Spatial filtering magnetic metasurface for misalignment robustness enhancement in wireless power transfer applications

In this paper, we present the design of spatial filtering magnetic metasurfaces to overcome the efficiency decay arising in misaligned resonant inductive Wireless Power Transfer systems. At first, we describe the analytical framework for the control of currents flowing on a finite-size metasurface, avoiding classical truncation effects on the periphery and opportunely manipulating, at the same time, the spatial magnetic field distribution produced by the closely placed RF driving coil. In order to validate the theoretical approach, we conceive a numerical test case consisting of a WPT system operating at 12 MHz. By performing accurate full-wave simulations, we prove that inducing a uniform current in the metasurface results in a more robust WPT system in terms of misalignment with respect to conventional configurations, also including standard metasurfaces. Therefore, while the use of metasurfaces in WPT systems has been already demonstrated to be beneficial in terms of efficiency enhancement, we confirmed that a proper control of the metasurfaces field filtering response can be advantageous also for the misalignment issue. Notably, the free space wavelength at the operating frequency (12 MHz) is 25 m, whereas the proposed metasurface dimensions are only 0.0024λ × 0.0024λ. Despite the extremely reduced dimensions, the spatial magnetic field distribution produced by the closely placed RF driving coil can be nevertheless opportunely manipulated. Finally, experimental measurements conducted on fabricated prototypes validated the numerical results, demonstrating the effectiveness of the proposed approach. These achievements can be particularly helpful in WPT applications where the position of driving and receiving coils frequently changes, as in consumer devices and biomedical implants.