The aim of the work described in this thesis is firstly to improve the collection of vibration energy for piezoelectric cantilever harvesters, by a mechanical technique, so that the devices can harvest energy over a wider bandwidth. Secondly to investigate a new circuit topology for achieving complex conjugate load matching to the piezoelectric harvester. The thesis has been divided into two parts - the mechanical approach and the electrical approach. For the mechanical approach, a novel multiresonant beam, comprising piezoelectric fiber composites on a clamped-clamped beam and side mounted cantilevers, was proposed. The side cantilevers are tuned by tip masses to be resonant at different frequencies. A Rayleigh-Ritz model was developed to predict the vibration response of the proposed model multiresonant beam. This model showed that the bandwidth of the multiresonant beam was increased over that of a single cantilever harvester. A multiresonant beam for energy harvesting was experimentally tested and compared with a single cantilever energy harvester. The transmissibility and voltage responses were investigated, the beam showed a wide frequency response between 14.5Hz and 31Hz, whereas the single cantilever only showed one resonant frequency. Therefore the multiresonant beam system is feasible for wide band energy harvesting. For the electrical approach, the task was to investigate complex conjugate impedance matching for the piezoelectric energy harvesters, so that the output impedance from the piezoelectric harvester can be reduced, and maximum energy extracted from the device with a possibility of frequency tuning. A new amplified inductor circuit was proposed to enable the capacitive output impedance of the piezoelectric device to be cancelled. Experimental and software simulations are provided to verify the theoretical predictions. A prototype amplified inductor circuit was simulated and tested. The results showed that a variable effective inductance was achieved. However the circuit is lossy due to imperfections within the system, and needs further work to eliminate these imperfections.

Motivado pela crescente demanda por fontes energéticas alternativas, este trabalho discute o uso do efeito piezelétrico para geração de energia a partir de vibrações estruturais, cujo caráter ubíquo as têm colocado em posição de destaque dentre outras fontes renováveis. O processo conhecido por piezoelectric energy harvesting ou scavenging é estudado utilizando-se de múltiplos graus de liberdade para maximização da energia gerada e aumento da faixa de frequência útil do gerador, permitindo melhores resultados em aplicações sujeitas a excitações aleatórias de larga banda de frequência. Diretamente relacionada ao custo do dispositivo, e por isso, à sua viabilidade comercial, a eficiência dos harvesters em função da quantidade de material piezelétrico utilizado também é amplamente discutida. Para o desenvolvimento do tema são realizadas simulações numéricas em MATLAB, primeiramente para modelos de parâmetros concentrados, com análises de sensibilidade da geração de energia em relação a características mecânicas, bem como disposição e propriedades dos elementos piezelétricos em soluções com dois e três graus de liberdade. Usando modelos de parâmetros distribuídos os estudos são replicados a uma solução construtiva do tipo viga \"L\", com validação do modelo matemático e das proposições levantadas através de ensaios em laboratório usando um protótipo do gerador. Ao final é feita uma análise crítica relativa ao piezoelectric energy harvesting através de geradores de múltiplos graus de liberdade, em que são confrontados e discutidos os resultados teóricos e experimentais obtidos. / Motivated by the increasing demand on alternative energy resources, this study discusses the usage of the piezoelectric effect for energy generation from structural vibrations, which stands out among other renewable energy resources by its ubiquitous essence. The process known as piezoelectric energy harvesting or scavenging is evaluated using multiple degrees-of-freedom to maximize the energy generated and broaden the useful frequency bandwidth of the harvester, enabling better outcomes in applications subjected to random broadband excitations. Due to its direct relation to costs, and therefore to market feasibility, the harvester efficiency based on the piezoelectric material quantity is widely discussed. Numeric simulation using MATLAB are performed for the subject development, firstly using lumped parameter models to conduct generation sensitivity analysis on the mechanical characteristics, piezoelectric properties and allocation of two and three degrees-of-freedom solutions. Using distributed parameter models the study is replicated to an L-shaped configuration, with validation of the theoretical model and the brought forward proposals through laboratory experiments using an energy harvester prototype. At the end, a critical analysis on piezoelectric energy harvesting through multiple degrees-of-freedom is conducted, comparing and discussing the theoretical and experimental results.

