Researchers at North Carolina State University have developed a soft underwater robot inspired by feather stars, a type of marine invertebrate.
The robot can move in three dimensions, including forward and backward, up and down, and rotation around its axis.
This range of motion is achieved with just two actuators, compared to the six typically required for similar three-dimensional movements.
The design leverages “mechanical intelligence” and relies on the robot’s structure and interaction with water to reduce the amount of control input required.
Feather star robot
Researchers at North Carolina State University have developed a soft underwater robot that performs three-dimensional movements using just two actuators, reducing the mechanical complexity normally required for this level of maneuverability.
Inspired by feather stars, marine invertebrates that can move and float in different directions by coordinating their limbs, the robot uses four elastic wings arranged around a central disc. The wings are monostable, meaning they can flex when actuated, but will naturally return to their original position when the force is removed. This mechanical design allows the robot to produce multiple types of motion without requiring a separate actuator for each direction.
According to the NCSU team, the system is based on the principle of “mechanical intelligence,” where a robot’s physical structure helps determine its behavior and reduces the amount of control input required from computers or human operators. Conventional systems capable of comparable three-dimensional movements would typically require at least six actuators.
“An exciting aspect of this work is that it shows how we can design robotic devices with an incredible range of motion with a minimal number of actuators by leveraging intelligent design techniques,” said Jie Yin, corresponding author of the study and a professor of mechanical and aerospace engineering at NCSU, in a statement.
Switches movement mode
The robot’s two actuators are housed in its central disc and control all four wings. When both actuators are activated, the wings snap down. As soon as the actuators are switched off, the elastic wings return upwards. By repeating this cycle quickly, the robot flaps its wings and moves upward through the water. When the wings beat slower, the robot can maintain its position and hover, a movement the researchers call “jellyfish mode.”
The same hardware can create horizontal movement by changing the way the actuators are controlled. By activating just one actuator, a wing flaps like a tail fin, pushing the robot forward or backward. The researchers call this “fish mode.”
The robot can also rotate around its central axis. By quickly switching between the two actuators, the researchers create asymmetrical wing movements that cause the robot to rotate. This “rotor mode” provides directional control and allows the robot to steer underwater.
Together, the three movement modes give the soft robot control over vertical, horizontal and rotational movements. The researchers demonstrated how the system explores underwater environments with a camera and lifts objects either individually or in coordination with other robots.
The design builds on the team’s previous work on a manta ray-inspired aquatic robot that could move quickly through water, but lacked comparable three-dimensional maneuverability. Instead of adding more motors and complex mechanisms, the researchers tried to achieve additional movement capabilities through the robot’s physical structure.
The team found that the compact design could provide a platform for underwater robotic applications where maneuverability and mechanical simplicity are important. Future work will focus on developing a fully wireless version and exploring potential applications in collaboration with researchers from other fields.
