New process improves timber grading
12 Jun 2001
Scientists from Australia's Commonwealth Scientific and Industrial Research Organisation (CSIRO) have added a new dimension to grading timber with the development of high speed scanning using microwave technology.
'SpeedGrader utilises microwaves to actually look inside the structure of air-dry timber that is up to 50 mm thick and measures the presence and size of natural features such as knots, slope of grain and juvenile wood,' said Dr Leicester of CSIRO Sustainable Materials Engineering.
'Timber with these features is prone to warp and twist with changes in humidity, an undesirable feature in manufactured timber products.
'Detection of this undesirable wood with microwave scanners helps remove headaches for end users, saving replacement materials and dealing with unhappy customers,' he added.
SpeedGrader can 'look' along a length of sawn timber at the comparatively high speeds of 15-20 kilometres an hour or even faster while it is being processed in a modern high speed mill.
Within mills that specialise in the production of structural timber, the SpeedGrader has the potential to increase by around 3%, the quantity of wood that is sorted into acceptable grades of structural timber.
For a large Australian mill with an output approaching a half a million cubic meters a year of structural timber, this could mean an increased profit of $ 4-6 million annually. The upgrading of timber to higher grades also has the potential to add a further $ 2-3 million a year.
On-line scanning has so far been dominated by mechanical stress grading machines. Stress grading involves bending timber to assess its stiffness and then using this property to assess strength and its structural grade. However, this method provides little other useful information.
The prototype commercial stress-grader includes two sets of microwave scanners comprising a knot detector (KD) and a slope-of-grain measurement unit (SOG). These units are placed in line with a conventional mechanical stress-grading (MSG) unit.
SpeedGrader scanners are low-powered at around 0.15 milliwatts and are said to be extremely safe in service.
However, one limitation in using this low power level is that the scanners are suitable only for timber with moisture content below 20 percent but, according to CSIRO, a modified scanning system can be developed if it were required to grade timber at higher moisture contents.
Dr Leicester said the most important (and most difficult) aspect of developing SpeedGrader was to produce an algorithm used by the computer to recognise the features being transmitted in the form of microwave signals.
These signals have different signatures depending on the type of material being encountered in the timber so a different algorithm is required for each timber species being graded.
