Stiffness Map Generation for Soft Materials Using Axis-Aligning Non-Contact Measuring Device

Abstract

This paper presents a novel non-contact sensing device for constructing stiffness distribution maps on soft-material surfaces. Since directly contacting and measuring each point is inefficient, the device applies an air jet to the surface to induce deformation, and the resulting displacement is measured with a laser displacement sensor. By employing a confocal laser displacement sensor and aligning illumination and collection on the same optical axis, the actuation point of the jet and the sensing point coincide, enabling stable measurements even on tilted or locally rough surfaces. The device is compact and lightweight, allowing it to be mounted on the wrist of a robot. We further introduce an estimation algorithm that quantitatively evaluates stiffness in both the normal and tangential directions of the surface. Experimental results demonstrate that, unlike conventional contact-based methods, the proposed approach can rapidly generate stiffness maps with sufficient accuracy even on elastic surfaces whose stiffness varies spatially.