Field of the Invention The present invention concerns improvements in and relating to servo valves. More particularly, the

invention concerns an improved pilot stage for a servo valve. The invention also concerns a servo valve

including such a pilot stage, a hydraulic system

including such a servo valve, a method of controlling a hydraulic system using such a servo valve and a 3D printing file for use in the production of such a valve.

Background of the Invention

Servo valves are used in a wide variety of

industries to control the movement of hydraulic or pneumatic actuations 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.

Additive manufacture, also known as 3D printing, is a term applied to processes whereby three-dimensional articles are manufactured by building up successive layers of material in different shapes. This is in contrast to traditional manufacturing techniques (known as subtractive manufacturing) such as milling or boring in which material is removed in order to create the final form of an article. The flexibility offered by additive manufacturing techniques allows the design of servo valves to be approached differently. Redesigning a servo valve taking into account the possibilities offered by flexible manufacturing has resulted in an improved valve design which overcomes a number of longstanding issues associated with servo valves.

A servo valve typically comprises a moving element (spool) and a fixed element (sleeve) . The relative movement of these two elements controls the flow of liquid through the valve in response to a mechanical or electrical input signal. The amount of flow passing through the spool, and the forces generated by said flow, are typically key factors in determining the amount of force required to move the spool, and therefore the size of the electric motor or power source.

A known method of reducing the force requirement is to include a first, low flow, pilot stage in the valve. However, including such a pilot stage may increase the complexity of the valve. Increasing the complexity of the valve may increase manufacturing costs and reduce reliability.

One prior-art pilot stage uses a jet-nozzle to direct a secondary flow to one of the ends of the spool. The pilot stage in such a valve, which is spaced apart from the primary stage, includes a flow divider and a moveable nozzle. A flow gallery connects each side of the flow divider to one end of the spool. In the null position, the nozzle is directed such that fluid is equally split either side of the flow divide. The equal division of flow means that the fluid pressure on the spool is balanced. In order to move the spool, the nozzle is moved to direct flow to one side of the flow divider such that more fluid is directed to one end of the spool than the other. The increase in fluid pressure at that end of the spool then causes the spool to move in a direction away from the region of higher pressure. In another prior-art pilot stage the jet-nozzle is replaced with an elongate member (known as a flapper) which swings to one side or the other under control of the motor in order to divert an incoming flow to either end of the spool .

Servo valves including nozzles or flappers in the pilot stage as described above require a continuous secondary flow, even when the spool is in the neutral position. The presence of the secondary flow may reduce the efficiency of the valve.

Pilot stages as described above may also be prone to blockage at the point at which the secondary flow is redirected. For example, contaminant particles may become lodged between the nozzle and receiver plate leading to a loss of control in the valve. Increasing the size of the nozzle/gap between the nozzle and

receiver plate to reduce this risk increases flow through the pilot stage and therefore may reduce the efficiency of the pilot stage.

In many applications in which servo valves are employed the space available for the servo valve is limited. It would therefore be advantageous to provide a more compact servo valve. Typically, reducing the size of the valve leads to a reduction in the maximum flow rate that can be achieved through the valve.

Consequently, it would be advantageous to produce a servo valve that has an increased flow rate in comparison to its size and weight.

In many applications in which servo valves are employed it is desirable that the hydraulic system responds quickly to an input control signal. Consequently, the frequency response of the servo-valve (i.e. the ability of the servo valve to rapidly adjust the flow to the hydraulic system) is also important. It would therefore be advantageous to provide a servo valve with an improved frequency response.

The present invention seeks to mitigate the above- mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved servo valve, in particular a servo valve having an improved pilot stage.

Summary of the Invention

According to a first aspect of the invention, there is provided a servo valve comprising a pilot stage and a primary stage, the pilot stage including a first spool mounted for movement between a first position and a second position, the primary stage including a second spool mounted for movement between a third position and a fourth position, and wherein the pilot stage and the primary stage are arranged such that, in use, movement of the first spool from the first position towards the second position causes movement of the second spool from the third position to the fourth position.

Using a spool to control the flow of fluid through the pilot stage of the servo valve may facilitate more compact servo valve designs. Using a spool rather than, for example a nozzle or flapper in the pilot stage, may facilitate changes in the internal geometry of the valve that reduce the risk of blocking by contaminants, improve frequency response and allow for a simplified design, thereby improving the performance and reliability of the servo valve. Using a spool to control the flow of fluid through the pilot stage may reduce parasitic leakage compared to pilot stages incorporating nozzles or

flappers which require a continuous secondary flow.

