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Experimental Investigation of 3D Metamaterial for Mid-range Wireless Power Transfer

A.L.A.K.Ranaweera, Thuc Phi Duong, Byoung-Suk Lee, and Jong-Wook Lee Department of Electronics and Radio Engineering

Kyunghee University 1, Socheon-dong, Giheung-gu, Yongin-si, Gyeonggi-do, South Korea

*jwlee@khu.ac.kr

Abstract— We investigate a metamaterial structure to investigate the potential of extending the WPT to mid-range distances. Three-dimensional (3D) metamaterial structure is realized using 4×5×1-array of three-turn spiral resonators. By using the designed metamaterial structure, we achieve 33% and 7.3% efficiency improvements at 1.0 and 1.5 m distance between Tx/Rx resonators, respectively. EM simulations are also performed and verify that proposed metamaterial structure is capable of enhancing the efficiency at mid-range distances.

Keywords- wireless power transfer, metamaterial, three turn spiral resonatoe (3T-SR)

I. INTRODUCTION

The concept of Wireless Power Transfer (WPT) began more than a century ago, with Nicola Tesla’s demonstration in 1899 [1]. Nowadays, WPT has become one of the fast moving areas of research all over the world. This trend of research was boosted by the reestablishment of Nicola Tesla’s original idea by a group of researchers at MIT in 2007[2] as well as the consumer demand for WPT products during the last few years. Magnetic resonance coupling is the promising technology behind most of the mid range WPT systems. Metamaterials are artificial materials that can be flexibly engineered to have EM properties such as negative refraction which are not exhibited by naturally occurring materials [3]. In [4], Yaroslav et al. presents a theoretical analysis on the possibility of using metamaterials to enhance the mutual coupling between magnetic dipoles and thereby the efficiency of WPT system based on simplified assumptions. In [5], Bingnan Wang et al. showed that metamaterial could be utilized to enhance the evanescent-wave coupling for enhancing power transfer. At a moderate distance of 0.5 m, they were able to increase the transfer efficiency from 17% to 35% using 3D metamaterial. However, experimental study on using metamaterial to enhance WPT at midrange distance is still lacking.

In this paper, we experimentally show the potential of utilizing three-turn spiral resonator (3T-SR) based 3D metamaterial for increasing the power transfer range of WPT system. To show the efficiency improvement experimentally, we utilized WPT system designed in [6] operating at 6.5 MHz. The experimental results with the proposed metamaterial

shows increased efficiency (33% and 7.3% improvements) at 1.0 and 1.5 m distances. The designed metamaterial structure can be potentially utilized in extending the WPT into mid range distances.

II. DESIGN AND SIMULATION

To enhance the transfer efficacy of WPT, it is sufficient to use magnetic metamaterial which has only negative permeability but positive permittivity [5]. There are several approaches to realize negative permeability metamaterial. When metamaterials are used for WPT enhancement, a higher Q factor metamaterial is desired for better efficiency and longer transfer distance. According to [7], spiral resonator based metamaterial are shown to have a high-Q factor. In this work, we utilize three-turn spiral resonator (3T-SR) to achieve negative permeability. Our designed metamaterial has the advantage of small electrical size and easy fabrication compared to previously reported metamaterial for WPT in [5]. In order to adjust the frequency, we add high-Q variable capacitor to the spiral resonator. Figure 1 shows the structure of designed 3T-SR and real part of permeability extracted from EM simulation results. The method described in [8] is employed to extract the numerical value of permeability.

2 3 4 5 6 7 8 9 10-10-8-6-4-202468

101214

Perm

eabi

lity

(μ)

Frequency (MHz)

real(μ)

Figure 1. 3T-SR structure and extracted permeability.

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By adjusting the value of variable capacitor added to the spiral resonator, we are able to get permeability of -1 at 6.5 MHz. The result verifies the designed structure is operating as magnetic metamaterial.

