Development And Experimental Research Of Surface Knot Detection Equipment For The Structural Timber

Visually grading structured timber is an important tool for efficient use of it Visual classification is based on grading rules for the classification of wood by visual observation of the effects of various defects in the wood.However,there are problems such as low efficiency and large subjectivity.In view of the above problems,this paper develops a device that can automatically detect the surface knots and classify the timber.The equipment based on visual grading rules uses the machine vision inspection technology to collect and analyze the image of structural timber,and classifies it.In this paper,according to the functional characteristics of the surface knots detection equipment,the overall structure of the equipment is designed and a three-dimensional model is established.The optical system model of the surface knots detection equipment for timber is discussed and studied,and the pretreatment method of the timber surface image was analyzed.This paper puts forward a simulation of the dead-spot evaluation of open-hole simulations for visual inspection of structural timber,and conducts experimental studies on the mechanical properties of structural timber of three tree species under the situation of opening holes.Finally,the stability of the performance of the equipment was verified by experiments.The following are main findings and innovations(1)The study found that the key technical difficulties of the timber surface knots detection,designed the equipment according to the working principle and the working process,established a three-dimensional solid model and optical system model,explored the relationship between the image gray and the camera-light source installation position.Besides,explored the effect of vibration on the image gray during the movement of timber.The results show that the installation position of the light source and the camera and the vibration of structural timber have an effect on the grayscale of the image.(2)Image processing methods such as grayscale transformation,median filtering,and histogram equalization were used to process the structural timber image.Several common methods for image segmentation were introduced.This paper proposes to simulate dead zone using open hole to explore the change trend of mechanical properties of open hole structural timber under pressure,and provide the evaluation rule for mechanical properties of visually graded structural timber,so as to achieve the purpose of combining mechanical stress classification and visual inspection.(3).Experimental study has been conducted timber to explore the mechanical properties under the open hole condition.In this paper,open severe dead knots were replaced by opening holes,three-point bending tests were conducted on Douglas-fir and spruce with opening holes.The results showed that openings in both spruce and Douglas fir had a greater impact on the material’s bending strength.Both of them exhibited linear elasticity features before failure.The mean values of bending strength of the open-hole spruce samples were lower than the mean value of bending strength of the spruce samples without open-hole by 5.32%,14.16% and 17.08% respectively.Similarly,the mean value of bending strength of the Douglas fir samples were 13.86%,21.97% and 23.37% lower than the samples without open-hole respectively,but showed different failure modes due to the difference in material.(4)Three-point bending test of poplar LVL open-hole specimens was performed by finite element simulation.The results showed that the damage mechanism of poplar LVL was similar to that of Douglas fir and spruce.Structural timber is subjected to a great compressive stress at the indenter and two fulcrums,and the bottom is also subjected to tensile stress.Therefore,during the test,a compressive damage zone appears first in the vicinity of the indenter.At the same time,cracking at the bottom of the specimen often occurs.(5)The experimental study on the performance and stability of the equipment was carried out,the scheme for equipment improvement and optimization was proposed,and the relationship between equipment operating speed and image quality was explored.Tests have shown that when the scan rate is greater than the motor speed,the scanned image will be stretched,otherwise,the image will be compressed.At the same time,with the number of knots as the research object,the stability of the equipment detection system was verified.The results show that the detection rate of the device to the number of knots can reach 96%,and the accuracy is high.