Research On The Hybrid Piezoelectric Electromagnetic Broadband Generator With Magnetic Force Tuning

In recent years, with the miniaturization and integrated development of wireless sensor networks nodes,MEMS sensor and portable intelligent devices,the wireless sensor nodes show a lot of advantages such as quantity and wide distribution,working in various environment, however it is difficult for maintenance. One of the reasons is the replacement of batteries that is inconvenient and enven impossible.The self-powered system will expect to breakthrough this power supplying obstacle.Recently, energy harvester is being widely researched to replace the conventional chemical batteries, which works stably and last for a long time. The compact structures can be easily integrated, environmental protection without pollution.This paper explains the working mechanism about the various types of environmental energy harvesting technology. Multi-form energy harvesting devices are introduced at domestic and abroad. The random environmental vibration shows the characteristics that the frequency is low,the amplitude is small and vibrating in several directions. Now the environment vibration energy harvesting devices mainly showed as single natural frequency model, so the higher output power only produce in the resonant state which is influenced by exciting frequency. The main factors include the stiffness of cantilever beam and the effective vibration mass of system vibration especial the picking sensitive structure. The research objective is concentrated to extend frequency band and match the random environmental vibration frequency.Firstly,a mathematic model is established by numerical method to analyse the hybrid piezoelectric-electromagnetic broadband power generation system with magnetic force tuning. The cantilever stiffness will automatically adjust with the magnetic force from a pair of permanent magnets to adapt the environment vibration frequency and improve the energy conversion efficiency. The output power based on the structure in this paper drops tremendously once the environment vibration frequency shifts away from natural frequency (47Hz) of harvesting system. The resonance frequency expands from 42 to 51 Hz with the magnetic adjustment.Meanwhile, the output power keeps 6-10mW. Then, the output power characteristics about the piezoelectric and electromagnetic system were respectively simulated with the software of ANSYS and Ansoft Maxwell. Finally, an experiment prototype hybrid generator was set up to test the system characteristics with the magnetic force tuning. When the vibration acceleration is 0.4g and there is not frequency modulation,the maximum open-circuit voltage output of the piezoelectric power generation system could reach to 5.5V (simulation result is 5.1 V) at the resonance frequency of 54Hz (simulation result is 64Hz). Meanwhile,untuned standalone electromagnetic power generation system achieved to peak open-circuit voltage of 410mV(simulation result is 350mV) at the resonance frequency of 76Hz (simulation result is 64Hz). And the hybrid power generation device could obtain a peak open-circuit voltage of 5.8V at the resonance frequency of 60Hz. In addition, when the movement distance of the piezoelectric cantilever beam varied up or down from 0 to 15mm respectively, the resonance frequency expanded from 45 to 76Hz and the adjustable range reached 31 Hz. Similarly, when the horizontal moving distance of the cantilever beam was 0-3 0mm, the resonance frequency was successfully tuned from 51 to 70Hz and the adjustable range reached 19Hz. It demonstrates that the broadband energy harvesting system could match the low-frequency environment random vibration to mach the low-power system.