Fibre optic sensors help robotic hand measure pressure for picking up objects
Researchers at Carnegie Mellon University have created a three-fingered robotic hand, which has 14 stretchable fibre optic strain sensors embedded into each finger. As each optical fibre can contain multiple sensors, the fingers are capable of measuring the loss of light as it picks up an option. Using the light measure, the fingers can calculate the amount of force it should apply when picking up an object.

Carnegie Mellon University researchers have developed a three-fingered soft robotic hand with multiple embedded fiber optic sensors and a novel type of stretchable optical sensor.

The researchers used fiber optics to embed 14 strain sensors into each of the fingers in the robotic hand. The new stretchable optical sensing material could potentially be used in a soft robotic skin to provide even more feedback.

Yong-Lae Park, assistant professor of robotics, explained that human skin contains thousands of tactile sensory units only in the fingertip and a spider has hundreds of mechanoreceptors on each leg. However, even a state-of-the-art humanoid such as NASA’s Robonaut has only 42 sensors in its hand and wrist.

Adding conventional pressure or force sensors is problematic because wiring can be complicated, prone to breaking and susceptible to interference from electric motors and other electromagnetic devices.

But a single optical fiber can contain several sensors; all of the sensors in each of the fingers of the CMU hand are connected with four fibers, although, theoretically, a single fiber could do the job, Park said. And the optical sensors are impervious to electromagnetic interference.

Each of the fingers on the robotic hand mimics the skeletal structure of a human finger, with a fingertip, middle node and base node connected by joints. The skeletal ‘bones’ are 3D-printed hard plastic and incorporate eight sensors for detecting force.

Each of the three sections is covered with a soft silicone rubber skin embedded with a total of six sensors that detect where contact has been made. A single active tendon works to bend the finger, while a passive elastic tendon provides opposing force to straighten the finger.

The hand incorporates commercially available fiber Bragg grating (FBG) sensors that detect strain by measuring shifts in wavelength of light reflected by the optical fiber.

Park noted that despite their advantages, conventional optical sensors don’t stretch much. Glass fibers stretch hardly at all and even polymer fibers stretch typically only 20-25 percent. That is a limiting factor in a device such as a hand, where a wide range of motion is essential.

Park has previously developed highly stretchable microfluidic soft sensors, membranes that measure strain via liquid-conductor-filled channels. But they are difficult to make and can cause a mess if the liquid leaks out.

To overcome these problems, the researchers developed a highly stretchable and flexible optical sensor, using a combination of commercially available silicone rubbers. These soft waveguides are lined with reflective gold. As the silicone is stretched, cracks develop in the reflective layer, allowing light to escape. By measuring the loss of light, the researchers are able to calculate strain or other deformations.

This type of flexible optical sensor could be incorporated into soft skins, enabling them to not only detect contact, but also measure force.

Images courtesy of Carnegie Mellon University