Hydraulic stage

Abstract

A hydraulic stage includes a hydraulic element located between and sealing a first and second chamber, wherein the first chamber comprises at least one aperture through which fluid is arranged to flow into or out of the first chamber; and at least one piezoelectric element which is positioned adjacent to the at least one aperture and is arranged to deform in response to an applied potential difference such that it blocks or obstructs the at least one aperture to a varying degree according to the level of deformation, so as to control fluid flow into or out of the first chamber. The level of deformation of the piezoelectric element thus reduces or increases an effective size of the inlet or outlet aperture to which it is adjacent, restricting or permitting an increase in fluid flow accordingly.

Claims

1. A hydraulic stage comprising: a hydraulic element located between and sealing a first and second chamber, wherein the first chamber comprises at least one aperture through which fluid is arranged to flow into or out of the first chamber; at least one piezoelectric element which is positioned adjacent to the at least one aperture and is arranged to deform in response to an applied potential difference such that it blocks or obstructs the at least one aperture to a varying degree according to the level of deformation, so as to control fluid flow into or out of the first chamber; and a mechanical position feedback system including a feedback member arranged to mechanically deform the at least one piezoelectric element; wherein the hydraulic stage is configured to measure a potential difference generated by the at least one piezoelectric element and to use the measured potential difference to estimate a position of the hydraulic element.

2. The hydraulic stage as claimed in claim 1 wherein the at least one aperture comprises an inlet aperture through which fluid may be introduced to the first chamber, and an outlet aperture through which fluid may exit the first chamber.

3. The hydraulic stage as claimed in claim 2 wherein the piezoelectric element is positioned adjacent to the outlet aperture and is arranged to control fluid flowing therethrough.

4. The hydraulic stage as claimed in claim 1, wherein the piezoelectric element is arranged to restrict fluid flow into or out of the first chamber when in a neutral position.

5. The hydraulic stage as claimed in claim 1, wherein the piezoelectric element comprises a piezoelectric bimorph.

6. The hydraulic stage as claimed in claim 5 wherein at least one end of the bimorph is fixed in place.

7. The hydraulic stage as claimed in claim 1, further comprising a second piezoelectric element, arranged to control fluid flow into or out of the second chamber.

8. The hydraulic stage as claimed in claim 7 wherein the first piezoelectric element is arranged to control fluid flow out of the first chamber and the second piezoelectric element is arranged to control fluid flow out of the second chamber.

9. The hydraulic stage as claimed in claim 1, claim wherein the hydraulic element comprises a piston or a spool.

10. The hydraulic stage as claimed in claim 1, wherein the hydraulic element comprises a component of a secondary hydraulic stage.

11. The hydraulic stage as claimed in claim 1, further comprising an electronic position feedback system comprising an electronic position sensor coupled to the hydraulic element.

12. The hydraulic stage as claimed in claim 1, wherein the mechanical feedback system is configured to provide negative feedback.

13. A method of operating a hydraulic stage, the hydraulic stage including a hydraulic element located between and sealing a first and second chamber, wherein the first chamber comprises at least one aperture through which fluid is arranged to flow into or out of the first chamber, at least one piezoelectric element which is positioned adjacent to the at least one aperture, and a mechanical position feedback system including a feedback member arranged to mechanically deform the at least one piezoelectric element, the method comprising: controlling the fluid flow into or out of the first chamber by applying a potential difference to the at least one piezoelectric element such that it deforms so as to block or obstruct the at least one aperture to a varying degree according to the level of deformation; and measuring a potential difference generated by the at least one piezoelectric element and using the measured potential difference to estimate a position of the hydraulic element.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) One or more non-limiting examples of the present disclosure will now be described with reference to the accompanying Figures, in which:

(2) FIG. 1 shows a cross sectional view of a hydraulic stage according to an example of the present disclosure;

(3) FIGS. 2 and 3 illustrate the operation of the hydraulic stage shown in FIG. 1;

(4) FIGS. 4A-4C illustrate the behaviour of a piezoelectric bimorph;

(5) FIG. 5 shows a cross sectional view of a hydraulic stage with electronic position feedback; and

(6) FIGS. 6 and 7 illustrate the operation of a hydraulic stage with mechanical position feedback.

DETAILED DESCRIPTION

(7) FIG. 1 shows a hydraulic stage 2 according to an example of the present disclosure. More particularly, the hydraulic stage shown here is an electrohydraulic servo valve. The stage 2 comprises a housing 4 defining an elongate cavity 6. A spool 8 is disposed within the cavity 6 and defines and seals a first chamber 10 and a second chamber 12. For simplicity, the details of the construction of the spool 8 have been omitted, but it will be appreciated that the spool 8 would typically be operatively connected to another element to control movement thereof. For example, spool 8 might in some examples have other annular chambers formed thereon which make or break fluid connections with other fluid passages depending on the axial position of the spool 8.

