Characterization of a rotary piezoelectric energy harvester based on plucking excitation for knee-joint wearable applications

Abstract

Wearable medical and electronic devices demand a similarly wearable electrical power supply.Human-based piezoelectric energy harvesters may be the solution, but the mismatch between thetypical frequencies of human activities and the optimal operating frequencies of piezoelectricgenerators calls for the implementation of a frequency up-conversion technique. A rotarypiezoelectric energy harvester designed to be attached to the knee-joint is here implemented andcharacterized. The wearable harvester is based on the plucking method of frequency up-conversion,where a piezoelectric bimorph is deflected by a plectrum and permitted to vibrate unhindered uponrelease. Experiments were conducted to characterize the energy produced by the rotary piezoelectricenergy harvester with different electric loads and different excitation speeds, covering the rangebetween 0.1 and 1 rev s −1 to simulate human gait speeds. The electrical loads were connected to thegenerator either directly or through a rectifying bridge, as would be found in most power managementcircuits. The focus of the paper is to study the capability of energy generation of the harvesterfor knee-joint wearable applications, and study the effects of the different loads and differentexcitation speeds. It is found that the energy harvested is around 160–490 µJ and strongly dependson the angular speed, the connected electric loads and also the manufacturing quality of theharvester. Statistical analysis is used to predict the potential energy production of a harvestermanufactured to tighter tolerances than the one presented here.