3-DOF X-structured piezoelectric harvesters for multidirectional low-frequency vibration energy harvesting

Nonlinear energy harvesters with specially-designed nonlinear characteristics (e.g., quasi-zero stiffness, multi-stability, and internal resonance) have been extensively studied to develop broadband harvested energy at low and ultra-low frequencies. Nevertheless, most developed energy harvesting devices were restricted to unidirectional energy harvesting, which is a limitation in many applications. This paper proposes a novel 3-degrees-of-freedom (3-DOF) X-structured piezoelectric harvesting system, enabling simultaneous & enhanced multidirectional low-frequency energy harvesting. It is shown that the developed 3-DOF X-harvester can produce useful & powerful motion coupling effect and unique band-pass-like property of two peaks and large frequency range due to nonlinear dynamics incurred by the specially designed X-mechanism, and can thus offer greater flexibility in tuning structural parameters to achieve broadband and higher low- and ultralow-frequency energy harvesting in multi-directions, compared to many other literature results. In experiments, with 300 kΩ and two piezoelectric beams connected in-series, the prototyped 3-DOF X-harvester achieved peak power outputs of 2.5 mW in the z-DOF at 9 Hz and 2 mm, 41.2μW in the x-DOF at 13.5 Hz and 2 mm, and 86.2μW in the ψ-DOF at 13.5 Hz and 1.7 mm, much better than many other literature results. In real-world applications with multidirectional vibrations, the harvested power will definitely have a wider bandwidth and higher peak outputs due to the motion coupling effect, allowing for enhanced ultralow-frequency energy harvesting.