PASSIVE FREQUENCY DOUBLING ANTENNA SENSOR FOR WIRELESS STRAIN SENSING

Proceedings of te ASME 212 Conference on Smart Materials, Adaptive Structures and Intelligent Systems SMASIS212 September 19-21, 212, Stone Mountain, Georgia, USA SMASIS212-7923 PASSIVE FREQUENCY DOUBLING ANTENNA SENSOR FOR WIRELESS STRAIN SENSING Xiaoua Yi Cunee Co Yang Wang Scool of Civil and Environmental Eng., Georgia Institute of Tecnology, Atlanta, GA 3332, USA Benjamin S. Cook James Cooper Rusi Vyas Manos M. Tentzeris Scool of Electrical and Computer Eng., Georgia Institute of Tecnology, Atlanta, GA 3332, USA Roberto T. Leon Department of Civil and Environmental Eng., Virginia Polytecnic Inst. and State Univ., Blacksburg, VA 2461, USA ABSTRACT Tis paper presents te design, simulation, and preliminary measurement of a passive (battery-free) frequency doubling antenna sensor for strain sensing. Illuminated by a wireless reader, te sensor consists of tree components, i.e. a receiving antenna wit resonance frequency f, a transmitting antenna wit resonance frequency 2f, and a matcing network between te receiving and transmitting antennas. A Scottky diode is integrated in te matcing network. Exploiting nonlinear circuit beavior of te diode, te matcing network is able to generate output signal at doubled frequency of te reader interrogation signal. Te output signal is ten backscattered to te reader troug te sensor-side transmitting antenna. Because te backscattered signal as a doubled frequency, it is easily distinguised by te reader from environmental reflections of original interrogation signal. Wen one of te sensor-side antennas, say receiving antenna, is bonded to a structure tat experiences strain/deformation, resonance frequency of te antenna sifts accordingly. Troug wireless interrogation, tis resonance frequency sift can be measured by te reader and used to derive strain in te structure. Since operation power of te diode is arvested from te reader interrogation signal, no oter power source is needed by te sensor. Tis means te frequency doubling antenna sensor is wireless and passive. Based on simulation results, strain sensitivity of tis novel frequency doubling antenna sensor is around -3.84 khz/µε. INTRODUCTION In order to accurately assess deterioration of civil, mecanical, and aerospace structures, a large volume of researc in structural ealt monitoring (SHM) as been inspired over past few decades [1]. Sensors can be used to measure various structural responses and operating conditions, including strain, displacement, acceleration, umidity, temperature, etc. Among te measurements, strain can be an important indicator for stress concentration and damage development. Metal foil strain gages are currently among te most common solutions, due to teir low-cost, simple circuitry, and acceptable reliability in many applications. However, wen applied to large structures, traditional metal foil strain gages require lengty cable connections for power and data acquisition, wic can significantly increase installation time and system cost [2]. To avoid cabling difficulty associated wit metal foil strain gages, wireless strain sensors ave recently been developed. For example, a wireless strain sensor is proposed based on inductive coupling principle involving two adjacent inductors [3-5]. However, te interrogation distance acieved by inductive coupling is usually limited to several inces, wic is inconvenient for practical applications. In order to increase interrogation distance, electromagnetic backscattering tecniques ave been exploited for wireless strain sensing [6]. Since te electromagnetic resonance frequency of a planar antenna is related to te antenna s pysical dimension, te resonance frequency canges wen te antenna is under strain. Tis relationsip between resonance frequency and strain can be used for stress/strain measurement of a structure to wic te planar antenna is bonded. For example, a as been designed for wireless strain sensing [7], were a pototransistor is adopted for signal modulation of te RF 1 Copyrigt 212 by ASME

