Optimization of the law of vaiation of shunt egulato impedance fo Poximity Contactless Smat Cad Applications to educe the loading effect. Catheine Maechal, Dominique Paet. Laboatoie LIT ESIGETEL, ue du Pot de VALVINS - 770 Avon, FANCE Email : catheine.maechal@esigetel.f Abstact. These pape deals with FID system and in paticula with the loading effect. We often confuse the load-modulation with the loading effect. If the load modulation is well known, it is not the case of the loading effect. We focus in paticula on the loading effect due to the shunt egulato in the tag. We seach a law of these shunt egulato to impove and/o educe the loading effect. Intoduction. FID (adio Fequency Identification) systems ae moe and moe pesent commecially. Contactless identification systems involve a base station / inteogato communicating with tags/ badges/ tanspondes/ smat cads/ via optical/infa ed o adiofequency (HF, UHF) links. This pape deals with inductive coupling F link between base station and tansponde. In this pape, as an example, we descibe a poximity contactless smat cad in accodance with the ISO 4 443 standad. The load modulation and the loading effect have the same physical basis. The cuent in the antenna of the base station vaies with the load of the tag. In the fist case, with the load modulation, we want this vaiation to assue the communication between the tag and the base station thanks to the etomodulation of the load. The loading effect is due to diffeent factos: the nominal load : nominal consumption of the IC, communications ate vaiation of the dynamic applicative load : embedded cypto copocessos consuming moe enegy duing phases of activity, dynamic vaiation of the communication ate the envionment : pesence of coppe, feite o a hand last but not least, a paallel shunt egulato acting as a function of the distance a) on one hand, as a voltage egulato to supply the smat cad IC b) on the othe hand as a powe vaiation dampe against incoming powe fom the base station. This loading effect will be emphasized in the coming yeas because futue applications will need:
Catheine Maechal, Dominique Paet. lage memoies sizes, lage calculation facilities using highe calculation / clock speeds equiing highe consumption fo a highe communication ate moe sophisticated secuity (embedded cypto copocessos) consuming moe enegy duing phases of activity new electomagnetic envionments due to the use of NFC (Nea Field Communication) fo Mobile Phone applications. In the applications, sometimes, in specific cases, some base stations communicate coectly with a smat cad at full ange ( 5 cm), but not at shot distance (0 to 3 4 cm), due to the «loading effect». This esults of a lack of tansmitted powe due to stong influence of the load of the cad to the base station. Pape [] poposes an adaptive matching netwok at the base station in ode to impove poo powe tansfe efficiency. The goal of the pape is devoted to study an othe way to educe the loading effect that is to find a paticula law of the shunt impedance as a function of the distance between the base station and the smat-cad. The pape summaizes the physical oigins of the loading effects and shows, in a diffeent way that in [] [] [6], the base station antenna cuent equation - including shunt egulato influence - as a function of the distance between the base station and a smat cad. In fact the magnetic field H ceated by the base station is popotional to the cuent value flowing into the base station antenna. A modeling of the system (including magnetic coupling) of the base-station and smat cad is used to calculate the loaded/unloaded (with o without cad) base station antenna cuent atio (I /I 0 ) as a function of the distance between the base station and the smat cad. The simulation shows that the cuent (I 0 ) flowing into the base station antenna and the elated magnetic field ceated by the base station could be damatically educed when the smat cad is vey close to the base station. This explains why communication can cease due to a lack of enegy tansfe. Fo moe pecision, we simulate also the magnetic field at the smat cad at a distance d of the base station. esults ae discussed below. Loaded/unloaded base station antenna cuent atio. Modeling of magnetic coupling between base station and contactless smat cad Fig. shows the modeling of the magnetic coupling between base station and a contactless smat cad [].
