US3922955A

US3922955A - Fail-fixed servovalve

Info

Publication number: US3922955A
Application number: US437667*A
Authority: US
Inventors: Howard Berdolt Kast
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1974-01-29
Filing date: 1974-01-29
Publication date: 1975-12-02
Anticipated expiration: 1992-12-02
Also published as: GB1489321A; BE824920A; FR2259263B1; DE2503067C2; FR2259263A1; JPS604364B2; IT1031166B; JPS50107387A; DE2503067A1

Abstract

A fail-fixed servovalve includes a jet pipe for discharging a pressurized liquid wherein the jet pipe may be selectively signaled to deflect and unbalance the pressures at opposing ends of a spool which would otherwise remain centered within a sleeve by resilient means at opposing ends thereof. The spool includes a plurality of circumferentially relieved areas interspaced between a plurality of lands with interconnecting passages therebetween wherein a servopiston may be actuated to move by pressurized liquid selectively received from the sleeve and spool such that a sudden loss of either the deflecting signal or the pressurized liquid would operate to block the flow of liquid to the servopiston, locking it in place.

Description

United States Patent Kast 1 Dec. 2, 1975 I FAIL-FIXED SERVOVALVE 75 J Inventor: Howard Berdolt Kast, Fairfield, "9' Maslousky Ohio Attorney, Agent, or F1rm lames W. Johnson, Jr.;

Derek P. Lawrence [73] Assignee: General Electric Company, Lynn,

Mass" 57 ABSTRACT Filedi Jan- 29, 1974 A fail-fixed servovalve includes a jet pipe for discharg- [211 App]. NO: 437,667 ing a pressurized liquid wherein the jet pipe may be selectively signaled to deflect and unbalance the pressures at opposing ends of a spool which would other- 37/ 137/625-68 wise remain centered within a sleeve by resilient l5 Clmeans at pposing ends thereof The spool includes a Fleld of Search t plurality of circumferentially relieved areas inter- 91/461 spaced between a plurality of lands with interconnecting passages therebetween'wherein a servopiston may References Clted be actuated to move by pressurized liquid selectively UNITED STATES PATENTS received from the sleeve and spool such that a sudden 2.796.851 6/1957 Ziskal l37/625.68 loss of either the deflecting signal or the Pressurized 3 133330 9 3 Rcitmunm" 25 51 liquid would operate to block the flow of liquid to the 3.282283 11/1966 Takeda 91/461 servopiston, locking it in place. 3,472,278 [0/1969 Arfelt 137/6242 3,528,446 9/1970 Horn l37/625.6 x 7 Clam, 5 Drawmg F'gures Ara/21;;

US. Patent Dec. 2, 1975 Sheet 1 of 2 3,922,955

Jar/4y KER/Z US. Patent Dec. 2, 1975 Sheet 2 of2 3,922,955

- aaa FAIL-FIXED SERVOVALVE BACKGROUND OF THE INVENTION This invention relates to a fail-fixed servovalve and, more particularly, to a fail-fixed servovalve which remains fixed in place upon loss of either an input signal or hydraulic liquid, and which may function as either a digital or analog device depending upon the frequency of the input signal.

Servovalves of the electrohydraulic type have been used broadly as the interface between an electrical control signal and different types of actuating and metering devices. Servovalves may also be directly applicable to the fuel control of a gas turbine engine. For example, in a gas turbine engine fuel control system, there may be an electrical signal generated by a control which compares a reference engine speed with an actual operating speed. This electrical signal may then be connected to the input of a servovalve which in turn controls a servopiston wherein the mechanical output of the servopiston is connected to a fuel metering valve. Thus the fuel flow of the gas turbine engine can be varied as a function of the electrical signal in order to maintain the reference engine speed. Such a system would provide a highly stable and accurate control of the engine speed.

Todays applications for servovalves, particularly in gas turbine engines, demand that the servovalve be failfixed. By fail-fixed, it is meant that the mechanical output of the servopiston, as may be provided by an actuator, will be locked in place immediately following a loss of either the electrical input signal or the hydraulic lines. Present day servovalves may have a mechanical bias which permits the servopiston to move in a preselected direction upon loss of electrical signal. Thus in the event of such a failure, the servopiston will be driven at a predetermined velocity in a preselected direction to the end of the piston stroke. In the case of a fuel metering valve, the preselected direction would effect either a complete shutoff or maximum flow of fuel.

