Modeling and Stiffness-Based Continuous Torque Control of Lightweight Quasi-Direct-Drive Knee Exoskeletons for Versatile Walking Assistance

• Published in: IEEE transactions on robotics Vol. 38; no. 3; pp. 1 - 18
• Main Authors: Huang, Tzu-Hao, Zhang, Sainan, Yu, Shuangyue, MacLean, Mhairi K., Zhu, Junxi, Di Lallo, Antonio, Jiao, Chunhai, Bulea, Thomas C., Zheng, Minghui, Su, Hao
• Format: Journal Article
• Language: English
• Published: United States IEEE 01.06.2022; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Actuation; Actuators; Biomechanics; Controllers; Exoskeletons; Flexors; Force/torque control; Gait; Gear ratios; Joints (anatomy); Knee; knee exoskeleton; Legged locomotion; Lightweight; Modelling; Muscles; quasi-direct-drive actuation; Stiffness; stiffness control; Torque; Walking; force/torque control; quasi-direct drive actuation; stiffness control; Knee exoskeleton
• ISSN: 1552-3098; 1941-0468
• DOI: 10.1109/TRO.2022.3170287

State-of-the-art exoskeletons are typically limited by the low control bandwidth and small-range stiffness of actuators, which are based on high gear ratios and elastic components (e.g., series elastic actuators). Furthermore, most exoskeletons are based on discrete gait phase detection and/or discrete stiffness control, resulting in discontinuous torque profiles. To fill these two gaps, we developed a portable, lightweight knee exoskeleton using quasi-direct-drive (QDD) actuation that provides 14 N·m torque (36.8% biological joint moment for overground walking). This article presents 1) stiffness modeling of torque-controlled QDD exoskeletons and 2) stiffness-based continuous torque controller that estimates knee joint moment in real-time. Experimental tests found that the exoskeleton had a high bandwidth of stiffness control (16 Hz under 100 N·m/rad) and high torque tracking accuracy with 0.34 N·m root mean square error (6.22%) across 0-350 N·m/rad large-range stiffness. The continuous controller was able to estimate knee moments accurately and smoothly for three walking speeds and their transitions. Experimental results with eight able-bodied subjects demonstrated that our exoskeleton was able to reduce the muscle activities of all eight measured knee and ankle muscles by 8.60%-15.22% relative to the unpowered condition and two knee flexors and one ankle plantar flexor by 1.92%-10.24% relative to the baseline (no exoskeleton) condition.