Optimization of the law of vaiation of shunt egulato impedance fo Poximity Contactless Smat Cad Applications to educe the loading effect. 3 I -jmωi -jmωi shunt I I Fig.. Modeling of the coupling between base station and contactless smat cad. Whee the components ae descibed below with thei value fo the simulations. Fo the base-station Capacitive coupling netwok antenna to the base station s output stage (Ca 43pf, Ca 33 pf); Base station antenna inductance and esistance (L 500nH, 0.7 Ω); Output impedance of base station amplifie (30 Ω) ; Equivalent Thevenin voltage of the base station s amplifie Fo the smat cad Smat cad antenna inductance and esistance (L 4 µh, 0 Ω); Total smat cad capacito (IC equivalent input capacito + antenna stay capacito + packaging) (C 4 pf); IC equivalent load esistance (this value depends on the IC opeating conditions) (L 50 kω); IC modulation esistance fo data communication. (aveage value M 5 kω); M : mutual inductance between antennae; Shunt esistance to avoid ovevoltage, shunt. The pupose of this pape is to define and optimize this shunt esistance law. The diect esolution of the equations issued fom the oiginal diagam (Fig. ) leads to a complicated equation. Fo this eason, this diagam is tansfomed into a new one pesented in Fig.. U B jx B jx -jmωi -jmωi Tag I I Fig.. Simplified diagam of the system base station / smat cad with BS eq + () jx BS + jceqω jl ω () tag + 7 (3)
4 Catheine Maechal, Dominique Paet. jx Tag + jlω jc ω 7 5 6 + Qc (4) (5) whee 6 shunt + L + M (6) Q c 6Cω (7) And with Cω C 5ω + Q c (8) Zeq eq + jx eq jc ω + a jcaω + + jc ω jc ω Now let s have a look on unloaded and loaded situations. unloaded - Without smat cad We note I 0, the unloaded base station antenna cuent. a U I0 BS + jx BS a (9) (0) loaded - With a smat cad in the communication ange. ( BS + jx BS ) U jm ω I () I jmω I ( I () Tag + jx Tag ) Fom this equation we find the loaded (I ) /unloaded(i 0 ) base station antenna cuent atio : I + BS jx BS I0 M ω + + M ω BS Tag j X + BS XTag Tag XTag Tag + XTag (3) with M k L L (4)
Optimization of the law of vaiation of shunt egulato impedance fo Poximity Contactless Smat Cad Applications to educe the loading effect. 5 and k µ 0 n 3 ns [3, p3] + d L L ( ) whee µ 0 4π0-7 H/m magnetic pemeability, : base station antenna adius (0.03m). n : base station antenna tun numbe (n ) n : smat cad antenna tun numbe. We simulate with diffeent values of n. s : smat cad antenna one tun aea (0.004 m ) d : distance of the smat-cad fom the base station. Shunt esistance Shunt aleady intoduced into calculation by the way of tag and X tag, gives a vey complex fomulation of equation (3). Shunt is vey difficult to be isolated fom this equation in ode to define and optimize its liteal law of vaiation. So, to find the shunt esistance vaiations as a function of distance in ode to obtain a cuent atio as high as possible, we simulate the cuent atio I /I 0 as a function of both a) the distance between smat cad and base station, and b) shunt esistance value. (5) Simulations We aleady intoduced the values used fo simulations. Simulation with diffeent values of n. We simulate the cuent atio with seveal numbe of tuns of the smat-cad antenna, (n, 4, 8), distance between base station and smat cad fom 0m to 0.m (0 cm) and shunt esisto fom 30 Ω to 00 kω. (fig 3 to 5). Theses figues show that when the smat cad is fa fom the base station (0 cm), the cad has no effect on the cuent flowing into the base station antenna (loaded/unloaded base station antenna cuent atio equals ). Fo vey shot distance (below cm), the cuent atio dops. The highe the tun numbes of smat-cad antenna, the moe impotant is this dop so the moe impotant is the loading effect. Figues show also the benefice of having a shunt esisto to minimize the global loading effect. Fo shot distance, the cuent atio value is highe with the pesence of a shunt esisto of small value.
6 Catheine Maechal, Dominique Paet. Fig. 3. : Cuent atio as a function of the distance and the shunt esisto fo n tun numbe of the smat-cad antenna. Fig. 4. Cuent atio as a function of the distance and the shunt esisto fo n 4 tun numbe of the smat-cad antenna. Fig. 5. Cuent atio as a function of the distance and the shunt esisto fo n 8 tun numbe of the smat-cad antenna.
Optimization of the law of vaiation of shunt egulato impedance fo Poximity Contactless Smat Cad Applications to educe the loading effect. 7 Simulation of the magnetic field atio at the smat cad. To complete this study, we simulate the magnetic field H located at a distance d fom the base station without and with a smat-cad. The magnetic induction B without a cad is given in [3, p]. B( d) µ Without a cad 0 n 3 I ( + d ) Note : without cad I 0 is a constant. So at the base station, the magnetic induction is B ( 0) µ 0 n I 0 0 (6) (7) We simulate the atio B B ( d ) 3 ( 0) 3 ( ) Without cad + d (8) as a function of the distance (Fig.6). 0.9 0.8 Field atio without smat cad 0.7 0.6 0.5 0.4 0.3 0. 0. 0 0 0.0 0.0 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0. Distance Fig. 6. Magnetic field atio as a function of the distance without smat-cad. The magnetic induction with a cad is: B( d) µ n With a cad 3 I ( + d ) Whee I is the cuent in the antenna of the base station with a cad at a distance d. I is acting as a function of d. We simulate the atio (9)
8 Catheine Maechal, Dominique Paet. B B ( d ) With a cad ( 0) ( ) 3 I0 Without a cad 3 + d I (0) The cuent atio I I0 is given in equation (3) Fig. 7 shows the esults fo n. The loading effect is small and the system will pobably wok nea the base station. In many cases, with same s, in ode to incease the magnetic flux, the antenna tag tun numbe is lage than. Fig. 7. Magnetic field atio as a function of the distance and the shunt esisto fo n tun numbe of the smat-cad antenna. Fig. 8 shows the magnetic field atio with a tun numbe of the tag s antenna of 4. The magnetic field is vey small and we can wonde how the system base station/tag wok. Fig. 8. Magnetic field atio as a function of the distance and the shunt esisto fo n 4 tun numbe of the smat-cad antenna. In fact, fo a same magnetic induction, same elementay suface, the induced voltage in the tag is geate as the tun numbe of the antenna is geate (equation ).