There may also be an electrical failure wherein the servovalve receives a constant maximum current signal of either a positive or negative polarity. Present day servovalves which fail under this condition will provide for a maximum flow to the servopiston so as to drive the piston at maximum velocity in a direction determined by the polarity of the failure.

Either extreme is obviously unsatisfactory and the better result would be for the fuel metering valve to hold its initial position immediately prior to the failure as would happen if the servovalve were fail-fixed.

Electrohydraulic stepping motors provide many of the features of servovalves and can be made fail-fixed; however, they are also relatively inefficient and incur many of the difficulties associated with servomotors.

Therefore it is a primary object of this invention to provide a fail-fixed servovalve, the output of which will be locked in place immediately following the failure of either an electrical signal or hydraulic line.

It is also an object of this invention to provide a failfixed servovalve which retains the desirable features of conventional servovalves so that past experience and manufacturing techniques may be made applicable.

It is a further object of this invention to provide a failfixed servovalve which may be used in the fuel control of a gas turbine engine to convert an electrical input signal to a mechanical output.

SUMMARY OF THE INVENTION These and other objects and advantages will be more clearly understood from the following detailed description and drawings, all of which are intended to be representative of, rather than in any way limiting on, the scope of invention. The servovalve of this invention includes a deflecting means and a jet pipe for discharging a jet of pressurized liquid wherein the deflecting means is secured to the jet pipe so as to deflect the jet upon an input signal to the deflecting means. A sleeve is provided with a plurality of ports therethrough, one of which receives an inlet flow of pressurized liquid. A spool is translatably disposed within the sleeve wherein the spool is also centered within the sleeve by resilient means at opposing ends of the spool. The spool also includes a plurality 'of circumferentially relieved areas interspaced between a plurality of circumferential lands with flow communication selectively established between certain relieved areas by passages internal to the spool. A pair of receiver passages are disposed to accept an equal amount of liquid and equal recovered pressure from the jet pipe when in the non-deflected position wherein the receiver passages connect with opposite ends of the sleeve. There is also provided a servopiston having a piston translatably disposed within a bore, both sides of which communicate with ports in the sleeve.

An input signal to the deflecting means will operate to deflect the jet pipe so that the receiver passages receive unequal amounts of liquid and unbalance the pressure at opposing ends of the spool to translate the spool during which time a pulse of pressurized liquid is ported to one side of the piston which is brought into momentary flow communication with the pressurized liquid entering the sleeve by an internal spool passageway. At the same time a pulse of high pressure liquid is also ported away from the opposing side of the piston by another internal spool passageway thus having the net effect of moving the piston a discreet distance within the bore.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be better understood upon reading the following description of the preferred embodiment in conjunction with the accompanying drawings.

FIG. 1 shows a cross-sectional view of the fail-fixed servovalve of this invention.

FIG. 2 shows a cross-sectional view of a portion of the fail-fixed servovalve of FIG. 1 in another mode of operation.

FIG. 3 shows a cross-sectional view of a portion of the fail-fixed servovalve of FIG. 1 in still another mode of operation.

FIG. 4 shows a cross-sectional view of a portion of the fail-fixed serovalve of FIG. 1 in still another mode of operation.

FIG. 5 shows a cross-sectional view of a portion of the fail-fixed servovalve of FIG. 1 in still another mode of operation.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, there is shown a fail-fixed servovalve 10 comprising a flexible jet pipe 12 together with a pivot seal 13 mounted in a housing 14. The jet pipe 12 receives a pressurized liquid, which may be any suitable servo fluid, for discharge through a relatively small area nozzle 16 into a chamber 18. The chamber 18 has an outlet 20 which connects by way of a return conduit 22 to a reservoir of low pressure liquid (not shown). The pressure drop across the nozzle 16 of the jet pipe 12 causes a discharge of a high velocity jet of liquid into the chamber 18. A pair of

receiver passages

24, 26 are disposed to accept an equal amount of the high velocity liquid jet when the jet pipe 12 is at its illustrated neutral position. The

receiver passages

24, 26 connect with opposite ends of a sleeve 28 in which a spool 30 is slidably disposed. Means are provided to deflect the jet pipe 12 and are herein shown as a torque motor 32 which is made responsive to electrical signals furnished through lines 36. An armature 34 of the torque motor 32 is secured to the jet pipe 12 and the pivot seal 13, and exerts a bending moment thereon when differential current is applied to the lines 36. The jet pipe 12 and pivot seal 13 exert a resisting moment which causes its displacement to be directly proportional to the magnitude of the differential current.

