ELECTRO-HYDROSTATIC ACTUATION SYSTEM

Abstract

An electro-hydrostatic actuation system and a method for driving a hydraulic actuator, e.g. a hydraulic cylinder, are described, wherein the system comprising a leakage branch, and wherein preferably an additional pump is arranged. The system further comprises a source for providing hydraulic liquid; a high-pressure circuit to direct the hydraulic liquid to a hydraulic actuator, such as e.g. a hydraulic cylinder; a low-pressure circuit having several branches; a main pump for hydraulic liquid arranged in the high-pressure circuit, comprising a housing having a high-pressure section and a low-pressure section, separated by gap sealings, wherein the high-pressure section comprises a first outlet and a second outlet to provide the hydraulic liquid flow in the high-pressure circuit; and wherein the low- pressure section comprises a leakage outlet; an electric motor driving the main pump.

Claims

1. An electro-hydrostatic actuation system for driving a hydraulic actuator, comprising: a source for providing hydraulic liquid; a high pressure circuit to direct the hydraulic liquid to a hydraulic actuator; a low pressure circuit having multiple branches; a main pump for hydraulic liquid arranged in the high pressure circuit; the main pump comprising a housing having a high pressure section and a low pressure section, separated by sealing gaps, wherein the high pressure section comprises a first outlet and a second outlet to provide the hydraulic liquid flow in the high pressure circuit; and wherein the low pressure section comprises a leakage outlet; an electric motor driving the main pump; a leakage branch connecting the hydraulic leakage outlet of the low pressure section of the housing of the main pump to the low pressure circuit, wherein an additional pump is arranged; and a flushing branch connecting a flushing inlet of the low pressure section of the housing of the main pump to the low pressure circuit, and having a hydraulic connection with the leakage branch, wherein a unidirectional check valve is arranged in the flushing branch.

2. An electro-hydrostatic actuation system according to claim 1, comprising a first valve and a second valve separating the high pressure circuit from the low pressure circuit.

3. An electro-hydrostatic actuation system according to claim 2, wherein the first valve and the second valve are check valves or control valves.

4. (canceled)

5. (canceled)

6. An electro-hydrostatic actuation system according to claim 1, comprising an additional flushing branch connecting the flushing inlet of the low pressure section of the housing of the main pump to the high pressure circuit, wherein an additional valve is arranged, having a hydraulic connection with the leakage branch, and wherein the hydraulic connection between the high pressure circuit with the leakage branch comprises additional pressure-controlled valves.

7. (canceled)

8. (canceled)

9. An electro-hydrostatic actuation system according to claim 1, wherein the additional pump has a delivery volume larger than a leakage volume of the main pump occurring in the low pressure section of the housing of the main pump.

10. An electro-hydrostatic actuation system according to claim 1, comprising an additional valve is arranged in hydraulic connection with the leakage branch, before the additional pump, and having a hydraulic connection with the low-pressure circuit.

11. An electro-hydrostatic actuation system according to claim 1, comprising a filter unit having a hydraulic connection with the leakage branch to filter the hydraulic liquid volume delivered through the additional pump.

12. An electro-hydrostatic actuation system according to claim 1, comprising a cooling unit having a hydraulic connection with the leakage branch to cool down or heat up the hydraulic liquid volume delivered through the additional pump.

13. (canceled)

14. An electro-hydrostatic actuation system according to claim 1, wherein the electric motor has a variable speed and the main pump has a constant volume, or the electric motor has a constant speed and the main pump is a variable displacement pump, or the electric motor has a variable speed and the main pump is a variable displacement pump.

15. An electro-hydrostatic actuation system according to claim 1, wherein the additional pump is operatively configured to controls the pressure in the low pressure section of the housing of the main pump.

16. An electro-hydrostatic actuation system according to claim 1, wherein an operative pressure difference between the pressure in the low pressure circuit and the low pressure section of the housing of the main pump does not fall below a predefined value.

17. An electro-hydrostatic actuation system according to claim 16, wherein the predefined value of the pressure difference between the pressure in the low pressure circuit and the low pressure section of the housing of the main pump is in a range from 0.2 to 20 bar.

18. An electro-hydrostatic actuation system according to claim 1, wherein an operative pressure in the low pressure section of the housing of the main pump is defined by a difference between a flow of the additional pump and a leakage flow deriving from the sealing gaps separating the high pressure section from the low pressure section of the housing of the main pump and a hydraulic resistance in the flushing branch.

