By David L. Chandler | MIT News Agency
An underwater structure that can change its shape dynamically, as fish do, pushes water far more efficiently than a conventional rigid hull. But creating a deformable device that can change the curves of its body shape while maintaining a smooth profile is a long and arduous process. MIT RoboTunafor example, it is made up of about 3,000 different parts and took about two years to design and build.
Now, researchers at MIT and colleagues — including one from the original RoboTuna team — have devised an innovative approach to building deformable underwater robots, using simple repeating substructures instead of unique components. The team has demonstrated the new system in two different sample configurations, one like an eel and the other like a wing-like hydrofoil. The principle itself, however, allows for nearly unlimited variations in shape and scale, the researchers say.
Existing approaches to soft robotics for marine applications are generally built on small scales, while many useful real-world applications require devices in meter scales. The new modular system the researchers proposed could easily be expanded to such a size and beyond, without the kind of retooling and redesign required to upgrade current systems.
“Scalability is a strong point for us,” said Parra Rubio. Given the low density and high rigidity of the lattice-like pieces, called voxels, that make up their system, he says, “we have more room to continually improve,” whereas today’s most widely used technologies “rely on high-density materials that faced drastic problems” in moving to a larger size.
The individual voxels in the team’s experimental proof-of-concept set are mostly hollow structures made of narrowly supported pieces of cast plastic in complex shapes. The box-like shape is weight bearing in one direction but soft in another, an unusual combination achieved by blending rigid and flexible components in different proportions.
“Treating soft versus hard robotics is a false dichotomy,” says Parra Rubio. “It’s something in between, a new way to build things.” Gershenfeld, head of MIT Bit and Atom Centeradding that “it’s a third way that marries the best elements of the two”.
“The flexibility of the smooth body surfaces allowed us to apply flow control which reduced drag and increased propulsion efficiency, resulting in substantial fuel savings,” said Triantafyllou, who is the Henry L. and Grace Doherty Professor in Ocean Science and Engineering, and was part of the the RoboTuna team.
Credit: Researchers’ award.
In one of the kits produced by the team, voxels are attached end to end in long rows to form a meter-long snake-like structure. The body consists of four segments, each consisting of five voxels, with an actuator in the center that can pull wires attached to each of the two voxels on either side, contracting and causing the structure to bend. The entire 20-unit structure is then covered with support structures such as ribs, and then a tight-fitting, water-resistant neoprene skin. The researchers used the structure in the MIT crane’s tank to demonstrate its efficiency in the water, and showed that it was indeed capable of generating enough forward thrust to propel itself forward using a undulating motion.
“There were a lot of snake-like robots before,” says Gershenfeld. “But they are generally made of bespoke components, as opposed to simple scalable building blocks.”
For example, says Parra Rubio, the snake-like robots built by NASA are made up of thousands of unique pieces, whereas for snakes of this group, “we show that there are about 60 pieces.” And compared to the two years it spent designing and building the MIT RoboTuna, the device was assembled in about two days, he said.
Another device they demonstrated was a wing-like shape, or hydrofoil, which consists of the same array of voxels but is able to change the shape of its profile and therefore control the lift-to-drag ratio and other properties of the wing. Such a wing-like shape could be used for a variety of purposes, from generating power from the waves to helping increase the efficiency of ship hulls — an urgent demand, as shipping is a significant source of carbon emissions.
The wing shape, unlike that of a snake, is covered with an overlapping arrangement of scale-like tiles, which are designed to press against each other to maintain a watertight seal even as the wings change their curvature. One possible application is the addition of hull profiles which could reduce the formation of tug-inducing eddies and thereby increase their overall efficiency, a possibility the team is currently exploring with collaborators in the shipping industry.
Ultimately, the concept could be applied to whale-like submarines, using their shapeshifting bodies to create propulsion. Such an aircraft can escape bad weather by remaining below the surface, but without the noise and turbulence of conventional propulsion. The concept can also be applied to other parts of ships, such as racing yachts, where having a keel or rudder that can bend gently when turning instead of staying straight can provide an added advantage. “Instead of being stiff or just having closure, if you can actually bend like a fish, you can change your course more efficiently,” says Gershenfeld.
The research team included Dixia Fan from Westlake University in China; Benjamin Jenett SM ’15, PhD ’20 from the Discrete Lattice Industry; Jose del Aguila Ferrandis, Amira Abdel-Rahman and David Preiss from MIT; and Filippos Tourlomousis of the Greek Democritos Research Center. The work was supported by the US Army Research Laboratory, CBA Consortium funding, and the MIT Marine Grants Program.