| With the rapid growth of various types of consumer electronics, electric vehicles and implantable medical sensors, wireless power transfer(WPT) based on near-field magnetic resonant coupling has become a hot spot of research in this field as its moderate transmission distance, relatively high efficiency and the relatively small influence to organism and environment in recent years. However, the theory and experiment show that the energy transmission efficiency decreases sharply with the increasing of transmission distance. We can improve the energy transfer efficiency by improving the quality factor of the coils. To overcome this problem, the other approach is to increase the mutual coupling strength of the source and the receiver coils which can be achieved by the control of the near field distribution, especially, the field distribution can be modified by medium material lens. Metamaterial which has negative refractive index, evanescent wave enhancement, perfect lens, singular physical characteristic of electromagnetic is expected to solve the problem of controlling near electromagnetic which could not achieve by using traditional materials. In this paper, we mainly studied the mechanism of negative permeability metamaterials in WPT system and summarized WPT system transmission characteristics with different types of negative-permeability metamaterials slab(NPMs). At last, a kind of miniaturized low-frequency wireless energy transmission system is designed, fabricated and measured based on the enhancement of loading with indefinite negative-permeability metamaterial slab(IMs).The main contents of this paper are shown as follows:Firstly, a typical WPT system based on near-field magnetic resonant is analyzed with circuit theory. The definition of the system transmission efficiency is also given here. Using the magnetic dipole approximation model, the calculation formula of the mutual inductance between the two coils is deduced and verified by numerical simulation. The results show that, when the whole system is in the sub wavelength conditions, the theoretical value and the simulation results are in good agreement.Secondly, using the multi physical field simulation software CMOSOL,the influence of loading with different types of NPMs were numerically simulated here by evaluating the enhancement factor ? which is defined as the ratio between the mutual inductance enhanced with metamaterial lens and the mutual inductance in vacuum. For isotropic definite negative-permeability metamaterial(NMs), we summarized the finite size effect and the loss effect. Anisotropic definite negative-permeability metamaterial(ANMs) is also studied based on the optical transformation theory at the case which does not satisfy the condition of perfect lens. We concluded the change of ? by using IMs which has different parameter. Theoretical simulations show that the indefinite metamaterials lens outperforms isotropic negative metamaterial lens in terms of coupling efficiency and lens size when parameters for the finite slab size and permeability are optimized due to the Fabry-Perot resonances(FPR).Finally, we propose a miniaturized WPT system loaded with ultra-subwavelength IM at high frequency. The enhancement of efficiency between the two coils and the impact of the finite size of the IM lens were evaluated with the 3D electromagnetic simulation software CST. Then we present the design principles of the geometric structure and electromagnetic parameters that can achieve the best enhancement effect. Through this design, we can achieve the best enhancement effect. A highly sub-wavelength IM lens is proposed and realized on a thin slab using dual layer planar spiral resonators. The WPT experiments show that the efficiency can be enhanced significantly when the system is loaded with indefinite-permeability metamaterial lens. Besides, the proposed IM lens is demonstrated to extend transmission distance in coil axis directions and provide better transmission in a middle range distances. |
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