PO.248
PitchandRollAngleCalibrationforScanningLiDARs
toReduceUncertaintiesinMeasurementHeight
Abstract
TheapplicationofscanningLiDARsforthe
measurementofwindspeedisarelativelynew
technologyinthewindenergysectorandalsohas
potentialrelevanceforvarioustopics,(e.g.
measuringthewindspeedforpowerperformance
testforoffshorewindturbines).ScanningLiDARs
offeracustomizablescanninggeometryandare,
therefore,abletomeasurewindspeedandwind
directionatapositionthatisnotcenteredonits
ownposition.Forsuchsituations,theknowledge
oftheelevationangleiscrucial,asitdirectly
influencesthemeasurementheight.Forinstance,
forpowerperformancetestsofoffshorewind
turbinesoftypicalsize,anaccuracyhigherthan
0.1° isnecessary.Independentfromthedisplayed
elevationangleofthedevice,theinclinationofthe
devicemustbeknown.
Objectives
UweLohbeck,KlausFranke,AiltWiardJanssen
DeutscheWindGuardConsultingGmbH
Blockageofthelaserbeamoccurswhenthe
LiDAR’scarrier-to-noiseratio(CNR)decreases
significantlyfortherangegateinapproximatelythe
distanceofthescreenandrangegatesbehind.The
CNRismonitoredinreal-timewithalaptop
connectedtotheinstrument.
Device
Device1
Device2
Device3
Device4
δh
α
h
δα
RollAngle
Slope
Offset
[-]
[°]
-0.97
0.18
-0.99
0.03
-1.00
0.27
-0.99
0.05
Table1:ResultsofcalibrationsperformedatDeutscheWindGuard
thusfar,MethodAapplied.ApplyingMethodBisresultinginvery
similarcorrectionfactors.
L
h0
PitchAngle
Slope
Offset
[-]
[°]
0.98
-0.13
1.02
-0.39
0.98
-0.07
1.03
-0.30
Detailedcalibrationresultsareshownforthepitch
angleofdeviceno.2.
Parameter
MeasuredInclinationasgivenbyLiDAR
ScanningDirection
AzimuthAngle[°]
ZenithAngle[°]
TransversalInclination
Name
DisplayedValue[°]
LDistanceLiDAR-Screen[m]
DateofCalibration
Variateleglengthtovariateα
Figure1:Schematicsideviewofthemeasurementsetup.
Calibrationtargetisthepitchangleα.Theinfraredlaser(red
dashedline)canbedetectedwiththegapinthescreenatdistance
LoftheLiDAR.Thevisiblelaser(greensolidline)isapproximately
paralleltotheinfraredlaser,i.e.theangleδαandtheoffsetδhare
fixedduringthemeasurements.Thedottedblacklineistheview
directionofthetheodolitetothevisiblelaseronthescreen.The
referenceheighth0 isdefinedbytheexitpointofthelaserbeam
atthecentreofthelens.
Value
Pitch
0
90
Roll
0.000
83.85
2015-07-03
Table2:Measurementconfigurationofthepitchcalibration
RotatingLaser
Twobuilt-inelectronicinclinometersmeasureits
ownorientationwithintheearth’sgravitational
fieldduringoperation.
Inordertoachieveahighaccuracyofthese
measurands,thesesensorsmustbecalibrated
priortothemeasurement.Acalibrationprocedure,
includingdetaileduncertaintyassessment,has
beendevelopedinthetestinglaboratoryof
DeutscheWindGuard.
Visable
LiDAR
Infrared
δy
δx
Theodolite
Screen
Figure2:Topviewofmeasurementsetup.Therotatinglaser
transfersthereferenceheighth0fromtheLiDARtothetheodolite
(MethodA)andthescreen(MethodB)
Figure4:CalibrationresultsofthepitchangleoftheLiDAR.The
referenceinclinationwasmeasuredbyMethodA,i.e.directangle
measurementswithatheodolite.Thebluemarkersarethe
referencepitchasfunctionofthepitchgivenbytheLiDAR.The
errorbarsdenotethestandarduncertaintyinpitch.Thered
diamondsgivetheresiduals,i.e.thedeviationbetweentheshown
calibrationlineandthemeasuredvalues.Thedottedlinesdenote
thestandarduncertaintyofthecalibrationontheaxisofthe
residuals
MeasurementSetup
Toassesstheaccuracyofthepitchandroll
inclinometersoftheLiDAR,theelevationangleα
ofthelaserbeaminagivenconfigurationofthe
instrumentiscomparedtotheoutputofthe
inclinometer.Figure1andFigure2show
schematicoverviewsofthemeasurement
configuration.Figure3showsaphotographofthe
setup.
Theelevationangleαcanbemeasuredbytwo
independentmethods:
•
Directmeasurementoftheangleαwitha
theodolite(MethodA)
•
Indirectmeasurementoftheangleαwith
therelationtan α=h/L,withhbeingthe
verticaldisplacementofthelaserbeamata
horizontaldistanceLfromtheexitlens
(MethodB)
Bothmeasurementsareappliedduringthe
calibration.
Commontobothmethodsisthechallengeof
findingtheinvisiblelaserbeam[1].The
wavelengthofthelaserisapproximately
1550 nm,i.e.thelaseremitsintheinfrared.
Thereforedirectdetectionoftheoutgoinglaser
wasnotfeasible.
Thelocationofthelaserbeamwasidentifiedby
iterativelyblockingandunblockingthelaserbeam
withthewoodenconstructionshowninFigure3.
Conclusions
Figure3:Pictureoftheexperimentalsetup.Theinsetshowsthe
screen.Itisconstructedtoidentifythelocationoftheinfrared
laserbeamoftheLiDAR.Thewhitepanelsaremoveduptothe
pointatwhichtheyjustdonotblockthelaser.Therotatinglaser
definesareferenceheightacrossthemeasurementsite
Inordertoworkoutcalibrationfactorsintermsof
slopeandoffset,bothmethodswereappliedforup
tosixdifferentinclinationangleswhicharevaried
byadjustingthelegsofthedevice.Procedurewas
conductedforpitchandrollanglebyrotatingthe
deviceby90°.
Results
Atthisstage,calibrationoffourscanningLidarshas
beenperformedinthefacilitiesofDeutscheWind
Guard.Itturnedout,thattheinclinometersare
moreinfluencedbyanoffsetthantheslopefactor.
ResultsofallcalibrationsarelistedinTable1.
windeurope.org/summit2016
#windeuropesummit2016
Anewmethodofcalibratingpitchandrollanglefor
scanningLiDARshasbeendeveloped.The
challengeofidentifyingtheinvisiblelaserbeamis
avoidedbytheapplicationofanadjustablevisible
laserbeam.Redundantmeasurementprinciples
verifythereliabilityoftheresults.
Theresultsofdevicestestedthusfarleadtothe
assumptionthatcalibrationofthebuilt-in
inclinometersisrecommended,inordertoachieve
ahighaccuracyinelevationangleand
consequentlyinmeasurementheight.Foratypical
measurementconfigurationforapowercurvetest
ofanoffshorewindturbine,adeviationofthe
elevationangleof0.1° willresultinanerrorof
measurementheightofabout0.5m.
References
1. Calibratingnacellelidars,DTUWindEnergyE-0020,January
2013
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