HYDRAULIC ACTUATOR

Abstract

A hydraulic actuator including a cylinder housing surrounding a cylinder cavity. A piston is in the cylinder cavity and separates the cylinder cavity into a first and a second pressure chamber. A connection between the first and the second pressure chamber for a hydraulic fluid is provided. A piston rod extends from the piston along an actuation direction. A locking device can be selectively switched between a locking mode and a non-locking mode, wherein in the locking mode the locking device prevents movement of the piston and in the non-locking mode does not prevent movement of the piston. A first arrangement of piezoelectric elements is inside the first pressure chamber which first arrangement of piezoelectric elements is adapted to selectively contract or expand to change the volume of the first pressure chamber occupied by the piezoelectric elements of the first arrangement of piezoelectric elements.

Claims

1. A hydraulic actuator comprising: a cylinder housing, a piston, a piston rod and a locking device; wherein the cylinder housing surrounds a cylinder cavity, wherein the piston is in the cylinder cavity, separates the cylinder cavity into a first pressure chamber and a second pressure chamber and is movable along an actuation direction, wherein a volume of the first and the second pressure chamber changes, when the piston moves along the actuation direction, wherein a connection between the first and the second pressure chamber for a hydraulic fluid is provided, wherein the piston rod extends from the piston along the actuation direction through the cylinder housing to an exterior of the cylinder housing, and wherein the locking device can be selectively switched between a locking mode and a non-locking mode, wherein in the locking mode the locking device prevents movement of the piston and in the non-locking mode does not prevent movement of the piston, and a first arrangement of piezoelectric elements is inside the first pressure chamber and the first arrangement of piezoelectric elements is adapted to selectively contract or expand to change the volume of the first pressure chamber occupied by the piezoelectric elements of the first arrangement of piezoelectric elements.

2. The hydraulic actuator according to claim 1, wherein a first diaphragm is in the first pressure chamber, wherein the first diaphragm separates the first arrangement of piezoelectric elements from a part of the first pressure chamber which can be filled with a hydraulic fluid.

3. The hydraulic actuator according to claim 1, wherein a second arrangement of piezoelectric elements is inside the second pressure chamber and the second arrangement of piezoelectric elements is adapted to selectively contract or expand to change the volume of the second pressure chamber occupied by the piezoelectric elements of the second arrangement of piezoelectric elements.

4. The hydraulic actuator according to claim 3, wherein a second diaphragm is in the second pressure chamber, wherein the second diaphragm separates the second arrangement of piezoelectric elements from a part of the second pressure chamber which can be filled with a hydraulic fluid.

5. The hydraulic actuator according to claim 1, wherein the connection between the first and the second pressure chamber for a hydraulic fluid are is provided by one or more internal connections, wherein each internal connection is formed by an aperture in the piston, and/or wherein the connection between the first and the second pressure chamber for a hydraulic fluid is provided by one or more external connections, wherein each external connection is formed by a fluid line connecting the first and the second pressure chamber.

6. The hydraulic actuator according to claim 5, wherein each connection between the first and the second pressure chamber for a hydraulic fluid comprises a flow control for selectively varying a flow of a hydraulic fluid through the respective fluid connection, wherein the flow control comprises active MEMS valves or active one-way disc-valves or magnetorheological valves.

7. The hydraulic actuator according to claim 5, wherein each connection between the first and the second pressure chamber for a hydraulic fluid is provided with a throttle or a hydraulic screen locally reducing the flow through the respective connection between the first and the second pressure chamber for a hydraulic fluid.

8. The hydraulic actuator according to claim 5, wherein the connections between the first and the second pressure chamber for a hydraulic fluid only are formed by a first set of fluid lines or apertures only enabling a flow of a hydraulic fluid from the first pressure chamber to the second pressure chamber and wherein a second set fluid lines only enabling a flow of a hydraulic fluid from the second pressure chamber to the first pressure chamber.

9. The hydraulic actuator according to claim 1, wherein the locking device is formed by a multi-disk brake engaging the piston rod, wherein the multi-disk brake is switched between the locking and the non-locking mode by a brake arrangement of piezoelectric elements, wherein the piezoelectric elements of the brake arrangement of piezoelectric elements preferably switches the multi-disc brake via a hydraulic enhancer.

10. The hydraulic actuator according to claim 1, wherein the locking device is a hydraulic locking device, wherein the hydraulic locking device is formed by a brake cylinder housing forming a brake cylinder cavity, wherein the piston rod extends along the actuation direction through the brake cylinder cavity, wherein a brake piston is attached to the piston rod, the brake piston being inside the brake cylinder cavity and separating the brake cylinder cavity into a first brake pressure chamber and a second brake pressure chamber, wherein a connection between the first brake pressure chamber and the second brake pressure chamber for a hydraulic fluid is provided, and comprising flow control for selectively preventing and enabling flow of a hydraulic fluid between the first and the second brake pressure chamber.

