Hybrid piezoelectric-electromagnetic energy harvesting from ultra-low-frequency human motion via magnetic frequency up-conversion

Efficiently harvesting ultra-low-frequency vibration energy from human motion to power electronic devices remains challenging. This study presents a compact magnetically coupled energy harvester (MCEH) that integrates piezoelectric (PEH) and electromagnetic (EEH) modules through a shared reciprocating center magnet. The center magnet simultaneously produces electromagnetic induction and excites the piezoelectric cantilevers through nonlinear magnetic repulsion, enabling dual-path energy conversion and magnetic frequency up-conversion from excitation below 10 Hz. A coupled lumped-parameter electromechanical model was established and evaluated using numerical simulations and experiments, including frequency-response, load-matching, human-motion, and sensor-powering tests. At 6.5 Hz, the PEH reaches an RMS voltage of 3.56 V with magnetic coupling, versus only 0.0383 V for the no-center-magnet reference, showing an approximately 93-fold enhancement. Under the matched loads, the system delivers a total output of 1,147.88 μ W and a normalized power density of 31.89 μ W/ cm 3 . The prototype also powers a temperature-humidity sensor under the tested motion conditions, demonstrating the feasibility of the proposed approach for low-power wearable sensing.