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Investigation of Wireless Power Transfer in Through-Wall Applications
Matt Hoang, Young-Sik Seo, Zachariah Hughes, Deena Isom, Minh Nguyen, Smitha Rao, and J.-C. Chiao
NSF REU: Sensors and Applications, The University of Texas at Arlington, Arlington, Texas 76019
Abstract
5cm Coil Set Tuning Results
Results
In this work, a through-wall wireless power transfer system is
investigated to observe the effects of various materials on maximum
power transfer as the medium between the transmitter and receiver
circuits. The wireless power transfer system utilizes inductive coupling
with a fixed frequency of 1.3MHz resonance. Materials such as wood,
brick, and drywall were tested at two different thicknesses. In addition,
two sets of identical radial antenna were used and compared.
It is concluded that the power attenuation with spacing distance
dominates the output power and transfer efficiency, while tuning could
counteract the parasitic effects in the material and recover the power lost
in deviation from resonance. Tests were performed on randomly chosen
walls to verify the system performance.
Transfer efficiency was calculated from the output power in the
receiver load over the input power in the transmitter. For each set of
coils, the following procedures to reach a maximum output power
were repeated:
1. The system was first tested with air as the medium.
2. The transmitter circuit was tuned with matching capacitance until a
maximum voltage was achieved (resonance).
3. Then the operating frequency was fine-tuned to increase the load
voltage
4. Next, tuning the receiver capacitances to reach maximum output
voltage.
5. The same tuning procedures 1-4 were repeated for brick, wood and
drywall as the media, each with two different thicknesses of 4.4 cm
and 8.5 cm.
Introduction
Wireless power transfer has been utilized for lower energy level
applications such as passive RFID, and higher energy level ones such as
charging of portable electronic devices. Inductive coupling has been
utilized due to its relatively simple operation mechanism and availability
of affordable electronic parts. The technique provides the ability to
transfer high amounts of power. The operating limitation for inductive
coupling is on the attenuation of electromagnetic energy with the distance
between transmitter and receiver coils, reducing the power delivered and
efficiency. In this work, we proposed a through-wall inductive coupling
system to transfer electric energy. An example of the proposed system can
be seen in figure 1.
Wall
CC2 1
AC
RR21
L2L1
R21
R
LL21
CC12
RLL
R
Figure 3. This graph shows the maximum efficiency and output power vs.
matching capacitance in air. Shows that maximum efficiency and power
occur at different capacitances.
Figure 1. Proposed
Wirelessly Powered
Lighting System
0.5
The system configuration is shown in Fig. 1. The carrier frequency was
set at 1.3MHz and the load was 500 Ω. The input signal was a square
wave with 6 Vpp amplified by a class-E amplifier consisting of an nchannel power MOSFET with a 50% duty cycle. Output voltage was
measured on the load resistor for output power while input voltage and
current were measured to obtain input power. Two sets of antennas were
utilized in our experiments. In the coil set #1, both coils had a radius of 5
cm and turn numbers of 17 and 16 for the transmitter and receiver,
respectively. Coil set #2 consisted of similar coils of radius 15cm with
the same wire lengths of coil set #1 for the transmitter and receiver. The
experimental setup can be seen in Fig. 2 below.
Load Power (W)
4.4cm
Materials and methods
Wood
Drywall
Spacing, cm
4.4
8.5
4.4
8.5
4.4
8.5
Tuned (Air)
0.449
0.165
0.392
0.163
0.406
0.157
Untuned (Medium)
0.424
0.157
0.388
0.155
0.399
0.154
Tuned (C1)
0.428
0.165
0.388
0.161
0.399
0.157
Tuned (Freq.)
0.428
0.165
0.388
0.162
0.399
0.157
Tuned (C2)
0.428
0.165
0.388
0.162
0.399
0.157
Table 1. 5cm Coil Results from Retuning with Various Wall Materials.
These coils have closer optimal distance around 4.4cm. Power drops
significantly if the distance is almost doubled. From these results the
retuning method proposed in the procedure, we have managed to
scavenge some energy back in 4.4 cm. At 8.5 cm we are able to return to
the same or close to the output power when it was in air.