11. The hydraulic actuator according to claim 1, comprising a hydraulic fluid reservoir, wherein a connection between the first and/or the second pressure chamber for a hydraulic fluid is provided such that a hydraulic fluid can flow from the hydraulic fluid reservoir to the first and/or the second pressure chamber or from the first and/or the second pressure chamber to the hydraulic fluid reservoir, and wherein a third arrangement of piezoelectric elements is inside the hydraulic fluid reservoir for actively providing hydraulic fluid to the first and/or the second pressure chamber or actively removing hydraulic fluid from the first and/or the second pressure chamber.

12. The hydraulic actuator according to claim 1 further comprising a control configured to control the first arrangement of piezoelectric elements and the locking device according to a following sequence of steps: in a first step the locking device is controlled to switch to the non-locking mode; in a second step the first arrangement of piezoelectric elements is controlled to expand such that the volume of the first pressure chamber occupied by the first arrangement of piezoelectric elements is increased; in a third step the locking device is controlled to switch to the locking mode; and in a fourth step the first arrangement of piezoelectric elements is controlled to contract such that the volume of the first pressure chamber occupied by the first arrangement of piezoelectric elements is reduced.

13. The hydraulic actuator according to claim 12, wherein a second arrangement of piezoelectric elements is inside the second pressure chamber and the second arrangement of piezoelectric elements is adapted to selectively contract or expand to change the volume of the second pressure chamber occupied by the piezoelectric elements of the second arrangement of piezoelectric elements, wherein the control is further adapted to control the second arrangement of piezoelectric elements, wherein in the second step the second arrangement of piezoelectric elements is controlled to contract such that the volume of the second pressure chamber occupied the second arrangement of piezoelectric elements is reduced, and wherein in the fourth step the second arrangement of piezoelectric elements is controlled to expand such that the volume of the second pressure chamber occupied by the second arrangement of piezoelectric elements is increased, and wherein in the second step and in the fourth step the first arrangement of piezoelectric elements and the second arrangement of piezoelectric elements are controlled such that during the second and the fourth step the absolute value of the change of volume occupied by the first arrangement of piezoelectric elements over time corresponds to the absolute value of the change of volume occupied by the second arrangement of piezoelectric elements over time.

14. The hydraulic actuator according to claim 12, wherein the connection between the first and the second pressure chamber for a hydraulic fluid are is provided by one or more internal connections, wherein each internal connection is formed by an aperture in the piston, and/or wherein the connection between the first and the second pressure chamber for a hydraulic fluid is provided by one or more external connections, wherein each external connection is formed by a fluid line connecting the first and the second pressure chamber, wherein each connection between the first and the second pressure chamber for a hydraulic fluid comprises a flow control for selectively varying a flow of a hydraulic fluid through the respective fluid connection, wherein the flow control comprises active MEMS valves or active one-way disc-valves or magnetorheological valves, wherein the control is further adapted to control the flow control of each connection between the first and the second pressure chamber for a hydraulic fluid, wherein in the first step the flow control is controlled to prevent fluid flow of the first pressure chamber to the second pressure chamber, and wherein in the third step the flow control is controlled to enable fluid flow from the second pressure chamber to the first pressure chamber.

15. The hydraulic actuator according to claim 12, wherein the connection between the first and the second pressure chamber for a hydraulic fluid are is provided by one or more internal connections, wherein each internal connection is formed by an aperture in the piston, and/or wherein the connection between the first and the second pressure chamber for a hydraulic fluid is provided by one or more external connections, wherein each external connection is formed by a fluid line connecting the first and the second pressure chamber, wherein each connection between the first and the second pressure chamber for a hydraulic fluid is provided with a throttle or a hydraulic screen locally reducing the flow through the respective connection between the first and the second pressure chamber for a hydraulic fluid, wherein the absolute value of the change of volume occupied by the first arrangement of piezoelectric elements over time in the second step is controlled to be greater than the absolute value of the change of volume occupied by the first arrangement of piezoelectric elements over time in the fourth step.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] In the following various exemplary embodiments of a hydraulic actuator according to the disclosure herein and a method for operating a hydraulic actuator according to the disclosure herein will be described with reference to the drawings, wherein:

[0053] FIG. 1 shows a first exemplary embodiment of a hydraulic actuator according to the disclosure herein;

[0054] FIG. 2 shows a second exemplary embodiment of a hydraulic actuator according to the disclosure herein;

[0055] FIG. 3 shows a third exemplary embodiment of a hydraulic actuator according to the disclosure herein;

[0056] FIG. 4 shows a fourth exemplary embodiment of a hydraulic actuator according to the disclosure herein;

[0057] FIG. 5 shows a fifth exemplary embodiment of a hydraulic actuator according to the disclosure herein;

[0058] FIG. 6 shows exemplary expansion and contraction rates of a first and a second arrangement of piezoelectric elements used in the exemplary embodiment of a hydraulic actuator of FIG. 5;

[0059] FIG. 7 shows in an exemplary manner the relationship between flow rate through a hydraulic screen and a throttle, respectively, and the pressure difference over the hydraulic screen or the throttle;

[0060] FIG. 8 shows a first exemplary embodiment of a locking device for use in a hydraulic actuator according to the disclosure herein;

[0061] FIG. 9 shows a second exemplary embodiment of a locking device for use in a hydraulic actuator according to the disclosure herein;

[0062] FIG. 10 shows a sixth exemplary embodiment of a hydraulic actuator according to the disclosure herein;

[0063] FIG. 11 shows a seventh exemplary embodiment of a hydraulic actuator according to the disclosure herein;

[0064] FIG. 12 shows a flow chart for a first exemplary embodiment of a method for operating a hydraulic actuator according to the disclosure herein; and

[0065] FIG. 13 shows a flow chart for a second exemplary embodiment of a method for operating a hydraulic actuator according to the disclosure herein.

