This application claims priority to European Patent Application No. 22461651.6 filed Dec. 23, 2022, the entire contents of which is incorporated herein by reference.
The present disclosure is concerned with servo valves and, in particular, two-stage servo valves.
Servo valves are mechanisms which control fluid flow to effect driving or control of another part e.g. an actuator, and find a wide range of applications for controlling oil, air, fuel or other fluid flows.
Various types of servo valves are known, examples of which are described in UK Patent Application No. GB 2104249A, U.S. Patent Application Publication No. 2015/0047729 and U.S. Pat. No. 9,309,900, all of which are incorporated herein by reference.
Generally, a servo valve transforms an input control signal into movement of a spool to vary fluid flow through the servo valve. Conventionally, servo valve systems operate by obtaining pressurised fluid from a high pressure source, e.g. via a pump. In the case of two-stage servo valves, a first stage with a motor e.g. an electrical or electromagnetic force motor or torque motor is controlling a moveable member, typically a flapper, or a jet pipe, is deflected by action of an armature connected to the motor away from or towards nozzles, which controls the fluid flow through the nozzles. The motor can operate to position the moveable member in response to an input drive signal or control current, to control flow through the first, pilot, stage which controls fluid flow to drive the second stage valve member e.g. the spool valve by controlling the flow of fluid acting on the spool. Movement of the spool causes alignment between the ports and fluid channels to be changed to define different flow paths for the control flow which, in turn, can control flow of hydraulic fluid and by creating a pressure imbalance on an element that is controlled by the servo valve e.g. an actuator, enables precise movement or force control.
Such mechanisms are used, for example, in various parts of aircraft where the management of fluid/air flow is required, such as in engine fuel control, oil flow, engine bleeding systems, anti-ice systems, air conditioning systems and cabin pressure systems, as well as landing gear systems, braking systems and primary flight control surfaces. Servo valves also are widely used to control the flow and pressure of pneumatic, fuel and hydraulic fluids to an actuator, e.g. to control moving parts such as fuel or air systems. Some examples of applications are aircraft, automotive systems and in the space industry.
Whilst such servo valves have been found to be effective in many applications, they are relatively large, bulky and heavy parts and the component parts have to be machined and assembled with high accuracy, which increases the cost and time of manufacture and assembly. In aircraft, and other fields, there are restrictions on permitted size and weight of parts and cost and efficiency is also often a limiting factor. There is, therefore, a need for a servo valve that can be manufactured and assembled more quickly and at lower cost, is smaller, simpler and lighter and enables reliable and effective operation.
According to the present disclosure, there is provided a two-stage servo valve comprising a first, drive stage and a second fluid transfer stage, the fluid transfer stage comprising a housing having a plurality of ports and a spool axially moveable within an axial cavity defined within the housing to control flow of fluid between the plurality of ports according to the axial position of the spool, wherein the drive stage is configured to cause axial movement of the spool; wherein the housing comprises three ports, a first port being a supply port fluidly connected to a second port, being a control port, via a supply channel and a first control channel via the spool cavity, and a third port being a return port fluidly connected to the control port via a return channel and a second control channel via the spool cavity, and wherein the spool comprises a middle portion and first and second end portions, a first opening defined between a first edge of the middle portion and the first end portion and a second opening defined between a second edge of the middle portion and the second end portion, the first and second openings being sized such that in a neutral position, the first opening does not overlap both the supply channel and the first control channel and the first edge K1 is positioned to prevent fluid flow between the supply channel and the first control channel and the second opening does not overlap both the return channel and the second control channel and the second edge L1 is positioned to prevent fluid flow between the second control channel and the return channel; in a first axial position, the first edge K1 is positioned such that the first opening overlaps at least a portion of both the supply channel and the first control channel to allow fluid flow between the supply channel and the first control channel, and the second opening does not overlap both the return channel and the second control channel and the second edge L1 is positioned to prevent fluid flow between the second control channel and the return channel; and in a second axial position, the second edge L1 is positioned such that the second opening overlaps at least a portion of both the return channel and the second control channel to allow fluid flow between the return channel and the second control channel, and the first opening does not overlap both the supply channel and the first control channel and the first edge K1 is positioned to prevent fluid flow between the supply channel and the first control channel.
Embodiments will now be described, by way of example only, with reference to the Figures.
With reference to
The servo valve has a fluid flow system. This provides for the flow of a working fluid, such as a hydraulic fluid, for operating the servo valve 10. Hydraulic fluid is, for example, fuel or oil. The fluid flow system includes a fluid supply (not shown) from which fluid is provided to the servo valve via an inlet fluid port 17. The fluid supply is a common fluid supply to both the first stage 11 and the second stage 12.
The first stage 11 of the servo valve comprises a flexible moveable member 16 which, in this example, is a flapper, which is actuated by the electric motor 13. Other examples may use a jet pipe as the moveable member, through which fluid is provided. The armature of the electric motor 13 causes the flapper to be deflected. The first stage 11 comprises two axially aligned, opposed first stage nozzles 18a, 18b. The first stage nozzles are housed within a nozzle chamber 19 and comprise fluid outlets 20a, 20b which are spaced apart from each other. Working fluid is received at the first stage nozzles from the fluid supply via the inlet port 17 and via respective channels 21a, 21b. The flapper is received between the fluid outlets of the first stage nozzles. The flapper interacts with the fluid outlets of the first stage nozzles to alternately block the nozzles and provide metering of fluid from the fluid outlets, according to the axial position of the flapper as controlled by the drive stage. Blockage of each fluid outlet of the first stage nozzles causes a pressure differential between different sides of the first stage 11 of the servo valve 10 which is provided to the second stage 12 at corresponding spool end ports 22a, 22b to control operation of the second stage 12. The second stage comprises a movable spool 30 axially moveable within cavity 42 defined by a spool sleeve 40 in the housing. Spool end ports 22a, 22b are provided at each end of the cavity 42 to provide pressurized fluid to act on the ends of the spool to cause it to move axially within the sleeve, due to pressure imbalance.
