Piezoelectric energy harvesters convert vibrational energy into electrical energy, thereby reducing the dependence of wireless devices on batteries. However, most existing harvesters have a limited bandwidth, generating usable power only within a narrow range of excitation frequencies. Buckled beam harvesters address this issue by intentionally introducing nonlinear behaviour, but systematic design strategies for such devices are still missing due to the absence of versatile models and limited understanding of their dynamics.
In this thesis, these challenges are addressed by developing a model that combines finite element analysis with lumped-parameter equations and validating it experimentally. The resulting framework captures higher-order vibration modes and nonlinear dynamic phenomena such as secondary resonances and softening behaviour. It is demonstrated that these effects can be exploited to optimize the design and broaden the bandwidth of buckled beam piezoelectric energy harvesters. Based on the improved physical understanding, several practical design improvements are proposed. ... Read more

Vibration energy harvesting is a growing field of research. Harvesting energy from vibrations can be of use in devices in hard to reach places, such as sensors on a train track or a pacemaker. These devices makes use of batteries, which need to be replaced. Using energy harvesting this battery life could be improved.
The difficulty lies in harvesting low frequency vibrations. These vibrations are hard to harvest using a transducer such as a piezo. A solution could be using a frequency up converter, which can raise a vibrations frequency. This thesis shows the design of a Bi-stable Impact-driven Snap-through frequency up converter (BISup). Enhancing the behaviour of this bi-stable design could improve the energy harvesting capabilities. Different designs are made using ortho planar spring design, these designs are simulated and compared using ANSYS. These simulations are experimentally verified.
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Limited stroke automatic watch winding poses a challenge due to the proof mass range being smaller than the input displacements. Traditionally the proof mass is connected to the mainspring by a linear reduction transmission however this setup only functions effectively for specific accelerations. This paper proposes to use a nonlinear transmission between the proof mass and the mainspring improve the power output. This transmission will us a singularity to have a mechanical advantage of zero in the middle of its motion range, and increasing the further it moves This improves the range in which the automatic winding device can operate especially the lower accelerations. A quasi static model of the system is made to estimate the efficiency of the mechanism for different accelerations which is verified by a demonstrator. These efficiencies combined with a human motion analysis suggest it could increase the energy generated to the mainspring by 52% compared to the linear transmission. ... Read more

Vibration energy harvesting has proven to be a durable source of energy for a wide variety of applications, however not all of the positions these applications are placed at are suitable for conventional vibration energy harvesting. For some of these applications a frequency up-converted energy harvester could increase the power harvested. These systems are analysed in this thesis project, by first gaining insight in previously performed. After which a new method is designed and modelled. The design was then fabricated and a proof of concept was tested, of which the results are presented in this thesis project report.

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Energy harvesting from renewable energy sources has become more popular in the last decades than ever before. New energy sources consisting of human input vibrations are hopeful alternatives for powering wearable low power electronics. These sensors currently rely on the lifetime of the battery and come along with high maintenance costs in case of a replacement. Many designs found within literature are based on the working principles of linear resonant energy harvesters, consisting of high output generation when they are exited on their resonance frequency. However, if the vibration energy harvester is not accurately tuned to the input signal of the real world, poor output performance can be expected. Bistability, consisting of a unique double well potential energy curve is an interesting alternative. The oscillation between the two potential wells contribute to higher output performance in comparison with resonant configurations. However, these potential wells are segregated by an energy barrier and the motion between the two wells will not always occur during excitation. A solution is found to reduce the energy barrier by means of mechanical end-stops. A mechanical model based upon beam theory is created in ANSYS and their stiffness characteristics are used as an input parameter for the dynamical model. To confirm this model a prototype is constructed and investigated. A mechanical analysis is carried out using a quasistatic forcedeflection measurement and the dynamical analysis is performed on a linear air bearing stage, consisting of a maximum stroke of half a meter being able to reproduce low frequency input excitations. It could be observed that participation of the desired trajectory between the two potential wells is enhanced and occur at lower input accelerations, as the oscillators motion is confined by means of hard mechanical end-stops. Therefore, the integration of mechanical end-stops as a design parameter for bistable energy harvesters can be considered as a viable solution to capture the kinetic energy induced by human input motion with the use of bistable mechanisms. ... Read more

Vibration energy harvesters can help in different areas. These vary from supplying power to medical implants or wireless sensors that can aid in predicting natural disasters. However, the problem is that they are not able to generate power across a large bandwidth of frequencies. Bistable energy harvesters can solve this problem when these are able to oscillate between their stable equilibria. A bistable mechanism that has these benefits is a post-buckled beam. However, a potential energy barrier needs to be overcome before this interwell motion can occur. If there is no interwell motion, the power output will be severely reduced. A topology optimization is performed on a beam so that the buckling loads approach almost equality. This leads to lowering of the potential barrier and thus interwell motion is eased. Numerical simulations and experimental measurements show that the stiffness can be reduced by a factor of 10 by removing material. Another topology optimization is performed to increase the stiffness. It has been numerically and experimentally verified that removing material can increase the stiffness of the post-buckled beam by 20%. ... Read more

