Thierry (troz@emmarin.ch) Vincent Fuentes (vfuentes@emmarin.ch) MICROE

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Thierry (troz@emmarin.ch) Vincent Fuentes (vfuentes@emmarin.ch) MICROE

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Using power transponders tags RFID applications.
Thierry (troz@emmarin.ch) Vincent Fuentes (vfuentes@emmarin.ch) MICROELECTRONIC MARIN 2074 MARIN Switzerland
Abstract
general description transponder families given aiming real RFID applications such animal objects identification, access control, immobilization, electronic purse, laundry tagging, ticketing. High security identification multi-tag identification applications addressed. Some parameters system discussed such antenna resonant frequencies adjustment, detuning effect, communication distance range system, transponder cost performances.
Introduction
Regarding rapid increase RFID market, many companies really specialized system design, having experience real time processing software hardware design tend such technology their applications where wireless communication greatly improve user comfort. transponder prices went down, high volume applications arose such immobilizing system, animal identification, retailing simplicity, only Frequency band systems (typically 125kHz) will addressed however these guidelines applied Medium Frequency band systems (typically 13.56MHz). RFID systems composed three components reader (also called transceiver), transponder (also called tag), computer data processing system. easily found such components products antenna circuits often provided they application dependent. first problem engineers faced which
antenna geometry should reach 50cm with this transponder integrated circuit third chapter gives some general rules guidelines calculate initial parameters RFID system such reader antenna geometry, transponder antenna geometry, emitted power fourth chapter will address called Read Only applications such access control, object tagging, anti-theft system. main principle these transponders that they have factory programmed identification code stored read-only Memory. fifth chapter will addressed called Read/Write applications where transponder carry store some software programmable data volatile memory. Typically, retail, cylinder identification, ticketing, pass applications need such feature. sixth chapter will focus cost RFID system, mainly target cost system.
System parameters
wireless communication achieved band based magnetic coupling resonant circuits (also called inductive circuits) tuned frequencies close possible. reader emits magnetic field when transponder passes through powers begins transmit on-chip stored data. signal generated reader usually provides timing information well enough energy power tag. communication range much smaller than signals wavelength simple model used forgetting what system designers commonly using such scattering parameters, reflection coefficient, impedance matching
Transceiver field signal Receiver Received Data VLtx k(d) Ltag Itag VLtag Ctag RLtag
Transponder Ztag
Vtag
Rtag
DATA MODULATION
Figure RFID system front-end equivalent circuit lossy transformer theory will used conjunction with magnetic field theory evaluate RFID systems performances. Figure gives schematic simplified image RFID system front-end including transceiver transponder view. represents reader antenna Ltag transponder antenna. Currently, Ctag Rtag given parameters transponder Generally coil used both sides, except immobiliser, animal identification where transponder antenna based windings running small ferrite. This increases antenna factor concentrates field force lines order compensate coupling factor reduction very small size transponder antenna. band, today, RFID market uses batteryless transponder, also called passive transponders, which means transponder derives power supply clock source from induced sine wave voltage antenna. first communication range parameter identified transponder powering distance mainly dependent reader antenna current circuit (Itx, Ltx, Ctx, Rtx) transponder parameters (Ltag, Ctag, Rtag). Calculating power received transponder done combining magnetic field formula (1), Lenz lossy transformer system equations (3).
Ltag
parameters meaning antenna windings voltage resistivity antenna current antenna radius distance between antennas antenna surface mutual inductance antenna inductance antennas angle (parallel evaluate main electrical parameters (received power, transponder modulation index induced reader antenna) antenna coupling factor known. This parameter depends only antennas geometry, distance between orientation. Combining (1), (2), leads coupling factor formula given (5). minimum coupling factor commonly used, this give first solution antennas geometry these parameter usually application constrained.
