The new type of stretchable optical sensing material was not embedded in the robotic hand in this study but could be used in soft robotic skin provide even more feedback in future bionic devices.
For this time, the CMU team embedded existing fiber optic sensors in robotic hands. “Fiber Bragg grating — or FBG — sensors have been widely used in civil engineering for structural health monitoring, but not much in robotics,” says Yong-Lae Park, assistant professor of robotics at CMU. His team found a way to use this existing sensor technology in robotics to get force and tactile information that is useful for dexterous manipulation.
What is more, the University also introduced highly stretchable optical skin sensors. “Our new optical sensor uses a soft waveguide made of hyperelastic, clear silicone rubber, which is encapsulated by a very thin reflective metal layer, such as gold,” explains Park. “At rest, the gold layer reflects the light from one end to the other end without loss, using internal reflection. However, if the material is stretched or deformed, the gold layer makes many very small micro-cracks and allows part of the light to escape through. Then, you have an optical power loss that can be detected by a photo-detector in the other end.”
Traditionally, force sensing in robots is accomplished with external sensors attached to an existing structure, resulting in limited sensing capabilities. “However, if you look at biology — humans, animals, insects — they are covered in hundreds to even hundreds of thousands of mechanoreceptors that make them really responsive to external world,” Park says. “This responsiveness is one of the key factors to a real autonomous system. By embedding fiber optics sensors, we can make the entire robot structure sensorized like human skin.”
Furthermore, Park elaborates that fiber optic sensors eliminate the problem of electromagnetic interference (EMI), one of the most serious problems of conventional electric sensors, since fiber optic sensors use only optical signals and electrically passive. Also, since they are very tiny and flexible, it is easy to embed many sensors in a small structure, allowing small and complicated designs of robots or robotic parts. “All these advantages expand your material selections to a variety of plastics, including 3D printing materials while traditional robots mostly use only metals in their structures.”
Robot fingers protect, prevent optical power loss
Since
the fiber optic sensors the team used in this current set of experimental
robotic hands are an already existing, commercially available technology, their
biggest challenge was figuring out how to design a robot structure that allows
them to easily embed optical fibers and efficiently detects the contact forces.
“We had to design our robot fingers that cannot only physically protect the
fibers but also prevent any potential optical power loss,” Park says.Easily sterilizable, such fiber optic sensors could one day be used in other biomedical or surgical devices. Adds Park: “We also plan to use our optical soft skin senosors for robot skin that can cover existing robots to provide better sensing capability.” This approach has the advantage of “not having to redesign or remake all the existing robots.”
Another area of application Park and his colleagues are currently exploring is a wearable skin suit for humans, using the optical soft skin they have developed. “When you wear this skin suit,” Park says, “it cannot only monitor your body motions but also detect any external contacts to your body.” Such functionalities, for example, could be useful for monitoring oneself during athletic activity or rehabilitation exercises for improvements. Park adds that the optical soft skin could even detect any risk of injuries during physical activity.”
Continuing with the research project, Park plans to embed more sensors for increased sensitivity of the robotic hand. “We also plan to do more experiments for controlling our hands using the fiber optic sensors for demonstration of dexterous manipulation,” he adds. “For optical skin sensors, we are currently working on further miniaturizing the sensors thickness combining with optical fibers while still maintaining the same high stretchability and physical compliance.”
Finally, the the CMU team will put both their innovations together and integrate this sensitive skin with the robotic fingers that contain the fiber optic sensors (FBGs).
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