Mechatronics for Safety, Security and Dependability in a New Era - Arai and Arai Part 12 docx

KivikoskiTampere University of Technology, Institute of Electronics,Rauma Research Unit, Kalliokatu 2, FI-26100 Rauma, FINLANDABSTRACT In passive radio frequency identification systems R

PASSIVE RFID SYSTEM

An RFID system consists of tags, readers, and an application host The readers communicatewirelessly with the tags to obtain the information stored on them The data sent by the reader ismodulated and backscattered from a number of tags The cheapest RFID tags with the largestcommercial potential are passive, harvesting energy from the reader's communication signal to power

up their operation and communication with the reader (Auto-ID Labs 2001), (Vogt 2002) RFIDcommunication consists of a number of communication cycles Each cycle consists of three sections:first, the reader sends an activation field to the tags Then, the reader sends a command to the tags, andfinally it sends a CW field that the tags modulate and backscatter back to the reader The reader'scommand field defines the content of the tags' replies Communication bit rates are 70.18 kb/s forforward link and 140.35 kb/s for backward link (Auto-ID Labs 2001)

ANTICOLLISION ANALYSIS

This chapter analyses EPC tree algorithm and Aloha protocols EPC tree algorithm (Auto-ID Labs2001) is chosen according to its wide popularity Aloha protocols are included in ISO 18000-6standard and also used by some RFID manufacturers (Vogt 2002)

EPC tree algorithm

EPC tree algorithm defined by Auto-ID labs (Auto-ID Labs 2001) goes through all possible codecombinations as a binary tree It optimises the number of required time slots by ignoring those leavesthat do not respond without any further requests Moreover, any collisions between replying tags donot interfere with the identification procedure as the reader does not need to know the contents oftags' replies, only whether any replies occur or not This is because of the well-synchronized replywindow: it has eight slots and each tag will modulate the requested 3-bit section of its identificationcode to one slot Chose of the slot is based on the content of the reply The eight slots allow eachdifferent set of the tree bits to occupy different slots (2 = 8) The actual duration of the totalidentification procedure of a number of tags lies between the maximum and minimum curves,depending on the alignments of the identification codes of the tags in the current binary tree Thesemaximum and minimum curves are presented in Figure 1 64 bit tag identifiers were used incalculations The derivation of these curves is presented in publication (Penttila et al 2004)

T3 (ft

Q

20 40 60 Number of tags

Number of tags

Figure 1 Maximum (left) and minimum (right) identification duration with EPC tree algorithm

Aloha family protocols

0 10 20 30 40

of identified tags as a function of the size of tag population for each measurement cases, where linescorrespond squares and dashed lines correspond triangles, coloured correspondingly

80 85 90 95 100

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20 30 40 50 Number of tags (with 8*8 grid)

an impact to the reliability As the distance between tags increases, the tags will interfere less witheach other

Item specific antennas and tag attachments will become an essential factor when designing fast,passive RFID with the option of multiple object identification Specific limitations of multiple objectidentification with passive RFID technology lie within object and tags mutual alignments.Furthermore, as the electromagnetic fields easily reflect from metallic surfaces and attenuate to non-conducting materials the fabrication material of objects to be identified has a great influence to theidentification accuracy

ACKNOVLEDGEMENTS

The authors would like to thank the Finnish National Technology Agency and Nokia Foundation forfinancing the research done for this paper

REFERENCES

Auto-ID Labs 860 MHz - 930 MHz Class I Radio Frequency Identification Tag Radio Frequency &

Logical Communication Interface Specification Published on 14 Nov 2001

Lim A., Mok K A study on the design of large-scale mobile recording and tracking systems IEEE

Proceedings of the 3 P' Hawaii International Conference on System Sciences, 6 - 9 Jan.1998 Kohala

Coast, HI USA Vol.7, pages 701-710

Penttila, K., Sydanheimo L., Kivikoski M Analysis of Multiple Object Identification with Passive

RFID Proceedings of the 5 th International Conference on Machine Automation, ICMA 24-26 Nov.

