ROTARY SERVO VALVE

Description

Field of the Invention

[0001] The present invention concerns improvements in and relating to servo valves. More particularly, the present invention concerns an improved spool and housing design for a rotary servo valve and an improved two-stage valve using such a rotary servo valve.Background of the Invention

[0002] Servo valves are used in a wide variety of industries to control the movement of hydraulic or pneumatic actuators in response to an input signal and are employed in industries where precise control of an actuator is required, for example in the aerospace industry. Servo valves alter the flow of a fluid through the valve in order to control the position, velocity, acceleration or force generated by an actuator, for example a hydraulic or pneumatic cylinder or motor.

[0003] EP0102884A1 discloses a direct drive servo valve that includes a rotary torque motor having an output shaft connected to a spool member forming part of a hydraulic control valve. The valve further includes a housing having conduits connected to high and low pressure sides of a fluid pressure source and to opposite sides of an actuator. A sleeve in the housing includes a plurality of passageways connecting the conduits with channels in the sleeve to direct the working fluid as required to cause the associated actuator to move in a desired direction. The sleeve is formed of a plurality of disks having the desired internal patterns such that when they are assembled, brazed together, and machined to the desired dimensions, the passages are formed with openings as required. A torque tube fastened between the shaft and the sleeve housing acts as a centering spring and seals operating fluid from the motor windings.

[0004] WO2016 / 075491A1 discloses a servo actuator comprising an actuator housing; an actuator member located within the actuator housing and at least one spool located in a cavity formed within the actuator housing. The housing also comprises a first set of internal ports including an inlet port, an outlet port and a pair of control ports, the inlet port being arranged for connection to a first pressurised supply and a second set of internal ports comprising an inlet port, an outlet port and a pair of control ports, the inlet port being arranged for connection to a second pressurised supply. In use, movement of the spool alters the flow path of fluid through the first and second set of internal ports to control the movement of the actuator member.

[0005] US2946348A relates to hydraulic valves in hydraulic control systems of the kind comprising a source of liquid under pressure, an actuator such as a hydraulic jack or motor and a valve for connecting one or other side of the actuator to the source of supply of liquid under pressure and to a low pressure outlet respectively.

[0006] A servo valve typically comprises a moving element (spool) and a fixed element (for example a housing, sleeve or manifold) including fluid inlets and outlets. The relative movement of these two elements controls the flow of fluid through the valve.

[0007] A rotary servo valve typically comprises a spool mounted for rotation relative to the fixed element defining the fluid inlets and outlets. By rotating the spool relative to the fluid inlets and outlets a flow path between the fluid inlets and outlets may be provided or altered thereby allowing control of the flow of fluid through the valve.

[0008] A linear servo valve typically comprises a spool mounted for axial displacement relative to the fixed element defining the fluid inlets and outlets. By moving the spool back and forth relative to the inlets and outlets a flow path between the fluid inlets and outlets may be provided or altered thereby allowing control of the flow of fluid through the valve.

[0009] Rotary servo valves commonly have either a "single-stage" design, in which the spool is driven by a motor (usually an electric motor) coupled, usually via reduction gearing, to the spool, or a "two-stage" design, in which the spool is driven by a separate pilot stage. An advantage provided by single-stage valves compared to two-or-more-stage valves, is lack of sensitivity to changes in supply pressure or viscosity. Single-stage valves are often useful in applications where pilot flow for first-stage operation is not available; however, they cannot typically generate the high spool driving forces of piloted two-stage valves.

[0010] Use of rotary servo valves has been limited in high-power fluid systems because rotary servo valves tend to be less efficient than linear servo valves as a result of greater operating friction. However, they provide the advantage of being easier to couple to rotary electrical motors.

[0011] A known rotary servo valve design has a housing including pairs of ports diametrically opposed around a central rotating spool. The pressure in each port is duplicated and supplied to the diametrically opposed port. As the spool rotates, different pathways are progressively opened and closed between ports at different pressures.

[0012] A problem with such a design is that the pressurised fluid inflow acts to produce radial loads on the spool, generating imbalances and causing energy losses through operating friction. In addition, torsional loads are created on the spool in accordance with the "Bernoulli effect" as fluid flows around the spool. The need to withstand these loads may dictate the sizing of various components in the valve, and accordingly it would be advantageous to reduce these loads to enable a reduction in the size and / or weight of the valve.

[0013] The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved servo valve.Summary of the Invention

[0014] The present invention provides, according to a first aspect, a rotary servo valve according to claim 1. In a second aspect, there is provided a method of controlling fluid flow using a rotary servo valve, accordingly to claim 14. Optional but preferred features are set out in the dependent claims.

[0015] Providing three pairs of ports, wherein the ports of each pair are on opposite sides of the cavity may allow the radial forces on the spool to be better balanced. The spool and port arrangement described above may allow the flow along opposing indented sides to be in opposite directions, thereby producing a clockwise torque on one side and an anticlockwise torque on the other side which leads to a reduction in overall torque loads on the spool for a given flow as the opposing torques may counteract each other. Providing a spool within two indented sides that define the flow paths through the cavity / spool may allow for a reduction in torque loads for a given flow as the pressure force generated by flow over the indented side is located close to the axis of rotation of the spool. Reducing loading on the spool may allow for a given flow rate to be achieved with a smaller and / or lighter valve (for example by reducing the size of motor required to counteract the torque and / or allowing for smaller components) and / or may increase the life of a valve. Each pair of ports comprises two ports of the same type, two return ports, two pressure ports and two service ports. With two types of service ports in a valve, for example two first service ports and two second service ports, a pair comprises two service ports of the same type.