19. A method of using the electro-hydrostatic actuation system according to claim 1, comprising the step of driving self-sufficient or autonomous axles.

20. An electro-hydrostatic actuation system according to claim 1, wherein the hydraulic actuator is selected from a group consisting of a hydraulic cylinder, a double-rod or synchronous cylinder, a pivoting drive, a hydraulic rotary drive and/or a differential cylinder.

21. An electro-hydrostatic actuation system according to claim 6, wherein the additional pressure-controlled valves of the hydraulic connection between the high pressure circuit with the leakage branch comprise pressure-controlled unidirectional check valves.

22. An electro-hydrostatic actuation system according to claim 17, wherein the predefined value of the pressure difference between the pressure in the low pressure circuit and the low pressure section of the housing of the main pump is in a range from 0.5 to 10 bar.

23. An electro-hydrostatic actuation system according to claim 22, wherein the predefined value of the pressure difference between the pressure in the low pressure circuit and the low pressure section of the housing of the main pump is in a range from 1 to 5 bar.

24. An electro-hydrostatic actuation system according to claim 14, wherein the electric motor is a servo-motor and the main pump is static, or the electric motor is a constant-motor and the main pump is a variable displacement pump, or the electric motor is a servo-motor and the main pump is a variable displacement pump.

Description

[0079] FIG. 1a shows a cross-sectional view of the internal structure of a pump with external leakage oil connection, suggesting the pressure conditions within the housing. The pump comprises a first outlet 300; a second outlet 350, in hydraulic connection with, and defining, a high-pressure circuit in an electro-hydrostatic actuation system or hydraulic circuit such as 100 as shown in FIGS. 2-8; gap sealings/seals 320, on which an additional pump may act for regulating the low pressure in the low-pressure section of the main pump housing; a shaft seal 310 limiting the permissible pump housing pressure; and external leakage outlet 340 in hydraulic connection with a leakage branch. In the low-pressure section of the pump housing 330 the pressure values equal the pressure of the hydraulic liquid leakage. The pump comprises a drive shaft 360 for connection with the electric motor.

[0080] FIG. 1b shows the derating curve for an electro-hydrostatic actuation system, wherein the pressure in the housing of the main pump is plotted against the speed at which the system is operated. In particular, as discussed above, the housing pressure at speeds higher than 1800 r/min decreases at increasing speeds. In fact, this means that the operation of an electro-hydrostatic actuation system at the maximum allowable pressure in the low-pressure section of the housing of the main pump in the system is limited by the process dynamics. Nevertheless, the shaft seal of the pump has a pressure limitation that, for example, in the case of a rotary shaft seal, allows a maximum pressure of 10 bar in the low-pressure section of the main pump, which decreases, e.g. down to 4 bar at 4500 RPM, when the system is run at higher speeds, wherein higher speeds is intended e.g. as speeds starting from 1800 RPM. Therefore, in the systems found in the state of the art, the pressure in the low-pressure section of the main pump, when the motor-pump unit is operated up to 4500 RPM, is limited e.g. to a maximum of 4 bar, and the rotary shaft seal is pressurized at said 4 bar.

[0081] In a self-contained axis system, a lower pressure (such as e.g. the above-mentioned 4 bar) in the low-pressure section of the main pump requires a larger low-pressure hydro-accumulator, compared to a system with a higher pressure (e.g. 10 bar) in the low-pressure section of the main pump. Owing to the need for a larger low-pressure hydro-accumulator resulting from the lower low pressure in the low-pressure section of the main pump, such systems are less compact.

[0082] Furthermore, owing to the lower pressure in the low-pressure section of the main pump, the actuator of a self-contained axis is less clamped. As a result, the elastic modulus of the axis is smaller, leading to a smaller natural frequency, and consequently to a far less effective control of the axis.

[0083] FIG. 2 shows an electro-hydrostatic actuation system or hydraulic circuit 100 according to one embodiment of the present invention. In the figure, the actuation system 100 is represented in connection with a hydraulic actuator, e.g. a hydraulic cylinder 101. The actuation system or hydraulic circuit 100 comprises a source or accumulator 102, an electric motor 112 driving the main pump 107. The main pump 107, comprising the outlet 108 and the outlet 109 to provide hydraulic flow of the hydraulic liquid in the high-pressure circuit 103, is provided with a leakage outlet 110 in hydraulic connection with the low-pressure circuit 104 through the leakage branch 113. An additional pump 114 is arranged on the leakage branch 113 in order to promote the leakage flow through the low-pressure circuit 104. A first valve 105 and a second valve 106 are arranged in the low-pressure circuit 104 in order to provide parametric control of the hydraulic flow in the actuation system or hydraulic circuit 100.

