Against the background of inaccurately measured wood particle dimensions, applying the three-dimensional (3-D) image analysis–based particle size characterization system Partimac 3D XL in pre- liminary tests, metal platelets with various aspect ratios of the three main axes are employed to understand and explain the observed limitations of the measuring principle. It was found that particle width and thickness interact increasingly with a decreasing aspect ratio and, thus, the digital replica and subsequent determination of the dimensions become incorrect. This was ascribed to the random orientation of the particles during image acquisition, which cannot be overcome with a ﬁnite number of cameras in the system. 

Benthien, Jan T

Friehmelt, Rainer

Ludtke, Jan

Schafer, Michael

Wood and Fiber Science (E-Journal)

TECHNICAL NOTE: LIMITATIONS OF A 3-D IMAGE ANALYSIS–BASEDPARTICLE SIZE MEASURING SYSTEM FOR WOOD PARTICLEDIMENSION MEASUREMENTJan T. Benthien*Research AssociateE-mail: jan.benthien@thuenen.deJan LüdtkeResearch Associate/Dr.Thünen Institute of Wood Research21031 Hamburg, GermanyE-mail: jan.luedtke@thuenen.deRainer FriehmeltResearch Manager/Dr.E-mail: rainer.friehmelt@basf.comMichael SchäferResearch Manager/Dr.BASF SE 67056 Ludwigshafen, GermanyE-mail: michael.schaefer@basf.com(Received September 2017)Abstract. Against the background of inaccurately measured wood particle dimensions, applying the three-dimensional (3-D) image analysis–based particle size characterization system Partimac 3D XL in pre-liminary tests, metal platelets with various aspect ratios of the three main axes are employed to understandand explain the observed limitations of the measuring principle. It was found that particle width andthickness interact increasingly with a decreasing aspect ratio and, thus, the digital replica and subsequentdetermination of the dimensions become incorrect. This was ascribed to the random orientation of theparticles during image acquisition, which cannot be overcome with a finite number of cameras in the system.Keywords: 3-D particle size measurement, image analysis, particleboard manufacturing.INTRODUCTIONIn the manufacturing of particleboards, the prop-erties of the wood particles have a significant effecton panel quality. The particles also impact thecost-related optimization of quality with respect tothe required amount of adhesive. In this context,the important wood particle characteristics are thedistributions of particle length, width, and thick-ness, and the particle surface area onto which theadhesive is spread. Both determine the effectivelyavailable area for the formation of adhesive con-nections between the particles (Dunky and Niemz2002).The particle size is influenced by the woodspecies used for particle manufacture and thedetails of the chipping process. In the case of thechipping process, the geometry and the wear ofthe cutting tool edges are of particular interest.Because the wear of the cutting tool edges and thewood quality vary permanently during production,a detailed knowledge of the actually producedparticle size distribution is required to operate ina highly optimized process and, thus, with max-imal efficiency.Commonly applied sieve analysis is inadequateas it is limited to providing only one-dimensionalsize information. This information cannot beclearly attributed to one of the main particle axes.* Corresponding authorWood and Fiber Science, 50(3), 2018, pp. 358-362https://doi.org/10.22382/wfs-2018-034© 2018 by the Society of Wood Science and TechnologyEven conventional two-dimensional (2-D) im-age analysis–based particle size characterization(PSC) is of limited informative value. Only twoof the particles’ main axes (length and [usually]width) and just one of the theoretically infinitenumber projection areas can be thus determined(Sun et al 2014). To measure particle thicknessand, consequently, obtain information on par-ticle volume and surface area, an additionalmeasurement (eg manual thickness gauging) isrequired, as stated by Plinke (1987). The dy-namic development of digital image acquisi-tion, and image and data processing techniquesshould