The spool 30 is maintained at the median position within the sleeve 28 by a pair of opposing springs 38, which are respectively engaged by one end of the sleeve 28 and by a plug 29 threadably inserted at the other end of the sleeve. The spool 30 includes a plurality of circumferentially relieved

areas

41, 42, 43, 44, 45, 46 and 47 which are interspaced between a plurality of

circumferential lands

48, 49, 50, 51, 52, 53, 54 and 55. Flow communication is provided between the

relieved areas

43, 44 and by a passageway 58 which is internal to the spool 30. In like manner, flow communication is provided between the

relieved areas

41 and 42 by the passageway 56 and between the

relieved areas

46 and 47 by the passageway 60, both passageways of which are formed internal to the spool 30. An inlet conduit 62 furnishes a supply of pressurized liquid from a source (not shown) wherein the pressurized liquid enters the space defined between the relieved area 44 and the sleeve 28 by way of an inlet port 64. The pressurized liquid thereupon enters the space defined between the

relieved areas

43, 45 and the sleeve 28 by way of the internal passageway 58. Pressurized liquid also exits from the space defined between the relieved area 44 and the sleeve 28 by way of an outlet port 66 which communicates with a conduit 68 for supplying the high pressure liquid to the jet pipe 12. There are also provided two

outlet ports

70, 72 in the sleeve 28 which communicate respectively with the spaces defined between the

relieved areas

41, 47 and the sleeve 28, whereby low pressure liquid is returned to a reservoir (also not shown). As is readily apparent, the return conduit 22 is in flow communication with the outlet port by way of the space defined between the relieved area 41 and the sleeve 28.

There is also provided a servo piston shown generally at as including a piston 84 disposed for translation within a bore 86 from which extends a connecting rod 88 in integral connection with the piston 84. The head side of the piston 84 receives an inlet flow of pressurized liquid from an inlet port 81 which communicates with a port 76 in the sleeve 28 by way of an interconnecting conduit 78. In like manner, the connecting rod side of piston 84 receives an inlet flow of pressurized liquid from an inlet port 79 which communicates with a port 74 in the sleeve 28 by way of an interconnecting conduit 77. O-ring seals 90 may be provided to insure that the piston 84 and connecting rod 88 both sealingly engage the bore 86.

During operation, thejet pipe 12, when in the neutral position, as shown in FIG. 1, directs a high velocity liquid jet at both

receiver passages

24, 26 so that the pressures on opposite ends of the spool 30 are equal. The distance between the nozzle 16 and the

receiver passages

24, 26 is such that substantially all of the kinetic energy of the jet is converted to pressure in the passages. When the

passages

24, 26 are full, the excess liquid flows through the outlet 20 to the conduit 22 from whence the liquid is passed to a low pressure reservoir (not shown).

When a differential current is applied to the torque motor 32, the jet pipe 12 is deflected an amount directly proportional to the magnitude of the current differential. The high velocity jet from the pipe 12 then impinges to a greater extent on one of the receiver passages to increase the pressure on one end of the spool 30 and urge it into motion. Assuming now that a step input of positive maximum rated differential current is applied to the torque motor 32, the jet pipe 12 will then deflect in the direction of the receiver passage 26 which in turn will increase the pressure on one side of the spool causing the spool 30 to translate in the direction as shown in FIGS. 2 and 3. Before the spool 30 engages the end of the sleeve 28, as shown in FIG. 3, there will be a momentary alignment, as shown in FIG. 2, wherein a pulse of high pressure liquid is ported to the connecting rod side of the piston 84 from the inlet port 64 which is brought into momentary flow communication with the port 74 by the internal spool passageway 58. In like manner, a pulse of high pressure liquid is ported away from the head side'of the piston 84 by way of the port 76 which is brought into momentary flow communication with the outlet port 72 by the internal spool passageway 60. As is readily apparent, once the positive rated current is applied and the spool 30 begins to translate due to the deflection of the jet pipe 12, it will not stop until engaging the end of the sleeve 28, as shown in FIG. 3, and thus the position as shown in FIG. 2 is illustrative of only the instantaneous alignment assumed as the spool translates toward the end of the sleeve.

