User manual

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 DocumentMT0100P.N
©XsensTechnologiesB.V. MTiandMTxUserManualandTechnicalDocumentation
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However,forsomeapplicationsthisassumptiondoesnothold.Forexampleanacceleratingautomobilemay
generate significant accelerations for time periods lasting longer than the maximum time the MT’s rate
gyroscopescanreliablykeeptrackoftheorientation.Thiswillseverelydegradetheaccuracyoftheorientation
estimates with XKF‐3, because the use scenario (application) does not match the assumptions made. Note
however, that as soon as the movement does again match the assumptions made, XKF‐3 will recover and
stabilize.Therecoverytooptimalaccuracycantakesometime.
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NOTE:Tobeabletoaccuratelymeasureorientationsaswellaspositioninapplicationswhichcanencounter
longtermaccelerationsweofferasolutionthatincorporatesaGPSreceiver(theMTi‐G).
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3.2.2 UsingtheEarthmagneticfieldtostabilizeHeading(Yaw)
By default, the heading is stabilized using the local (earth) magnetic field. In other words, the measured
magneticfieldisusedasacompass.IfthelocalEarthmagneticfieldistemporarilydisturbed,XKF‐3willtrack
this disturbance instead of incorrectly assuming there is no disturbance. However, in case of structural
magneticdisturbance(>10to20s)thecomputedheadingwillslowlyconvergetoasolutionusingthe'new'
localmagneticnorth.Notethatthemagneticfieldhasnodirecteffectontheinclinationestimate.
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In the special case the MTi or MTx is ri gidly strapped to an object containing ferromagnetic materials,
structural magnetic disturbances will be present. Using a so‐called 'magnetic field mapping' (i.e. a 3D
calibration for soft and hard iron effects), these magnetic disturbances can be completely calibrated  for,
allowingtheMTi/xtobeusedasifitwouldnotbesecuredtoanobjectcontainingferromagneticmaterials.
Seesection7.2.3formoredetails.
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3.2.3 Initialization
TheXKF‐3algorithmnotonlycomputesorientation,butalsokeepstrackofvariablessuchassensorbiasesor
propertiesofthelocalmagneticfield.Forthisreason,theorientationoutputmayneedsometimetostabilize
oncetheMTisputintomeasurementmode.Timetoobtainoptimalstableoutputdependsonanumberof
factors.Animportantfactordeterminingstabilizingtimeisdeterminedbythetimetocorrectforsmallerrors
onthebiasoftherategyroscopes.Thebiasof therategyroscopemayslowlychangeduetodifferenteffect
suchastemperaturechangeorexposuretoimpact.To reducestabilizingtime,thelastcomputedgyroscope
bias can be stored in the sensor unit non‐volatile memory. If the MTi/x is used after only a short period of
power‐offthe gyrobiases will generallynothave changedalotand thestabilizingtime will typicallybeless
than10seconds.Furthermore,XKF‐3willconvergefasterandreachoptimalrobustnessfasterifitisstartedin
anareawithoutmagneticdisturbances.
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