(Editor’s Note: In his final regular column, Alan Froome continues his discussion of sawmill ideas and methods and how they evolved, along with a review of some recent developments in technology. This series of columns is intended to be a generic overview of different methods used in the sawmill industry in response to readers’ requests.)
Previous articles discussed the processes used to produce rough lumber boards, cants, and so on, and then finish them by drying and planing. Now we examine scanning and optimization technology.
Computer Decisions
Scanning and computer optimization in the sawmill is a huge subject. Almost every stage in the production of lumber and other wood products has been automated by scanning and optimization to some extent. We will focus on the basic types, their uses and where research and development is headed.
In general, the decision-making ability of a machine operator is being replaced by the ubiquitous computer. Many experienced operators have found it hard to accept that a scanner and computer can do their job better and faster. In fact, most high recovery softwood canter lines are shut down if the scanner and optimization system stops working for any reason — until it is fixed. The potential drop in lumber recovery factor (LRF) is so great under manual control, and machine controls have become so complex.
The first scanners were developed in Scandinavia in the 1960s in order to scale and sort raw logs. Sawmills in Sweden and Finland run differently from North America; a mill may sort logs into 70 or more categories, and then run for several hours, processing logs of only one category. Accurate log sorting, therefore, is very important. They do not set the machines for each individual log, as sawmills do in the U.S. and Canada.
Personnel and working conditions in the frozen north may have had something to do with the growth of this technology in Scandinavia, too. By the 1970s, practically every mill in the Nordic countries was using scanners this way for sorting logs. Other scanning applications came later in conjunction with optimization computer software development in both Scandinavia and North America.
LED Shadow Scanners
Some U.S. companies first adopted scanning to scale logs by cubic volume. These first scanners used the ‘shadow’ method of scanning the logs. Basically, they used an array of light emitting diodes (LED) mounted on one side of a log conveyor and a matching set of detectors on the opposite side. As the log passes through the light beams, it blocks a number of the diodes, which gives the diameter of the log at that point. A rotary pulse generator driven by the conveyor is used in conjunction with the LEDs so that the diameter can be measured at known intervals along the log to determine its taper and length. Many simpler log sorting scanning systems still use this method today, but the disadvantage is they assume all logs are straight, like telephone poles.
Carriage Scanners
After log scaling and sorting, the next application for scanning was at the head rig carriage. The technology gave the sawyer more accurate log lengths and diameters than he could guess by ‘eyeball.’
This gave rise to a much larger transverse version of the ‘shadow’ scanner, namely the ‘light curtain’ scanner. At first this method used long-range photo-electric cells mounted on the rafters over the carriage loading zone and matching detectors below the carriage knee level. Again the number of cells blocked by the log provided the sawyer with the basic measurements – diameter, length and taper. The main problem with this method is that bark and debris fall on the bottom detectors, so they have to be cleaned constantly.
Later lasers and detectors were used instead of photo-electric cells because they perform better in dusty, dirty conditions. For a time, conventional video cameras and digital images were used for carriage scanning. Several cameras were mounted overhead, each viewing part of the log below. Log data was arrived at by counting the pixels or ‘dots’ from each image, which was added to the next to complete the picture. This was state-of-the-art for a time but soon was replaced by more reliable methods.
Now the most advanced carriage scanners use lasers in conjunction with CCD cameras; both lasers and cameras are high above the carriage — nothing below. The computer optimization software interprets the scan data to produce a digital 3D image of the log. The optimization process considers the log’s sweep and crook, as well as diameter, length and taper. This more complete information provides better cutting solutions and higher recovery.
Some laser carriage scanners use military-type terrain mapping technology (LASAR), which may give even higher density 3D images (more data points). The processing computer ‘horsepower’ required is higher, however, and likely to be more expensive to install.
Optimized Log Bucking
Optimized log bucking is used to buck logs 60 feet or longer in the mill yard. The alternative is cut-to-length logging in the forest by a harvesting machine. However, these machines cannot take into account log sweep and crook.
Optimized bucking is common in most mills in northern Canada and many mills on the West Coast of the U.S. The scanning method used is lineal with either a LED type or laser-camera type straddling the infeed log conveyor. Depending on the conveyor, some can also scan for shape and sweep (stable log is essential). The computer software, using the scan data and length from a rotary encoder counting pulses, calculates the optimum bucking solution (highest value), and outputs it to the buck saw and log stops.
Since there may be several hundred different bucking solutions for any given tree and each has a different value, an operator doesn’t stand much of a chance of getting it right with manual controls. This is also the first stage in processing the log to affect the overall LRF for the mill, so optimum bucking certainly has value in a high production mill.
Canter Line Scanners
At the canter line, logs are fed lineally (end-wise), not transversely, so a different scanner configuration is used. The LED scanner was the first and is still used in many mills, but higher scan density laser units have replaced many of them. As with the carriages, ‘true’ or real log shape information allows the machine infeed and cutter heads to be positioned for processing logs of various shapes, not just ‘telephone poles.’
More sophisticated optimization software continues to be developed as ever-faster computers become available. This means that the optimizer can now ‘crunch’ all the numbers in the data the modern scanner sees — not just some of it, as was necessary only a few years ago.
Edger Optimizers
Sideboards have been edged by some kind of optimizer for several years, and many different systems are available on the market. Some early systems imported from Sweden used strobing car headlights and linear array cameras to capture the board data and set the machine; sunglasses became normal wear in that part of the mill. Today the systems offered use a wide range of different scanners, such as infra-red photocells, LEDs, lasers and cameras. Most systems used today scan the incoming board transversely and position it on a slat-bed under press rolls before it is sent end-ways through the Edger. The ability to accurately scan for wane is important to achieve the best results, and the software should be able to take every advantage of the wane rules.
Cant Optimizers
After the head rig has made the first cuts, cants may be sent to a gang edger. At first these machine were all straight-fed, and the scanners basically were the same as for the board edgers. Now that much higher speed computers are available, the modern cant gang is likely to be a curved or ‘real shape’ machine. It uses a scanner and computer to swing and set the machine infeed as the curved cants are fed through. Higher LRF is again the motivation to install these expensive machines, which can make straight boards from curved cants by sawing along the grain. Some systems use a transverse scanner set-up, and some use a lineal unit. Either way, the laser-camera type of scanner is preferred to provide the necessary high density data.
Trim Optimizers
Trimmer optimizing systems use the similar transverse type of scanners as the board edgers. The computer software is quite different, of course, and can also include some grading functions. Besides trimming waney ends, some systems using laser-camera scanners and high-speed computers also can scan for common defects, including knots, splits and rot. This usually is done using a combination of color and shape recognition. Some wood species with large color variations, like red cedar, make this grading feature less reliable.
Future Scanners
Besides the scanning methods described above, development continues to achieve more improvements. The general direction this is going is to scan wood for quality as well as for dimension. This means scanning internally as well as from the outside.
CAT scans (Computerized Axial Tomography) as used in hospitals and NMR (Nuclear Magnetic Resonance) scanning, also used in medical applications, have both been tried successfully to find internal rot and knots in wood.
X-ray systems have also been tested. MacMillan Bloedel (now part of Weyerhaeuser) had a system working on raw logs on Vancouver Island for some years. It was rumored to have cost several million dollars at the time. Cost is the main reason these more exotic methods are still in the experimental stage.
There is a practical X-ray system designed to grade boards in the planing mill. This system, called an ‘X-ray lumber gauge,’ uses technology similar to baggage X-ray machines at airports and measures the density along the length of the board. This data is used to grade and predict the strength and stiffness of the piece and can be used on softwood boards from 2×3 to 2×12.
