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Title Two- and Three-Dimensional Omnidirectional Wireless Power View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by HKU Scholars Hub Two- and Three-Dimensional Omnidirectional Wireless Power Title Transfer Author(s) Ng, WMR; ZHANG, C; Lin, D; Hui, SYR IEEE Transactions on Power Electronics, 2014, v. 29, p. 4470- Citation 4474 Issued Date 2014 URL http://hdl.handle.net/10722/199099 Rights Creative Commons: Attribution 3.0 Hong Kong License 4470 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 9, SEPTEMBER 2014 Letters Two- and Three-Dimensional Omnidirectional Wireless Power Transfer Wai Man Ng, Member, IEEE, Cheng Zhang, Deyan Lin, Member, IEEE, and S. Y. Ron Hui, Fellow, IEEE Abstract—Nonidentical current control methods for 2- and 3-D omnidirectional wireless power systems are described. The omni- directional power transmitter enables ac magnetic flux to flow in all directions and coil receivers to pick up energy in any position in the proximity of the transmitter. It can be applied to wireless charging systems for low-power devices such as radio-frequency identifica- tion devices and sensors. Practical results on 2-D and 3-D systems have confirmed the omnidirectional power transfer capability. Index Terms—Inductive power, magnetic resonance, wireless power transfer. I. INTRODUCTION IRELESS power pioneered by Tesla a century ago can W be classified as radiative and nonradiative. In communi- cation, omnidirectional antenna is a class of antenna that radiates signals in all directions [1]. For nonradiative applications, most of the low-power [2]–[4] and medium/high-power [5] wire- less power applications have their power flow guided by coil Fig. 1. Use of three separate orthogonal coils A, B, and C connected in series resonators. In many low-power applications such as sensors as a transmitter structure for wireless power transfer reported in [10]. and radio-frequency identification (RFID) devices, replacing the flow) or two directions on the same plane (i.e., 2-D power flow). batteries has been a maintenance problem in industry. A well- However, two recent reports explore the possibility of omni- designed omnidirectional wireless charging system is therefore directional wireless power. In [9], the use of orthogonal coils a highly attractive and economic option for charging a multiple is proposed to reduce the effect of small mutual inductance of devices simultaneously. Based on 2-D magnetic structures, when the receiver coil is perpendicular to one of the transmit- the studies in [6] and [7] have described how magnetic flux can ter coil (see Fig. 1). Their approach considers the open-ended be controlled in different directions on a 2-D plane. In this letter, coils as antennas, and use the parasitic coil inductance and ca- a patent-pending control methodology [8] for omnidirectional pacitance to form an equivalent LC circuit. Because the coils wireless charging systems is presented. It has been studied with are considered as antennas, such a design approach (based on the aid of time-domain finite-element magnetic field simulations impedance matching or the maximum power transfer (MPT) and practically tested in hardware prototypes. theorem) would have the following limitations. II. OMNIDIRECTIONAL WIRELESS POWER SYSTEMS WITH 1) The length of the wire used to implement the resonant ORTHOGONAL COILS circuit is comparable to the wavelength at the resonance frequency. Both of the transmitter and receiver coils are So far, the majority of the nonradiative wireless power sys- one quarter of the wavelength at the resonant frequency. tems have the power flow either in one direction (i.e., 1-D power This approach is therefore dimension dependent and is restrictive in terms of the relative sizes of transmitter and Manuscript received September 13, 2013; revised November 30, 2013; receiver coils. accepted January 9, 2014. Date of current version April 30, 2014. This work was supported by the Hong Kong Research Grant Council under 2) Due to the usually low parasitic capacitance in open- Project HKU-712913E. Recommended for publication by Associate Editor ended coil, the resonant frequency and therefore the oper- M. A. E. Andersen. ating frequency is usually high. High-frequency ac power W. M. Ng, C. Zhang, and D. Lin are with the Department of Electrical and Electronic Engineering, The University of Hong Kong, Pokfulam, Hong sources are usually more expensive than low-frequency ac Kong (e-mail: [email protected]; [email protected]; deyanlin@ power sources. eee.hku.hk). In [9], two separate orthogonal coils are driven by a single S. Y. Ron Hui is with the Department of Electrical and Electronic Engineer- ing, The University of Hong Kong, Pokfulam, Hong Kong and also with the power source with the same ac current (i.e., the two separate coils Department of Electrical