Abstract:
The invention relates to a method and arrangement for measuring the quality of wood. The wood comprises not only a pure body but also components of bark and knots, having optical properties different from those of the pure body. Barked trees ( 308 ) are turned into wood powder ( 322 ). The wood powder ( 322 ) is illuminated with optical radiation and the radiation is received by means of a camera ( 330 ). The camera ( 330 ) transmits a signal consistent with the radiation reflected from or passed through the wood powder ( 322 ) to a computer ( 332 ) which, by means of the optical properties of the wood powder ( 322 ), determines the amount of bark, knots, and/or defective wood present in the wood.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a 371 of PCT/FI99/00899 Oct. 27, 1999 now WO 00/25115. 
    
    
     FIELD OF INVENTION 
     The present invention relates to the measurement of wood, especially by means of optical radiation. 
     BACKGROUND OF INVENTION 
     In mechanical and chemical forestry, one of the most important wood handling processes is the barking of logs. In the production of energy as well, the handling of logs is usually begun by barking and the amount of bark has an impact on the burning process in terms of its regulation and efficiency. In the production of pulp, the barked trees are first chipped, whereafter the chips proceed to a pulp digester. The bark remaining on chips deteriorates the quality of pulp being produced and causes a need of adjustment in the digestion process. Mechanical pulp is produced by grinding or refining. In grinding, the barked log is pressed against a grindstone. In refining, the barked logs are first chipped and the chips are refined between two rotary wheels. However, it is not worth while to bark the logs too thoroughly, as a result of this is the removal of pure wood material along with the bark, leading to losses of energy and material. It has been a common attempt to regulate the barking process in view of optimizing the amount of bark in wood chips. The amount of bark typically accepted in a pulp mill is less than 0.5%-1%, and in a paper mill even less than 0.1% of the total mass of refined or ground mechanical pulp. The adjustment of a barking process requires information about the purity grade or thoroughness of barking. 
     In prior art solutions, the purity grade of barking is measured by imaging the logs or chips to be barked and by applying various computer-based image processing programs for assessing the respective proportions of wood and bark. Indeed, there is such a distinction between bark and pure wood body that bark is usually darker than pure wood body. A problem in this type of method is that it is difficult to distinguish the dark bark for example from shadows. Moisture causes reflections, impeding the detection of bark material present in the chips or body. In addition, when measuring tree trunks, it is difficult to make a distinction between the pure wood material and the bark as a result of the geometrical patterns of a wood surface. The problem is particularly pronounced when the amount of bark is small. 
     In another prior art solution, the purity grade of barking has been measured by estimating the proportion of pure body material in barking refuse. However, the measurement does not correlate particularly well with the bark remaining affixed to a tree trunk, since the barking process makes it necessary to break dry wood more than moist wood. Thus, the barking refuse of dry wood contains a greater amount of pure body material than the barking refuse of moist wood, even though both logs would have an equal amount of bark affixed thereto. 
     BRIEF DESCRIPTION OF INVENTION 
     The invention seeks to provide such a method, and such an arrangement for implementing the method, that the above problems can be solved. 
     It is an object of the invention to provide a method for measuring the quality of wood, in which method the wood is constituted by timber which, in addition to a pure body, includes at least a bark component and knot components, which differ from the pure body in terms of optical properties thereof, the method involving the barking of logs. Furthermore, the method of the invention comprises turning at least some of the barked trees into wood meal or powder; exposing the wood meal to optical radiation; and measuring the quality of wood by means of the optical radiation. 
     It is another object of the invention to provide a measuring arrangement for the quality of wood, wherein the wood refers to timber which, in addition to a pure body, includes at least a bark component and a knot component, having optical properties different from those of the pure body, said measuring arrangement comprising a stripper for the barking of logs. Furthermore, in the arrangement of the invention, the measuring arrangement is adapted to produce wood meal or powder from barked trees; the measuring arrangement comprises a detector responsive to optical radiation; the measuring arrangement comprises a measuring device; and the detector is adapted to receive optical radiation coming from the direction of wood meal or powder and to carry a signal responsive to the optical radiation to the measuring device, and the measuring device is adapted to measure the wood meal by means of an optical-radiation based signal coming from the detector, and to determine the quality of wood. 
     A number of benefits are gained by the method and system of the invention. The amount of bark, knots, and/or defective wood in timber can be reliably measured without being interfered by the shape, shadows, or moisture (moisture-caused reflections) of tree trunks. Furthermore, the barking process and for example the digestion of chemical pulp can be optimally controlled according to the quality of wood. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     The invention will now be described more closely in conjunction with preferred embodiments, with reference made to the accompanying drawings, in which 
     FIG. 1 shows a piece of wood, 
     FIG. 2 shows a measuring arrangement, 
     FIG. 3 shows a measuring arrangement, 
     FIG. 4 shows the matrix surface of a detector, and 
     FIG. 5 shows a detector. 
