Coupled exoskeleton assistance simplifies control and
maintains metabolic benefits: a simulation study
Nicholas A. Bianco
1*
, Patrick W. Franks
1
, Jennifer L. Hicks
2
, Scott L. Delp
1,2,3
,
1 Department of Mechanical Engineering, Stanford University, Stanford, California,
United States of America
2 Department of Bioengineering, Stanford University, Stanford, California, United
States of America
3
Department of Orthopaedic Surgery, Stanford University, Stanford, California, United
States of America
* nbianco@stanford.edu
Abstract
Assistive exoskeletons can reduce the metabolic cost of walking, and recent advances in
exoskeleton device design and control have resulted in large metabolic savings. Most
exoskeleton devices provide assistance at either the ankle or hip. Exoskeletons that
assist multiple joints have the potential to provide greater metabolic savings, but can
require many actuators and complicated controllers, making it difficult to design
effective assistance. Coupled assistance, when two or more joints are assisted using one
actuator or control signal, could reduce control dimensionality while retaining metabolic
benefits. However, it is unknown which combinations of assisted joints are most
promising and if there are negative consequences associated with coupled assistance.
Since designing assistance with human experiments is expensive and time-consuming,
we used musculoskeletal simulation to evaluate metabolic savings from multi-joint
assistance and identify promising joint combinations. We generated 2D muscle-driven
simulations of walking while simultaneously optimizing control strategies for simulated
lower-limb exoskeleton assistive devices to minimize metabolic cost. Each device
provided assistance either at a single joint or at multiple joints using massless, ideal
actuators. To assess if control could be simplified for multi-joint exoskeletons, we
simulated different control strategies in which the torque provided at each joint was
either controlled independently or coupled between joints. We compared the predicted
optimal torque profiles and changes in muscle and whole-body metabolic power
consumption across the single joint and multi-joint assistance strategies. We found
multi-joint devices–whether independent or coupled–provided 50% greater metabolic
savings than single joint devices. The coupled multi-joint devices were able to achieve
most of the metabolic savings pro duced by independently-controlled multi-joint devices.
Our results indicate that device designers could simplify multi-joint exoskeleton designs
by reducing the number of torque control parameters through coupling, while still
maintaining large reductions in metabolic cost.
Introduction 1
Wearable robotic exoskeletons that reduce the metabolic cost of walking could improve
2
mobility for individuals with musculoskeletal or neurological impairments and assist 3
April 6, 2021 1/26
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The copyright holder for this preprint (whichthis version posted April 18, 2021. ; https://doi.org/10.1101/2021.04.16.440073doi: bioRxiv preprint
