Scanning options for the optimized edger evaluated
As way of an explanation to readers on scanning systems on offer. Transverse board scanners are either differential (top and bottom) or top only. Boards are normally conveyed thru the transverse scanner on a lugged chain or between hook stops on a slick chain. The most common scan heads have multiple point lasers being viewed by a line image camera that is synchronized to the scan chain by an encoder. Some configurations of the multipoint scanners include a thru beam light-curtain with beams every 5 to 10mm to improve width and length resolution. An alternate transverse scanning method uses a line laser profiling heads. After the flitch is scanned, it's positioned and skewed above the feed chain, and the edger positions its saws to the determined cutting patterns. Press-rolls then force the flitch onto the chain. The flitch is then driven straight through the edger. At the edger out-feed there are a pickers that allow the boards to be kept, and the edgings to be dropped out. A slewing lineal scanned edger takes a different approach. The flitch is fed onto a belt without any precision positioning. The flitch is then scanned as it travels on a conveyor belt towards the edger. The scanning is generally done by a pair line laser profiling scan heads. The optimizer determines the optimal cutting pattern, and the saws are angled and positioned. The board remains on the belt right up to the edger in-feed. The edger then slews the saws to match the angle of the cut as the flitch moves through the saws. At the edger out-feed, the pickers are also slewed to remain under the boards. In a lineal system, the sensors are a small part of the total system costs, and the designs are driven more by simplicity, reliability, scan density and accuracy. The scanning technology is almost exclusively laser line profiling. The basic technology is little changed from what was used 15 years ago, but over the years it has become less expensive, faster, more accurate and more reliable. Most lineal scanning systems are top only, and require the flitches to be turned wane up before being loaded onto the belt. The scan zones consist of a pair of scanners looking toward the belt in a V formation. It is important that the board have been loaded on the belt far enough upstream that it is fully steady before entering the scan zone. Recently there has been a strong push for short in-feed lineal scanning. This is accomplished by adding additional scan zones. Costs have limited how many zones could be affordably used. Recently, multiple line sensors have been used to provide cost effective systems with full profile scanning in as little as 150mm of travel. There have been other attempts to work around the problems of transverse scanned edger without requiring the long scan belt and slewing edger of a lineal system. One that has been around for some time is the clamp and scan. Another interesting technique to be presented by Crosby Machine at ScanTech 2008 scans the flitch in place at the edger in-feed and then positions it. The very latest developments in the mill will be discussed with most major sawmilling companies in both New Zealand and Australia shortly. For more information on this two yearly programme, see www.scantech2008.com | ||
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