Pneumatic servo valve with adjustable metering members

A servo valve for an actuator is described. The servo valve comprises a housing and a sleeve provided within the housing having a plurality of metering holes for communication with a cavity within. The servo valve further comprising a metering rod provided with metering members on a central rod for metering flow into and out of the cavity through the metering holes. The metering members comprise annular elements which are located axially on the central rod by an interference fit. The position of the metering members can be adjusted through using heat to thermally expand the metering member relative to the central rod to release the hold of the friction fit, allowing the metering member to be repositioned by being slid along the central rod.

FOREIGN PRIORITY

This application claims priority to European Patent Application No. 17461540.1, filed May 31, 2017, the entire contents of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to pneumatic servo valves.

BACKGROUND

Single stage pneumatic servo valves are known for use in aircraft air management systems such as: engine bleed, cabin air conditioning, pressurization or wing and cowl anti ice protection. Such servo valves are required to operate at various pressures and temperatures depending on their location in the aircraft air management system. They are typically disposed to engage with an actuator, the actuator being further disposed to engage with an air valve such as a butterfly valve. For fast acting air valve actuators, relatively large flows are required depending on size of the actuator and the air valve slew rate. Servo valves that engage fast acting air valve actuators need to handle larger flows of fluid and provide desired output (pressure recovery) proportional to control current.

In order to handle high flow rates larger orifice areas within the servo valve are required. For flapper-nozzle type servo valves, when dealing with high flow rates, higher flow forces act in the direction of flapper movement along the nozzle orifice axis. A torque motor used in such valves is required to overcome them and this causes issues with performance stability of the flapper-nozzle servo valves.

Typical single stage pneumatic servo valves used in aircraft air management systems also experience difficulties associated with the calibration of the servo valve.

SUMMARY OF THE DISCLOSURE

According to a first aspect the present disclosure can be seen to provide a servo valve comprising a housing, a sleeve provided within the housing having a plurality of metering holes for communication with a cavity within, the servo valve further comprising a metering rod provided with metering members on a central rod for metering flow into and out of the cavity through the metering holes, wherein the metering members comprise annular elements which are located axially on the central rod by an interference fit.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the metering rod may comprise at least a first metering member and a second metering member, and the sleeve may comprise at least a first metering hole in communication with a pressure supply port in the housing and at least a second metering hole in communication with an exhaust port in the housing, the first and second metering holes being opened and closed through sealing engagement of the first and second metering members respectively.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the sleeve may comprise a pair of first metering holes and a pair of second metering holes, each metering hole being arranged opposite the other of the pair.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the metering holes of each pair may be equidistant from a port that the metering holes are in communication with.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the sleeve may further include a hole for communication with a control pressure port in the housing.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the metering rod may reciprocate along an axis within the cavity of the servo valve. The metering members may be arranged such that at least one metering hole is being opened while another metering hole is being closed off.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the metering members may have substantially the same dimensions.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the annular elements providing the metering members may comprise flats or other forms of recess in a perimeter thereof to allow pneumatic fluid to pass from one side of each of the metering members to the other.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the metering members may have an elongate, stadium-like cross section in an axial direction, each metering member having two opposed flat surfaces following opposed chords of an otherwise circular perimeter.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the metering rod may be suspended within the cavity by a pair of spring seals spaced axially either side of the metering members, the spring seals being sealingly engaged with the metering rod to seal off the cavity.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, each of the spring seals may sealingly engage with both the metering rod and one of the sleeve or the housing.

In embodiments where the spring seals sealingly engage the sleeve, the sleeve may be pre-assembled with the seals in place and fitted into the housing of the servo valve as a cartridge.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the sleeve may be cylindrical and the metering holes may be arranged to communicate with a port in the housing via an annular recess provided in the outer surface of the sleeve.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the metering holes may comprise circumferentially extending slots in such an annular recess.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the sleeve may be secured within the housing with an interference fit.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the servo valve may comprise a drive unit to reciprocate the metering rod along the axis.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the drive unit may be a direct single solenoid drive.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the servo valve is a three way single stage pneumatic servo valve.

According to another aspect, the present disclosure can be seen to provide an air management system comprising an actuator and a servo valve as described in any of the above statements.

According to another aspect, the present disclosure can be seen to provide a method of making a servo valve comprising: forming a housing; forming a sleeve for location within the housing, the sleeve comprising an axis, an inner surface defining a cavity for the servo valve and a plurality of metering holes for communication with the cavity, assembling a metering rod which comprises metering members positioned along a central rod for metering flow into and out of the cavity through the metering holes, wherein the metering members comprise annular elements that are slid into position along the central rod and held axially by an interference fit.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the method may include a calibration step where an axial position of one or more of the metering members is adjusted to calibrate the servo valve.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the axial position of the one or more metering members may be adjusted through applying mechanical force on the metering member to displace it axially along the central rod.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the axial position of the one or more metering members may be adjusted through using heat to thermally expand the metering member relative to the central rod to release the hold of the friction fit, allowing the metering member to be repositioned.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, during calibration of the servo valve, the metering members may be positioned along the metering rod such that a surface of the first metering member closest to a first end of the sleeve and a surface of the metering member closest to a second end are separated by a distance equal to a separation between a mid-point of a first at least one metering hole along the axis and a mid-point of a second at least one metering hole along the axis.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, during calibration of the servo valve, the metering members may be positioned along the metering rod such that a distance between a surface of the first metering member closest to a first end of the sleeve and a surface of the metering member closest to a second end is equal to a length of a land extending axially along an inner surface of the sleeve between a first metering hole and a second metering hole plus an axial dimension of the metering holes.

