Design and Control of a Novel Semi-Passive Knee Exoskeleton

This letter presents a novel semi-passive knee exoskeleton designed to provide running assistance. It incorporates an energy-efficient clutch mechanism activated by a mini servomotor which engages and disengages the spring that supports the leg during running. The exoskeleton extracts energy during the running phase when the muscles are acting as brakes (negative power), stores it in the spring, and then returns this energy during the positive power phase (when the muscles are acting as motors). The exoskeleton controller implements an inertial measurement unit (IMU) sensor to estimate the shank orientation that determines when to engage and disengage the spring. Two experiments designed to probe the functionality of the exoskeleton were conducted to evaluate its control performance and actuation, and the exoskeleton's biomechanical impact on three subjects. The findings showed that the control mechanism could be engaged and disengaged in real time. The maximum moment created on the knee muscles was 17 Nm, although the device could supply 28 Nm. The ratio of the consumed servo energy consumption to the subjects’ saved energy was 1:160 (0.1W input to 16W saved). This study thus paves the way for the development of lightweight, inexpensive exoskeletons that can contribute to their greater availability for a broader range of individuals.