Abstract:
A testing device tests a mat that is moved in one direction and that is made of biomass particles for manufacturing boards. On one side of the mat, radiation sources are positioned with a transversely spaced-apart relationship transverse to the direction of motion. On the other side of the mat, a line of detector elements is arranged beneath each of the radiation sources. A fan-shaped beam impinges on said detector elements. The beam passes either through one standard body, through the mat, or through neither the standard body nor the mat and is received by the detector elements and converted into electric output signals. The output signals are transferred via lines to an evaluating circuit that controls a device for removing mat portions that contain unwanted matter or the weight per unit area of which is too low.

Description:
BACKGROUND 
   Mats to be tested consist of fibers and/or wood chips and are actually preferably formed into chip boards, fiber boards, oriented strand boards (“OSBs”) and similar boards in continuous double band presses. Today&#39;s current, continuous double band presses are isochoric, i.e., they operate by maintaining a predetermined distance between the press plates. The double band presses are provided with steel belts that run in opposite directions relative to one another and that compress the mat to achieve a final thickness. Foreign matter and zones in the mat that cannot be compressed to this final thickness may cause bulges, cracks or even cuts to be formed in the steel belts and may even damage the roller bar or roller chain systems supporting the steel bands and the heating plates. 
   To avoid such damage, it has become known in board manufacturing to make use of metal detectors which sense magnetizable and non-magnetizable metal pieces in the mat. Upon detection of such metal pieces, the forming belt, which is divided transverse to the direction of motion, is parted in the direction of motion and the defective portion of mat is evacuated into a discharge chute. After closure of the forming belt, the manufacturing process is carried on with a flaw free mat. Magnetizable metal pieces are also removed from the mat by magnets. 
   CA 1 202 431 A teaches to dispose, on one side of a plate-shaped product, one radiation source the output beam of which is fan-shaped. The width of the beam extends transverse to the direction of motion of the product. On the other side of the product, detectors are arranged in only one row on an arc of a circle, the center of which is the radiation source. The detectors are mounted in alignment with the fan-shaped beam. A distant positioned detector receives radiation from a radiation source that has not penetrated the product. This distant positioned detector serves to automatically calibrate the known device. This device is intended for determining the weight per unit area of the product. If the density of the product is constant, the thickness may be determined from the weight per unit area. On the other hand, if the thickness of the product is constant, the density of the product may be deduced from the weight per unit area. 
   SUMMARY 
   It is the object of the invention to detect, aside from metallic pieces, other foreign matter of undesired high density in the mat and to avoid that this foreign matter is brought into the double band press connected downstream where it may lead to local concentrations of too high density. 
   The solution to this object is achieved by a method of testing a mat moved in one direction and made of biomass particles, more specifically of fibers and/or wood chips for manufacturing boards, wherein, on a first side of the mat, there is provided an array of detector elements extending transverse to the direction of motion of the mat. The detector elements receive the beam that originates from a second side of the mat located opposite the first side thereof and that has passed through the mat. Each detector element produces electrical output signals that are proportional to the received beam and that these signals are entered into an evaluating circuit. The detector elements are arranged on a plane parallel to the mat. On the second side of the mat, there is provided an array of radiation sources that extends transverse to the direction of motion. The radiation sources are positioned with a transversely spaced-apart relationship transverse to the direction of motion of the mat. The beam originating from every radiation source is shared into a fan-shaped beam, a width of each fan-shaped beam extending transverse to the direction of motion of the mat. The distal ends of neighboring fan-shaped beams are disposed in overlapping relation with one another transverse to the direction of motion of the mat. The overlapping is performed over at least one thickness of the mat. One line of detector elements of the array of detector elements is radiated by each fan-shaped beam. Neighboring fan-shaped beams and the respective one of the associated lines of detector elements are longitudinally spaced a distance from each other in the direction of motion of the mat. 
