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Embedded optical sensors could make robotic hands more dextrous

30 September 2015

Carnegie Mellon University researchers have developed a three-fingered soft robotic hand with multiple embedded fibre optic sensors.

The fingertips are 3D-printed hard plastic and incorporate eight sensors for detecting force (photo: Park research team/Carnegie Mellon University)

By using fibre optics, the researchers were able to embed 14 strain sensors into each of the fingers in the robotic hand, giving it the ability to determine where its fingertips are in contact and to detect forces of less than a tenth of a Newton. A new stretchable optical sensing material developed by the team - not incorporated in this version of the hand - could potentially be used in a soft robotic skin to provide even more feedback.

“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,” says Yong-Lae Park, assistant professor of robotics. “Human skin contains thousands of tactile sensory units only in the fingertip and a spider has hundreds of mechano-receptors 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.”

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 fibre can contain several sensors; all of the sensors in each of the fingers of the Carnegie Mellon University hand are connected with four fibres, although, theoretically, a single fibre could do the job, Park said. And the optical sensors are impervious to electromagnetic interference.

Each of the fingers on the robotic hand mimic 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 fibre Bragg grating (FBG) sensors, which detect strain by measuring shifts in the wavelength of light reflected by the optical fibre.

Despite their advantages, conventional optical sensors don’t stretch much – glass fibres stretch hardly at all and even polymer fibres 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.

Park's team have created 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.

Park said this type of flexible optical sensor could be incorporated into soft skins. Such a skin would not only be able to detect contact, as is the case with the soft components in the Carnegie Mellon University hand, but also measure force.

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