CARL-Bot Catches an Underwater Wave

Small autonomous underwater vehicles, like the drones of the sea, could be very useful for studying the depths of the ocean and monitoring its changing conditions. But such nautical mini bots can be easily overpowered by turbulent ocean currents.

Caltech scientists led by John Dabiri (PhD '05), the Centennial Professor of Aeronautics and Mechanical Engineering, have been taking advantage of the natural ability of jellyfish to traverse and plumb the ocean, outfitting them with electronics and prosthetic "hats" with which the creatures can carry small payloads on their nautical journeys and report their findings back to the surface. These bionic jellyfish must contend with the ebb and flow of the currents they encounter, but the brainless creatures do not make decisions about how best to navigate to a destination, and once they are deployed, they cannot be remotely controlled.

"We know that augmented jellyfish can be great ocean explorers, but they don't have a brain," Dabiri says. "So, one of the things we've been working on is developing what that brain would look like if we were to imbue these systems with the ability to make decisions underwater."

Now Dabiri and his former graduate student Peter Gunnarson (PhD '24), who is now at Brown University, have figured out a way to simplify that decision-making process and help a robot, or potentially an augmented jellyfish, catch a ride on the turbulent vortices created by ocean currents rather than fighting against them. The researchers recently published their findings in the journal PNAS Nexus.

For this work, Gunnarson returned to an old friend in the lab: CARL-Bot (Caltech Autonomous Reinforcement Learning roBot). Gunnarson built the CARL-Bot years ago as part of his work to begin incorporating artificial intelligence into such a bot's navigation technique. But Gunnarson recently figured out a simpler way than AI to have such a system make decisions underwater.

"We were brainstorming ways that underwater vehicles could use turbulent water currents for propulsion and wondered if, instead of them being a problem, they could be an advantage for these smaller vehicles," Gunnarson says.

Gunnarson wanted to understand exactly how a current pushes a robot around. He attached a thruster to the wall of a 16-foot-long tank in Dabiri's lab in the Guggenheim Aeronautical Laboratory on Caltech's campus in order to repeatedly generate what are called vortex rings—basically the underwater equivalents of smoke rings. Vortex rings are a good representation of the types of disturbances an underwater explorer would encounter in the chaotic fluid flow of the ocean.

Gunnarson began using the CARL-Bot's single onboard accelerometer to measure how it was moving and being pushed around by vortex rings. He noticed that, every once in a while, the robot would get caught up in a vortex ring and be pushed clear across the tank. He and his colleagues started to wonder if the effect could be done intentionally.

To explore this, the team developed simple commands to help CARL detect a vortex ring's relative location and then position itself to, in Gunnarson's words, "hop on and catch a ride basically for free across the tank." Alternatively, the bot can decide to get out of the way of a vortex ring it does not want to get pushed by.

Dabiri points out that this process includes elements of biomimicry, stealing a page from nature's playbook. Soaring birds, for example, will often take advantage of strong winds to save energy rather than attempt to fly against them. Experiments have also shown that fish may allow themselves to be carried by the ocean's swirling currents to help conserve energy. However, in both natural cases, the systems are using relatively sophisticated sensory input and a brain to accomplish this.

"What Peter has figured out is that basically with a single sensor, this one accelerometer, and relatively simple control laws, we can achieve similar advantages in terms of using the energy in the environment to go from point A to point B," Dabiri says.

Looking to the future, Dabiri hopes to marry this work with his hybrid jellyfish. "With the jellyfish, we can have an onboard accelerometer measure how this system is getting pushed around," he says. "Hopefully, we can demonstrate a similar capability to take advantage of environmental flows to move more efficiently through the water."

The PNAS Nexus paper is titled "Surfing vortex rings for energy-efficient propulsion." The work was supported by the National Science Foundation.