High-Power Density Inertial Energy Harvester Without Additional Proof Mass for Wearables

Inertial energy harvesters have great potential to sustainably power wearable sensors for the Internet of Things. However, the bulky proof mass used to capture kinetic energy limits the power density of such an energy harvester. Targeted for high-power density, we propose an inertial energy harvester without additional proof mass to efficiently scavenge the kinetic energy of human limb swing. By adopting a planetary structure, the power generation unit, comprising the base, coils, rotor, and magnets, serves as an eccentric weight of the system, so no additional proof mass is needed. Excited by limb swing, the power generation unit oscillates along the sun gear so that the rotor is actuated by the sun gear to spin and produce electricity. We build a theoretical model to predict the average output power under limb swing excitations. We fabricate a miniature prototype to experimentally characterize the energy harvester under pseudowalking excitation and evaluate its performance in real walking. The results show that the prototype generates a maximum power of 1.46 mW and power density of $454.82~{\mu }\text{W}$ /cm³ in pseudowalking testing, which are over ten times those of its counterparts. In the real walking test, the prototype performed even better, achieving 1.84 and 2.95 mW when worn on the wrist and ankle, respectively. After power regulation, the energy harvester can fully power a pedometer at various walking speeds. Finally, simulations using real walking data demonstrate that the proposed device reaches higher power density compared with conventional structures and that introducing the additional proof mass unnecessarily increases power density.