Rock analysis on the moving conveyor belt
In order to produce greater transparency in the raw material preparation process, and obtain more dependable information regarding the particle size distribution in the feed and discharge flows of crushing processes, a reliable measurement and monitoring system is required. With the VisioRock™ automated visual rock size monitoring system from Metso, headquartered in Helsinki/Finland it is possible to perform monitoring and evaluation of visible rock during the production process. The system measures and monitors the feed material for different crushing stages or mills. VisioRock™ combines reliable particle size measurement with the possibility to directly influence the crushing gap adjustment if the particle size deviates from specific requirements. Especially in the case of mills, the setting of an appropriate feed quantity is directly related to the particle size of the material.
The rock analysis focusses on rock size, colour, texture and particle shape (Fig. 1). The process monitoring system VisioRock™ comprises a video (IP) network camera in a stainless steel housing (protection rating IP67), including LED lighting and mounting system. The hardware is mounted above the conveyor. The camera itself is equipped with a dust-proofing system to which an air compressor can optionally also be connected. When mounting the camera, care must be taken to shield the optics from stray light – a precaution which is necessary because large sections of conveyor systems are typically located outside of buildings.
The VisioRock™ software can be administered via an ordinary PC user interface, but can also be incorporated into LAN and WAN structures via an OPC server and TCP/IP. The system is usually operated from the control station. The advantage of the software is precise analysis of the video stream delivered by the camera, from which up to 10 images per second can be taken and evaluated.
In the initial stage of image processing, optical transmission weaknesses are compensated by the use of filters or automatic contrast corrections, before the software starts with the analytical segmentation (Fig. 2). Sections of the conveyor belt that are not carrying any material are identified by the software as “empty” and are not taken into consideration in the analysis process. Image information from areas outside the edge of the conveyor belt is not included in the calculation.
Any fines that are in the size range below the resolution of the screen are assigned by the software to the fine fraction. In accordance with individual specifications, the software represents the particle size distribution in 2D. The operator of the plant can concentrate entirely on control tasks, while the intelligent algorithm of the program filters out and automatically displays the current particle size distribution data for him. “Non-critical particle sizes” are displayed in colours ranging from green to blue in accordance with the respective particle size distribution, while “critical” particle sizes (those above a specified tolerance limit), appear in red. Via the plugin OCS 4D® Image Miner, the operator can at any time obtain pictures with a time stamp and specific search criteria information from the database.
It is possible to install six or more cameras (Fig. 3) whose images can be synchronously displayed on a terminal. Adjustment of the cameras is carried out remotely via the computer in the control station. Film sequences can be replayed as desired, and logs with historical data can be retrieved at any time. The system is equipped with the standard interfaces DDE or OPC and can be integrated without difficulty into almost any network.
Overall, the VisioRock™ system from Metso is a high-performance instrument for increasing the stability of material cycles in treatment processes and for improving the management of material feed streams.
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