Design and Robust Torque Control of a Variable Transmission Series Elastic Actuator for Hip Exoskeletons

Precision in multi-torque-speed characteristics, disturbance resistance, transparency, and back-drivability are expected in exoskeleton actuators' design and control. The introduction of series elastic actuators (SEA) for lower-extremity exoskeletons has recently attracted more attention owing to their special merits compared to traditional actuators in high torque control accuracy, favorable output compliance, and unique shock tolerance. In this work, a SEA with a variable transmission ratio for the hip exoskeleton is developed for different working conditions requiring variable torque-speed characteristics. A crank-slide mechanism that actively changes the torque transmission path based on different tasks to meet the torque-speed requirements for the low-extremity exoskeleton. The high-precision torque delivery is the main concern in SEA control. Also, the crank-slider mechanism added in SEA will bring a new control difficulty with the frequent switching of the transmission ratio which may cause severe mechanical vibration and unsafe factors to users. Thus, a novel torque control architecture, based on disturbance observer integrated with a three-mass modeling method for VTSEA is proposed. Simulations are carried out to validate that the developed controller can restrain disturbance and provide accurate assistive torque tracking effectively. This work serves as a fundamental for employing and developing hip exoskeleton for locomotion assistance.