Inline detection of defects on free formed metal pressings using a single shot inverse fringe projection approach

Optical inspection of surface topographies typically relies on the acquisition of several images. However, in production measurement technology, there are many applications where the use of multiple images is prohibitive. One of those applications is the detection of constrictions on deep-drawn parts. Especially vibrations, short cycle times, varying lighting conditions and the limited installation space issue big challenges when integrating an optical inspection system into a press. However, as only the deviation from an a priori known topography is of interest, and as the device under test is well-aligned within the press, this is a perfectly suited application for “inverse fringe projection”. The key idea is to pre-distort the projected pattern based on the known shape of the part. Thus, the camera only detects variations of the fringes at defect areas. In our approach, the analysis of the resulting pattern is carried out using the Fourier-method introduced by Takeda. We present simulation, hardware implementation and first measurement results for such a system. To qualify the developed solution in the field, it is tested in a production environment in cooperation with BMW.