Ph.D Thesis


Ph.D StudentNadav Cohen
SubjectTheoretical and Experimental Study of Double-Well
Energy Harvesters
DepartmentDepartment of Mechanical Engineering
Supervisor Full Professors Bucher Izhak
Full Thesis textFull thesis text - English Version


Abstract

The study of vibration energy-harvesting has begun in the 1990’s by investigating the feasibility of using resonant-oscillator to convert mechanical to electrical energy.

 The present research proposes methods to overcome the inherent limitation of nature energy sources that exhibit unsteady low frequencies motions.

A system designed to resonate at low frequencies and high acceleration inputs, requires large base displacements imposing an unrealistic constraint on the size of the enclosure, leading to devices working in a sub-resonance regime, resulting in low power outputs.

The thesis studies the potential benefit of incorporating a nonlinear bistable oscillator in an energy harvester (NBEH). The advantage in adding a nonlinear potential barrier (PB) into a conventional resonating harvester is studied and explained by means of analytical and numerical models and validated via extensive laboratory experiments.

An NBEH excited by low frequencies exhibits vibrations characterized by a combination of slow and non-stationary fast oscillations. A non-parametric response decomposition approach is employed separating the slow and fast parts. This decomposition provides insight into the effect of the nonlinear PB by identifying the evolution of instantaneous amplitude and frequency. The components are utilized to identify the backbone-curves of the two asymmetric potential-wells and the nonlinear stiffness of the system. The proposed decomposition and analysis allow to examine the transient dynamics near the PB without resorting to averaging or perturbation based methods. This approach and the subsequent analysis are demonstrated via numerical and experiment results.

Analytical models representing such a system are formalized. Through the use of the models, the orbital-stability of the NBEH is studied, explaining most of the period-doubling and symmetry-breaking bifurcations arising when such an oscillator is excited. A simplified model is developed to obtain analytical prediction of the harvested power in this case. The proposed model is verified through analytical and numerical analysis and some experimental results.

The NBEH is also investigated when excited by random, band-limited vibrations. It is shown that nonlinear NBEHs perform considerably better than their linear counterpart under band-limited excitation in certain regions. A sharp increase in performance is observed for band-limited random excitation along a well-defined region in the input-level and bandwidth plane. The results of simulated, partially analytical and experimental studies are compared and analyzed to perform mutual validation.

In addition to the stated above, the concept of utilizing NBEHs having more than one degree of freedom is studied. Two such prototypes are designed and studied via experimental investigation.