Stagnation point flow toward a stretching/shrinking sheet with a convective surface boundary condition

The steady two-dimensional stagnation point flow toward a stretching/shrinking sheet with the bottom surface of the sheet heated by convection from a hot fluid is considered. The governing partial differential equations are transformed into ordinary differential equations, before being solved numerically. Results for the skin friction coefficient and the local Nusselt number as well as the temperature profiles are presented for different values of the governing parameters. Effects of the governing parameters on the heat transfer characteristics are thoroughly examined. Different from a stretching sheet, it is found that the solutions for a shrinking sheet are non-unique.     

多功能摩擦磨损试验机结构设计

对不同固体材料在不同条件下的摩擦磨损实验要求,开发设计了一种多功能摩擦磨损试验机。实现对试验数据的在线或离线采集分析处理,对各种工况可进行选择性模拟试验。能完成往复式摩擦磨损、旋转式摩擦磨损以及滚动式摩擦磨损实验一体化设计,即在同一试验台可实现三种运动方式的试件磨损试验。应用静电感应信号对摩擦磨损状态实施监控观察,并可应用于多种形状、材料夹具摩擦磨损试验,解决了目前摩擦试验机只能试验单一摩擦类型且精度不高的缺点。     

A theory of brakes,an example of a theoretical study of wear

The bases for a theoretical study of wear by friction were given by Th. Reye. Starting from the obvious assumption that the more work done by the friction forces the more material is removed from the worn surface, he was able first to investigate theoretically the mechanics of different types of bearings. The importance of wear by friction is in some mechanical organs, particularly brakes, so great, that the ideas which Reye published 80 years ago still form today the theoretical basis for their design. The present paper is an attempt to go further in these investigations by taking into account two factors which up to now have been neglected: (1) the elastic deformation of the body subjected to wear and (2) the continuous change of its shape in consequence of the wear. The study, which is carried out in a case where the wear effect is particularly evident, the case of brakes, leads to formulas for the determination of the working features of this organ. It explains also in a very clear manner why a certain geometric position of the braking organs is necessary in order to obtain a progressive braking action and why this becomes worse as the wear goes on.     

静压桩与天津滨海新区土体滑动摩擦的试验研究

静压桩贯入地基时,桩侧面与土之间会产生滑动摩擦.采用改进直剪仪进行试验,研究了静压桩与天津滨海新区土体间的滑动摩擦及其时效性,得出了静压桩与土的外摩擦角及其变化规律,并总结了静压桩与土摩阻力的时效性机理.研究发现滑动摩阻力与土的类型、法向应力有关,且与法向应力近似呈直线关系;随着时间的增长,粘土和淤泥质土与桩之间的摩阻...     

摩擦表面形貌研究中的分形理论

摩擦表面的几何特征采用形貌参数描述,基于分形理论及ｎ维分形布朗运动,提出用分形数来描述摩擦表面的几何特征,并证明其反映了摩擦表面形貌的地序程度。     

Modeling and control analysis of electric driving system considering gear friction based on dual-inertia system

In the electric driving system, the measurement of the motor speed error becomes more and more important, which has an impact on the system vibration suppression. In this paper, based on the single-neuron adaptive PID control method, the dual-inertia system considering gear friction torque is modeled and studied. Firstly, the dual-inertia system with gear friction is established, and dynamic differential equations of it are derived; Then, the comprehensive meshing stiffness and the time-varying friction torque of the gear system are deduced; Next, the Ziegler-Nichlos frequency domain response method is adopted to obtain the parameters of the PID controller. The control methods including the PID, Fuzzy-PID with DOB and single-neuron adaptive PID are utilized to adjust the motor speed of the system; Finally, the effects of gear friction, the moment of inertia of load and control methods on motor speed and system error are analyzed.     

