1.Institute of Solid State Physics, Shanxi Datong University, Datong 037009, China 2.Shanxi Provincial Key Laboratory of Electromagnetic Functional Materials for Microstructure, Datong 037009, China 3.School of Mathematics and Physics, Suzhou University of Science and Technology, Suzhou 215009, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant Nos. 51607119, 11874245, 11604186) and the Shanxi Science and Technology Innovation Team of Microstructural Functional Materials, China (Grant No. 201805D131006)
Received Date:02 June 2019
Accepted Date:07 August 2019
Available Online:01 November 2019
Published Online:05 November 2019
Abstract:In order to improve the efficiency of wireless power transfer (WPT) system, the spatial fields are regulated on a two-non-resonant-coil WPT system by hexagon artificial magnetic conductors (AMC). In our configuration, the AMC is located by the side of the two-non-resonant-coil WPT system and close to the transmitter coil. The AMC structure consists of small hexagon copper patches periodically arranged on the dielectric substrate. Each patch is grounded by a via passing through its center hole. Chip capacitors are soldered in the gaps between the adjacent patches. We can design the working frequency of WPT system through the capacitance of these chip capacitors. The results show that the electromagnetic fields are changed between the transmitter coil and the receiver coil in WPT system due to the introducing of the AMC structure. There are two main reasons. First, many resonant modes are excited by near magnetic fields on the AMC structure. Second, near magnetic fields are shielded by the AMC structure. The variation of space electromagnetic field improves the transmission efficiency of WPT system. When the working frequency is 27 MHz and the transmission distance is 3 cm, the experiment verifies that the transmission efficiency increases by 22% in the WPT system with the AMC structure compared with the WPT system without the AMC structure. Simultaneously, the transmission efficiency is raised by 25% at different transmission distances. The simulation results are almost consistent with the experimental results. There is a little difference that the number of resonant modes is different between the simulation and the experiment due to the resistance loss of the chip capacitors in experiment. Therefore, we correct the simulation results under consideration of resistive loss. In addition, the excited multiple resonant modes can supply multiple and adjustable working frequencies in the WPT system with the AMC structure. In practical applications, AMC is low in cost and easy to implement. Keywords:artificial microstructure materials/ artificial magnetic conductors/ wireless power transfer
全文HTML
--> --> --> 1.引 言家居、城市、交通和太空智能等物联网器件[1]的在线, 需要灵活实时的能量供应, 这类能量供应的需求迅速增加和扩大了对无线能量传输(wireless power transfer, WPT)技术的应用需求和范围. 例如, 无线充电技术[2]已经被应用在生物医学植入[3]、消费电子产品[4]、电动汽车[5]、无线传感器网络[6]等领域. 1914年, Tesla[7]提出WPT的最初设想; 1963年, Brown[8]实现了微波远场的WPT; 2007年, Kurs等[9]验证了磁谐振耦合感应式WPT, 其可以适用于近距离和中距离的能量传输; 随后有多种谐振子和传输路径设计方案被用来提高WPT的传输效率[10]. 2011年, Wang等[11]将人工微结构材料引入WPT系统中, 利用超材料(metamaterials)构造的特异表面结构放置在发射线圈和接收线圈的中间, 放大电磁场传输过程中的倏逝波, 从而增强了WPT系统的传输效率. 为了提高WPT的传输效率, 有多种微结构材料被应用在WPT系统[12—17], 进行近电磁场调控. 人工磁导体(artificial magnetic conductors, AMC)是由人工设计的特异表面结构[18—24], 通过覆铜板刻蚀后实现, 同时也是一种高阻抗表面结构. 该结构被应用于天线, 由于高阻抗表面有抑制表面波和零相位反射的性质, 很大程度地提高了天线的方向增益性, 同时有利于天线体积微型化[22—24]. 2013年, Wu等[25]提出将理想磁导体作为反射面用在WPT系统中, 仿真计算得到了传输效率增强的结果. 2015年, Lawson等[26]将加载了电容的AMC应用于磁感应式WPT系统中, 证实AMC结构具有电磁屏蔽的效果. 本文将加载贴片电容的正六边形AMC结构引入到WPT系统中, 通过仿真和实验相结合的方式, 研究了WPT的传输效率, 得到了效率增强的结果, 并从物理上解释其原因. 相较于文献[25, 26], 我们不仅在实验上验证了效率的提升, 同时分析了引入AMC结构后空间电磁场的变化, 找到了WPT效率增强的物理原因. 2.结构设计文中采用的WPT系统结构设计如图1(a)所示, 从上而下依次是接收线圈、发射线圈和AMC, 两个线圈的直径相同, 且AMC的表面与线圈平行放置. 线圈的线直径是2 mm, 环直径是150 mm. AMC结构属于蘑菇型结构, 是由介质基板上周期排列的正六边形小贴片组成, 每个小贴片在其中心均通过镀铜过孔接地, 并在相邻单元的间隙焊接贴片电容. 取横向周期数为8, 在倾斜方向取两个单元为一个复式单元, 在纵向此复式单元的周期数为5; 正六边形铜贴片的边长为20 mm, 相邻贴片间的缝隙宽度为1 mm; 贴片电容的电容值均为4.7 nF; 镀铜过孔直径是1 mm; 电路板(printed circuit board, PCB)所用介质为介电常数4.4的FR4, 厚度为3 mm, 如图1(b)所示. 这里需要强调一点, PCB介质的介电常数对该结构在此应用的性能影响很小, 所以可以选择成本最低的材料. 图 1 (a) WPT结构示意图; (b) AMC单元结构示意图 Figure1. (a) Schematic of the WPT structure; (b) schematic of the AMC unit cell structure.
图 4 仿真效率随距离的变化 Figure4.S21 as a function of distance from the coil obtained from the simulation.
为了更好地说明效率提升的物理原因, 图5(a)—(e) 5幅图分别给出了图3中标注的5个峰所对应频率的磁场侧面分布图, 图5(f)给出了无AMC结构时磁场侧面分布图. 从图5(f)可以看到, 非共振双线圈WPT系统中, 无AMC结构时, 能量有很大一部分分布在发射线圈的另一侧, 到达接收线圈的能量很有限. 然而, 当加入AMC结构时, 近磁场不同程度地被该结构屏蔽, 但是磁场屏蔽并不是传输效率提升的根本原因, 而是由于AMC结构使附近空间的磁场分布发生了变化, 如图5(a)和图5(f)所示. 从图5(a)—(e)还可以看到, 对应不同的共振频率处, 对近磁场的屏蔽程度不同, 而且5个模式分布对应着正六边形AMC结构的不同阶模式, 所以传输效率不同. 值得一提的是, AMC结构有屏蔽MHz频段磁场的功能, 而传统磁材料一般低于1 MHz, 且现有的MHz磁屏蔽材料造价昂贵. 图 5 (a)?(e)分别对应图3中的5个共振频率处的磁场侧面分布图; (f)对应无AMC结构时磁场侧面的分布图 Figure5. (a)?(e) Side distribution of the magnetic field associated with the five resonance frequencies shown in Figure 3; (f) the side distribution of the magnetic field in the absence of an AMC structure.
图 1 (a) WPT结构示意图; (b) AMC单元结构示意图
图 2 AMC结构的等效电路模型
图 3 仿真的效率图
图 4 仿真效率随距离的变化
图 5 (a)?(e)分别对应图3中的5个共振频率处的磁场侧面分布图; (f)对应无AMC结构时磁场侧面的分布图
图 6 实验配置图
图 7 实验的传输效率图
图 8 实验效率随距离的变化
图 9 贴片电容与电阻串联时的仿真效率