GB2044961A - Servo valve - Google Patents

Abstract

A servo valve is constructed and arranged to be of relatively compact form and to afford a significant reduction in the cost of production. The valve comprises a casing 1, a valve element 2 which is movable in at least one direction away from a closed position and fluid-pressure-operable means 3, for moving the element in said one direction. A fluid-pressure-chamber 4 is operably associated with the means 3. Flow-restrictive inlet means 5 is adapted to connect the chamber 4 with a fluid-pressure source 6, and flow- restrictive outlet means 7 is adapted to connect that chamber to exhaust. Heating means 8 are associated with at least one of said flow-restrictive means. <IMAGE>

Description

SPECIFICATION Servo valve This invention relates to a servo valve.

Hitherto, certain servo valves have included input means, requiring only low power input signals for their operation, a fluid-pressure-operable bridge circuit and a displaceable valve element so arranged in combination that when the input means are in their"null" condition the bridge circuit is maintained in a balanced condition, thus holding the valve element in a balanced, closed, position and when the input means are adjusted, by suitable signals transmitted thereto, away from their "null" condition, the bridge circuit is thereby so unbalanced as to cause the valve element to move away from its closed position to a position in which it permits flow of fluid under pressure through the valve to a point of usage.

Such input means have comprised a solenoid having associated therewith a pair of opposed exhaust nozzles leading from the bridge circuit and a flapper element disposed between the nozzles, which is operated by the solenoid. These components are relatively expensive to produce.

The invention as claimed is intended to provide a remedy. It solves the problem of how to design a servo valve which is so constructed and arranged as to be of relatively compact form and as to afford a significant reduction in the cost of production thereof.

According to the invention a servo valve comprises a casing, a valve element which is movable in at least one direction away from a closed position, fluid-pressure-operable means for moving said element in said one direction, a fluid-pressure chamber operably associated with said means, flowrestrictive inlet means adapted to connect said chamber with a fluid-pressure source, flowrestrictive outlet means adapted to connect said chamber to exhaust, and heating means associated with at least one of said flow-restrictive means.

Where said element is movable in both directions away from its closed position, two of said fluidpressure-operable means may be provided and a respective said fluid-pressure chamber be operably associated with each said fluid-pressure-operable means, a respective said flow-restrictive inlet means and a respective said flow-restrictive outlet means being connected to each said fluid-pressure chamber.

In this case said heating means may be associated with both of said flow-restrictive inlet means and/or with both of said flow-restrictive outlet means.

The advantages offered by the invention are mainly that a relatively-inexpensive, compact and robust servo valve is provided which maintains desired performance characteristics, as well as avoiding hysteresis and being substantially insensitive to vibration.

Two ways of carrying out the invention are described in detail below with reference to diagrammatic drawings in which: Figure 1 is a servo valve in accordance with the firct nmhariimPnt n n rl.

Figure 2 is a servo valve in accordance with the second embodiment.

The figures show servo valves, which in their basic design each comprise a casing 1, a valve element 2 which is movable in at least one direction away from a closed position, fluid-pressure-operable means 3 for moving the element in that direction, and a fluid-pressure chamber 4 operably associated with the means 3.

In accordance with the invention flow-restrictive inlet means 5 is provided which is adapted to connect the chamber 4 with a fluid-pressure source 6. Fldw restrictive outlet means is provided which is adapted to connect the chamber 4 to exhaust, and heating means 8 are associated with at least one of said flow-restrictive means.

In accordance with the embodiment shown in Figure 1, the valve element 2 is shown in its closed, neutral, position and is movable in both directions away from that position. Therefore, two of the fluid-pressure-operable means 3, 3' are provided, one at each end of the valve element, and a respective fluid-pressure chamber 4,4' is operably associated with each means 3, 3'. A respective flow-restrictive inlet means in the form of a passage 5, 5' leads into each chamber 4,4' and a respective flow-restrictive outlet means in the form of a capillary tube 7,7' leads out from each of those chambers. The heating means 8, which are of electrical resistance type, are associated with the two capillary tubes 7, 7' and include a respective heating element 9,9' for each tube.The means 8 also include a device 10 for adjusting electrical current caused to pass through the elements 9,9'.

Each element 9, 9' is disposed around the exterior surface of its respective tube 7,7' and the device 10 comprises a potentiometer, the wiper 11 of which is connected to a source of electrical current. One end of the winding 12 of the potentiometer is connected to one end of the element 9 and the other end of that element is connected to earth. The other end of the winding 12 is connected to one end of the other element 9' and the other end of that element is connected to earth. The potentiometer is thereby so arranged in relation to the two elements 9, 9' as to be capable of differentially controlling the current flowing through the two elements.

In this embodiment the two passages 5, 5' are each provided with an orifice 13, 13'.