The pilot stage may be defined as comprising those elements of the servo valve which are arranged to control the movement of the primary stage (i.e. second) spool.

The primary stage may be defined as those elements of the servo valve which are arranged to control the flow of fluid into and out of the hydraulic system to which the servo valve is connected. It will be appreciated that the flow rate in the pilot stage is typically significantly less than the flow rate in the primary stage. For example, the flow rate through the pilot stage may be less than 10% of the flow rate through the primary stage, for example the flow rate through the pilot stage may be less than 5%, for example less than 2%, for example less than 1% of the flow rate through the primary stage.

Each spool may include a surface having at least one groove formed therein. Fluid may flow through the servo valve via the grooves formed in the spools. It may be that fluid flowing via the at least one groove in the first spool controls the movement of the second spool. It may be that fluid flowing via the at least one groove in the second spool controls the movement of an actuator.

The at least one groove may extend circumferentially around the spool. Alternatively, the groove may be curvilinear. That is to say, the groove may follow a curved path on the surface of the spool. Thus, the start point and the end point of the groove may be offset both longitudinally and angularly with respect to each other.

The servo valve may be an electrohydraulic servo valve .

The first spool may be mounted for translational movement, for example axial movement, between the first position and the second position. Thus, it may be that movement of the first spool from the first position towards the second position includes a translation of the spool. The first spool may be mounted for rotational movement between the first position and the second position. Thus, it may be that movement of the first spool from the first position towards the second position includes a rotation of the spool.

The second spool may be mounted for translational movement, for example axial movement, between the third position and the fourth position. Thus, it may be that movement of the first spool from the first position towards the second position includes a translation of the spool .

Each stage may include a fluid inlet and a fluid outlet. It may be that, in the first position, the surface of the first spool closes off at least one of the fluid inlet and the fluid outlet. It may be that, in the second position, the at least one groove in the surface of the first spool is aligned with the fluid inlet and the fluid outlet such that fluid may flow from the fluid inlet to the fluid outlet via the first spool. It may be that, in the third position, the surface of the second spool closes off at least one of the fluid inlet and the fluid outlet. It may be that, in the fourth position, the at least one groove in the surface of the second spool is aligned with the fluid inlet and the fluid outlet such that fluid may flow from the fluid inlet to the fluid outlet via the second spool.

The first position may be a null position. The second position may be a null position. A null position may be defined as a position in which fluid does not flow via the spool. The servo valve may include a fluid manifold. The fluid manifold may include a cavity. The cavity may be substantially cylindrical. The cavity may be defined by an inner surface of the manifold. The first and/or second spools may be located in the cavity.

A stage may include a plurality of internal ports associated with the spool. An internal port may be said to be associated with a spool if fluid flows into or out of the at least one groove on each spool via the port. The internal ports may be located in the inner surface of the manifold.

Each internal port may have a non-circular cross- section. Each internal port may be of elongate shape. That is to say that the internal port may extend over a greater circumferential distance than an axial distance. It will be understood that the circumferential and axial distances are relative to the manifold. Each internal port may have a substantially rectangular cross-section. Each internal port may be a substantially rectangular slit in the inner surface of the manifold which defines the cavity. The circumferential extent of a port may vary with respect to the axial length of the port.

Varying the geometry of the internal ports may allow the displacement of the first spool between the first

position and the second position and between the first position and the fifth position to be advantageously reduced compared to the displacement by the nozzle/ flapper in a prior art pilot stage. Smaller movements may be advantageous in that they may be quicker and therefore lead to an improved frequency response as compared to the prior art valves. An internal port that has an elongate shape may also be less prone to blocking by contaminant particles, and may therefore lead to improved reliability of the servo valve.

Categories of internal port may include supply ports, tank ports, pilot ports and control ports. Fluid may flow into the at least one groove on each spool via a supply port. Each supply port may be in fluid

communication with a pressurized supply such that fluid from the pressurized supply may flow via the supply port to the first and/or second spools. Fluid may flow out of the at least one groove via a tank port. A tank port may be connected to an unpressurised tank such that fluid may flow to tank via the tank port from the first and/or second spools. Each stage may include at least one supply port. Each stage may include at least one tank port.