   Transmitters are particularly suitable radiation sources. The mat may be tested over the entire surface thereof in this manner. Each detector element may be provided with several detector cells. Depending on the number of detector cells having any degree of resolution, information about the density of the mat is obtained. The optimum number of radiation sources may be used in each case. The spacing between the radiation sources is preferably adjustable. The fan-shaped beams allow for a compact construction and high operational reliability. The overlaps serve on the one hand to reliably acquire data over the entire width of the mat and on the other hand to improve the evaluation of the acquired data. Through the longitudinal spacing it is made certain that the beam of each radiation source only impinges on the corresponding line of detector elements. 
   According to another embodiment of the invention, values may be calculated normal to the surface of the mat. For example, the various distances between the radiation source and the mat due to the fan-shaped path of the rays, the various beam paths through the mat and the variously radiated surfaces of the detector elements are trigonometrically compensated. Further, the various orientations of the density measurements of the mat, which are due to the fan-shaped oath of the rays, are converted on the basis of variously oriented double determinations in the overlaps of neighbouring units by means of computing models relying on the well known technique of digital laminography and tomosynthesis. 
   According to another embodiment of the invention, during testing, foreign matter such as metal pieces, lumps of glue, plastic pieces and overdense particle aggregates, is detected in the mat and corresponding electrical output signals are entered into the evaluating circuit. A device for removing a portion of the mat containing the foreign matter is controlled through the evaluating circuit. All of the foreign matter encountered in practical operation may be detected and removed together with the corresponding portion of the mat. Despite the greatest care exercised in carrying out the process, foreign matter of various kind and size are repeatedly encountered in the mats. This foreign matter includes metal pieces from the raw wood or from previous processing stages, metal and plastic parts originating from possible admixtures of waste material, solidified or cured lumps of glue from the binder applicator or overdense particle aggregates which may form at the various stages of the process. Said foreign matter form invisible overdense sites in the mat. As wood cannot be compressed beyond its bulk density of approximately 1,500 kg/m 3 , these overdense sites in the mat cannot be compressed to reach the final thickness of the finished plate set at the hot press and their density cannot be increased. In order not to damage the hot press, it is therefore of considerable advantage if all of the foreign matter can be removed from the mat. The device for removing a portion of the mat containing foreign matter may be provided, in a manner well known in the art, with a forming belt that is divided transverse to the direction of motion thereof and may be parted to temporarily form a slot. A discharge chute into which the defective portion of mat is cast is arranged downstream of the slot. Then, the slot of the forming belt is closed again and the process is resumed. 
   According to another embodiment of the invention, during testing, the weight per unit area of the mat is continuously determined in kg/m 3  for the entire surface of the mat from the output signals entered into the evaluating circuit by way of the evaluating circuit. The density data obtained also permit determining the weight per unit area of the entire surface of the mat. It is thus also possible to monitor the mat with regard to undesired variations in the weight per unit area occasioned by the scattering machine and to make corrections where needed. 
   According to another embodiment of the invention, at one longitudinal border of the mat at least an outer portion of an outer unit of an outer radiation source and of an outer line of detector elements is adjusted to a region located outside the longitudinal border of the mat. A standard body with a known weight per unit area and at least one associated outer detector element are disposed in a first part of each region located outside the longitudinal border of the mat. At least one outer detector element onto which the beam impinges directly is disposed in a second part of each region. A calibration of the associated outer unit is performed using each standard body and the output signals of the outer detector elements. At least one of the other units radiating through the mat and neighbouring every outer unit is calibrated from a respective one of the radiation sources and of the lines of the detector elements using the output signals of the outer detector elements. Accordingly, simple and reliable calibration of the units consisting of radiation source and associated detector elements is made possible. The standard body is preferably arranged on only one of the two long borders of the mat. 