rLtx rLtag .cosq rLtx rLtag
rLtx
rLtx rLtx Ltag .cosq Ltag Ltag
example, figure shows coupling factor reader antenna radius 10cm transponder antenna radius with angle transponder power consumption normally specified through minimum power supply voltage associated current consumption. Therefore, interesting electrical parameter look voltage Vtag induced transponder. derived combining transponder circuit which gives continued page
4.50% 4.00% 3.50% coupling factor 3.00% 2.50% 2.00% 1.50% 1.00% 0.50% 0.00% Distance
Figure Coupling factor antennas spacing
18.00 16.00 Vtag, Transponder voltage 14.00 12.00 10.00 8.00 6.00 4.00 2.00 0.00 Ltag, Transponder antenna inductivity "Rtag=1meg" "Rtag=500k" "Rtag=100k"
Figure Transponder voltage transponder antenna value
Ltag Ltag Vtag Ltag Ltag 0tag Ltag Ltag Rtag
Ltag
Ltag Ltag
assuming (8), perfect tuning between transponder antenna circuit resonant frequency (w0) magnetic field frequency (w).
assuming (10), perfect tuning between field frequency reader antenna circuit resonant frequency (w0tx).
0tag
Ltag Ctag
(10)
Rtag Rtag Ctag
(11)
detuning between these frequencies induces reduction Vtag voltage. Figure shows voltage induced transponder antenna depending distance between antennas different transponder load assuming following parameters k=0.5%, Ltx=0.35mH, Itx=100mA, constant transponder factor Qtag=20, w0=2p.125kHz. practice, transponder antenna voltage will never reach values will limited chip voltage clamping elements. Using these curves, goal determined optimum value transponder antenna. Importance transponder power consumption clearly shown. Some general rules extracted from this first calculation maximize powering distance transponder maximize coupling factor increasing antenna geometry much application permits. maximize reader antenna current increasing reader power supply voltage maximize reader antenna factor much transponder data rate permits, much antenna drivers permit. maximize transponder antenna circuit Qtag factor power voltage operated transponder. second RFID system parameter which influences overall system performance reader sensitivity. This parameter divided factors input sensitivity demodulation chain transponder modulation induced reader antenna Factor given reader specific performances will discussed purpose this paper. Factor must maximized will derived with following calculation. main reader electrical parameter which contains transponder modulation antenna current (Itx). derivation using (3), gives
transponder modulation induced through Ztag variations. higher variations, longer communication range. Hence, right hand side term denominator variations have maximized. When transponder modulating state, Rtag maximum only depends transponder current consumption, hence Ztag expression (11) approximated follow
Ctag
(12)
Replacing Ztag with (12) gives minimum (13), assuming perfect tuning between transponder antenna circuit resonant frequency (w0) magnetic field frequency (w).:
(min)
Ltag
(13)
Ltag
Regarding (13), seen that Itx(min) does depend Ltag value Ltag over RLtag independent number windings. last assumption holds true normal coil using single conductor wire when skin effect proximity effect negligible. When transponder modulating state, Rtag minimum depends transponder modulator impedance, kilo-ohms quite common. this case, Ztag should possible negligible compared RLtag w.Ltag. this case, maximum given (14)
(max)
(14)
view (13) (14), following general guidelines used maximize transponder modulation induced reader antenna maximize transponder Qtag factor (RLtag min) maximize antennas coupling factor maximize transponder antenna value Ltag maximize reader antenna factor(RLtx min) much transponder data rate permits, much antenna drivers permit.
When having determined Ltag, antenna design phase start. complete this study, give (15) empirical formula antenna design which gives good agreement between calculation measurements.
0.08.
where number windings, coil diameter (cm) coil width (cm) coil thickness (cm)
(15)
Finally, this shows importance maximize antenna factor both sides, reader transponder there some limitations. frequency bandwidth reader antenna should forgotten function factor, transponder data rate must included this frequency bandwidth. Also, each antenna circuit frequency tuning tolerances component variations. reminded that simplify previous calculation, perfect frequency tuning between field frequency antennas resonant circuit been assumed. Therefore, after first system parameters been calculated using above formulas, second iteration process should done including effect frequency detuning check components tolerances meet overall system performance requirements.