If the differential current applied to the torque motor 32 is returned to zero current, then the jet pipe 12 will also return to the neutral position so that the pressures on the opposing ends of the spool 30 again become equalized. Thus, the spool 30 will be translated by the coaction of the

springs

38, 40 back to the median position as shown in FIG. 1, passing once again through the momentary alignment as shown in FIG. 2. A second pulse of high pressure liquid will be applied to the connecting rod end of the piston 84 from the inlet port 64, which is brought into momentary flow communication with the port 74 by the internal spool passageway 58. In like manner, a second pulse of high pressure liquid will be ported away from the head side of the piston 84 by way of the port 76, which is brought into momentary flow communication with the outlet port 72 by the internal spool passageway 60. Thus it becomes immediately apparent that a step input of maximum rated differential current to the torque motor 32 will operate to cause the piston 84 to translate a discreet distance as determined by the velocity of the spool, the area of the ports, and the liquid pressure differential. In other words, if the electrical input to the torque motor 32 were a series of square waves of current, stepping from zero to the maximum positive rated current and back to zero, then the piston 84 would move in small incremental steps in one linear direction.

The direction of the servovalve piston 84 can be reversed by applying a negative maximum rated differential current to the torque motor 32. A negative differential current operates to deflect the jet pipe 12 in the direction of the receiver passage 24, thus increasing the pressure of the liquid on one side of the spool 30 so as to translate the spool in the direction shown in FIGS. 4 and 5. FIG. 4 shows the instantaneous alignment between the spool 30 and sleeve 28 which allows a pulse of high pressure liquid to enter the head side of piston 84 from the inlet port 64 which is brought into momentary flow communication with the port 76 by the interconnecting spool .passageway 58. In like manner, a pulse of high pressure liquid is ported from the connecting rod side of piston 84 by way of the port 74 which is brought into momentary flow communication with the outlet port 70 by the internal spool passageway 56.

If the differential current input to the torque motor is returned to zero, the jet pipe 12 will again return to the neutral position as shownin FIG. 1, thus equalizing the pressure on both sides of the spool 30. The

springs

38 and 40 will then operate .to return the spool 30 to the median positions as shown in FIG. 1 whereupon a second pulse of high pressure liquid will be applied to the head side of piston 84, and a corresponding pulse of high pressure liquid will be ported from the connecting rod side of piston 84 when the spool 30 momentarily aligns within the sleeve 28 as shown in FIG. 4. Thus it is apparent that if the differential current is in steps from zero to the maximum negative rated differential current and then back to zero, the servopiston 80 will move so as to extend the connecting rod 88 a discreet distance as determined by the velocity of the spool, the area of the ports, and the liquid pressure difference. It is also apparent that if the input signal to the torque motor is a series of square waves of current, stepping from zero to the maximum positive rated current and back to zero, or stepping from zero to the maximum negative rated current and then back to zero, the servopiston will move in either direction in a series of small discreet steps.

As previously discussed, servovalves of this type have broad application in the fuel controls of gas turbine engines wherein they may act as the interface between a digital electrical control and a metering valve or mechanical actuator. For example, in a gas turbine engine fuel control system, the current input to the torque motor 32 may be an electrical signal generated by a control which compares a reference engine speed with an actual operating speed. The piston 84 may be connected through the rod 88 to a fuel meteringcontrol valve (not shown). Thus the fuel flow to the gas turbine engine can be varied as a function of the electrical signal from the control to maintain the reference engine speed. Such a control system would provide a highly stable and accurate control of the engine speed.

