Servo valve

A servo valve comprising first and second nozzles and first and second piezoelectric actuators arranged to control fluid flow through the first and second nozzles respectively. A first fluid flow path is defined between the first nozzle and the first piezoelectric actuator and a second fluid flow path is defined between the second nozzle and the second piezoelectric actuator. The first and second piezoelectric actuators are arranged such that applying a voltage to the first and second piezoelectric actuators causes a change in dimension thereof, which acts to open or restrict said first and second fluid flow paths respectively.

FOREIGN PRIORITY

This application claims priority to European Patent Application No. 17461648.2 filed Dec. 22, 2017, the entire contents of which is incorporated herein by reference.

TECHNICAL FIELD

This disclosure relates to a servo valve. This disclosure also relates to an actuation system and a method of controlling a servo valve.

BACKGROUND

Servo valves are well-known in the art and can be used to control the flow of hydraulic fluid to an actuator via a spool valve. Typically, a flapper is deflected by an armature connected to an electric motor away or towards nozzles, which control fluid flow to the spool valve. Deflection of the flapper can control the amount of fluid injected from the nozzles, and thus the amount of fluid communicated to the actuator via the spool valve. In this way, servo valves can allow precise control of actuator movement.

SUMMARY

From one aspect, the present disclosure relates to a servo valve in accordance with claim1.

The piezoelectric actuators may be configured such that the change in dimension fully opens the fluid flow paths (i.e. the nozzles are unobstructed by the piezoelectric actuators) or fully closes the fluid flow paths (i.e. the nozzles are completely blocked/restricted by the piezoelectric actuators). Additionally or alternatively, the piezoelectric actuators may be configured to only partially block/restrict the nozzles and/or only partially open the nozzles.

The first and second nozzles may be in fluid communication with a spool valve that controls a hydraulic actuator. The spool position may be varied by restricting or opening the first and/or second fluid flow paths.

In an embodiment of the above servo valve, the first and second piezoelectric actuators are arranged such that the change in dimension thereof increases or decreases a gap between the first and second piezoelectric actuators and the first and second nozzles respectively. The increase or decrease in gap can open or restrict the first and second fluid paths respectively. The gap can be increased until the fluid flow paths are “fully open” and decreased until the fluid flow paths are “fully closed”, or to some other (intermediate) degree of open or closed.

In a further embodiment of either of the above servo valves, the first and second nozzles define first and second nozzle axes. The first and second piezoelectric actuators extend along the first and second nozzle axes respectively, and the change in dimension of the first and second piezoelectric actuators causes the first and second piezoelectric actuators to expand or contract along the first and second nozzle axes respectively. The first and second nozzle axes may be aligned with each other. The nozzle axis may be defined as the axis along which fluid flows through nozzle outlet openings of the nozzles (i.e. a central axis of the nozzle outlet openings).

In a further embodiment of any of the above servo valves, each piezoelectric actuator comprises a piezoelectric element and a blocking element. The blocking element is at a first axial end of the piezoelectric element facing the respective nozzle. The blocking element comprises a surface for engaging a nozzle outlet opening in the respective nozzle. The surface may be planar in a plane perpendicular to the nozzle axis/fluid flow path out of the nozzle outlet opening (i.e. the central axis of the nozzle outlet openings).

In a further embodiment of any of the above servo valves, the servo valve further comprises a servo valve housing including a pair of nozzle cavities. Each nozzle cavity houses a nozzle and a piezoelectric actuator.

In a further embodiment of the above servo valve, the piezoelectric actuators are retained in the nozzle cavities by contact between a second axial end of each piezoelectric actuator opposite a or the first axial end facing the nozzle, and a wall defined by the respective nozzle cavity. The wall may be a common wall shared by the first and second cavities. The wall may separate the first and second cavities.

In a further embodiment of any of the above servo valves, the servo valve further comprises a first pair of caps each configured to hermetically seal a respective nozzle cavity from the exterior of the servo valve housing. The caps are removably secured to the servo valve housing to allow access to the nozzles and the piezoelectric actuators.

In a further embodiment of any of the above servo valves, the servo valve housing further includes a spool cavity housing a spool. The spool has a central spool axis, a first axial end and an opposing second axial end. The spool is configured to translate axially along the spool axis in response to a fluid bias being placed on the spool between the first and second axial ends of the spool. The servo valve further comprises a pair of opposing spool biasing members in contact with the first and second axial ends of the spool respectively. The biasing members are configured to oppose axial translation of the spool along the central spool axis.