DETAILED DESCRIPTION

[0066] In the following figures like reference numerals designate like elements.

[0067] FIG. 1 shows a first exemplary embodiment of a hydraulic actuator 1 according to the disclosure herein. The hydraulic actuator 1 comprises a cylinder housing 3, a piston 5, a piston rod 7 and a locking device 9. The cylinder housing 3 surrounds a cylinder cavity 11 which is separated by the piston 5 into a first pressure chamber 13 and a second pressure chamber 15. In the exemplary embodiment shown in FIG. 1 the hydraulic actuator 1 is a synchronous actuator with the piston rod 7 extending likewise from the surface of the piston 5 facing the first pressure chamber 13 and the surface of the piston 5 facing the second pressure chamber 15. As can be clearly seen in FIG. 1, the piston rod 7 extends through the cylinder housing 3 into the exterior 17 of the hydraulic actuator 1.

[0068] The locking device 9 is only shown schematically in FIG. 1. It is adapted to switch between a locking mode and a non-looking mode. In the locking mode the locking device 9 prevents movement of the piston 5 by engaging the piston rod 7. In the non-looking mode the locking device enables movement or does not prevent movement of the piston 5 by disengaging the piston rod 7.

[0069] In each of the first and the second pressure chamber 13, 15 a respective first and second arrangement of piezoelectric elements 19, 21 is arranged. Each arrangement of piezoelectric elements 19, 21 is formed by one or more stacks of piezoelectric elements. A stack of piezoelectric elements may comprise a plurality of piezoelectric elements arranged adjacent to one another. When the piezoelectric elements arranged in a stack are operated in a synchronized manner, the entire stack preferably expands or contracts simultaneously. Thereby, the overall distance the stack expands or contracts corresponds to the sum of the individual expansion or contraction of the separate piezoelectric elements.

[0070] The first arrangement of piezoelectric elements 19 is arranged in the first pressure chamber 13 and firmly attached to the cylinder housing 3 at an end of the first pressure chamber 13 facing away from the piston 5. In other words in an actuation direction 23 defined by the direction in which the piston 5 is movable in the cylinder cavity 11 the first arrangement of piezoelectric elements 19 is arranged furthest away from the piston 5. A first membrane or diaphragm 25 seals the part of the first pressure chamber 13 in which the first arrangement of piezoelectric elements 19 is arranged from the remainder of the first pressure chamber 13. Thereby, the first arrangement of piezoelectric elements 19 is protected from any hydraulic fluid used for operating the hydraulic actuator 1. When the first arrangement of piezoelectric elements 19 expands, the volume of the first pressure chamber 13 occupied or filled by the first arrangement of piezoelectric elements 19 increases. When the first arrangement of piezoelectric elements 19 contracts, the volume of the first pressure chamber 13 occupied or filled by the first arrangement of piezoelectric elements 19 decreases.

[0071] The second arrangement of piezoelectric elements 21 is arranged in a corresponding position in the second pressure chamber 15. Thus, it is firmly attached to the cylinder housing 3 at that end of the second pressure chamber 15 which is furthest away from the piston 5 in the actuation direction 23. A second membrane 27 is provided for separating the second arrangement of piezoelectric elements 21 from the remainder of the second pressure chamber 15 and, in particular, for sealing the second arrangement of piezoelectric elements 21 from hydraulic fluid which is used to operate the hydraulic actuator 1. When the second arrangement of piezoelectric elements 21 expands, the volume of the first pressure chamber 15 occupied or filled by the first arrangement of piezoelectric elements 21 increases. When the second arrangement of piezoelectric elements 21 contracts, the volume of the second pressure chamber 15 occupied or filled by the second arrangement of piezoelectric elements 21 decreases.

[0072] The first and the second pressure chamber 13, 15 are connected such that a hydraulic fluid can flow from one pressure chamber 13, 15 to the other pressure chamber 13, 15. In the exemplary embodiment three alternative connections 29, 31, 33 are shown which connect the first and the second pressure chamber 13, 15. The three alternative connections 29, 31, 33 may also be used in combination.

