Actuators are the components of a machine that generate force or torque, and in conventional robotic design they are the primary currency of mobility. A robot that can translate in three dimensions and reorient itself typically requires at least six independently controlled actuators to achieve that full range of motion. Jie Yin, corresponding author of the paper and a professor of mechanical and aerospace engineering at NC State, explains that the feather star robot achieves the same degrees of freedom with a fraction of that hardware. The key, he says, is intelligent design: by shaping the robot’s body so that its structure and its interaction with the surrounding water govern its behavior, the researchers dramatically reduced the input required from computers or human operators. The robot’s capabilities emerge from its geometry and material properties rather than from an elaborate control system.

The inspiration for the design came from feather stars, crinoid echinoderms that can move in any direction or hold themselves stationary in the water column by coordinating the movements of their flexible, feather-like limbs. The robot distills that biological principle into a remarkably simple architecture: four elastic wings protrude from a central disk that houses the two actuators. Each wing is monostable, a term from mechanics describing a structure that can be bent out of shape but will always snap back to a single preferred resting position. This snap-through behavior is the heart of the robot’s locomotion strategy, converting smooth actuator input into rapid, impulsive wing motion that pushes effectively against the water.

The robot’s repertoire is organized into three distinct modes, each produced by a different pattern of actuator activation. When both actuators are switched on, all four wings snap downward simultaneously; when the actuators are turned off, the monostable wings snap back up. Rapidly cycling the actuators on and off makes the robot flap all four wings in quick succession, driving it upward through the water column. Switching the actuators off entirely stops the flapping and lets the robot descend. And by flapping the wings slowly rather than rapidly, the robot can hover in place, a behavior the researchers have nicknamed jellyfish mode, since the slow pulsing motion and stationary hovering recall the swimming style of a jellyfish.

Horizontal travel relies on an even more economical trick. To move forward or backward, the researchers activate only one of the two actuators. This causes a single wing to flutter, functioning like a tailfin and pushing the robot in the opposite direction. The team calls this fish mode, and it means that steering and translation along the horizontal plane require no additional hardware whatsoever. The third mode, rotor mode, comes from alternating rapidly between the two actuators, which sets the robot spinning on its axis. By combining jellyfish mode, fish mode, and rotor mode, the operators can maneuver the robot through all three dimensions, climbing, descending, translating, hovering, and turning with a machine that carries only two sources of actuation.

The project grew out of an earlier effort that illustrates exactly what mechanical intelligence is meant to solve. Haitao Qing, first author of the paper and a postdoctoral researcher at UC Berkeley who began the project as a Ph.D. student at NC State, previously helped create a soft aquatic robot inspired by the manta ray. That robot could move quickly through the water, but it lacked three-dimensional maneuverability. The goal of the new work was to design a robot with far greater agility without resorting to unduly complicated mechanisms. The feather star concept delivered that agility by shifting the burden from actuator count to structural design, letting the elastic wings and their snap-through dynamics generate a rich set of behaviors from minimal input.

The practical implications of reducing actuator count extend well beyond elegance. Every actuator on an underwater robot adds mass, wiring, power draw, and a potential point of failure, and in marine environments where maintenance is difficult and expensive, simplicity translates directly into reliability and endurance. A two-actuator platform is lighter, easier to seal against water intrusion, cheaper to manufacture, and simpler to control than a six-actuator equivalent. Because the robot’s modes are selected by simple activation patterns rather than complex coordinated control of many motors, the computational demands are correspondingly modest, which matters for small autonomous vehicles with limited onboard processing and battery capacity.

The researchers also demonstrated concrete uses for the robot. In tests, the machines were shown exploring underwater spaces while carrying a camera, and lifting objects underwater, either operating alone or working in concert with other robots. That cooperative capability hints at a future in which fleets of inexpensive, simply actuated soft robots swarm through shipwrecks, aquaculture pens, coral reefs, or submerged infrastructure, performing inspection and light manipulation tasks that would currently require larger, costlier, and more fragile vehicles. Qing describes the design as a versatile platform for integrating with other technologies across a range of underwater applications, and notes that the team’s future directions include developing a fully wireless version of the robot, freeing it from the tether that currently connects it to its power and control source.

