GB1598571A

GB1598571A - Servo valves

Info

Publication number: GB1598571A
Application number: GB39686/77A
Authority: GB
Original assignee: Sperry Ltd
Current assignee: Unisys Group Services Ltd
Priority date: 1977-09-23
Filing date: 1977-09-23
Publication date: 1981-09-23
Also published as: IT7851187A0; DE2841366A1; IT1181590B

Description

(54) IMPROVEMENTS IN OR RELATING TO SERVO VALVES (71) We, SPERRY LIMITED (formerly SPERRY RAND LIMITED), a British Company of Sperry House, 78 Portsmouth Road, Cobham, Surrey, Kit11 lJZ do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to servo valves.

British Patent Specification No. 1 245 137 discloses an electro-hydraulic servo comprising a flapper disc assembly having two flapper discs respectively cooperable with two nozzles forming the outlets from pressure chambers disposed at respective ends of a slidable valve spool. In the quiescent state of the valve, the flapper discs are spaced slightly from the respective nozzles so that hydraulic flow (leakage) through the chambers is necessary to maintain the valve in the quiescent state. Further, springs centre the spool with respect to the flapper disc assembly so that failure in the hydraulic fluid supply to the chambers would not necessarily cause the valve to "fail safe" to the closed condition. An object of the present invention is to provide a valve in which these disadvantages are overcome.

According to the invention a servo valve comprises a valve member which is movable in order to alter the flow of a fluid through the valve and which in use is subjected to the pressure of a control fluid, resilient means which act to maintain the valve member in or urge the valve member towards a first position, a solenoid, a slidable flapper disc cooperable with a nozzle, and spring means which urge the flapper disc to a closed position in which the flapper disc closes the nozzle and thereby maintains the pressure in the control fluid, energisation (or deenergisation) of the solenoid causing the flapper disc to move against the influence of the spring means and open the nozzle to cause a reduction in pressure in the control fluid so as to cause the valve member to move, against the influence of the resilient means, to a second position, de-energisation (or energisation) of the solenoid causing the flapper disc to revert to the closed position and the valve member to the first position, the valve also comprising a further solenoid, a further flapper disc cooperable with a further nozzle and further spring means which urge the further flapper disc into a closed position in which the further flapper disc closes the further nozzle and thereby maintains pressure in the control fluid such that the valve member remains in the first position or moves to the second position, energisation (or de-energisation) of the further solenoid causing a reduction is pressure in the control fluid to cause the valve member to move, against the influence of the resilient means, to a third position, with the first position being a centre position disposed between the second and third positions which are offset positions.

The control fluid may be a fluid different from the fluid the flow of which is altered by movement of the valve member, but in a preferred embodiment both fluids are hydraulic fluid supplied to the valve under pressure from a common source.

The two nozzles preferably form outlets of respective chambers which are positioned at respective ends of the valve member and which are supplied with hydraulic fluid from the source.

The flapper discs may be integral with or connected to respective guide rods slidable in respective bores extending from respective ends of the valve member, and the ftst-mentioned spring means and the further spring means may be provided by respective helical compression springs accommodated in respective counterbores extending from the ends of the valve member.

An electro-hydraulic servo valve according to the invention will now be described, by way of example, with reference to the accompanying drawings which is a longitudinal sectional view of the valve.

The valve has a body 1 with a central bore 2 accommodating a valve member in the form of a valve spool 3 slidable within the bore 2. Over a central portion of its length the spool 3 has a reduced-diameter waist 4 which is surrounded by an annular inlet port 5 when the valve spool 3 is in the centre position (corresponding to a closed condition of the valve) shown in the drawing. When the valve spool 3 moves to the left as viewed in the drawing, the inlet port 5 is placed in communication with a first annular outlet port 6 via the waist 4, and when the valve spool 3 moves to the right the inlet port 5 is placed in communication with a second annular outlet port 7 via the waist 4, the outlet ports 6,7 being connectible to a Ioad. Hence, hydraulic fluid fed under pressure to the inlet port 5 may be diverted either to the port 6 or the port 7.

Movement of the valve spool 3 is effected by a pilot stage comprising a control supply of hydraulic fluid under pressure applied to an inlet passage 8, and two solenoids 9 and 9a disposed at respective ends of the valve. The inlet passage 8 communicates with a control orifice 10 leading through passages 12 and 13 to an end pressure chamber 14 into which the corresponding end 15 of the valve spool 3 projects.