The pilot stage may further include at least two pilot ports. Fluid may flow into the at least one groove on the first spool via a pilot port. Fluid may flow out of the at least one groove on the first spool via a pilot port. Thus, a pilot port may act as a fluid inlet or a fluid outlet depending on the position of the second spool. Fluid flowing out of a groove formed in the surface of the first spool via the first pilot port may flow to a region adjacent to a first end of the second spool. Fluid flowing out of a groove formed in the surface of the first spool via the second pilot port may flow to a region adjacent to a second end of the second spool. It will be appreciated that fluid flowing via the first spool to a region adjacent to either the first or second end of the second spool will cause an increase in fluid pressure in that region. Thus, fluid flowing via the first spool and the associated pilot ports may cause the second spool to move. The primary stage may further include at least two control ports. Fluid may flow into the at least one groove on the second spool via a control port. Fluid may flow out of the at least one groove on the second spool via a control port. A control port may act as a fluid inlet or a fluid outlet depending on the position of the second spool. Fluid flowing out of a groove in the surface of the second spool via a first control port may cause the actuator to move in a first output direction. Fluid flowing out of a groove in the surface of the second spool via a second control port may cause the actuator to move in a second output direction.

The pilot stage may include at least one supply port, at least one tank port and at least two pilot ports. The primary stage may include at least one supply port, at least one tank port and at least two control ports. Each stage may include further supply ports.

Each stage may include further tank ports. The pilot stage may include further pilot ports. The primary stage may include further control ports.

The internal ports associated with the first spool may be grouped separately from the internal ports associated with the second spool along the longitudinal axis of the spool. A port may be said to be associated with a spool if fluid may flow to or from the spool via that port.

The manifold may include a sleeve, for example an integral sleeve. The sleeve may be located within the cavity of the manifold. The sleeve may be substantially cylindrical in shape. The sleeve may be hollow. The first and/or second spool, sleeve and manifold cavity may be concentric. The first and/or second spool may be located within the sleeve. The sleeve may divide the annular cavity surrounding the first and/or second spool into at least two concentric annular zones. It may be that the sleeve does not extend along the majority of the length of the spool. The sleeve may divide the annular cavity surrounding the spool into at least two concentric annular zones over a portion of the length of the spool. Proximate to each inlet port there may be a high pressure zone. Proximate to each control port there may be a high pressure zone. Each high pressure zone may have a sleeve. The sleeve may act to pressure-balance the interface between any incoming fluid and the spool. The use of sleeves in servo valves is well known and will not be discussed further here.

The region adjacent to an end of the spool may be defined at least in part by that end of the spool and the inner surface of the manifold. Thus fluid flowing to a region adjacent to an end of the spool may increase the fluid pressure in the cavity at that end of the spool. The region adjacent to an end of the spool may include a portion of the sleeve.

The first spool may be mounted for movement in a first direction from the first position towards the second position. The second spool may be mounted for movement in a second direction from the third position towards the fourth position. The second direction may be opposite to the first direction. The first spool may be mounted for movement in a third direction, opposite to the first direction. The second spool may be mounted for movement in a fourth direction, opposite to the second direction. The first spool may be mounted for movement in the third direction from the first position towards a fifth position. The second spool may be mounted for movement in the fourth direction from the third position towards a sixth position. The first spool may be mounted for movement between the first and fifth positions. The second spool may be mounted for movement between the third and sixth positions.

Each stage may include a further fluid inlet and/or a further fluid outlet. It may be that, in the fifth position, the at least one groove in the surface of the first spool is aligned with one of the further fluid inlet and/or the further fluid outlet. Thus, fluid may follow a different flow path via the first spool when the first spool is in the fifth position as compared to the flow path when the first spool is in the second position. It may be that, in the sixth position, the at least one groove in the surface of the second spool is aligned with one of the further fluid inlet and/or the further fluid outlet. Thus, fluid may follow a different flow path via the second spool when the second spool is in the sixth position as compared to the flow path when the second spool is in the fourth position.

It may be that, when the first spool is in the second position fluid from a fluid inlet flows via a groove in the surface of the first spool and out of the first pilot port to the first end of the second spool. It may be that, when the first spool is in the fifth position fluid from a fluid inlet flows via a groove in the surface of the first spool and out of the second pilot port to the second end of the second spool.

Thus, moving the first spool from the second position to the fifth position may change the direction in which the second spool moves.

