servo valve operation principle

8/9/2019 Servo Valve Operation Principle

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Servovalve, Hydraulic - Description

A hydraulic servovalve is a servo (see Servo, Hydraulic – Description) with a device (either flapper nozzleor jet pipe) used to position the servo. The ter electrohydraulic servovalve is often used !ecause

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servovalves are controlled throu"h an electrical si"nal. Servovalves are norally used when accurateposition control is re#uired, such as control of a priary fli"ht control surface. $osition control is achievedthrou"h a closed loop control syste, consistin" of coand sensor, feed!ac% sensor, di"ital or analo"controller, and the servovalve. Servovalves can !e used to control hydraulic actuators or hydraulicotors. &hen a servoactuator is used to control an actuator, the servovalve and actuator co!ination areoften referred to as a servoactuator. The ain advanta"e of a servovalve is that a low power electricalsi"nal can !e used to accurately position an actuator or otor. The disadvanta"e is cople'ity and costwhich results fro a coponent consistin" of any detail parts anufactured to very ti"ht tolerances.Therefore, servovalves should only !e used when accurate position (or rate) control is re#uired. or oreinforation on closed loop position control usin" electrohydraulic servoactuators, see Servovalve,Hydraulic – $osition ontrol. Standard servoactuator terinolo"y is provided at the end of this section.

A scheatic of a servoactuator is shown in i"ure *. The actuator is included to show how the servovalveand actuator coponents wor% to"ether. The priary coponents in a servovalve are a tor#ue otor,flapper nozzle or jet pipe, and one or ore spools. The flapper+nozzle (alternatively jet pipe) and the spoolvalve are considered sta"es-. A sta"e provides hydraulic force aplification flapper+nozzle or jet pipe"oes fro low power electrical si"nal to spool /p and the spool valve aplifies /p on the actuator. Theservovalve shown in i"ure * is a 0 sta"e servovalve. Alost all servovalves are 0 sta"e, !ut soe 1sta"e desi"ns e'ist. A 1 sta"e servo has an additional spool valve !etween the *st spool valve and theactuator. The *st spool valve provides a spool /p to the 0nd spool valve.

The servovalve shown in i"ure * uses a flapper nozzle. A servovalve has a hydraulic pressure inlet andan electrical input for the tor#ue otor. The input current controls the flapper position. The flapper positioncontrols the pressure in ha!ers A 2 3 of the servo. So, a current (4 or 5) will position the flapper,leadin" to a delta pressure on the servo, which cause the servo to ove in one direction or the other.6oveent of the servo ports hydraulic pressure to one side of the actuator or the other, while portin" theopposite side of the actuator to return. 7peration of a servovalve is descri!ed in ore detail !elow.


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i"ure * lapper 8ozzle Servoactuator

lapper 8ozzle Syste

lapper position is controlled !y the electroa"netic tor#ue otor (see top portion of i"ure *). Thetor#ue developed !y the tor#ue otor is proportional to the applied current. urrents are "enerally sall,in the illiap ran"e. A tor#ue otor consists of two peranent a"nets with a coil windin" attached to aa"netically perea!le arature. The arature is part of the flapper piece. &hen a current is applied tothe coils, a"netic flu' actin" on the ends of the arature is developed. The direction of the a"netic flu'(force) depends on the si"n (direction) of the current. The a"netic flu' will cause the arature tips to !eattracted to the ends of the peranent a"nets (current direction deterines which a"netic pole isattractin" and which one is repellin"). This a"netic force creates an applied tor#ue on the flapperasse!ly, which is proportional to applied current. 9n the a!sence of any other forces, the a"netic force


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would cause the arature to contact the peranent a"net and effectively loc% in this position. However,other forces are actin" on the nozzle, such that flapper position is deterined throu"h a tor#ue !alanceconsistin" of a"netic flu' (force), hydraulic flow forces throu"h each nozzle, friction on the flapper hin"epoint, and any sprin" (wire) connectin" the flapper to the spool (which is alost always installed used inservovalves to iprove perforance and sta!ility).

As the applied current is increased, the arature and flapper will rotate. As the flapper oves closer toone nozzle, the flow area throu"h this nozzle is decreased while the flow area throu"h the other nozzleincreases. The flapper "enerally rotates over very sall an"les (: ;.;* rad) and the "ap (< in the fi"ure)is around ;.;;0 – ;.;;1 inches. 9f the "ap,


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will displace to the ri"ht. Hi"h pressure fluid will then flow to the $ A actuator cha!er while the $3 actuator cha!er is ported to return. Dependin" on the size of the flapper and nozzles, the /p across the pilotspool is liited in a"nitude (0;;51;; l! ran"e for ediu size aerospace applications).

