R&D PULSE
Robotic Eyes Boast
Three Degrees of Freedom
A team at Shanghai University has developed a unique bionic eye structure that allows a robotic eye to look in all directions.
Written by Kayt Sukel
THE HUMAN EYE IS A COMPLEX STRUCTURE. While only a small part is visible to the outside world, around and within the eyeball, you will find millions of moving parts, from cells to sinews, which allow us to perceive what’s happening in the world around us.
While there have been great advances in computer vision algorithms—including the design of innovative bionic eye systems—to support new robotic applications, one thing has been missing for some time: the ability for a bionic eyeball to move in three degrees of freedom. While some engineering teams have leveraged electric motors to support movement in multiple degrees of freedom, they can be somewhat clunky. And do not mimic the smooth, effortless movement seen in the human eye.
To create a bionic eye system that could support three degrees of freedom, including yaw (left to right), pitch (up and down), and roll (around), Yonghao Xie, a researcher in the School of Computer Engineering and Science at China’s Shanghai University, and his colleagues looked to the design of the human eye. They hoped to simulate its movement characteristics in a manmade system to provide this kind of flexible movement.
A 3D model of the bionic eye simulation platform and schematic diagram of the artificial muscle flexible connection. Image: Yonghao Xie
Structural diagram of human extraocular muscles. Image: Yonghao Xie
The team’s research, “Design of Three-Degrees-of-Freedom Bionic Eye Driven by Artificial Muscle Based on the Law of Human Eye Movement,” was recently published in the ASME Journal of Mechanisms and Robotics.
The eye has seven specialized muscles to help control eye and eyelid movement—allowing the eyeball to move in various directions, as well as to raise the upper eyelid. These muscles—four rectus, two oblique, and one levator palpebrae superioris—all work together, in a coordinated fashion, to help each of us not only track what’s happening in the visual field but sync up our two eyes to achieve binocular vision.
Xie and colleagues undertook a “biomimetic replication” of these muscles, utilizing flexible materials and structures to achieve more complex eye movements. He said the biggest challenge was identifying a material that combined good elastic modulus with high flexibility to create the necessary artificial muscles. That material would also have to ensure manufacturing precision to meet the demands of complex movements, he added.
The research team’s experimental platform for its bionic eye. The bionic eye terminal is fixed with four optical spheres for capturing the terminal trajectory. Image: Yonghao Xie
“We selected dual-component vulcanized silicone rubber as the material for the flexible muscles,” Xie said. “We used mold-casting technology to fabricate the bionic eyeball and flexible muscles, ensuring manufacturing precision.”
The team then determined the new design’s kinetic flexibility through a series of tensile experiments. Relying on Listing’s Law, an ophthalmological principle that states any eye position can be achieved by rotating it around an axis lying on a specific plane perpendicular to the eye’s primary orientation, the researchers determined the insertion and control points of the vulcanized silicone rubber muscles. They then conducted forward and inverse kinematic analyses of their engineered system.
“The experimental results aligned with our expectations,” Xie said. “The tensile experiments successfully determined the flexibility equation of the artificial muscles, providing a foundation for subsequent control.”
“We selected dual-component vulcanized silicone rubber as the material for the flexible muscles.”
—Yonghao Xie, a doctoral student at Shanghai University
The research team then followed up these initial experiments by building a prototype of the bionic eye system. When that was tested, they found similar results: high precision in continuous reciprocating motion and trajectory tracking movements. In an eight-shaped trajectory tracking experiment, for example, they found only a 5.07 percent difference between the actional motion trajectory and the theoretical one.
Xie said the team’s next steps are to optimize the structural design of this new bionic eye system to improve system integration as well as reduce its volume and weight. This study represents as an important step for adding more flexible eye movements into robotic vision systems, medical assistive devices, virtual reality, and augmented reality applications, Xie noted. But he hopes that others will build upon this work and continue new advances in bionic eye systems.
“We hope that other engineers recognize the importance of bionic design and the value of interdisciplinary collaboration in solving complex engineering problems,” he concluded.
Kayt Sukel is a technology writer and author in Kansas City.
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