Over the past few years, the interest of remote wireless sensor networks has increased with the growth of Internet of Things technology. The wireless sensor network applications vary from tracking animal movement to controlling small electrical devices. Wireless sensors deployed in remote areas where the grid is unavailable are normally powered by batteries, inducing a limited lifespan for the sensor. This thesis work presents a solution to implement solar energy harvesting to a wireless sensor network. By gathering energy from the environment and using it in conjunction with an energy storage, the lifetime of a sensor node can be extended while at the same time reducing maintenance costs. To make sensor nodes in a network energy efficient, an adaptive controller of the nodes energy consumption can be used. A network consisting of a client node and a server node was created. The client node was powered by a small solar cell in conjunction with a capacitor. A linear-quadratic tracking algorithm was implemented to adaptively change the transmission rate for a node based on its current and previous battery level and the energy harvesting model. The implementation was done using only integers. To evaluate the system for extended run-times, the battery level was simulated using MATLAB. The system was simulated for different weather conditions. The simulation results show that the system is viable for both cloudy and sunny weather conditions. The integer linear-quadratic algorithm responds to change very abruptly in comparison to a floating point-version.

This thesis presents novel research in the area of energy harvesting from broadband vibra-tions. The aim of energy harvesting is to recover energy wasted or unused in the environmentto power low-consumption devices on the order of hundreds of microwatts to milliwatts. The motivation is twofold. In providing a localized, self-contained power source, device reliability, flexibility of installation location can be improved, and maintenance costs can be reduced. Furthermore, reduced reliance on batteries will mitigate the environmental impact associated with resource extraction, and disposal. To this end, this thesis investigates bistable laminates with piezoelectric transduction as broadband energy harvesters. Hitherto, a wealth of literature exists in which narrowband energy harvesters have been studied and optimized to operate over a small frequency interval. While these have been successful to the point of having devices commercially available, many situations exist where the dominant frequencies from which energy is to be harvested change with respect to time, or may be dominated by noise, thus not having a truly dominating frequency. Energy harvesters with nonlinear frequency responses have attracted substantial research interest because of their ability to respond over a broaderfrequency band. Due to complexities of the response of these harvesters, particularly when the intensity of the vibrational input is high, modeling their behavior is difficult. Designing these harvesters is therefore challenging as the relationships between the various design parameters and power output can be highly involved, or require numerical solutions as analytical solutions may not be possible. This thesis helps to address this knowledge gap. Bistable laminates ofboth cantilever and plate configuration are studied. Parametric studies are undertaken to empirically demonstrate the relationship between power output and parameters such as resistance load, proof mass addition, operation orientation, different shapes, ply angles, and introduction of adjustable magnetic compression. Modeling work is also undertaken to capture the mainfeatures of the nonlinear response such as subharmonics, superharmonics, and snap-through. A study is also carried out to quantify the differences of performance between a linear harvester and an equivalent bistable counterpart. As a practical demonstration, some plate-type harvesters are subjected to excitation patterns based on measured train data. Ultimately, thisthesis provides an in depth understanding of bistable shape, layup, and design on harvesting performance.

Asymmetric composite laminates can have a bistable response to loading. The potentially large structural deformations which can be achieved during snap-through from one stable state to another with small and removable energy input make them of interest for a wide range of engineering applications. After 30 years of research effort the shapes and response to applied loads of laminates of general layup can be quantitatively predicted. With attention switching to the incorporation of bistable laminates for practical applications, tools for the design and optimisation of actuated bistable devices are desirable. This thesis describes the analytical and experimental studies undertaken to develop novel modelling and optimisation techniques for the design of actuated asymmetric bistable laminates. These structures are investigated for practical application to morphing structures and the developing technology of piezoelectric energy harvesting. Existing analytical models are limited by the need for a numerical solver to determine stable laminate shapes. As the problem has multiple equilibria, convergence to the desired solution cannot be guaranteed and multiple initial guesses are required to identify all possible solutions. The approach developed in this work allows the efficient and reliable prediction of the stable shapes of laminates with off-axis ply orient at ions in a closed form manner. This model is validated against experimental data and finite element predictions, with an extensive sensitivity study presented to demonstrate the effect of uncertainty and imperfections in the laminate composition. This closed-form solution enables detailed optimisation studies to tailor the design of bistable devices for a range of applications. The first study considers tailoring of the directional stiffness properties of bistable laminates to provide resistance to externally applied loads while allowing low energy actuation. The optimisation formulation is constrained to guarantee bistability and to ensure a useful level of deformation. It is demonstrated that 'cross-symmetric' layups can provide stiffness in an arbitrary loading direction which is five times greater than in a chosen actuation direction.