6ost servovalves incorporate a feed!ac% sprin" (wire) !etween the pilot spool and the flapper. This wireis shown as a dotted !lue line in i"ure *. ?'ainin" i"ure *, if the flapper oves to the left, the /p on

the pilot spool oves the spool to the ri"ht. The feed!ac% wire will then pull the flapper !ac% towards theneutral position. Hence the feed!ac% wire provides a sta!ilizin" force to the flapper and helps iprovesta!ility and response of the flapper syste. This sae affect can !e done electronically !y puttin" afeed!ac% sensor (usually a linear varia!le differential transducer) on the pilot spool. The output of thesensor is fed !ac% electronically to reduce the current coand and allow the flapper to ove !ac% to theneutral position.

@et $ipe

Another ethod to control the pilot spool is to use a jet pipe confi"uration. The jet pipe is an alternative tothe flapper nozzle syste however, a siilar tor#ue otor is used to control the jet pipe position. A

scheatic of a jet pipe servoactuator is shown in i"ure 1.


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i"ure 1 @et $ipe Servoactuator

The jet pipe converts %inetic ener"y of the ovin" fluid into static pressure. &hen the jet pipe is centered!etween the 0 receiver holes in a receiver !loc%, the pressure on the servo is e#ual. However, when the

jet pipe is rotated toward one of the receiver holes, the pressure at this receiver hole is "reater than theother receiver hole, thus creatin" a load i!alance on the servo. i"ure B shows a scheatic of the jetpipe illustratin" how pressure varies !etween the receiver holes as the jet pipe is rotated.


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(a) @et $ipe entered (!) @et $ipe =otated to =i"ht

i"ure B @et $ipe 7peration

The sta"nation pressure at the tip of the jet pipe is "iven !y 3ernoulliCs e#uation as

This is the sta"nation pressure at the idstrea of the flow and would represent the a'iu pressure"iven !y the jet pipe. ro the center of the jet strea, the pressure drops off as shown in the i"ureB(a). 9n the i"ure B(a) confi"uration, the pressures on !oth sides of the servo are e#ual (/p>;). 9n i"ure

B(!), the jet pipe has !een rotated to the ri"ht. This has the effect of increasin" the pressure on the ri"htside of the servo and reducin" pressure on the left side. The servo will then ove to the left. As a "eneralrule, oveent of the jet pipe is sufficiently sall such that the differential pressure will vary linearly overthe ran"e of jet pipe travel.

7ptiization of nozzle perforance is done !y e'perientation. There is a relationship !etween thenozzle diaeter and receiver hole diaeters, which usually ust !e developed throu"h testin". Also, thedistance fro the nozzle e'it to the receiver is iportant ( > 0 Dn has !een su""ested in literature) aswell as the distance !etween the receiver holes. 9n "eneral, the receiver holes should !e as closeto"ether as possi!le, to %eep $* and $0 as hi"h as possi!le. 9t is desira!le to %eep receiver holes as lar"eas possi!le to prevent containation issues. The "oal of a jet pipe desi"n is to achieve the necessarya'iu /p across the pilot spool and aintain ti"ht position control (no different fro a flapper nozzledesi"n).

The !i""est advanta"e of the jet pipe valve over the flapper valve is less sensitivity to containation. @etpipe orifices are "enerally lar"er than flapper nozzle orfices at the e'pense of ore lea%a"e flow. Aclo""ed flapper nozzle orifice will cause a servovalve to "o hardover in one direction. A jet pipe valve will"enerally fail neutral or operate slu""ish if the inlet nozzle plu"s. However, !oth confi"urations are stillused today and !oth have proven to !e relia!le and accurate in service.


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Servo

The servo part of the valve is e'actly the sae as any servo or spool valve. The function of the servo isthe sae for either a flapper nozzle or a jet pipe servoactuator. The relationship !etween flow and /pthrou"h the servo valve is "overned !y the orifice flow e#uation. Servo position is deterined !y a force!alance on the spool, which includes the /p created fro the flapper nozzle or jet pipe, friction forces,

sprin" forces and flow forces actin" on the spool. or a coplete description of a servo, see Servo,Hydraulic – Description.

