PAPER Reliable Data Transmission for Resonant-Type Wireless Power Transfer

Transcription

1 298 IEICE TRANS. FUNDAMENTALS, VOL.E96 A, NO.1 JANUARY 2013 PAPER Reliable Data Transmission for Resonant-Type Wireless Power Transfer Shinpei NOGUCHI a), Student Member,MamikoINAMORI b), and Yukitoshi SANADA c), Members SUMMARY Wireless power transfer research has been receiving a great deal of attention in recent years. In resonant-type wireless power transfer, energy is transferred via LC resonant circuits. However, system performance is dependent on the circuit components. To transfer power efficiently and safely, information, such as frequency, required power and element values, need to be transmitted reliably in the system. This paper investigates data communication using orthogonal frequency division multiplexing (OFDM) modulation in resonant-type wireless power transfer systems. The equivalent circuit used in the transmitting and receiving antennas is a band pass filter (BPF) and its bandwidth is evaluated through circuit simulations and experimental measurements. Numerical results obtained through computer simulation show that the bit error rate (BER) performance is affected by the splitting resonant frequency. key words: OFDM, Wireless power transfer, BPF, communication, control 1. Introduction Recent interest in wireless power transfer has been attracting a great deal of attention. Wireless power transfer will enable advances in the use of electronic devices such as mobile phones, portable computers, etc.. Wireless power transfer is currently achieved via three techniques, with each system possessing different characteristics in terms of distance and power transfer efficiency. The three techniques are electromagnetic induction, coupled radio frequency power transmission, and resonant coupling. In electromagnetic induction, the magnetic flux induces the electric current, thus power is transferred wirelessly to the received coil [1]. The efficiency of power transfer varies between 60 98% over a distance of several millimeters. To achieve coupled radio frequency power transmission, electromagnetic waves are converted to direct currents, which provides power [2]. The efficiency of power transfer is less than 50% over a distance of several meters. In the resonant coupling technique, two coils are tuned at the same resonant frequency. Power transfer efficiency is approximately 50% over a distance of several tens of centimeters [3]. In 2006, MIT released WiTricity, which applies this resonant induction [4]. In this paper, the magnetic resonant coupling system is modeled for wireless power transfer. Transmitting and receiving antennas in the resonant coupling system need to induce a non-radiative magnetic field. A practical implementation can be applied by using loop antennas, in which changes to induced magnetic field are affected by the number of turns [5]. However, the self-resonant coils rely on the interplay between distributed conductance and distributed capacitance, which effects the power transfer efficiency. The power transfer efficiency is affected by the change of load at the receiver [6]. To enable fast reaction, it is desirable to have the ability to change the power signal according to the request of the receiver when providing power at the transmitter. When change is detected at a transmitter, information such as frequency, required power and element values should be transmitted shortly and adapt to the desired power signal accordingly [6], [7]. Therefore, it is very important for wireless power transfer systems to transmit these data fast and reliably. This paper investigates the data transmission process. The equivalent circuit used in the transmitting and receiving antennas is a band pass filter (BPF) and its bandwidth is evaluated through circuit simulations and experiments. In this paper, the transfer function S 21 as measured experimentally and calculated from the circuit model are evaluated. The bandwidth to transmit the data information is then decided. To satisfy the conditions for the high speed communication and reliability, orthogonal frequency division multiplexing (OFDM) is applied as a modulation scheme. Bit error rate (BER) is calculated through MATLAB simulations. The required BER is set to 10 4, which is reliable for data communication [8]. This paper is organized as follows. Section 2 introduces the system model and Sect. 3 outlines the experimental setup. In Sect. 4, numerical results obtained through computer simulations are presented. Section 5 gives our conclusions and directions for future work. 2. System Model 2.1 Single Antenna In this paper, a 3-turn coil as shown in Fig. 1 is used as a transmitting and receiving antenna. The equivalent circuit is Manuscript received June 29, The authors are with the Dept. of Electronics and Electrical Engineering, Keio University, Yokohama-shi, Japan. a) anoguchi@snd.elec.keio.ac.jp b) inamori@elec.keio.ac.jp c) sanada@elec.keio.ac.jp DOI: /transfun.E96.A.298 Fig. 1 Single antenna. Copyright c 2013 The Institute of Electronics, Information and Communication Engineers

