Study On Broad Band Energy Harvesting With Additional Rolling Balls

The piezoelectric energy harvester is a device that converts the energy of vibration in the surrounding environment,into electricity.With the development of micro-and nano-electromechanical systems,the energy harvester has become one of the alternatives to traditional replacement batteries.The piezoelectric energy harvester based on cantilever has simple structure and is easy to design and process.Moreover,the whole structure has a low natural frequency and is easy to resonate with the vibration of the surrounding environment,which has become the focus of research.Adding a proof mass to the cantilever end can enhance the energy capture efficiency.However,the energy harvester can only work around its natural frequency,and is difficult to adapt to the capture device under various external excitation frequencies.Therefore,the design of a structure that can adjust its natural frequency and broaden the bandwidth of the energy harvester is a great significance for the piezoelectric energy harvester to work at complex excitation frequencies.In order to widen the working frequency range of the energy harvester,two new types of additional rolling sphere piezoelectric energy harvesters are proposed.One is to use the tunable function brought by the change of the position of the rolling sphere,the other is to use the impact force brought by the rolling sphere to widen the bandwidth of the energy harvester.The main research contents are as follows:(1)Based on the traditional cantilever plate model,a cantilever plate structure is designed,with two boxes added at two free edges along the fixed-free direction and one rolling ball placed in each boxes respectly.The natural frequency of the structure can be adjusted by changing the position of the rolling balls in the boxes,and the range of adjustment which contains from the maximum(upper limit)to minimum(lower limit)frequency,is defined as the bandwidth.The mode function of tunable cantilever plate is given according to deflection curve.Considering the rigid body motion of boxes and the composite motion of the balls,the Hamilton principle and the Hertz contact theory are taken into account to derive the vibration equations of the system.The experiment is carried out to verify the correctness of the theoretical mode and the dynamic equations.The effects of structural parameters,such as the thickness of plate,the mass of balls,the length and the material of the boxes on natural frequency and bandwidth are analyzed.The responses of the structure under different excitation frequencies and the different initial position of the balls in the boxes are calculated by using MATLAB software.(2)Based on the structure of tunable cantilever plate with additional rolling balls proposed in this paper,a broad band piezoelectric energy harvester is designed.A layer of piezoelectric material is laid on the surface of the plate structure,and the upper and lower poles of the piezoelectric layer are drawn out and connected to the external circuit.The dynamic equation and electrical equation of the energy harvester are solved by using MATLAB software.The output voltage of the energy harvester under different external excitation frequencies is calculated.The response of device under different external excitation accelerations is given.The effects of different mass of ball and length of box on the bandwidth of the energy harvester are studied.The variation of power with external resistance load under different conditions is analyzed.(3)Based on the double cantilever beams model,a steel ball is set between two piezoelectric composite beams,and a track is designed to make the ball move only between the two beams.When the ball and the double beams collide in contact,a greater contact force will be generated between them,which can increase the displacement of the beam and improve the efficiency of energy harvester.Based on the classical plate theory,Hertz contact theory and Hamilton principle,the dynamic equation of piezoelectric energy harvester with impact double beam structure is established.The dynamic response of the system is calculated by using MATLAB software,and the amplitude-frequency response curve of the system is solved.The response of the system under resonant and non-resonant conditions is given.The feasibility of double beam impact structure in low frequency operation is analyzed.