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. 2021 Apr 9;9:615358. doi: 10.3389/fbioe.2021.615358

FIGURE 1.

FIGURE 1

A powered ankle exoskeleton using neuromuscular model (NMM) based control across a range of virtual reflex settings (A) Block diagram of a neuromuscular model (NMM) based controller to generate torque output of a powered ankle exoskeleton. The user’s ankle joint angle drives the length change of a virtual muscle-tendon unit (MTU) that uses a positive force feedback reflex loop to stimulate a virtual Hill-type muscle contractile element with force-length and force velocity properties similar to the human plantarflexors. The virtual muscle produces a force that is transmitted through a virtual tendon and then applied through a virtual moment arm to generate a virtual ankle moment which is scaled to produce a desired exoskeleton torque. (B) Test conditions for NMM reflex Gain and Delay parameter sweeps included an unpowered or zero-torque condition (NoPwr) (gray) along with powered ankle exoskeleton conditions using controllers with increasing virtual reflex Gain = 0.8, 1.2, 1.6, and 2.0 all with a 10 ms Delay (G0.8, G1.2, G1.6, and G2.0, respectively) (green); increasing virtual reflex Delay = 10, 20, 30, and 40 ms all with a 1.2 reflex Gain (D10, D20, D30, D40, respectively) (blue); and a high-Gain-high-Delay condition (G2.0 D40) (black). All tests were conducted during treadmill walking at 1.25 m/s.