Development of Low-Frequency Broadband Vibration Energy Harvester based on Soft Magneto-Sensitive Elastomer

Abstract

Vibration energy harvester (VEH) is an electromechanical device that can convert the ambient vibrational energy into useful electricity. This technology is believed to be a promising alternative to conventional batteries as energy source for small electronics. However, the traditional VEHs are usually designed as a linear oscillator with narrow frequency bandwidth and constructed by inflexible structure. These properties may significantly limit their applications in many scenarios, where the ambient vibrations are confined in low frequency domain and contain multiple frequency components. In this thesis, we endeavour to develop a light-weight broadband VEH operating in low frequency domain (< 10 Hz) and small acceleration (<0.2 g) conditions. This VEH is constructed by a cantilever based on soft magneto-sensitive elastomer (SMSE) material. This material is fabricated by mixing some iron particles in a soft polymer base before crosslinking. The SMSE is quite flexible, so that a low resonance frequency could be easily achieved without largely increasing the mass. Moreover, the SMSE cantilever exhibit a unique softening nonlinearity, which could be applied to broaden the frequency bandwidth of the VEH. Experimental and theoretical studies are conducted for fabrication, charactering the SMSE material and analysing its unique nonlinearity. A piezoelectric VEH is developed with the SMSE cantilever and a polyvinylidene fluoride (PVDF) layer. This harvester can achieve a broad frequency bandwidth in low frequency domain and low acceleration levels, but its energy harvesting efficiency is extremely low due to the fact that the soft substrate of SMSE is difficult to deform the PVDF layer with relatively high stiffness, apart from the low coupling coefficient of PVDF itself. An electet-based VEH (eVEH) is then developed using the SMSE cantilever. The voltage generation mechanism of the eVEH is studied in view of its charge migration process. The impact-based frequency up-conversion technology is utilized in the eVEH and an asymmetrical frequency response profile of the SMSE cantilever is proposed to mitigate the side effect of the impacts and preserve the broadband behaviour of the eVEH. Efficient broadband energy harvesting is achieved with such an SMSE eVEH device. According to the experimental results, the proposed broadband eVEH based on SMSE cantilever can deliver an average power output of 40.32 μW at its optimal load resistance and the effective working frequency bandwidth is 4.0 Hz to 8.2 Hz.