US3680594A

US3680594A - Servovalve with accumulator means on drain cavities

Info

Publication number: US3680594A
Application number: US45654A
Authority: US
Inventors: Niel R Petersen
Original assignee: MTS Systems Corp
Current assignee: MTS Systems Corp
Priority date: 1970-06-12
Filing date: 1970-06-12
Publication date: 1972-08-01
Anticipated expiration: 1989-08-01
Also published as: DE2129241A1; JPS5564502U; JPS5817122Y2

Abstract

A servovalve assembly used with hydraulic shakers comprising accumulator means in close association with all low pressure drain and/or return areas of the servovalve to prevent cavitation in the drain cavities and return lines. More accurate dynamic response from the valve to an input program is thus obtained. In the drain or return lines from the main controls spool, accumulators are mounted directly in the valve block to eliminate pressure spikes or peaks caused by cavitation in these normally low pressure areas. The opposite ends of the main spool are also equipped with resilient diaphragm type accumulator means to prevent pressure peaks in these areas. On the pilot spool, the drain area cavities are provided with small O-ring accumulators wherein the O-ring groove is provided with relief areas so that when the spool movement causes pressure fluctuations, the O-rings will deflect into the provided relief areas to prevent high peak pressures. The valve then is capable of operating without disruptive pressure peaks during dynamic operation.

Description

United States Patent Petersen [451 Aug. 1, 1972 [54] SERVOVALVE WITH ACCUMULATOR Primary Examiner-William R. Cline MEANS ON DRAIN CAVITIES AttorneyDugger, Peterson, Johnson & Westman [72] Inventor: Niel R. Petersen, Hopkins, Minn. [57] ABSTRACT [73] A tg fi fiz' Corporation A servovalve assembly used with hydraulic shakers p comprising accumulator means in close association [22] Filed: June 12, 1970 with all low pressure drain and/or return areas of the servovalve to prevent cavitation in the drain cavities [211 App! 45654 and return lines. More accurate dynamic response from the valve to an input program is thus obtained. 'i E2 In the drain or return lines from the main controls [58] Fieid 69 625 61 spool, accumulators are mounted directly in the valve 137 625 25'1/324 5 block to eliminate pressure spikes or peaks caused by 138/DIG 6 cavitation in these normally low pressure areas. The I opposite ends of the main spool are also equipped with resilient diaphragm type accumulator means to [56] References Clted prevent pressure peaks in these areas. On the pilot UNITED STATES EN spool, the drain area cavities are provided with small O-rmg accumulators wherein the O-nng groove is pro- 2,6l4,793 10/1952 Sto rm ..l37/525 vided with relief areas so that when the spool mve 3,191,626 6/1965 Lelbfritz ..137/625.69 causes pressure fluctuations, the wings will 3,477,464 ll/l969 Ryan ..137/525 X V deflect into the provided relief areas to prevent i 2,973,746 3/1961 Jupa ..l37/625.63 peak pressures The valve then is capable of operating 3,189,050 6/1965 Heckrnann ..l37/625.63 without disruptive pressure peaks during dynamic 3,209,782 10/1965 Wolpin et a1...,.....137/625.69x operation 3,456,688 7/1969 Clark ..137/625.63

16 Claims, 6 Drawing Figures PATENTEDAUB H912 SHEET 1 [IF 4 FIE: .'Z

SPLACEM TRANSDUCER CONTROLS INVENTOR. N IEL R. PETERSEN RESERVOIR y ATTORNEYS minnows 1 I912 3.680.594

- sum 2 OF 4 ATTORNEYS PATENTEDAus {I912 3.680.594

. sum 3 on} INVENTOR.

