Dynamic responses of the 2DOF electromagnetic vibration energy harvester through different electrical coil connections

This work investigated the dynamic responses and evaluated the performance of a two-degree-of-freedom (2DOF) coupled coil-magnet electromagnetic vibration harvester excited at low frequencies. Three different connection modes with individual, in-series, and in-parallel configurations of the transduction coils were considered. Such approach was taken to identify the most suitable mode for usage because different sensor nodes have different power, current and impedance ratings and the optimum load associated with each connection mode differs. The above reasons also lead to the need for impedance matching of the sensor and the harvesters optimum load, as otherwise, the power deliverable to the sensor would be significantly lower than those achievable under a matched impedance condition. The mathematical model was established and verified experimentally, and a 97.8 % agreement is shown across the connection types at resonance of 9.69 Hz and 0.30 g excitation level. At optimum loads, the individual and series connections output the highest power at 497.50 mW and 410.40 mW, respectively. The optimum power peaked at densities of 223.13 Wm³, 1216.36 Wm³, 1009.03 Wm³, and 658.38 Wm³ for coils 1 and 2 (individual), series and parallel connections, respectively. Overall analysis concludes that the optimum harvestable power using in-series and individual coil 2 connection is higher by 47.41 % and 36.25 %, respectively, over the in-parallel connection and 81.77 % and 77.90 % over individual 1 connection. A comparison of the 2DOF harvester with an equivalent two SDOF harvesters at same resonance, excitation level and mechanical damping coefficient shows that that 2DOF series and individual coil 2 connections perform consistently well in preference to the equivalent two SDOF systems in terms of the power harvested, bandwidth, and normalized power density while associated optimum load resistance dichotomized the practical operational impedance into the high and low on the series and individual coil 2, respectively in the operational bandwidth corresponding to each design.