# Miniature Coil for Wireless Power and Data Transfer through Aluminum +
## 1 Introduction  
Wireless power and data transfer through metal enables a new set of applications in critical fields such as sensing of temperature and pressure behind sealed metal containers in aircraft fuselages, vehicle armor, and isolated chambers [1, 2, 3, 4]. Inductive coupling is key to non-contact or wireless power and data transfer through metal surfaces [5, 6]. Research in the area of wireless power and data transfer through metal has been mainly focused on large coils for power transfer [7, 8]. In [9], authors employed a coil with an outer diameter of \(30\,\mathrm{mm}\) enclosing a metal pipe to transfer \(30\,\mathrm{\SIUnitSymbolMicro W}\) through stainless steel (\(1.6\,\mathrm{mm}\)). In [4], a solenoid coil of \(234\,\mathrm{mm}\times 63\,\mathrm{mm}\times 61\,\mathrm{mm}\) was used with an even larger core to transfer \(1.2\,\mathrm{W}\) at \(10\%\) efficiency through stainless steel (\(10\,\mathrm{mm}\)). In [10, 11, 12], a helical stacked coil of \(49\,\mathrm{mm}\) outer diameter and \(50\,\mathrm{mm}\) height demonstrated power transfer of \(5\,\mathrm{W}\) at \(9\%\) efficiency through aluminum (\(3.1\,\mathrm{mm}\)). Only [9] has partially demonstrated power and data transfer through metal using the same coil structure, but information on the coil size and data rate were not available. In general, the size of a coil used for wireless power transfer through metal ranges between \(30\,\mathrm{mm}\) to \(400\,\mathrm{mm}\)[4, 9, 10, 11, 12, 13]. The large coil size poses a limitation for certain applications [14].  
This journal article is an extended version of the paper \"Miniature Coil Design for Through Metal Wireless Power Transfer\" presented in the 2021 IEEE Wireless Power Transfer Conference, San Diego, CA, USA, 1-4 June 2021 [15]. In our conference paper, we demonstrated the optimization of a wire gauge for building a planar circular geometry coil. The planar circular geometry coils were used in an inductive link to demonstrate \(100\,\mathrm{mW}\) harvested power through a \(1\,\mathrm{mm}\)-thick aluminum plate without any data communication.  
In this paper, we present the development of a miniature coil for through metal power and data transfer using inductive coupling. Our new coil has a very small size of15 mm \(\times\) 13 mm \(\times\) 6 mm. Our developed coil deviates from the traditional flat circular or square geometries [16] to maximize the effect of its ferrite core to obtain superior harvested power in through metal scenarios. To fabricate the coil, 24 AWG copper wire was tightly wound around a rectangular ferrite core. The winding covered the entire core, with more than 25 turns per layer and four layers in total. Two identical coils placed on both side of a 1 mm-thick aluminum plate were used to form an inductive link for energy transfer. The experimental results demonstrate that 440 mW was harvested through the 1 mm aluminum plate using our miniature coils. The coil can operate up to 2 kHz for aluminum and stainless steel metal barriers. We report the measured harvested power and efficiency of the new coil for stainless steel and aluminum barriers with different thicknesses. We developed a through-metal communication system where the same pair of coils is used for both energy transfer and wireless communications. Furthermore, the inside communication subsystem (inside a metal box) only uses harvested power for operation. We have demonstrated a successful bidirectional communication at a minimum of 100 bps using 75 mW harvested energy for the inside subsystem through a 1 mm aluminum barrier. Communication through aluminum is significant since is a light-weight material.