Burkhard Plinke; Xavier Le Fur; Peter Meinlschmidt; Friedrich Schlüter

Transcription

1 CHARACTERIZATION OF STRUCTURE PROPERTIES OF OSB: ONLINE MAT MONITORING THERMOGRAPHIC ASSESSMENT OF DENSITY DISTRIBUTION OPTICAL SURFACE TOPOGRAPHY MEASUREMENT Burkhard Plinke; Xavier Le Fur; Peter Meinlschmidt; Friedrich Schlüter Fraunhofer WKI, Braunschweig, Germany SUMMARY The quality of OSB (oriented strand boards) is mainly determined by three process steps: flaking, mat forming, and pressing. Strand geometry, influenced by the flaking process, strand orientation and homogeneity during mat forming, and the surface evenness are crucial properties which determine the board quality. Fraunhofer WKI in Braunschweig developed off-line and on-line evaluation methods based on image processing, thermography, and optical 3D measurement. Strand orientation during the mat forming process can be monitored using image processing: A camera continuously acquires gray-value images of a part of the mat surface moving from the mat forming station towards the press. After a transformation of the image into the spatial frequency domain (via FFT) and the application of an algorithm suitable for segmentation of the strands in the top layer one can derive indices for the mat quality: The deviation of the orientation angles of the strands from the production direction, an index for the quality of orientation, and an index for the size distribution of the strands. The software using these methods is ready for integration into statistical process control concepts. As a matter of fact the geometry of the strands influences the quality of the OSB, and here in particular the strength properties. Another software for statistic laboratory analyses of the strand geometry was developed allowing the evaluation of size and outline parameters. Thus, for example, the post-disintegration and smoothing of the particle shapes by the drying process could be verified. Thermography transforms the temperature distribution on a surface into gray-value images (thermograms). Thermal images of boards leaving the press reflect the local density distribution of the board. Therefore, thermographic inspection of the boards after leaving the press may be a novel technique for on-line density monitoring. One surface property of OSB is the evenness (or roughness) characterized by the frequency of small gaps (typically some millimeters wide and not deeper than one millimeter) appearing in the surface near crossing strands. For laboratory purposes a measuring technique based on fringe-projection and image processing has been tested to evaluate different surface qualities. As a result, the OSB surface can be visualized as a topographic map together with statistics on amount and size of gaps. Most of the measuring techniques presented here have already been tested in the industry or under comparable conditions. They are ready for implementation to close some gaps in process measurement technology. INTRODUCTION For several surface properties of OSB, on-line measurement methods are still missing, and therefore these properties are usually evaluated by visual assessment of the mat or the board. On the other hand it can be noted that today modern OSB plants are equipped with process optimization systems [Berrnardy et al. 2004], receiving however so far only a part of the quality parameters as inputs. In particular, on-line acquired data with reference to the

2 particle and surface geometry are missing. A recently completed research project of the WKI deals with the development of suitable optical measuring techniques. ONLINE MAT MONITORING The inspection of the OSB mat by evaluation of images of a camera over the mat forming line was already reported in a previous presentation for EPPS [Plinke 2004]. In the meantime an inspection system is available whose substantial functions are described here briefly. The software StrandSize acquires image data of a mat or singular flakes and processes each image. There are three main features: Mat monitoring for orientation angle and quality StrandSize computes indices from the gray-value image of the mat (Fig. 1a) using a Fast Fourier Transform (FFT) and vizualizes the result as a wind rose (Fig. 1b). As indices the mean orientation angle of the strands in each image (normally 90 ) and the orientation ratio (relation between vertical and horizontal components in the image for good orientation this value should exceed 2.0) are logged. Size and shape measurement of singular strands StrandSize can process images of isolated strands generated using for example low-cost flatbed scanners or a CCD camera. The result is a list of shape data (e.g. area, length, width and further indices for the shape roughness and slenderness ) for all objects in the image and can be logged like the data of the mat monitoring. The shape data can also be exported for further statistical analysis (see below). Mat monitoring to achieve size indices of flakes on top Moreover, images from an OSB mat can be processed with a Watershed algorithm. In this case the software tries to isolate the strands visible in the top layer as far as possible (Fig. 1c), and computes the shape data. These data reflect changes in the size distribution but have to be calibrated to match the distribution of the real dimensions. Fig. 1a, b, c: Screen output of the Software StrandSize (1a - left) Top view of the OSB mat with object borders and numbers (1b - middle) Visualization of the strand orientation computed by Fast Fourier Transform (1c - right) Segmentation of strands in the top layer using a Watershed algorithm

