Mini Cheetah Is the First Four-Legged Robot to Do a Backflip

Estimated reading time: 6 minutes

The engineers ran the mini cheetah through a number of maneuvers, first testing its running ability through the hallways of MIT’s Pappalardo Lab and along the slightly uneven ground of Killian Court.

In both environments, the quadruped bound along at about 5 miles per hour. The robot’s joints are capable of spinning three times faster, with twice the amount of torque, and Katz estimates the robot could run about twice as fast with a little tuning.

The team wrote another computer code to direct the robot to stretch and twist in various, yoga-like configurations, showcasting its range of motion and ability to rotate its limbs and joints while maintaining balance. They also programmed the robot to recover from an unexpected force, such as a kick to the side. When the researchers kicked the robot to the ground, it automatically shut down.

“It assumes something terrible has gone wrong, so it just turns off, and all the legs fly wherever they go,” Katz says.

When it receives a signal to restart, the robot first determines its orientation, then performs a preprogrammed crouch or elbow-swing maneuver to right itself on all fours.

Katz and co-author Jared Di Carlo, an undergraduate in the Department of Electrical Engineering and Computer Science (EECS), wondered whether the robot could take on even higher-impact maneuvers. Inspired by a class they took last year, taught by EECS Professor Russ Tedrake, they set about programming the mini cheetah to perform a backflip.

“We thought it would be a good test of robot performance, because it takes a lot of power, torque, and there are huge impacts at the end of a flip,” Katz says.

The team wrote a “giant, nonlinear, offline trajectory optimizations” that incorporated the robot’s dynamics and actuator capabilities, and specified a trajectory in which the robot would start out in a certain, right-side-up orientation, and end up flipped 360 degrees. The program they developed then solved all the torques that needed to be applied to each joint, from each individual motor, and at every time period between start and end, in order to carry out the backflip.

“The first time we tried it, it miraculously worked,” Katz says.

“This is super exciting,” Kim adds. “Imagine Cheetah 3 doing a backflip — it would crash and probably destroy the treadmill. We could do this with the mini cheetah on a desktop.”

The team is building about 10 more mini cheetahs, each of which they plan to loan out to collaborating groups, and Kim intends to form a mini cheetah research consortium of engineers, who can invent, swap, and even compete with new ideas.

Meanwhile, the MIT team is developing another, even higher-impact maneuver.

“We’re working now on a landing controller, the idea being that I want to be able to pick up the robot and toss it, and just have it land on its feet,” Katz says. “Say you wanted to throw the robot into the window of a building and have it go explore inside the building. You could do that.”

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