On the parametric power maximisation of a velocity-amplified electromagnetic energy harvester

Recently, the “Smart City” has become the leading concept for the development of a new kind of urban space: each city will become a connected, sensitive and responsive environment, which will communicate with the citizen. This trend is reflected by the increasing number of monitoring and data analysis applications related to urban services. However, the development of ubiquitous sensor networks is largely restrained by the lack of a reliable power source with an infinite life-time. Energy harvesting arose a few years ago as the solution to the powering issue by converting the energy present in the ambient into electrical energy. This thesis focuses on vibrational energy harvesting, for which the simplest configurations are based on spring-mass resonator systems: this linear conversion is characterised by high efficiency when excited at its resonance frequency, which drops drastically when out-of-resonance. Non-linear devices have been proven to be more efficient if excited with wide-band signals, which makes them more suitable for any practical application which does not rely on single frequency oscillations. However, the major issue for any vibrational energy harvesters is the low power that each device is able to generate: power optimisation is a major need for any practical application, and it can be very complex for multi-Degree of Freedom non-linear devices where the analytical representation is impossible. This thesis deals with two Degree-of-Freedom (2DoF) vibrational electromagnetic energy harvesting, focusing on the power optimisation of non-linear devices. In this thesis, the parametric analysis of a non-linear velocity-amplified harvester is investigated, taking into account geometrical, mechanical and electrical parameters, in order to obtain a better understanding of the influence of each one on power optimisation. The study is conducted both from the modelling and from the experimental points of view, analysing different set-ups for the device. A detailed model is realised, by integrating finite element and dynamical equation-based modelling, in order to replicate the complex shape of the magnetic field within the device in the numerical integration of the equation-based model. Analytical expressions are found to describe the effect of magnetic springs in the device, and to model the change in total height of the harvester (a key parameter which influences non-linearity). In addition, the optimal set-up under real vibrations is discussed, enlightening the different contributions to the power maximisation in this case: particular focus is given to peak power amplitude and band-widening, which are demonstrated to be the parameters which play the most important role with broad-band excitation. The optimisation of the device is demonstrated, showing a best-in-class volumetric Figure of Merit at low frequencies for the prototype: a discussion on Figure of Merits (FoM) as representative parameters for non-linear tunable devices is also presented. Finally, a second improved device is shown, which resulted from addressing key issues of the previous harvester, and which confirms the findings of the earlier analysis regarding the key parameters required to optimise the dynamics of such devices for real-world, non-harmonic applications. A wideband response and a new state-of-the-art FoM are presented, demonstrating the efficacy of the improvements brought to the first prototype.