Towards industrial applications of optical in-line measurement of geometry

Abstract

Automatic control of the geometry of manufactured components is a cornerstone in the Industry 4.0 transfer that still constitutes an elusive goal. There are however some full-field optical techniques that in combination with optimized software have started to make the transition from the measurement room out to the production line and are able to generate real-time measures of sufficient quality. In this presentation two such techniques are reviewed together with challenges that still remains to be solved before a full implementation is possible. Short-range photogrammetry is a technique that utilizes the explosive development of high-resolution detectors to measure shape from differences in perspective observed in two or more cameras. As a non-interferometric technique, it relies on the presence of structure on the surface of the component that can be identified from different views. With present day technology it has been shown that it is possible to reach a precision of roughly 10-4 of the measurement volume in serial production. Another technique that has the potential to make the same transition is multiplexed multi-wavelength holography. Historically phase wrapping has been seen as an obstacle to apply dual-wavelength holography on generally shaped objects, which has pushed the technique towards wavelength scanning or coherence-gating and scanning. However, new approaches in software have made multiplexed multi-wavelength holography attractive for use also in an industrial environment, which is demonstrated by the use of a specially designed three-wavelength pulsed laser system. The precision achievable is comparable or slightly better than for photogrammetry and the robustness is sufficient. Despite the fundamental differences in technology both techniques require for its realization the same software steps to generate absolute measures, which include object identification, alignment, virtual fixturing and calculation of absolute measures. In the presentation the role of each of these steps are described in some detail.