3 The program StrandSize is working in an OSB plant since more than one year. For the users additional data representing the mat quality are of particular interest for the process monitoring. Therefore, a data interface to text files, MS Excel and to an open-source database system was implemented. Thus, orientation angle and ratio can now be continuously monitored and used for process optmization. OFF-LINE STRAND SIZE AND SHAPE ANALYSIS The existence of a correlation between different dimensions of strands and the mechanical properties of the final OSB board, in particular the bending strength, is well known, but not exactly described because of a lack of suitable analysis methods. To fill this gap, WKI developed a second software component SizeStat. The data obtained with StrandSize were evaluated to obtain useful and concrete statistical distributions for industrial purposes. Materials and Methods For the investigations different collectives of strands were used: uniform particles, manufactured with a disk flaker at the small-scale plant of WKI industrial particles taken before and after drying. The particles were isolated manually, spread on the glass of a flat bed scanner, and scanned. Depending on the size up to 50 particles could be recorded per scanning run. Afterwards, the image series were evaluated using the software StrandSize, so that a data record with shape parameters was available for each particle. These data were further evaluated using the software SizeStat. A - Strands from industrial production B - uniform strands A and B mixed: 580 Strands Figure 2: Distribution of the wood volume of three amounts of strands according to the particle length (assuming constant thickness). The diagrams clearly show the preferred length produced by the flaker. Exploitation of shape data The frequency distributions of strand dimensions as well as some 2-D size factors (e.g. the roughness degree i.e. the ratio of the real perimeter and convex perimeter) are suitable to describe phenomena occurring during the process. In manufacturing and processing of the strands it is more important to know the volume of strands of a certain size range rather than

4 their number. This is considered by SizeStat: Assuming a constant thickness and weighting the area by the length, a concrete representation of the total volume of wood according to the main dimensions of the disintegrated elements can be obtained. A further option of the software allows a selection of the measured strands by specifying the range of the strand dimensions to be considered in the statistics. Results Graphic representations and statistical indices to characterize several amounts of strands could be obtained. An important observation is that length and width, weighted by volume, are not distributed normally but appear at least as bimodal distributions (see Fig. 2, left). Fig. 2 also shows that the flake length determined by the flaker s adjustment is the main peak in the frequency distribution. In mixtures of two types these geometric properties appear like a fingerprint in the statistic (Fig. 2c). DRYING Figure 3: Examples of statisticsfor on the geometry of strands top: before drying, bottom: after drying (3a left) Normalized area distribution (3b middle) Scatter diagram for the main size parameters (3c right) Distribution of the roughness of shape (ratio perimeter convex perimeter) The effect of processing steps such as the disintegration during drying as well as other phenomena such as the polishing effect can be precisely shown: Fig. 3 shows the area distribution (Fig. 3a), a length/width scatter diagram (Fig. 3b; length measured in the main direction of each flake; width, or breadth, measured perpendicular to that direction), and the distribution of the roughness (Fig. 3c; shape roughness defined as the relation of convex to