As shown, the valve element 2 comprises a spool 14 having lands 15,16, 17 and 18. The outer end faces of the lands 15 and 18 form the respective fluid-pressure-operable means 3;3'. Afluid-pressure supply passage 19 which, as shown, is in communication with the source 6, is arranged to be permanently in communication with the annulus 20 between lands 16 and 17. In the neutral position of the spool service passages 21 and 22, connectible for example to a fluid-pressure-operable double-acting jack 23, are respectively closed by the lands 16 and 17. Exhaust passages 24 and 25 are arranged to be permanently in communication with the annuli 26, 27 respectively formed between the lands 15,16; 17, 18 and connect with a reservoir 28. Capillary tubes 7, 7' likewise discharge to this reservoir as shown.

Spring means in the form of coil springs 29,29' are provided in each of the chambers 4, 4' to bias the spool 14 to its neutral position.

When, in operation of the servo valve, a fluid, in this embodiment hydraulic liquid under pressure, is supplied thereto from the source 6, this liquid passes into the annul us 20 and, by way of the passages 5,5' and the orifices 13, 13' therein, into the chambers 4, 4'. This liquid passes from the chambers 4,4' through the tubes 7,7' to the reservoir 28. Provided the wiper 11 of the potentiometer 10 is in its central position as shown with respectto its winding 12, the currents passing through the heating elements 9, 9' are equal in value. Thus these two elements have the same heating effect upon the hydraulic liquid passing out from the chambers 4,4' through the respective tubes 7,7'.Hence the viscosity of the liquid in these tubes is the same, or substantially the same, and the hydraulic bridge circuit formed by the passages 5, 5', tubes 7,7' and the chambers 4,4' is, in consequence, in a condition of balance. Thus the spool is held in its neutral position as shown and the jack 23 is held stationary.

If, for example, the wiper 11 of the potentiometer 10 is moved in the clockwise direction, the current flowing through the element 9' is increased whilst the current flowing through the element 9 is correspondingly reduced. Thus the element 9' increases in temperature and the element 9 reduces in temperature.

Since the liquid passing through the tube 7' from the chamber 4' now therefore becomes less viscous its flow rate increases. Consequently a pressure drop occurs across the orifice 13' and the pressure in the chamber 4' falls. Simultaneously, since as a result of the temperature reduction in the element 9 the liquid passing through the tube 7 from the chamber 4 becomes more viscous its flow rate decreases.

Consequently a pressure rise occurs across the orifice 13 and the pressure in the chamber 4 increases.

Thus the spool 14 is hydraulically unbalanced and moves to the right in the drawing against the effort of the spring 29' so that liquid under pressure present in the annulus 20 passes into the passage 22 and thus to the right-hand side of the jack 23. Since the left-hand side of the jack is in communication with the reservoir 28 by way of the passage 21, annulus 26 and passage 24, the jack is able now to contract.

Suitable feed-back means may be provided between the jack 23 and the potentiometer 10 and/or between the spool 14 and the potentiometer for returning the servo valve to its neutral position upon the jack attaining its selected movement.

If the wiper 11 of the potentiometer 10 is instead moved in the anti-clockwise direction, the element 9 is increased in temperature and the element 9' is correspondingly reduced in temperature with the result that the spool 14 moves to the left in the drawing to cause the jack 23 to extend.

In the construction of the second embodiment illustrated in Figure 2 the servo valve is basically the same as that described with reference to Figure 1 and like reference numerals apply. However, the passages 5, 5' are not provided with fixed orifices but are instead in the form of capillary tubes which are provided around their exterior surfaces each with a respective heating element 30,30'. Also the right-hand end of the winding 12 of the potentiometer 10 is instead directly connected to one end of the element 30 while the other end of that element is conneced by way of the element 9' to earth. As in Figure 1 the left-hand end of the winding 12 of the potentiometer is connected to one end of the element 9, but the other end of that element is connected to one end of the element 30' and the other end of that element 30' is connected to earth.

When, during operation of the servo valve the wiper 11 is in its central position, equal current passes through each of the elements 9, 9', 30 and 30'. However, when for example, the wiper 11 is moved in the clockwise direction, the current in the element 30 and in the element 9' is increased, while the current in the element 9 and the element 30' is reduced. Consequently, the viscosity of the liquid passing through the tube 5 and the tube 7' is reduced, resulting in a higher rate of flow of liquid into the chamber 4 and a higher rate of flow out from the chamber 4'. Simultaneously, the viscosity of the liquid passing through the tube 7 and the tube 5' is increased, resulting in a lower rate of flow of liquid out of the chamber 4 and a lower rate of flow into the chamber 4'.Hence the pressure in the chamber 4 rises and the pressure in the chamber 4' falls so that the spool moves to the right to cause contraction of the jack 23. If the wiper 11 is instead moved in the anti-clockwise direction the current in the element 30 and in the element 9' is reduced, while the current in the element 30' and in the element 9 is increased, resulting in movement of the spool 14 to the left and extension of the jack 23.

As with the construction of Figure 1, suitable feed-back means may be provided between the jack 23 and the potentiometer 10 and/or between the spool 14 and the potentiometer.

Further means (not shown) may be provided in association with the constructions of Figure 1 and Figure 2 to compensate for any variation in the temperature of the liquid supplied from said source, and for any variation in ambient temperature.

