reading of uhf band passive rfid tags using a reflectorap-s.ei.tuat.ac.jp/isapx/2011/pdf/[frd4-5]...
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Reading of UHF band passive RFID tags
using a reflector #Yuki Oowashi
1, Shigeki Takeda
1 Kenichi Kagoshima
1
1 Ibaraki University
4-12-1 Nakanarusawa, Hitachi, Ibaraki, 316-8511 Japan, Email:obote@mx.ibaraki.ac.jp
Abstract Due to the high cost of tags which are designed for metallic objects, they have not been
commonly used compared to the conventional ones. On the other hand, although reading distance
becomes short, a recognition technique using a dipole tag is very effective to reduce introduction
cost of RFID. In this paper, we have proposed the technique for improving the recognition
performance of UHF RFID which is used in close proximity of metallic objects. We have employed
a waveguide like square shaped reflector in order to be able to read tags attached any position
including the reverse side of a metallic objects which is assumed to be a cylinder. It is clarified that
optimum electromagnetic distribution is similar to TM01 mode of a coaxial waveguide and that the
validity of this property is evaluated by method of moment based electromagnetic simulation and
experiment.
Keywords : UHF RFID tag, Reflector, Reverse side
1. Introduction Radio frequency identification (RFID) is noncontact recognition technology to send and
receive information by wireless. Recently, the RFID technology becomes to be widely used.
However, RFID tags have weakness that their performance is easily affected by metallic objects.
We will present a performance of RFID tags which are used in the close proximity of a
metallic objects. Fig.1 shows the introduction example. In general, when the tag is got close to
metal, the recognizing performance is greatly deteriorated because the impedance of tag is changed.
Especially, receiving power of tags attached to reverse side of metallic object is lower than those
which are attached to front or lateral side. Thus, it is difficult to recognize tags.
In this paper, we have proposed a technique for improving the recognition performance of
UHF RFID which is used in close proximity of metallic objects. We have employed a waveguide
like square shaped reflector in order to be able to read tags attached to reverse side of metallic
objects and assumed a shape of a metallic object to be cylinder. It will be clarified that optimal
electromagnetic distribution is similar to TM01 mode of coaxial waveguide and validity of this
property is evaluated by method of moment based electromagnetic simulation and experiment.
Fig.1 Example of practical use.
2. Problem of recognizing tags attached to reverse side of metallic objects
at free space Reading characteristics of RFID tags which is attached to front, lateral and reverse side of a
metallic cylinder are evaluated in free space. Geometry of a experimental set up is shown in Fig.2.
Since radius of a metallic cylinder is 34.45 mm, distance between the reader and tag is started from
50 mm, and it is varied up to 200 mm with intervals of 50 mm in each three tags position of front,
lateral, and reverse side. Furthermore, measurement has been carried out for different z of 0 mm and
100 mm. z of 100 mm corresponds to the edge of the reader antenna. The reader is RF-ATLP002 by
Mitsubishi Electric Corporation [1]. Width, height and thickness are 200 mm, 200 mm and 25 mm,
respectively. Transmitting power is 30 dBm. A UHF band RFID tag is RF-TGP005-W (Fig.3) [2].
metallic object
direction of movement metallic plate
reader
UHF RFID tagbelt conveyer
waveguide like reflector
metallic object
direction of movement metallic plate
reader
UHF RFID tagbelt conveyer
waveguide like reflector
Experimental results are shown in Fig.4. From Fig.4, the edge located tags have better
characteristic than the center located one in the front side in x=50 mm, because the distance of the
reader and the metallic cylinder is narrow. Moreover, electric field Z-component Ez is attenuated in
reverse side of metallic object from Fig.5. So, the tag in reverse side cannot be recognized.
dUHF RFID tag
metallic
cylinder
Z=0Z
x
reader
68.9
reverse side
lateral side
front side
dUHF RFID tag
metallic
cylinder
Z=0Z
x
reader
68.9
reverse side
lateral side
front side
Fig.2 Experimental environment in free space. Fig.3 UHF band RFID tag (RF-TGP005-W).
Fig.4 Recognition performance as a function of distance between the metallic cylinder and reader.
(a) Ez in x-y plane (z=0)
(i)x=50 [mm] (ii)x=100 [mm] (iii)x=150 [mm] (iv)x=200 [mm]
(b) Ez in x-z plane (y=0)
Fig.5 Electric field distribution (Ez component) in free space.