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The invention offers a solution which is particularly applicable to wood handling processes used in mechanical and chemical forest industry. In addition, the invention provides a solution, which is applicable in sawmills and wood-consuming power production facilities requiring accurate knowledge about the quality of wood being used. 
     Reference is first made to a block of wood, which is shown in FIG.  1 . The block of wood may be for example a log coming into the barking plant of a pulp mill. The block of wood comprises a pure body  100 , a bark  102 , a branch  104 , a knot  106 , and a defective wood  108 . The exemplified block of wood is for example a log of pine or spruce. The knot  106  refers to a zone affected by the branch  104 . It is essential for the solution of the invention that the bark  102  or any wood material other than the pure body  100  be optically distinct from the pure body  100 . Typically, the knot  106  referred to as a zone affected by the branch  104  is optically perceivable from the pure body  100 . Likewise, the defective wood  108  has an optical behaviour different from that of the pure body  100 . The defective wood  108  may be mechanically damaged or it may be sick, for example rotten. 
     The solution of the invention will be examined now in general terms, with reference made to FIG.  2 . The solution of the invention comprises a feeder  200 , possibly a conveyor belt, which carries incoming unbarked logs  202  to a stripper  204 . The stripper  204  may be for example a barking drum. A second feeder  206 , for example another conveyor belt, is used for carrying barked logs  208  to a sawdust refiner  210  for turning the logs  208  into a powder or meal  212 . Regarding the classification of its size, the powder is preferably not above the centimeter class. The powder  212  is preferably the same type as sawdust or cutter chips. The powder  212  has a virtue of being homogeneous and the powder  212  reflects optical radiation almost totally diffusively, thus avoiding the problems caused by reflection. From the sawdust refiner  210  the powder or meal  212  progresses onto a measuring deck  214 , which may be a conveyor belt or a stationary platform. As soon as the powder  212  reaches a measurement site  216 , the powder  212  is lighted with an illuminator  218 . The illuminator  218  comprises preferably a fluorescent tube, but the illuminator can be constituted by one or more identical or different sources of optical power, which may be any narrow-or broadband, continuous or pulse-repeated sources of optical power, such as for example filament lamps, LEDs, and lasers. Since the measurement is preferably conducted indoors, for example in an industrial hall, the sample is most preferably lighted with the very same ceiling-mounted indoor illuminator that is used for lighting the entire hall. It is also possible to use daylight for lighting. At the measurement site  216 , the powder  212  is measured for at least one optical property, such as for example darkness, colour, and spectral distribution. In the proximity of the measurement site  216  is located a detector  220 , which is responsive to optical radiation transmitted by the illuminator  218 . The powder  212  reflects optical radiation to the detector  220 , which supplies a measuring block  222  with a signal proportional to optical radiation received thereby. Typically, the detector  220  comprises a video camera or a line camera, which is trichromatic, black-and-white, digital, or analogical. If the detector  220  is analogical, the measuring block  222  requires a digitizing board for facilitating digital signal processing. The signal transmitted by the detector  220  is used by the measuring block  222  for measuring the powder  212  for its reflection density or darkness, colour, and/or spectral distribution with an image processing program. In a solution of the invention, the measuring block  22  is preferably used for controlling the stripper  204 , as well. If, for example, the powder  212  has a reflection density or degree of darkness which is far too light (hypothesis: the bark  102  is dark and the pure body  100  is light), the stripper  204  will have its barking efficiency reduced. On the other hand, if the powder  212  has an excessively high reflection density, the barking efficiency will be increased (in a barking drum, the barking time is increased). 
     The solution of the invention will now be studied as applied in conjunction with a pulping process, as shown in FIG.  3 . In this case, as well, unbarked logs  302  are delivered for example on a conveyor belt  300  to a stripper  304 . Barked logs  308  are carried on a conveyor belt  306  to a chipper  310  for turning the barked trees into chips. For the most part, the chips are carried along a transfer line  312  to a pulp digester  314 . In practice, the transfer line  312  may be provided with an intermediate storage (not shown in FIG.  3 ), but this is not essential as far as the invention is concerned. A chip sample  319  is picked up from the moving chips at a point  316  onto a sample line  318 . The chip sample  319  constitutes a representative sample of the barked logs  308  as the chipper has chipped the trees into smallish chip fragments and mixed the chip fragments thoroughly. The chip sample is carried to a sawdust refiner  320 , which is typically a mechanical, chip-breaking device. The sawdust refiner  320  turns the chips  319  into a powder or meal  322  suitable for a measurement, which is transferred onto a measurement deck  324 . The measurement deck  324  is preferably a conveyor belt overlaid with a uniform layer of the wood meal or powder  322 . Upon reaching the end of the conveyor belt, the wood meal  322  is dropped onto a heap of wood meal  340 . The wood meal  322  is illuminated with optical radiation at least at a measurement site  326 . The optical radiation is generated by an illuminator  328 , which has already been described in conjunction with FIG.  2 . It is a detector  330  which receives radiation reflected from or passed through the wood meal  326  and transmits, in accordance with the impact caused by the radiation, a signal to a measuring device  332 , comprising at least a PC-computer  336 . The detector  330  has already been described in conjunction with FIG.  2 . Especially, if the detector  330  is an analogic camera, the measuring device  332  requires not only the PC-computer  336  but also a digitizing board  338  for converting an analogical signal to a digital one. The PC-computer  336  is provided with software for determining the wood meal  322  in terms of its reflection density, colour, and/or spectral distribution. In addition, the PC-computer  336  is functionally linked to the stripper  304  and/or the pulp digester  314 , such that the information regarding the quality of wood obtained by measuring at least one optical property of the wood meal  322  could be used for driving or controlling the stripper  304  and/or the pulp digester  314  for the achievement of a more optimal barking result and/or pulping process. At its simplest, the fact is that the barking purity of the stripper  304  can be controlled by a measurement of the wood meal  322  for its reflection density, since the wood meal  322  has its reflection density varying essentially as a function of the amount of bark. For the most part, the same applies to the control of a pulping process, as well. In pulping, however, it is also important to have knowledge about the number of branches and the amount of defective wood. This information is obtained by measuring the wood meal or powder  322  for its colour. The number of branches and the amount of defective wood can be assessed even more accurately by measuring the wood meal  322  for its spectrum. 