5 real perimeter) for strands before (Fig. 3 top) and after drying (Fig. 3 bottom). All parameters follow a trend to smaller and smoother shapes which can be explained by disintegration of flakes or polishing of outstanding fibres. Discussion and outlook The software SizeStat has been proven to be effective in analyzing numerous shape indices to compare amounts of strands and to determine the influence of the drying process on the particle geometry. Various further analyses are possible: Wear of the flakers knives, subsequent disintegration caused by transportation and screening processes, influence of the gluing, origin and type of the raw wood, season, etc. The third step of these experiments is to use statistics to correlate geometrical parameters of strands with moisture-related and mechanical properties of the OSB. Both software components, StrandSize and SizeStat, are ready for use but still under development for extensions regarding shape characteristics and more statistical evaluations. Method THERMOGRAPHIC ASSESSMENT OF DENSITY DISTRIBUTION The intensity of infrared radiation emitted from a surface is transformed into gray-value images (thermograms). If the emissivity can be assumed as homogeneous, the thermogram corresponds to the temperature distribution. For particle boards and OSB the density distribution and heat capacity of the particles can be assumed as more or less homogeneous, therefore thermal images of boards leaving the press reflect structural properties of the board. Thermal images of OSB show the strand orientation not only in the surface but also from some inner layers, and even small local density variations like annual rings within single strands. Tests at WKI s laboratory press have been performed with boards including artificial defects. Insufficient bonding zones of the core layer (voids) are visible as cold or hot spots, when using a passive or an active approach, respectively [Meinlschmidt 2003]. Another interesting feature of this technique is the fact that the gray value distribution in thermal images correlates well with the local density distribution of the board. Experiments This relationship well-known already for other types of particle boards [Mehlhorn and Hilbert 1989] could be proven for OSB as well. In addition, laboratory boards with an inhomogeneous density distribution were manufactured. After leaving the laboratory press images were taken from above for several minutes during the cooling procedure by an IR camera. At a certain time after opening the press an inhomogeneous heat distribution on the surface could be observed (Fig. 4b, c). After the cooling the board was divided in a raster of 50 mm by 50 mm and the density was measured gravimetrically for each partial area. The result is shown in Fig. 4a as diagram in the same scale as the thermal image. The zones of lower and/or higher density recognizable in Fig. 4a correspond to the warmer and/or cooler zones appearing in the thermal image. This is due to differences of the local heat capacity: Zones with higher density appear warmer because the cooling process by radiation and convection performs slower and the heat is kept longer.

6 Fig. 4: Density distribution of an OSB panel compared with thermal images (4a - top) Density, measured in a 50 mm x 50 mm grid, visualized and smoothed (4b bottom left) Thermal image (gray values) (4c - bottom right) Thermal image (pseudo-colorized) Discussion and outlook It was shown that infrared thermography can be used to check the homogeneity of the density of OSB. The thermographic measurements can be calibrated so that a density measurement could be effected. This requires however stationary conditions during the cooling process, and different parameters such as board thickness, transportation speed, distance between the press and the measuring point and the surface texture have to be taken into account. Therefore, thermographic inspection of the boards leaving the press may be a novel technique for on-line density and internal bond monitoring. OPTICAL SURFACE TOPOGRAPHY MEASUREMENT OSB surfaces are not ideally smooth even after optimal mat formation: The outlines of the strands as well as the gaps between the strands appear as three-dimensional structures. The evenness affects the suitability of OSB regarding water absorption and the following coating steps.

7 The area and depth of the gaps needs a quantitative description to become another parameter of surface quality. The contact stylus method usually applied for roughness measurements acquires only the height profile along one line but usually does not cover a complete area of the surface. In order to acquire data to characterize the surface, a measurement technique is needed, which allows to capture an entire surface area with an adequate accuracy. Measuring method To this end the contactless measuring process of the fringe projection, already applied at WKI for other purposes [Meinlschmidt et al. 2003], appears suitable: The surface to be scanned is successively illuminated with fringe patterns of different width and subsequently imaged by a CCD video camera. From the recorded image sequences a 3-D profile of the surface is calculated in several steps from the geometrical distortion of the fringe patterns and made available for quantitative evaluations. Large-scale deformations such as cup deformations are detected by means of a suitable polynomial fit function. Thus the measured (local) 3-D structures are represented relatively to a curved surface of the OSB board as reference plane. The fringe processor system is a commercial device and supplies the topographic data. Further statistical evaluations and visualizations are performed with software developed at WKI. Results Fig. 5a shows results for a typical OSB section (size 20 cm x 30 cm). Fig. 5a is a normal photograph of the surface. Fig. 5b displays the measured small-scale topography perpendicular to the surface (z-axis) according to the colour bar on the right side. The largescale deformation represented in Fig. 5c was used as the reference plane. The geometrical resolution amounts to approx. 0.6 mm per pixel in the surface plane and approx 0.1 mm per height unit in direction of the z-axis. Fig. 5: Topographical measurement of an OSB section (5a left) Photograph, Size approx. 20 cm x 30 cm (5b middle) Small-scale topography, colour-coded, showing surface roughness (5c right) Large-scale topography, colour-coded, showing deformation

