Yongseok Lim

An Adaptive Impedance-Matching Network Based on a Novel Capacitor Matrix for Wireless Power Transfer

In a wireless power transfer (WPT) system via the magnetic resonant coupling, one of the most challenging design issues is to maintain a reasonable level of power transfer efficiency (PTE), even when the distance between the transmitter and the receiver changes. When the distance varies, the PTE drastically decreases due to the impedance mismatch between the resonator of the transmitter and that of the receiver. This paper presents a novel serial/parallel capacitor matrix in the transmitter, where the impedance can be automatically reconfigured to track the optimum impedance-matching point in the case of varying distances. The dynamic WPT matching system is enabled by changing the combination of serial and parallel capacitors in the capacitor matrix. An interesting observation in the proposed capacitor matrix is that the resonant frequency is not shifted, even with capacitor-matrix tuning. In order to quickly find the best capacitor combination that achieves maximum power transfer, a window-prediction-based search algorithm is also presented in this paper. The proposed resonance WPT system is implemented using a resonant frequency of 13.56 MHz, and the experimental results with 1W power transfer show that the transfer efficiency increases up to 88 % when the distance changes from 0 to 1.2 m.

Impedance-Matched Wireless Power Transfer Systems Using an Arbitrary Number of Coils With Flexible Coil Positioning

This letter presents a new analytic design method for impedance-matched wireless power transfer (WPT) systems using an arbitrary number of coils. For deriving the impedance matching condition, we have used an equivalent circuit of the WPT system. It allows us to avoid solving voltage-current equations for the actual circuit. For verifying the proposed design method, an impedance-matched WPT system with one repeater coil has been designed and fabricated. This letter also shows that the distances between the coils can be varied maintaining the impedance-matching performance, and the distances can be determined analytically using the presented design method. Application of the presented design method can be found in designing WPT systems that require flexible coil positioning.

A Novel Phase-Control-Based Energy Beamforming Techniques in Nonradiative Wireless Power Transfer

Recent efforts to increase the energy transfer efficiency in magnetic resonant coupling-based wireless power transfer (WPT) systems, have been focused on improving quality factor, precision of impedance matching, and position alignment between the resonators. Although those approaches are effective to increase transfer efficiencies, the transferred energy can easily be wasted due to leakage flux of nondirectional fields. In this paper, we present a novel magnetic field shaping technology for improving the energy efficiency in a near-field WPT system. In this study, the beamforming techniques that have been used for radio frequency systems are efficiently exploited in a WPT system to improve the transfer efficiencies by minimizing unnecessary leakage flux. The optimal antenna structure for energy forming is first determined through mathematical analysis. Using the proposed crossed antennas, the phase-control method is effectively used to form magnetic fields in particular directions. The proposed energy forming-based WPT system using crossed antennas is implemented with the phase control of three-power stack transmitters. The experimental results matches well with the theoretical analysis, and the energy-forming approach for synthesizing the magnetic fields achieves average improvements of the transfer efficiency and transfer distance of up to 20.1% and 30%, respectively, over the conventional nonradiative energy transfer approach at 1 m distance.

Design of Physical Layer for Magnetic Field Area Network

This paper introduces the concept of a Magnetic Field Area Network (MFAN), which uses low-frequency magnetic-field signals by loop antennas. It has two layers, data link layer and physical layer. The physical properties of magnetic field signals enable these systems to operate through soil, rock, water and medium boundary as well as air although magnetic field strength falls off rapidly. Additionally, Loop antennas make it possible to implement low-frequency communication systems. We have designed physical layer functions and a system including ARM7TDMI, FPGA, and several peripheral devices.

Analysis of Antenna Structure for Energy Beamforming in Wireless Power Transfer

The technology to send a magnetic field in a particular direction, known as energy beamforming, has been recently introduced as a magnetic field shaping technology in nonradiative wireless power transmission. In general, one of the most efficient conditions for energy beamforming is that the magnetic fields induced by each antenna should be synthesized to head the same direction. To synthesize the magnetic fields induced at each antenna, interference by mutual inductance that can occur between transmitting (TX) antennas should be minimized. In addition, energy should not be exchanged between the TXs, otherwise it lowers the transmission efficiencies of TX and receiving antenna. In this paper, we present an optimal antenna structure that minimizes the mutual inductance between two TX antennas. First, we have analyzed the mutual inductance between TX antennas that have asymmetric sizes with different antenna lengths and arrangement angles. The directivity of the magnetic field vector is also investigated through an experimental analysis of an antenna structure. Finally, it has been verified that the optimal TX antennas for energy beamforming should be symmetric, which means that all the length of antennas are same and disposed perpendicular to each other. The experimental results show that the deviations of magnetic field directivity for symmetric and asymmetric antennas are 0.045 and 0.355, respectively, which shows that the symmetric structure shows 8.2 times larger consistency over the asymmetric structure.

Design of the Protocol for Wireless Charging of Mobile Emotional Sensing Device

In order to supply emotion service depending on users emotional change in a mobile environment, various researches have been carried. This paper discusses a protocol for wireless charging and an embedded platform of the mobile emotional sensing device which supports that. Wireless charging process relieves users vexatious task to charge the emotional sensing device. To support wireless charging, there are one basestation and several mobile devices. Basestation coordinates and controls the devices over wireless communication, as well as supplies energy. For 1:N communication we defines the network whose superframe is classified into four categories: a network join superframe, a charging request superframe, a charging superframe and an inactive superframe. Physical layer provides how to supply energy to the devices and communicate physically. Mobile device is equipped with energy charged circuits, which correspond with the defined energy supplying method, as well as bidirectional communication circuits. Mobile device monitors and analyzes its own battery status, and is able to send a request packet to basestation. Therefore, it can be charged before its battery is exhausted without users perception.

An ambient RF energy harvesting and backscatter modulating tag system enabling zero-power wireless data communication

This paper describes an Internet of Things (IoT) tag using an RF energy harvesting and a backscatter wireless communicating system in ambient wireless network environment. We propose a method to transmit data from the tag to Wi-Fi device without power consumption using Wi-Fi signal. This tag system is composed of RF energy harvesting module, backscatter communicating module and sensor. This tag operates to generate power from wireless signal and to transfer date by modulating the receiving sensitivity of Wi-Fi packet. We implemented the zero-power communication tag and tested in Wi-Fi network. As a result, an output voltage above 0.9 V for operating the tag is obtained starting from a received power of about -16 dBm. The maximum power conversion efficiency reaches 25% at 2.4 GHz band. At the communication test, the data transfer rate is achieved 30 kbps at 3 m distance between Wi-Fi devices.

The Study of the Controlling Resonant Wireless Power Transfer Using Bluetooth Communication

Wireless power transfer technology has become a recent big issue for mobile phones industry, which can be classified into two types: inductive type and resonant type. Inductive type usually has higher efficiency but requires short distance between the transmitter and receiver. And resonant type has much better freedom from distance.