Instruction Manual



GNAV540UserManual
7430‐0808‐01Rev.B
Page27


VGFunction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
sfilteredy
associatedwithyawrate,theunitmaintainsalow‐pas aw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
turnswitchmaintainsbetterattitudeaccuracyduringshort‐termdynamicsituations,butcaremustbetakento
ensurethatthedutycycleoftheturnswitchgenerallystaysbelow10%duringthevehiclemission.Ahighturn
switchdutycycledoesnotallowthesystemtoapplyenoughratesensorbiascorrectionandcouldallowtheattitude
estimatetobecomeunstable.
TheVGFunction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
softwareStatus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, ,willforcethealgorithmintothereset
AR
phase.
InadditiontothescaledsensorpacketsdescribedintheIMUFunctionsection,theVGFunctionhasadditional
measurementoutputpacketsincludingthedefault
A2AnglePacketwhichoutputstherollangle,pitchangle,and
digitalIMUdata.
N0andN1packetsarealsoavailableforusewithanexternalGPSreceiver.Formoreinformation,
referto
Chapter9.CommunicatingwiththeGNAV540UnitandChapter10.ProgrammingGuidelinesforpacket
descriptions.
VGFunctionAdvancedSettings
Inadditiontotheconfigurablebaudrate,packetrate,axisorientation,andsensorlow‐passfiltersettings,VG
unctionprovidesadditionaladvancedsettingsthatareselectablefortailoringtheunittoaspecificapplication
equirements.Thesettingsarelistedin
F
r

Table8below.
Table8VGFunctionAdvancedSettings
Setting DefaultValue Comments
BaudRate 38,400baud 9600,19200,57600alsoavailable
PacketType A2 S1,S2,N0,N1alsoavailable
PacketRate 25Hz ThissettingsetstherateatwhichselectedPacketType,packetsareoutput.

Ifpolledmodeisdesired,thenselect
Quiet.IfQuietisselected,theunitwillonly
sendmeasurementpacketsinresponsetoGPcommands.
Orientation
See
Figure4on
page
24.
Toconfiguretheaxisorientation,selectthedesiredmeasurementforeachaxis:
NAV‐VIEW2.2willshowthecorrespondingimageoftheunit,soiteasyto
visualizethemodeofoperation.Referto
OrientationFieldonpage86forthe
twentyfourpossibleorientationsettings.Thedefaultsettingpointsthe
connectorAFT.