A C-battery scale energy harvester, part b: Transducer optimization and modeling

A two Degree-of-Freedom (2DoF) nonlinear electromagnetic energy harvester, which employs velocity amplification, with a volume of 26.7cm 3 and 25.6 cm3 (25.5mm diameter and 52.4mm height) is investigated in this work. These dimensions are very close to those of a C-battery (26.2mm diameter and 50mm length, for a volume of 27.8cm3) , making the harvester suitable to be integrated in electronic devices. The harvester consists of a Halbach array of magnets oscillating inside a set of seven coils. The use of magnetic springs and the impacts between the two masses, leads to nonlinear harvester behaviour, broadening the harvester's spectral response. Moreover, the impacts exploit velocity amplification on the secondary (smaller) mass, improving the electromagnetic conversion. The aim of this work is to optimize the performance of the electromagnetic transducer through analytical and numerical methods and to experimentally verify the optimization methods. This paper discusses the magnetic configuration that maximizes the variation of flux density and an analytical model is presented that predicts the optimal number of turns and wire diameter for the coils. A finite element simulation takes the output from the initial optimization calculations and predicts the output voltage of the harvester. Experimental results are then presented where various coil designs are tested and comparisons are made to the numerical results to validate the models. The experimental results also show a high volumetric Figure of Merit (FoMV ), which highlights the benefits of the optimisation methods used. Finally, in order to give the reader an understanding of the system performance under realworld vibrations, the system was tested under excitation generated by human motion.