NIEL R. PETERSEN BY I ATTORNEYS PKTENTEDAU; I 1912 SHEET Q [If 4 I ATTORNEYS SERVOVALVE WITH ACCUMULATOR MEANS ON DRAIN CAVITIES BACKGROUND OF THE INVENTION has been done as shown in the article entitled Hydrau- 5 lie Vibrators by John A. Dickie appearing in Product Engineering, Design Edition, December 9, 1957, pages 94-98. Shake tests usually involve commanding the servohydraulic actuators with a sine wave signal of the desired frequency. However in the past, servohydraulic shakers have been plagued by very high distortion (frequently over 50 percent total harmonic distortion). The result is that whereas the desired test may be programmed at a certain frequency and amplitude of acceleration, other excitation factors are present which may do substantial specimen damage or otherwise negatively influence the result of the test due to distortion caused by the behavior of the hydraulic fluid under the oscillating flows.

In addition, hydraulic shakers have in the past tended to exhibit a response jump phenomena in which the amplitude of the output does not increase smoothly with the amplitude of the command signal. The result is that there will exist frequency-amplitude combinations at which the shaker will not be stable. If some sort of automatic excitation level control is used with the system, operation of the shaker may result in a varying load even if the program is for a constant force level.

Distortion has been found to be combinations of several basic problems. These include:

Spikes on the load waveform caused by collapse of fluid cavitation bubbles in the return lines connecting the servovalve return to the pump reservoir. The acceleration of the fluid flowing in the return line causes this cavitation after the valve return ports are opened for return flow and then are momentarily closed for reversing the direction of operation of the valve;

Subharmonic oscillations of the output also are caused by cavitation within the drain cavities of the servovalve.

This drain cavity cavitation is caused by the motion of the internal pilot and main stage valve spools of the assembly.

The main stage spool requires drain flow at its stub ends to be ported back and forth rapidly as the spool oscillates during operation. To prevent the possibility of separation in the present device a large diameter cross drilled port is used together with elastic membranes acting as accumulators to absorb rapid fluid acceleration pulses.

In the pilot stage, the ends of the pilot stage spool are at the full diameter of the spool. A large cross port drilled hole is included and an accumulator effect is accomplished by distorting the sealing O-rings used on the drain cavities into cut-outs opening to the O-ring.

Standing waves in the pressure line supplying hydraulic fluid to the servovalve result in undesirable load waveform shapes at moderate frequencies.

These are minimized by the inclusion of a pressure accumulator with a very large throat size on the pressure line. The pressure line accumulators are used to provide a nearly constant supply pressure to the servovalve pressure metering orifices, as distinguished from pressure line accumulators previously used only to suppress upstream hydraulic line transients.

Thus while the use of accumulators has been well known on hydraulic devices, the positioning of the accumulators in a proper relationship to eliminate distortion and non-linear operation has not previously been done.

SUMMARY OF THE INVENTION The present invention relates to a servovalve construction wherein distortion and non-linearities are greatly reduced. Small accumulators are placed directly adjacent the servovalve, in all pressure, return and drain areas. In particular, the main return line for the servovalve leading to a reservoir is equipped with accumulators positioned very closely adjacent to the main valve spool return orifices which will prevent cavitation of the hydraulic fluid flowing to the reservoir and will eliminate pressure spikes on the valve.

Accumulator means having outer shells fixed directly to the main servovalve block and opening directly into the return orifices of the servovalve are provided so that the accumulators are as close as possible to and practically overlie the return orifices of the main valve spool. Further, the drain cavities of the main valve spool have diaphragm type accumulators and at the drain cavities of the pilot valve, where O-rings are used for seal, the O-rings are mounted in grooves having reliefs or scallops which permit the O-ring to deflect slightly in a direction so that it does not interfere with the scaling function of the O-ring, but forms a small accumulator to smooth out operation. This invention also comprises the forming of small accumulators with 0- rings without disturbing the sealingproperties of the rings.

It is an object of the present invention to reduce nonlinearities in a servovalve by proper location accumulator means.

It is a further object of the invention to present a new small volume O-ring accumulator for general use in hydraulic devices.

