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 torque motor controlled by a control current, a pressure divider, and a hydraulic amplifier composed of a spool and sleeve control valve 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 an electro/hydraulic transducer, which commands an actuator, that changes the position of a valve's flow modulating feature.

Such mechanisms are used, for example, in various parts of an 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> or <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. With a two-stage servo valve, in a first stage typically a flapper is deflected by action of an armature connected to the motor away or towards nozzles, which controls the fluid flow through the nozzles. The nozzles cooperating with the flapper are connected to the valve pressure supply port via constant nozzles. Four nozzles with appropriate hydraulic conductivity together with the flapper, build a hydraulic pressure divider system with a modulated pressure ratio proportional to the position of the flapper in relation to the variable nozzles. Deflection of the flapper can control the amount of fluid injected from the nozzles, and thus control of a movable spool in a second stage. The second stage acts to control an actuator. In this way, servo valves allow precise control of actuator movement. During use, contaminants in the fluid passing through the servo valve may cause blockages. Blockages are known to occur in the first stage due to the small gaps between the flapper and the nozzles. Accordingly servo valves are typically provided with particulate filters to filter contaminants in the fluid passing through the servo valve. Various locations for filters to filter the first stage fluid are known. Such filters may be provided in the supply fluid line or in the supply fluid channels which direct a lower pressure supply fluid to the first stage nozzles. It can be difficult to design filters that can be easily and reliably assembled into the fluid flow path without substantially adding to the overall size and weight of the system.

One conventional filter assembly, that will be described below with reference to <FIG>, uses a filter member in the form of a perforated tube that is press-fit in the space between opposing constant orifice nozzles or restrictors that are provided in the fluid flow path between the supply port and the variable nozzles adjacent the flapper in the first stage. Because the filter member is assembled by being press-fit between the nozzles, it needs to be robust and thick to avoid it being deformed or collapsing due to the pressing force on assembly. The use of a thick material, however, means that it is more difficult to form the perforations through the material, resulting in more costly and time intensive manufacture of the filter. Also, the perforations are likely to have a conical shape which may adversely affect the filtering performance. Further, in order the limit the force that needs to be applied to assemble the filter, the interference fit should be limited - i.e. the space into which the filter member is pressed should be as large as possible whilst still providing a securing function to hold the filter member. The manufacturing tolerances are, therefore, very tight. The accuracy required also increases the time and cost of manufacture. Another known filter assembly is disclosed in <CIT>.

The inventors have identified a need for an improved filter assembly for filtering first stage fluid in a servo valve system.

According to this invention, there is provided a servo valve as claimed in claim <NUM>.

Also provided is a method of forming a filter assembly for a servo valve according to 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.

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

The servo valve <NUM> is a two-stage servo valve comprising a first stage <NUM> and a second stage <NUM>. The servo valve <NUM> comprises an electric motor <NUM>, typically a torque motor. The first stage <NUM> is provided between the electric motor <NUM> and the second stage. The motor <NUM> and first and second stages <NUM>, <NUM> of the servo valve <NUM> are in a housing <NUM>.

The electric motor <NUM> comprises permanent magnets <NUM>, coils <NUM>, and an armature <NUM> (not shown). The coils electrically communicate with an electrical supply (not shown) and, when activated, interact with the permanent magnets to create movement of the armature. In the example shown, two sets of coils are provided to provide redundancy in case one set fails. This is particularly important in safety-critical applications such as in aircraft.

The servo valve works by the flow of a working fluid, such as a hydraulic fluid. Hydraulic fluid is, for example, fuel or oil. The system includes a fluid supply (not shown) which provides pressurized supply fluid <NUM> via a supply port <NUM> and to the first and second stages.

The first stage <NUM> of the servo valve comprises a flapper <NUM> which is actuated by the electric motor <NUM>. The armature <NUM> of the electric motor <NUM> causes the flapper <NUM> to be deflected in the direction indicated by arrow F according to a control signal that determines the size and direction of the applied current.

The first stage <NUM> comprises two axially aligned, opposed first stage nozzles <NUM>, <NUM>. The first stage nozzles <NUM>, <NUM> are housed within a nozzle chamber <NUM> and comprise fluid outlets <NUM>, <NUM> which are spaced apart from each other. The distal end <NUM> of the flapper <NUM> is located between the fluid outlets of the two nozzles <NUM>, <NUM>.

<FIG> shows the flapper <NUM> in the neutral position with its distal end equidistant from each nozzle outlet <NUM>, <NUM>.

In operation, supply fluid <NUM> flows through the supply fluid port <NUM> and into the interior of the spool <NUM> of the second stage <NUM>. Supply fluid also flows via channels <NUM>, <NUM> into the nozzle chamber <NUM> at either end of the nozzles, via restrictors or constant orifice nozzles <NUM>, <NUM>. The fluid flows out of the nozzle chamber via the nozzles <NUM>, <NUM> via their fluid outlets <NUM>, <NUM> through a return line and out of a return port. When the flapper <NUM> is in the neutral position, the pressure of fluid on both ends of the servo valve is the same and the spool <NUM> of the second stage <NUM> is held in an axially centered position in the housing. In this position, an outlet port of the spool is aligned with the return line and so fluid flows through the spool to the return port.

