Analysis, design and testing of a rolling magnet harvester with diametrical magnetization for train vibration

•A new rolling magnet harvester is proposed as a building block of transportation IoT.•The device features a mono- and tri-stable potential to broaden the response band.•Rolling magnet slip and its rolling and sliding effects are theoretically described.•The system can sever as a self-powered node for passenger ride comfort monitoring. Harvesting energy from ambient vibrations, particularly vibrations of a train, is very useful to build a transportation Internet of Things. This paper presents an electromagnetic vibration energy harvester (VEH) based on a cylindrical rolling magnet (RM) and one or three fixed magnets (FMs). This VEH can reach mono-stable and tri-stable potential energy states by changing the number of FMs, and enhance the nonlinearity of the system by reducing the vertical gap between the RM and FMs. Firstly, a theoretical model of the proposed VEH is established. During model construction, the commonly used dipole moment method (DMM) is found unsuitable for calculating the magnetic restoring force acting on the RM since the DMM cannot consider influences of magnet dimensions. Thus, the finite element method is applied. A position-varying factor is exploited to accomplish the electromagnetic coupling of the system. The occurrence of RM slip and the differences between its rolling and sliding effects are investigated. Then, frequency-sweeping tests are performed to understand the dynamical characteristics and energy harvesting capacity of the VEH. The experimental and simulated results under small-magnitude excitations exhibit similar trends, demonstrating the suitability of the theoretical model. The VEH with one FM achieves its maximum instantaneous power of 40.6 mW at 8.0 Hz (9.8 mW at 7.5 Hz for the VEH with three FMs). Finally, the application of this VEH for scavenging energy from the lateral vibration of a metro car body is explored in the laboratory, which suggests its high potential for powering a self-contained onboard monitoring node on a metro train.