Optimization of Wireless Power Transmission through Resonant Coupling

426 29 COMPATIBILITY AND POWER ELECTRONICS CPE29 6TH INTERNATIONAL CONFERENCE-WORKSHOP Optimization of Wireless Power Transmission through Resonant Coupling Yong-Hae Kim, Seung-Youl Kang, Myung-Lae Lee, Byung-Gon Yu, and Taehyoung Zyung Electronics and Telecommunications Research Institute (Daejeon, Korea) yhakim@etri.re.kr Abstract-We investigate the vaious experimental setups to find the optimum condition for the wireless power transmission through resonant coupling. We can transmit the power with 2% efficiency up to 2 m distance and light the LED lamp by transmitting 3 watt. I. INTRODUCTION In the beginning of 2 th century, Nokola Tesla carried out his experiments on power transmission by radio waves instead of the wire grid. However, his typical embodimemts involved undesirably large electric fields which was radiating their power in every direction. To circumvent the energy loss, microwave power transmission using very short wavelengths and optical reflectors of practical dimensions was investigated. This technology had lead to the microwave-powered helicopter and the solar-power satellite concept []. However, such power transmission requires the existence of an uninterruptible lineof-sight and a complicated tracking system in the case of mobile objects. The inductive conpling was a traditional way of power transmission. A surge in the use of autonomous electronic devices usually require the rechargeable batteries. Those batteries sometimes shows the failure during the power recharging from the mehanical contact. Energy transfer utilizing inductive coupling can overcome this contact failure problem and makes it convenient [2]. Electrical circulatory assist devices such as total artificial heart generally use a brushless dc motor as their pump. Because these devices are inside the body, it is desirable to transfer electrical energy to these circulatory assist devices transcutaniously without breaking the skin [3]. There is a demand for developing new power-transmission methods for a multitude of electronic devices such as sensor networks. Those ubiquotous sensing will be used in our daily life to enhance security, safety, and conveniency. T. Sekitani et al. made the large-area wireless power transmission system using inductive couplig and MEMS technology [4]. However, inductive coupling is effective only for short distance. The wireless power transmission through the resonant coupling is recently developed [,6]. A. Kurs et al. showed the wireless power transmission through the resonant coupling by lighting the bulb and explained by the coupled mode theory. In the present paper, we will investigate the various experimental setups to find the optimum condition for the wireless power transmission through resonant coupling. We will confirmed the wireless power transmission through the resonant coupling in the high power range by connecting an array of the light emitting diode (LED). II. EXPERIMENTAL DETAILS Fig. shows the schematic diagram of the experimental setup for the wireless power transmission and fig. 2 shows the photographes of (a) a power coil (load coil) and (b) a send coil (receive coil). The system is consisted of the arbitrary function generator (AFG32, Tektronics), the wideband RF power amplifier (8FE, OPHIR), the four kinds of coils, and the spectrum analyzer (86E, Agilent). The coils are composed of a power coil, a send coil, a receive coil, and a load coil. A power coil (PC) and a load coil (LC) have the same shape and are made of copper with.3 m radius and 3 mm cross-sectional radius wound into a open circle. A send coil (SC) and a receive coil (RC) have the same shape and are made of 9.89 m total length and 3 mm crosssectional radius wound into a helix of.2 turns,.3 m radius, and.2 m height. The wireless power transmission between the power coil and the send coil is transmitted with the inductive coupling. Because the inductive coupling is effective at short distance, the distance between the PC (RC) and the SC (LC) is maintained within mm. The power between the send coil and the receive coil is transmitted wirelessly through resonant coupling. Because the resonant coupling can transmit the power at long distance, we varied the distance between the SC and the RC upto 2 m. In the measurement of the power transmission efficiency, the output of the power amplifier was set to mw and the load Fig.. Schematic diagram of the experimental seutp for the wireless power transfer. 978--4244-286-4/9/$2. 29 IEEE

