FI85769C - X-ray tomographic procedure for observing faults and twigs in wood - Google Patents

Description

'85769

X-ray GENTOMOGRAPHIC METHOD FOR DETECTION OF DEFECTS AND BRANCHES ON WOOD

The present invention relates to a method for locating defects in a log tree by means of a layer imaging device based on the use of X-rays, which device comprises at least one X-ray source and the necessary X-ray detectors for generating a tomography image.

In the sawmill industry, the aim is to map the defects contained in the saw log before sawing in order to optimize the sawing result. The main defects to be located are branches, 10 cracks, burrows and larval holes. In order for a device suitable for this purpose to be usable, it should be able to detect details of about 1 cm 2, i.e. cross-sectional images must be taken every 1 cm. However, the high speed of the saw log: (1 - 3 m / s) has prevented the application of standard imaging devices 15, which can be used to photograph the saw log in three -:. dimensionally, because current tomography technology achieves *: * imaging speeds of one image per second, i.e. imaging: the speed is up to 1000 times too slow. There is a need for similar detection of wood defects in other wood processing steps 20 such as the plywood industry. Manufacturers of chemical and mechanical pulp of wood are also interested in wood from which branches have been removed.

The layer imaging1 uses an X-ray tube 25 as the radiation source, which moves around the object during the imaging. In order to be able to describe the above-mentioned saw log, this device should be able to describe 100 to 300 cross-sections per second.

- · However, the rotational speed of the X-ray tube is not sufficient: ‘large i.

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A sufficiently fast imaging time can be achieved in imaging devices in which the X-ray tube has been replaced by an electron gun and the jet from the cannon is electronically directed to a fixed anode surrounding the subject. Such devices are not yet in use and are projected to be too expensive for such applications. Another, also generally known, means is to place a number of individual X-ray tubes around the object and opposite them on the opposite side of the object with the necessary amount of radiation detectors, so that the X-ray sources and not the radiation detectors need to be moved.

It is known that the branches of a tree can be separated in the image of a conventional layer imaging device when the object has been photographed from, for example, 2k0 from different directions. Other defects that differ from their environment in terms of radiation absorption properties can also be distinguished. In this case, however, the time spent calculating the image becomes too large and the cost too high. For the device to be usable. The only way is to minimize the number of shooting directions (about 3 ~ 6). To form Figure 20 when the imaging directions are few, several algorithms are known. *: Several algorithms, including ART (algebra 1 reconstruction method) and maximum entropy methods. As the number of shooting directions: decreases, the resolution of the equipment also deteriorates and the branches or other defects of the log cannot be seen without a different method. The main reason for this is the water contained in the wood, which reduces the difference in absorption of the branches compared to other wood, making them more difficult to detect because the amount of water is also not nearly evenly distributed across the cross section of the wood.

30 Differential imaging is commonly used in medicine, for example to describe blood vessels. In this method, the blood vessels are visualized by first taking a normal X-ray of the patient. A contrast agent is then injected into the blood vessels, and a new image is taken. When the images are subtracted from each other, 35 an image of the contrast-filled blood vessels remains. However, this method is not applicable to wood description.

The object of the present invention is to obviate the above-mentioned drawback and to provide a method by means of which the branches of a tree can be located in three dimensions, even if the description of the tree is made from only a few directions.

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The object of the invention is achieved by a method which is characterized by what is stated in the claims.

