FEM Information technology industries PZT Taguchi method bimetallic effect broadband diamagnetically stabilized levitation electromagnetic electromagnetic energy harvester electromagnetic generator (EMG) energy harvester energy harvesting flowing water high performance human body kinetic energy hybrid energy harvester low frequency magnetic coupling maximum gap modeling nonlinear optimization pattern search piezoelectric piezoelectric energy harvester piezoelectric vibration energy harvester piezoelectricity power density improvement resonant frequency stable levitation