Study On Piezoelectric Energy Harvester Under Random And Low-frequency Excitation

The last two decades have witnessed several advances in microfabrication technologies and electronics,leading to the rapid development of low-power wireless sensor systems,which has been proposed as an integral part of a plethora of applications including environmental or military monitoring devices,biomedical implants and wearable instrumentation.Conventional battery is the main power source of such devices but it has some limitations including finite lifetime,bulky nature,periodic replacement,low energy density and environment pollutions.And hence conventional battery could not keep pace with the demands of these low-power wireless sensor systems.As a result,researchers are paying increasing attention to vibratory energy harvester,which could scavenge energy from ambient vibration,as a suitable alternative to provide a continuous power supply for low-power wireless sensor systems.Among vibratory energy harvesters,piezoelectric energy harvester(PEH)offers the advantage of higher power densities,high electromechanical coupling,no external source requirement,simple structure and ease of miniaturisation.Hence,it has been the most popular research focus of multiple groups in recent years.Cantilevered piezoelectric energy harvester operates on the basic principle of linear resonance and it usually has narrow bandwidth and high natural frequency.Besides,its output power reaches maximum when the natural frequency matches with the excitation frequency.Thus,this configuration has strong dependence on the source excitation.However in the majority of practical cases,it has been indicated that vibration in the environment is totally random and has rather low frequency.Consequently,on the one hand,in order to lower the natural frequency,improve the output performance and enhance the environmental adaptability,this thesis presents three new energy harvesting structures respectively by optimizing piezoelectric vibratory beam structure,combining nonlinearity and hybrid harvesting mechanisms,and introducing frequency up-conversion technology.On the other hand,in order to provide accurate prediction of energy harvesting vibration characteristics in the random and low-frequency environment,based on White Gaussian noise(which accords with general excitation),colored noise(which is consistent with characteristics of specific narrow band excitation),and random distribution of rotation excitation in the practical environment respectively,for these three new configurations,this thesis obtains innovative achievements from random vibration theory.Through theoretical analysis,numerical simulation and experimental verification,responses of these three structures under random and low-frequency excitation are obtained,which provides a basis for improving the output performance of PEH in the practical excitation.The main contributions of random vibration theory in this thesis present as follows:(1)A doubly-clamped PEH is designed.By optimizing piezoelectric vibratory beam structure,the output performance has been improved.Using the concept of equivalent resistance and based on Fokker-Planck equation,this thesis proposes a new theoretical analysis to obtain output characteristics of white Gaussian noise excited doubly-clamped PEH.Monte Carlo simulation and experimental results exhibit qualitative agreement with Fokker-Planck theory,showing that optimizing PZT length could improve output characteristics of random excited doubly-clamped piezoelectric energy harvester considerably.Besides,it also demonstrates that changing vibratory beam shape will not only lower natural frequency but increase power output effectively.Moreover,mean output power increases linearly with acceleration's spectral density and load resistance could affect output performances for doubly-clamped PEH under stochastic excitation.(2)Combining nonlinearity and hybrid harvesting mechanisms,a nonlinear hybrid piezoelectric(PE)and electromagnetic(EM)energy harvester is proposed.Compared with their linear counterparts,nonlinear hybrid energy harvesters have smaller natural frequencies and better output performances,which are beneficial in terms of capturing energy from low-frequency and random ambient environment.Using the concept of equivalent resistance,this thesis gives theoretical modeling of nonlinear hybrid energy harvester under these two stochastic excitations respectively.Stochastic nonlinear vibration theory for single mechanism energy harvester is expanded and applied to nonlinear hybrid energy harvester.Based on Fokker-Planck equation and Fokker-Planck approximate equation respectively,analytical expressions of mean output power for nonlinear hybrid piezoelectric and electromagnetic energy harvester subjected to white Gaussian excitation and colored excitation are presented.Along with Monte Carlo(MC)simulation and experimental analysis,it proves that PE and EM load resistances could be optimized for the designed nonlinear hybrid PE and EM energy harvester and total mean output power increases linearly with the increase of acceleration's spectral density.Especially when this structure excited by colored noise,output responses also involve with center frequency and bandwidth of colored noise.It is found that the improvement in output power is mostly pronounced for a wider bandwidth.Moreover,within different bandwidth ranges,optimal ratios of the excitation's center frequency to the system's natural frequency for maximizing output are different.(3)Aiming at the rotation excitation in the practical environment and based on frequency up-conversion,a non-contact rotational frequency up-convertion PEH is presented.It could obtain high output power from random and low-frequency rotation excitation.Considering random distribution characteristics of rotation excitation in the practical environment,taking rotation excitation driven by random wind load as an example,theoretical model of non-contact rotating frequency up-convertion PEH under rotation excitation is established.Expressions of mean output power of the rotational up-converted PEH driven by random wind load are presented as well.Partially validated by experiment and fully verified by simulation,it indicates that the designed energy harvester could obtain high mean output power and energy efficiency under random and low-frequency rotation excitation in the practical environment.The distance between magnets and load resistance could have an effect on the output performance.Moreover,it demonstrates that when the rotational frequency driven by wind load ranges from 1 Hz to 10 Hz,the mean output voltage and power are both linearly increase with the average wind speed.