In addition to one or more features described above, or as an alternative to any of the foregoing embodiments, the metering members may slide over to occlude and reveal the metering holes, to prevent and allow communication with a supply pressure port and an exhaust pressure port in the housing, wherein the metering members have perpendicular planar faces and the metering holes comprise circumferentially extending slots with planar edges so that the revealed area of the metering holes increases and decreases linearly with reciprocation of the metering rod within the cavity.

DETAILED DESCRIPTION

The present disclosure relates to pneumatic servo valves. In particular, it may be seen to relate to 3-way single stage electro pneumatic servo valves utilized to control air valves that manage air distribution through variety of aircraft air management systems. It describes an improved way of handling high flow rates and high ambient and supply air temperatures, while keeping the design compact and of a relatively small size. It is an alternative for traditional shear orifice type designs, and at least in the illustrated embodiments can be seen to be offering improved calibration with no manufacturing gaping/matching required for setting the operation of the metering members.

As shown inFIG. 2, an aircraft air management system1comprises a servo valve2. The servo valve2may be, for example, a 3-way single stage electro pneumatic servo valve2, as shown.

The servo valve2is shown enlarged and in more detail inFIG. 1and comprises a housing4, the housing4having a supply pressure port6, a control pressure port8and an exhaust pressure port10. The servo valve2also comprises a cavity16(seeFIG. 1), arranged generally centrally within the servo valve2and defining an operational axis A. The cavity16may be cylindrical in shape, extending along the axis A, as shown.

The ports6,8,10are provided for communicating pneumatic lines with the cavity16, for example, a supply pressure Ps, a control pressure Pcand an exhaust pressure Pe, respectively. Depending on the operation of the servo valve2and the pressure within the cavity16at a particular time, each of the ports6,8,10may act as an inlet for introducing a pneumatic fluid, such as air or other pneumatic gas, at a given pressure to the cavity16, or as outlet for allowing the release of the fluid.

Although in the illustrated embodiment only one supply pressure port6, one control pressure port8and one exhaust pressure port10is depicted, it is contemplated that a plurality of supply pressure ports, control pressure ports and/or exhaust pressure ports could be provided, as required for a particular situation. In addition, the ports6,8,10may be of any shape which allows a fluid to flow into and out of the cavity16. Circular ports6,8,10, can be fabricated easily, for example, by drilling holes in the housing4.

The ports6,8,10may be provided all on one side of a housing4as shown for ease of fabrication and for ease of connection to a pneumatic supply and actuator11. Other arrangements are also envisaged where one or more of the ports6,8,10are out of alignment with the others when the servo valve2is viewed along the axis A. Such an arrangement may be desirable where a pneumatic line is coming from a different component or direction.

Proximate the outer ends of the ports6,8,10, where the ports6,8,10are to connect with a pneumatic supply, a seal3is provided. In the embodiment shown, a gasket seal3is disposed on an outer surface of the housing4, providing a common seal3to each of the ports6,8,10, sealing the pneumatic lines from the environment. The gasket seal3is also configured to seal off the supply pressure port6, the control pressure port8and the exhaust pressure port10from one another. The gasket seal3may be made of a compound having greater thermal properties than silicone rubber, which is traditionally used for O-ring seals allowing broad temperature tolerances. Such materials may comprise ceramic fibres in a matrix. Through the provision of such a gasket seal3and other changes, O-ring seals can be avoided in the servo valve2, enabling the servo valve2to be used in higher temperature environments.

The supply pressure port6and the control pressure port8may be connected to an actuator11as shown inFIG. 2. The actuator11depicted inFIG. 2is a half area actuator and is merely exemplary of one type of actuator11that may be used in combination with the servo valve2. The actuator11could take a number of other forms, for instance the actuator11could be a single acting proportional actuator, e.g., for low load applications where servo pressure acts on a single piston against a return spring. In such arrangements the spring stiffness can be set to match the supply pressure variation range. Thus the actuator11is not limited to the actuator configuration in the figures.

The illustrated actuator11has a chamber5with a piston7disposed therein. The piston7has a first face f1and a second face f2. The aircraft air management system1and the actuator11are configured such that fluid entering the supply pressure port6also enters a first side of the actuator11that is defined by the first face f1of the piston7. The aircraft air management system1and actuator11are further configured such that fluid is able to pass between the control pressure port8and a second side of the actuator11defined by the second face f2of the piston7.

