Abstract
In this study, small multilayer planar spiral coils were analyzed and optimized to wirelessly charge an in-ear wearable bio-signal monitoring device in a wine-glass-shaped transmitter (Tx) based on magnetic resonance wireless power transfer (MR-WPT). For analysis of these coils, a volume filament model (VFM) was used, and an equivalent circuit formulation for the VFM was proposed. The proposed method was applied to design effective multilayer coils with a diameter and height of 6 and 3.8 mm, respectively, in the wearable device. For the coils, a printed circuit board having a 0.6 mm thick dielectric substrate and a 2 oz thick copper metal was used. Moreover, the coils on each layer were connected in series. The dimensions of the double-, four-, and eight-layer coils were optimized for the maximum quality factor (Q-factor) and coupling efficiency. The operating frequency was 6.78 MHz. The optimal dimensions for the maximum Q-factor varied depending on the number of coil layers, pattern width, and turn number. For verification, the designed coils were fabricated and measured. For the four-layer coil, the coupling efficiency and Q-factor using the measured resistance and mutual inductance were 58.1% and 32.19, respectively. Calculations showed that the maximum Q-factor for the four-layer coil was 40.8 and the maximum coupling efficiency was 60.1%. The calculations and measurement were in good agreement. Finally, the entire system of the in-ear wearable bio-signal monitoring device, comprising a wine-glass-shaped transmitter, the designed receiving coil, and a monitoring circuit, was fabricated. The measured dc-dc efficiency of the MR-WPT system was 16.08%.
Highlights
A small multilayer planar spiral coil was analyzed using the volume filament model (VFM) in the MHz frequency range to assess the applicability of the coil for wearable bio-signal monitoring devices
By considering the size limitation of wearable devices, a multilayer coil was optimized for maximum coupling efficiency
The multilayer planar spiral coils are considered to be equivalent to the concentric multiloop coil, as shown in Figure 2, and t = 2 oz (~70 μm), T = 0.6 mm, and w0 = 0.15 mm are used
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The pattern width and metal thickness were normalized by the skin depth These methods are inapplicable for multilayer planar spiral coils. In [10], a method deriving a resonant frequency and the Q-factor of a four-layer printed spiral coil, using coil dimensions such as turn number, line width, and number of layers, was provided. A small multilayer planar spiral coil was analyzed using the volume filament model (VFM) in the MHz frequency range to assess the applicability of the coil for wearable bio-signal monitoring devices. The operating frequency was 6.78 MHz. By considering the size limitation of wearable devices, a multilayer coil was optimized for maximum coupling efficiency. The complete bio-signal monitoring device is integrated and tested
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