Stretchable Optical Waveguide Sensor Capable of Two-Degree-of-Freedom Strain Sensing Mediated by a Semidivided Optical Core

With the rising demand for flexible strain sensors for soft robots, optical sensing solutions have been proven to be electrically safe, stable, and precise. This study presents a highly stretchable optical waveguide sensor that extends existing solutions to two degrees of freedom while maintaining low manufacturing complexity and part count. Through casting in a 3D-printed mold, a polymer waveguide is manufactured, featuring a semidivided core cross section. The light from a near-infrared LED light source is guided to two phototransistors connected to each chamber. The connection between both core chambers makes the power throughput amplitude and ratio at the two outputs dependent on the strain amplitude as well as its location or direction. The proposed waveguide is experimentally compared with single and dual core designs in four different strain modes, to sense elongation, local deformation amplitude and position across length, twisting angle and direction, as well as bending amplitude and direction in a soft finger. While influences of the manual manufacturing process are apparent, the results verify that the presented waveguide sensor can be effectively applied in these four strain scenarios. We also demonstrate that information about superimposed strain states can be obtained from the time signal.