Efficient point-by-point engine calibration using machine learning and sequential design of experiment strategies

Modern engines are controlled by electronic control units, which operate all the engine actuators based on the signals from various sensors in the engine. Traditionally, the control parameters of the actuators are obtained through huge amount of trial-and-error experiments. However, using traditional approach to calibrate these parameters becomes more challenging with the increasing incorporation of new technologies into advanced engines. In order to reduce the number of experiments required in the calibration process of modern engines, a novel point-by-point engine calibration approach based on machine learning methods is proposed in this study. It is an iterative procedure that, for a given operating point, sequential design of experiment (DoE) strategy is utilized to measure the responses of different engine sensors corresponding to different actuator signals, and a machine learning algorithm called initial-training-free online extreme learning machine is utilized to incrementally learn the relationship between the sensors and actuators based on the measurement acquired. In each iterative cycle, meta-heuristic optimization is performed on the machine-learning-based model to search for the best parameters, which are then used as the initial parameters for generating DoE plan of the next cycle. The iteration is repeated until the optimal parameters of that operating point are found. To verify the effectiveness of the proposed approach, experiments on both simulation engine in commercial software and real engine in test bench have been conducted. The results show that the engine calibration can be carried out with significant fewer experiments and time by using the proposed approach.     

浅析并置理论视角下的机器翻译

近些年来,由于国内外信息交流量增大、交流速度增加,人们或多或少都会使用一些翻译软件以便快速地获取信息。然而无论是以词汇为主的机器翻译系统还是以句法为主的机器翻译系统或是以语义为主的机器翻译系统都或多或少存在一些不尽如人意的地方。本文从语义学的并置理论出发,利用一些翻译软件对一些搭配用法的翻译结果进行搜索,然后发现问题并提出改进方法,以期探寻出机器翻译未来改进的方向,使机器翻译的译文质量得到提高。     

Investigation of the heat transfer of a non-Newtonian fluid flow in an axisymmetric channel with porous wall using Parameterized Perturbation Method (PPM)

In the present article Parameterized Perturbation Method (PPM) is used to obtain the solutions of momentum and heat transfer equations of non-Newtonian fluid flow in an axisymmetric channel with porous wall for turbine cooling applications. The validity of the results of PPM solution were verified by comparison with numerical results obtained using a fourth order Runge–Kutta method. These comparisons reveal that Parameterized Perturbation Method is a powerful approach for solving this problem. The analytical investigation is carried out for different governing parameters namely, Reynolds number, injection Reynolds number, Prandtl number and power law index. The results show that skin friction coefficient increases with increase of Reynolds number, especially at high Reynolds numbers. Also it can be found that Nusselt number has direct relationship with Reynolds number, Prandtl number and power law index.     

基于技术生命周期的企业技术管理能力评价研究

基于技术生命周期理论,考察技术生命周期不同阶段技术任务的变化,根据技术管理系统运作过程识别技术管理能力的关键活动,并据此设计技术管理能力评价指标体系。选择支持向量机方法构建技术管理能力评价模型,并以A企业为例进行实证分析。结果表明:在不同技术生命周期阶段,技术管理的重点发生变化,技术管理能力的评价指标体系不同;技术管理能力评价的支持向量机模型能够对技术管理能力进行有效评价。     

Single cell rheometry with a microfluidic constriction: Quantitative control of friction and fluid leaks between cell and channel walls

We report how cell rheology measurements can be performed by monitoring the deformation of a cell in a microfluidic constriction, provided that friction and fluid leaks effects between the cell and the walls of the microchannels are correctly taken into account. Indeed, the mismatch between the rounded shapes of cells and the angular cross-section of standard microfluidic channels hampers efficient obstruction of the channel by an incoming cell. Moreover, friction forces between a cell and channels walls have never been characterized. Both effects impede a quantitative determination of forces experienced by cells in a constriction. Our study is based on a new microfluidic device composed of two successive constrictions, combined with optical interference microscopy measurements to characterize the contact zone between the cell and the walls of the channel. A cell squeezed in a first constriction obstructs most of the channel cross-section, which strongly limits leaks around cells. The rheological properties of the cell are subsequently probed during its entry in a second narrower constriction. The pressure force is determined from the pressure drop across the device, the cell velocity, and the width of the gutters formed between the cell and the corners of the channel. The additional friction force, which has never been analyzed for moving and constrained cells before, is found to involve both hydrodynamic lubrication and surface forces. This friction results in the existence of a threshold for moving the cells and leads to a non-linear behavior at low velocity. The friction force can nevertheless be assessed in the linear regime. Finally, an apparent viscosity of single cells can be estimated from a numerical prediction of the viscous dissipation induced by a small step in the channel. A preliminary application of our method yields an apparent loss modulus on the order of 100 Pa s for leukocytes THP-1 cells, in agreement with the literature data.     

