Researchers use fiber optics to improve robotic sensing

New robotic hands with advanced sensing capabilities, developed in part by researchers at Carnegie Mellon University, have brought humans closer than ever to robots. This project, led by assistant professor of robotics Yong-Lae Park and developed with researchers at Intelligent Fiber Optic Systems Corp., with support from NASA, aims to create structurally sensorized, three-fingered soft robotic hands that are more similar to human or animal limbs, which have thousands of nerve endings. Using fiber optics, the researchers embedded 14 strain sensors into each of the fingers in the robotic hand, which allows the hand to determine where its fingertips are in contact and detect forces of less than a tenth of a newton.

Traditionally, industrial robots are only equipped with a few sensors, which are usually resistive or capacitive. This small number, however, limits the ability of robots to operate autonomously without instructions from humans.

“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,” said Park in a university press 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.”

“We wanted to build a more sensorized robot, that would be more autonomous,” Parks said. Another benefit of robots with more sensors is that they would be better equipped to interact with humans, where safety concerns require more emphasis on tactile and force sensing.

Adding additional pressure or force sensors, however, is difficult since they are prone to breaking and interference from electrical motors and other electromagnetic devices. In order to resolve this problem, the team turned to optical sensors. A single optical fiber can contain several sensors, which allowed the researchers to incorporate a larger number of sensors with fewer limitations.

The robotic hand, developed in part by senior mechanical engineering students Leo Jiang and Kevin Low, includes Fiber Bragg Grating sensors in the rigid bone structure that detect strain by measuring the wavelength of light reflected by the optical sensors. Each of the fingers of the team’s hand is composed of four optical fibers that connect all of the sensors in the finger.

Structurally, the robotic fingers are similar to human fingers, with a fingertip, middle node, and base node connected by joints. A single active tendon can bend the finger while a passive elastic tendon straightens the finger by providing an opposing force. The sensors detect strain caused by small structural deformations, which allows robots to react to changes in their environment.

There are, however, limitations to this model. Despite their advantages, conventional optical sensors don’t stretch much. Glass fibers barely stretch at all and polymer fibers only stretch approximately 20 to 25 percent, which is a problem for the device as it requires a large range of motion.

In order to address this issue, Park, along with Celeste To, a masters student in mechanical engineering at Carnegie Mellon, and Tess Lee Hellebrekers from the University of Texas, developed a stretchable optical sensor using commercially available silicon rubbers. This new optical sensor is able to measure strain and deformation, and could potentially be used in future versions of the robotic hands.

Integrating the fiber optic signals is also a challenge. The electronic signal detector, which is located in the back of the robot, is quite bulky and awkward. With continuing advancements in technology, a more compact interrogator would make these robotic hands more useful in a variety of applications, including human friendly robots.

The researchers discussed the robotic hand and a report on the highly stretchable optical sensors on September 29 at the IEEE International Conference on Intelligent Robots and Systems (IROS 2015) in Hamburg, Germany.