The aircraft air management system1may further comprise an air duct15e.g., in the form of a section of pipe which is connected into the air supply. The actuator11is configured such that the piston7engages with a valve13disposed within the air duct15. The valve13is optionally a butterfly valve13that can move from an open position, whereby fluid is allowed to pass freely through the air duct15, to a closed position, whereby fluid is prevented from passing through the air duct15by the butterfly valve13. The piston7is arranged to move the butterfly valve13between the open and closed positions, e.g., such that when the piston7extends fully in a direction toward the second side of the actuator11the butterfly valve13is in the open position and when the piston7extends fully in a direction toward the first side of the actuator11the butterfly valve is in the closed position.

The aircraft air management system1, the actuator11and the air duct15are configured such that fluid entering the supply pressure port6and the first side of the actuator11also enters a first end17of the air duct15. Returning toFIG. 1, the servo valve2can be seen to further comprise an elongate, cylindrical sleeve12disposed about the axis A within the housing4. The sleeve12has an inner surface14which defines a radial extremity of the cavity16within the servo valve2. It also has an outer surface18, at least a portion of which may be in contact with the housing4. As shown inFIG. 3, the sleeve12may have annular recesses20,22defined in its outer surface18that extend around a circumference of the sleeve12. Each recess20,22may connect with one of the ports6,8,10as shown, for example, the supply pressure port6or the exhaust pressure port10, providing a path for fluid to flow around the sleeve12to reach the cavity16or escape therefrom.

Although the sleeve12is described as being cylindrical, it is envisaged that the sleeve might take other elongate shapes, for example, oval or a polygonal prismatic shape such as hexagonal, cuboid or triangular faced prism, with the internal surface of the housing4shaped accordingly. While a cylindrical sleeve12within a cylindrical cavity of a housing4offers a simple fabrication route through machining, other forms of production such as 3D printing are becoming more widespread and may offer opportunities for other shapes.

Optionally, the sleeve12is installed as a sliding interference fit (e.g., a light interference fit) within the housing4and can be removed from the housing4so as to improve upon the line replacibility of the servo valve2. A cylindrical sleeve12would make such sleeve replacement simpler. The outer surface18of the sleeve12may be dimensioned to provide a tight fit within the housing4and may require thermal expansion of the housing4to install the sleeve12within the housing4. Alternatively, the sleeve12may be locked in position via a locking mechanism locking mechanism within the housing4. Both types of installation allow for the sleeve12to be removed from the housing4and so also improves upon the line replacibility of the servo valve2.

The sleeve12has a plurality of holes24,26,28,30,32, e.g., five holes defined therein (as shown more clearly inFIGS. 3 and 5), four of which may be metering holes26,28,30,32. Each hole24,26,28,30,32penetrates through the sleeve12from the inner surface14to the outer surface18such that each hole24,26,28,30,32is in communication with one of the supply pressure port6, the control pressure port8and the exhaust pressure port10, and the respective supply pressure, control pressure and exhaust pressure lines Ps, Pc, Pe.

Whilst in the illustrated embodiment, five holes are present, it is contemplated that the sleeve12may comprise any number of holes provided there is at least one hole in communication with each of the ports6,8,10, e.g., the supply pressure port6, control pressure port8and the exhaust pressure port10. Some of the holes, for example, the metering holes26,28,30,32may be arranged as pairs, one of the pair disposed opposite another, e.g. a pair of metering holes26,28in communication with the supply pressure port6and a pair of metering holes30,32in communication with the exhaust pressure port10.

A first hole24is arranged to be in communication with the control pressure port8. It may be disposed such that the centre of the first hole24is equidistant from a first end38and a second end40of the cavity16.

The first hole24is disposed adjacent the control pressure port8such that the first hole24is generally the same size as and aligns with the control pressure port8. The first hole24may be circular as shown. The first hole24may be effectively a continuation of the control pressure port8, though may also be slightly smaller than the control pressure port8to provide precise control over the flow of pneumatic fluid through the control pressure port8. For example, as will become apparent later, the sleeve12can be machined or otherwise fabricated to a high level of precision and the housing4can be machined or otherwise fabricated to a lower level of precision, reducing production costs of the final servo valve2. The first hole24and the control pressure port8are configured such that fluid exiting the control pressure port8enters the sleeve12through the first hole24.

As can be seen inFIG. 3, the first hole24has a larger area than the other holes26,28,30,32, which are metering holes. This larger area allows for larger flow rates of pneumatic fluid to pass through, into and out of the cavity16. The smaller holes, i.e., the metering holes26,28,30,32, are configured to control the flow of fluid, to meter the flow into and out of the cavity16.

As shown inFIG. 3, a second hole26and a third hole28are disposed to be in communication with the supply pressure port6at a position proximate the first end38of the cavity16. The second and third holes26,28are a first pair of metering holes and are provided in the first annular recess20in order to communicate with the supply pressure port6. In this embodiment, both of these metering holes26,28are in the form of axially aligned, circumferentially extending slots, in particular having an arcuate shape with a rectangular opening projected onto the inner surface14of the sleeve12. However, other shapes may also be appropriate.