signal backscattered from te antenna sensor. As a result, signal backscattered from te sensor can be distinguised from environmental reflections. However, te ligt-switcing mecanism is not practical for outdoor application, were ligt intensity is usually so strong tat te pototransistor is constantly activated and tus, loses ability of switcing. To avoid tis difficulty, a low-cost off-te-self radiofrequency identification (RFID) cip is adopted as a simple mecanism for signal modulation [8]. Since te RFID cip is powered by wireless interrogation signal, te RFID-based strain sensor is wireless and passive (battery-free). Te prototype RFID antenna sensor as sown a strain measurement resolution of 2 µε in laboratory experiments, and can measure large strains up to 1, µε [9]. Previous studies demonstrated tat if operating frequency of te wireless strain sensor is increased, strain sensitivity can be improved and sensor size can be reduced. However, te RFID cip only functions in te frequency band of 86-96MHz. Alternative approaces need to be exploited in order to operate at iger frequencies. Tis paper presents a novel wireless strain sensor design tat adopts a frequency doubling sceme to enable sensor operation at a ig frequency. Te basic concept is to let te sensor double te frequency of reader interrogation signal (f) and backscatter signal at te doubled frequency (2f). Because environmental reflections to reader interrogation signal are concentrated at f, te reader only receives signal at 2f backscattered from te sensor. Te frequency doubling operation is implemented troug a Scottky diode, wic is a nonlinear circuit device and can generate output signal wit frequencies at multiples of input frequency. Diode frequency multipliers ave been adopted for energy arvesting [1], insect tracking [], among oters. In [12], a ig efficiency diode frequency doubling device is designed using a GaAs Scottky diode tat provides 1% conversion efficiency at -3 dbm input power. Neverteless, te autors are not aware of oter literature on using Scottky diode to enable frequency doubling for wireless passive strain sensing. In our study, te diode-enabled frequency doubling mecanism is investigated and incorporated wit two patc antennas to form a wireless strain sensor. A low-cost Scottky diode (SMS7621-79LF) from Skyworks Solutions, Inc. is adopted. A wit resonance frequency at 2.9GHz is designed as a receiving antenna of te wireless strain sensor. Meanwile, anoter wit resonance frequency at 5.8GHz is designed to serve as a transmitting antenna of te wireless strain sensor. For connection between tese two patc antennas, te simulation model of a Scottky diode is first verified troug experimental measurement, and ten used to design te matcing network between te two s. Finally, te tree components, i.e. te receiving and transmitting antennas and matcing network, are combined togeter to form a frequency doubling antenna sensor. Since operation power of te diode is arvested from wireless interrogation signal, te frequency doubling antenna sensor is wireless and passive (battery-free). Strain sensing simulation sows tat te Reader side Function generator Spectrum analyzer Transmitting reader antenna Receiving reader antenna proposed frequency doubling sensor can acieve a strain sensitivity of -3.84 khz/µε. Te rest of tis paper is organized as follows. Strain sensing mecanism of te antenna sensor is first presented. Designs of te 2.9 GHz and 5.8 GHz s are ten described. Next, verification experiment for te Scottky diode simulation model is presented, followed by te diode-integrated matcing network design. Combining all tree components, simulation results are presented to demonstrate strain sensing performance of te proposed frequency doubling antenna sensor. Finally, a summary and discussion is provided. STRAIN SENSING MECHANISM OF FREQUENCY DOUBLING ANTENNA SENSOR Fig. 1 illustrates te operation mecanism of a frequency doubling antenna sensor. Te sensor consists of tree main components, i.e., a receiving antenna (wit resonance frequency f ), a transmitting antenna (wit resonance frequency 2f ), and a diode-integrated matcing network between receiving and transmitting antennas. During operation, a wireless interrogation signal is emitted from te reader side by a function generator and troug a transmitting reader antenna. If interrogation frequency f is in te neigborood of f, resonance frequency of te receiving at sensor side, interrogation power is captured by te sensor-side receiving and transferred to te matcing network. Te diode ten generates output signal at doubled frequency 2f. Te output signal at 2f is backscattered to reader troug sensor-side transmitting (resonance frequency at 2f ). A spectrum analyzer finally measures te backscattered signal at reader side. Frequency of backscattered sensor signal is at 2f, and te unwanted environmental reflections to original reader interrogation signal remains at f. Terefore, it is easy for te f 2f Sensor side Receiving (resonance frequency f ) Diode and matcing networks Transmitting (resonance frequency 2f ) Fig. 1 Operation mecanism of a frequency doubling antenna sensor system 2 Copyrigt 212 by ASME

