Sun. Oct 2nd, 2022
Researchers build a swimming robot that works in the Mariana Trench

Remotely operated vehicles have changed the way we explore and exploit the ocean. They can operate much longer than human-occupied vehicles, go to areas where risk would avoid humans, and reach depths where very few craft can carry a human. Yet much of the hardware is taken up by an enclosure that can protect things like batteries and electronics from the pressures of the deep.

But that may not be entirely necessary, according to a report in Today’s Nature. In it, a team of Chinese researchers describes modifying hardware so that it could control a soft-body robot in the deep ocean. The researchers then gave the robot a 10-kilometer ride down the Mariana Trench and showed that it worked.

getting soft

Mention robots, and for many people the first thing that comes to mind is the collections of metal and cables that make up things like Boston Dynamics’ dancing atlases. But over the past decade, many researchers have shown that all that rigid hardware isn’t strictly necessary. Soft-bodied robots also work and can do interesting things, such as squeeze through tight spaces or incorporate living cells into their structure.

The lines between biology and robotics can be blurred in other ways, too, with the shape and behavior of these robots often directly inspired by biology.

That seems to have happened with the new piece in Nature. The researchers say they were inspired by species of snail fish found in deep-sea gullies more than five miles from the surface. The snailfish seem to have an unusual adaptation to the crushing pressures found there: Their skulls don’t close all the way. With less stiff material to crush, the fish seems to be able to adapt better to the conditions in the depth.

A soft-bodied robot seems to allow for just this kind of flexibility. But robots need things like power sources, actuators, control electronics, and other solid hardware. While some soft-bodied versions keep all the rigid material on the outside and only send power and control signals to the robot, here the researchers tried to create something self-contained — something closer to a remote-controlled vehicle. This requires rethinking how the hardware part of a soft-body robot is put together.

To make adjustments

Apparently lithium batteries have no special problems with pressure. But that doesn’t apply to electronics. In general, we have put everything needed – processors, power converters, control hardware, and so on – on one circuit board. Testing one of these boards under pressure showed that hardware failures were common, and those failures usually occurred at the junction between components on the board.

So the researchers divided the board as much as possible. They placed different components on separate boards, placed them in different locations in the robot’s body and connected them with flexible wires. And for components that cannot be placed on separate boards, the researchers designed larger boards so that the components have more space between them. Overall, this reduced the stresses on the board by a factor of six.

The other thing that needed some overhaul was the actuator, which acts like the soft robot’s muscles, making it flap its fins. As with some other soft robots, these are made of a flexible polymer that expands and contracts in response to an applied current. A typical material used for this reacts to currents with a contraction of almost 20 percent of its length. Unfortunately, put the same material into the high pressures and low temperatures typical of the deep ocean, and that figure drops to less than 3 percent, which isn’t enough to actually power the robot’s fins.

The researchers found a related material that normally doesn’t work as efficiently (the strain response is only 13 percent). But that response doesn’t decrease as much in a cold, high-pressure environment, down to 7 percent. While not great, it’s still enough to power the robot’s fins.

testing, testing

Overall, the design has all the electronics inside the robot’s body, with a tail extending behind it that’s about as long as the body. Each side has two large fins that can bend up and down; a thin, flexible membrane attached to the fin provides propulsion. In total, the robot is about 22 cm long and 28 cm from fin tip to fin tip. The “muscle” of the fish is placed at the junction between the body and the fin; the contraction causes the fin to be pulled downward relative to the body.

You can see it in action in the video below. While it doesn’t actually shoot through the water, it clearly moves away from the point where it was released.

The robot slides through a lake.

That test was done at about 70 meters depth in a lake; most initial tests were done there or in a high-pressure tank at a university. But when it passed, the robot was lowered 3 km into the South China Sea and was allowed to demonstrate its swimming skills. Then, in the ultimate stress test, the robot drove an equipment sled to the bottom of the Mariana Trench, more than 10 km from the ocean surface. While the researchers didn’t risk letting it swim freely, they did show that the actuators worked as expected, and the fins flapped in a way that would have provided propulsion had it been released.

This isn’t close to replacing some of the solid, pressure-resistant hardware we currently have across the oceans. First, the robot’s built-in battery doesn’t even support an hour’s worth of operations. And, so far, the control circuitry hasn’t been asked to do anything more complicated than make it swim in circles (which can be done simply by flapping one of the fins more often than the other). Finally, the researchers have not built in any capabilities to control the depth of the robot.

Still, it works, and it’s a demonstration that a soft body doesn’t mean these robots should be confined to temperate environments.

Nature, 2021. DOI: 10.1038/s41586-020-03153-z (About DOIs).

By akfire1

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