[0016] It will be appreciated that, as used herein, the radius refers to the radial distance from the centre of the spool to the outer surface of the spool.

[0017] Each side of increased radius may have a radius no smaller than the maximum radius of the indented side. Each side of increased radius may have such a radius along the whole of the length of the side. Thus, at any point on a side of increased radius, the radius may be no less than the maximum radius of the indented side.

[0018] Each side of increased radius may have a radius that is constant along the length of the side. Thus, each side of increased radius may have a constant radius. Each side of increased radius may be a convex side, for example an arced side.

[0019] Each side of increased radius may have a radius that varies along the length of the side, provided that variable radius is no less than the maximum radius of the indented sides.

[0020] Each indented side may define a portion of the flow path between two ports. Thus, in use, fluid may flow over at least part, for example all, of the indented side. Each indented side may define a fluid flow control surface, on which the fluid flow properties can be varied to control the loads acting on the spool.

[0021] Each indented side may have a radius that varies along the length of the side. The radius of each side may reduce from a maximum radius adjacent at the end of the side (i.e. in the region adjacent to a side of increased radius), to a minimum radius located mid-way along the side. Each indented side may be a concave side, for example the rate of variation of the radius with respect to length may decrease from a maximum at the end of the side to a minimum mid-way along the side. Such a side may provide improved fluid flow, for example with reduced turbulence.

[0022] Each indented side may be a straight side. Each indented side may be defined by a chord of a notional cylindrical spool portion, that is to say a straight line whose endpoints both lie on the circumference of the notional spool. The two indented sides may extend parallel to each other. The two indented sides may be symmetrical.

[0023] Each side of increased radius extends between the two indented sides. Thus, the circumference of the spool may comprise two sides of increased radius and two indented sides only.

[0024] Each set of three pairs of ports may be referred to as a layer. Thus, the housing portion defines a layer of ports, each layer of ports having only three pairs of ports. The three pairs of ports are arranged around the circumference of the cavity. The three pairs of ports are located at substantially the same axial position along the length of the spool and cavity.

[0025] When the spool is in the open position, a fluid flow path is provided between the first pair of service ports and a pair of the remaining two pairs of ports. The other of the remaining two pairs of ports may be blocked, for example by the spool, for example by the sides of increased radius.

[0026] The spool portion may be located within the cavity such that there is substantially no gap between the surface of the spool where the indented side is not present and the inner surface of the cavity. The majority of the surface of the spool may be in contact, as herein defined, with the inner surface of the cavity. "in contact" as herein defined means that any gap between the inner surface of the cavity and the surface of the spool is small enough that internal leakage of fluid is less than 5% of the flow through the valve. Therefore fluid flow around the spool, other than via the indented sides, may be prevented. Thus, contact between the spool and the inner surface of the housing that defines the cavity may be defined as the spool and the inner surface of the housing being sufficiently close together to prevent significant flow between the inner surface of the cavity and the surface of the spool. For example, the clearance between the spool and the inner surface may be 5 µm or less. In this way precise control of the fluid flow through the valve is achievable, as the amount of flow is the result of the degree of alignment between the groove and the fluid inlet / outlet.

[0027] A pressure port may be defined as a port that, in use, is in fluid communication with a pressurised supply of fluid. The pressure port may be connected to the pressurised supply via the housing and / or a manifold (if present).

[0028] A return port may be defined as a port that, in use, is in fluid communication with a fluid return having a lower pressure than the pressurised supply. The return port may be connected to the return via the housing and / or a manifold (if present).

[0029] A service port may be defined as a port that, in use, is in fluid communication with the system element to be controlled by the servo valve. The system element may be a second stage valve, a hydraulic actuator and / or a hydraulic motor. Depending on the nature of the actuator and the state of operation of the valve, a service port may be a fluid inlet or a fluid outlet. The service port may be connected to the system element via the housing and / or a manifold (if present). The servo valve may comprise a single type of service port (a first service port). Thus, a single service connection may be provided by the valve and the servo valve may be a three-way valve. Alternatively, the servo valve may comprise two types of service port (first service ports and second service ports. Thus, two service connections may be provided and the servo valve may be a four-way valve. It may be that the supply of fluid via one of said service ports to the system element causes the system element to which the valve is connected to move in a first direction, while the supply of fluid via the other of said ports to the system element causes the system element to move in a second, opposite, direction.

[0030] When the spool is rotated to the first position, a fluid flow path may be provided from the pressure ports to the service ports. It may be that when the spool is rotated to the first position the return ports may be blocked.

[0031] The spool may be arranged to rotate in a second, opposite, direction from said neutral position to a second open position in which a fluid flow path is provided between each service port and another of the remaining ports, each fluid flow path being defined at least in part by one of the indented sides.

[0032] The rotary servo valve may be further configured such that those ports that were open (i.e. connected to the service ports via the fluid flow path) in the first open position are closed in the second position. Thus, in the first open position a fluid flow path may be provided between the pressure ports and the first service ports, in the second open position a fluid flow path may be provided between the first service ports and the return ports, or vice versa.

[0033] The fluid may be hydraulic fluid.

[0034] The rotary servo valve may be configured to be directly driven by an electric motor. A directly driven configuration may provide additional efficiency gains, compared to use of a separate amplifier pilot stage.

[0035] The rotary servo valve may be symmetrical about a centre line, such that a similar flow is provided on either side of the spool.

[0036] The housing may be a hollow cylindrical tube, with a maximum diameter of less than 50 mm.

