Bifurcation Identification from Magnetic Flux Distribution by using TMR Sensor-Based Wireless Power Transfer System

Wireless power transfer (WPT) system consists of at least two weakly mutual coupled resonant circuits which are tuned to be operated at the same resonant frequency. Since the parameters (i.e. coupling coefficient, load resistance, etc.) could be varied, the multiple zero-phase angle (ZPA) frequencies would be occurred. This phenomenon is known as bifurcation. Some literatures avoid the bifurcation due to its instability issues. Additionally, the bifurcation cannot be easily handled by using the simple variable frequency controller. However, it offers main advantage of coupling-independent operation, allowing the constant voltage or current control regardless of variations in coupling coefficients. Typically, the critical coupling coefficient (k_cri) is primarily employed for bifurcation criterion during operation. Unfortunately, the exact value of k_cri is rather difficult to be obtained in the real system. In this paper, the bifurcation identification from magnetic flux distribution is therefore proposed. Thanks to the unique patterns of magnetic flux distribution at the resonant and bifurcation frequencies, the TMR sensors array can be properly installed to detect the bifurcation through the measured magnetic flux density. The simulated studies and results of finite element method (FEM), using COMSOL Multiphysics, are included for verifications. In addition, the proposed method is implemented and validated basing on the 400W Series-Series WPT hardware prototype. The experimental results show that the sensing voltages from TMR sensors can be effectively used for the detection of bifurcation and lateral misalignment position.