Video: Yin Lab@NCSU
They are swimming like a butterfly: researchers have developed the fastest aquatic soft robots to date using a sophisticated drive concept. The “butterfly bots”, which are also inspired by rays, are driven by tensioned elements that snap into a different position when the pressure of small air sacs changes, thereby providing propulsion. This allows the robots to reach an impressive speed of almost four body lengths per second – with little energy consumption.
Conventional underwater vehicles are rigid, massive and powered by whirring propellers. Animals, on the other hand, move smoothly in the watery medium: soft body parts such as fins or wing structures in rays ensure gliding propulsion with little energy consumption. These nature patents have served as models for researchers for some time. They are developing so-called soft robots, which consist of soft components and have locomotion concepts that are based on those of fish, jellyfish or rays. Compared to conventional systems, they can score with a high level of energy efficiency and they are also suitable for special applications - for example as natural-looking and flexible scouts in the underwater world.
But compared to their natural models, many of the developments in aquatic soft robotics still leave a lot to be desired. “So far, swimming soft robots have not been able to swim faster than one body length per second. Marine animals - like manta rays - on the other hand, can move much faster," says senior author Jie Yin of North Carolina State University in Raleigh. That's why he and his team have now developed a concept that is intended to literally help aquatic soft robots along the way.
Bistable wing state
Their designs derive buoyancy from wing-like structures that exhibit a tense, bistable state. This means that they can jump from one position to another with just a small amount of force. The wing can be compared to a hair clip: these structures are stable until a certain amount of energy is applied by bending them slightly with finger pressure. When the amount of energy exceeds a critical point, the bobby pin then snaps into its second stable form. In the Butterfly Bots, the barrette-inspired bi-stable wings are formed from a frame of flexible polyester strips covered by a plastic membrane. The strips are connected at their ends in a way that creates a curved, bistable stress state of the wing.
The structure, reminiscent of the shape of a ray, is attached to a soft silicone element that allows it to snap into the other state. Inside are two air chambers arranged one above the other, which can be filled as desired: if the upper chamber expands, the silicone element curves downwards, and if the lower chamber is filled, it curves upwards. This puts the tensioned wing under pressure until it snaps over. Thus, as the chambers alternately inflate and deflate, the wing flexes up and down. In order to increase the propulsion generated by this fluttering, the researchers attached an additional membrane element to the wings of the construction. Because the locomotion is jerky, reminiscent of the butterfly swimming style, the researchers gave the design the name Butterfly Bot.
Efficient on the go
“Most previous attempts to develop wing-flapping robots have focused on using motors to drive these units directly. In comparison, the use of bi-stable wings, which are passively driven by the movement of a central element, allows for a simplified design that reduces weight,” explains Yin. As the team reports, their 2.8 gram prototype can now swim more than four times faster than previously developed soft robots: It reached an average speed of 3.74 body lengths per second. According to the researchers' calculations, it has an energy efficiency that can be compared to that of marine animals when swimming.
However, the fast version of the Butterfly Bot cannot be controlled and only swims straight ahead. The team therefore also developed a steerable model. It has two drive units connected side by side. Thanks to this construction, the wings on both sides can be controlled independently of one another in order to bring about a change of direction with a lateral flapping of the wings. However, maneuverability comes at the expense of speed. But even the manoeuvrable prototype still reaches a speed of 1.7 body lengths per second.
The researchers say, however, that development work is still required before the concept can be used in practice. “Most obviously, the current prototypes are still constrained by thin tubes that we use to pump air into the core elements. However, we are currently working on developing a version that can swim freely and autonomously," says Yin.
Source: North Carolina State University Professional article: Science Advances, doi: 10.1126/sciadv.add3788