Triaxial 3-D-Channeled Soft Optical Sensor for Tactile Robots

Soft optical transducers have the potential to fulfill the need for advanced tactile sensing in robotics. We present a fingertip-shaped soft sensor with optically transparent channels that relies on soft materials and sensor morphology to measure an applied triaxial force. The proposed 3D-channeled sensor has a volume of 2.5 cm3, and the experimental results reveal a 15-fold increase in voltage compared to its bulk analogous, showcasing a sensitivity of 0.34 and 0.09 N/mV to tangential and normal forces. A prototype with a diameter of 2 mm (<inline-formula> <tex-math notation="LaTeX">$0.4\times $ </tex-math></inline-formula>) indicates the feasibility of scaling down the sensor. Force magnitude is estimated with a linear model and then decomposed into its <inline-formula> <tex-math notation="LaTeX">${F}_{\textit {xy}}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">${F}_{z}$ </tex-math></inline-formula> with an <inline-formula> <tex-math notation="LaTeX">${R}^{{2}}$ </tex-math></inline-formula> of 0.93 and 0.98 within a sensing range of 4.05 and 8.50 N, respectively. A coordinate transformation from a covariant to a Cartesian reference frame is used to retrieve the direction of the tangential component of the force. The sensor was integrated into a compliant robotic hand as a proof-of-concept to demonstrate real-time operation in typical grasping tasks. The results of this work show new possibilities for scalable optical soft sensors to provide complete local information about the interaction forces in soft/rigid robots.