What other tests or uses are scheduled for the robot in the near future?

In April, Crabster has deployed to the site of Sewol ferry disaster to assist rescue operations. But originally, Crabster was supposed to explore ancient shipwreck in west sea of Korea during that time. So, we are planning again to explore the ancient shipwreck in next spring.

What are the key mechanisms that make Crabster work?

Crabster is the robot for underwater exploration. So the most important key technology is the water proof and pressure resistant mechanism. Crabster is designed to be operated in 200 meters depth so it has to endure minimum 20 bar of the water pressure. The second is the harmonious and stable control of many motors in six legs. Crabster has six legs and thirty BLDC motors in the legs. To walk on the seafloor or explore underwater, Crasbster has to be control the motors harmoniously and stably. All joints are connected to the computer via CAN bus. The computer computes commands for the harmonious control of all joints by solving kinematics and dynamics of all the legs. Crabster can walk with various gait algorithms to explore on the seafloor. Crabster can perform posture control to survey or work on the seafloor.

What were the chief challenges to designing Crabster?

One of the challenges was to design a hydrodynamically streamlined body. The body shape has to minimize the fluid drag while securing sufficient space for the exploring equipment. The body also has to be lightweight but structurally strong. We decided to adopt composite material to satisfy the competing demands. We designed the frame using carbon fiber reinforced plastic and the skin using glass fiber reinforced plastic.

Another challenge was to design an underwater joint mechanism. The joints of legs have to be powerful, no backlash, no exposure of wiring, watertight and compact. We adopted frameless BLDC motors and harmonic drives for the joint mechanism. More than ten thousands parts were used in a front leg and the leg has more than 60 O-rings for watertight.

Will Crabster help out at the ferry site in the future?

I hope so, but I don’t know. We reported our survey results to government and government will make decision.

Was it ever difficult to coordinate Crabster's work along with all the other ships, robots and human divers operating at the ferry site?

Hundreds of ships were float around the ferry site and dozens of boats were float around the barge on the ferry site. It was not easy to work along with other ships because the velocity and direction of current were changed continuously. During the Sewol survey, our ship drifted to the barge and Crabster’s tether cable touched the anchor line of barge. We had to have help from a tug boat to escape the situation.

How did Crabster compare with the robotic submersibles (AUVs and ROVs) that were deployed at the ferry site?

Because Crabster weighs 150~ 200 kg in water, it can keep its posture and position more stably on seafloor even in current. Crabster has acoustic instrument against turbid water. In addition to the default sonars, we installed additional sonars when we were waiting in pier. We had additional scanning sonar for vertical scan and additional multi-beam sonar for real-time image in more range. Our team developed AUVs and ROVs for last 20 years, those propeller-driven subs tends to drift by current which is resulted to entangle the tether cable with mooring line of other ships or barge. I think the ROVs deployed at ferry site were not different.

Did Crabster provide any advantages for the search and salvage operation over human divers? Was it able to work together with human diver teams?

Since the visibility at the site was only 0.2 meters, divers were hard to search underwater. Crabster can stay deeper and longer, Crabster can see farther. But Crabster cannot go into the ship. We wanted to work together with human divers, but we had no chance to do.