A knee energy harvester with variable transmission ratio

Utilizing human motion as a source of energy is a potential method to extend the battery life of portable electronics. A number of energy harvesters have been developed in recent years to harvest energy over the knee joint. However, the overall efficiency of these devices is often low due to the noticeable increase in user effort. This increase is the result of the extra effort required to carry the device, as well as the inability of the harvester to match its resultant torque with the users’ natural knee torque. Here, a lightweight cable driven energy harvester over the knee with variable transmission ratio is introduced to harvest energy with minimal user effort. The variable transmission ratio is achieved with a variable radius drum (VRD) cable mechanism utilized to match the user’s and harvester’s effective knee torque over the gait. Power output and gait analysis testing are conducted with VRDs producing 20% to 100% of the natural knee torque, as well as without VRD. The results show a linear relationship between the 20-80% VRDs and power output, with the 80% VRD producing the highest power output at 0.35 W. Without a VRD, power output is around 0.22 W. Gait analysis shows the 20% VRD slightly reduces the impact of the harvester with a RMSE of 5.53^o compared to 5.86^o without VRD.