[0073] The first connection 29 is an external connection 29 and comprises two fluid lines 35, 37 each comprising an active flow control in form of a one-way valve 39, 41. Thus, if the valve 39 is open a hydraulic fluid may flow from the second pressure chamber 15 through the first fluid line 35 to the first pressure chamber 13. If the valve 39 is closed, no hydraulic fluid can flow through the fluid line 35 from the second pressure chamber 15 to the first pressure chamber 13. As the second fluid line 37 of the first connection 29 comprises a one-way valve 41 which only allows flow of a hydraulic fluid from the first pressure chamber 13 to the second pressure chamber 15, by closing the valve 39 in the first fluid line 35 no hydraulic fluid can flow through the entire first connection 29 from the second pressure chamber 15 to the first pressure chamber 13. On the other hand only if the second valve 41 in the second fluid line 37 is open, a hydraulic fluid may flow from the first pressure chamber 13 through the first connection 29 to the second pressure chamber 15. The external first connection 29 has the advantage that the valves 39, 41 are easily accessible and can be easily maintained.

[0074] The second connection 31 is formed by two apertures 43, 45 in the piston 5. Each aperture 43, 45 comprises a valve which is not a designated with a reference numeral to keep the drawing comprehensible. As in the example of the first connection 29, the first aperture 43 comprises an active flow control in form of a one-way valve which only allows flow from the second pressure chamber 15 to the first pressure chamber 13 and blocks flow in the opposite direction. The second aperture 45 also comprises a one way valve, this one only allowing flow from the first pressure chamber 13 to the second pressure chamber 15. Hence, the observations provided with regard to the first external connection 29 also apply to the second internal connection 31 and the apertures 43, 45 providing the internal connection 31. The second internal connection 31 has the advantage that it allows a hydraulic actuator 1 with particularly compact dimensions and no external fluid lines which reduces the risk of loss of hydraulic fluid.

[0075] Finally, the third external connection 33 comprises only one fluid line 47. However, the fluid line comprises two parallel active flow control in form of one-way valves 49, 51, each allowing flow only in one direction. Thus, when the one-way valve 49 is open, the third external connection 33 allows flow from the second pressure chamber 15 to the first pressure chamber 13. Only when the second one-way valve 51 is open, hydraulic fluid can flow from the first pressure chamber 13 to the second pressure chamber 15 through the third external connection 33. Such an external connection has the advantage that only two openings in the cylinder housing are required, the valves 49, 51 are easily accessible and the dimensions of the hydraulic actuator 1 can be kept relatively compact.

[0076] The valves 39, 41, 49, 51 are preferably selectively operable one-way MEMS-valves, one-way disc valves or magnetorheological valves. All of these valves have the advantages that they can be switched in very short time frames between an opened and a closed state.

[0077] Finally, the hydraulic actuator 1 comprises a control 53, for example, in form of a microprocessor with a memory. A program for operating the microprocessor is stored in the memory. The control 53 is functionally connected to all previously described elements of the hydraulic actuator 1 that can be operated: the first and second arrangement of piezoelectric elements 19, 21, the locking device 9 and the valves 39, 41, 49, 51.

[0078] For moving the piston 5 in the actuation direction 23 such that the volume of the first pressure chamber 13 is increased, the control 53 is adapted to control the controllable elements of the hydraulic actuator according to the following method. The steps of operating the control will be described with further reference to the flow chart in FIG. 12.

[0079] In a first step 55 the control operates the locking device 9 to switch into the non-locking mode such that the piston 5 and the piston rod 7 are movable in the actuation direction. Further, also in the first step 55 all valves 39, 41, 49, 51 and, in particular, the valves 41, 51 allowing flow from the first to the second chamber are set to a position in which no flow is allowed through the connections 29, 31, 33 from the first to the second pressure chamber 13, 15.

[0080] In a second step 57 the first arrangement of piezoelectric elements 19 is controlled to expand. Simultaneously the second arrangement of piezoelectric elements 21 is controlled by the control 53 to contract. The rate at which the first arrangement of piezoelectric elements 19 expands and the second arrangement of piezoelectric elements 21 contracts are preferably chosen such that the absolute change of volume of the respective arrangements of piezoelectric elements 19, 21 over time is the same at any point in time during the second step 57. As the first arrangement of piezoelectric elements 19 expands, the first arrangement of piezoelectric elements 19 occupies more volume of the first pressure chamber 13 and hydraulic fluid in the first pressure chamber 13 is displaced. None of the connections 29, 31, 33 for a hydraulic fluid allows flow of hydraulic fluid out of the first pressure chamber 13. Therefore any displaced hydraulic fluid creates a pressure force acting on the piston 5 and pushing the piston 5 away from the first arrangement of piezoelectric elements 19. Thereby, the overall volume of the first pressure chamber 13 is increased. Simultaneously, the second arrangement of piezoelectric elements 21 contracts such that it fills or occupies less volume of the second pressure chamber 15. As all connections 29, 31, 33 for a hydraulic fluid are closed, no hydraulic fluid can flow into the second pressure chamber 15. Hence, the contracting second arrangement of piezoelectric elements 21 creates a vacuum which pulls the non-locked piston 5 in the actuation direction 23 towards the second arrangement of piezoelectric elements 21.