The solenoid 9 has a coil 16 surrounding a longitudinally movable armature rod 17 which extends, with radial clearance, through a cylindrical nozzle 18 and which is integrally formed with a flapper disc 19. The flapper disc 19 is guided for longitudinal sliding movement in a central bore 20 in the end 15 of the valve spool 3 by a cylindrical guide rod 21 which is, in effect, an extension of the rod 17.

The flapper disc 19 is urged into engagement with the nozzle 18 by means of a helical compression feedback spring 22 positioned between an external shoulder 23 on the flapper disc 19 and the base of a counterbore 24 formed in the end 15 of the valve spool 3. The diameter of the guide rod 21 is made equal to the internal diameter of the nozzle 18 to achieve a pressure balance on the solenoid armature. Instead of being integrally formed, the rod 17, flapper disc 19 and guide rod 21 may be separately formed,but rigidly interconnected components.

When the flapper disc 19 is in the closed position shown, corresponding to a de-energised condition of the solenoid 9, the flapper disc 19 seals the nozzle 18. When the coil 16 of the solenoid 9 is energised, the armature rod 17 moves to the right, thereby pushing the flapper disc 19 (against the influence of the spring 22) out of engagement with the nozzle 18 so as to open the latter, allowing hydraulic fluid to pass from the chamber 14, through the radial clearance gap between the nozzle 18 and the rod 17 and into passages 25 and 26. The passage 26 leads to a hydraulic tank or reservoir (not shown) via a passage 27 surrounding the central portion of the valve spool 3. Deenergisation of the solenoid 9 causes the flapper disc 19 to revert to the closed position under the action of spring 22.

The inlet passage 8 also communicates with a further control orifice 10a leading through passages 12a and 13a to a further end chamber 1 4a having a further nozzle 1 8a and flapper disc 19a controlled by the solenoid 9a in a similar manner to that previously described for the solenoid 9. Corresponding parts associated with the two solenoids 9 and 9a bear the same reference numerals, with the parts associated with the right-hand solenoid having the suffix a. Two helical compression springs 28 and 28a disposed in the two chambers 14 and 1 4a act to maintain the spool 3 in (or urge the spool 3 towards) the centre position shown in the drawings in which the valve is closed.

When hydraulic fluid passes through the nozzle 18 on energisation of the solenoid 9, the reduction in fluid pressure in the chamber 14 causes the valve spool 3 to move to the left against the influence of the spring 28 because the force imparted to the spool 3 by the hydraulic fluid in the chamber 14a exceeds the force imparted to the spool 3 by the hydraulic fluid in the vented chamber 14. The spool 3 comes to rest in an offset position in which the centring force of the compressed spring 28 balances the difference of the force applied to the spool 3 by the fluids in the chambers 14 and 14a. The inlet port 5 is then placed in communication with the outlet port 6. At the same time, the outlet port 7 is connected to the hydraulic tank or reservoir (not shown) via the passage 27 since a further waist 29 on the spool 3 then connects the two.In this way, one end of say a double-acting hydraulic cylinder is vented whilst the other end has pressure fluid supplied to it.

On displacement of the spool 3 to the left as described, the feedback spring 22 is compressed and thus acts against the force applied by the solenoid armature to give an overall force balance. The flow of hydraulic fluid through the gap between the flapper disc 19 and the nozzle 18 causes a pressure drop across the orifice 10, this pressure drop balancing the force applied by the spring 28 and the Bernouilli flow forces. The pressure drop is typically in the range of 3 to 8 bar. De-energisation of the solenoid 9 causes the armature rod 17 to move to the left under the influence of the spring 22 so as to re-close the flapper nozzle 18 and re-establish the pressure in chamber 14 equal to that in chamber 14a.

Thus there is no net fluid pressure acting on the spool 3 which is centred by the spring 28 so that the valve reverts to the closed condition.

In a similar manner, energisation of the solenoid 9a causes a reduction in fluid pressure in the chamber 14a as a result of fluid passing through the nozzle 18a, the passages 25a and 26a and thence into the tank. As a result, the valve spool 3 moves to the right and compresses the spring 28a, thereby placing the inlet port 5 in communication with the outlet port 7 and the outlet port 6 in communication with the reservoir via a further spool waist 30 and the passage 27. The spring 28a pushes the spool 3 back to the centre position on de-energisation of the solenoid 9a. The direction of movement of both flapper discs 19 and 19a is aligned with the direction of sliding movement of the valve spool 3. It will be realised that the spool 3 will revert to the centre position on electrical failure or failure of the hydraulic fluid supply, which is a very desirable "fail safe" feature.