Other objects will become apparent as the description proceeds.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a part schematic representation of a typical shake testing arrangement showing a servovalve made according to the present invention installed in the system;

FIG. 2 is an enlarged sectional view taken as on line 2-2 in FIG. 1 with parts broken away;

FIG. 3 is a sectional view taken as on line 3-3 in FIG. 2;

FIG. 4 is a sectional view taken as on line 4-4 in FIG. 2;

FIG. 5 is an enlarged side elevational view of a portion of the seal for a drain cavity of the pilot spool of the servovalve assembly showing a construction to get fluid absorption with this device; and

FIG. 6 is a fragmentary sectional view taken as on line 6-6 in FIG. 2.

A servovalve assembly illustrated generally at 10, as shown, is used in combination with a shake test machine. The set-up as shown schematically includes a seismic base 12 which supports a double acting, hydraulic actuator 17. The actuator has a rod 13 supporting a specimen to be tested shown schematically at 14. The actuator assembly 17 has an internal piston attached to the rod 13. Substantially non-compressible hydraulic fluid under pressure supplied to opposite ends of the hydraulic actuator 17 acts on the piston and thereby moves the rod. ln shake testing the device is programmed to cyclically move the rod in an oscillating motion shown by double arrow 15. The actuator is cycled at the desired frequency, usually as shown from 1 to 1,000 cycles persecond, and at acceleration levels from to 100 G5. Although the application shown is a shaker testing system the invention described herein can be adapted to other specimen testing configurations. The servovalve assembly is programmed to direct fluid under pressure to the hydraulic actuator at the desiredfrequency. A hydraulic pump 20 for supplying hydraulic fluid under pressure receives the hydraulic fluid from a reservoir 21 and supplies this fluid under pressure through a pressure line 22 to a pressure port 23 of a valve block or body 24 of the servovalve assembly. The servovalve assembly 10 then is operated to direct fluid under pressure either to a first actuator pressure line 25 or a second actuator pressure line 26 leading from controlled ports in the valve block to the actuator by way of a matching manifold 58, to move the rod 13 in the desired direction. When one of the actuator pressure lines is under pressure, the other is simultaneously connected to return.

A displacement transducer 27 is used in conjunction with the actuator 17 and delivers a controllable signal indicating actuator rod displacement along a line 28 to a control console 31 that is of a desired type to initiate drive signals for appropriate excitation of the servovalve.

The controls for the servovalve are commonly used and supply a sinusoidal signal to cause oscillatory motion. The displacement transducer provides a feedback signal to ensure that the actuator operates at its desired displacement.

In addition to the hydraulic lines previously mentioned, a hydraulic pressure line 33 comes from the pump to the valve block 24 and leads through passageways as shown in FIG. 2 to a pilot spool valve assembly generally shown at 35. The pilot valve assembly is operated in response to electrical signals coming from the control console 31 to open or close the pilot stage pressure ports to direct fluid under pressure through provided passageways (shown in dotted lines) in the usual manner up to the slave or main spool assembly shown generally at 40. The slave spool assembly, including a spool 49 which is the main spool of the hydraulic servovalve, and the actuating chambers 41 .and 42 for the main spool 49, is perhaps best shown v 'its sleeve 46. Return or centering springs 45 are positioned at opposite ends of the pilot spool. Note that a conical member is integral with the pilot spool and has an outer flange portion that carries the moving coil. The wires which carry current to the moving coil are shown attached to the conical portion. The pilot spool valve is a four-way valve, and when, for example, the

spool 44 is shifted in one direction longitudinally, for example upwardly when viewed in FIG. 2, (away from the field coil end) hydraulic fluid under pressure from the pressure line 33 will flow through the provided ports in the sleeve 46 for the pilot spool 44 through a passageway 47 indicated in dotted lines in FIG. 6 to the actuating chamber .41 at one end of the main spool 49. This chamber 41 is between an outer sleeve 48 and the stub end of the main spool 49. Simultaneously the chamber 42 'will be ported to return through a passageway and ports of the pilot valve and to the reservoir by way of the pilot return line 34.

