Patent Description:
Servo valves find a wide range of applications for controlling air, fuel, oil or other fluid flows to effect driving or control of another part, e.g., an actuator or in fuel control systems.

A servo valve assembly may include a drive assembly such as a motor controlled by a control current which controls fluid flow to or from an actuator. Generally, a servo valve transforms an input control signal into movement of an actuator cylinder. The actuator controls another component which, in some examples, may be a valve. In other words, a servo valve acts as a controller, which commands the actuator, which changes the position of a valve's flow modulating feature.

Such mechanisms are used, for example, in various parts of aircraft where the management of fluid/air flow is required, such as in engine fuel control, oil flow, engine bleeding systems, anti-ice systems, air conditioning systems and cabin pressure systems. Servo valves also are widely used to control the flow and pressure of pneumatic and hydraulic fluids to an actuator, e.g. to control moving parts such as flight control surfaces, flaps, landing gear, and in applications where accurate position or flow rate control is required. Some examples of applications are aircraft, automotive systems and in the space industry.

Conventionally, servo valve systems operate by obtaining pressurised fluid from a high pressure source which is transmitted through a load from which the fluid is output as a control fluid. Various types of servo valves are known, examples of which are described in UK Patent Application No. <CIT>, <CIT>, <CIT>, <CIT>, <CIT> and <CIT>.

Electrohydraulic servo valves can have a first stage with a motor, e.g. an electrical or electromagnetic force motor or torque motor, controlling flow of a hydraulic fluid to drive a valve member e.g. a spool valve of a second stage, which, in turn, can control flow of hydraulic fluid to an actuator for driving a load. The motor can operate to position a moveable member, such as a flapper, in response to an input drive signal or control current, to drive the second stage valve member e.g. a spool valve by controlling the flow of fluid acting on the spool. Movement of the spool causes alignment between the ports and fluid channels to be changed to define different flow paths for the control flow. Such systems are known in the art and will not be described further in detail.

Such conventional systems will be described in more detail below with reference to <FIG>.

Servo valves are often required to operate at various pressures and temperatures and so components parts need to be large enough to handle the large amounts of fluid needed to operate under such conditions. For example, in fast acting air valve actuators, relatively large amounts of fluid are required depending on the size of the actuator and the valve slew rate. For such high flow rates, however, large valve orifice areas are required. For 'flapper' type servo valves, problems arise when dealing with large flows due to the fact that flow force acts in the direction of the flapper movement and the motor is forced to overcome the flow forces. For clevis-like metering valves such as those described in <CIT> and <CIT>, the flow forces, which are proportional to the flow, act simultaneously in opposite directions so that the clevis is balanced and centered. The clevis, however, needs to be big due to the requirement for bigger orifices to handle larger flows.

Jet pipe servo valves are types of valves that provide an alternative to 'flapper'- type servo valves. Jet pipe servo valves are usually larger than flapper type servo valves but are less sensitive to contamination. In jet pipe systems, fluid is provided via a jet pipe to a nozzle which directs a stream of fluid at a receiver. When the nozzle is centered - i.e. there is no current from the motor so it is not caused to turn, the receiver is hit by the stream of fluid from the nozzle at the centre so that the fluid is directed to both ends of the spool equally. If the motor causes the nozzle to turn, the stream of fluid from the nozzle impinges more on one side of the receiver and thus on one side of the spool more than the other, which causes the spool to shift. The spool shifts until the spring force of a feedback spring produces a torque equal to the motor torque. At this point, the nozzle is centred again, pressure is equalized on both sides of the receiver and the spool is held in the centered position. A change in motor current moves the spool to a new position corresponding to the applied current. Conventional systems are fairly large, bulky systems with a complex construction of several moving parts and channels, which means that there are several potential points of failure. The fluid flow channels and long fluid paths slow down the response time for the position of the spool to change in response to changes in the control signal, and can also become blocked and unreliable.

There is a need for improved servo valve arrangements that can handle large fluid flows effectively and at high operation frequency, but with fewer expensive and complex parts and which are simple to manufacture and assemble, whilst retaining a compact and reliable, responsive design.

The present invention provides a servo valve assembly as defined in claim <NUM>.

Also provided is a method of driving a valve spool of a servo valve as defined in claim <NUM>.

Preferred embodiments will now be described with reference to the drawings.

Servo valves are generally used when accurate position control is required, such as, for example, control of a primary flight surface. Servo valves can be used to control pneumatic or hydraulic actuators or motors. They are common in industries which include, but are not limited to, automotive systems, aircraft and the space industry.

A known type of servo valve has a flapper and nozzle arrangement.

<FIG> shows generally a known arrangement of a flapper and nozzle servo valve.

