Numerical and experimental performance study of two-degrees-of-freedom electromagnetic energy harvesters

Energy harvesting is a rising technology able to replace conventional batteries in supplying low-power devices. Researchers are studying the use of energy harvesters in Autonomous Internet of Things (AIoT) systems to create a wireless network of nodes for real-time monitoring of assets. Electromagnetic energy harvesters exploiting ambient vibrations for electric power generation are used in monitoring applications for sensorized industrial vehicles or mechanical systems. This paper shows a design methodology for two-degrees-of-freedom gravitational electromagnetic energy harvesters (2DOF GEMEHs) along with prototype testings. The main purpose of this non-linear two-degrees-of-freedom system is to improve conversion efficiency and bandwidth broadening through the introduction of a second resonance frequency. The proposed harvester devices could be suited for vehicle monitoring and in particular railway monitoring applications. The novelty of the configuration is the use of two magnetic springs and the series connection of two induction coils. The system design achieves long-lasting performances since there are no mechanical parts involved in the dynamics, thus being compatible with low maintenance requirements. 2DOF GEMEHs can have the two resonance frequencies tuned to two fundamental frequencies of the vehicle harvested vibrations for power enhancement. Infreight trains applications the system resonance frequencies may be tuned to the two natural frequencies of the bogie when the railcar is in tare and loaded conditions. The working principle, configuration and analytical model of these devices are described in a detailed way. The numerical modeling approach consists of a combination of FEM analyses in Ansys Maxwell and dynamic simulations in Simulink for evaluation of stiffness and damping characteristics of the system. Experimental laboratory tests on harvester prototypes are compared to numerical results of dynamic simulations for the validation of the proposed model through error estimation. Performance improvements of the 2DOF GEMEH are evaluated through the definition of a merit factor based on output power and bandwidth. The use of a 2DOFsystem is justified by comparing its efficiency respect to the 1DOF configuration, leading to an overall harvesting performance improvement of 10%.