(a)

(b)

Figure 2. Comparison of H-field distribution of WPT system (a) with (b) without proposed 3D metamaterial structure.

Figure 2 shows the H-field distribution of the WPT

system obtained with EM simulation with and without proposed metamaterial structure. The increased H-field intensity at the receiver results an improvement of transfer efficiency of the system.

III. EXPERIMENTAL RESULTS

The 3T-SR is fabricated on acrylic (ε=3) substrate of thickness 1 mm using copper strips with thickness 0.2 mm. Strip width is 7 mm, space between nearby strips is 3 mm and outermost radius of the spiral is 7 cm. Unit cell size is 15 cm. Variable capacitor is loaded into the spiral resonator to adjust the resonance frequency. Using network analyzer (Agilent 8714ES) resonant frequency is measured. Unit cells are arranged in cubic cell arrangement and they are assembled as 4×5×1-array to make 3D metamaterial having overall size of

60 cm×75 cm×15cm. At the operating frequency, wavelength to unit cell size ratio is approximately 300 such that the designed metamaterial is highly sub-wavelength metamaterial.

In order to show the enhanced WPT operation, 4 coil WPT system designed in [6] has been utilized. The system contains source loop, load loop, Tx and Rx coils resonating at 6.5 MHz. To demonstrate that the proposed metamaterial structure is capable of extending the range of WPT system, power transfer efficiency at midrange distances of 1.0 and 1.5 m is measured. Figure 3 shows the experimental setup including the metamaterial structure placed between the Tx and Rx resonators.

Figure 3. Experimental Setup including metamaterial structure

We measure the power transfer efficiency using S-

parameter measurements for the cases with and without metamaterial structure in between the Tx and Rx. The introduction of metamaterial structure into the WPT system changes the matching condition. To remove the effect of change of impedance matching, the distance between source loop and TX resonator is adjusted to match the input impedance to 50 Ω so that the |S11| at the resonance is very small. (See Figure 4 and Figure 6) Then the relative power transfer efficiency is evaluated using |S21|2.

5.5 6.0 6.5 7.0 7.5-50

-40

-30

-20

-10

0

S11

with

Met

amat

eria

l (dB

)

Frequency (MHz) Figure 4. Measured |S11| as a function of frequency at 1.0 m distance between

Tx and Rx

Figure 5 shows the experimental results of WPT system when the distance between Tx and Rx resonators are kept at 1.0 m. Metamaterial structure is placed in the midpoint

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between the Tx and Rx resonators. WPT efficiency is increased from 21% to 54% showing nearly 33% improvement of efficiency by the introduction of 3D metamaterial structure.

5.0 5.5 6.0 6.5 7.0 7.5 8.00

10

20

30

40

50

60

70

Effi

cien

cy(%

)

Frequency (MHz)

w/o metamaterial with metamaterial

Figure 5. Comparison of power transfer efficiency at 1.0 m distance

between TX and Rx coils.

5.0 5.5 6.0 6.5 7.0 7.5 8.0-16

-14

-12

-10

-8

-6

-4

-2

0

S11

with

Met

amat

eria

l (dB

)

Frequency (MHz) Figure 6. Measured |S11| as a function of frequency at 1.5 m distance

between Tx and Rx coils.

5.0 5.5 6.0 6.5 7.0 7.5 8.0

0

5

10

15

Effic

ienc

y(%

)

Frequency (MHz)

Efficiency without Metamaterial Efficiency with Metamaterial

Figure 7. Comparison of power transfer efficiency at 1.5 m distance between

TX and Rx coils. Figure 7 shows WPT efficiency enhancements at a Tx/Rx

distance of 1.5 m. At this distance WPT efficiency is increased

from 4.2 to 11.5% showing an improvement of 7.3% in efficiency. This verifies that the designed structure has potential to extending the WPT system into mid-range distances. In addition to efficiency improvement in both the cases, the bandwidth of WPT operation range is also increased. This is due to the mutual coupling between nearby 3T-SRs in the metamaterial structure.

