Servohydraulic Drive

Abstract

A servohydraulic drive includes a hydrostatic displacement machine, an electric machine that is mechanically speed-coupled with the displacement machine, a hydraulic cylinder that is fluidically connected to the displacement machine via first and second working lines, a hydraulic accumulator, and a supply unit. The displacement machine has a stroke that is adjustable via a hydraulic adjustment device. The cylinder is configured to be activated by reversal of the fluid flow through the displacement machine in opposite directions. The accumulator is preset to a low pressure and is fluidically connected via a valve assembly in each case to the lower pressure working line. The supply unit is configured to supply the adjustment device with pressurized fluid under the necessary pressure for the adjustment regardless of the present pressure in the working lines such that the displacement machine is configured for an active and load pressure-independent adjustment of its stroke volume.

Claims

1. A servohydraulic drive, comprising: a hydrostatic displacement machine having a stroke volume configured to be adjusted by a hydraulic adjustment device; an electric machine that is mechanically speed-coupled with the displacement machine; a hydraulic cylinder that is fluidically connected to the displacement machine via a first working line and a second working line, the hydraulic cylinder configured to be activated by reversal of the fluid flow through the displacement machine in opposite directions; a low-pressure hydraulic accumulator that is preset to a low pressure and fluidically connected via a valve assembly in each case to one of the first and second working lines in which the lower pressure prevails; and a supply unit configured to supply the adjustment device with pressurized fluid under the necessary actuating pressure for the adjustment regardless of the present pressure in the first and second working lines.

2. The servohydraulic drive according to claim 1, further comprising a switching valve that has (i) a first switching position in which the hydraulic adjustment device is subjected to actuating pressure and (ii) a second switching position in which the hydraulic adjustment device is relieved of pressure to a pressure that is less than a minimum actuating pressure.

3. The servohydraulic drive according claim 2, wherein, in the second switching position, a first port of the switching valve connected to the hydraulic adjustment device is open to a second port of the switching valve connected to a relief fluid path leading to the low-pressure hydraulic accumulator.

4. The servohydraulic drive according to claim 3, wherein the relief fluid path leads to the low-pressure hydraulic accumulator while bypassing a leakage port of the displacement machine.

5. The servohydraulic drive according to claim 2, wherein the switching valve is integrated in the supply unit.

6. The servohydraulic drive according to claim 1, wherein the supply unit includes an auxiliary pump and an electric motor configured to drive the auxiliary pump, the auxiliary pump having (i) a pressure port by which the hydraulic adjustment device is configured to be supplied with pressurized fluid and (ii) a suction port that is connected to the low-pressure hydraulic accumulator.

7. The servohydraulic drive according to claim 6, wherein the pressure port of the auxiliary pump is configured to be connected to a relief fluid path leading to the low-pressure hydraulic accumulator.

8. The servohydraulic drive according to claim 7, wherein one or more of a filter device and a cooling device configured for the pressurized fluid is situated in the relief fluid path.

9. The servohydraulic drive according to claim 7, wherein the pressure port of the auxiliary pump is connected directly to the hydraulic adjustment device and is configured to be connected via the switching valve to the relief fluid path.

10. The servohydraulic drive according to claim 3, wherein the supply unit includes an actuating pressure hydraulic accumulator, wherein the switching valve has a third port that is connected to the actuating pressure hydraulic accumulator, and wherein the first port of the switching valve is connected to (i) the third port in the first switching position and (ii) the second port in the second switching position.

11. The servohydraulic drive according to claim 6, further comprising a switching valve that has (i) a first switching position in which the hydraulic adjustment device is subjected to actuating pressure and (ii) a second switching position in which the hydraulic adjustment device is relieved of pressure to a pressure that is less than a minimum actuating pressure, wherein, in the second switching position, a first port of the switching valve connected to the hydraulic adjustment device is open to a second port of the switching valve connected to a relief fluid path leading to the low-pressure hydraulic accumulator, wherein the supply unit includes an actuating pressure hydraulic accumulator, wherein the switching valve has a third port that is connected to the actuating pressure hydraulic accumulator, wherein the first port of the switching valve is connected to (i) the third port in the first switching position and (ii) to the second port in the second switching position, and wherein the actuating pressure hydraulic accumulator is configured to be charged by the auxiliary pump.

