RADIATION MONITORING OF ROUND TIMBER ASSORTMENTS FOR RATIONAL SAWING PATTERNS

V. A. Grudinin, N. N. Soldatenko

(Presented by the scientific seminar of the Research Institute of Electronic Introscopy)

Sawing round timber into lumber is one of the important stages in wood processing. The greatest yield of high-quality lumber is the goal of the woodworking industry.

The quality of lumber depends on visible and hidden defects: knots, cracks, rot, wormholes, etc. Consequently, by taking into account the existing defects and sawing the assortments in an optimal manner, it is possible to obtain a yield of high-quality lumber. A rational sawing pattern is the orientation of the round assortment and the saw setting on the basis of the result of solving the problem of the maximum qualitative yield of lumber, taking defects into account.

Labor productivity can be increased by using devices for automatic objective monitoring and computer technology.

At present, the assessment of the parameters of round timber by diameter, length, stem curvature, taper, as well as by the location of existing visible and hidden defects that determine the yield of commercial products and their grade, is carried out subjectively by the operator. A subjective assessment of the parameters of the timber and its defects cannot serve as the basis for an optimal decision on a rational sawing pattern.

It is difficult, and at times impossible, for the operator to evaluate all parameters visually, especially in flow-line production. To solve this problem, it is necessary simultaneously to store all information about the parameters of the assortments and their defects and to solve the problem of the rational sawing pattern using electronic computing equipment. To obtain the data entered into the computer, a nondestructive monitoring device is required that would automatically provide information about the parameters of defects in the assortments.

The physical properties of round timber determine the choice of the monitoring method for an installation intended to detect defects and determine their coordinates. Wood is a material of biological origin and, as an object of monitoring, is quite distinctive. It should be noted that wood does not have definite values of bulk density inherent only in “sound” or “affected” wood. A qualitative and quantitative analysis of the densities of the wood of the principal Siberian species forming assortments shows that, in freshly felled wood, the deviation of the density values of sound wood for different species reaches 40–50%, within a single species up to 20%, and between butt, middle—

between the butt and top parts in the same stem is 3–10%. Studies have established that, in the same stem, at a distance of 0.5–2.0 meters along the length of the trunk, the average density of sections of sound portions is approximately the same. The presence of rot and knots gives certain differences between the densities of the defects and the surrounding sound wood, which makes it possible to detect them. The presence of bark, taper of the diameter, the distinctive distribution of moisture over the diameter and length of the trunk, and the wide range of thicknesses being inspected require a careful study of the influence of these factors on the detectability of the principal assortment-forming defects [1].

The choice of a method for inspecting round timber was made on the basis of an analysis of the applicability of nondestructive methods for monitoring wood quality. Mechanical methods, ultrasonic and acoustic methods, magnetic and electromagnetic methods, and X-ray and gamma methods were considered. As a result of the analysis, it became clear that methods for inspecting wood based on the use of gamma radiation have substantial advantages over the others. Their principal advantages are noncontact operation and high inspection speed. This makes it possible to use them for inspecting assortments in a technological flow [2–4].

Gamma radiation can be used in several ways. There is a method involving direct transillumination of an object by a beam of gamma radiation with registration of the radiation intensity beyond the object, and a method involving registration of backscattered radiation—“albedo.” For the conditions of detecting defects in wood and determining their coordinates, the method of direct transillumination proved acceptable, since the transverse dimensions of the beam can be made smaller than the dimensions of the smallest defect to be detected. In this case, a certain accuracy in determining the defect coordinates is achieved.

The source of gamma radiation for quality inspection is a radioactive isotope. The most acceptable are isotopes with radiation energies of 80–600 keV. These are thulium-170, cerium-144, europium-155, and cesium-137. Small dimensions are the main advantage of using isotopes.

In round timber, defects may be located in different ways, but for some of them certain regularities exist. Knots are located in the radial direction and have an angle of attachment of 30–60°. Radial cracks (shakes) are located in a plane along the log and in the radial direction; butt and internal trunk rots are located in the center of the log. Knots, shakes, and rot are among the most frequently encountered defects.

The experimental installation for quality control of round-timber samples, created at the Research Institute of Electronic Introscopy, makes it possible to detect knots, shakes, and rot of the 3rd–4th stage. The coordinates of knots and shakes in the perpendicular cross section of the log are expressed in a polar coordinate system. The origin of coordinates is the central axis of the log. The convenience of using the polar system is explained by the fact that, in rational positioning, it is necessary to rotate the assortment around the central axis by the required angle. It is not necessary to convert the values of defect coordinates from one system to another.

The narrow beam of gamma radiation in the experimental installation is directed diametrically through the center of the sample in the plane of the perpendicular cross section. The sample has translational and rotational motion. The detector registers the intensity of the radiation that has passed through the object, which depends on the thickness transilluminated and the average density. When a defect is transilluminated by the beam, the value of the average density changes; accordingly, the registered ...

radiation intensity. The information is recorded on a self-recording instrument.

Experimental studies carried out on birch specimens to verify this method of radiation monitoring showed that, in principle, it is possible to detect knots, pith flecks, and their coordinates. The sensitivity of the method depends on changes in the thickness penetrated, due to irregularities of the bark and the surface of the log, as well as on the wood species.

Conclusions

1. The most acceptable method for monitoring round timber under the conditions of the sawing-process flow line is the method using gamma radiation from radioactive isotopes.

2. The sensitivity of the monitoring method, when the radiation beam is directed diametrically through a rotating object, depends on the wood species, the type of defect, and changes in the thickness penetrated.

References

1. Yu. I. Shchetinin, L. N. Isaeva. Properties of wood as an absorber of gamma (X-ray) radiation. In: Investigation of the Properties of Wood and Wood Materials. Krasnoyarsk: Publishing House of the Institute of Forest and Wood, Siberian Branch, Academy of Sciences of the USSR, 1969.

2. A. N. Karmadonov, Yu. I. Litovchenko, V. A. Grudinin et al. On the choice of a method for monitoring wood quality. In: Investigation of the Properties of Wood and Wood Materials. Krasnoyarsk: Publishing House of the Institute of Forest and Wood, Siberian Branch, Academy of Sciences of the USSR, 1969.

3. V. E. Vasilenko et al. Detection of wood flaws and defects using radiation. Woodworking Industry, 1966, No. 2.

4. B. K. Lakatosh. Defectoscopy of Wood. Moscow: Lesnaya Promyshlennost, 1966.