3D printed fingers and fiber optical sensors could make robotic hands more dexterous

Researchers at Carnegie Mellon University have developed a three-fingered soft robotic hand with multiple 3D-printed fiber optic sensors, together with a new type of stretchable optical sensor. The robotic hand, supported by NASA, is able to detect forces of less than a tenth of a newton.

Using fiber optics, the team of researchers placed 14 strain sensors into each of the robot’s fingers, which are closely modelled on the skeletal structure of a human finger. Each finger has a 3D-printed plastic fingertip, middle node and base node connected by joints and covered in a silicon rubber skin. This technology provides the robotic hand with the ability to determine where its fingertips are in contact and to detect even minuscule forces. Although the new stretchable optical sensing material has not been incorporated in the current version of the hand, researchers are hopeful that it could be used in a future soft robotic skin to provide greater feedback.

The use of conventional pressure or force sensors can be problematic. This is because wiring can get complicated, the sensors are prone to breaking, and they are extremely susceptible to interference from electric motors and other electromagnetic devices. A single optical fiber can, on the other hand, contain several sensors. All of the sensors in each of the fingers of the partially 3D-printed robotic hand are connected with four fibers, and are completely resistant to electromagnetic interference.

The researchers say that the technology is intended to increase robot autonomy. “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,” explained Yong-Lae Park, assistant professor of robotics at Carnegie Mellon. “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 developed the robotic hand with the help of mechanical engineering students Leo Jiang and Kevin Low. The device incorporates commercially available fibre Bragg grating (FBG) sensors, which detect strain by measuring shifts in the wavelength of light reflected by the optical fibre. The finger is bent by a single active tendon, while a passive elastic tendon provides opposing force to straighten the finger.But what of the aforementioned new type of stretchable optical sensor? The team hope that the sensor could be used on future versions of the hand. Since conventional optical sensors lack flexibility—glass fibers barely stretch and even polymer fibers stretch typically only 20-25 percent—their use value in moving structures is limited. However, using a combination of silicon rubbers lined with reflective gold, researchers are able to escaping light when pressure is placed on the sensor, which allows them to measure force. Park believes that this type of sensor could both detect contact and measure force.

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