It may be that the surface of the first and/or second spool includes more than one groove. It may be that when the first spool is in the second and/or fifth position a first flow path is formed via a first groove and a second flow path is formed by a second groove. It may be that when the second spool is in the third and/or sixth position a first flow path is formed via a first groove and a second flow path is formed by a second groove .

The servo valve may include at least one flow gallery arranged such that fluid may flow via the first spool to a region adjacent to an end of the second spool. The fluid manifold may include at least one flow gallery arranged such that the fluid outlet associated with the first spool is in fluid communication with a region adjacent to a first end of the second spool. The manifold may include a flow gallery arranged such that the fluid outlet associated with the first spool is in fluid communication with a region adjacent to a second end of the second spool. Fluid flowing via the first spool and either of the flow galleries may increase the fluid pressure in a region adjacent to either the first or second end of the second spool. Thus, fluid flowing via the first spool may control the direction in which the second spool moves. The at least one flow gallery may be curvilinear.

Each spool may have a longitudinal axis. The longitudinal axis of the first spool may extend

substantially parallel to the longitudinal axis of the second spool. It may be that the first and second spools are coaxial. The first and/or third direction may be parallel to the longitudinal axis of the spool. The second and/or fourth direction may be parallel to the longitudinal axis of the spool.

Each spool may be located in a cavity formed within the manifold of the servo valve. It may be that the first spool and the second spool are located in the same cavity formed within the manifold of the servo valve. Locating the first and second spools in the same cavity may facilitate the design of more compact servo valves. Alternatively, it may be that the first spool and the second spool are located in different cavities formed within the manifold of the servo valve.

In the case where the first and second spools are coaxial, at least a portion of the first spool may overlap a portion of the second spool. That is to say, a portion of the second spool and a portion of the first spool may share the same axial position with respect to the second spool. Overlapping the first and second spools may facilitate more compact servo valve designs while maintaining the flow rate through the valve.

The first spool may be located radially outside a portion of the second spool. The second spool may be located radially outside a portion of the first spool. It may be that a portion of one of the first or second spool is formed around the other of the first and second spools. It may be that a portion of one of the first or second spool is located within the other of the first and second spools. The first spool may overlap the second spool along the entirety of the length of the first spool. The second spool may overlap the first spool along the entirety of the length of the second spool. The surface of one of the first or second spool may include a recess, for example a region of reduced

diameter, in which the other of the first or second spools is located. The first and/or second spool may be hollow. The first and/or second spool may include an axial bore extending along its longitudinal axis. The region of reduced diameter of one of the first or second spool may extend through the axial bore of the other of the first and second spools. The first spool may be centrally located along the longitudinal axis of the second spool. The second spool may be centrally located along the longitudinal axis of the first spool.

The servo valve may include at least one resilient member. The resilient member may be arranged between the first and second spools such that relative movement of the first and second spools exerts a force on the

resilient member. The resilient member may be a spring, for example a helical spring. The longitudinal axis of the resilient member may extend parallel to the

longitudinal axis of the first and/or second spools. It may be that the resilient member directly connects the first and second spools. The servo valve may include at least one resilient member connected at each end of the first spool. The first resilient member may be arranged such that movement of the first spool in the first direction compresses the resilient member. The second resilient member may be arranged such that movement of the first spool in the first direction stretches the resilient member. The first resilient member may be arranged such that movement of the first spool in the third direction stretches the resilient member. The second resilient member may be arranged such that

movement of the first spool in the third direction compresses the resilient member. The first resilient member may be arranged such that movement of the second spool in the second direction compresses the resilient member. The second resilient member may be arranged such that movement of the second spool in the second direction stretches the resilient member. The first resilient member may be arranged such that movement of the second spool in the fourth direction stretches the resilient member. The second resilient member may be arranged such that movement of the second spool in the fourth direction compresses the resilient member. Thus, the at least two resilient members may provide a feedback system which acts to return the first spool to the first position.

In some embodiments, the feedback system which acts to return the first spool to the first position may be an electrical feedback system. For example, the servo valve may include a transducer on the first and/or second spools. The servo valve may include a feedback control arranged to control the electric motor to move the first and/or second spools in response to the transducer output .

