This paper presents a novel radio-frequency identification (RFID) sensing system using enhanced surface wave technology for battery exchange stations (BESs) of electric motorcycles. Ultrahigh-frequency (UHF) RFID technology is utilized to automatically track and manage battery and user information without manual operation. The system includes readers, enhanced surface wave leaky cable antennas (ESWLCAs), coupling cable lines (CCLs), and small radiation patches (SRPs). The RFID sensing system overcomes the electromagnetic interference in the metallic environment of a BES cabinet. The developed RFID sensing system can effectively increase the efficiency of BES operation and promote the development of electric vehicles which solve the problem of air pollution as well as protect the environment of the Earth.
1. Introduction
Air pollution, especially CO, HC, and NOx, generated by vehicles, such as motorcycles and cars, is a very serious problem in many countries. The quantity per year of polluted air exhausted by vehicles continues to reach record highs. Therefore, the replacement of traditional petroleum vehicles with electric vehicles is becoming a global trend. However, batteries and cost are the most important challenges for electric motorcycles and cars. A rechargeable battery is a low cost solution for electric vehicles. As far as the electric motorcycles are concerned, battery exchange stations (BESs) or rapid-charging batteries are required [1, 2].
Battery information is of critical importance for the management of the BES. A barcode attached to a battery is one approach to identify each battery in the BES; however, it consumes manpower and time to gather the battery information.
In recent years, radio-frequency identification (RFID) technology [3–15] has been widely used in different applications because of the contactless, long reading distance and multiread characteristics [16–26]. RFID applications include asset management, health care, logistics, security, and so on [27–38]. During long-distance RFID signal transmission, signal interference might occur and result in erroneous or missed readings. In the metallic environment of a BES, electromagnetic radiation is disturbed in a small space. The traditional antenna of an RFID reader radiates far-field electromagnetic waves and occupies a large area. These features are not suitable for a BES which has a closed and small space.
Leaky cable antennas (LCAs) [39–46] have attracted much attention in regard to communication applications. The conventional LCA is widely used for the mobile communications in subways and tunnels.
In this paper, a novel ultrahigh-frequency (UHF) RFID sensing system integrating a flexible enhanced surface wave leaky cable antenna (ESWLCA) with a coupling cable line (CCL) and a small radiation patch (SRP) is proposed incorporating the enhanced surface wave technology in order to overcome the narrow metallic environment of the BES and to lower the manufacturing cost. The ESWLCA has been successfully implemented in a UHF RFID sensing system for the BESs of electric motorcycles.
2. System Structure
Rechargeable batteries for electric motorcycles are installed in a BES, as shown in Figure 1. A BES is a closed metallic cabinet, in which a large number of dense metal brackets and wires disturb far-field electromagnetic radiation. In addition, the electromagnetic noise reflected by the metallic BES cabinet might cause the saturation and malfunction of the receiver of a reader.
BES.
The system structure of the proposed RFID sensing system is shown in Figure 2. The system includes readers, ESWLCAs, CCLs, SRPs, and tags. The ESWLCA transmits the enhanced surface wave through the CCL to the SRP which is close to the tag. The SRP at the end of the CCL radiates the electromagnetic wave to activate the RFID tag attached to the battery. The physical picture of the RFID sensing system with an ESWLCA, a CCL, and an SRP for the BES is shown in Figure 3. The ESWLCA has several advantages, such as flexibility, a slim form factor, and low radiation, to avoid interference in the metal-rich environment and to be easily placed in suitable positions for detecting tags. Figure 4 shows the structure of the ESWLCA. There is an open slot on the ESWLCA.
System structure of RFID sensing system.
RFID sensing system with ESWLCA, CCL, and SRP for BES.
Structure of ESWLCA.
The operation of the system is described as follows. First, one of the ESWLCA ports is connected to the output port of the UHF reader and the other port is terminated with 50 ohms. When the reader turns on the RF power, the RF signal will be fed into the ESWLCA. Some signals will radiate into the air from the slot aperture and some will flow through the cable surface from the slot aperture to produce surface waves along the cable.