When fluid under pressure is introduced to the chamber 41, the main spool 49 is moved in axial direction indicated by the arrow 52. Fluid under pressure is then directed from the internal pressure port 53 shown in FIG. 3 into actuator line passageway 54 which is connected to line 26, for example. The spool movement will cause fluid under pressure to flow into the actuator 17 by way of the manifold 58 from the pump and mounted directly onto the valve block 24 with a suitable clamping'assembly illustrated generally at 61 including bolts 62. The accumulator has an interior bladder 65 and has a wide-mouth in which a perforated bladder restrainer 63 is sealingly positioned. The restrainer 63 permits free passage of hydraulic fluid, but holds the bladder in the outer bottle.

The accumulator 60 is actually open directly to the return passageway 56 inside the valve block. Thus,

when the return oil flows through the line 25 of the actuator, in the passageway 55, and through the, main spool assembly and out the passageway 56, the accumulator, which is recharged at a relatively low air pressure setting will provide a compressible fluid cushion over the substantially non-compressible hydraulic fluid. When the spool 49 closes off flow through passage 56 suddenly as the valve cycles, the accumulator prevents cavitation in the return line caused by stopping flow of the hydraulic oil in direction toward the reservoir.

Compressed fluid in the accumulator expands to.

prevent separation and cavitation of the hydraulic fluid in the return lines. The accumulator mouth is open directly to the passageway so the accumulator acts to prevent any cavitation even right next to the main valve.

To move the main valve spool 49 in the direction opposite from the arrow 52, a signal comes from the controls 31 through the line 32, which drives the voice coil 39 to move the pilot spool 44 in direction so that fluid will flow from the pressure line 33 through the passageways and past the lands provided on the pilot valve spool, through a passageway 69 shown in dotted lines, into the main spool actuating chamber 42. At the same time, the pilot valve opens passageways to drain chamber 41 will pass back through the pilot drain line 34 to the reservoir. Thus the pressure in chamber 42 will move the main spool in opposite direction from the arrow 52. The pilot spool controls the main spool so that the small movements of the pilot spool control the fluid under pressure going to the actuator.

When the main valve spool changes position because of a reverse actuation signal to the pilot valve, the main spool 49 will move very rapidly in opposite direction from arrow 52 and will then very quickly shut off the flow through passageway 56 to the reservoir. The velocity of the hydraulic fluid flowing through the lines 57 tends to keep the fluid moving for a short while even after the valve is closed. This causes separation in a closed system. The accumulator 60 will prevent this cavitation and will thus prevent high force peaks which occur when the cavitation bubbles collapse as the velocity of the oil flowing back to the reservoir drops. The pressure spikes in the return line reflect back to the actuator cavities causing spikes on the actuator wave form.

In cyclic operation, after the main valve spool 49 moves in opposite direction from that indicated by the arrow 52, hydraulic fluid under pressure from the pump ports 53 will be admitted past a land on the spool to the passageway 55, and thus into the line 25 to move the actuator 17 in opposite direction from the previous direction. The passageway 54 will be opened by a land on spool 49 to the return passageway 66 which opens to the return line 67. Fluid then will flow from the base end of actuator 17, through line 26 and out through the passageway 66 into the return line 67 back to the reservoir. As shown, passageway 66 is open through a wide port to a second accumulator 72 held in place with the same framework as the accumulator 60. The accumulator 72 includes an interior bladder 73. This accumulator is open to the passageway 66 on the return side of the main spool 49 to prevent cavitation and surges of pressure in the return lines 67 and 68 caused by the velocity of the oil in the return lines during very rapid main valve spool actuation. The accumulator 72 is operating at a low precharge pressure, about 15 psi gage, to keep a suitable compressible fluid pressure against the bladder 73. The accumulator will prevent pressure peaks due to collapsing bubbles caused from cavitation when the main spool 49 is closed and again operated in direction as indicated by the arrow 52. The main spool actuation is at a desired frequency which can be quite high, for example in the range of 250 cycles per second, and this can give very high pressure and force peaks and undesirable pressure wave forms which might otherwise reflect on the motion of the actuator piston rod 17 causing distortion. A perforated bladder restrainer screen 74 is also mounted at the mouth of the accumulator 72.

