The humble razor clam inspires underwater innovation
|The RoboClam anchoring device
Images: Donna Coveney
|RoboClam, the metal device on the right, next
to the inspirational razor clam to the left
Dubbed “the Ferrari of underwater diggers,” the tenacious razor clam has inspired the invention of a new MIT robot by a team lead by Anette “Peko” Hosoi, associate professor in the Department of Mechanical Engineering. Hosoi and Amos Winter, a graduate student in her lab, along with engineers at Bluefin Robotics Corp, are collaborating to explore the performance capabilities of clam-inspired digging.
“Our goal was to develop a lightweight anchor that you could set and then easily unset, something that’s not possible with conventional devices,” Hosoi says. RoboClam may indeed lead to the development of a “smart” anchor that burrows through the ocean floor to reposition itself and even reverse, making it easier to recover.
Such a device could be useful, for example, as a tether for small robotic submarines that are routinely repositioned to monitor variables, such as currents and temperature. Further, a device that can burrow into the seabed and be directed to a specific location could be useful as a detonator for buried underwater mines. Winter presented the team’s results at a recent meeting of the American Physical Society.
Novel propulsion mechanisms
For several years, Hosoi’s research has focused on propulsion mechanisms inspired by nature. So when faced with the anchor problem, the team thought, “Is there an animal that’s well adapted to moving through sediment on the seafloor?” The first stage of the research, Winter says, involved “looking at all the organisms I could find that dig into the ocean bottom, stick to it, or cling to it mechanically.” His search uncovered the razor clam. About seven inches long by an inch wide, the razor clam can move at approximately a centimeter a second. “You have to dig fast to catch them,” says Winter, who became a licensed clam digger as a result of the research.
Another reason the razor clam makes a good model for novel anchors: it can dig deeply (upto about 70 centimeters). Most important, in a measure of anchoring force—how hard you pull before an anchor rips out of the soil compared to the energy required to embed it—the razor clam reigns supreme. “It beats everything, including the best anchors,” Winter explains, “by at least a factor of 10.”
Research subject in hand, one of the team’s first tests gave perplexing results. They pushed a clam shell cast in epoxy into sand composed of glass beads and compared the amount of force necessary to do so to the razor clam’s performance. They found a major discrepancy between the two. “They’re much too weak to do what they do,” Hosoi says, “so we knew they were doing something tricky.” To test the discrepancy, Winter created a glass-sided box filled with water and beads, added a living clam, and watched the animal burrow. It turns out to be a multistep process. The animal wiggles its tonguelike “foot” down into the sand, then makes a quick up-and-down movement while opening and closing its shell. Together, these movements propel it.
By filming the movement of the beads, Winter made a startling discovery. The clam’s quick up and-down and opening-and-closing movements turned the waterlogged sand around it into a liquid-like quicksand. Experiments showed that moving through a fluidized substrate (the quicksand) rather than a packed granular medium (ordinary sand) drastically reduces the drag force on the clam’s body, bringing it to a point within the animal’s strength capabilities.
Over the summer, Winter completed construction of the RoboClam itself. Although only about the size of a lighter, it is supported by a large apparatus of pressure regulators and pistons that control such variables as how hard the robot is pushed in each direction.
This work was sponsored by Bluefin, Battelle, and Chevron.
Excerpted from the MIT Tech Talk article by Elizabeth Thomson