[0037] The maximum diameter of the spool may be less than half the maximum diameter of the housing, and preferably may be less than one third the maximum diameter of the housing.

[0038] The housing may be formed using additive manufacturing. Manufacturing using additive manufacturing techniques may facilitate the provision of a housing portion having the ports required by the present invention as it allows more flexibility in terms of the layout of the flow galleries within and leading to the housing as compared with traditional (subtractive) commercial manufacturing techniques which may be limited to, for example, straight bores.

[0039] The housing may be a metal housing, for example a steel housing. The housing may be mounted in and / or form part of a manifold.

[0040] A metering element may further comprise a flow divider positioned inside each service port.

[0041] The flow divider may comprise a body located immediately upstream of the service ports, for example within the housing, spaced apart from the port formed in the surface defining the cavity, and arranged such that in use fluid flows on either side of the divider before passing through the port. The flow divider may comprise a curved protrusion which extends into the port from the port inner side wall (i.e. the portion of the housing defining the port). The flow divider may function to split the fluid flow, thereby reducing the flow area and increasing the fluid speed (providing a consequent reduction in pressure) adjacent the fluid control surface. Such a feature may therefore act to reduce the magnitude of the loads acting on the spool.

[0042] The spool may comprise one or more through conduits extending from one indented side to the opposite indented side. The conduits may be arrange to provide a flow path between the two indented sides. Such a feature may facilitate equalisation of local pressures across the spool.

[0043] The rotary servo valve may be configured to provide three-way control. That is, to connect pressure and return (via the pressure ports and return ports respectively) with a single service (via a first set of service ports). In the case of three-way valves there is provided more than one set (or layer) of ports in the housing and a corresponding spool portion in order to increase the flow rate through a valve of a given size.

[0044] The rotary servo valve may be configured to provide four-way control. That is, to connect pressure and return (via the pressure ports and return ports respectively) with a first and second service (via the first and second service ports respectively). The first and second service may be either side of an actuator (or system element). The provision of more than one set (or layer) of ports and a corresponding spool portion facilitates the provision of four way control in valves in accordance with the present invention.

[0045] When the spool portions of the first and second layer are in the first open position, fluid may flow in the first layer between the pressure ports and the first pair of service ports, and in the second layer between the second pair of service ports and the return ports. Similarly, when the spools portions of the first and second layer are in the second open position, fluid may flow in the first layer between the first pair of service ports and the return ports, and in the second layer between the pressure ports and the second pair of service ports.

[0046] The servo valve may comprise further layers and further spool portions. For example, the servo valve may comprise four layers and four corresponding spool portions.

[0047] The service ports may be larger than the pressure and / or return ports. The surface area of the service ports (at the opening of said port into the cavity) may be twice the surface area of the pressure and / or return ports.

[0048] The servo valve may comprise a spool comprising the first spool portion and the second spool portion and any further spool portions (if present). The spool may be integrally formed. The servo valve may comprise a housing comprising the first housing portion, the second housing portion and any further housing portions (if present). The housing may be integrally formed.

[0049] The ports of each layer may be radially aligned with (but axially separated) from the ports of any other layer within the housing. In operation, the rotational position of the first spool portion with respect to the housing may therefore be the same as the rotational position of the second spool portion with respect to the second housing.

[0050] The method may further comprising the step of rotating the spool portion from the neutral position to a second open position in which fluid flows between each of the service ports of the first pair and a corresponding port of one of other remaining pair of ports, the fluid passing over an indented side as it flow between said ports.

[0051] There may be provided a two-stage servo valve for controlling fluid flow wherein the first stage valve is a rotary servo valve as described above, and the second stage valve is a linear valve mounted coaxially with the rotary valve.

[0052] Arranging the first and second stages coaxially may facilitate the production of a more compact valve for a given flow rate.

[0053] The two-stage valve of the present invention may provide a second stage flow in the region of 10-200 times larger than the first stage flow. The two-stage valve may optionally include further stages, for example a third stage.

[0054] The fluid may be hydraulic fluid. The first stage may be coupled, for example hydraulically coupled, to the second stage, such that a rotary displacement of the first stage valve produces a corresponding linear displacement in the second stage valve.

[0055] The second stage valve may comprise a second stage spool. The second stage spool may comprise an elongate, generally cylindrical, body. The second stage valve may comprise a sleeve. The sleeve may define a cylindrical cavity and one or more layer of ports. The sleeve may be mounted in a manifold comprising a plurality of flow galleries in fluid communication with the ports. The second stage spool may be mounted concentrically within the sleeve. The second stage spool may be mounted for movement along the longitudinal axis of the sleeve.

[0056] The first stage valve may comprise a first stage spool. The spools of the first and second stage valves may be mounted coaxially. That is to say, the rotational axes of the spools of the first and second stage may be on a common axis. The first and second stage spools may be mounted concentrically. That is to say, at least a portion of the first and second stage spools may overlap such that at least a portion of the first stage spool is located within a portion of the second stage spool (or vice versa).

[0057] The first stage spool may be mounted within a housing. The housing may be mounted on or form part of a first stage manifold.

[0058] The second stage value may include a first cavity and a second cavity, each cavity being arranged such that fluid in said cavity exerts a pressure on the second stage spool such that, in use, the second stage spool moves with respect to the sleeve as a result of the pressure difference between the first and second cavities. The two-stage valve may include a sensor for providing second stage spool position feedback. Fluid in the first and / or second cavity may act (i.e. exert pressure) on a corresponding portion of the surface of the spool. The surface area of the corresponding portion of the first cavity may differ from the surface area of the corresponding portion of the second cavity. Thus, it may be that for the same pressure in each cavity a different force is applied to the second stage spool. Alternatively, the surface area of the corresponding portions of the surface of the spool of the first and second cavities may be the same. The surface area of the corresponding portion may be determined by the diameter of the spool and / or the cross-sectional area of the cavity.

