Huge Six-Legged Robot Crabster Goes Swimming

Next time you're basking on the beach, watch out for giant crab robots.

The Crabster CR200, a huge six-legged underwater robot, took the plunge earlier this month for the first time. Developed at the Korean Institute of Ocean Science and Technology (KIOST), the Crabster is an alternative to propeller-driven remotely-operated vehicles (ROVs) and autonomous underwater vehicles (AUVs), which are ill-equipped to deal with strong tidal currents in shallow seas.

Robotic underwater vehicles play an important role in marine research, environmental operations, deepwater exploration, and search-and-rescue missions. Most ROVs and AUVs rely on propellers, which allow them to maneuver swiftly and dive to great depths. But there are certain places these machines can't easily access because of strong currents. To overcome this hurdle, researchers at the Korean Institute of Ocean Science and Technology (KIOST) sought inspiration not from fish but from legged sea creatures like crabs and lobsters. The result was Crabster.

Its first major underwater tests took place at KIOST's South Sea Research Institute, located in Geoje City, South Korea. That's about seven hours away from the lab where it was built, but luckily the 600 kg (1322 lb) robot and its remote control station were designed to be easily transported in a pair of shipping containers. In total the robot is 2.42 m long, 2.45 m wide, and 2 m high (7.9 ft x 8 ft x 6.5 ft).

Unlike ROVs and AUVs, the Crabster is designed to be lowered by crane to around 200 meters (650 feet) below the surface, where it will walk along the sea floor on six legs powered by 30 joints. Moving on legs will hopefully prove more stable, and won't stir up as much debris as propellers. And like a crab or a lobster, the robot's two front legs are equipped with manipulators that can grasp objects that can be stored in a frontal compartment. The researchers also designed the robot's shell to deflect strong currents by adjusting its overall posture.

It takes four people to operate the Crabster. The pilot controls the robot's walking and posture while a co-pilot works its manipulators, cameras, and lights. A navigator plans its movement and keeps track of its position, while a sonar engineer monitors the scanning sonar and other sensors. This is all accomplished from the remote control station inside one of the shipping containers.

The Crabster can remain on the sea floor for days at a time if necessary, as it is tethered to an external power source. It's equipped with a high resolution scanning sonar, acoustic camera, acoustic doppler current profiler (ADCP), and several optical cameras. The goal is to explore submerged ships in currents moving at 1.5 meters per second, which are strong enough to rip the oxygen mask off of a scuba diver's face.

crabster underwater robot

After a few days of set-up and on-land tests, it was time to get the Crabster's feet wet. For the initial tests the robot was only submerged 5 to 7 meters (16 to 23 feet) depending on the tide, so the team could check that everything was watertight and test its main capabilities and ballast condition. Then over the next several days the researchers sent the robot diving, gathering data from its sensors (including 15 m and 100 m sonar scans) and setting various control parameters.

"I was very excited, but on the other hand I was nervous during the test," says Bong-Huan Jun, a principal research scientist at KIOST. He says the team had to be cautious because Crabster is packed with complex  systems, and this is its first test in real-world conditions. "There are many mechanical and electrical parts inside the legs, such as the electrical motors, reduction gears, amplifiers, and interface boards," he explains. "If even one sealing is faulty, seawater will enter the container which could cause shorts and corrosion. Any single minor problem would require a long time to fix."

Crabster performed well during its first dive. There were some minor problems but the Korean researchers were satisfied with the results. "We have brought a multi-legged underwater robot to reality," says Bong-Huan Jun. In future tests, the plan is to assess all of Crabster's capabilities, as well as identify possible upgrades. Then early next year, the team wants to send the robot to investigate a submerged ship in a strong tide.

The Crabster team, led by Bong-Huan Jun, includes researchers from five universities: Seoul National University of Science and Technology developed the gait algorithm, using Little Crabster as a test-bed; Mokpo National University performed hydrodynamic analysis and estimation of current load on the robot; Chungnam National University helped with optimal joint path planning for swimming motion; Pukyong National University studied how to implement fault tolerance gait; Kookmin University contributed with numerical modeling and simulation of the robot; and Korea Marine University developed the master arm and remote control of manipulators.

[ KIOST ]

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