3. Reading technique of RFID tags attached to metallic objects 3.1. Method of moment analysis As shown in Fig.4 and Fig.5, it is very hard to recognize a tag in the reverse side. Therefore,
we propose a square shaped reflector to improve reading characteristics of tags attached to a reverse
side of a metallic cylinder. Fig.6 shows the analytical model. In this way, the technique for
improving the recognition rate of tag on reverse side was examined. The input impedance of the
patch antenna is 47.0-j8.02 ohm in free space at 953MHz and source impedance is set to be
complex conjugate of this input impedance. Transmitting power of the reader is set to be 30 dBm.
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7.3 mm
2.2 mm
Unit:mmx
zy
x
zy
(a)d=4 [mm] (b)d=6 [mm]
metallic
cylinder patch
antenna
Simulation results are obtained by EEM-MOM ver.1.5 [3] that is based on method of
moment.
3.2. Simulation results Electric field Ez that is parallel to a tag is shown in Fig.7. Fig.7 (a) and (b) show z
component of electric field in x-y plane (z=0) and x-z plane (y=0), respectively. The width of the
square shaped waveguide is varied from 340 mm to 400 mm every of 20 mm. From these results,
we can see uniform electric field distribution around the metallic cylinder is obtained at width of
360 mm. This kind of electric field distribution is desirable, because tags attached to front, lateral
and reverse sides must be recognised. This electric field distribution is similar to TM01 mode of a
coaxial waveguide [4]. On the other hand, complex electric field distribution appears for larger
width of the square shaped reflector than 360 mm. The complex electric field distributions results in
dead spots. The dead spot means the position where recognition rate is lower than 100 %. Therefore,
we can conclude that TM01 mode electric field distribution is optimum for RFID systems. In the
experiment presented in the next section, the size of copper reflector will be chosen to be 360×360
×400 mm.
patch antenna
metallic cylinder
a
a
400
square shaped
waveguide
68.9z
x
y
0
200
200
10
140
140
patch antenna
35
patch antenna
metallic cylinder
a
a
400
square shaped
waveguide
68.9z
x
y
0z
x
y
0
200
200
10
140
140
patch antenna
35
Fig.6 Analysis model.
(a) Electric field in x-y plane (z=0)
(i)a=340 [mm] (ii)a=360 [mm] (iii)a=380 [mm]
(b) Electric field x-z plane (y=0)
Fig.7 Electric field distribution (Ez component).
4. Experimental results 4.1 Experimental set up The width and the height of the reflector are 360 mm and 400mm, respectively. These are
determined based on the simulation in the preview section. Fig.8 shows the photos of the
experimental set up.
Unit:mm
metallic
cylinder
square shaped
waveguide patch
antenna
4.2 Experimental results Fig.9 shows experimental results in terms of recognition rate with or without the reflector
when distance of d is 4 mm and 5 mm. The recognition performance of the tag on lateral and
reverse side has been improved by the reflector. Especially, the recognition performance on the
reverse side is considerably improved.
When d is grater than or equal to 3 mm, recognition rate on reverse side at z of 0 becomes
to be 100%.
(a)Perspective view (b)Internal structure
Fig.8 An experimental environment.
Fig.9 Recognition performance with or without the reflector.
5. Conclusion We have proposed the recognition technique of UHF band RFID tags attached to a
cylindrical metallic object. In order to recognize tags attached to the reverse side of the metallic
object, we have introduced the square shaped reflector. The square shaped reflector and cylindrical
metallic object generate electric field distribution similar to a coaxial waveguide. It is clarified that
TM01 mode electromagnetic distribution is optimum to recognize tags attached on front, lateral and
reverse side of the cylindrical metallic objects.
Derivation of optimum size of a square shaped reflector for various kind of metallic objects
will be a further study.
References [1] http://www.mitsubishielectric.co.jp/device/rfid/products/index.html#02
[2] http://www.mitsubishielectric.co.jp/device/rfid/products/index.html#03
[3] http://www.imsab.co.jp/eem/
[4] N. Marcnvitz, “Waveguide Hand Book,” Mc Graw-Hill Co., Inc., pp.39-81, 1951.
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UHF RFID reader UHF RFID
tag
z
xy
z
xy
x
y
zx
y
z
(a)d=4 mm (b) d=5 mm
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