[0059] One or more of the first and / or second stage: spool, housing, sleeve and / or manifold may be formed using additive manufacturing techniques. The maximum diameter of the second stage spool may be less than 100 mm. The maximum diameter of the first stage spool may be less than 50 mm. The maximum diameter of the second stage sleeve may be less than 150 mm. The maximum diameter of the first stage housing may be less than 100 mm. The minimum diameter of the second stage sleeve may be greater than 20 mm.

[0060] The first stage valve may be configured to be directly driven. The first stage valve may be coupled directly to a rotary motor. Such a feature may provide the benefit of minimising quiescent fluid leakage (i.e. power loss). The first stage valve may be bi-directional.

[0061] The second stage valve may be configured to receive a pressure source fluid flow in one of the first and second cavity. The second stage valve may be configured to receive a service fluid flow in the other of the first and second cavity. The service fluid flow may be received from the first stage valve, for example via one of more flow galleries formed in the first and / or second stage manifold. The second stage spool may comprise one or more flow galleries configured to provide fluid to the second cavity. The cross-sectional area of the cavity which receives a service flow from the first stage valve may be twice the cross-sectional area of the cavity which receives a supply pressure.

[0062] Each of the first and / or second cavity may extend into the spool from a respective distal end. The or each cavity may extend in a direction parallel to the longitudinal axis of the second stage spool.

[0063] The first stage spool may extend into the first cavity. The first stage valve and second stage valve may be mounted such that the first stage manifold and / or housing extends into the first cavity of the second stage spool. The first stage manifold and / or housing may be configured such that the first stage manifold and / or housing fits within the first cavity to seal the first cavity.

[0064] The cross-sectional area of the second cavity may differ from the cross-section area of the first cavity such that for the same pressure in each cavity a different force is applied to the second stage spool. For example, the cross-sectional area of the first cavity may be twice the cross-sectional area of the second cavity (or vice versa).

[0065] The second stage may be configured such that pressurized fluid in the first and / or second cavity acts on a respective distal end of the spool. The diameter of the spool at one distal end may be different to the diameter of the spool at the other distal end such that for a given pressure in each cavity a different force is applied to the second stage spool.

[0066] The rotary valve may comprise two, or more, layers of ports as discussed above. Thus, the rotary valve may provide four-way control. In such a four-way configuration, each layer includes a pair of service ports, a pair of pressure ports and a pair of return ports.

[0067] The second stage valve may be configured to receive a first service fluid flow in the first cavity, and a second, different, service fluid flow in the second cavity. Each of the first and second service flow may be received from the first stage valve. In such a configuration, a variable pressure may be applied in the first cavity by the first service fluid flow, and a second variable pressure may be applied in the second cavity by the second, different, service fluid flow.

[0068] The valve may comprise a control system configured to control operation of the valve. The control system may comprise one or more magnets configured to bias the spool towards the neutral position. The control system may comprise a first magnet mounted on the spool and / or housing and a second magnet or magnetic (e.g. ferromagnetic) material located on the other of the spool and / or housing. In use, the attraction or repulsion of the magnets (depending on whether they are of the same or opposite polarity) may act to move the spool towards the neutral position. Similarly, the attraction of the magnet and the magnetic material may act to move the spool towards the neutral position.

[0069] The two-stage valve may comprise a magnetic torsion spring. The valve may be configured such that the magnetic torsion spring acts to return the spool to the neutral position. The magnetic spring may comprise a first magnet mounted on the spool and a second magnet mounted on the housing. Each magnet may have a north pole and a south pole. The valve may be configured such that when the spool is in the neutral position the poles of the first magnet are aligned with the opposite poles of the second magnet (e.g. north aligned with south). The valve may be configured such that, as the spool is moved away from the neutral position, the poles of the first magnet are moved towards the like poles (e.g. north towards north), and / or away from the opposite poles of the second magnet. Thus, the magnetic force will seek to return the spool to the neutral position. The first magnet may be in the form of a cylinder. The second magnet may be annular. The first magnet may be mounted concentrically within the second magnet.

[0070] The two-stage servo valve may further comprise an electrical transducer, configured to feedback information relating to the position of the second stage valve.

[0071] The two-stage valve may further comprise an electrical transducer configured to feedback information relating to the position of the first stage valve.

[0072] There may be provided a method of operating a two-stage servo valve comprising a first (pilot) stage valve as described above and a second stage valve, the method comprising rotating (or causing to rotate) the spool of the first stage valve such that an axial displacement of the spool of the second stage is produced. The axial displacement of the second stage spool may result in a flow path being created or altered between the ports of the second stage, as described above.

[0073] The method may be such that the displacement of the spool of the second stage (and the subsequent creation or alteration of flow path) causes a movement in a system element (for example an actuator).

[0074] The method may comprise rotating the first stage spool using a direct drive motor.

