By By Mark Williamson, Journalist Engineer, Thornhill, Ontario, Canada
BRUSSELS,
Aug. 24, 2009
(RISI) -
Operators on M-real Husum's PM8 and off-machine coater can now see the same flaws at the same precise location in the parent reel and relate them to how the sheet runs through the coater, and where it breaks. With precise fault classification and exact stopping of the re-reeler, the critical faults can be isolated and patched so the re-reeling process is more effective at improving line efficiency.
It is self-evident that paper web faults from the papermaking process cause breaks in an off-machine coater. But the real challenge to papermakers has been to precisely locate, classify and define the root causes of those faults and eliminate the troublesome ones that really cause runnability problems and customer quality problems. It hasn't been so easy in the past. If the sheet tail could be found, some clues might lead to the source. However, until recently, it has been a bit a guessing game.
M-real's Husum mill in Sweden is well on its way to solving these longstanding problems with the installation of a unified web fault recognition and classification system together with web break detection cameras on its PM8 coated magazine paper line. The origins of OMC breaks are now well defined quickly, the loop is closed and the guessing game at Husum is over.
The unified system, supplied by Viconsys Oy, consolidates the web fault inspection and break analysis functions on the PM8 base paper machine and the off-machine coater. The company, based in Jyväskylä, Finland, introduced the unified system technology in 2006 and has delivered over 180 systems since then.
Operators on the PM8 paper machine and on the off-machine coater can now see the same flaws at the same precise location in the parent reel and relate them to how the sheet runs through the coater, and where it breaks. Moreover, with precise fault classification and exact stopping of the re-reeler, the critical faults can be isolated and patched so the re-reeling process is more effective at improving line efficiency. The line produces coated magazine papers with a total grammage of 80 to 130 g/m2.
Improved fault detection and classification
The decision to purchase a new web fault inspection system was based on reasons typical to many mills. The technology in the older web inspection system had been eclipsed by newer, more precise and detailed detection and classification techniques. Maria Koitrand, Production Engineer, outlines the reasons for an upgrade in technology: "We wanted to improve the reliability of the detection of faults. The image quality of the older system was not perfect; we could not detect all the defects and some coater breaks could not be explained." As result, it was difficult to determine the optimum patching strategy that would provide the best OMC efficiency. Moreover, some troublesome faults might have been invisible to the older inspection system.
She continues, describing the desired end results: "Our final objective was to improve efficiency of the re-reeling process and, following that, the efficiency of the coater. To improve the patching strategy on the re-reeler, the operators needed to know the effects of certain types of faults on OMC runnability and, most importantly, what to patch and what does not need patching." Of course, the elimination of faults at their source on the paper machine was the ultimate objective, together with ensuring consistent customer quality.
Better operator communications
In their pursuit of higher line efficiency the mill staff concluded that improved communication between all the operators was very important. It is an extremely long production line with many intermediate processing steps. "We wanted to address the long distances by providing better operator communications," says Koitrand. The mill staff evaluated the integrated approach offered by Viconsys that would tie together fault web fault detection and web break detection on PM8 and the OMC and, at the same time, improve the effectiveness of the re-reeler patching strategy. This was a new concept introduced by Viconsys that showed a lot of potential to the mill staff. And they were willing to accept the potential. "We try to use new, promising technology. To be the best, you need to by daring," adds Koitrand.
The Viconsys fault detection capability was evaluated by Husum staff at M-real's Äänekoski mill in Finland, where a previous system had been installed, and by sending the Husum mill's paper fault samples to the competitive suppliers. Tommy Norberg, Automation Engineer, says they were impressed by the ability of the Viconsys system to see very small defects, including pinholes. He says the ability to combine two detection techniques- web break cameras and web fault inspection cameras - in a single system with a common operator interface was a big plus in their estimation. This gives operators the ability to track web quality faults and breaks in the same system and to determine the exact relationship between them. This capability is important on an off-machine coated paper line since some faults cause OMC breaks and some don't. Nevertheless, all possible faults must be detected first. Then their impact can be evaluated.
Precise imaging, exact fault location
The new integrated system was installed in May, 2007. On PM8, a new fault inspection (Quality Vision) detection beam with 24 digital matrix cameras detects the CD and MD position of web faults before the reel and 8 existing Process Vision cameras, delivered previously by another supplier, detect web breaks. On the OMC, 8 Quality Vision web inspection cameras are integrated with 16 previously-installed Process Vision cameras which detect web breaks on the four coating stations. Image processing hardware for the 56 web inspection and break cameras and 4 edge code reading cameras are housed in two cabinets. Camera images are transmitted from the machines by fiber optics for low signal noise and good image quality.
When the system was installed, one of the first tasks was to evaluate the improved detection capability of the new web inspection (Quality Vision) cameras. The new digital camera beam with strobed LED lights was installed next to the older system. The improvement in detection capability was noticeable immediately. Many new and previously undetectable flaws were now visible. So it gave machine operators much more valuable information to work with to solve web fault problems.
