【作者】 尹大勇；

【导师】 程放；

【作者基本信息】 中国林业科学研究院 ， 材料加工工程， 2009， 硕士

【摘要】 近些年来,我国的人造板工业迅猛发展,在国民经济中也发挥着越来越重要的作用。人造板产品的质量性能与其剖面密度分布密切相关,对人造板的剖面密度分布进行精确检测,有助于改进热压工艺参数,提高人造板产品的质量,节能降耗,从而提高人造板生产企业的经济效益,促进我国木材工业的健康和可持续发展。本研究主要解决人工X射线人造板剖面密度仪研制中的几个关键问题:确定剖面密度仪使用的X射线管、确定X射线管高压源的技术指标及评定仪器的特性,同时本研究还涉及系统其他方面的设计,包括X射线探测器和步进电机的选用、X射线的防护、编程和系统测试。通过理论探讨和实验分析,得到如下结论:1、确定剖面密度仪使用的X射线管用X射线检测平均密度在0.40-0.90g/cm3之间、尺寸为50mm×50mm的刨花板试件的剖面密度分布,选用20-50keV之间的X射线可使检测的灵敏度达到最佳,以前利用放射性同位素241Am发出的60keV的γ射线进行检测,检测的灵敏度并未达到最佳;选用20-50keV之间的X射线进行检测时,刨花板含水率和树种这两个因素的变化均会对其质量衰减系数有极显著的影响。用20-30keV的X射线进行检测时,施胶量因素对刨花板的质量衰减系数有极显著的影响,当X射线能量增加到40keV时,其影响处于显著水平,50keV时,其影响不显著;2、确定X射线管高压源的技术指标X射线管高压源的输出电压有三个不稳定因素——温度漂移、时间漂移和纹波电压,其中温度漂移和时间漂移这两个不稳定因素不会影响检测结果的不确定度,只有纹波电压能影响剖面密度检测结果的不确定度;检测人造板剖面密度的X射线能量分别为20 keV、30 keV、40 keV和50 keV时,X射线管高压源的纹波系数必须分别低于1.5%、0.79%、0.60%和0.52%,才可以期望在(ρ? 0.01g /cm3 ,ρ+0.01g/cm3)的区间内包含了剖面密度ρ检测结果可能值的较大部分。3、系统设计本研究中剖面密度仪的X射线探测器选用塑料闪烁体与光电倍增管组成的闪烁探测器,步进电机选用混合式两相步进电机,外壳使用1mm厚的铅板制成;用VB6.0语言编程实现从X射线强度计数到剖面密度值的转换,程序主要由以下几部分构成:自检、启动射线管、填写试件数据并设置步进速度、扫描计数、计算剖面密度、绘制密度曲线和退出程序;X射线管的管电压和管电流处于40kV-1.0mA和50kV-1.0mA两种状态,是X射线管的两个标准工作状态;当X射线管工作在40kV-1.0mA状态时,X射线管出射射线的强度计数为1910,在确定试件边缘时,用强度计数1756作为边界值;X射线管工作在50kV-1.0mA状态时,出射X射线的计数为3415,确定试件边缘的计数边界值为3226;当设置步进电机在1s时间间隔内的步长为1421时,可以认为其步进速度为0.10mm/s;当设置步进电机在1s时间间隔的步长为712时,步进速度为0.050mm/s。4、仪器评定根据JJF1094-2002《测量仪器特性评定》国家标准评定剖面密度仪的重复性和灵敏度特性。利用基本方法、最大残差法和极差法三种方法评定剖面密度仪的重复性时,仪器检测发现试件边缘个别点的重复性相对较差,而内部大多数点的重复性较好;X射线管的管电压和管电流工作在50kV-1.0mA状态时,剖面密度仪的重复性优于其工作在40kV-1.0mA状态;当X射线管工作在50kV-1.0mA状态时,用三种方法计算得到的实验标准差的平均值分别在0.012-0.024、0.011-0.018和0.012-0.021之间。评定人工X射线人造板剖面密度仪的灵敏度发现,检测不同试件时,仪器的灵敏度由剖面密度、X射线穿透的试件长度和人造板试件的质量衰减系数三个变量共同决定;检测同一试件时,灵敏度只随剖面密度变化,且二者成负指数关系,因此仪器在密度低的位置检测灵敏度较高,而在密度高的位置检测灵敏度较低。更多还原