数控机床维修与排故

在现代工业制造系统中,数控技术是关键技术,正向开放式的智能数控系统发展。因之,作为关键设备的数控机床就成日常操作和使用中的核心问题,而就此而言,维护是长期的,维修和排故是偶然的和不确定的。这样,维修和排故在日常使用中的地位和作用就愈益突出。正是基于此,笔者结合实践从维护、维修和排故三个方面作了思考和探讨,以供商榷。     

Design and implementation of a new sliding mode controller on an underactuated wheeled inverted pendulum

In this paper, a sliding mode controller (SMC) is proposed for control of a wheeled inverted pendulum (WIP) system, which consists of a pendulum and two wheels in parallel. The control objective is to use only one actuator to perform setpoint control of the wheels while balance the pendulum around the upright position, which is an unstable equilibrium. When designing the SMC for the WIP system, various uncertainties are taken into consideration, including matched uncertainties such as the joint friction, and unmatched uncertainties such as the ground friction, payload variation, or road slope. The SMC proposed is capable of handling system uncertainties and applicable to general underactuated systems with or without input coupling. For switching surface design, the selection of the switching surface coefficients is in general a sophisticated design issue because those coefficients are nonaffine in the sliding manifold. In this work, the switching surface design is transformed into a linear controller design, which is simple and systematic. By virtue of the systematic design, various linear control techniques, such as linear quadratic regulator (LQR) or linear matrix inequality (LMI), can be incorporated in the switching surface design to achieve optimality or robustness for the sliding manifold. To further improve the WIP responses, the design of reference signals is addressed. The reference position for the pendulum is adjusted according to the actual equilibrium of the pendulum, which depends on the size of the friction and slope angle of the traveling surface. A smooth reference trajectory for the setpoint of the wheel is applied to avoid abrupt jumps in the system responses, meanwhile the reaching time of the switching surface can be reduced. The effectiveness of the SMC is validated using intensive simulations and experiment testings.     

数控机床维修与排故

在现代工业制造系统中,数控技术是关键技术,正向开放式的智能数控系统发展。因之,作为关键设备的数控机床就成日常操作和使用中的核心问题,而就此而言,维护是长期的,维修和排故是偶然的和不确定的。这样,维修和排故在日常使用中的地位和作用就愈益突出.正是基于此,笔者结合实践从维护、维修和排故三个方面作了思考和探讨,以供商榷。     

中国和发展中国家贸易摩擦的特点及对策分析

近两年,发展中国家与中国的贸易摩擦有五大特点趋势:与中国发生贸易摩擦案件的数量不断增长;对中国产品反倾销诉讼的涉案金额越来越大;对中国产品实施贸易保护措施的概率太大;对中国产品征收反倾销税的税率过高;印度是与中国发生贸易摩擦最多的发展中国家。     

斜盘发动机滚动摩擦性能分析

摩擦学特性对发动机功效的发挥起着重要作用,对其进行分析和探讨有助于改善发动机工作性能。基于斜盘发动机基本结构,建立了其内部组件的球滚动摩擦模型和柱滚动摩擦模型,并推导了相应的滚动摩擦力和滚动摩擦系数公式。然后结合某斜盘发动机具体参数,分别计算了球滚动摩擦力、摩擦系数和柱滚动摩擦力、摩擦系数,通过对比,认为柱滚动摩擦力、摩擦系数远小于球滚动摩擦力、摩擦系数。最后分析了摩擦生热、高温环境对发动机摩擦性能的影响,并指出了下一步研究的方向。     