Besides system level analyses, some implementation issues on switching interface circuits are also investigated. These interfaces show a great potential on harvesting efficiency improvement. Based on the experimental observation, it is found that there is a voltage reversion after every inversion in SSHI, which weakens the harvesting performance. This influence is caused by the dielectric loss in piezoelectric material. A revised model as well as detailed analysis are proposed to evaluate the influence of dielectric loss over the harvesting power degradation. / Considering the practical implementation, a modified self-powered switching interface circuit is proposed. It can achieve better isolation among components and involve less dissipative components. Improved analysis on this self-powered switching interface circuit is also provided. It is shown that the higher the excitation level, the more beneficial for replacing the SEH interface with the self-powered switching interface; meanwhile, the closer between the performances of self-powered and ideal (external powered) switching interfaces. / Owing to the great reduction on power consumption of integrated circuits (ICs) and miniaturization during the past decades, the energy harvesting technique has gained much interest recently with the inspiration that more devices in wireless sensor networks as well as mobile electronics could power themselves by scavenging the ambient energy in different forms. Piezoelectric energy harvesting (PEH) is one of the most widely studied techniques to scavenge energy from ambient vibration sources. With the electromechanical nature, a PEH device can be divided into mechanical and electrical parts. The two parts are linked by the piezoelectric transducer. Literatures on PEH are reviewed and discussed. In the research of PEH, generally there are four different research foci on: mechanical part, electrical part, piezoelectric transduction, and system. / This thesis provides new insight into the research of piezoelectric energy harvesting from some systematic viewpoints. The modeling process of a single degree-of-freedom (SDOF) PEH system is firstly discussed. It shows how the model of a PEH device is built from the material level to element level, and then to device level. In the systematic analysis to PEH devices, the energy flow and impedance based analysis are highlighted. A detailed analysis on the energy flow within the PEH system provides good understanding on the system. However, up to now, most of the researches on PEH have been mainly concerned with the absolute amount of energy that can be harvested from vibrating structures; the detailed energy flow within the system as well as its effect on the vibrating structure, were seldom discussed. By studying the energy flow within three applications of standard energy harvesting (SEH), resistive shunt damping (RSD), and synchronized switching harvesting on inductor (SSHI), it can be concluded that, in a PEH system, the two functions of energy harvesting and dissipation are coexistent. Both of them bring out structural damping. New factors are defined to give a more comprehensive evaluation on the energy flow in PEH systems. / To enhance the harvesting power by using the impedance matching is not new; yet, previous literatures on impedance matching for PEH oversimplified the problem. Without clarification on the energy flow in the PEH system, their objectives on power optimization were ambiguous. Some literatures even assumed that the harvesting interfaces, which are nonlinear in nature, can be equalized to linear loads, and the load impedance can be arbitrarily set. With the understanding on energy flow within piezoelectric devices, we clarify the objective of impedance matching, and further demonstrate that the range of equivalent impedance of existing harvesting interfaces is in fact constrained, rather than unlimited. The analyses on system level provide guideline to improve the harvesting performances. Improvements can be made with innovative designs in either mechanical configuration, piezoelectric transducer, or interface circuit. / Liang, Junrui. / Adviser: Wei-Hsin Liao. / Source: Dissertation Abstracts International, Volume: 73-03, Section: B, page: . / Thesis (Ph.D.)--Chinese University of Hong Kong, 2010. / Includes bibliographical references (leaves [145]-155). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Electronic reproduction. [Ann Arbor, MI] : ProQuest Information and Learning, [201-] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese.

