Non-Contact Detection of Surface Quality of Knot Defects on Eucalypt Veneers by Near Infrared Spectroscopy Coupled with Soft Independent Modeling of Class Analogy

Abstract

A knot is a natural defect that degrades the quality of softwood and hardwood veneer. To improve efficiency, the plywood industry needs a rapid, inexpensive method of knot identification that is easy to operate and industrialize. Although a non-contact knot-detection technology based on NIR spectroscopy and soft independent modeling of class analogy (SIMCA) has been successful in detecting softwood knots, it has not yet been explored in eucalypt (hardwood) veneer. This study investigated the interaction between knot size, spectral pretreatment methods, and wavelength range selections on this model's classification accuracy of knots and normal eucalypt wood. The study found that classification results were accurate up to 94.4% for large knot samples (10 to 15 mm in diameter) and up to 100% for knot-free samples. Spectral data for small knots (< 5 mm in diameter) impeded the model's classification accuracy because of confusion between small knots and both large knots and normal wood. Calibration models developed with second-derivative spectra exhibited the highest accuracy, followed by models built with first-derivative spectra, models based on spectra transformed by vector normalization, and the model based on the raw spectroscopy. Wavelength ranges of 1100 to 2500 nm enabled greater classification accuracy than wavelength ranges of 780 to 1100 nm or 780 to 2500 nm.

Detection of Knot Defects on Coniferous Wood Surface Using Near Infrared Spectroscopy and Chemometrics

“…The knot samples were found to have higher absorption amplitude than normal samples for each wood species. These changes in spectra were consistent with previous experiments by and Yang et al (2015Yang et al ( , 2016. This may be due to the difference in the orientation of the microfibrils within fibers between knots and normal wood, as the microfibrils of a knot's cells was substantially parallel to the NIR light incidence angle Yang et al 2015).…”

“…The absorbance near 1195 nm was bound up with the second overtone of C-H stretching vibration from lignin or cellulose (Schwanninger et al 2011). The strong peak at approximately 1462 nm was primarily attributed to the O-H stretching from water or phenolic groups of lignin (Schwanninger et al 2011;Yang et al 2015).…”

“…The results showed that NIRS coupled with SIMCA could identify knots. Yang et al (2012Yang et al ( , 2015 assessed the potential of visible-NIR spectroscopy for identifying knots in Pinus massoniana veneer and eucalypt veneer, with good classification results. Yang et al (2016) also investigated the feasibility of NIRS coupled with partial least squares discriminant analysis (PLS-DA) to discriminate knots on the surface of poplar, eucalypt (hardwood), and masson pine (softwood) veneers.…”

Rapid Detection of Knot Defects on Wood Surface by Near Infrared Spectroscopy Coupled with Partial Least Squares Discriminant Analysis

“…In a recent study, the ability of NIR spectroscopy coupled with SIMCA was investigated to detect the surface quality of knots detected on eucalypt veneers and results showed that the method had a classification accuracy of 94.4% for knot samples and 100% accuracy for knot-free samples (Yang et al 2015). In the present study, PLS-DA was selected for analyzing the data from NIR spectroscopy.…”