Design and Analysis of a Bent Antenna-coil for a HF
RFID Transponder
Florian Ohnimus*#1, Ivan Ndip#, Stephan Guttowski#, Herbert Reichl*#
#
Fraunhofer-Institut für Zuverlässigkeit und Mikrointegration (IZM)
Gustav-Meyer-Allee 25,13355, Berlin, Germany
1
florian.ohnimus@izm.fraunhofer.de
*
Technische Universität Berlin
Straße des 17. Juni 135, 10623 Berlin, Germany
Abstract— In this work, the design of a bent antenna-coil for a Therefore, in this work, the design of a bent coil for a 13.56 13.56 MHz RFID transponder is presented. The tag is integrated MHz RFID transponder is presented, which is integrated into into a plastic ballpoint pen, optimally making use of the marginal a plastic ballpoint pen, optimally making use of the marginal volume inside the cylindrical geometry of the casing. The coil is volume inside the cylindrical geometry of the casing. Quasi-designed with a 50 % higher inductance in its planar state, so
static and full-wave methods for the electromagnetic
that the required inductance of nearly 6 µH to ensure resonance
is obtained when the bent tag is inside the pen. Quasi-static and simulation of the coil are compared and an analysis of the full-wave methods for the electromagnetic simulation of the coil influence of the bend radius on the inductance of the coil is are compared and an analysis of the influence of the bend radius conducted. on the inductance of the coil is conducted. Furthermore, the
II. OVERVIEW OF HF RFID TRANSPONDERS designed tag was manufactured using two technologies
comprising copper windings on a polyimide substrate and A 13.56 MHz passive RFID transponder consists of three conductive polymer paste on a polyester foil for experimental components: a transponder IC, a highly inductive coil (often verification. With a mid-range RFID reader configuration, read-with an additional discrete off-chip tuning capacitor) and a ranges of up to 23 cm and 12 cm, respectively, were obtained
carrier substrate. The coil of the transponder is used not only with the bent tag inside the pen.
I. INTRODUCTION
Radio Frequency Identification (RFID) systems have become widespread in many application areas, such as asset tracking and access control. The 13.56 MHz band is available worldwide and has been exploited by passive inductively-coupled near-field systems.
To date, much research has revolved around the design of coils for inductively-coupled RFID transponders. E.g., in [1], the design and modeling of planar single-layer and double-layer tags are presented. Design procedures for calculating the optimum inductance values for reader and transponder coils, so as to maximize the read-range, are discussed in [2]. Furthermore, coil designs targeting different applications scenarios have been proposed. E.g., in [3], a coil is designed for mounting on metal surfaces. Concerning the influence of conductor materials on the performance of tags, a comparison of coils for smart-card inlays with different conductive polymer pastes is conducted in [4].
However, little attention has been paid to the design of bent RFID transponder coils that operate sufficiently well in their deformed state, i.e. achieve the required inductance after geometrical deformation. In [5], it is shown that the read-range of printed polymer-based tags on curvilinear surfaces is dramatically reduced of up to 50% if the tag is bent around a surface.
However, often application scenarios require bent coils, so as to optimally make use of the available space or to ensure conformity with the envisaged environment.
for the data transfer, but must also generate enough current from the oscillating magnetic field, generated by the reader, to supply the IC with energy. Generally, it occupies by far the largest area (or volume), compared to the other components. The read-range of the transponder can be maximized by minimizing the required magnetic interrogation field strength of the transponder. This can be achieved as follows [6]: • The coil and transponder IC form a resonant circuit at
13.56 MHz so as to maximize the induced current from the oscillating magnetic field of the reader. • The effective coil area is maximized. This minimizes
the required magnetic field strength to produce the same magnetic flux in the coil.
The use of an additional tuning capacitor to compensate a low coil inductance is not used in many applications so as to reduce the required assembling components to only one transponder IC. Hence, the coil should provide the full inductance required to ensure resonance at 13.56 MHz together with the integrated IC-capacitance (and additional parasitic capacitances of the coil).
The following diagram shows a typical equivalent circuit model of an RFID transponder.
Fig. 1 Equivalent circuit of a RFID transponder
978-2-87487-006-4 © 2008 EuMA75October 2008, Amsterdam, The Netherlands
The values of the equivalent parallel circuit of the transponder IC, RIC and CIC, are usually specified by the manufacturer. L is the coil inductance, R the coil resistance and C the parasitic capacitance of the coil. The required inductance to achieve resonance can easily be found:
•
The decrease of weff also results in a decrease of the effective coil area. This increases the minimum required interrogation field strength Hmin of the transponder.
Hmin~
L=
decrease of L is strongly dependent on the bend radius of the coil. As a result, the design of the bent coil must ensure that III. CONCEPT OF A BENT TRANSPONDER COIL
the required inductance is reached when the coil is in its bent
In our envisaged application, a 13.56 MHz RFID
state.
transponder is integrated into a ballpoint pen. To optimally make use of the available space inside the pen, i.e. to IV. ELECTROMAGNETIC MODELING OF HF RFID COILS maximize the effective coil area, a bent tag is designed which
Due to the complexity in analysis of a bent coil, analytical
can be manufactured on a flexible substrate carrier. Therefore,
methods cannot be used to accurately extract the required
in the following, the concept of a bent coil, illustrated in
electrical parameters. Hence, numerical techniques, providing
Figure 4, will be briefly examined.
an approximate solution to Maxwell’s equations, are used. In this work, we applied Ansoft Q3D to provide solutions based on quasi-static assumptions and Ansoft HFSS to provide full-wave solutions. In both cases, the adaptive mesh refinement was used to ensure stability of the computed solution. The model of the coil is depicted in the figure below.
