Strain gauge

Strain gauge info

1. StrainGauge
Astraingaugeisa passivetransducer,thatconverts
mechanicaldisplacementintothechangeofresistance.
Astraingaugesensorisathinwafer-likedevicethatcanbe
attachedtoavarietyofmaterialstomeasureappliedstrain.
Theseareusedasafundamentalsensorinmanytypesof
sensorslikepressuresensors,loadcells,torquesensorsetc.

2. WorkingPrinciple:-
Here =Resistivity
Theresistanceofthefoilchangesasthematerialtowhich
thegaugeisattachedundergoestensionorcompressiondue
tochangeinitslengthanddiameter.Aslengthchangesthe
ResistanceChangesduetotheaboveformula.
Thischangeinresistanceisproportionaltotheapplied
strain.
Asthischangeinresistanceisverysmallinmagnitudesoits
effectcanbeonlysensedbyaWheatstonebridge.Thisisthe
basicstraingaugeworkingprinciple.
AcircuitdiagramisshowninFigure#2.Inthiscircuit
diagram,astraingaugeisconnectedintoaWheatstone
bridge.
Thiscircuitissodesignedthatwhennoforceisappliedto
thestraingauge,R1isequaltoR2andtheresistanceofthe
straingaugeisequaltoR3.
InthisconditiontheWheatstonebridgeisbalancedandthe
voltmetershowsnodeflection.

3. Butwhenstrainisappliedtothestraingauge,theresistance
ofthestraingaugesensorchanges,theWheatstonebridge
becomesunbalanced,acurrentflowsthroughthevoltmeter.
Sincethenetchangeintheresistanceisproportionaltothe
appliedstrain,therefore,resultantcurrentflowthroughthe
voltmeterisproportionaltotheappliedstrain.
So,thevoltmetercanbecalibratedintermsofstrainorforce.
Intheabovecircuit,wehaveusedonlyonestraingauge.
Thisisknownas‘quarterbridge’circuit.
Wecanalsousetwostraingaugesorevenfourstraingauges
inthiscircuit.
Thenthiscircuitiscalled‘halfbridge’and‘fullbridge’
respectively.
Thefullbridgecircuitprovidesgreatersensitivityandleast
temperaturevariationerrors.
GaugeFactorofStrainGauge:-
Thegaugefactorofstraingaugeisdefinedastheunit
changeinresistanceperunitchangeinlength.
gaugefactorGf=(∆R/R)/(∆l/l)
where,R=nominalgaugeresistance,
∆R=changeinresistance,
l=lengthofthespecimeninanunstressedcondition,
∆l=changeinspecimenlength.
Itcanbeprovedmathematically,

4. Gaugefactor,Gf=1+2v+(∆ρ/ρ)/(∆L/L)
Ifthechangeinresistivityduetostrainisalmostnegligible,then
gaugefactorofstraingauge,Gf=1+2v
Where,visPoisson’sratio.
Itmaybedefinedastheratioofstraininthelateraldirection
tothestrainintheaxialdirection.
ThePoisson’sratioformostmetalsliesintherangeof0to
0.5andthisgivesagaugefactorof2approximately.

5. WirelessStrainSenSpot™sensors
WirelessStrainSenSpot™sensorsofferabreakthrough
technologyforreal-timeandlong-termstructuralhealth
monitoring.
SenSpot™straingaugeusesResensys’sproprietarysensing,
schedulingandultra-lowpowersynchronizationtechnology.
SenSpot™isdesignedtooperatemaintenance-freeformore
thanadecade.Afterinstallation,SenSpot™doesnotneed
calibration,batteryreplacement,oranyothermaintenance
duringitsentireservicelife.
Features
1.Longlifetime:(minimumexpectedlifewithoutbattery
replacement10years)
2.Lightweight
：
about147g(5.19oz.)
3.Wirelesstransmitter:120g(4.23oz.)
4.Cable(1ft):10g(0.35oz.)
5.Strainsensingelement:17g(0.6oz.)
6.Easymounting
a)Self-adhesive,nodrillingisrequired(e.g.steel)
b)Flange-mount,drillingisrequired(e.g.concrete)
c)Quickinstallation,1-2minutes
7.Highaccuracy:1-µStrainresolution
8.Wideworkingtemperature:-40to+65°C(-40°Fto+150°F)
9.Longcommunicationrange:1.0kmfreespace

6. References:-
1.http://www.resensys.com/senspot-
strain.html?gclid=CjwKCAiAl-
6PBhBCEiwAc2GOVL_k7fgHEOciMFCqiNAewcjJk_snv
W3g4Pr5kbjSpt_GRQBM3xOOghoCC_4QAvD_BwE
2.https://www.electrical4u.com/strain-gauge/