[0084] FIG. 3 shows an electro-hydrostatic actuation system or hydraulic circuit 100 according to another embodiment of the present invention. In the figure, the actuation system 100 is represented in connection with a hydraulic actuator, e.g. a hydraulic cylinder 101. The actuation system or hydraulic circuit 100 comprises a source or accumulator 102, an electric motor 112 driving the main pump 107. The main pump 107, comprising the outlet 108 and the outlet 109 to provide hydraulic flow of the hydraulic liquid in the high-pressure circuit 103, is provided with a leakage outlet 110 in hydraulic connection with the low-pressure circuit 104 through the leakage branch 113. An additional pump 114 is arranged on the leakage branch 113 in order to promote the leakage flow through the low-pressure circuit 104. A first valve 105 and a second valve 106 are arranged in the low-pressure circuit 104 in order to provide parametric control of the hydraulic flow in the actuation system or hydraulic circuit 100. In FIG. 3 an additional branch or flushing circuit 200 is shown, comprising a flushing branch 115 connecting the flushing inlet 111 arranged in the low-pressure section of the main pump 107 to the low-pressure circuit 104. A valve 116 is arranged on the flushing branch in order to exert parametric control of the hydraulic flow throughout the circuit.

[0085] FIG. 4 shows an electro-hydrostatic actuation system or hydraulic circuit 100 according to another embodiment of the present invention. In the figure, the actuation system 100 is represented in connection with a hydraulic actuator, e.g. a hydraulic cylinder 101. The actuation system or hydraulic circuit 100 comprises a source or accumulator 102, an electric motor 112 driving the main pump 107. The main pump 107, comprising the outlet 108 and the outlet 109 to provide hydraulic flow of the hydraulic liquid in the high pressure circuit 103, is provided with a leakage outlet 110 in hydraulic connection with the low-pressure circuit 104 through the leakage branch 113. An additional pump 114 is arranged on the leakage branch 113 in order to promote the leakage flow through the low-pressure circuit 104. A first valve 105 and a second valve 106 are arranged in the low-pressure circuit 104 in order to provide parametric control of the hydraulic flow in the actuation system or hydraulic circuit 100. In FIG. 4 an additional branch or flushing circuit is shown, comprising a flushing branch 215 connecting the flushing inlet 111 arranged in the low-pressure section of the main pump 107 to the high-pressure circuit 103 through the control branches 217 and 218. A valve 216 is arranged on the flushing branch in order to exert parametric control of the hydraulic flow throughout the circuit. The control branches 217 and 218 comprise a first valve 219 and a second valve 220 and are arranged in order to establish and maintain a safe hydraulic connection between the flushing branch 215 and the high-pressure circuit 103.

[0086] FIG. 5 shows an electro-hydrostatic actuation system or hydraulic circuit 100 according to yet another embodiment of the present invention, which comprises the arrangement shown first in FIG. 3. Additionally to the embodiment of FIG. 3, the embodiment of FIG. 5 further comprises an additional valve 117, which is arranged before the additional pump 114 and has a hydraulic connection with the low-pressure circuit 104.

[0087] FIG. 6 shows an electro-hydrostatic actuation system or hydraulic circuit 100 according to yet another embodiment of the present invention, which comprises the arrangement shown first in FIG. 5. Additionally to the embodiment of FIG. 5, the embodiment of FIG. 6 further comprises a filter unit 118 having a hydraulic connection with the leakage branch 113 to filter the hydraulic liquid volume of delivered through the additional pump 114.

[0088] FIG. 7 shows an electro-hydrostatic actuation system or hydraulic circuit 100 according to yet another embodiment of the present invention, which comprises the arrangement shown first in FIG. 6. Additionally to the embodiment of FIG. 6, the embodiment of FIG. 7 further comprises a cooling unit 119 having a hydraulic connection with the leakage branch to cool down or heat up the hydraulic liquid volume delivered through the additional pump 114; this ensures the thermal stability of the system through providing temperature regulation of the hydraulic liquid flow.

