Geometrical Design of a Scalable Overlapping Planar Spiral Coil Array to Generate a Homogeneous Magnetic Field

We present a design methodology for an overlapping hexagonal planar spiral coil (hex-PSC) array, optimized for creation of a homogenous magnetic field for wireless power transmission to randomly moving objects. The modular hex-PSC array has been implemented in the form of three parallel conductive layers, for which an iterative optimization procedure defines the PSC geometries. Since the overlapping hex-PSCs in different layers have different characteristics, the worst case coil-coupling condition should be designed to provide the maximum power transfer efficiency (PTE) in order to minimize the spatial received power fluctuations. In the worst case, the transmitter (Tx) hex-PSC is overlapped by six PSCs and surrounded by six other adjacent PSCs. Using a receiver (Rx) coil, 20 mm in radius, at the coupling distance of 78 mm and maximum lateral misalignment of 49.1 mm ( 1/√3 of the PSC radius) we can receive power at a PTE of 19.6% from the worst case PSC. Furthermore, we have studied the effects of Rx coil tilting and concluded that the PTE degrades significantly when θ > 60^° . Solutions are: 1) activating two adjacent overlapping hex-PSCs simultaneously with out-of-phase excitations to create horizontal magnetic flux and 2) inclusion of a small energy storage element in the Rx module to maintain power in the worst case scenarios. In order to verify the proposed design methodology, we have developed the EnerCage system, which aims to power up biological instruments attached to or implanted in freely behaving small animal subjects' bodies in long-term electrophysiology experiments within large experimental arenas.

I. Introduction

Wireless power transmission using magnetic fields has recently received considerable attention as one of the attractive methods to recharge mobile consumer electronics, such as smartphones and laptops, by simply placing them over flat charging surfaces without power cords [1]. As a result, an industrial consortium has been formed to standardize such devices, and several companies are now offering new products that operate on the basis of inductive power transmission [2]–[6]. To prevent overheating and interference with other nearby devices, in compliance with the Federal Communications Commission (FCC) rules, the power transmission efficiency (PTE) should be maintained at a high level [7].