The method for locating branches utilizes the internal structure of the tree 10. Studies have shown that the absorption properties of flawless and branchless wood remain practically constant in the longitudinal direction of the wood when looking at a short part of the wood, and the cross-sectional area of the wood does not change. The position of the branches of the tree in the longitudinal direction of the log can be detected on the basis of the change (increase) in radiation absorption. By measuring the absorption profiles (projections) at the branchless point of the tree, information is obtained on the density distribution of the tree in the branchless tree. By subtracting this projection from the projection measured at the knotty point, it is possible to form a difference projection 20 which contains only the absorption caused by the branches. However, direct reduction does not occur because the cross-sectional shape of the tree changes and the diameter increases at the branches. Projection U; in connection with the measurement, the shape of the season is also measured across the tree as accurately as possible. The measured absorption profiles give twice the number of measured values. the edges of the wood1 le as there are sources of radiation. The cross-sectional measurement accuracy is improved when the edge of the wood is also measured from other directions, for example with an ultrasonic sensor. The shape of the edge is now obtained by combining the edge points thus obtained with some smooth interpolation function, for example the spline function1a.

in relation to the diameters of the sections and taking into account • · the effect of the difference in absorption distances 11 85769 caused by the 35 variable cross-sections on the measurements. In connection with the application of the projection, it is advantageous to utilize the annual rings of the wood, which can be separated in the measured projections as absorption maxima, and to match the projections to be deducted accordingly.

5 In this way a separation projection is formed which contains only the absorption caused by the branches. The branches are now locally formed by a cross-sectional view of these projections, for example by one of the algorithms mentioned above. Such an examination also allows for a rapid reconstruction of the images 10 from the measurement results, since the bottom of the next image is guessed across1 from the changes in the contours of the cut-off image.

Figure 1 shows a cross-sectional drawing of a tomograph. The cut basically runs along the shooting plane. In practice, the X-ray-15 imaging directions are sequential with respect to the direction of travel of the log to allow the positioning of the detectors. In processing the image, the cross-section of the log is taken into account when forming projections from different directions from the same place on the tree, so the imaging takes place as if all the projections and edge point measurements were in the same 20 imaging planes.

Figure 1 shows X-ray sources 1, 6, symmetrically around the wood to be imaged 3 · The wood passes perpendicularly through the plane of the paper. There are, for example, 200 X-ray detectors 2 for each radiation beam of the radiation source 25 on the opposite side of the tree. The figure shows the value added to the wood surface u 11sound path i autumn sensors i · X-rays 1 y i lmai s imim define 2 boundary points from the edge surface of the log, always 2 from each shower. Utilizing these points 5 30 (12 pcs.) And the points 6 (6kp1) of the distance sensors, the exact shape of the surface of the wood can be accurately solved by the computer 7 with suitable fitting instructions 11a. Using this boundary curve, a differential image analysis is then performed on the review described in the claims to reveal defects in the tree in its 35 longitudinal three-dimensional image, which is assembled from approximately 1 sms of false cross-sectional images.

The invention is not limited to the application example shown, but can be modified within the scope of the claims. The measurement of the boundary curve 5 can be done in several different ways in addition to the ultrasonic measurement, such as optically or if the number of X-ray projections is sufficient solely by examining these interfaces from each row of X-ray detectors.

Claims (4)

6 85769 1. X-ray tomographic method for detecting defects and branches of a tree and determining their shape and location in the tree trunk before its use for industrial purposes, characterized in that fixed X-ray projection measurements 11a (1) and (2) 5 are defined by cross-sectional view of logs (3) using standard imaging techniques an accurate image of the cross-sectional interface of the tree, when the imaging has progressed and a few pieces of faultless tree can be imaged, separating images for each of the next 10 images, stretching or shrinking according to the change of its outer surface radially based on the previous image deviations appearing in the section, such as branches and holes, are thus continued along the entire length of the tree and the three sections thus formed give a three-dimensional image of the tree. Method according to Claim 1, characterized in that the interface points of the tree are determined by means of floating sound distance sensors (M) so that the interface of the tree at the section 20 can be measured as accurately as possible. A method according to claim 1, characterized in that the interface points of the wood section are measured optically by means of illuminated shadow images from different directions and these points 25 are used to determine the interface of the section as accurately as possible. A method according to claim 1, characterized in that the interface of the wood section is measured by mechanical contact from different sides and on the basis of this contact distance measurement the interface of the section is determined as accurately as possible. 7 85769