Vibration energy harvesters have been proposed as a solution to increase the lifetime of wireless and portable medical devices. One example of implantable medical devices for which energy harvesters could be interesting, are pacemakers. With a lifespan of about 6 to 12 years, the battery must be replaced after this period of time. Using an energy harvester instead of a battery is therefore seen as an interesting alternative. However, the human heart rate is usually between 0.6-2Hz and consists of low acceleration peaks (<1g). The use of resonance at low frequency is extremely difficult, especially when the motion amplitude is larger than the device itself. A solution is sought in the non-resonant bistable energy harvesters. When enough force is applied to overcome the potential energy barrier, snap through motion is induced, resulting in a significant increase in power output. However, large threshold accelerations are limiting the usability of these systems. Therefore, stiffness compensation is required. A prototype was fabricated in which buckled flexures were used to add negative stiffness to a piezoelectric cantilever, resulting in a stiffness compensated bistable energy harvester suitable for energy harvesting from low frequency and low force excitations. The dynamical behaviour and practical performance of the prototype was studied in relation to a heartbeat, sawtooth wave and sine waves. The output power of the non-resonant prototype was compared to a resonant device, which in all cases showed that the non-resonant prototype outperformed the resonant device. This shows that stiffness compensated bistable energy harvesters can be used in order to make energy harvesting for low force and low frequency excitations, such as a heartbeat, possible. ... Read more

Vibration energy harvesters are especially interesting to use in an environment where there is one dominant vibration frequency present because then the harvesters can be designed to resonate at that specific frequency. To spread out the power yield over more frequencies a multi-modal harvester can be used which can resonate at multiple frequencies. A vibration with more than one sine wave can be manifested in a number of ways. The two frequencies can be present simultaneously, or they can alternate each other. How the energy harvesters react to these different vibration inputs is researched in this paper. Two fundamentally different multi-modal energy harvesters are used here. One which can be described by a coupled system of equations and one uncoupled. Two prototypes of an uncoupled and one coupled device are made and tested on an electromagnetic shaker. The vibration signals are sent to the shaker and the power output of the energy harvesters is measured using piezoelectric transducers mounted to the mechanisms. The results show that a phaseshift in the sine wave input signal generally results in a increase in power, where a decrease was assumed beforehand. When switching the input vibration from the first to the second eigenfrequency the power output does drop significantly, but the coupled mechanism has a substantially higher power output than the uncoupled device. And when the mechanisms are excited by a vibration with two eigenfrequencies at the same time no significant difference between the two can be observed, nor does the power output drop significantly. While the comparison between these two mechanisms is probably accurate, the quantitative conclusions must be taken with a grain of salt as it was noticed in a later stage of the research that the vibration signals were not consistent over the entire time period. At this point it is unclear if an overall better mechanism can be picked between the coupled and uncoupled one. However, it is shown that both have their distinct advantages where they outperform their counterpart, which can be used for designing a better energy harvester in future applications ... Read more

A pacemaker runs on a conventional battery that lasts for approximately 6-12 years, after which the pacemaker must be replaced. Converting the heart wall vibrations into electricity through a vibration energy harvester has been considered a promising solution to this problem. However, the complexity of the heart signals on which the energy harvester has to operate is a challenge. The human heart signal is a broadband signal, consisting of a varying acceleration amplitude at low frequencies. Most of the testing signals used in the labs are harmonic signals, Gaussian white noise or Gaussian coloured noise. These signals do not have the same characteristics as a human heart signal. In addition, the dynamical behaviour of an energy harvester differs per input signal. Therefore, it is important to test energy harvesters on the operation signal, in this case human heart acceleration signals. A heart signal differs per person depending on, for instance, someone's age, sex and health. This means that multiple human heart input signals are needed. Ethical requirements make the measurement of these signals with the necessary details a challenge in itself. In order to meet this demand and to avoid this ethical issue, a heart signal generator is developed as a first step towards the testing of energy harvesters on an approximation of human heart signals. Three different sources of heart signals are combined in order to obtain a new source of heart signals, an approximation of reality, which can be used for the testing. Speckle Tracking Echocardiography signals, open-chest pig heart acceleration signals and human chest motion acceleration signals are analysed and their characteristics are used as the source for the heart signal generator. This heart signal generator is able to mimic multiple heartbeats and the influence of the heart rate on the amplitude and signal duration. The disadvantages of accelerometer measurements are compensated with the advantages of Speckle Tracking Echocardiography measurements, and vice versa, in order to obtain an accurate and detailed heart signal. The output of the heart signal generator is a one-dimensional acceleration signal. An energy harvester is tested on multiple generated heart signals for a heart rate range of 120-200 bpm. It was observed that the mean power output and the efficiency of the energy harvester differs per heart signal. This shows that testing on multiple heart signals is crucial in order to validate that enough power is generated for charging the battery. ... Read more

