Recently, various energy harvesters have been investigated for efficiently harvesting energy in rotational motion. However, some challenging issues are still existing in current harvesters, such as narrow operating bandwidth, low-efficiency energy harvesting performance, and so on. To solve these issues, this paper proposes a self-tuning nonlinear piezoelectric energy harvester (PEH) in rotational motion. The corresponding distributed-parameter theoretical model is derived to describe the dynamic response characteristics and predict the energy harvesting performance. Based on this, the principle of a passively self-tuning harvester, which is realized via the centrifugal force acting on the tip mass of the harvester, is theoretically analyzed. Furthermore, experiments are conducted to validate the performance of different nonlinear PEHs under various constant rotational speeds. The experimental results demonstrate that the tri-stable PEH can obtain better energy harvesting performance than the bi-stable one in the low rotational speed range (60–300 rpm). In the high rotational speed range (more than 440 rpm), the bi-stable and tri-stable PEHs achieve the passive self-tuning effect in the ranges of 440–720 rpm and 990–1300 rpm, respectively. These different self-tuning rotational speed ranges are caused by the different rotational radiuses in rotational motion. Overall, this paper provides a theoretical framework to achieve a self-tuning energy harvester in rotational motion.

With the rapid development of Internet of Things (IoT) and the popularity of wireless sensors, using internal permanent or rechargeable batteries as a power source will face a higher maintenance workload. Therefore, self-powered wireless sensors through environmental energy harvesting are becoming an important development trend. Among the many studies of energy harvesting, the research on rotational energy harvesting still has many shortcomings, such as rarely working effectively under low-frequency rotational motion or working in a narrow frequency band. In this article, a rotational magnetic couple piezoelectric energy harvester is proposed. Under the low-frequency excitation (16 Hz), it can solve the condition that the piezoelectric beam cannot respond in time by frequency down-conversion. Therefore, the energy harvester still has a certain degree of energy harvesting ability (18–22 Hz and 29–31 Hz) under high-frequency conditions. Meanwhile, corresponding theoretical analyses and experimental verifications were carried out to investigate the dynamic characteristics of the harvester with different excitation and installation directions. The experimental results illustrate that the proposed energy harvester has a wider working bandwidth benefiting from the frequency up-conversion mechanism and frequency down-conversion mechanism. In addition, the forward beam will have a wider bandwidth than the inverse beam due to the softening effect. In addition, the maximum powers of the forward and inverse beams at 310 rpm (15.5 Hz) are 93.8 μW and 58.5 μW, respectively. The maximum powers of the two beams at 420 rpm (21 Hz) reached 177 μW and 85.2 μW, respectively. The self-powered requirement of micromechanical systems can be achieved. Furthermore, this study provides the theoretical and experimental basis for rotational energy harvesting.

This paper presents a centrifugal softening impact energy harvester with the bistability using flextensional transducers. The bistability is firstly demonstrated to further enhance the advantages of the centrifugal softening effect in improving the impact energy output at low rotational speeds. In the harvester, two flextensional transducers are impacted by a centrifugal softening driving beam, which is experiencing the magnetic repulsive force at the same time. The flextensional transducers are adopted for their high electromechanical coupling coefficient and robustness under the large impact force. A theoretical model is built and validated by experiments. Experimental results show that the bistable harvester can generate higher energy output than the non-linear monostable and linear harvesters at the rotational speed ranging from 60 rpm to 360 rpm and a certain clearance of 1.07 mm. Its maximum instantaneous power and RMS voltage at 60 rpm are respectively increased by 323.1% and 184.3% compared with the non-linear monostable one, and 899.9% and 304.2% compared with the linear one. Such significant improvement cannot be achieved by changing the clearance in the linear harvester while it can be achieved by adding the bistability. Therefore, our proposed method facilitates the effective energy harvesting from widely-distributed low-speed rotations.