Measurement of Wireless Power Transfer

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1 Measurement of Wireless Power Transfer Andi Sudjana Putra #1, Sriharsha Vishnu Bhat #2, Vinithra Raveendran #3 # Engineering Design and Innovation Centre (EDIC), ational University of Singapore (US) Block E1A-03-03, 1 Engineering Drive 2, Singapore engpas@nus.edu.sg 2 bhat.sriharsha@gmail.com 3 vini.raveendran@gmail.com Abstract Wireless power transfer offers the potential to redefine the usage of electricity. The advancement of near field magnetic resonance technology makes wireless power transfer viable nowadays. Measurement of power, however, remains an inherent challenge that derives from the difficulty of measuring the voltage and current of a battery. This paper presents our current development of wireless power transfer technology and focuses on the power measurement aspect of such technology. The continuous nature of the wireless power transfer is employed to provide accurate measurement of power, decoupled from the inherent problem of battery measurement. We have applied this technology in a model for the future transportation application. I. INTRODUCTION Wireless power transfer is the process of transferring power from one circuit to another without passing any manmade conductive elements interconnecting them [1]. Wireless power has shown great potential in the consumer market [2]. The absence of hard-wire provides convenience to the users and is compatible with the increasing number of portable devices. Wireless power transfer has held allure since the turn of the 20 th century, when Nikola Tesla attempted to realize longrange wireless power transmission before the advent of the transmission line [3]. Although Tesla s grandiose efforts did not succeed, the idea of wireless electricity still lingers because of its potential in terms of allowing mobility and convenience. Wireless power transfer often employs the principle of electromagnetic induction. The progress in the areas of radio frequency transmission and magnetic resonance, however, turn out to be implementable, too, for wireless power transfer. While electromagnetic induction offers high efficiency of power transfer at short range, the same reduces significantly at long range. Far-field radio frequency transmission offers high efficiency over long distance through the use of high frequency electromagnetic waves, but has an inherent tradeoff between directionality and transmission efficiency. Nearfield magnetic resonance, on the other hand, offers high efficiency at shorter distance through the use of resonant inductive coupling between two charging loops [4]. When two self-resonant loops are tuned to resonate with each other, highest efficiency is achieved and maximum power transfer takes place. Indeed, there are studies done in this area. Self-resonant coils have been used in a strongly coupled regime, resulting in a non-radiative power transfer over distances up to 8 times the radius of the coils [5]. Design of a wireless power transfer for high power moving applications has been proposed using transformer-based design [6]. Variations of such designs and experiments have been performed [7]-[15]. The sound implementation of such wireless power transfer system will benefit many applications such as high power moving objects and magnetic levitation systems [6]. Our work in developing a wireless power transfer system has been focused on the usability of such system. Yet there are technological development and advancement to be reported as a contribution to the scientific community. This study provides our recommendation for methodology of measuring/evaluating the power transfer in an installation that employs wireless transfer technology. In this paper, we describe the measurement aspect of a wireless power transfer environment that has been developed in our facility. The challenge of power measurement in a wireless power transfer system has been known to be complex and compounded by the battery, which is typically present in such system. The power measurement, therefore, has to consider not only the voltage and the current, but also the temperature and the chemical composition of the battery. Complicating the problem further, the discharge voltage curve of a large number of batteries, such as alkaline-based, is flat and as such cannot be used to infer its power or charging state. In relation to wireless power transfer, the challenge is compounded by at least the following two factors: Inaccuracy of voltage measurement when the battery is being charged or discharged due to voltage polarisation Constant attachment of load to the battery during voltage measurement that typically needs detachment of load We exploit the continuous nature of wireless power transfer by measuring the net power transfer, i.e. decoupling such measurement from the measurement of the battery power. Our implementation is based on measuring the transfer of power (in a wireless power transfer installation) instead of the available power in a battery, hence simplifying the measurement of power to achieve higher accuracy. We will first present the overview of the wireless power transfer installation in Section II to give an idea about the technical configuration of the installation. We will then zoom into the discussion about the concept and implementation of the measurement of wireless power transfer in Section III, with discussions about the design and arrangement of the power measurement while alleviating the abovementioned /13/$ IEEE 351

