Remote-controlled vehicles have changed the way we explore and use the ocean. They can be used for much longer than human-occupied vehicles, penetrate areas where risk dictates human avoidance, and reach depths where very few vehicles can accommodate a human. Even so, much of the hardware is housed in an enclosure capable of protecting things like batteries and electronics from the pressures of the deep.
However, it may not be strictly necessary based on a report in the nature of today. In it, a team of Chinese researchers describes how hardware is adapted in such a way that it can operate a robot with a soft body in the deep sea. The researchers then drove the robot 10 kilometers into the Mariana Trench and showed that it worked.
Mention robots, and for many people the first thing that comes to mind is the collection of metal and cables that make up things like Boston Dynamics' dancing atlases. But over the past decade, many researchers have shown that all of this rigid hardware is not strictly necessary. Soft-bodied robots work too and can do interesting things like squeezing through tight spaces or drawing living cells into their structure.
The lines between biology and robotics can also become blurred in other ways, as the shape and behavior of these robots are often directly inspired by biology.
That seems to be what happened to the new piece in Nature. The researchers say they were inspired by species of snailfish found in deep-sea trenches over five miles from the surface. The snail fish seem to have an unusual adaptation to the pressure prevailing there: their skulls do not close completely. With less rigid material to be crushed, the fish seems to be better able to adapt to conditions at depths.
A soft-bodied robot seems to allow this kind of flexibility. However, robots need things like power sources, actuators, control electronics, and other solid hardware. While some soft-bodied versions retain all of the rigid material on the outside and only send power and control signals to the robot, here the researchers tried to create something of their own – something that comes closer to a remote-controlled vehicle. This requires rethinking how the hardware part of a soft body robot is put together.
Apparently, lithium batteries don't have any particular printing issues. But that does not apply to electronics. We've generally put everything we need – processors, power converters, control hardware, and so on – on a single circuit board. Testing one of these cards under pressure showed that hardware failures were common and these failures tended to be at the juncture between components on the card.
So the researchers split the board as much as possible. You put different components on separate circuit boards, place them in different places in the robot's body and connect 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 loads on the board by a factor of six.
The other thing that needed to be reworked was the actuator, which acts as the muscles of the soft robot, causing it to flap its fins. As with a number of other soft robots, these are made from a flexible polymer that expands and contracts in response to an applied current. A typical material used for this responds to currents with a contraction of almost 20 percent of its length. Unfortunately, the same material is subjected to the high pressures and low temperatures typical of the deep sea, and that number drops below 3 percent, which is not enough to actually propel the robot's fins.
The researchers found a related material that doesn't normally work as efficiently (its stretching response is only 13 percent). In a cold, high pressure environment, however, this response doesn't drop nearly as much and is 7 percent. While not great, it is still enough to propel the robot's fins.
Overall, the design has all of the electronics in the robot's body, behind which extends a tail that is roughly the length of the body. Each side has two large fins that can bend up and down. A thin, flexible membrane attached to the fin provides propulsion. Overall, the robot is about 22 cm long and 28 cm wide from fin tip to fin tip. The "muscle" of the fish is located at the junction between the body and the fin; Its contraction pulls the fin downward relative to the body.
You can see it in action in the video below. While it's not shooting straight through the water, it's moving significantly away from where it was released.
The robot glides through a lake.
This test was carried out approximately 70 meters in a lake; Most of the first tests were done there or in a high pressure tank at a university. But when it happened, the robot was brought 3 km into the South China Sea and was allowed to demonstrate its swimming skills there. 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 sea surface. While the researchers didn't risk releasing it to swim free, they showed that the actuators worked as expected and the fins fluttered in a way that would have provided propulsion if it had been released.
This is far from replacing some of the solid, flameproof hardware we currently have in the oceans. For one thing, the robot's on-board battery doesn't even support an hour of operation. And so far, at least, the control circuit has not been asked to do anything more complicated than make it swim in circles (which can be done simply by flicking one of the fins more often than the other). Finally, the researchers did not use any functions to control the depth of the robot.
Even so, it works, and it's a demonstration that a soft body doesn't mean these robots need to be restricted to temperate environments.
Nature, 2021. DOI: 10.1038 / s41586-020-03153-z (About DOIs).