Wireless Power Transfer for IoT Bolts in Railways: Overcoming Coupling Challenges due to Conductive and Ferromagnetic Rails

Internet-of-Things (IoT) bolts, which incorporate sensors and are embedded within rail fasteners, are used for monitoring environmental and train status in railway. Wireless power transfer (WPT) is a promising solution for powering these sensors due to its maintenance-free and highly reliable characteristics. However, the large number of bolts makes it costly to adopt the traditional approach where every receiver (Rx) coil is paired with its own transmitter (Tx) coil. To charge multiple Rx coils with a single transmitter, a dynamic-Tx-to-static-Rx (DTSR) WPT approach is proposed. Two key challenges remain. First, conductive and ferromagnetic rails might interfere with the coupling between Tx and Rx coils—a phenomenon that has not been explored previously. Second, due to high-speed trains, charging time is limited to milliseconds. This article is the first to quantify rail interference using a third-order circuit model. On this basis, the challenges and design requirements for DTSR-WPT systems are summarized. Multilayer asymmetrical coils and optimized frequency selection are employed to mitigate rail effects and maximize the energy transferred within a given time. A joule-level WPT prototype with capacitive loads is built. Experimental results indicate that capacitive voltage reaches 80% and 95% of steady-state levels within 6.4 and 9.6 ms, respectively.