Output, W
LED
lighting
M
M
Brick
8.5cm
0.4
0.3
1)
2)
3)
4)
0.2
0.1
Air (tuned)
Brick
Brick (C1) Brick (Freq.) Brick (C2)
(Untuned)
Tuning Methods
Brick
Wood
Drywall
Spacing, cm
4.4
8.5
4.4
8.5
4.4
8.5
Tuned (Air)
0.354
0.422
0.305
0.423
0.370
0.439
Untuned (Medium)
0.236
0.346
0.297
0.388
0.339
0.411
Tuned (C1)
0.305
0.371
0.297
0.404
0.349
0.415
Tuned (Freq.)
0.306
0.375
0.299
0.406
0.352
0.415
Tuned (C2)
0.308
0.375
0.322
0.405
0.352
0.415
Table 2. 15cm Coil Results from Retuning with Various Wall Materials.
These coils have a closer optimal distance around 8.5cm. This is different
from the 5cm radii coils, possible due to some kind of near-field
interference. However, the difference from 4.4 to 8.5cm is not as drastic
from the first coil set. The results from this test are similar to the first coil
set. Some differences to note is that the power loss from the brick is more
pronounced as we are unable to scavenge as much power in this case as
the others.
In order to test and validate the proposed system, a random wall test
was conducted on various walls of the testing facility at UTA
Nedderman Hall. The transmitter antenna was attached to one side
of the wall and the receiver was attached to the opposite side as seen
in Fig. 6. Preliminary results have shown that the 5cm coil set will
be unable transfer sufficient power from a distance of the walls
which were measured to be 30cm. Because of this, only the 15cm
coils were tested. An output power of 25mW with an efficiency of
2% was achieved after tuning which agreed with the results
measured in air.
Coil taped on wall
Brick
wall
In this work, we proposed and investigated a through-wall wireless
power transfer system that utilizes inductive coupling for energy
scavenging from outdoor sources, such as solar panel or wind turbine,
for indoor applications, such as lighting or sensing. Different wall
materials have been tested with various thicknesses and coil
configurations. We concluded that the wall materials do not affect
significantly the output power or transfer efficiency due to the long
wavelength. The power attenuation with spacing dominates the
performance and determines design factors in applications. Tuning the
transmitter and receiver circuitry as well as the operating frequency to
overcome additional parasitic effects from the wall materials can help
in achieving optimal output powers.
Coil
Literature cited
J. Curty et al., “Remotely powered addressable UHF RFID integrated
system”, IEEE J. Solid-State Circuits, Vol. 40, No. 11, pp. 21932202, 2005.
H. Cao et al., “Remote Detection of Gastroesophageal Reflux Using an
Impedance and pH Sensing Transponder,” 2012 IEEE IMS,
Montreal, Canada, June 17-22, 2012.
Z. N. Low et al., "Design and Test of a High-Power High-Efficiency
Loosely Coupled Planar Wireless Power Transfer System," IEEE
Transactions on Industrial Electronics, Vol. 56, No. 5, pp. 18011812, May 2009.
Y.-S. Seo et al., “Wireless Power Transfer for a Miniature
Gastrostimulator,” 2012 EuMC, Amsterdam, Netherlands, Oct. 28
– Nov. 2 2012.
Acknowledgments
We thank NSF for their support for funding grant # EEC-1156801, Research
Experiences for undergraduates in Sensors and Applications at the University of
Texas at Arlington.
Also, we would like to thank Dr. Bredow, Dr. Alavi, and Mohammadreza
Jahangir Moghadam for their support and guidance as the REU: Sensors and
Application coordinators.
For further information
Please contact [email protected] for further information and
feedback.
Figure 4. Shows the first four steps of the tuning process for the case of
concrete brick. First, the circuit is tuned for air at 4.4 and 8.5cm. Once the
brick is inserted between the coils, there is a small decrease in output
power. By retuning the transmitter and receiver capacitance as well as the
frequency, we are able to reach or come close to the original output power
when in air. Numerical results are further detailed and explained in tables 1
and 2.
To view more information and past topics iMEMS group have
researched, please visit
http://www.uta.edu/faculty/jcchiao/index_frame.htm
Concrete block wall, painted
Figure 2. Experimental Setup
Summary and conclusions
15cm Coil Set Tuning Results
Sensor
Solar
panel
Output, W
Figure 5. Random Wall Experiment
Circuits