2004 Osaka, Japan, pp 559-564

Vogt H Efficient Object Identification with Passive RFID Tags International Conference on

Pervasive Computing Zurich, 2002.

Wieselthier J.E., Ephremides A., Michaels L.A An exact analysis and performance evaluation of

framed ALOHA with capture IEEE Transactions on Communications Feb 1989 Vol 37, issue 2,

pages 125-137

M M Keskilammi, L T Sydanheimo and M A KivikoskiTampere University of Technology, Institute of Electronics,Rauma Research Unit, Kalliokatu 2, FI-26100 Rauma, FINLAND

ABSTRACT

In passive radio frequency identification systems (RFID), data and power is transferred between areader and an identification device wirelessly by means of electromagnetic waves, Finkenzeller (2003).Antenna solutions, in both the identification device and the reader, are crucial to the performance ofradio frequency identification systems To improve the performance of these RFID systems applicationspecific antennas can be used for challenging items including metals, liquids or lossy material Thispaper describes the simulation model for radio wave attenuation in paper reel Simulated values forpropagation in different grades of paper are presented Theoretical background is also discussed

The dipole and folded dipole antennas generally used in radio frequency identification devices areusually omnidirectional, i.e they emit electromagnetic radiation in all directions However, theseantenna types have low amplification Furthermore, the frequency bands used by radio frequencyidentification devices have an officially regulated highest permitted transmission power, i.e directionalantenna structures can be used for improving the transmission of an identification device, if required.The use of directional, i.e amplifying antenna structures, such as a microstrip antenna or an antennaarray, allows the electromagnetic radiation power transmitted by the antenna to be directed moreefficiently in the desired direction This improves the coupling between the identification device andthe reader antennas in the direction of the maximum of the radiation beam of the directional antennacompared with omnidirectional antennas, whereas the coupling is weaker outside the radiation beamthan with omnidirectional antennas.

DIELECTRIC PROPERTIES OF PAPER

The relative permittivity in copy paper or in other paper qualities consisting mostly of wood fibers istypically from 2 to 4 decreasing with frequency In coated paper the permittivity increases even up to 8due to high amount fillers like CaCO3 added The change in the moisture content of paper doesn'tchange much the dielectric constant of paper itself, though the dielectric constant in water is 80 This isbecause in paper, water molecules are associated with polysaccharide chains and cannot rotate freely.Rotation is possible only if the field is parallel to the chain axis Because of the chain orientations inpaper are random, only a small fraction of the paper molecules have perfect alignment with the electricfield This makes the effective dielectric constant much smaller than it would be in liquid water,Niskanen (1998) However, the increase in moisture content increases dielectric losses in paper Inpaper with anisotropic fiber orientation, the dielectric constant is largest in the direction of the fiberorientation angle i.e typically in the planar directions In z-direction the dielectric constant is smaller.See Figure 1

The real part of the relative permittivity s t of paper increases with increasing density p and the

behavior follows with reasonable accuracy the Clausius-Mossotti relation, Niskanen (1998)

0 0.1 0.2 0.3 0.4 0.5 0.6

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—o- Coated paper machine direction

-a- Coated paper thickness direction

—•- Copy paper machine direction

—•- Copy paper thickness direction

0.4

—on Coated paper machine direction

-a- Coated paper thickness direction

—.6 Copy paper machine direction

—on Copy paper thickness direction ; X

) 40 ative humidity (%)

30 40 Relative humidity (%)

Figure 1: Electrical properties of the coated paper and copy paper as a function of relativehumidity at a frequency of 1 MHz a) Permittivity b) Loss tangent, Simula et al (1999).organic and inorganic ions, adsorbed ions, carboxyl groups, fiber morphology, polarization losses,rotation and oscillation of polar material, fine structure of cellulose and pulp components

SIMULATION RESULTS FOR THE COUPLING BETWEEN TWO DIPOLE ANTENNAS THROUGH THE PAPER REEL

For most paper reel identification applications the transponder should be attached to the core of thepaper reel This is how the identification of the reel can be done over its whole lifecycle However, theperformance of the communication link between the reader unit and the transponder is limited due tolosses in paper In the following the effect of loss tangent of the paper on the coupling between two

