Comparing neural control and mechanically intrinsic control of powered ankle exoskeletons

There are an infinite number of ways to control an assistive robotic device; however, there is little consensus on which ways are better than others and why. We designed this study to compare the control of powered ankle exoskeletons using neural measurements to drive control versus that using mechanically intrinsic measurements. The controller driven by neural measurements was a dynamic gain proportional myoelectric controller using user's soleus muscle activity for an actuation signal. The controller driven by mechanically intrinsic measurements was a timing-based controller using detected heel-strikes of the user to appropriately time actuation. We designed these two controllers in such a way that both had the same average actuation signal and tested them with 8 healthy subjects. Results show no significant difference in metabolic work rate between the two controllers. Both controllers resulted in reductions in metabolic work rate of 19% below walking in the devices unpowered. We found that subjects using the timing-based mechanically intrinsic controller exhibited less positive and negative total ankle power than when using the dynamic gain proportional myoelectric controller. This finding was coupled with a reduction of 11.8% in soleus muscle activity. We believe these finding can have large implications for applications in rehabilitation and lend insight to when one controller is more appropriate to use than another.