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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012

Positioning-Free Resonant Wireless Power
Transmission Sheet With Staggered
Repeater Coil Array (SRCA)
Kazuaki Mori, Hyunkeun Lim, Shunta Iguchi, Koichi Ishida, Member, IEEE, Makoto Takamiya, Member, IEEE,and Takayasu Sakurai, Fellow, IEEE(Invited Paper)

Abstract—A magnetically resonant wireless power transmission
sheet (WPTS) on a printed circuit board is developed. WPTS wirelessly supplies the power to electronic devices including the wearable wireless healthcare devices anywhere on the sheet with highefficiency. In order to increase a power transmission efficiency degraded by a misalignment of a receiver coil, a transmitter coil array
and a proposed staggered repeater coil array (SRCA) are stacked
in WPTS. One transmitter coil closest to the receiver coil and thesurrounding four repeater coils are selectively activated based on
the result of a position detection of the receiver coil. By adding
SRCA to the transmitter coil array, the measured power transmission efficiency at the worst-case misalignment increases from 4.8%
to 64% at a distance of 15 mm between the transmitter and receivercoils with a diameter of 40 mm. The measured results are verified
by electromagnetic simulations.
Index Terms—Magnetic resonance,repeaters, wireless communication.

powertransmission,

Fig. 1. Four types of coils used in coil arrays in Fig. 2. (a) Transmitter coil
(TXC). (b) Repeater coil (RepC). (c) Open coil. (d) Nine transmitter coils(9 TXCs).

I. INTRODUCTION

AMBIENT or ubiquitous charging of electronic devices
such as smartphones is increasingly needed because thesmartphone requires a frequent charging due to the increasedpower consumption. A wireless power transmission sheet(WPTS) [1]–[4] with a transmitter (TX) coil array and a selective activation of a TX coil based on the position detection of areceiver (RX) coil is one of the possible solutions for the ubiquitous charging. The WPTS embedded in a bed or clothes alsoenables the ubiquitous charging of wearable wireless healthcare
devices. A wireless power transmission distance of the sheet,however, is short (e.g., 4 mm distance at 10% efficiency [1])because an electromagnetic induction is used in the sheet. Inorder to increase the distance, a magnetic resonance [5] insteadof the electromagnetic induction is used in this letter. In thenewly developed magnetically resonant WPTS on a printed
circuit board (PCB), however, wireless power transmission
efficiency degradation due to a misalignment of the RX coil isstill large even if the magnetic resonance is used. In order toManuscript received December 14, 2012; accepted January 07, 2013. Date ofpublication January 23, 2013; date of current version January 31, 2013.
The authors are with the University of Tokyo, Tokyo 153-8505, Japan (e-mail:.kamori@iis.u-tokyo.ac.jp)
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LAWP.2013.2239600

Fig. 2. Three different coil arrays. (a) Conventional 1-TXC scheme [1], [3],RepC scheme.
[4]. (b) 9-TXC scheme [2]. (c) Proposed stacked 1 TXC

keep a high efficiency regardless of the misalignment, a staggered repeater coil array (SRCA) is proposed in this letter [6].
By using a WPTS with an SRCA, a positioning of the RX coil
is not required, thereby enabling the ubiquitous charging.
II. RESONANT WIRELESS POWER TRANSMISSION SHEETWITH SRCAA WPTS has transmitter coil arrays. In a WPTS, the target
size is 1–2 m , and the target thickness is less than 1 mm. In aWPTS, a transmitter coil is selectively activated based on theposition detection [1]–[4] of RX coil. The purpose of this letteris to propose the best configuration of the coil array that achieves
the highest wireless power transmission efficiency at the worst

1536-1225/$31.00 © 2013 IEEEMORI et al.: POSITIONING-FREE RESONANT WIRELESS POWER TRANSMISSION SHEET WITH SRCA

Fig. 3. Thickness of the coil arrays.

Fig. 4. Photograph of 3

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Fig. 5. Definition of distance ( ) and misalignment (a)and (b).

3 coil array fabricated on PCB.

TABLE ISUMMARY OF COIL FOR MAGNETIC RESONANCE

misalignment condition of RX coil. In order to find the best
configuration, the efficiency of three different coil arrays arecompared.
Fig. 1 shows three types of coils used in the coil arrays.
Marks for the three types of coils are defined. The transmittercoil (TXC) in Fig. 1(a) is a power feeding coil. The repeater
coil (RepC) in Fig. 1(b) is a short coil to extend the wirelesspower transmission distance [7]. The open coil in Fig. 1(c)is a nonactivated and unused coil. The nine transmitter coils(9 TXC’s) in Fig. 1(d) are nine coils sharing a single powersource.
Fig. 2 shows the three different coil arrays compared inthis letter. In the conventional 1-TXC scheme [1], [3], [4] inFig. 2(a), one TXC closest to the RX coil is activated basedon the result of a position detection of the RX coil, and all theother coils are open. In the 9-TXC scheme [2] in Fig. 2(b),nine TXCs are activated. In the proposed stacked 1 TXC
RepC scheme in Fig. 2(c), a bottom TX layer and a top repeater
layer are stacked, and one TXC in the bottom TX layer and theFig. 6. Measured frequency dependence of efficiency. (a) Conventional 1-TXC
RepC scheme.
scheme. (b) Proposed stacked 1 TXC

