Flexible robot fin automatically adapts to water currents

A research team from the University of Southampton has developed a robot fin that can detect changes in water flow and automatically adjust its shape to compensate for flow disturbances. Compared to autonomous underwater vehicles (AUVs) with rigid fins, the adaptive fins can increase the efficiency and maneuverability of underwater robots.

In designing the adaptive fin, the scientists at the University of Southampton were inspired by nature – specifically by the body's own senses in birds or fish. Birds perceive changes in airflow through their plumage via proprioception, while fish sense changes in the water flow around them via their lateral line system and fin rays. The latter refers to the rigid supporting elements of, for example, a dorsal fin.

For their robot fin, the engineers have developed an electronic skin that can detect the smallest changes in water flows, as described in the study “Harnessing proprioception in aquatic soft wings enables hybrid passive-active disturbance rejection,” published in npj.

Adaptive Fin

The fin essentially consists of silicone, into which flexible liquid metal wires are integrated. These wires react to deformations and emit corresponding signals as soon as the flexible fin is deformed by changes in water flow. The changes in resistance in the wires are used to control the stiffness and curvature of the fin. This is done by a hydraulic system housed in the body of an underwater robot, which transmits pressure via two hoses to the fin, thereby adapting its shape accordingly.

The researchers are thus addressing the problem that the rigid bodies and fins of AUVs require a lot of energy to counteract forces when they are caught by sudden waves and currents. The electronic skin quickly detects the flows and automatically controls the fins to increase the stability, responsiveness, and efficiency of the underwater robot – with less energy expenditure. In turbulent ocean environments, the UAV thus remains more stable and retains its maneuverability.

In tests, the scientists found that the adaptive fins reduced unwanted buoyancy from sudden water currents by 87 percent compared to a similar AUV with rigid fins. Self-stabilization also increased.

However, the researchers have also identified limitations with the technology. For example, scaling and integrating the technology into rigid hulls can be an issue. The durability of the technology also needs further work. The researchers believe that these difficulties can be partially overcome by using powerful, stable actuators.

(olb)