SPECIAL POWER ELECTRONICS SYSTEMS AND APPLICATIONS 427 Fig. 2. The photograhphs of (a) a power coil (load coil) and (b) a send coil ( receive coil). coil was connected to the spectrum analyzer. In the lighting experiment of LED lamp, the output of the power amplifier was set to W and the load coil is connected to the LED lamp. In the measurement of the resistance and the standing wave reflection (SWR), HF/VHF SWR Analyzer (MFJ-29B, MFJ Enterprises) was connected to the power coil. In the measuring the electic and the magnec field, we used the broadband isotropic field probe (HI-4433-LFH for magnetic filed, HI- 4433-STE for the electric field, ETS-Lindgren). III. RESULTS AND DISCUSSIONS Resistance (Ω) Standing wave reflection 4 3 2 Power coil only Power coil and send coil (a) 2 3 4 6 7 8 9 2 3 4 2 2 Power coil only Power coil and send coil (b) 2 3 4 6 7 8 9 2 3 4 Fig. 3. (a) Resistance and (b) standing wave reflection characteristics in the case of the power coil only existence and in the case of the power coil and send coil existence... E-3 E-4 Existence of Send & Receive Coil Both Receive Only Send Only Neither E- 2 3 4 6 7 8 9 2 3 4 Fig. 4. Output power transmission characteristics as a function of frequency according the existence of the send coil and the receive coil. Fig. 3 compares the (a) resistance and (b) SWR characteristics of the power coil only and of the power coil and send coil. The distance between the PC and the SC is maintained below mm. In the case of the power coil only, the resistance is always zero and the SWR is over 2 which is the measurement maximum. However, in the power coil and the send coil, the resonance is occurred near 9.7 MHz where the resistance shows the peak and the SWR goes down below 2 reaching nearly. This experiment means that the resonance frequency of the SC is near 9.7 MHz, but the resonance frequency of the PC does not exist within the measurement frequency range. To confirm the resonant coupling effect, we measured the power transmission by inserting or omitting the resonant objects (the SC and the RC). Fig. 4 shows the output power transmission characteristics as a function of frequency according to the existence of the SC and the RC with the m distance between the PC and the LC. The peak power transmission in every case is shown in fig.. The Both 3 2 2 Both Send only Receive only Neither Fig.. Peak power transmission according to the existence of the SC and the RC with the m distance between the PC and the LC.

428 29 COMPATIBILITY AND POWER ELECTRONICS CPE29 6TH INTERNATIONAL CONFERENCE-WORKSHOP.. E-3 E-4 [Amp. & Analyzer] [Power, Load] [Send, Load] [Powe, Receive] [Send, Receive]... m.6 m.7 m.8 m.9 m. m.2 m.4 m. m.6 m.7 m.8 m.9 m 2. m 2. m 2.2 m E- 2 3 4 6 7 8 9 2 3 4 Fig. 6. Output power transmission as a function of frequency according to the connection variation with m distance between the PC and the LC. condition means that the SC is located near the PC and the RC is located nearby the LC which is same with fig.. In the Receive only (Send only) condition, the RC (SC) is located nearby the PC (LC) but the SC (RC) is omitted in which the resonance conpling condition is broken. In the Neither condition, both the PC and the LC is omitted. The peak power transmission over 3 mw occurs in the Both condition at the 9.4 MHz which is near the resonance frequency. In the other conditions, the output power transmission is smaller than mw. To further confirm the resonant coupling rather than the radiation effect between the resonant objects, we used all coils but varied the RF power amplifier (spectrum analyzer) connection to coils. Fig. 6 shows the output power transmission characteristics as a function of frequency according to the connection variation with m distance between the PC and the LC. The peak power transmission in every case is shown in fig. 7. In the [Power, Load] condition, the power amplifier 3 2 2 [Power, Load] [Send, Load] [Power, Receive] [Send, Receive] Fig. 7. Peak power transmission according to the connection variation with m distance between the PC and the LC. E-3 2 3 4 6 7 8 9 2 3 4 Fig. 8. Output power transmission as a function of frequency according to the distance variation between the power coil and the load coil. (spectrum analyzer) is connected to the PC (LC) coil as shown in fig.. In the other condition, the power amplifier (spectrum analyzer) is connected to the SC (LC) in the [Send, Load] condition, to the PC (RC) in the [Power, Receive] condition, and to the SC (RC) in the [Send, Receive] condition. The [Send, Receive] condition is similar to the power transmission through radiation only. Because there are the SC and the RC in all cases, the peak power transmission occurs near 9.4 MHz. However the maximum peak power transmission over 3 mw is occurred in the [Power, Load] condition only. In the other conditions, the output power transmission is smaller than mw. Although the 9. MHz is the resonance frequency of the send and the receive coil, if we connect the RF power amplifier (spectrum analyzer) to the send coil (receive coil) directly, the peak power transmission is too small. This result means that to effectively transfer the power wirelessly, the free resonant objects (here, the send coil and the receive coil) are necessary. The following experiment is made with the [power, Load] condition as shown in fig.. Fig. 8 shows the output power transmission characteristics as a function of frequency with the distance variation between the power coil and the load coil. The peak power transmission is occurred near the 9. MHz when the distance is larger than m. However, as the distance between the PC and the LC is narrowed below m, the peak power transmission s frequency started to split. Fig. 9 shows the (a) peak power transmission frequency and (b) peak power transmission with the distance variation between the PC and the LC. The peak power transmission frequency is splitted at the m distance and the peak power transmission frequency is splitted to.7 MHz at. m distance. As the distance between the PC and the LC, the peak power transmission is increased and saturated near m distance. The peak frequency splitting is due to the strong interaction of the two coils which modify the normal modes of the individual coil [6]. Even though the peak power transmission frequncy splitting is zero, the power transmission

SPECIAL POWER ELECTRONICS SYSTEMS AND APPLICATIONS 429 Peak frequency (MHz) 9.8 9.6 9.4 9.2 9. 8.8 8.6 8.4 8.2 8...2.4.6.8..2.4.6.8 2. 2.2 Distance (m) 4 3 2 (a) (b)..2.4.6.8..2.4.6.8 2. 2.2 Distance (m) Fig. 9. (a) Peak power tramsmission frequency and (b) peak power transmission with the distance variation between the PC and the LC. is possible with 2 % efficiency up to 2 m through resonant coupling. Fig. shows the output power transmission characteristics as a function of frequency with the distance variation between 3 2 2 2 Distance (mm) Fig.. Peak power transmission according to the distance variation between the receive coil and the load coil. The distance between the power coil and the load coil is fixed at.4 m. the receive coil and the load coil. The distance between the power coil and the load coil is fixed at.4 m. The frequency of the peak power transmission is not changed but the shape of the output power transmission is sharped in the lower frequency region as the distance between the LC and the RC is increased. Fig. shows the peak power transmission with the distance variation between the load coil and the receive coil. The peak power showed the maximum at 28 mm. Because the distance between the LC and the PC is constant, the distance between the SC and the RC is decreased as the the distance between the RC and the LC is increased which intensify the coupling between the resonance objects. However as the distance between the load coil and the receive coil is increased, the inductive couplng between the RC and the LC is decreased. So there is an optimum distance, 28 mm. Fig. 2 shows the output power transmission characteristics as a function of frequency with the distance variation between.. E-3 E-4 E- mm 8 mm 2 mm 28 mm 3 mm mm 93 mm 3 mm 9 mm E-6 2 3 4 6 7 8 9 2 3 4 Fig.. Output power transmission characteristics as a function of frequency according to the distance variation between the receive coil and the load coil. The distance between the power coil and the load coil is fixed at.4 m... E-3 E-4 mm mm 2 mm 3 mm 4 mm mm 7 mm mm 3 mm mm 7 mm 2 mm E- 2 3 4 6 7 8 9 2 3 4 Fig. 2. Output power transmission characteristics as a function of frequency according to the distance variation between the send coil and the power coil. The distance between the power coil and the load coil is fixed at. m.