[0075] The method may comprise rotating the first stage spool in a first direction to a first open position to cause movement of the second stage spool in a first axial direction, and then rotating the first stage spool in a second, different, direction to a second open position to cause movement of the second stage spool in a second, different axial direction.Description of the Drawings

[0076] Various embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: Figure 1is an axial sectional view of part of a prior art rotary servo valve; Figure 2is an axial sectional view of a layer of a rotary servo valve according to a first example embodiment of the invention; Figure 3is an axial sectional view of the layer of Figure 2 according to the first example embodiment of the invention, showing a fluid flow path; Figure 4is an axial sectional view of a metering element of a rotary servo valve according to a second example embodiment of the invention; Figure 5is an axial sectional view of a first and a second layer of a rotary servo valve according to a third example embodiment of the invention, showing a fluid flow path to and from a piston; Figure 6is a perspective view of the housing of the rotary servo valve of the third example embodiment; Figure 7is a perspective view of a spool for use in embodiments of the invention; Figure 8is an axial sectional view of three layers of a rotary servo valve in a fourth example embodiment of the invention; Figure 9is a cross-sectional side view of a rotary servo including layers in accordance with the third embodiment; Figure 10is a schematic view of the first and second stages of a two-stage valve according to a fifth example embodiment of the invention; Figure 11is a schematic view of the first and second stages of a two-stage valve according to a sixth example embodiment of the invention; Figure 12is a cross-sectional side view of part of the two-stage valve of the fifth embodiment of the invention; Figure 13is a cross-sectional side view of a valve of the fifth embodiment; and Figure 14shows a magnetic torsion spring for use in example embodiments. Detailed Description

[0077] Figure 1 shows part of a prior art rotary servo valve 1, with a spool 3 rotatably mounted within a surrounding housing 5 (mountings not shown). The housing 5 is broadly cylindrical (shown in cross-section) and shaped to define a central cylindrical cavity 7, and eight ports (P1-P8) arranged around the circumference of, and opening into, the cavity 7. The ports are arranged in diametrically opposed pairs, P1 paired with P5, P2 paired with P6, P3 paired with P7 and P4 paired with P8, so that pressures are equalized across the spool 3.

[0078] The spool 3 is sized to be a close fit within the cylindrical cavity 7 such that there is no significant flow of fluid around the sides of the spool. The spool 3 is also broadly cylindrical, with four depressed sides 9 and four protruding arms 11 spaced equidistantly around the circumference of the spool 3. The spool 3 has fourth order rotational symmetry. As the spool 3 rotates within the cavity 7, the arms 11 move to open and close (cover and uncover) in turn the ports P1, P3, P5, P7. In the Figure, the spool 3 is shown in a neutral position, in which the ports P1, P3, P5 and P7 are blocked by the arms 11 of the spool 3. In that neutral position, there is no fluid flow path provided between the ports. The spool 3 may move clockwise, or anticlockwise, as is typical for a rotary servo valve, to expose different flow paths within the valve. If the spool 3 is moved for example a few degrees clockwise, a flow path is opened between ports P8 and P1, P2 and P3, P4 and P5 and P6 and P7. If the spool 3 is moved for example a few degrees anti-clockwise, a flow path is opened between ports P1 and P2, P3 and P4, P5 and P6, and P7 and P8. Ports P1, P3, P5 and P7 may be referred to as control ports as it is the degree to which these ports are opened which controls the flow along each flow path.

[0079] In operation, a command signal corresponding to a desired spool position is applied to displace the spool. A suitable flow path from pressure to return is provided, thus producing a flow of fluid to an actuator / motor which is pressurised according to the input signal. A closed loop feedback system may be employed, to feedback information relating to the spool position to a controller which varies the input signal accordingly, to minimise any error.

[0080] Rotary servo valves such as the one shown are typically produced using traditional subtractive manufacturing techniques which may limit their size, it being difficult to produce very small valves in this way.

[0081] Figure 2 shows a layer 101 of a rotary servo valve in accordance with a first example embodiment of the invention. It will be appreciated that in embodiments of the invention, the rotary servo valve comprises a first layer of ports and a second layer of ports. The valve is directly driven by an electric motor (not shown). The layer 101 has a spool 103 rotatably mounted within a surrounding housing 105 (mountings not shown). The housing 105 is broadly cylindrical (shown in cross-section) and shaped to define a central cylindrical cavity 107. In contrast to the housing 5 of the prior art, the housing 105 of the present embodiment defines six ports (D1-D6) opening into the cavity 107. The ports are positioned around the circumference of the cavity 107 in diametrically opposed pairs (D1 with D4, D2 with D5, and D3 with D6).

[0082] In contrast to the metering element 1 of the prior art, the components have been manufactured using additive manufacturing.

[0083] In contrast to the metering element 1 of the prior art, the spool 103 has second order rotational symmetry. The spool 103 is cylindrical; however, it has a portion of its length along which two parallel facets 113 have been formed on opposing sides of the spool 103 defining two indented portions. The facets can be cut using a cutting wheel, or other technique known in the art. In the first example embodiment the facets 113 are planar, however in an alternative embodiment they could be slightly concave. The facets 113 are joined by two sides of increased radius 115 (i.e. the remainder of the notional cylinder of the spool). The spool 103 is depicted in Figure 2 in a neutral position in which ports D1, D2, D4 and D5 are blocked by the protruding sides 115. In that neutral position, no fluid flow path through the metering element is provided.

[0084] Figure 3 shows the layer 101 of the first example embodiment of the invention having a service connection 'S 1 ' to ports D3 and D6, a pressure connection 'Pr' to D1 and D4, and a return connection 'Rt' to D2 and D5. Rotation of the spool 103 clockwise to the position shown in Figure 3 therefore provides a flow path between the service ports (D3, D6) and pressure ports (D1, D4), whilst rotation of the spool 103 in an anticlockwise direction provides a flow path between the service ports (D3, D6) and the return ports (D2, D5). In this configuration, the metering element may provide three way control. The flow of hydraulic fluid through the flow paths is illustrated by dashed lines in Figure 3. This flow creates surface pressures on the spool 103 along the facets 113, which are reacted by a torque on the spool 103 shaft.