Read Only transponder applications
When 1993, major German insurance company pressure either protect vehicles against massively increasing theft else renounce full insurance coverage, nobody believed that RFID then emerging technology would it's first major successes, still unbeaten numbers other application today. Objection raised reliability potential systems, their suitability automotive environment course, lack standards what then seemed mystic collection proprietary systems, seen major obstacle. extremely fast substantial development effort undertaken powerful automotive industry suppliers, resulting miniaturized "Inkey" system it's lock transceiver counterpart fitted cars early 1994 already. This first generation system, still fitted certain models most major makers, consisted read-only, brick type transponder, embedded ignition vehicle transceiver antenna with it's electronics package PCB, integrated around behind lock. functionality consists
safely cutting power supply starter, fuel pump, system ignition other system elements, required vehicle's operation during driving case, where presented does match prestored codes. Specifications miniaturized transponders were very stringent, requiring maximum read distance metal loaded environment over industry practice thermal ranges with only failure rate admitted! Three mainstream systems were adopted majority automotive industry have since been fitted millions cars with great success. Immobilizers today Europe part normal equipment cars whilst still standard rules neither application products, several systems have established facto standard wide public acceptance: What good automotive industry used many applications elsewhere! parallel, variety retrofit systems were developed protect vehicles already into circulation. Most these contactless systems rely same principle except that they keyholders that their transceivers mounted inside more convenient place dashboard. Their success market expected their need longer given, considering that almost European vehicles equipped with systems. possibility emulate code with unsophisticated reader/transponder blackbox hence circumvent immobilizer early 1995 origin development, aiming increased security features. widely felt, that barrier level high enough that target reduction thefts, mainly high-end models, could missed. Second generation products interface with several more motor management functions readwrite transponders feature encrypting communication with lock, rendering fraudulent attempts drive away with vehicle almost impossible. example, typical encrypted communication protocol called challenge/response described figure Statistics stolen cars, fitted with first generation immobilizers, showed however, that problem been dramatically reduced objective been exceeded. first RFID applications animal whilst this most useful application never came importance that should have done, still first domain with official standard. 11784/11785, released 1996/7 indeed down rules, applying variety products such implants, marks, bolus etc. intended establishing traceability, predominantly herd animals. standard
contested suppliers proprietary systems, used companionship animals, present upheld therefore still valid. standard prescribes frequency series communication protocols transponder level well numbering system embedded bits transponder memory. Criticism relate potential flaws uniqueness allocated numbers possibility fraud similar above described automotive fears, industry considers sophisticating application with additional security. however widely felt, that barrier read-only transponder quite sufficient reduce near insignificance
F(RN)
attempt circumvent target that additional features would only laden cost products, rendering application impossible agriculture. Current projects European Union clearly tend compliant system tested large scale several European Governments appear work setting-up necessary infrastructure. 1998 1999 will therefore another major application emerge read-only products, which will remain important product group years come.
IMMOBILIZER
G(RN)
Ltag
HIGH SECURITY TRANSPONDER
RLtag
Figure Challenge/Response authentication protocol example
Phase Immobilizer sends challenge message, formed Random number, followed date stream called f(RN) calculated with secret random number. Phase transponder upon reception challenge message checks f(RN). Phase challenge message passes transponder check, transponder calculates g(RN) number with random number same secret with another algorithm. This forms response message. Phase Immobilizer upon reception response message checks g(RN) declares authentication procedure successful.
turnstiles without having fumble skipass. access wrist watch this application equipped with kbit read write connected antenna within watch. system passive, does draw power from onboard battery, still operates within distance min. from under skidress reader station. Said transceiver will communicate with watch safely alter contents EEPROM memory reflect credit balance, access authority (for daily passes) etc. since watch remains with owner beyond skiing, application reduces waste generated with plastified cards like. Benefits resort include substantial increase data, captured throughout sites used statistical purposes, extension planning, punctual improvements etc. RFID technology accepted
Read Write transponder applications
September 1995, world's leading maker fashion watches Austrian leader electronic ski-ticketing systems agreed co-operation field contactless ski-ticketing. goal cooperation combine strength Austrians with innovative concept resulting advantages. Launched December 1995 Laax (Switzerland), concept today installed more than resorts worldwide. Hardly other application could underline better term hands-free operation readers practice skiing, have stood queue with gear their hands (and your very small children next you) minus 12°C will understand once advantages passing through skilift access
users same extent code and/or magnetic stripe tickets. watch application stands series opportunities, where digital interface between human beings machines greatly facilitates desired transaction; both time accuracy. fact that RFID watches perceived necessary, even fashionable accessory people will understanding public acceptance technique. Another important application field transponder multi-tag identification where sending information absolute requirement. applications like laundry automation, textile rental, inventory control, necessary identify count several transponders present same time within reader field. most known protocol called anticollision protocol developed CSIR (South Africa) licensed (United Kingdom). main principle this protocol that transponders messages very short randomly repeated during time. This supposes excellent chip random generator transponder Some additional features like temporary inhibition final inhibition commands allow reader mute identified transponder thus reducing number message collisions allowing higher number tags field.