Then, at suitable positions of the ESWLCA, the CCL will be connected to the ESWLCA by wiring, so that some surface waves will flow into the CCL from the ESWLCA.
Finally, the surface waves on the CCL will be fed into the port of the radiation patch and the RF energy will be transferred to the RFID tag. As the RF energy is higher than the threshold energy of the RFID tag, communication between the reader and the tag will happen.
3. System Theorem
An LCA has the functions of transmission and radiation of electromagnetic waves. When the electromagnetic waves pass the open slot of the LCA, some electromagnetic waves will leak through the open slot. The LCA has the advantages of wide operation frequency band, large sensing range, easy deployment, and low cost. Figure 5 shows the conventional LCA. The electric field within the area of the radius r of the LCA is determined by
(1)E(r,φ,xd)=M(η,r,φ)Z(xd)ejβxde-αxd,
where φ is the current density, xd is the distance along the x-axis, η is the magnetic flux density, β is the propagation constant of the electromagnetic wave, α is the attenuation constant of the electromagnetic wave, and M(η,r,φ) represents the integration of electromagnetic field for specific boundaries.
Conventional LCA.
The refection coefficient Γ, the voltage U(xd), and the current I(xd) along the x-axis can be represented, respectively, by
(2)Γ(xd)=ZL-Z0ZL+Z0e-j2βxd,U(xd)=Ui0ejβxd[1+Γ(xd)],I(xd)=Ui0ejβxdZ0[1-Γ(xd)],
where Ui0 is the amplitude of the incident wave at xd=0, which is the results of the addition of the characteristic impedance Z0 and load impedance ZL.
The theorems of the impedance, voltage, and current of the ESWLCA follow those of the conventional LCA. The ESWLCA along the z-axis does not need to consider the radiation of the electromagnetic waves. The surface electric field of the ESWLCA, Ep(r,ϕ,z), is a simplified periodic function of Z, as follows:
(3)E(r,ϕ,z)=Ep(r,ϕ,z)e-jkzz,(4)-mf1<f<-mf2,m<0,(5)f1=cP(εr+1),(6)f2=cP(εr-1),
where c is the velocity of light in free space, εr is the dielectric constant, and P is the period of slots. If the frequency conditions (4)–(6) are satisfied, the leaky cable produces radiation waves. Otherwise, the leaky cable generates surface waves. The suitable length of P and slot size will optimize the composition of radiation and surface waves for the desired applications.
Figure 6 shows the SRP structure. The substrate material is the FR4 circuit board with the dielectric constant εr of 4.4 and thickness h of 1 mm. The dimensions are Xw=50 mm, Yl=50 mm, xw=10 mm, and yl=20 mm. The signals are fed into the SRP at position O, as indicated in Figure 6.
SRP structure.
The electric field as a function of the position is as follows:
(7)Ez=Eocos(πxx).
The magnetic flow density on the patch of the SRP is
(8)J=-en·ez·Ez.
The minimum electric field required to activate the tag, Er, is derived as follows:
(9)Er=[Pr·(4·πλ2)·(377Gr)·(1τ)]0.5,(10)S=GtPt4πR2=E2377,(11)τ=4·Ric·Rant|Zant+Zic|2,
where Pr is the received power of the tag at a distance from the antenna of the reader, λ is the operation wavelength, Gr is the tag antenna gain, S is the electric field obtained per unit area, Gt is the reader antenna gain, Pt is the output power of the reader, R is the distance from reader antenna, E is the electric field, τ is the power transfer efficiency from the antenna of the RFID tag to the radio-frequency integrated circuit (RFIC) of the RFID tag, Ric is the real RFIC impedance, Rant is the real antenna impedance, Zant is the antenna impedance, and Zic is the RFIC impedance.
