Research On The Electromechanical Conversion Characteristic Of Piezoelectric Composite Beam And Multimode Wide Band Power Generation

Piezoelectric materials have widespread application prospects in smart structures,due to coupling effect in between mechanical deformation and electric field(or electromechanical coupling for short),are widely used to make functional components such as sensing elements,actuation elements and energy harvesting elements.Since vibrations are everywhere in our environment,getting the electrical energy by the conversion the vibration energy into electrical energy is considered to be a very important step in order to achieve an autonomous wireless nodes.Analytical expressions of the electric power generation performance were established for the piezoelectric composite cantileverd beams excited by harmonic excitation in this dissertation.The analysis methods of the equivalent model were given.Accurate analytical solutions to the piezoelectric composite beams considering the secondary piezoelectric effect were presented first in this dissertation,including output charges,output voltages and electric impedances,etc.Those provide the effective theory basis for the design of the piezoelectric vibtation generation devices.A piezoelectric vibration generator prototype with multi-frequencies vibrations response is proposed.The piezoelectric vibration energy harvester uses the former two order modes of a folded structure to convert the vibration energy into electric energy.Through the finite element computal simultation the optimization design of the prototype is performed.Fabrication method is put forward.The prototype successfully achieved the broadband vibrations energy conversion in tests.The main research contents and innovations of this dissertation are as follows:(1)We derived the dynamics analytical relations between excitation and response of the piezoelectric composite cantilever beam using procedures similar to those that Jan G.Smits et al.used.The varying exciting harmonical parameters include an external tip force,external moment,and an applied voltage to their response parameters;these response parameters include the generated tip deflection,tip angular slope,and the electric charge.The constitutive relation for piezoelectric layers considering the impact of the secondary piezoelectric effect on the electric field and the stress field within the piezoelectric material layer is given.The bending rigidity of the piezoelectric composited beam(PCB)is obtained.It is to make necessary adjustments to the bending rigidity obtained by the transformed cross section method of the composite beam.Then for their extraordinarily large piezoelectric coefficients novel materials the dynamic properties of PCB are obtained and the dynamic electrical admittance equation and resonance frequency equation of PCB as a function of frequency are presented.The results are verified.Larger piezoelectric coefficients and electromechanical coupling coefficients of piezoelectric materials are the greater impact the secondary piezoelectric effect has on the electric field and the stress field within the piezoelectric material layer.(2)The dynamic analytical expressions in the circular frequency ω domain for the piezoelectric cantilever composite beams were derivated.The relationships between the external excitation and response in the mathematical model are presented in a matrix.These characteristics relate the harmonically varying excitation parameters,namely,an external tip force,external moment,and applied harmonic voltage,to their response parameters,specifically the generated tip deflection,tip angular slope,and electric charge.The frequency responses functions drawn of PCBs can also be used for better analysis of the behavior of PCTBs with the piezoelectric materials being electrically in series connection or in parallel as a function of frequency.The finite element analyses(FEA)computational model using the commercial finite element program ANSYS is established.Theoretical and FEA analytical results of the PCB are good in agreement with each other.These analytical solutions are conducive to reveal the physical nature of electromechanical conversion characteristics and are as validation criteria to the numerical solution.(3)A novel piezoelectric vibration generation devices based on the folded structure is proposed.The proposed piezoelectric vibrator with a foled structure that serves as a vibration energy harvester was developed to work not only in the first resonant mode but also in the second.Two kinds of the procedures and methods for the multi-frequencies vibration energy conversion are given.One is array of multiple piezoelectric cantilever beams,and the other is vibration energy harvesting device based on a folded piezoelectric structure.Those devices are designed for overcoming the main shortcoming of such typical single-mode resonant piezoelectric harvesters with their narrow effective frequency band.After all,while the frequency of the vibration source deviated from the nature frequency of the energy harvesters,piezoelectric power generation device cannot effectively complish vibration energy conversion.The finite element analyses(FEA)computational model for the proposed piezoelectric energy harvesting device is builded.With the optimization of the finite element simulation,the dimensions of the folded vibrator were carefully selected based on modal analysis.The features of the former two order modes are analyzed.Meanwhile,the distributions of the relative stress in the upper piezoelectric layer attached to the middle slender beam were explored through finite element simulated modal analysis.Processing and manufacture technics for the piezoelectric folded vibrator were given,and a prototype energy harvester is also fabricated.Experimental study is made to test the generation property and electric power output.When the energy harvester was driven with a vibration of 0.7 g peak acceleration,the output power achieved two power peaks:6.64 mW at 120.9 Hz and 0.43 mW at 97 Hz.The output power remained above 20 μW within the operation frequency band that ranged from 88 Hz to 177 Hz.The normalized frequency bandwidth(NFB)was roughly 74%.The results show that the proposed piezoelectric generation devices can hopefully develop the winning generation devices and they can convert the vibration energy into electric energy from the multi-frequencies vibrations and improve overall efficiency of vibration energy conversion of the devices.The analysis theory and structural design strategies discussed in this dissertation will pormot the development of the vibration power generation devices using piezoelectric elements and have significance both in academiclly and practically.