1.35 in. spectrum analyzer to distinguis backscattered sensor signal from unwanted environmental reflections. Te receiving and transmitting antennas of te frequency doubling sensor are microstrip s. For a microstrip wit lengt L, widt W, and substrate tickness, effective dielectric constant of te antenna can be calculated for determining antenna electrical lengt [13]: 1 1 1 12 2 2 W r r were ε r is te substrate dielectric constant. Te resonance frequency (f ) of a at zero strain level can be estimated as: f c 2( L 2L) were c is te speed of ligt; ΔL is antenna lengt compensation due to fringing effect [14], wic is given empirically by: 1 2 W (. 3)(. 264) L. 412 W (. 258)(. 8) By defining coefficient k as: ΔL can be rewritten as: W (. 3)(. 264) k. 412 W (. 258)(. 8) (1) (2) (3) (4) L k (5) Wen te is under strain ε along te direction of patc lengt L, pysical dimensions of te cange accordingly. Tis cange causes sift in resonance frequency: c f 2 L( 1 ) 2k( 1 ) c 2 ( L 2k ) ( L 2k ) L 2k f 1 f S L 2k were ν is Poisson s ratio of substrate material; S represents strain sensitivity of te. According to Eq. (6), (6) wen strain is small, resonance frequency cange of te patc antenna as an approximately linear relationsip wit applied strain. Tis serves as strain sensing mecanism of te patc antenna. In tis paper, it is assumed tat only receiving antenna of a frequency doubling sensor is bonded to structural surface, wile matcing network and transmitting antenna are floating and stress/strain free. Troug te matcing network and transmitting, tis frequency sift causes cange in te backscattered signal, wic is to be captured by te reader. Tis cange in te backscattered signal is used to derive strain on te monitored structure. FREQUENCY DOUBLING ANTENNA SENSOR DESIGN Tis section presents preliminary designs for tree components of te frequency doubling antenna sensor, including te receiving antenna, te transmitting antenna, and te diode-integrated matcing network. Before diode-integrated matcing network is designed, measurement is conducted to verify a simulation model of te diode. Receiving and Transmitting Antenna Designs Substrate material adopted in te preliminary sensor design is Rogers/duroid R 588, a glass micro-fiber reinforced PTFE material. Te substrate tickness is cosen as 31 mil, a trade-off between increasing wireless interrogation distance and improving strain transfer ratio from structural surface to top layer of te microstrip [15]. Te dielectric constant ε r of te material is 2.2. In current design, te resonance frequency is set to f = 2.9GHz for te receiving antenna, and 2f = 5.8GHz for te transmitting antenna. Te two s are simulated in a commercial software package, Ansoft HFSS. Fig. 2 sows te poto of a fabricated 2.9GHz. Te dimension of te antenna is 1.75 in. 1.35 in. Fig. 3 sows te simulated scattering parameter S plot of te antenna. Te simulation sows te antenna as a resonance frequency at 2.9 GHz wit return loss 4 db. Te return loss is muc lower tan te -1 db return loss tresold for typical antennas, wic indicates good radiation performance. Fig. 3 also sows te measured S of a 1.75 in. Fig. 2 Poto of a fabricated 2.9GHz receiving 3 Copyrigt 212 by ASME

S S.7 in. piece of fabricated 2.9 GHz receiving antenna. Te measured resonance frequency is 2.893GHz, wic is about 7MHz (only.24% difference) lower tan te simulation result. Te discrepancy between te simulation and measurement is reasonably small, considering imperfections in bot simulation and fabrication. Te simulated radiation pattern is plotted in Fig. 4, sowing a peak gain of 5.4 db. Te alf-power beamwidt (HPBW) of te antenna is 76, wic allows flexible reader antenna positioning. Fig. 5 sows te poto of a fabricated 5.8 GHz transmitting. Te dimension of te antenna is.74 in..71 in. Fig. 6 sows te S plot of te 5.8 GHz transmitting patc antenna. Te simulation sows te antenna as a resonance frequency at 5.81 GHz wit return loss -23 db. Te.74 in. Fig. 5 Poto of a fabricated 5.8GHz transmitting patc antenna -1-5 -1 Simulation Experiment -2-15 -3 Simulation Experiment -4 28 285 29 295 3 Frequency(MHz) Fig. 3 Simulated and measured S of 2.9GHz receiving -2-25 56 57 58 59 6 Frequency(MHz) Fig. 6 Simulated and measured S of 5.8GHz transmitting Fig. 4 Simulated radiation pattern of 2.9GHz receiving Fig. 7 Simulated radiation pattern of 5.8GHz transmitting 4 Copyrigt 212 by ASME