A feedback signal is provided from the main spool with a suitable signal means illustrated generally at 75, and a connection leading to the line 76 which does back into the suitable controls 31 for determining the dynamic position of the main spool, this signal being used in the detailed function of the controls 31. Maximum spool travel is plus or minus a small fraction of an inch of the main spool 49 so that movements are very short, but high and rapidly changing flows are involved. This means that the length of return lines have a substantial amount of hydraulic fluid and above only a few cycles per second the inertia of the fluid in the return lines becomes important.

The actuation of the pilot spool 44 and the main spool 49 is conventional, but the addition of the

accumulators

60 and 72 onto the return passageways 56 and 66 prevents cavitation even at high frequencies of operation.

In order to further smooth out the operation of the valve assemblies, the drain areas at the ends of both the main spool and the pilot spool are provided with means for absorbing pressure peaks and thus greatly reducing the amplitudes of extraneous pressure spikes and smoothing out the cyclically actuated operation.

First, in the pilot spool area, it can be seen that the pilot spool 44 is integral with the frustoconical voice coil carrying member 75.

The voice coil carrying member 75 is in a chamber 76 defined in the valve block, which is at drain or return pressure, and suitable porting is provided for connecting this chamber in the valve block leading from this chamber 76 to the reservoir.

At the opposite end of the pilot spool, there is a sleeve 77 mounted in an opening in the valve block which houses the zero adjust screw 78 for the pilot stage valve. The sleeve is held in the valve block with a suitable member, and an O-ring 79 seals the sleeve with respect to the chamber that it is mounted in.

The sleeve 77 is made specially to form pulsation dampening in this pilot spool drain area chamber 81. The chamber 81 is formed between the inner end portions of the sleeve 77 and the interior surfaces of the pilot valve and pilot valve sleeve. The balancing spring 45 at this end of the pilot valve is mounted inside an interior opening of the sleeve 77. As shown perhaps best in FIG. 5, the end of the sleeve 77 is provided with short slots 82. The O-ring 79 is mounted in an annular groove 83 on the end portion of the sleeve. The O-ring here is sealing on the interior surface of the opening for the sleeve 77 and the inner bottom surface of groove 83. As shown, in order to provide for some pulsation dampening, the groove 83 has recesses 84 extending in direction along the longitudinal axis of the sleeve 77, defined therein on an opposite side of the O-ring 79 from the chamber 81. In ordinary operation the O-ring 79, which is under some tension, will lie along a radial plane. When pressure against the O-ring 79 from chamber 81 increases, the pressure will force the O- ring back toward the exterior end of the screw 78, and the sides of the O-ring 79 will deflect from the normal planar position slightly into these scallops or recesses 84, as shown in FIG. 5. The seal surfaces of the O-ring remain in contact with the inner surface of the chamber for sleeve 77 and groove 83 so the chamber 81 doesnt leak. Because of the low quantities of displaced oil in the chambers this is a sufficient amount of an accumulator action to prevent pressure spikes and cavitation from occurring within this chamber. When the pressure in the chamber 81 reduces, the elasticity of the O-ring returns to its normal position. Thus, the O-ring 79 itself serves as a peak pressure relief means at the drain area of this end of the pilot spool and serves as a seal as well. The recesses 84 are at atmospheric pressure and can be vented if desired.

