With heavy bodies and small flippers, seals aren’t the most graceful movers out of water. But despite appearances, a robot that imitates the way they flop over dry land might be effective in search and rescue operations where a wheeled robot would struggle, says the team that made it.
Dimuthu Kodippili Arachchige at DePaul University in Chicago and his colleagues created a robot that emulates the way pinnipeds – such as seals and sea lions – bounce and lunge on land, bobbing their heads and bodies to gain momentum while pushing along the ground with their flippers.
The robot consists of four identical limbs, each 24 centimetres long and 4 centimetres in diameter. Each limb is made of three silicone tubes that can be filled with liquid to become rigid, or drained to become soft, all wrapped in a hard plastic skin. By selectively filling one or more tubes, the robot can steer each limb in any direction.
In experiments, the robot was able to move forwards at almost 12 centimetres a second, but can go faster backwards, reaching almost 17 centimetres a second.
Arachchige says that having all the limbs identical makes the robot more adaptable, but it struggles to replicate the movement of a seal because it has less mass and is arranged in a different way.
“Most of a seal’s weight is concentrated towards the rear of their body, but due to the even distribution of weight in the robot, it becomes challenging to stay upright while moving forward,” he says. “On the contrary, when it’s moving backward, the robot’s body helps maintain the balance by countering torque produced by its movement.”
The way a real seal turns by bobbing and lurching sideways doesn’t work well for the robot either, and it also turns more effectively while moving backwards.
Despite this, Arachchige has resisted simply making the back of the robot the front, which would allow it to move faster and turn while travelling forwards. “If we make the back the front, the robot doesn’t resemble a pinniped anymore,” he says.