Magnetic Field Analysis of Curved-Surface Coils for High-Spatial-Freedom WPT

The MHz Wireless Power Transfer (WPT) is known to have the advantages of further transfer distance and extra convenience. A combination of multiple coils and some modulation methodologies provide improved spatial freedom. However, research on coil designs and array applications for confined areas still lacks consideration. This article studies the characteristics of curved-surface coils in magnetic field shaping. Focusing on a specific cylindrical surface, the curved-surface coils are analyzed and optimized, aiming to provide stable output inside the cylindrical space when both lateral and angular misalignment happen. The magnetic field density generated by a curved-surface coil is mathematically derived. Relevant simulations are conducted in HFSS to verify the field shaping effects. A 6.78 MHz system is setup to test the power transfer performance. One planar coil and one curved-surface coil with identical size are compared in the experiment. The results show improved stability of power transfer when both lateral and angular misalignment exist.