Red Rnge Anlysis of Pssive RFID Sysems fo Mnufcuing Conol Sysems M. Keskilmmi, L. Sydänheimo, P. Slonen, M. Kivikoski leconics Tmpee Univesiy of Technology P.O. Box 69, FIN-3311 Tmpee FINLAND Absc: - An RFID sysem cn be consideed s ieless communicion sysem becuse he ede device communices ih he nsponde by using elecomgneic ves dio fequencies. The pefomnce of his communicion link cn be sudied by deemining he ed nge fo bcksce RFID sysems. The ed nge o he disnce hee he ede uni noices he g depends on mny fcos. Fequency used fo idenificion, gin, oienion nd polizion of he ede nenn nd he nsponde nenn, nd he plcemen of he g on he objec o be idenified ill ll hve n impc on he RFID sysem s ed nge. [1] This ppe pesens n ppoch o nlyzing he ed nge of bcksce RFID sysem s funcion of fequency, nenn gin nd polizion mismch. Key-Wods: - RFID, Mnufcuing Conol, Tnspoion Sysems, Poducion Plnning 1 Inoducion In pssive RFID sysems he enegy needed fo he communicion is eniely fom he ede device nd i is no good sing ih bd nenns. To bsic pinciples fo d nsmission beeen pssive nsponde nd he ede device e piezoelecic nd bcksce o lod modulion mehods. In piezoelecic nspondes he eques signl fom he ede device is cpued by he g nenn nd conveed ino sufce ves. These sufce ves e hen efleced bck hough he convee o he ede device by using he g nenn. The idenificion numbe of he iem is coded ino hese efleced signls h cn be deeced. Pssive nspondes bsed on bcksce echnology use lod modulion fo communicion ih he ede. Moduled RF signl is emied fom he ede s nenn; popoion of hich eches he nsponde s nenn. Due o he RF field fom he ede nenn volge is induced he inpu eminls of he nsponde. This DC volge is used o chge cpcio o povide he bis fo he pocessing cicuiy (Figue 1. [3]). In he eun link fom nsponde o he ede popoion of incoming RF signl is bcksceed fom he nsponde s nenn bck o he ede nenn. The pocessing cicui of he nsponde chnges he RF impednce of he nsponde s nenn nd conols he moun of his sceed field. In his cse he modulion of he sceed field conins he idenificion infomion []. The nsponde is idenified hen he bcksceed field is eceived nd decoded by he ede uni. The ole of he nenns is emkble fo his kind of communicion sysem. Tx Tnsceive Rx Diecionl Couple Rede Moduled Reflecion R L Lod Resiso IC Tnsponde Figue 1. Bcksce RFID sysem Chceisics fo nsponde nenns used in RFID sysems e oulined in [5] s follos: - smll enough o be ched o he equied objec - hve omni diecionl o hemispheicl covege - mus povide mximum possible signl o he ASIC - hve polizion such s o mch he enquiy signl egdless of he physicl oienion of he poeced objec - obus - vey chep
Similly e cn define chceisics fo nenns used in RFID ede devices: - high dieciviy - high diion efficiency - lo side-lobe level o educe inefeence - diion pen opimized o he eding zone - obus - chep Thee e sevel ypes of nenns fo micove nge h cn be used fo RFID. Common nenn ypes fo his fequency nge e dipoles (ie, pined, folded), pches, PIFAs (Pln Inveed-F Anenn) nd helix nenns. The elecicl size of hese nenn ypes is compble o velengh of given fequency. Depending on he design used he gin nd he diion pen vies. The diion pen cn be omni diecionl ih pek gin of o dbi o diecionl hee he diion pen hs definie lobe nd he pek gin migh be sevel dbis [5]. The chceisics of nenns hve dicl effec on he ed nge of RFID sysems. In he folloing pgphs he effec of nenn popeies on ed nge of he pssive RFID sysems is sudied. Red Rnge Anlysis of Bcksce RFID Sysems Nex he ed nge of bcksce RFID sysems is sudied. In bcksce RFID sysems he eflecion of elecomgneic ves fom he objec is used fo he nsmission of d fom nsponde o ede [3]. The RF poe is popged in ll diecions by he nenn of he ede. The poe densiy S he locion of he nsponde is S = πr hee P is he nsmission poe of he ede, G is he gin of he nsmie nenn nd R is he disnce beeen ede nd nsponde. The nsponde eflecs poe P h is popoionl o he poe densiy S P = σ The d coss secion σ is mesue of n objec s biliy o eflec elecomgneic ves. Depending on he mching of he nsponde s nenn σ cn S (1) () vy fom o σ mx h is he cse hen he nenn eminls e sho cicuied of lef open. The