The first pair of metering holes26,28may be disposed at diametrically opposite sides of the sleeve12from one another in order to balance pressures on a metering member44arranged to meter flow into the cavity16. As can be seen inFIGS. 3 and 4when the sleeve12is viewed along the axis A, the second and third holes26,28are disposed perpendicular to the first hole24. In this way, flow rates from the supply pressure port6around each path of the first recess20to the respective second and third holes26,28may be, as far as possible, balanced.

It is, however envisioned that any spatial relationship between the first pair of metering holes26,28with respect to the first hole24may be suitable, as long as flow rates and flow pressures are reasonably balanced, e.g., by arranging metering holes symmetrically in the sleeve12.

The second hole26and the third hole28are configured such that fluid passing through the supply pressure port6enters the sleeve12in a controlled manner through the restricted openings of the metering hole26,28.

With continued reference toFIG. 3, a fourth hole30and a fifth hole32are disposed to be in communication with the exhaust pressure port10at a position proximate the second end40of the cavity16. The fourth and fifth holes30,32are a second pair of metering holes and may be disposed at diametrically opposite sides of the sleeve12from one another, in the same way as the first pair of metering holes26,28. The fourth and fifth holes30,32, may also be in the form of aligned, circumferentially extending slots, in particular of an arcuate, rectangular shape, which penetrate through the sleeve12from the inner surface14to the outer surface18of a second annular recess22in the sleeve12. However, as with the first pair of metering holes26,28, other metering hole shapes could be used.

The use of such slots allows the open area of the metering holes26,28,30,32to be increased linearly as they are uncovered by a metering member44, while at the same time minimising the distance that the metering member44must move to open and close off the metering holes26,28,30,32.

The first pair of metering holes26,28may be identical in shape and dimension to the second pair of metering holes30,32.

As seen in the figures, when the sleeve12is viewed along the axis A, the fourth and fifth holes30,32are disposed perpendicular to the first hole24. The second pair of metering holes30,32may mirror the position of the first pair of metering holes26,28; the servo valve2may have symmetry about the first hole24communicating with the control pressure port8. It is, however, envisaged that any spatial relationship between these metering holes30,32and the first hole24is suitable provided that flows are reasonably balanced.

The fourth hole30and the fifth hole32are configured such that fluid exiting the sleeve12through the restricted openings of these metering holes30,32does so in a controlled manner and then passes through the exhaust pressure port10. There may be a linear flow to displacement relationship, as with the first pair of metering holes26,28.

The area of the metering holes26,28,30,32described herein may be chosen dependent upon the desired flow capacity requirements of the servo valve2. In addition or alternatively, the total number of metering holes disposed within the sleeve12may be varied in order to provide a greater overall area for fluid inlet/outlet to the sleeve12in order to achieve a desired flow capacity through the servo valve2. Thus in some arrangements it may be desired to provide more than two metering holes26,28,30,32for the supply port6and the exhaust port10, for example, there may be three, four or other number of metering holes.

The exhaust pressure port10may lead to an exhaust pressure line Pcin a manifold as shown, or may vent straight to outside of the servo valve2, as desired. An advantage of this servo valve2is that it can be used in higher temperature environments. Under such conditions, it may be desirable to transport the heated exhaust fluid from the servo valve2to a different area of the engine or aircraft where the heated fluid can be utilised or vented, e.g., externally of the aircraft.

The servo valve2further comprises a metering rod42disposed in the cavity16that extends along the axis A. It may extend beyond the cavity16too, as shown inFIG. 1, extending through the first end38of the cavity16and the second end40of the cavity16, as will be discussed further below. The metering rod42is arranged for reciprocating movement along the axis A. The metering rod42comprises metering members44,46; namely a first metering member44and a second metering member46. Whilst the illustrated embodiment comprises two metering members44,46, it is contemplated that the metering rod42may comprise three or more metering members as necessary in a particular set up.

The first metering member44and second metering member46may be independent of one another and/or may be identical in dimension. For example, the metering members44,46may comprise individual annular elements, for example, substantially disc-shaped elements, that are mounted on a central rod42by an interference fit. The metering members44,46and central rod43may comprise any suitable high temperature metal. The interference fit allows the position of the metering members44,46to be adjusted, e.g. through applying mechanical force and/or through using thermal expansion to release the hold of the friction fit.

The metering members44,46may comprise apertures or flats which allow pneumatic fluid to pass from one side to the other within the cavity16. In addition or alternatively, one or more channels could be provided in the inner surface14of the sleeve12to allow pneumatic fluid to flow around the perimeter of the metering members44,46.

In a method of making a servo valve2, the method may include a step of adjusting the position of the metering members44,46on the metering rod42through thermally expanding the metering member44,46with respect to the metering rod42and sliding it along the metering rod42. The same operation may be performed for calibrating the servo valve2since this allows for fine adjustment of the servo valve2without having to resort to grinding or gaping the metering members44,46.