[0085] In use, the hydraulic fluid exerts static pressure forces (F1 and F1') on the spool, which can be resolved in axial and radial components. The axial components tend to be negligible. Since the fluid pressures are matched across the diametrically opposed pairs, the radial components generated on either side of the spool 103 are substantially equal and opposite, and the net radial force is close to zero (i.e. balanced).

[0086] In addition to forces arising from static pressures, Bernoulli forces (F2, F2') act on the spool as a result of the variation in the velocity of the hydraulic fluid as it passes across the faces of the spool on its way through the metering element. In contrast to the prior art, the arrangement of ports in valves in accordance with the present invention means that flow flows over the two indented facets 113 in opposite directions, and accordingly the resulting torque on the spools will balance each other. Aside from this balancing, the magnitude of the reaction torque is reduced since the forces act on a surface which is closer to the central axis of the spool.

[0087] Figure 4 shows a layer 201 of a rotary servo valve in accordance with a second embodiment, in which the pressure and return connections have been swapped, causing the flow direction to be reversed for the same spool rotation. It will be appreciated that in embodiments of the invention, the rotary servo valve comprises a first layer of ports and a second layer of ports. Several optional features are illustrated (which equally apply to the first embodiment). A flow divider 217 may be positioned in each service port S, to further direct and control the flow. In addition, the spool may be provided with conduits 218 across the spool 203 which allow fluid to flow from one indented face 213 to the other to equalise local pressures.

[0088] Figure 5 shows a schematic view of a third embodiment in which two layers 301, 301a are shown. Although depicted adjacent one another in Figure 5 it will be appreciated that the metering elements 301, 301a share a common spool 303 and common housing 305. Such a double layer configuration may be used to provide four way control when operating a piston 323 with the two services S 1 , S 2 arranged to provide fluid to either side of the piston 323. The Figure illustrates a configuration in which the spool is rotated clockwise from the neutral position. The fluid flow (indicated in dashed lines) passes from a pressure source Pr, through the first service S1 to one side of the piston 323 to move the piston 323 to the left. Fluid from the other side of the piston 323 flows via the second service S2 to the return Rt. To move the piston to the right, the spool must be rotated anticlockwise, to reverse the flow direction.

[0089] Figure 6 shows the housing 305 of the third example embodiment of the invention. The housing 305 surrounds a cylindrical cavity 307, and has ports in the form of bores into the cylindrical cavity 307 in two axially separated layers. The ports in Figure 6 each have the same height h. In other embodiments (not shown) the ports in different layers may have different heights.

[0090] Figure 7 shows a spool 803 for use with embodiments of the invention. The spool 803 is cylindrical; however, it has three segments (axially separated to correspond with the axial spacing of the layers of port in the housing) in which two parallel facets 813 have been cut on opposing sides of the spool.

[0091] Figure 8 shows a fourth embodiment in which three metering elements 501, 501a and 501b are mounted in series, the metering elements having a common spool and common housing. Still further metering elements having the common spool and common housing may be added in series, if desired.

[0092] Figure 9 shows a cross sectional view of a rotary servo valve 301 of the third example embodiment of the invention. Housing 305 is located within a manifold 333 which includes various fluid flow galleries 349 which link the cylindrical cavity 307 and the ports (D1-D6) to external ports 348 on the outside of the manifold 332. In use the external ports 348 are connect to a pressurised supply, a return or the service ports of an element (for example a piston) to be controlled by the valve. The spool 303 extends upwards through the housing 305 and through a motor 325 which in use, moves the spool 303 to control the flow of fluid through the valve. The manifold 333 may be produced from metal using additive manufacturing techniques.

[0093] A rotary servo valve as described above may be used as a pilot stage to drive a higher flow linear second stage. The electrical power source of a servo valve is typically small, for example no greater than 50V and 10A. When there is a requirement for a high bandwidth, high powered servo valve, a small electrically powered valve can be used to control the movement of a second larger valve which in turn controls the main fluid flow. The flow provided by the second stage is typically 10-200 times larger than the flow provided by the first stage.

[0094] Figure 10 shows a fifth example embodiment which uses a directly-driven single metering rotary servo valve 501 of the kind described in the first example embodiment of the invention, and three options for a second stage linear valve comprising a spool (550, 550a and 550b). It will be appreciated that in embodiments of the invention, the rotary servo valve comprises a first layer of ports and a second layer of ports. In each of the three options a supply pressure is provided to one end of the second stage spool (shown on the left in Fig. 10) and a service supply from the first stage 501 is provided to the other end of the second stage spool (shown on the right in Fig. 10). In the first option, the supply pressure acts on an end of spool 550 having a reduced diameter and service pressure on the full end diameter, the area ratio being approximately 1:2. In the second option, the supply pressure acts on the end of spool 550a having a reduced diameter of spool and the service pressure acts on the other end of spool 550b which has a larger (but not full in comparison to the main body of the spool) diameter, the area ratio being approximately 1:2. In the third option, an axially extending cavity is formed in each end of the spool 550b. The supply and service pressure acting on the bottom of that cavity at the relevant end. In the spool 550b, the cavity acted on by the supply pressure has a smaller diameter than the cavity acted on by the service pressure, the area ratio being approximately 1:2.

