The effects of metals on a transcutaneous energy transmission system

The effects of metal objects on the mutual inductance, self-inductance, and effective series resistance (ESR) of the coaxial coils of a transcutaneous energy transmission system (TETS) were investigated theoretically and experimentally. The theory considers a thin conducting sheet of infinite size aligned parallel to a current-carrying coil. Results of the theory indicate that coil parameters vary with the distance from the sheet to the coil. Changes in mutual and self inductance are independent of the conductivity and thickness of the sheet, with a larger percentage change for mutual inductance than for self inductance. Changes in ESR are proportional to the surface resistivity of the sheet. Experimental measurements using several aluminum sheets and a titanium alloy can in the presence of the TETS coils used for the Penn State artificial heart showed excellent agreement with the theory for inductance parameters and agreed within a factor of 2 for ESR measurements when skin effect was considered. It was generally observed that mutual inductance drops to 65% of its initial value as a metal sheet is moved to within 1-2 cm of the coil, while self-inductance drops to 80% of its initial value when the sheet is 2 cm from the coil. Measured changes in ESR tended to be higher than the calculated values with the discrepancy depending on distance to the metal object. Theory and measurements were extended to the case of lateral misalignment of the coils.