Research On Magnetic Coupled Piezoelectric-Electromagnetic Energy Harvester

In recent years, portable electronic devices, micro-electromechanical systems(MEMS) and wireless sensor networks have been widely used in civilian, military, medical and industrial production. Most of these microelectronic products are often powered by the conventional batteries. However, batteries have the limited power density and lifespan and they need to be periodically replaced or recharged. Moreover, they can cause the environmental pollution. Therefore, it is difficult to meet the power demand of the microelectronic products. How to energize the microelectronic products wirelessly becomes an urgent issue. Vibration energy is widely distributed in everyday life and engineering practice. It cannot be affected by the position and weather, and possesses higher energy density. As a result, more and more scholars and experts work on the vibration energy harvesters, which convert vibration energy into electricity to power microelectronics. Depending on the conversion mechanism, energy harvesters can be divided into five types: piezoelectric, electromagnetic, electrostatic, magnetostrictive and hybrid. Among them, piezoelectric and electromagnetic energy harvesters are most studied currently, which are considered as the promising alternative to the batteries.In the previous research papers, piezoelectric and electromagnetic energy harvesting devices are always simplified as single degree-of-freedom(SDOF) systems, whose operating frequency bandwidths are very narrow, so that the generating performances depend highly on the external vibration frequency. In order to increase the output power, broaden the bandwidth, and enhance the environmental adaptability of the harvesters, this thesis focuses on three different piezoelectric-electromagnetic hybrid energy harvesters(HEHs), namely, magnetic coupling tunable HEH, beam-spring multi-frequency HEH and dual-beam multi-frequency HEH. The electromechanical coupling models for the harvesters were established separately. Numerical simulation and experimental verification were performed and analyzed. The effects of structural parameters and load resistances on the generating characteristics were studied. The results provide the theoretical and experimental foundation for improving the generating performance of the HEHs.The frequency of some vibration sources in the environment varies around the fundamental frequency occasionally with a limited range, while the resonant frequency of the HEH is constant. Once the resonant frequency of the HEH does not match the excitation frequency, the output power will drop dramatically. In this paper, the electromechanical coupling model for the magnetic coupling tunable HEH was derived based on the Euler-Bernoulli beam theory, Rayleigh-Ritz approach and energy method. The effects of the mechanical damping, coil parameters and nonlinear electromagnetic coupling coefficient on generating characteristics were considered. This model was used to study the dynamic response and power characteristics of this HEH under the harmonic excitation. The effects of separation distance between two magnets, magnetization direction, load resistances, electromechanical coupling coefficients, mechanical damping ratios, coil parameters and excitation acceleration on the generating characteristics were analyzed, respectively. A prototype was developed. The experimental results verified the theoretical model. The research results show that magnetic interaction can be used to tune the resonant frequency, so that the output power and environmental adaptability can be improved. The effect of load resistance connected to the coil on the resonant frequency of piezoelectric oscillator can be ignored. The piezoelectric coupling coefficient determines the advantages of the hybrid energy harvesting mechanism. Under the excitation frequency of 100 Hz, the inductance of the coil can be ignored. When the inner radius of the coil is constant, increasing the coil height and decreasing the outer radius contribute to enlarging the electromagnetic coupling coefficient. When the separation distance is small enough and excitation level is high enough, the generator shows the hardening behavior, induced by nonlinear magnetic force. The bandwidth can be effectively broadened under increasing frequency sweep. In the experimental process, the load resistance connected to piezoelectric patches is matched firstly, and then the one connected to the coil.In the environment, some vibration sources have two or more discrete frequencies. However, the SDOF energy harvester only has one resonant frequency. Accordingly, it cannot effectively scavenge these vibration energy. Moreover, the natural frequency of the micro-scale harvester is so high that it is difficult to harvest the low-frequency vibration energy with the magnetic tuning method. Therefore, this paper proposed a beam-spring and a dual-beam multi-frequency HEHs. The corresponding electromechanical coupling models were established and used to analyze the effects of load resistance, mass ratio, natural frequency ratio, mechanical damping ratio and separation distance on the generating characteristics. The prototypes were developed. The experimental results validated the theoretical models. It is shown that magnetic coupling multi-frequency response of this device helps to harvest vibration energy with two discrete frequencies simultaneously and improve the generating performance in the low-frequency environment. The resonant frequencies can be adjusted by changing the distance between two magnets to enhance the environmental adaptability. The hybrid conversion mechanism contributes to broadening the bandwidth and improving the power generation performance.The beam-spring HEH is suitable for harvesting the multi-frequency vibration energy dominated by low-frequency. Its first two resonant frequencies are higher than that of the corresponding single-frequency energy harvesters; decreasing the separation distance can enhance the output power in the low-frequency range; at the excitation acceleration of 4m/s2, this HEH shows almost linear dynamic behavior. The dual-beam HEH is suitable for harvesting the multi-frequency vibration energy dominated by high-frequency. Its first two resonant frequencies of are lower than that of the corresponding single-frequency energy harvesters; the low-frequency magnetic oscillator, which is used for electromagnetic energy harvesting, can improve the peak output power. It is not always helpful to increase the peak output power for the piezoelectric oscillator, which is used for both piezoelectric and electromagnetic energy harvesting; the beam length of magnetic oscillator mainly affects the first resonant frequency and peak output power; at the excitation acceleration of 4m/s2, this HEH shows the hardening behavior at the first resonant frequency, while softening behavior at the second resonant frequency. The bandwidth can be effectively broadened under frequency sweep.