43 29 COMPATIBILITY AND POWER ELECTRONICS CPE29 6TH INTERNATIONAL CONFERENCE-WORKSHOP 3 3 2 2 2 Distance between Send & Power (mm) Fig. 3. Peak power transmission according to the distance variation between the send coil and the power coil. The distance between the power coil and the load coil is fixed at. m. the send coil and the power coil. The distance between the power coil and the load coil is fixed at. m. The frequency of the peak power transmission is not change but the shape of the output power transmission is sharped in the upper frequency region as the distance between the SC and the PC is increased. Fig. 3 shows the peak power transmission variation with the distance variation between the send coil and the power coil. The peak power tansmission showed the local minimum near 3 mm instead of the local maximum as in fig.. To increase the power efficiency, the distance between the power coil and the send coil should be narrowed as possible, but the distance between the load coil and the receive coil should be sought the optimum distance for the best power efficiency. Fig. 4 shows the lighting characteristics of light emitting device s (LED) connected to the load coil as a function of frequency. The distance between the power coil and the load coil is fixed at. m. The LED is turned on at 9. MHz and shows the maximum brightness at 9.4 MHz. And the LED is turned off again at 9.6 MHz. The lighting characteristics is Fig.. Lighting characteristics of LED lamp which is consisted of 22 LEDs. The distance between the power coil and the load coil is fixed at. m. completely accounted by the output power transmission characteristics. To check the wireless high power transmission compatibility, E (V/m) 3 2 2..6.7.8.9.. Position (m) (a) 2.. H (A/m).....6.7.8.9.. Position (m) (b) Fig. 4. Lighting characteristics of LED connected to the load coil as a function of frequency. The distance between the power coil and the load coil is fixed at. m. Fig. 6. (a) Electric and (b) magnetic field intensity along the position from the power coil to the load coil. The distance between the power coil and the load coil is fixed at. m. The forward power to the power load is 3 W.

SPECIAL POWER ELECTRONICS SYSTEMS AND APPLICATIONS 43 we connected the LED lamp which is consisted of 22 LEDs as shown in fig. 6. The output was reached as W. Fig. 6 shows (a) the elelctric and (b) the magnetic field intensity along the position from the power coil to the load coil where the load coil is connected to the 2 LED lamps and the output power transmision is 3 W. Instead of the monotically decreasing field intensity from the power coil, theh fields show the minimum at the middle position between the send coil and the receive coil which is the characteristics of the resonant coupling. IV. SUMMARY We will investigate the various experimental setups to find the optimum condition for the wireless power transmission through resonant coupling. Without the free resonant objects, we cannot obtain the high efficient wireless power transmission. There is a optimal distance between the load coil and the receive coil to maximize the power transmission efficiency. The wireless power transmission is possible upto 2 m with 2 % efficiency. We successfully light the LED lamp wirelessly using resonant coupling. REFERENCES [] W. C. Brown, The history of power transmission by radio waves, IEEE Trans. Microwave Theory Tech., vol. 32, pp. 23-242, September 984. [2] C. G. Kim, D. H. Seo, J. S. You, J. H. Park, and B. H. Cho, Design of a contactless battery charger for cellular phone, IEEE Trans. Ind. Electron., vol. 48, pp. 238-247, December 2. [3] G. B. Joung and B. H. Cho, An energy transmission system for an artificial heart using leakage inductance compensation of transcutaneous transformer, IEEE Trans. Power Electron., vol. 3, pp. 3-22, November 998. [4] T. Sekitani, M. Takamiya, Y. Noguchi, S. Nakano, Y. Kato, T. Sakurai, and T. Someya, A large-area wireless power-transmission sheet using printed organic transistors and plastic MEMS switches, Nature Mater., vol. 6, pp. 43-47, June 27. [] A. Kurs, A. Karalis, R. Moffatt, J. D. Joannopoulos, P. Fisher, and M. Soljacic, Wireless power transfer via strongly coupled magnetic resonances, Science, vol. 37, pp. 83-86, July 27. [6] A. Karalis, J. D. Joannopoulos, and M. Soljacic, Efficient wireless nonradiative mid-range energy transfer, Annal of Physics, vol. 323, pp. 34-48, 28