Coil Array Technique With Maximum Tolerance to 2-D Misalignment for Wireless Power Transfer Systems

In this article, we present a coil array technique for wireless power transfer (WPT) systems that provides maximum tolerance to misalignment in any direction. Our approach integrates assistant coils, consisting of a
$2\,\, {}\times {}2$
array of corner coils and a center coil, with the primary coil. By adjusting the varactors that load the assistant coils, optimum magnetic field is generated to maintain high efficiency regardless of misalignment. The corner coils steer the magnetic field to maintain the performance even with misalignment. Moreover, the center coil induces a virtual coil, which effectively compensates the degraded efficiency near the notorious transfer dead point where the net magnetic flux through the receiving coil becomes zero. Our experimental results, conducted at 6.78 MHz, demonstrate the efficacy of our approach. For a coil system with a vertical distance of 15 mm between Tx and Rx coils measuring
$100\times100$
mm2, we achieved an efficiency above 70% even at misalignments of up to 116% in the lateral direction and 103% in the diagonal direction, relative to the maximum coil dimension. Remarkably, our system maintains efficiency above 70.5% and 73.6% in the lateral and diagonal directions, respectively, even at 100% misalignment relative to the maximum coil dimension. Furthermore, our proposed system is asymmetrical, requiring assistant coils only on the transmitting (Tx) side. This greatly enhances practicality, particularly in mobile applications where coil volume expansion especially on the Rx side may not be feasible. Nevertheless, our system successfully overcomes the disadvantage of an asymmetric system and offers unparalleled tolerance to 2-D misalignment, regardless of the direction. Through this research, we provide a novel and more practical solution to enhance the performance of WPT systems in real-world conditions.