Mechanical Engineering - Doctoral Theses

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    Ultra-low and wide bandwidth vibrational energy harvesting using a statically balanced compliant mechanism
    (University College Cork, 2022-04-21) Liang, Haitong; Hao, Guangbo; Olszewski, Zbigniew; Horizon 2020
    The development of Internet of Things (IoT) in recent times has met with the challenges of powering numerous sensors in a wireless sensor network with traditional batteries, owing to their limited lifetime, environmental pollution, high maintenance cost, etc. Vibrational energy harvesting is an ideal and green powering solution due to the ubiquitous environmental vibrations and their sufficient power density (~0.3 μW/mm3). A systematic review on state-of-the-art structural methodologies of vibrational energy harvesters from the aspect of compliant mechanisms (CMs) is first carried out, focusing on the energy conversion mechanism by piezoelectric effect in particular. The frequency gap between the majority of energy harvesting devices (with narrow bandwidth in the high frequency range) and the accessible vibrational sources (at 1-10 Hz levels) is observed and is still to be filled. In this thesis, a structural solution of vibrational energy harvesters using a statically balanced compliant mechanism (SBCM) is proposed, theoretically characterised, and experimentally demonstrated to address this need. This SBCM is designed based on the concept of stiffness compensation between a linear positive-stiffness component (two double parallelograms in parallel) and a nonlinear negative-stiffness component (two sets of post-buckled fixed-guided compliant beams in parallel). A design guideline of the SBCM starting from using a rapid-design stiffness compensation equation is provided for a reasonable approximation of results. The whole-range nonlinear force-displacement relationship of the SBCM is obtained through nonlinear finite element analysis (FEA) simulations and a 5th order polynomial fit is chosen taking only odd terms into account. Subsequently, a dynamic analytical model of the displacement response of the SBCM to harmonic base excitations has been derived based on the averaging method. The accuracy of this analytical model is confirmed by numerical analysis and FEA simulations. Next, an SBCM prototype was fabricated and its applicability to piezoelectric vibrational energy harvesters (PVEHs) was demonstrated by integrating piezoelectric transducers, made of PVDF films, with compliant beams of the SBCM to generate electric outputs in response to bending of the beams. Static balancing was successfully tuned in the static experiments. Displacement responses and electric outputs were obtained from the preliminary SBCM-based PVEH in the ultra-low and wide frequency range with weak accelerations in the dynamic experiments. Two application cases of the SBCM in macro and micro scales in vibrational energy harvesting were investigated using FEA simulations. The integration of the SBCM into an oceanic drifter for harvesting vibrational energy from ocean waves was first explored. The SBCM is then miniaturized in the MEMS scale and its dynamic displacement under harmonic base excitation was then demonstrated. In conclusion, a novel SBCM structure is designed based on the stiffness compensation principle. It is verified analytically, numerically and experimentally that this SBCM structure responds to regular and irregular vibrations over ultra-low wide bandwidth frequencies (theoretically starting above 0 Hz) and low accelerations regardless of the dimensions and scales. It provides an effective structural solution to the ultra-low wide bandwidth vibrational energy harvesters. Future work of this research includes optimization of the SBCM structure and electric circuits, application of the SBCM-based PVEHs in real vibrational conditions, miniaturization of the SBCM in the MEMS scale, and integration of the SBCM with other nonlinear oscillators.