Laser line triangulation measurement on incandescent steel objects: methodologies to improve optical signal to noise ratio

Abstract

Integrating optical sensors into harsh industrial environments poses challenges and limits to the application. This paper discusses the implementation of a LLT (Laser Line Triangulation) sensor in a steel industry use case, designed to measure the straightness of steel bars at very high temperatures, up to 1000-1200 °C. Due to the bar incandescence it is challenging to use the optical instrument, as Signal to Noise Ratio (SNR) of the laser line projected onto the inspected object is lower than 2 dB. Both hardware and software solutions are investigated, in particular the choice of a very narrowband filter is selected to remove quite all the other spectral content coming from the bar irradiance. Even though this solution enhances SNR, the narrow band filter blocks the laser irradiance at the edges of the field of view: a reference ruler measured at ambient temperature has shown that 40.3 % of the length of the bar is not detected with respect to the measurement without the narrow band filter. To cope with this issue, a software solution is implemented which consists in the preprocessing of the image with a Region of Interest (ROI) around the laser line. This solution imposes the constraint of repeatable product positioning which has to be assured by the robot which picks up the product for the measurement. Applying the ROI, the Laser Line Extraction (LLE) algorithm detects clearly the laser line, with outliers reduction of 97.5 % with respect to the case without ROI introduction.