Wireless Energy Transfer System with Multiple Coils via Coupled … · 2016-01-08 · We expect...

A general equivalent circuit model is developed for a wireless energy transfer system composed of multiple coils via coupled magnetic resonances. To verify the developed model, four types of wireless energy transfer systems are fabricated, measured, and compared with simulation results. To model a system composed of n-coils, node equations are built in the form of an n-by-n matrix, and the equivalent circuit model is established using an electric design automation tool. Using the model, we can simulate systems with multiple coils, power sources, and loads. Moreover, coupling constants are extracted as a function of the distance between two coils, and we can predict the characteristics of a system having coils at an arbitrary location. We fabricate four types of systems with relay coils, two operating frequencies, two power sources, and the function of characteristic impedance conversion. We measure the characteristics of all systems and compare them with the simulation results. The flexibility of the developed model enables us to design and optimize a complicated system consisting of many coils.

Keywords: Energy transfer, wireless, magnetic resonance, model, multi-coils.

Manuscript received July 19, 2011; revised Feb. 29, 2012; accepted Mar. 23, 2012. This work was partly supported by the IT R&D program of MKE/KCC/KEIT [10035181-

2010-01, Development of RF energy Transmission under 100Watts and Harvesting Technology] and [12ZF1170, Future Technology Researches in the Fields of Informations and Telecommunications].

Sanghoon Cheon (phone: +82 42 860 1651, [email protected]), Yong-Hae Kim ([email protected]), Seung-Youl Kang ([email protected]), Myung Lae Lee ([email protected]), and Taehyoung Zyung ([email protected]) are with the Convergence Components Materials Laboratory, ETRI, Daejeon, Rep. of Korea.

http://dx.doi.org/10.4218/etrij.12.0111.0461

I. Introduction

In recent years, there has been an increasing interest in wireless power transfer technology. In particular, significant progress has been charted for inductively coupled systems [1]-[11]. Inductively coupled power transfer systems have been developed for a wide range of applications, including vehicle battery charging systems, and a very high end-to-end system efficiency of up to 80% has been documented [4], [8]. However, most studies have been restricted to close range, that is, typically shorter than 30% of the coil diameter. The transmission distance is generally close to 1 cm [4], and 15 cm is considered a fairly large distance [8]. Results at the mid-range (that is, more than twice the coil diameter) have not been reported.

Recently, MIT proposed a new scheme based on strongly coupled magnetic resonances, thus presenting a potential breakthrough for a mid-range wireless energy transfer [12], [13]. The fundamental principle is that resonant objects exchange energy efficiently, while non-resonant objects do not. Figure 1 shows the basic coil system composed of four coils: drive, transmit resonance, receive resonance, and load coils. The transmit resonance coil is coupled to the drive coil which is linked to a power amplifier that supplies energy to the system. The receive resonance coil is coupled with the load coil to provide the power to an external load. The scheme is carried with a power transfer of 60 W and has RF-to-RF coupling efficiency of 40% for a distance of 2 m, which is more than three times the coil diameter. We expect that coupled magnetic resonances will make possible the commercialization of a mid-range wireless power transfer.

Magnetic resonance coupling is a new concept in wireless energy transmission. The analyses used in early research were

Wireless Energy Transfer System with Multiple Coils via Coupled Magnetic Resonances

Sanghoon Cheon, Yong-Hae Kim, Seung-Youl Kang, Myung Lae Lee, and Taehyoung Zyung

ETRI Journal, Volume 34, Number 4, August 2012 © 2012 Sanghoon Cheon et al. 527

To verify the model, we fabricate four types of systems that have relay coils, two operating frequencies, two power sources, and the function of characteristic impedance conversion. We measure the characteristics of all systems and compare them with the simulation results.

Fig. 1. Schematic of wireless energy-transfer system usingcoupled magnetic resonances.

Drive coil

Transmit resonance coil

Receive resonance coil

Load coil

1 1

2

11

11

11

11

2

0

00

0/ ( ) /

0

0/ ( ) /

0

ii

ii i

ii

kk

kkk

kk

L C

L C

CL

LL S C i

LL

LL

C kLL

CL

i ii i

iiii i i j C Zii

iii j C Zii

ii

ω

ω

−−

++

−−

++

⎛ ⎞ ⎛ ⎞ ⎛⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟ −⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟+ =⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟ −⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟

⎜ ⎟⎜ ⎟ ⎝⎝ ⎠⎝ ⎠

.