1
(1) 22As will be examined in section V, the intensity of the 4πf(CIC+C)1
(3)
weffleff
Fig. 2 Different views of the transponder coil. 1. Top view of the planar coil. 2. Cross-sectional view of the planar coil. 3. Cross-sectional view of the bent coil.
Fig. 3 3D models of coil used in the electromagnetic simulations
TABLE I COIL PARAMETERS
Parameter Description Value / mm The total self inductance L is the quotient of interlinked
magnetic flux and electric current I. It is dependent on the geometry of the coil, as indicated in (2) which is based on a very loose approximation of a simple inductor model.
L(weff,leff,deff,N)~
weffleffdeff
N2 (2) WCw
srtLC
The precise relationship in the range of interest can be extracted by using mathematical parameterization based on a proper design of experiment (DOE).
N is the total number of coil windings. Each coil winding contributes to the magnetic flux density. When the coil is bent around the x-axis, two effects are observed.
• The effective width weff is decreased and the effective depth deff is increased while the effective length leff remains the same. Since L varies directly with weff and inversely with deff, it will decrease when the coil is bent.
Coil length Coil width Conductor width Conductor space Bend radius Conductor thickness 55 25 0.25 0.2 5.25 0.0175 The available physical dimensions inside the cylindrical casing of the ballpoint pen are approximately 55 mm of length and 10.5 mm of diameter allowing the maximum coil length LC to be 55 mm and the bend radius r to be 5.25 mm.
In order to obtain a low value for deff so as to achieve a high coil inductance, the line width and space was chosen to be 250 µm and 200 µm, respectively, which is compatible with
76
The skin depth dskin for copper at 13.56 MHz is ~18 µm. The conductor thickness for the copper coil is 17.5 µm. In contrast to that, the skin depth for conductive polymer paste, assuming a conductivity σ=2.5 MS/m, is ~86 µm. Hence, for the second case, the skin effect may be neglected. The other factor that results in an increased AC resistance of the coil is the proximity effect caused by the closely coupled lines.
The computed inductances of the bent coils obtained from
Q3D and HFSS are 5.8 µH and 5.9 µH, respectively, while the
computed planar inductances are 8.8 µH for both cases. In Fig. 4 Equivalent circuit T-model of the coil
addition to that, RA was computed as 17 Ω with copper as
The values of the T-model comprise the coil inductance LA, conductor material, resulting in a Qcoil of 29. Furthermore, CA parasitic capacitance CA (including bridge, winding and pad
was computed as 1.35 pF, which corresponds to a coil
capacitances) and coil resistance RA.
capacitance C of 0.35 pF when transformed parallel to CIC (see
HFSS, on the other hand, computes the S-parameters at the Figure 1). ports. From the computed input reflection coefficient S11 and As a result, the quasi-static approximation yields the port impedance Z0, the input impedance Zin of the coil can
satisfactory results for the coil. This is also plausible,
be calculated:
considering the maximum physical dimension Dmax of the coil
Z−Z0S+1in relationship to the wavelength λ of nearly 22 m at 13.56
(4) ↔Zin=Z011 S11=in
MHz. Zin+Z01−S11
Dmax<<λ/10 (12)
Since the coil capacitance is assumed very much smaller than the inductance, the following relationship can be used to Consequently, the displacement current density can be extract the series coil inductance LS and resistance RS. assumed negligible [7] compared to the electric current
density and, hence, not contribute significantly to the Zin≈RS+jωLS (5)
reactance to the coil.
standard flex manufacturing and screen printing technologies. It must be considered that with decreasing line width the coil resistance is also increased, reducing the overall Q-factor. An appropriate coil width Wc of 25 mm and a total number of 10 windings were determined via simulation. The position of the transponder IC d can be varied to fine-tune the inductance.
Q3D directly computes the inductance, capacitance and resistance values of the equivalent circuit T-model depicted in figure 5.
Another important consideration is the skin effect, which can be approximated by (11) with the permeability µ and conductivity σ.
dskin=
2σωμ (11) V. INFLUENCE OF THE BEND RADIUS ON THE INDUCTANCE The bend radius is the most critical parameter of the bent RS=Re{Zin} (7)
coil. As mentioned above, the inductance of the coil in its
The computed inductance from the full-wave simulation LS planar state is always larger than in its bent state. and from the quasi-static simulation LA can be equated to the inductance L from the equivalent circuit in figure 1, again assuming a negligible parasitic coil capacitance.
