Podcast: How Robocats Land on Their Feet

The physics of “cat-turning” has been a subject of fascination for hundreds of years, in part because a cat’s almost uncanny ability to land on its feet seems, at first glance, to violate the conservation of angular momentum.

“It sounds like a paradox when you first talk about it,” says Dr. Will Robertson of the University of Adelaide. “Obviously when the cat is falling, there’s no one else helping it to flip over in the air.”

Images of a falling cat published in Nature in 1894.
Image Credit: Nature via Wikimedia Commons

Robertson and his students have designed a cat-like robot to mimic one of the mechanisms used by real cats to reorient themselves in free fall. The trick is to consider the cat not as a rigid body, but as “lots of rigid bodies all chained together. So when we consider it as a whole, everything internal to the cat can do its own thing so long as the overall angular momentum remains constant.”

Image Credit: Ben Shields et al./University of Adelaide

As roboticist Dr. Jeff Bingham points out, “Angular momentum being conserved does not necessarily mean that motion is zero.” As the cat falls, its front and back halves rotate in opposite directions, keeping the total angular momentum constant. By twisting its spine and alternately tucking and extending its front and back legs, the cat can manipulate the different rotation rates of its front and back halves to flip over and land on its feet (see the video below).

During his time as a postdoctoral scholar at the Georgia Institute of Technology, Bingham designed a robot consisting of just three segments — a 2-dimensional “cat” — that can cycle through three configurations to achieve a net rotation and land in a favorable orientation. By confining the cat-bot’s movement to a plane, the group could let it slide down a frictionless air table rather than drop it, slowing its descent and giving it time to perform its contortions.

Image Credit: Bingham et al./Georgia Tech

Having the cat-like ability to reorient in mid-air is important for all sorts of applications, from robots to satellites and accurate video game animations. Apple even filed a patent last December proposing a “protective mechanism for an electronic device,” designed to manipulate your iPhone’s moments of inertia and save it from a bad fall – not unlike what a cat does to land on its feet.

Of course, cats aren’t the only animals that robot designers look to for inspiration, but when it comes to graceful landings, they’re purrrrfect role models.

-Podcast and blog post by Meg Rosenburg

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