Effects of Refraction Model on Binocular Visuotactile Sensing of 3-D Deformation

Abstract

This article presents a binocular visuotactile sensor (BVTS) and its model for reconstructing the 3-D deformation due to mechanical contact on its elastomer surface. The model explicitly accounts for the refraction effects of the elastomer on the depth map reconstructed from the imaged markers. To help gain physical insights into the refraction effects on the depth map reconstruction and experimentally analyze the effectiveness of the model-based BVTS, a prototype BVTS has been developed for experimental investigation. The refractive indexes of the elastomer, the acrylics, and their combination are experimentally determined from the model-based reconstruction and verified with that measured by a spectroscopic ellipsometer; the differences are within 3%. As a basis for evaluation, the reconstructed depth maps are compared with that of the two other commonly used practices, stereovision (SV) calibration without considering refraction and integration camera model that lumps the refraction effects with the SV parameters during calibration. By explicitly modeling the refraction physics using ray tracing, the model-based BVTS faithfully reconstructs the points on the actual height and is capable of dynamically tracking small torsional displacements with balls of different radii (3–15 mm) with a depth error below 4%.