make it possible to measure the thick-ness for a considerable number of particles, andbase the result on a reasonable statistical samplesize. At this time, however, such measurementsare not economically viable, and applicableindustrial measurement systems are not yetavailable.Preliminary tests with Partimac 3D XL (BASFSE, Ludwigshafen, Germany) (BASF SE et al2007) were used to transfer this technol-ogy from rather convex particles to biaxiallystretched wooden particles for process opti-mization. As was to be expected from theprincipal limitation of 3-D size characterizationof particles with arbitrary shapes (ie not simplyellipsoid-shaped or noncylindrical objects),size measurement was found to be faulty. Inparticular, the calculated density of samplewood particles was strongly undervalued. Thisleads, in consequence, to wrong conclusionsthrough an overvaluation of the particle’svolume. Satisfactory results were, in contrast,obtained at the Partimac’s actual area of ap-plication. Depending on the specific material,the volume of mainly convex granulates orextrudates is measured with an error of 5-20%.The intention of the present study was tocomprehend the findings of 3-D wood PSC onthe basis of reference samples (metal platelets)of defined and manually verifiable dimensions.The study shall also contribute to the ongoingeffort to develop an industrial technique tomeasure particle dimensions, in particular, particlethickness.MATERIALS AND METHODSTest Specimens and Experimental SetupNine metal platelets with variations in length,width, and thickness weremeasured approximately100 times each (by manual refeeding into the vi-brating feeder inlet) applying the 3-D imageanalysis–based PSC system Partimac 3D XL.To investigate the influence of particle length onthe overvaluation of particle dimensions, testspecimens’width (thickness) was kept constant at5 mm (1 mm), whereas length was 10, 15, and20 mm. The influence of particle width on theovervaluation of the particle dimensions was mea-sured using specimens of constant length (15 mm)and thickness (1mm) but variedwidth (1, 2.5, 5, and10 mm). Particle thickness was investigated as aninfluencing variable by applying metal platelets ofconstant length (15 mm) and width (5 mm) butvaried thickness (0.5, 1, 1.5, and 2 mm).The Shapiro–Wilk Test was applied to check eachdata set for normality. As no normal distributionoccurred (exclusively overvaluation of the particledimensions because of the overlaying main axiswhich results in a skew distribution), the non-parametric Kruskal–Wallis test was applied to testwhether all sample means are equal at the 95%significance level. The Kruskal–Wallis Test wasapplied as it is suitable for comparing two or morenonnormally distributed independent samples ofequal or different sample sizes. If the null hypothesis(no difference between the samples’ means) wasrejected, the samples were compared pairwise ap-plying the nonparametricmultiple Steel–Dwass test.The Steel–Dwass test was applied to show whethersample means are equal or, respectively, not equalto each other. For this purpose, groups of statistichomogeneity (groups with the same subset are notstatistically different—homogenous group) weredetermined afterward from the Steel–Dwass test’sresults and given in Table 1. Statistical analysis wasperformed by applying the analysis tool JMP fromSAS Institute (Cary, NC).Differences between measurements were calcu-lated by subtracting the real specimens’ dimen-sions (caliper measurement) from the PartimacBenthien et al—LIMITATIONS OF 3-D IMAGE ANALYSIS–BASED PARTICLE SIZE MEASURING SYSTEM 359measurement (mean) and expressed as percentagein respect to caliper measurement.Although presented coefficient of variance (CV)may be misleading for skewed distributions (as thedistribution is not symmetrical), they help to pointout the investigation’s goal in the present article.Particle Size Measuring SystemThe Partimac system consists of four digital cam-eras, each of them opposing a light field of lightemitting diodes and arranged with intermediateangles of 45° around the particles’ free fall path. Anoverhead vibrating feeder