4. System Design
The design of the RFID sensing system is based on a full-wave electromagnetic simulator, Ansoft HFSS. The system is designed to be operated at a UHF frequency band of 860–960 MHz. The output power of the RFID reader is 30 dBm. According to (9), the minimum electric field required to activate the RFID tag is 4.8 V/m. Figure 7 shows the three-dimensional (3D) electromagnetic model of the ESWLCA. Figure 8 shows the return loss characteristics of the ESWLCA.
3D electromagnetic model of ESWLCA.
S11 return loss characteristics of ESWLCA.
Figure 9 is the equivalent circuit of the Alien Higgs-3 RFIC, in which the parallel capacitance Cp is 1.3 pF and the parallel resistance Rp is 1.5 kohms. The RFIC impedance Zic can be determined by
(12)Zic=Rp1+ω2Cp2Rp2-jωCpRp21+ω2Cp2Rp2.
RFIC equivalent circuit.
The impedance of the Higgs-3 RFIC at 920 MHz is 31-j216 ohms.
The back-radiation power of the RFID tag of the battery, Pback, is
(13)Pback=Pthτ·Gr·|Zic-ZantZic+Zant|2,
where Pth is the threshold power to activate the RFIC.
The radar cross-section of the antenna of the tag, σ, is
(14)σ=Pback(Pt·Gt)/4πR2=((((Pt·Gt)/4πR2)·λ/4π·Gr)·Gr·|(Zic-Zant)/(Zic+Zant)|2(((Pt·Gt)/4πR2)·λ/4π·Gr))×((Pt·Gt)/4πR2)-1.
5. Results and Discussion
Figure 10 shows the environment of the RFID tag of the battery. A copper metal object with the size of 200 × 200 × 1 mm3 is 2 mm below the RFID tag.
Environment of RFID tag of battery.
Figure 11 shows the design diagram of the RFID tag with the dimensions of L1=17 mm, L2=5 mm, L3=4 mm, L4=6 mm, L5=3 mm, L6=13 mm, L7=9 mm, L8=10 mm, L9=10 mm, L10=11 mm, L11=17 mm, W1=54.25 mm, W2=2 mm, W3=3 mm, W4=1 mm, W5=1 mm, W6=1 mm, W7=1 mm, W8=1.5 mm, W9=1 mm, W10=5.5 mm, W11=8 mm, W12=10 mm, W13=20.5 mm, W14=18.5 mm, W15=6 mm, W16=5.25 mm, and W17=7 mm.
Design diagram of RFID tag.
Figure 12 shows the S11 return loss characteristics of the RFID tag of the battery. The tag is suitable for objects with metal surfaces. The S11 is −28.6 dB at 930 MHz. The antenna impedance is Zant=19+j186 ohms. Figure 13 shows the 3D pattern of the RFID tag of the battery. The Alien Higgs-3 turn on power sensitivity is −18 dBm.
S11 return loss characteristics of RFID tag of battery.
3D pattern of RFID tag of battery.
Figure 14 shows the two-dimensional (2D) pattern of the RFID tag of the battery. The maximum gain of the antenna of the RFID tag of the battery Gr is −13 dBm. According to (11), the power transfer efficiency from the antenna of the RFID tag of the battery to the Higgs-3 RFIC is τ=0.6.
2D pattern of RFID tag of battery.
The surface electric field distribution of the ESWLCA is analyzed as the RFID reader feeds 30 dBm output power into the ESWLCA. Figure 15 shows the surface electric field distribution of the ESWLCA at the location of 1 mm from the ESWLCA. The maximum electric field Emax is greater than 100 V/m and the minimum electric field Emin is 80 V/m. The RFID tag requires the minimum electric field of 4.8 V/m for operation.
Surface electric field distribution of ESWLCA.
Figures 16, 17, and 18 illustrate the electric field distribution at a distance H from the ESWLCA. Figure 16 shows the electric field distribution of the ESWLCA at a quarter of a wavelength from the ESWLCA (H=λ/4). The maximum electric field Emax is 15 V/m and the minimum electric field Emin is 2.8 V/m.
