Research ArticleEXOSKELETONS

Improving the energy economy of human running with powered and unpowered ankle exoskeleton assistance

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Science Robotics  25 Mar 2020:
Vol. 5, Issue 40, eaay9108
DOI: 10.1126/scirobotics.aay9108
  • Fig. 1 Experimental setup.

    (A) Exoskeleton emulator testbed. A participant runs on a treadmill while wearing bilateral ankle exoskeletons actuated by motors located off-board with mechanical power transmitted through flexible Bowden cables. (B) Ankle exoskeleton. The ankle exoskeleton attaches to the user by a strap above the calf, a rope through the heel of the shoe, and a carbon fiber plate embedded in the toe of the shoe. The inner Bowden cable terminates on a 3D printed titanium heel spur that is instrumented with strain gauges for direct measurement of applied torque. A magnetic encoder measures ankle angle. (C) Participant running on the treadmill with bilateral ankle exoskeletons. Metabolic data are collected through a respiratory system by measuring the oxygen and carbon dioxide content of the participant’s expired gasses.

  • Fig. 2 Metabolic results.

    Optimized spring-like and Optimized powered assistance resulted in metabolic reductions of 2.1 and 24.7%, respectively, compared with zero-torque mode, while running at 2.7 m s−1. Optimized powered assistance resulted in an improvement in running economy of 14.6% compared with running in normal shoes, whereas Optimized spring-like assistance resulted in an 11.1% increase in the energy cost of running. Error bars indicate SD. *P < 0.05.

  • Fig. 3 Optimized assistance patterns applied to both ankles.

    (A) Measured exoskeleton torque patterns resulting from Optimized controller settings for the powered assistance strategy are shown as solid, colored lines for each participant. A 50% scale representation of the torque typically produced by the biological ankle during normal, unassisted running at 2.7 m s−1 is shown as a dashed line (50). (B) Torque versus angle curves show a large amount of work performed by the exoskeletons for all participants under the powered condition. (C) Optimized torque patterns for spring-like assistance are shown as colored, solid lines. A 50% scale representation of the torque typically produced by the biological ankle during normal, unassisted running is shown as a dashed line (50). (D) Torque versus angle curves show minimal work performed on the ankle for all participants under the passive spring-like condition.

  • Fig. 4 Controller parameterizations.

    (A) Powered parameterization. Three nodes define the pattern of torque control under the powered condition. These nodes are connected by sinusoids and defined by four parameters: magnitude of peak torque, peak time, onset time, and off time. (B) Range and examples of possible powered torque patterns. A simple parameterization results in a wide range of possible torque patterns. (C) Spring-like parameterization. A quadratic function fitted to three nodes defines the relationship between ankle angle and applied torque. These nodes are defined by three parameters: the magnitude of torque applied at the maximum dorsiflexion angle; the ankle angle at which the spring engages; and a shape constant that determines the degree to which the spring is linear, stiffening, or softening. (D) Range and examples of possible spring-like torque patterns.

  • Table 1 Participant characteristics.

    ParticipantSexMass (kg)Age (years)Height (m)
    1M76.0241.77
    2M83.9361.83
    3M72.6241.80
    4M63.5221.75
    5M79.4211.83
    6M79.4301.84
    7M77.1211.80
    8M59.4201.75
    9M70.3231.73
    10M83.5371.70
    11M84.4211.80

Supplementary Materials

  • robotics.sciencemag.org/cgi/content/full/5/40/eaay9108/DC1

    Fig. S1. Accuracy of steady-state metabolic rate estimates during optimization.

    Table S1. Values of Optimized parameters for each participant.

    Table S2. Values of metabolic rate (watts per kilogram) for each participant under each condition.

    Movie S1. Video showing a participant running with exoskeletons in Optimized powered, Optimized spring-like, and zero-torque modes and with running shoes.

    Reference (62)

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. Accuracy of steady-state metabolic rate estimates during optimization.
    • Table S1. Values of Optimized parameters for each participant.
    • Table S2. Values of metabolic rate (watts per kilogram) for each participant under each condition.
    • Legend for movie S1
    • Reference (62)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Video showing a participant running with exoskeletons in Optimized powered, Optimized spring-like, and zero-torque modes and with running shoes.

    Files in this Data Supplement:

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