In a further embodiment of any of the above servo valve, the servo valve further comprises a supply port, a return port, and first and second control ports. The supply port is upstream of the first and second nozzle cavities in fluid communication with the first and second fluid flow paths via the first and second axial ends of the spool and respective first and second inlet orifices. The return port is downstream of the nozzle cavities in fluid communication with the first and second fluid flow paths and the spool. The first and second control ports are for providing fluid communication between the spool and a hydraulic actuator.

In a further embodiment of any of the above servo valves, the servo valve is configured such that each piezoelectric actuator is independently controllable. The piezoelectric actuators may be configured such that one expands/contracts by a different extent to the other (e.g. by supplying a different amount of voltage to the other or providing one piezoelectric actuator with a different piezoelectric coefficient to the other, to provide a differential rate of expansion/contraction for a given voltage). The first and second piezoelectric actuators may be configured such that they experience changes in dimensions in opposite directions, i.e. as one piezoelectric actuator expands in the axial direction, thus opening the nozzle/gap, the other contracts, thus closing the nozzle/gap (e.g. by supplying voltages of opposite polarity to the piezoelectric actuators). Alternatively, the piezoelectric actuators may be configured to act in the same manner at the same time, i.e. both expanding or contracting concurrently in response to independent control (e.g. by supplying the same voltage to each piezoelectric actuator).

From another aspect, the present disclosure relates to an actuation system comprising the servo valve according to the aspect, or any embodiment thereof, described above, and a hydraulic actuator in fluid communication therewith, such that the servo valve controls the actuator.

From yet another aspect, the present disclosure relates to an actuation system in accordance with claim11.

From yet another aspect, the present disclosure relates to a method of controlling a servo valve in accordance with claim13.

In an embodiment of the above method, the method further comprises supplying voltage to each piezoelectric actuator independently to change the axial dimension of each piezoelectric actuator independently.

In a further embodiment of the above method, the servo valve has a spool having a central spool axis, a first axial end and an opposing second axial end, the first nozzle is in fluid communication with the first axial end of the spool, and the second nozzle is in fluid communication with the second axial end of the spool, the method further comprising generating a fluid bias between the first and second axial ends of the spool by the opening or restricting of said first and second fluid flow paths by the first and second piezoelectric actuators respectively.

It is to be understood, in any of the above aspects or embodiments thereof, that the amount of dimensional change of the piezoelectric actuators varies given the amount of voltage supplied, and that the direction of dimensional change (expansion or contraction) will change depending on voltage polarity.

It is also to be understood that in any of the above aspects or embodiments thereof, independent control of the piezoelectric actuator means the first and second fluid flow paths can be independently opened and restricted/closed. In other words, each piezoelectric actuator can be actuated separately from the other. This may be achieved by having a separate voltage supply for each piezoelectric actuator or by providing a common voltage supply that allows independent switching on and off of each piezoelectric actuator, e.g. using an electronic control. The voltage supplied to each piezoelectric actuator may be positive or negative (i.e. of any polarity), depending on the dimension change desired (i.e. expansion or contraction).

DETAILED DESCRIPTION

With reference toFIG. 1, a prior art servo valve1is illustrated. Servo valve1comprises an electric motor4, flapper2, nozzles6and nozzle housing8. The electric motor4comprises coils4a, permanent magnets4band armature4c. The coils4aare in electrical communication with an electrical supply (not shown) and when activated, interact with the permanent magnets4bto create movement of armature4c, as is known in the art. Flapper2is attached to armature4c, and is deflected by movement of the armature4c. Nozzles6are housed within nozzle housing8via an interference fit and comprise a fluid outlet6aand fluid inlet6b. Housing8also has a port8a, which allows communication of fluid to the nozzles6. The flapper2comprises a blocking element2aat an end thereof which interacts with fluid outlets6aof nozzles6to provide metering of fluid from the fluid outlets6ato a fluid port8bin the housing8, which allows communication of metered fluid from the nozzles6to an actuator via a spool valve input (not shown). As is known in the art, the electric motor4is used to control deflection of the blocking element2aand vary the fluid delivered to the actuator from nozzles6, as required.