S 21 S measurement of a fabricated piece sows a resonance frequency of 5.81 GHz, i.e. about 9 MHz (only.16% difference) iger tan te simulation result. Te simulated radiation pattern of te antenna is plotted in Fig. 7, sowing a peak gain 6.6 db. Te HPBW of te antenna is 67, wic is also convenient for reader antenna positioning. Matcing Network Design Besides two s, diode-integrated matcing network between receiving and transmitting antenna is also designed. Te goal of matcing network design is to maximize output power during frequency doubling. A Scottky diode manufactured by Skyworks Solutions, Inc., te SMS7621-79LF diode, is adopted in tis design. A commercial simulation model for tis diode, wic operates in Advanced Design System (ADS) software package, is provided by Modelitics, Inc. Te matcing network design is performed in ADS, wic as te embedded diode model and built-in microstrip line models. To verify te accuracy of te Modelitics diode model, a diode testing board is first designed and mounted wit a Scottky diode (SMS7621-79LF). Performance of te diode is measured and compared wit simulation results. Fig. 8 sows te diode testing board for verifying accuracy of te simulation model. Te SMS7621-79LF Scottky diode is installed between two 5 Ω transmission lines. Scatteringparameters are measured by vector network analyzer (VNA) wen different input power levels are supplied to te board troug Port 1. Te simulation and experimental results are compared in Fig. 9(a) for S-parameter S, and Fig. 9(b) for S parameter S 21. A larger value of S 21 means tat a iger percentage of te power is transmitted troug te matcing network. Four different input power levels,, 1, 2, and 3 dbm are caracterized in te experiments, wic are identified in te legend as Exp dbm, Exp 1dBm, Exp 2dBm, and Exp 3dBm. Te simulated S-parameters are also plotted in Fig. 9(a) and Fig. 9(b), wic are identified in te legend as Sim S and Sim S 21, respectively. For tis comparison, a difference witin 1dB is regarded as very small, considering fabrication and measurement tolerances, and will ave minimum effect to design performance. Close matc 5 Sim S Exp dbm Exp -1dBm Exp -2dBm Exp -3dBm Port 1 Diode Port 2-5 25 3 35 4 45 5 55 Frequency(MHz) (a) S comparison Rogers 588 substrate (a) Diode testing board -5 VNA -1-15 Sim S 21 Exp dbm Exp -1dBm Exp -2dBm Exp -3dBm 25 3 35 4 45 5 55 Frequency(MHz) (b) S 21 comparison Diode testing board (b) Measurement setup Fig. 9 Verification for te diode simulation model Fig. 8 Experimental setup for diode caracterization test 5 Copyrigt 212 by ASME

4.25 in. Diode 2.2 in. between simulation and experimental results indicates te accuracy of simulation model provided by Modelitics, Inc. Te matcing network design integrating Scottky diode SMS7621-79LF is ten performed in ADS. Fig. 1 sows te poto of a fabricated matcing network. Te dimension of te matcing network is 2.2 in. 1.2 in. Because diode is a nonlinear circuit device, power loss of te diode-integrated matcing network is dependent on input power level. To verify te input power effect, different input power levels are simulated and te power loss results are summarized in Table 1. In te frequency doubling sensor design, input power of 1 dbm is adopted. At tis input power level, power loss due to matcing network and diode is te lowest. To caracterize backscattering beavior of te doubling sensor, sensor response for a neigborood frequency range needs to be analyzed. To tis end, power loss due to diodeintegrated matcing network is investigated by a armonic balance simulation in ADS. Te input power to te matcing network is set to -1dBm. Te output power at doubled frequency 2f is measured. Te results are plotted in Fig.. STRAIN SENSING PERFORMANCE OF FREQUENCY DOUBLING ANTENNA