The servovalve of this invention is also fail-fixed in that if the differential current applied to the torque motor 32 should, for some reason, fail to return to zero, the spool 30 will then remain at an end position within the sleeve 28 and the piston 84 will remain locked in position due to the alignment of the spool lands which block the flow of liquid from the

ports

74, 76. It will be further appreciated that the piston 84 remains locked in position regardless of the polarity of the differential,

current at the time of failure. In addition, should there be a loss of liquid pressure due to a rupture or break" in.

one of the conduits, the spool 30 will be returned to the medium position as shown in FIG. 1 by the co-action of the

springs

38, 40. Thus a failure in either the electrical or hydraulic systems will not result in unpredicted movements of the servopiston. I

If the input differential current to the torque motor 32 should be switched too rapidly and exceed the frequency response time of the servovalve, then the servovalve will operate as an analog device in the following manner. If the differential current input is varied rapidly from zero to its maximum rated value, at a frequency exceeding the frequency response time of the servovalve, then the spool 30 will assume the position as shown in FIG. 2 and allow a maximum continuous flow of liquid to the connecting rod side of piston 84, together with a maximum continuous flow of liquid away from the head side of piston 84. Piston 84 will thus move at a maximum continuous velocity as determined by the size of the ports and the pressure differen-' tial of the liquid. Should the amplitude of the differential current be less than that of the maximum rated value, then the jet pipe 12 will not deflect to its maximum distance and the pressure differential operating on the spool 30 will not be sufficient to achieve the full alignment of FIG. 2, as would be required for maximum flow. Thus the flow of liquid to the servovalve piston will be reduced, effecting a corresponding change in the velocity of the piston 84 which is an analog representation of the amplitude of the differential current applied during each cycle. Likewise, should there be a reduction in the actual time in which the rated differential current is applied to the torque motor, as would happen if the rated differential current were applied for less than one-half of each cycle, then there will also be a corresponding decrease in the pressure differential across the spool 30 wherein the spool will fall short of achieving the full alignment position as shown in FIG. 2. Thus the flow of liquid to theservopiston80 will again be reduced, causing a corresponding reduction in the velocity of the piston 84 which provides an analog output representative of the reduced time that the differential current is applied to the torque motor.

As will be readily appreciated, the differential current may also be switched to a negative polarity at a rate exceeding the frequency response time of the servovalve 10, in which case for a maximum rated current amplitude applied for the maximum time not to exceed one-half of each cycle, there is shown in FIG. 4 the full alignment position assumed by the spool 30. This position provides for the maximum continuous flow of liquid to the head side of piston 84 together with a maximum flow of liquid away from the connecting rod side of the piston 84. Thus the velocity at which the connecting rod 88 moves out of the servopiston 80 'becomes an analog representation of either the amplitude of the differential current applied to the torque motor or the actual time that the differential current is applied. Again, it should be readily appreciated that the servovalve can be operated only as an analog device when the input differential current to the torque motor is switched at a frequency exceeding the frequency response of the servovalve. Otherwise, the servovalve will operate as the digital stepping device first described.

Thus the output flow of liquid from the servovalve to the servopiston is proportional to either the amplitude of the high frequency current signal or the actual time the current is applied during each cycle. The servovalve therefore has a multiplication capability. For instance, one variable such as servopressure may be used to vary the actual time the current is applied during each cycle while a speed error signal could be used to vary the current amplitude during each cycle. The ability to simultaneously vary two input signals to the servovalve permits the use of one signal to vary the gain of the servovalve for different operating modes of the system.

Accordingly, while the preferred embodiment of the present invention has been depicted and described, it will be appreciated by those skilled in the art that many modifications, substitutions, and changes may be made thereto without departing from the inventions fundamental theme. For example, conventional servovalves generally have a feedback spring between the spool and jet pipe which could also be incorporated in the failfixed servovalve of this invention.

