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Innovative methods of non-destructive evaluation of log quality

Innovative methods of non-destructive evaluation of log quality For the sustainability of an important renewable resource, such as wood, it is important to significantly increase the efficiency of its processing. A large part of this raw material ends up in the wood processing industry, where it is used for the production of pulp, paper, construction and furniture timber, floors and others. Therefore, it is very important to gain the knowledge needed for optimal valuation of raw wood material, through quality detection and classification into quality classes. There are many defectoscopic methods working on different physical principles. The most familiar of these methods are semi-destructive and non-destructive, as they do not cause damage to the tree or wood during assessment. The aim of this article is to describe, assess and compare known semi-destructive and non-destructive methods for the assessment of wood properties. This article describes basic visual inspection, basic semi-destructive methods (Pilodyn, Resistograph) and advanced semi-destructive methods (SilviScan®, DiscBot®) as well. Non-destructive methods use mostly acoustic wave motion (acoustic, ultrasonic), high-frequency waves (using georadar, microwave) and methods based on visual evaluation (image, laser). At last, there are X–ray methods with the latest technology using three-dimensional (3D) computed tomography (CT). The implementation of modern non-destructive methods is of great importance for the application of principles of Industry 4.0, where these methods provide collecting of data on the material properties, in its entire production flow of log processing. Key words: X–ray method; acoustic methods; georadar methods; laser methods; industry 4.0, CT scanner Editor: Miloš Pánek important for the increasing yield. Globally, there is a 1. Introduction great shift in the intensity of forest management and log Today, under growing pressure from society, ecological processing, which requires significantly faster and more standards and the developing industry in our country and accurate evaluation of wood properties. Unlike concrete, in the world, there is an increasing emphasis on assessing bricks, steel, aluminium and most other construction the quality and origin of wood raw material. For these materials, wood has highly complex, anisotropic and var- reasons, the European Union has developed several iable internal structure. For the correct use of woodand instruments aimed at the ecology, regulation and legali- products based on it (plywood, KVH prisms, timber, OSB sation of harvesting. In 2003, “Forest Law Enforcement, boards, etc.), it is necessary to evaluate the properties of Governance and Trade” (FLEGT) was created to prevent this material. illegal timber trade and encourage investment in legal There are a number of methods for evaluating wood. harvesting in developing countries. “Reducing emissions They can be divided, according to the degree of destruc- from deforestation and degradation” (REDD) offers a tion of the evaluated material, into: 1. destructive, 2. non- new method how to reduce CO emissions by paying for destructive, 3. semi-destructive methods. Non-destructive actions, to prevent deforestation or forest degradation. and semi-destructive methods are more important in the Rising social and ecological standards have given rise to evaluation of wood. Non-destructive evaluation is the FSC (Forest Stewardship Council) certic fi ation, which is identification of the physical and mechanical properties a trusted system for forest certification and certification of a material without altering its end use and, using the of the processing (consumer) chain of wood processing identified properties, decides on its appropriate appli - worldwide. cations (Ross et al. 1998). In contrast, semi-destructive Evaluation of the quality of wood raw material and techniques for evaluating wood properties are defined its sorting has a significant effect on maximising yield. as test procedures that are non-destructive in relation Financial, quantitative, but also ecological reasons are to the structural element but destructive in relation to *Corresponding author. Vojtěch Ondrejka, e-mail: ondrejka.vojtech@gmail.com V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 the extracted sample (Kasal et al. 2013). In the forestry 2.1. Semi-destructive methods context, non-destructive evaluation of wood (NDE) is Semi-destructive methods damage the material only considered attractive. Authors Schimleck, et al. (2019) partially without affecting its further use. They are used described the main reasons why the NDE has grown rap- mainly to evaluate the properties of wood, logs, or his- idly over the last 20–25 years: Protection of investment in toric roof constructions. A new approach to the historic wood raw material; potential to reduce wood processing roof structures properties assessment is described in the costs; easier to use for e fi ld measurements; fast real-time publication (Kloiber et al. 2015). The authors describe data collection; the ability to identify the most suitable recently developed assessment methods such as tensile measurement application and reduce the variability of strength of small samples, tensile Young’s modulus of product classes. mesospecimens, compression strength of cores, com- This article focuses on the description and mutual pression strength in a drilled hole, mechanical resistance comparison of non-destructive and semi-destructive to pin pushing, Young’s modulus derived by measuring methods of wood evaluation, working on various physi- the hardness and shear strength of screw withdrawals. cal principles, described from basic methods to today‘s They do not belong to the industrially used methods, they most modern. The methods will be examined in relation are used mainly for scientific work. They are described to their use in forestry and the wood processing industry, here for their use in evaluating the quality of trees, which as these sectors are significantly interrelated. helps in the classification of wood according to quality, already in the first phase (standing tree). p ilodyn is a portable tool for evaluating the density 2. evaluation methods for wood of trees or wooden constructions. The tool was originally The main goal of wood evaluation is to assess qualita- developed in Switzerland to obtain quantitative data on tive properties, sometimes supplemented by quantita- the degree of soft rot in wooden poles. The tip penetrates tive ones. For qualitative evaluation, it is necessary to the surface of the material, measuring the depth of pen- determine the mechanical and physical properties of etration (Cown 1978). The penetration depth of the pin wood. Non-destructive methods determine, for exam- negatively correlates with the density of the wood. Three ple, mechanical properties (moduli of elasticity, impact different rods (diameter 2.0, 2.5 and 3.0 mm) enable to strength etc.) and physical (moisture, density, acous- adapt the use of Pilodyn to the density of tested wood tic properties, visual properties etc.). Another part of (Gao et al. 2017). In practice, the accuracy of Pilodyn is the qualitative evaluation is the determination of wood somewhat limited, so it is mainly used to estimate aver- defects (knots, cracks, rot, irregularity of the wood age values (Cown 1978). It is one of the least invasive structure, foreign objects, traces of biological damage, sampling techniques, but unfortunately it cannot pen- etc.). The identified properties significantly affect the etrate the middle layers of the trunk. It is also assumed subsequent sorting of wood products, yield and thus the that tested wood is above the b fi er saturation point (FSP), as the penetration of the tip decreases with decreasing financial appreciation of wood. moisture content below the FSP (Llana et al. 2018). The basic, still widely used evaluation is visual inspec- The resistograph began to develop in the early 1990s tion. The method is very fast, with lower costs. It allows (Rinn et al. 1996). It was utilised for improving the iden- to obtain basic information in situ about the assessed tic fi ation of rot in trees and poles. The tool drives a special tree or log. Visual inspection allows only the detection drill (needle) with a diameter of 3 mm through a tree at a of external, visible defects and mechanical damage to the given feed speed and rotation speed (rpm) and measures wood. Another disadvantage is the higher demand on the the resistance to rotation (torque). The trace represents expertise of the assessor, which can increase the overall the resistance prol fi e every 0.1 mm and the radial change cost. Visual inspection is also the basis for in situ visual in wood density (Rinn et al. 1996; Downes & Lausberg classification according to the quality classes of logs 2016). Gao et al. (2017) concluded that compared to and classification of construction timber into strength other semi-destructive techniques, Resistograph was a classes. Visual evaluation is strongly represented in supplier-customer relations in log trading. Here, the cheaper and faster method for collecting wood density log is most often sorted according to the standards STN data. The key features of this tool are its low cost for field EN 1316–1 – 3 for broadleaf raw material and STN EN use, digital data collection and relatively high-resolution data (Schimleck et al. 2019). Research by Kloppenburg, 1927–1 – 3 for coniferous raw material. These standards (2018) focused on the possibility of using the Resisto- determine the evaluation parameters of raw logs for clas- sification into four classes (A, B, C, D). Results