⎞⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟

⎠

(1)

based on pure physical theory and failed to provide tangible findings for electrical engineers [12]-[18]. The reports [8]-[23] do not clearly present practical design methods. Reports [24]-[26] accurately present the characteristics of a coupled magnetic resonance system, and these reports can be practically applied to the design. Through the circuit analysis of an equivalent model, the system transfer function can be obtained as a function of frequency, and several key parameters of the system can be analyzed [24]. Also, the coupled mode theory [12] can be extended for an analysis of other systems [25]. The proposed methods are easily applied to a system composed of several coils and having negligible weak coupling. However, if the system is composed of many coils and weak coupling affects the system characteristics, it is very difficult to apply the methods for analysis.

II. Modeling for System Composed of n-coils

1. Node Equation and Equivalent Circuit Model RF engineering is also a candidate method for the analysis of

a coupled magnetic resonance system. In [26], the equivalent circuit model with a compensation source is developed to take into account both weak coupling and loss. The model has been established in an RF simulator and the parameters for the model are extracted through a measured S-parameter. Simulation results show strong agreement with the measurement results [26].

In general, the wireless power transfer system has three types of coils: a coil with a power source, a floating coil, and a coil with a load. The resonance frequency of floating coils is an operating frequency. Figure 2 shows the proposed equivalent circuits of three such types of coils. The equivalent circuit of a system composed of n-coils has a number of resonant circuits, n, and each resonant circuit has a number of compensation sources of n–1. The compensation source represents the effect of mutual inductance related to coupling with other coils [26]. Here, Li,j,k is a self-inductance, and Ci,j,k

is a capacitance of the coil.

Previous studies have analyzed the system consisting of four coils. In this paper, the node equation in the form of a matrix is written for a system composed of n-coils by expanding the node equation of [26]. The general model of the coupled magnetic resonance system is established using an electric design automation tool, and the model enables us to design a system composed of several coils, power sources, and loads.

From the equivalent circuit of each coil, we can obtain (1) and (2) in the form of a matrix by writing the current in each branch at the node whose voltage is Vi,j,k. This matrix can be

1 12 13 1 1 1

21 2 23 2 2 2

31 32 3 3 3 3

1 2 3

1/ 0 0 00 1/ 0 00 0 1/ 0

0 0 0 1/

n L

n L

n L

n n n n Ln

j L j M j M j M i j Cj M j L j M j M i j Cj M j M j L j M i j C

j M j M j M j L i j

ω ω ω ω ωω ω ω ω ωω ω ω ω ω

ω ω ω ω

⎛ ⎞ ⎛ ⎞⎜ ⎟ ⎜ ⎟⋅ ⋅ ⋅ ⋅ ⋅ ⋅⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟

⋅ ⋅ ⋅ × ⋅ = ⋅ ⋅ ⋅⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅⎜ ⎟ ⎜ ⎟

⋅ ⋅ ⋅ ⋅ ⋅ ⋅ ⋅⎜ ⎟ ⎜ ⎟⎜ ⎟ ⎜ ⎟⋅ ⋅ ⋅ ⋅ ⋅ ⋅⎝ ⎠ ⎝ ⎠

1

2

3

.

C

C

C

n Cn

iii

C iω

⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟

× ⋅⎜ ⎟⎜ ⎟ ⋅⎜ ⎟

⋅⎜ ⎟⎜ ⎟⎝ ⎠

⎛ ⎞⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠

(2)

528 Sanghoon Cheon et al. ETRI Journal, Volume 34, Number 4, August 2012

Fig. 2. Equivalent circuits of three types of coils: (a) coil withpower source, (b) floating coil, and (c) coil with load.

Li

M1

Mn–1

M2

Z0 Ci

is

iL iC

Vi Vj

iL iC

Lj

Cj

iLiC

Lk

Ck

Vk

(a) (b) (c)

+ – + –

+ –

+ – + –

+ –

+–+–

+–

M1

Mn–1

M2

M1

Mn–1

M2

Z0

Fig. 3. Coupling constant as function of distance between coils.