(8) The spool 8 is able to slide freely within the cavity. When the spool 8 moves to the right it reduces the volume of the first chamber 10 and increases the volume of the second chamber 12. Correspondingly, when the spool 8 moves to the left it increases the volume of the first chamber 10 and decreases the volume of the second chamber 12.

(9) The first chamber 10 comprises a first inlet 14 and a first outlet 16. Similarly, the second chamber comprises a second inlet 18 and a second outlet 20. The first and second inlets 14, 18 are both connected to a fluid reservoir 22, from which fluid is supplied at a fixed pressure. Alternatively, first and second inlets 14, 18 may be supplied by a pump or a pressurized line.

(10) The first and second outlets 16, 20 are connected to a fluid drain 24, to which fluid can drain from the first and second chambers 10, 12 at a rate limited only by the size of the respective outlets 16, 20.

(11) A first bidirectional bi-morphic piezoelectric element 26 is positioned to partially obstruct the first outlet 16 when in a neutral state (i.e. with no potential difference applied thereto). A second bidirectional bi-morphic piezoelectric element 28 is positioned to partially obstruct the second outlet 20 when in a neutral state. The first and second bi-morphic piezoelectric elements 26, 28 are connected to a control unit 30 which is operable to apply a potential difference to neither, either or both elements 26, 28. The control unit 30 comprises an input 31 to which control signals may be sent to operate the hydraulic stage (e.g. from aircraft flight controls).

(12) A detailed cross sectional view of the first bi-morphic element 26 in the neutral state is shown in FIG. 4A (and it will be appreciated that the second bimorphic element 28 has the same construction in mirror-image). The bi-morphic element 26, 28 comprises a first piezoelectric layer 402 and a second piezoelectric layer 404 which are attached at their respective ends 406. As mentioned above, the piezoelectric element 26 is positioned to partially obstruct the first outlet 16 (fluid flow is illustrated in FIG. 4A using dashed arrows).

(13) The operation of the hydraulic stage 2 will now be described with reference to FIGS. 2-3.

(14) FIG. 2 shows a first state of operation of the hydraulic stage 2 in which a positive potential difference is applied by the control unit 30 to the first piezoelectric element 26. FIG. 4B shows the piezoelectric element in the same configuration as in FIG. 4A but with arrows showing the contraction of first layer 402 and the expansion of second layer 404 caused by the electric potential difference applied thereto. FIG. 4C shows the result of the deformation. As seen in FIGS. 4B and 4C, the potential difference causes the first piezoelectric layer 402 to contract and the second piezoelectric layer 404 to expand. This causes the piezoelectric element 26 (which is fixed at its two opposite ends to the housing 4) to bend towards the first outlet 16. This further obstructs the first outlet 16, reducing its effective size and thus the rate at which fluid can flow therethrough.

(15) The reduced outflow rate from the first chamber 10, coupled with the constant inflow pressure from the first inlet 14, results in the pressure within the first chamber 10 increasing. Contrastingly, the pressure in the second chamber 12 is unaffected and remains constant. As a result of the pressure differential between the first and second chambers 10, 12, the spool 8 experiences a net force to the left, and begins to accelerate in that direction (towards the second chamber 12).

(16) As shown in FIG. 3, once the spool 8 has moved to the required position, the control unit stops applying a potential difference to the first piezoelectric element 26. The first piezoelectric element 26 can now return to its neutral shape and the effective size of the first outlet 16 can return to its initial state. As the outflow from the first chamber 10 is then no longer restricted compared to the outflow from the second chamber 12, the pressures within the first and second chambers 10, 12 equalise and there is no longer a net force on the spool 8. Now due to the absence of a differential pressure between the two chambers the spool 8 stops moving and remains in the required position indefinitely.

(17) In other words, to operate the hydraulic stage 2, a potential difference is applied to the piezoelectric element 26. This induces a deformation of the element 26 and consequently a variation of the effective size of the first outlet 16. This causes a change of flow rate through the first outlet 16 causing a pressure differential to arise between the first and second chambers 10, 12. This displaces the spool 8.

(18) As mentioned above, both the first and second bi-morphic piezoelectric elements 26, 28 are bidirectional and are connected to the control unit 30 such that it can apply a potential difference in any direction to neither, either or both elements 26, 28. As explained below, this adds redundancy to the hydraulic stage, in that desired movement of the spool 8 can be achieved even if one of the piezoelectric elements 26, 28 were to fail and become inoperative.