The spool 3 can be driven from one offset position, through the centre position to the other offset position by de-energising one solenoid and energising the other solenoid and the solenoids can be controlled in Class A or Class B fashion.

The flow of hydraulic fluid through the gap between the flapper disc 19 and the nozzle 18 constitutes the normal leakage flow through a spool valve which flow has to be kept to a minimum in known valves since it takes place in the quiescent state of the valve. However, generally speaking, the greater the leakage flow, the greater the dynamic response of the valve. Since according to the present invention there is no leakage flow through the pilot stage in the quiescent state of the valve, then there is no restraint on the magnitude of the leakage flow from this standpoint, whereby the leakage flow can be increased to give the valve a high dynamic response and this is a significant advance in the art.However, there is some limitation on the magnitude of leakage flow that can be accommodated but the dynamic response of the valve can be increased by arranging for the solenoid armature 17 to be centred by relatively stiff resilient means which may be a compression spring (as shown at 31) or one or more diaphragms, for example, The use of relatively stiff centring means increases the natural frequency of the pilot stage of the valve, thereby increasing the dynamic response. This use also means that the solenoid 9 is operated over only a very small portion of the available air gap which further aids in increasing the dynamic response of the valve.

In a typical valve, the flapper disc (and hence solenoid armature) may move only 0.001 to 0.002 of an inch whereas the spool may move ".

The valve may have several operating modes depending upon the degree of sophistication necessary in a given system, namely: a. Open loop with electrical input signal.

b. Open loop with pressure compensator module to provide pressure compensated flow control, the compensator giving constant pressure drop which would otherwise vary with load.

c. With pressure transducers and electrical feedback to provide pressure compensated flow control.

d. As an electro-hydraulic servo valve in a closed loop system.

The advantages of the described valve are that: 1. It provides an electrically modulated flow control.

2. The pilot stage consists of independent linear solenoids and nozzle flapper discs.

3. It fails safe to the centre position under action of the centering springs.

4. No leakage is required in the null position or quiescent state, whereby a high dynamic response for the valve can be obtained.

5. The valve body could be a standard directional control valve.

6. Very small movement is required from the solenoids which are therefore operating under the most desirable conditions.

7. Travel of the spool is not limited by solenoid characteristics.

8. The spool, when in other than an extreme end position, can be powered in either direction by electronically splitting the power to both solenoids.

9. Sealing between the high pressure chambers and the solenoids, in the closed positions of the flapper discs, is obtained through the nozzles and flapper discs.

10. Force feedback enables greater spool displacement to be obtained.

11. A greater spool travel reduces the percentage threshold effect such as resolution and hysteresis.

WHAT WE CLAIM IS: 1. A servo valve comprising a valve member which is movable in order to alter the flow of a fluid through the valve and which in use is subjected to the pressure of a control fluid, resilient means which act to maintain the valve member in or urge the valve member towards a position, a solenoid, a slidable flapper asC cooperable with a nozzle, and spring means which urge the flapper disc to a closed position in which the flapper disc closes the nozzle and thereby maintains the pressure in the control fluid, energisation (or de-energisation) of the solenoid causing the flapper disc to move against the influence of the spring means and open the nozzle to cause a reduction in pressure in the control fluid so as to cause the valve member to move, against the influence of the resilient means, to a second position, de-energisation (or energisation) of the solenoid causing the flapper disc to revert to the closed position and the valve member to the first position, the valve also comprising a further solenoid, a further flapper disc cooperable with a further nozzle and further spring means which urge the further flapper disc into a closed position in which the further flapper disc closes the further nozzle and thereby maintains pressure in the control fluid such that the valve member remains in the first position or moves to the second position, energisation (or de-energisation) of the further solenoid causing a reduction in pressure in the control fluid to cause the valve member to move, against the influence of the resilient means, to a third position, with the first position being a centre position disposed between the second and third positions which are offset positions.

3. A servo valve according to Claim 2, wherein the nozzles form outlets of respective chambers which are positioned at respective ends of the valve member and which are supplied with the control fluid.