At the voice coil end of the pilot spool, as shown, the chamber 76 is sealed by the housing 86 for the field coil that bolts against the surface of the block 24 for the servovalve. An O-ring 88 is provided and is placed in an annular groove that extends annularly around the outer periphery of the chamber 76. In this particular instance, referring to FIG. 6, the O-ring recess is provided with radially extending recesses 87. The O-ring 88 can thus deflect elastically out of its normal annular shape into these recesses 87. The O-ring 88 here seals on its lateral sides and the sealing surfaces remain in contact with the O-ring 88 as it deflects into the recesses 87. At both ends of the pilot spool the O-ring grooves are provided with recesses that open to the respective grooves. Note that the

recesses

84 and 87 are partially defined by surfaces that form a continuation of one sealing surfaces for the respective O-ring. This permits O-ring deflection without losing the sealing properties. Because the chamber 76 is ported to the reservoir, and because the dynamic volume involved is quite low,the small deflections of the O-ring 88 into the recesses 87 provide a suflicient accumulator action. The scallops or recesses'84 and 87 are vented to atmospheric pressure and do not have to be sealed.

The drain chambers for the pilot spool are connected together with a passageway 89, shown in FIG. 6. The drain chambers also are connected to the reservoir 21 by suitable passageways.

' The required drain areas at opposite ends of the main spool extend outwardly beyond the stub ends of the main spool and are shown as

chambers

90, and 91, respectively. These chambers are connected with an internal passageway 101. The passageway 101 leads into the reservoir through a suitable internal passageway and the return lines leading from the valve block. The operation of the main spool 49 requires a rapid oscillation of oil in these drain areas. When the main spool 49 is operating with an oscillating motion along its longitudinal axis, the oil in these drain chambers will be pulsating back and forth along with the movement of the spool. The drain chamber 90 at one end of the main spool is defined in part by a resilient elastomeric member 92 forming a surface held in place with a clamp 93 that is bolted onto a main portion of the feedback mechanism. The chamber 90 is thus able to change in volume by deflecting the member 92 with respect to a chamber 94 defined in the cap 93. This chamber 94 is filled with a compressible fluid such as air that is vented to atmosphere, and so the membrane 92 provides a small accumulator effect in this drain area.

At the opposite end of the main spool, chamber 91 is defined in part by a resilient elastomeric member 95 held in place with a cap 96. The chamber 91 is thus in part defined by the member 95 that will deflect. The cap 96 has an interior chamber 97 defined therein that is vented to atmosphere, and the member 95 then can deflect into the chamber 97 to give an accumulator effeet to prevent cavitation and pressure spikes from occurring in the drain area at this end of the main spool. Even rapid oscillations, where the inertia and accelerations of the fluids in the

drain area chambers

90 and 91 becomes high, the resilient elastomeric members 92 and will deflect into their respective atmospheric chamber areas to cushion rapid changes in pressure and prevent cavitation by deflecting suitably to accommodate fluctuations in pressure.

In addition, a suitable accumulator 100, shown schematically, is placed onto the pressure line 25 leading to I the servovalve 10 in order to have a full cushioned servovalve operation. The accumulator 100 shown schematically on the pressure line, is a conventional type accumulator including an outer wall having an inner fluid barrier that is subjected to pressure, and open to the hydraulic line. This accumulator, however, has a .unit thus becomes a fully conditioned servovalve thus accumulator means at the main drain lines, on the pressure lines, and at the non-working drain cavities where fluid is oscillated on the interior of the unit.

in previous devices where accumulators were attached to the return lines but at some distance from the block, there was sufficient hydraulicfluid volume in the return lines between the servovalve and the accumulators so that cavitation could occur between the internal passages of the servovalve spool and the accumulator. The present structure prevents this because the accumulators are open right directly to the spool passageways from the servovalve and prevent separation and cavitation and resulting peak pressures attendant thereto. in 4 addition, where a multi-stage servovalve is utilized such as in the present invention, the

low pressure drain areas at opposite ends of both the pilot valve spool and the main valve spool are provided with small deflectable means to prevent cavitation by absorbing, fluid pulses. in the small flow areas, distorting of the sealing O-ring grooves either into radially extending recesses or in axially extending recesses equally as small which do not disturb the sealing properties of the O-ring but permit some deflection into these cavities under higher pressures, provide the necessary damping effect on fluid pulses. Note that the O-rings have some surface areas open to the chambers they seal. Thus, the

chambers

81 and 76 are partially defined by a deflectable elastomeric member.

