Owner's manual

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UserManual
7430‐0131‐01Rev.F
Page25
NAV440
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NOTE:Ifthecoordinatesystemisconfiguredtoanon‐standardorcustomconfiguration,applytheappropriate
rotationandconfigurethefiltersettingsaccordingly.
VG440(VerticalGyroscope)Function
TheVG440providesdynamicrollandpitchmeasurements,aswellasallIMU440datafunctions.Thedynamicroll
andpitchmeasurementsarestabilizedbytheusingtheaccelerometersasalong‐termgravityreference.TheVG440
canalsooutputafreeintegratingyawanglemeasurementthatisnotstabilizedbyamagnetometerorcompass
heading).
Atafixed100Hzrate,theVG440continuouslymaintainsboththedigitalIMU440dataaswellasthedynamicroll
andpitchdata.Asshowninthesoftwareblockdiagram(
Figure2onpage21),aftertheSensorCalibrationblock,the
IMU440dataispassedintoan
IntegrationtoOrientationblock.(IfusingexternalGPS,refertoFigure4on
page
22.)TheIntegrationtoOrientationblockintegratesbodyframesensedangularratetoorientationatafixed100
ndtoav eswhendealingwiththe ix,a
t top epro
timespersecond.Forimprovedaccuracya oidsingulariti cosinerotationmatr
quaternionformulationisusedinthealgori hm rovideattitud pagation.
Alsoshowninthesoftwareblockdiagram(
Figure2,page21)theIntegrationtoOrientationblockreceivesdrift
correctionsfromthe
ExtendedKalmanFilterorDriftCorrectionModule.Ingeneral,ratesensorsandaccelerometers
sufferfrombiasdrift,misalignmenterrors,accelerationerrors(g‐sensitivity),nonlinearity(squareterms),andscale
factorerrors.
Thelargesterrorintheorientationpropagationisassociatedwiththeratesensorbiasterms.TheExtendedKalman
Filter(EKF)moduleprovidesanon‐the‐flycalibrationfordrifterrors,includingtheratesensorbias,byproviding
correctionstotheIntegrationtoOrientationblockandacharacterizationofthegyroscopebiasstate.IntheVG440,
theinternallycomputedgravityreferencevectorprovidesareferencemeasurementfortheEKFwhentheunitisin
quasi‐staticmotiontocorrectrollandpitchangledriftandtoestimatetheXandYgyroscoperatebias.Becausethe
gravityvectorhasnohorizontalcomponent,theEKFhasnoabilitytoestimateeithertheyawangleerrorortheZ
gyroscoperatebias.
VG440adaptivelytunestheEKFfeedbackinordertobestbalancethebiasestimationandattitudecorrectionwith
distortionfreeperformanceduringdynamicswhentheobjectisacceleratingeitherlinearly(speedchanges)or
centripetally(falsegravityforcesfromturns).Becausecentripetalandotherdynamicaccelerationsareoften
associatedwithyawrate,theunitmaintainsalow‐passfilteredyawratesignalandcomparesittotheturnSwitch
thresholdfield(useradjustable).
Whentheplatformtheunitisattachedtoexceedsthe
turnSwitchthresholdyawrate,theunitlowersthefeedback
gainsfromtheaccelerometerstoallowtheattitudeestimatetocoastthroughthedynamicsituationwithprimary
relianceonangularratesensors.ThissituationisindicatedbythesoftwareStatus→turnSwitchstatusflag.Usingthe
turnSwitchmaintainsbetterattitudeaccuracyduringshort‐termdynamicsituations,butcaremustbetakento
ensurethatthedutycycleoftheturnSwitchgenerallystaysbelow10%duringthevehiclemission.Ahigh
turnSwitchdutycycledoesnotallowthesystemtoapplyenoughratesensorbiascorrectionandcouldallowthe
attitudeestimatetobecomeunstable.
TheVG440algorithmhastwomajorphasesofoperation.Thefirstphaseofoperationistheinitializationphase.
Duringtheinitializationphase,theunitisexpectedtobestationaryorquasi‐staticsotheEKFweightsthe
accelerometergravityreferenceheavilyinordertorapidlyestimatetherollandpitchangles,andX,Yratesensor
bias.Theinitializationphaselastsapproximately60seconds,andtheinitializationphasecanbemonitoredinthe
softwareStatusBITtransmittedbydefaultineachmeasurementpacket.Aftertheinitializationphase,theunit
operateswithlowerlevelsoffeedback(alsoreferredtoasEKFgain)fromtheaccelerometerstocontinuously
estimateandcorrectforrollandpitcherrors,aswellastoestimateXandYratesensorbias.Toresetthealgorithm
orre‐entertheinitializationphase,sendingthealgorithmresetcommand,
AR,willforcethealgorithmintothereset
phase.