Qing also extends an open invitation to other disciplines. The team members are mechanical engineers, he notes, and they have produced a novel design for aquatic robotics; they would welcome collaborations with experts in other fields to explore what the platform might accomplish in practice. That collaborative posture is fitting, because the underlying idea, mechanical intelligence, is relevant far beyond one robot. It suggests a broader design principle in which morphology, elasticity, and environmental interaction substitute for actuation and computation, a lesson that could inform everything from search-and-rescue vehicles to biomedical devices that must navigate constrained, fluid-filled spaces.

The paper, titled Minimal-Actuation Feather Star Inspired Soft Swimmers for Multimodal 3D Maneuverability, was published on October 7 in Science Advances. In addition to Yin and Qing, the co-authors are Caizhi Zhou and Haoze Sun, both Ph.D. students at NC State; Yuanhang Zhu of the University of Virginia and the University of California, Riverside; and Jiacheng Guo, Haibo Dong, and Daniel Quinn of the University of Virginia. The research was supported by the National Science Foundation under grants 2126072 and 2329674, and by the Office of Naval Research under MURI grant N00014-22-1-2616. For a field that has often equated capability with complexity, the feather star robot makes a quietly radical argument: sometimes the smartest machine is the one whose body already knows what to do.

Subject of Research: A feather star-inspired soft underwater robot achieving 3D maneuverability with two actuators through mechanical intelligence

Article Title: Feather star-inspired underwater robot gets 3D maneuverability from only two actuators

Article References: Feather star-inspired underwater robot gets 3D maneuverability from only two actuators. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: soft robotics, feather star, underwater robot, mechanical intelligence, actuators, bioinspired design, marine robotics, Science Advances, North Carolina State University, 3D maneuverability, monostable wings, swimming robot

Oarfish are enormous, reaching lengths that can exceed several meters, and they are rarely encountered alive by humans. Their bodies are long and laterally compressed, and running along the top of the body is a continuous dorsal fin supported by hundreds of bone-like fin rays connected by a thin, webbed membrane. When the fish swims in its characteristic mode, these rays move in coordinated, wave-like patterns, rippling the membrane and pushing water in a way that drives the animal forward or backward. Because the propulsion comes from the fin rather than from side-to-side bending of the trunk, the oarfish can travel without the large body undulations that most elongated fishes depend on, a capability with important consequences for how it hunts and avoids detection in the open water column.

Part of the reason the fin rays have remained so poorly understood is the fragility of the animals themselves. When oarfish die, their long bodies tend to break into segments, and intact specimens suitable for detailed anatomical work are exceptionally rare. That scarcity has left scientists with a limited picture of how the fish’s unusual propulsion actually works at the tissue level. The Cornell team overcame this obstacle by combining several lines of evidence: careful dissections, histological sectioning of tissues, x-rays of a specimen held at the Smithsonian Institution, computed tomography scans made at Cornell, and frame-by-frame analyses of movies showing the dorsal fin rays in motion. Together, these methods allowed the researchers to reconstruct the anatomy of a structure that few laboratories have ever been able to examine in depth.

What the dissections and scans revealed is an architecture unlike anything described before in such detail. Each fin ray is attached at its top to the thin fin membrane. Below, at the base of the ray, the anatomy becomes more elaborate: the ray connects to cartilage, to a series of muscles, and to a ball-and-socket joint. It is this joint that grants each ray its remarkable freedom of movement, allowing it to swivel through a complete rotation rather than merely swinging back and forth in a single plane. In effect, every one of the hundreds of rays is a small, individually controlled actuator, and the fin as a whole functions as a dense array of independently steerable oars, which is fitting for a fish named for the implement it so closely resembles in action.