of the graph to accurately assess the wood density of tropical visual inspection form the basis from which subsequent hardwoods. The tool has proven to be suitable, but the non-destructive testing (NDT) can be planned (Piaza et sharpness of the tool has a significant effect on accuracy. al. 2008). To evaluate the quality of wood, a large number Specific devices are SilviScan® and DiscBot®. Both of devices have been developed that can assess the prop- of these devices are developed more for laboratory evalu- erties of timber logs, veneers and other wood products. ation of wood (logs, timber). They are more invasive, as it 4 V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 is necessary to take a sample (a prism, a disc) for evalua- 2.2. Non-destructive methods tion. The r fi st of these, the SilviScan®, is a special labora- Non-destructive evaluation is the identification of the tory device designed to measure wood quality parameters physical and mechanical properties of a material with- such as the density and angle of microfibrils (Schimleck out altering its end use and, using the identified proper- et al. 2002). The device was created by Dr. Robert Evans ties, decides on its appropriate applications (Ross et al. and developed with his CSIRO team as early as 1992. So 1998). Previously, non-destructive methods were applied far, 3 versions of this device have been created. The main to evaluate very valuable (e.g. exotic) wood species. Cur- components of SilviScan are (Schimleck et al. 2019): rently, these methods are increasingly preferred, because – Optical cell scanner (radial and tangential tracheid they do not degrade the raw material (wood), they are and fiber diameters, vessel size and their position, a accurate and are fast. This is applied mainly in automated boundary and orientation of annual rings); modern sawmills, where the identified parameters help – X–ray densitometer (conditioned density profile, to better evaluate the processed raw material. The most fiber tilt, annual ring transition); commonly used physical principles in non-destructive – X–ray diffractometer (microb fi ril angle, tracheid and evaluation are: acoustic, ultrasonic, microwave, imaging, fiber 3D orientation, cellulose crystallisation). laser and X–ray. A stiffness estimate generated by SilviScan is based on the diffractometric and densitometric properties of wood, calibrated by the acoustic resonance technique (Ilic 2001). The device has been tested in many studies 2.2.1 Acoustic and ultrasonic methods and has proven to be very accurate (Buksnowitz et al. Acoustic methods of wood evaluation are among the 2008; Schimleck et al. 2002). oldest methods of evaluating the properties of wood. Another special device is the DiscBot, developed by As defined in (Lipta el al. 1972), acoustic velocity is in the New Zealand company SCION. The device utilises a form of transitional elastic waves generated by the scanning technologies, which can determine a large rapid release of energy in a material. Thus, the source number of wood properties affecting the resulting qual- is an acoustic impulse, such as a hammer impact, or an ity of timber and other final products. The scanner uses excited acoustic signal (sound). Therefore, scanning the automatic motion of wooden discs under various sen- speed of acoustic waves in wood is most often used for sors that capture information on wood density, microfi- evaluation. Another non-destructive acoustic method bril angle and chemical composition. The first sensor is is ultrasonic measurement. Ultrasound is a mechani- a camcorder for capturing a high-quality colour image cal wave with a frequency generally greater than 20 kHz (in an RGB spectrum), which allows the identification (Wang et al. 2002). The diagnostic tools operate with of wood defects such as knots, resin pockets and com- frequencies in the range from 20 kHz to 500 kHz. Dur- pression wood. A spectrograph fitted with an infrared ing the measurement, ultrasonic waves to the object are (NIR) camcorder (900–1700 nm) applied in several generated and subsequently the transmitted waves are studies (Jones et al. 2006; Thumm et al. 2010) is used measured to identify the transmission properties of wood to determine the chemical composition and properties (Österberg 2009). To increase the contact of ultrasonic of lignin and cellulose. The wood density is measured receivers and transmitters, it has proven appropriate to using X–rays. The strength of wood is evaluated using immerse the examined wood into a water bath (McDon- an ultrasonic device when the wave passes through the ald 1978; Han & Birkeland 1992). wood along the fibers. Fig. 1. Principle of measurement by acoustic tools: FAKOPP, a TreeSonic™ model and Microsecond Timer. 5 V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 The development of acoustic devices for the evalua- of elasticity by acoustic tools, different values may occur tion of standing trees opened the way for assessing the when repeating the measurement, which causes some properties of tree wood before their harvesting. This facil- inaccuracies in the results, see research: Lindström et itates the management, planning of harvest and wood al. (2009), Simic et al. (2019). processing in a way that maximizes the value extracted from the source (Riggio et al. 2008; Carter 2017). In the last few decades, acoustic technologies have become well-established devices for material evaluation in the wood processing industry. They have become a widely accepted tool for quality control and product classifica - tion (Wang et al. 2004), but also for breeding research, where they are utilised for evaluation of young trees (Lenz et al. 2013). Most acoustic devices that evaluate the properties of trees are modelled for radial measurement (perpendicu- lar to the trunk axis). For example, these devices: Arbor- Sonic 3D Acoustic Tomograph, Hitman Resonance Tool, IML Impulse Hammer, and FAKOPP company tools Fig. 3. Illustration of the principle of ultrasonic tomography. (Microsecond Timer, Resonance Log Grader, ArborE- lectro Impedance Tomograph) (Fig. 1). In contrast to All the mentioned devices are difc fi ult to apply to rapid these tools, the FAKOPP TreeSonic™ is developed for industrial production. That is why fast acoustic tools are measuring in the longitudinal direction of the trunk (Fig. being developed today, measuring at a feed speed of 200 2). In acoustic systems, the source of the impulse is most pieces per minute. An example is the Swedish company often the hammer impact. DYNALYSE, which offers Dynagrade and Precigrade devices for evaluating timber properties. Others are devices for evaluating logs before cutting them. A com- pany based in New Zealand, Fiber-gen, has developed the HITMAN PH330 measuring tools implemented into a forest harvester, the HITMAN LG640 for log processing lines and the HITMAN HM200 manual tool. 2.2.2 Microwave methods The microwave frequency in the electromagnetic spec- trum is between the frequencies of infrared radiation and high-frequency waves, i.e. approximately between the frequencies 1 GHz and 100 GHz, or a wavelength between 1 mm to 1 m (Fuller 1990). However, the bound- ary between microwaves and infrared or radio waves has not been precisely defined. The microwave scan- ning method has been under development for several years. The first tools were created to measure moisture and were also applied in industrial production (Tiuri & Heikkilä 1979). At present, research using microwave tomography is known, where the authors describe the detection of internal wood defects with higher accuracy Fig. 2. Acoustic system for measuring standing trees in the (Pastorino et al. 2015; Boero et al. 2018). Using this longitudinal direction. technique, it is possible to determine properties such as: density and moisture of wood, deflection and direction of fibers or detection of wood defects as well. Ultrasonic systems evaluate the material mainly per- Microwave induced thermo-acoustic tomography pendicular to the b fi ers (Fig. 3). The most used ultrasonic (MITAT) combining microwave imaging with ultra- devices are FAKOPP IltraSonic Timer, Arborsonic Decay sonography. This special technology is mainly used in Detector, Sylvatest, V-Meter MK IV System, Proceq Pun- biomedicine and is still under development and the sub- dit Lab +, Pundit 250 Array and Tico. Most tools evaluate ject of much research (Ku & Wang 2001; Meaney et al. the quality of wood using the determined dynamic modu- 2012; Rosenthal et al. 2012; Wang et al. 2013). Research lus of elasticity and density. When evaluating the moduli 6 V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 using this technology to determine the properties of wood cal and algorithmic bases for achieving automatic visual is known. Authors Zhang et al. (2019) use the thermo- understanding (Sonka et al. 2014). Image technology has acoustic principle combining microwaves and ultrasonic also become a part of most of these non-destructive scan- waves for spatial imaging of a hole in a wood sample. The ning methods. method proves fast and efficient. Many authors deal with evaluation methods by means of image technology that utilises analysis of shadows, shades of grey or colour spectrum (e.g. RGB) obtained with the help of camcorders (Hu et al. 2004; Sandak & 2.2.3 Georadar methods (GPR) Tanaka 2005; Faria et al. 2008). Most of the systems used This method utilises electromagnetic radiation in the in the wood processing industry can be considered as 2D microwave band, high-frequency radio waves (10 MHz systems. These systems are mainly used for timber evalu- – 2.6 GHz) and detects reflected signals from subsur - ation. Today, more and more 3D scanning technologies face structures of materials. (July 2008). This technol- using lasers