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Laser Sensors Help Reduce Waste at Wood Fiberboard Plant

Photonics Spectra
Mar 1997
Ruth A. Mendonsa

Eliminating waste is a prime concern for any manufacturing plant, not only from the financial standpoint, but also from the environmental one.
At a Swanboard AB wood fiberboard manufacturing plant in Larvik, Norway, the problem was how to cost-effectively prevent wet fiberboard from being pressed into sheets of uneven thickness on a high-speed production line. Similar to the paper-making process, the wood fiber stock, destined for use in the building and construction industry, is smoothed and drained on an endless wire gauze conveyor belt. The nature of the mixture used and the direction of the motion created a tendency for the wet stock to slough up unevenly on the wire gauze, resulting in an unacceptable amount of waste.
To solve this problem, Swanboard needed to accurately measure the thickness of the wet stock, which moves on-line at speeds of 5 m/min. The ocular monitoring system in use was unable to do the job reliably.
The company bought two Selcom SLS 5000 sensors from Selcom AB in Göteborg, Sweden. Based on the principle of triangulation and designed for high-speed applications in sawmills and in the general wood industry, the SLS 5000 series sensors feature high-speed resolution with accuracy from ±0.14 to 0.4 mm of the measurement range, a data sampling rate of 16 kHz, a bandwidth of 2 kHz and a stand-off distance of up to 300 mm. The sensors have a totally integrated sensor within the head and a built-in signal processor for data averaging and filtering.
In Swanboard's application, two sensors were fastened to a crossbeam and placed at right angles over the wire gauze carrying the stock along. Thickness measurements within an accuracy of approximately ±0.25 mm are taken near the edges of the board, with operators making necessary adjustments to rollers if the wood stock needs to be smoothed out and pressed more evenly.
Swanboard was so pleased with the results from the first two sensors that it installed a second set: one additional sensor was installed where the board enters the dryer, and the second immediately after the dryer. At this point, although the boards are moving at 20 m/min, the accuracy of the sensors is not affected by the high line speed.
"Both applications exceeded our measurement expectations," said Swanboard's Stefan Edberg, who helped install the two Selcom sensors. "In fact, Swanboard is so pleased that it is now considering using the sensors in an automatic control system at the Larvik plant."
A representative of Selcom says the sensors also have numerous applications in the steel, foundry, metalworking, rubber, food packaging and robotic welding industries.

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