The Trouble with Exoskeletons
They’re heavy and they hurt, but researchers are developing a more comfortable model.
By Rosemarie D. Wesson and Hao Su
Owning an Iron Man set of armor is the dream of millions of superhero fans. Engineers have been working for several decades to turn this science-fiction fantasy into reality. Under pioneering professor Homayoon Kazerooni, the Robotics and Human Engineering Laboratory at the University of California, Berkeley in 2009 developed the Human Universal Load Carrier, the first energetically autonomous, orthotic, lower extremity exoskeleton, providing the ability for its user to carry 200-pound weights. Five years later, more than a billion people around the globe watched as a paraplegic youth wearing a thought-controlled, robotic exoskeleton kicked off the opening match of the FIFA World Cup in São Paolo, Brazil. Beyond their role in gait restoration and rehabilitation, exoskeletons since 2016 have attracted the interest of automakers Ford, BMW, Toyota, Hyundai, and Honda for their potential to reduce workers’ musculoskeletal injuries and augment human capabilities.
Two challenges researchers continue to confront are the combined weight of the electric motors and the metallic structure and the misalignment of robotic and human biological joints. Both cause discomfort, but beyond that, the Centers for Disease Control and Prevention has found that the weight of these devices could inadvertently increase the load on the musculoskeletal system, making long-term use of exoskeletons especially difficult for wearers. A comfortable and safe resolution of these problems is critical.
A team of researchers is working to tackle these dawbacks by developing new enabling technologies. Collaborators from the City College of New York and the Kessler Foundation, a leader in rehabilitation science, are developing soft, powerful, and comfortable exoskeletons for patients who suffer from spinal cord injury and stroke. Led by Hao Su, an assistant professor of mechanical engineering at the City College of New York, the team is developing high-torque-density electric motors and bioinspired mechanisms to make the exoskeleton system more lightweight and give it a smoother operation.
Conventional exoskeletons are typically heavy, weighing 25 to 50 pounds, making it hard for a wearer to maintain good balance. By reducing the weight (by about seven pounds) the high-torque density actuator significantly improves stability. The actuator, when combined with a low-gear-ratio transmission, reduces mechanical impedence and increases response time of the exoskeleton. So a wearer’s walking motion is faster and more natural and fluid.
Su has also invented a bioinspired mechanism that mimics the biological knee joint with rolling contact between femur and tibia. The mechanism minimizes discomfort caused by the misalignment of the robot joints and human biological joints. With the reduced mass and minimized joint misalignment, the new exoskeleton is expected to be much more comfortable and reach a broader user population.
Developing high-torque-density motors is not only useful for today’s exoskeletons but may help prevent injury in the future when humans work alongside collaborative robots, also called co-robots. Conventional motors offer extremely high speed and low torque output, the opposite of the human motion and torque profile, making human-machine coordination difficult. High-torque-density motors will allow for a better match. As the research evolves, it is expected to bring improved ease of operation and safety in an array of co-robots, including industrial robots, legged robots, and humanoid robots, as well as exoskeletons and robotic prostheses, orthoses, and rehabilitation devices.
Rosemarie D. Wesson, Ph.D., is associate dean for research at the Grove School of Engineering, City College of New York, where Hao Su, Ph.D., is an assistant professor of mechanical engineering.