Modelling and characterisation of a novel nonlinear velocity-amplified electromagnetic vibrational energy harvester

The use of sensors that can communicate wirelessly with each other (Internet of Things) is becoming widespread in everyday life. One of the main issues with this kind of technology, however, is the limited lifetime of batteries that are typically used to power sensor nodes. A practical solution to power these sensors comes from vibrational energy harvesting, in which the kinetic energy of ambient vibrations is converted into electrical energy.

To overcome the problems of narrow bandwidth and high resonant frequency at small scale for conventional linear vibrational energy harvesters (VEHs), a nonlinear two degree-of-freedom (2-Dof) velocity-amplified VEH is presented in this thesis. Electromagnetic induction was used as the conversion mechanism. The harvester consisted of two masses relatively oscillating, one inside the other, between four sets of magnetic springs. A movable cap retained one of the mag?nets of the magnetic springs, in order to vary their effective elastic constant. The outer mass was made up of seven coils while the inner mass consisted of five magnets in a Halbach configuration. Collisions between the masses could occur, and they transferred momentum from the heavier to the lighter mass, increasing the velocity of the latter and so enhancing the output power. Magnetic springs introduced a hardening nonlinearity in the system, which was exploited to broaden the bandwidth of the device and to allow the masses to resonate in the sub 15 Hz range.

Experimental and theoretical analyses were carried out for improved insight into the nonlinear dynamics of the device. Harmonic excitation and frequency sweeps were used to study the effects of cap height and external acceleration on the electrical response of the harvester. Hysteresis and a shift in the resonant peak, both caused by the hardening potential of magnetic springs, were observed in the experimental and theoretical data for increasing acceleration or for decreasing cap height. Bispectral analysis was carried out and identified period-doubling and couplings between modes, effects related to the use of magnetic springs.

The possibility to predict couplings and the knowledge achieved through the analysis and characterisation of the nonlinear 2-Dof VEH were used to build a wideband 2-Dof VEH. This second prototype showed how the experimental and theoretical analysis of the nonlinear 2-Dof VEH were fundamental to develop a device that could work over a wide range of frequencies. The prototype was shown to exploit nonlinear effects to harvest significant levels of energy in regions far from resonance. Both prototypes exhibited best-in-class values of volumetric figure of merit in the sub 15 Hz frequency range.