Wireless micro-sensors can enjoy popularity in biomedical drug-delivery treatments and tire-pressure monitoring systems because they offer in-situ, real-time, non-intrusive processing capabilities. However, miniaturized platforms severely limit the energy of onboard batteries and shorten the lifespan of electronic systems. Ambient energy is an attractive alternative because the energy from light, heat, radio-frequency (RF) radiation, and motion can potentially be used to continuously replenish an exhaustible reservoir. Of these sources, solar light produces the highest power density, except when supplied from indoor lighting, under which conditions the available power decreases drastically. Harnessing thermal energy is viable, but micro-scale dimensions severely limit temperature gradients, the fundamental mechanism from which thermo piles draw power. Mobile electronic devices today radiate plenty of RF energy, but still, the available power rapidly drops with distance. Harvesting kinetic energy may not compete with solar power, but in contrast to indoor lighting, thermal, and RF sources, moderate and consistent vibration power across a vast range of applications is typical. Although operating conditions ultimately determine which kinetic energy-harvesting method is optimal, piezoelectric transducers are relatively mature and produce comparatively more power than their counterparts such as electrostatic and electromagnetic kinetic energy transducers. The presented research objective is to develop, design, simulate, fabricate, prototype, test, and evaluate CMOS ICs that harvest ambient kinetic energy in periodic and non-periodic vibrations using a small piezoelectric transducer to continually replenish an energy-storage device like a capacitor or a rechargeable battery. Although vibrations in surrounding environment produce abundant energy over time, tiny transducers can harness only limited power from the energy sources, especially when mechanical stimulation is weak. To overcome this challenge, the presented piezoelectric harvesters eliminate the need for a rectifier which necessarily imposes threshold limits and additional losses in the system. More fundamentally, the presented harvesting circuits condition the transducer to convert more electrical energy for a given mechanical input by increasing the electromechanical damping force of the piezoelectric transducer. The overall aim is to acquire more power by widening the input range and improving the efficiency of the IC as well as the transducer. The presented technique in essence augments the energy density of micro-scale electronic systems by scavenging the ambient kinetic energy and extends their operational lifetime. This dissertation reports the findings acquired throughout the investigation. The first chapter introduces the applications and challenges of micro-scale energy harvesting and also reviews the fundamental mechanisms and recent developments of various energy-converting transducers that can harness ambient energy in light, heat, RF radiation, and vibrations. Chapter 2 examines various existing piezoelectric harvesting circuits, which mostly adopt bridge rectifiers as their core. Chapter 3 then introduces a bridge-free piezoelectric harvester circuit that employs a switched-inductor power stage to eliminate the need for a bridge rectifier and its drawbacks. More importantly, the harvester strengthens the electrical damping force of the piezoelectric device and increases the output power of the harvester. The chapter also presents the details of the integrated-circuit (IC) implementation and the experimental results of the prototyped harvester to corroborate and clarify the bridge-free harvester operation. One of the major discoveries from the first harvester prototype is the fact that the harvester circuit can condition the piezoelectric transducer to strengthen its electrical damping force and increase the output power of the harvester. As such, Chapter 4 discusses various energy-investment strategies that increase the electrical damping force of the transducer. The chapter presents, evaluates, and compares several switched-inductor harvester circuits against each other. Based on the investigation in Chapter 4, an energy-investing piezoelectric harvester was designed and experimentally evaluated to confirm the effectiveness of the investing scheme. Chapter 5 explains the details of the IC design and the measurement results of the prototyped energy-investing piezoelectric harvester. Finally, Chapter 6 concludes the dissertation by revisiting the challenges of miniaturized piezoelectric energy harvesters and by summarizing the fundamental contributions of the research. With the same importance as with the achievements of the investigation, the last chapter lists the technological limits that bound the performance of the proposed harvesters and briefly presents perspectives from the other side of the research boundary for future investigations of micro-scale piezoelectric energy harvesting.