The first and second spools may be of an integral construction. Thus, the first spool and the second spool may be provided as a single piece. The first spool, second spool and at least one resilient member may be of an integral construction. Thus, the first spool and the second spool, and the feedback means between the two spools, may be provided as a single piece. Providing the first spool, second spool and, if present, a resilient member, as a single piece may simplify manufacture and thereby reduce costs.

The servo valve may comprise an electric motor arranged to move the first spool from the first position towards the second position. The electric motor may be arranged to move the first spool from the first position towards the fifth position. The servo valve may comprise a drive shaft arranged to transmit the motion of the electric motor to the first spool. The electric motor may be a torque motor. The servo valve may include a valve centring mechanism arranged to bias the torque motor towards a valve-centre position. That is to say the torque motor may be biased towards a position in which the first spool is in the first position. The valve centring mechanism may comprise a resilient member, for example a spring. The resilient member of the valve centring mechanism may be connected to the drive shaft.

The servo valve may have a volume in the range of 0.1 litres to 5 litres, for example between 1 litre and 3 litres. The servo valve may have a mass in the range of 0.1 kg to 5 kg, for example 1 kg to 5 kg. Movement of the first spool from the first position to the second position may include a movement of less than 1 mm, for example less than 0.5 mm but greater than 0.001 mm.

Movement of the second spool from the third position to the fourth position may include a movement of less than 1 mm, for example less than 0.5 mm but greater than 0.001 mm.

According to a second aspect of the invention there is provided a hydraulic system comprising a servo valve in accordance with the first aspect.

The servo valve may be connected to a hydraulic system. The hydraulic system may include an actuator. The actuator may be a pneumatic cylinder. The actuator may be a hydraulic cylinder. The actuator may be a hydraulic motor. The servo valve may be connected to the hydraulic system such that the servo valve controls the flow of fluid to the actuator. The hydraulic system may include a pressurised supply. The hydraulic system may include an unpressurized tank. A servo valve may be connected to the hydraulic system such that fluid flows from the pressurised supply to the actuator via the servo valve. A servo valve may be connected to the hydraulic system such that fluid flows from the actuator to tank via the servo valve.

According to a third aspect of the invention, there is provided a method of controlling a hydraulic system, the system comprising a servo valve, the servo valve comprising a pilot stage having a first spool and a primary stage having a second spool, the method

comprising the step of moving the first spool in a first direction such that fluid flowing via the first spool increases fluid pressure in a region adjacent to a first end of the second spool.

Moving the first spool in the first direction may create a first flow path via the first spool to the first end of the second spool. Thus, moving the first spool in the first direction may allow pressurised fluid to flow to the first end of the second spool. The consequent increase in fluid pressure at the first end of the second spool may then cause the second spool to move in the second direction.

The method may include a step of moving the first spool in a second direction, the second direction being opposite to the first direction, such that fluid flowing via the first spool increases fluid pressure in a region adjacent to a second end of the second spool.

Moving the first spool in the second direction may create a second flow path via the first spool to the second end of the second spool. Thus, moving the first spool in the first direction may allow pressurised fluid to flow to the second end of the second spool. The consequent increase in fluid pressure at the second end of the second spool may then cause the second spool to move in the fourth direction. The method may include a step of rotating the first spool in the first and/or third directions. The method may include a step of translating the first spool in the first and/or third directions.

The method may include a step of coaxially moving the first and second spools.

The method may include a step of producing the first spool and/or the second spool using an additive

manufacturing process. The method may include a step of producing the servo valve manifold using an additive manufacturing process. Using an additive manufacturing process may allow for more efficient, for example

commercial, production of servo valves as compared to traditional subtractive manufacturing processes.

According to a fourth aspect of the invention, there is provided a computer-readable medium having computer- executable instructions adapted to cause a 3D printer to print at least part of a servo valve in accordance with the first aspect. For example, the instructions may be adapted to cause a 3D printer to print one or more of a servo valve manifold, a first spool and a second spool in accordance with the first aspect.

It will of course be appreciated that features described in relation to one aspect of the present invention may be incorporated into other aspects of the present invention. For example, the method of the invention may incorporate any of the features described with reference to the apparatus of the invention and vice versa .

Description of the Drawings Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which: shows a schematic view of a hydraulic system including a servo valve according to a first embodiment of the invention.

shows a cross-sectional schematic view of part of a servo valve according to the first embodiment of the invention;

shows a cross-sectional view of a servo valve according to a second embodiment of the invention.