2 NOGUCHI et al.: RELIABLE DATA TRANSMISSION FOR RESONANT-TYPE WIRELESS POWER TRANSFER 299 Fig. 4 Equivalent circuit of the power transfer system. Fig. 2 Equivalent circuit of antenna. Fig. 3 Resonant coupling system with two coils. Fig. 5 Coupling coefficient k vs. distance between two antennas dz. Table 1 Parameters in single antenna. R r L C 0 C 1 36 mω 50 μh 5.7 pf 47 pf shown in Fig. 2 [5]. In Fig. 2, L represents self-inductance, R r represents the radiation resistance of the coil, C 0 represents the stray capacitance, and C 1 represents the load capacitance. The conductor losses are ignored in the circuit model. From Neumann s formula, the self inductance, L, is given by L = μ 0 dl 1 dl 2, (1) 4π 1 2 r 12 where dl 1 and dl 2 are small line elements on a coil, and r 12 is the thickness of the coil [9]. The radiation resistance of a coil R r is given by ( ) 4 2π ( R r = 20 πp 2 G ) 2 (2) λ where G is the number of turns, p is the radius of a coil. The corresponding free space wavelength λ = c/ f where c = m/s and f represents the source frequency [10]. Figure 3 shows the antenna model. The diameter of the coil, D, is 30 cm and the thickness is 5 cm in the experimental model. In this paper, the parameter values, L, C 0 and R L measured in the experimental model are applied. Following which, the load capacitance, C 1, is determined for the resonant frequency of 10 MHz [11]. The values of each parameter in this model are shown in Table Resonant Coupling System The equivalent circuit of this system is shown in Fig. 4 [5]. The load impedance, Z 0, is set to 100 Ω. From Neumann s formula, the mutual inductance between the antennas is given by and M = μ 0 4π 1 2 dl 1 dl 2, (3) dz M = k L 1 L 2, (4) where dz is the distance between antennas, k is the coupling coefficient, and L 1, L 2 are the self inductance of the receiving antenna as given in Eq. (1) [9]. From Eqs. (3) and (4) with assuming L = L 1 = L 2, the coupling coefficient, k, is calculated as shown in Fig. 5. With the value for the coupling coefficient, k, and Eq. (4), the transfer function, S 21, which represents the power transfer efficiency, is calculated for the circuit model. 2.3 Communication Model In the communication model for data transmission, the equivalent circuit used in the transmitting and receiving antennas is regarded as a BPF, which has to be custom designed in order not to cause interference. To satisfy these constraints, OFDM is applied for data transmission in the power transfer system. Suppose the information symbol on

3 300 IEICE TRANS. FUNDAMENTALS, VOL.E96 A, NO.1 JANUARY 2013 the kth subcarrier is s[k] (k = 0,..., N 1), the OFDM symbol is given by u[n] = 1 N 1 N k=0 s[k]e j 2πnk N, (5) where n (n = 0,..., N 1) is the time index and N is the number of subcarriers. The guard interval is added before the data transmission. The baseband signal at the output of the filter is given by x(t) = P 1 n=0 u[n]c t(t nt s ), where C t (t) is the impulse response of the transmitting filter, P is the length of the impulse response, and T s is the OFDM symbol duration. In this system, the antennas are fixed and multipath fading is not assumed. The received signal is given by P 1 y(t) = u[n]h(t nt s ) + v(t), (6) n=0 where v(t) is the additive white Gaussian noise (AWGN), h(t) is the impulse response of the composite channel and is given by In this experimental system, S 21 was measured with the vector network analyzer (VNA). It is assumed that the antenna is only moved along the z-axis. The experimentally measured inductance of the coil at the transmitting and receiving antenna, Lˆ 1, is applied to the values in the circuit model, L 1 and L Simulation Results 4.1 S 21 Characteristic Figures 7 and 8 show S 21 characteristic as measured experimentally and calculated from the equivalent circuit model. The distance between the transmitting and receiving antenna on the experimental measurements, dz, is set to 10 cm and 40 cm, the coupling coefficient, k, on the circuit simulator is set to 0.16 and 0.07 from Fig. 5, respectively. In Fig. 7, when dz = 10 cm, both the theoretical curve based on the circuit model and the experimental measurement curve show the splitting of resonant peak. As the coupling between the coils h(t) = C t (t) C r (t), (7) where denotes convolution and C r (t) is the impulse response of the receiving filter. The frequency response of channel in the communication model, H, is equivalent to S 21 in the power transfer system. 3. Experimental Measurement The experimental single antenna with 3-turn coil is shown in Fig. 6. The measurement equipment is shown in Table 2. Fig. 7 Transfer function S 21 (k = 0.16, dz = 10 cm). Fig. 6 Experimental antennas. Table 2 Equipment Vector network analyser VNA control software Circuit simulator Tx antenna Rx antenna Measurement equipment. Specification Agilent 8753ET Agilent technology Intuilink (Version 1.3) PSpice circuit simulator Loop antenna (D = 30 cm) Loop antenna (D = 30 cm) Fig. 8 Transfer function S 21 (k = 0.07, dz = 40 cm).