12. The servohydraulic drive according to claim 11, further comprising a second switching valve that has (i) a third switching position in which the auxiliary pump is connected to the actuating pressure hydraulic accumulator via the second switching valve of the pressure port and (ii) a fourth switching position in which the auxiliary pump is connected to the relief fluid path leading to the low-pressure hydraulic accumulator via the second switching valve of the pressure port.

13. The servohydraulic drive according to claim 10, wherein the actuating pressure hydraulic accumulator is configured to be charged from one of the first and second working lines in which the higher pressure is conducted.

14. The servohydraulic drive according to claim 10, wherein the actuating pressure hydraulic accumulator is configured as a pressure booster having (i) a large piston face that is adjacent to a low-pressure chamber and (ii) a small piston face that is adjacent to a high-pressure chamber and mechanically connected to the large piston face, and wherein the low-pressure chamber is connected to the low-pressure hydraulic accumulator and the high-pressure chamber is connected to the third port of the switching valve.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Four exemplary embodiments of a servohydraulic drive according to the disclosure are represented as a hydraulic circuit in the drawings. The disclosure shall now be explained more closely with the aid of these drawings.

[0019] In the figures:

[0020] FIG. 1 shows the first exemplary embodiment, in which an auxiliary pump directly supplies the adjustment device with pressure fluid,

[0021] FIG. 2 shows the second exemplary embodiment, in which an actuating pressure hydraulic accumulator is chargeable from the auxiliary pump,

[0022] FIG. 3 shows the third exemplary embodiment, in which an actuating pressure hydraulic accumulator is charged from the working lines, and an actuating pressure hydraulic accumulator is chargeable from the auxiliary pump,

[0023] FIG. 4 shows the fourth exemplary embodiment, in which an actuating pressure hydraulic accumulator is designed as a pressure booster.

DETAILED DESCRIPTION

[0024] In all four exemplary embodiments, the servohydraulic drive comprises a hydrostatic displacement machine 10, which can be operated both as a pump and also as a motor and whose stroke volume can be adjusted with the aid of an adjustment device 11 between a minimum value and a maximum value. The displacement machine has a first working port 12 and a second working port 13 and is for example an axial piston machine in swash plate design, in which the stroke volume is adjusted by a pivoting of the swash plate. To the displacement machine there is mechanically coupled, without an interposed gearing, a variable-speed electric machine 14, wherein, by means of this coupling, the electric machine when operating as a motor can drive the displacement machine in both directions of rotation in its operation as a pump, and when operating as a generator can be driven in both directions of rotation by the displacement machine operating as a motor. The rotary speed of the machine combination consisting of the displacement machine and the electric machine is detected by a speed sensor 15.

[0025] The servohydraulic drive furthermore comprises a hydraulic cylinder 16, which is designed as a differential cylinder having a piston 17 and a one-sided piston rod 18 and is thus designed with a piston-rod-side, ring-shaped cylinder chamber 19 and with a piston-rod-averted cylinder chamber 20 shaped as a circular disk in cross section. The cylinder chamber 20 is connected without an interposed valve directly via a working line 21 to the working port 12 of the displacement machine 10. In a working line 22 between the working port 13 of the displacement machine 10 and the cylinder chamber 19 of the hydraulic cylinder 16 there is situated a lowering brake valve 23, which combines in itself multiple functions, namely, the holding of a load pulling on the piston rod 18 of the hydraulic cylinder, the limiting of the pressure occurring in the cylinder chamber 19, the controlling for a desired lowering of a load, and a check valve function for the lifting of a load. The lowering brake valve 23 divides the working line 22 into a section near the displacement machine 24 and a section near the cylinder 25.