0 100 200 300 400 500 600 7000.0

0.1

0.2

0.3

0.4

0.5

Distance (mm)

Coil-1 vs. coil-1 Coil-1 vs. coil-A

Cou

plin

g co

nsta

nt (A

. U.)

established in Advanced Design System (ADS), which is design software widely used for RF and microwave applications [27].

2. Parameter Extraction for System Composed of n-Coils

To model a system composed of n-coils, we have to extract the following parameters. Each coil has the parameters of capacitance, self-inductance, and RLoss. Also, to model the compensation sources, we need coupling constants for the possible combinations of coils. As a result, there is a total of 3n+n(n–1)/2 parameters. For example, a system composed of 4 coils has 18 parameters, and a system composed of 6 coils has 33 parameters.

To simulate the system characteristics as the distance between coils, we need a function whose output is a coupling constant and whose input is the distance between coils. Figure 3 shows an extracted graph with such a function. We have measured the coupling constant according to the distance and extracted coefficients of (3) by fitting with the measurement results.

Table 1. Extracted parameters.

A B C

Coil-1 vs. coil-1 2.72E+07 –3.64 138

Coil-1 vs. coil-A 1.14E+06 –3.19 97

( ) ,BA x Cκ = + (3)

where x is the distance between the coils and κ is the coupling constant. This procedure is performed for the possible combinations of coils. Table 1 shows the extracted coefficients. Each coil used in Table 1 has a diameter of 15 cm, and the diameter of the conductor section is 1 mm. Coil-1 has twelve turns and a pitch of 3 mm and coil-A has one turn.

We can predict the characteristics of a system in which the coils have arbitrary locations by establishing (3) in the simulator. The completed model requires just the location of each coil for simulation.

III. Model Validation and Experiment Results

Using the developed model, we predict the characteristics of various systems, measure their characteristics, and compare them with the simulation results. Each system has relay coils, two operating frequencies, two power sources, and the function of characteristic impedance conversion. The system configuration is very useful when applying it to a commercial product of wireless power transfer.

1. Relay System

Although the technology based on strongly coupled magnetic resonances is able to transmit energy over a much longer distance than the traditional method can, degradation in efficiency is unavoidable in a system in which the transmitter is away from the receiver. In this case, we can improve the efficiency by adding coils to create a relay device. Figure 4(b) shows such a relay system. A system without relay coils, shown in Fig. 4(a), shows an efficiency of –18 dB (1.6%). Coil-A and coil-1 in Table 1 are used for feeding and resonator coils, respectively. The resonance frequency of coil-1 is 11 MHz.

To improve the efficiency of the system in Fig. 3(a), we can use the relay coils, which are the same as coil-1. The number of the required coils and their location can be determined through simulation using the developed model. Because of the fast simulation time, we can easily obtain the transfer characteristics according to the position of the relay coils, and

ETRI Journal, Volume 34, Number 4, August 2012 Sanghoon Cheon et al. 529

the optimum position for maximum efficiency can be determined. Figure 5(a) shows the improved transfer characteristics achieved through such a process. After adding two relay coils, the efficiency is improved to –4.2 dB (38%). Figure 5(b) shows the measurement and simulation results,

Fig. 4. Photographs of system (a) without and (b) with relay coils.

(a)

(b)

50 cm

13.2 cm 17 cm 13.2 cm

Fig. 5. (a) Measured results without and with relay coils and (b)simulation and measurement results of system inFig. 4(b).

4M 6M 8M 10M 12M 14M 16M–100

–80

–60

–40

–20

0

Measurement (relay) Measurement (no relay)

Frequency (Hz)

Gai

n (d

B)

(a)

6M 8M 10M 12M 14M 16M–60

–50

–40

–30

–20

–10

0 Model Measurement

Frequency (Hz)

Gai

n (d

B)

(b)

simultaneously. The agreement between the model and experiment data is fairly good. This example shows that we can predict and optimize the characteristics of the system through the developed model without experimentation.

The proposed relay scheme overcomes the mid-range limitation of the present wireless electric system, allowing more flexible power transmission without sacrificing efficiency.