Although in the embodiment above-described with reference to Figure 1 the flow-restrictive inlet means 5,5' are provided with orifices 13, 13', in alternative embodiments of the invention the means 5,5' may, like the outlet means 7,7', be in the form of capillary tubes. In other embodiments the means 5, 5' may remain as passages which include orifices 13, 13' but the means 7,7', instead of being in the form of capillary tubes, may, like the means 5, 5', be in the form of passages provided with orifices.

Further, although in the embodiment abovedescribed with reference to Figure 2 the flowrestrictive means 5, 5', 7 and 7' are all in the form of capillary tubes, in alternative embodiments of the invention the means 5,5' may be in the form of passages provided with orifices as in Figure 1,while the means 7,7' remain in the form of capillary tubes, or vice versa. In yet other embodiments all of the flow-restrictive means 5, 5', 7 and 7' may be in the form of passages provided with orifices and thus none of them in the form of capillary tubes.

Although in the two embodiments abovedescribed with reference to the drawings each heating element is disposed around the exterior surface of its respective flow-restrictive means, in other embodiments certain at least of such elements may instead be disposed within its or their flowrestrictive means, or, alternatively, may be formed as a part of the wall or walls of the flow-restrictive means.

Again, although in the embodiment abovedescribed with reference to Figure 1 the elements 9, 9' are associated with the flow-restrictive outlet means 7,7', in alternative embodiments of the invention they may instead be associated only with the flow-restrictive inlet means, said device 10 being suitably connected to said elements so that when operated it produces the desired controlling effect upon the pressures in the chambers 4,4'.

Although in the two embodiments abovedescribed with reference to the drawings, the fluid used is a liquid, in other embodiments a gaseous fluid may instead be used.

Further, although in the two embodiments abovedescribed with reference to the drawings, fluidpressure-operable means 3, 3' are provided at both ends of the spool 14, in other embodiments such means may be provided only at one end thereof. In this case only one fluid-pressure chamber 4 is therefore provided, this with a single flow-restrictive inlet means either in the form of a passage with an orifice or in the form of a capillary tube taken thereto and with a single flow-restrictive outlet means either in the form of a passage with an orifice or in the form of a capillary tube taken therefrom. At least one of these flow-restrictive means may be provided with a heating element having an associated device, the operation of which effects appropriate change in the current passing through the element or elements in such a manner as to result in an increase or a decrease ofthe pressure in the single fluidpdressure chamber and thus adjustment of the spool against, for example, the effort of a coil spring.

Claims (17)

1. A servo valve comprising a casing, a valve element which is movable in at least one direction away from a closed position, fluid-pressure-operable means for moving said element in said one direction, a fluid-pressure chamber operably associated with said means, flow-restrictive inlet means adapted to connect said chamber with a fluidpressure source, flow-restrictive outlet means adapted to connect said chamber to exhaust, and heating means associated with at least one of said flow-restrictive means.

2. A valve as claimed in Claim 1, wherein said valve element is movable in both directions away from its closed position.

3. A valve as claimed in Claim 2, wherein two of said fluid-pressure-operable means are provided and a respective said fluid-pressure chamber is operably associated with each said fluid-pressureoperable means, a respective said flow-restrictive inlet means and a rspective said flow-restrictive outlet means being connected to each said fluidpressure chamber.

4. A valve as claimed in Claim 3, wherein said heating means are associated with both of said flow-restrictive inlet means and/or with both of said flow-restrictive outlet means.

5. A valve as claimed in any one of the preceding claims, wherein said heating means are of electrical resistance type.

6. A valve as claimed in Claim 5, wherein said heating means include a respective heating element associated with the or each said flow-restrictive inlet means and/or with the or each said flow-restrictive outlet means.

7. A valve as claimed in Claim 6, wherein said heating means also include a device for adjusting electrical current caused to pass through said element or elements.

8. Avalve as claimed in any one of Claims 5 to 7, wherein the or each said element is disposed externally of its respective said flow-restrictive means.

9. A valve as claimed in any one of Claims 5 to 7, wherein the or each said element is disposed internally of its respective said flow-restrictive means.

10. Avalve as claimed in any one of the preceding claims, wherein the or each said flow-restrictive inlet means comprises a passage having an orifice therein.

11. A valve as claimed in any one of Claims 1 to 9, wherein the or each said flow-restrictive inlet means comprises a capillary tube.

12. Avalve as claimed in any one of the preceding claims, wherein the or each said flow-restrictive outlet means comprises a passage having an orifice therein.

13. Avalve as claimed in any one of Claims 1 to 11, wherein the or each said flow-restrictive outlet means comprises a capillary tube.

14. A valve as claimed in any one of Claims 3 to 13, wherein said valve element comprises a spool which is so movable in said casing in either direction away from its closed, neutral, position as to be capable of directing fluid, supplied under pressure to said casing, either into the one or into the other of two service passages which are connectible to a service controlled by said valve.

15. Avalve as claimed in Claim 14, wherein said spool and said two fluid-pressure-operable means are integrally formed.

16. A servo valve substantially as hereinbefore described with reference to Figure 1 oftheaccom- panying drawings.

17. A servo valve substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.