[0026] The servohydraulic drive furthermore comprises a hydropneumatic accumulator 29, called for short in the following a hydraulic accumulator, which is designed for example as a bladder accumulator or membrane accumulator and which is preset to a low pressure of, for example, 5 bar. The hydraulic accumulator 29 is connected to a low-pressure collecting line 30. This is fluidically connected to the working line 21 via a check valve 31, opening toward said working line 21, and to the section of the working line 22 situated between the working port 13 and the lowering brake valve 23 via a check valve 32 opening toward said working line 22. Between the low-pressure collecting line 30 and the working line 21 are furthermore situated a pressure limiting valve 33, by which the pressure in the working line 21 and thus that in the cylinder chamber 20 is limited to a maximum value, and a releasable check valve 34, which opens toward the working line 21 and is releasable by a pressure in the section 24 of the working line 22. After a releasing of the releasable check valve 34, pressure fluid can flow from the working line 21 to the low-pressure collecting line 30 and thus to the hydraulic accumulator 29. Between the low-pressure collecting line 30 and the section 24 of the working line 22 there are furthermore situated a pressure limiting valve 35, by which the pressure in the section 24 of the working line 22 and thus in the cylinder chamber 19 is limited to a maximum value, and a releasable check valve 36, which opens toward the section 24 of the working line 21 and is releasable by a pressure in the working line 21. After a releasing of the releasable check valve 36, pressure fluid can flow from the section 24 of the working line 21 to the low-pressure collecting line 30.

[0027] The check valve 31 and the pressure limiting valve 33 as well as the check valve 32 and the pressure limiting valve 35 are usually in each case combined into a so-called pressure replenishing valve.

[0028] Finally, a leakage port 37 of the displacement machine 10 is fluidically connected via a check valve 38 opening toward the low-pressure collecting line 30 and thus toward the hydraulic accumulator 29 to the hydraulic accumulator 29.

[0029] Various sensors serve for the monitoring and the control of the servohydraulic drive. With the speed sensor 15, as already mentioned, the rotary speed of the hydrostatic displacement machine 10 and the electric machine 14 is detected. With a temperature sensor 46, the temperature of the leakage fluid flowing from the displacement machine 10 to the low-pressure collecting line 30 is detected. With a pressure sensor 47, the pressure in the low-pressure collecting line 30 and thus in the hydraulic accumulator 29 is detected. With a pressure sensor 48, the pressure in the cylinder chamber 20 is detected. With a pressure sensor 49, the pressure in the cylinder chamber 19 is detected. With an acceleration sensor 50, the acceleration of the piston 17 of the hydraulic cylinder 16 is detected. From the acceleration, the velocity can be calculated by integration, and by a further integration the position of the piston 17.

[0030] The adjustment device 11 for the stroke volume of the displacement machine 10 comprises an actuating cylinder 55, in which an actuating piston 57 adjacent to an actuating chamber 56 can move in the lengthwise direction. The actuating piston 57 is mechanically connected to the swash plate of the displacement machine 10 in such a way that a pressure force can be exerted by it on the swash plate in order to turn the swash plate about a swivel axis and move it into the position with small stroke volume. Acting counter to the force exerted by the actuating piston 57 on the swash plate, and in the opposite swivel direction of the swash plate, are the force of a return spring 58, which, contrary to the schematic illustration in the figures, is supported not against the activating piston but instead directly against the swash plate, and, on account of an eccentric arrangement of the swivel axis relative to the axis of rotation of the drive shaft of the displacement machine 10, in particular the power unit forces. A supply unit 59 serves for the supplying of the adjustment device 11 with pressure fluid. For this, in the exemplary embodiment per FIG. 1, an auxiliary pump 60 with a constant stroke volume is provided, which can be driven by a simple electric motor 61 in a single direction of rotation. The auxiliary pump 60 has a suction port 62, which is connected via a manually activatable valve 63 to the low-pressure collecting line 30, and a pressure port 64, to which the actuating chamber 56, a pressure limiting valve 65, and a first port 66a of a 2/2-way valve 66 situated in the bypass past the pressure limiting valve 65 are connected. The directional valve 66 under the action of a compression spring assumes an opened rest position and can be switched electromagnetically into a blocked position. Downstream from the pressure limiting valve 65 and a second port 66b directional valve 66, there are situated in a relief fluid path 72 in series with each other a filter device 67 having a filter element 68 and having a bypass check valve 69 situated in the bypass past the filter element, which bypass check valve opens when the pressure drop across the filter element exceeds a certain value, and a cooling device 70 downstream from the filter device 67. Pressure fluid cooled in the cooling device 70 flows via a manually activatable valve 71 situated in the relief fluid path 72 into the low-pressure collecting line 30.