2. Two-Tone System

Some applications of wireless energy transfer require two or more receiving devices, and magnetic resonance coupling is expected to support multiple receivers at the same time. However, if the resonance frequencies of receiving devices are the same, interference caused by a coupling of receiving coils makes the implementation very difficult. When multiple receivers are in close proximity to each other, there is a coupling interaction between the receivers, which makes the resonant peak split into separate peaks [28].

We overcome this difficulty through the use of resonant coils with different resonance frequencies. The coupling interaction occurs only when coils have the same resonant frequency. Thus, if the resonance frequencies of coils are different, the resonant peak frequency does not split and it can be maintained.

Figure 6 shows the experimental setup for a system with two receiving devices. The two coils of the transmit device in the middle of the system have different resonance frequencies, and the two receiving coils on both sides have corresponding resonance frequencies. A coil with a 3 mm pitch has a resonance frequency of 9.8 MHz, and a coil with a 4 mm pitch has a resonance frequency of 11.3 MHz. The number of turns of each coil is 11. Figure 7 shows the measurement and simulation results of power transferred to two receiving devices at both ends. The peak frequency of gain to each receiver is shifted slightly from the resonance frequency of the resonant coil, from 9.8 MHz to 10.9 MHz at the left receiver and from 11.2 MHz to 13.2 MHz at the right receiver. This means that the coupling interaction is small but still exists despite the different resonant frequencies of 9.8 MHz and 11.2 MHz.

In the proposed system, wireless power transmission to two

Fig. 6. Photograph of experimental setup for two-tone system.

Res. freq.=9.8 MHz

Res. freq. =11.2 MHz


Fig. 7. Comparison of experiment data to simulated results:transmitted to (a) left and (b) right devices.

8M 10M 12M 14M 16M–50

–40

–30

–20

–10

0 Model Measurement

Frequency (Hz)

Gai

n (d

B)

(a)

8M 10M 12M 14M 16M–50

–40

–30

–20

–10

0

Frequency (Hz)

Model Measurement

Gai

n (d

B)

receivers can be done at the same time by generating a two-tone signal at the transmitter. However, the transmitter of the system needs as many resonant coils as receivers and a multi-tone signal should be generated, which makes it difficult to implement.

Ultimately, it is possible to simulate such a system using the developed model. As shown in Fig. 7, the predictions of the model are in agreement with the measurement results.

3. System with Two Transmitting Devices

As an alternative to including relay coils in the system to improve transmission efficiency, we can add more transmit coils to the system to improve transmission efficiency. Figure 8 shows a photograph of such a system. To improve the efficiency of the normal system in Fig. 8(a), we can place a transmit coil at the other side, as shown in Fig. 8(b). After adding the coil, we achieve an efficiency improvement of 2.6 dB. Figure 9(a) shows the measurement results, and Fig. 9(b) shows a comparison with the simulation. It also shows that we can predict the system characteristics through simulation using

Fig. 8. Photographs of system for charging-zone experiment: (a)normal transfer system. Device on the right is receiver andhas load coil inside resonance coil. (b) System composedof two transmitting devices and one receiving device.

30 cm

30 cm 30 cm

(a)

(b)

Fig. 9. (a) Measurement results with one transmitting device andtwo transmitting devices and (b) comparison withsimulation results of system in Fig. 7(b).

10.0M 10.5M 11.0M 11.5M–30

–20

–10

0

Frequency (Hz)

After adding sending coil

Gai

n (d

B)

–9.018 dB

–6.392 dB

(a)

8M 10M 12M 14M 16M–80

–60

–40

–20

0

Model Measurement

Gai

n (d

B)

Frequency (Hz)

(b)

Two transmitting devices One transmitting device


Fig. 10. Photograph of system for charging-zone experiment.Receiving device is placed in an arbitrary location.

X

30 cm 30 cm

Fig. 11. Efficiency vs. location of receiving coil. Red line showscorresponding frequency.

–27 –18 –9 0 9 18 27–10

–5

0

8

9

10

11

12

X (cm)

Gai

n (d

B)

Peak

freq

uenc

y (M

Hz)

the developed model. We can find out the expression for the coupling coefficient in terms of angle between two coils. If we add the expression to the developed model, the characteristics of a more complicated system can be predicted by simulation.