915 MHz dipole antennas is studied

The height of the simulated reel is 1200 mm and the diameter of the reel is 1000 mm The corediameter inside is 76 mm In the simulations one dipole was inserted inside the reel in the middle ofthe reel core while the other dipole was outside the reel The axial position of both dipoles in relation

to the reel was 600 mm The separation of dipoles was 538 mm

The free space loss can be evaluated using Friis transmission formula, Balanis (1997)

P=P,GG X

where is P r , P t , G r and G t are the received and transmitted powers and antenna gains respectively The

term is called the free space loss factor where X is the wavelength used and r is the separation of the antennas For two dipoles (G r - G t — 1.63) with 538 mm antenna separation in 915 MHz frequency (A = 328 mm) the coupling in free space is -22.05 dB.

In Figure 2, the effect of loss tangent of the paper on coupling between dipoles is presented In thesimulation the relative permittivity of paper was 2.0 The value of loss tangent varied from 0.05 to 0.5.The coupling between the antennas is decreased from the free space coupling with increasing losstangent value The simulation results agree well with previous studies with two dimensional layermodel, Keskilammi et al (2000)

600 700 800 900 1000 1100 1200

MHzFigure 2: Coupling between two 915 MHz dipole antennas through paper as a function

of frequency for eight loss tangent values Relative permittivity of paper is 2.0

SIMULATION RESULTS FOR DIPOLE ANTENNA INSIDE THE PAPER REEL

To find out the effect of change in dielectric properties of paper to the properties of dipole antenna thefollowing set-up was simulated The dipole antenna was inserted between the reel core and bulk paper.The dipole antenna was set in the middle of axial height of the reel The height and the diameter of thereel were as in the previous simulation The reel core inner diameter was 76 mm with wall thickness of

16 mm

The Effect of Permittivity

First relative permittivities of 2.0, 2.5, 3.0, 3.5, 4.0, 5.0 and 6.0 for paper were simulated In theFigure 3a) the change in return loss due to change in relative permittivity of paper is presented Theloss tangent for the simulations is tanS = 0.1

In the Table 1 the resonance frequency, return loss and bandwidth as a function of relative permittivity

of paper are presented

TABLE 1SIMULATED RESULTS FOR DTPOLE ANTENNA

568 703

= 135 536 659

= 123 512 624

= 112 492 595

= 103 474 568

= 94 446 519

= 73 424 478

= 54

Bandwidth (%)

b) Effect of loss tangent (tan0.05, 0.1, 0.2, 0.3 and 0.4).

As the relative permittivity of both paper and the reel core increases the resonant frequency of dipoleantenna decreases noticeably as expected Also the -10 dB bandwidth narrows, as the return loss getsworse with the increasing relative permittivity

The effect of loss tangent

The effect of loss tangent on properties of dipole antenna was studied for values 0.05, 0.1, 0.2, 0.3 and0.4 The simulations were repeated for three different values of relative permittivity 2.0, 4.0 and 6.0.The relative permittivity of the reel core was 3.0 and the loss tangent 0.1 In the Figure 3b) the change

in return loss due to change in loss tangent of paper is presented

In the Table 2 the resonance frequency, return loss and bandwidth as a function of loss tangent ofpaper are presented

TABLE 2THE EFFECT OF LOSS TANGENT ON RESONANCE FREQUENCY, RETURN LOSS AND BANDWIDTH OF A DIPOLE ANTENNA.

