Pulse Density Modulation Based Mutual Inductance and Load Resistance Identification Method for Wireless Power Transfer System

In wireless power transfer (WPT) system, the information of mutual inductance and load resistance is usually needed for the front-end to regulate power and efficiency. To acquire the values of these two parameters without wireless communication system and without affecting the output power, this article proposes an identification method based on the pulse density modulation (PDM) technique. The underlying principle is to obtain a range of interharmonics by altering the sequence of PDM. First, the PDM strategy brings in interharmonics, which can be identified through fast Fourier transmission (FFT). Then, these interharmonics are used to established multiple sets of equation related to front-end impedance and the two unknowns. Finally, with the least-square approximation, mutual inductance and load resistance can be estimated. This article notices that the sequence of PDM strategy could affect the amplitudes of interharmonics, and selects the sequences to maximize the magnitudes for less sensitivity to measurement errors. Experimental results show that the relative errors of identification are less than 5% when the nominal values of capacitors are the same as actual values, and reach 7.40% when considering the nominal values may deviate from the actual values. Besides, the output power and efficiency are not affected during the identification process. This proposal requires only magnitudes of voltage and current in the transmitter side, no other hardware is needed, and works with the fixed frequency, which is suitable for any frequency range.