Results of proposed system is compared with similar WPT systems reported previously at mid range distance. The comparison highlights the potential of proposed metamaterial in extending the WPT distance. Result of comparison is listed in Table I. Because of the different size of coil structures, normalized distance between Tx/Rx is used for the comparison. In [9], 4 coil WPT systems working at 7.65 MHz, similar to one we used here is utilized. In [10], helical shape coil system with impedance matching network is utilized. The results show that our proposed metamaterial structure has a good potential for extending the range of WPT system into mid range.

TABLE I. EFFICIENCY COMPARISON

Efficiency at normalized distance of 3.3

Efficiency at normalized distance of 5.0

[9] 49 % -

[10] 24 % 11.4 %

[6] 21 % 4.2 % [6] with

metamaterial* 54 % 11.5 % * The proposed WPT system in this paper

IV. CONCLUSION In this study, we experimentally investigate the possibility

of extending the WPT distance in to midranges using a 3T-SR based 3D metamaterials. By introducing the designed metamaterial structure into the WPT system, we could obtain 33% and 7.3% improvement in WPT efficiency at Tx/Rx distances of 1.0 m and 1.5 m respectively. Even though the bulkiness of the designed metamaterial is a disadvantage, this study will have high impact on the WPT application working on mid-range distances. Reducing the bulkiness while keeping the efficiency at the same level will be the subject of future study.

ACKNOWLEDGMENT This research was supported by the Basic Science Research

Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2012-001327).

REFERENCES

[1] L.I.Anderson, “Nikola Tesla on his work with alternating currents and their application to wireless telegraphy, telephony and transmission of Power,” Twenty First Century Books, pp. 88-147, 2002

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[2] A. Kurs, A. Karalis, R. Moffatt, J. D. Joannopoulos, P. Fisher, and M. Soljacic, “Wireless power transfer via strongly coupled magnetic resonances”, Science, vol. 317, pp. 83–86, July 2007.

[3] W. J. Padilla, D. N. Basov, and D. R. Smith, “Negative refractive index metamaterials”, Materials Today, vol. 9, pp 28 – 35, July 2006.

[4] Y. Urzhumov and D. R. Smith, “Metamaterial-enhanced coupling between magnetic dipoles for efficient wireless power transfer”, Physical Review,B83, pp May 2011

[5] B. Wang, K.H. Teo, T. Nishino, W. Yerazunis, J. Barnwell, and J. Zhang, “Experiments with wireless power transfer with metamaterials”, Appl. Phys.Lett., vol. 98, p. 254101, June 2011

[6] T. P. Duong and J.-W. Lee, “Experimental results of high-efficiency resonant coupling wireless power transfer using a variable coupling method,” IEEE Microwave Wireless Compon. Lett., vol. 21, no 8, pp. 442-444, Aug. 2011.

[7] T. Hao, C.J. Stevens, and D.J. Edwards, “Optimization of metamaterials by Q factor” , IEE Electron Lett., Vol 41, No 11, 26th May 2005

[8] R.W. Ziolkowski, “Design, fabrication, and testing of double negative metamaterials”, IEEE Trans. on Antenna and Propagation, vol. 51, no 7, pp 1516- 1529, July 2003

[9] A. P. Sample, D. A. Meyer, and J. R. Smith, ‘‘Analysis, experimental results, and range adaptation of magnetically coupled resonators for wireless power transfer,’’ IEEE Trans. Ind. Electron., vol. 58, no. 2, pp. 544---554, Feb. 2011.

[10] H.J. Choi, E.H. Ahn, S. Y. Park, and J. R. Choi, “Portable Battery Charging circuits for enhanced magnetic resonance wireless power transfer (WPT) System”, PRIME 2013, Villach, Austria , pp 273-276.

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