Electric field distribution of ESWLCA at H=λ/4.
Electric field distribution of ESWLCA at H=λ/2.
Electric field distribution of ESWLCA at H=λ.
Figure 17 shows the electric field distribution of the ESWLCA at half a wavelength from the ESWLCA (H=λ/2). The maximum electric field Emax is 5 V/m and the minimum electric field Emin is 1.3 V/m.
Figure 18 shows the electric field distribution of the ESWLCA at a full wavelength from the ESWLCA (H=λ). The maximum electric field Emax is 2.8 V/m and the minimum electric field Emin is 0.62 V/m.
Figure 19 shows the measurement characteristics of the surface wave power as a function of the diameter of the CCL (DCCL) at a source power of 0 dBm. The surface wave power increases with the increasing DCCL.
Measurement characteristics of surface wave power.
Figure 20 shows the completed sensing system for the BES. The ESWLCA is suitable for the BES which is a small space in a metallic cabinet. The leaky wave is radiated from the open slot in the transmission line of the ESWLCA. The electromagnetic wave propagates along the surface of the transmission line and transmits to the SRP through the bendable CCL. The CCL can still easily transmit the electromagnetic wave even in a closed metallic environment where the barcode approach is not applicable.
Completed RFID sensing system for BES.
6. Conclusion
The state-of-the-art UHF RFID sensing system for the BES of electric motorcycles has been developed. The ESWLCA, CCL, and SRP are designed to overcome the metallic environment in a BES cabinet. The RFID sensing system demonstrates excellent characteristics and shows great potential for the modern BES of electric vehicles.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgment
This work was supported in part by the Ministry of Science and Technology of Taiwan, R.O.C. under Contracts NSC 101-2218-E-018-001, NSC 102-2218-E-018-002, and MOST 103-2221-E-018-021.
MirchandaniP.AdlerJ.MadsenO. B. G.New logistical issues in using electric vehicle fleets with battery exchange infrastructure201410831410.1016/j.sbspro.2013.12.815WangY.-W.Locating battery exchange stations to serve tourism transport: a note200813319319710.1016/j.trd.2008.01.0032-s2.0-41849144086LaiY.-L.ChenC.-C.An intelligent RFID fall notification system201176313331452-s2.0-79956199250ChengC.-S.ChangH. H.ChenY.-T.LinT. H.ChenP. C.HuangC. M.YuanH. S.ChuW. C.Accurate location tracking based on active RFID for health and safety monitoringProceedings of the 3rd International Conference on Bioinformatics and Biomedical Engineering (iCBBE '09)June 2009Beijing, China1410.1109/ICBBE.2009.51622662-s2.0-72749092666SchneiderC. W.TautzJ.GrünewaldB.FuchsS.RFID tracking of sublethal effects of two neonicotinoid insecticides on the foraging behavior of Apis mellifera20127119e3002310.1371/journal.pone.00300232-s2.0-84855704065ChangC.-H.LaiY.-L.ChenC.-C.Implement the RFID position based system of automatic tablets packaging machine for patient safety20123663463347110.1007/s10916-011-9799-62-s2.0-84867868336WambaS. F.NgaiE. W. T.Importance of the relative advantage of RFID as enabler of asset management in the healthcare: results from a Delphi studyProceedings of the 45th Hawaii International Conference on System Sciences (HICSS '12)January 2012Maui, Hawaii, USA2879288910.1109/HICSS.2012.3152-s2.0-84857970152LaiY.