channel for particledosage and separation supplies the particles. Theydrop through the field of view of the cameras andthus fall freely during image acquisition (BASF SEet al 2007; Aßmann 2015). The Partimac was usedas 3DXL variant, which means a camera resolutionof 54 µm per pixel and an object field of 41.2mm31.4 mm. The image information is used to builda voxel model of the particle and to derive particlelength, width, and thickness approximated by thethree orthogonal axes of inertia.RESULTS AND DISCUSSIONIt can be seen from the data displayed in Table 1that the Partimac basically found the varieddimensions of the metal test specimens (italic inTable 1) to be significantly different from eachother (subsets are different for the groups ofvaried dimension). Length measurements con-sistently show low CV, whereas thickness andcertain width measurements consistently holda much higher CV level. CV was found to becomparatively high for slender (thin) specimensand lower for wide (thick) specimens. It seems asif the repeatability decreased with decreasingwidth and thickness, respectively. No comparablerelationship was observed at varied specimenlength. Notwithstanding these details, measure-ment accuracy (difference between caliper andPartimac measurement) was unacceptable formost width, and all thickness, measurements.Although differences between caliper and Parti-mac length measurement was low (0-2%), widthmeasurements differed up to 52%. Thicknessmeasurements were found to be between 40%and 243% greater than the caliper measurement.In addition to dimension-related variability, in-terdependencies were observed between speci-mens’ dimensions and the measuring accuracy.Variation of specimens’ lengths showed no in-fluence on the accuracy of width and thicknessmeasurements. Specimens’ width was over-valued on an acceptable level (7-9% difference)and thickness more than at least twice (120%Table 1. Coefficient of variance (CV) and homogenous group (HG) of sample means (different letters within a columnindicate statistical distinguishability) as well as differences of caliper and measuring system measurement.Spec. no.Nominal specimen size (mm) CV and HG of measuring system measurementDifference between caliper andmeasuring system measurement (%)l w t l w t l w tInfluence of particle length1 10 5 1 2.5 A 6.8 A 31.2 A 2 7 1202 15 5 1 1.8 B 5.8 A 26.4 A 1 8 1203 20 5 1 2.6 C 4.3 B 23.9 A 0 9 121Influence of particle width4 15 1 1 1.9 A 28.1 A 12.2 A 0 52 405 15 2.5 1 2.0 AB 10.2 B 19.5 B 1 13 61(2) 15 5 1 1.8 A 5.8 C 26.4 C 1 8 1206 15 10 1 1.6 B 2.8 D 31.9 D 0 2 187Influence of particle thickness7 15 5 0.5 2.0 A 4.7 AB 34.2 A 0 5 243(2) 15 5 1 1.8 A 5.8 AC 26.4 B 1 8 1208 15 5 1.5 2.2 AB 5.4 BD 18.5 C 1 9 669 15 5 2 1.8 B 4.6 CD 15.1 D 1 11 51italic: varied dimension. l, length; w, width; t, thickness.WOOD AND FIBER SCIENCE, JULY 2018, V. 50(3)360difference) at all tested specimen lengths.Varying the specimen’s width, a high level ofmeasuring accuracy (2% difference) was foundfor wide specimens (10 mm), whereas the level ofmeasuring accuracy decreased with decreasingwidth. For specimens with a width of at least1 mm, a difference of 52% was found betweenPartimac and caliper measurements. Thicknesswas determined to vary at different specimenwidths. Thickness overvaluation increased from40% (1-mm-wide specimens) to nearly 190%(10-mm-wide specimens). This means both thick-ness and width determination are impaired by in-creasing particle width. The same holds true whenvarying the specimen thickness: measurement ac-curacy improves with increasing specimen thick-ness at a fixed width (lowering CV). However,differences between Partimac and caliper mea-surement were found to be very high (243% dif-ference) for thin (0.5 mm) and wide specimens.The difference between caliper and Partimacwidth and thickness measurements is obviouslycaused by the mechanical transport configurationof the Partimac 3D XL system. This system is notadapted for measuring flat particles (length >width >> thickness), particularly if the twosmaller dimensions are considerably different.During image acquisition, the particles