2 inherent issues. We close with conclusions and future work in Section IV. II. THE WIRELESS POWER TRANSFER SYSTEM We have set up a wireless power transfer installation based on resonant inductive coupling principle [16]. The resonant charging circuit has 2 parts: Primary circuit, which consists of an oscillation power source and a self-resonant loop Secondary circuit, which consists of a coupled-resonant loop and a load Using this principle, wireless power transfer occurs due to electromagnetic induction, but its best efficiency occurs if the primary and secondary circuits are tuned such that their oscillating frequencies are resonant with each other. For resonance to take place, an oscillating current is passed through a coil (inductor) that generates an oscillating magnetic field. The presence of a capacitor loading the inductor makes the primary circuit ring at a specific frequency, generating a resonant magnetic field, with energy in the coil dying away much slower than pure induction. If a second coil is brought near the first, at the same resonant frequency, highly efficient energy transfer takes place. The self-resonance of the loop is generated via an inductorcapacitor (LC) circuit. Both the self-resonant and coupledresonant loop is tuned to the same frequency by choosing the values of inductors and capacitors according to the desired frequency. A. Overview of the Wireless Power Transfer Installation The backbone of our wireless power transfer is a Colpitt s oscillator [17], feeding alternating current to a primary LC circuit (Fig. 1), coupled to a secondary LC circuit that powers a resistive load [5] (Fig. 2). The primary LC circuit draws power from a power supply at the infrastructure side, and has an oscillator to convert it to a sinusoidal current that goes to a resonant circuit consisting of inductors and capacitors in parallel. The secondary circuit, residing at the device side, is coupled with the primary circuit at the same frequency, and has a rectifier that converts the oscillating current to DC to charge a battery. The matching of the primary and secondary frequency is done by tuning the inductance and capacitance of the primary and secondary circuit; and hence creating a resonance. Fig. 1. Primary circuit in the wireless power transfer installation Fig. 2. Secondary circuit in the wireless power transfer installation B. Design and Calculation The model of the wireless power transfer is developed based on the parallel circular coil arrangement [18] as presented in Fig. 3. In such arrangement, mutual inductance is only along the z axis. As such, only the z-direction magnetic flux density is considered in our design as follows (referring to Fig. 2): B = μ ρkm Em, (1) where B z is the magnitude of the z-direction magnetic flux density, µ o the absolute permeability, I the current, D the distance between two coils, and: Km=1 m sin θ dθ, (2) Em= 1 m sin θ dθ, (3) = Fig. 3. Parallel circular coil configuration. (4) The magnetic flux density of a circular coil at the points of the same ρ is identical and the total magnetic flux linkage is obtained by summing the flux of a central circular area and each circular subdivision. Taking into account the single loop hollow tube as the primary coil, we have =. As a note, we have = ; i.e. symmetric primary and secondary coil. The angle alignment between the primary and secondary coils affects the efficiency of power transfer in the manner specified in a few studies [19], where it is also discovered that compensation of misaligned coils can be accomplished by rotating the coils to specific angles. The power in the alternating current circuit consists of the real power and the reactive power; the latter is due to the presence of the inductive and capacitive loads in the circuit /13/$ IEEE 352

3 C. Implementation Example The design above is used to explore various scenarios in a future transportation system, as presented in Fig. 4. This area of implementation is one of a few interest areas pursued in our centre to address challenging urban problems in the society. we have = = = = =6 nf, =100 nf, and = = = =BYV27. Fig. 5. Charging circuit B. Voltage and Current Measurement Continuous monitoring is done by measuring the voltage and the current out of the rectifying circuit prior to the load. Such implementation, as presented in Fig. 6, is aimed at decoupling the measurement from the battery and hence directed at the net power transfer from the charging coil only. Furthermore, an isolation circuit is included to provide a high impedance input and minimize the loading effect. Fig. 6. Measurement circuit Fig. 4. Model of transportation system III. MEASUREMENT As the device is being charged, the status of its current and voltage is monitored, from which the power status will become available. A. Battery Charging Circuit The power is transmitted from the primary coils on the infrastructure to the secondary coils on the device via inductive coupling. A full-bridge rectifier is used to convert the transmitting alternating current into direct current to charge the device s battery. The schematic of the battery charging circuit is presented in Fig. 5. In our implementation, The voltage is measured directly using a voltage divider circuit. This circuit is also used to step down the voltage to a readable level by the microprocessor. We realise such circuit using the resistances of R 1 and R 2. An optional variable resistance R 3 is used to calibrate R 1 and R 2 component tolerance. We choose a large value (more than 1MΩ) of resistance to minimise the current losses due to the voltage divider. The current is measured indirectly using a resistive component current sensor and then measuring the voltage drop across this known resistive sensor. In our experimental setup, an 8-bit PIC18F772 microprocessor is used to process the measurement signal. This measurement technique for a wireless power transfer system is versatile, in that it can be applied for a wide range of application that might use a wide range of battery. C. System Implementation The voltage signal was processed by the microprocessor to determine the measured voltage and current. Resistance R 1 and R 2 were chosen at a ratio of 1:3 to 1:4 to step down the voltage at the battery node to be compatible to the input requirements of the microprocessor /13/$ IEEE 353

