Embedded optical sensors could add dexterity to robotic hands

Researchers at Carnegie Mellon University have developed a stretchable optic sensor that, when embedded into a soft robotic hand, could be used to improve feedback for the device.

The robotic hand, which was developed together with researchers at Intelligent Fiber Optic Systems Corp., has three fingers and is embedded with 14 fiber optic strain sensors that allow the hand to determine where the fingertips are in contact and to detect forces of less than a tenth of a newton, according to a university press release.

“If you want robots to work autonomously and to react safely to unexpected forces in everyday environments, you need robotic hands that have more sensors than is typical today,” Yong-Lae Park, PhD, assistant professor of robotics, said in the release. “Human skin contains thousands of tactile sensory units only in the fingertip and a spider has hundreds of mechanoreceptors on each leg, but even a state-of-the-art humanoid such as NASA’s Robonaut has only 42 sensors in its hand and wrist.”

Park said optical sensors provide an advantage over force sensors because they are impervious to electromagnetic interference and may require fewer connections since one fiber can contain multiple sensors. Along with mechanical engineering students Leo Jiang and Kevin Low, Park incorporated commercially available fiber Bragg grating (FBG) sensors. The sensors “detect strain by measuring shifts in the wavelength of light reflected by the optical fiber,” according to the release.

Each finger on the robotic hand developed at Carnegie Mellon University (CMU) mimics the structure of a human finger, with a fingertip, middle node and base node connected by joints. Each of these three sections are covered with soft silicone rubber skin that is embedded with six sensors. The hand’s “bones” are made of 3-D printed plastic and force detection sensors. The hand also contains an active tendon, which works to bend the finger, and a passive elastic tendon which provides opposing force to straighten the finger.

To overcome the limitations of conventional optic sensors, which do not stretch very much, Park worked with mechanical engineering students Celeste To from CMU and Tess Lee Hellebrekers from the University of Texas to create a stretchable and flexible optic sensor. The sensor is made from a combination of commercially available silicone rubbers. The sensor is lined with reflective gold that cracks to allow light to escape when the silicone is stretched. Researchers can calculate strain and deformations by measuring the loss of light.

Park said a flexible optical sensor could be used to create a soft skin that can detect contact and measure force.

Reference: www.cmu.edu

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