(19) In the operation described above, the control unit 30 applies a positive potential difference to only the first piezoelectric element 26, in order to move the spool 8 towards the second chamber 12. However, this result may also be achieved by applying a negative potential difference to the second piezoelectric element 28. Because the second piezoelectric elements 28 is bidirectional, this causes the second piezoelectric element 28 to bend away from the second outlet 20. This reduces the obstruction of the second outlet 20, increasing its effective size and thus the rate at which fluid can flow therethrough.

(20) The pressure within the second chamber 12 thus decreases, while the pressure in the first chamber 10 is unaffected and remains constant. As before, the spool 8 experiences a net force to the left, and begins to accelerate in that direction (towards the second chamber 12).

(21) In addition, it is possible to deform both the first and second piezoelectric elements 26, 28 simultaneously (e.g. by applying a positive potential difference to one, and a negative potential difference to the other), to generate an increased pressure differential between the first and second chambers 10, 12. This increases the net force on the spool 8 which can speed up its movement and/or increase the size or mass of spool 8 which may be used.

(22) As shown in FIG. 5, the hydraulic stage 2 may comprise a position feedback system comprising an electronic position sensor 32. The electronic position sensor 32 is coupled to the spool 8 and is connected to the control unit 30. The electronic position sensor 32 is arranged to output a signal indicative of the position of the spool 8. This signal provides the control unit 30 with feedback on the current position of the spool 8. This enables the control unit 30 to control the piezoelectric elements 26, 28 so as to move the spool 8 into a desired positioned with high accuracy. It also enables the control unit 30 to smooth the motion of the spool 8, by dynamically adjusting the force applied to the spool 8 through continuous adjustment of the potential difference applied to the piezoelectric elements 26, 28 based on the current and desired positions of the spool 8 (e.g. to reduce steadily the force on the spool 8 as it approaches a desired position).

(23) FIG. 6 shows an example of the hydraulic stage 2 comprising a mechanical position feedback system. The mechanical position feedback system comprises a lever 33 comprising a cylinder 34 (although it will be appreciated that a sphere or other shape could be used) with an off-axis pivot 35, located a distance G above the centre 37 of the cylinder 34. The centre 37 of the cylinder 34 is located exactly between the first and second outlets 16, 20 (with the off-axis pivot 35 located the distance G above).

(24) The lever 33 further comprises an arm 36 which extends from the cylinder 34 and is coupled to the spool 8, such that movement of the spool 8 within the cavity causes the cylinder 34 to rotate about the off-axis pivot 35. The lever 33 has a length L.

(25) Because the pivot 35 is off-axis, rotation of the cylinder 34 causes the cylinder 34 to move towards either the first or second piezoelectric elements 26, 28. Thus, when the spool 8 moves towards the second chamber 12, for example, the cylinder 34 rotates clockwise about the pivot 35 and moves towards the second piezoelectric element 28.

(26) The cylinder 34 is sized such that even a small rotation causes it to contact and apply a force to the piezoelectric element 26, 28 towards which it rotates. The force applied causes the piezoelectric element 26, 28 to deform towards the corresponding outlet 16, 20, restricting outflow therethrough and increasing the pressure in the corresponding chamber 10, 12.

(27) The mechanical position feedback system thus provides negative feedback to any movement of the spool 8. For example, as shown in FIG. 7, when the spool 8 moves towards the second chamber 12 due to a potential difference being applied to the first piezoelectric element 26 (and pressure increasing in the corresponding first chamber 10), the resultant rotation of the cylinder 34 deforms the second piezoelectric element 28, resulting in an increase in pressure in the second chamber 12. When the spool 8 has moved a critical distance towards the second chamber 12 the cylinder 34 has rotated to a point at which the increased pressure in the first chamber 10 (caused by piezoelectric deformation) is balanced by the increased pressure in the second chamber 12 (caused by mechanical deformation) and an equilibrium is reached, preventing further movement of the spool 8. If the potential difference then ceases to be applied to the first piezoelectric element 26, the piezoelectric element 26 returns to its neutral state and the pressure in the first chamber 10 returns to its neutral level. However, the mechanical deformation to the second piezoelectric element 28 remains, and the pressure in the second chamber 12 is thus greater than that in the first (now unrestricted) chamber 10. This pressure differential causes the spool 8 to move back towards a neutral position.

(28) It is possible, by exploiting the property of piezoelectric materials to generate voltage proportional to their deformation when subjected to an external force, to estimate the position of the spool 8 by measuring the potential difference generated by the mechanical deformation to the second piezoelectric element 28. This may be used along with the lever ratio L/G to calculate the position of the spool 8.

(29) When mechanical feedback is implemented as described herein, it may be preferable to use only mono-directional piezoelectric elements 26, 28.