The Study On Harvesting Vibration Energy Based On Multi-stable Structure

The natural environment owns enormous energy,which can be converted into electricity to meet the power supply demand of various micro-electronic devices and replace traditional chemical batteries.This provides a new way to solve the power problem of wireless sensor network nodes.In recent years,the piezoelectric energy harvester has become a research hotspot due to its simple structure,high flexibility,no electromagnetic interference,easy integration,miniaturization and many other advantages.Most piezoelectric energy structure adopt linear design.Therefore,it reaches the high power transform efficiency only when the excitation frequency of external vibration source is near the natural frequency of the structure.However,the energy in environment usually is distributed over a wide spectrum or even in the random form,which will significantly decrease the efficiency of the linear piezoelectric energy harvester.The proposed bistable and tristable systems have been explored to open up a new way to improve the energy harvesting performance.Because they not only have large inter-well motion,but also have a wide response bandwidth at low frequency.The bistable and tristable systems are modeled so as to analyze their potential energy functions for exploring the dynamical characteristics.As a result,it may play some guiding roles in the design and application of the piezoelectric energy harvester.The works of this thesis are organized as follows:1.The piezoelectric energy harvester with a tip mass is proposed to scavenge vibration energy of bridge caused by moving vehicle,which could be simplified as a wheel-springdamper-body model.The vibration differential equation of the bridge under moving load is derived by the theory of plate-shell vibration.The piezoelectric and mechanical coupling equations excited by bridge vibration is derived by Euler-Bernoulli beam theory and Kirchhoff’s first law.The characteristics of piezoelectric energy harvester are analyzed for different parameters.The results show that the maximum deflection of the beam is closely related to the vehicle speed when the vehicle passes the bridge.Moreover,there exists a critical speed in which the bridge have the maximum deflection and the piezoelectric energy harvester has a maximum voltage output.Furthermore,the output can also be optimized with respect to power generation efficiency by adjusting the mass of the end of piezoelectric cantilever beam and the thickness of substrate.2.The static bifurcation and stability of the bistable piezoelectric energy-harvesting system are studied so as to investigate the bistable characteristics.The influences of external excitation parameters on response characteristics and output power are analyzed by the Harmonic Balance Method.The necessary condition for the occurrence of large-amplitude inter-well motion is obtained.The results show that the excitation amplitude has a great influence on intra-well and inter-well responses,and both of them have two energy states,the high and the low ones.When the excitation amplitude exceeds a critical value,the system keeps the high energy state and produced a high energy harvesting efficiency.3.The kinetic characteristics of tristable and bistable energy harvesting systems under harmonic excitation are studied.The motion equations of the systems are established by the energy method to study the response characteristics of system.The results found that the tristable system under harmonic excitation has more advantages than bistable one.The tristable scenario not only has a lower threshold for inter-well jumping,but also has frequent jumping phenomena.4.The dynamic responses and power generation characteristics of bistable and tristable systems under random excitation are studied.The tristable configuration can exhibit tristable characteristics by adjusting the space between the magnet at the end of cantilever beam and the fixed one.From the numerical simulation and experimental research,for a relatively weak external excitation,the bistable system can only oscillated around one of two stable equilibrium points.In contrast,the tristable system can jump between three equilibrium points at this excitation level.When the external excitation intensity is fairly large,the tristable system can realize more frequently jumping between potential wells and give a higher voltage output than the bistable one.Therefore,the tristable system has more advantages in energy harvesting under random excitation.5.A Y-shaped bistable piezoelectric energy harvester with two curved wings is presented to scavenge wind energy.The experimental results show that it can take frequently inter-well jumping over a wide range of wind speed,thereby generating a high-voltage output.From the spectra of the linear and bistable systems,it is evident that the spectrum of bistable system has a dominant nearly periodic component in the low frequency region,suggesting that it is an effective way to imporve the harvesting performance by introducing bistability.6.The Melnikov method is used to investigate the chaotic response of a nonlinear piezoelectric bistable energy-harvesting system under the combination of bounded noise and harmonic excitation.The occurrence conditions for chaotic motion of the nonlinear harvesting system are given theoretically and is verified by simulation.The optimal impedance could be obtained to improve the energy harvesting performance.It is found that noise can improve the probability of escaping from potential wells,which is beneficial for improving the energy harvest efficiency.In summary,the dynamical characteristics and mechanism for the power generation of multi-stable structure are comprehensively analyzed,which could play some guiding roles in the design and optimization of nonlinear piezoelectric energy harvester.