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外文文献CurrentRFIDTechnologyThissectiondescribesoutofwhichpartsRFIDtagsconsistof,howtheyworkinprinciple,andwhattypesoftagsdoexist.Itfocusesonhowtagsarepoweredandwhatfrequencyrangesisused.Thesectionconcludesbycoveringafewimportantstandards.RFIDtransponders(tags)consistingeneralof:Microchip,Antenna,Case,Battery(foractivetagsonly)ThesizeofthechipdependsmostlyontheAntenna.Itssizeandformisdependentonthefrequencythetagisusing.Thesizeofatagalsodependsonitsareaofuse.Itcanrangefromlessthanamillimeterforimplantstothesizeofabookincontainerlogistic.Inadditiontothemicrochip,sometagsalsohaverewritablememoryattachedwherethetagcanstoreupdatesbetweenreadingcyclesornewdatalikeserialnumbers.ARFIDtagisshowninfigure1.Theantennaisclearlyvisible.Assaidbeforetheantennahasthelargestimpactofthesizeofthetag.Themicrochipisvisibleinthecenterofthetag,andsincethisisapassivetagitdoesnothaveaninternalpowersourceInprincipleanRFIDtagworksasfollows:thereadingunitgeneratesanelectro-magneticfieldwhichinducesacurrentintothetag'santenna.Thecurrentisusedtopowerthechip.Inpassivetagsthecurrentalsochargesacondenserwhichassuresuninterruptedpowerforthechip.Inactivetagsabatteryreplacesthecondenser.Thedifferencebetweenactiveandpassivetagsisexplainedshortly.Onceactivatedthetagreceivescommandsfromthereadingunitandrepliesbysendingitsserialnumberortherequestedinformation.Ingeneralthetagdoesnothaveenoughenergytocreateitsownelectro-magneticfield,insteaditusesbackscatteringtomodulate(reflect/absorb)thefieldsentbythereadingunit.Becausemostfluidsabsorbelectro-magneticfieldsandmostmetalreflectthosefieldsthereadingoftagsinpresenceofthosematerialsiscomplicated.Duringareadingcycle,thereaderhastocontinuouslypowerthetag.Thecreatedfieldiscalledcontinuouswave,andbecausethestrengthofthefielddecreaseswiththesquareofthedistancethereadershavetousearatherlargepower.Thatfieldoverpowersanyresponseatagcouldgive,sothereforetagsreplyonside-channelswhicharelocateddirectlybelowandabovethefrequencyofthecontinuouswave.1.EnergySourcesWedistinguish3typesofRFIDtagsinrelationtopowerorenergy:Passive,Semi-passive,ActivePassivetagsdonothaveaninternalpowersource,andtheythereforerelyonthepowerinducedbythereader.Thismeansthatthereaderhastokeepupitsfielduntilthetransactioniscompleted.Becauseofthelackofabattery,thesetagsarethesmallestandcheapesttagsavailable;howeveritalsorestrictsitsreadingrangetoarangebetween2mmandafewmeters.Asanaddedbenefitthosetagsarealsosuitabletobeproducedbyprinting.Furthermoretheirlifespanisunlimitedsincetheydonotdependonaninternalpowersource.Thesecondtypeoftagsissemi-passivetags.Thosetagshaveaninternalpowersourcethatkeepsthemicrochippoweredatalltimes.Therearemanyadvantages:Becausethechipisalwayspowereditcanrespondfastertorequests,thereforeincreasingthenumberoftagsthatcanbequeriedpersecondwhichisimportanttosomeapplications.Furthermore,sincetheantennaisnotrequiredforcollectingpoweritcanbeoptimizedforbackscatteringandthereforeincreasingthereadingrange.Andlastbutnotleast,sincethetagdoesnotuseanyenergyfromthefieldthebackscatteredsignalisstronger,increasingtherangeevenfurther.Becauseofthelasttworeasons,asemi-activetaghasusuallyarangelargerthanapassivetag.Thethirdtypeoftagsisactivetags.Likesemi-activetagstheycontainaninternalpowersourcebuttheyusetheenergysuppliedforboth,topowerthemicrochipandtogenerateasignalontheantenna.Activetagsthatsendsignalswithoutbeingqueriedarecalledbeacons.Anactivetag'srangecanbetensofmeters,makingitidealforlocatingobjectsorservingaslandmarkpoints.Thelifetimeisupto5years.2.FrequencyBandsRFIDtagsfallintothreeregionsinrespecttofrequency:Lowfrequency(LF,30-500kHz),Highfrequency(HF.10-15MHz),Ultrahighfrequency(UHF,850-950MHz,2.4-2.5GHz,5.8GHz)Lowfrequencytagsarecheaperthananyofthehigherfrequencytags.Theyarefastenoughformostapplications,howeverforlargeramountsofdatathetimeataghastostayinareadersrangewillincrease.Anotheradvantageisthatlowfrequencytagsareleastaffectedbythepresenceoffluidsormetal.Thedisadvantageofsuchtagsistheirshortreadingrange.Themostcommonfrequenciesusedforlowfrequencytagsare125-134.2kHzand140-148.5kHz.HighfrequencytagshavehighertransmissionratesandrangesbutalsocostmorethanLFtags.Smarttagsarethemostcommonmemberofthisgroupandtheyworkat13.56MHz.UHFtagshavethehighestrangeofalltags.Itrangesfrom3-6metersforpassivetagsand30+metersforactivetags.Inadditionthetransmissionrateisalsoveryhigh,whichallowstoreadasingletaginaveryshorttime.Thisfeatureisimportantwheretaggedentitiesaremovingwithahighspeedandremainonlyforashorttimeinareadersrange.UHFtagsarealsomoreexpensivethananyothertagandareseverelyaffectedbyfluidsandmetal.ThosepropertiesmakeUHFmostlyusefulinautomatedtollcollectionsystems.Typicalfrequenciesare868MHz(Europe),915MHz(USA),950MHz(Japan),and2.45GHz.FrequenciesforLFandHFtagsarelicenseexemptandcanbeusedworldwide;howeverfrequenciesforUHFtagsdifferfromcountrytocountryandrequireapermit.3.StandardsThewiderangeofpossibleapplicationsrequiresmanydifferenttypesoftags,oftenwithconflictinggoals(e.g.lowcostvs.security).Thatisreflectedinthenumberofstandards.Ashort