Detection of surface defects on sheet metal parts by using one-shot deflectometry in the infrared range

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 special, trained experts, which means subjective and qualitative results. 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.