The assembly comprises a torque motor subsystem <NUM> and a flapper-nozzle subsystem <NUM>. ln more detail, the assembly comprises a flapper <NUM> disposed in a flapper cavity <NUM> , a pair of nozzles <NUM> disposed in a nozzle housing, and an electromagnet <NUM> surrounding an armature <NUM>. The armature has opposed tips, which protrude through gaps in a housing surrounding the electromagnet, and which are arranged to leave spaces between the armature and the housing.

The electromagnet is connected to an electrical input (not shown) and the armature <NUM> is connected in a perpendicular manner to the flapper <NUM>, or is an integral part of the flapper - the integral part being perpendicular to the flapper. The electromagnet includes coils that surround the armature and a set of permanent magnets that surround the coils. When a current is applied to the coils from the electrical input, magnetic flux acting on the ends of the armature is developed. The direction of the magnetic flux (force) depends on the sign (direction) of the current. The magnetic flux will cause the armature tips to be attracted to the electromagnet (current direction determines which magnetic pole is attracting and which one is repelling) thus varying the size of the spaces. This magnetic force creates an applied torque on the flapper, which is proportional to applied current. The flapper rotates and interacts with the nozzles.

Nozzles <NUM> are housed within a respective nozzle cavity in the housing, and comprise a fluid outlet and fluid inlet. Housing also has a port, which allows communication of fluid to the nozzles. The flapper comprises a blocking element at an end thereof which interacts with fluid outlets of nozzles to provide metering of fluid from the fluid outlets to a fluid port in the housing. Fluid port in turn allows communication of fluid pressure downstream to a spool valve and actuator arrangement (not shown). The positioning of the flapper between nozzles (controlled by the movement of the armature via electromagnet) will control the amount of fluid pressure communicated to the spool valve and actuator arrangement (not shown), which can be used to control actuator movement.

Although the flapper and nozzle type of servo valve arrangement shown in <FIG> can be effective at controlling an actuator, it has been found that limitations nevertheless exist. For example: in order to provide the correct limitations on flapper and armature movement, the spaces must be calibrated to very tight tolerances, as must the spacing of the nozzles from the flapper. Moreover, there is also a general desire to reduce servo valve weight and simplify its manufacture, construction and operation, as well as improve the operational pressures and frequencies that may be realised with such servo valve arrangements.

The apparatus of the present invention eliminates many of the problems of existing assemblies. This will be described with reference to <FIG>.

The servo valve assembly comprises a drive assembly <NUM> and a spool assembly.

The spool assembly comprises a body <NUM> and a spool <NUM> extending from the drive assembly into the body <NUM>.

The body <NUM> has a supply port <NUM>, a control port <NUM> and a return port <NUM> providing ports into/out of the body interior. End plugs <NUM>,<NUM> seal the ends of the body <NUM>. The supply port <NUM> connects a supply fluid to a first chamber AA of the body <NUM>, sealed at the outer end by end plug <NUM>. The return port <NUM> provides a fluid outlet from a chamber BB at the other end of the body <NUM> closed by end plug <NUM>. Chamber AA and chamber BB are fluidly connected by a chamber CC defined by the spool <NUM> extending into the body <NUM> between chambers AA and BB as will be described further below. Chamber AA is fluidly connected to chamber CC via a first channel <NUM> and a first spool opening <NUM> in the spool <NUM>. Chamber BB is fluidly connected to chamber CC via a second channel <NUM> and a second spool opening <NUM> in the spool <NUM>.

When the first spool opening <NUM> is aligned with the first channel <NUM>, fluid flows from the supply port through chamber AA, through channel <NUM> into chamber CC and out of the control port <NUM>. When the second spool opening <NUM> is aligned with the second channel <NUM>, a fluid flow path is defined between the control port <NUM> and the return port <NUM> via chambers CC and BB and channel <NUM>. Thus, fluid flow through the valve can be controlled as required to control actuators or valves such as, in a fuel supply system, a hydraulic cylinders or a throttle in a fuel inlet channel (not shown).

The first and second spool openings <NUM>,<NUM> are formed in the spool <NUM>, that extends into the body <NUM> from the drive system <NUM>, as can be best seen in <FIG>, such that when opening <NUM> is aligned with channel <NUM> and so channel <NUM> is open, second opening <NUM> is not aligned with channel <NUM> and so channel <NUM> is closed, and vice versa. According to the desired fluid flow, the spool <NUM> is rotated by the drive assembly as will be described further below, to align the appropriate spool opening and channel. Arrows X,Y show rotation of the spool.

The preferred drive mechanism for rotating the spool <NUM> will now be described with particular reference to <FIG> and <FIG>. As can be best seen in <FIG>, the drive assembly <NUM> is mounted on top of the body <NUM>. The spool <NUM> extends from the drive assembly into the body <NUM> and is caused to rotate within the body as shown by arrows X and Y relative to the body <NUM>.

The preferred drive assembly comprises permanent magnets <NUM> alternating with coils <NUM> around the drive assembly. The coils <NUM> each comprise a winding <NUM> around a respective core <NUM>. An air gap <NUM> is provided between each core and the adjacent permanent magnet <NUM>.