are coming to the fore, with their use mainly ogy is most often used for the evaluation of soils, rocks for scanning logs in sawmills. Studies dealing with the and building elements (concrete, brick walls). However, scanning of standing trees using LiDAR scanners to many studies have pointed to the possibility of using this achieve the evaluation of surface defects before their har- ground-penetrating technology to evaluate internal wood vesting, is known (Pirouz et al. 2015; Sauter et al. 2017). defects (Nicotti et al. 2003; Muller 2003; Hislop et al. In spatial (3D) laser scanning, controlled operation of 2009), where microwave radiation with a frequency of laser beams is combined with a laser rangefinder. Using 1.5 – 2.5 GHz is recommended. For the evaluation of specialized software, the shape of the surface of objects wood, mainly devices for construction industry are used. can be quickly captured. Pirouz et al. (2015) in their research compares image, The laser was first utilised to irradiate samples when laser, X–ray scanning and technologies using GPR. He measuring wood in the 1980s. The laser and camcorder uses a device by a Sensors & Software company, namely were first combined to measure the dimensions. Later, a Pulse EKKO® type intended for the evaluation of inter- dynamic laser dispersion was also used, e.g. to measure nal rot. grain orientation (Österberg, 2009). Authors Kowal et al. Further research uses similar devices by a GSSI com- (2012) and Sioma (2015) deal with the use of 3D images pany, namely SIR 3000, SIR 4000 (Senalik et al. 2016). in automatic detection and localization of defects on the The only device designed directly for the evaluation of wood surface using the laser triangulation method. The wood, or on trees, is TRU™ (Tree Radar Unit) by the work of Thomas et al. (2007) is aimed at scanning the sur- American company TreeRadar™ (Wen et al. 2016). face of hardwood logs to determine their external defects. The study (Halabe et al. 2009) showed that GPR There are several variants of a laser scanner, some can be used to accurately identify internal defects such with multiple laser beams and one camcorder, while as knots, cracks, and metal nails that are not commonly other scanners have two camcorders and one laser. For visible. Scanning speed, good repeatability and the ability example, following manufacturers offer the laser scan- to detect wood moisture are the advantages of this tech- ning method: Cognex (a model: 3D-A5000, In-Sight nology. The disadvantage is the limited ability to detect 9000), JoeScan (a model: JS-25-x-SERIES), LMI Tech- small defects and the complexity of data processing as nologies (a model: chroma + scan series, Gocator 2100 algorithms for evaluation are still under development. Series); SICK (a model: TriSpector 100). The technology is proving very promising for use in the sawmill indust. 2.2.5 X–ray methods 2.2.4 Image and laser methods In the second half of the 1980s, X–ray image methods were in the research phase. Methods developed for use in The ever-increasing capacities of sawmills and the medicine have been transferred to the wood processing speed of log processing are forcing wood processors to industry (Österberg 2009). Efficient commercial X–ray introduce efficient and fast methods of material evalua- devices for log and timber evaluation are currently avail- tion. Various machine vision systems (machine vision) able. The device can detect internal defects of wood, hid- in frequent combination with laser, acoustic and other den foreign objects, measure properties such as wood systems have proven to be suitable solutions. Today, structure, moisture and density. However, due to its these systems are among the most widely used in mod- ern industrial production, also in the wood processing high cost, X–ray computed tomography (CT) is used industry. Computer vision, which deals with the acquisi- only in the largest sawmills. Conventional X–ray scan- tion, processing, analysis and understanding of digital ners are usually based on discrete X–ray scanning in 1–4 images most often obtained from cameras, camcorders, directions (sometimes 6), where the object or source of X–rays does not rotate (Grundberg & Grönlund 1997). 3D scanners and other scanning devices, is considered the basis of this method. Its task is to develop theoreti- The difficulty is that even an expensive scanner with six 7 V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 X–ray directions cannot provide an accuracy similar to et al. 2019). Determining internal properties such as that obtainable by the CT scanner (Oja 1997). For these pith, core, knots, cracks and others is another their use reasons, CT X–ray scanners are coming to the fore today. (Stängle et al. 2015; Rais et al. 2017). CT wood scanning There are four generations of CT scanner technology technology is also used for determining the content and that are known. The first and second generation scan in flow of water in wood (Hansson et al. 2017; Longuetaud parallel, the third and fourth generation use rotary fan- et al. 2017). A substantial part of the research is devoted shaped scanning, where an object or source of an X–ray to mathematical algorithms for evaluating the acquired beam moves. First generation scanners (Fig. 4a) use a images. Images for the above mentioned research are single X–ray detector. The X–ray beam passes through obtained mainly by means of CT scanners intended for the scanned object and measures X–ray intensities medicine, but there are also modern industrial devices through parallel paths in that object (Schmoldt et al. intended for the wood processing industry. 1998). They are very simple, affordable but slow. The The best known manufacturers of medical macro CT second generation (Fig. 4b) uses a detector system con- scanners are Siemens and GE Healthcare. There are sev- sisting of multiple X–ray detectors for scanning. It’s also eral manufacturers of micro CT scanners on the market, simpler, faster, but collects unnecessary large amounts which are designed for smaller objects that they can scan of data with frequent noise. The third generation of CT with higher resolution. These are, for example, BRUKER scanners (Fig. 4c) uses an array of detectors with many (SkyScan series), ZEISS (Xradia series), Nikon (Model detectors usually placed in an arc. These devices are more XT H 225). The Belgian Ghent University (models: complex for data processing, but are more accurate. For EMCT, Hector, Herakles, Nanowood) also developed its smaller objects, they collect unnecessary amounts of data devices for micro CT scanning. Other device designed and are expensive. The fourth generation (Fig. 4d) has mainly for timber evaluation uses X–rays in combination a larger number of detectors placed in a circle. Here, an with other methods (laser, image scanning). Well-known X–ray source revolves around a scanned object, which is manufacturers are: WEINIG Gruppe (Model CombiS- placed on a table. It can produce very accurate 3D images. can +), Innovativ Vision (Model WoodEye 5), Microtec The fourth generation has the same disadvantages as the (Goldeneye series). third generation but owing to the large number of detec- There are very few manufacturers of industrial X–ray tors it is more expensive. scanners designed to scan logs. One of the manufacturers Fig. 4 Generations of CT scanners: a) 1st generation, b) 2nd generation, c) 3rd generation, d) 4th generation. Most modern CT scanners (4th generation) are is the German Jörg Elektronik with its JORO-X model equipped with the possibility of spiral CT scanning, with two X–ray sources. The second is a 3D laser scan- which provides almost perfect volumetric reconstruc- ner with multi-sensor camcorders and an X–ray scan- tion created from CT images (Beaulieu & Dutilleul 2019). ner placed in two positions, produced by the MICRO- Studies by Schmoldt et al. (1998), Gupta et al. (2004) Te C® company (Logeye series). The Italian company deal with the design of a tangential CT scanner, where MICROTEC® is the best known and most advanced when scanning the detector field is placed parallel to the company dedicated to the field of CT scanners for the axis of rotation of the object, parallel to the length of the wood processing industry. Its CT log device (Fig. 5) is log. Unlike a typical scanning, where the beam is parallel the only known device in the world designed for 3D log to the width of the log. This technology is patented (Gupta scanning. The device scans and digitally reconstructs the 1997), but its use in industry is not known. internal properties of the log what allows to optimise the CT X–ray scanning is becoming more and more cutting plan in real time (Microtec 2019). The developed affordable, so there is a quantity of research arising for software optimises the bending of the log, determines the this technology. They are utilised to determine the density best cutting plan to achieve the highest possible quality of wood and the size of annual increments in coniferous of the final product. So far, 8 of these devices have been and broadleaf species (Longuetaud et al. 2017; Jacquin installed in the world (Table 1). 