Energy harvesters collect and convert energy available in the environment into useful electrical power to satisfy the power requirements of autonomous systems. Vibration energy is a prevalent source of waste energy in industrial and built environments. Vibration-based energy harvesters, or vibration-based micro power generators (VBMPGs), utilize a transducer, a mechanical oscillator in this application, to capture kinetic energy from environmental vibrations and to convert it into electrical energy using electromagnetic, electrostatic, or piezoelectric transduction mechanisms. A key design feature of all VBMPGs, regardless of their transduction mechanism, is that they are optimally tuned to harvest vibration energy within a narrow frequency band in the neighborhood of the natural frequency of the oscillator. Outside this band, the output power is too low to be conditioned and utilized. This limitation is exacerbated by the fact that VBMPGs are also designed to have high quality factors to minimize energy dissipation, further narrowing the optimal operating frequency band. Vibrations in most environments, however, are random and wideband. As a result, VBMPGs can harvest energy only for a relatively limited period of time, which imposes excessive constraints on their usability. A new architecture for wideband VBMPGs is the main contribution of this thesis. The new design is general in the sense that it can be applied to any of the three transduction mechanisms listed above. The linear oscillator is replaced with a piecewise-linear oscillator as the energy-harvesting element of the VBMPG. The new architecture has been found to increase the bandwidth of the VBMPG during a frequency up-sweep, while maintaining the same bandwidth in a frequency downsweep. Experimental results show that using the new architecture results in a 313% increase in the width of the bandwidth compared to that produced by traditional architecture. Simulations show that under random-frequency excitations, the new architecture collects more energy than traditional architecture. In addition, the knowledge acquired has been used to build a wideband electromagnetic VBMPG using MicroElectroMechanical Systems, MEMS, technology. This research indicates that a variety of piecewise-linear oscillators, including impact oscillators, can be implemented on MPG structures that have been built using MEMS technology. When the scale of the MPGs is reduced, lower losses are likely during contact between the moving oscillators and the stopper, which will lead to an increase in bandwidth and hence in the amount of energy collected. Finally, a design procedure has been developed for optimizing such wideband MPGs. This research showed that wideband MPGs require two design optimization steps in addition to the traditional technique, which is used in all types of generators, of minimizing mechanical energy losses through structural design and material selection. The first step for both regular and wideband MPGs minimizes the MPG damping ratio by increasing the mass and stiffness of the MPG by a common factor until the effect of size causes the rate at which energy losses increase to accelerate beyond that common factor. The second step, which is specific to wideband MPGs, tailors the output power and bandwidth to fit the Probability Density Function, PDF, of environmental vibrations. A figure of merit FoM was devised to quantify the quality of this fit. Experimental results show that with this procedure, the bandwidth at half-power level increases to more than 600% of the original VBMPG bandwidth.

Energy harvesters collect and convert energy available in the environment into useful electrical power to satisfy the power requirements of autonomous systems. Vibration energy is a prevalent source of waste energy in industrial and built environments. Vibration-based energy harvesters, or vibration-based micro power generators (VBMPGs), utilize a transducer, a mechanical oscillator in this application, to capture kinetic energy from environmental vibrations and to convert it into electrical energy using electromagnetic, electrostatic, or piezoelectric transduction mechanisms. A key design feature of all VBMPGs, regardless of their transduction mechanism, is that they are optimally tuned to harvest vibration energy within a narrow frequency band in the neighborhood of the natural frequency of the oscillator. Outside this band, the output power is too low to be conditioned and utilized. This limitation is exacerbated by the fact that VBMPGs are also designed to have high quality factors to minimize energy dissipation, further narrowing the optimal operating frequency band. Vibrations in most environments, however, are random and wideband. As a result, VBMPGs can harvest energy only for a relatively limited period of time, which imposes excessive constraints on their usability. A new architecture for wideband VBMPGs is the main contribution of this thesis. The new design is general in the sense that it can be applied to any of the three transduction mechanisms listed above. The linear oscillator is replaced with a piecewise-linear oscillator as the energy-harvesting element of the VBMPG. The new architecture has been found to increase the bandwidth of the VBMPG during a frequency up-sweep, while maintaining the same bandwidth in a frequency downsweep. Experimental results show that using the new architecture results in a 313% increase in the width of the bandwidth compared to that produced by traditional architecture. Simulations show that under random-frequency excitations, the new architecture collects more energy than traditional architecture. In addition, the knowledge acquired has been used to build a wideband electromagnetic VBMPG using MicroElectroMechanical Systems, MEMS, technology. This research indicates that a variety of piecewise-linear oscillators, including impact oscillators, can be implemented on MPG structures that have been built using MEMS technology. When the scale of the MPGs is reduced, lower losses are likely during contact between the moving oscillators and the stopper, which will lead to an increase in bandwidth and hence in the amount of energy collected. Finally, a design procedure has been developed for optimizing such wideband MPGs. This research showed that wideband MPGs require two design optimization steps in addition to the traditional technique, which is used in all types of generators, of minimizing mechanical energy losses through structural design and material selection. The first step for both regular and wideband MPGs minimizes the MPG damping ratio by increasing the mass and stiffness of the MPG by a common factor until the effect of size causes the rate at which energy losses increase to accelerate beyond that common factor. The second step, which is specific to wideband MPGs, tailors the output power and bandwidth to fit the Probability Density Function, PDF, of environmental vibrations. A figure of merit FoM was devised to quantify the quality of this fit. Experimental results show that with this procedure, the bandwidth at half-power level increases to more than 600% of the original VBMPG bandwidth.

