The researchers, who’ve been working on RoboClam since 2006, describe the mechanics behind this process in a paper to be published in Bioinspiration and Biomimetics
In use, RoboClam could be used to dig itself into the ground to bury anchors or destroy underwater mines, claimed its developer, Amos Winter, the Robert N. Noyce Career Development Assistant Professor of Mechanical Engineering at MIT.
Despite its rigid shell, the Atlantic razor clam can move through soil at a speed of 1cm per second. ‘The clam’s trick is to move its shells in such a way as to liquefy the soil around its body, reducing the drag acting upon it. This means it requires much less force to pull its shell into the soil than it would when moving through static soil,’ Winter said in a statement.
To develop a robot that replicates this capability, Winter and his co-developer, Anette Hosoi, professor of mechanical engineering and applied mathematics at MIT, needed to understand how the clam’s movement causes the soil to liquefy, or turn into quicksand, around its shell.
When the razor clam begins to dig, it first retracts its shell, releasing the stress between its body and the soil around it. This causes the soil to begin collapsing, creating a localised landslide around the animal. As the clam continues to contract, reducing its own volume, it sucks water into this region of failing soil. The water and sand particles mix, creating a fluidized substrate (quicksand).
If the clam were to move its shell too slowly, the sand particles would collapse around the animal without fluidising, Winter said. However, if the clam moved too quickly, it would not give the sand particles enough time to mix with the water flowing past, and they would remain stationary. ‘Our data showed that there was a very abrupt transition from being able to fluidize the soil to not moving the soil particles at all,’ he said.
To develop a low-energy anchoring system that can create quicksand around itself in this way, the researchers built a mechanical puppet clamshell, consisting of two halves that can move together and apart. The puppet clam is connected to a rod, which can open and close the shell and push it up and down, creating the same contractions as the animal can achieve.
To make it easier to test their RoboClam prototype in salt water, the researchers used a compressed air system to power the expansion and contraction of the shells. Winter’s team is now developing an electronic version, which will make it compatible for use with underwater vehicles developed by the team’s sponsor, Bluefin Robotics, an MIT spinout.
Winter first began developing the RoboClam for his PhD research, alongside Hosoi. The researchers wanted to find a way to anchor autonomous underwater vehicles to a seabed or riverbed without consuming a great deal of energy. Robotic vehicles have limited battery power, so any energy consumed by the anchoring system would reduce the device’s operating time.
‘You might be operating these vehicles in a current, and need them to be stationary – for example, to monitor a biological situation, or for military purposes,’ Winter said. ‘You wouldn’t want the vehicle constantly spinning its propellers in order to stay in one place because that just wastes energy, so it would be nice if you could just deploy an anchor and maintain your position without expending any energy.’