrical line

75

electrical line

76

electrical line

77

electrical line

78

electrical line

79

electrical line

Claims (12)

1. Method for controlling a lowering pressure of an electrohydraulic power lift which has at least one working cylinder (10) having a piston (11), a lifting side (13) which the piston (11) delimits by a lifting-side effective area (15), said lifting-side effective area (15) being supplied with pressurizing media for lifting a load, and in which lifting-side effective area (15) a lifting pressure is prevalent, and having a lowering side (14) which the piston (10) delimits by a lowering-side effective area (16), said lowering-side effective area (16) being supplied with pressurizing media for lowering a load, and in which lowering-side effective area (16) the lowering pressure is prevalent; and a first pressure sensor (19) for detecting the lifting pressure, and a second pressure sensor (20) for detecting the lowering pressure; and an adjustable pressure valve (35) for adjusting a lowering pressure; and an electronic control unit (70) which is supplied the pressures detected by the pressure sensors (19, 20) and by way of which the pressure valve (35) is actuatable, characterized by the following steps:

   continuously determining an equivalent lowering pressure by forming in the electronic control unit (70) the difference between the lifting pressure multiplied by k as the minuend, and the lowering pressure as the subtrahend, where k is the ratio between the size of the lifting-side effective area (15) and the size of the lowering-side effective area (16) of the piston (11);

   forming a pressure value as the difference between a setpoint pressure adjusted at the electronic control apparatus (70) on the lowering side as the minuend, and the equivalent lowering pressure as the subtrahend;

   actuating the pressure valve (35) with an actuating signal corresponding to the pressure value.
2. Method according to Patent Claim 1, wherein for forming the pressure value the equivalent lowering pressure is set to zero if the continuous determination of the equivalent lowering value results in a negative value.
3. Method according to Patent Claim 1 or 2, wherein the pressure valve (35) is actuated so as to correspond to the lowest pressure adjustable thereon if a negative pressure results.
4. Method according to one of the preceding patent claims, wherein the equivalent lowering pressure is attenuated by an electronic filter.
5. Method according to one of the preceding patent claims, wherein for compensating a pressure drop caused by the volumetric flow between the pressure valve (35) and the lowering side, the actuating signal for the pressure valve (35) is increased or decreased as a function of the current volumetric flow and its algebraic sign.
6. Method according to Patent Claim 5, wherein the actuating signal for the pressure valve (35) is increased or decreased according to a characteristic curve.
7. Method according to Patent Claim 5 or 6, wherein a load-compensating LS directional valve (60) is connectable to the lifting side (13) of the working cylinder (10), and a volumetric flow flowing between the pressure valve (35) and the lowering side (14) of the working cylinder (10), in particular a volumetric flow flowing away from the lowering side, is determined by way of the actuating signal by which the LS directional valve (60) is actuated.
8. Method according to Patent Claim 5, 6 or 7, wherein a volumetric flow flowing between the pressure valve (35) and the lowering side (14) of the working cylinder (10) is determined by way of the derivation of the signal of a laser sensor (18) assigned to the working cylinder (10) .
9. Method according to one of the preceding patent claims, wherein different setpoint pressures are defined by the electronic control apparatus (70) as a function of the algebraic sign and/or value of the determined equivalent lowering pressure.
10. Method according to one of the preceding patent claims, wherein a shut-off valve (45) is disposed between the pressure valve (35) and the lowering side (14) of the working cylinder (10), and wherein pressurizing media from the lowering side (14) of the working cylinder (10) are able to be discharged via a relief valve (50) while bypassing the pressure valve (35), and wherein the shut-off valve (45) is closed and the relief valve (50) is opened when the setpoint volumetric flow to the lifting side (13), and thus the setpoint velocity for lifting, exceeds a specific value and the pressure value undershoots a specific value.
11. Method according to one of the preceding claims, wherein the pressure valve (35) is a pressure control valve.
12. Electrohydraulic power lift which has at least one working cylinder (10) having a piston (11), a lifting side (13) which the piston (11) delimits by a lifting-side effective area (15), said lifting-side effective area (15) being supplied with pressurizing media for lifting a load, and in which lifting-side effective area (15) a lifting pressure is prevalent, and having a lowering side (14) which the piston (10) delimits by a lowering-side effective area (16), said lowering-side effective area (16) being supplied with pressurizing media for lowering a load, and in which lowering-side effective area (16) the lowering pressure is prevalent; and a first pressure sensor (19) for detecting the lifting pressure, and a second pressure sensor (20) for detecting the lowering pressure; and an adjustable pressure valve (35) for adjusting a lowering pressure; and an electronic control unit (70) which is supplied the pressures detected by the pressure sensors (19, 20) and by way of which the pressure valve (35) is actuatable, characterized in that the electronic control unit (70) is designed to carry out a method according to one of Patent Claims 1 to 11.

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