In the illustrated embodiment, the metering members44,46are positioned along the metering rod42such that a surface of the first metering member44closest to the first end38of the sleeve12and a surface of the metering member46closest to the second end40are separated by a distance equal to the separation between a mid-point of the second and third holes26,28along the axis A and a mid-point of the third and fourth holes30,32along the axis A. To put this another way, the distance between a surface of the first metering member44closest to the first end38of the sleeve12and a surface of the metering member46closest to the second end40may be equal to the length of the land between the pairs of metering holes plus the axial dimension of one pair of the metering holes26,28,30,32.

In this way, as the metering rod42reciprocates along the axis A, one pair of metering holes26,28is being opened while another pair of metering holes30,32is being closed off, and vice versa.

The purpose of the metering members44,46is to meter, as precisely as possible, the flow into and out of the cavity16through the two pairs of metering holes26,28,30,32. This is achieved by the metering members44,46sliding over to occlude and reveal these metering holes26,28,30,32, thereby preventing and allowing communication respectively with the supply pressure port6and the exhaust pressure port10. For reasons of calibration, it can be advantageous for the revealed area of the metering holes26,28,30,32to increase and decrease linearly with reciprocation of the metering rod42.

The metering members44,46are not pistons which seal off one portion of the cavity16from another; instead paths are provided for the pneumatic fluid to flow around the edge of or through the metering members44,46, from one side of a metering member44,46to another.

As shown inFIG. 4, the metering members44,46may appear to have an elongate, almost stadium like cross section, where each metering member44,46has two opposed flat surfaces s1, s2following opposed chords of an otherwise circular perimeter. The flat surfaces s1, s2create gaps through from one side of the metering member44,46to the other where the perimeter of the metering member44,46departs from the inner surface14of the sleeve12. Two opposed curved surfaces c1, c2are arranged at either end of these flat surfaces s1, s2, the two curved surfaces conforming to the shape of the inner surface14of the sleeve12such that the curved surfaces c1, c2contact the inner surface14of the sleeve12to occlude and reveal the respective metering holes26,28,30,32.

The metering rod42may be movably sealingly positioned within the cavity16of the servo valve2without polymeric seals and without sliding seals. As can be seen inFIG. 3, the servo valve2may further comprise a first spring seal34disposed within the housing4at a first end38of the sleeve12and a second spring seal36disposed within the housing4at a second end40of the sleeve12. The spring seals34,36effectively replace conventional arrangements where O-ring seals would have been used to seal the metering rod42and provide the working cavity16within a valve housing4. As a result, better operating temperature ranges can be achieved.

The first spring seal34may be attached to the metering rod42at a first point42aand the second spring seal36may be attached to the metering rod42at a second point42b. The ends of the metering rod42may extend beyond the first and second points42a,42b, into an internal space of the housing.

Each of the spring seals34,36may sealingly engage with both the metering rod and one of the sleeve16or the housing4. In embodiments where the first and second spring seals34,36are connected to the sleeve12for closing off the ends of the cavity16, the sleeve12can be slid into the housing4as a replaceable cartridge with the spring seals34,36already in place.

The first spring seal34is arranged to close off the first end38of the sleeve12and the second spring seal36is arranged to close off the second end40of the sleeve12, such that the cavity16is fluidly sealed at the first and second ends38,40by the first and second spring seals34,36, respectively.

The first and second spring seals34,36are configured and arranged such that the spring forces from the internal pressure within the cavity16are balanced. For example, the first and second spring seals34,36may have substantially the same spring constant in a direction along the axis A. The first and second spring seals34,36may comprise metal diaphragms. The metal may have high elasticity, for example, it might be spring steel, in particular a non-magnetic spring steel. The first and second spring seals34,36may be of equal area and have matching profiles, the second spring seal36reversed with respect to the first spring seal34.

Thus the first spring seal34exerts a component of spring force on the metering rod42along the axis A resulting from the internal pressure within the cavity16that is equal and opposite to a component of spring force exerted by the second spring seal36on the metering rod42along the axis A resulting from the same internal pressure within the cavity16.

Displacement of the metering rod42along the axis A by a drive unit48will generate other components of spring force which will urge the metering rod42to return back to its neutral position. Additional forces may also be present from pressure on a butterfly valve associated with the servo valve2.

The spring seals34,36may comprise a range of different profiles to provide a diaphragm between the metering rod42and the sleeve12. As demonstrated byFIG. 9, the two spring seals34,36may, optionally, be in the form of bellows. The spring seals34,36, may for example, comprise an undulating profile in a radial direction of the servo valve2. It may comprise a sinusoidal profile of reducing displacement in the radial direction. In this way, with the undulating profile, the spring deformation of the spring seals34,36along the axis A may give rise to a substantially linear relationship with spring force as shown inFIG. 8. As can be seen in exemplary embodiment ofFIG. 8, the spring constant (force/displacement) of the spring seals34,36may be between 28000 N m−1-52000 N m−1for certain applications By way of example, the spring constant of the spring seals34,36may be 40000 N m−1.

