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Abstract—If the wireless power transfer system is located to a
person; the magnetic fields produce by the wireless power
transfer system will vary in the region occupied by the human
body. According the Institute of Electrical and Electronic
Engineers (IEEE) and International Commission on
Non-Ionizing Radiation Protection (ICNIRP), the predicated or
the measured values have to be spatially averaged in an area
representing the dimension of the human body and compared
with the adopted reference level of exposure. Although spatial
averaging is better approximation compared to point
measurement, the standard of how to perform such an
assessment does not exist. This paper describes the method for
spatial averaging magnetic field. We analyzed the difference of
spatial averaging value (1-and 2-dimensional templates consists
of different number of position) and compared it with the
standard uncertainty for measurement drift. The proposed
methods are given to present the effectiveness of the choice of
measurement position.
Index Terms—Wireless power transfer system, magnetic
field, exposure, human, spatial average, reference level,
uncertainty.
I. INTRODUCTION
As wireless power transfer technology advances, a variety
of applications such as mobile consumer electronics,
automotive, biomedical devise and industrial system are
developed. Although the wireless power transfer
technologies provide the public with convenience and safety
which wired power connection couldn’t provide, there is
another potential danger arising from the system s form of
electromagnetic field(EMF). Just like the EMI is one of the
most import criteria of electronic system, EMF is a
significant measure of safety for human body and also for
other electric devices.
Exposure guidelines and standards[1]-[4] from regarding
the protection of human from electromagnetic fields adopt
basic restrictions of exposure to such fields. The reference
levels are provide due to difficulties involved in assessing the
basic restriction for a particular exposure condition. The field
value for single point is required for comparions with the
reference value. However, When the sources of
electromagnetic fields(EMFs) is close to the body, the EMFs
will vary depending on the position at which it is measure. To
address this, exposure guidelines and standards recommend
that the reference levels must be compared with the root
Manuscript received October 25, 2012; revised November 29, 2012. This
work was supported in part by the KCC(Korea Communicaitons
Commission), Koea, under the R&D program supervised by the KCA(Korea Communication Agency)” (KCA-2012-08921-01304)
The authors are with the Electronics and Telecommunications Research
Institute, Daejeon, Korea (e-mail: [email protected], [email protected],
[email protected], [email protected]).
mean square(RMS) fields quantities spatially averaged over a
specific area representing the dimensions of the human body
in the absence of a person. Although spatial averaging is a
better approximation compared to point measurements, a
harmonized standard of how to perform such an assessment
does not exist.
The purpose of this paper is to propose spatial averaging
methods and to assessment for strength and distribution of
magnetic field from wireless power transfer system.
II. MEASUREMENT AND ANALYSIS
We performed measurements of the magnetic field
strength generated by an wireless power transfer system of
desktop computer. The wireless power transfer system of
desktop computer shows is Fig 1. The wireless power transfer
system of desktop computer is used the coupled magnetic
resonance method, the resonance frequency and transmitting
power is 1.71 MHz and 85 W, respectively.
Taking into account that the exposure standards in use
require the reference levels to be compared with the
maximum expected RMS values spatially averaged over a
volume representing the human body, many spatial averaging
techniques have been proposed[5]-[8], but a harmonized
standard of how to perform such an assessment does not exist.
The magnetic field strength was examined in constricted
volumes corresponding approximately to the dimension of
the human body. The plane with width of 40 cm and the
height of 180 cm was chosen to be dimension (4x18, 10 cm
grid step).The measurement distance and spacing is 50 cm
and 10 cm, respectively. We used EHP-200 instrument
(Narda STS, Germany). The measurements were repeated 3
times at each position.
The Fig. 2 respresent the measured distribtuons of
magnetic field strength. The magnetic field strength of
wireless power transfer system of desktop computer are
0.012 A/m (minimum), 0.402 A/m (maximum), and the total
spatially averaged value(𝐻𝑎𝑙𝑙 ) is 0.213 A/m, which don’t
exceeds the reference level, 0.43 A/m of ICNIRP’s
guideline[1].
The spatial averaging measurement takes considerable
evaluation time. Therefore, we suggest the improved
measurement method to reduce the evaluation time for the
spatial averaging measurement. The reduction of the number
of measurement positions was investigated by creating 1- and
2- dimensional templates consisting of 3, 6, 9, 18 and 27
positions (see Fig. 3).
The spatially averaged value of different templates
(𝐻𝑡𝑒𝑚𝑝𝑙𝑎𝑡𝑒 ) were comparing averaged value (𝐻𝑎𝑙𝑙 ) from all
90 measured positions. The mean value ( 𝐷𝑠𝑚 ) of this
differenence value were calculated
Seon-Eui Hong, Hyung-Do Choi, Jeong-Ik Mom, and Seong-Min Kim
Evaluation Method of Electromagnetic Field Exposure
Levels from Wireless Power Transfer System
International Journal of Computer and Electrical Engineering, Vol. 5, No. 3, June 2013
334DOI: 10.7763/IJCEE.2013.V5.726
1.8
m
0.4 m
10 cm10 cm10cm
10cm
10 cm
10 cm
Fig. 1. Wireless power transfer system of desktop computer and measurement
position for whole body
Fig. 2. Magnetic field strength distribution for wireless power transfer system of Desktop computer
TABLE I: TEMPLATE SCHEMES
Template
Horizontal axis
Interval
(cm)
vertical axis
Interval
(cm)
Total
points
Surface a
b
20
20
20
30
27
18
Line*
d
e
f
-
20
30
50
9
6
3
* Line is center line of whole body
We compared these differences (𝐷𝑠𝑚 ) with the standard
uncertainty for measurement drift. That is, we tried to find the
maximum difference value, which is less than the standard
uncertainty for repeated measurement. The standard
uncertainties by repeated measurement were in 0.74
dB(student t-distribution; 𝑡0.05 = 4.30, degree freedom 2).