For mobile devices, it is very practical that the charging position is not fixed and has a degree of freedom. Figure 8(b) shows the possibility of a one-dimensional charging zone consisting of two transmit coils. In a real situation, the receiving device would be placed in any position between two transmit coils, as shown in Fig. 10. Figure 11 shows the measured efficiency versus location, and we obtain the available charging area between two transmit coils. The red line in Fig. 11 is a peak frequency of gain corresponding to the efficiency. The change of peak frequency is due to the coupling interaction between the resonant coils in the transmitter and receiver. We must change the operating frequency to achieve optimum efficiency in Fig. 11, depending on the receiver position.

As there is an increase in portable devices, including cellular phones, people will need a charging area, not a charging pad made by induction technology. Figure 10 is a good example of needing such a requirement.

4. Conversion of Characteristic Impedance

Depending on the structure of the power source or value of

Fig. 12. Photograph of system for impedance conversionexperiment.

1 turn 2 turn

Fig. 13. Measured S-parameter of system in which impedance of input port is 50 Ω and output is 50 Ω: (a) S11 and (b) S21.

S(2, 2)

Freq. 6.100 MHz to 16.10 MHz

m1

m1 Freq.=10.93 MHz S(2, 2)=0.447/5.784 Impedance=Z0*(2.578+j0.290)

8M 10M 12M 14M–40

–30

–20

–10

0

Frequency (Hz)

Gai

n (d

B) –2.45 dB@

10.93 MHz

(a)

(b)

the load impedance, we may wish to design the characteristic impedance of the system with a value other than 50 Ω. In this case, an inductor and capacitor can be used for the matching function. However, there may be a drawback, such as an increase in cost and heat loss at the lumped elements. We can solve these problems using a feeding coil and load coil with different inductance values. Equation (4) is an analytic result ignoring the weak coupling and heat loss in a conventional system composed of four coils [26]. From (4), input impedance from the power source is changed in proportion to the


Fig. 14. Measured S-parameter of system in which impedance of input port is 50 Ω and output is 150 Ω: (a) S11 and (b) S21.

(a)


S(2, 2)

m1

m1 Freq.=10.93 MHz S(2, 2)=0.092/142.488 Impedance=Z0*(0.859+j0.097)

8M 10M 12M 14M–40

–30

–20

–10

0

Frequency (Hz)

Gai

n (d

B)

–1.52 dB @10.93 MHz

(b)

inductance ratio of the feeding and load coils.

40 1 0 0 4

1

4 40 0 1 0

1 0 1

40

1

1

1

.

inL

Y j C Y j CL j

L LY j C j C

L j LL

YL

ω ωω

ω ωω

⎛= + ⋅ + +⎜

⎝⎛ ⎞

= + + +⎜ ⎟⎝ ⎠

≈

0 4

41

L

L

⎞⎟⎠

(4)

Figure 12 shows a photograph of the system used for the impedance change experiment. The feeding coil has one turn and an inductance of 0.467 μH. The load coil has two turns and an inductance of 1.505 μH. The inductance of the load coil is three times that of the feeding coil. Therefore, we can obtain maximum efficiency if the load impedance is three times the characteristic impedance of the power source.

Figures 13 and 14 are the measurement results of the system, in which the port impedance is 50 Ω to 50 Ω and 50 Ω to 150 Ω, respectively. The port impedance of 50 Ω to 150 Ω shows well-matched results without reflected power (S11) and higher gain (S21). Figure 15 shows the measurement and

Fig. 15. Comparison with simulation results of system in whichimpedance of input port is 50 Ω and output is 150 Ω,from Fig. 12: (a) S11 and (b) S21.

(a)

(b)

Model Measurement


6M 8M 10M 12M 14M 16M–50

–40

–30

–20

–10

0

Model Measurement

Frequency (Hz)

Gai

n (d

B)

simulation results for the system with a port impedance of 50 Ω to 150 Ω. We can verify the usefulness of the developed model once again from a well-predicted value of S21.

IV. Conclusion

It is clear that a field simulation is the most obvious method for the analysis of wireless power transfer via coupled magnetic resonances, such as HFSS or CST. However, it takes a long time to perform a field simulation, and it is not possible to perform such a simulation when there is a variety of different coil configurations and characteristics according to the distance between the coils. For example, it takes about 10 min to solve one case with the MIT system using HFSS. Moreover, it shows different results depending on boundary conditions.

