Ambient Vibration Energy Harvesting

The recent rapid development of smart electronic devices, including wearable electronic devices, Internet of Things (IoT) devices, implantable medical devices and remote smart monitors, has raised great research interests on optimising battery power consumption in small-scaled and remotely operational electronic components. For the past ten years, there has been many research works in vibration-based energy harvesting, i.e., converting ambient vibration sources into electrical energy via different transduction methods. In dealing with environmental vibration sources, the most pressing issue for vibration-based energy harvesters (VEHs) from mechanical and structural perspectives is whether the device can efficiently output an optimised power level under realistic (i.e., non-stationary, time-dependent, distributed in frequency spectrum, etc.) excitation sources. Additionally, the performance of VEHs is also restricted in size, complexity of design, and power density issues. Under such design criteria, introducing nonlinearities into VEHs attracted wide attention. This research work aims to investigate the bandwidth performance of vibration-based energy harvesting by subjecting nonlinear phenomena/techniques with internal and externally induced dynamic behaviours into the systems, hence broadening the operational bandwidth and optimising the power level under a wide range of frequencies. The outcomes of this research work yield five peerreviewed journal papers and several international conference papers. The five journal papers are presented in five chapters (Chapter 3 to Chapter 7) as the main contributions of this thesis. Paper 1 presents a magnetic VEH using combined primary and parametric resonances. In order to merge the resonant regions of fundamental primary resonance and parametric resonance as one continuously operational bandwidth, the motion limiter is utilised to induce external hardening effects that aim to eliminate the off-resonance regime between the two resonances. Theoretical investigations, including the finite element method (FEM) using ANSYS and an averaging perturbation technique, are performed to explain the experimental results. System parameter studies are also conducted to highlight the limiter gap distance, type and orientation angle effects. The paper has been published in Energy Conversion and Management (Q1; Impact Factor =9.7; journal rank: 2 out of 135 in Mechanics). Paper 2 aims to investigate the effects of the motion limiter on directly and parametrically excited energy harvesters based on our first paper. An array-type energy harvester is presented with piezoelectric transducers and three configurations of motion limiters. The bandwidth performance of the device under direct and parametric excitation directions is tested, and the results imply an enhanced power level and a continuous operational bandwidth. The results also demonstrate that the optimised performance is achieved by a duo-type motion limiter configuration. The effects of piezoelectric layers and nonlinear cubic stiffness terms in the governing equations are also studied and validated with experimental results. This paper has been accepted for publication in International Journal of Non-Linear Mechanics. Paper 3, further investigates the feasibility of parametrically excited devices and focuses on reducing the level of the potential barrier and initial threshold amplitude in bistable and parametrically excited devices, respectively. From our first two papers, the results imply that triggering the parametric resonance needs a certain excitation level. By considering the joint restraint between bistable and parametrically excited energy harvesters, we propose a broadband magnetically coupled bistable VEH via parametric excitation, to reduce both the initiation threshold amplitude and the potential barrier for large-amplitude oscillations. The mathematical expression of magnetic coupling effects is presented to predict the dynamics of the system and verify the experimental results. The paper has been published in Energy Conversion and Management (Q1; Impact Factor =9.7; journal rank: 2 out of 135 in Mechanics). Paper 4 is inspired by our previous studies on parametric resonance. The principal parametric resonance locates at twice the fundamental primary resonance (in general, the first transverse mode). A system can also possess a two-to-one internal resonance if the natural frequencies are commensurable or nearly commensurable (i.e., 1:2 frequency ratio between the first two natural frequencies). This paper presents theoretical modelling and experimental validation of a U-shaped piezoelectric energy harvester. The double-jump phenomena extend the resonance regime in a certain range, and the energy exchanges between the two transverse modes enable the motions of all segments (i.e., three cantilever beams) for high-efficient energy harvesting. Paper 5 provides an insight into coupling bending and torsional modes on an Lshaped energy harvester. An internal resonance phenomenon between bending and torsional modes is achieved. In paper 4, we study a two-to-one internal resonance between the first two transverse modes. To optimise the strain rate-voltage transduction efficiency, we consider the out-of-plane motions in the internal resonance phenomena in this paper. In the presence of modal interactions, the power level can be greatly improved as the strain changes in both bending and torsional directions can be harvested under the same excitation frequency.Thesis (Ph.D.) -- University of Adelaide, School of Mechanical Engineering, 202