&hen the spool is in the neutral position, the servovalve is in the null position. 9n soe applications, acopression sprin" is installed on each side of the servo to help %eep the servo centered. 9n otherapplications (spoiler panel servovalves, for e'aple), a sprin" is installed in one side only which will pushthe servo in one direction. or fli"ht spoilers the sprin" would !ias the actuator to the retract position. So,in the a!sence of electrical coands, the sprin" pushes the servo towards the retract coand positionallowin" hydraulic fluid to flow to the retract cha!er. The applied current re#uired to overcoe the sprin"force and return the servo to the null (no flow) position is referred to as the null !ias. The null !ias currentwill drift in service due to chan"es in supply pressure, operatin" teperatures, wear and other factors.


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where i + ia' is the applied current e'pressed as a ratio of a'iu current, is the density, /p is thepressure drop throu"h the servo flow ports and # is the flow rate throu"h the servo. The servo valve willhave a hi"her "ain around null then at the end of the spool travel.

i"ure G Servo low haracteristics – ontrol low vs. oad $ressure

i"ure I shows the how control flow varies with input current. The relationship is linear for a constant /pacross the servo, as can !e seen fro e#uation (*). Also shown in i"ure I is the effect of pressure dropthrou"h the servo. Siilar to i"ure G, flow increases for hi"her /p. i"ure I represents an ideal servo,with no hysteresis or friction effects.


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i"ure I Servo low haracteristics – ontrol low vs. 9nput urrent

i"ure J shows the effects of hysteresis on the ideal flow curve. As current is cycled fro ; to a'iupositive value, then to a'iu ne"ative value, then !ac% to zero, the control flow will follow the !luecurve. Hysteresis results fro electroa"netic affects in the tor#ue otor as well as friction in the valve.The electroa"netic effects have a nonlinear characteristic where the width of the hysteresis loop varieswith the input si"nal. riction !ehaves ore li%e a !ac%lash nonlinearity which tends to !e ore constantover the ran"e of oveent (when ovin" at constant speed). Servovalve hysteresis is easured atslow speeds where valve dynaic (inertia) effects are ne"li"i!le.


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i"ure J Servo low haracteristics – low urve with Hysteresis

The last curve, i"ure K, repeats the curve in i"ure J, !ut adds in a null !ias offset. A null !ias is createdwith an un!alanced sprin" on the servo. The sprin" will force the servo to not !e centered (offset in one

direction). A certain aount of tor#ue otor current is re#uired to offset the sprin" force, which has theaffect of shiftin" the flow curve. 9n the e'aple of i"ure K, * illiap (positive) is re#uired to hold thevalve in the no flow (actuator stationary) position. The ain reason for a null !ias offset is when current islost the actuator will !e powered in a desired direction (usually to retract).

i"ure K Servo low haracteristics – low urve with Hysteresis 2 8ull 3ias

Servoactuator Terinolo"y

9n the aerospace industry, the docuent that "overns servoactuator desi"n is SA? Aerospace =ecoend $ractice(A=$) BL;. A copy of this docuent is availa!le for Society of Autootive and Aerospace ?n"ineers (SA?).

9portant definitions are provided !elow.

Control Flow 5 low throu"h valve ports (to actuator). ontrol flow is referred to as no load flow when there iszero load pressure drop (i.e., pressure due to load on actuator piston is ;).

Flow Gain 5 Slope of the control flow vs. input current curve in any specified operatin" re"ion (see previousfi"ures). Typically this refers to the noral flow re"ion which is outside of the nullre"ion and !efore saturation near the end of travel.


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Hysteresis 5 Difference in the valve input currents re#uired to produce the sae valve output durin" a sin"lecycle of valve input current, e'pressed as a percent of rated current (see previousfi"ures). A noinal value for hysteresis is E1F of rated current.

Linearity 5 De"ree to which noral flow curve confors to noral flow "ain line with other operational varia!lesheld constant.

Load Pressure Drop 5 Differential pressure !etween control ports. ontrol ports are the ports to the actuator.

Null 5 ondition where the valve supplies zero control flow at zero load pressure drop (actuator is heldstationary at the null position).

Null Bias 5 9nput current re#uired to !rin" the valve to null, e'cludin" the effects of valve hysteresis.

Null Region 5 =e"ion a!out null where effects of lap in the output sta"e predoinate.