【Abstract】 Recent years, the wood-based panel industry of China has been developing rapidly, and playing a more and more important role in the country’s economy. The quality of wood-based panel products correlates closely to its vertical profile density. So to measure the vertical profile density of wood-based panel accurately, would be propitious to ameliorating the hot-pressing technical parameters, improving the quality of wood-based panel products, saving wood material and decreasing the energy consumption. And this would help the wood-based panel corporations to increase benefits and promot the healthy and sustainable development of China’s wood industry.In this study, some key technologies were solved in developing the X-ray profile density analyzer for wood-based panel. The key technologies were to select the X-ray tube used in the instrument, to calculate the technical parameters of the high-voltage source connected to the X-ray tube, and to evaluate the characteristics of the instrument. And in this study, designing of other parts of the system were also refered, including selection of X-ray detector and stepping motor, X-ray radiological protection, programming and test of the system. After discussing in theory and analyzing the experiment data, the main results are summarized as following:1. Selecting the X-ray tube used in the instrumentWhen X-ray among 20-50keV was used in measuring the vertical density profile of wood-based panel samples,whose density was among 0.4-0.9g/cm3 and dimension was 50mm×50mm,the best sensitivity would be got. The sensitivity of former measurement using 60keVγ-ray emitted from 241Am radioactive isotope wasn’t the best.When measuring using 20-50keV X-ray, the effects of the two factors--moisture content and wood species of the panel--on the mass attenuation coefficient were both extremely significant. When measuring using 20-30keV X-ray, the effect of resin content on the mass attenuation coefficient was extremely significant. If the X-ray energy increased to 40keV, its effect was significant, and 50keV, its effect was not significant.2. Calculating technical parameters of the high-voltage source connected to the X-ray tube There are three factors related to the stability of the voltage output from the high-voltage source connected to the X-ray tube: temperature-drift, time-drift and ripple voltage, among whom the temperature-drift and time-drift don’t impact the uncertainty of the results. And only the ripple voltage can impact the uncertainty of the results.If the X-ray energy used in measuring were 20keV, 30 keV, 40 keV, or 50 keV, the corresponding ripple voltage coefficient of the high-voltage source connected to X-ray tube must be lower than 1.5%,0.79%,0.60% and 0.52% so as to ensure most of the possible results can be included in (ρ? 0.01g /cm3 ,ρ+0.01g/cm3).3. Designing systemIn this study, scintillation detector made up of plastic scintilla and photomultiplier was selected as X-ray detector of the profile density analyzer. And hybrid two phases stepping motor was selected as its stepping motor. The shell of the instrument was made of 1mm-thick-lead.VB6.0 language was used to programme to realize transition from X-ray intensity value to vertical density profile. The programme was made up of the following parts:self-check, startup of X-ray tube, filling-in data of sample,setting stepping speed,scanning sample and counting, computing vertical density profile, plotting the density curve and exiting the programme.Tube voltage and current of X-ray tube being 40kV-1.0mA and 50kV-1.0mA are two normal working state of the tube. When X-ray tube working at 40kV-1.0mA state,the intensity value of X-ray from X-ray tube is 1910, and 1756 as intensity value is selected as boundary in determining sample’s edge. When X-ray tube working at 50kV-1.0mA state,the value of X-ray is 3415, and 3226 is selected as boundary in determining sample’s edge.When setting step length of stepping motor in one second to 1421, the stepping speed can be considered as 0.10mm/s. When setting step length of stepping motor in one second to 721, the stepping speed is 0.050mm/s.4. Evaluating the characteristics of the instrumentIn this study, the characteristics of repeatability and sensitivity of the instrument measuring the vertical density profile, were evaluated according to JJF 1094-2002 National Standard“Evaluation of the Characteristics of Measuring Instruments”. It was found after evaluating the repeatability of the X-ray profile density analyzer using basic method, largest residual error method and range method that, the repeatability of the instrument is bad when the few points near the edge of samples are measured, and the repeatability is good when instrument is measuring the points inside the samples. When the tube-voltage and tube-current of X-ray tube are working at 50kV-1.0mA state, the repeatability of the instrument is better than working at 40kV-1.0mA state. And when X-ray tube working at 50kV-1.0mA state, the average experiment standard deviations calculated in the three methods are 0.012-0.024、0.011-0.018 and 0.012-0.021.It was found after evaluating the sensitivity of the X-ray profile density analyzer that, when measuring different samplesm, the sensitivity of the instrument is related to three factors: vertical density profile, length of X-ray traverses through in the samples and mass attenuation coefficients of samples. When measuring one sample, the sensitivity is only related to the vertical density profile, and their map is negative exponent. So sensitivity of the instrument is higher when it is measuring the lower density points, and the sensitivity is lower when it is measuring the higher density points.更多还原