The Fascination of Technology Soft Robot Maneuvers in All Directions Using Just Two Actuators

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In our “Fascination with Technology” section, we showcase impressive research and development projects for design engineers every week. Today: A bionic underwater robot that achieves full 3D mobility using just two actuators, thanks to mechanical intelligence.

A research team at North Carolina State University has achieved a breakthrough in the development of maritime soft robotics. Their latest robot navigates effortlessly in all three spatial directions, yet requires only two actuators to do so. This enormous reduction in drive and control components is made possible by the concept of “mechanical intelligence.”The complexity of locomotion is not solved by complex computer calculations or sensor technology, but is instead incorporated into the robot’s physical structure itself. The machine’s behavior is largely dictated by its design and its physical interaction with the water. 

What Does “Mechanical Intelligence” Mean in this Context?

In conventional robotic systems, movement is controlled by software, sensor inputs and numerous individual actuators. “Mechanical intelligence” shifts this complexity out of the computer and directly into the hardware architecture. The system’s behavior is determined primarily by its structural design and its physical interaction with the environment—in this case, the water. In the feather-star robot, this is evident in its four arms: they are designed to be elastic and monostable. The two central actuators bend them downward, but as soon as the actuators are switched off, the arms’ own elastic tension automatically returns them to their original position. The structure itself provides the restoring motion. This greatly reduces the computational input required—and eliminates the need for additional actuators.

Normally, you would need at least six actuators to move in three dimensions—that is, up and down, forward and backward, and in a rotational motion. The fact that we can achieve this range of motion with just two actuators impressively demonstrates just how efficient intelligent design can be.

Jie Yin, Professor of Mechanical and Aerospace Engineering and corresponding author of the study

Three Modes for Maximum Mobility

The robot’s hardware architecture is as simple as it is effective: Four flexible, monostable “wings” extend from a central disc that houses the two actuators. These wings can be bent but always snap back to their original position. By controlling the motors differently, three modes of locomotion are possible:

• Jellyfish Mode: When both actuators are activated and deactivated in sync, all four wings flap. At a fast cycle, the robot rises; when the actuators are deactivated, it sinks; and at a slow cycle, it hovers steadily in place.
• Fish Mode: If only one propulsion system is activated, a single wing flutters like a tail fin. This propels the robot in the opposite direction (forward or backward).
• Rotor Mode: Rapid switching between the two actuators causes the robot to rotate around its own axis. This allows it to be precisely oriented in any desired direction. By combining these three modes, the system is able to move completely freely in three-dimensional space, according to Yin.

From the Speedy Ray to the Agile Brittle Star

The new model builds on past findings. Previously, the team had developed a robot modeled after a manta ray. “Although it was very fast in the water, it lacked 3D maneuverability,” summarizes Haitao Qing, first author and postdoctoral researcher at UC Berkeley. The goal of the current project was therefore to drastically increase mobility without having to resort to complicated mechanics. 
Marine brittle stars, known for their coordinated, gliding swimming behavior, served as the model. Initial demonstrations confirm the practical utility of the concept: Equipped with cameras, the soft robot can explore underwater spaces or—either individually or in a swarm—lift objects.
In the future, the engineers plan to develop a fully wireless version. In addition, the team is actively seeking interdisciplinary collaboration partners to further develop the hardware concept for specific underwater applications.

Initial demonstrations have shown the practical benefits of the concept: Equipped with cameras, the soft robot can explore underwater spaces or—either individually or in a swarm—lift objects.

The results were published under the title “Minimal-Actuation Feather Star–Inspired Soft Swimmers for Multimodal 3D Maneuverability” in the open-access journal *Science Advances* (DOI: 10.1126/sciadv.aeg9211). The research was funded by, among others, the National Science Foundation (NSF) and the Office of Naval Research. A corresponding patent application is pending.