<p>Within the past decade, advances in small-scale electronics have reduced power consumption requirements such that mechanisms for harnessing ambient kinetic energy for self-sustenance are a viable technology. Such devices, known as energy harvesters, may enable self-sustaining wireless sensor networks for applications ranging from Tsunami warning detection to environmental monitoring to cost-effective structural health diagnostics in bridges and buildings. In particular, flexible electroelastic materials such as lead-zirconate-titanate (PZT) are sought after in designing such devices due to their superior efficiency in transforming mechanical energy into the electrical domain in comparison to induction methods. To date, however, material and dynamic nonlinearities within the most popular type of energy harvester, an electroelastically laminated cantilever beam, has received minimal attention in the literature despite being readily observed in laboratory experiments. </p><p>In the first part of this dissertation, an experimentally validated first-principles based modeling framework for quantitatively characterizing the intrinsic nonlinearities and moderately large amplitude response of a cantilevered electroelastic generator is developed. Nonlinear parameter identification is facilitated by an analytic solution for the generator's dynamic response alongside experimental data. The model is shown to accurately describe amplitude dependent frequency responses in both the mechanical and electrical domains and implications concerning the conventional approach to resonant generator design are discussed. Higher order elasticity and nonlinear damping are found to be critical for correctly modeling the harvester response while inclusion of a proof mass is shown to invigorate nonlinearities a much lower driving amplitudes in comparison to electroelastic harvesters without a tuning mass.</p><p>The second part of the dissertation concerns dynamical systems design to purposefully engage nonlinear phenomena in the mechanical domain. In particular, two devices, one exploiting hysteretic nonlinearities and the second featuring homoclinic bifurcation are investigated. Both devices exploit nonlinear magnet interactions with piezoelectric cantilever beams and a first principles modeling approach is applied throughout. The first device is designed such that both softening and hardening nonlinear resonance curves produces a broader response in comparison to the linear equivalent oscillator. The second device makes use of a supercritical pitchfork bifurcation wrought by nonlinear magnetic repelling forces to achieve a bistable electroelastic dynamical system. This system is also analytically modeled, numerically simulated, and experimentally realized to demonstrate enhanced capabilities and new challenges. In addition, a bifurcation parameter within the design is examined as a either a fixed or adaptable tuning mechanism for enhanced sensitivity to ambient excitation. Analytical methodologies to include the method of Harmonic Balance and Melnikov Theory are shown to provide superior insight into the complex dynamics of the bistable system in response to deterministic and stochastic excitation.</p> / Dissertation

Electromechanical modeling efforts in the research field of vibration-based energy harvesting have been mostly focused on deterministic forms of vibrational input as in the typical case of harmonic excitation at resonance. However, ambient vibrational energy often has broader frequency content than a single harmonic, and in many cases it is entirely stochastic. As compared to the literature of harvesting deterministic forms of vibrational energy, few authors presented modeling approaches for energy harvesting from broadband random vibrations. These efforts have combined the input statistical information with the single-degree-of-freedom (SDOF) dynamics of the energy harvester to express the electromechanical response characteristics. In most cases, the vibrational input is assumed to have broadband frequency content, such as white noise. White noise has a flat power spectral density (PSD) that might in fact excite higher vibration modes of an electroelastic energy harvester. In particular, cantilevered piezoelectric energy harvesters constitute such continuous electroelastic systems with more than one vibration mode. The main component of this thesis presents analytical and numerical electroelastic modeling, simulations, and experimental validations of piezoelectric energy harvesting from broadband random excitation. The modeling approach employed herein is based on distributed-parameter electroelastic formulation to ensure that the effects of higher vibration modes are included. The goal is to predict the expected value of the power output and the mean-square shunted vibration response in terms of the given PSD or time history of the random vibrational input. The analytical method is based on the PSD of random base excitation and distributed-parameter frequency response functions of the coupled voltage output and shunted vibration response. The first one of the two numerical solution methods employs the Fourier series representation of the base acceleration history in a Runge-Kutta-based ordinary differential equation solver while the second method uses an Euler-Maruyama scheme to directly solve the resulting electroelastic stochastic differential equations. The analytical and numerical simulations are compared with several experiments for a brass-reinforced PZT-5H cantilever bimorph under different random excitation levels.In addition to base-excited cantilevered configurations, energy harvesting using prismatic piezoelectric stack configurations is investigated. Electromechanical modeling and numerical simulations are given and validated through experiments for a multi-layer PZT-5H stack. After validating the electromechanical models for specific experimentally configurations and samples, various piezoelectric materials are compared theoretically for energy harvesting from random vibrations. Finally, energy harvesting from narrowband random vibrationsusing both configurations are investigated theoretically and experimentally.