8 V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 defects. To a greater extent, these technologies are now used in large sawmills focused on coniferous raw mate- rial. However, they are of great importance for cutting more expensive hardwood species. In terms of applicability, we can divide non-destruc- tive methods into two main categories, i.e. used only for research and applicable for research and industrial practice. Semi-destructive methods are used mainly for research. Most of them are applied in dendrology and forestry for the evaluation of young and older trees. The Pilodin and Resistograph devices are often used in Central Europe. Their utilization is also in wood degra- dation assessment of roof structures. SilviScan® and DiscBot® devices are very specific not being in Central Europe. Their use in rapid industrial production is very Fig. 5. CT Log scanner. © Microtec (Microtec 2019). limited. Other methods (microwave methods, methods using georadar) are still under development and further Table 1. Overview of CT Log device installations. research will be needed for their use in industry, ideally in State Number of CT Logs Company Canada 1 Interforest Ltd. collaboration with practice. However, there are a number Danzer Services, Inc. USA 2 of publications appearing that prove the applicability of Idaho Forest Group Chile 1 Arauco – Horcones these methods. Piveteaubois France 2 Described technologies enable different levels of Siat Braun Germany 1 HIT Holzindustrie Torgau non-destructive evaluation. Österberg (2009) created a Sweden 1 Norra Timber table (Table 2), which is suitable for a clear comparison of individual methods. Indicates the level of technology acquisition costs, application level, safety, and suitability 3. Applicability and efficiency of methods for identifying selected features. It can be deduced from the table that the X–ray Current trends, such as rising costs and limited supplies method is the most suitable for the evaluation of most of quality logs, are causing the increase in efforts to max- parameters. These devices are less wide-spread due to imise yields in the wood processing industry processing their high prices. In Europe, there are only 4 industrial coniferous and hardwood tree species. The production CT scanners located in France, Sweden and in Germany of timber is considerably complicated, due to the high in terms of the closest distance (see Table 1). Other types demands on the operation, complexity of the sorting, and of technologies are laser and image ones. Considering the requirements for the accuracy of the sawmill equip- that these two technologies are combined in most indus- ment. For these reasons, in today‘s wood processing trial devices, we can also consider them very suitable for industry, all process stages are increasingly being mod- wood evaluation. Laser, image and also acoustic methods ernized and automated. A great benefit for improving are therefore the most used in industrial practice. Their yield by automatic cut optimisation is also the introduc- great advantage is affordability as well as advanced tion of various scanning systems to detect internal wood Table 2. Evaluation of individual technologies according to their possibilities. Laser IR Visible light Acoustics Radio frequencies µ-waves X-ray Cost ++ ++ +++ ++ + + --- Applicability +++ - +++ + - ++ + Safety - +++ +++ +++ ++ + -- Performance: 3D-shape +++ +++ +++ --- --- - +++ Rot and decay --- + - ++ - -- +++ Colour defects --- --- ++ --- -- --- --- Compression and tension wood --- - + + + + +++ Cracks and splits --- + + ++ - + ++ Resin pockets --- - -- -- - + ++ Foreign objects --- - -- + ++ ++ +++ Knots --- + + - + ++ +++ Wood density --- - ++ ++ + ++ +++ Strength ++ + ++ + + + +++ Annual ring width --- - +++ --- --- --- + Grain orientation +++ --- + --- + ++ ++ Moisture content --- ++ --- -- + ++ ++ Bark content ++ -- +++ --- --- --- +++ Fiber properties --- --- - --- --- --- - 1 2 when combined with image analysis; is not thermography, i.e. no spectral content is utilised; -- the technology is completely unsuitable for determining the mentioned characteristics; +++ the technology is suitable for determining the mentioned characteristics; Commercially known techniques. 9 V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 computer vision technology for image analysis. This ers. Currently, some non-destructive methods are only results in the possibility of accurate evaluation at high applicable to scientific activities and their introduction speeds, applicable in fast automated production. Laser, into industrial practice is the future. The most widely image and acoustic methods, often combined with IR used non-destructive methods in industry are image, radiation, are used in all large, automated sawmills in laser and acoustic methods. These technologies are now Europe. Gradually, however, they started to be used in more available and allow very fast evaluation of wood medium-sized sawmills. These devices are produced e.g. properties. However, the possibilities of these technolo- by WEINIG Gruppe, Innovativ Vision, MICROTEC®. gies are limited mainly to surface defects, with the excep- MICROTEC® equipment is used in the Czech Republic tion of acoustic ones. The X–ray method, mainly using e.g. by sawmills Mayr-Melnhof Holz Paskov Ltd., Holz computed tomography, is certainly the most accurate Schiller Ltd., Danzer Ltd. In Slovakia there are PRP Ltd. method. Most X–ray devices create 2D images of the and Rettenmeier Tatra Timber. From these three meth- scanned material. The CT Log by Microrec® company ods, only the acoustic method has the ability of the evalu- is considered the most advanced device as it enables 3D ation of internal wood structures. Investment in these X–ray scanning of the log in real time. It can thus detect methods is very important for market competitiveness, internal defects with high accuracy, and then choose the especially for large and medium-sized sawmills. Return optimal cutting pattern to achieve the highest quality of on investment ranges from 2 to 8 years. The most accu- the final product (timber). rate non-destructive method for evaluating properties For modern sawmills of the third millennium, it is is certainly the X–ray method with computed tomogra- necessary to invest in modern non-destructive methods phy (CT). These devices are slowly applied in modern of wood evaluation to increase the yield of a quality final wood processing plants. The big disadvantage is the high product. These technologies can significantly increase purchase price, which allows the use of this technology competitiveness. The use of Industry 4.0, or modern mainly in medium and large plants. Gergeľ et al. (2019) automated log cutting lines, is certainly the future in the confirmed this from the existing research and concluded wood processing industry. Streamlining of production that there is a demonstrable increase in the profit gained under Industry 4.0 anticipates achieving almost zero from processed wood from 11.3% to 23.7% for conifer- production downtime and transparency. This requires ous logs and by 24% for broadleaf logs. In processing the processing of huge amounts of data from a variety broadleaf raw material, the return period for large saw- of sensors and transducers. To process this data, special mills is about 3 years and for medium sawmills 8 years. In systems using machine learning, neural networks and processing coniferous raw material, the return period is artificial intelligence are applied. The Microrec® com - about 4 years for large sawmills and 8 years for medium pany applied this method and developed its own Sawmill sawmills. 4.0 system – Digital Fingerprint. This technology enables The Italian Microrec® company, which offers tech- to create a separate record with all the data obtained for nologies using all the mentioned methods (acoustic, each board from scanning the log to the final product. image, laser, X–ray), is the important company in the This facilitates a significant increase in the degree of development of these automation technologies. In all automation on sawmills. These processes are the basis technological industries, the Industry 4.0 application for creating a reliable system that will monitor the flow comes to the fore, or the modernization and automation of wood from its harvest to the final product. of production lines, associated with the need to obtain a The implementation of these technologies within the large amount of data on material. The Microrec® com- European area is very different. Sweden, Finland and, in pany has developed the Sawmill 4.0 – Digital Fingerprint the area of Central Europe, Germany and Austria have system, which applies their non-destructive wood evalu- a significant competitive advantage in the utilization of ation technologies to achieve the greatest possible auto- these technologies. These countries are significantly mation of coniferous and broadleaf wood cutting. Combi- innovating and automating the entire log processing nation of their devices (CT Log, Logeye, Goldeneye) and process. Scanning technologies are used only at the the implementation of artic fi ial intelligence allows to cre - two largest sawmills in Slovakia, but they are gradually ate a record for each board that is monitored throughout started to be used by medium-sized log processors. It the process. This makes it possible to qualitatively classify is strongly recommended to support this trend, e.g. by the final products and thus make the most of them. subsidies, because these technologies mean significant savings and improved quality of wood raw material. 4. Conclusion Acknowledgements Analysis of existing methods for evaluating the quality This publication is the result of the project implementation: Cen- of wood properties, using semi-destructive and non- tre of Excellence of Forest-based Industry, ITMS: 313011S735 destructive technology. Semi-destructive technologies supported by the Research & Development Operational Pro- can be used mainly for scientific activities and for grow- gramme funded by the ERDF. and with the support of MARDSR, 10 V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 item 08V0301 – Research and development to promote forestry Gupta, N. K., Hughes, S. H. 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Innovative methods of non-destructive evaluation of log quality

