Avoiding spurious reflections from shiny surfaces on a 3D real-time machine vision inspection system

Abstract

Spurious reflections from shiny surfaces are one of the main problems in machine vision inspection systems based on triangulation of structured light This paper presents a practical way to solve this problem using reference points developed off-line and stored in a Look Up Table (LUT). First, a non-contact real-time 3D machine vision inspection system is described. Its accuracy is +/-2 /spl mu/m using a structured laser light sheet imaged by a machine vision camera. Based on the triangulation method, the system uses a single image and a fixed sheet of light combined with known position of the object being inspected The inspection locations on the object's surfaces are created off-line. To increase the inspection speed and eliminate the influence of spurious reflections from shiny surfaces, inspection of subsequent objects checks only those previously found locations. The system's repeatability depends on improving the laser centroid algorithm and avoiding reflections from shiny surfaces, A new algorithm is presented which calculates the center of the diffusely reflected laser line using a machine vision camera. The system verifies that this algorithm is more robust and accurate than traditional ones, To avoid spurious reflections from shiny surfaces, the object is first scanned off-line, finding as many points as possible where the laser line crosses the object Then the 3-space locations of these points are found using the standard triangulation algorithm. Next the object's surfaces are reconstructed using the scanned 3D points, breaking the object into various simple surface primitives such as planes, cylinders, cones, spheres and splined surfaces. The influence of the spurious reflections from shiny surfaces is eliminated by deleting those points which do not fall on the constructed surfaces. Based on the reconstructed surfaces, the system creates the object's inspection locations which is used later for fast and accurate on-line inspection,.