In this paper, a novel circuit model for a wireless power transfer system via coupled magnetic resonances was proposed. We built general node equations of the system, composed of n-coils, and established an equivalent circuit model in ADS [27]. The model has the data of a coupling constant according to the


distance between the coils, and we can simulate a system in which coils are placed at arbitrary points. As a result, we can perform a very fast simulation for the different coil configuration composed of many coils.

To verify the model, we made four types of systems, measured their S-parameters, and made a comparison with the simulation results. The configuration of a system is very significant when developing a mid-range wireless power transfer system where the magnetic resonance is expected to be the most advantageous.

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[27] Agilent | Overview: Advanced Design System (ADS). Available: http://www.home.agilent.com/agilent/product.jspx?cc=US&lc=eng& ckey =1297113&nid=-34346.0.00&id=1297113, 2000.

[28] B. L. Cannon et al., “Magnetic Resonant Coupling as a Potential


Means for Wireless Power Transfer to Multiple Small Receivers,” IEEE Trans. Power Electron., vol. 24, no. 7, July 2009, pp. 1819-1825.

Sanghoon Cheon received his BS, MS, and PhD in electronics engineering and computer science from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Rep. of Korea, in 1993, 1995, and 2001, respectively. In 2001, he joined Knowledge*on Inc. in Korea, where he was involved in the 6-

inch InGaP/GaAs HBT foundry service business. He worked on large signal modeling for microwave devices from 2001 to 2003 and the development of a power amplifier for the WCDMA system from 2004 to 2005. In 2006, he moved to ETRI, Daejeon, Rep. of Korea, where he has been involved in the development of micro-bolometer for infrared focal plane arrays. His current research interests include wireless power transfer systems and meta-materials for antennas.

Yong-Hae Kim received his BS and PhD in physics from the Korea Advanced Institute of Science and Technology (KAIST), in 1993 and 1997, respectively. From 1997 to 2000, he worked with SK Hynix Semiconductor Inc. and developed the 0.13 μm DRAM technology. He joined ETRI, Daejeon, Rep. of Korea, in 2000,

and he has been involved in flexible display technology, such as low temperature poly-Si TFT on plastic substrates and active matrix OLED. His research interests include digital paper, application of meta-material, and wireless power transmission.

Seung-Youl Kang received his BS in physics from Seoul National University, Seoul, Rep. of Korea, in 1987 and his MS and PhD in Physics from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Rep. of Korea, in 1990 and 1994, respectively. Since he joined ETRI, Daejeon, Rep. of Korea, in 1994,

he has been involved in flexible display technology, such as organic TFTs on plastic substrates and electronic paper. Currently, his research interests include electronic paper, flexible electronics, applications of meta-materials, and wireless power transfer systems.

Myung Lae Lee received his BS in physics from Dong-A University, Busan, Rep. of Korea, in 1989 and his MS and PhD in physics from the Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Rep. of Korea, in 1992 and 1998, respectively. Since 1998, he has been working for ETRI, Daejeon, Rep. of

Korea. His research interests include the design, fabrication, and characterization of MEMS devices with signal processing ICs for applications in optics, RF, Bio, and ubiquitous sensor networks (USNs). In recent years, he has branched his study into several areas, including wireless power transfer by a magnetic resonance phenomenon, meta-materials for small antennas, and highly-sensitive gas sensors using nanogold particles.

Taehyoung Zyung graduated from Seoul National University, Seoul, Rep. of Korea, in 1977 and worked at the Korea Institute of Science and Technology (KIST), Seoul, Rep. of Korea, as a researcher for three and a half years from 1978. He received his PhD at Texas Tech University, Lubbock, TX, USA, in 1986. He

performed post-doctoral study as a research associate at the University of Illinois at Urbana-Champaign, Urbana and Champaign, IL, USA, from 1986 to 1989. He joined ETRI, Daejeon, Rep. of Korea, in 1989 and was a director and/or an executive director of the future technology research division from 2000 to 2008. His work has been published in more than 150 SCI journals, and he has filed about 30 patents.


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