Electronic-compass-sensor

ElectronicCompassSensorRobertRacz,ChristianSchott,SamuelHuberSentron,aMelexisCompany,Switzerland,[e-mail],[e-mail],[e-mail],wedescribeanewtworespectivethreeaxisintegratedIMCHall-ASICthatcanbeusedasasinglechipelectroniccompassandappliedinportablelowcostelec-tronicequipmentlikewristwatchesandmobilephones.Theelectroniccompasschipconsistsofringshapedinte-gratedferromagneticconcentrators(IMC),aCMOSinte-gratedcircuitandanexcitationcoilmanufacturedbydi-rectbondingontothechipsurface.TheIMCHallASICmeasuresthetwoin-planmagneticfieldcomponentsBxandByandtheverticalcomponentBzoftheearthmag-neticfield.TheIMCHallsensorexhibitsanexcellentsensi-tivityof8000V/TfortheXandYaxiswhichyieldstoanin-planeheadingaccuracyof+/-1°,whereasthefieldintheZ-axisismeasuredwithalowersensitivityof800V/T.ThehighsensitivitycharacteristicsareobtainedbyintegratingmagneticfluxconcentratorsdirectlyonHall-ASIC,whichamplifiestheearthmagneticfieldbyafactorof10.Furthermorethecompasssensorsprovidesalowenergymethodtoeliminatethepermingeffect(remnantfieldeffectaftersub-jectingtostrongmagneticfields)KeywordsMagnetometer,electroniccompass,earthmagneticfield,Hallsensors,IMC-HallASIC,Triaxis-TechnologyINTRODUCTIONMagneticsensorshavebeeninusefordirectionfindingornavigationforcenturies.Theearthmagneticfieldisaweakthree-dimensionalfieldwithanintensityof50-60µT(0.5-0.6Gauss)thatcanbeapproximatedbyadipolemodel.BydefinitiontheX-Ycomponentofthefieldvectorliesparal-leltotheearthsurfaceandpointstowardsthemagneticnorthpole.Foralatitudenotclosetotheequatorthemajor-ityoftheearthmagneticfieldliesalongtheZ-axisandpointsatthenorthernhemisphereintotheground.How-ever,thehorizontaldirectionoftheearthfieldisalwayspointingtowardmagneticnorthandisusedtodeterminethecompassdirection.Becauseofthisthecompasshastobeheldparalleltotheearth’ssurfaceorhastobegimbaledinordertobeaccurate.Hence,atwo-axismagnetometerheldparalleltotheearthsurfaceistheminimumconfigurationformagneticcompassheading.Foranelectroniccompassappliedinportablelowcostequipment,themagneticfieldsensorhastohavethefollow-ingfeatures:•tomeasureatleasttwoorthogonalaxesXandYoftheearthmagneticfieldwithanaccuracyofbetterthan1%.•tobeinsensitivetothermalandmagneticshock(per-ming)•tobesmallinsizeandhavelowpowerconsumption•tobemass-manufacturableatlowcost.Today,therearevarioustypesofelectroniccompassesavailable.Themostcommonlyusedmagneticfieldsensorsforcompassapplicationsarebasedonthemagnetoresistive(AMR)effect,onthefluxgateeffectoronmagneto-inductiveeffect.Becauseoftheweaknessoftheearthmag-neticfieldHall-effectsensorswerehardlyappliedduetotheirpoorsensitivity.ItisgenerallybelievedthatHallsensorsareonlyapplicableatmagneticfieldhigherthan0.5mT,andthatthemeasuredfieldmustbeperpendiculartothesensorssurface.Ontheotherhandmagnetoresistive(AMR)andfluxgatesensors(FG)areconsideredtobehighlysensitiveandwelladaptedforlowfields.However,AMRandFGsensorshavesomedrawbackscomparedtoHall-sensors.Theyareeithernotfullycompatiblewithmassfabricationatlowcostorwithlowpowerrequirements.ForexampleAMRsensorsrequireanintegratedset-resetcoilforlowfieldDCmeasurementsandarenotfullycompatiblewithconventionalCMOStech-nology.FluxgatesensorswererecentlyfullyintegratedonsingleCMOSchip[4]butduetotheprincipleofchoppingthemagneticfield,thesaturationofanopenloopcorere-quiresasignificantcurrentnotsuitableforlowpowerap-plication.Aneffectthatarisesformagneticfieldsensorscontainingofferromagneticmaterialisthattheoffsetofthesensorchangesafterexposuretoahighmagneticfield.Thiseffectisknownaspermingandcanbeexplainedbytheremnantfieldeffectofthematerial.Permingisacriticalissueforlowfieldmeasurementsandhastobeconsideredforcom-passapplications.InthispaperwepresentaconceptofahighsensitivityHall-sensorconsistingofCMOS-HallASICandIMC.StandardCMOStechnologyallowsforembeddingamplificationandnoisereductioncircuit,theA/Dconversion,calibrationandinterfacingonthesamechipaswellasforlowcostmassproduction.TheIMCfunctionsaspassivemagneticfieldamplifieranddramaticallyimprovesthesensorsperform-ance.0-7803-8692-2/04/$20.00©2004IEEE.1446Figure1:TheIMClocallyrotatesandamplifiesthemagneticfieldsothatitcanbemeasuredbystan-dardHallelementsX1andX2.Thefocusofthispaperisnotonlowpowerconsumption,butratherondemonstratingthatIMC-technologycanboostthemagneticsensitivityofCMOS-Hallsensorssufficientlyforcompassapplications.Anadditionalkeycharacteristicofthenewcompasssensoristhatitprovidesforalow-energymethodtovirtuallyeliminatethepermingeffect.IMCHall-SensorsTheideaofintegratingmagneticfluxconcentratorsdirectlyonHall-ASICswasfirstdescribedabout10yearsago[1].Theintegratedconcentratorconvertstheexternalmagneticfieldparallelwiththechipsurfacelocallyintoafieldper-pendiculartotheplane,whichissensedbyconventionalplanarHallelements.Figure1showsthecrosssectionofaconventionalCMOS-Hallchipcombinedwithaplanarmagneticconcentrator.ThecombinationofIMCandHallfeatureshighermagneticsensitivity,lowerequivalentoffsetandoffsetdriftcomparedtoconventionalHallsensors,sothattheweekearthmagneticfi