7-Linear & Rotational Sensors – III


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Sensors& Actuators
Dr. Hassan SHARABATY
BY
Aleppo –April 2012
Department ofMechatronicsEngineering
EEM304 Mechatronics
7-Linear & Rotational Sensors -III
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
2/26
Linear & Rotational Sensors
Themostcommonmotionsinmechanicalsystemsarelineartranslationalongafixedaxisandangularrotationaboutafixedaxis.
Morecomplexmotionsareusuallyaccomplishedbycomposingthesesimplermotions.
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – III 3/26
Inductive Sensors
dt
d
E N

   0
In 1831, Michael Faraday in England and Joseph Henry
in the U.S.A. discovered one of the most fundamental
effects of electromagnetism: an ability of a varying
magnetic field to induce electric current in a wire.
Faraday’s law of induction says that the induced
voltage, or electromotive force (e.m.f.), is equal to the
rate at which the magnetic flux through the circuit
changes. If varying magnetic flux is applied to a
solenoid, e.m.f. appears in every turn and all these
e.m.f. must be added.
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – III 4/26
Inductive Sensors
dt
d
E N

   0
Where
N is the number of turns,
B is the amplitude of magnetic field,
and A is the area of the circuit where is in the magnetic field.
The induced voltage is
dt
d B A
E N
( . )
0   
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – III 5/26
Inductive Sensors
dt
dx
E N.B.l 0  
Ex.
– Magnetic Motion detector.
– Geophone (Measurement of seismic signals).
A  l.x
dt
d B A
E N
( . )
0   
How to got magnetic field ?
1. By using a permanent magnet and a
movable plate.
Ps. Permanent magnets can be made from
ferromagnetic materials.
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – III 6/26
Inductive Sensors
How to got magnetic field ?
2. The magnetic field can also produced
by electric current in a solenoid coil.
B .n.I
dt
d B A
E N
( . )
0   
Ex.
– Position and displacement sensors.
– Inductive proximity Sensors.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
7/26
PS.Variable-inductancesensorsthatuseanon-magnetizedferromagneticmediumtoalterthereluctance(magneticresistance)ofthefluxpathareknownasvariable-reluctancetransducers.
Inductive Sensors
How to got magnetic field ?
2.Themagneticfieldcanalsoproducedbyelectriccurrentinasolenoidcoil.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
8/26
Abasicinductivesensorconsistsofamagneticcircuitmadefromaferromagneticcorewithacoilofnnumberofturnswoundonit.
Thecoilactsasasourceofmagnetomotiveforce(mmf)whichdrivesthefluxΦthroughthemagneticcircuitandtheairgap.
Inductive Sensor with Variable Gap, ISVG
Thepresenceoftheairgapcausesalargeincreaseincircuitreluctanceandacorrespondingdecreaseintheflux.Hence,asmallvariationintheairgapresultsinameasurablechangeininductance.
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – III 9/26
Self inductance
of the coil 



.
. .
2
1 2
0
N
L
Z  jwL

Z
Magnetic reluctance
Inductive Sensor with Variable Gap, ISVG
Where
μ0 = the permeability of free space (= 4π × 10–7 H/m)
δ = the air gap.
N = the number of the turns of the coil.
Φ = the flux through the magnetic circuit. (weber)
 The relationship between the magnetic
reluctance Z and the air gap δ is not linear
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
10/26
Differential Inductive Sensor
Thisarrangementovercomestheproblemofnonlinearityinherentinsinglecoilsensors.
Avariable-differentialreluctancesensorconsistsofanarmaturemovingbetweentwoidenticalcoresseparatedbyafixeddistance.Thearmaturemovesintheairgapinresponsetoamechanicalinput.Thismovementaltersthereluctanceofcoils1and2,thusalteringtheirinductiveproperties.
Dr. Hassan SHARABATY Sensors & Actuators – Linear & Rotational Sensors – III 11/26
[ X = 0 ]  [ Z1 = Z2 = Z0 ]
Z Z Z
Z Z Z
  
