Hip exoskeleton assistance delays impact joint mechanics but not energy savings
Effects of Assistance Delay on Joint Mechanics and Energetics in Biological Torque Control of a Hip Exoskeleton
Robotics
Summary
The paper examines how delays in hip exoskeleton assistance affect walking on different surfaces. The authors studied how timing delays in delivering supportive force influence the wearer’s joint work and energy use during level ground, uphill, and downhill walking. They found that while delays changed how the joints moved and worked mechanically, the overall energy savings for the wearer stayed about the same. However, assistance that works well for uphill or flat walking may not help as much for downhill walking, which requires different joint movements.
What this means in practice
- •For rehabilitation engineers: Design hip exoskeletons that adjust timing delays to optimize assistance for various walking tasks including ramps and level surfaces.
- •For physical therapy device makers: Improve exoskeleton control systems to provide consistent metabolic benefits despite mechanical assistance timing variations.$Commercial implications: Enables development of advanced hip exoskeletons with improved energy efficiency marketed to rehabilitation and mobility aid sectors.
Authors
Jimin An, Ryan Lee, Jingshu Peng, Eni Halilaj, Inseung Kang
Abstract
Biological torque control directly maps an estimated human joint moment to exoskeleton assistance, providing a task-agnostic strategy for supporting diverse locomotor activities. However, it remains unclear whether a fixed state-to-torque mapping provides effective assistance across biomechanically distinct tasks. We examined how assistance delay affected hip exoskeleton performance during level-ground (LG), ramp-ascent (RA), and ramp-descent (RD) walking. Eight participants completed a zero-torque baseline condition and five active assistance conditions with delays ranging from 40 to 320 ms. Across tasks and active delays, assistance reduced net metabolic rate by 5.24%, positive biological hip joint work by 5.86%, and total lower-limb positive joint work by 1.68% (all p < 0.05). Assistance delay affected both joint-work outcomes (both p < 0.001) but not net metabolic rate. Mechanical unloading generally decreased with increasing delay, whereas metabolic benefits remained comparatively stable. Relative to the zero-torque condition, net metabolic rate decreased by 9.75% during LG and 7.20% during RA but increased by 1.23% during RD. We did not detect task-dependent differences in the delay response. Our findings indicate that biological torque mappings should be evaluated based on the target outcome and mechanical role of the assisted joint, and that predominantly positive-power assistance may not generalize to negative-work-dominant locomotion without modification.