   Other embodiments of the invention provide a convenient way to proceed for calibration. For example, according to one embodiment, all of the other units are calibrated successively or simultaneously using the output signals of the outer detector elements. Further, according to another embodiment: at least one first of the other units is calibrated using the output signals of the outer detector elements; a second of the other units is next calibrated using output signals of the line of detector elements of the first other unit; and all of the remaining other units are calibrated in an analogous manner. 
   It is also the object of the present invention to control the scattering machine arranged upstream. The solution to this object is achieved by the features recited in another embodiment of the resent invention. This embodiment addresses a method of testing a mat moved in one direction and made of biomass particles, more specifically of fibers and/or wood chips for manufacturing boards. On a first side of the mat, there is provided an array of detector elements extending transverse to the direction of motion of the mat. Detector elements receive the beam that originates from at least one radiation source disposed on a second side of the mat located opposite the first side thereof and that has passed through the mat. Each detector element produces electrical output signals that are proportional to the received beam and that these signals are entered into an evaluating circuit. The output signals of the detector elements are arranged in successive groups of output signals over the width of the mat. The output signals of each group, which each represent the density of a longitudinal strip of mat, are processed together in the evaluating circuit. Each thus processed group of output signals yields a controlled variable for a reaction mechanism that is associated to the corresponding longitudinal strip and takes place in a scattering machine scattering the biomass particles to form the mat. The density data which have been obtained for the entire surface of the mat may also be advantageously used to control the scattering machine arranged upstream. Each longitudinal strip may have a width ranging e.g., from 10 to 20 cm. The width of the longitudinal strips may be adjusted to the width of the reaction mechanisms in the scattering machine. Various such reaction mechanisms are known which permit to scatter the corresponding longitudinal strip of the mat so that it is more or less dense. 
   According to another embodiment of the invention, the evaluating circuit controls a device for removing a portion of mat the weight per unit area of which is too low, i.e., mat portions of too low a density may be removed. 
   Another embodiment of the invention addresses a device for testing a mat moved in one direction and made of biomass particles, more specifically of fibers and/or wood chips for manufacturing boards. On a first side of the mat, there is provided an array of detector elements extending transverse to the direction of motion of the mat. The detector elements receive the beam that originates from a second side of the mat located opposite the first side thereof and that has passed through the mat. Each detector element is adapted to produce an electrical output signal that is proportional to the received beam and that this signal is enterable into an evaluating circuit. The detector elements are arranged in a plane parallel to the mat. An array of radiation sources extending transverse to the direction of motion is provided on the second side of the mat. The radiation sources are positioned with a transversely spaced-apart relationship transverse to the direction of motion of the mat. The beam originating from every radiation source is shaped into a fan-shaped beam, a width of each fan-shaped beam extending transverse to the direction of motion of the mat. Distal ends of neighbouring fan-shaped beams are disposed in overlapping relation with one another transverse to the direction of motion of the mat. The overlay occurs over at least one thickness of the mat. One line of detector elements of the array of detector elements is radiated by each fan-shaped beam. Neighboring fan-shaped beams and the respective one of the associated lines of detector elements are longitudinally spaced a distance from each other in the direction of motion of the mat. 
   According to another embodiment of the invention, an aperture angle of each fan-shaped beam ranges from approximately 30°to 60°, and preferably amounts to 44°. The number of radiation sources and the evaluation of the electrical output signals obtained can be optimised in function of the type of mat that is to be tested. 
   According to another embodiment of the invention, all of the radiation sources are configured in the same manner and are arranged on a plane parallel to the mat. Such an arrangement provides operational and cost advantages. 
   These and further advantages and characteristics of the present invention will become apparent in the following description of an exemplary embodiment that is explained in more detail with reference to the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic flow diagram illustrating the manufacturing of the mats and boards, 
       FIG. 2  is the enlarged detail II of  FIG. 1 , 
       FIG. 3  is a side view of a testing device, 
       FIG. 4  is an enlarged sectional view taken along the line IV—IV of FIG.  3  and 
       FIG. 5  is a schematic view taken along V—V of  FIG. 3  with associated circuit array. 
