The measurement of the particle size distribution plays an important role in mineral processing. Due to the high costs and time-consumption of the screening process, modern machine vision methods based on the acquisition and analysis of recorded photographic images. But the image analysis methods used so far, do not provide information on the three-dimensional shape of the grain. In the coal industry, the application scope of these methods is substantially limited by the low reflectivity of the black coal particle surface. These circumstances hinder proper segmentation of coal stream surface image. The limited information contained in two-dimensional image of the raw mineral stream surface, makes it difficult to identify proper size of grains partially overlapped by other particles and skewed particles. Particle height estimation based on the shadow length measurement becomes very difficult in industrial environment because of the fast movement of the conveyor belt and because of spatial arrangement of these particles, usually touching and overlapping. Method of laser triangulation connected with the movement of the conveyor belt makes it possible to create three-dimensional depth maps. Application of passive triangulation methods (e.g. stereovision) can be impeded because of the low contrast of the black coal on the black conveyor belt. This forces the use of active triangulation methods, directly identifying position of the analyzed image pixel. High contrast of the image can be obtained by a direct pointwise laser lighting. For the simultaneous identification of the entire section of the raw material stream it is useful to apply a linear laser (a planar sheet of the laser light). There have been presented basic formulas for conversion of pixel position on the camera CCD matrix to the real-word coordinates. A laboratory stand has been described. This stand includes a linear laser, two high-definition (2Mpix) cameras and stepper motor driver. The triangulation head moves on the rails along the belt conveyor section. There have been compared acquired depth maps and photographic images. Depth maps much better describe spatial arrangement of coal particles, and have a much lower noise level resulting from the specular light reflections from the shiny fragments of the particle surface. This makes possible an identification of the coal particles partially overlapped by other particles and obliquely arranged particles. It enables a partial elimination or compensation of image disturbances affecting the final result of the estimated particle size distribution. Because of the possibility of the reflected laser beam overriding by other particles it is advantageous to use a system of two cameras. Results of the experimental research confirmed the usefulness of the described method in spite of low reflectance factor of coal surface. The fast detection of changes in particle size distribution makes possible an on-line optimization of complex technological systems - especially those involving coal cleaning in jigs - thus leading to better stabilization of quality parameters of the enrichment output products. An additional application of the described method can be achieved by measuring the total volume of the stream of the transported materials. Together with the measurement signal from the belt conveyor weight it makes possible to estimate the bulk density of the raw mineral stream. The low complexity of the signal processing in the laser triangulation method is associated with the acquisition of high contrast images and analysis based on simple trigonometric dependencies.
Bączek A., Chudek M., Cierpisz S., Heyduk A., Jendryś. J., Joostberens J., Kleta H., 2013. Visualization-assisted method of the assessment of the technical condition and safety of the mineshaft lining using a digital image analysis. Monografia nr 482, Wydawnictwo Politechniki Śląskiej, Gliwice.
Heyduk A., 2005. The influence of lighting conditions on the image segmentation in machine vision system of particle size analysis. Mechanizacja i Automatyzacja Górnictwa, R. 43, nr 10, s. 21-29.
Heyduk A., Pielot J., 2014. Economical efficiency assessment of an application of on-line feed particle size analysis to the coal cleaning systems in jigs. Inżynieria Mineralna, R. 15, nr 2, s. 217-228.
Koh T.K, Miles N., Morgan S., Barrie Hayes G., 2007. Image Segmentation of Overlapping Particles in Automatic Size Analysis Using Multi-Flash Imaging. IEEE Workshop on Applications of Computer Vision, p. 47-52, Pielot J., 2010. An analysis of effects of coal jigging after changes in the grain composition of a feed. Arch. Min. Sci., Vol. 55, No 4, p. 827-846.
Rafajlowicz E, Rafajłowicz W., Rusiecki A., 2009. Image processing algorithms and an introduction to work with an OpenCV library. Wydawnictwo Politechniki Wrocławskiej, Wrocław.
Stefańczyk M., Kornuta T., 2014. Acquisition of RGB-D images - methods. Pomiary, Automatyka, Robotyka, R. 18, nr 1, s. 82-90.
Tosun A., Konak G., Toprak T., Karakus D., Onur A., 2014. Development of the Kuz-Ram Model to Blasting in a Limestone Quarry. Arch. Min. Sci., Vol. 59, No 2, p. 477-488.
Trybalski K., 2013. Control, modeling and optimization of technological processes of the ore processing. Wydawnictwa AGH, Kraków.
Szponder-Kołakowska D.K., Trybalski K., 2014. Modern methods and measuring devices in the study of raw materials and mineral wastes. Wydawnictwa AGH, Kraków.