Analysis of Simultaneous WPT in Ultra-Low-Power Systems with Multiple Resonating Planar Coils

This paper analyses the conceptual application of a wireless power transfer (WPT) system with multiple resonators supplying outdoor sensors using a mobile charger. The solution is based on the idea of using sensors, located in open space, to monitor environmental parameters. Instead of the typical two-coil WPT with a single charger, energy transfer is realized simultaneously, using a group of identical planar coils as transmitters and receivers connected to the independent power supply circuits of each sensor and microcontroller. By isolating these charged circuits, a higher reliability and powering flexibility of the weather station can be achieved. The concept of the proposed system was discussed, and it was proposed to include the main devices in it. A theoretical analysis was performed considering all mutual couplings and the skin effect; hence, the system is characterized by a matrix equation and sufficient formulae are given. The calculations were verified experimentally for different frequencies, two possible distances between the transmitters and receivers, and equivalent loads. Both the efficiency and load power are compared and discussed, showing that this solution can provide power to ultra-low-power devices, yet the efficiency must still be improved. At the small distance between the transmitting and receiving coils (5 mm), the maximum efficiency value was about 40%, with a load resistance of 10 Ω. By doubling the distance between the coils, the efficiency of the WPT system decreased by three times.