4 NOGUCHI et al.: RELIABLE DATA TRANSMISSION FOR RESONANT-TYPE WIRELESS POWER TRANSFER 301 at the transmitting and receiving antenna becomes stronger, the peak splits into two. Moreover, the theoretical curve based on the circuit model does not fit the experimental measurement curve. It is due to the mismatch of derived values on the experimental measurement model and equivalent circuit model, which are chosen from parameters such as resistances, stray capacitances and self inductances. The parameters used in the transmitter and the receiver are assumed to be the same in the circuit model as described in Sect. 3. Therefore, the mismatch comes from the difference of the coupling coefficient, k, between the circuit model and the experimental measurement. From Fig. 8, when dz = 40 cm, both the theoretical curve based on the circuit model and the experimental measurement curve show that the peak does not split. In the experimental measurement, the asymmetry curve is observed. The reason is that the transmit and receive antennas are not physically identical in this experimental model. 4.2 Impulse Response To investigate the influence of the transmitting and receiving antennas, the impulse responses of S 21 is shown. Figures 9 and 10 display the impulse response of the channel from the experimental measurement in the delay domain between the antennas when dz = 10 cm. In the data transmission system, OFDM is employed for the 2nd modulation, and the bandwidth of OFDM is designed to fit the relatively large impulse response of the channel in the guard interval period. Thus, the number of the subcarriers is derived to satisfy this condition: N/T s W. (8) Here, W is the bandwidth of the composite filters, which is measured at half-power points (3 db) from the peak. 4.3 BER Performance Simulation Model BER performance is evaluated through computer simulation. The simulation model is shown in Fig. 11 and the simulation conditions are shown in Table 3. Information bits are modulated with quadrature phase shift keying (QPSK) or 64 quadrature amplitude modulation (QAM) on each subcarrier. The number of discrete Fourier transform (DFT) points is set to 32 and 16,which are fit to W given from experimentally measured S 21 characteristic as shown in Figs. 7 and 8, respectively. The guard interval is set to 8 and 4, which are 1/4 of the number of subcarrier N. The channel model used is a quasi-static multipath channel, which is the path between the transmit antenna and the receive antenna. The phase compensation is assumed to be perfect Simulation Results From Figs. 12 to 15, the BER performances on the AWGN Fig. 9 Impulse response of S 21 from the experimental measurement (k = 0.16, dz = 10 cm). Fig. 11 Simulation model. Fig. 10 Impulse response of S 21 from the experimental measurement (k = 0.07, dz = 40 cm). Table 3 Simulation conditions. Modulation scheme 1st : QPSK/64QAM 2nd : OFDM Bandwidth 1.33 MHz 0.36 MHz FFT size Number of data subcarriers Number of guard interval 8 4 Channel model quasi-static multipath channel Number of OFDM symbols 10, 000, 000 dy 0cm dz 10 cm 40 cm

5 302 IEICE TRANS. FUNDAMENTALS, VOL.E96 A, NO.1 JANUARY 2013 Fig. 12 BER performance of QPSK (k = 0.16, dz = 10 cm). Fig. 14 BER performance of QPSK (k = 0.07, dz = 40 cm). Fig. 13 BER performance of 64QAM (k = 0.16, dz = 10 cm). Fig. 15 BER performance of 64QAM (k = 0.07, dz = 40 cm). model with QPSK and 64QAM are shown, when dz is 10 cm and 40 cm. In Figs. 12 and 13, BER performances of dz = 10 cm appear degraded between 2 4 db at 10 4 when compared to the AWGN theoretical curve due to the frequency selective channel, which is caused by the unbalanced splitting resonant frequency used. Moreover, the BER curve with experimental frequency response is worse than that of the calculated frequency response from the circuit model. This is because the OFDM signal is affected by the splitting resonant frequency in the experimental measurement. On the other hand, in Figs. 14 and 15, when dz = 40 cm, both BER curves with circuit simulation and experimental measurement are close to the AWGN theoretical curve. This is because the splitting resonant frequency is not observed and the bandwidth of OFDM is fit to W in Fig. 8. When dz = 10 cm, either one of the peak frequencies can be chosen as the passband for data communication. In such a case, the BER performance is similar to the results shown in Figs. 14 and 15, because a narrow bandwidth is used at dz = 40 cm and it is not affected by the splitting resonant frequency. 5. Conclusions In this paper, data transmission for power transfer systems has been investigated. Resonant coupling is used to deliver power from one coil to another coil wirelessly. In the wireless power transfer system, information, such as frequency, required power and element values, need to be transmitted initially to ensure safe power transfer. The equivalent circuit of the antenna is BPF, and the transfer function S 21 is regarded as the impulse response of the channel in the data transmission. In this paper, the transfer function S 21 is given from calculation on the circuit model and from experimental measurements. In the data transmission model, OFDM is used as the 2nd modulation and the impulse response of S 21 in the time domain is designed to fit within the guard interval pe-