[0081] In a third step 59 the locking device is switched by the control 53 into the locking mode such that the piston 5 and the piston rod 7 cannot move any further in the actuation direction 23. At the same time (though this may happen sequentially) at least those valves 39, 49 of the connections 29, 31, 33 are opened which may allow flow of a hydraulic fluid from the second pressure chamber 15 to the first pressure chamber 13. In other words, after the third step 59 is completed a hydraulic fluid can flow from the second pressure chamber 15 to the first pressure chamber 13.

[0082] Finally, in the fourth step 61 the first arrangement of piezoelectric elements 19 is controlled by the control 53 to contract and the second arrangement of piezoelectric elements 21 is controlled by the control 53 to expand. In consequence, the volume occupied by the first arrangement of piezoelectric elements 19 in the first pressure chamber 13 is reduced and the volume occupied by the second arrangement of piezoelectric elements 21 in the second pressure chamber 15 is reduced. Thus, in the first pressure chamber 13 a vacuum is created which is filled by hydraulic fluid from the second pressure chamber 15. Here, the expanding second arrangement of piezoelectric elements 21 displaces hydraulic fluid which is therefore actively pumped through the connection 29, 31, 33 into the first pressure chamber 13. As the piston 5 is locked into position by the locking device 9, any force due to the pressure or vacuum created by one of the arrangements of piezoelectric devices 19, 21 is used to pump hydraulic fluid from one pressure chamber 15 to the other pressure chamber 13.

[0083] For further moving the piston 5 in the actuation direction 23, the control 53 and the method may continue at the first step 55.

[0084] For moving the piston in the opposite direction essentially the same method can be used with the caveat that the order of the second and the fourth step 55, 61 needs to be reversed, i.e., the control implements a method with the following sequence of steps: first step 55, fourth step 61, third step 59 and second step 57. It is, however, important to note that in the first step 55 all valves 39, 41, 49, 51 are switched to a closed position and in the third steps 59 all valves 39, 41, 49, 51 are switched back to the opened position.

[0085] FIG. 2 shows a second exemplary embodiment of a hydraulic actuator 63 according to the disclosure herein. The hydraulic actuator 63 shown in FIG. 2 is similar to the hydraulic actuator 1 is shown in FIG. 1. Thus, in the following only the differences to the hydraulic actuator 1 will be discussed in more detail.

[0086] The key difference between the hydraulic actuators 1, 63 of FIGS. 1 and 2 is the use of different valves 39, 41, 49, 51, 65, 67, 69 in the connections 29, 31, 33, 71, 73 between the first and the second pressure chamber 13, 15 for a hydraulic fluid. The valves 39, 41, 49, 51 used in the embodiment shown in FIG. 1 are one-way valves 39, 41, 49, 51. These valves 39, 41, 49, 51 only permit flow of a hydraulic fluid either from the first pressure chamber 13 to the second pressure chamber 13, 15 or in the opposite direction. However, valves 65, 67, 69 used in the connections 71, 73 between the first and second pressure chamber 13, 15 in the second embodiment are two-way valves 65, 67, 69 which permit flow in both directions. This has the advantage, that one connection 71, 73 is sufficient for allowing flow of a hydraulic fluid in both directions.

[0087] For example, the embodiment shown in FIG. 2 only comprises an external connection 71 with one fluid line 75 providing a flow path between the first and the second pressure chamber 13, 15 in both directions. Additionally or alternatively the embodiment also comprises an internal connection 73 formed by two apertures 77, 79 in the piston 5. Each of the apertures 77, 79 comprises a valve 67, 69 allowing flow in both directions.

[0088] The hydraulic actuator 63 can be operated according to the same method as the actuator 1. Hence, a similarly adapted control 53 can be used to operate the actuator 63

[0089] A further exemplary embodiment of a hydraulic actuator 81 according to the disclosure herein is shown in FIG. 3. Again, the features of this hydraulic actuator 81 are very similar to the features of the hydraulic actuator 1 shown in FIG. 1. Thus, only the differences to the preceding embodiment will be described in more detail.

[0090] The hydraulic actuator 81 differs from the hydraulic actuator 1 shown in FIG. 1 in that the first and second arrangement of piezoelectric elements 83, 85 are not rigidly attached to the walls of the cylinder housing 3 but instead to the piston 5. Further, as the arrangements of piezoelectric elements 83, 85 take up most of the surface area of the piston 5 towards the first and second pressure chamber 13, 15 the hydraulic actuator 81 does not comprise any internal connections between the first and second pressure chamber 13, 15 for a hydraulic fluid. Instead external connections 29, 31 are provided which are identical to the connections 29, 31 of the exemplary embodiment of a hydraulic actuator 1 shown in FIG. 1.

[0091] The hydraulic actuator 81 shown in FIG. 3 is operated in the same manner as the hydraulic actuator 1 shown in FIG. 1. Thus, the control 53 operates according to the same method as previously described.

[0092] A fourth exemplary embodiment of a hydraulic actuator 87 is shown in FIG. 4. Again, only the differences to the previously described embodiments will be discussed in more detail.

