Low frequency energy harvesting with impact based frequency up-onversion

Abstract

Harvesting low frequency vibration energy poses significant challenges: difficulty in resonant frequency matching, large displacement amplitude and low electromechanical coupling. The aim of this project was to develop, model and experimentally validate a new vibration energy harvesting structure utilising a novel mechanical frequency up-conversion method to address these challenges. A new method for mechanical frequency up-conversion was proposed leveraging a non-linear effect which has not previously been studied in energy harvesting. In this method an oscillation in a cantilever beam may be induced at a higher frequency than both the cantilever resonant frequency and excitation frequency through impact with a high restitution end-stop placed near the neutral position of the beam, when exposed vibration excitations below the resonant frequency of the cantilever. A structure to implement this method was developed, mathematically modelled and experimentally validated. Simulation indicated that, relative to a simple cantilever structure, the RMS velocity of the proof mass may be increased, while the displacement of the mass is simultaneously decreased, resulting in an improved energy density and this was confirmed by experimental results. The frequency up-conversion mechanism was used to implement an energy harvester using electromagnetic transduction. The energy harvester was modelled mathematically and this model was validated experimentally. Simulation results suggested a potential increase in the power output of the harvester at low electromagnetic damping factors and were confirmed by experimental results. The output voltage was, however, very low and was determined to be insufficient for a stand-alone harvester. Adopting the frequency up-conversion mechanism using piezoelectric transduction was considered and a new soft piezoelectric transducer utilising PVDF and a PDMS central shim was proposed, modelled and experimentally investigated. The transducer demonstrated potential for high voltage outputs, however at the cost of low power output and high resonant frequency. It was therefore determined unsuitable for implementation in the mechanical frequency upconversion mechanism. Finally a new approach using a hybrid electromagnetic-piezoelectric scheme was investigated and a structural implementation was proposed, modelled and experimentally validated. With this structure, the high power output of the electromagnetic transducer was retained, while an auxiliary output at a high voltage level, suitable for powering a low-power conditioning circuit, was produced by the piezoelectric transducer.