经济型数控机床加工中误差来源分析及其对策

经济型数控机床是由普通机床数控改造而成，主要目的就是提高机床的加工精度，在我国使用非常普遍。但是由于受到多方面的影响，其加工精度长期以来也成为困扰其应用的主要问题。本文以经济型数控车床CAK6150P为例，对国产经济型数控机床影响加工精度的误差进行分析，针对走行误差、工件定位误差、对刀误差提出提高的途径及补偿方法。另外，根据日常使用经验，针对控制系统误差进行分析，有利于制造商改进控制系统，提高控制系统精度。通过研究这些问题，可提高经济型数控机床的制造、使用和维护的水平。     

Design and analysis of a novel cascade control algorithm for braking-by-wire system based on electromagnetic direct-drive valves

In order to improve the response speed and control precision of the braking system with parameters uncertainty and nonlinear friction, a braking-by-wire system based on the electromagnetic direct-drive valve and a novel cascade control algorithm was proposed in this paper. An electromagnetic linear actuator directly drives the valve spool and rapidly adjusts the pressure of braking wheel cylinders. A dynamic model of electromagnetic direct-drive valve considering improved LuGre dynamic friction is established. A novel cascade control algorithm with an outside loop pressure fuzzy controller and an inside loop electromagnetic direct-drive valve position controller was proposed. An adaptive integral robust inside loop controller is designed by combining friction compensation adaptive control law, linear feedback, and integral robust control. The uncertainty parameters and the friction state are estimated online. The stability of the cascade controller is proved by the Lyapunov method. Then a multi-objective opitimizemization design method of control parameters is proposed, which combines a multi-objective game theory and a technique for order preference by similarity to ideal solution (TOPSIS) based on entropy weight. The results show that the pressurization time of cascade control is less than 0.09 s under the 15 MPa step target signal. The control precision is improved effectively by the cascade controller under the ARTEMIS condition.     

变频技术与电动机调速

目前,我国电动机总装机容量达4.5亿千瓦,电动机总装机容量占目前总用电量的70%以上,其中近70%的电机拖动的负载是风机、泵类、压缩机等,而其中又有近70%的电机适合进行调速,即有约两亿千瓦的电机处在浪费运行的状态。因此如何实现电动机节能就显得非常重要。文章根据目前应用比较广泛的交流电动机,简单地介绍它的特点,结合以往的调速方式以及各自调速方式的优劣性,分析为什么要采用变频调速,而且如何实现变频调速,以及采用变频调速以后节能的效果。     

常用皮肤消毒液的临床应用研究

目的:探讨使用常用皮肤消毒液(酒精、碘伏、安尔碘)时,蘸有消毒液的无菌棉签在皮肤上的涂擦力度、螺旋消毒方式,以及皮肤消毒面的干燥情况对皮肤消毒效果的影响,及使用常用皮肤消毒液2次涂擦消毒后,消毒处皮肤的干燥时间。方法:随机选取符合纳入标准的80名志愿者,随机分为A、B组各40名,A、B组均接受三种常用皮肤消毒液进行皮肤涂擦消毒,但涂擦力度、螺旋消毒方式、消毒面待干与否均有不同。结果:A、B组志愿者使用不同皮肤消毒液后,手背皮肤菌落数均减少,均可达到皮肤消毒效果标准要求。但只有使用碘伏和安尔碘消毒皮肤时涂擦力度不同,A、B组志愿者手背菌落数差异有统计学意义(P<0.05)。结论:三种皮肤消毒液均可达到良好的消毒效果;涂擦力度的轻重对消毒效果有影响,建议在临床实际中消毒皮肤时使用力度重的消毒手法,而螺旋消毒方式和消毒面待干与否对消毒效果无影响。