Theoretical and Experimental Study of Arc-shaped Beams for Piezoelectric Energy Harvesting


Student thesis: Doctoral Thesis

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Award date4 Jan 2022


Enabling technologies for harvesting ambient vibration energy have attracted considerable attention in research communities from various disciplines over the past few decades. Self-powered electronics possess a distinctive superiority over their battery-operated counterparts from the perspective of environmental friendliness and service life span. Most existing piezoelectric energy harvesters (PEHs) are constructed with straight cantilever beams. These harvesters have been widely investigated, from designs to modeling, simulation, and experiments. This study aims to extend piezoelectric energy harvesters to 3D curved-beam structures and to develop a theoretical framework for this endeavor. To demonstrate the application of the arc-shaped energy harvesters, this study also aims to propose self-powered wrist-worn wearables. Specifically,

First, the dissertation starts with an introduction of the distributed-parameter electromechanical coupling models and experimental methods for piezoelectric energy harvesters with non-uniform thickness. Compared to composite beam configuration with a conventional uniform thickness, the thickness-variable harvesters are beneficial to improving its performance. The model is used to unveil the beneficial effect of evenly distributed strain on the performance enhancement. The experiment verifies an improvement in energy conversion efficiency for the thickness-variable device.

Second, this research proposes curved piezoelectric energy harvesters to alleviate the profile constraint. Whereas previous studies focused on structures with circular configurations, this dissertation extends the devices to the arc-shaped structures with a continuously variable curvature. A distributed-parameter electromechanical coupling model for curvature-variable device is built, based on Euler-Bernoulli beam theory. The governing equations are solved via the Rayleigh-Ritz method. The convergence and accuracy of the model are validated by finite element (FE) simulation and experiments. Based on the developed model, the effects of parameters including the proof mass, Young’s modulus of the substrate, the thickness ratio of the substrate to the total thickness, the curvature of the substrate, and the piezoelectric patch on the mechanical and electrical responses of the structure are discussed in detail.

Third, for the discontinuously variable curvature beam case, a segmented arch-shaped harvester is constructed. The curvature discontinuity of the harvester renders the difficulty of developing the distributed-parameter electromechanical model and solving it using Rayleigh-Ritz method. The distributed-parameter electromechanical coupling model for such a configuration is built using Timoshenko beam theory and solved using the mode expansion method. The mechanical and electrical responses are obtained from the model, and the analytical results are verified by experimental trails and the FE method. A parametrical study using the proposed model is also presented to examine the effects of the geometric and material parameters on the system’s mechanical and electrical responses.

Finally, to illustrate the application of the non-conventional arc-shaped energy harvesters, an experimental study for wrist-worn wearables is conducted. A woodpecker-mimic two-layer band piezoelectric energy harvester for a fully autonomous wearable is proposed. The energy harvester has a unique two-band layer mimicking the structure of the woodpecker’s head. The outer band is constructed in an arc-shaped configuration. The harvester operates in both movement mode and impact mode. The FE model proposed for curvature-variable composite beams is used to optimize the design. With an AC-DC rectifier, the electrical and mechanical responses of the proposed harvester are characterized under the in-plane excitations and hand-tapping excitations. The demonstrations show that the energy harvester can power a commercial watch consecutively when walking or tapping the prototype.

In summary, this research develops the distributed-parameter electromechanical models for arc-shaped PEHs with variable curvature and demonstrates the application of the arc-shaped PEH in wearable devices. The framework helps to extend the design scope of energy harvesters from 2D straight beams with flat piezoelectric elements to a broader domain of 3D curved structures. Furthermore, curved PEHs provide more alternatives for addressing the space limitation of diverse applications. We believe that this analytical and experimental study provides insights for understanding and predicting the mechanical and electrical response of the PEHs with curved configurations and offers guidance for designing and optimizing new PEHs.