-L.ChengJ.A 2.45-GHz RFID wireless-sensor-network location tracking systemProceedings of the IEEE 17th International Symposium on Consumer Electronics (ISCE '13)June 2013Hsinchu, Taiwan133134JabbarH.JeongT.HwangJ.ParkG.Viewer identification and authentication in IPTV using RFID technique200854110510910.1109/TCE.2008.44700312-s2.0-41649111900ChenC.-L.LaiY.-L.ChenC.-C.DengY.-Y.HwangY.-C.RFID ownership transfer authorization systems conforming EPCglobal class-1 generation-2 standards201113141482-s2.0-79953072900LaiY.-L.ChengJ.A cloud-storage RFID location tracking system20145073501004RazaN.BradshawV.HagueM.Applications of RFID technologyProceedings of the IEE Colloquium on RFID TechnologyOctober 1999London, UK1/11/5SteinerG.ZanglH.FulmekP.BrasseurG.A tuning transformer for the automatic adjustment of resonant loop antennas in RFID systems2Proceedings of the IEEE International Conference on Industrial Technology (ICIT '04)December 2004Hammamet, Tunisia9129162-s2.0-27944466466HennonC. C.HelmsC. N.KnappK. R.BowenA. R.An objective algorithm for detecting and tracking tropical cloud clusters: implications for tropical cyclogenesis prediction20112881007101810.1175/2010JTECHA1522.12-s2.0-80052360271LinI.-C.YangC.-W.TsaurS.-C.Nonidentifiable RFID privacy protection with ownership transfer201065234123512-s2.0-77953018435BucknerM.CrutcherR.MooreM. R.WhitusB.MICLOG RFID tag program enables total asset visibility2Proceedings of the Military Communications Conference (MILCOM '02)October 2002Anaheim, Calif, USA1422142610.1109/MILCOM.2002.1179691YuW. D.RayP.MotocT.A RFID technology based wireless mobile multimedia system in healthcareProceedings of the 8th International Conference on e-Health Networking, Applications and Services (HEALTHCOM '06)August 2006New Delhi, India1810.1109/HEALTH.2006.2464822-s2.0-42549110506YunD.-G.LeeJ.-M.YuM.-J.ChoiS.-G.Agent-based user mobility support mechanism in RFID networking environment200955280080410.1109/TCE.2009.51744572-s2.0-68949175638YenY.-S.LinF.ChaoH.-C.Integrated residential gateway: easy IA management with P2P community using RFID200551382483010.1109/TCE.2005.15104902-s2.0-27144502638TesorieroR.GalludJ. A.LozanoM. D.PenichetV. M. R.Tracking autonomous entities using RFID technology200955265065510.1109/TCE.2009.51744352-s2.0-68949194704TesorieroR.GalludJ. A.LozanoM.PenichetV. M. R.Using active and passive RFID technology to support indoor location-aware systems200854257858310.1109/TCE.2008.45601332-s2.0-48749118721ManapureS. S.DarabiH.PatelV.BanerjeeP.A comparative study of radio frequency-based indoor location sensing systems2Proceedings of the IEEE International Conference on Networking, Sensing and ControlMarch 2004Taipei, Taiwan126512702-s2.0-2942661628HontaniH.NakagawaM.KugimiyaT.BabaK.SatoM.A visual tracking system using an RFID-tagProceedings of the SICE Annual ConferenceAugust 2004Sapporo, Japan272027232-s2.0-12744274500CaoT.ShenP.Cryptanalysis of two RFID authentication protocols200991951002-s2.0-77949553535GarfinkelS. L.JuelsA.PappuR.RFID privacy: an overview of problems and proposed solutions200533344310.1109/MSP.2005.782-s2.0-20844459862SieglaffJ. M.HartungD. C.FeltzW. F.CronceL. M.LakshmananV.A satellite-based convective cloud object tracking and multipurpose data fusion tool with application to developing convection201330351052510.1175/JTECH-D-12-00114.12-s2.0-84875650513HanS.LimH.LeeJ.An efficient localization scheme for a differential-driving mobile robot based on RFID system20075463362336910.1109/TIE.2007.9061342-s2.0-52949100457LeiH.CaoT.RFID protocol enabling ownership transfer to protect against traceability and DoS attacksProceedings of the 1st International Symposium on Data, Privacy and E-CommerceNovember 2007Chengdu, China508510LuJ.