[0031] It shall now be assumed that, in the exemplary embodiment of FIG. 1, the displacement machine 10 is set at maximum stroke volume and pressure fluid is being delivered into the working line 21 and thus into the cylinder chamber 20. The load pressure is at first low. The directional valve 66 is open. The auxiliary pump 60 is driven by the electric motor 61 and delivers pressure fluid from the low-pressure collecting line 30 via the directional valve 66, the filter device 67 and the cooling device 70 back to the low-pressure collecting line, while the pressure at the pressure port 64 and thus in the actuating chamber 56 is only slightly above the low pressure, due to the hydraulic resistances of the filter device 67 and the cooling device 70. If the resistance to a further extending of the piston rod 18 and thus the load pressure increases, the displacement machine should be adjusted to the minimum stroke volume. For this, the directional valve 66 is closed. The pressure at the pressure port 64 of the auxiliary pump 60 and thus in the actuating chamber 56 increases and reaches the level necessary for the adjustment, such as 50 bar, so that the actuating piston 57 extends and the swash plate swivels against the force of the return spring 58 and against the power unit forces. The pressure in the actuating chamber 56 is limited by the pressure limiting valve 65 to a value slightly above the maximum necessary actuating pressure.

[0032] If the displacement machine is to be adjusted once more to the maximum stroke volume, the directional valve 66 is opened. As a result, the actuating chamber 56 is relieved of pressure and the return spring and the power unit forces are able to swivel the swash plate back to maximum stroke volume.

[0033] Whereas in the exemplary embodiment of FIG. 1 the pressure port 64 of the auxiliary pump 60 is connected directly to the actuating chamber 56, the exemplary embodiment of FIG. 2 comprises an actuating pressure hydraulic accumulator 75 in the pressurized fluid supply for the adjustment device 11. A 3/2-way switching valve 76 is present, which connects the pressure port 64 of the auxiliary pump 60, in a rest position which said valve assumes under the action of a spring, to the series circuit of the filter device 67 and the cooling device 70, bypassing a pressure limiting valve 65 present as in the exemplary embodiment of FIG. 1, and, in an activated switching position, to the actuating pressure hydraulic accumulator 75. The pressure limiting valve 65 is connected to this connection between the directional valve 76 and the actuating pressure hydraulic accumulator 75 and limits the accumulator pressure.

[0034] A further 3/2-way switching valve 77 is present, which connects the actuating chamber 56 via a first port 77a and a second port 77b, in a rest position, to the bypass past the pressure limiting valve 65 and via the first port 77a and a third port 77c, in a switched position, to the actuating pressure hydraulic accumulator 75.