With reference toFIG. 2, a servo valve10is illustrated, in accordance with an embodiment of the present disclosure. Servo valve10comprises a pair of opposed nozzles12a,12b, a pair of opposed piezoelectric actuators14a,14b, and a servo valve housing18. The nozzles12a,12bare axially spaced apart and aligned along a common nozzle axis C. Each nozzle12a,12bhas an outlet opening13a,13bcentred on the nozzle axis C. The piezoelectric actuators14a,14bare co-axial with the nozzle axis C. Each piezoelectric actuator14a,14bcomprises a piezoelectric element15a,15band a blocking element16a,16battached at an axial outer end thereof relative to the nozzle axis C, such that each blocking element16a,16bis positioned adjacent a respective one of the nozzle outlet openings13a,13b, between the nozzle outlet opening13a,13band the piezoelectric element15a,15bit is attached to. The piezoelectric actuators14a,14bare used to interact with the nozzle outlet openings13a,13b, as will be discussed below.

Although in the depicted embodiment, blocking elements16a,16bare separate components attached to the piezoelectric elements15a,15b, within the scope of this disclosure, blocking elements16a,16bcould also be integrally formed with the piezoelectric elements15a,15b.

Although the nozzles12a,12b, outlet openings13a,13band piezoelectric actuators14a,14bare depicted in the illustrated embodiment as co-axial along a common nozzle axis C, it is to be understood, that within the scope of this disclosure, this need not be the case. For instance, in other embodiments, the pair of nozzles12a,12bneed not be axially aligned with each other or axially spaced apart from each other. Instead, each nozzle12a,12b(and respective outlet opening13a,13b) may have separate nozzle axes, which are not aligned or co-axial with the other. Piezoelectric actuators14a,14bmay be aligned with a respective one of each separate nozzle axes. In this manner, more flexibility is provided for the positioning of each nozzle12a,12band piezoelectric actuator14a,14b.

The servo valve housing18comprises first and second nozzle cavities18a,18b, which are axially separated from each other by an axially and radially extending wall18c, which is common to the nozzle cavities18a,18band axially centred between them relative to the nozzle axis C. The first nozzle cavity18ahouses the first nozzle12aand the first piezoelectric actuator14a, whilst the second nozzle cavity18bhouses the second nozzle12band the second piezoelectric actuator14b. In this manner, the nozzles12a,12band the piezoelectric actuators14a,14bform pairs, in a respective nozzle cavity18a,18b. In embodiments where the nozzles12a,12bdo not share a common axis, the cavities18a,18bmay be separated by one or more different walls.

The nozzles12a,12bare held in place by respective nozzle retainers11a,11b, which are secured against respective axially extending internal surfaces18a′,18b′ defined by each nozzle cavity18a,18b, which are positioned radially outward from and extend parallel to the nozzle axis C. The piezoelectric actuators14a,14bare retained by the wall18c, and more specifically, by contact between respective first axial ends of the piezoelectric elements15a,15band radially extending internal surfaces18a″,18b″ on opposite axial sides of the wall18c. The blocking elements16a,16bare attached to the piezoelectric elements15a,15bat opposite axial ends of the piezoelectric actuators14a,14bthan those that are in contact with the wall18c(i.e. the axial ends adjacent the nozzle outlet openings13a,13brather than adjacent the wall18c).

Nozzle cavities18a,18bextend through the servo valve housing18from the exterior of the housing18. Caps19a,19bare used to hermetically seal the cavities18a,18bfrom the exterior of the servo valve housing18. In embodiments, the caps19a,19bare removable, such that the nozzle cavities18a,18bcan be accessed easily for maintenance purposes. For instance, caps19a,19bmay be in screw threaded engagement with the servo valve housing18

The piezoelectric actuators14a,14beach comprise at least one piezoelectric material, and are configured such that an electrical signal can be supplied to each piezoelectric actuator14a,14bseparately, for instance, using separate power supplies (not shown). In the depicted embodiment, the piezoelectric actuators14a,14bcomprise a piezoelectric stack. As will be appreciated by one skilled in the art, application of an electrical signal to the piezoelectric actuators14a,14bwill result in a change in dimension in the piezoelectric material, which can be used to move blocking elements16a,16bin an axial direction parallel and co-axial with the nozzle axis C. In this manner, energisation of the piezoelectric actuators14a,14ballows axial translation of the blocking elements16a,16balong the nozzle axis C. As will be understood by the skilled person, the degree of axial translation can be adjusted by varying the amount of voltage/current used to energise the piezoelectric actuators14a,14b. In this manner, piezoelectric actuators14a,14bcan be used to adjust the axial distance between the first blocking element16aand the first nozzle outlet opening13a, and between the second blocking element16band the second nozzle outlet opening13b, in order to control the amount of fluid flow through the nozzles12a,12b. To control the fluid flow more accurately, the first and second blocking elements16a,16bcomprise protrusions16a′,16b′ that extend axially therefrom relative to the nozzle axis C towards the nozzles12a,12b, and which have a planar (i.e. flat) surface in a plane perpendicular to the nozzle axis C, facing the respective nozzle12a,12b. Within the scope of this disclosure, any suitable type of piezoelectric actuator, including any suitable material piezoelectric material, may be used.