Detailed Description

FIG. 1 shows a schematic view of a hydraulic system 3 including a servo valve 1 in accordance with a first example embodiment. The servo valve 1 comprises a primary spool 2 and a pilot spool 10. The primary spool 2 and pilot spool 10 are both substantially cylindrical and are coaxially located (see FIG. 2 for more detail) . The servo valve also includes control electronics 26 and an electric motor 28. The control electronics 26 are connected to the electric motor 28 by a signal line 30. The electric motor is connected to the pilot spool 10 by a drive shaft 32. The hydraulic system 3 also includes a pressurised supply P and an unpressurised tank T to which the servo valve 1 is connected. The hydraulic system 3 also includes an actuator 34, to which the servo valve is connected via control flow-paths CI and C2. The

manifold includes a plurality of flow galleries (not shown) linking the pressurised supply P and tank T with control flow-paths CI and C2 via the primary spool 2. The manifold also includes a plurality of flow galleries (not shown) linking the pressurised supply P and tank T with the pilot spool 10.

FIG. 2 shows a more detailed cross-sectional view of part of the servo valve of FIG. 1. The primary spool 2 and pilot spool 10 are coaxially located in a cylindrical cavity 18 formed in the manifold 4 of the servo valve 1. The primary spool 2 has an area of reduced diameter 2a midway along its length. Six circumferential grooves 20 are formed in the surface of primary spool 2 either side of the area of reduced diameter 2a; three grooves 20 are located at the left-hand end of the spool 2 and three grooves 20' are located at the right-hand end of the spool 2. The pilot spool 10 is located midway along the length of the primary spool 2. The pilot spool 10 has an axial bore 24 extending along its length. The area of reduced diameter 2a of the primary spool 2 extends through the axial bore 24 formed in the pilot spool 10. A circumferential groove 22 is formed in the surface of the pilot spool 10 near its left-hand end. A second circumferential groove 22' is formed at the other end of the pilot spool 10. Two springs 12, 12' extend between the two spools (pilot spool 10 and primary spool 2), one each side of pilot spool 10. A plurality of ports 6, 8 are formed in the surface of the manifold 4 which defines the cavity 18. A first group of six ports 6 are

associated with the pilot spool 10 and are spaced apart along the longitudinal axis of the pilot spool 10. Three of the first group of ports 6 are located in the region of the left-hand end of the pilot spool 10. The other three internal ports 6' are formed in the region of the right-hand end of the pilot spool 10. A second group of ten internal ports 8 are associated with the primary spool 2 and are spaced apart along the longitudinal axis of the primary spool 2. Five of the ports 8 are located in the region of the left-hand end of the primary spool 2. The other five internal ports 8' are formed in the region of the right-hand end of the primary spool 2. The ports 8, 8' associated with the primary spool 2 are spaced either side of the pilot spool 10. Each port 6, 8 extends around the whole of the circumference of the corresponding spool 2, 10 and appears as two rectangles either side of the spool when viewed in cross-section in Fig. 1.

The coaxial arrangement of the pilot spool 10 and primary spool 2 results in a more compact valve than prior art valves. The circumferential nature of the ports 6 associated with the pilot spool 10 reduces the risk of a contaminant completely blocking a port and may therefore increase the reliability of the servo valve. Additionally, as the port extends circumferentially around the spool a given flow rate may be achieved with a relatively small longitudinal extent. Pilot stages in accordance with the present embodiment may therefore allow for smaller movements of the pilot spool 2 in order to control flow through the pilot stage and may therefore have a higher-frequency response than prior art valves.

Each group of ports associated with the pilot spool 10 includes a supply port 6p, a tank port 6t and a control port 6c. A flow gallery 14 shown as a dashed line in Fig. 1, connects the control port 6c (at the left-hand end of the pilot spool 10) to the region of the cavity 18 adjacent to the right-hand end of the primary spool 2. A second flow gallery 14' connects the control port 6'c (at the right-hand end of the pilot spool 2) to the region of the cavity adjacent to the left-hand end of the primary spool 2.

Each group of ports associated with the primary spool 2 includes two supply ports 8p, a tank port 8t and two control ports 8c. Flow galleries (not shown) connect the control ports 8c to the actuator 32 (see FIG. 1) .

In use, the position of the spools 2, 10 relative to the ports 6, 8 as shown in FIG. 2 means that no fluid can flow via the grooves, 20, 22 in either the primary 2 or pilot spools 10. Thus, in servo valves in accordance with the present embodiment, there is no parasitic flow through the pilot stage when the valve is in the null position .