[0035] It shall now be assumed once again that, in the exemplary embodiment of FIG. 2, the displacement machine 10 is set to maximum stroke volume and pressure fluid is being delivered to the working line 21 and thus to the cylinder chamber 20. The load pressure is at first low. The directional valves 76 and 77 are in their rest positions, as shown in FIG. 2, so that the auxiliary pump bypasses the pressure limiting valve 65 and delivers to the filtering and cooling circuit, and the actuating chamber 56 is relieved of pressure. If the resistance to a further extending of the piston rod 18 and thus the load pressure increases, the displacement machine should be adjusted to the minimum stroke volume. For this, the directional valve 77 is switched over, so that the actuating chamber 56 is connected to the actuating pressure hydraulic accumulator 75. The pressure in the actuating chamber 56 increases to the accumulator pressure, so that the actuating piston 57 extends and the swash plate swivels against the force of the return spring 58 and against the power unit forces. The directional valve 76 is switched over for the charging of the actuating pressure hydraulic accumulator 75, so that the actuating pressure hydraulic accumulator 75 is connected to the pressure port of the auxiliary pump 60. This charging of the actuating pressure hydraulic accumulator 75 can occur at any time within a movement cycle of the hydraulic cylinder 16. The pressure in the actuating pressure hydraulic accumulator 75 can be limited by the pressure limiting valve 65 to a value making possible one or more setting sequences with no additional charging.

[0036] If the displacement machine is to be adjusted once more to the maximum stroke volume, the directional valve 77 is placed in its rest position as shown in FIG. 2. As a result, the actuating chamber 56 is relieved of pressure and the return spring and the power unit forces are able to swivel the swash plate back to maximum stroke volume.

[0037] In the exemplary embodiment of FIG. 3, no auxiliary pump and no filter device and no cooling device are provided. The actuating pressure hydraulic accumulator 75 designed as a bladder accumulator or as a membrane accumulator as in the exemplary embodiments of FIGS. 1 and 2 is charged via a selector circuit, consisting of two check valves 78 and 79, respectively from the working port 12 or 13 of the displacement machine 10 in which the higher pressure is present relative to the other working port. The pressure limiting valve 65 in the exemplary embodiment of FIG. 3, with its inlet connected to the actuating pressure hydraulic accumulator 75 and with its outlet connected to the low-pressure collecting line 30, serves for the mere safeguarding of the accumulator and should as far as possible not be triggered, since when the pressure limiting valve is triggered pressure fluid would be removed needlessly from the working lines. As in the exemplary embodiment of FIG. 2, a 3/2-way switching valve 77 is present. The actuating chamber 56 of the adjusting device 11 is connected in the rest position of the directional switching valve 77 to the low-pressure collecting line 30 and thus is relieved of pressure. In the activated switching position of the directional switching valve 77, the actuating chamber 56 is connected to the actuating pressure accumulator 75 and thus subjected to the high accumulator pressure. Hence, by switching over the directional valve 77, the displacement machine is adjusted from minimum stroke volume to maximum stroke volume and vice versa.

[0038] In the exemplary embodiment of FIG. 4, the actuating pressure hydraulic accumulator is designed as a pressure booster 80 having a large piston face 81, adjacent to a low-pressure chamber 82 connected to the low-pressure collecting line 30 and thus to the low-pressure accumulator, and having a small piston face 83, adjacent to a high-pressure chamber 84 and mechanically connected to the large piston face 81. The piston faces 81 and 83 are located on a stepped piston 85, with the space at the step being relieved to the atmosphere. As in the exemplary embodiment of FIG. 3, the high-pressure chamber 84 of the pressure booster 80 is charged via a selector circuit consisting of two check valves 78 and 79 respectively from the working port 12 or 13 of the displacement machine 10 in which the higher pressure prevails compared to the other working port.

[0039] As in the exemplary embodiment of FIGS. 2 and 3, a 3/2-way switching valve 77 is present. The actuating chamber 56 of the adjustment device 11 in the rest position of the directional switching valve 77 is connected to the low-pressure collecting line 30 and thus relieved of pressure. In the activated switching position of the directional switching valve 77, the actuating chamber 56 is connected to the high-pressure chamber 84 of the pressure booster 80 and thus subjected to high pressure. Hence, by switching over the directional switching valve 77, the displacement machine is adjusted from minimum stroke volume to maximum stroke volume and vice versa.

