Electromagnetic Energy Harvester With Flexible Coils and Magnetic Spring for 1–10 Hz Resonance

This paper presents an energy harvester with microfabricated flexible coils (rolled and aligned to a magnet array for maximum magnetic flux change) and magnetic spring to generate electrical power from human body motion. The magnet array is suspended by a magnetic spring for a resonant frequency of several hertz. An analytical model of vibration-driven energy harvester with magnetic spring through magnet and coil arrays is developed to explore the power generation with various magnet ranges and vibration amplitudes. Experimental results show that the electromagnetic energy harvester with six 7-turn microfabricated coils (occupying 3.8 cc and weighing 8.5 gram) generates an induced electromotive force (EMF) of V
_rms
=6.7 mV with 0.53-μW power output (into 21-Ω load) from 0.27-g acceleration at 8 Hz (corresponding to 1.05-mm vibration amplitude). Its larger-scale version with sixteen 200-turn wire-wound coils (occupying 26 cc and weighing 98 gram) generates an EMF of V
_rms
=1.3$ V with 4.3-mW power output (into 100-Ω load) from 0.5-g acceleration at 5.5 Hz (corresponding to 4.1-mm vibration amplitude). When the larger-scale version of the energy harvester is placed in a backpack of a human walking at various speeds, the power output is increased as the walking speed is increased from 0.45 m/s (slow walking) to 2.68 m/s (slow running), and reaches 14.8 mW at 2.68 m/s.