   

   DETAILED DESCRIPTION 
     FIG. 1  schematically shows a line  1  for the continuous manufacturing of boards  2  made of biomass particles that are fed to a hopper  4  in the direction shown by an arrow  3 . From hopper  4 , the biomass particles, more specifically the fibers and/or wood chips, are transferred to a binder applicator  5  from where they are fed to a scattering machine  6 . 
   In a manner well known in the art, the scattering machine  6  scatters the biomass particles on a forming belt  8  being moved in a direction of motion  7 . A nonwoven sheet or mat  9  made of biomass particles is thus produced on the forming belt  8 . 
   The mat  9  is then preferably precompressed in a continuous cold press  10 . Next, the precompressed mat  11  is conveyed through a testing device  12 . On the one hand, the testing device  12  tests the mat  11  for foreign matter like metal pieces, lumps of glue, plastic pieces, overdense particle aggregates and similar pieces. On the other end, the testing device permits to additionally determine the weight per unit area of the mat  11  over the entire surface thereof if necessary. 
   In the region of a device  13 , the forming belt  8  is divided transverse to the direction of motion  7  and may be parted to form a gap  14  when the testing device  12  detects a flaw in the mat  11 . A portion  15  of mat  11 , in which the flaw was found, may thus be cast into a discharge chute  16 . As soon as this has happened, the forming belt  8  is caused to join again and the gap  14  is closed. Thereupon, the mat  11  is transferred further in the direction of motion  7  to a continuous hot press  17 . In the hot press  17 , the precompressed mat  11 , which is now rid of the flaws mentioned, is compressed to form the finished plate  2  by the application of pressure and heat. The thermoactive binder applied to the biomass particles inside the binder applicator  5  thereby cures and causes the particles to bond together and the finished board  2  to solidify. 
   Hot press  17  preferably is a conventional double band press in which the board  2  is compressed between an upper press belt  18  and a lower press belt  19 . The press belts  18 ,  19  consist of steel bands of e.g., 2.5 mm thick that extend over the entire width of the board  2 . 
   Further details of the hot press  17  are shown in FIG.  2 . The press belts  18 ,  19  abut on upper roller bars  20  and on lower roller bars  21  which in turn are supported by an upper heating plate  22  and a lower heating plate  23 . The upper heating plate  22  abuts on a press plate  24  whereas the lower heating plate  23  rests on a press table  25 . Pressing forces P are applied to the system in a manner well known in the art. 
   When the precompressed mat  11  according to  FIG. 2  contains one or several unwanted high-density pieces of foreign matter  26  that cannot be compressed beyond the bulk density of wood, which approximately amounts to 1,500 kg/m 3 , said pieces of foreign matter  26  cannot be compressed to the final thickness of the finished board  2  ( FIG. 1 ) set at the hot press  17  and the density thereof cannot be increased any further. As a matter of fact, the same applies to metallic foreign matter  26 . Besides metallic foreign matter  26 , foreign matter  26  in the form of lumps of glue loosening from the binder applicator system may get into the mat  11 . Foreign matter in the form of metal and plastic pieces resulting from admixtures of waste material are also to be found. Further possible foreign matter  26  to be encountered are high-density fiber lumps with a high share of glue that form sometimes in the scattering machine. Such pieces of foreign matter  26  may have different sizes. In MDF boards, the pieces of foreign matter  26  may have a size of 2 to 3 mm, in OSB, the foreign matter  26  may be of a much larger size and reach up to 5 cm. 
   Since known hot presses  17  are isochoric, i.e., they operate by maintaining a predetermined distance between the press plate  24  and the press table  25 , the press belts  18 ,  19  cannot avoid the foreign matter  26  and are easily damaged by the foreign matter  26 . These damages may take the form of bulges, cracks or even perforations occurring in the press belts. At the worst, even the roller bars  20 ,  21  and the heating plates  22 ,  23  may become damaged. It is therefore of particular importance and a substantial object of the present invention to ensure that no unwanted foreign matter  26  is still left in the mat  11  when said precompressed mat  11  enters the hot press  17 . 