The smoother servovalve response occasioned by eliminating the abrupt non-linearities caused by internal cavitation when the fluid mass following the spools tends to separate, provides for the valve output amplitude to be modulated smoothly and this allows for closer electronic control of the level of output put on by the actuator 17 at high frequencies. 5

It should be noted that the controls 31 are nowcommercially available for use in conjunction with servovalves controlled shake test machines from MTS Systems Corporation, Eden Prairie, Minnesota, and include means for generating the necessary control signal for the servovalve from the desired program, and feed back means for maintaining the displacement and frequency at the programmed level. The system uses a hydraulic oil for the actuating fluid.

The small O-ring accumulators can be used in many types of hydraulic devices, as long as the O-ring will deflect elastically into an open area under pressure pulsation and then elastically return to its normal position when the pressure reduces. The O-ring accumulator finds special usage in the drain areas of servovalves where cyclic loading is present.

What is claimed is:

1. In a hydraulic device having an oscillating control element, drain cavity means formed in said hydraulic device and open to at least one portion of said oscillating element and to a drain, said drain cavity means including a groove defined in said device and having two facing sealing surfaces, an elastomeric ring member positioned in said groove means and engaging the sealing surfaces, a wall portion of said elastomeric ring defining a wall portion of said drain cavity means, and relief cavity means defined in said hydraulic device and open to said groove at spaced locations on an opposite side of said ring member than the wall portion of said ring defining a wall portion of said drain cavity means, said relief cavity means being of size to permit said ring member to deflect under increased elastomeric tension due to pressure in said drain cavity means while remaining in contact with the sealing surfaces to keep said drain cavity sealed.

2. The combination as specified in claim 1 wherein said groove means for said ring member comprises an annular groove generated about an axis, and said relief cavity means extend radially outwardly from said groove with respect to said axis at spaced intervals around said annular groove.

3. The combination as specified in claim 1 wherein said groove means for said ring member comprises an annular groove generated about an axis, and said relief cavity means is open to said groove at spaced intervals around said ring and extend in generally parallel to said axis direction.

4. The combination as specified in claim 1 wherein said oscillating control element comprises a valve spool, and said drain cavity means is open to at least one end of said spool.

5. The combination as specified in claim 4 wherein said drain cavity means is open to opposite ends of said valve spool and passage means to permit movement of hydraulic fluid between said opposite ends as said spool oscillates.

6. In a hydraulic device comprising a valve member having valve spool control means movable in opposite directions to control flow of substantially noncompressible hydraulic fluid, chamber means on at least one end of said spool, said chamber means being maintained at drain pressure and providing drain means for said spool, said chamber means being defined at least in part by an elastomeric member forming at least a portion of the wall of said chamber means and being movable from a rest position only under pressure differentials thereon which overcome elastomeric resistance of said member, and at least a portion of an opposite side of said elastomeric member from said chamber means being open to a compressible fluid to permit said elastomeric member to deflect and compress said compressible fluid to absorb transient pressure changes in the noncompressible hydraulic fluid in said chamber engaging said elastomeric member.

7. The combination as specified in claim 6 wherein said elastomeric member is a disc-like member and forms a substantial portion at one wall of said chamber.

8. The combination as specified in claim 6 wherein said elastomeric member is an annular ring member, and is open to said chamber along one portion of said ring type member. 1

9. The combination as specified in claim 8 wherein the portions of said elastomeric ring member open to a compressible fluid pressure comprise relief chambers spaced around the annular periphery of said ring member on a side opposite of said ring member from said chamber.