[0095] In the example embodiment, the first stage 501 is mounted co-axially with the second stage (550, 550a, 550b). Control is provided by rotary feedback of the first stage and linear feedback from the second stage. Feedback may be provided by mechanical or electrical means. In some embodiments, a magnetic spring may be used to provide rotary feedback. Fig. 14 shows a schematic example of a magnetic torsion spring 770. The spring comprises a cylindrical magnet 772 co-axially mounted on the spool (not shown in Fig. 14) concentrically located within an annular magnet 774. As shown in Fig. 14, the left-hand half 772a of cylindrical magnet 772 is a south pole and the right-hand half 772b is a north pole while the left-hand half 774a of annular magnet 774 is a north pole and the right-hand half 774b is a south pole. The spring is shown with the spool in its neutral position in Fig. 14. As the spool, and consequently cylindrical magnet 772 rotates away from the neutral position, poles of like polarity (e.g. 772a and 774b) are brought closer together. Accordingly, the repulsion between said like poles acts against the rotation, producing a force which acts to return the magnet 772, and consequently the spool to which it is connected, to the neutral position. While such a magnetic control system may be less precise than other mechanical control systems, there may be applications where the mechanical simplicity provided by using a torsional magnetic spring as a control system is desirable.

[0096] Figure 11 shows a schematic view of sixth example embodiment comprising a servo valve having a first stage comprised of a directly-driven rotary servo valve 601 of the kind described in the third example embodiment which is capable of providing four way control. The second stage comprises a spool 650, with a first service supply S 11 provided from the first stage valve to one end of the spool 650, and a second service supply S12 provided from the first stage valve to the other end of the spool 650. Again, three options are shown for the second stage spool (650, 650a and 650b). In the first option 650 the diameter of the second stage spool 650 is equal at both ends, and therefore S 11 and S 12 act on the same sized surface area. In the second option 650a the diameter of the spool differs at the two ends, such that S 11 and S 12 act on different sized surfaced areas of the spool, the area ratio being approximately 1:2. In the third option, a cavity is formed in each end of the spool 650b, the internal diameter of the two cavities being similar, such that S 11 and S 12 act on similarly sized internal surface areas of the spool.

[0097] In the example embodiment, the first stage 601 is mounted co-axially with the second stage (650, 650a, 650b) (the two metering elements of the first stage also being mounted co-axially). Control is provided by rotary feedback from the first stage 601 and linear feedback from the second stage (650, 650a, 650b).

[0098] Figure 12 shows a close up view of a portion of a valve similar to the valve of Figure 10 under the third option (i.e. with supply pressure acting on a reduced internal diameter of the second stage spool, and service on a larger internal diameter). The three-way first stage 501 is shown, coupled to a rotary brushless motor 531. The first stage valve 501 comprises a first stage spool 503 within a housing 505. The first stage spool 503 and housing 505 are together mounted within a first stage manifold 533 which contains flow galleries which provide fluid connections to the various ports of the first stage. In another embodiment, the housing may be integrally formed with the manifold (i.e. there may be no separate housing).

[0099] The first stage 501 is mounted co-axially with a second stage comprising a cylindrical spool 550b. The second stage spool 550b is housed inside a sleeve 535. The sleeve is located within a second stage manifold 543 (see Fig. 13) comprising a plurality of flow galleries 534 that provide fluid connections to the ports in sleeve 535. Adjacent the sleeve the galleries 534 are provided in five layers, each layer having eight rectangular galleries spaced around the circumference of the sleeve 535.

[0100] The second stage spool 550b is mounted concentrically within the sleeve 535. The second stage spool 550b includes two cavities (537, 539) one of said cavities extending inwards from each end of the spool 550b, the two cavities being located either side of a central bridging portion 538. The cavity 537 closest to the first stage valve 501 has a cross-sectional area approximately twice the cross-sectional area of the other cavity 539.

[0101] The first stage valve 501 is also mounted concentrically within the sleeve 535, a portion of the spool 503 and housing 505 of the first stage valve extending into the upper cavity 537 formed in the second stage valve spool 550b such that there is an overlap in an axial direction between the spool 503 and housing 505 of the first stage valve 501 and the second stage spool 550b.

[0102] The second stage spool 550b is mounted such that it moves linearly (i.e. in an axial direction) with respect to the first stage manifold 533. The first stage manifold 533 seals the cavity 537 closest to the first stage valve 501, providing a chamber into which pressurised fluid may flow, under the control of the first stage valve. A constant supply pressure is provided to the smaller cavity 539, whilst a variable service pressure, from the first stage valve, is applied in the larger cavity 537, and the difference in pressure between the two cavities 537, 539 causes movement of the second stage spool 500b relative to the first stage valve manifold 533, housing 505 and spool 503.

[0103] The two-stage valve 502 includes a Hall Effect sensor 541 for providing second stage spool position feedback.

[0104] Figure 13 shows the valve 502 of Figure 12 in the complete manifold 543 including flow galleries 543, control electronics 545 and connector 547. It will be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.