So we selected the value of 0.74 dB as the criterion for
measurement point reduction. And another selection criterion
for measurement position is the least number of measurement
points.
We selected the measurement position as the suitable
spatial averaging for human exposure measurement from
wireless power transfer system of desktop computer. The
selected the measurement positions are shown Fig. 3e.
III. CONCLUSION
To evaluate exposure compliance with reference levels,
the spatial averaging process is applied. The spatial averaging
measurement takes considerable evaluation time. We suggest
the improved measurement method to reduce the evaluation
time for the spatial averaging measurement. If the difference
spatial averaging value between reduced measurement
position and measurement position for whole body dense is
less than the standard uncertainty for repeated measurement,
the reduced measurement position is considered sufficient to
evaluate human exposure to EMF from wireless power
transfer system of desktop computer.
TABLE II: THE SPATIALLY AVERAGED VALUE AND THE DIFFERENCE
BETWEEN AVERAGES MEAN VALUE FOR DIFFERENT TEMPLATES AND
AVERAGED VALUE FROM ALL 90 MEASURED POSITIONS
Template Spatially averaged Value (Htemplate )
[A/m]
Dsm
(dB)
Surface a
b
0.189
0.181
0.45
0.83
line
d
e
f
0.197
0.188
0.205
0.30
0.60
0.23
1.8
m
0.4 m
0.2 m
0.2 m
0.2 m
0.2 m
1.8
m
0.4 m
0.2 m
0.3 m
0.3 m
0.2 m
(a) (b)
1.8
m
0.2 m
0.2 m
1.8
m
0.3 m
0.3 m
0.3 m
0.8 m
1.3 m
(c) (d) (e)
Fig. 3. Template schemes
REFERENCES
[1] A. Ahlbom, U. Bergqvist et al.,“Guidelines for limiting exposure to
time-varying electric, magnetic, and electromagnetic fields (up to 300 GHz),” Health Phys., vol. 74, no. 4, pp. 494-522, Apr. 1998..
[2] A. Ahlbom, U. Bergqvist et al., “For limiting exposure to
time-varying electric, magnetic, and electromagnetic fields (1Hz - 100 kHz),” Health Phys., vol. 99, no. 6, pp. 818-836, 2010.
[3] IEEE Standard for Safety Levels With Respect to Human Exposure to
Radio Frequency Electromagnetic fields, 3 kHz to 300 GHz, IEEE Std. C95.1-2005, 2006..
[4] Technical Requirements for the human Protection against
Electromagnetic waves, KCC Notice 2012-2. [5] Electronic Communications Committee (ECC), ECC Recommendation
(02)04 (revised electronic Communications Committee (ECC), ECC
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180
Mag
ne
tic
fie
ld s
ten
gth
[A
/m]
Vertial plane height [cm]
International Journal of Computer and Electrical Engineering, Vol. 5, No. 3, June 2013
335
Recommendation (02)04 (Revised Bratislava 2003, Helsinki 2007):
Measuring Non-Ionizing Electromagnetic Radiation (9 kHz–300 GHz),
2007. [6] Limits of Human Exposure to Radiofrequency Electromagnetic Fields
in the Frequency Range From 3 kHz to300 GHz, Canada, Safety Code
6, 1999. [7] IEEE Recommended Practice for Measurements and Computations of
Radio Frequency Electromagnetic Fields With respect to Human
Exposure to Such Fields, 100 kHz- 300 GHz, IEEE Std. C95.3 2002, 2002.
[8] IEC 62369-1, “Evaluation of human exposure to electromagnetic fields
from short range devices (SRDs) in various applications over the frequency range 0 GHz to 300 GHz -Part 1: Fields produced by devices
used in electronic article surveillance, radio frequency identification
and similar systems,” 2008.
Seon-Eui Hong was born in Daejon, Korea, in 1995. She received the B.S and MS degrees in radio science and
engineering from chungnam National University, Daejeon,
in 1997 and 1999, respectively. Since 1999, she has been with the Electronics and Telecommunications Research
Institute, Daejeon, Korea, Where is current a Senior
Member of the radio technology group. Her current research interests include evaluation of human exposure
levels from Wireless power transfer system and the usage equipment in
workplace.
Hyung-Do Choi received the M.S. and Ph.D. degrees
in material sciences from Korea University, Seoul, in
1989 and 1996, respectively. Since 1997, he has been with the Electronics and Telecommunications Research
Institute, Daejeon, Korea, where he is currently a
Senior Member of the radio technology group. His current research interests include electromagnetic
compatibility countermeasures, electromagnetic wave
absorber design, and absorbing and shielding materials.
Jung Ick Moon was born in Daegu, South of Korea, in 1975. He received the B.S. degree in electrical
engineering from Yeugnam Univ. in 1996, and the M.S.
degree and Ph.D. degree in electrical and electronics from KAIST, South of Korea in 2000, 2004,
respectively. Since 2004, he has worked as a
Researcher in Electronics and Telecommunications Research Institute (ETRI), South of Korea. His main
interests in research are broadband antenna design, antenna measurement
system and wireless power transmission.
Sung Min Kim was born in Daegu, South of Korea, in
1973. He received the B.S. and M.S. degree in
electronics engineering from Kyungpook National Univ. in 1997, 2009, respectively. Since 2002, he has worked
as a Researcher in Electronics and Telecommunications
Research Institute (ETRI), South of Korea. His main interests in research are RF circuit design and wireless
power transmission.
International Journal of Computer and Electrical Engineering, Vol. 5, No. 3, June 2013
336