In response to an input signal the control

electronics 26 drive the electric motor 28 via signal line 30 (all shown in FIG. 1) . The motor 28 then moves the pilot spool 10 towards the left-hand side of FIG. 2. As the pilot spool 10 moves relative to the manifold 4 a flow path is formed between the supply port 6'p and the control port 6'c via the circumferential grooves 22' at the right-hand end of the pilot spool 10. Fluid

therefore flows from the pressurized supply P (as shown in FIG. 1) connected to supply port 6'p, via groove 22', control port 6'c, and fluid gallery 14' to the left-hand end of the primary spool 2. Moving the pilot spool 10 to the left also forms a flow path at the left-hand end of the pilot spool 10 such that fluid can flow from the region adjacent to the right-hand end of the primary spool 2 via flow gallery 14, control port 6c, groove 22 and tank port 6t to the unpressurised return T (as shown in FIG. 1) . Increased pressure at the left-hand end of the primary spool 2 due to an influx of fluid from the pressurized supply P pushes the primary spool 2 to the right. As the primary spool moves to the right fluid is pushed out of the cavity 18 from the region of the right- hand end of the primary spool 2 via the pilot spool 10.

The movement of the primary spool 2 relative to the manifold 4 in the right-hand direction forms a flow path between the supply port 8p (which is connected to

pressurised supply P, see FIG. 1) and the first control port 8cl and between the tank port 8t and the second control port 8c2. These flow paths are also created at the right-hand end of the primary spool. Pressurized fluid flowing out of the first control port 8cl and via control flow path CI (see FIG. 1) causes the actuator 34 connected to the servo valve 1 to move in a first

direction. Movement of the actuator in the first

direction pushes fluid from within the actuator 34 back to the servo valve 1 and to the unpressurised return T via control flow path C2 and the primary spool 2.

The leftwards movement of the pilot spool 10 and the rightwards movement of the primary spool 2 compress the spring 12 and stretch the spring 12' . The springs 12, 12' therefore exert a force on the pilot spool 10 acting towards the right-hand side of fig. 1. Thus, servo- valves in accordance with the present embodiment include a feedback system (springs 12, 12') that acts to return the pilot spool 10 to its original ^ull' position (as shown in FIG . 2 ) .

FIG. 3 shows a cross-sectional view of a servo valve in accordance with a second example embodiment. Like components have been labelled with like references. Only those aspects of the present embodiment which differ with respect to the first embodiment will be discussed here. The ports 106, 108 are spaced around the circumference of the cavity 118 in the embodiment of FIG. 3 and thus only some of the ports 106, 108 appear in the cross-sectional view of FIG. 3. Spacing the ports 106, 108 around the circumference of the cavity 118 may allow for more efficient packing of the servo valve, thereby reducing the size of the valve. The width of the circumferential grooves 120, 122 is very much greater than the width of the corresponding port 106, 108. The reduced width of the ports 106, 108 in comparison to the grooves 120, 1229 allows a small movement of the spool to have a

significant impact on flow via the associated spool 102, 110. Thus, valves in accordance with the present

embodiment may provide improved control and frequency response. The ports 108 and grooves 120 associated with the primary spool 102 are much larger than the ports 106 and grooves 122 associated with the pilot spool 110. The larger groves 120 and ports 108 associated with the primary spool 102 allow for higher flow rates via that spool. Thus, servo valves in accordance with the present embodiment may offer increased flow rates and a better frequency response in comparison with prior art designs.

Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different

variations not specifically illustrated herein. By way of example only, certain possible variations will now be described .

For example, in the above embodiments, no sleeve has been shown. Sleeves are well known in servo valve design and are used in servo valves to ensure an even pressure distribution around the internal ports of the servo valve. It will be appreciated that a sleeve may be used with a two-stage servo valve in accordance with the present invention. In the above embodiments, the first and second spools are shown coaxially mounted in the same cavity. It will be appreciated that some of the

advantages discussed above may be obtained when the first spool is located separately from the second spool. For example, the pilot stage may be spaced apart from the primary stage as in conventional servo valves. The embodiments shown above have a first spool mounted for axial translation. It will be appreciated that the first spool may be mounted for axial rotation, particularly in embodiments where the first spool is spaced apart from the second spool.

Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also 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.