cost system design
Particular emphasis given system design cost since most RFID applications become price sensitive shortly after start deployment phase; when number installed items and/or quantities become significant. System cost shall therefore mean application development cost, cost hardware including tags, readers, network hardware other peripherals well computing facilities, their installation commissioning finally cost system operation throughout it's planned lifecycle. Since quantifiable elements expected gains often difficult demonstrate beyond doubt, such system cost have kept possible, without attempting unprudently save money during development. Assuming majority development cost within proper translation performance requirements, i.e. definition intended functioning, creation software, setting-up operational procedures elements finally testing system components, lump available funding already gone. Unless application intended turnkey package sold many times "without adjustments" alternatively truly high
volume niche application with useful lifetime several years, development price system represents enlarge major cost application project, relegating cost transponders readers well below level attention that these items usually given. recent emergence single chip reader ASSP greatly improved potential cost, handheld, medium range readers band. Several semiconductor producers offer comparable solutions with auto-frequency adjust features (PLL loops) design simple, rugged transceiver units. Some these reader ASSP truly power simply interface with microcontroller designers choice, hence leaving vast room feature incorporation. Provided that designers want push communication performance beyond reason, customized readers developed less than months. increasing number products, ranging from boards through handheld computers with RFID reading capability RFDC infrared) interface readily available market their prices still falling. Often, system cost requirement driven cross-subsidizing policy, applied several system houses overall product/system pricing. consists squeezing cost "consumables" below publicly perceived level item value order maximize margins. lure consists overpricing mainly transponders tags with view recover underpriced application development costly adjustments other, associated hardware items. This proceeding increasingly dangerous with emerging component producers, entering market selling their hardware offthe-shelf commodity; mainly when "consumables" mainstream technology therefore system compatible. design LF-transponders, i.e. tags range make use, least, actually heart features, connected copper antenna coil, sometimes even capacitor. Miniaturized type transponders used animal implantable carriers furthermore ferrite essence, single most costly item remains therefore, considerations wire optimization, bobbins etc. ignored purpose this paper. Designs nowadays incorporate single chip solutions, mounted PCB, TAB, lead frame even better, connected without further carrier structure directly antenna. trade-off between onboard capacitor (single chip) multicomponent subassembly, instance required half duplex systems clearly favors terms cost reliability products, that minimize number built-in components. Finally, consideration given packaging transponders line
with intended lifetime, even with respect ultimate disposal after it's useful life. Widespread cheap plastic encapsulation, including laminates well reduce item cost later become unacceptable. Analyzing straightforward Access Control Application with add-on benefit shopfloor data collection smaller company employees) would show some transceiver stations area protection data collection maybe tags employees. system would have wired company's facility off-the-shelf software adjusted company's particular situation (Logo display etc.) prices $400.- transceiver $5.- badges estimation installation wiring $2000.-, would invest $7000.- such system with semiconductor content less that $300.- course, time takes issue badges (with personal photo?), re-engineer operation explain (and re-explain) system each employee would have added. enough attention given determination saving potential productivity increases, resulting RFID application. widely felt that many current installations fruit "imperative" situation some "gadget" influential group people, insisting their "solution". Overall system cost these cases minimum that away with needless that such systems operate with restrictions that reputation auto solutions suffers.
Bertin, Faroux, Renault, DUNOD collection. P4022, Contactless Identification Device preliminary data sheet, CSIR, EM-Microelectronic MARIN V4070, high security authentication Contacless Device EM-Microelectronic MARIN
Conclusions
simple theory been developed allow specialist build RFID systems using products like transponder RFID reader board. Some major applications have been described showing large potential RFID market. Very high volume applications about emerge requiring optimized cost solutions like Retail contactless smart card applications.
Etienne, Lecteurs Access ASULAB Internal report 1997 RFID works profit from Technologies. http ://www.id-tech.com Buesser, private communication, EM-Microelectronic MARIN
References

Thierry (troz@emmarin.ch) Vincent Fuentes (vfuentes@emmarin.ch) MICROE

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