-C.ChenY.-Y.WangJ.-M.JanJ.-K.ChenC.-C.LaiY.-L.Study and implementation of RFID eseals for power metersProceedings of the 2nd International Conference on Innovations in Bio-Inspired Computing and Applications (IBICA '11)December 2011Shenzhan, China35235510.1109/IBICA.2011.932-s2.0-84862934933ToiruulB.LeeK.An advanced mutual-authentication algorithm using AES for RFID systems200669B156162HenriciD.MüllerP.Hash-based enhancement of location privacy for radio-frequency identification devices using varying identifiersProceedings of the 2nd IEEE Annual Conference on Pervasive Computing and Communications Workshops (PERCOMW '04)March 2004Orlando, Fla, USA14915310.1109/PERCOMW.2004.12769222-s2.0-2942625823JuelsA.Strengthening EPC tags against cloningProceedings of the 4th ACM Workshop on Wireless Security (WiSe '05)September 2005Cologne, Germany67762-s2.0-33749028998KoralalageK. H. S. S.RezaS. M.MiuraJ.GotoY.ChengJ.POP method: an approach to enhance the security and privacy of RFID systems used in product lifecycle with an anonymous ownership transferring mechanismProceedings of the ACM Symposium on Applied Computing (SAC '07)March 2007Seoul, Republic of Korea27027510.1145/1244002.12440692-s2.0-35248846439MolnarD.SopperaA.WagnerD.A scalable, delegatable pseudonym protocol enabling ownership transfer of RFID tags20063897276290Lecture Notes in Computer Science10.1007/11693383_19OsakaK.TakagiT.YamazakiK.TakahashiO.An efficient and secure RFID security method with ownership transferProceedings of the International Conference on Computational Intelligence and Security (CIS ’06)November 2006Guangzhou, China1090109510.1109/ICCIAS.2006.2954302-s2.0-38949198716SaitoJ.RyouJ.-C.SakuraiK.Enhancing privacy of universal re-encryption scheme for RFID tags20043207879890Lecture Notes in Computer Science10.1007/978-3-540-30121-9_84SeoY.AsanoT.LeeH.KimK.A lightweight protocol enabling ownership transfer and granular data access of RFID tagsProceedings of the Symposium on Cryptography and Information Security (SCIS '07)January 2007Sasebo, Japan16WeisS. A.SarmaS. E.RivestR. L.EngelsD. W.Security and privacy aspects of low-cost radio frequency identification systems20042802201212Lecture Notes in Computer ScienceZhangY. P.Indoor radiated-mode leaky feeder propagation at 2.0 GHz200150253654510.1109/25.9230652-s2.0-0035268746WangJ. H.MeiK. K.Theory and analysis of leaky coaxial cables with periodic slots200149121723173210.1109/8.9824522-s2.0-0035721063FosterP. R.BurberryR. A.Antenna problems in RFID systemsProceedings of the IEE Colloquium on RFID TechnologyOctober 1999London, UK3/13/510.1049/ic:19990676KimS.-T.YunG.-H.ParkH.-K.Numerical analysis of the propagation characteristics of multiangle multislot coaxial cable using moment method199846326927910.1109/22.6617142-s2.0-0032026858WaitJ. R.Electromagnetic field analysis for a coaxial cable with periodic slots1977EMC-1917132-s2.0-001908524710.1109/TEMC.1977.303516HuangJ.WeiT.WuS.LanX.FanJ.XiaoH.Coaxial cable bragg grating sensors for structural health monitoring2012553383422-s2.0-84866892513KawanishiT.IzutsuM.Coaxial periodic optical waveguide200071102210.1364/OE.7.0000102-s2.0-0011793344InomataK.YamaguchiY.YamadaH.TsujitaW.ShikaiM.SumiK.Accuracy of 2-dimensional object location estimation using leaky coaxial cables20115962396240310.1109/TAP.2011.21436612-s2.0-79957970712