A footwear harvester needs to fulfill both technical and practical functionalities, including but not limited to energy extracting and storage, easy implantation and durability. The very limited space in a shoe is the key challenge to the design of a footwear harvester. This paper presents the design, optimization, modeling and testing of an embedded piezoelectric footwear harvester for energy scavenging from human walking. A force amplification frame is designed and optimized to transmit and amplify the vertical heel-strike force to the inner piezoelectric stack deployed in the horizontal direction. Two heel-shaped aluminum plates are employed to gather and transfer the dynamic force over the heel to the sandwiched force amplification frames. The dynamic force at the heel is measured to design, optimize and simulate the piezoelectric footwear harvester. A numerical model is developed and validated to be capable of precisely predicting the electrical outputs of the harvester. Two prototypes, respectively including eight and six stacks, are fabricated and tested on a treadmill at different walking speeds and external resistances. The numerical simulations agree well with experiments. The harvester with fewer piezoelectric stacks could produce more power at the same walking speed and matched resistance. Experimental results manifest that the footwear harvesters with eight and six stacks, respectively, have 7 mW/shoe and 9 mW/shoe average power outputs at the walking speeds of 3.0 mph (4.8 km/h). Simulation results from the validated numerical model show that the harvester with four piezoelectric stacks could harvest 14 mW/shoe and 20 mW/shoe average power at 3.0 mph (4.8 km/h) and 3.5 mph (5.6 km/h), respectively.
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