[0040] If it is assumed that the ratio between the large piston face 81 and the small piston face 83 is equal to 10 and the low pressure in the low-pressure hydraulic accumulator 29 is 5 bar, then at equilibrium of forces there is a pressure of 50 bar on the stepped piston 85 in the high-pressure chamber 84. If the pressure on one of the working ports 12 or 13 of the displacement machine is higher than 50 bar, pressure fluid will flow via the corresponding check valve 78 or 79 to the high-pressure chamber. The stepped piston 85 will be displaced, under decreasing volume of the low-pressure chamber 82, as far as an end stop, so that the pressure in the high-pressure chamber will increase, under compression of the pressure fluid in the high-pressure chamber 84, up to a maximum pressure which is present at the working port with the higher pressure. After this, the displacement machine 10 can be adjusted from the large stroke volume to the small stroke volume, even if the actuating pressure needed for the adjustment is present neither at the working port 12 nor at the working port 13 of the displacement machine. During an adjustment, compression pressure fluid will at first be consumed, before then, with further consumption of pressure fluid during the present or a subsequent adjustment, the stepped piston 85 of the pressure booster moves so as to reduce the volume of the high-pressure chamber and maintaining a pressure of 50 bar in the high-pressure chamber 84 of the pressure booster.

[0041] In the two exemplary embodiments of FIGS. 3 and 4, the charging of the actuating pressure hydraulic accumulator 75 or 80 is only possible in defined positions of the hydraulic cylinder 16, especially in the positions in which the piston rod 18 is fully retracted or fully extended, and/or when a sufficiently high pressure (load pressure) is present at the two check valves.

LIST OF REFERENCE NUMBERS

[0042] 10 Hydrostatic displacement machine

[0043] 11 Adjustment device for stroke volume

[0044] 12 First working port of 10

[0045] 13 Second working port of 10

[0046] 14 Electric machine

[0047] 15 Speed sensor

[0048] 16 Hydraulic cylinder

[0049] 17 Piston of 16

[0050] 18 Piston rod of 16

[0051] 19 Piston-rod-side cylinder chamber of 16

[0052] 20 Piston-rod-averted cylinder chamber of 16

[0053] 21 Working line

[0054] 22 Working line

[0055] 23 Lowering brake valve

[0056] 24 Section of 22

[0057] 25 Section of 22

[0058] 29 Hydraulic accumulator

[0059] 30 Low-pressure collecting line

[0060] 31 Check valve

[0061] 32 Check valve

[0062] 33 Pressure limiting valve

[0063] 34 Releasable check valve

[0064] 35 Pressure limiting valve

[0065] 36 Releasable check valve

[0066] 37 Leakage port of 10

[0067] 46 Temperature sensor

[0068] 47 Pressure sensor

[0069] 48 Pressure sensor

[0070] 49 Pressure sensor

[0071] 50 Acceleration sensor

[0072] 55 Actuating cylinder

[0073] 56 Actuating chamber

[0074] 57 Actuating piston

[0075] 58 Return spring

[0076] 58 Supply unit

[0077] 60 Auxiliary pump

[0078] 61 Electric motor

[0079] 62 Suction port of 60

[0080] 63 Valve

[0081] 64 Pressure port of 60

[0082] 65 Pressure limiting valve

[0083] 66 2/2-way valve

[0084] 66a First port of 66

[0085] 66b Second port of 66

[0086] 67 Filter device

[0087] 68 Filter element of 67

[0088] 69 Bypass check valve of 67

[0089] 70 Cooling device

[0090] 71 Valve

[0091] 72 Relief fluid path

[0092] 75 Actuating pressure hydraulic accumulator

[0093] 76 3/2-way switching valve

[0094] 77 3/2-way switching valve

[0095] 77a First port of 77

[0096] 77b Second port of 77

[0097] 77c Third port of 77

[0098] 80 Pressure booster

[0099] 81 Large piston face of 80

[0100] 82 Low-pressure chamber of 80

[0101] 83 Small piston face of 80

[0102] 84 High-pressure chamber

[0103] 85 Stepped piston of 80