Effects of non-linear stiffness on performance of an energy harvesting device

Vibration-based energy harvesting devices have received much attention over the past few years due to the need to power wireless devices in remote or hostile environments. To date, resonant linear generators have been the most common type of generators used in harvesting energy for such devices. Simple tuning and modelling methods make it a more favourable solution theoretically if not practically. This thesis considers the limitations of resonant linear devices and investigates two non-linear generators to see if they can outperform the linear devices in certain situations. So far, in most of the literature, the energy harvester is assumed to be very small dynamically compared to the source so the source is not affected by the presence of the device. This thesis considers how the dynamics of the source is affected by the device if its impedance is significant compared to the source. A tuning condition for maximum power transfer from the source to the device is derived. This tuning condition converges to the one presented in most of the literature when the impedance of the device is assumed to be very small compared to that of the source i.e. tuned so that the natural frequency of the device equals the excitation frequency. For the case when the impedance of the device has a negligible effect on the source, the performance of the device is only limited to a narrow frequency band and drops off rapidly if mistuned. To accommodate the mistuning limitations, new types of generators are proposed mainly by using a non-linear mechanism.These mechanisms are made up of a non-linear spring connected together with a mass and a linear viscous damper i.e. the energy harvesting component. The analysis of the fundamental performance limit of any non-linear device compared to that of a tuned linear device is carried out using the principal of conservation of energy. The analysis reveals that the performance of a non-linear device in terms of the power harvested is at most 4x greater than that of a tuned linear system and is strongly dependent upon the type of the non-linearity used. Two types of non-linear mechanisms are studied in this thesis. The first one is a non-linear bi-stable mechanism termed a snap-through mechanism which rapidly moves the mass between two stable states. The aim is to steepen the displacement response curve as a function of time which results in the increase of velocity for a given excitation, thus increasing the amount of power harvested. This study reveals that the performance of the mechanism is better than a linear system when the natural frequency of the system is much higher than the excitation frequency. The study also shows that the power harvested by this mechanism rolls off at a slower rate compared to that of the linear system. Another non-linear mechanism described in this thesis uses a hardening-type spring. The aim of this mechanism is to provide a wider bandwidth over which the power can be harvested. This thesis presents numerical solutions and approximate analytical solutions for the bandwidth and effective viscous damping of a non-linear device employing a hardening-type stiffness. Unlike the linear system, in which the bandwidth is only dependent on the damping ratio, it is found that the bandwidth of the nonlinear device depends on both the strength of the nonlinearity and the damping ratio. Experimental results are presented to validate the theoretical results. This thesis also investigates the benefits of the non-linear device for a low frequency and high amplitude application using the measured vibration inputs from human motion such as walking and running. The effect of harmonics on the power harvested is also studied. Numerical simulations are carried out using measured input vibrations from human motion to study the best placement of the natural frequency of the device across the range of harmonics

Theoretical Model Of Absorption Coefficient Of An Inhomogeneous MPP Absorber With Multi-Cavity Depths

Micro-perforated panel (MPP) absorber has been known as an alternative absorber to classical porous material given its facile installation, long durability, environmental friendliness and attractive appearance. Extensive studies of MPP absorber proposing the improvement of its absorption frequency bandwidth have been published. This study presents a MPP absorber introduced with inhomogeneous perforations and with multi-cavity depths. The MPP is divided into two sub-area, where each area has different hole diameter, perforation ratio and a separated backed cavity depth. The acoustic impedance is modelled using electrical equivalent circuit and the absorption coefficient is calculated under normal-incidence of sound. It is found that the inhomogeneous MPP can have good bandwidth of absorption by designing the sub-MPP of smaller perforation ratio with large hole diameter and the one having the larger perforation ratio with smaller hole diameter. The absorption bandwidth can be conveniently controlled by adjusting the cavity depth of each sub-MPP. The results from the experimental work show good agreement with the theoretical model

CHARACTERIZATION OF A STRUCTURE- BORNE SOURCE USING THE RECEPTION PLATE METHOD

This paper presents the characterization of vibration strength obtained from reception plate method by applying the mobility concepts. It describes a laboratorybased measurement procedure, which determines the strength of a vibration source in terms of the total squared free velocity of the source. The source used in the experiment is the small electric fan motor installed on high mobility aluminum panel in order to neglect the influence of the source mobility. The complexity of the mobility at the contact points are reduced using the single value of effective mobility. The aim is to validate the data obtained from the reception plate method with one from the direct measurement. Thus, this research is expected to give a simple but accurate way to determine input power from a structure-borne sound source

A Combined Softening and Hardening Mechanism for Low Frequency Human Motion Energy Harvesting Application

This paper concerns the mechanism for harvesting energy from human body motion. The vibration signal from human body motion during walking and jogging was first measured using 3-axes vibration recorder placed at various places on the human body. The measured signal was then processed using Fourier series to investigate its frequency content. A mechanism was proposed to harvest the energy from the low frequency-low amplitude human motion. This mechanism consists of the combined nonlinear hardening and softening mechanism which was aimed at widening the bandwidth as well as amplifying the low human motion frequency. This was realized by using a translation-to-rotary mechanism which converts the translation motion of the human motion into the rotational motion. The nonlinearity in the system was realized by introducing a winding spring stiffness and the magnetic stiffness. Quasi-static and dynamic measurement were conducted to investigate the performance of the mechanism. The results show that, with the right degree of nonlinearity, the two modes can be combined together to produce a wide at response. For the frequency amplification, the mechanism manages to increase the frequency by around 8 times in terms of rotational speed

