Snakes on Mars

Video shows robot snakes climbing better than ever before

Is nowhere safe?

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Despite their cute faces and little tongues, snakes are far from a widely loved animal. For many, these slithering serpents invoke fear of poisoning, strangulation or simply of them falling from airplane storage compartments. But when scientists from Johns Hopkins University looked at these legless beasts, they instead saw robotic inspiration.

It's no secret that nature still does things best and borrowing from nature to improve the design and implementation of technology is nothing new. In recent years scientists have borrowed the texture of shark scales to create bacteria resistant materials and crafted hefty running robots in the likeness of dogs and cheetahs for reconnaissance. When it comes to snakes, however, scientists are particularly interested in how these slender animals are able to easily traverse rough terrain and how such tactics could be implemented in search and rescue robots or even exploration of an extraterrestrial surface like Mars.

But before engineers could design such nimble robots, they first had to learn what makes snakes so good at this creepy, crawling.

In a study published Tuesday in the journal Royal Society Open Science, a team of researchers set out to study how the kingsnake is able to climb smooth, vertical planes (like a tree or stairs) without losing its balance or slipping off. The researchers paid close attention to how real snakes maneuvered their bodies when climbing stairs of different heights. From analyzing videos of these trials they were able to determine a few key characteristics they allowed these snakes to be successful scaling heights nearly half their own body length.

The team observed that real snakes partitioned their bodies into three different sections when traveling up the stairs. There was an oscillating section at the snake's front and back to propel the movement, and then a rigid middle section that scaled the height of the stair. With these biological characteristics in mind, the team then set out to design a robot to mimic them.

The robotic snake they designed was 107 cm long (about 42 in,) 2.36 kg (or a little more than 5 lbs) and was composed of 19 different segments equipped with motors that either pitched the robot (angling it up) or created yaw motion (left and right.) This robot was then tasked with slithering up stairs with heights ranging from 33 to 43 cm (13 to 17 in) and raising its own body up and over the precipice.

The team reports that the snake was fairly successful in these first trials. It could traverse steps one-third of its length with 90 percent probability, but the researchers also found that this success rate plummeted as the steps grew in height. Part of the problem, they found, was that the robotic snake (unlike its live counterpart) was predisposed to either getting stuck during its climb or knocking itself over by failing to time the oscillations of its upper and lower body partitions.

This robot snake was designed to wiggle its front and back in order to help it climb steps like a real snake.JHU/Will Kirk

After returning to videos of the kingsnake, the researchers determined that part of what their robotic model lacked was something called body compliance, meaning how its body could dynamically adapt to its environment. To account for this, the team added small suspension systems to the robot snake's body that allowed it compress itself against the surface when needed to prevent falling.

Qiyuan Fu, the first author of the study and Ph.D. candidate at Johns Hopkins University's Terradynamics Lab, tells Inverse that body compliance is something we use every day without even thinking about it.

"In general we refer to body compliance as anything that can deform easily when there are some external forces," says Fu. "For example, animals, including humans, [have] muscles, skin, tendons. In robots that can be many different combined components, the most common ones [are] springs or other soft materials such as rubber."

With these adjustments, the robot was able to better scale the stairs. Not only with greater agility, but with speed better than most other robotic snakes and close to that of the kingsnake itself. The authors do note in the paper, however, that the added suspension did increase the robot's energy usage and also resulted in it becoming stuck more often thanks to reduced clearance.

As researchers continue to advance these kinds of robots in the future, the authors of this study write that it will be important to investigate how to make these robots even more dynamic and compliant. For example, real snakes are able to use sensory input to change how they move in their environments, but as of now, robotic snakes have no such advantage. Looking into designing tactile sensory feedback systems could help make these robots even more realistic and effective.

Fu tells Inverse that this could look like adding pressure-sensitive scales to their robots, adding eye-like cameras or using infrasound sensors.

Abstract: Snakes can move through almost any terrain. Although snake locomotion on flat surfaces using planar gaits is inherently stable, when snakes deform their body out of plane to traverse more complex terrain, maintaining stability becomes a challenge. In arboreal environments and on sandy slopes, snakes can grip branches or brace against depressed sand to maintain stability. However, how snakes stably surmount large, smooth obstacles like boulders that lack such “push points” remains less understood. Similarly, snake robots must maintain stability when traversing similar obstacles like rubble for search and rescue. Our recent study discovered that snakes combine lateral body oscillation and cantilevering to traverse large step obstacles stably. Here, to further understand the stability principles of snakes surmounting such large, smooth obstacles, we developed a snake robot and used it as a physical model to study how its step traversal depended on step height and body compliance. As step height increased, the robot with a rigid body failed more frequently due to more frequent rolling and flipping over, which was rarely observed in the snake that has a compliant body. Adding body compliance reduced the robot’s roll instability via improved contact with the surface. Besides advancing fundamental understanding of snake locomotion, our snake robot achieved high step traversal speed surpassing most previous snake robots and approaching that of snakes, while maintaining high traversal probability.

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