January 2021

A Quieter, More Energy-Efficient Prosthetic Leg

A student tests the robotic leg at UT Dallas. The strong motors powering the knee and ankle can propel the user’s body while allowing the knee to swing freely, with regenerative braking to extend battery life. Image courtesy of UT Dallas.

A new robotic prosthetic leg prototype offers a more natural gait while also being quieter and more energy efficient than other designs. The key is the use of new small and powerful motors, originally designed for a robotic arm on the International Space Station. The streamlined design offers 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 1 charge per day.

“Our prosthetic leg consumes approximately half the battery power of state-of-art robotic legs, yet can produce more force,” said Robert Gregg, PhD, an associate professor of electrical and computer engineering at the University of Michigan (U-M) and a member of the U-M Robotics Institute, who led the study while previously at The University of Texas at Dallas (UT Dallas).

Using conventional prostheses, 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 more comfortable gait, but one of their drawbacks is stiffness in the joints.

Additionally, motors in robotic legs need to fit into the space that an ordinary limb would inhabit. 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 freely. Gregg’s group surmounted this by incorporating two of those stronger space station motors, one powering the knee and the other powering the ankle, and by reducing the number of gears in the transmission.

Among the benefits to using fewer gears include enabling the free-swinging knee as well as reducing the noise level 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, which reduces pain for the user.

The amputees who tested the prosthesis in Gregg’s lab reported that they could feel the leg helping them push off the ground as they walked. “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,” said Gregg. “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.

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