  
2 0
1 0
[Z0 = jw L0 ]
0
0 2 Z
V Z
E in 
 
Differential Bridge Conditioning Circuit
[ X >0 ] 
Differential Inductive Sensor
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
12/26
Avariable-reluctancetachogeneratorisasensorwhichisbasedonFaraday’slawofelectromagneticinduction.Itconsistsofaferromagnetictoothedwheelattachedtotherotatingshaftandacoilwoundontoapermanentmagnetextendedbyasoftironpolepiece.
Thewheelrotatesincloseproximitytothepolepiece,thuscausingthefluxlinkedbythecoiltochange.Thechangeinfluxcausesanoutputinthecoilsimilartoasquarewaveformwhosefrequencydependsonthespeedoftherotationofthewheelandthenumberofteeth.
A variable-reluctance tachogenerator
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
13/26
Thebasicarrangementofamulti-inductiontransducercontainstwocoils-primaryandsecondary.Theprimarycarriesacexcitation(Vref)thatinducesasteadyacvoltageinthesecondarycoil.Themovementofanobjectmadeofferromagneticmaterialwithinthefluxpathaltersthecouplingbetweenthecoils.Consequently,themagneticfluxcouplingbetweentwocoilsisconvertedintovoltage.
LVDT and RVDT
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
14/26
LVDTisbasedonelectromagneticinduction.
Theprimarycoilisdrivenbyasinewave(excitationsignal)havingastabilizedamplitude.
Anacsignalisinducedinthesecondarycoils.
Acoremadeofaferromagneticmaterialisinsertedcoaxiallyintothecylindricalopeningwithoutphysicallytouchingthecoils.
LVDT and RVDT
1-LVDT (linear variable differential transformer)
Thetwosecondariesareconnectedintheopposedphase.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
15/26
Whenthecoreispositionedinthemagneticcenterofthetransformer,thesecondaryoutputsignalscancelandthereisnooutputvoltage.
LVDT and RVDT
1-LVDT (linear variable differential transformer)
Movingthecoreawayfromthecentralpositionunbalancestheinducedmagneticfluxratiobetweenthesecondaries,developinganoutput.Asthecoremoves,thereluctanceofthefluxpathchanges.
Thelinearrangeofmeasurementisabout±40owithnon-linearityerrorof1%
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
16/26
Hence,thedegreeoffluxcouplingdependsontheaxialpositionofthecore.Atasteadystate,theamplitudeoftheinducedvoltageisproportional,inthelinearoperatingregion,tothecoredisplacement.
Consequently,voltagemaybeusedasameasureofadisplacement.
TheLVDTprovidesthedirectionaswellasmagnitudeofthedisplacement.
Thedirectionisdeterminedbythephaseanglebetweentheprimary(reference)voltageandthesecondaryvoltage.
Excitationvoltageisgeneratedbyastableoscillator.
LVDT and RVDT
1-LVDT (linear variable differential transformer)
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
17/26
FortheLVDTtomeasuretransientmotionsaccurately,thefrequencyoftheoscillatormustbeatleast10timeshigherthanthehighestsignificantfrequencyofthemovement.Fortheslow-changingprocess,stableoscillatormaybereplacedbycouplingtoapowerlinefrequencyof60or50Hz.
LVDT and RVDT
1-LVDT (linear variable differential transformer)
LVDTcanbeusedtomeasurethedisplacement,deflection,position.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
18/26
LVDT and RVDT
2-RVDT (rotary variable differential transformer)
TheRotationalVariableDifferentialTransformer(RVDT)isusedtomeasurerotationalanglesandoperatesunderthesameprinciplesastheLVDTsensor.
WhereastheLVDTusesacylindricalironcore,theRVDTusesarotaryferromagneticcore.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
19/26
LVDT and RVDT
Pros and Cons
AdvantagesoftheLVDTandRVDTarethefollowing:
1.Relativelowcostduetoitspopularity.
2.Nofrictionresistance,sincetheironcoredoesnotcontactthetransformercoils,resultinginanverylongservicelife.
3.Negligiblehysteresis.
4.Highsignaltonoiseratioandlowoutputimpedance.
5.thereislowsusceptibilitytonoiseandinterferences;
6.Solidandrobust,capableofworkinginawidevarietyofenvironments.
7.extremelysmallresolutionispossible.
8.NopermanentdamagetotheLVDTifmeasurementsexceedthedesignedrange
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
20/26
ThemaindisadvantageoftheLVDTandRVDTisthecoremustbeincontact(directlyorindirectly)withthemeasuredsurfacewhichisnotalwayspossibleordesirable.
LVDT and RVDT
Pros and Cons
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
21/26
AlthoughtheLVDTisadisplacementsensor,manyotherphysicalquantitiescanbesensedbyconvertingdisplacementtothedesiredquantityviathoughtfularrangements.
DiaphragmPressure Gage
LVDT and RVDT
Applications
Exampel1
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
22/26
LVDT and RVDT
Applications
ThicknessVariationofWorkpieces:
-dimensiongages,
-thicknessandprofilemeasurements,
-productsortingbysize.
Profile Gage
Exampel2
AlthoughtheLVDTisadisplacementsensor,manyotherphysicalquantitiescanbesensedbyconvertingdisplacementtothedesiredquantityviathoughtfularrangements.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
23/26
Fluid Level gage
LVDT and RVDT
Applications
Exampel3
AlthoughtheLVDTisadisplacementsensor,manyotherphysicalquantitiescanbesensedbyconvertingdisplacementtothedesiredquantityviathoughtfularrangements.
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
24/26
Theinductiveproximitysensorisanelectronicproximitysensor,whichdetectsmetallicobjectswithouttouchingthem.
Thesensorconsistsofaninductionloop(anelectromagneticcommunicationanddetectionsystem).Theinductanceoftheloopchangesaccordingtothematerialinsideitandsincemetalsaremuchmoreeffectiveinductorsthanothermaterialsthepresenceofmetalincreasesthecurrentflowingthroughtheloop.Thischangecanbedetectedbysensingcircuitry,whichcansignaltosomeotherdevicewhenevermetalisdetected.
Inductive proximity sensor
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
25/26
Commonapplicationsofinductiveproximitysensorsincludemetaldetectors,trafficlights,carwashes,andahostofautomatedindustrialprocesses.Itcanbemountedontextilemachines,transferlines,transportsystems,packagingequipment,andintheautomobileindustry.
Inductive proximity sensor
Inductiveproximitysensorsofferanattractivealternativetootherproximitysensorswhenitisrequiredtocoverlargeareasandtooperateoveranextendedtemperaturerangeundertheinfluenceofstronginterferences,suchaswind,acousticnoise,fog,dust,moisture,andsoforth.Itcanalsousedeveninthemostdifficultworkingconditionsforexampleinthepresenceofoils,powders,liquidsandvibrationswhichdonothaveanyeffectontheirsecurefunctioning.
Applications
Dr. Hassan SHARABATY
Sensors& Actuators -Linear & Rotational Sensors -III
26/26
END

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