nenn hus eflecs ll of he enegy iving i nd cs s eflecing sufce. σ mx = λ G π The folloing equion descibes he poe densiy S h finlly euns bck o he nenn of he ede. S σ λ G = = 3 ( π ) R ( π ) R The mximum eceived poe h cn be dn fom n nenn, given opiml lignmen nd coec polizion, is popoionl o he poe densiy of n incoming ve. The popoionliy fco denoed s he effecive e A e of he nenn is popoionl o is gin nd is he sme s σ mx nd is defined s A = λ π e G The ecepion poe P of he ede nenn is hen P = S A e = λ G ( π ) R The equion demonses h he ed nge of such bcksce RFID sysem is popoionl o he fouh oo of he nsmission poe of he ede. If ll he ohe hings being equl e mus muliply he nsmission poe by sixeen o double he nge. The ed nge R is solved fom he equion (6). λ R = π P G.1 ffec of Anenn Gin I is obvious h he nenn gin effecs on he ed/ie nge of he RFID sysem. Nex he effec of nenn gin on he ed nge is sudied The ed nge fo hee diffeen nsponde nenn ypes s funcion of ede nenn gin is illused in Figue. The ed nge is clculed (3) () (5) (6) (7)
Red nge (m) 3 5 15 1 5 Tg A (pch G=6.7 dbi) Tg B (folded dipole G=3.7 dbi) Tg C (dipole G=. dbi) 6 8 1 1 1 16 18 Rede nenn gin (dbi) Figue. ffec of nenn gin on ed nge (f =.5 GHz) fom he equion (7) fo he sysem in hich he sensiiviy of he ede P is 7 dbm nd he nsmied poe P dbm he fequency.5 GHz (λ =.1 m). Used vlues e ypicl fo bcksce RFID sysems opeing.5 GHz ISM bnd. Clculed vlues in Figue. e heoeiclly mximum ed nges hee he ede uni cn deec he eflecion fom he nsponde nenn unde idel condiions. Reflecions fom conducive sufces, bckgound noise nd nenn lignmen cu he ed nge don o les hlf of he heoeicl mximum hen he RFID sysem is opeing in el envionmen.. ffec of Fequency The fequency nges of opeion fo common bcksce RFID sysems e 915 MHz,.5 GHz nd 5.8 GHz. Wvelenghs fo hese fequencies e.38 m,.1 m nd.51 m coespondingly. In he equion (7) he ed nge is diecly popoionl o he velengh used. By using loe fequency, in ohe ods longe velengh, he ed nge cn be incesed. The ed nge s funcion of ede nenn gin fo 915 MHz,.5 GHz nd 5.8 GHz RFID sysems is pesened in Figue 3. The nsponde nenn used fo ed nge clculions is folded dipole nenn (G = 3.7 dbi). In some cses he RF signl fom he ede o he nsponde hs o popge hough n bsobing meil. The fequency used hs n effec on he popgion losses in he meil In Figue. he effec of fequency on dio ve enuion in ppe eel is pesened.[6] The enuion beeen o iple-bnd nenns s fis mesued in i Red nge (m) 5 5 35 3 5 15 1 5 915 MHz.5 GHz 5.8 GHz 6 8 1 1 1 16 18 Rede nenn gin (dbi) Figue 3. ffec of fequency on ed nge nd fe h hough he ppe eel so h he nsmiing nenn s inside he eel coe. The chceisic fequencies of he iple-bnd nenn used ee 5 MHz, 9 MHz nd 19 MHz. As e cn see fom he Figue. he enuion in he ppe eel inceses s he fequency ises. The enuion in he mesued ppe eel is 8 db @ 5 MHz, 1 db @ 9 MHz nd 16 db @ 19 MHz. db -1 - -3 - -5-6 -7-8 Ai: mesued Ppe A: mesued.3.5.7.9 1.1 1.3 1.5 1.7 1.9 GHz Figue. ffec of fequency on dio ve enuion in ppe eel Becuse he dimensions of nenns e popoionl o he velengh used, he loe fequency nd longe velengh mens inevibly lge nsponde size. In mos cses he size of n nenn is limiing fco fo miniuizing nspondes in RFID sysems. The lengh of folded dipole nenn h is commonly used s nsponde nenn is λ /. This mens h he size of nsponde in 915 MHz sysem is ppoximely 16 mm hile he sme ype of nsponde nenn in.5 GHz sysem fis ino 61 mm..3 ffec of Anenn Polizion Mismch The cse hee he polizion of he eceiving nenn is no he sme s he polizion of he
