Download LVDT - Newmark Structural Engineering Laboratory

INTRoDUCTIoN
The purposeof this rnanualis to provide the basic principlesof theory and installation
associatedrvith linear variable diffcrcntial transfbrmersto users of the Nervmark Structural
EngineeringLaboratory(NSEL). The materialpresentedin this manualis basedon 50 yearsof'
ofpast
listedat the endof thisnranual.Also, the experience
expcrience
ofNSI:L and ret'erences
qucstions
have
askcd
on
Many
been
*as
u,hile
working
on
this
manual.
usersol'NSEL
used
wererecomrrendedin rvhichansu'crscould
rlhich eitlierdirectansu'ersrveregivcn or ref-erences
fon.nat. Any
be tbund. That is w'hy this manualwas partially rvrittenin thc question-ansu'er
criticalcommentsand remarksof currentuscrsare also welcomcdand they rvill be usedin a
and
future revision. This manual is divided into threc scctions:Theory, lnstrumentation,
Installation.
I HEORY
What is a lincar variabledit-ferential
transforrner?The linear variablc dill'erential
of
rvhich allolvs the measurement
transducer,
transfbrmer(LVDT) is an elcctromcchanical
displacernent
of any movablepafi. The measuredamountof displacementis proportionalto the
elcctricaloutput of the transduceras a resultantof an inductiveinteractionbetrvccna magnetic
core and a set of windings: prinraryrvindingwhich is polveredby high frequencylorv voltage
'fhc
current,and trvo secondaryrvindingsusuallysymrnetricallylocatedagainstthe primary one.
secondarywindingsgeneratethc electricaloutputcorrelatedto the positlonof the magncticcore.
Figure I shorvsan examplcof a commonlyusedLVDT. The primary rvinding (8) and
the secondaryu,indings(9) are separated
by high-densityglossfilled polyrnercoil fbrm (4) and
'fhis
in cpoxy (3).
assemblyis covcrcdby stainlesssteelhousing(l),
both are encapsulatcd
sleeve(6), and end caps(5). The h igh-pcrneabilitymagneticshell (2) is placedbetweenthc
'l'he
(3), and separators
(zl).
nickelhigh-permeability
housing(l), and polyrnerencapsulation
(6).
sleeve
Carricr(10),
(7),
is
placed
inside
of
the
usuallythreadedfiom both ends,
iron core
rvhich must bc non-magnetic,
transfersthe linear motion of an object to the core (7) and
changestlie electricaloutputof the [-VDT.
consequently
Horv doesthe I-VDT work? Figure2 illustrateswhat happcnsrvhenthe core of'the
by a
LVDT is in differentaxialpositions.The primaryrvinding,(P) o1'theLVDT is energized
(AC)
is
coupled
by
source.The magneticflux thus developed
constantamplitudcalteredcurrent
(Sr) and (Sz). If the core is locatcdmidivay
tlie core to the adjaccntsecondaryr.vindings,
(S1) and (S2)(Figure2b), equalmagneticflux is coupledto eachsecondarywinding so
bctr.veen
the voltages,(E1) and (82) inducedin eachwindingare equal. At this midrvaycoreposition,
voltageoutput,(Er - Ez) is ellectivelyzcro. As
to as the null point,the difl'erential
ref-errsd
rvhile
shorvnin Figure2a, if the coreis movedcloserto (Sr), the inducedvoltage(Ey) incrcases
if the coreis moved
(E2)decreases,
voltage(Er E:) Conversely,
resultingin the diff-erential
(52)
(S1),
(E2)is incrcased
as
and
and lessto
closerto (S2)(Figure2c), more11uxis coupledto
(E1)is decreased,
voltage(E: - Er). Figure3a showsthe n.ragnitude
resultingin the differential
ofthe diff'erentialoutputvoltage(Eecour)variesrvith coreposition. The valueof E466u1at