[0093] This embodiment of a hydraulic actuator 87 is based on the embodiment shown in FIG. 3. In fact, it shows a combination of the embodiments of FIG. 3 and FIG. 2 where the arrangement of piezoelectric elements 83, 85 are arranged on the piston 5 as in the embodiment of FIG. 3 and there is only one external fluid line 71 as in the embodiment of FIG. 2. The single external fluid line 71 provides a two-way connection between the first and second pressure chamber 13, 15 for a hydraulic fluid, i.e., the valve 65 is a two-way valve 65.

[0094] The embodiment of FIG. 4 is like the previously discussed hydraulic actuators 1, 63, 81 operated using a control 53. The control 53 is adapted to operate the locking device 9, the arrangements of piezoelectric elements 83, 85 and the valve 65 according to the previously described method.

[0095] A fifth embodiment of a hydraulic actuator 89 is shown in FIG. 5. This embodiment is based on the exemplary embodiment of a hydraulic actuator 63 shown in FIG. 2. As already practiced with regard to the previous embodiments, only differences to the hydraulic actuator 63 shown in FIG. 2 will be discussed in more detail.

[0096] The embodiment of FIG. 5 differs from the embodiment in FIG. 2 in that the valves 65, 67, 69 in the first and second connection 71, 73 between the first and the second pressure chambers 13, 15 for a hydraulic fluid are replaced with throttles 91, 93, 95. In other words, the external fluid line 75 providing the first connection 71 between the first and second pressure chamber 13, 15 of the cylinder housing 3 comprises a throttle 91 which locally limits the cross-section of the fluid line 75. Similarly, in the apertures 77, 79 provided in the piston 5 as internal fluid connection 73 between the first and the second pressure chamber 13, 15 for a hydraulic fluid throttles 93, 95 have been arranged. These throttles 93, 95 locally limit the cross-section of the apertures 77, 79 and reduce the flow through the latter. Using a throttle 91, 93, 95 instead of an active flow control such as a valve advantageously reduces the number of movable parts in the hydraulic actuator 89.

[0097] A method of operating the fifth exemplary embodiment of a hydraulic actuator 89 will now be described with reference to FIG. 13. Operation can be effected by implementing in the control 53 the following sequence of method steps, i.e., by adapting the control 53 to operate according to the following method steps. The method described in the following paragraphs is based on the method previously described the reference to FIG. 12. Thus, only the differences to the previously described method will be described in more detail.

[0098] The first step 97 corresponds to the first step 55 and the locking device 9 is set into the non-looking mode in which the piston 5 is not locked in position, i.e., once the first step 97 has been completed the piston 5 and the piston rod 7 may freely move in the actuation direction 23. Note that the open cross-section of the throttles 91, 93, 95 is not modified in any of the method steps. Hence, the connections 71, 73 between the first and the second pressure chamber 13, 15 for a hydraulic fluid are permanently kept open in this method, i.e., the first and the second pressure chamber 13, 15 are permanently in fluid communication.

[0099] As in the previously described method, in the second step 99 the first arrangement of piezoelectric elements 19 is controlled by the control 53 to expand at a first rate of expansion. Simultaneously, the second arrangement of piezoelectric elements 21 is controlled by the control 53 to contract at a first rate of contraction. A rate of expansion and a rate of contraction both quantify the change of volume occupied by the respective arrangement of piezoelectric elements 19, 21 over time. While a rate of expansion refers to an increase in volume occupied by an arrangement of piezoelectric elements 19, 21, a rate of contraction refers to a reduction in volume occupied by an arrangement of piezoelectric elements 19, 21. In the second step 99, the first rate of expansion corresponds to the first rate of contraction, i.e., the absolute value of the ratio of change in volume over time is the same for the first and the second arrangement of piezoelectric elements 19, 21.

[0100] In the third step 101 the locking device 9 is switched into the locking mode such that it prevents any movement of the piston 5 and the piston rod 7 in the actuation direction 23. Again, the open cross-section of the throttles 91, 93, 95 is not modified.

[0101] Finally, in the fourth step 103 the first arrangement of piezoelectric elements 19 is controlled by the control 53 to contract at a second rate of contraction. Simultaneously, the second arrangement of piezoelectric elements 21 is controlled by the control 53 to expand at a second rate of expansion. In the fourth step 103 the second rate of expansion corresponds to the second rate of contraction, i.e., the absolute value of the ratio of change of volume over time is the same for the first and the second arrangement of piezoelectric elements 19, 21. The second rate of contraction is smaller than the first rate of contraction and the second rate of expansion is smaller than the first rate of expansion. In other words, in the fourth step 103 the second arrangement of piezoelectric elements 21 expands at a slower rate than the first arrangement of piezoelectric elements 19 in the second step 99. Likewise, in the fourth step 103 the first arrangement of piezoelectric elements 19 contracts at a slower rate than the second arrangement of piezoelectric elements 21 in the second step 99.

