Effects of Animals on Performance of UHF RFID

Tags in Animal Traceability Application

#Pornanong Pongpaibool RFID Program, National Electronics and Computer Technology Center 112 Thailand Science Park, Klong Luang, Pathumthani 12120, Thailand

Email: pornanong.pongpaibool@nectec.or.th

1. Introduction Recently, the radio frequency identification (RFID) technology is popular for automatically identifying the objects such as pallets, books, animals, or human beings. In the RFID system, the data is transferred between a transponder (tag) and an interrogator (reader) using magnetic or electromagnetic fields [1]. The magnetic coupling technique is used in the low frequency (LF) and high frequency (HF) bands, where the read range is limited. On the other hand, the electromagnetic backscattering technique is used in the ultra-high frequency (UHF) band where a longer read range can be achieved. The UHF RFID is widely used in many applications where tags cannot be moved close to the reader, such as logistics or airline baggage system. However, the performance of the UHF RFID system is strongly affected by the dielectric bodies and the metals nearby. Various studies have been proposed for improving the read range and performance of the UHF RFID system [2-5] Due to the animal disease outbreak in recent years such as the avian influenza, the system that can immediately identify the origin of animals is urgently required. The RFID technology is one of the suitable solutions for a quick track and trace [6]. The RFID Program at National Electronics and Computer Technology Center (NECTEC) implemented a pilot project of poultry traceability system [7]. In this system, the UHF RFID is used for tracing the cage containing about 6-8 chickens instead of using LF RFID for tracing the individual chicken. However, the RFID tag cannot be directly attached to the cage. The package is needed for protecting the RFID tag from water, sterilizing agent, and bumping. The dielectric bodies of package, cage, and poultry have a significant effect on the UHF RFID system performance. In order to study the performance of the RFID tag placed inside a protective package, we have examined the effects of the air gap size between the RFID tag and the package by neglecting the poultry inside the cage [8]. The results in [8] indicated that high performance RFID tags can be achieved by using the FR4 substrate and increasing the air gap size to at least 2 mm, both on top and bottom of the RFID tag.

In this paper, we study the effects of the animal inside the cage on the performance of the RFID tag. The poultry is modeled as a simple cube and sphere with the dielectric property of a muscle. We vary the size of the poultry model and the air space between the model and the cage to examine the UHF RFID tag performance, the resonant frequency, and the read range.

2. Models and Method

2.1 Calculation Models Figure 1 shows the antenna used for evaluating the RFID tag performance. The meandered dipole antenna has the same size as in [8] for the 2-mm air gap size between the antenna substrate and the package, but the shape is modified to be able to fit into the rectangular cells. This antenna is tuned to match with the impedance of the IC chip (XRAG2, STMicroelectronics) [9] at 922.5 MHz, which is the center frequency of the UHF band used in Thailand, 920-925 MHz. The dimensions of these antennas are: l1=60 mm, s1=18 mm, s2=s4=4 mm, s3=2 mm, s5=10 mm, w1=1 mm.

Figure 1: Antenna model.

W1

W2

W3

H1H2

H4

H3

Package

Poultry cage

Antenna substrate

y

z

x

Muscle tissue

CD

(a) 3-D view. (b) Vertical cross-sectional view.

Figure 2: Dielectric model.

Table 1: Properties of Dielectric Model. Model Material Relative permittivity (εr)

Package Acrylonitrile butadiene styrene (ABS) 2.6 Poultry Cage High density polyethylene (HDPE) 2.4

Poultry Muscle tissue 55 Antenna Substrate FR4 4.3

The dielectric model of the package, the cage, and the poultry is shown Fig. 2. The antenna substrate is 1.6-mm FR4. In this study, the poultry is modelled as a cube and a sphere of muscle tissue. The dimensions of these dielectric bodies are: L1=104 mm, L2=140 mm, H1=5.6 mm, H2=9.6 mm, H3=5 mm, W1=70 mm, W2=74 mm, W3=100 mm. We vary the air space between the poultry cage and the poultry model (H4), the side length of a cube (C), and the diameter of a sphere (D). The center of the poultry model on the XY plane is at the antenna feed point. The dielectric property of these dielectric materials is listed in Table 1.

