Study On Nonlinear Widening Frequency Band Of Electromagnetic Vibration Energy Harvester

With the rapid development of microelectronics technology and lower power consumption of electronic devices,energy harvesting technology could be widely applied in wireless sensors,implanted medical devices and wearable devices.Vibration energy harvesters convert ambient kinetic energy into electrical energy mainly based on the transduction mechanisms of piezoelectric,electrostatic and electromagnetic.According to Faraday's law of induction,the electromotive force is produced due to the relative motion between the magnet and coils.Comparing with piezoelectric and electrostatic energy harvester,the obvious advantage of electromagnetic energy harvester is the relatively low output impedance and high output current.The main issues of vibration energy harvesting are limited working bandwidth and low electromechanical conversion efficiency.Traditional vibrational energy harvesters are designed as linear resonators to achieve optimal performance by matching their resonance frequencies with the ambient excitation frequencies,which have obvious disadvantage of narrow working frequency bandwidth.If the excitation frequency of the ambient vibration shifts slightly,the output power of the energy harvester would drastically drop.Thus,a number of approaches have been utilized to address this limitation,including resonant frequency tuning,multimodal energy harvesting and nonlinear behavior.Nonlinearity,which has attracted much attention,leads frequency response curves to bend thus to widen the response bandwidth of energy harvesters.This dissertation mainly focuses on nonlinearity design for electromagnetic energy harvesting.Two nonlinear electromagnetic energy harvesting methods have been proposed in order to broaden working bandwidth.The main work and the conclusion of this dissertation are in the following:1.A stretching-based nonlinear electromagnetic energy harvesting method has been proposed in order to broaden the frequency bandwidth.Static and dynamic model have been simulated and validated.Prototype has been fabricated by integrated manufacturing technology and characterized.Combining the merits both of the guided-beam structure and mechanical properties,stretching in addition to bending occurs under large deformation and leads to strong hardening nonlinearity.The stretching-based nonlinear method has advantage of small volume and suitable for integrated process based on MEMS technology.Test results show that the energy harvester has good repeatability without any destruction under large deformation condition.As the excitation acceleration increases,the amplitude increases and the peak frequency drifts upward,with which the working bandwidth of the energy harvester broadens.At the acceleration of 0.5g,very large bandwidths of 129 Hz and 59 Hz are obtained for displacement and output voltage.Power output of 3.4 ?W and normalized power density of 125.92 ?W cm-3 g-2 are achieved with the load resistance of 38 ohm.2.Based on stretching-nonlinearity,a novel independent nonlinear stiffness tuning method through sophisticated geometrical design has been proposed for large range control of peak frequency and working bandwidth without influence on linear resonant frequency.By carefully controlling the thickness to length ratio of the planar spring beams,the nonlinear stiffness coefficient can be intentionally and precisely adjusted with the thickness,while the linear stiffness coefficient can be kept unchanged.Static and dynamic simulation were conducted to validate our method and experimental prototypes for validation have been fabricated.At the acceleration of 0.5g,the energy harvesting device is able to tune the operating bandwidth of the output voltage from 37 Hz to as wide as 75 Hz.This design can easily be realized by tuning the length and thickness of the spring beams without changing the volume of the whole energy harvester device,which is compatible with MEMS batch fabrication process.3.A magnetic force nonlinear electromagnetic energy harvesting method has been proposed based on strong magnetic force coupling between magnets and soft material iron,which introduces nonlinearity and leads to broadened operation bandwidth.The force between magnets and soft material introduces either monostable(single potential well)or bistable(double potential well)nonlinearity into the system,which can be controlled by tuning the gap between the permanent magnets and soft magnetic material.With the gap of 1.5 mm between magnets and soft material,the harvester prototype exhibits monostable behavior.Wide working bandwidth of 7.3 Hz or 11.28% of peak frequency is obtained in the case of up frequency sweep at an acceleration of 5 m/s2.With the gap reducing to 0.5 mm,the harvester prototype exhibits bistable behavior.Under up frequency sweep at an acceleration of 10 m/s2,large amplitude interwell oscillation occurs and wider working bandwidth of 15.5 Hz or 21.5% of peak frequency is obtained.Compared with open magnetic circuit,closed magnetic circuit can effectively improve the output voltage and power.With the gap of 0.5 mm,maximal output voltage of energy harvester with closed magnetic circuit could be 118 m V,which is 2.6 times that of 45 m V with open magnetic circuit condition;the maximal load power of 8.25 ?W is obtained,which is 9.8 times larger than that of 0.84 ?W with open magnetic circuit condition.