U-M Professor Taps Space Tech to Develop Robotic Legs That Regenerate Energy

The space program has been spinning off technology to the ground-based world since its inception. Computer miniaturization and lightweight materials like titanium were perfected for space farers.
robotic prosthetic leg using Space Station technology
CULTINE: Small but powerful motors designed for use on the International Space Station are helping to make this prototype robotic prosthetic leg more comfortable for amputees. // Photo courtesy of the University of Texas at Dallas

The space program has been spinning off technology to the ground-based world since its inception. Computer miniaturization and lightweight materials like titanium were perfected for space farers.

Now, technology from the International Space Station could soon help those needing a prosthetic leg.

A new robotic prosthetic leg prototype, developed by a team led by Robert Gregg, an associate professor at the University of Michigan in Ann Arbor while he was at the University of Texas at Dallas, uses small and powerful motors originally designed for a robotic arm on the ISS.

The prototype prosthetic offers a more natural gait, is quieter, and more energy efficient than other designs, says Gregg.

It is designed to offer a free-swinging knee and regenerative braking, which charges the battery with energy captured when the foot hits the ground. This feature enables the leg to more than double a typical prosthetic user’s walking needs with one charge per day, according to Gregg.

“Our prosthetic leg consumes approximately half the battery power of state-of-art robotic legs, yet can produce more force,” says Gregg, who also is a member of the U-M Robotics Institute.

Using conventional prosthetics, amputees must raise their hips to lift the prosthetic foot from the floor and swing the leg forward. This unnatural gait takes more energy than ordinary walking, causes extra stress and pain in the hips and lower back, and eventually damages the joints.

Robotic legs have the potential to provide a much more comfortable gait, but one of their drawbacks is stiffness in the joints.

“We designed our joints to be as compliant, or flexible, as possible,” says Toby Elery, first author of the study and recent doctoral graduate from UT Dallas. “Our robotic leg can perform and even react like a human joint would, enabling a naturally free-swinging knee and shock absorption when contacting the ground.”

Motors in robotic legs need to fit into the space that an ordinary limb would take up. In the past, this has meant using small motors that spin quickly, and then using a series of gears to convert the fast spin into a more powerful force.

The problem is that the gears are noisy, inefficient, add weight, and make it harder for the joints to swing. Gregg’s group solved this issue by incorporating two of those stronger space station motors, one powering the knee and the other powering the ankle.

There are many benefits to using fewer gears. In addition to enabling the free-swinging knee, removing gears brought the noise level down from the scale of a vacuum cleaner to a refrigerator. Also, the regenerative braking absorbs some of the shock when the prosthetic foot hits the ground.

“If the joints are stiff or rigid, the force is transferred to the residual limb, and that can be painful,” Gregg said. “Instead, we use that force to charge the battery.”

The amputees who test drive the prosthetics in Gregg’s lab say they can feel the leg helping them push off the ground as they walk.

“In some cases, they have observed that they feel like muscles in their hips and back are working less with our leg, compared to their conventional leg,” he says. “We’re able to reduce compensations at the hips.”

The team’s next step is to improve the control algorithms that can help the leg automatically adjust to different terrain, changes in pace, and transitions between different types of activity.

The study is published in the journal IEEE Transactions on Robotics. It was funded by the National Institutes of Health, the National Science Foundation, and the Burroughs Wellcome Fund.

To review the abstract of the study, visit here.

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