The researchers say the 9kg mini cheetah is designed to be “virtually indestructible”, recovering with little damage, even if a backflip fails.
The four-legged powerpack can bend and swing its legs wide, enabling it to walk either right-side up or upside down, it can also trot over uneven terrain at 5mph.
Each of the robot’s legs is powered by three low-cost electric motors that the researchers engineered using off-the-shelf parts. Each of these motors can be swapped out for a new one.
“You could put these parts together, almost like Legos,” said Benjamin Katz, a technical associate in MIT’s Department of Mechanical Engineering. “A big part of why we built this robot is that it makes it so easy to experiment and just try crazy things, because the robot is super robust and doesn’t break easily, and if it does break, it’s easy and not very expensive to fix.”
Each leg is powered by three motors, to give it three degrees of freedom and a huge range of motion. The lightweight, high-torque, low-inertia design enables the robot to execute fast, dynamic manoeuvres and make high-force impacts on the ground without breaking gearboxes or limbs.
Katz explains: “When it’s running, its feet are only on the ground for something like 150 milliseconds at a time, during which a computer tells it to increase the force on the foot, then change it to balance, and then decrease that force really fast to lift up. So, it can do really dynamic stuff, like jump in the air with every step, or run with two feet on the ground at a time. Most robots aren’t capable of doing this, so move much slower.”
The team wanted to put the robot through higher-impact manoeuvres than righting itself after being pushed by the researchers. So, they set about programming the mini cheetah to perform a backflip.
Katz said: “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.”
The team wrote “giant, nonlinear, offline trajectory optimisations” that incorporated the robot’s dynamics and actuator capabilities and specified a trajectory in which the robot would start out in a right-side-up orientation and end up flipped 360 degrees. The program they developed 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, to carry out the backflip.
“The first time we tried it, it miraculously worked,” Katz said.
The team is building about 10 more mini cheetahs, each of which they plan to loan out to collaborating groups, intending 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 the ability for the mini cheetah to land on its feet after being thrown or dropped.