"Two years ago, we demonstrated an aquatic soft robot that was able to reach average speeds of 3.74 body lengths per second," said Jie Yin, associate professor of mechanical and aerospace engineering at NC State and corresponding author of the study. "Our new soft robot is more energy efficient and reaches a speed of 6.8 body lengths per second. In addition, the previous model could only swim on the surface of the water. Our new robot is capable of swimming up and down throughout the water column."
The robot's design incorporates manta ray-like fins, made from a material that remains stable when the fins are fully extended. These fins are attached to a flexible silicone body containing an air chamber. By inflating and deflating the chamber, the robot mimics the up-and-down fin movements of a manta ray, propelling itself forward. A video showcasing the robot's capabilities can be viewed at https://youtu.be/pXB9Ip7qa0.
"Pumping air into the chamber introduces energy into the system," explained Haitao Qing, the study's first author and a Ph.D. student at NC State. "The fins want to return to their stable state, so releasing the air also releases the energy in the fins. That means we only need one actuator for the robot and allows for more rapid actuation."
The research team closely studied manta ray fluid dynamics to optimize the robot's vertical movement.
"We observed the swimming motion of manta rays and were able to mimic that behavior in order to control whether the robot swims toward the surface, swims downward, or maintains its position in the water column," said Jiacheng Guo, a co-author of the paper and Ph.D. student at the University of Virginia. "When manta rays swim, they produce two jets of water that move them forward. Mantas alter their trajectory by altering their swimming motion. We adopted a similar technique for controlling the vertical movement of this swimming robot."
The robot's design leverages its ability to generate differing jet forces. "Simulations and experiments showed us that the downward jet produced by our robot is more powerful than its upward jet," said Yuanhang Zhu, an assistant professor at the University of California, Riverside, and co-author. "If the robot flaps its fins quickly, it will rise upward. But if we slow down the actuation frequency, this allows the robot to sink slightly in between flapping its fins - allowing it to either dive downward or swim at the same depth."
The team also noted how buoyancy influenced its functionality. "We are powering this robot with compressed air," Qing said. "When the fins are at rest, the air chamber is empty, reducing buoyancy. In contrast, rapid fin flapping keeps the air chamber full, increasing buoyancy."
The robot demonstrated its capabilities in two key tests: navigating a complex obstacle course in a water tank and hauling a payload while untethered on the water's surface.
"This is a highly engineered design, but the fundamental concepts are fairly simple," Yin noted. "With only a single actuation input, our robot can navigate a complex vertical environment. We are now working on improving lateral movement, and exploring other modes of actuation, which will significantly enhance this system's capabilities. Our goal is to do this with a design that retains that elegant simplicity."
Research Report:Spontaneous snapping-induced jet flows for fast, maneuverable surface and underwater soft flapping swimmer
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