A Comparative Study of Pulley and Bowden Transmissions in a Novel Cable-Driven Exosuit, the Stillsuit

Abstract

Cable-driven exosuits assist users in ambulatory activities by transmitting assistive torques from motors to the actuated joints. State-of-the-art exosuits typically use Bowden cable transmissions, albeit their limited efficiencies (40–60 %) and non-linear response in curved paths. This paper evaluates the efficiency and responsiveness of a new cable-pulley transmission compared to a Bowden transmission, using both steel and Dyneema cables. The analysis includes three experiments: a test bench simulating a curved transmission path, followed by a static and dynamic experiment where six unimpaired participants donned an exosuit featuring both transmissions across the hips and knees. Our findings demonstrate that the pulley transmission consistently outperformed the Bowden's efficiency by absolute margins of 18.77 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\pm 7.29\%$</tex> using a steel cable and by 40.60 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\pm 6.76\%$</tex> using a Dyneema cable across all experiments. Additionally, the steel cable was on average 19.19 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\pm 5.29\%$</tex> more efficient than the Dyneema cable in the pulley transmission and 41.02 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\pm 6.34\%$</tex> in the Bowden tube. These results led to the development of the Stillsuit, a novel lower-limb cable-driven exosuit that uses a pulley transmission and steel cable. The Stillsuit sets a new benchmark for exosuits with 87.56 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\pm$</tex> 3.92 % transmission efficiency, generating similar biological torques to those found in literature (16.4% and 19.0% of the biological knee and hip torques, respectively) while using smaller motors, resulting in a lighter actuation unit (1.92 kg).