Research On The Influence Of Metal Foreign Object On Wireless Power Transfer Performance And Its Detection

With the application of wireless power transfer technology in more and more fields,various power supply problems in the era of wired power transfer have been greatly improved.Magnetically coupled resonant wireless power transfer(MCR-WPT)technology,as one of the most promising technologies in wireless power transfer technology,often works in high frequency conditions,meaning that high frequency electromagnetic field exists in space.In high frequency electromagnetic field,eddy effect and magnetic effect of non-ferromagnetic metals and ferromagnetic materials will change the equivalent parameters of the system,affect the system performance and cause safety hazards.At present,the work related to wireless power transfer is mainly focused on the case without metal foreign bodies,and the detection of metal foreign bodies is mainly focused on the case of large-size foreign bodies,so it is very important to analyze the influence of metal foreign bodies on wireless power transfer system and study the detection method of small size foreign bodies.Firstly,the influence of eddy effect and magnetic effect on the parameters of MCR-WPT system is verified by electromagnetic field theory and finite element simulation.The eddy effect reduces the coil's equivalent self-inductance and mutual inductance between coils,and the resonance point shifts to the right.The magnetic effect increases the equivalent self-inductance of the coil,and the resonance point shifts to the left,and its influence on the mutual inductance needs to consider the size and position of the foreign object.At the same time,the relationship between the mutual inductance and the relative position between coils and some performance indexes(output power,transmission efficiency,input current)are given.Secondly,based on the electromagnetic shielding theory,the influence of the metal casing on the MCR-WPT system is analyzed.In the longitudinal non-transmission space,non-ferromagnetic aluminum plates attenuate the magnetic fields in almost all space,and the double-layer structure of aluminum plates and ferrite can increase the magnetic field in the transmission space while ensuring effective shielding of the magnetic field in non-transmission space.Aluminum plate in the side non-transmission space can effectively reduce the edge effect of longitudinal shielding metal and the magnetic field in the side non-transmission space,and the larger the height and width of the side aluminum plate,the better the shielding effect.For the metal casing on the transmission space,the cutting radius can be reduced and the mutual inductance can be increased when using the cut-off cutting scheme.Thirdly,the detection sensitivity of the detection coil to metal foreign objects is analyzed,and a two-layer balanced detection coil set with high sensitivity is proposed.In order to increase the frequency,a double resonance structure is adopted at the transmitting end,so that the transmitting current includes the 9th component for foreign object detection while the receiving current does not.Filtering the fundamental component of the induced voltage of the detection coil group through the conversion circuit ensures that the detection voltage is independent of the receiving current,and solves the problem that the metal foreign object detection fails when the transmitting and receiving coils are not aligned.In addition,this paper uses BP neural network to achieve more accurate receiving coil positioning with a simple detection coil set.Finally,the MCR-WPT experimental platform was built to verify the effect of metal objects on self-induction and transmission performance.And it is shown that the double-layer detection coil set and the higher harmonic current can effectively improve the detection sensitivity of small-size metal objects,and the accurate positioning of the error of 3mm is realized with the help of BP neural network.