In plywood mills finished panels are manually graded and sorted based on specific defects. The panel edges are especially difficult to grade by human visual inspection due to the small nature of different defects, especially at higher line speeds. This can result in misread errors that can be costly.
In this project a prototype scanner, based on 3D laser profilometry, was developed by FPInnovations and demonstrated in two Canadian plywood mills for automatic edge grading. At both mills, panels were scanned on the production line in real time, collecting full length, 3-dimensional edge profiles in the x, y and z coordinate fields that were then analyzed by computer software sub-routines to identify defects for each panel. The data was processed to categorize groups of data points, depending on the edge profile depth (z-axis) variation, length (x-axis) and height (y-axis) into the specific defect categories of core, top and bottom edge void, core gap and core overlap, based on the values of predetermined edge profile thresholds.
Results from the two mill tests showed that the scanner was effective with a correct identification rate greater than 80%. The lack of panel hold-downs at each mill resulted in extreme height variation of the panel edges and this limited the defect detection accuracy. Based on the tests, the technology for automated edge grading is feasible. Longer term mill evaluations are recommended with adequate panel hold-downs in place before confirming that this technology is ready for commercialization.