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Abstract

For the sustainability of an important renewable resource, such as wood, it is important to significantly increase the efficiency of its processing. A large part of this raw material ends up in the wood processing industry, where it is used for the production of pulp, paper, construction and furniture timber, floors and others. Therefore, it is very important to gain the knowledge needed for optimal valuation of raw wood material, through quality detection and classification into quality classes. There are many defectoscopic methods working on different physical principles. The most familiar of these methods are semi-destructive and non-destructive, as they do not cause damage to the tree or wood during assessment. The aim of this article is to describe, assess and compare known semi-destructive and non-destructive methods for the assessment of wood properties. This article describes basic visual inspection, basic semi-destructive methods (Pilodyn, Resistograph) and advanced semi-destructive methods (SilviScan®, DiscBot®) as well. Non-destructive methods use mostly acoustic wave motion (acoustic, ultrasonic), high-frequency waves (using georadar, microwave) and methods based on visual evaluation (image, laser). At last, there are X–ray methods with the latest technology using three-dimensional (3D) computed tomography (CT). The implementation of modern non-destructive methods is of great importance for the application of principles of Industry 4.0, where these methods provide collecting of data on the material properties, in its entire production flow of log processing. Key words: X–ray method; acoustic methods; georadar methods; laser methods; industry 4.0, CT scanner Editor: Miloš Pánek important for the increasing yield. Globally, there is a 1. Introduction great shift in the intensity of forest management and log Today, under growing pressure from society, ecological processing, which requires significantly faster and more standards and the developing industry in our country and accurate evaluation of wood properties. Unlike concrete, in the world, there is an increasing emphasis on assessing bricks, steel, aluminium and most other construction the quality and origin of wood raw material. For these materials, wood has highly complex, anisotropic and var- reasons, the European Union has developed several iable internal structure. For the correct use of woodand instruments aimed at the ecology, regulation and legali- products based on it (plywood, KVH prisms, timber, OSB sation of harvesting. In 2003, “Forest Law Enforcement, boards, etc.), it is necessary to evaluate the properties of Governance and Trade” (FLEGT) was created to prevent this material. illegal timber trade and encourage investment in legal There are a number of methods for evaluating wood. harvesting in developing countries. “Reducing emissions They can be divided, according to the degree of destruc- from deforestation and degradation” (REDD) offers a tion of the evaluated material, into: 1. destructive, 2. non- new method how to reduce CO emissions by paying for destructive, 3. semi-destructive methods. Non-destructive actions, to prevent deforestation or forest degradation. and semi-destructive methods are more important in the Rising social and ecological standards have given rise to evaluation of wood. Non-destructive evaluation is the FSC (Forest Stewardship Council) certic fi ation, which is identification of the physical and mechanical properties a trusted system for forest certification and certification of a material without altering its end use and, using the of the processing (consumer) chain of wood processing identified properties, decides on its appropriate appli - worldwide. cations (Ross et al. 1998). In contrast, semi-destructive Evaluation of the quality of wood raw material and techniques for evaluating wood properties are defined its sorting has a significant effect on maximising yield. as test procedures that are non-destructive in relation Financial, quantitative, but also ecological reasons are to the structural element but destructive in relation to *Corresponding author. Vojtěch Ondrejka, e-mail: ondrejka.vojtech@gmail.com V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 the extracted sample (Kasal et al. 2013). In the forestry 2.1. Semi-destructive methods context, non-destructive evaluation of wood (NDE) is Semi-destructive methods damage the material only considered attractive. Authors Schimleck, et al. (2019) partially without affecting its further use. They are used described the main reasons why the NDE has grown rap- mainly to evaluate the properties of wood, logs, or his- idly over the last 20–25 years: Protection of investment in toric roof constructions. A new approach to the historic wood raw material; potential to reduce wood processing roof structures properties assessment is described in the costs; easier to use for e fi ld measurements; fast real-time publication (Kloiber et al. 2015). The authors describe data collection; the ability to identify the most suitable recently developed assessment methods such as tensile measurement application and reduce the variability of strength of small samples, tensile Young’s modulus of product classes. mesospecimens, compression strength of cores, com- This article focuses on the description and mutual pression strength in a drilled hole, mechanical resistance comparison of non-destructive and semi-destructive to pin pushing, Young’s modulus derived by measuring methods of wood evaluation, working on various physi- the hardness and shear strength of screw withdrawals. cal principles, described from basic methods to today‘s They do not belong to the industrially used methods, they most modern. The methods will be examined in relation are used mainly for scientific work. They are described to their use in forestry and the wood processing industry, here for their use in evaluating the quality of trees, which as these sectors are significantly interrelated. helps in the classification of wood according to quality, already in the first phase (standing tree). p ilodyn is a portable tool for evaluating the density 2. evaluation methods for wood of trees or wooden constructions. The tool was originally The main goal of wood evaluation is to assess qualita- developed in Switzerland to obtain quantitative data on tive properties, sometimes supplemented by quantita- the degree of soft rot in wooden poles. The tip penetrates tive ones. For qualitative evaluation, it is necessary to the surface of the material, measuring the depth of pen- determine the mechanical and physical properties of etration (Cown 1978). The penetration depth of the pin wood. Non-destructive methods determine, for exam- negatively correlates with the density of the wood. Three ple, mechanical properties (moduli of elasticity, impact different rods (diameter 2.0, 2.5 and 3.0 mm) enable to strength etc.) and physical (moisture, density, acous- adapt the use of Pilodyn to the density of tested wood tic properties, visual properties etc.). Another part of (Gao et al. 2017). In practice, the accuracy of Pilodyn is the qualitative evaluation is the determination of wood somewhat limited, so it is mainly used to estimate aver- defects (knots, cracks, rot, irregularity of the wood age values (Cown 1978). It is one of the least invasive structure, foreign objects, traces of biological damage, sampling techniques, but unfortunately it cannot pen- etc.). The identified properties significantly affect the etrate the middle layers of the trunk. It is also assumed subsequent sorting of wood products, yield and thus the that tested wood is above the b fi er saturation point (FSP), as the penetration of the tip decreases with decreasing financial appreciation of wood. moisture content below the FSP (Llana et al. 2018). The basic, still widely used evaluation is visual inspec- The resistograph began to develop in the early 1990s tion. The method is very fast, with lower costs. It allows (Rinn et al. 1996). It was utilised for improving the iden- to obtain basic information in situ about the assessed tic fi ation of rot in trees and poles. The tool drives a special tree or log. Visual inspection allows only the detection drill (needle) with a diameter of 3 mm through a tree at a of external, visible defects and mechanical damage to the given feed speed and rotation speed (rpm) and measures wood. Another disadvantage is the higher demand on the the resistance to rotation (torque). The trace represents expertise of the assessor, which can increase the overall the resistance prol fi e every 0.1 mm and the radial change cost. Visual inspection is also the basis for in situ visual in wood density (Rinn et al. 1996; Downes & Lausberg classification according to the quality classes of logs 2016). Gao et al. (2017) concluded that compared to and classification of construction timber into strength other semi-destructive techniques, Resistograph was a classes. Visual evaluation is strongly represented in supplier-customer relations in log trading. Here, the cheaper and faster method for collecting wood density log is most often sorted according to the standards STN data. The key features of this tool are its low cost for field EN 1316–1 – 3 for broadleaf raw material and STN EN use, digital data collection and relatively high-resolution data (Schimleck et al. 2019). Research by Kloppenburg, 1927–1 – 3 for coniferous raw material. These standards (2018) focused on the possibility of using the Resisto- determine the evaluation parameters of raw logs for clas- sification into four classes (A, B, C, D). Results of the graph to accurately assess the wood density of tropical visual inspection form the basis from which subsequent hardwoods. The