2.2 Calculation Method In [8], we simulated the antennas using the Advanced Design system (ADS) momentum analysis software, which is based on the method of moment (MoM) [10]. However, the ADS recommend that the substrate thickness should be less than half wavelength. The wavelength of high-dielectric-constant muscle tissue is approximately 4 cm, which is much smaller than the actual poultry size. Therefore, the finite difference time domain (FDTD) method [11] is used in this study in order to simulate the thick dielectric body with high dielectric constant. The minimum cell size is set to 1 mm according to the antenna width and the 8-cell perfectly matched layer absorbing boundary condition is used. The RFID tag performance is evaluated using the read range, which is obtained from the Frii’s transmission equation [2] as follows.

th

rtt

P

GGPr

τπλ4

= (1)

λ is the wavelength, Pt is the power delivered to the transmitting reader antenna, Gt is the gain of the transmitting reader antenna, Gr is the gain of the receiving tag antenna, Pth is the minimum threshold power required to power up the IC chip, and τ is the power transmission coefficient given by τ = 4RcRa/|Zc+Za|

2. Zc = Rc + jXc is the impedance of IC chip, and Za = Ra + jXa is the impedance of RFID tag antenna.

(a) Normalized read range. (b) Resonant frequency shift.

Figure 3: Simulation results. In this study, the read range is determined by normalizing the read range of the RFID tag antenna with the evaluating dielectric model to that of the antenna in the package without poultry model. For the same RFID reader and IC chip, the normalized read range r’ can simply be calculated as follows.

refref

ee

ref

e

G

G

r

rr

ττ

==' (2)

Subscripts e and ref indicate the evaluating poultry model and the reference model of the package and the cage without the poultry model, respectively. Ge and Gref are the antenna gain at θ = 0. Moreover, the resonant frequency shift is determined by the difference of the resonant frequency of the evaluating poultry model and that of the reference model.

3. Simulation Results Fig. 3 shows the normalized read range and the resonant frequency shift of the meandered dipole antenna with different air space between the cage and the animal model (H4); 0, 20, 50, and 100 mm. The read ranges are normalized to the read range of the tag placed in the package without the poultry model. Two different side length of cubic poultry model (50 and 100 mm) and three different diameter of spherical poultry model (50, 100, and 150 mm) are used.

For the cubic model, the read range decreased when the space H4 is 0 and 100 mm. The read range of spherical model with the diameter of 50 and 100 mm decreased in all cases. However, the read range of the 150-mm-diameter sphere decreased when the space H4 decreased. We can see that, both cubic and spherical model, except 150-mm-diameter sphere, result in the same level of read range when the space H4 is 0 or 100 mm. The average read range of these models also results in the same level at both space size, and higher than 100% when the space H4 is 20 and 50 mm. For the frequency shift in Fig. 3 (b), except the 100-mm cubic model, the resonant frequency shifted to the higher frequency and less than 20 MHz. However, the resonant frequency of the 100-mm cubic model shifted to the lower frequency when the space is 0, 20 and 50 mm. The higher in resonant frequency shift of this model is considered as the effect from the larger surface area. The average resonant frequency shift of these models is approximately 10 MHz. According to these results, we can see that a larger surface area model results in a higher resonant frequency shift. Moreover, the read range can be slightly increased by providing the distance between the cage and the animal to approximately 20-50 mm.

4. Conclusions In this paper, we study the effects of the animal to the performance of the UHF RFID tags in poultry traceability application. The meandered antenna is used and placed in the protective package attached to the poultry cage. The poultry is modelled as a cubic and spherical shape of

muscle tissue. The air space between the poultry cage and the poultry model is varied to examine the effects of the poultry model to the read range and the resonant frequency. The numerical simulation results show the effects of the shape and the size of poultry models. The larger surface area model results in the higher resonant frequency shift. Moreover, the average results indicated that the read range in the same level as the read range of the tags placed in the package in free space can be obtained by providing the distance between the cage and the poultry to approximately 20-50 mm.

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[6] U. Ketprom, C. Mitrpant, and P. Lowjun, “Closing digital gap on RFID usage for better farm management,” Portland International Center for Management of Engineering and Technology Proceeding, pp. 1748-1755, Aug. 2007.

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[8] P. Pongpaibool, “A study on performance of UHF RFID tags in a package for animal traceability application,” Proceeding of ECTI-CON, pp. 741-744, May 2008.

[9] STMicroelectronics, Evaluating the XRAG2 impedance for antenna design purposes [online] Available URL: http://www.st.com/stonline/products/literature/tn/13927.pdf.

[10] R. F. Harrington, Field Computation by Moment Method, The Macmillan Co., 1968. [11] A. Taflove, Computational Electrodynamics: The Finite-Difference Time-Domain Method,

Artech House, 1996.