In recent years, energy harvesting has been generating great interests among researchers, scientists and engineers alike. One of the major reasons for this increased interest sterns from the desire to have autonomous perpetual power supplies for remote monitoring sensor nodes utilizing some of the already available and otherwise wasted energy in the environment in a very innovative and useful way (and at the same time, maintaining a green environment). Scientists and engineers are constantly looking for ways of obtaining continuous and uninterrupted data from several points of interests especially remote or dangerous locations, using sensors coupled with RF transceivers, without the need of ever replacing or recharging the batteries that power these devices. This is now made possible through energy harvesting technologies which serve as suitable power supply substitutes, in many cases, for low power devices. With the proliferation of wireless energy in the environment through different radio frequency bands as well as natural sources like solar, wind and heat energy, it has become a desirable thing to take advantage of their availability by harvesting and converting them to useful electrical energy forms. The energy so harnessed or harvested could then be utilized in sensor nodes. Now, since these energy sources fluctuate from time to time, and from place to place, there is the need to have a form of energy accumulation, conversion, conditioning and storage. The stored energy would then be reconverted and used by the sensors nodes and/or RF transceivers when needed. The process through which this is done is referred to as energy management. In this research work, many types of energy harvesting transducers were explored including – solar, thermal, electromagnetic and piezo/vibration. A proof of concept approach for an on-demand electromagnetic power generator is then presented towards the end. While most, if not all, of the energy harvesting techniques discussed needed some time to accumulate enough charge to operate their respective systems, the on-demand energy harvester makes energy available as at and when needed. In summary, a system level design is presented with suggested future research works.

Piezoelectric energy harvesting is the process of taking an external mechanical input and converting it directly into electrical energy via the piezoelectric effect. To determine the power created by a piezoelectric energy harvester, a specific application with defined input and design constraints must first be chosen. The following thesis established a concept design of a hydrokinetic energy harvesting system, the piezoelectric artificial kelp (PAK), which uses piezoelectric materials to harvest coastal ocean waves while having a beneficial impact on the surrounding environment. The harvester design mimics the configuration of sea-kelp, a naturally occurring plant that anchors to the ocean floor and extends into the water column. Underwater currents caused by wave-action result in periodic oscillations in the kelp. In order to determine the average power generated by this design concept, predictive tools were devised that allowed for the determination of the optimized average power produced by the piezoelectric energy harvester. For a stiff energy harvester, the linear differential equations were analytically solved to find an equation for the average power generated as a function of design parameters. These equations were used to compare the effect on power output of the design configuration and piezoelectric material choice between a piezopolymer (PVDF) and a piezoceramic (PZT). The homogeneous bimorph was found to have the optimal design configuration and it was shown that a harvester constructed using PVDF would produce approximately 1.6 times as much power as one using PZT. For a flexible energy harvester, an iterative nonlinear solution technique using an assumed polynomial solution for the local curvature of the energy harvester was used to verify and extend the analytic solutions to large deflections. An energy harvester was built using off-the-shelf piezoelectric elements and tested in a wave tank facility to validate experimentally the voltage and average power predicted by the analytical solution. The iterative code showed the PAK harvester to produce volumetric power on the order of other energy harvesting concepts (17.8 micro [mu]W/cm³). Also, a full-scale PAK harvester approximately ten meters long in typical wave conditions was found to produce approximately one watt of power.

Thesis (M.S.)--California Polytechnic State University, 2010. / Mode of access: Internet. Title from PDF title page; viewed on March 15, 2010. Major professor: Dr. David Braun. "Presented to the faculty of California Polytechnic State University, San Luis Obispo." "In partial fulfillment of the requirements for the degree [of] Master of Science in Electrical Engineering." "March 2010." Includes bibliographical references (p. 65-69).