Nonlinear Frequency Broadening Approaches For The Vibration-based Energy Harvesters

Energy harvesting technology converts the renewable energy in the environment such as solar energy,ocean energy,and vibration energy,etc.,into electrical energy to power miniature low-power electronic equipment,which can be widely used in military,transportation,life,medical and other fields.Among them,vibration energy stands out due to its advantages of wide distribution and easy access,and vibration energy harvesting technology has also received extensive attention.However,currently the resonance frequency of the vibration energy harvester is generally high,and the output performance of the vibration energy harvester is the best only when the resonance frequency is consistent with the external excitation frequency.In general,the vibration frequency of the environmental vibration source is low,the frequency variation range is wide and random.Therefore,it is required that the vibration energy harvester has a low operating frequency and a wide operating bandwidth.Aiming to overcome the problems of high operating frequency and narrow bandwidth of vibration energy harvesters,this paper conducted the research on nonlinear frequency broadening approaches.The theoretical models of the impact-based and bistable energy harvesters were established,and the influence of the structural parameters on the output performance of the harvesters was analyzed systematically.Two types of nonlinear vibration energy harvesters suitable for low-frequency vibration environment were designed,which were piezoelectric energy harvester(PEH)based on impact frequency broadening approach and piezoelectric-electromagnetic hybrid energy harvester(PE-EM HEH)based on bistable frequency broadening approach.The piezoelectric beam of the impact-based PEH was fabricated using MEMS technology to achieve miniaturization of the device.The collision between the low-frequency flexible beam and the high-frequency piezoelectric beam was used to widen the operating bandwidth to 12.8Hz.The bistable PE-EM HEH combined piezoelectric and electromagnetic energy conversion mechanisms to increase the output power of the harvester to 95.1mW,and achieved an ultra-wide operating bandwidth of 24.2Hz through bistable vibration.The main research contents of this paper are as follows:(1)The theoretical model of nonlinear vibration system was established,and the influence of system parameters on the frequency response characteristics of nonlinear vibration was analyzed.The feasibility of nonlinear frequency broadening approaches was demonstrated.Then the bandwidth broadening principle of the impact-based PEH was analyzed,and the output theoretical model of the piezoelectric beam and the concentrated parameter equivalent model of the PEH were established.Finally,the theoretical model of piezoelectric and electromagnetic energy conversion mechanism of the PE-EM HEH and the dynamic model of the bistable piezoelectric beam were established,and the vibration characteristics of the bistable piezoelectric beam were analyzed.(2)A PEH based on the impact frequency broadening approach was designed.The main structure included a low-frequency flexible beam and a high-frequency piezoelectric beam.The piezoelectric beam was fabricated using MEMS piezoelectric thick film technology.According to the dynamic model of the impact-based system,the structural parameters such as the mass and resonance frequency of the flexible beam,the distance between the piezoelectric beam and the flexible beam were optimized by Simulink simulation.The experimental results show that the impact-based PEH can work in the low frequency excitation between 20?30Hz.Under a sinusoidal excitation with an acceleration of 1g,the maximum output power of the impact-based PEH at a distance of 4mm is 8.6?W,which is 78 times that of the non-impact piezoelectric beam.When the distance is 1mm,the maximum bandwidth of the PEH is 12.8Hz.It is verified that the impact-based frequency up-conversion mechanism has the function of improving output power and broadening frequency bandwidth.(3)A PE-EM HEH based on the bistable frequency broadening approach was designed.By introducing a high-permeability iron core,the bistable vibration of the piezoelectric beam was realized,and the induced voltage of the coil was increased.The magnetic flux density distribution between the magnet and the iron core was analyzed by COMSOL simulation,and a nonlinear magnetic force function was fitted.Simulink simulations were then performed based on the established theoretical models of piezoelectric and electromagnetic energy conversion mechanisms and the dynamic models of bistable piezoelectric beams.Parameters such as the thickness of the copper substrate of the piezoelectric beam,the distance between the magnet and the core,the size of the coil were optimized by simulation.Then the output performance of the optimized bistable PE-EM HEH was tested experimentally.The output power reaches 95.1mW when the acceleration is 1.5g.And the harvester is able to maintain high output performance in an ultra-wide frequency range of 18.8?43Hz,and the operating bandwidth reaches 24.2Hz,which indicates that the bistable PE-EM HEH has the characteristics of high output power and wide operating bandwidth.