Wireless charging technology is safer and more convenient than traditional wired charging, and has been widely used in the field of electric vehicles. However, wireless charging systems have the characteristic of separating the ground end and the vehicle end, and there are issues with the selection and measurement of power metering points. The mainstream approach is to set the measurement position of the wireless charging system on the transmitting coil. However, the current and voltage of the transmitting coil are relatively high, so that directly using sensors for measurement can lead to high sensor costs. To address the aforementioned issues, this paper proposed a non-contact measurement method for output power of transmitting coil of wireless charging systems using multi coil collaboration. This method did not require specialized high-power high-frequency current and voltage sensors, and adapted well to practical scenarios such as different power levels, horizontal and vertical ground displacement of cars, and shielding materials, and had high measurement accuracy.

The paper utilized the law of electromagnetic induction to measure the output power of transmitting coil by setting sensing coils. Firstly, it was inferred that there was a relationship between the output power of transmitting coil and the voltage product of the sensing coil. Secondly, the fitting coefficient was used to fit this relationship, and it was derived that when the positions of detection coils and transmission coil were fixed, the fitting coefficient did not shift with the receiving coil. After obtaining the fitting coefficient, only the terminal voltage of the sensing coil needed to be measured. Finally, by constructing the voltage matrix of sensing coils and the standard output power of transmitting coil matrix, the fitting coefficients were obtained, and a coupling matrix model was established between the voltage phasor of each sensing coil and the output power of transmitting coil considering horizontal offset, vertical offset, and power variation.

The experimental results show that under different power levels, the maximum measurement error of the proposed method is within 1.5% when the wireless charging system undergoes offset in both horizontal and vertical ground directions. Under the condition of 2 square sensing coils, the maximum error in power measurement was 47%. When the number of sensing coils increased to 6, the maximum error decreased to within 1.5%. This result indicates that as the number of sensing coils increases, the accuracy will further improve. Meanwhile, research has found that when the transmitting and receiving coil are square, using a square sensing coil results in more ideal accuracy. In addition, with the use of 6 detection coils, 3, 5, and 9 power sampling points were used within the measured power range. The maximum errors in power measurement were 7.8%, 3.6%, and 1.5%, respectively, indicating that the more sampling points are, the more accurate the model is. Finally, further exploration is conducted on the practical scenario of placing multiple sensing coils horizontally, which can avoid the problem of vertical stacking height affecting the short distance power measurement.

This method has been proven to be effective through simulation and experimental analysis. Through comparative analysis of the results, the following conclusions can be drawn: (1) The size and quantity of detection coils are key to ensuring the accuracy of the model. A sufficient number of sensing coils will bring higher model accuracy, but at the same time, it will also increase the number of samples in the model solving process. Therefore, in practice, a reasonable selection can be made based on the measured power error requirements. (2) For coupling devices where both the transmitting and receiving coils are square coils, the measurement accuracy using square sensing coils is more ideal compared to circular sensing coils. (3) A sufficient number of sampling points in model solving can also affect the accuracy of the entire system model. In practice, a reasonable selection can be made based on the measured power error requirements. (4) Horizontal placement of multiple sensing coils can achieve high-precision power measurement, just like vertical stacking. In practical scenarios, placing multiple sensing coils horizontally can avoid the problem of vertical stacking height affecting the short distance measurement of the transmitting and receiving coils.