Self-tuning Harvester Research Articles

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.

Analytical and experimental investigation of the centrifugal softening and stiffening effects in rotational energy harvesting

Depending on its mounting orientation, the rotational piezoelectric harvester can be subjected to the centrifugal softening or stiffening effects, which may pose a positive or negative influence on the energy harvesting performance. This paper firstly presents the comprehensive comparison of the centrifugal softening and stiffening harvesters with a unified dimensionless analytical model. The analytical expressions of the optimum short-circuit and open-circuit rotational frequencies, the optimum resistances and the optimum centrifugal coefficients are derived to reveal the influence mechanism of the centrifugal effects. The proposed model is validated experimentally and used for case studies to compare the centrifugal softening and stiffening harvesters. Results illustrate that the centrifugal softening harvester has a superior energy harvesting performance at low rotational frequencies while the centrifugal stiffening harvester is better at high rotational frequencies. Furthermore, the peak power of the centrifugal softening harvester is higher while the frequency bandwidth of the centrifugal stiffening harvester is wider. In addition, it is found that the centrifugal stiffening harvester is more sensitive to the centrifugal coefficient. Based on the centrifugal stiffening effect, the self-tuning harvester can be achieved with a wide frequency bandwidth. Meanwhile, this requires that the centrifugal coefficient is precisely determined, and the fundamental natural frequency of the harvester without the centrifugal force should be low enough when operating at low rotational frequencies. Compared with the centrifugal stiffening effect, the centrifugal softening effect may be more suitable to be used in MEMS/NEMS applications since it can reduce the fundamental resonant frequency of the harvester during the rotation.