Move with the Current

Jellyfish, manta rays, eels, and other sea creatures appear to move effortlessly in the water, easily stabilizing themselves against the underwater currents that move against them. Most autonomous underwater vehicles (AUVs), however, are stiff and unyielding—and those more rigid bodies mean they must expend a great deal of energy to counteract the motion of the water. 

“Most of these AUVs are designed in a straight line, torpedo-style, and can move forward very efficiently but that’s it,” said Leo Micklem, a roboticist who formerly worked at the University of Southampton in the United Kingdom and is currently at Portland State University. “If you want some level of maneuverability, you are relying on what are basically big boxes with propellers on all sides to move it around in the water. It’s just not efficient. So, we started to think about how we could get better biological control, introducing the idea of proprioception to the system.” 

Mimicking proprioception, the body’s internal sense of position, movement, and force, is a challenging task. But Micklem, who had been working on the design of a soft robotic wing while at the University of Southampton, and colleagues teamed up with researchers at the University of Edinburgh to make use of an innovative new electronic skin (e-skin) they had designed to enhance robotic gripping systems. The e-skin acts somewhat like nerve cells, sensing changes in the air to guide the way the grippers move in space. This research, “Harnessing proprioception in aquatic soft wings enables hybrid passive-active disturbance rejection,” was recently published in Nature.

While there are a range of one-dimensional sensors on the market that can capture one bending mode, Micklem said, the e-skin offered the team the potential to perceive forces underwater, helping them achieve a high-resolution response to the “complex shapes and geometries” seen in moving water. 

“We knew that the system worked in air,” he said. “But we needed to make it work underwater—and if we could, we knew it would give us a lot more stability than other approaches.” 

Translating the e-skin’s capabilities in air to an underwater environment, Micklem said, was “quite tricky,” requiring the research group to rethink how to train the model that governed movement.  

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“The way the sensors work is you measure relative capacitance between the various lengths of liquid metal wires. When you’re underwater, those signals are always changing,” he explained. “So, we had to train the model underwater, using image capture from a single camera…it was a real iterative process but one that ultimately allowed us to show that this works as a proof of concept.” 

When the team tested the new wing by subjecting it to underwater disturbances of different shapes and magnitudes, they found it performed better than both rigid wings and other types of soft wing designs. For example, they saw it could respond up to four times faster than similar AUV soft wings without the e-skin. In addition, the soft wing/e-skin design consumed five times less energy than systems that currently use thermal energy to change shape in response to environmental changes. While the wing has only, thus far, been tested in the laboratory, the results potentially translate into huge improvements in stability, responsiveness, and efficiency for underwater systems, Micklem said. 

When asked what’s next, Micklem said he and his colleagues continue to improve upon the design—and consider how to scale it for use in a real underwater environment. 

“How do we integrate this on to an underwater vehicle and start testing it out in real flow conditions? At the moment, we’ve tested this design in only experimental facilities with four sensors. Everything has been super precise and measured,” he explained. “And, in soft robotics, we see moving from proving concepts to implementation in real world applications is quite a difficult step.” 

He is, however, optimistic that this concept can improve AUVs in the future—thanks to its bio-inspired design. And he hopes that other engineers who are working on a challenging design problems remember there is a “huge amount of inspiration in the world around us.” 

“If we’re given the space and freedom to be creative with natural inspiration, rather than just replicate what [different animals] do, and take the concepts, the physics, and the ideas into our research space, we can come up with some really interesting solutions that have the capacity to even outperform their biological counterparts,” he said. “Always remember there’s not just one way to move forward.” 

Kayt Sukel is a technology writer and author in Kansas City.