Robotic manta rays and turtles, like the MantaDroid and naro-tartaruga, may solve current limitations to AUVs as the next generation of marine robots copy biological designs to become more efficient, adaptable and maneuverable.
Imagine a future where underwater robots that swim and move like real animals are navigating and assessing our waterways and oceans. While it may sound like science fiction, this robotic future is happening now. Engineers looking to make underwater robots more efficient, adaptable and maneuverable are drawing inspiration from nature, replicating manta rays, turtles, sea snakes and, of course, fish.
Biomimicry inspires many science and technology fields, including medicine, materials, aviation, and architecture. Biomimetic autonomous underwater vehicles (BAUVs) mimic blueprints found in marine animals to overcome the limitations of traditional propeller-driven AUVs. Evolution, a slow and iterative process, is the perfect testbed to develop hydrodynamic prototypes. This gives engineers a head start when developing underwater robots which can perform better in the dynamic ocean environment.
Initial attempts at biomimetic underwater robots imitated the locomotion of fish, which vary based on the amount of body movement: whole body (anguilliform), half to one third (carangiform and sub-carangiform) to just the tail (thunniform). Years of foundational research went into understanding drag and turbulence along these flexible-hulled underwater vehicles. While many BAUVs continue to resemble fish, an obvious choice in water, the next generation also are looking at other animals for inspiration.
Using only one motor per fin, the MantaDroid uses passive dynamics to move through the surrounding water with ease. Video Credit: National University of Singapore.
In the underwater realm, manta rays have perfected efficient locomotion, being almost 90 percent efficient – the ray’s interaction with water is a flawless study in fluid dynamics. Associate Professor Chew Chee Meng and his team at the National University of Singapore have built on this to develop MantaDroid, a zippy 35 cm long biomimetic robot.
With a background in bipedal, or humanoid, robots, Chew knew the most optimal design would not fully mimic the manta ray. For efficient two-legged locomotion, the best approach is passive dynamics, where swinging motions naturally occur by the interaction between gravity and the leg. His team used the same approach for MantaDroid.
“We tried to not fight against hydrodynamics – the interaction between the wing and the water,” said Chew. “If you try to fight against hydrodynamics you waste a lot of energy in creating unnecessary turbulence.”
Efficient locomotion is crucial to AUV research, to conserve battery power and ensure long deployment times. Forward thrust is generated in marine animals when either the body bends or paired fins move to create a propulsive wave. This design pushes the animals, and now robots, through the water.
By keeping the design simple, with one motor controlling the stiffer leading edge of the flexible PVC wing, MantaDroid moves elegantly through the water using principles similar to a kite. The wing design mimics the oscillatory (or mobuliform) locomotion of a live ray, which produces vortices of water to generate forward thrust, allowing the robot to be propelled forward at around two body lengths per second.
“Simplicity is the best approach for any engineering solution. If you start with a complex system, it’s very difficult to build upon and add in complexity later. You make the system intractable,” said Chew.
The current MantaDroid is too small to cope with conditions at sea, though Chew’s team is already working on the second iteration. The next droid will be fully autonomous, allowing it to interact and adapt with the world around it. The droid also will be twice the size, with room for more sensors along its belly. By adding sensors, BAUVs are one step closer to becoming platforms for science.
Cédric Santana-Siegenthaler is one researcher looking to make BAUVs into more stable science platforms via the naro-tartaruga, a mechanical turtle which resides at the Swiss Federal Institute of Technology in Zurich (ETH Zurich).
“The main goal is a robot body that is more usable for actual research. On a fish, you have a hard time placing sensors and components because the whole body is moving,” explained Santana-Siegenthaler. “On a turtle, we can keep the fin locomotion but use a subject that has a rigid body.”
While naro-tartaruga’s predecessor, naro-original, copied the geometry and thunniform locomotion of a tuna, naro-tartaruga is equivalent in weight, form and movement of a small leatherback turtle, with a 1.7-meter wing span and weighing in at 75 kg. The powerful pectoral fins provide a high degree of maneuverability.