[0102] This is exemplified in the two graphs 105, 107 shown in FIG. 6. Both graphs 105, 107 show on the abscissa or horizontal axis 109 the time. The ordinate of the top graph 105 shows on the ordinate or vertical axes 111 the volume of the first arrangement of piezoelectric elements 19 and the ordinate of the lower graph 107 shows on the ordinate or vertical axes 113 the volume of the second arrangement of piezoelectric elements 21. On the abscissa 109 reference numerals 99 and 103 indicate which method step is carried out in the respective time sections, i.e., if a section of the abscissa is indicated with reference numeral 99, a second method 99 step is carried out and if a time sections indicated with reference numeral 103, a fourth step 103 is carried out. The two graphs 105, 107 clearly show how in the second method step 99 the respective first rate of expansion or contraction is greater then in the fourth method step 103. Using a smaller rate of expansion/contraction during the fourth step 103 increases the absolute volume of the hydraulic fluid that is pumped from the second pressure chamber 15 to the first pressure chamber 13 compared to as if the same rates would be used in the second and the fourth step.

[0103] In a preferred modification of the embodiment of FIG. 5 hydraulic screen with a central opening and a length to diameter ratio of less than 1.5 are used instead of the throttles 91, 93, 95. Replacing throttles by a hydraulic screen (similar to a photographic aperture) has the advantage that the effect of using a slow expansion/contraction rate in the pumping step (fourth step) compared to a higher expansion/contraction rate in the actuation step (second step) for pumping more hydraulic fluid is even more pronounced.

[0104] This is underlined by FIG. 7 which shows two graphs 115, 117 representing the flow rates of a hydraulic screen and a throttle as a function of the pressure differential across the hydraulic screen and the throttle, respectively. The ordinate 121 depicts the flow rate V and the abscissa the pressure differential p. As can be seen in FIG. 7 graph 115 shows that the flow rate V varies for the hydraulic screen with the square root of the pressure differential p whereas it is linear in the pressure differential p for the throttle (graph 117).

[0105] FIG. 8 shows a first exemplary embodiment of a locking device 123 which can be used in a hydraulic actuator 1, 63, 81, 87, 89 according to the disclosure herein. The locking device 123 is a hydraulic locking device 123. It is formed by a brake cylinder housing 125 which forms a brake cylinder cavity 127. The piston rod 7 of the hydraulic actuator 1, 63, 81, 87, 89 extends in the actuation direction 23 through the brake cylinder housing 125. A brake cylinder piston 129 is attached to the piston rod 7 such that it is arranged inside the brake cylinder cavity 127 and separates the letter into a first brake pressure chamber 131 and a second brake pressure chamber 133.

[0106] The first and second brake pressure chamber 131, 133 are in fluid connection. In the exemplary embodiment shown in FIG. 8 two different kinds of fluid connections 135, 137 are shown. It is contemplated that these connections 135, 137 can be used alternatively or in combination. The first connection 135 is an external connection comprising a fluid line 139 which provides a connection for a hydraulic fluid between the first brake pressure chamber 131 and the second brake pressure chamber 133. The fluid line 139 comprises a magnetorheological valve 141 for selectively allowing and blocking flow of a magnetorheological hydraulic fluid through the fluid line 139. The second connection 137 is formed by two apertures 143, 145 extending through the brake piston 145. Each aperture 143, 145 provides a connection for a hydraulic fluid between the first and the second brake pressure chamber 131, 133. More or fewer than two apertures 143, 145 can be used. Each aperture 143, 145 comprises a magnetorheological valve 147, 149, where a magnetorheological valve 141, 147, 149 is essentially a way or means for generating a magnetic field that can be selectively turned on or off for interacting with the magnetorheological hydraulic fluid in the brake cylinder cavity 127.

[0107] The hydraulic locking device 123 can be switched to a locking mode by closing the magnetorheological valves 141, 147, 149. If the valves 141, 147, 149 are closed. The brake piston 129 cannot move in the brake cylinder cavity 127. However, as the brake piston 129 is rigidly connected to the piston rod 7 of the hydraulic actuator it also prevents movement of the piston 5 of the latter. For switching the hydraulic locking device 123 into a non-looking mode the magnetorheological valves 141, 147, 149 are deactivated such that the hydraulic fluid can freely flow from the first brake pressure chamber 131 to the second brake pressure chamber 133 and back. The hydraulic locking device 123 is preferably controlled using the same control 53 which is also used for operating and controlling the hydraulic actuator 1, 63, 81, 87, 89.

[0108] While other valves can be used instead of the magnetorheological valves 141, 147, 149 for operating the hydraulic locking device 123, magnetorheological valves 141, 147, 149 are preferred as they can be switched rapidly between a blocking and a non-blocking setting.

[0109] A second exemplary embodiment of a locking device 151 which can be used in a hydraulic actuator 1, 63, 81, 87, 89 according to the disclosure herein is shown in FIG. 9. This locking device 151 is also controlled by the same control 53 that is used to control the hydraulic actuator 1, 63, 81, 87, 89. The locking device 151 comprises a multi-disk brake 153 which directly acts on the piston rod (not shown) of the hydraulic actuator 1, 63, 81, 87, 89. The multi-disc brake 153 is actuated by a brake arrangement of piezoelectric elements 155 which acts on to the multi-disc brake 153 via a hydraulic enhancer 157. The latter is formed by an enhancer cavity 159 filled with a hydraulic fluid acting on an enhancer piston 161 having a reduced cross-section towards the hydraulic fluid as compared to the cross-section of the brake arrangement of piezoelectric elements 155 towards the hydraulic fluid.

