Instruction Manual

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GNAV540UserManual
7430‐0808‐01Rev.B
Page29
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AHRSFunction
TheAttitudeHeadingReferenceSystem(AHRS)Functionutilizesa3‐axismagnetometer(internalorexternal)in
additiontotheaccelerometersandgyroscopes,aswellastheassociatedsoftwarerunningontheDSPprocessor.
Thisenablesthecomputationofdynamicheading,aswellasdynamicrollandpitch.AHRSFunctionprovides
dynamicheading,roll,andpitchmeasurementsinadditiontotheVGFunctionandIMUFunctiondata.Thedynamic
headingmeasurementisstabilizedusingthe3‐axismagnetometerasamagneticnorthreference.Thedynamicroll
andpitchmeasurementsarestabilizedusingtheaccelerometersasalong‐termgravityreference.Theunitcanbe
configuredtoturnonandoffthemagneticreferenceforuserdefinedperiodsoftime.
Thisfunctionutilizesdatafromcalibratedsensors,thegyroscopes,theaccelerometersandamagnetometer
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(internalorexternal).FordetailsoftheIMUfunctions,referto n
IMUFunctio onpage
functions,referto
25.FordetailsoftheVG
.
VerticalGyroscope(VG)Functiononpage26
Inadditiontothefeaturesdescribedintheprevioussections( and ),
theAHRSalgorithmhastwomajorphasesofoperation.Thefirstphaseofoperationisthehigh‐gaininitialization
phase.Duringtheinitializationphase,theunitisexpectedtobestationaryorquasi‐staticsotheEKFweightsthe
accelerometergravityreferenceandEarth’smagneticfieldreferenceheavilyinordertorapidlyestimatetheX,Y,
andZratesensorbias,andtheinitialattitudeandheadingoftheunit.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andmagnetometerstocontinuouslyestimateandcorrectforroll,pitch,and
IMUFunction VerticalGyroscope(VG)Function
heading(yaw)errors,aswellastoestimateX,Y,andZratesensorbias.
TheAHRSFunctionprovidesthesamescaledsensorandanglemodepacketsoftheVGFunction.TheAHRSFunction
defaultstothe
A1AnglePacketwhichoutputstherollangle,pitchangle,yawangle,anddigitalIMUdata.Inthe
AHRSFunction,the
A0andA1packetscontainaccuratemagnetometerreadings.Formoreinformation,referto
Chapter9.CommunicatingwiththeGNAV540UnitandChapter10.ProgrammingGuidelinesforpacketdescriptions.
NOTE:Forproperoperation,theunitreliesonmagneticfieldreadingsfroma3‐axismagnetometer.Theunitmust
beinstalledcorrectlyandcalibratedforhard‐ironandsoftironeffectstoavoidanysystemperformance
degradation.RefertoChapter4.MagnetometerCalibrationandAlignmentGuidelinesforinformationabout
magneticcalibration;reviewthatsectionbeforeusingtheAHRSFunction.
NOTE:TheGNAV540unitandtheexternalmagnetometer(ifused)mustbemountedatleast24”awayfromlarge
ferrousobjectsandfluctuatingmagneticfields.Failuretolocatetheunitinacleanmagneticenvironmentwill
affecttheattitudesolution.
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AHRSAdvancedSettings
Inadditiontotheconfigurablebaudrate,packetrate,axisorientation,andsensorlow‐passfiltersettings,theunit
providesadditionaladvancedsettingswhichareselectablefortailoringtheunittoaspecificapplication
requirements.TheAHRSadvancedsettingsarelistedin
Table9below:
Table9AHRSSeriesAdvancedSettings
Setting DefaultValue Comments
BaudRate 38400 9600,19200,57600alsoavailable
PacketType A1 S0,S1,S2,A0,A2,N0,N1alsoavailable
PacketRate 25Hz
ThissettingsetstherateatwhichselectedPacketType,packetsareoutput.If
polledmodeisdesired,thenselectQuiet.