6 NOGUCHI et al.: RELIABLE DATA TRANSMISSION FOR RESONANT-TYPE WIRELESS POWER TRANSFER 303 riod. As the distance between antennas was fixed, the channel is assumed to be AWGN. From simulation results, BER performance of dz = 10cm is degraded compared to the AWGN theoretical curve due to the splitting resonant frequency. This is because the shorter distance between antennas make mutual inductance strong. In this paper, it is assumed that the system parameters are transmitted as information in the system. Therefore, this research is valid for low data rate transmission with narrow bandwidth. Further work will consider the data transmission with power transferring for control system adjustments. Acknowledgement This work is supported in part by a Grant-in-Aid for Young Scientists (B) under Grant No from the Ministry of Education, Culture, Sport, Science, and Technology and Keio Gijuku Academic Development Funds in Japan. Shinpei Noguchi was born in Tokyo, Japan in He received his B.E. degrees in electronics engineering from Keio University, Japan in Since April 2011, he has been a graduate student in School of Integrated Design Engineering, Graduate School of Science and Technology, Keio University. His research interests are mainly concentrated on control system for wireless power transfer. Mamiko Inamori was born in Kagoshima, Japan in She received her B.E., M.E., and Ph.D. degrees in electronics engineering from Keio University, Japan in 2005, 2007, and 2009, respectively. Since October 2009, she has been an assistant professor in Keio University. She received the Young Scientist Award from Ericsson Japan in Her research interests are mainly concentrated on software defined radio. References [1] W. Zheng, X. Pi, X. Zheng, H. Liu, X. Xiao, and C. Peng, A wireless transmission system based on electromagnetism induction for remote controlled capsule, Proc. World Automation Congress 2008, pp.1 4, Sept [2] K. O Brien, R. Teichmann, and H. Gueldner, Magnetic field generation in an inductively coupled radio-frequency power transmission system, Proc. 37th IEEE Power Electronics Specialists Conference, pp.1 7, June [3] A. Karalis, Efficient wireless non-radiative midrange energy transfer, Annals of Physics, vol.323, pp.34 48, April [4] A. Kurs, Wireless power transfer via strongly coupled magnetic resonances, Science Express, vol.317, no.5834, pp.83 86, [5] Y. Hiraiwa, N. Kikuma, H. Hirayama, and K. Sakakibara, A consideration on transmission efficiency characteristics of wireless power transfer using proximity coils, IEICE Technical Report, A P , Sept (in Japanese). [6] D. Wageningen and T. Staring, The Qi wireless power standard, 14th International Power Electronics and Motion Control Conference, [7] W.X. Zhong, X. Liu, and S.Y.R. Hui, A novel single-layer winding array and receiver coil structure for contactless battery charging systems with free-positioning and localized charging features, IEEE Trans. Ind. Electron., vol.58, no.9, pp , Sept [8] J.D. Griffin and G.D. Durgin, Gains for RF tags using multiple antennas, IEEE Trans. Antennas Propag., vol.56, no.2, pp , Feb [9] S.I. Babic and C. Akyel, New analytic-numerical solutions for the mutual inductance of two coaxial circular coils with rectangular cross section in air, IEEE Trans. Magn., vol.42, no.6, pp , June [10] B.L. Cannon, J.F. Hoburg, D.D. Stancil, and S.C. Goldstein, Magnetic resonant coupling as a potential means for wireless power transfer to multiple small receivers, IEEE Trans. Power Electron., vol.24, no.7, pp , July [11] I. Awai, BPF theory-based design method for wireless power transfer system by use of magnetically coupled resonators, IEEJ Trans. EIS, vol.130, no.12, pp , June 2010 (in Japanese). Yukitoshi Sanada was born in Tokyo in He received his B.E. degree in electrical engineering from Keio University, Yokohama Japan, his M.A.Sc. degree in electrical engineering from the University of Victoria, B.C., Canada, and his Ph.D. degree in electrical engineering from Keio University, Yokohama Japan, in 1992, 1995, and 1997, respectively. In 1997 he joined the Faculty of Engineering, Tokyo Institute of Technology as a Research Associate. In 2000 he joined Advanced Telecommunication Laboratory, Sony Computer Science Laboratories, Inc, as an associate researcher. In 2001 he joined Faculty of Science and Engineering, Keio University, where he is now a professor. He received the Young Engineer Award from IEICE Japan in His current research interests are in software defined radio, cognitive radio, and ultra wideband systems.