The development of self-powered wireless sensor networks for structural and machinery health monitoring has attracted considerable attention in the research field during the last decade. Since the low-duty-cycle wireless sensor networks have significantly reduced the power requirements to the range of tens to hundreds of microwatts, it is possible to harvest environmental energy as the power supply instead of using batteries. Vibration energy harvesting using piezoelectric materials has become the most popular technique, which has a good potential to generate adequate power. However, there is a limitation for the conventional beam-shaped harvester designs in real applications due to their limited bandwidth. In order to overcome this limitation, the essential objective of this thesis is to develop harvesters with multi-resonance structures. The multi-resonance harvester with good broadband performance can achieve close resonance frequencies and relatively large power output in each vibration mode. The main tasks and contributions of this thesis are summarised as follows: • A parametric analysis is presented to determine how the modal structural and electromechanical performances of cantilevered beam harvesters are affected by two modal factors designated as mass ratio and electromechanical coupling coefficient (EMCC). The modal performance of using rectangular, convergent and divergent tapered configurations with and without extra masses are systematically analysed by geometric variation using the finite element analysis (FEA) software ABAQUS. • A modal approach using the two modal factors to evaluate the modal performance of harvesters is introduced and a configurational optimization strategy based on the modal approach is developed to pre-select the configurations of multi-resonance harvesters with better modal structural performance and close resonance frequencies in multiple modes. Using this optimization strategy obviates the need to run the full analysis at the first stage. • A novel two-layer stacked harvester, which consists of a base cantilevered beam that is connected to an upper beam by a rigid mass, is developed. By altering the dimensions and the locations of the masses, the two-layer harvester can generate two close resonance frequencies with relatively large power output. The effects of using rectangular, convergent and divergent tapered beam configurations are systematically analysed. • Multi-layer stacked harvesters with up to five layers are developed. The three-layer harvesters with different mass positions, which can generate three close resonance frequencies, are optimized using the configurational optimization strategy. • A novel doubly-clamped multi-layer harvester, which is able to generate five close resonance frequencies with relatively large power output, is developed and thoroughly analysed. • An experimental study of the multi-layer stacked harvester is presented to validate the simulated results and the configurational optimization strategy. • An experimental study of the two-layer stacked harvester using high performance single crystal piezoelectric material PIMNT is presented. The harvester using PIMNT can generate nearly 10 times larger power output and 3.5 times wider bandwidth than using PZT. Besides, by modifying the location of the piezoelectric layer, anti-resonances between two adjacent modes can be eliminated.

Harvesting energy from ultra-low-power vibration energy sources typically employs a rectifier circuit as the first power conditioning stage. The Schottky diode has a 0.15 V - 0.2 V threshold voltage and can not extract energy efficiently at low voltage. Other technologies such as MOSFET bridge or active diode are designed to minimize the voltage drop to reduce the conduction loss. However, these designs require either additional power supplies to operate comparators or have a larger threshold turn-on voltage than Schottky. Therefore, most rectifiers have an unresponsive or significant low-efficiency zone when the input power is low. This dissertation will elaborate on a backward diode based self-powered micro-circuit diode that will operate in the extremely weak or low alternating source applications, where the existing approaches offer poor outcomes. This proposed micro-circuit diode was compared to a Schottky diode in several experiment setup. The micro-circuit based half-wave rectifier circuit harvested 3.1 mV DC at a 239.5 Ohm load when the input magnitude is 50 mV while the Schottky diode was unable to convert this ultra-low AC power. This dissertation also provides the analysis of two alternating sources, the oscillatory electromagnetic generator and the piezoelectric energy harvester, to conduct experiments in a more realistic context. The micro-circuit diode shows excellent advantages in electromagnetic generator experiment, the micro-circuit based half-wave rectifier circuit harvested 5.16 mV DC at a 0.5 kOhm load when the input magnitude is 40 mV. However, due to the large leakage current in negative resistance region, this micro-circuit is unable to show advantages in piezoelectric energy harvester applications.