The new matrix camera technology is the key to the image clarity and fault detail provided by the new system. The matrix cameras take an instantaneous snapshot of the full field of view of each camera, so all pixels are captured at the same time. The light capture efficiency is very high as the cameras capture about 95% of the light from the strobed LED light source. The beam of LED lights is pulsed at a very fast rate, which is adjustable down to 3 microseconds. This light pulsing freezes the sheet image to minimize the smearing effects of sheet movement. The Quality Vision cameras on PM8 can detect web faults as small as 0.24 mm. Even pinholes can be spotted.
A Ryeco edge marking system, activated by the Viconsys system, provides reference marks indicating parent reel web length, sheet direction and the location of specific flaws defined by the customer. The edge reference marks are detected on the re-reeler and the OMC by Viconsys digital matrix cameras. The system automatically compensates for slab-off or break losses. Therefore, it never loses track of the exact location of faults. A fault seen on the OMC is in exactly the same position as in the original parent reel.
Operator interface terminals, combining web fault maps and web break images, are installed in the control rooms of PM8 and the OMC. In addition, two production floor terminals are provided for the operators of the re-reeler. One terminal indicates the patching requirements of the current parent reel. Another screen indicates the patching requirements of parent reels in inventory. In this way, operators can see what is coming and adjust patching strategies accordingly.
Precision stopping of re-reeler
Using the edge mark reference measurements, the system is programmed to automatically stop the re-reeler within 0.5 meters so certain previously-defined fault types can be repaired by the operators. Decelerating and stopping a re-reeler with a speed approaching 2500 m/min was one of the project's most challenging tasks which required the cooperation of the Viconsys engineers and the original PLC supplier for the re-reeler. However, the 0.5 meter stopping precision was achieved. The operators can quickly locate and patch a fault in the parent reel with the aid of an array of lights embedded in the floor under the re-reeler. This light array was designed by mill engineers. As a fault is approaching, the CD location of the fault is indicated by a flashing light. The patched faults are noted in the system by the re-reeler operator so they can be seen by the OMC operators.
Freezing break events
Another Quality Vision array of digital matrix cameras inspects the sheet as it is unwinding on the OMC. Edge mark references applied at the paper machine are measured by a digital camera. With this absolute position referencing the faults from the paper machine and the patches made by the re-reeler operators are synchronized to the web inspection cameras and the web break cameras on the OMC. The synchronization must be very exact, as the operating speed of the coater is approximately 1700 m/min. Literally every fault from the paper machine can be seen as the sheet is running through the OMC. Diverse faults such as holes of various dimensions, felt hairs, slime spots, thin spots and many others can be seen. Through the neural network learning capability built into the system there are now over 40 categories.
By analyzing web faults and break events, OMC operators can see which flaws have caused breaks on the coater and in which coating stations. A break event initiates a freezing of web inspection and break camera images. By using the synchronized web break (Process Vision) cameras and relating them to fault images, the operators can see how a break has developed through the coating stations. The area of paper currently running through the coater is indicated on the screen by a green band.
Operators learning and adapting
How are the PM8 line operators adapting to the new, much more detailed and complete defect information? Nordberg indicates that re-reeler operators are patching more than before but doing it in smarter way with the more complete information. He refers to the fault filtering capabilities built into the system which allow operators to define the most critical faults which must be eliminated. This is one tool in a wide-ranging plan to develop a patching strategy which is consistent for all line operators and represents the best practices for achieving high line efficiency. This consistent operating strategy is important, so all six shifts with six operators per shift are approaching fault elimination in the same way.
Koitrand adds her thoughts: "Regarding patching strategy, the goal or main objectives with the strategy is to only patch the pre-selected defects from the paper machine. The operator should not add any own defects that look suspicious. The Quality Vision system should be trimmed so we can rely on it 100%. With this strategy, we should still obtain the runnability on the coater without any breaks caused by base paper related defects." She also says that the patching strategy should be dynamic as the OMC is often "more forgiving" of faults after a few days running time.
Ongoing improvements
On the paper machine, some recurring faults identified by the new system have been dealt with or are under investigation. A problem called "horshoeing", a small-scale formation defect with a horseshoe shape, was traced to the operation of the PM8 shoe press and solved. A recurring problem with debris from doctor blades is under study and a new strategy for applying doctors will be developed. "We didn't know we had doctoring problems before," says Koitrand. But finding a previous unknown problem like this is a big plus when it leads to a solution.
In summary, Koitrand believes the new system has allowed them to learn more about the process and to know where to look if a problem arises. "Before, we could only guess; now we know," she states. "We believe the Viconsys system, among other improvements, is a very successful factor in achieving OMC runnability," she concludes.
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