In the field of vibration energy harvesting, vibration energy is tranduced to electrical energy to power small, low powered devices. Many energy harvesters are produced in the form of a resonator, where resonant amplification enables efficient operation of the energy harvester. At low frequencies below 10 hz, input motions quickly increase for a constant input acceleration and resonant amplification results in energy harvesters that are too large to implement them. A solution is sought in creating a nonresonant energy harvester. The stiffness of a strongly coupled piezoelectric beam is compensated by adding negative stiffness to bring it to a near statically balanced state. This negative stiffness is embodied by attracting magnets. To model the dynamics and voltage output of the harvester, a modal analysis based distributed parameter model is used and further developed by including negative stiffness and force-displacement measurements of the stiffness compensated piezo. To investigate the mechanical behaviour of a compensated piezo, force-displacement measurements are carried out at different deformation speeds and load resistances. From these measurements, it has been observed that the stiffness of the compensated piezo strongly depends on the connected load resistance and the deformation speed. Furthermore, memory effects in piezoelectric hysteresis found in actuators such as curve alignment and wipeout have also been confirmed in force-displacement measurements. The performance of the harvester has been evaluated by exciting it on a linear air bearing stage. It has been found that for excitations between 2 and 6 hz, the error in RMS power between simulation and measurement remains below 10%. For its range of excitation, this harvester has been observed to be the most efficient with respect to prior art from literature. Therefore, stiffness compensation of a piezoelectric energy harvester can be considered as a successful method to improve the efficiency at low frequency excitation. ... Read more

Vibration energy harvesting is a promising step towards a more sustainable society. The world is getting more and more connected through electronic devices, which all require electrical energy. A battery can deliver electrical energy; however, such a battery needs to be replaced or recharged; this is where energy harvesters become interesting. Energy harvesters convert energy from ambient sources to electricity; this source can be, for example, solar or thermal energy but also vibrational energy. Extensive research has been done in vibrational energy harvesting so far. The subject of human motion energy harvesting is increasing interest. An energy harvester for human motion can be used to power health monitoring devices. However, there is one significant problem; human motions are dominantly low-frequency with high amplitude motions, while energy harvesters tend to work better on high frequencies. A limiting factor for successful experimental research is the equipment. The lack of sufficient stroke, controlled and low-frequency excitation impede research regarding human motion. In this research, a new test setup is developed for experimental research. A linear air-bearing stage is used to reproduce the human motions with an amplitude up to 500 mm. The air-bearing stage has an incremental encoder to ensure a precision of at least 20 µm. This stage may be used for many different testing situations ranging from vibration testing to impact testing. The stage can reproduce motions that were impossible to reproduce with a shaker, for example. The research is expanded with a nonlinear oscillator to assess its performance on large amplitude motions. A transducer can be attached to the oscillator to transduce the vibrational energy into electrical energy. By using an oscillator, the frequency is increased, causing a higher energy output at low frequencies. The dynamics of the nonlinear oscillator are numerically calculated. For which a new method is proposed to simulate the bouncing behavior in the spring numerically, called the bounce loss coefficient. The new method shows better results than the traditional model used to simulate the bouncing behavior (coefficient of restitution). The numerical model is experimentally verified on the newly developed testing setup. It was shown that using a new model for the bouncing behavior, the dynamical behavior of the nonlinear oscillator can be reproduced when excited at a large amplitude motion. ... Read more

Bistable vibration energy harvesters are an interesting alternative to their linear counterparts. They allow for large amplitude oscillations between their stable equilibria, from which much energy can be generated. However, the stable equilibria are separated by a potential energy barrier that has to be overcome. Therefore, we cannot guarantee these oscillations, and the performance advantage diminishes. As a solution to this, a novel method of stiffness compensation in compliant bistable mechanisms is explored to lower the potential barrier. This method makes use of interaction between the buckling modes. Whereas this phenomenon is most undesired in structures due to their increasing proneness to catastrophic failure, we cleverly use it to our advantage. During the deflection required for the large amplitude oscillations, a transition between these buckling modes occurs, causing the increase in potential energy. By bringing the corresponding buckling loads closer together, the transition is eased and the potential barrier is lowered. An analytical framework was set up as a fundamental test of this method. Using a discrete analytical model of a bistable buckled four-bar linkage with torsion springs, it was shown that the potential barrier can be flattened upon matching the first two critical buckling loads, resulting in static balancing. This was achieved by making two torsion springs three times stiffer with respect to the other two springs. To put theory into practice, three compliant mechanisms were designed using the ratio between the first two buckling loads. Their force-deflection characteristics were experimentally determined and it was shown that the stiffness may be tuned according to the ratio between the buckling loads. Furthermore, it was shown that in designs having the first two buckling loads equal to each other, near zero stiffness is achieved. Hence, this method is proven a successful addition to the arsenal of methods in stiffness compensation and static balancing of compliant mechanisms. ... Read more

