A Lightweight Hip Exoskeleton with High Torque-to-Mass Ratio: Design, Gait-Synchronized Control, and Physiological Validation
Tzy-Qian Pai, Shi Mou Lin, Chao-Chieh Lan
AI summary
Problem
Practical wearable hip exoskeletons remain limited by excessive weight, complex control requirements, and the need for repeated user-specific calibration, hindering everyday mobility support.
Approach
The authors developed a lightweight, single-piece stainless steel hip exoskeleton using quasi-direct drive actuators and implemented a delayed output feedback control strategy that proactively synchronizes assistive torque with the wearer's gait cycle.
Key results
- Achieves a 5.71 Nm/kg torque-to-mass ratio with a 2.1 kg total system mass
- Identifies a consistent optimal time delay (~0.4 s at 5 km/h) that generalizes across users and speeds
- Reduces vastus medialis and vastus lateralis muscle activation by 35–45% at optimal delay
- Maximizes positive power transfer while minimizing resistive effects during walking
Why it matters
Delivers a practical, energetically efficient, and physiologically validated hip assistance solution for everyday mobility support in aging or mobility-impaired populations.
Abstract
This paper presents the design, control, and experimental validation of a lightweight hip exoskeleton for walking assistance. By integrating quasi-direct drive actuators, single-piece stainless steel frames, and passive revolute joints, the device achieves a high torque-to-mass ratio while maintaining a compact and lightweight structure. A delayed output feedback control strategy synchronizes assistive torque with the gait cycle by actively leading the wearer's hip motion, with user studies identifying a consistent optimal phase difference across participants and walking speeds, eliminating repeated calibration. Surface electromyography validates the assistance, demonstrating substantial reductions in activation of the vastus medialis and vastus lateralis at the optimal time delay. Power analysis further confirms that this setting maximizes positive power transfer while minimizing resistive effects. The proposed exoskeleton delivers physiologically meaningful and energetically efficient hip assistance suitable for everyday mobility support.