The coordinated motion of these rays is what turns the fin into an engine. By rotating in sequence, the rays create waves that travel along the connecting membrane, and those waves push against the water to generate thrust. The system is not limited to a single direction of travel. A section of the membrane can wave toward the posterior of the animal while another section simultaneously waves toward the anterior, allowing the oarfish to fine-tune its propulsion with a subtlety that conventional body-driven swimming cannot match. This means the fish can generate forward and backward thrust, and modulate each along different parts of the fin, all without moving its trunk laterally.

Willy Bemis, a retired Cornell University ichthyologist and one of the study’s authors, placed the finding in the context of how long-bodied fishes normally move. Most fishes with elongated bodies swim like eels, he explained, propelling themselves through a series of undulations that press water back and forth. Oarfish can do that as well, but they also do something remarkable: they use the dorsal fin to propel themselves without moving their bodies laterally. According to Bemis, the animals are continuously able to change the pattern of the dorsal fin rays and to do so very quickly, and the fin rays are, in his words, incredibly mobile. That combination of speed and flexibility in reconfiguring the wave patterns gives the oarfish a level of locomotor control that sets it apart from nearly every other fish of comparable size.

The biological payoff of this system appears to be stealth. Because the fin-driven mode of swimming does not require vigorous bending of the whole body, the oarfish can move through the water while presenting a much quieter profile than an eel-like swimmer of the same dimensions. The researchers note that this lets the animal silently stalk prey without having to swim hard to propel its entire body. For a large predator living in the open ocean, where there is little cover and where prey may be alert to disturbances, the ability to approach slowly and quietly using only a rippling fin could be a decisive advantage. It may also help explain why oarfish are so seldom seen making sudden, conspicuous movements when encountered by divers or remotely operated vehicles.

The research did not begin as a purely zoological inquiry. Rob Shepherd, a roboticist at Cornell, became interested in the oarfish’s unusual locomotion while working on a project to design a large yet quiet swimming robot, supported by a grant from the Office of Naval Research. The machine he envisioned would serve as an ocean monitoring platform, housing a range of instrumentation, and would need to swim silently forward and backward to monitor ocean health without scaring off fish and other aquatic creatures. The Navy shared an interest in such a platform. Recognizing that the oarfish had already solved, through millions of years of evolution, the problem of large-scale, low-disturbance propulsion, Shepherd consulted Bemis, and the collaboration that followed produced the anatomical study now published.

The intersection of the two disciplines is part of what makes the work notable. Engineers building bioinspired underwater vehicles have long drawn on the swimming of fishes, but most designs have focused on body undulation or on the motion of tail fins and pectoral fins. The oarfish offers a different template: a long, continuous fin along the back, driven by hundreds of independently rotating rays, capable of producing waves in either direction along its length. Translating that arrangement into a robotic actuator array could yield vehicles that maneuver precisely at low speed, hold position, and travel in reverse, all while generating minimal hydrodynamic noise. For an ocean monitoring platform meant to observe marine life rather than disturb it, those are exactly the properties a designer would want.

For now, the study stands as the first full anatomical description of the system that makes the oarfish’s swimming possible, documenting the chain from fin membrane to cartilage, muscle, and ball-and-socket joint that gives each ray its joystick-like rotation. It also underscores how much remains to be learned about large deep-sea animals simply because intact specimens are so hard to obtain. The oarfish, an animal that has haunted maritime folklore for centuries, turns out to carry in its dorsal fin a propulsion mechanism of remarkable sophistication, one that a team of anatomists and roboticists has only now begun to describe, and one that may soon ripple outward from ichthyology into the design of the next generation of quiet underwater machines.

Subject of Research: The anatomy and mechanics of oarfish dorsal fin ray rotation and its role in fin-driven swimming and bioinspired underwater robotics

Article Title: Researchers uncover the mechanics behind the oarfish's unusual swimming style

Article References: Researchers uncover the mechanics behind the oarfish's unusual swimming style. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: oarfish, fin rays, dorsal fin, swimming mechanics, Cornell University, Ichthyology and Herpetology, ball-and-socket joint, bioinspired robotics, underwater robot, deep-sea fish, locomotion, marine science