Piezoelectric transducers are attractive for harvesting vibrational energy from rotational motion to help power wireless sensor networks because they are small and lightweight and have high energy density. But their performance remains largely ineffective because their high resonant frequencies are not compatible with the low operational frequencies of ambient rotational energy sources, such as wind turbines and automobile tires.
Fang et al. have developed a transducer prototype that exploits the static instability caused by the centrifugal softening effect (CSE) to address this problem.
From the literature, the researchers knew that inverting a rotational piezoelectric cantilever beam to create CSE could dramatically decrease the resonant frequency of the transducer to increase the output power at low rotational frequencies. But as the frequency exceeds a critical value, static instability occurs, causing the inverted beam to overbend and eventually snap.
The researchers found that introducing a large impact force would offset the negative equivalent structural stiffness of the deflected rotational beam caused by CSE. They also applied the impact force on two rigid piezoelectric beams. As a result, energy generation takes place as the impact force continuously knocks the system out of the divergence state, thus exploiting the instability for energy production.
In addition, the researchers adjusted the impact gaps between the rotational driving beam and the upper and lower generating beams. The more they increased the impact gap, the higher the maximum output power became while causing the operating rotational frequency bandwidth to narrow. With an impact gap of 2.14 millimeters, the maximum output power of the harvester increases by up to 682.8 percent.
Source: “Exploiting the advantages of the centrifugal softening effect in rotational impact energy harvesting,” by Shitong Fang, Suo Wang, Shengxi Zhou, Zhichun Yang, and Wei-Hsin Liao, Applied Physics Letters (2020). The article can be accessed at https://doi.org/10.1063/1.5140060.
