Turtle-inspired robot can swim under the sand and detect obstacles

Roboticists designed an untethered bot that can detect obstacles in the subterranean when swimming through sand and can effectively engage in search and rescue missions.
Shubhangi Dua
The robot is equipped with flipper-like appendages which allow it to swim under sand. The two black fins on its nose ensure that it remains submerged under the sand.
The robot is equipped with flipper-like appendages which allow it to swim under sand. The two black fins on its nose ensure that it remains submerged under the sand.

David Baillot / University of California San Diego 

While a considerable amount of research has gone into robots being able to swim in water, and fly in the air, it is only in the last 15 years or so that scientists have turned their attention to movement across 'deformable ground' like sand, snow, soil and extraterrestrial regolith, where robots are in most demand.

However, a new study has been undertaken to build a streamlined robot that can effectively 'swim' under sand, inspired by the movements of turtle hatchling. 

A team of roboticists at the University of California San Diego conducted comprehensive stimulations and testing to make this one-of-a-kind robot that can travel in the sand at a depth of five inches.

The researchers say that the robot can further travel at a speed of 1.2 millimeters per second - roughly 13 feet (4m) per hour.​ It may seem slow, but it is relatively fast when compared to other subterranean animals like worms and clams.

Turtle power

The innovation draws inspiration from the features of baby turtles. Equipped with two front limbs that closely mimic the flippers of turtle hatchlings, the robot demonstrates remarkable capabilities in 'swimming' through sand and effectively digging itself out.

Additionally, the device is furnished with force sensors at the end of its limbs. This component allows the robot to detect obstacles when in motion. 

“It can operate untethered and be controlled via WiFi,” the researchers said.

The primary obstacle encountered by roboticists was the development of a robot that could effectively move through sand, which presented higher force compared to movement in air or water. Such devices tend to damage quickly, so something robust was required. 

Sensing obstacles

Regardless of the challenges, such an invention has the potential to solve locomotion in the sand. The scientists state that the turtle-like device could inspect grain silos, measure soil contaminants, dig seafloors, conduct extraterrestrial exploration, and engage in search and rescue missions.

The friction in sand behaves like liquid, as well as a solid, depending on the context. This friction makes it difficult for robots to sense obstacles, so the scientists conducted several experiments ahead of successfully producing a prototype.

Shivam Chopra, lead study author said, “We needed to build a robot that is both strong and streamlined.”

The team overcame the challenge by observing animals. At first, they considered designing worms, but eventually settled on sea turtle hatchlings due to their enlarged front fins, which allow them to surface after hatching.

“Turtle-like flippers can generate large propulsive forces; allow the robot to steer; and have the potential to detect obstacles,” they said, “the bot detects obstacles by monitoring changes in the torque generated by the movement of its flippers. It can detect obstacles above its body, but not below or directly in front of it.”

Terrafoils enable lift function

Researchers devised two foil-like surfaces which they refer to as terrafoils, to maintain the robot at a consistent depth in the sand. These specially designed terrafoils have enabled the team to regulate the lift function, ensuring that the robot remains level and stable during its movement through the sand.

It was a necessary step in the development of the bot, as it was inclined to keep its nose pointed towards the surface, the study noted. 

The machine was tested in a five feet long tank in the lab and at the – La Jolla Shores, a beach near the UC San Diego campus. They found that the robot slowed down in wet sand, which offers more resistance. 

Researchers are further undertaking steps to increase the bot’s speed, “allowing it to burrow into sand, in addition to digging itself out of the sand,” they said. 

The study was published on 12 May in the journal, Advanced Intelligent Systems


Granular environments, such as sand, are one of the most challenging substrates for robots to move within due to large depth-dependent forces, unpredictable fluid/solid resistance forces, and limited sensing capabilities. An untethered robot is presented, inspired by biological diggers like sea turtles, which utilize underactuated appendages to enable propulsion and obstacle sensing in granular environments. To guide the robot's design, experiments are conducted on test appendages to identify the morphological and actuation parameters for forward thrust generation. Obstacle sensing is observed in granular media by measuring the increased force on the moving appendage caused by changes in the granular flow around it. These results are integrated into an untethered robot capable of subsurface locomotion in a controlled granular bed like natural, loosely packed sand. The robot achieves subsurface “swimming” at a speed of 1.2 mm s−1, at a depth of 127 mm, faster than any other reported untethered robot at this depth, while also detecting obstacles during locomotion via force sensors embedded in the appendages. Finally, subsurface robot locomotion in natural sand at the beach is demonstrated, a feat no other robot has accomplished, showcasing how underactuated structures enable movement and sensing in granular environments with limited limb control.

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