User Manual

3
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AUTOMATION & PROCESS
TECHNOLOGIES
Gemco Magnetostrictive Technology Made Easy
In a Gemco position sensor,acurrentpulseis
sentdownamagnetostrictivewireinaspecially
designedwaveguide(FigureA).Theinteractionof
thiscurrentpulsewiththemagneticeldcreatedby
themovablemagnetassemblyproducesatorsional
strainpulseonthewire,whichtravelsatsonic
speedalongthewire(FigureB).Thestrainpulse
travelingupthewireissensedbyasmallinduction
pickupcoilintheheadassemblyoftheLDT.The
positionofthemovablemagnetisdeterminedwith
highprecisionbymeasuringthetimebetweenthe
launchingofthecurrentpulseandthearrivalofthe
torsionalstrainpulse.Theresultishighlyaccurate
non-contactpositionsensingwithabsolutelyno
wearonthesensingelement.
Hysteresis
Thedifferenceinindicatedpositionwhenthesame
pointisreachedfromtwodifferentdirections.
Repeatability
Thedeviationinindicatedpositionwhenapoint
alongastrokelengthisapproachedrepeatedlyfrom
thesamedirection.
Magnetostriction
Amagneticeldproducesasmallchangeinthe
physicaldimensionsofferromagneticmaterialson
theorderofseveralpartspermillionincarbonsteel
and,conversely,aphysicaldeformationorstrain(or
stresswhichcausesstrain)producesachangeof
magnetizationinthematerial.
Non-Linearity
Thedegreethattheindicatedpositionofthe
magnetatpointsalongthestrokevariesfromthe
actualphysicalposition.Non-linearityofanLDTis
expressedinabsoluteerrororasapercentageof
theactivestrokelength.
Resolution
Resolutionisthesmallestincrementalchangein
positionalongthestrokelengthofthesensorthat
canbedetectedandindicatedinanoutput.When
usingLDTswithanalogoutput(i.e.,voltageor
currentoutputs),resolutionislimitedbytheamount
ofpowersupply-inducedoutputrippleandthe
sensitivityand/ordesigncapabilitiesofthereceiver
electronics.Digitalsystemresolutionisdenedbya
specicvalue.

Recirculation
Amethodusedtoimprovetheresolutionofa
systemusingdigitalLDTs.Theontimeofthe
pulsewidthoutputismultipliedbyaspecicfactor.
Thismultiplicationprovidesmorecountingtime
forthecounterinthecustomer’selectronics,thus
improvingtheresolution.