[0110] FIG. 10 shows a further embodiment of a hydraulic actuator 163 according to the disclosure herein. The hydraulic actuator 163 is based on the hydraulic actuator 1 shown in FIG. 1. Thus, only the differences as compared to FIG. 1 will be described in more detail.

[0111] Other than the hydraulic actuator 1 of FIG. 1 the hydraulic actuator 163 of FIG. 10 is a differential actuator having a piston 165 with different cross-sectional areas towards the first and the second pressure chamber 13, 15. The different cross-sectional areas result from the fact that the piston rod 167 only extends from the piston 165 towards and through the first pressure chamber 13 while there is no piston rod extending through the second pressure chamber 15. In order to have sufficient hydraulic fluid in second pressure chamber 15 when moving the piston 165 towards the first arrangement of piezoelectric devices 19 and for taking up supplemental hydraulic fluid when moving the piston 165 towards the second arrangement of piezoelectric elements 21 a hydraulic fluid reservoir 169 is provided.

[0112] The hydraulic fluid reservoir 169 is connected to the second pressure chamber 15 via a fluid line 171. The fluid line 171 comprises a valve arrangement 173 which is made up of two one-way valves 175, 177 which can also be controlled using the control 53. One of the one-way valves 175 enables a flow from the second pressure chamber 15 to the hydraulic fluid reservoir 169 and the other one-way valve 177 only allows flow from the hydraulic fluid reservoir 169 to the second pressure chamber 15. The valve arrangement 173 is controlled such that whenever more hydraulic fluid is required in the second pressure chamber 15 than what can be provided by the first pressure chamber 13 hydraulic fluid is taken from the hydraulic fluid reservoir 169. On the other hand, whenever more hydraulic fluid is to be expelled from the second pressure chamber 15 than what can be taken up by the first pressure chamber 13, this supplemental hydraulic fluid can be pumped to the hydraulic fluid reservoir 169.

[0113] The hydraulic fluid reservoir 169 preferably comprises a third arrangement of piezoelectric elements 179 which moves along the wall of the hydraulic fluid reservoir 169 in a crawling manner to change the volume of the hydraulic fluid reservoir 169 and allow it to act like a pump.

[0114] Finally, a further differential hydraulic actuator 181 according to the disclosure herein is shown in FIG. 11. Features of this hydraulic actuator 181 which are similarly or identically present in the previous embodiment will not be described in more detail.

[0115] Hydraulic actuator 181 differs from all previously described embodiments in that it comprises only a first arrangement of piezoelectric elements 183 but no further arrangement of piezoelectric elements. This first arrangement 183 is arranged in the first pressure chamber 13 of the cylinder housing 3 (note that the first and second pressure chamber are reversed in the drawing as compared to the previously discussed embodiments). The first and the second pressure chamber 13, 15 are both connected via fluid lines 185, 187 to a hydraulic fluid reservoir 189. The hydraulic fluid reservoir 189 is similar to the hydraulic fluid reservoir 169 of the previous embodiment discussed with regard to FIG. 10. In each fluid line 185, 187 a valve arrangement 191, 193 comprising two one-way valves is arranged. The valve arrangements 191, 193 are controlled by the control 53.

[0116] The hydraulic actuator 181 is operated in a similar manner as all previous hydraulic actuators 1, 63, 81, 87, 89, 163 with the addition that when the first arrangement of piezoelectric devices 183 expands for moving the piston 165 away from the first arrangement of piezoelectric devices 183, the valve arrangement 191 between the first pressure chamber 13 and the hydraulic fluid reservoir 189 is switched such that hydraulic fluid can be pushed out of the second pressure chamber 15 into the hydraulic fluid reservoir 189. Further, when the first arrangement of piezoelectric devices 183 is controlled to contract for moving the piston 165 towards the first arrangement of piezoelectric devices 183, additional hydraulic fluid needs to be supplied from the hydraulic fluid reservoir 189 to the second pressure chamber 15. In addition, if the first arrangement of piezoelectric devices 183 is controlled to expand for pumping hydraulic fluid out of the first pressure chamber 13, the valve arrangements 193 need to be switched such that the hydraulic fluid can be taken up by the hydraulic fluid reservoir 189.

[0117] While at least one exemplary embodiment of the invention(s) herein is disclosed herein, it should be understood that modifications, substitutions and alternatives may be apparent to one of ordinary skill in the art and can be made without departing from the scope of this disclosure. This disclosure is intended to cover any adaptations or variations of the exemplary embodiment(s). In addition, in this disclosure, the terms comprise or comprising do not exclude other elements or steps, the terms a or one do not exclude a plural number, and the term or means either or both. Furthermore, characteristics or steps which have been described may also be used in combination with other characteristics or steps and in any order unless the disclosure or context suggests otherwise. This disclosure hereby incorporates by reference the complete disclosure of any patent or application from which it claims benefit or priority.