A connector <NUM> is mounted across the drive assembly in engagement with the permanent magnets <NUM> so as to be moved with movement of the magnets. The spool <NUM> is fixedly connected to and extends from the connector <NUM>. The spool <NUM> is preferably connected to the connector <NUM> via a washer <NUM> and seal ring <NUM> so that the connector and spool rotate together. In the preferred embodiment, the spool is connected to the connector via a washer slider <NUM> which means that the components can be press-fit and brazing or welding is not required. The seal <NUM> is preferably a Simering seal as is known for sealing rotary joints, but other designs are possible. A spring <NUM> is provided to bias the connector and spool in the neutral position against the force of the magnets.

When the valve is to be operated, current (in a direction and amount determined according to the desired fluid flow, and according to a control command from a controller (not shown) is applied to the drive assembly coils <NUM>. The coil core <NUM> magnetises and according to its polarity, attracts the permanent magnet adjacent to one end of the core and repels the permanent magnet adjacent the opposite end of the core. This causes the connector <NUM> and, hence, the spool <NUM> to rotate in a direction determined by the applied current. The rotation of the spool will then align either the first channel and the first spool opening or the second spool opening and the second channel to create the desired fluid flow path through the valve body. By changing the polarity of the coil, the direction of rotation (X or Y) is changed.

The connector is biased to its neutral position by the spring <NUM> which counteracts the magnetic force from the coils.

The assembly may also be provided with a coil and spring lock arrangement <NUM> to ensure positioning of the spool <NUM> and air gap adjustment. In the example shown, the coil and spring lock is a sliding element <NUM> mounted on the body <NUM>. The element is mounted e.g. by screws with a small amount of clearance, such that it is able to move slightly to compensate for spool rotation and determine the required air gaps.

Setting the spool in the neutral position is required during a calibration process when both flow channels <NUM>, <NUM> are half-closed. The connector and spring should be configured such that in the neutral position the flow is the same in both directions. This can be achieved by rotating the connector and spool and fixing the coil and spring lock <NUM> at the appropriate neutral position in a calibration procedure. The fluid flow is checked and when flow in both directions is equal, the coil and spring lock is secured e.g. by tightening the screws that fasten it to the body <NUM>. Next in the calibration process, the air gaps need to be set. This is done by adjusting the position of the coils. When the air gaps are correct, the position of the coils is secured e.g. by fastening screws or the like.

Further preferred elements of the assembly can be seen in, in particular, <FIG>. For example, filter components <NUM> can be provided at the ports <NUM>, <NUM>, <NUM> to prevent debris or contaminants in the fluid path. The filter components may be retained by screen rings <NUM>.

The figures also show a pin <NUM> that may be provided to assist in positioning the assembly onto another part such as a pump or manifold e.g. of a fuel system.

The coil wires <NUM> are preferably led out of the body <NUM> as shown in <FIG> and <FIG>.

The drive assembly is preferably closed by a cover <NUM>.

The arrangement of this invention provides a servo valve assembly with fewer expensive and complex parts and which is simpler to manufacture and assemble.

Claim 1:
A servo valve comprising:
a fluid transfer valve assembly comprising a supply port (<NUM>) and a control port (<NUM>);
a valve spool (<NUM>) arranged to regulate flow of fluid from the supply port (<NUM>) to the control port in response to a control signal; and
a drive means (<NUM>) configured to move the valve spool relative to the fluid transfer assembly in response to the control signal to regulate the fluid flow;
wherein the drive means is arranged to rotate the spool relative to the fluid transfer assembly, the spool provided with first and second openings (<NUM>,<NUM>) arranged to selectively align with or block respective first and second flow channels (<NUM>, <NUM>) in the fluid transfer assembly according to the direction and degree of rotation of the spool; wherein the fluid transfer valve assembly extends in a first direction between a first end and a second end and comprises a first chamber (AA) in fluid flow engagement with the supply port and, via first flow channel (<NUM>) and first opening (<NUM>) via the valve spool (<NUM>), with the control port (<NUM>), and a second chamber (BB) in fluid flow engagement with the supply port and a return port (<NUM>) via the second flow channel (<NUM>) and second opening (<NUM>) via the valve spool (<NUM>); and wherein the valve spool (<NUM>) comprises a tubular member defining a fluid chamber (CC) and extending between the first chamber and the second chamber in a direction substantially perpendicular to the first direction, such that, the first chamber (AA) is provided between the first end and the tubular member, along the first direction, and the second chamber (BB) is provided between the tubular member and the second end, along the first direction, and such that rotation of the spool is about an axis substantially perpendicular to the first direction and such that in a first position of rotation of the valve spool, the first opening aligns in the first direction with an opening from the first chamber to define a first fluid flow path from the supply port, into the first chamber and, through the first opening, into the fluid chamber, and then through the control port.