Servo valve10further comprises a spool valve assembly. Spool valve assembly includes a spool20having a central spool axis S. Servo valve housing18houses the spool20in a spool cavity22that is in fluid communication with the nozzle outlet openings13a,13bvia first and second opposing axial ends of the spool20. Spool20is configured to translate axially along the spool axis S, within the spool cavity22, in response to a fluid bias being placed on the spool20between the first and second axial ends due to the changes in fluid flow communicated through the nozzles12a,12b.

In the same manner as the nozzle cavities18a,18b, spool cavity22also extends from the exterior of the housing18, and a second pair of caps24a,24bare used to hermetically seal opposing ends of the spool cavity22from the exterior of the servo valve housing18. Caps24a,24bmay also be removable, for instance, by being in screw threaded engagement with the servo valve housing18.

A pair of opposing spool biasing members26a,26bare housed in the spool cavity22, and each biasing member26a,26bis disposed and retained between the first and second axial ends of the spool20and caps24a,24brespectively. The spool biasing members26a,26bare configured to bias the spool20to a central “neutral” axial position in the spool cavity22relative to the spool axis S. In this manner, biasing members26a,26bresist axial translation of the spool20from the central “neutral” position.

Servo valve10further comprises a supply port21for supplying fluid to the nozzle outlets13a,13bfrom a fluid supply (not shown), a return port23for returning fluid from the nozzle outlet openings13a,13bback to the fluid supply, and first and second control ports25a,25bfor delivering fluid from the spool cavity22to an actuator (not shown).

The supply port21is fluidly connected to the nozzle outlets13a,13bby respective channels31a,31b, that extend through the servo valve housing18. Channels31a,31bpass through the spool cavity22between the outer axial ends of the spool20and the caps24a,24b. A third channel31calso allows fluid to be directly communicated from the supply port21to the spool cavity22, without passing through the nozzles12a,12b. Inlet orifices27a,27bare placed in the channels31a,31bto help communicate a fluid pressure volume to the nozzles12a,12b. The return port23is fluidly connected to the nozzle outlet openings13a,13bby respective channels33a,33b, which each extend from the nozzle cavities18a,18bto the spool cavity22, allowing fluid communication thereto from the nozzle outlet openings13a,13b.

As will be understood by the skilled person, by adjusting the axial positioning of the blocking elements16a,16bby selectively energising the piezoelectric actuators14a,14b, the fluid pressure bias communicated to the spool20can be controlled. This, in turn, controls the axial positioning of the spool20in the spool cavity22, which controls the amount of fluid communicated to the actuator via the control ports25a,25b. In this manner, a relatively small movement of the piezoelectric actuators14a,14band blocking elements16a,16bcan produce a highly amplified movement of the actuator.

It is to be appreciated that by replacing the electric motor4and flapper2of the prior art with the piezoelectric actuators14a,14bof this disclosure, a much more compact “pilot stage” of a servo valve can be realised, which reduces weight, size and complexity. Such reductions in weight and size are particularly advantageous in aerospace applications. In addition, the use of piezoelectric actuators14a,14ballows for a more sensitive servo valve10that can make finer and more accurate adjustments than an assembly controlled by an electric motor4and flapper2system. Moreover, the ability to control individual piezoelectric actuators14a,14bto dictate the fluid injected from individual nozzles12a,12b, not only allows flexible positioning of the nozzles12a,12band actuators14a,14bwithin the servo valve10, but also allows even finer calibration of the fluid bias exerted on the spool20. In particular, it is known that the flow characteristics through the nozzles can be different when a current supplied to a piezoelectric actuator is increased compared to when the current supplied is instead decreased. By having two individual piezoelectric actuators14a,14b, such differences can be compensated for.