In the illustrated embodiment, the first spring seal34and the second spring seal36are depicted as being identical in surface area, having an outer radius determined by the size of the sleeve12and an inner radius determined by the cross-section of the metering rod42at points42aand42bwhere the spring seals34,36are mounted. In theory, the cavity16could vary in diameter and the spring seals34,36could differ in size, provided that the spring constants of the first spring seal34and the second spring seal36are adjusted accordingly, so that the first and second spring seals34,36always provide an equal and opposite spring force component resulting from the internal pressure to the metering rod42. One or both of the spring seals34,36could instead connect to the housing4, as desired.

The servo valve2is configured to meter pneumatic fluid to the actuator11via movement of the metering rod42along the axis A. Three non-limiting positions of the metering rod42with respect to the sleeve12and housing4will now be described to demonstrate how the servo valve2is able to meter fluid to the actuator11.

In a first position, as demonstrated inFIG. 5, when an operating solenoid coil of the drive unit48is de-energised, the metering rod42is disposed along the axis A toward the first end38of the cavity16such that the first metering member44overlaps (occludes) and seals the first pair of metering holes26,28. Thus, the pneumatic fluid that is supplied through the pressure supply port6is prevented from entering the cavity16by the first metering member44blocking its path. In the first position, the second metering member46is simultaneously displaced toward the first end38of the cavity16relative to the second pair of metering holes30,32(i.e., they are open) such that fluid in the cavity16can pass freely into the exhaust pressure port10.

When the metering rod42is in the first position the first spring seal34and the second spring seal36are at an equilibrium position such that the first spring seal34and the second spring seal36have no relative bias along the axis A.

Whilst the metering rod42is in the first position, fluid in the second side of the actuator11is free to pass through the control pressure port8into the cavity16and then, subsequently, out of the cavity16through the second pair of metering holes30,32into and through the exhaust pressure port10. As the fluid passes from the second side of the actuator11into the cavity16, a pressure corresponding to the exhaust pressure is reached in the cavity16and applied to the first spring seal34and the second spring seal36by this fluid. The same pressure is experienced by both spring seals34,36, and as such, the force acting on the first spring seal34from the introduction of this fluid is equal and opposite to the force acting on the second spring seal36from the introduction of this fluid. Thus, there is no net force acting on the metering rod42as a result of the pressure within the fluid, and as result, there is also no net movement of the metering rod42arising from the pressure within the fluid.

Also in the first position, the fluid in the second side of the actuator11is allowed to pass into the exhaust pressure port10via the cavity16. Thus, there is a reduced fluid pressure in the second side of the actuator11. As a result, the force applied to the second face f2of the piston7is also reduced. The force acting on the first face f1of the piston7is larger than the force acting on the second face f2of the piston7, so the piston7is urged to extend in a direction toward the second side of the actuator11. In addition to the forces acting on the piston7from the fluid pressure acting on its faces f1, f2, the supply pressure Psentering the first side17of the air duct15simultaneously acts on the butterfly valve13. The force produced by the interaction of the supply pressure Pswith the butterfly valve13in turn acts on the piston7. The output force from the actuator may increase with the supply pressure and match the increase in load on the butterfly valve. Due to the2to1area ratio in the exemplary embodiment, servo pressure (control pressure) is one-half of the supply pressure in the balanced mode (neglecting butterfly loads). In a control mode, servo pressure may be modulated to overcome the butterfly loads, causing the valve to move in response to exerted current to desired butterfly openings. In a second position, as demonstrated byFIG. 6, when a solenoid coil of the drive unit48is energised by 50%, the metering rod42is disposed such that the first metering member44overlaps with and seals 50% of the area of the first pair of metering holes26,28. Thus, fluid in the supply pressure port6can pass into the cavity16(and build up pressure in this cavity16) via the second and third holes26,28at a flow rate determined by the area of the metering holes26,28that has been uncovered. In the second position, the second metering member46is simultaneously positioned such that it overlaps with and seals 50% of the area of the second pair of metering holes30,32.

Thus, fluid in the cavity is able to pass into the exhaust pressure port10at a flow rate determined by the uncovered area of the fourth and fifth holes30,32. As the areas of the metering holes26,28,30,32are equal, when the metering rod42is in the second position the flow rate of fluid supplied to the cavity16through the first pair of metering holes26,28(second and third holes26,28) is equal to the flow rate of fluid exiting the cavity16through the second pair of metering holes30,32(fourth and fifth holes30,32). Since fluid enters the cavity16from the supply pressure port6at a rate that is equal to fluid exiting the cavity16through the exhaust pressure port10, the pressure within the cavity16is equal to the pressure of the fluid in the second side of the actuator11. By controlling the position of the metering rod42around this 50% position, the pressure can be modulated through alternating the balance of the flow through the different pairs of metering holes26,28,30,32.