surrounding four RepC’s in the top repeater layer are activated.
The repeater layer is called SRCA because the centers of theTX coils and the centers of the repeater coils are staggered witheach other. Fig. 3 shows thickness of the coils of the setup.
Fig. 4 shows a photograph of the 3 3 coil array fabricatedon FR4 PCB. The diameter of the octagonal coil is 40 mm, andthe coil pitch ( ) is 44 mm. “Position O,” “Position A,” and“Position B” of the RX coil are defined to evaluate the efficiency
at the misalignment conditions. Table I summarizes the coil forthe magnetic resonance. The number of turns of the coil is nine,
and the thickness of the metal is 35 m. Chip capacitors are used
to resonate the coil for the magnetic resonance.
III. EXPERIMENTAL RESULTS
In order to compare the three different coil arrays in Fig. 2,
the dependence of the efficiency on the frequency, the distance,
and the misalignment are measured. One TX coil is driven. The

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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 11, 2012

Fig. 7. Measured distance dependence of WPT efficiency in three different coilarrays. (a) Position O. (b) Position B.

coils are matched to 50 .
is measured with the VNA andconverted into the efficiency. Fig. 5 shows a definition of thedistance and the misalignment. The distance is defined as . The
misalignment on the -axis and -axis are defined asand, respectively.
Fig. 6 shows a measured frequency dependence of theefficiency. Fig. 6(a) and (b) shows the conventional 1-TXC
scheme and the proposed stacked 1 TXC
RepC scheme,is varied. The frequency thatrespectively. is 15 mm, andachieves the highest efficiency at the worst misalignment condition ofmm (Position B) is used in the following
measurements for each of the three different coil arrays.
Fig. 7 shows a measured distance dependence of the efficiency in the three different coil arrays. Fig. 7(a) and (b) showsthe efficiency at Position O and Position B, respectively. The
target in this letter is 10–20 mm. At Position O, the conventional 1-TXC scheme achieves the highest efficiency at– mm. In contrast, at Position B, the proposed stacked
1 TXC
RepC scheme achieves the highest efficiency atmm. The 15-mm distance that achieves the highest efficiency
RepC scheme at Position B
in the proposed stacked 1 TXC
is used in the following measurements.
Fig. 8 shows a measured misalignment dependence of the efficiency in the 3 different coil arrays. Fig. 8(a) and (b) shows
anddependence, respectively. The horizontal axis isnormalized by . is 15 mm. As
orincreases, the efficiency decreases except for the proposed stacked 1 TXC
RepC scheme. Therefore, Position O (in Fig. 8(a)andin Fig. 8(b)) are the best-case misalignments,in Fig. 8(a)] and Position B
while Position A [Fig. 8. Measured misalignment dependence of WPT efficiency in three difdependence. (b)dependence.
ferent coil arrays. (a)TABLE IISUMMARY OF FREQUENCY AND WPT EFFICIENCY[in Fig. 8(b)] are the worst-case misalignment. The efficiency of the proposed stacked 1 TXC
RepC
scheme is more than 64% at any misalignment.
Table II summarizes the frequency and the efficiency at Position O, Position A, and Position B in the three different coilarrays. Compared to the conventional 1-TXC scheme, the efficiency of the proposed stacked 1 TXC
RepC scheme increases from 37% to 71% at Position A, and from 4.8% to 64%
at Position B. The increase of the efficiency at the worst-casemisalignment by adding SRCA is very important because theend-product performance of WPTS is determined by the worst
minimum efficiency.

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Fig. 9. Simulated frequency dependence of efficiency. (a) Conventional
RepC scheme.
1-TXC scheme. (b) Proposed stacked 1 TXC

IV. VERIFICATION WITH ELECTROMAGNETIC SIMULATIONSIn order to validate the measured results, 2.5-dimensional
electromagnetic simulations are performed with Momentum(Agilent). Fig. 9 shows a simulated frequency dependenceof the efficiency, which corresponds to the measured resultsin Fig. 6. Fig. 9(a) and (b) shows the conventional 1-TXC
scheme and the proposed stacked 1 TXC
RepC scheme,respectively. The simulated frequency dependence in Fig. 9
correlates with the measured results in Fig. 6.
Fig. 10 shows a simulated misalignment dependence of theefficiency in the conventional 1-TXC scheme and the proposed
stacked 1 TXC
RepC scheme, which corresponds to theandmeasured results in Fig. 8. Fig. 10(a) and (b) shows
dependence, respectively. The simulated misalignment dependence in Fig. 10 correlates with the measured results in Fig. 8,
and the maximum simulation error is 47%.
V. CONCLUSIONSRCA is proposed to enable the positioning-free WPTS for
the ubiquitous charging of the electronic devices including thewearable wireless healthcare devices. By stacking SRCA andthe transmitter coil array, the measured efficiency at the worstcase misalignment condition increases from 4.8% to 64% at
15 mm distance. The improvement of the worst
minimum
efficiency is very important because the end-product performance of WPTS is determined by the worst efficiency, regardless of the reduction of the efficiency of the best case due to theFig. 10. Simulated misalignment dependence of WPT efficiency in two difdependence. (b)dependence.
ferent coil arrays. (a)absorbed power by the repeater coils. The electromagnetic simulations prove the validity of the measurements.
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