A small wrist-watch-like wearable electric energy harvester which can extract electricity from swinging motions of people’s arms while walking has been developed newly. The harvester consists of multiple vibrating piezoelectric cantilevered thin beams attached to a round central hub structure radially with tip masses. The cantilevers are made of a polycarbonate substrate beam, PMN-PT piezoelectric material on its both sides, and a high density tungsten tip mass. The swinging of a human arm with the harvester causes the bending deformations in each blade while walking and then produces electricity from strains in two piezoelectric layers. The swinging motion was formulated mathematically and kinematically in terms of swinging angles, angular velocities and accelerations. Finite element analysis was used to model the cantilevered beams and calculate the voltage output. The optimum shape of piezoelectric layers were calculated on the basis of the topology optimization method specialized for piezoelectric materials.

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