Secrets of the sidewinder

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Secrets of the sidewinder
Face-to-face photo with the unwrapped snake robot sitting coiled and looking curious, with a metal head.
Not science fiction: this robot snake charges up hill and spills a snake’s secrets.
Photo: Nico Zevallos and Chaohui Gong

By now, you know about robots that roll, fly, swim and walk, insect-like, on six legs. Are you ready for a robot that climbs a sandy hill in the fashion of the sidewinder rattlesnake?

In research in this week’s Science, Daniel Goldman and company described using a robot to explore exactly how the sidewinder achieves the rare feat of climbing a steep, sandy slope.

“For years, we’ve spent a lot of time on problems involving animals doing interesting things in granular material, including lizards that swim in sand and turtles that run across sand,” says Goldman, a professor of physics at Georgia Institute of Technology. “Then we asked what is the weirdest animal that moves on sand, and it’s the sidewinder. People studied it 50 years ago, but nothing was done on a sandy slope.”

The name tells it all: the sidewinder winds sideways up the hill. In tests at the Atlanta zoo, live snakes were able to climb a 31° sandy slope. That’s 4° above the angle at which disturbed sand starts to avalanche. “It can get quite precarious,” Goldman says. “Snakes, by doing God knows what, can ascend almost to the point where the material is cascading.”

Slow-motion top and side views of a sidewinder rattlesnake moving up a 20° sandy incline.
Video: Henry Astley

When the snake climbs a 20° sandy slope, the tracks show no signs of slippage.

The sidewinders, Goldman adds, are a “wonderful study subject. You put the snake in, and it climbs, it does what you want.”

In the lab, Goldman’s group sought to understand the snake’s secrets by studying a robot created by Howard Choset at Carnegie Mellon University. “Our idea was to use robots … as a physical model, to test parameters that are inconvenient to test in the real world,” meaning in a live snake. “We answered the questions by drag-force experiments on inclines, which had not been done before.”

Sidewinding vs. slithering

Two illustrations of snake locomotion compare direction of motion and ground contact of 'sidewinders' and 'slitherers.'
A sidewinder snake propels itself with two waves that lift sections of its body and push it forward. Turn off the vertical wave, keep the full snake on the surface, and you get slithering.
Screenshot modified from AAAS/Science

Mastering the incline required fine-tuning the robot’s movement. The video above shows an early effort that failed to climb the sandy slope. Below: After some tweaks, success!
Video credit: Henry Astley

How?

Goldman sees sidewinding as a hybrid of two synchronized waves: a horizontal wave down the body (best seen from above), and a vertical wave (best seen from the side). “These waves are phased, the robot and the snake both space the waves appropriately. The part of the body that contacts the ground is gently placed down, and peeled up gently.”

The force that pushes the sidewinder uphill “comes from the body that is pressed down so the material is not destabilized,” Goldman says. As it ascends, the snake subtly lifts sections of its body that would otherwise counter its uphill movement. “On a granular surface, there can be a small difference in how you move that dictates success or failure.”

Anybody who has trudged up a steep sandy slope knows what happens to two-foots: We take two steps forward, and slide back one step (or more). Physically speaking, this occurs because sand and other granular materials “can transition from a solid to a fluid; it can slip out from under you,” Goldman says.

Snakes must also avoid changing the surface to make their job more difficult. If they push away sand, they may deprive themselves of purchase. Or they may create ridges that turn into obstacles.

The live sidewinders seem to move effortlessly, especially when compared to the artificial one, but the point was not to create a cave-exploring robot but rather to figure out how flesh-and-blood sidewinders do it. “It’s incredible, beautiful,” says Goldman. “I’m a physicist by training. I had a love of herpetology as a kid, but I had no idea that locomotion was interesting … from the perspective of physics. I tend to think you strike a ball and it moves, but here’s an internal mechanism that can manipulate the environment so it can move.”

Photo of a sidewinding rattlesnake coiled with head up and tongue out.
A sidewinder rattlesnake curled on sand.
Photo: Tim Nowak

In recent years, many robot-makers, including Choset, creator of the artificial snake under study, have been fascinated with animals’ astonishing ability to walk, run, climb, fly and swim. Here, the goal was different, Goldman says. “We think of it as biological physics, looking for fundamental principles. We are not using animals explicitly to inspire a better strategy in robots. We are using the robot to understand the animals at a higher level. It’s a scientific enterprise, not a design or engineering enterprise.”

– David J. Tenenbaum

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Kevin Barrett, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer

Bibliography

  1. Sidewinding with minimal slip: Snake and robot ascent of sandy slopes; H. Marvi et al, Science 10 October, 2014.
  2. Bionic kangaroo demonstrates big leap in robotics
  3. Dinobots, ping-pong-playing bugs and tiny cheerleaders: The latest innovations in robotics go on display in Japan.
  4. Robotics at a Snail’s Pace.
  5. Wearable robotics to minimize energy required to make physical movement.