nsmiing nenn o he incoming ve cn be sed s polizion mismch. The moun of poe exced by he nenn fom he incoming signl ill no be mximum becuse of he polizion loss. [7] Assuming h he elecic field of he incoming ve cn be expessed s i = ρˆ hee ρˆ is he uni veco of he ve, nd he polizion of he elecic field of he eceiving nenn cn be ien s = ρˆ hee ρˆ is is polizion veco, he polizion loss cn hen be ken ino ccoun by inoducing polizion loss fco. Polizion fco (PLF) is defined s PLF = ρˆ ρˆ i = cosψ hee ψ p is he ngle beeen he uni veco of inciden ve nd he nenn polizion veco (Fig. 5.). ρˆ ρˆ ρˆ ψ p ρˆ p ρˆ ψ p (ligned) (oed) (ohogonl) Figue 5. Polizion of nenn nd inciden ve The polizion loss fco PLF expessed in decibels PLF(dB) = 1log1 PLF is illused in Figue 6. fo polizion mismch fom º o 9º. Fo nenn mislignmens unde 5º he poe loss is less hn 3 db. If he ngle of polizion mismch inceses he poe loss ss o incese significnly. ρˆ (8) (9) (1) (11) PLF (db) -5-1 -15 - -5-3 1 3 5 6 7 8 9 ψp (degees) Figue 6. PLF(dB) s funcion of polizion mismch ngle. Using Cicul Polizion Cicul polizion cn be obined in n nenn by feeding i ih o ohogonl, line field componens hving he sme mgniude nd ime phse diffeence of odd muliples of 9º. [7] φ = φ y φ x x + = - = y ( ½ + n) ( ½ + n) π, n =,1,,... π, n =,1,,... (1) (13) In some cses he use of cicully polized nenns on RFID ede impoves he sysem pefomnce. In h cse he effec of polizion mismch cn be negleced nd he ngle beeen he ede nenn nd he nsponde nenn hs no effec on ed nge. Hoeve if he nsponde nenn is linely polized nd he ede nenn is cicully polized hee is 3 db poe loss iespecive of ngle beeen he nenns comped o he cse hee he polizion mched linely polized nenns on boh he ede nd he g e used. This is due o he fc h he cicully polized field consiss of o line fields hving 9º phse shif nd he linely polized nenn in he nsponde noices h p of he field h mches is polizion. 3 Conclusion In his ppe he bsic pinciple of pssive RFID sysem using bcksceed ves s pesened. In pssive RFID sysems he enegy needed fo he communicion is eniely fom he ede device. Pssive RFID sysems e esy o pply o mnufcuing nd logisics conol sysems becuse he nspondes e chep, smll nd esy o fix o he objec o be idenified.
The effec of nenn popeies on ed nge nd he pefomnce of pssive bcksce RFID sysem s lso nlyzed. The ed nge in bcksce RFID sysems depends on he nsmied poe, he fequency used, he gin of he ede nd he g nenns nd he sensiiviy of he eceive. The uhoiies egule he nsmied poe fo cein fequency nge nd i cnno be exceeded. To impove he pefomnce of he RFID sysem e cn concene on nenns. By using high gin nenns, nenn ys o muliple nenns conneced o he ede uni he ed nge cn be incesed. The ed nge is diecly popoionl o he velengh used. By using loe fequency, in ohe ods longe velengh, he ed nge cn be incesed. If he nsponde is loced inside he objec o be idenified he enuion of he RF signl cn be decesed by using loe fequencies. Hoeve, he size of he nsponde is in mos cses he limiing fco fo he nenn sucue echniclly fesible. The size of he nsponde lso limis he fequency used becuse he size of he nenn is popoionl o he velengh. In mny pplicions fo mnufcuing conol he posiion of he objec o be idenified on conveyo is knon. In hese cses polizion mched linely polized nenns cn be used o mximize he ed nge nd he RFID sysem pefomnce. If he posiion nd he ngle beeen he nenns duing he idenificion even e no knon hee my be losses due o he polizion mismch. The effec of polizion mismch cn be negleced in hese cses by using cicully polized nenns in he RFID ede. Hoeve, if he nsponde nenn is linely polized nd he ede nenn is cicully polized he mximum ed nge is in ny cse less hn he ed nge fo polizion mched linely polized nenns. Refeences: [1] K. V. S. Ro, D. W. Dun, H. Heinich, On he Red Zone Anlysis of Rdio Fequency Idenificion Sysems ih Tnspondes Oiened in Abiy Diecions, Micove Confeence, 1999, Asi Pcific 1999. [] J. Sidén, P. Jonsson, T. Olsson, G. Wng, Pefomnce Degdion of RFID Sysem Due o he Disoion in RFID Tg Anenn, 11 h Inenionl Confeence Micove & Telecommunicion Technology 1 1 Sepembe 1, Sevsopol Cime, Ukine. [3] K. Finkenzelle, RFID Hndbook, John Wiley & Sons Ld. 1999. [] K. V. S. Ro, An Ovevie of Bck Sceed Rdio Fequency Idenificion Sysem (RFID), Micove Confeence, 1999, Asi Pcific 1999. [5] P. R. Fose, R. A. Bubey, Anenn Poblems in RFID Sysems, I Colloquium on RFID Technology, Ocobe 5, 1999, London, UK. [6] M. Keskilmmi, P. Slonen, L. Sydänheimo, M. Kivikoski, Rdio Wve Popgion Modeling in Ppe Reel fo Novel Rdio Fequency Idenificion Sysem, JmCon Technology fo conomic Developmen, Aug. 11-13,, Ocho Rios, Jmic. [7] C. A. Blnis, Anenn Theoy: Anlysis nd Design, nd ed., John Wiley & Sons Inc., USA, 1997