     Whatever measurements can be conducted with the measuring device  332  depends on the detector  330 , which is now examined with reference to FIGS. 4 and 5. The actual detection surface of a detector  500 , for example a video or line camera, can be constituted by a pixel matrix  400  or a pixel line. In the inventive solution, the reflection density of wood powder can also be determined without imaging optics  502 , but most preferably the camera is provided with an objective constituted by lenses for making a real image of the wood meal or powder on the pixels of the detector  500 . For the determination of the reflection density of wood meal or pieces  6 f bark or other such pieces distinguishable from sound wood it is sufficient to use a black-and-white camera, while the acquisition of colour information requires a colour camera. 
     In a preferred embodiment of the invention, a detector surface  506  is formed with an image, wherein the per se known pattern recognition methods applied in image processing are used for separating domains that are exceptional or different from sound wood in terms of the darkness, tint, or spectrum thereof. The surface area of such exceptional domains are compared with that of the entire image or the detector surface  506  for a result proportional to the amount of bark or the like. Thus, the quality of wood is determined by measuring the wood quality in terms of pixels. In the visualization of FIG. 4, the exceptional or extraordinary matter is found in three sites  404 ,  404 , and  406 . The exceptional doamin  402  covers effectively 4 pixels, the exceptional domain  404  covers a single pixel, and the exceptional doamin  406  covers two pixels. Thus, the exceptional domains have a total area of about 7 pixels. Since the matrix has a total area of 10×14=140 pixels, the exceptional domain covers a share of the total which is 7/140=0.05. In reality, the measuring area of a matrix surface can be for example 500×500 pixels. If the size of an image is e.g. 500×500 pixels, the image field has a “surface area” of 250000 pixels. The image processing methods are readily capable of separating domains with a minimum size of 1-4 pixels from an image taken of a homogeneous matter. Consequently, the method has a theoretical responsivity which in the case of a single image is better than 0.0016% (=4/250000). In addition, the responsivity can be imporved further by increasing the number of images. 
     Information regarding the spectrum of wood powder is obtained by means of spectroscopy. For example, the solution shown in FIG. 5 has been implemented by using a spectrograph, such as a.o. SPECIM, a spectrograph called ImSpector, manufactured by Spectral Imaging Ltd. The apparatus comprises a detector surface  506  constituted by a pixel matrix for visualizing an object to be imaged by means of an objective  502 . The detector surface  506  is functionally linked with an electronic circuit  508 , which produces an electrical signal consistent with optical radiation received by the detector surface  506  to be forwarded to a measuring device. Between the imaging objective  502  and the pixel matrix  506  of the camera is a prism-lattice-prism component  504  for diffusing the object-emitted optical radiation into a spectrum. The actual image of an object is a single row matrix line (e.g. the x axis of an orthogonal xy coordinate system), and the spectrum of each aligned pixel spreads out onto pixels located laterally of the matrix (e.g. the y axis of an orthogonal xy coordinate system). As wood meal or powder is progressing on a conveyor belt, it is possible to image random parts of the wood meal at random moments, whereby the wood meal can be analyzed statistically for its reflection density, colour, and/or spectrum by using an automated data processing program of a computer. 
     In a solution of the invention, it is also possible that some of the software typically housed in the measuring block  332  be allocated also to the detector  330 ,  500 . Thus, for example, the smart camera  330 ,  500  is used for selecting optical bands from the spectrum, which are transferred to the measuring device  332  for processing. 
     Although the invention has been described above with reference to the example shown in the drawings, it is obvious that the invention is not limited thereto, but it can be subjected to a multitude of modifications within the inventive concept set forth in the annexed claims.