A lightweight biomechanical energy harvester with high power density and low metabolic cost

Generating electricity by a biomechanical energy harvester whose input energy is human joint mechanical energy can reduce user requirements to carry backup batteries. In this work, a lightweight cable-pulley harvester mounted on knee joint has been designed. In order to reduce the weight and raise the center of mass of the harvester, we use a cable-pulley mechanism instead of the gear train. The cable-pulley mechanical transmission converts the flexion and extension of the knee joint to the line movement of the nut of a ball screw. The ball screw operates in the inverse mode and converts the line movement of the nut to the rotor shaft of a generator. The transmission structure from flexion and extension to line to rotation consists of a speed up mechanism that can convert low velocity and high torque at knee joint into high velocity and low torque at the rotor shaft of the generator. A dynamic model of the electromagnetic torque is established for choosing the optimal mechanical parameters. Being different from other lower limb single joint biomechanical energy harvesters that operate in the generative braking mode, the cable-pulley harvester needs not gait phase detection system and operates in the full cycle generation mode that makes the harvester more reliable and applicable. The experimental results show that the cable-pulley harvester produces average 4.1 W of electricity with the lightest mass and the highest power density, and the growth rate of user metabolic power remains low.