Desert snakes that slide on the sand at night can encounter obstacles such as plants or twigs that change the direction of their course. By studying this movement to learn how legless animals control their bodies in such environments, researchers have found that snakes colliding with these obstacles mimic aspects of light or subatomic particles when they encounter a network. of diffraction.
The effect of this "mechanical diffraction" allowed the researchers to observe how the trajectories of snakes were modified by passive mechanisms governed by the skeletal and muscular dynamics of the propagating body waves. The researchers studied living snakes as they passed an obstacle made up of six rigid, force-sensitive stakes that looped the bodies of animals and changed their course in a predictable manner.
The results, described on February 25 in the newspaper Proceedings of the National Academy of Sciences, indicate that Eastern Hog-nosed Snakes (Chionactis occipitalis) do not change direction deliberately when encountering obstacles when running at full speed over sand. Understanding the movement of these limbless animals could help engineers improve the control of autonomous search and rescue robots designed to work on sand, grass and other complex environments.
"The basic idea of passive dynamics is that the animal changes the shape of its waveform entirely because of the passive properties of its body," said Perrin Schiebel, a recent PhD. graduated from the physics school of the Georgia Institute of Technology. "Instead of sending a signal to activate a muscle, the interaction of snake bodies with the outside environment is what causes the shape change. The forces of the obstacles push the snake bodies towards a new form. "
The colorful shovel snake normally uses an S-shaped sinusoidal wave to move through the deserts of the southwestern United States. Falling on rigid stakes in a laboratory environment does not cause him to actively modify this waveform, which Schiebel and his colleagues studied using high-speed video cameras with eight different animals.
In a study funded by the National Science Foundation, the Army Research Bureau, the Defense Advanced Projects and a National Defense Graduate Scholarship in Science and Engineering, researchers used 253 snake travels to create a diffraction pattern. Remarkably, the pattern also revealed that the diffusion directions were "quantized", so that the probability of finding a snake behind the painting could be represented in a pattern mimicking the interference of the waves. A computer model was able to capture the model, showing how the direction of snakes would be altered by encountering obstacles via the passive buckling of the body.
"A problem with robots moving in the real world is that we do not yet have any principles to understand how to best control these robots on grainy surfaces such as sand, dead leaves, rubble or the grass, "said Daniel Goldman, Dunn Family Professor at the Georgia Tech School of Physics. "The purpose of this study was to try to understand how armless locomotives, which have long bodies that can bend in interesting ways using potentially complicated neuromechanical control schemes, manage to move over complicated terrain. . "
The Snake experiment was suggested by a robotic study conducted by Jennifer Rieser, postdoctoral fellow, who discovered similar behavior in robots encountering obstacles.
"The robot tends to have aspects that mimic the characteristics of the subatomic world – the quantum world," said Goldman. "When it hits barriers, a robot spreads through these barriers using body bending waves, its trajectory deviates as it emerges from the barriers, and many repeated tests reveal a pattern of dispersion." "lumpy", analogous to the experiments.We realized that we could use this phenomenon surprising and beautiful, classical physics but with self-propulsion a key feature, as a broadcast experiment to interrogate the control scheme used by the snakes. "
On an experimental basis, researchers used a "snake arena" covered with a long-haired carpet to mimic sand. Alex Hubbard and Lillian Chen, undergraduates, released the snakes one at a time in the arena and encouraged them to sneak through the network.
The eyes of desert snakes are naturally covered with scales to protect them. The researchers used a children's painting to "temporize the eyes" of animals so that they would not be distracted by the researchers. The painting did not harm animals.
"When we put the snakes in the arena, they started moving using the same waveform that used on the desert sand," Schiebel explained. "They would then meet the grid of the ankle, cross it and continue on the other side always using this waveform."
Instead of continuing to cross the arena in a straight line, the snakes came out from a different angle, although they did not grab the poles or use them to help them move. Schiebel worked with Zeb Rocklin, an assistant professor of physics at Georgia Tech, to model the changes in direction. The model showed how simple interactions between the snake wave pattern and the lattice produce patterns of preferred diffusion directions.
"We believe that the snake is essentially operating in a pattern that control engineers would consider an" open loop "," Goldman said. "He installs a particular motor program on his body, which generates the characteristic wave type and, when he hits the obstacle, his body mechanics allows him to deform and move the poles without degrading his speed. "
Goldman believes this work could help snake-like robot developers improve their control systems.
"We believe that our findings on the role of passive dynamics in the snake can facilitate the design of new snake robots that will allow them to move more smoothly in complex environments," he said. "The goal would be to build search and rescue robots that can penetrate these complex environments and help first responders."
And as a bonus, Goldman said, "We find that the richness of the interactions between self-propelled systems such as snakes and robots and their environment is fascinating from the point of view of the physics of the" active ingredient "."
Smooth and thin snake beats the short, squat lizard in the sand while swimming
Perrin E. Schiebel et al. Mechanical diffraction reveals the role of passive dynamics in a creeping snake, Proceedings of the National Academy of Sciences (2019). DOI: 10.1073 / pnas.1808675116