Abstract
The optical inspection of wrought hot workpieces between subsequent forming steps of a multistage process chain can yield diverse advantages. Deficient components can be detected in an early forming stage. Moreover, the eliminated cooling economizes heating energy. The present workpiece temperature can be exploited in the following chain steps. Challenges arise due to the heat input into the air surrounding the workpiece, as triangulation techniques rely on homogeneous optical conditions. The effect of an inhomogeneous refractive index field (RIF) in air on a 3-D geometry measurement by optical triangulation is modeled exemplary by a virtual measurement of a hot cylinder. To our knowledge, this is the first simulation approach that fully considers both light deflection from the illumination unit to object and from object to camera. Simulated measurement results in a homogeneous and an inhomogeneous RIF are compared. The presented approach predicts measurement deviations in inhomogeneous optical media and can help to design actuated or computer-assisted compensation routines in order to reduce deflection effects when measuring hot objects.
Highlights
The optical triangulation method is a state-of-the-art technique to acquire geometry data of complex freeform geometries and used in different scales.[1]
The triangulation sensor is not rotated around the cylinder axis (β 1⁄4 0 deg) to realize a measurement from above under full influence of the refractive index field (RIF)
A virtual triangulation setup based on the light section method is presented, using a matrix camera with entocentric lens as detection unit and a telecentric laser line generator as illumination unit
Summary
The optical triangulation method is a state-of-the-art technique to acquire geometry data of complex freeform geometries and used in different scales.[1] A common industrial application is the inspection of formed metal sheets in the automotive sector by fringe projection systems,[2] whereas endoscopic systems for confined spaces with small measurements heads are being investigated for in-situ inspection tasks (e.g., the restoration of turbine blades[3]). The simulations are performed with the software Comsol Multiphysics,[14] as the software provides both a simulation module for numerical heat transfer calculations, as well as a ray tracing module
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