tool has proven to be suitable, but the non-destructive testing (NDT) can be planned (Piaza et sharpness of the tool has a significant effect on accuracy. al. 2008). To evaluate the quality of wood, a large number Specific devices are SilviScan® and DiscBot®. Both of devices have been developed that can assess the prop- of these devices are developed more for laboratory evalu- erties of timber logs, veneers and other wood products. ation of wood (logs, timber). They are more invasive, as it 4 V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 is necessary to take a sample (a prism, a disc) for evalua- 2.2. Non-destructive methods tion. The r fi st of these, the SilviScan®, is a special labora- Non-destructive evaluation is the identification of the tory device designed to measure wood quality parameters physical and mechanical properties of a material with- such as the density and angle of microfibrils (Schimleck out altering its end use and, using the identified proper- et al. 2002). The device was created by Dr. Robert Evans ties, decides on its appropriate applications (Ross et al. and developed with his CSIRO team as early as 1992. So 1998). Previously, non-destructive methods were applied far, 3 versions of this device have been created. The main to evaluate very valuable (e.g. exotic) wood species. Cur- components of SilviScan are (Schimleck et al. 2019): rently, these methods are increasingly preferred, because – Optical cell scanner (radial and tangential tracheid they do not degrade the raw material (wood), they are and fiber diameters, vessel size and their position, a accurate and are fast. This is applied mainly in automated boundary and orientation of annual rings); modern sawmills, where the identified parameters help – X–ray densitometer (conditioned density profile, to better evaluate the processed raw material. The most fiber tilt, annual ring transition); commonly used physical principles in non-destructive – X–ray diffractometer (microb fi ril angle, tracheid and evaluation are: acoustic, ultrasonic, microwave, imaging, fiber 3D orientation, cellulose crystallisation). laser and X–ray. A stiffness estimate generated by SilviScan is based on the diffractometric and densitometric properties of wood, calibrated by the acoustic resonance technique (Ilic 2001). The device has been tested in many studies 2.2.1 Acoustic and ultrasonic methods and has proven to be very accurate (Buksnowitz et al. Acoustic methods of wood evaluation are among the 2008; Schimleck et al. 2002). oldest methods of evaluating the properties of wood. Another special device is the DiscBot, developed by As defined in (Lipta el al. 1972), acoustic velocity is in the New Zealand company SCION. The device utilises a form of transitional elastic waves generated by the scanning technologies, which can determine a large rapid release of energy in a material. Thus, the source number of wood properties affecting the resulting qual- is an acoustic impulse, such as a hammer impact, or an ity of timber and other final products. The scanner uses excited acoustic signal (sound). Therefore, scanning the automatic motion of wooden discs under various sen- speed of acoustic waves in wood is most often used for sors that capture information on wood density, microfi- evaluation. Another non-destructive acoustic method bril angle and chemical composition. The first sensor is is ultrasonic measurement. Ultrasound is a mechani- a camcorder for capturing a high-quality colour image cal wave with a frequency generally greater than 20 kHz (in an RGB spectrum), which allows the identification (Wang et al. 2002). The diagnostic tools operate with of wood defects such as knots, resin pockets and com- frequencies in the range from 20 kHz to 500 kHz. Dur- pression wood. A spectrograph fitted with an infrared ing the measurement, ultrasonic waves to the object are (NIR) camcorder (900–1700 nm) applied in several generated and subsequently the transmitted waves are studies (Jones et al. 2006; Thumm et al. 2010) is used measured to identify the transmission properties of wood to determine the chemical composition and properties (Österberg 2009). To increase the contact of ultrasonic of lignin and cellulose. The wood density is measured receivers and transmitters, it has proven appropriate to using X–rays. The strength of wood is evaluated using immerse the examined wood into a water bath (McDon- an ultrasonic device when the wave passes through the ald 1978; Han & Birkeland 1992). wood along the fibers. Fig. 1. Principle of measurement by acoustic tools: FAKOPP, a TreeSonic™ model and Microsecond Timer. 5 V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 The development of acoustic devices for the evalua- of elasticity by acoustic tools, different values may occur tion of standing trees opened the way for assessing the when repeating the measurement, which causes some properties of tree wood before their harvesting. This facil- inaccuracies in the results, see research: Lindström et itates the management, planning of harvest and wood al. (2009), Simic et al. (2019). processing in a way that maximizes the value extracted from the source (Riggio et al. 2008; Carter 2017). In the last few decades, acoustic technologies have become well-established devices for material evaluation in the wood processing industry. They have become a widely accepted tool for quality control and product classifica - tion (Wang et al. 2004), but also for breeding research, where they are utilised for evaluation of young trees (Lenz et al. 2013). Most acoustic devices that evaluate the properties of trees are modelled for radial measurement (perpendicu- lar to the trunk axis). For example, these devices: Arbor- Sonic 3D Acoustic Tomograph, Hitman Resonance Tool, IML Impulse Hammer, and FAKOPP company tools Fig. 3. Illustration of the principle of ultrasonic tomography. (Microsecond Timer, Resonance Log Grader, ArborE- lectro Impedance Tomograph) (Fig. 1). In contrast to All the mentioned devices are difc fi ult to apply to rapid these tools, the FAKOPP TreeSonic™ is developed for industrial production. That is why fast acoustic tools are measuring in the longitudinal direction of the trunk (Fig. being developed today, measuring at a feed speed of 200 2). In acoustic systems, the source of the impulse is most pieces per minute. An example is the Swedish company often the hammer impact. DYNALYSE, which offers Dynagrade and Precigrade devices for evaluating timber properties. Others are devices for evaluating logs before cutting them. A com- pany based in New Zealand, Fiber-gen, has developed the HITMAN PH330 measuring tools implemented into a forest harvester, the HITMAN LG640 for log processing lines and the HITMAN HM200 manual tool. 2.2.2 Microwave methods The microwave frequency in the electromagnetic spec- trum is between the frequencies of infrared radiation and high-frequency waves, i.e. approximately between the frequencies 1 GHz and 100 GHz, or a wavelength between 1 mm to 1 m (Fuller 1990). However, the bound- ary between microwaves and infrared or radio waves has not been precisely defined. The microwave scan- ning method has been under development for several years. The first tools were created to measure moisture and were also applied in industrial production (Tiuri & Heikkilä 1979). At present, research using microwave tomography is known, where the authors describe the detection of internal wood defects with higher accuracy Fig. 2. Acoustic system for measuring standing trees in the (Pastorino et al. 2015; Boero et al. 2018). Using this longitudinal direction. technique, it is possible to determine properties such as: density and moisture of wood, deflection and direction of fibers or detection of wood defects as well. Ultrasonic systems evaluate the material mainly per- Microwave induced thermo-acoustic tomography pendicular to the b fi ers (Fig. 3). The most used ultrasonic (MITAT) combining microwave imaging with ultra- devices are FAKOPP IltraSonic Timer, Arborsonic Decay sonography. This special technology is mainly used in Detector, Sylvatest, V-Meter MK IV System, Proceq Pun- biomedicine and is still under development and the sub- dit Lab +, Pundit 250 Array and Tico. Most tools evaluate ject of much research (Ku & Wang 2001; Meaney et al. the quality of wood using the determined dynamic modu- 2012; Rosenthal et al. 2012; Wang et al. 2013). Research lus of elasticity and density. When evaluating the moduli 6 V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 using this technology to determine the properties of wood cal and algorithmic bases for achieving automatic visual is known. Authors Zhang et al. (2019) use the thermo- understanding (Sonka et al. 2014). Image technology has acoustic principle combining microwaves and ultrasonic also become a part of most of these non-destructive scan- waves for spatial imaging of a hole in a wood sample. The ning methods. method proves fast and efficient. Many authors deal with evaluation methods by means of image technology that utilises analysis of shadows, shades of grey or colour spectrum (e.g. RGB) obtained with the help of camcorders (Hu et al. 2004; Sandak & 2.2.3 Georadar methods (GPR) Tanaka 2005; Faria et al. 2008). Most of the systems used This method utilises electromagnetic radiation in the in the wood processing industry can be considered as 2D microwave band, high-frequency radio waves (10 MHz systems. These systems are mainly used for timber evalu- – 2.6 GHz) and detects reflected signals from subsur - ation. Today, more and more 3D scanning technologies face structures of materials. (July 2008). This technol- using lasers are coming to the fore, with their use mainly ogy is most often used for the evaluation of soils, rocks for scanning logs in sawmills. Studies