Design Principles for Compact, Backdrivable Actuation in Partial-Assist Powered Knee Orthoses

• Published in: IEEE/ASME transactions on mechatronics Vol. 26; no. 6; pp. 3104 - 3115
• Main Authors: Zhu, Hanqi, Nesler, Christopher, Divekar, Nikhil, Peddinti, Vamsi, Gregg, Robert D.
• Format: Journal Article
• Language: English
• Published: United States IEEE 01.12.2021; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Acoustic noise; Actuation; Actuator design; Actuators; Brushless motors; Coils (windings); exoskeletons; Gear ratios; Gear trains; Gears; Knee; Medical robotics; Orthoses; Patient rehabilitation; Permanent magnet motors; Principles; rehabilitation robotics; Rotors; Stator windings; Thermal environments; Torque
• ISSN: 1083-4435; 1941-014X
• DOI: 10.1109/TMECH.2021.3053226

This article presents the design and validation of a backdrivable powered knee orthosis for partial assistance of lower-limb musculature, which aims to facilitate daily activities in individuals with musculoskeletal disorders. The actuator design is guided by design principles that prioritize backdrivability, output torque, and compactness. First, we show that increasing the motor diameter while reducing the gear ratio for a fixed output torque ultimately reduces the reflected inertia (and thus backdrive torque). We also identify a tradeoff with actuator torque density that can be addressed by improving the motor's thermal environment, motivating our design of a custom brushless dc motor with encapsulated windings. Finally, by designing a 7:1 planetary gearset directly into the stator, the actuator has a high package factor that reduces size and weight. Benchtop tests verify that the custom actuator can produce at least 23.9-N<inline-formula><tex-math notation="LaTeX">\,\cdot\,</tex-math></inline-formula>m peak torque and 12.78-N<inline-formula><tex-math notation="LaTeX">\,\cdot\,</tex-math></inline-formula>m continuous torque, yet has less than 2.68-N<inline-formula><tex-math notation="LaTeX">\,\cdot\,</tex-math></inline-formula>m backdrive torque during walking conditions. Able-bodied human subject experiments (<inline-formula><tex-math notation="LaTeX">N=3</tex-math></inline-formula>) demonstrate reduced quadricep activation with bilateral orthosis assistance during lifting-lowering, sit-to-stand, and stair climbing. The minimal transmission also produces negligible acoustic noise.