Combining magnet-induced nonlinearity and centrifugal softening effect to realize high-efficiency energy harvesting in ultralow-frequency rotation

Few studies have successfully achieved high-efficiency energy harvesting in ultralow-frequency rotational motion where the rotational frequency is less than 2 Hz (120 rpm). In order to solve this challenge, this paper proposes a novel energy harvester with the centrifugal softening effect (CSE) and the magnet-induced nonlinearity. In a rotational coordinate system, the corresponding theoretical model is derived to reveal the mechanism of the CSE and to analyze the influence of the nonlinear magnetic force on the output power of the proposed harvester. Furthermore, the mechanism of the centrifugal stiffening effect (C_stiffen_E) is also theoretically analyzed. The analysis results indicate that based on the C_stiffen_E, the self-tuning harvester with a wider bandwidth can be obtained, and the CSE is better than the C_stiffen_E in enhancing the ultralow-frequency rotational energy harvesting. Meanwhile, the derived model is experimentally validated via comparisons among different piezoelectric energy harvesters (PEHs) including the linear inverse PEH, the nonlinear inverse PEH and the nonlinear forward PEH. The experimental results show that the nonlinear inverse PEH combining the magnet-induced nonlinearity and the CSE is capable of realizing high-efficiency energy harvesting in the rotational speed range of 60-120 rpm. The corresponding RMS output voltage is larger than 4 V, which is promising to provide the continuous power source for a wireless sensor. In addition, the parametric studies are conducted to provide theoretical guidance for the further optimization design of the proposed PEH.