In the event that the servo valve is to supply pressurized fluid to a control output <NUM>, a control signal is generated (not shown) to cause a control current to be applied to the motor <NUM>. This causes the flapper <NUM> to deflect relative to the first stage nozzles <NUM>,<NUM> in a direction determined by the applied current. Current applied in one direction will cause the flapper distal end <NUM> to deflect towards nozzle <NUM>, current in the opposite direction will cause the flapper end to deflect towards nozzle <NUM>. If the end of the flapper closes off the outlet <NUM> of nozzle <NUM>, fluid in the chamber <NUM> behind that nozzle, and in channel <NUM>, will not be able to flow out of the nozzle and so the pressure at that end of the first stage will increase compared to the lower pressure on the other side of the valve where the fluid can still flow through nozzle <NUM>. The increase of pressure will result in an increase in pressure at the corresponding end A of the second stage spool <NUM> causing the spool <NUM> to move axially in the housing in direction B. This movement will misalign the interior of the spool with the return port and, instead, will align it with a control port <NUM> to output pressurized fluid to e.g. an actuator. If the current applied to the motor causes the flapper to move towards nozzle <NUM>, the spool <NUM> will move axially in direction B'. A control wire <NUM> is attached to the spool (here closely fits in a slot in the spool) and therefore moves with the spool. The control wire sends a signal back to a controller (not shown) to feedback the axial position of the spool.

As mentioned above, and as seen in <FIG>, the supply fluid enters the nozzles <NUM>, <NUM> via restrictors or constant orifice nozzles <NUM>, <NUM> spaced apart to feed the fluid at a lower pressure to the respective nozzle <NUM>, <NUM>.

A filter assembly <NUM> is disposed between the two constant orifice nozzles <NUM>, <NUM>. The filter assembly <NUM> filters the working fluid provided to the first stage from the fluid supply. The filter assembly <NUM> prevents contaminants from entering the first stage <NUM>, and, in particular from reaching the first stage nozzles and the flapper of the first stage <NUM>. Because the nozzles <NUM>, <NUM> are so small and the spacing between the nozzles <NUM>, <NUM> and the flapper end <NUM> is also very small, the effect of any contaminants or particles here can have substantial adverse consequences on the operation of the servo valve.

The filter assembly <NUM> is press-fitted in the space <NUM> between the two restrictor nozzles <NUM>, <NUM>.

Referring now to <FIG>, the filter assembly <NUM> will now be described in detail. The filter assembly <NUM> comprises a filter member <NUM> and the nozzles <NUM>, <NUM>. The filter member <NUM> is tubular and has an array of holes <NUM> formed therethrough. The array of holes <NUM> are usually formed by laser drilling, but could be formed in other ways e.g. by photo-etching.

Upon assembly of the filter assembly <NUM>, with reference to <FIG>, the tube <NUM> will be press-fit into the opposing, spaced apart nozzles <NUM>, <NUM>. In this way, supply fluid <NUM> will pass through the perforated filter member <NUM> before flowing through the nozzles <NUM>, <NUM> to the first stage variable nozzles <NUM>, <NUM>.

As mentioned above, although such a filter works well once assembled, and does not require much space in the servo valve, it needs to be made thick enough to withstand the force applied by pressing to secure the filter member <NUM> in place between the opposing nozzles <NUM>, <NUM>. The required thickness is, however, difficult to perforate using known techniques such as laser drilling.

The filter assembly of the present invention is shown, for example in <FIG>. Here, the constant nozzles <NUM>', <NUM>' are provided as cylindrical bodies that abut against each other to define the constant nozzles in a single tubular unit. The tubular unit has, between its ends, across the fluid flow ports, a recessed area <NUM> extending around the periphery. The recessed area <NUM> receives the filter member <NUM> which is a perforated sheet extending around the tubular unit, covering the fluid flow ports.

Because the constant nozzles <NUM>', <NUM>' form a single unit, they provide support for the filter member <NUM> as it is assembled so that it cannot collapse or deform when pushed into position. Because of this support, the filter member can be made of a thinner material that was previously the case. The perforations can be quickly and easily formed in the filter material by known techniques such as laser drilling.

<FIG> and <FIG> show the fluid flow path for the supply fluid <NUM>. Supply fluid for the second stage flows across the outer surface of the filter member <NUM> and along channels <NUM>, <NUM> into the spool <NUM> of the second stage. Fluid for the first stage flows across the filter member via the perforations through the nozzles <NUM>', <NUM>' (<FIG>) from where it flows to the variable flow nozzles <NUM>, <NUM> and through the outlets <NUM>, <NUM> depending on the position of the flapper <NUM>. The supply fluid for the second stage performs a washing function of the filter member.

The material used for the filter member may be the same as that used for the nozzles so that both components have the same coefficient of thermal expansion, e.g. A286 alloy or CRES which works well in combination with an aluminum housing in terms of thermal expansion. Different materials can, however, be used.

The filter assembly provides effective filtering without the need for additional parts and can be easily, quickly and reliably assembled using lighter, thinner materials than for the known systems.

Claim 1:
A servo valve comprising:
a fluid supply;
a first stage (<NUM>) of the servo valve;
a second stage (<NUM>) of the servo valve; and
a filter assembly;
wherein the filter assembly filters fluid from the fluid supply to the first stage of the servo valve; the filter assembly comprising:
a first fluid nozzle (<NUM>') defining a first fluid passage therethrough between a first fluid inlet and a first fluid outlet;
a second fluid nozzle (<NUM>') defining a second fluid passage therethrough between a second fluid inlet and a second fluid outlet; characterized by:
the first and second fluid nozzles arranged in end to end abutment with each other to form a single tubular fluid nozzle unit in which the first and second fluid inlets are adjacent each other, and the first and second fluid passages align to form a common fluid passage through the tubular fluid nozzle unit, the first fluid outlet being at a first end of the tubular fluid nozzle unit and the second fluid passage being at a second, end of the tubular fluid nozzle unit;
a recess (<NUM>) formed between the first end and the second end and around the periphery of an outer surface of the tubular fluid nozzle unit in a region where the first and second fluid inlets are located; and
a perforated filter member (<NUM>) provided in the recess around the tubular fluid nozzle unit and across the first and second fluid inlets.