10. The combination as specified in claim 6 wherein said valve member is an electro-hydraulic servovalve having a pilot valve spool control and a main valve spool, and wherein said servovalve has main valve drain chambers defined at opposite ends of said main valve spool, passageway means connecting said main valve drain chambers, and said pilot valve has pilot valve drain chambers at opposite ends of said pilot valve spool, passageway means connecting said pilot valve drain chambers, and all of said drain chambers comprising said chamber means.

11. A servovalve having a valve block mounting a pilot stage valve and a main stage valve, said main stage valve being operated in opposite directions in response to said pilot stage valve to control flow of substantially noncompressible hydraulic fluid to and from an actuator, said valve block further including internal drain passage means controlled by said main stage valve to permit drain fluid flow from said actuator through said passage means to the reservoir as said actuator is operated and accumulator means in said drain passage means including a barrier member open to the drain passage means on a surface thereof, and compressible fluid on an opposite surface of said barrier from the drain passage means to permit said barrier to move under pressure transients in said noncompressible fluid, said accumulator means opening to the drain passage means closely adjacent said internal drain passage means of said servovalve.

12. The combination as specified in claim 11 wherein said main stage of said servovalve is defined in a valve block, and wherein said accumulator means for each of said internal drain passage means comprise separate accumulator members having outer housings, and having accumulator inlets, said accumulator inlets being mounted to open through said valve block to said drain passage means.

13. The combination as specified in claim 12 wherein remote drain lines for hydraulic fluid are connected to said drain passage means at ports on said valve block, and wherein said accumulator means open through different areas of said valve blocks to said drain passage means than said ports.

14. In a servovalve assembly for controlling substantially noncompressible hydraulic fluid under pressure and having a linearly movable pilot valve means, a linearly movable main stage valve means, and return line means leading to reservoir from said main stage valve means, and having hydraulic fluid drain cavities at opposite ends of said linearly movable main stage valve means and said linearly movable pilot valve means, the improvement comprising a pneumatic accumulator means open to said return line means closely adjacent said main stage valve, and separate accumulator means open to each of the drain cavities in said ser vovalve to absorb transient pressure peaks caused by inertia of the hydraulic fluid in said drain cavities when the respective valve means move linearly.

15. The combination as specified in claim 14 wherein said pilot stage comprises a pilot stage valve spool, and wherein there are drain cavities at opposite ends of said pilot stage valve spool, and wherein said accumulator means on the drain cavities at opposite ends of said pilot stage valve spool comprises elastic ring means normally sealing said pilot stage cavities, and relief cavities adjacent and open to said elastic ring means and into which said elastic ring may elastically deflect when pressure in said pilot stage drain cavities exceeds a predetermined amount.

16. The combination as specified in claim 14 and a I source of hydraulic fluid under pressure, and wherein said servovalve has pressure outlet port pressure lines leading from the source of fluid under pressure to said servovalve and wide-mouth accumulator means being sufficient to provide substantially constant supply pressure to the servovalve pressure inlet ports as the servovalve is operated.

Claims

8. The combination as specified in claim 6 wherein said elastomeric member is an annular ring member, and is open to said chamber along one portion of said ring type member.

14. In a servovalve assembly for controlling substantially noncompressible hydraulic fluid under pressure and having a linearly movable pilot valve means, a linearly movable main stage valve means, and return line means leading to reservoir from said main stage valve means, and having hydraulic fluid drain cavities at opposite ends of said linearly movable main stage valve means and said linearly movable pilot valve means, the improvement comprising a pneumatic accumulator means open to said return line means closely adjacent said main stage valve, and separate accumulator means open to each of the drain cavities in said servovalve to absorb transient pressure peaks caused by inertia of the hydraulic fluid in said drain cavities when the respective valve means move linearly.

16. The combinaTion as specified in claim 14 and a source of hydraulic fluid under pressure, and wherein said servovalve has pressure outlet port pressure lines leading from the source of fluid under pressure to said servovalve and wide-mouth accumulator means being sufficient to provide substantially constant supply pressure to the servovalve pressure inlet ports as the servovalve is operated.