摘要
机油冷却器是一种典型的紧凑式散热器,具有传热效率高、体积小、重量轻、形状设计及安装位置自由等优点,广泛应用于汽车、工程机械等发动机中的机油冷却,其质量的好坏直接影响发动机的运行效果。而另一方面,机油冷却器所处的工作环境恶劣(发动机的工作周期长、震动剧烈、润滑油不够清洁、润滑系统回路中存在瞬态冲击力等),在交变载荷重复作用下易造成其发生疲劳破坏现象。因此,在机油冷却器的开发和生产过程中,对其进行疲劳性能测试是十分必要的。
本文采用集散型控制系统DCS机理设计和开发散热器脉冲疲劳试验台。其中,试验压力的控制精度对疲劳试验结果具有重要影响。此外,在机油冷却器的疲劳试验过程中,一个试验台要能够产生多种试验波形如三角波、正弦波、梯形波等,对多种型号的产品进行试验；试验过程中也会出现液压控制元件特性变化以及其它形式的外部扰动；这就要求该试验台具有良好的自适应性和鲁棒性。为了满足对试验压力的控制性能要求,搭建了脉冲疲劳试验系统中压力生成和控制部分的软、硬件环境,采用重复控制理论设计建立试验台脉冲压力重复控制系统,并利用Matlab/simulink对其控制性能进行仿真和分析。本论文的另一特色是采用计算流体力学CFD技术分析机油冷却器内部压力的分布情况。因为在疲劳试验系统的设计中,对于这类封闭式结构实体,通常只能获得其进、出口端试验压力。对此进行研究,除了可以预测其内部的试验压力大小,为试验系统分析设计提供依据外,还可以用于指导机油冷却器本身结构上的改进设计。
本论文的主要工作内容如下：
(1)介绍机油冷却器的功用和疲劳试验、疲劳试验系统的基本原理和重要性,概述了电液比例控制技术的结构及特点,并且总结了一些近代先进的电液控制技术。
(2)学习和研究脉冲疲劳试验台的系统设计方案,并搭建能够实现产生和控制试验压力的基本软、硬件环境,用于试验压力精度控制策略的研究。本试验台的设计方案对于其他类型疲劳试验台的搭建具有借鉴意义。
(3)学习机油冷却器内部的阻力特性、CFD技术和多孔介质模型理论。本文将机油冷却器翅片区域简化为多孔介质模型,并提出了一种确定多孔介质模型中阻力系数的方法。
(4)在Gambit中建立机油冷却器的仿真模型,运用Fluent软件对机油冷却器内部的压力场进行仿真试验,在Matlab中对试验数据进行拟合,总结出压力分布规律。
(5)针对本散热器脉冲疲劳试验台的试验要求和特性,基于重复控制理论设计控制系统,学习和研究该基本控制理论以及其在伺服控制中的应用。(6)建立本伺服控制系统中试验压力的数学模型,并且确定本试验系统的相关性能参数,建立系统的仿真模型。(7)采用状态空间法设计重复控制器,建立本试验台的重复控制系统,并在Matlab环境中运行各种仿真试验,包括对试验波形的精度跟踪性能,对梯形波、三角波、正弦波、合成波形等多种波形以及不同波形周期的自适应能力、对不确定参数和干扰的鲁棒性,分析系统试验压力的跟踪控制效果。对重复控制器设计过程中参数的确定方法提出了一些指导性结论,可作为重复控制器在其他脉冲疲劳试验台、甚至其他控制领域应用的参考。
Oil cooler is a kind of compact heat exchanger, and it has a lot of advantages, such as high heat-conducting capacity, small volume, low weight, easy to design and fix. So it has been applied widely to cool down the lubricant in automobile and construction engines. And its quality determines a lot to the performance of engines. On the other hand, the working environment of oil cooler is rather tough, such as long term work of the engine, intense vibration, contaminated lubricant, transient pulse force in lubricating system, etc. As a result, the heat exchanger tends to break down due to alternating load. It is quite necessary to carry out fatigue life-span test on oil cooler during its development and production.
A fatigue test machine for oil cooler was developed based on DCS (Distribute Control System) in this paper. Pressure accuracy in the system contributes greatly to the test results. In addition, the device is expected to be versatile, so it can fit for experiments on series of products and different test parameters. To meet this demand, the test machine must have a strong self-tuning ability. Furthermore, the test machine must also be robust to some uncertain parameters and disturbance. A repetitive control system was designed to achieve above requests, the simulation model was created in Matlab to check out its work. Another feature of this thesis is to investigate the inner pressure in heat exchanger. It's hard to get the data in common test machines, for pressure sensors can only be fixed at the inlet or outlet of closed frame test piece. Thanks to computational fluid dynamics, it can be predicted based on a mathematical model derived from simulation results. The study on the inner pressure of heat exchanger can also serve to its development, which has a great influence on heat-conducting capability and construction safety.
Main contents of this thesis are listed as below:
(1) Function of oil cooler and basic principles of fatigue life-span and fatigue test machine were introduced, and main construction and features of electro-hydraulic control system were summarized, as well as some cutting-edge electro-hydraulic servo control strategies.
(2) A group of fruits of other fatigue test machines were studied as reference to this one. In order to check the work of advanced control strategy, main structure of the test machine was built up.
(3) Inner friction property of heat exchanger, CFD technology and multi-porous media theory were learned, and then oil cooler cores were transformed and treated as multi-porous media model.
(4) Simulation model of heat exchanger was built up in Gambit, and simulation was carried out in Fluent. The pressure distribution law was derived by analyzing the results and polyfitting the data in Matlab.
(5) One particular advanced control strategy was appointed according to demands and properties of the test machine, next step was to learn its theory and applications in servo systems. Repetitive control was an ideal candidate for the test machine in this thesis.
(6) Model of electro-hydraulic system and some properties were gained, and simulation model was created in Matlab.
(7) Repetitive controller was created in state-space method, and then repetitive control system was also built up in Matlab, and then some simulation experiments were done to check the functions of the controller. Some experiences in design and analysis of repetitive were summarized, which can be referred by other applications.
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