A spring-assisted adaptive bistable energy harvester for high output in low-excitation

This work proposes a spring-assisted adaptive bistable energy harvester (briefly as SABEH) with two degrees of freedom by adding a spring to the external magnet from conventional bistable energy harvester (BEH) perpendicular to the vibration direction. The spring enables the energy barrier to increase when the cantilever oscillates to stable equilibrium points and decrease when the cantilever goes down to the middle position (unstable point), which realizes adaptive potential during the vibration. We find that SABEH with a suitable spring stiffness can realize interwell oscillations frequently in low-excitation where BEH cannot. As a result, subjected to both harmonic and random excitations, higher outputs from SABEH with suitable spring stiffness, compared to those from BEH, are demonstrated by both experimental and analytical results, especially in low-excitation. The governing electromechanical equations and potential function have been derived and analytical studies have been developed for further analysis. We observe that the magnet interval at stable equilibrium points and spring stiffness codetermine the output of SABEH. Meanwhile, it is proved that the spring stiffness can not only contribute to broadening the harvesting bandwidth, but also influence the system characteristics.