SENSOR Tree components of te frequency doubling sensor are 1.2 in. combined togeter and te overall design drawing is sown in Fig. 12. Currently not optimized for size reduction, dimension of te overall frequency doubling antenna sensor is about 4.25 in. 1.77 in. Since te pysical dimension of a is inversely proportional to operating frequency, footprint of te sensor can be easily scaled down by increasing te operating frequency in future designs. Te sensor as a power flow mecanism as illustrated in Fig. 13. P represents te received power at te 2.9 GHz receiving antenna. P 1 and P 2 denote te power before and after te matcing network, respectively. Finally, P represents te power being backscattered to te R T reader. As sown in Fig. 13, S and S are scatteringparameters of te receiving and transmitting antennas, R T respectively. Te S and S plots of current antenna designs M Loss S 21-12 -14-16 -18-2 -22 5.4 5.6 5.8 6 Frequency (GHz) Fig. Power loss of diode-integrated matcing network (input power at frequency f, output power at 2f as marked on te x-axis) Fig. 1 Poto of matcing network in te frequency doubling sensor 5.8 GHz Table 1 Simulated power loss under different input power Input power (dbm) Output power (dbm) Power loss due to matcing network and diode -16.932 16.932-5 -19.855 14.855-1 -23.62 13.62-15 -28.712 13.712-2 -35.533 15.533-25 -43.287 18.287 1.75 in. Matcing network 2.9 GHz Fig. 12 Design drawing of frequency doubling antenna sensor 6 Copyrigt 212 by ASME

Frequency (MHz) P/P M ave been sown in Fig. 3 and Fig. 6. S is forward 21 transmission coefficient of te matcing network, wic determines te ratio of output power at Port 2 over te input power at Port 1 (as sown in Fig. ). Te overall power P transmitted back to te reader can be estimated as: -13.5-14 R 2 M 2 T 2 21 P P (1 [ S ] )[ S ] (1 [ S ] ) (7) Te P/P ratio, wic varies wit frequency, quantifies te backscattering performance of te antenna sensor. Combining R T M te individual results for S, S, and S, te P/P 21 versus frequency relationsip is plotted as te solid line (µε, i.e. zero strain level) in Fig. 14(a). A clear resonance is demonstrated in te plot, and te corresponding frequency is named as te resonance frequency of te entire sensor. Power loss of te frequency doubling antenna sensor at 5.8 GHz is around 13.6 dbm (for zero strain level). Maximum wireless interrogation distance of te sensor can be estimated according to Friis equation. For tis estimation, it is assumed tat te interrogation power emitted by te reader-side function generator is 15 dbm, and te lowest backscattered power detectable by te spectrum analyzer is 7dBm. If bot transmitting and receiving antennas at te reader side ave a gain of 1 db, te acievable interrogation distance is estimated to be 2.2m. Te distance can be increased during actual testing, e.g. by adopting iger reader interrogation power or reader antennas wit iger gain. Wen te sensor is under strain, cange in te backscattering performance can be interrogated by a wireless reader and used for detecting strain. For simulating te strain sensing performance of te frequency doubling sensor, we studied te scenario wen te 2.9 GHz is bonded to te monitored structure and stretced under strain. All oter parts of te sensor, including te matcing network and te 5.8 GHz transmitting antenna, are strain free. Four more strain levels, from 5µε to 2, µε wit 5 µε increment, are simulated. Te relationsip between power ratio P/P and frequency at eac strain level is again plotted in Fig. 14(a). Resonance frequency of te entire sensor is sown to decrease as strain increases. Te resonance frequency at eac strain level P 1 ( S ) P R 2 M 2 1 P2 ( S21 ) P1 P P P 1 2 S R 2.9 GHz receiving patc antenna M S21 Port 1 Port 2 Matcing network and diode T 2 P 1 ( S) P2 P S T 5.8 GHz transmitting patc antenna Fig. 13 Power flow of frequency doubling antenna sensor -14.5 5 1 15 2-15 5.74 5.76 5.78 5.8 5.82 Frequency (GHz) (a) Power ratio P/P versus frequency relationsip for multiple strain levels 5792 579 5788 5786 5784 f R = -.384 + 579.4 R 2 =.9897 5782 5 1 15 2 Strain () (b) Resonance frequency (of te entire sensor) versus strain relationsip Fig. 14 Strain sensing simulation results for te frequency doubling antenna sensor is extracted from Fig. 14(a), and plotted against strain in Fig. 14(b). Te figure sows a simulated