Light nerves to equip soft robots

December 29, 2016 // By Julien Happich
Being compliant, soft robotics offer promising solutions to emulate human limbs in prosthetic applications, where touch and force sensing ought to be combined with full prehension capabilities.

Though, reliable sensor implementation can be difficult on flexible extremities such as finger-like grips. This is what a team of researchers from Cornell University (Ithaca, NY) proposed to address with a novel type of soft waveguide-based light sensors readily embedded into deformable grips.

Their paper "Optoelectronically innervated soft prosthetic hand via stretchable optical waveguides" published in Science Robotics details the fabrication and operation of chemically inert stretchable and flexible optical waveguides made up of an optically transparent core (2dB/cm propagation loss at 860nm) clad into a light blocking elastomer. Once fitted with a LED on one end and a photodiode on the other end, these elastomeric optical waveguides can be monitored for any deformation (stretching, bending, compression) affecting light propagation.

Fabricated using cheap custom molds obtained through 3D printing, the elastomeric optical waveguides unveiled in this paper had an overall square profile of 3mm by side, with an inner core 1mm wide. Several of them could be accommodated within the fingers of a pneumatically-actuated soft prosthetic hand to actually try their sensing capabilities in a real application context.

The process steps for fabricating a waveguide (with the corresponding cross section for each step).

The actual waveguides in a curved shape.

While typically on traditional mechanical hands, proprioceptive sensing is performed through motor motion encoders combined with bulky and rigid multiaxial force/torque load cells, here only one type of continuous flexible sensor was effectively innervating the soft prosthetic hand.

In their soft robotic hand, each finger featured three waveguides bent into a U-shape to detect axial strain throughout the finger. Fitted with a stiff plate in a neutral bending plane where there is no axial strain, one of the waveguides also served as a touch sensor at the fingertip.