In a steady state, that is when the butterfly is at a desired position and is not modulating, there is no flow to or from the actuator11via the pressure control line Pc. This means that whatever amount of fluid enters cavity16, the same amount will leave it. When there is 50% overlap the flow rate through the valve2(from supply port6to exhaust port10) is highest. As the metering rod42is displaced along the axis A in one or other direction to make the overlap closer to 40% or 60% for either pair of metering holes26,28,30,32, the flow rate decreases. Flow rate through the servo valve2(from the supply port6to the exhaust port10, with the pressure in the control port8in equilibrium with the cavity pressure) as a function of control current may have a generally parabolic profile.

When the metering rod42is in the second position the first spring seal34and the second spring seal36are also deformed against their spring bias in a direction along the axis A away from the first end38of the cavity16. The first spring seal34and the second spring seal36are deformed by an equal displacement along the axis A such that the volume of the cavity16remains constant.

The pressure applied to the first spring seal34by the fluid in the cavity16and second side of the actuator11is equal to the pressure applied by the fluid to the second spring seal36. As such, the force acting on the first spring seal34from the internal pressure of this fluid is equal and opposite to the force acting on the second spring seal36. Thus, there is no net force acting on the metering rod42as a result of the pressure in the fluid, and so there is no displacement of the metering rod42as a result of the internal pressure within the cavity16.

In the second position, fluid in the second side of the actuator11applies a control pressure to the second face f2of the piston7which is different to the supply pressure in the first side of the actuator11applied to the first face f1. The piston7is, hence, forced to move in a direction toward the second side of the actuator11until the force applied to the first face f1of the piston7is equal to the force applied to the second face f2of the piston7(neglecting butterfly loads). I.e., the piston is forced to move to a position where the below equation is satisfied:
Af1Ps=Af2Pc
where Af1is the area of the first face f1of the piston7, Af2is the area of the second face f2of the piston, Psis the supply pressure supplied to the first side of the actuator11, Pcis the control pressure in the second side of the actuator11.

In a third position, as demonstrated byFIG. 7, when a solenoid coil of the drive unit48is energised 100%, the metering rod42is moved along the axis A toward the second end40of the cavity16such the second metering member46overlaps and seals the second pair of metering holes30,32(the fourth and fifth holes30,32). Thus, the fluid that is in the cavity16is prevented from exiting the cavity16through the exhaust pressure port10by the second metering member46. In the third position, the first metering member44is simultaneously positioned toward the second end40of the cavity16relative to the first pair of metering holes26,28, such that fluid in the supply pressure port6can freely pass into the cavity16. When the metering rod42is in the third position the first spring seal34and the second spring seal36are extended in a direction along the axis A away from the first end38of the cavity16. The first spring seal34and the second spring seal36are displaced an equal distance relative to one another along the axis A such that the volume of the cavity16remains constant. The displacement of the first spring seal34and the second spring seal36in the third position is greater than the displacement of the first spring seal34and the second spring seal36in the second position

When the metering rod42is in the third position, fluid is free to pass from the supply pressure port6into the cavity16via the second and third holes26,28, then through the control pressure port8via the first hole24, and subsequently into the second side of the actuator11. Hence, the pressure in the first side of the actuator11and the second side of the actuator11is equal; it is the supply pressure Ps.

As the pneumatic fluid passes from the supply pressure port6to the cavity16, the supply pressure Pswithin the cavity16exerts a force on the first and the second spring seals34,36. The pressure applied to the first spring seal34by this fluid is equal to the pressure applied by the fluid to the second spring seal36. As such, the force acting on the first spring seal34from the supply pressure Psis equal and opposite to the force acting on the second spring seal36from the supply pressure Ps. Thus, there is no net force acting on the metering rod42from the pressure within the cavity16, and so no there is no associated displacement of the metering rod42as a result of the pressure in the fluid.

In the third position, as pneumatic fluid is allowed to pass from the supply pressure port6to the second side of the actuator11, the pressure of the fluid in the second side of the actuator11is equal to the pressure of the fluid in the supply pressure port6, the cavity16and the first side of the actuator11. Consequently, the pressure applied to the first face f1of the piston7is equal to the pressure applied to the second face f2of the piston7. The second face f2of the piston7is larger than the first face f1of the piston7, at least in the illustrated embodiment, hence the force acting on the second face f2of the piston7is larger than the force acting on the first face f1of the piston7and so the piston7extends fully in a direction toward the first side of the actuator11.

Whilst the function of the servo valve2has been described with reference to the metering rod42in three different positions, the servo valve2may take any position in between the first and third positions to achieve the desired actuator extension.

With reference again toFIG. 1, a drive unit48is provided to reciprocate the metering rod42along the axis A. The drive unit48may be a direct single solenoid drive48. In this way, improved calibration and control can be achieved through a linear relationship of excitation and displacement. However, it is envisaged that other driving mechanisms may be suitable for driving the metering rod42into the different positions.

