Optimal design and experimental verification of piezoelectric energy harvester with fractal structure

The important issues in the conventional rotating vibration energy harvester are that the energy transfer efficiency and the fatigue life were limited because considering only the simply transferred force direction. Thus, this work focuses on the improving energy efficiency of piezoelectric energy harvester with a novel fractal structure. The harvester is based not only on an optimization model analysis, but also on an experimental investigation of coupling coefficients (structural and piezoelectric coupling) that can significantly affect efficiency performance. A model with a single-degree of freedom is developed and then optimized a structure model, yielding fractal-form design and verifications of performance analysis. In order to verify the optimized model, experimental tests are undertaken on a partial fractal structure energy harvester with each of the transmitted force angles. The harvester efficiently converts all responses transmitted at various frequencies. However, by incomplete harvester assembly conditions, shows unusual hysteresis curve tendencies. Especially, the nonlinearity by a fractal structure in single-mode tends to average about 40% in vertical vibration, while the nonlinearity by tangent direction tends to decrease to about 20%. When the harvester is applied to the bearing system, it is observed that the stiffness and damping ratios of the bearing-rotor system by the elastic capacity increase according to the electromechanical characteristic of the energy harvester. As a result, it generates up to 7 mW with the force transmitted by the bearing vibration. It is suggested that the optimal harvester design for increasing energy efficiency of bearing applications, and the importance of nonlinearity study (hysteresis behavior by a friction effect) for harvester performance.