rotatefreely and their main axes are most often un-aligned to one of the optical camera axes. Eachcamera captures the projected area of the particlesin relation to the camera axis. The projection areais most often a superposition of all particle mainaxes. An evaluation algorithm can only correctthis geometrical effect if the projected dimensionsof an axis are seen in different pictures. For flatparticles, the projection of the longest axis is seenin nearly every image. The reconstruction algo-rithm can thus correctly evaluate particle length.The projection of width and thickness nearlyalways superpose, and cannot be detected sepa-rately. This is why especially thickness, but alsowidth, is too highly estimated. As a consequence,volume and surface area are also mostly over-valued. Figure 1 shows that overvaluation ofthickness is a linear function of the relation ofwidth to thickness.CONCLUSIONSMetal platelets with various aspect ratios inlength-to-width, width-to-thickness, and length-to-thickness were employed as placeholders ofwood particles. The platelets had defined andmanually verifiable dimensions. They help tounderstand and explain limitations in the mea-surement accuracy of 3-D wood PSC with imageacquisition on free-falling sample material. It wasfound that measurement inaccuracies are not anindividual shortcoming of any one instrument,but rather more of the measuring principle itself.Thus, they will likewise affect 2-D PSC systemswith image acquisition on free-falling samplematerial as well. It was found that particle lengthwas measured with sufficient accuracy, whereasthickness measurement becomes increasinglyinaccurate with increasing relation of width-to-thickness. From these findings, the limitations forwood particles can be anticipated. When mea-suring wood particles, length measurement willbe minimally affected as the ends of the particlestaper to nearly a point (when wood particles areimagined to be an ellipsoid) so that almost nosuperposition will occur with the particle width orthickness.Possible actions to reduce the effect of aspectratios and, thus, increase the accuracy of mea-surement would be to increase the number ofcameras during image acquisition in an airbornestate or possibly to employ a transport unit basedon particle alignment to a plane surface. As soonFigure 1. Measurement error in relation to width-to-thickness ratio.Benthien et al—LIMITATIONS OF 3-D IMAGE ANALYSIS–BASED PARTICLE SIZE MEASURING SYSTEM 361as these improvements are implemented, 3-DPSC will become of increasing interest becausefurther processing and, thus, cost optimizationrequire a detailed knowledge of the distribution ofall particle dimensions.REFERENCESAßmann J (2015) Kostendruck macht erfinderisch: Innova-tionen für die Holzwerkstoffindustrie [Cost pressure is themother of invention: Innovations for the wood-based panelindustry]. Pages 26-34 in Anonymous, eds, Proc. 11thHolzwerkstoffkolloquium, 10-12 December 2015, Dres-den, Germany. Institut für Holztechnologie Dresdengemeinnützige GmbH (IHD), Dresden, Germany (InGerman).BASF SE, Schäfer M, Ettmüller J, Ziegler S, Reindel K(2007) Device and method for automatically determiningthe individual three-dimensional shape of particles. WorldIntellectual Property Organization (WIPO), Publicationnumber WO/2007/060127.Dunky M, Niemz P (2002) Holzwerkstoffe und leime—Technologie und einflussfaktoren [Wood-based panels andadhesives—Technology and influencing factors]. Springer-Verlag, Berlin/Heidelberg. 954 pp (In German).Plinke B (1987) Rechnergestützte partikelmessung anmaterialien der holzwerkstoffindustrie [Computer-aidedparticle size measurement on materials of the wood-based panel industry]. Holz Roh Werkst 54:315-318 (InGerman with summary in English).Sun Y, Indraratna B, Nimbalkar S (2014) Three-dimensionalcharacterisation of particle size and shape for ballast.Géotech. Lett. 4(3):197-202.WOOD AND FIBER SCIENCE, JULY 2018, V. 50(3)362

Limitations of a 3-D Image Analysis-Based Particle Size Measuring System for Wood Particle Dimension Measurement

Abstract

Similar works

Full text

Available Versions

Wood and Fiber Science (E-Journal)