Chipping away

Laser transmitters help oriented strand board plant hike productivity.

Louisiana Pacific's manufacturing plant in Hayward, Wise., had a problem. Because the company wanted to get more productivity out of its production process for oriented strand board (OSB), the process required a more precise level measurement of wood chips in large wet storage bins. The problem is the environment in these eight bins contains a high degree of moisture as well as dust from the chips, depending on the time of year.

Louisiana Pacific was using electrooptical sensors, or ultrasonic "through beams" with a photo eye in the bins to measure the level and density of the bins. The technology used a half-light that looked across the bin and produced readings in 25% increments (25, 50, 75, and 100%). These devices had performed well as point level switches, and detected if the bin was either open or blocked.

This method, however, did not provide the desired continuous level output required in the production process.

Louisiana Pacific had limited visibility in identifying whether the bins were gaining or losing density; when they did find out, it was at 25% increments. By that time, the bin density had shifted too much in one direction and created problems up and down the production line. Oftentimes, if the reading determined that the bins were low and production was losing ground, they would have to start up the burners again, which slowed the production process and increased production costs.

Louisiana Pacific needed something that would stand up to the equipment in the wet bins, produce an accurate measurement, and improve consistency and flow in the production process.

OSB is a layered, mat-formed panel product made of strands, flakes, or wafers sliced from small diameter, round wood logs and bonded under heat and pressure. OSB's strength comes mainly from the uninterrupted wood fiber, interweaving of the long strands or wafers, and degree of orientation of strands in the surface layers.

As a part of the process, wet wood chips feed into the center of the wet bin where a conveyor drops the chips through an opening near the end of the bin. As the chips fill the bin, the conveyor pulls the chips horizontally through the bin (approximately 60 feet) until the bin is full. These chips will then leave the bin and proceed into a dryer, another conveyor, and then to a dry bin where the chips mix with resin. At the end of the process, boards can end up being as large as 12 by 24 feet or smaller depending upon the requirements of the end user.

"We're an OSB mill, and in part of our process we take a log and shave it into flakes 1.5 by 4.5 inches long and 15 thousandths thick," said Dave Skopek, an electrical lead man at the plant in Hayward. "We put these flakes in a 60-foot-long horizontal bin, and we need a way to accurately measure the level across the entire length of the bin."

"The challenge is that we have moving rakes in there, raking these piles back," Skopek said. "With equipment moving within the bin, we can't use a sonic device that measures a 3-foot square area."

After looking at a multitude of possibilities, Louisiana Pacific decided to install laser transmitters in the wet bins at the Hayward plant.

These laser transmitters have an all-digital design that provides speed and accuracy in a small, tight unit. The system architecture allows the software to perform distance measurements quickly, then dedicate more time for sophisticated signal processing, data analysis, and distance display. Last pulse detection means the transmitter can select true signals from among the many that reflect from dust clouds, moving machinery parts, or other obstacles. Color compensation corrects for weak or excessive return signals. Variable gain allows the system to select the best return signal in quickly changing visibility conditions, such as moderate dust, vapors, and materials of different colors. Such capabilities are especially useful for underground mines and smelters where high dust levels and excessive vibration can render other noncontact technologies inoperable.

In production, the laser transmitter measures the distance of wood chips horizontally across the bin. Process temperature ranges from 40°F to 140°F and the pressure is atmospheric.

A laser near the top of the bin fires a short pulse of light down to the process surface where it reflects back to a detector at the top of the bin. A timing circuit measures the elapsed time and calculates the density. The transmitter alerts the operators when the bin becomes full. The noncontact aspect of the transmitter is useful because no components inside the bin can suffer any damage. In addition, due to the bin height, the laser has a minimal beam width, which ensures there is no chance of receiving false echoes.

The transmitter takes measurements using a laser with an extremely small wavelength of ~1 micrometer. As a result, the I unit has virtually no angle divergence, even over long distances.

Operators can narrowly focus it to ensure it does not pick up interference from surrounding objects. The system can also measure oblique angles with pinpoint accuracy. This allows it to operate within complex vessels, even if they contain internal structures. The transmitter is also immune to reflections and echoes, thus providing consistent performance on solids and total immunity to "cone up" and "cone down" variations in the measured material.