Automatic detection of pith location along norway spruce timber boards on the basis of optical scanning
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ORIGINAL ARTICLE
Automatic detection of pith location along norway spruce timber boards on the basis of optical scanning Tadios Habite1 · Anders Olsson1 · Jan Oscarsson1 Received: 10 February 2020 © The Author(s) 2020
Abstract Knowledge of annual ring width and location of pith in relation to board cross-sections, and how these properties vary in the longitudinal direction of boards, is relevant for many purposes, such as assessment of shape mechanical properties and stability of sawn timber. Hence, the present research aims at developing a novel method and an algorithm, based on data obtained from optical surface scanning, by which the pith location along the length of sawn timber boards can be determined accurately and automatically. The first step of the method is to identify clear wood sections, free of defects along boards. Then time-frequency analysis, using the continuous wavelet transform, is applied to detect the surface annual ring width distribution of the four sides of the selected sections. Finally, the pith location is estimated by comparing annual ring width distributions on the different surfaces, and assuming that annual rings are concentric circles with the pith in the centre. The proposed algorithm was applied to a total sample of 104 Norway spruce boards. Results indicate that optical scanners and the suggested automatic method allow for accurate detection of annual ring width and location of pith along boards. For a sample of boards with the pith located within the cross-section, a mean error of 2.6 mm and 3.2 mm in the depth and thickness direction, respectively, was obtained. For a sample of boards of which 60% with pith located outside the cross-section, a mean discrepancy between automatically and manually determined pith locations of 3.9 mm and 5.8 mm in depth and thickness direction, respectively, was obtained.
1 Introduction 1.1 Background Different mechanical and physical properties of clear wood from softwood species can be related to the distance from the pith of the log. For Norway spruce (Picea abies), density, longitudinal modulus of elasticity (MOE), and modulus of rupture (MOR) increase significantly in the radial direction from pith to bark, whereas the longitudinal shrinkage coefficient and annual ring width decrease from pith to bark (Blouin et al. 2007; Ormarsson et al. 1999). Mechanical properties of sawn structural timber depend both on clear wood properties and on occurrence of knots, the latter being oriented in the direction from the pith and outward with larger knot diameter at larger distance from the pith (Kliger et al. 1998; Johansson 2003), meaning that relationships * Tadios Habite [email protected] 1
Department of Building Technology, Linnaeus University, Växjö, Sweden
between different properties of sawn timber are not identical to those valid for clear wood. Nonetheless, the pith location and annual ring width, and how these change in the longitudinal direction of a board, are relevant for assessment of stiffness and strength (Hu et al. 2016)
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