The drive unit48may comprise a drive unit housing50as shown inFIG. 1. The drive unit housing50may be made of metal such as a non-magnetic stainless steel. Such a material provides improved high temperature strength offering mechanical protection to the other components of the direct single solenoid drive48. The drive unit housing50may also be made of any other material that offers suitable thermal properties.

At a first end52of the drive unit48a soft magnetic adjustable core54is disposed. The adjustable core54has a first portion56and a second portion58. The first portion56of the adjustable core54is threaded with a thread59and partially disposed through the drive unit housing50. A corresponding thread51is provided in the drive unit housing50. Thus, by rotating the first portion of the adjustable core54about the axis A, the adjustable core54can be moved along the axis A in a direction either closer to the cavity16or away from it, depending on the direction of rotation.

The drive unit48further comprises a soft magnetic plunger62, having a first end64spaced from the second portion58of the adjustable core54. The space between the second portion58and the first end64of the plunger62can be adjusted through rotation of the adjustable core54about axis A. By adjusting the (air) gap between the first end64of the plunger62and the second portion58of the adjustable core54the resistance within the drive unit48can be altered. Thus, the current/displacement characteristics of the solenoid can be influenced by adjusting said gap.

For example, an end of the adjustable core54extending out of the drive unit housing50may be provided with a torque transferring feature78, for example for applying torque using a tool such as an allen key, a screw driver, a socket driver, etc., to rotate the adjustable core with respect to the screw thread and change the relative position of the adjustable core54with respect to the solenoid68.

The soft magnetic plunger62may have a frusto-conical portion or tip76at one end which overlaps with a frusto-conical recess74of the core54. The other end of the plunger62is configured to attach to an end of the metering rod42extending through the second spring end40of the cavity16. Thus the plunger62may comprise a threaded recess63for threaded engagement with an end of the metering rod42. The soft magnetic plunger62is arranged for reciprocating movement along the axis A.

A solenoid68is also disposed within the drive unit housing50. The solenoid68is arranged to surround an extent of the plunger62and a portion of the core54. The solenoid68is configured so that when it is energised, the solenoid68urges the plunger62away from the cavity16of the servo valve2(i.e., pulled to the right of the figure inFIG. 1) against the bias of the spring seals34,36. The solenoid68may have ceramic insulation around the windings to improve thermal insulation of the coils disposed therein. The solenoid68may be energised by any desired command current so as to achieve the necessary flow requirements within the servo valve2.

A soft magnetic pole70is provided surrounding the second portion58of the adjustable core54.

A soft magnetic pole72is provided to surround part of the plunger62.

The poles70,72, the plunger62and the core54may all comprise soft magnetic materials. The drive unit48may comprise no permanent magnets, thus giving it improved performance at higher temperatures where permanent magnets typically demonstrate a drop in their magnetic properties.

When the metering rod42is in the first position the solenoid68is de-energised. No electromagnetic forces act on the soft magnetic plunger62leaving it positioned toward the cavity16due to the bias of the first spring seal34and the second spring seal36in their equilibrium positions.

When the metering rod42is in the second position a command current has been applied to the solenoid68to energise it to a 50% state. An electromagnetic force is produced by the solenoid68that acts on the plunger62, urging it into an intermediate position such that the plunger is moved away from the cavity16to open the metering holes by 50%.

When the metering rod42is in the third position a command current has been applied to the solenoid68to energise it to a 100% state. An electromagnetic force is produced by the solenoid68that acts on the plunger62, urging it to a fully extended position such that the plunger is moved further away from the cavity16.

Whilst the direct single solenoid drive is described here at three different solenoid energy states, the coil may be energised to any state desired so as to move the metering rod42to any position to meter a desired flow of fluid within the servo valve2. In addition, while the drive unit48is shown as one which returns the metering rod42to a position where the first pair of metering holes26,28are closed off when the power is switched off, the drive unit48could be set up to overcome a biasing force of a spring arrangement which returns the metering rod42to an alternative position in the event of no electrical power. The drive unit48could also be arranged on the other end of the cavity to push the metering rod42rather than pull it.

Thus at least in the illustrated embodiments, the direct drive assembly may be protected from the ambient by a stainless steel cover. The servo valve, due to lack of O-rings and use of ceramic insulation on wire of the coils, may be far more resistant to higher temperatures than known servo valves. It may be utilized to control the airflow in the engine bleed system where high temperatures occur. A lack of permanent magnets also helps in such high temperature environments (where usually magnetic properties drop with increase in temperature). Further by changing the size of the metering holes in the sleeve, one can change the flow capacity of the servo valve to any desired value, including high flow rates, without increasing the overall size of the assembly. The servo valve may maintain a compact size compared to known valves. It may provide a single stage pneumatic servo valves for use in aircraft air management systems, such as but not limited to engine bleed, cabin air conditioning, pressurization or wing and cowl anti ice protection. It has a modular design and offers the possibility of using different sleeves with various metering hole sizes. The position of the metering members can be adjusted to allow easier calibration.