     FIG. 3  shows details of the testing device  12 . The device  12  is provided with a frame  27  having an upper tie bar  28  and a lower tie bar  29 . 
   In  FIG. 3 , the direction of motion  7  ( FIG. 1 ) is oriented normal to the plane of the drawing. An outer unit  31  and other units  32  are mounted on the tie bars  28 ,  29  transverse to said direction of motion  7 . The outer unit  31  is provided with an outer radiation source  33  and an outer line  34  of detector elements  44 ,  44 ′,  51  ( FIG. 5 ) that extends transverse to the direction of motion  7 , said detector elements being mounted on the lower tie bar  29 . Each other unit  32  consists of a radiation source  35  on the upper tie bar  28  and of a line  36  of detector elements  51  ( FIG. 5 ) that extends transverse to the direction of motion  7 , said detector elements being in turn mounted on the lower tie bar  29 . Each detector element  44 ,  44 ′,  51  is provided with a line of e.g., 128 detector cells, i.e., pixels (not shown). The output signals may be for example periodically retrieved from the detector cells in the form of data of density values and be evaluated. 
   The beam emitted by each radiation source  33 ,  35  is formed into a fan  37  with an aperture angle  38  that ranges between 30° and 60° and preferably amounts to 44°. A width  39  of each fan-shaped beam  37  extends transverse to the direction of motion  7  of the mat  11  and is flush with the corresponding line  34 ,  36  of detector elements. According to  FIG. 3 , distal ends of neighbouring fan-shaped beams  37  are disposed in overlapping relation with one another transverse to the direction of motion  7 . This is more specifically shown for the two other units  32  on the right side of FIG.  3 . When the mat  11  is radiographed, a double information about the density of mat  11  is obtained for the triangular overlap  40  of neighbouring fan-shaped beams  37 . Said double information may be used to calculate, in a manner to be described later on, the weight per unit area of the mat  11 . It is for example advantageous when the height of the overlap  40  is at least equal to a thickness  41  of the mat  11 . As a result thereof, the information about the density may be obtained for the entire width of the mat. 
   As shown in  FIG. 3 , a left portion of the fan-shaped beam  37  of the outer unit  31  is directed past a longitudinal border  42  of mat  11  and penetrates a standard body  43 , the weight per unit area of which is known, that is located on the outer line  34  of detector elements  44 ,  44 ′,  51 . The portion of the beam, which has not penetrated through the mat  11  but through the standard body  43  only, is received by at least one outer detector element  44  (see also  FIG. 5 ) of the outer line  34  of detector elements and is converted into electrical output signals. According to  FIG. 5 , said output signals are transferred via a line  45  to an evaluating circuit  46  where they are used to calibrate the outer unit  31  and the other units  32 . An outer portion of the left fan-shaped beam  37  in  FIG. 3  is directed past the standard body  43  and is received by at least one outer detector element  44 ′ of the outer row  34  of detector elements. In  FIG. 3 , the outer detector element  44 ′ may be arranged on the left side (as shown) or on the right side of the outer detector element  44 . The portion of beam received by the outer detector element  44 ′ is converted into reference output signals that are supplied via a line  45 ′ ( FIG. 5 ) to the evaluating circuit  46  and are used to calibrate the outer unit  31 . All of the other units  32  may be aligned with the outer unit  31 . It is also possible though to first calibrate the other unit  32  which neighbours the outer unit  31  according to outer unit  31  and to then calibrate one after the other all of the remaining other units  32  accordingly. 