What is claimed is:

l. A servovalve comprising:

a jet pipe for discharging a jet of pressurized liquid;

deflecting means responsive to an input signal for deflecting the jet pipe in a direction determined by the input signal; 7

a sleeve having a plurality of ports therethrough one of which receives an inlet flow of pressurized liquid;

a spool translatably disposed within the sleeve and centered therein by resilient means at opposing ends thereof wherein the spool includes a plurality of circumferentially relieved areas interspaced between a plurality of circumferential lands and wherein selected relieved areas may be placed in flow communication with selected ports in the sleeve by translation of the spool;

a pair of receiver passages in flow communication with opposite ends of the sleeve and disposed to accept an equal amount of liquid from the jet pipe when the jet pipe is in the non-deflected position and an unequal amount of liquid when the jet pipe is in the deflected position whereby an input signal to the deflecting means operates to deflect the jet pipe causing the receiver passages to receive unequal amounts of liquid and thereby unbalance the pressure at opposing ends of the spool to cause the spool to translate in the direction of lower pressure; servopiston having a piston translatably disposed within a bore each side of which communicates with separate ports in the sleeve; passage means internal to the spool and interconnecting selected relieved areas for delivering a pulse of pressurized liquid to a first side of the piston and porting away pressurized liquid from the opposite side of the piston when the spool is translated in one direction and delivering a pulse of pressurized liquid to the opposite side of the piston and porting away pressurized liquid from the first side of the piston when the spool is translated in the opposite direction such that there is no fluid flow to the servopiston when the spool is fully translated in either direction.

2. The servovalve of claim 1 wherein the spool includes: a first centrally relieved area for receiving the inlet flow of pressurized liquid, second and third relieved areas each of which is adjacent on opposing end of the centrally relieved area and spaced apart therefrom by first and second respective lands wherein the second and third relieved areas are in flow communication with the centrally relieved area by a first internal spool passageway, fourth and fifth relieved areas adjacent the second relieved area and spaced apart therefrom by a third land with a fourth land interspaced between the fourth and fifth relieved areas wherein flow communication is established between the fourth and fifth relieved areas by a second internal spool passageway, and sixth and seventh relieved areas adjacent the third relieved area and spaced apart therefrom by a fifth land with a sixth land interspaced between the sixth and seventh relieved areas wherein flow communication is established between the sixth and seventh relieved areas by a third internal spool passageway.

3. The servovalve of claim 2 wherein the sleeve includes:

a first port in flow communication with the space between the centrally relieved area and the sleeve for receiving the inlet flow of pressurized liquid;

a second port spaced axially apart from the first port and in flow communication with one side of the servopiston wherein translation of the spool toward one end of the sleeve and back again operates to establish momentary flow communication between the second port and the space between the second relieved area and sleeve while translation of the spool in the opposing direction toward the other end of the sleeve and back again operates to establish momentary flow communication between the second port and the space between the fourth relieved area and sleeve; third port spaced axially apart from the first and second ports in flow communication with the other side of the servopiston wherein translation of the spool toward one end of the sleeve and back again operates to establish momentary flow communication between the third port and the space between the third relieved area and sleeve while translation of the spool in the opposing direction toward the other end of the sleeve and back again operates to establish momentary flow communication between the third port and the space between the sixth relieved area and sleeve; fourth outlet port spaced axially apart from the first, second, and third ports and in flow communication with the space between the fifth relieved area and sleeve, and

a fifth outlet port spaced axially apart from the first,

second, third and fourth ports and in flow communication with the space between the seventh relieved area and sleeve.

4. The servovalve of claim 3 wherein the resilient means are springs disposed at opposing ends of the spool and the deflecting means is a torque motor having an armature secured to the jet pipe so as to exert a bending moment thereon when differential current is applied to the torque motor.

5. The servovalve of claim 1 wherein the input signal to the deflecting means is switched at a frequency exceeding the frequency response time of the servovalve such that the spool assumes a position within the sleeve offset from the central position, but not abutting either end of the sleeve wherein a continuous flow of liquid is conducted to one side of the servopiston while a continuous flow of liquid is ported away from the other side of the servopiston so as to impart a continuous movement to the servopiston, the velocity of the servopiston being an analog representation of the magnitude of the input signal applied to the deflecting means multiplied by the actual time the signal is applied.

6. The servovalve of claim 1 wherein the return to zero of the input signal to the deflecting means causes the jet pipe to return to the central position so that the receiver passages again receive equal amounts ofliquid and balance the pressure at opposing ends of the spool such that the spool is returned by the resilient means to the center position within the sleeve during which time a second pulse of pressurized liquid is ported to the same side of the piston previously pulsed which side is once again brought into momentary flow communication with the pressurized liquid entering the sleeve by an internal spool passageway while at the same time a pulse of high pressure liquid is again ported away from the opposing side of the piston by another internal spool passageway thus moving the piston a second discreet distance within the bore.

