By Zoltán Sárosi, Wolfgang Knapp, Andreas Kunz, Konrad Wegener, IWF, ETH Zurich, Switzerland
The early detection of surface defects on raw sheet metal parts is a critical problem in the automotive industry. Typical defects like dents, bumps, and waviness are invisible at the early production stages because of the parts' rough surfaces. The defects become visible and disturbing only after the later production steps in which the parts get painted and varnished, meaning that they become specular-reflecting. The later the defect gets detected, the more the correction will cost. Currently, the raw sheet metal parts are only randomly tested by specially trained experts, which mean the results are subjective and qualitative. To overcome this problem, we propose a method that can be used on-line, on each part directly at the production line. The system employs a one-shot deflectometry method in the infrared range. At sufficiently long wavelengths, the sheet metal's surface becomes specular-reflecting, thus enabling the use of the deflectometry method. For this method, a device capable of displaying a reference pattern at the infrared wavelengths was developed. This single infrared reference pattern is reflected from the surface of the raw sheet metal object and the reflected image is captured by a thermo-camera. The distortion of the reflected pattern due to the surface's geometry and defects is analyzed by the Fourier-transform method. Due to the single exposure method, vibrations during the measurement are not disturbing and the alignment problem, that is characteristic for multi-exposure methods, is nonexistent in this case.