NETWORK-OF-REMOTE-SENSORS-FOR-MAGNETIC-DETECTION

NETWORKOFREMOTESENSORSFORMAGNETICDETECTIONA.Sheinker1,N.Salomonski1,B.Ginzburg1,A.Shkalim1,L.Frumkis2,B.Z.Kaplan21R&DIntegratedSystemsSection,PropulsionDivision,SoreqNRC,Yavne81800,Israel2DepartmentofElectricalandComputerEngineering,Ben-GurionUniversityoftheNegevP.O.Box653,Beer-Sheva84105,IsraelAbstractAnetworkofremotemagneticsensorsisusedfordetectionofferromagneticobjects.Themagneticsensorsarearrangedingradiometricpairsinordertocancelbackgroundnoise.Foraneffectivenoisecancellationthesensors'readingsaresynchronizedbyasmartsynchronizationtechnique.Datafromthesensorsaretransmittedviaawirelesslink,managedbyanintelligentaccessprotocol.Thedataarecollectedbyawirelessacquisitionunitwhichtransfersitseriallytoamonitoringstationforfurtherprocessing.ThemonitoringstationconsistsofaPCthatrunsanadvanceddetectionalgorithmwhichextractsasignalofmagneticdipoleoutofabackgroundnoise.Anadditionalfeatureistheabilitytodistinguishbetweentargetsandfalsealarms.Keywords:MagneticAnomalyDetection(MAD),Sensornetwork.1.IntroductionMagneticAnomalyDetection(MAD)[1]isapassivetechniquefordetectionofferromagneticobjects.ThemagneticfieldproducedbytheobjectismodeledasafieldofadipolewhichgeneratesananomalyintheambientEarthmagneticfield.WehavedevelopedanetworkofremotemagneticsensorsinordertodetecttheanomalyinEarthmagneticfield.Thesensorsarearrangedingradiometric[2]pairsinordertocancelcommoncomponentsoffarmagneticnoisesourcessuchasgeomagneticnoise[3].Eachremotesensorconsistsofasingle-axisfluxgatemagnetometer,aninterfacemodulewitha50Hz/60Hzdigitalnotchfilter,RFcommunicationcontroller,andabattery.Thedatafromthesensorsarecollectedbyawirelessacquisitionunit,whichtransfersitseriallytoamonitoringstationforfurtherprocessing.ThemonitoringstationconsistsofaPCthatrunsanadvancedalgorithmfordetectionofthemagneticanomaly.Thesystemarchitectureisdepictedinfigure1.2.MagneticAnomalyDetectionFartherawayfromtheobject,themagneticfieldproducedbytheobjectmaybeconsideredasamagneticdipolefield:()−⋅=3504),(rmrrrmrmBrrrrrrrπμ(1)Figure1Sensorsnetworkforremotedetectionofferromagneticobjects2whereBristhemagneticfieldthatisproducedbyanobjectwithamagneticmomentmratdisplacementrr.Here,BrcanbetreatedasanomalyintheambientEarthmagneticfield,EBr,asisgivenby:EmBBBrrr+=(2)3.TheRemoteMagneticSensingUnitTheRemoteMagneticSensingUnit(RMSU)consistsofasingle-axisfluxgatemagnetometer,aninterfacetothemagnetometerwitha50Hz/60Hzdigitalnotchfilter,RFcommunicationcontrollerandanenergysource.ThearchitectureoftheRMSUisdepictedinfigure2.Figure2TheinsidestructureoftheRMSUDuetopowersavingconsiderationswepreferredtouseasingle-axismagnetometerratherthanathree-axismagnetometer,althoughthelatteroneprovidesmoreinformationaboutthetarget.Thesingle-axismagnetometerhasbeeninstalledinthedirectionofEarthmagneticfieldinordertoreducemagneticinterferencescausedbyorientationinstability.WheninstalledinthedirectionofEarthmagneticfield,thereadingsofasingle-axismagnetometerresemblethereadingsofatotalfieldmagnetometer[2].Thechosensingle-axismagnetometerhastwoanalogoutputs.Adirectoutputwhichthevoltageisproportionaltothesensedmagneticfield,andafilteredoutputwhichisobtainedbyprocessingthedirectoutputbyaBandPassFilter(BPF),inordertoeliminatetheDCcomponent.TheRMSUisequippedwithalowpowernarrowbandRFtransceiver.Communicationrangeoffewhundredmetersmaybeachievedusingaλ/4antenna.Inordertoextendcommunicationrange,ahighergainantennaisavailable.Theoutputofthemagnetometerissampledatarateof500samplesperseconds.Thesamplesareprocessedthroughadigitalnotchfilterinordertosuppress50Hz/60Hzinterferences.Thefilterdecimatesthesamplesdowntoarateof10samplespersecond,whichissufficientforobjectdetectionaswellasforgradiometricnoisecancellation.Thedecimatedsamplesarestoredinabufferuntiladatarequestcommandisreceived,thenthebufferdataaretransmittedovertheRFcommunicationlink.CarefulmechanicalandelectricaldesignwasmadeinordertoeliminatemagneticinterferencescausedbythemodulesoftheRMSU.4.TheWirelessAcquisitionUnitTheWirelessAcquisitionUnit(WAU)coordinatesthedataflowfromtheRMSUsusinganadvancedaccessprotocolthatwasdesignedtomeettheparticularneedsofthesystem.ForeffectivebackgroundnoisecancellationthereadingsfromtheRMSUsshouldbesynchronized.ThesynchronizationisachievedthroughtheadvancedaccessprotocoloftheWAU.TheWAUscanstheRMSUssuccessivelybyinitiatingadatarequestcommandviatheRFcommunicationlink.OnlytheRMSUwiththeaddressedIDrespondsbytransmittingitsbufferdataovertheRFcommunicationlink.ThecommunicationbetweentheWAUandtheRMSUisorganizedinpacketswhichincludeapreamble,asynchronization3symbol,header,filteredreadingsfromthemagnetometer,andachecksumbyte.TheWAUisconfiguredtoresendimmediatelyaseconddatarequestcommandinthecaseofapacketloss.Ifthereisnovaliddatareceptionafterresendingdatarequestcommand,theninthenextscanadoubleamountofdatawillbesentfromtheRMSU.AsynchronizationcommandisgeneratedperiodicallybytheWAUforalltheRMSUsinordertosynchronizeallRMSUsclocks.ThedatafromalltheRMSUsarecollectedbytheWAUandtransferredseriallyforfurtherprocessingbythemonitoringstation.5