word文檔可自由復(fù)制編輯word文檔可自由復(fù)制編輯word文檔可自由復(fù)制編輯外文文獻(xiàn)CurrentRFIDTechnologyThissectiondescribesoutofwhichpartsRFIDtagsconsistof,howtheyworkinprinciple,andwhattypesoftagsdoexist.Itfocusesonhowtagsarepoweredandwhatfrequencyrangesisused.Thesectionconcludesbycoveringafewimportantstandards.RFIDtransponders(tags)consistingeneralof:Microchip,Antenna,Case,Battery(foractivetagsonly)ThesizeofthechipdependsmostlyontheAntenna.Itssizeandformisdependentonthefrequencythetagisusing.Thesizeofatagalsodependsonitsaofuse.Itcanrangefromlessthanamillimeterforimplantstothesizeofaboocontainerlogistic.Inadditiontothemicrochip,sometagsalsohaverewritablememoryattachedwherethetagcanstoreupdatesbetweenreadingcyclesornewdatalikeserialnumbers.ARFIDtagisshowninfigure1.Theantennaisclearlyvisible.Assaidbeforetantennahasthelargestimpactofthesizeofthetag.Themicrochipisvisibleincenterofthetag,andsinceisapassivetagitdoesnothaveaninternalthi powersourceInprincipleanRFIDtagworksasfollows:thereadingunitgeneratesanelectro-magneticfieldwhichinducesacurrentintothetag'santenna.Thecurrentusedtopowerthechip.Inpassivetagsthecurrentalsochargesacondenserwhichassuresuninterruptedpowerforthechip.Inactivetagsabatteryreplacesthecondenser.Thedifferencebetweenactiveandpassivetagsisexplainedshortly.Onceactivatedthetagreceivescommandsfromthereadingunitandrepliesbysendingitserialnumberortherequestedinformation.Ingeneralthetagdoesnothaveenoughenergytocreateitsownelectro-magneticfield,insteaditusesbackscatteringtomodulate(reflect/absorb)thefieldsentbythereadingunit.Becausemostfluidsabsorbelectro-magneticfieldsandmostmetalreflectthosefieldsthereadingoftinpresenceofthosematerialsiscomplicated.Duringareadingcycle,thereaderhastocontinuouslypowerthetag.Thecreatedfieldiscalledcontinuouswave,andbecausethestrengthofthefielddecreaseswithesquareofthedistancethereadershavetousearatherlargepower.Thatfieldoverpowersanyresponseatagcouldgive,sothereforetagsreplyonside-channelswhicharelocateddirectlybelowandabovethefrequencyofthecontinuouswave.1.EnergySourcesWedistinguish3typesofRFIDtagsinrelationtopowerorenergy:Passive,Semi-passive,ActivePassivetagsdonothaveaninternalpowersource,andtheythereforerelonthepowerinducedbythereader.Thismeansthatthereaderhastokeepupitsfielduntiltransactioniscompleted.Becauseofthelackofabattery,thesetagsarethesmallestandcheatagsavailable;howeveritalsorestrictsitsreadingrangetoarangebetween2mmandafewmeters.Asanaddedbenefitthosetagsarealsosuitabletobeproducedbyprinting.Furthermotheirlifespanisunlimitedsincetheydonotdependonaninternalpowersource.Thesecondtypeoftagsissemi-passivetags.Thosetagshaveaninternalpowersourcethatkeepsthemicrochippoweredatalltimes.Therearemanyadvantages:Becausethechipisalwayspowereditcanrespondfastertorequests,thereforeincreasingthenumberoftagsthatcanbequeriedpersecondwhichisimportanttosomeapplications.Furthermore,sincetheantennaisnotrequiredforcollectingpowitcanbeoptimizedforbackscatteringandthereforeincreasingthereadingrange.Alastbutnotleast,sincethetagdoesnotuseanyenergyfromthefieldthebackscatteredsignalisstronger,increasingtherangeevenfurther.Becauseofthelastreasons,asemi