The fastest two-legged robot with knees can now step gracefully down an 8-inch, stair-height platform—blind.
MABEL, which stands for Michigan Anthropomorphic Biped with Electric Legs, has no cameras. The drop she negotiates in this video is a total surprise to her, like when the curb sneaks up on you. And if the curb were 8 inches high, many humans wouldn't be able to recover and hold their footing as well as MABEL does. They'd stumble or twist an ankle, says robotics researcher Jessy Grizzle, a University of Michigan electrical engineering professor and MABEL's owner.
This isn't just cool. It's a useful skill that we'd all expect from the rescue robots of the future. There probably wouldn't be a clear path for machines you might send into a burning building to make sure everyone had escaped. They might have to traverse stairs or maybe step over toys in the living room, Grizzle says.
MABEL has improved on her previous scores in what the robotics community calls the "step down test." It's a standard measure of how stable a robot's gait is. The best that others can do is 5% of their leg length, but MABEL can now handle 20%.
Here's how she does it: Ph.D. student Hae Won Park reprogrammed her so that a shock absorbing mechanism kicks in milliseconds after her leg hits the ground at the bottom of the step.
"The knee of the leg that impacts the ground is programmed to buckle at a controlled rate to dissipate the impact energy," Grizzle said. "In addition, the torso of the robot is programmed to, roughly speaking, go with the flow and not fight the impact. That prevents overreaction and oscillation. All of this happens in less than half a second, so the human eye cannot see it. Two steps after the sudden drop in height, the robot is walking again under its normal control algorithm."
Grizzle is pretty much floored by MABEL's accomplishments, which he says are really the accomplishments of his grad students Hae-Won Park, Koushil Sreenath, and, most recently, Alireza Ramezani.
"Walking over wildly uneven ground? Running a 9-minute mile? Walking backward just as gracefully as she walks forward? They have MABEL doing things that her developer Jonathan Hurst and I may have dreamed about," Grizzle said, "but only in our wilder moments."