In operation, fluid from the supply passes (usually through filters) through apertures in the sleeve into the cavity and also passes to the nozzles via the channels 21a, 21b and to the spool end ports 22a, 22b. If, for example and with reference to
The spool 30 is formed to have a number of spool portions 31, 32, 33, 34 along the axial length of the spool that define edges K, L, M, N extending across the radial extent of the cavity in which the spool moves and which define chambers 35, 36 in the cavity between the edges. As seen, in this example, the spool 30 has opposing end portions 31, 34 defining, respectively, inner edges K and N, and two inner portions 32, 33 which abut at their axially inner sides and define axially outer edges L, M. A first chamber 35 is defined between edges K and L and a second chamber 36 is defined between edges M and N. All four edges are needed to define the flow paths through the servo valve.
The sleeve 40 is provided with a number of openings 43, 44, 45, 46 which align with ports in the servo valve housing. The position of the spool portions relative to the openings determines whether fluid can flow between the ports or not. Typically, a servo valve will have four ports—a supply port for providing the fluid from the supply to the servo valve as described above, a return port via which fluid flowing through the nozzles is returned to the fluid supply, and two control or output ports to provide control fluid to the device e.g. actuator to be driven by the servo valve.
Movement of the spool 30 in the sleeve moves the spool portions and edges relative to the openings in the sleeve. If the spool is moved such that a spool portion lies over an opening, the opening is blocked by the spool and fluid cannot flow through it, from the cavity, or supply port. In an example, the spool may be moved to the right so that one control port is connected to the supply and the other control port is connected to return. Movement of the spool in the opposite direction connects the first control port to return and the other control port to the supply.
The spool, and particularly its spool portions and edges, as well as the sleeve and the position of the apertures and the four ports in the housing all have to be manufactured and assembled with great precision which is expensive and time consuming.
The servo valve according to this disclosure is a two-stage servo valve that, in principle, operates in a manner similar to that of the conventional servo valve described above, but using a simpler spool and housing design that is simpler, smaller, lighter, quicker and less expensive to manufacture and assemble.
Embodiments of the servo valve according to the disclosure will be described, by way of example only, with reference to
In the example shown, the drive stage and the first stage of the servo valve are the same as in the known design described above with reference to
The second stage of the servo valve according to this disclosure has a simplified housing having only three ports rather than the four ports of the known designs, since the control port is fluidly connected to a single chamber, rather than having two control ports connected to two chambers. Further, the servo valve has a simplified spool having only two control edges K′, L′ that need to be positioned to control fluid flow through the valve, rather than four, as in the known designs. The simplified design is based on the premise that many applications for servo valves do not actually require all four of the ports of the standard designs.
The housing 140 houses the first 11 and second 12 stages of the servo-valve. The first stage is conventional and will not be described further.
The second stage includes a spool 50 mounted within the housing (similar to the known design) for axial movement within a cavity 142 defined by the housing 140. In the example shown, opposing axial ends of the cavity 142 are sealed by respective plugs 60. The spool 50 defines first and second openings 52, 54 that move axially with respect to three axially spaced ports in the housing, namely a supply port 70, a control port 80 and a return port 90. The first and second openings 52, 54 are defined between a respective edge K1, L1 of a central portion 53 of the spool and respective spool ends 51a, 51b, edges K1, L1 defining control edges as described below.
The three ports formed in the housing are fluidly connected to each other via the first and second openings 52, 54, via flow channels sc, cc1, cc2 and rc, as seen in
In a manner similar to that described above in relation to
In this example, the control port 80 is fluidly connected to control movement of an actuator e.g. a piston 200 mounted in a cylinder 250. The piston head 220 extends radially across the cylinder to divide the cylinder into two chambers 222, 224. The first chamber 222 is fluidly connected to the control port 80 of the servo valve and the second chamber 224 is fluid connected to the fluid supply (here a pump 100).
When the servo valve is positioned as in
When, as shown in
By having a simplified spool and housing, where the spool only has two control edges and the housing only has three ports, the servo valve of the disclosure is much smaller, lighter and less expensive and time consuming to manufacture. As the precise machining of the spool gives rise to a large part of the manufacturing costs of a spool valve, simplifying this design provides considerable cost savings.
Further, by having a smaller spool, which provides less resistance to movement (due to a smaller contact area between the spool and the sleeve) the servo valve of the present disclosure has improved dynamic behavior A smaller hydrodynamic force is generated when fluid flows through the new spool. The servo valve can, therefore, respond more quickly to the command signals.
The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, element components, and/or groups thereof.
While the present disclosure has been described with reference to an exemplary embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this present disclosure, but that the present disclosure will include all embodiments falling within the scope of the claims.
Number | Date | Country | Kind |
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22461651.6 | Dec 2022 | EP | regional |