4. A servo valve according to Claim 2 or 3, wherein the flapper discs are guided for movement towards and away from the associated nozzles by guide rods slidable in respective bores formed in respective ends of the valve

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. solenoid and energising the other solenoid and the solenoids can be controlled in Class A or Class B fashion. The flow of hydraulic fluid through the gap between the flapper disc 19 and the nozzle 18 constitutes the normal leakage flow through a spool valve which flow has to be kept to a minimum in known valves since it takes place in the quiescent state of the valve. However, generally speaking, the greater the leakage flow, the greater the dynamic response of the valve. Since according to the present invention there is no leakage flow through the pilot stage in the quiescent state of the valve, then there is no restraint on the magnitude of the leakage flow from this standpoint, whereby the leakage flow can be increased to give the valve a high dynamic response and this is a significant advance in the art.However, there is some limitation on the magnitude of leakage flow that can be accommodated but the dynamic response of the valve can be increased by arranging for the solenoid armature 17 to be centred by relatively stiff resilient means which may be a compression spring (as shown at 31) or one or more diaphragms, for example, The use of relatively stiff centring means increases the natural frequency of the pilot stage of the valve, thereby increasing the dynamic response. This use also means that the solenoid 9 is operated over only a very small portion of the available air gap which further aids in increasing the dynamic response of the valve. In a typical valve, the flapper disc (and hence solenoid armature) may move only 0.001 to 0.002 of an inch whereas the spool may move ". The valve may have several operating modes depending upon the degree of sophistication necessary in a given system, namely: a. Open loop with electrical input signal. b. Open loop with pressure compensator module to provide pressure compensated flow control, the compensator giving constant pressure drop which would otherwise vary with load. c. With pressure transducers and electrical feedback to provide pressure compensated flow control. d. As an electro-hydraulic servo valve in a closed loop system. The advantages of the described valve are that:

1. It provides an electrically modulated flow control.

3. It fails safe to the centre position under action of the centering springs.

5. The valve body could be a standard directional control valve.

7. Travel of the spool is not limited by solenoid characteristics.

10. Force feedback enables greater spool displacement to be obtained.

WHAT WE CLAIM IS:

1. A servo valve comprising a valve member which is movable in order to alter the flow of a fluid through the valve and which in use is subjected to the pressure of a control fluid, resilient means which act to maintain the valve member in or urge the valve member towards a position, a solenoid, a slidable flapper asC cooperable with a nozzle, and spring means which urge the flapper disc to a closed position in which the flapper disc closes the nozzle and thereby maintains the pressure in the control fluid, energisation (or de-energisation) of the solenoid causing the flapper disc to move against the influence of the spring means and open the nozzle to cause a reduction in pressure in the control fluid so as to cause the valve member to move, against the influence of the resilient means, to a second position, de-energisation (or energisation) of the solenoid causing the flapper disc to revert to the closed position and the valve member to the first position, the valve also comprising a further solenoid, a further flapper disc cooperable with a further nozzle and further spring means which urge the further flapper disc into a closed position in which the further flapper disc closes the further nozzle and thereby maintains pressure in the control fluid such that the valve member remains in the first position or moves to the second position, energisation (or de-energisation) of the further solenoid causing a reduction in pressure in the control fluid to cause the valve member to move, against the influence of the resilient means, to a third position, with the first position being a centre position disposed between the second and third positions which are offset positions.

member.

5. A servo valve according to Claim 4, wherein each guide rod is cylindrical and has a diameter equal to the internal diameter of the respective nozzle.

6. A servo valve according to any of Claims 2 to 5, wherein each solenoid has a corresponding slidable armature rod which extends with radial clearance through the associated nozzle and which is connected to or integral with the corresponding flapper disc.

7. A servo valve according to Claims 5 to 6, wherein each armature rod is aligned with the corresponding guide rod along the direction of sliding movement of the valve member.

8. A servo valve according to any of Claims 2 to 7, wherein the first-mentioned spring means and the further spring means are provided by respective helical compression springs.

9. A servo valve according to Claim 8 wherein each helical compression spring acts between the corresponding flapper disc and the base of the counterbore formed in the respective end of the valve member.

10. A servo valve according to any of the preceding claims, wherein the armature of the or each solenoid is centred by relatively stiff resilient means.

11. A servo valve according to Claim 10, wherein the relatively stiff resilient means is in the form of a compression spring.

12. A servo valve according to Claim 10, wherein the relatively stiff resilient means is in the form of one or more diaphragms.

13. A servo valve according to any of the preceding claims wherein the control fluid and the fluid the flow of which is altered by movement of the valve member are a hydraulic fluid supplied to the valve under pressure from a common source.

14. A servo valve according to any of Claims 1 to 10, wherein the control fluid is a fluid different from the fluid the flow of which is altered by movement of the valve member.

15. A servo valve constructed and arranged to operate substantially as herein particularly described with reference to the accompanying drawing.