Generating Electricity During Locomotion Modes Dominated by Negative Work via a Knee Energy-Harvesting Exoskeleton

The capability to generate electricity from human motion can reduce the battery requirements for wearable devices. The key challenge faced by wearable energy harvesters is the generation of sufficient power without interfering with the wearer. To our knowledge, currently available joint-motion energy harvesters are all for level walking (LW). In this study, we developed an energy-harvesting exoskeleton that can generate electricity during LW, downhill walking (DW), and stair descent (SD) without sensors. Bi-directional knee motion is transformed to the unidirectional rotation of the generator by one-way bearing and appropriate gear train. The average electricity generated by each leg for the three modes of walking is $5.4 \pm 0.8{\rm{\ W\ }}({\text{LW},\ 5{\rm{\ km}}/\mathrm{h}})$, $6.5 \pm 0.6{\rm{W\ }}({\text{DW},\ 5{\rm{\ km}}/\mathrm{h}})$, and $8.2 \pm 0.4{\rm{W\ }}({\text{SD},4{\rm{\ stairs}}/\mathrm{s}})$, respectively. The Pearson coefficients of the knee angle under exo-on and no-exo conditions are 0.995 (LW), 0.996 (DW), and 0.999 (SD), respectively. The cost of harvesting is -0.006 (LW) and -0.01 (DW). The increase in the metabolic rate in energy harvesting on condition is merely 2.3% (LW) and 1.6% (DW) compared to the no-exo condition. There is no significant increase in the metabolic cost while generating electricity $(P\ = \ 0.363,$ LW; $P\ = \ 0.662,$ DW). In summary, the knee energy-harvesting exoskeleton developed in this study can generate substantial and sustainable electricity with little extra effort, which indicates its potential as a means to charge-powered portable exoskeletons and prosthetics.