L≈LA≈LS (8) The Q-factor of the coil Qcoil can be approximated with the
following equation:
⎧Z⎫
LS=Im⎨in⎬ (6)
⎩2πf⎭
GG
ωεE< ωL R (9) The total Q-factor Qtot of the coil and transponder IC can be calculated as follows: ⎛11⎞Qtot=⎜+⎟ (10) QQIC⎠⎝coil Fig. 5 Influence of the bend radius on coil the inductance −1 77 The above figure shows the computed inductance values from the low Q-factor of the screen printed coils (~6.5 for the for different bend radii. It can be seen, that the inductance of 75 Ω coils) resulting in a high resonant circuit bandwidth. the coil in its planar state is 50 % larger than the inductance in its designated bent state inside the pen with a bend radius of 5.25 mm. As a result, it is of vital importance that the coil inductance is optimized for its envisaged bend radius. VI. EXPERIMENTAL VERIFICATION AND COMPARISON OF RESULTS As can be seen in Table II, impedance measurements (using Agilent E4294) were conducted on the manufactured copper coils in their planar and bent (inside the ballpoint pen) states. A good correlation between measurement and simulation results was obtained. Fig. 6 Manufactured and integrated transponder tag VII. CONCLUSIONS In this work, a bent antenna-coil for a HF RFID TABLE II transponder was designed and integrated into a plastic COMPARISON BETWEEN MESUREMENT AND SIMULATION ballpoint pen. The coil was designed with a 50 % higher Technique Electrical Parameters inductance so that the required inductance is obtained when Planar LAC Bent LAC the tag is in its bent state inside the pen. Furthermore, quasi-Quasi-static simulation ~8.8 µH ~5.8 µH static and full-wave simulation techniques for computing the Full-wave simulation ~8.8 µH ~5.9 µH coil parameters were compared and verified with impedance Impedance measurement ~8.7 µH ~5.7 µH measurements. An analysis of the influence of the bend radius on the inductance of the coil was also conducted. Different prototypes with assembled transponder IC were also manufactured and integrated into the ballpoint pen. Apart ACKNOWLEDGMENT from the copper coils, other polymer-based coils were The designed RFID pen was presented at the Productronica manufactured with conductive paste (Dupont 5029) on a 2007 in Munich. The authors wish to thank Harald Pötter and polyester foil. The tags were not laminated after fabrication, Dr. Klaus-Dieter Lang for the coordination as well as yielding a higher resistivity. Christine Kallmayer, René Vieroth, Bettina Otto and Jörg A RFID reader configuration (module ISC.MR100-A with Gwiasda for the manufacturing of test samples. We would the ISC.ANT 340/240 pad antenna from Feig) was set up and also like to acknowledge Infineon, Dupont and Coveme for the transponders were tested. The obtained results are shown their input. in Table III below. REFERENCES TABLE III COMPARISON OF PERFORMANCE BETWEEN DIFFERENT RFID TAGS [1] Conductor Material Cu (17.5 µm thick) Ag-paste (~30 µm thick) RDC Read-range ~5 Ω ~23 cm [2] ~50 Ω ~12 cm ~75 Ω ~9 cm [3] [4] A read-range of 23 cm was obtained with a midrange reader configuration (specified 30 cm) with the bent copper tag inside the ballpoint pen. In comparison, the coil in its planar state has a read range of only 12 cm. This is because the inductance 50 % too high when the coil is not bent. The high inductance detunes the resonant circuit and makes the tag less sensitive to the oscillating magnetic field from the reader. The polymer-based coils achieved a read-range of 12 cm and 9 cm, respectively, depending on the resistivity of the paste. It was also found, that these coils are less sensitive to the influence of the bend radius i.e., similar read-ranges were obtained for different bend radii. It is believed that this results [5] [6] [7] S. S. Basat, L. Kyutae, J. Laskar, and M. M. Tentzeris, \"Design and modeling of embedded 13.56 MHz RFID antennas,\" presented at Antennas and Propagation Society International Symposium, 2005 IEEE, 2005. N. Rueangsri and A. Thanachayanont, \"Coil Design for Optimum Operating Range of Magnetically-Coupled RID System,\" presented at Communications and Information Technologies, 2006. ISCIT '06. International Symposium on, 2006. S. Bovelli, F. Neubauer, and C. Heller, \"A Novel Antenna Design for Passive RFID Transponders on Metal Surfaces,\" presented at Microwave Conference, 2006. 36th European, 2006. S. Cichos, J. Haberland, and H. Reichl, \"Performance analysis of polymer based antenna-coils for RFID,\" presented at Polymers and Adhesives in Microelectronics and Photonics, 2002. POLYTRONIC 2002. 2nd International IEEE Conference on, 2002. S. Y. Y. Leung and D. C. C. Lam, \"Performance of Printed Polymer-Based RFID Antenna on Curvilinear Surface,\" Electronics Packaging Manufacturing, IEEE Transactions on, vol. 30, pp. 200-205, 2007. K. Finkenzeller, RFID Handbook, 2 ed. West Sussex, England: John Wiley & Sons, Ltd., 2003. H. Henke, Elektromagnetische Felder-Theorie und Anwendung, 2. Auflage, Springer-Verlag, 2004. 78 因篇幅问题不能全部显示，请点此查看更多更全内容