strain sensitivity at 3.84 khz/µε, wic is about five times iger tan te RFID-based antenna sensor presented in [8]. CONCLUSIONS In tis researc, a passive frequency doubling antenna sensor is designed and fabricated. A Scottky diode is integrated in te sensor design for generating output signal at doubled frequency of te input signal, wic allows te reader to easily distinguis backscattered sensor signal from unwanted environmental reflections. Because operation power of te diode is captured from wireless interrogation signal emitted by a reader, no oter power source is needed by te sensor, wic means te antenna sensor is wireless and passive (battery-free). 7 Copyrigt 212 by ASME

Two s, serving as 2.9GHz receiving antenna and 5.8GHz transmitting antenna, are designed. A Scottky diode simulation model is first verified by measurement, and ten used for designing te diode-integrated matcing network. Te tree components of te frequency doubling antenna sensor are combined togeter and te strain sensing performance is simulated. Based on simulation results, a strain sensitivity of 3.84 khz/µε is acieved. Tensile testing will be performed in te future to evaluate te strain sensing performance of te frequency doubling sensor. ACKNOWLEDGMENTS Tis material is based upon work supported by te Federal Higway Administration under agreement No. DTFH61-1-H- 4. Any opinions, findings, and conclusions or recommendations expressed in tis publication are tose of te autors and do not necessarily reflect te view of te Federal Higway Administration. Te autors would like to acknowledge te Modelitics models utilized under te University License Program from Modelitics, Inc., Tampa, FL. REFERENCES [1] H. Son, C. R. Farrar, F. M. Hemez, D. D. Sunk, D. W. Stinemates, and B. R. Nadler, "A Review of Structural Healt Monitoring Literature: 1996-21," Report No. LA- 13976-MS, Los Alamos National Laboratory, Los Alamos, NM, 23. [2] M. Çelebi, "Seismic Instrumentation of Buildings (wit Empasis on Federal Buildings)," Report No. -746-6817, United States Geological Survey, Menlo Park, CA, 22. [3] S. Jon C., G. Jun, C. Nattapon, K. Wen H., and Y. Darrin J., "Wireless, passive, resonant-circuit, inductively coupled inductive strian sensor," Sensors and Actuators A, vol. 12, pp. 61-66, 22. [4] K. J. Lo, J. P. Lync, and N. A. Kotov, "Inductively coupled nanocomposite wireless strain and ph sensors," Smart Structures and Systems, vol. 4, pp. 531-548, 28. [5] Y. Jia, K. Sun, F. J. Agosto, and M. T. Quinones, "Design and caracterization of a passive wireless strain sensor," Measurement Science and Tecnology, vol. 17, pp. 2869-2876, 26. [6] K. Finkenzeller, RFID Handbook, 2nd ed., Jon Wiley & Sons, New York, 23. [7] S. Desmuk and H. Huang, "Wireless interrogation of passive antenna sensors," Measurement Science and Tecnology, vol. 21, pp. 3521, 21. [8] X. Yi, T. Wu, Y. Wang, R. T. Leon, M. M. Tentzeris, and G. Lantz, "Passive wireless smart-skin sensor using RFIDbased folded s," International Journal of Smart and Nano Materials, vol. 2, pp. 22-38, 2. [9] X. Yi, T. Wu, G. Lantz, J. Cooper, C. Co, Y. Wang, M. M. Tentzeris, and R. T. Leon, "Sensing resolution and measurement range of a passive wireless strain sensor," Proceedings of te 8t International Worksop on Structural Healt Monitoring, Stanford, CA, USA, 2. [1] S. Berndie and C. Kai, "5.8-GHz cricularly polarized rectifying antenna for wireless microwave power transmission," IEEE Transactions on Microwave Teory and Tecniques, vol. 5, pp. 187-1876, 22. [] C. Bruce G. and B. Gilles, "Harmonic radar transceiver design: miniature tags for insect tracking," IEEE Transactions on Antennas and Propagation, vol. 52, pp. 2825-2832, 24. [12] P. Suzette M., W. Tomas M., S. Steven, and R. Micael, "Hig efficiency diode doubler wit conjugate-matced antennas," Proceedings of te 37t European Microwave Conference, Munic, 27. [13] C. A. Balanis, Antenna Teory: Analysis and Design, 2nd ed., Jon Wiley & Sons, New York, 1997. [14] I. J. James and P. S. Hall, Handbook of Microstrip Antennas, Peter Peregrinus Ltd., 1989. [15] X. Yi, T. Wu, G. Lantz, Y. Wang, R. T. Leon, and M. M. Tentzeris, "Tickness variation study of RFID-based folded s for strain sensing," Proceedings of SPIE, Sensors and Smart Structures Tecnologies for Civil, Mecanical and Aerospace Systems, San Diego, CA, USA, 2. 8 Copyrigt 212 by ASME