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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3 MTiandMTxSystemOverview

3.1 Overview

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NOTE:NotallpinsavailableontheconnectorofallversionsoftheMT.Pleaserefertosection6.4fordetails.

3.2 XsensKalmanFilterforMTiandMTx

The orientation of the MTi and  MTx is computed by Xsens Kalman Filter for 3 degrees‐of‐freedom (3DoF)

orientation(XKF‐3).XKF‐3usessignalsoftherategyroscopes,accelerometersandmagnetometerstocompute

a statistical optimal 3D orientation estimate of high accuracy with no drift for both static and dynamic

movements.

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ThedesignoftheXKF‐3algorithmcanbeexplainedasasensorfusionalgorithmwherethemeasurementof

gravity (by the 3D accelerometers) and Earth magnetic north (by the 3D magnetometers) compensate for

otherwise slowly, but unlimited, increasing (drift) errors from the integration of rate of turn data (angular

velocityfromtherategyros).Thistypeofdriftcompensationisoften calledattitudeandheadingreferenced

andsuchasystemisoftencalledanAttitudeandHeadingReferenceSystem(AHRS).

3.2.1 Usingtheaccelerationofgravitytostabilizeinclination(roll/pitch)

XKF‐3stabilizestheinclination(i.e.rollandpitchcombined,alsoknownas“attitude”)usingtheaccelerometer

signals.Anaccelerometermeasuresgravitationalaccelerationplusaccelerationdueto themovementofthe

objectwithrespecttoitssurroundings.

XKF‐3 uses the assumption that on average the acceleration due to the movement is zero. Using this

assumption,thedirectionofthegravitycanbeobservedandusedtostabilizetheattitude.Theorientationof

theMTinthegravityfieldisaccountedforsothatcentripetalaccelerationsorasymmetricalmovementscan

notcauseadegradedorientationestimateperformance.Thisassumptionissurprisinglypowerful,almostall

moving objects undergo accelerations if they are moving, but in most cases the average acceleration with

respecttotheenvironmentduringsomeperiodoftimeiszero.Thekeyhereistheamountoftimeoverwhich

theaccelerationmustbeaveragedfortheassumptiontohold.Duringthistime,therategyroscopesmustbe

abletotracktheorientationtoahighdegreeofaccuracy.Inpractice,thislimitstheamountoftimeoverwhich

theassumptionholdstrue.FortheclassofminiatureMEMSrategyroscopesusedintheMTthisperiodoftime

isabout10‐20secondsmaximum.

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