7. The servovalve of claim 2 wherein the servopiston remains fail-fixed in the event that the signal to the deflecting means remains at its full deflected position causing the spool to remain at an end position within the sleeve; said servopiston remaining locked in position due to the alignment of the spool lands which block the flow of liquid to and from the servopiston where the spool is in the end position.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENT NO. 1 3,922,955 DATED December 2 1975 INVENTOR(5) I Kast, Howard Berdolt It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 8, line 6, change "on' to an Column 10, line 17, change 'vvtnere" to "When-- Signed and Scaled this second Day Of March 1976 [SEAL] Attest:

Claims

1. A servovalve comprising: a jet pipe for discharging a jet of pressurized liquid; deflecting means responsive to an input signal for deflecting the jet pipe in a direction determined by the input signal; a sleeve having a plurality of ports therethrough one of which receives an inlet flow of pressurized liquid; a spool translatably disposed within the sleeve and centered therein by resilient means at opposing ends thereof wherein the spool includes a plurality of circumferentially relieved areas interspaced between a plurality of cirCumferential lands and wherein selected relieved areas may be placed in flow communication with selected ports in the sleeve by translation of the spool; a pair of receiver passages in flow communication with opposite ends of the sleeve and disposed to accept an equal amount of liquid from the jet pipe when the jet pipe is in the nondeflected position and an unequal amount of liquid when the jet pipe is in the deflected position whereby an input signal to the deflecting means operates to deflect the jet pipe causing the receiver passages to receive unequal amounts of liquid and thereby unbalance the pressure at opposing ends of the spool to cause the spool to translate in the direction of lower pressure; a servopiston having a piston translatably disposed within a bore each side of which communicates with separate ports in the sleeve; passage means internal to the spool and interconnecting selected relieved areas for delivering a pulse of pressurized liquid to a first side of the piston and porting away pressurized liquid from the opposite side of the piston when the spool is translated in one direction and delivering a pulse of pressurized liquid to the opposite side of the piston and porting away pressurized liquid from the first side of the piston when the spool is translated in the opposite direction such that there is no fluid flow to the servopiston when the spool is fully translated in either direction.

3. The servovalve of claim 2 wherein the sleeve includes: a first port in flow communication with the space between the centrally relieved area and the sleeve for receiving the inlet flow of pressurized liquid; a second port spaced axially apart from the first port and in flow communication with one side of the servopiston wherein translation of the spool toward one end of the sleeve and back again operates to establish momentary flow communication between the second port and the space between the second relieved area and sleeve while translation of the spool in the opposing direction toward the other end of the sleeve and back again operates to establish momentary flow communication between the second port and the space between the fourth relieved area and sleeve; a third port spaced axially apart from the first and second ports in flow communication with the other side of the servopiston wherein translation of the spool toward one end of the sleeve and back again operates to establish momentary flow communication between the third port and the space between the third relieved area and sleeve while translation of the spool in the opposing direction toward the other end of the sleeve and back again operates to establish momentary flow communication between the third port and the space between the sixth relieved area and sleeve; a fourth outlet port spaced axially apart from the first, second, and third ports and in flow communication with the space between the fifth reLieved area and sleeve, and a fifth outlet port spaced axially apart from the first, second, third and fourth ports and in flow communication with the space between the seventh relieved area and sleeve.

6. The servovalve of claim 1 wherein the return to zero of the input signal to the deflecting means causes the jet pipe to return to the central position so that the receiver passages again receive equal amounts of liquid and balance the pressure at opposing ends of the spool such that the spool is returned by the resilient means to the center position within the sleeve during which time a second pulse of pressurized liquid is ported to the same side of the piston previously pulsed which side is once again brought into momentary flow communication with the pressurized liquid entering the sleeve by an internal spool passageway while at the same time a pulse of high pressure liquid is again ported away from the opposing side of the piston by another internal spool passageway thus moving the piston a second discreet distance within the bore.