Null Shift 5 han"e in null !ias due to chan"es in supply pressure, teperature and other operatin" conditions,includin" wear.

Polarity 5 =elationship !etween direction of control flow and direction of input current.

Pressure Gain 5 =ate of chan"e of load pressure with input current at zero control flow (psi+a, for e'aple).

Rated Current 5 Specified input current (illiap) of either polarity to produce rated flow.

Rated Flow 5 Specified control flow correspondin" to rated current and specified load pressure drop.

Servovalve 5 A electrical input, flow control valve, which can provide continuous control. 7utput sta"e flow is adirect function of current.

Stage 5 Any device where hydraulic aplification occurs in a servovalve. Three ost coon sta"es areflapper valve, jet pipe and spool valve. Servovalves can !e sin"le5sta"e or two5sta"e.9n rare cases, a servovalve ay have 1 sta"es.

Threshold 5 9ncreent of input current re#uired to produce a chan"e in valve output, e'presses as apercent of rated current. A noral value of threshold is E*F of rated current.

Selectin" a Servovalve or ServoActuator

Servoactuators are inherently coplicated with nonlinear !ehavior. 9n addition, servoactuators areassociated with electronic controllers and closed loop position control loops. &hen selectin" a servovalveor servoactuator a lar"e nu!er of factors need to !e e'ained. ro a perforance perspective, thecritical ites to loo% at are covered in Servovalve, Hydraulic – Sizin". 3eyond perforance factorscovered in Servovalve, Hydraulic – Sizin", other ites would include

Pressure Rating – a%e sure valve is rated for your syste pressure

Teperature Rating – valve should !e rated for fluid teperatures and applica!le environentalteperatures


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Rated Flow – catalo" data for servovalves will provide data for sin"le operatin" condition which consistsof flow rate, inlet pressure and applied current. This data is a desi"n operatin" point for the valve and can!e used for initial sizin" studies.

Rated Pressure 5 catalo" data for servovalves will provide data for sin"le operatin" condition whichconsists of flow rate, inlet pressure and applied current. This data is a desi"n operatin" point for the valve

and can !e used for initial sizin" studies.

Rated Current 5 catalo" data for servovalves will provide data for a sin"le operatin" condition whichconsists of flow rate, inlet pressure and applied current. This data is a desi"n operatin" point for the valveand can !e used for initial sizin" studies.

Closed Loop Syste !ntera"tions – when usin" a servo in a closed loop syste, the closed loop systeinteractions (!oth sta!ility and closed loop perforance) should !e evaluated (see Servovalve, Hydraulic

– $osition ontrol)

#alve $aterials – should !e sufficient to pass proof and !urst testin", not !e suscepti!le to corrosion andother environental considerations, and not cause any pro!les under teperature e'trees

Failure $odes – the ain failure odes are servo valve jain" in any position fro full closed to fullopen and a hardover coand. Servovalves are norally closely onitored throu"h the electroniccontroller for any a!noral !ehavior.

%le"tri"al !nterfa"es – electronic controller output si"nals need to !e copati!le with tor#ue otorre#uireents. ?lectrical connectors should !e hi"h standard (such as ilitary standard) to preventdeterioration in service (which can lead to hardover failures).

%nvelope&$ounting – servoactuator ust fit in the availa!le space and have ountin" features that allowthe unit to !e ounted to priary support structure

Hydrauli" Fittings – hydraulic connections ust !e copati!le. 9nlet and outlet ports will often have

different size connectors to prevent cross connectin" hydraulic lines durin" installation.

Applications of Servoactuators

Servoactuators will !e used for any hydraulically actuated control where precise position or rate control isre#uired. The ost coon application for servoactuators is on priary fli"ht control surfaces. Aservoactuator will !e used on any priary fli"ht control surface that is hydraulically actuated. 7n priaryfli"ht control surfaces, servoactuators will !e part of an overall power control unit ($M). 6ore inforationon power control units can !e found in $ower ontrol Mnits, Hydraulic – Description. 7ther applicationsfor servovalves in coercial aircraft would !e flap or slat control, spoiler panels and nosewheel steerin".7n ilitary applications, servovalves can !e used to control any other systes on the aircraft such as

thrust vectorin".

Servoactuator+Servovalve Nualification

See Nualification 5 Hydraulic oponents for discussion on servovalve and servoactuator #ualificationand re#uired certification testin".


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servo valve operation principle

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