If someone asked you to move like a robot and you responded with the fluid art of ballet, your audience would be baffled, yet technically, you would be right. Robots are famous for their characteristic rigid movement, which is useful in some applications but can hinder adaptability. Now, researchers have developed a robotic wing that moves like no other.
Using a combination of soft robotics and biomimicry, a team of researchers from the University of Southampton, the University of Edinburgh, and Delft University of Technology has developed a robotic wing that moves with remarkable fluidity underwater. The wing has a skin that can “feel” and adapt to disruption.
Robots have a much harder time moving underwater than on land. For starters, water is 800 times denser than air. This density amplifies forces such as drag and added mass, making movement slower, more energy-intensive, and harder to control. On top of that, water bodies are rarely calm, with the speed and direction of water around the vehicle often changing very quickly and unpredictably.
For remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) that are trying to follow a path or hold position while carrying out inspections or performing repairs – for example – these disturbances can cause them to suddenly lose stability and go off course. Engineers have traditionally addressed these challenges using rigid, streamlined vehicles with active control systems. Soft material systems have also been explored to passively absorb environmental forces.
However, these solutions have their own problems. The more aggressively a robot must counter disturbances, the more power it consumes. Furthermore, the mechanical systems that repeatedly move wings or joints can also suffer wear and fatigue. Without integrated sensing or feedback, soft-only systems are limited in their ability to react to rapid changes and maintain precise maneuverability. In summary, existing solutions either react too slowly, require too much energy, or cannot adapt smoothly enough to the constantly changing flow conditions found underwater.
On the other hand, fish and birds thrive under the same conditions, gracefully frolicking through the chaos. How? The team of researchers found the answer in proprioception - the ability of animals to sense and respond to fluid forces. Fish and birds can sense the position and deformation of their own wings or fins and adjust them in real time to maintain stability.
Drawing inspiration from this ability, the team developed a soft robotic wing that can sense its own shape as it moves through water. The system is built around a flexible wing made of soft materials, allowing it to bend and deform under fluid forces. Unlike rigid hydrofoils that fight against sudden currents, this compliant structure simply flexes, passively absorbing part of the disturbance and reducing the destabilizing forces acting on the vehicle.
“Instead of building ‘tougher’ robots designed to fight the ocean’s power, we are moving toward smarter, softer machines that work in synergy with the environment,” says Leo Micklem, the paper’s lead author.
To give the wing “self-awareness” and active control, the team integrated a proprioceptive electronic “skin” directly into the structure. This thin silicone layer contains liquid-metal electrodes arranged in line patterns that act like nerves. When the wing bends, the spacing between these electrodes changes, altering their electrical capacitance and allowing the system to sense the wing's real-time deformation.
Two pressurized hydraulic tubes inside the wing's body respond to this sensory feedback, automatically adjusting the wing's stiffness and camber whenever its shape deviates from the desired state. The result is a hybrid passive-active system: the wing’s natural flexibility automatically absorbs part of the disturbance, while the sensing skin and actuators correct what remains, maintaining stable motion.
During testing, the team subjected the wing to flow fluctuations of varying shapes and magnitudes, comparing the results against a standard rigid-wing design and a basic soft-wing design without proprioceptive capabilities.
The results, published in the journal npj Robotics, were impressive. In addition to consistently maintaining smoother trajectories, the proprioceptive soft wing reduced the unwanted lift impulse over the disturbance by 87% compared with its rigid counterparts on conventional AUVs. Rigid wings experienced abrupt destabilization, while passive soft wings without sensing and control struggled to recover from larger flow perturbations.
So, why is the proprioceptive robotic wing something to be excited about? With the added stability the wings provide, AUVs can navigate and perform multiple underwater tasks, from repair to surveillance and inspection, more efficiently and accurately. Furthermore, the wing reduces the power requirements of AUVs, enabling engineers to design more compact AUVs. Essentially, this technology brings robotic systems closer to the adaptability and robustness of nature, opening the door to safer, more efficient, and more capable autonomous robots in real-world conditions.
Source: University of Southampton