dealing with the and building elements (concrete, brick walls). However, scanning of standing trees using LiDAR scanners to many studies have pointed to the possibility of using this achieve the evaluation of surface defects before their har- ground-penetrating technology to evaluate internal wood vesting, is known (Pirouz et al. 2015; Sauter et al. 2017). defects (Nicotti et al. 2003; Muller 2003; Hislop et al. In spatial (3D) laser scanning, controlled operation of 2009), where microwave radiation with a frequency of laser beams is combined with a laser rangefinder. Using 1.5 – 2.5 GHz is recommended. For the evaluation of specialized software, the shape of the surface of objects wood, mainly devices for construction industry are used. can be quickly captured. Pirouz et al. (2015) in their research compares image, The laser was first utilised to irradiate samples when laser, X–ray scanning and technologies using GPR. He measuring wood in the 1980s. The laser and camcorder uses a device by a Sensors & Software company, namely were first combined to measure the dimensions. Later, a Pulse EKKO® type intended for the evaluation of inter- dynamic laser dispersion was also used, e.g. to measure nal rot. grain orientation (Österberg, 2009). Authors Kowal et al. Further research uses similar devices by a GSSI com- (2012) and Sioma (2015) deal with the use of 3D images pany, namely SIR 3000, SIR 4000 (Senalik et al. 2016). in automatic detection and localization of defects on the The only device designed directly for the evaluation of wood surface using the laser triangulation method. The wood, or on trees, is TRU™ (Tree Radar Unit) by the work of Thomas et al. (2007) is aimed at scanning the sur- American company TreeRadar™ (Wen et al. 2016). face of hardwood logs to determine their external defects. The study (Halabe et al. 2009) showed that GPR There are several variants of a laser scanner, some can be used to accurately identify internal defects such with multiple laser beams and one camcorder, while as knots, cracks, and metal nails that are not commonly other scanners have two camcorders and one laser. For visible. Scanning speed, good repeatability and the ability example, following manufacturers offer the laser scan- to detect wood moisture are the advantages of this tech- ning method: Cognex (a model: 3D-A5000, In-Sight nology. The disadvantage is the limited ability to detect 9000), JoeScan (a model: JS-25-x-SERIES), LMI Tech- small defects and the complexity of data processing as nologies (a model: chroma + scan series, Gocator 2100 algorithms for evaluation are still under development. Series); SICK (a model: TriSpector 100). The technology is proving very promising for use in the sawmill indust. 2.2.5 X–ray methods 2.2.4 Image and laser methods In the second half of the 1980s, X–ray image methods were in the research phase. Methods developed for use in The ever-increasing capacities of sawmills and the medicine have been transferred to the wood processing speed of log processing are forcing wood processors to industry (Österberg 2009). Efficient commercial X–ray introduce efficient and fast methods of material evalua- devices for log and timber evaluation are currently avail- tion. Various machine vision systems (machine vision) able. The device can detect internal defects of wood, hid- in frequent combination with laser, acoustic and other den foreign objects, measure properties such as wood systems have proven to be suitable solutions. Today, structure, moisture and density. However, due to its these systems are among the most widely used in mod- ern industrial production, also in the wood processing high cost, X–ray computed tomography (CT) is used industry. Computer vision, which deals with the acquisi- only in the largest sawmills. Conventional X–ray scan- tion, processing, analysis and understanding of digital ners are usually based on discrete X–ray scanning in 1–4 images most often obtained from cameras, camcorders, directions (sometimes 6), where the object or source of X–rays does not rotate (Grundberg & Grönlund 1997). 3D scanners and other scanning devices, is considered the basis of this method. Its task is to develop theoreti- The difficulty is that even an expensive scanner with six 7 V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 X–ray directions cannot provide an accuracy similar to et al. 2019). Determining internal properties such as that obtainable by the CT scanner (Oja 1997). For these pith, core, knots, cracks and others is another their use reasons, CT X–ray scanners are coming to the fore today. (Stängle et al. 2015; Rais et al. 2017). CT wood scanning There are four generations of CT scanner technology technology is also used for determining the content and that are known. The first and second generation scan in flow of water in wood (Hansson et al. 2017; Longuetaud parallel, the third and fourth generation use rotary fan- et al. 2017). A substantial part of the research is devoted shaped scanning, where an object or source of an X–ray to mathematical algorithms for evaluating the acquired beam moves. First generation scanners (Fig. 4a) use a images. Images for the above mentioned research are single X–ray detector. The X–ray beam passes through obtained mainly by means of CT scanners intended for the scanned object and measures X–ray intensities medicine, but there are also modern industrial devices through parallel paths in that object (Schmoldt et al. intended for the wood processing industry. 1998). They are very simple, affordable but slow. The The best known manufacturers of medical macro CT second generation (Fig. 4b) uses a detector system con- scanners are Siemens and GE Healthcare. There are sev- sisting of multiple X–ray detectors for scanning. It’s also eral manufacturers of micro CT scanners on the market, simpler, faster, but collects unnecessary large amounts which are designed for smaller objects that they can scan of data with frequent noise. The third generation of CT with higher resolution. These are, for example, BRUKER scanners (Fig. 4c) uses an array of detectors with many (SkyScan series), ZEISS (Xradia series), Nikon (Model detectors usually placed in an arc. These devices are more XT H 225). The Belgian Ghent University (models: complex for data processing, but are more accurate. For EMCT, Hector, Herakles, Nanowood) also developed its smaller objects, they collect unnecessary amounts of data devices for micro CT scanning. Other device designed and are expensive. The fourth generation (Fig. 4d) has mainly for timber evaluation uses X–rays in combination a larger number of detectors placed in a circle. Here, an with other methods (laser, image scanning). Well-known X–ray source revolves around a scanned object, which is manufacturers are: WEINIG Gruppe (Model CombiS- placed on a table. It can produce very accurate 3D images. can +), Innovativ Vision (Model WoodEye 5), Microtec The fourth generation has the same disadvantages as the (Goldeneye series). third generation but owing to the large number of detec- There are very few manufacturers of industrial X–ray tors it is more expensive. scanners designed to scan logs. One of the manufacturers Fig. 4 Generations of CT scanners: a) 1st generation, b) 2nd generation, c) 3rd generation, d) 4th generation. Most modern CT scanners (4th generation) are is the German Jörg Elektronik with its JORO-X model equipped with the possibility of spiral CT scanning, with two X–ray sources. The second is a 3D laser scan- which provides almost perfect volumetric reconstruc- ner with multi-sensor camcorders and an X–ray scan- tion created from CT images (Beaulieu & Dutilleul 2019). ner placed in two positions, produced by the MICRO- Studies by Schmoldt et al. (1998), Gupta et al. (2004) Te C® company (Logeye series). The Italian company deal with the design of a tangential CT scanner, where MICROTEC® is the best known and most advanced when scanning the detector field is placed parallel to the company dedicated to the field of CT scanners for the axis of rotation of the object, parallel to the length of the wood processing industry. Its CT log device (Fig. 5) is log. Unlike a typical scanning, where the beam is parallel the only known device in the world designed for 3D log to the width of the log. This technology is patented (Gupta scanning. The device scans and digitally reconstructs the 1997), but its use in industry is not known. internal properties of the log what allows to optimise the CT X–ray scanning is becoming more and more cutting plan in real time (Microtec 2019). The developed affordable, so there is a quantity of research arising for software optimises the bending of the log, determines the this technology. They are utilised to determine the density best cutting plan to achieve the highest possible quality of wood and the size of annual increments in coniferous of the final product. So far, 8 of these devices have been and broadleaf species (Longuetaud et al. 2017; Jacquin installed in the world (Table 1). 