Biomechanical energy harvesting using a knee mounted generator

Abstract

The design of a knee mounted energy harvesting device for USB charging was investigated. With the current longevity issues of lithium batteries in portable electronics and the reliance for consistent access to a standard electrical socket for charging, a need for a portable and sustainable energy source was identified. Human biomechanical energy from movement was identified as an emission-free and untapped source of power. A rotational generator utilizing a knee-mounted apparatus was selected as the most likely candidate for generating enough power for USB charging (5 VDC and 0.1 A minimum). Because the device was to be attached to the body, emphasis was placed on developing a design that provided for minimal hindrance to the user's normal gait. Following this decision, several energy harvesting products using rotational generators were purchased and benchmarked. A brushless motor was purchased to act as a generator in the knee mounted system; this decision was made for its low mechanical resistance to motion when spinning the shaft and high back EMF constant. To actuate the generator of the system at a velocity which could provide adequate power, a gear train was designed to amplify the 1 Hz input from human gait; the gear train utilized a ratcheting freewheel, which allows for conservation of angular momentum in the forward direction between actuations of the gear train. Parallel to the mechanical design process, an electrical system was designed to rectify the three phase AC voltage produced by the generator. A DC/DC switching regulator was used to condition the rectified output to 5 VDC. An alpha prototype is currently being fabricated. Projections indicate the system should produce 5 VDC at a range of output currents from 0.1 A to 1 A depending on how fast the user is moving.