[0089] FIG. 8 shows an electro-hydrostatic actuation system or hydraulic circuit 100 according to yet another embodiment of the present invention, which comprises the arrangement shown first in FIG. 7. Alternatively to the embodiment of FIG. 7, the embodiment of FIG. 8 shows a pressure-reducing valve 120 arranged in the flushing branch 115. The pressure-reducing valve 120 is used to regulate the housing pressure of the main pump in the actuation system to a constant low value, independently of the resulting external leakage and independently of low-pressure level/value.

[0090] FIG. 9 illustrates the hydraulic liquid volume flows in pumps with external leakage oil connection using a schematic. In the schematic V.sub.theo indicates the theoretical displacement in a variable displacement pump; the pressure values p.sub.108 and p.sub.109 are associated with the high-pressure outlets of the main pump, defining and connected with the high-pressure circuit, a third pressure value pLe represents the pressure at the leakage outlet of the main pump where from a hydraulic leakage flow is indicated as Q.sub.L, and which is separated as Q.sub.Lext or external leakage of hydraulic liquid and Q.sub.Lint or internal leakage of hydraulic liquid. The solid arrows indicate the direction of the flow of the hydraulic liquid.

[0091] FIG. 10 shows the simulation circuit utilized to test the robustness of a preferred embodiment of the present invention, as described in FIG. 7. Example 1 reports the conditions of the simulation and the results obtained under two different setups.

[0092] FIG. 11 consists of a graphical representation showing the results of the simulation executed according to the simulation circuit in FIG. 10, based on the embodiment as described in FIG. 7, for the lower pressure in the accumulator and in the housing of the main pump with a hydraulic liquid flow from the additional pump of 7 l/min and high actuator forces. The pressure reduction is of ca. 2.8 bar.

[0093] FIG. 12 consists of a graphical representation showing the results of the simulation executed according to the simulation circuit in FIG. 10, based on the embodiment as described in FIG. 7, for the lower pressure in the accumulator and in the housing of the main pump with a hydraulic liquid flow from the additional pump of 7 l/min and low actuator forces. The pressure reduction is of ca. 2.7 bar.

EXAMPLE 1—Simulation

[0094] A calculation of the system behaviour according to the embodiment described in FIG. 7 has shown robustness of this solution against load variations and resulting different leaks. The results are shown in FIGS. 10, 11, 12.

[0095] The simulation has been carried out using the software Simulation X and the following boundaries conditions: [0096] Variable speed of the electro-hydraulic actuation system with pump size 19 cm.sup.3 rotates with sine 2 Hz +/−4500 rpm; [0097] External leakage modelled according to Moog measurements (about 2.5 l/min at 350 bar); [0098] Variable speed of the electro-hydraulic actuation system goes through all 4 quadrants; [0099] Dimensions of synchronous cylinder: piston diameter 110 mm, bar diameter 50 mm each; [0100] Cylinder stroke: 50 mm; [0101] Preload in the system: approx. 8 bar; [0102] Hydraulic storage volume: 0.5 l; [0103] Cooling/filter pump constant with 7 l/min; [0104] Opening pressure check valve: 1 bar.

[0105] Simulation no. 1: load on the cylinder with sine wave 1 Hz +/−90 kN=>pressure on HP (high pressure) side adjusts to approx. 130 bar

[0106] Simulation No. 2: load on the cylinder with sine wave 1 Hz +/−0.09 kN=>Pressure on HP side adjusts to approx. 10 bar

[0107] Both simulations provided nearly identical results for the resulting low pressures and the reduced housing pressure.

TABLE-US-00001 List of elements 100 actuation system or hydraulic circuit 101 hydraulic actuator, e.g. a hydraulic cylinder 102 source or accumulator 103 high-pressure circuit 104 low-pressure circuit 105 first valve 106 second valve 107 main pump 108 first outlet 109 second outlet 110 leakage outlet 111 flushing inlet 112 electric motor 113 leakage branch 114 additional pump 115 flushing branch 116 valve 117 additional valve 118 filter unit 119 cooling unit 120 pressure-reducing valve 200 flushing circuit 215 flushing branch 216 valve 217 control branch 218 control branch 219 first valve 220 second valve 300 first outlet 310 shaft seal 320 gap sealing/seal 330 low-pressure section of the pump housing 340 leakage outlet 350 second outlet 360 drive shaft to the electric motor 510 low-pressure values in low-pressure circuit 520 lower low-pressure values in pump housing 530 low-pressure values in low-pressure circuit 540 lower low-pressure values in pump housing