Wideband Sound Absorption Of A Double-Layer Microperforated Panel With Inhomogeneous Perforation

Micro-perforated panel (MPP) absorber is increasingly gaining popularity as an alternative sound absorber in buildings compared to the well-known synthetic porous materials. A single MPP has a typical feature of a Helmholtz resonator with a high amplitude of absorption but a narrow absorption frequency bandwidth. To improve the bandwidth, a single MPP can be cascaded with another single MPP to form a double-layer MPP. This paper proposes the introduction of inhomogeneous perforation in the double-layer MPP system (DL-iMPP) to enhance the absorption bandwidth of a double-layer MPP. Mathematical models are proposed using the equivalent electrical circuit model and are validated with experiments with good agreement. It is revealed that the DL-iMPP produces a wider half-absorption bandwidth compared to the conventional homogeneous double-layer MPP. The absorption bandwidth to higher frequencies can be effectively controlled by reducing the air cavity between the iMPPs and to the lower frequencies by increasing the back cavity depth at the second layer

Normal incidence of sound transmission loss of a double-leaf partition inserted with a micro-perforated panel

A double-leaf partition in engineering structures has been widely applied for its advantages i.e. in terms of its mechanical strength as well as its lightweight property. In noise control, the double-leaf also serves to be an effective noise barrier. Unfortunately at low frequency, the sound transmission loss reduces significantly due to the coupling between the panels and the air between them. This paper studies the effect of a micro-perforated panel (MPP) inserted inside a double-leaf partition on the sound transmission loss performance of the system. The MPP insertion is proposed to provide a hygienic double-leaf noise insulator replacing the classical abrasive porous materials between the panels. It is found that the transmission loss improves at the troublesome mass-air-mass resonant frequency if the MPP is located closer to the solid panel. The mathematical model is derived for normal incidence of acoustic loading

Characterisation of Structure-Borne Sound Source Using Reception Plate Method

A laboratory-based experiment procedure of reception platemethod for structure-borne sound source characterisation is reported in this paper. he method uses the assumption that the input power from the source installed on the plate is equal to the power dissipated by the plate. In this experiment, rectangular plates having high and low mobility relative to that of the source were used as the reception plates and a small electric fan motor was acting as the structure-borne source. he data representing the source characteristics, namely, the free velocity and the sourcemobility, were obtained and compared with those fromdirectmeasurement. Assumptions and constraints employing this method are discussed

Normal incidence of sound transmission loss of a double-leaf partition inserted with a micro-perforated panel

A double-leaf partition in engineering structures has been widely applied for its advantages i.e. in terms of its mechanical strength as well as its lightweight property. In noise control, the double-leaf also serves to be an effective noise barrier. Unfortunately at low frequency, the sound transmission loss reduces significantly due to the coupling between the panels and the air between them. This paper studies the effect of a micro-perforated panel (MPP) inserted inside a double-leaf partition on the sound transmission loss performance of the system. The MPP insertion is proposed to provide a hygienic double-leaf noise insulator replacing the classical abrasive porous materials between the panels. It is found that the transmission loss improves at the troublesome mass-air-mass resonant frequency if the MPP is located closer to the solid panel. The mathematical model is derived for normal incidence of acoustic loading

Parameter optimization of nonlinear piezoelectric energy harvesting system for IoT applications

The vibrational energy harvesting has been essentially applied to power up low-power electronics, microsystems, and wireless sensors especially in the areas of Internet of Things (IoT) devices. This paper investigates the prospect of incorporating nonlinearity in a unimorph piezoelectric cantilever beam with a tip magnet placed under a harmonic base excitation in IoT enabled environment. An empirical and theoretical analysis on the impact of various parameters such as spacing distance between magnets, presence of magnetic tip mass and positioning of vibrational source on the frequency response output was performed. It was observed that the largest spectrum of frequency can be produced when at the lowest resonant frequency of the cantilever. The positioning of vibrational source deeply impacts the hysteresis region and frequency range in realizing broadband energy harvesting. The inclusiveness of vibration source on both the cantilever beam as well as the external magnets impacts the energy harvester in terms of frequency range and the minimal distance for bistable condition

Experimental Characterization And Performance Of Dynamic Vibration Absorber With Tunable Piecewise-Linear Stiffness

Dynamic vibration absorber (DVA) is one of the ways to control the level of vibration. Among many mechanisms and control strategies of the DVA, passively tuned DVA emerges as one of the most efective absorbers due to its simple mechanism. However, the performance of the passive tuned DVA sufers from narrow suppression bandwidth. This paper introduces a nonlinear dynamic vibration absorber (NDVA) with an adjustable piecewise-linear stifness mechanism which has a characteristic almost similar to hardening stifness to broaden the vibration suppression bandwidth. The mechanism is made of a cantilever beam constrained by two vertically and horizontally adjustable limit blocks on either side of the beam’s equilibrium position. The static and dynamic properties of the proposed NDVA were frst investigated for diferent positions of the limit blocks. Then the performance of the NDVA was compared with the linear DVA in terms of the vibration suppression bandwidth, separation of resonance frequencies and the ability to cope with mistuning. Overall, the NDVA seems to outperform the linear DVA, and it is more forgiving in the case of mistuning