Hybridizing linear and nonlinear couplings for constructing two-degree-of-freedom electromagnetic energy harvesters

The efficient exploitation of ubiquitous low-frequency mechanical excitations through electromagnetic induction is important for implementing self-sustained low-power electronics. Conventional electromagnetic energy harvesters (EMEHs) have been usually designed as a 1-degree-of-freedom (1-DOF) linear system with a high resonant frequency, resulting in poor performance under low-frequency excitations. To solve this key issue, this paper presents four 2-DOF cylindrical EMEHs with various configurations. Theoretical models for the four EMEHs are built and then validated by experiment. With the theoretical models, a parametric study is carried out to reveal the energy harvesting performance of the four 2-DOF EMEHs. The results indicate that supporting a 1-DOF EMEH within a large tube using springs or magnetic coupling to construct a 2-DOF EMEH can lower the operating frequency, endowing the 2-DOF EMEH with improved performance under low-frequency excitations. Moreover, the 2-DOF EMEH can always provide higher power outputs than the corresponding 1-DOF EMEH except the linear 2-DOF EMEH configuration with overly stiff inner springs. Furthermore, for the nonlinear 2-DOF EMEH, the output power can be optimized by adjusting the spring stiffness and the length of the inner tube. In addition, increasing the mass of the inner center magnet can enhance the power output and in the meanwhile make the operational frequency shift toward the left (lower frequency).