Motivado pela crescente demanda por fontes energéticas alternativas, este trabalho discute o uso do efeito piezelétrico para geração de energia a partir de vibrações estruturais, cujo caráter ubíquo as têm colocado em posição de destaque dentre outras fontes renováveis. O processo conhecido por piezoelectric energy harvesting ou scavenging é estudado utilizando-se de múltiplos graus de liberdade para maximização da energia gerada e aumento da faixa de frequência útil do gerador, permitindo melhores resultados em aplicações sujeitas a excitações aleatórias de larga banda de frequência. Diretamente relacionada ao custo do dispositivo, e por isso, à sua viabilidade comercial, a eficiência dos harvesters em função da quantidade de material piezelétrico utilizado também é amplamente discutida. Para o desenvolvimento do tema são realizadas simulações numéricas em MATLAB, primeiramente para modelos de parâmetros concentrados, com análises de sensibilidade da geração de energia em relação a características mecânicas, bem como disposição e propriedades dos elementos piezelétricos em soluções com dois e três graus de liberdade. Usando modelos de parâmetros distribuídos os estudos são replicados a uma solução construtiva do tipo viga \"L\", com validação do modelo matemático e das proposições levantadas através de ensaios em laboratório usando um protótipo do gerador. Ao final é feita uma análise crítica relativa ao piezoelectric energy harvesting através de geradores de múltiplos graus de liberdade, em que são confrontados e discutidos os resultados teóricos e experimentais obtidos. / Motivated by the increasing demand on alternative energy resources, this study discusses the usage of the piezoelectric effect for energy generation from structural vibrations, which stands out among other renewable energy resources by its ubiquitous essence. The process known as piezoelectric energy harvesting or scavenging is evaluated using multiple degrees-of-freedom to maximize the energy generated and broaden the useful frequency bandwidth of the harvester, enabling better outcomes in applications subjected to random broadband excitations. Due to its direct relation to costs, and therefore to market feasibility, the harvester efficiency based on the piezoelectric material quantity is widely discussed. Numeric simulation using MATLAB are performed for the subject development, firstly using lumped parameter models to conduct generation sensitivity analysis on the mechanical characteristics, piezoelectric properties and allocation of two and three degrees-of-freedom solutions. Using distributed parameter models the study is replicated to an L-shaped configuration, with validation of the theoretical model and the brought forward proposals through laboratory experiments using an energy harvester prototype. At the end, a critical analysis on piezoelectric energy harvesting through multiple degrees-of-freedom is conducted, comparing and discussing the theoretical and experimental results.

Vibration converters based on piezoelectric materials are currently becoming increasingly important for powering low-power wireless sensor nodes and wearable electronic devices. Piezoelectric materials generate variable electrical charges under mechanical stress, requiring an energy management interface to meet load requirements. Resonant interfaces like Parallel Synchronized Switch Harvesting on Inductor (P-SSHI) are highly efficient and robust to energy sources and loads variations. Nevertheless, SSHI circuits require synchronous switch control for efficient energy transfer. At irregular excitation, SSHI circuits may not perform optimally because the resonant frequency of the circuit is typically tuned to match the frequency of the energy source, which in the case of footsteps can be irregular and unpredictable. In addition, the circuit may also be susceptible to noise and interference from irregular excitations, which can further affect its performance. The aim is to design a self-powered energy management solution that can operate autonomously even at low frequencies and for irregular chock excitations, while at the same time allowing higher energy flow to the energy storage device and maintaining high levels of energy efficiency. To evaluate the performance of the proposed circuit, a piezoelectric shoe insole is designed and used for testing with different storage capacitance values and loads as a proof of the circuit’s adaptability to various loading conditions.:1 Introduction 2 Theoretical background 3 State of the art of piezoelectric energy harvesting interfaces 4 Novel approach of SP-PSSHI piezoelectric energy harvesting interface 5 Experimental investigations 6 Conclusions and Outlook