A Novel RFID Sensing System Using Enhanced Surface Wave Technology for Battery Exchange Stations

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.


Introduction
Air pollution, especially CO, HC, and NO  , 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 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.

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.
The system structure of the proposed RFID sensing system is shown in Figure 2. The system includes readers, ESWL-CAs, 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.
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.

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  of the LCA is determined by where  is the current density,   is the distance along the -axis,  is the magnetic flux density,  is the propagation constant of the electromagnetic wave,  is the attenuation constant of the electromagnetic wave, and (, , ) represents the integration of electromagnetic field for specific boundaries.
The refection coefficient Γ, the voltage (  ), and the current (  ) along the -axis can be represented, respectively, by where  0 is the amplitude of the incident wave at   = 0, which is the results of the addition of the characteristic impedance  0 and load impedance   .The theorems of the impedance, voltage, and current of the ESWLCA follow those of the conventional LCA.The ESWLCA along the -axis does not need to consider the radiation of the electromagnetic waves.The surface electric field of the ESWLCA,   (, , ), is a simplified periodic function of , as follows: where  is the velocity of light in free space,   is the dielectric constant, and  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  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   of 4.4 and thickness ℎ of 1 mm.The dimensions are   = 50 mm,   = 50 mm,   = 10 mm, and   = 20 mm.The signals are fed into the SRP at position , as indicated in Figure 6.
The electric field as a function of the position is as follows: The magnetic flow density on the patch of the SRP is The minimum electric field required to activate the tag,   , is derived as follows: where   is the received power of the tag at a distance from the antenna of the reader,  is the operation wavelength,   is the tag antenna gain,  is the electric field obtained per unit area,   is the reader antenna gain,   is the output power of the reader,  is the distance from reader antenna,  is the electric field,  is the power transfer efficiency from the antenna of the RFID tag to the radio-frequency integrated circuit (RFIC) of the RFID tag,  ic is the real RFIC impedance,  ant is the real antenna impedance,  ant is the antenna impedance, and  ic is the RFIC impedance.

System Design
The design of the RFID sensing system is based on a fullwave 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.Figure 9 is the equivalent circuit of the Alien Higgs-3 RFIC, in which the parallel capacitance   is 1.3 pF and the parallel resistance   is 1.5 kohms.The RFIC impedance  ic can be determined by The impedance of the Higgs-3 RFIC at 920 MHz is 31 − 216 ohms.
The back-radiation power of the RFID tag of the battery,  back , is where  th is the threshold power to activate the RFIC.The radar cross-section of the antenna of the tag, , is  Figure 12 shows the  11 return loss characteristics of the RFID tag of the battery.The tag is suitable for objects with metal surfaces.The  11 is −28.6 dB at 930 MHz.The antenna impedance is  ant = 19 + 186 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.  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   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  = 0.6.

Results and Discussion
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  max is greater than 100 V/m and the minimum electric field  min is 80 V/m.The RFID tag requires the minimum electric field of 4.8 V/m for operation.
Figures 16, 17, and 18 illustrate the electric field distribution at a distance  from the ESWLCA.Figure 16 shows the         Figure 19 shows the measurement characteristics of the surface wave power as a function of the diameter of the CCL ( CCL ) at a source power of 0 dBm.The surface wave power increases with the increasing  CCL .
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.

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.

Figure 2 :
Figure 2: System structure of RFID sensing system.

Figure 10 :
Figure 10: Environment of RFID tag of battery.

Figure 11 :
Figure 11: Design diagram of RFID tag.

Figure 12 :
Figure 12:  11 return loss characteristics of RFID tag of battery.

Figure 13 :
Figure 13: 3D pattern of RFID tag of battery.

Figure 14 :
Figure 14: 2D pattern of RFID tag of battery.

Figure 17 D
Figure17shows the electric field distribution of the ESWLCA at half a wavelength from the ESWLCA ( = /2).The maximum electric field  max is 5 V/m and the minimum electric field  min is 1.3 V/m.

Figure 19 :
Figure 19: Measurement characteristics of surface wave power.

Figure 20 :
Figure 20: Completed RFID sensing system for BES.

Figure 18
Figure18shows the electric field distribution of the ESWLCA at a full wavelength from the ESWLCA ( = ).The maximum electric field  max is 2.8 V/m and the minimum electric field  min is 0.62 V/m.Figure19shows the measurement characteristics of the surface wave power as a function of the diameter of the CCL ( CCL ) at a source power of 0 dBm.The surface wave power increases with the increasing  CCL .Figure20shows 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 Figure 16: Electric field distribution of ESWLCA at  = /4.