579 685

= 106 568 703

= 135 548 718

= 170 530 725

= 195 515 723

= 208

Bandwidth (%)

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CONCLUSIONS AND FUTURE WORK

The electrical properties of material, the identified object is made of, change the characteristics of thetransponder antenna fastened to the object To maintain the performance of the RFID system in thevicinity of challenging materials the antenna element has to be tuned according to application To testthe performance of these application specific antennas for the paper reel application theelectromagnetic model for the reel was created The dielectric properties for the model were takenfrom the literature Little information of dielectric properties of paper at higher frequencies isavailable

First the field attenuation in paper was studied by simulating the coupling between two dipoles withpaper in between The attenuation increased from 3 dB to 25 dB from the free space attenuation value

as the loss tangent value increased from 0.05 to 0.5 In the means of antenna separation in free spacethis means that the distance between the antennas is increased from 0.53 meters to 0.8-8 meters.Second the effect of change in dielectric properties of paper to the properties of dipole antenna insertedinside the paper reel was analyzed Increasing the relative dielectric constant of the paper lowered theresonant frequency of the dipole antenna The change in loss tangent of the paper did not affect theresonant frequency remarkably, but the change in antenna matching was noticeable

In the future the research will concentrate on testing new application specific antenna geometries forthe paper reel RFID transponders with the proposed model

REFERENCES

Finkenzeller K (2003), RFID Handbook, 2nd Ed., John Wiley & Sons Inc., New York, USA Niskanen K (1998), Papermaking Science and Technology, Book 16: Paper Physics, Fapet,

Helsinki, Finland

Simula S., Varpula T., Ikalainen S., Seppa H., Paukku A., Niskanen K (1999), Measurement of

the Dielectric Properties of Paper, Journal of Imaging Science and Technology, 43:5, 472-477.

Matsuda S (2002), Handbook of Physical and Mechanical Testing of Paper and Paperboard,

2nd Ed., Dekker, New York, USA

Balanis C.A (1997), Antenna Theory, Analysis and Design, 2nd Ed., John Wiley & Sons, Inc.,

USA

Keskilammi M., Salonen P., Sydanheimo L and Kivikoski M (2000), Radio Wave Propagation

Modeling in Paper Reel for Novel Radio Frequency Identification System, IEEE, JamCon2000,

Technology for Economic Development, Aug 11-13, 2000, Ocho Rios, Jamaica

'Tampere University of Technology, Institute of Electronics, Rauma Research Unit,

Kalliokatu 2, FI-26100 Rauma, FinlandMassachusetts Institute of Technology, Auto-ID Labs,

77 Massachusetts Avenue, Bldg 35-205, Cambridge, MA 02139, USA

ABSTRACT

This paper presents a comparison of the performances of two different passive tag antenna designsattached to cigarette cartons The aluminium foil in the cigarette packs makes the identification ofcigarette cartons difficult using passive RFID technology Therefore, a novel microstrip patch-type tagantenna for passive RFID of cigarette cartons was designed The performance of the novel tag antenna

is compared to the performance of a label-fabricated folded dipole-type tag antenna The maximumread ranges of a single tagged carton and two tagged cartons are measured and compared The effect

of the aluminium foil in the cigarette packs is studied by carrying the measurements out also usingcigarette packs without the foils and an empty carton The novel tag antenna performed superior to thefolded dipole tag antenna on full cartons of cigarettes

1999, Glidden et al., 2004) Other emerging applications of RFID are identification of paper rolls andnumerous applications in health care industry (Raza et al., 1999) RFID system consists of a readerunit, reader antenna, host computer and a transponder (i.e tag) A tag contains a microchip and anantenna The microchip stores the identification data of the tag Passive RFID tags have no internalsource of energy and thereby they get all the energy for functioning from the electromagnetic fieldsent by the reader Communication between the tag and the reader is based on backscattering: reader

RFID Reader Reader Antenna Application

(PC, Host Computer)

Data Clock Energy

Transponder (i.e Tag) Tag’s Antenna

Object

Figure 1: The components of an RFID system

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sends commands to tag which then responds to the reader by backscattering its identification data Theidentification data is modulated into the backscattered electromagnetic wave using load impedancemodulation (Finkenzeller, 2003) The components of an RFID system are presented in Figure 1

