Vision System Automates Mining Bridge Conveyor

  • October 30, 2014
  • Cognex Corporation
  • Case Study

October 2014 - In the past, operators of a boring machine in an underground potash mine were responsible for controlling a device called a bridge conveyor that drops ore onto a multiple kilometer long main conveyor that hauls it to the loading pocket.  From there, it is hauled to the surface by the skip. The operators had many other jobs to do so the conveyor was often loaded unevenly, which could create spills that took hours to clear. Another problem was that the mine operators had no way of knowing how much potash was loaded on the conveyor. This is important because the skip is the bottleneck of the mining process. The lack of knowledge of how much ore was on the conveyor (plus in the loading pocket) makes it challenging to plan shift changes when there was sufficient ore on the conveyor to avoid skip downtime. Recently, the mining company installed a machine vision system developed by HINZ, a Rockwell Automation company based in Saskatoon, Saskatchewan, that automatically measures the amount and distribution of potash on the main conveyor as it is loaded by the bridge conveyor. A laser projects a line of light across the main conveyor and a Cognex In-Sight Micro 1050 vision system reads the profile of the line three times every second. This information is used by the mine control system to automatically move the bridge conveyor to correct any imbalance. The greatest benefit of the vision system is that it avoids spills while freeing up mining operators to concentrate on their primary task. Another benefit is that the mining company now knows exactly how much potash is on the conveyor at any point in time and so can plan shift breaks to avoid skip downtime.

bridge conveyor with camera on left end

Need to watch bridge conveyor Several operators are responsible for running the mining machine. In the past, the operators were also expected to simultaneously watch the bridge conveyor in order to evenly load the 4 foot wide main conveyor. The main conveyor is a continuous belt that sits on rollers that form the belt into a channel shape. The bridge conveyor is moved back and forth in order to load the main conveyor in its center. If the load should become concentrated on one side of the main conveyor the main conveyor could tip over and spill part of its load at some point in the tunnel. This forces the mine to shut down for several hours while workers drive trucks on a road that runs parallel to the conveyor in the tunnel to get to the spill and then clean it up with shovels. The skip taking ore out of the mine has the lowest capacity of any element in the conveyance chain from the mining machine to the surface, making it critical to carefully manage its operation. In an ideal world, the skip would run 24 hours by 7 days including through shift changes. However, keeping the skip running throughout a complete shift change requires that sufficient ore be available in the loading pocket, plus the ore loaded over the length of the main conveyor to operate the skip through the change. Up to now, mines have had no way of knowing how much ore was loaded on the main conveyor. Mines have tried various methods such as load cells to provide this information but have never been able to produce more than rough estimate of the volume of ore contained by a long conveyor.

In operation long exposure

Machine vision challenge Efforts in the past to address these challenges with machine vision have run up against very difficult lighting conditions underground. For example, one second the vision system will be operating with only its controlled lighting. The next second the controlled lighting will be overwhelmed by the bright headlights of a truck or the headlamp of a miner. The mine is also a very dusty and harsh environment where vibrations driven by powerful equipment or rock falls can easily destroy all but the toughest vision systems. “To the best of our knowledge, no one has ever developed a successful load profiling machine vision application in an underground mine prior to this application,” said Scott Huckerby, Senior Process Consultant for HINZ. “We took on the challenge because we felt that advancements in machine vision systems and lighting technology finally made it doable. Our concept was to project a laser light onto the surface of the ore. The laser light would be a carefully selected wavelength that was not present in the mine environment. The machine vision system would be equipped with a filter so that it would only see this particular wavelength.  We also needed to ensure that the wavelength selected would reflect off the belt materials, as well as the ore” HINZ engineers worked to find a laser with a color that would reflect off the ore and would be immune to interference in the mine. They selected a Lasiris solid state laser from Coherent Inc. “We selected Cognex Micro 1050 vision systems because they are the only product that met all of our requirements,” Huckerby said. “The Micro 1050 is completely self-contained, rugged enough to withstand the mining environment, communicates well with Rockwell Automation programmable logic controllers (PLCs), and provides a powerful programming environment that we were able to easily use to determine the complete profile of the top of the ore pile.” In-Sight Micro vision systems provide a complete vision system in a 30 mm by 30 mm by 60 mm package. Cognex Connect enables out-of-the-box connectivity between Cognex In-Sight® vision systems and virtually any major PLC, robot, HMI and Fieldbus system commonly used in factory automation. Cognex Connect supports open standard industrial Ethernet communications protocols such as MC Protocol, Ethernet/IP and PROFINET for trouble free connection to a wide range of PLCs including Mitsubishi, Rockwell and Siemens.

In operation showing dust in the air (with flash)

Developing the vision application HINZ engineers used the Cognex EasyBuilder interface to develop the machine vision application. They set up the laser so it would project a line onto the top of the pile of ore and set up the vision system so that it pointed at that line. Then engineers connected a notebook computer to the vision system and then followed wizards to trigger the vision system and set up the scale and nonlinear calibrations. The engineers then divided the line projected by the laser into 40 different windows on the vision system. They dragged a “line finder” tool into each window that reported the position of the line. They repeated the process with an empty conveyor in order to report the position of the line with an empty conveyor. The vision system subtracts the position of each section of the actual line from the position of the line with an empty conveyor to determine the amount and distribution of ore on the conveyor. Additional line finding tools were used to track the position of the edge of the conveyor belt to provide an alert if the belt is no longer centered on the rollers. A point and click communications setup was then used to configure the vision system to transmit this information to the PLC that controls the mine equipment. The PLC determines if the current load on the conveyor is unbalanced and if so, issues a command to the bridge conveyor to dump more ore on the lighter side. Cognex’s VisionView operator interface panel enables mining machine operators to monitor the condition of the conveyor. The picture on the screen shows the conveyor with the laser line and vision tools overlaid so operators can see at a glance both the distribution and volume of the load. Operators can easily review previous images to, for example, determine the exact point at which a problem began.

Installation of camera and laser

Surviving the underground environment The vision system has already demonstrated its ability to survive in the rugged mining environment without downtime. “The camera is mounted on the bridge conveyor and while I was underground commissioning the vision system a big hunk of rock went through the conveyor and become lodged in the chute that feeds the main conveyor,” Huckerby said. “The mine operators utilized the hydraulics on the bridge conveyor, itself a 10 ton piece of equipment, to slam the bridge conveyor with vision system attached repeatedly into the ceiling of the mine to clear the rock. After the dust settled, the vision system continued operation without any interruption.”

Installation of visionview - camera on other side at the end

The vision system provides major benefits to the mine operator. First of all, automating the control of the bridge conveyor has eliminated spills and associated downtime that often happened in the past when operators were too busy with other tasks to pay attention to the bridge conveyor. Second, operators who have been freed from the task of watching now have more time to focus on their primary responsibility of operating the mining machine. Third, now that the mining company knows the amount of ore on the main conveyor it can plan shift breaks to avoid downtime on the skip, which substantially increases throughput of the mine. “This technique could be used anywhere, underground or above ground, where you need to know what is loaded on a conveyor,” Huckerby concluded.

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