Piezoelectric energy harvesting from low frequency structural vibration signals: modeling and experimental analysis

Abstract

This research proposal aims to study vibration based piezoelectric energy harvesting systems with multiple degrees of freedom and focusing on low frequency applications. The project falls within the in the general area of smart structures and uses piezoelectric materials in the mechanical-to-electrical energy conversion process. Such a material presents suitable electromechanical properties in the development of technological solutions in either actuation for structural control or sensing in structural health monitoring processes. Particularly, the major goal here is to investigate different design configurations of electromechanical devices capable of converting ambient structural vibration signals, that usually are wasted into usable electric energy specially in situations where the spectral contents of the disturbing signal presents frequency components falling within $0-100$ Hz, that usually covers most of the structural vibration phenomena encountered in practical applications. The well-known cantilever beam model partially of fully covered by piezoelectric layers is one of the simplest and most exploited models for piezoelectric energy harvesting. Despite its usefulness, the cantilever model presents some limitations, the most important being related to the frequency range of operation of the device when volume restrictions are concerned. Smaller cantilever energy harvesters tend to present higher levels of stiffness and lower values for the equivalent mass, what inevitably leads to high values of the natural frequency of the fundamental mode shape as well as for the higher modes, generally well above the aforementioned target frequency range. Since the performance of vibration based energy harvesting systems depends on the tuning of the resonant frequencies of the device to the main frequency components of the input signal, it is evident that if the resonant frequencies fall outside the frequency range of interest, a much lower efficiency of the electromechanical energy conversion process is certainly expected. A possible way of overcoming this difficulty when reduced size harvesting devices are required is to combine multiple cantilever beams such that the combined system presents lower equivalent stiffness coefficients leading to lower values for the natural frequencies of the first mode shapes. Hence, the main goal of this research proposal is to perform an analytical modeling and later experimental verification of a piezoelectric energy harvester composed of multiple cantilever beams forming a fan-folded structure. Euler-Bernoulli theory with suitable boundary and equilibrium conditions is employed in developing the multi degree of freedom electromechanical analytical model of the energy harvester. The analytical model is used in numerical simulations using MATLAB in order to check its feasibility with respect to frequency selectivity and energy generation. Particularly, numerically simulated echo-cardiac signals are used to feed the harvester numerical model in order to verify its implementation in a more realistic dynamic environment. A detailed experimental survey is performed on a prototype of the energy harvesting system in order to verify the analytical model. As the main outcome of this project, it is expected that a broad understanding of the main physical processes that occur in the electromechanical conversion process be obtained and that concept solutions may be devised as for example the use of the model in the development of cardiac pacemakers, such that the device could be partially driven by the patient heart beat, what certainly can contribute for volume reduction as well as battery replacement. (AU)