4 As a note of implementation, the battery used in our experimental setup ranges from 0 12 V. This backbone system is implemented in the wireless charging installation explained in Section II. Fig. 7 presents the user interface of such indicator. This user interface serves as means to display the measured power status of the wirelesscharged battery. When the battery is in a discharging state (i.e. being used to power a device), a static battery is displayed with the highlighted green portion representing the percentage of the remaining battery charge. When the battery is in the charging state (i.e. being charged), a dynamic battery (with a moving green bar) is displayed. IV. CONCLUSIONS In this paper, we have discussed a reliable and versatile system of measurement of wireless power transfer by decoupling the measurement from the battery. We have also demonstrated the successful implementation of this measurement system in the form of a user interface for the wireless power transfer installation. Given the ease with which the setup can be adapted for a range of voltage or current specifications, there is much potential for this system to be adopted for continuous power measurement in a variety of applications. This method of power measurement enables many charging applications that require continuous measure of power. ACKNOWLEDGMENT The authors would like to acknowledge the excellent contribution of Fan Yizhong, Deng Xijia, and Leow Poh Jin in this work. The authors would also like to thank the Engineering Design and Innovation Centre (EDIC) for supporting the work presented in this paper. Fig. 7. Battery level display In the subsequent implementation, another user interface is added to allow the user to control the charging of the battery; as presented in Fig. 8. Fig. 8. Charging initiation control The decoupling concept is realisable using multi-cell system, too, where multiple batteries are present. Each battery cell is charged independently, while the other cells continue to drive various systems in the vehicle. This provides better isolation of the power measurement, in that the load is also decoupled from the power measurement system. REFERENCES [1] V. Prasanth, Wireless power transfer for E-mobility, Delft University of Technology, [2] B. Johns, An introduction to the wireless power consortium standard and Texas Instruments compliant solutions, Power Management, pp , 1 st quarter, [3] N. Tesla, US Patent 1,119,732, [4] I. J. Yoon and H. Ling, Realizing efficient wireless power transfer in the near-field region using electrically small antennas, Wireless Power Transfer - Principles and Engineering Explorations, K. Y. Kim (ed.), ISBN: , InTech, [5] A. Kurs, A. Karalis, R. Moffatt, J. Joannopoulos, P. Fisher, and M. Soljacic, Wireless power transfer via strongly coupled magnetic resonances, Science, vol. 317(5834), pp , July [6] S. Hasanzadeh and S. Vaez-Zadeh, Design of a wireless power transfer system for high power moving applications, Progress in Electromagnetics Research M, 28, pp , [7] Y. Kim and H. Ling, Investigation of coupled mode behavior of electrically small meander antennas, IET Electronics Letters, 43, pp , [8] H. C. Jing and Y. E. Wang, Capacity performance of an inductively coupled near field communication system, IEEE Antennas Propag. Int. Symp. Dig., [9] Y. Kim and H. Ling, On the coupled mode behavior of electrically small antennas, Proc. URSI at. Radio Sci. Meeting, [10] S. Pan, D. R. Jackson, J. Chen, and P. Tubel, Investigation of wireless power transfer for well-pipe applications, Proc. URSI at. Radio Sci. Meeting, [11] E. M. Thomas, J. D. Heebl, and A. Grbic, Shielded loops for wireless non-radiative power transfer, IEEE Antennas Propag. Int. Symp. Dig., [12] Y. K. Jung and B. Lee, Metamaterial-inspired loop antennas for wireless power transmission, IEEE Antennas Propag. Int. Symp. Dig., [13] 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 Electronics, 27, pp , [14] A. Kurs, R. Moffatt, and M. Soljacic, Simultaneous mid-range power transfer to multiple devices, App. Phys. Lett., 96, pp , [15] J. J. Casanova, Z. N. Low, and J. Lin, A loosely coupled planar wireless power system for multiple receivers, IEEE Trans. Industrial Electronics, 56, pp , /13/$ IEEE 354

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