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MAE researchers develop new lightweight exoskeleton

Person wearing an exoskeleton on legs while walking outside in a natural area.

By Jack Boden
Department of Mechanical and Aerospace Engineering

Researchers with the Department of Mechanical and Aerospace Engineering (MAE) at NC State University have developed a new portable, lightweight, and highly-compliant knee exoskeleton that helps wearers reduce muscle effects during walking.

MAE Professor Hao Su, along with his students and fellow researchers Sainan Zhang, Shuangyue Yu, Junxi Zhu, and Antonio Di Lallo, set out to enhance mobility in daily life with their exoskeleton designs, either for rehabilitation or augmentation purposes.

“Many exoskeletons are still heavy, bulky, non-compliant, and need to switch among multiple discrete states using hand-crafted rules that cause unnatural motion, abrupt torque assistance, and discomfort to the wearers. They also have difficulty adapting to changes in walking speeds.” a summary of the team’s work written by Zhang and Dr. Su states.

To improve upon these weaknesses, the team created a new actuation paradigm, i.e., quasi-direct drive (QDD) actuator that is composed of high-torque motors and small ratio gears, to create a portable and very lightweight exoskeleton. The exoskeleton designed by Prof. Su’s team is more compliant and 35% more lightweight than state-of-the-art robots, weighing only 3.5 kg. Prof. Su’s team also developed a stiffness-based continuous torque controller that estimates the biological torque in real-time and is adaptable to different walking speeds.

The goal was for to reduce the muscle effect (or amount of energy exerted) for able-bodied subjects during walking. As an assistive device, this exoskeleton is designed to supplement rather than replace the impaired functions of the limbs.

“Our results with 8 able-bodied subjects are promising as it demonstrated that the exoskeleton was able to reduce the muscle activities of all 8 measured knee and ankle muscles by 8.60%-15.22% relative to unpowered condition and reduced two knee flexors and one ankle plantar flexor by 1.92%-10.24% relative to baseline (no exoskeleton) conditions,” The summary states. “The electromyography (EMG) of the ankle plantar flexors medial gastrocnemius (MG) and lateral gastrocnemius (LG) was slightly increased during the powered condition by 3.55% and 2.18%, respectively, compared with no exoskeleton condition. The results demonstrate that the knee exoskeleton can reduce the muscle effect, i.e., helping wearers save effort during the level-ground walking.”

The developed exoskeleton and the associated stiffness-based continuous torque controller are intended not only for able-bodied individuals (e.g., workers who perform heavy lifting) but also for individuals with mobility impairments (e.g., subjects with knee osteoarthritis and stroke with stiff knee gait). As for the next step, the team is currently collaborating with Dr. Thomas Bulea at the National Institutes of Health to expand the usage of this exoskeleton to cover children with cerebral palsy. The team envisions developing an even lighter version of the current exoskeleton tailored to children’s needs to improve their mobility.

This work is published in IEEE Transactions on Robotics, the top journals in robotics. The research was supported by the National Science Foundation CAREER award, the National Institutes of Health (NIH R01EB029765), and the National Institute on Disability, Independent Living, and Rehabilitation Research (NIDILRR 90DPGE0011) in collaboration with Dr. Thomas Bulea at the NIH Clinical Center and Dr. Minghui Zheng at the University at Buffalo.

Click here to watch a video on the team’s work.

This post was originally published in the Department of Mechanical and Aerospace Engineering.