At present, one of the biggest challenges is tagging objects that are totally made of or containconductive materials in their structure Conductive materials next to antennas operating at UHFspectrum affect the performance of the antennas for example by lowering the radiation efficiency andchanging the resonance frequency Conductive materials also reflect the electromagnetic waveradiated by the antenna and therefore they affect the radiation pattern and radiation directions of theantenna (Raumonen et al., 2003) In addition, the conductive materials attenuate the incidentelectromagnetic wave and therefore the electromagnetic wave does not propagate well or at allthrough the conductive material (Reitz, Milford & Christy, 1993)

Cigarette cartons, which contain ten individual cigarette packs, have been a difficult object to identifyusing RFID technology because the individual packs are wrapped with aluminium foil This foil

contains thin layer of paper coated with 0.25 /.im thick layer of pure aluminium that is highly

conductive The aluminium layer is coated with very thin polyester layer The structure of a cigarettecarton and an individual cigarette pack are presented in Figure 2

RFIDReader

F

Application (PC,Host Computer)

_ _ _

ciga rette packs

—

Figure 2: A cigarette carton and a cigarette pack

Conventional label-fabricated RFID tags do not work with enough reliability when attached to acigarette carton Therefore, a novel tag design using microstrip patch antenna integrated on a cigarettecarton was designed The cigarette carton with individual packs was used as a substrate material of themicrostrip patch antenna and the dimensions of the antenna structure were designed to fit on thecarton and at the same time to achieve the 915 MHz resonance frequency 915 MHz is the UHF centerfrequency used in RFID in North and South America

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2 THE ANTENNA DESIGN

As previously noted, metallic structures near antennas affect their performance in many ways Placing

a conductive surface near an antenna has advantages and disadvantages In some cases a metallic platenear an antenna can act as a reflector causing the directivity to increase Also a number of antennatypes need a conductive ground plane to function properly In these cases a metallic surface can beused to improve the performance of the antenna

On the other hand, if the antenna does not use a ground plane in its function, the wave radiated by theantenna is almost totally reflected from the metallic surface since metal is highly conductive Whenelectromagnetic wave reflects from metallic surface a 180 degree phase shift occurs (Cheng, 1993).This reflected wave cancels the incoming wave and therefore the radiation efficiency of the antennadecreases These negative effects are strongest when the antenna is very near (for example at adistance of a couple of millimetres or less) the metallic surface (Raumonen et al, 2003)

The basic structure of the antenna design is shown in Figure 3 The patch-type tag needs a groundplane to function, and the metallic ground plane makes the antenna more stable and well functioningeven though the cigarette carton contains conductive aluminium foil Also, a folded extension of theground plane is added to improve the performance of the antenna The dimensions of the antenna wereoptimised using a computer simulation tool based on finite element method (FEM) The simulatedradiation pattern of the antenna, which is typical for microstrip patch-type antennas, is presented inFigure 4 The simulated bandwidth of the antenna is 236 MHz and the return loss (SI 1) at the 915MHz resonance frequency is -17.52 dB These values indicate a relatively wide bandwidth andsufficient impedance matching

The simulated input impedance of the antenna at 920 MHz is Z = (927 - J4.84) Q which is,considering the use of a microstrip patch-type antenna, relatively close to the impedance of theidentification microchips (Alien Technology's straps) The matching impedance of the straps is Z =(1200 — j 145) Q The relatively high matching impedance of the patch antenna leads to sufficientpower transfer from the microchip to the antenna and vice versa

Antenna Directivity Pattern vs Theta at 916 MHz, surface = abc-surface

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3 READ RANGE MEASUREMENTS

Read range measurements were carried out using a ThingMagic reader unit (Mercury 2, version 1.2.9software) and a linearly polarised reader antenna The read range measurement set up is shown inFigure 5 When the maximum read range was measured, the criteria for reliable identification was thatthe reader continuously identified the tag for at least one minute at the maximum reading distance Tostudy the effect of the aluminium foil on the read range, the measurements were carried out usingpacks with the foils, packs without foils and an empty carton

