Robotic fish could perform underwater-recovery missions

Researchers at Northwestern University have created a robotic fish that can move from swimming forwards and backwards to swimming vertically almost instantaneously by using a sophisticated, ribbon-like fin.

The robot — developed after observing and creating computer simulations of the black ghost knifefish — could pave the way for robots that could perform underwater recovery operations or long-term monitoring of coral reefs.

The black ghost knifefish, which hunts for prey at night in rivers of the Amazon basin, uses a weak electric field around its entire body, moving both forwards and backwards using a ribbon-like fin on the underside of its body.

Malcolm MacIver, associate professor of mechanical and biomedical engineering at Northwestern’s McCormick School of Engineering and Applied Science, has studied the knifefish for years. Working with Neelesh Patankar, associate professor of mechanical engineering, he has created mechanical models of the fish in the hope of better understanding how the nervous system sends messages throughout the body to make it move.

Planning for the robot — called GhostBot — began when graduate student Oscar Curet observed a knifefish suddenly moving vertically in a tank in MacIver’s lab. Further observations revealed that while the fish only uses one travelling wave along the fin during horizontal motion (forward or backward depending on the direction on the wave), while moving vertically it uses two waves. One of these moves from head to tail, and the other moves tail to head. The two waves collide and stop at the centre of the fin.

The team then created a computer simulation that showed that when these ’inward counterpropagating waves’ are generated by the fin, horizontal thrust is cancelled and the fluid motion generated by the two waves is funnelled into a downward jet from the centre of the fin, pushing the body up. The flow structure looks like a mushroom cloud with an inverted jet.

The group then hired Kinea Design, a design firm founded by Northwestern faculty that specialises in human interactive mechatronics, and worked closely with its co-founder, Michael Peshkin, a professor of mechanical engineering, to design and build a robot.

The company fashioned a forearm-length waterproof robot with 32 motors giving independent control of the 32 artificial fin rays of the Lycra-covered artificial fin.

The group then took the robot to Harvard University to test it in a flow tunnel in the lab of George Lauder, a professor of ichthyology. The team measured the flow around the robotic fish by placing reflective particles in the water, then shining a laser sheet into the water. That allowed them to track the flow of the water by watching the particles, and the test showed the water flowing around the biomimetic robot just as computer simulations predicted it would.

The robot is also outfitted with an electrosensory system that works in a similar fashion to that of the knifefish, and MacIver and his team now hope to further improve the robot so it can autonomously use its sensory signals to detect an object and then use its mechanical system to position itself near it.