8 Fig. 6 shows two examples of OSB surfaces with different smoothness (Fig. 6a and 6c). The frequency distribution of the z-values is apparently a Gaussian distribution (Fig. 6b and 6d). For statistical evaluations the usual parameters such as median, mean value, and standard deviation are suitable indices. Fig. 6: Statistical evaluation of surface topography (6a -top left) Panel made from industrial strands, small-scale topography (6b - top right) Frequency distribution of height values (6c - bottom left) Panel made from uniform strands, small-scale topography (6d - bottom right) Frequency distribution of height values Discussion and outlook Using the fringe projection technology, the topography of OSB surfaces can be acquired with high geometrical resolution and subsequently evaluated quantitatively. For example, one could define a certain acceptable span around the mean value and evaluate parts of the surface where the z-values lies outside the span. Further developments for these evaluations and appropriate visualizations are under way at WKI. Attempts are likewise intended for the on-line measurement of surface textures in the pass. If it becomes possible to determine representative parameters for the evenness of the surface at

9 least for a part of the boards, this would be a step towards the monitoring of a further important surface property of OSB. CONCLUSIONS As to some problems of the quality assurance in the OSB production solutions are on the horizon: The mat inspection software StandSize, which is commercially available, can be used for the continuous monitoring of the particle orientation, of the orientation angle, and of an index for the particle size and also for exporting the process data. By means of statistical analyses of the strand shape parameters it is possible to show influences of the drying process and the transport on strand geometry. The thermographic inspection of the board surfaces behind the press in principle is a suitable tool for the detection of inhomogeneities in the material density. Using the fringe projection and statistic analyses of the surface topography the evenness of OSB surfaces can be measured. All measuring techniques described here have the potential to be used on-line. ACKNOWLEDGEMENTS The project was funded by the Arbeitsgemeinschaft Industrieller Forschungsvereinigungen (AiF), Project No N, the internationaler Verein für Technische Holzfragen (ivth) and the German Ministry of Economics and Labour (BMWA). The author is indebted to the members of the project committee for their suggestions and to Mr. Thieme of WKI who performed the implementation of the software and experiments. REFERENCES BERNARDY G, LINGEN A, and SANDWEG T (2004): Systematic Reduction of Operating Costs in the Derived Timber Product Industry. Proc. Eighth European Panel Products Symp., Llandudno, Wales, UK, Oct , 2004, pp LE FUR X, and PLINKE B (2005): Characterisation of an amount of OSB strands Statistical distribution of shape indices. WKI Short Report No. 3/2005 MEHLHORN L, and HILBERT T (1989): Verfahren zur Dichtebestimmung von plattenförmigen Holzwerkstoffen. German Patent No MEINLSCHMIDT P, SCHLÜTER F, and BOTHE T (2003): Fringe Projection for 3-D- Measurements of Surfaces. WKI Short Report No. 19/2003 MEINLSCHMIDT P, SCHLÜTER F, MÄRGNER V, and EL-ABED H (2003): Online- Detection of Bonding Defects in Paper Laminating using Thermography. Proc. Seventh European Panel Products Symp., Llandudno, Wales, UK, Oct. 9-10, 2003, pp PLINKE B, and SCHLÜTER F (2004): Measurement of surface topography of OSB boards using fringe projection. WKI Short Report No. 19/2004

10 PLINKE B (2004): Measurement of OSB Strand Size and Orientation Image Processing Methods, Implementation, and Experiences with Production Monitoring. Proc. Eighth European Panel Products Symp., Llandudno, Wales, UK, Oct , 2004, pp