and Electronic Engineering, Imperial College London, are connected in series). This is why the receiver coil can pick up London, U.K. (e-mail: [email protected]). maximum power at an angle of 45◦ between the two orthogonal Color versions of one or more of the figures in this paper are available online ◦ at http://ieeexplore.ieee.org. transmitter coils. This result is reasonable because at 45 ,the Digital Object Identifier 10.1109/TPEL.2014.2300866 vectorial sum of the two coaxial magnetic field vectors from 0885-8993 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications standards/publications/rights/index.html for more information. IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 29, NO. 9, SEPTEMBER 2014 4471 the two orthogonal coils is maximum if the two coil currents are identical. Wang et al. [9] also suggest the extension to the 3-D structure based on three separate orthogonal coils that are connected in series and fed by the same current. In [10], a three-coil receiver structure with three orthogonal open-ended coils is placed inside a similar but larger three-coil transmitter structure also with open-ended coils. The three or- thogonal transmitter coils are connected in series and driven with the same ac current. It was demonstrated that wireless power transfer to the three-coil receiver unit can be achieved regardless of the orientation of the receiver unit inside the trans- mitter structure. However, this orientation-insensitive feature is only possible if the receiver has three orthogonal coils. For RFID tags applications, it is more likely to have a single planar coil in the RFID tag as a receiver coil. So, the approach in [10] is not suitable for a single-coil receiver. In summary, the magnetic resonance techniques were used in [9] and [10]. If impedance matching based on the MPT method is adopted, according to [4] and [11], the system en- Fig. 2. Trajectory of the magnetic field vector from viewpoint [1 1 1] of a 3-D omnidirectional wireless power transmitter under the current amplitude ergy efficiency will not exceed 50%. So, this project employs modulation control. the maximum energy efficiency (MEE) approach. The use of the same current in the orthogonal coils (i.e., identical current con- π π trol) also does not generate magnetic field vector that points in I3 = Im sin ω2 t + sin ωt + . (6) all directions in a 3-D manner—which is an essential feature for 2 2 true omnidirectional wireless power transfer. The next section In this particular case, the magnitude of I1 is constant. The will introduce the nonidentical current control methods. magnitudes of I2 and I3 vary with two sinusoidal envelopes that are 90◦ out of phase. This system is studied with the cou- III. CONTROL METHODS FOR OMNIDIRECTIONAL WIRELESS pled circuit model used in [12]–[14] and an electromagnetic POWER SYSTEMS field solver [15]. The current values and the phase angles are Fig. 1 shows a typical winding structure of a 3-D omnidi- calculated and sampled over several excitation cycles at the fun- rectional transmitter comprising three orthogonal coils in the damental angular frequency ω. The datasets obtained in a time x-, y-, and z-plane. In practice, each coil is connected to a se- sequence are recorded and the magnetic fields are plotted so ries capacitor to form a coil resonator. Each resonator is driven that the peaks of the magnetic field vectors can be obtained. The by an ac power source. For genuine omnidirectional wireless movements of the peak magnetic field vectors are then traced power transfer, it is necessary for the orthogonal coils current to form the trajectory. The trajectory of the magnetic field vec- to be nonidentical [8]. The three coil currents can generally be tors for several excitation cycles is plotted in Fig. 2. It can be expressed as follows: seen that the magnetic field vectors rotate periodically in all di- rections, confirming the omnidirectional nature of the wireless I1 = Im 1 sin (ωt) (1) power transmission. I = I sin (ωt + α) (2) Second, the frequency modulation control can be achieved 2 m 2 for (1)–(3) in various ways. For example, by putting I3 = Im 3 sin (ωt + β) (3) α = kβ (7) where ω is the angular frequency of the currents, t is the time | | variable, Imx is the current magnitude of coil-x (for x = 1, α = αm sin (ω2 t) (8) 2, 3), and α and β are two angular displacements. To achieve where k is a real number, ω is the angular frequency of the omnidirectional wireless power transmission, rotating magnetic 2 phase-angle variation, and α is the constant coefficient. Third, field vectors can be generated by either: 1) current amplitude m for phase angle control, one can simply excite the system with modulation; 2) phase angle control; or 3) frequency modulation the three orthogonal coil currents with the same current mag- [8].
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