Tagged carton or cartons

Figure 5: The read range measurement setup

3.1 Read Range Measurements of One Carton and Comparison to the Measurements with a Conventional Tag

Read range measurements were carried out with two individual integrated patch-type tags and twoconventional folded dipole-type tags The read ranges are an average value of the read rangesachieved with the two individual tags The folded dipole-type tag is presented in Figure 6

Figure 6: Folded dipole-type tagTable 1 shows a comparison of the read ranges achieved with both tag types It can be observed thatwhen the aluminium foil is not removed from the packs the carton cannot be identified when foldeddipole tag is used However, with the integrated patch-type tag a read range of 1.05 m is achieved.When the foils are removed from the packs, the read range of the integrated patch-type tag almostdoubles The read range of an empty carton is only slightly longer than the read range of a carton withpacks without the foils When the foils are removed from the packs or an empty carton is measured,typical read ranges of over 2 m are achieved with the folded dipole tag

TABLE 1COMPARISON OF MAXIMUM RF.AD RANGES OF THE TWO TAG TYPES

Tag antenna

Integrated patch

Folded dipole

Packs withfoils1.05 m

0 m

Packs without

foils1.95 m2.40 m

Empty carton

2.00 m2.75 m

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3.2 Read Range Measurements of Two Cartons

To study the effect of multiple carton identification on read ranges, the tags were identified in pairsnext to each other and on top of each other The reading positions are shown in Tigures 7 and 8 Both

of the tags had to be read reliably at the maximum reading distance

Vertical position

Figure 7: Reading position with the cartons

next to each other

of each other is more reliable than when the folded dipole tag is used The folded dipole tag does notneed a substrate material for functioning, and therefore the cigarette packs between the tags attenuatethe incident wave and thereby shorten the read range In general it can be stated that increasing thenumber of tags to be read simultaneously shortens the read range

TABLE 2MAXIMUM READ RANGE COMPARISON OF TWO CARTONS NEXT TO EACH OTHER

Cartons next toeach otherIntegrated patchFolded dipole

Packs withfoils0.50 m

0 m

Packs without

foils1.20 m1.80 m

Empty carton1.20 m1.80 m

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TABLE 3MAXIMUM READ RANGE COMPARISON OF TWO CARTONS ON TOP OF EACH OTHER

Packs without

foils0.65 m0.45 m

Empty carton0.50 m1.25 m

4 CONCLUSIONS

This paper presents a case study of identification of cigarette cartons with passive RFID technology.Two types of tags are tested and the achieved read ranges are compared The aluminium foil in thecigarette packs makes the identification of the cartons difficult Therefore, a novel microstrip patch-type tag antenna for passive RFID of cigarette cartons was designed

The performance of the new tag antenna design was compared to that of the conventional, foldeddipole-type tag antenna It was observed that the aluminium foil in the cigarette packs affects the readranges significantly With the novel patch-type tag antenna the maximum read range was 1.05 m whenthe foils were in the cigarette packs When the folded dipole tag was tested, the read range was 0 mwhen the foils were in the packs Removing the foils from the cigarette packs lengthens the readranges to approximately 2 m It was also observed that two cartons next to or on top of each other can

be read simultaneously Reading both tagged cartons simultaneously shortens the read ranges

Foster P R and Burberry R A (1999) Antenna Problems in RFID Systems TEE Colloquium on

RFID Technology (Ref No 1999/123), pp 3/1-3/5.

Glidden R et al (2004) Design of Ultra-Low-Cost UHF RFID Tags for Supply Chain Applications

IEEE Communications Magazine, 42:8, pp 140-151.

RazaN., Bradshaw V and Hague M (1999) Applications of RFID Technology TEE Colloquium

on RFID Technology (Ref No 1999/123), pp 1/1-1/5.

Raumonen P., Sydanheimo L., Ukkonen L., Keskilammi M., and Kivikoski M (2003) Folded Dipole

Antenna Near Metal Plate Proc IEEE Antennas and Propagation International Symposium, 1, pp.

848-851

Reitz J R., Milford F J and Christy R W (1993) Foundations of Electromagnetic Theory,

Addison-Wesley, pp 454-462