   According to  FIG. 4 , the units  31 ,  32  are provided in multiple rows  59 ,  60  such that neighbouring fan-shaped beams  37 ,  37  (and their associated lines  34 ,  36  of detector elements) are longitudinally spaced a distance 47 of e.g., 50 mm from each other in the direction of motion  7  of mat  11 . As a result, the fan-shaped beams of radiation emitted by the radiation sources  35  in  FIG. 3  appear to overlay in the direction of motion  7  of the mat  11 . 
   Conventional X-ray tubes are preferably used as radiation sources  33 ,  35 , said tubes acting in principle like point emitters. The fan-shaped beams  37  are formed by collimator ducts  48 , each collimator duct  48  having an upper collimator slot  49  and a lower collimator slot  50 . 
   As schematically shown in  FIG. 5 , the outer line  34  of detector elements is provided, aside from the outer detector elements  44 , with detector elements  51 . The other lines  36  of detector elements also consist of such detector elements  51 . Each detector element  51  is connected to the evaluating circuit  46  by way of a line  52 . For the sake of simplification,  FIG. 5  illustrates only some of said detector elements  51  and of the connecting lines  52  thereof.  FIG. 5  also clearly shows how the fan-shaped beam  37  of each unit  31 ,  32  is aligned with its line  34 ,  36  of detector elements. In this way, the electrical output signals of all of the detector elements  44 ,  44 ′,  51  are transferred to the evaluating circuit  46  where they are processed. The evaluating circuit  46  is connected to an input/output unit  54  with monitor  55 . 
   If the testing device  12  detects foreign matter  26  ( FIG. 2 ) in the mat  11 , the evaluating circuit  46  controls via line  56  the device  13  for removing the portion  15  of mat  11  containing the foreign matter  26 . 
   The detector elements  51  supply electrical output signals that are proportional to the density of the radiated-through mat  11 . Due to the chosen linear array of lines  34 ,  36  of detector elements, the distances from the associated radiation source  33 ,  35  to the detector elements  51  vary over the length of each line  34 ,  36 . The individual rays of each fan-shaped beam  37  further have beam paths of various lengths in the mat  11  and impinge differently onto the areas of the associated detector elements  51 . However, the effects of this varying geometric situation may be compensated by simple trigonometrical conversions, the angle between the respective one of the rays of the fan-shaped beam  37  and the normal being taken into consideration. 
   The correction calculations suffice to detect foreign matter  26  in mat  11  as only the presence and the weight of the foreign matter are of crucial interest and not the accurate localization thereof. 
   To determine the weight per unit area of mat  11 , a further data processing step needs to be carried out in the evaluating circuit  46 , though. The weight per unit area of mat  11  is to be indicated for vertically oriented portions of mat  11 . The measurements, which are oriented in different ways on account of the fan-shaped beam path, must therefore be converted to corresponding results obtained from vertical averages. The conversion is based on the double measurements from various directions in the triangular overlap  40  ( FIG. 3 ) of neighbouring units  31 ,  32 ;  32 ,  32 . Suitable computing models rely on the well known technique of digital laminography and tomosynthesis, the interested reader being referred to the essay of S. Gondrom and S. Schröpfer, FhG ITFP, Saarbrücken, Germany, entitled “Digital computed laminography and tomosynthesis—functional principles and industrial applications”, published in NDT.net—July, 1999, vol. 4, no. 7. 
   In the evaluating circuit  46 , the output signals of the detector elements  51  are preferably arranged in successive groups of output signals over the width of the mat  11 . The electrical output signals of the detector elements  51  of each group, which each represent the density of a longitudinal strip of mat  11  oriented parallel to the direction of motion  7 , are processed together in the evaluating circuit  46 . Each thus processed group of output signals yields a controlled variable that is used through a line  57  (see also  FIG. 1 ) for controlling a reaction mechanism of the scattering machine  6  that is associated to the corresponding longitudinal strip of mat  11 . 
   The priority document here, German Patent Application DE 1 01 60 398.3 filed Dec. 10, 2001, is hereby incorporated by reference.