人体振动特性研究

本文实验测试了人体对振动的动态响应。１５位受试者以坐姿承受了０～１００Ｈｚ垂直向随机振动,振动强度为０．５,１．０,１．５ｇ（ｒｍｓ）;测得了头、肩、骼部相对于振动台的响应。结果表明,人体各个部位的频率响应特征是相近的,但是具体的共振频率在个体之间稍有差异,并且观察到共振频率有随着振动刺激强度的增加而降低的现象。人体响应的非线性是明显的。利用最小二乘法以单自由度和双自由度模型拟合人体的响应特征曲线,表明头部和骼部用双自由度模型,肩部用单自由度模型更为合适。这些实验是进一步研究人体振动防护的基础性工作。     

西藏林芝云杉针叶净光合速率对环境因子的响应

使用便携式光合系统仪Li 6200测定了西藏林芝云杉净光合速率的日变化及其对环境因子如量子辐射、相对湿度和CO2浓度的响应。研究结果表明:林芝云杉的净光合速率在11:00点左右达到最大值,然后逐渐下降,到19:00点左右净光合速率降到0以下,转入暗呼吸。林芝云杉树冠上层的针叶由于受光充分,净光合速率较下层针叶大;阴生叶与阳生叶的净光合速率,上层差别不大,下层以阴生叶为大。在气温20℃左右、相对湿度70%±5%、CO2浓度为305mg·kg-1条件下,林芝云杉的光补偿点约为107 5μmol·m-2·s-1,光饱和点约为770μmol·m-2·s-1;在光强800μmol·m-2·s-1、气温20℃左右、相对湿度70%±5%条件下,林芝云杉的CO2补偿点约为157 2mg·kg-1,CO2饱和点约为3811 33mg·kg-1。     

The Spring Change in Hydraulic Architecture Characteristics in Some Woody Plants

Ten healthy tree species with regular management were selected on the campus of Beijing Forestry University, and they belong to tall tree, shrub and liana, respectively. Water potential and hydraulic architecture parameters of one-year-old twigs were measured in sunny day in the last ten days of March to the middle ten days of May in 2002. The results show that the daily change in water potential of tree species examined appears convex, i.e. the water potential is higher in the morning and evening, and lower in the midday. The change trend of water potential is consistent among different months. The seasonal change trend of water potential appears lower in March than that in April and May. There is a similar relationship between the daily change trend of water potential and special conductivity in spring, i.e. the higher the water potential, the higher the special conductivity, but this trend of change is not entirely synchronic. The seasonal change of special conductivity of conifer species is not obvi     

Evaluating root growth potential of tree seedlings with an automated measuring system

Root growth of tree seedlings was quantified as change in root area index by using a video camera and digitizing area meter system that sums scanning line and root intersections. Number and total length of roots 0.5 cm and area index change were measured over a wide range of values. Linear regressions indicated that the automated and manual quantification methods yield essentially the same result. The automated method reduces measurement time, minimizes observer subjectivity, and estimates all new root growth. However, the equipment is costly, and the method does not provide information on the origin or size of new roots.     

Outdoor exposure tests of structural laminated veneer lumber (II): evaluation of the strength properties after nine years

To investigate the durability of structural laminated veneer lumber (LVL), outdoor exposure tests have been conducted since 1990 at a field-testing site at the Forestry and Forest Products Research Institute. This article is the second interim report on the results after 9 years of exposure. Seven kinds of structural LVL with no preservative treatment were subjected to the tests. Almost all the exposed specimens were decayed by a kind of brown rot fungi (Pseudomerulius aureus (Fr.) Julich). The degree of decay varied with wood species; grand fir and western hemlock LVL in particular showed weak resistance against the decay. All the specimens were stored for more than 1 year in a testing room conditioned at 20°C and 65% relative humidity. We then measured the ultrasonic velocity of the specimens by the Pundit method, penetration depth by the Pilodyn method, and bending strength by a conventional bending test. Correlation between nondestructive measurement factors and the density was strong even on LVL with many adhesive layers. The nondestructive testing method was found to be applicable to LVL as well as solid lumber. After the nondestructive measurements, each LVL was cut into three types of specimen (top: T, middle: M, and bottom: B) for the bending tests. The bending strength varied with the type of specimens. Correlation between modulus of elasticity and modulus of rupture was strong even in the decayed specimens.     

Research on Seismic Behavior of Top-bear Arch Bridge with Viscoelastic Damping Braces

This paper proposes a new method of structure control that to set damping braces in a top - bear arch bridge. Lumped damping can be introduced to structures to reduce seismic response. The FEM analysis of Nimu bridge which is braced with viscoelastic dampers had done. The results show that the viscoelastic damping braces can remarkably decrease displacement within the arc plane, which is an efficient structure control method.     

Robust Control System for the Optical Extra-precise Vibration Isolation System

The robust control theory is used to solve the uncertainty of the optical extra-precise vibration isolation system. The wavelet theory is used to analyze the random vibration signal in the time-frequency domain and get the low frequency signal from the random vibration signal. The robust controller designed by the robust control theory is used to restrain the low frequency vibration. The active vibration isolation exploits the theory of robust control, which overcomes the uncertainty of model and the disturbance, and makes the control system to restrain the uncertainty of the model and the disturbance of the vibration. The simulation results prove that this method has shown good robustness and control precision in the vibration isolation of the optical extra-precise instrument and have good performance on restraining the low frequency vibration.     

Analytic Method for the Free Vibration of Plane Frame Structure

The analytic method for the free vibration of plane frame structure is presented. It is achieved by transforming the general eigenvalue problem of natural frequency and vibration mode of continuously distributed property system into typical boundary value problem of ordinary differential equation (ODE). A series of ODE, corresponding to the general eigenvalue problem, is formed, and then is solved by an general ordinary differential equation solver-COLSYS. Each component of a frame structure is regarded as an element in the analytic method. This makes it more efficient than that of finite element method which needs to increase elements to get the value of high-class natural frequency and vibration mode accurately. Some applications of the method show that it can solve the free bending vibration of plane-frame structure with various types of displacement constraint. The result indicates that this method is precise and efficient.     

Applications of SOM Neural Network in Multiple-faults Diagnosis of Turbogenerator Set

The turbogenerator vibration faults have the character of variety. Many faults often occur synchronously. The traditional BP neural network can diagnose the single fault effectively. If we diagnose the multiple faults by using the BP neural network, we must train all samples of multiple faults, which is will increase the number of training samples and the burden of learning greatly. So the diagnosis can not be performed easily. This paper introduces a method based on SOM neural network, which is studied by using the single sample and diagnosing the multiple faults according to the position of output nerve cell. By analyzing the examples, the method is proved to be available for diagnosing the multiple faults of Turbogenerator set.     

土壤塑性变形的混沌振动压路机动力学仿真研究

基于土壤变形的一种塑性模式,提出了混沌振动压路机压实过程的动力学模型。借助计算机技术对其过程进行了数值仿真,探讨了不同激振频率和不同路面条件下的振动轮运动规律,并进行了混沌特征的识别。仿真结果对混沌振动压路机的设计有参考价值。