“The benefit of whole fin locomotion is that you can adjust the movement based on travel speed,” said Santana-Siegenthaler. “You can optimise speed relative to motion whereas propellers are tuned to be very efficient at one specific speed, but lose performance over a big range.”
Increased maneuverability will be key to future AUVs, where a propeller won’t cut it for slow-speed inspection of vessel hulls, pipelines, or downed shipping containers. By combining this with long range endurance, future BAUVs may have the ability to combine wide scale search, with localised missions.
For the next stage in naro-tartaruga’s evolution, a teardrop shell will be attached to provide a more hydrodynamic shape, a more realistic look and, importantly, space for additional payload – cameras, side-scan sonar, velocity loggers. These sensors will allow naro-tartaruga to understand its location in time and space, as well as provide important data about the underwater environment. Slow-speed maneuverability isn’t the only thing researchers have in mind however – they’re also hoping to make BAUVs faster.
When it comes to speed, salmon are model candidates for studying, as they combine exceptional forward propulsion with agile maneuverability. RoboSalmon is closer to the original fish-like BAUVs. Developed by Euan McGookin, senior lecturer at the School of Engineering at the University of Glasgow, RoboSalmon is a replica of the North Atlantic salmon. It has sub-carangiform locomotion, meaning more than a third of its body bends to generate the force to move forward.
Both body- or fin-based locomotion are also far quieter than traditional propellers and this, combined with its propulsive capacity makes RoboSalmon the ideal platform for studying fish schools. These qualities may circumvent fish-schools’ avoidance of large noisy objects.
“The original idea behind RoboSalmon was to evaluate the energy requirements for fish using fish ladders. It was also used to count the number of fish that it encountered during operation in order to obtain a more accurate estimate of wild salmon stocks,” said McGookin. By recreating salmon in mechanical form, a better understanding of how fish and fish schools interact with their environment and each other also is possible.
RoboSalmon has yet to venture into the open ocean. “RoboSalmon has been tested in shallow waters, replicating rivers rather than open sea. The current version of RoboSalmon would be able to operate at depths of about 20 meters,” said McGookin.
Venturing into the open ocean is the goal for these robots, though they have to contend with a much harsher environment than their testing pools – faster currents and deeper depths. To make that move, a hybrid biomimetic-propeller based approach will be required, with other researchers delving into this option.
The Eelume takes the hybrid approach by combining the flexibility of an articulated snake with the propulsion of a propeller-based AUV. This combination of flexibility and thrusters means Eelume will have enough power to overcome fast currents and conduct underwater operations.
Eelume is modular, linked by flexible joints, allowing for different payload combinations and thruster configurations. Different sensors and tools can be attached along its length and ends, meaning not only inspection but light intervention is possible.
This design is intended to mitigate the need for a remotely operated vehicle (ROV). “Eelume is designed to live on the seafloor and operate autonomously. This removes the need for a crewed vessel, making them more cost efficient even if they lack the power of a traditional ROV,” says Arne Kjørsvik, CEO of Eelume Subsea.
This is not a traditional BAUV however, as even though it resembles the anguilliform locomotion of an eel, the flexible body provides little forward propulsion. This motion has other benefits though, aside from looking cool. Its slender body can reach awkward places, unattainable by standard AUVs.
“We have a very high degree of maneuverability because the whole body can move in all directions. For maintenance and inspection work, we need to be able to maneuver into confined spaces and this vehicle is capable of doing that,” explained Kjørsvik.
Eelume also can go deep. It’s currently rated to 500 meters, more than enough to cover the average depth of the North Sea, its testing ground and eventual work environment. Expected to be complete as an ROV by the end of 2018, by 2019 Eelume will be operating autonomously, without any tether.
While propeller-driven AUVs remain the workhorse of underwater exploration, the results and techniques from these niche bio-robots will enable more efficient, agile, and robust AUVs in the future. In the meantime, work into replicating fish, turtles and manta rays continues, so that these bio-robots might soon be making a break into the open ocean.
Erica Spain is a Ph.D. candidate at the Institute for Marine and Antarctic Studies in Hobart, Australia. She’s using AUVs to explore extreme environments in the Southern Ocean and Antarctic. Follow her on Twitter @xSmerica.