Piezoelectric energy harvester using impact-driven flexible side-walls for human-limb motion

We present a human-limb driven piezoelectric energy harvester using two mass-loaded unimorph piezoelectric beams clamped on two flexible sidewalls. Since vibration generated by human-limb motion has low-frequency and high amplitude characteristics, the energy harvester has been designed to up-convert the low-frequency human-limb vibration by mechanical impact of a spring less spherical metallic ball. However, instead of direct mechanical impact on the power generating elements (unimorph piezoelectric beams), the ball impacts on the bases (flexible sidewalls) of each beam to avoid mechanical wear of the piezo-materials. While excited by human-limb motion, the ball impacts consecutively on the flexible sidewalls which transfer impulsive forces to the loaded mass of the respective unimorph beam. The beam vibrates at its own resonant frequency and causes voltage generation by virtue of piezoelectric effect. A proof-of-concept prototype has been fabricated and tested. At optimum load condition, each unimorph piezoelectric generator generates 96 µW average power while excited at 4.96 Hz frequency and ~2g acceleration. The device with series connected generators is capable of generating maximum 175 µW average power. Improved design and further optimization would be able to increase its power generation capability (as well as power density) to be used in wearable devices applications.