Transmitter Coil Optimization for Free-Positioning Wireless Power Transfer Based on Surface Current Density Modes

In inductive coupling-based wireless power transfer (WPT), free orientation of the receiver (Rx) coil can be enabled by employing the 3-D omnidirectional method, where three self-decoupled coils are superimposed to generate a steerable magnetic field. However, conventional optimizations of the transmitter (Tx) coils begin with a limited set of regular shapes, resulting in suboptimal solutions in terms of efficiency and uniformity. This article proposes a novel optimization framework for obtaining the optimal Tx designs without a predetermined coil topology. The coil is modeled as linear expansions of basis surface current density (SCD) modes, and a circuit-electromagnetic (EM) field combined analysis is proposed to associate the transmission efficiency with the EM fields. With the proposed scheme, the optimal tradeoff between transmission efficiency and magnetic field (B-field) spatial uniformity can be found, and customizable according to application needs. A demonstrative platform for freely behaving small animals is set up to validate the proposed method, where three Tx coils are optimized for maximum efficiency with B-field uniformity of 90% along x- and y-axes and 97% along the z-axis. About 5%–10% transmission efficiency can be achieved inside the spherical shell using a single Rx coil at the measured points. The magnetic field measurement is in good agreement with the simulation results. The proposed method is suitable for the optimization of Tx coil designs for wireless headstage platforms and other consumer applications.