Mechanical Efficiency Investigation of an ankle-assisted robot for human walking with a backpack-load

Link to bioRxiv paper:
http://biorxiv.org/cgi/content/short/2020.11.18.388066v1?rss=1
Authors: Longhan, X., Wang, Z., Huang, G., Liu, B., Zhou, Z., Liang, B., Liu, Y.
Abstract:
Lower limb assistive robots have a wide range of applications in medical rehabilitation, hiking, and the military. The purpose of this work is to investigate the efficiency of wearable assistive devices under different weight-bearing walking conditions. We designed an experimental platform, with a lightweight ankle-assisted robot weighing 5.2 kg and carried mainly on the back. Eight subjects were tested in three experimental conditions: free walk with load (FWL), power-off with load (POFL), and power-on with load (PONF) for different levels of force at a walking speed of 3.6 km/h. We recorded the metabolic expenditure and kinematics of the subjects under three levels of weight-bearing (equal to 10%, 20%, and 30% of body mass). The critical forces from the fit of the assistive force and metabolic depletion curves were 130 N, 160 N and 215 N at three different load levels. The intrinsic weight of our device increases mechanical work at the ankle as the load weight rises, with 2.08 J, 2.43 J, 2.73 J for one leg during a gait cycle. The ratio of the mechanical work input by the robot to the mechanical work output by the weight of the device decreases (0.904, 0.717, and 0.513 with different load carriages), verifying that the walking assistance efficiency of such devices decreases as the weight rises. In terms of mechanical work in the ankle joint, our results confirm that the efficiency of the ankle-assisted walking robot decreases as weight bearing increases, which provides important guidance for the lightweight design of portable weight-bearing walking robots.
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