Stingray-inspired robot cracks the mystery of how rays swim
To help understand what makes stingrays so unique and unusual, a team of mechanical engineers from the University of California, Riverside (UCR) created a wavy robotic fin. After submerging the robot in underwater tunnels designed to mimic swimming near the seafloor, their tests indicate that different types of ray species may have evolved alternative swimming techniques better suited to their environments. Specifically, the results suggest that some species of rays swimming near the seafloor adjust the way their fins move and tilt to counter a downward force that would otherwise pull them toward the ground.
It turns out that the stingrays that gracefully glide along the waves near the seabed don’t do it to look cool. Instead, fancy kicking is likely an evolutionary adaptation for stability and durability while swimming. The team behind the mechanical fin believes these same principles could one day be applied to designing energy-efficient underwater mapping robots. And they are not the only ones to admire the rays. Other researchers are already trying to use the knowledge gained from stingray swimming to develop stealthier, next-generation underwater vehicles.
The study of robotic fins was published this week in the Journal of the Royal Society interface.
Putting stingray swimming to the test
When it comes to swimming, not all species of rays are the same. Massive manta rays and other pelagic ray species tend to hover near the ocean surface using a flapping motion. Benthic rays, like stingrays that spend their time in shallower water, rely on a different wave motion that often resembles the motion of the waves in which they swim. This second undulating swimming style in particular has fascinated scientists with its apparent simplicity and effectiveness. Previous research on this swimming method has shown that the wave-like motion used by stingrays actually appears to recycle energy from the surrounding water more efficiently than beating the fins by brute force.
Yuanhang Zhu, a UCR mechanical engineer and co-author of the paper, had a hunch that the divergence in swimming styles might come from the different environments in which stingray species live. To test this theory in controlled environments, the team decided to create the robotic fin. By testing the fin under different conditions, the researchers were able to observe how physical forces in the water affected its movement. The final fin design measured just 9.5 millimeters (about 0.4 inches) thick and was molded from silicone rubber. They also built a large water tunnel designed to simulate ocean flow.
During their experiments, the team placed the robot both near the surface of the tunnel and lower down, closer to the artificial seabed. In both cases, they were looking to see how different levels of ocean flow affected the lift transmitted to the fins. Understanding lift is important because it plays a key role in determining whether or not objects moving through space can stay level. For example, birds flying close to the ground experience positive lift them up, keeping them more level and stable. The researchers expected to see something similar happen for the robotic stingray swimming near the seafloor. Instead, the exact opposite happened. Their robot was being sucked down.
Robotic stingray fin
āThis was not what we expected,ā Zhu said in a UCR blog post. āInstead of gaining additional lift close to the ground, the spokes were pulled downward.
Surprised by the results, the team made slight adjustments to the robot to try to compensate for the negative lift. They found that the downward force could be reduced simply by tilting the robot’s fin a few degrees upward. Extrapolating from this, the researchers suggest that stingrays and other benthic rays naturally swim with a slight upward fin angle, which was previously unclear. During testing, the stingray-like wave motion also maintained a better distance from the seafloor than the flapping motion used by pelagic ray species.
āNature seems to have already solved the problem,ā Zhu added.
Robots and underwater vehicles of the future
This isn’t the first time engineers have attempted to apply a ray’s unique biology to the world of robotics. In 2018, engineers at UCLA designed a 10-millimeter-long stingray-like robot made from a mix of heart cells and flexible electrodes. Harvard researchers created an even stranger stingray biohybrid robot in 2017, powered by rat muscles and propelled forward by a light-triggered propulsion system.
Elsewhere, researchers at the University of Washington are already exploring ways to apply ray swimming techniques to next-generation underwater vehicles. Ultimately, they hope to adapt the structural features of the spokes to create vehicles that are both more energy efficient and quieter than current submarines and submersibles.
When it comes to designing the mechanisms of the future, the natural world remains unconquered.
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