8 V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 defects. To a greater extent, these technologies are now used in large sawmills focused on coniferous raw mate- rial. However, they are of great importance for cutting more expensive hardwood species. In terms of applicability, we can divide non-destruc- tive methods into two main categories, i.e. used only for research and applicable for research and industrial practice. Semi-destructive methods are used mainly for research. Most of them are applied in dendrology and forestry for the evaluation of young and older trees. The Pilodin and Resistograph devices are often used in Central Europe. Their utilization is also in wood degra- dation assessment of roof structures. SilviScan® and DiscBot® devices are very specific not being in Central Europe. Their use in rapid industrial production is very Fig. 5. CT Log scanner. © Microtec (Microtec 2019). limited. Other methods (microwave methods, methods using georadar) are still under development and further Table 1. Overview of CT Log device installations. research will be needed for their use in industry, ideally in State Number of CT Logs Company Canada 1 Interforest Ltd. collaboration with practice. However, there are a number Danzer Services, Inc. USA 2 of publications appearing that prove the applicability of Idaho Forest Group Chile 1 Arauco – Horcones these methods. Piveteaubois France 2 Described technologies enable different levels of Siat Braun Germany 1 HIT Holzindustrie Torgau non-destructive evaluation. Österberg (2009) created a Sweden 1 Norra Timber table (Table 2), which is suitable for a clear comparison of individual methods. Indicates the level of technology acquisition costs, application level, safety, and suitability 3. Applicability and efficiency of methods for identifying selected features. It can be deduced from the table that the X–ray Current trends, such as rising costs and limited supplies method is the most suitable for the evaluation of most of quality logs, are causing the increase in efforts to max- parameters. These devices are less wide-spread due to imise yields in the wood processing industry processing their high prices. In Europe, there are only 4 industrial coniferous and hardwood tree species. The production CT scanners located in France, Sweden and in Germany of timber is considerably complicated, due to the high in terms of the closest distance (see Table 1). Other types demands on the operation, complexity of the sorting, and of technologies are laser and image ones. Considering the requirements for the accuracy of the sawmill equip- that these two technologies are combined in most indus- ment. For these reasons, in today‘s wood processing trial devices, we can also consider them very suitable for industry, all process stages are increasingly being mod- wood evaluation. Laser, image and also acoustic methods ernized and automated. A great benefit for improving are therefore the most used in industrial practice. Their yield by automatic cut optimisation is also the introduc- great advantage is affordability as well as advanced tion of various scanning systems to detect internal wood Table 2. Evaluation of individual technologies according to their possibilities. Laser IR Visible light Acoustics Radio frequencies µ-waves X-ray Cost ++ ++ +++ ++ + + --- Applicability +++ - +++ + - ++ + Safety - +++ +++ +++ ++ + -- Performance: 3D-shape +++ +++ +++ --- --- - +++ Rot and decay --- + - ++ - -- +++ Colour defects --- --- ++ --- -- --- --- Compression and tension wood --- - + + + + +++ Cracks and splits --- + + ++ - + ++ Resin pockets --- - -- -- - + ++ Foreign objects --- - -- + ++ ++ +++ Knots --- + + - + ++ +++ Wood density --- - ++ ++ + ++ +++ Strength ++ + ++ + + + +++ Annual ring width --- - +++ --- --- --- + Grain orientation +++ --- + --- + ++ ++ Moisture content --- ++ --- -- + ++ ++ Bark content ++ -- +++ --- --- --- +++ Fiber properties --- --- - --- --- --- - 1 2 when combined with image analysis; is not thermography, i.e. no spectral content is utilised; -- the technology is completely unsuitable for determining the mentioned characteristics; +++ the technology is suitable for determining the mentioned characteristics; Commercially known techniques. 9 V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 computer vision technology for image analysis. This ers. Currently, some non-destructive methods are only results in the possibility of accurate evaluation at high applicable to scientific activities and their introduction speeds, applicable in fast automated production. Laser, into industrial practice is the future. The most widely image and acoustic methods, often combined with IR used non-destructive methods in industry are image, radiation, are used in all large, automated sawmills in laser and acoustic methods. These technologies are now Europe. Gradually, however, they started to be used in more available and allow very fast evaluation of wood medium-sized sawmills. These devices are produced e.g. properties. However, the possibilities of these technolo- by WEINIG Gruppe, Innovativ Vision, MICROTEC®. gies are limited mainly to surface defects, with the excep- MICROTEC® equipment is used in the Czech Republic tion of acoustic ones. The X–ray method, mainly using e.g. by sawmills Mayr-Melnhof Holz Paskov Ltd., Holz computed tomography, is certainly the most accurate Schiller Ltd., Danzer Ltd. In Slovakia there are PRP Ltd. method. Most X–ray devices create 2D images of the and Rettenmeier Tatra Timber. From these three meth- scanned material. The CT Log by Microrec® company ods, only the acoustic method has the ability of the evalu- is considered the most advanced device as it enables 3D ation of internal wood structures. Investment in these X–ray scanning of the log in real time. It can thus detect methods is very important for market competitiveness, internal defects with high accuracy, and then choose the especially for large and medium-sized sawmills. Return optimal cutting pattern to achieve the highest quality of on investment ranges from 2 to 8 years. The most accu- the final product (timber). rate non-destructive method for evaluating properties For modern sawmills of the third millennium, it is is certainly the X–ray method with computed tomogra- necessary to invest in modern non-destructive methods phy (CT). These devices are slowly applied in modern of wood evaluation to increase the yield of a quality final wood processing plants. The big disadvantage is the high product. These technologies can significantly increase purchase price, which allows the use of this technology competitiveness. The use of Industry 4.0, or modern mainly in medium and large plants. Gergeľ et al. (2019) automated log cutting lines, is certainly the future in the confirmed this from the existing research and concluded wood processing industry. Streamlining of production that there is a demonstrable increase in the profit gained under Industry 4.0 anticipates achieving almost zero from processed wood from 11.3% to 23.7% for conifer- production downtime and transparency. This requires ous logs and by 24% for broadleaf logs. In processing the processing of huge amounts of data from a variety broadleaf raw material, the return period for large saw- of sensors and transducers. To process this data, special mills is about 3 years and for medium sawmills 8 years. In systems using machine learning, neural networks and processing coniferous raw material, the return period is artificial intelligence are applied. The Microrec® com - about 4 years for large sawmills and 8 years for medium pany applied this method and developed its own Sawmill sawmills. 4.0 system – Digital Fingerprint. This technology enables The Italian Microrec® company, which offers tech- to create a separate record with all the data obtained for nologies using all the mentioned methods (acoustic, each board from scanning the log to the final product. image, laser, X–ray), is the important company in the This facilitates a significant increase in the degree of development of these automation technologies. In all automation on sawmills. These processes are the basis technological industries, the Industry 4.0 application for creating a reliable system that will monitor the flow comes to the fore, or the modernization and automation of wood from its harvest to the final product. of production lines, associated with the need to obtain a The implementation of these technologies within the large amount of data on material. The Microrec® com- European area is very different. Sweden, Finland and, in pany has developed the Sawmill 4.0 – Digital Fingerprint the area of Central Europe, Germany and Austria have system, which applies their non-destructive wood evalu- a significant competitive advantage in the utilization of ation technologies to achieve the greatest possible auto- these technologies. These countries are significantly mation of coniferous and broadleaf wood cutting. Combi- innovating and automating the entire log processing nation of their devices (CT Log, Logeye, Goldeneye) and process. Scanning technologies are used only at the the implementation of artic fi ial intelligence allows to cre - two largest sawmills in Slovakia, but they are gradually ate a record for each board that is monitored throughout started to be used by medium-sized log processors. It the process. This makes it possible to qualitatively classify is strongly recommended to support this trend, e.g. by the final products and thus make the most of them. subsidies, because these technologies mean significant savings and improved quality of wood raw material. 4. Conclusion Acknowledgements Analysis of existing methods for evaluating the quality This publication is the result of the project implementation: Cen- of wood properties, using semi-destructive and non- tre of Excellence of Forest-based Industry, ITMS: 313011S735 destructive technology. Semi-destructive technologies supported by the Research & Development Operational Pro- can be used mainly for scientific activities and for grow- gramme funded by the ERDF. and with the support of MARDSR, 10 V. Ondrejka et al. / Cent. Eur. For. J. 67 (2021) 3–13 item 08V0301 – Research and development to promote forestry Gupta, N. K., Hughes, S. H. 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Journal

Forestry Journal – de Gruyter

Published: Mar 1, 2021

Keywords: X–ray method; acoustic methods; georadar methods; laser methods; industry 4.0; CT scanner

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