-activetaghasusuallyarangelargerthanapassivetag.Thethirdtypeoftagsisactivetags.Likesemi-activetagstheycontainanintepowersourcebuttheyusetheenergysuppliedforboth,topowerthemicrochipandtogenerateasignalontheantenna.Activetagsthatsendsignalswithoutbeingqueriedarecalledbeacons.Anactivetag'srangecanbetensofmeters,makingitidforlocatingobjectsorservingaslandmarkpoints.Thelifetimeisupto5years.2.FrequencyBandsRFIDtagsfallintothreeregionsinrespecttofrequency:Lowfrequency(LF,30-500kHz),Highfrequency(HF.10-15MHz),Ultrahighfrequency(UHF,850-950MHz,2.4-2.5GHz,5.8GHz)Lowfrequencytagsarecheaperthananyofthehigherfrequencytags.Theyarefastenoughformostapplications,howeverforlargeramountsofdatathetimeataghaststayinareadersrangewillincrease.Anotheradvantageisthatlowfrequencytagsleastaffectedbythepresenceoffluidsormetal.Thedisadvantageofsuchtagsistshortreadingrange.Themostcommonfrequenciesusedforlowfrequencytagsare125-134.2kHzand140-148.5kHz.HighfrequencytagshavehighertransmissionratesandrangesbutalsocostmorethanLFtags.Smarttagsarethemostcommonmemberofthisgroupandtheyworkat13.56MHz.UHFtagshavethehighestrangeofalltags.Itrangesfrom3-6metersforpassivetagsand30+metersforactivetags.Inadditionthetransmissionrateisveryhigh,whichallowstoreadasingletaginaveryshorttime.Thisfeatureisimportantwheretaggedentitiesaremovingwithahighspeedandremainonlyforashorttimeinareadersrange.UHFtagsarealsomoreexpensivethananyothertagandareseverelyaffectedbyfluidsandmetal.ThosepropertiesmakeUHFmostlyusefulinautomatedtollcollectionsystems.Typicalfrequenciesare868MHz(Europe),915MHz(USA),950MHz(Japan),and2.45GHz.FrequenciesforLFandHFtagsarelicenseexemptandcanbeusedworldwide;howeverfrequenciesforUHFtagsdifferfromcountrytocountryandrequireapermit.3.StandardsThewiderangeofpossibleapplicationsrequiresmanydifferenttypesoftags,oftenwithconflictinggoals(e.g.lowcostvs.security).Thatisreflectedintheofstandards.AshortlistofRFIDstandardsfollows:ISO11784,ISO11785,ISO14223,ISO10536,ISO14443,ISO15693,ISO18000.Notethatthislistisnotexhaustive.SincetheRFIDtechnologyisnotdirectlyInternetrelateditisnotsurprisingthatarenoRFCsavailable.TherecenthypearoundRFIDtechnologyhasresultedinanexplosioninpatents.Currentlythereareover1800RFIDrelatedpatentsissued(fro1976to2001)andover5700patentsdescribingRFIDsystemsorapplicationsarebacklogged.4.RFIDSystemsARFIDreaderandafewtagsareingeneraloflittleuse.Theretrievalofasenumberdoesnotprovidemuchinformationtotheusernordoesithelptokeeptrackofitemsinaproductionchain.TherealpowerofRFIDcomesincombinationwithabackendthatstoresadditionalinformationsuchasdescriptionsforproductsandwhereandwhenacertaintagwasscanned.IngeneralaRFIDsystemhasastructureasdepictedinfigure2.RFIDreadersscantags,andthenforwardtheinformationtthebackend.Thebackendingeneralconsistsofadatabaseandawelldefinedapplicationinterface.Whenthebackendreceivesnewinformation,itaddsittothedatabaseandifneededperformssomecomputationonrelatedfields.Theapplicationretrievesdatafromthebackend.Inmanycases,theapplicationiscollocatedwithtreaderitself.Anexampleisthecheckoutpointinasupermarket(NotethatthegivexampleusesbarcodesinsteadofRFIDtagssincetheyaremorecommon;however,thesystemwouldbehaveinexactlythesamewayiftagswereused).Whenthereaderscansthebarcode,theapplicationusesthederivedidentifiertolookupthecurrentprice.Inaddition,thebackendalsoprovidesdiscountinformationforqualifyingproducts.Thebackendalsodecreasesthenumberofavailableproductsofthatkindandnotifiesthemanageriftheamountfallsbelowacertainthreshold.ThissectiondescribeshowRFIDtagsworkingeneral,whattypesoftagsexistandhowtheydiffer.ThethreefrequencyrangesthatRFIDtagstypicallyuseareLFHF,andUHF.Alsothedifferencebetweenpassive,semi-passive,andactivetagswasexplainedandtheiradvantagesanddisadvantageswerecompared.Thesectionconcludedbylookingatdifferentstandardsandshowedthegreatinterestoftheindustrybycountingthenumberofissuedandbackloggedpatents[USPatentOffice].翻譯：當(dāng)前的RFID技術(shù)該節(jié)描述的是RFID標(biāo)簽由哪些部分組成、工作原理和確實(shí)存在的標(biāo)簽類型，關(guān)注標(biāo)簽的供電方式和使用頻率范圍。這部分也總結(jié)了一些重要的標(biāo)準(zhǔn)。RFID應(yīng)答器的一般組成:微芯片、天線、線圈和電池(僅適用于有源標(biāo)簽)。芯片的大小主要取決于天線，它的規(guī)模和形式的取決于標(biāo)簽的使用頻率，也取決于它的使用面積。它的大小范圍可從不到一毫米的植入體大到一本關(guān)于集裝箱物流的書。除了微型芯片,有些標(biāo)簽也附有可重寫內(nèi)存，這樣標(biāo)簽就可儲(chǔ)存更新閱讀周期之間的或新的數(shù)據(jù)，如序號(hào)。如圖1所示的RFID標(biāo)簽。天線清晰可見。正如前面所說(shuō)的，天線對(duì)標(biāo)簽大小的影響最大。在標(biāo)簽的中心可看見的是芯片。因?yàn)檫@是一個(gè)無(wú)源標(biāo)簽所以無(wú)內(nèi)部的能源。RFID標(biāo)簽工作原理是如下:閱讀單元產(chǎn)生電磁場(chǎng)引導(dǎo)電流流進(jìn)標(biāo)簽的天線。該電流用以給芯片提供能源。在無(wú)源標(biāo)簽中該電流還為冷凝器充電，以保證芯片的不間斷供電。在有源標(biāo)簽中電池取代了冷凝器。有源和無(wú)源標(biāo)簽的區(qū)別是短期內(nèi)的信息闡釋。一旦被激活的標(biāo)簽收到閱讀的命令它就可以發(fā)送序列號(hào)或所要求的信息?？偟膩?lái)說(shuō),標(biāo)簽沒有足夠的能量來(lái)創(chuàng)造自己的電磁場(chǎng),相反它可以采用反向散射調(diào)制（反映/吸收）來(lái)產(chǎn)生由閱讀單元發(fā)射的電磁場(chǎng)。由于大多數(shù)流體吸收電磁場(chǎng)和大多數(shù)金屬反射這些場(chǎng)，故可使用的標(biāo)簽閱讀材料是復(fù)雜的在一次循環(huán)解讀中,閱讀器不得不持續(xù)給標(biāo)簽供電。它所建立的場(chǎng)將產(chǎn)生連續(xù)波,因?yàn)榇艌?chǎng)的強(qiáng)度隨距離的平方而減少，故閱讀器必須有一個(gè)相當(dāng)大的能源。該場(chǎng)迅速響應(yīng)標(biāo)簽給的任何指示,因此標(biāo)簽位于正下方的側(cè)渠道可以響應(yīng)上述連續(xù)波的頻率。1.能源我們辨別三種不同的RFID電子標(biāo)簽的能量或能源：被動(dòng)、半被動(dòng)和主動(dòng)。被動(dòng)式標(biāo)簽沒有內(nèi)部電源,因此它們的能量來(lái)源于閱讀器。這意味著閱讀器必須保持磁場(chǎng)直到轉(zhuǎn)換完成。由于沒有電池,故這些都是可用的最小和最便宜的標(biāo)簽。但它的閱讀范圍可從2毫米和幾米。這些標(biāo)簽的另一個(gè)好處是適用于印刷生產(chǎn)。此外，因?yàn)樗灰蕾囉趦?nèi)部電源，所以它們的壽命是無(wú)限的。第二種類型是半被動(dòng)式標(biāo)簽。這些標(biāo)簽都有內(nèi)部電源可在任何時(shí)候都給微芯片供電。它有許多優(yōu)點(diǎn):由于芯片在持續(xù)帶電的情況下反應(yīng)迅速,因此可以增加每秒查詢的標(biāo)簽數(shù)量，這是非常重要的應(yīng)用。此外,由于天線不需要收集能量，故可以優(yōu)化用以反向散射和回歸來(lái)增加閱讀范圍。最后但并非不重要，因?yàn)闃?biāo)簽不使用任何磁場(chǎng)能量所以反向散射的信號(hào)越強(qiáng)，閱讀范圍更廣。由于最后兩個(gè)原因，半被動(dòng)標(biāo)簽通常比被動(dòng)標(biāo)簽應(yīng)用范圍更廣泛。第三種類型是主動(dòng)式標(biāo)簽。類似于半主動(dòng)標(biāo)簽，它的內(nèi)部也有能源但它的能源用于兩個(gè)方面：給微芯片供電和使天線產(chǎn)生信號(hào)。主動(dòng)式標(biāo)簽發(fā)送信號(hào)而不被質(zhì)疑,這被稱為信標(biāo)。主動(dòng)標(biāo)簽可查詢的范圍是幾十公尺,從而使其適宜于定位對(duì)象或理想標(biāo)志點(diǎn)。壽命長(zhǎng)達(dá)5年的。2.頻帶RFID電子標(biāo)簽按照頻率分為三個(gè)部分:低頻(LF,30-500千赫)、高頻(HF、10-15兆赫)、超高頻(UHF),（850-950兆赫,2.4-2.5兆赫,5.8兆赫)。低頻標(biāo)簽比任何高頻率的標(biāo)簽都便宜。對(duì)于大多數(shù)應(yīng)用程序來(lái)講,它們的響應(yīng)速度很快。但是，留在閱讀器的大量標(biāo)記數(shù)據(jù)的時(shí)間范圍將增加。另一個(gè)優(yōu)點(diǎn)是低頻標(biāo)簽由于流體的存在或金屬的存在而受到的影響最小。這類標(biāo)簽的缺點(diǎn)是它們識(shí)別范圍很短。最常見的低頻標(biāo)簽頻率是125-134.2千赫和140-148.5千赫。高頻標(biāo)簽有更高的傳輸速率和更廣的范圍，成