Most people know the crab for its famous sideways walk. As savvy snorkelers and aquatic roboticists know, however, the often observed laterigrade scurrying is only one of many tricks in the crustacean’s bag of maneuverability. For instance, with their springy legs, crabs can navigate rough underwater terrain, and easily cling to rock, despite challenging currents.
These fascinating abilities make the crab attractive to roboticists eager to explore the ocean floor. Marcello Calisti, a professor of bioengineering at the BioRobotics Institute of the Sant'Anna School in Italy, desires more than exploration, though; he wants to clean it. His machine—a plastic-grabbing crab bot called Silver 2 (Seabed-Interaction Legged Vehicle for Exploration and Research)—made a first plunge in the Ligurian Sea this past summer.
While the Dutch-based robotics project, The Ocean Cleanup, targets large pieces of plastic and trash floating near the water’s surface, Calisti and his students are aiming to collect microplastics that settle near the ocean floor, polluting oceanic ecosystems and entering, consequently, the food chain.
Submarines and other traditional aquatic robots are unable to reach this sunken sea junk; as they approach the seabed—or rest on it—they inadvertently disturb the sand, burying or pushing away microplastics and other trash.
Additionally, hovering above the sea floor requires too much effort to make the task’s completion worthwhile. “It was natural to turn to the crab,” Calisti said. “They run quickly inside and outside the water, and they can stand on wet rocks—no problem—without manipulating the environment at all.”
Silver 2 robot is not the first to draw inspiration from a crab. The Korean Institute of Ocean Science and Technology unveiled their gigantic 1,400-pound Crabster several years ago, also designed to stroll across the ocean floor.
Though its design—a helmet-like body with six legs—clearly drew from the infraorder Brachyura species, the Crabster and other similar robots do not imitate the 10-legged beast accurately, limited to ambling along flat swaths of sand. “Underwater terrains have rocks, slopes, corals—elements which prevent you from moving in the typical way a crab walks outside water, on flat ground,” Calisti said.
Calisti used high speed cameras to capture crab's locomotion. His version of the robot crab has articulated legs, each with three degrees of free motion. Also, the legs are spring-loaded, as a crab’s leg springiness largely affects its ability to navigate rocky terrain. Thanks to the accuracy of its imitation, Silver 2 can cling to rocks for hours or inspect coral reefs without damaging them.
The crab has six legs for locomotion and can dive to a depth of 200 meters. Researchers at the BioRobotics Institute are continuing to improve their prototype: A trash-collecting robotic arm will be added soon. In Calisti’s vision the robot will forage for garbage and deposit whatever it collects in underwater docking stations.
Currently, the crab bot must be controlled by someone from a boat at the water’s surface. Someday, however, Calisti hopes his robot will roam the ocean floor, gathering trash, entirely on its own. “That’s my dream for the future…to make it totally autonomous,” he said.
Michael Abrams is a technology writer based in Westfield, New Jersey.
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