Why Do So Few Robots Walk Like People?

In the sci-fi universe, if you have a robot, there's a very good chance it walks on two legs. "Star Wars" has C-3PO, "Futurama" has Bender, "Transformers" has the Autobots — even the titular robots in "I, Robot" walk with one foot in front of the other. So why, now that real robots are actually a thing, do so few walk on two legs? It's because evolution took a long time to get us here, and it'll take roboticists more than a few decades in the garage to make a convincing copy.

One Small Step For...Oh Dear

Primates, a group to which humans belong, are one of only four groups of mammals that walk upright — and the other three hop around like kangaroos. Animals that walk on four, six, or eight legs are far more common, and for good reason. Four legs are kind of nature's minimum for stability. Sure, there are sturdy tables with three legs, but nature prefers things to have two matching sides, so odd numbers are out.

So when you start with the plan to make a two-legged robot, you're already at a disadvantage stability-wise. Humans maintain their balance with a complex system of kinesthetic awareness and proprioception that works together to control more than 600 individual muscles.

Robotic sensors that judge orientation and other spatial elements are quickly advancing, but they're still nowhere near what's in the human body. Combine that with the fact that our best motors aren't as strong or efficient as human muscles, and you get bipedal robots that are slow, lumbering, and frankly hilarious. Check out this video if you don't believe us.

A Compilation of Robots Falling Down at the DARPA Robotics Challenge

On Their Own Two Feet

We've been using "tracked" robots on wheels for decades, and four-legged robots are making bigger strides than two-legged ones lately (pun intended). So why waste energy on making a humanoid robot at all? Is our love of sci-fi tradition really worth the effort?

Roboticists say yes, and here's why. We live in a world we made for ourselves. There's a reason cats can't open closed doors and dogs look so funny walking downstairs. Walking upright gives humans the freedom to use their other two limbs to hold boxes, use tools, and turn doorknobs. Stairs are much easier when you only have two feet to keep track of. So when there's an earthquake, a nuclear meltdown, or another disaster in a building made for humans, a human-like robot is the best one to navigate the scene. Likewise, personal robots like the ones that might provide customer service or become our assistants should be able to go where we go.

And we're getting there. In February of 2017, Agility Robotics unveiled what might be the most practical bipedal robot yet. Named Cassie, after the ostrich-like cassowary, the headless, armless robot has reversed knees that make it walk around like a bird. But thanks to spring elements it uses in lieu of clunky motors, it walks at a respectable pace of about three meters per second and stays upright over all sorts of terrain. Sure, it's more "ostrichoid" than humanoid, but it's a start.

Cassie, the Two-Legged Robot

Big Bad Robots Panel Discussion