Optimization of the law of vaiation of shunt egulato impedance fo Poximity Contactless Smat Cad Applications to educe the loading effect. 9 Whee dbs dbs e n dt dt e is the induced voltage in the tag B the magnetic induction n the numbe of tun of the antenna of the tag s the suface of one tun of the antenna of the tag To take into account the incease of induced voltage with the numbe of the tun of the antenna of the tag we multiplied the magnetic induction / magnetic field atio with a cad by the numbe of the tun n (Fig. 9). With n 4, the induced voltage is above the induced voltage without a cad. If the tag is well designed the system should wok at any distance in its opeating ange and this one will be geate than if the tag would have only one tun (see fig. 7). () Fig. 9. Magnetic field atio*n as a function of the distance and the shunt esisto fo n 4 tun numbe of the smat-cad antenna. If we incease the numbe of tuns n of the antenna of the smat-cad, we can see that the system may wok between 6 and 8 cm, but not below 4 cm (Fig. 0). The same esult can be find in []. Fig. 0. Magnetic field atio*n as a function of the distance and the shunt esisto fo n 8 tun numbe of the smat-cad antenna.
0 Catheine Maechal, Dominique Paet. Optimization of shunt esistance vaiations To optimize the law of the vaiation of the shunt, we use the gaph that gives the induced voltage atio (Fig. 9 and 0) Fo a given distance between the base station and the tag, we take the value of the shunt egulato that gives the atio of the induced voltage the neaest of. If thee is the same induced voltage fo a given distance, we take the highe value of the shunt esisto to avoid high consumption. The pojection of all these points on the plan shunt, d (see figue ) gives an example of the gaph of the optimized law of shunt as a function of the distance d between smat cad and base station Fig.. Example of optimum law of the vaiation of intenal shunt impedance of the smat cad vesus distance. Conclusion This pape deals with the loading effect that will be emphasized in the coming yeas. Taking into account the shunt esistance influence we modelized the coupling between base station and smat cad at 3.56 MHz. This modeling leads to the simulation of the loaded/unloaded base station cuent atio as a function of vaious paametes (shunt esistance, n, and distance). We show the dop of the cuent in the antenna of the base station (so the dop of the magnetic field adiated at the base station) fo shot distance between the eade and the tag At this level, fist esults seem to show that it is not desiable to incease the antennae tun numbe of the tag. The moe impotant is this tun-numbe, the moe impotant is the dop of the cuent in the base station. In addition, taking into account the numbe of tuns in ode to obtain a sufficient induced voltage e acoss the coil of the tag IC, we simulate the magnetic field at a distance d with a smat-cad and we showed that a tun numbe of 4 fo the smat-cad s antenna gives good esults, that is the induced voltage is moe impotant than with only one tun. The communication ange is inceased. A too impotant numbe of the smat-cad s antenna may lead to
Optimization of the law of vaiation of shunt egulato impedance fo Poximity Contactless Smat Cad Applications to educe the loading effect. behavio of the system which may wok fo long distances but not fo shot ones. To conclude, in despite of pue mathematical esolutions, using ti-dimensional simulated cuves induced voltage, e f(d, shunt esistance) pesented in this pape, it is easy to define gaphically the optimum shunt esistance law vaiations vesus opeational distance in ode to minimize loading effects fo Poximity Contactless Smat Cad Applications. Fo a given distance between the base station and the tag, we take the value of the shunt egulato that gives the less dop of the induced voltage. If thee is the same induced voltage fo a given distance, we take the highe value of the shunt esisto to avoid high consumption. efeences [] Bing Jiang, Joshua. Smith, Matthai Philipose, Sumit oy, Kishoe Sundaa-ajan and Alexande V. Mamishev, Enegy Scavenging fo Inductively Coupled Passive FID Systems IMTC 005 Instumentation and Measuement Technology Confeence, Ottawa, Canada, 7-9 May 005 [] Stefan Babu, Simon Elhabi, Chistian ipoll, Geneviève Baudoin, Conception d antennes de tanspondeu pou les systèmes FID à 3,56 MHz avec optimisation de la télé-alimentation., 5ième Colloque su le Taitement Analogique de l Infomation, du Signal et ses Applications, TAISA 004, Lausanne, Suisse. [3] Dominique Paet, Applications en identification adioféquence et cates à puce sans contact, Dunod,Pais, 003 ISBN -0-005778- [4] Dominique Paet, FID and Contactless Smat Cad Applications, John Wiley, London, 004, ISBN: 978-0-470-095-9. [5] Klaus Finkenzelle, FID Handbook: adio-fquency Identification Fundamentlas and Applications, John Wiley, 999, ISBN 0-47-9885-0. [6] Caucheteux, D.; Beigne, E.; enaudin, M.; Cochon, E. AsyncFID: fully asynchonous contactless systems, poviding high data ates, low powe and dynamic adaptation Asynchonous Cicuits and Systems, 006. th IEEE Intenational Symposium on Volume, Issue, 3-5 Mach 006 Page(s): 0 pp.