Claims

1. A rotary servo valve for controlling fluid flow, the servo valve comprising: a first housing portion (305) defining a cylindrical cavity (307) and a first layer (301) of ports, the first layer of ports having only three pairs of ports, being a first pair of first service ports, a pair of pressure ports and a pair of return ports, the ports of each pair being arranged so as to be diametrically opposed about the cylindrical cavity, the three pairs of ports of the first layer being located at substantially the same axial position along the length of the cavity; and a first spool portion (303) comprising two opposing indented sides and two opposing sides having an increased radius relative to the indented sides, each side of increased radius extending between the two indented sides, and wherein the first spool portion (303) is mounted for rotation relative to the cylindrical cavity, from a neutral position in which each side of increased radius of the first spool portion blocks at least one port of the first layer so as to prevent fluid flow through the valve, to an open position in which a fluid flow path is provided between each first service port of the first pair of the first layer and a corresponding one of the remaining ports of the first layer, each fluid flow path being defined at least in part by one of the indented sides of the first spool portion; a second housing portion (305) defining a cylindrical cavity (307) and a second layer of ports (301a), the second layer having only three pairs of ports being a pair of pressure ports, a pair of return ports and a second pair of service ports being either a pair of first service ports or a pair of second service ports; the ports of each pair arranged so as to be diametrically opposed about the cylindrical cavity, the three pairs of ports of the second layer being located at substantially the same axial position along the length of the cavity; and a second spool portion (303) comprising two opposing indented sides and two opposing sides having an increased radius relative to the indented sides, each side of increased radius extending between the two indented sides, and wherein the second spool portion (303) is mounted for rotation relative to the cylindrical cavity, from a neutral position in which each side of increased radius of the second spool portion blocks at least one port of the second layer so as to prevent fluid flow through the valve, to an open position in which a fluid flow path is provided between each service port of the second pair of the second layer and a corresponding one of the remaining ports of the second layer, each fluid flow path being defined at least in part by one of the indented sides of the second spool portion.

2. A rotary servo valve according to claim 1, wherein each side of increased radius has a radius that is constant along the length of the side.

3. A rotary servo valve according to claim 1 or claim 2, wherein each indented side is a straight side or a concave side.

4. A rotary servo valve according to any previous claim, wherein the second pair of service ports is a pair of second service ports, and when the first and second spool portions are in the open position said fluid flow path is provided in the first layer between the pressure ports and the first pair of first service ports and the return ports of the first layer are blocked, and said fluid flow path is provided in the second layer between the pair of second service ports and the return ports and the pressure ports of the first layer are blocked.

5. A rotary servo valve according to any previous claim, wherein the second pair of service ports is a pair of second service ports, wherein the first and second spool portions (303) are arranged to rotate in a second, opposite, direction from said neutral position to a second open position in which a fluid flow path is provided in the first layer between the first pair of first service ports and the return ports, and a fluid flow path is provided in the second layer between the pressure ports and the pair of second service ports, each fluid flow path being defined at least in part by one of the indented sides of the spool portion associated with that layer.

6. A rotary servo valve according to any previous claim, wherein each of the ports in two of the three pairs of ports of a layer have a first smaller minimum width, whilst each of the ports in the other pair of ports of the layer have a second greater minimum width.

7. A rotary servo valve according to claim 6, wherein each port having a first smaller minimum width is a channel with a stepped profile.

8. A rotary servo valve according to claim 6 or claim 7, wherein each port having a second greater minimum width is a channel with straight sides.

9. A rotary servo valve according to any previous claim, further comprising a housing, the housing comprising the first housing portion and the second housing portion, and wherein the housing (305) is formed using additive manufacturing.

10. A rotary servo valve according to any previous claim, further comprising a flow divider (217) positioned inside each service port.

11. A rotary servo valve according to any previous claim, wherein the spool (303) comprises one or more through conduits extending from one indented side to the other indented side.

12. A rotary servo valve according to any previous claim, wherein the rotary servo valve is configured to be directly driven by an electric motor.

13. A rotary servo valve according to claim 1, wherein the first and second spool portions (303, 303a) are integrally formed to provide a common spool, and / or the first and second housing portions (305) are integrally formed to provide a common housing.

14. A method of controlling fluid flow using a rotary servo valve, the servo valve comprising: a first housing portion (305) defining a cylindrical cavity (307) and a first layer (301) of ports, the first layer of ports having only three pairs of ports, being a first pair of first service ports, a pair of pressure ports and a pair of return ports, the ports of each pair being arranged so as to be diametrically opposed about the cylindrical cavity, the three pairs of ports of the first layer being located at substantially the same axial position along the length of the cavity; and a first spool portion (303) comprising two opposing indented sides and two opposing sides having an increased radius relative to the indented sides, each side of increased radius extending between the two indented sides, and wherein the method comprises rotating the first spool portion (303) relative to the cylindrical cavity, from a neutral position in which each side of increased radius of the first spool portion blocks at least one port of the first layer so as to prevent fluid flow through the valve, to an open position in which fluid flows between each of the first service ports of the first pair of the first layer and a corresponding one of the remaining ports of the first layer, the fluid passing over an indented side of the first spool portion as it flows between said ports of the first layer; a second housing portion (305) defining a cylindrical cavity (307) and a second layer of ports (301a), the second layer having only three pairs of ports being a pair of pressure ports, a pair of return ports and a second pair of service ports being either a pair of first service ports or a pair of second service ports; the ports of each pair arranged so as to be diametrically opposed about the cylindrical cavity, the three pairs of ports of the second layer being located at substantially the same axial position along the length of the cavity; and a second spool portion (303) comprising two opposing indented sides and two opposing sides having an increased radius relative to the indented sides, each side of increased radius extending between the two indented sides, and wherein the method further comprises rotating the second spool portion (303) relative to the cylindrical cavity, from a neutral position in which each side of increased radius of the second spool portion blocks at least one port of the second layer so as to prevent fluid flow through the valve, to an open position in which a fluid flow path is provided between each service port of the second pair of the second layer and a corresponding one of the remaining ports of the second layer, the fluid passing over an indented side of the second spool portion as it flows between said ports of the second layer.