Vibration energy harvesting can become a durable source of energy for wireless
sensors or other low power applications like pacemakers. Huge savings in ecological footprint, production and maintenance costs can be achieved by replacing batteries for vibration energy harvesters. Most of the time, newly developed energy harvesters are tested in a lab environment on an electrodynamic shaker. The problem is that the standard lab experiments in the form of a sinusoidal or Gaussian noise signal excitation are not representative for the real world applications. In a classification of ambient vibrations it was observed that most vibrations found in the real world consist of a series of dominant frequencies, shocks and noise. It was also seen that among real world vibrations, there is a lot of variation in the power distribution among the classes. In the aim to bring the vibration energy harvester performance tests closer to the real world applications, an experimental benchmarking of energy harvester performance has been conducted. An energy harvester is designed and applied in the real world on the engine of two different cars. Successively, three different lab experiments are performed on an electrodynamic shaker, each experiment with its own type of vibration control. It is found that only taking the FFT data of a real world vibration is not sufficient. Using a sinusoidal excitation matching a single amplitude and frequency, or even a noise excitation matching the entire power spectrum, results in an under or overestimation of 50% compared to the real world performance. Therefore, to accurately predict the performance of an energy harvester in the real world, simulation or experimental testing need to be performed on the actual or a replication of the intended real world vibration. ... Read more

Vibration energy harvesting is the solution for powering on-road sensor measurements. Various techniques to harvest the most energy from a certain application are found in literature. An electromagnetic energy harvester was found to be the best option for transport applications.
In this work, the potential benefits of a coupled oscillator electromagnetic vibration energy harvester compared to a single degree of freedom vibration energy harvester is explored. This comparison is made based on the steady-state power output when the harvester is excited at its eigenfrequencies. The harvester concepts are compared based on two cases: one where two frequencies are continuously present, and one where two frequencies are alternately present. These cases are derived from on-road container transport measurements.
A single degree of freedom and an array of two single degree of freedom harvesters are used as a benchmark. Three configurations of the coupled oscillator harvester concept are presented, which have been optimized with respect to the magnitude of the electromagnetic damping and the ratio between the two masses.
It was found that a coupled harvester with two electromagnetic dampers performs as good as an array of two single degree of freedom harvesters. When using the same proof mass for all concepts, a coupled oscillator harvester with only one electromagnetic damper generates less power than one with two dampers.
A prototype has been built to validate the simulations. Good correspondence between simulations and experiments was found, both in terms of output power and optimum electromagnetic damping. ... Read more

There is a great amount of wave power in earth's oceans. The amount of power harvested for electricity is however very small in comparison to solar and wind. One of the reasons for this is the lack of consensus on the best design of wave energy converters. This thesis develops a novel system design for a wave energy converter array that has promise and implements, evaluates and compares control for it. The most important benefits of this new design versus the most used type of devices are that it does not rely on a connection to the ocean floor for energy harvesting and it is space efficient, meaning that devices lie close together. Control has never been designed for the type of device in this study to the best of this author's knowledge. The wave energy converter array consists of floating pontoons that are connected to each other. The wave energy is harvested through power take-off mechanisms in these connections. The most important requirement on the system design is survivability, as the ocean is a harsh environment. The kinematics are thus designed in such a way that forces on the connections can be set by the damping and stiffness coefficients of the connections. The array is optimized for efficient energy harvesting by its design, while keeping cost effectiveness in mind when possible. Not only the array itself is optimized for efficiency, the control is optimized for this as well. This means that the control problem is to maximize energy capture. Reactive and Resistive control are implemented and compared. A distributed version of these algorithms is investigated as well and improves on computation time for large arrays. Reactive control can improve upon Resistive control up to three times in terms of energy capture, depending on the efficiency of the power take-off mechanism. The reason for this great improvement is that Reactive control makes it possible for the array to reach resonance with the waves. The array performs average in terms of energy capture in comparison to other wave energy converters when Resistive control is used. ... Read more