An Ultra-low Frequency Ball-impacted Potential-variable Nonlinear Energy Harvester

With the miniaturization,low power consumption and intelligence of electronic devices,wireless sensing networks have been widely used in various fields such as modern military equipment,intelligent IOT,environmental monitoring,medical devices,structural health monitoring and space exploration.As the energy part in sensing nodes,traditional chemical batteries have gradually failed to meet the needs in various complex environments.By harvesting the vibration energy in the environment,it can make the sensing nodes passive to ensure the stable operation of wireless sensing networks for a long time,so vibration energy harvesters have become a hot research topic nowadays.Since most of the vibration excitations present in the environment are low frequency,or even ultra-low frequency vibrations below 10Hz,it is difficult for existing vibration harvesters to achieve response frequency matching,and effective energy harvesting of ultra-low frequency vibrations is still a great challenge.Although the current nonlinear energy harvesters have a wide frequency range of harvesting response,it is difficult to extend the frequency range to the ultra-low frequency range due to the limitation of the stiffness of the harvesters’vibration pickup structure.The energy conversion unit of the typical ultra-low frequency energy harvesters is still based on linear resonators,which makes it difficult to further improve the harvesting efficiency.In this thesis,a novel ultra-low frequency ball-impact potential-variable nonlinear energy harvester（abbreviated as BPNEH）is proposed for the first time by combining the ball-impact mechanism and the potential-variable nonlinear mechanism,aiming at harvesting more energy under the ultra-low frequency vibration excitation.The structural design of the BPNEH is firstly proposed,and the working principle of the BPNEH is explained in detail by decomposing the motion process.Through the magnetic force of the top magnet,the horizontal beams and the vertical beams are coupled.When the BPNEH is excited by the ultra-low frequency,the horizontal beams achieve large amplitude back and forth vibration through the impact of the ball,and at the same time the horizontal beams generates variable potential energy due to the vibration of the coupled vertical beams,which converts mechanical energy into electrical energy through the piezoelectric layer on the horizontal beam.A new theoretical model based on beam vibration and piezoelectric theory is developed to predict the motion state and piezoelectric output of the BPNEH by a lumped parameter model.The theoretical model is imported into MATLAB for the numerical simulation of BPNEH,then an experimental test platform is built and a BPNEH prototype was fabricated for the experimental study.The results of both the numerical simulation and the experimental study demonstrate a double peak phenomenon.In contrast to the conventional potential-variable nonlinear energy harvester（abbreviated as PNEH）,the BPNEH has a new sub-peak in the ultra-low frequency range to the left of the first-order resonance peak,which at the experimental level improves the RMS voltage output of the BPNEH by 78.4%in the 1-8 Hz range,and at the simulation level,this improvement reaches 209.6%.For the potential-variable mechanism a control probe is performed and the variable potential also brought a 7.7%improvement to the RMS output compared to the fixed potential configuration.In addition,the main factors affecting the sub-peaks,such as magnetic force,ball mass,excitation acceleration amplitude,and channel length,are considered and discussed in the observed double peak phenomenon in order to harvest more energy from the BPNEH.By studying the dynamic response of the sub-peak range,the four vibrational modes of the BPNEH are identified,among which the period-2 mode has the highest piezoelectric output.Finally,the power output of the BPNEH prototype is also tested experimentally.The BPNEH prototype generates a normalized average power density of 27.7μWcm^-3g^-2,which is nearly 9 times higher compared to the PNEH,and a preliminary demonstration of the energy harvesting effect of the BPNEH is demonstrated by an LED light illumination.The BPNEH proposed in this thesis along with the double peak phenomenon is considered suitable for ultra-low frequency scenarios,especially for engineering fields with large amplitudes,such as wearable devices,structural health monitoring systems,etc.