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英文文献Linearized Dynamic Models

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毕业论文范文题目：英文文献Linearized Dynamic Models，论文范文关键词：英文文献Linearized Dynamic Models
英文文献Linearized Dynamic Models毕业论文范文介绍开始：
X. HONG. AND C.J. HARRIS.
Image, Speech and Intelligent Systems Group, Department of Electronics and Computer Science, University of Southampton, Southampton SO17 1BJ, UK
1.1 Introduction
In the first part of this chapter, after a general introduction, the concepts of open-loop and closed-loop control are discussed in the context of a water level control system. This example is then used to introduce fundamental considerations in control system analysis and design.
In the second part of the chapter, Laplace transforms are discussed and used to define the transfer function of a system. This is a linearized model of the dynamic behavior of the system that will serve as the basis for system analysis and design in most of this book. Block diagram reduction is used to obtain the transfer function of a system consisting of interconnected subsystems. This completes the framework necessary for Chapter 2 , in which transfer functions are derived for a variety of physical system.
1.2 Examples and Classifications of Control Systems
Control systems exist in a virtually infinite variety, both in type of application and level of sophistication. The heating system and the water heater in a house are systems in which only the sign of the difference between desired and actual temperatures is used for control. If the temperature drops below a set value, a constant heat source is switched on, to be switched off again when the temperature rises above a set maximum. Variations of such relay or on-off control systems, sometimes quite sophisticated, are very common in practice because of their relatively low cost.
In the nature of such control systems, the controlled variable will oscillate continuously between maximum and minimum limits. For many applications this control is not sufficiently smooth or accurate. In the power steering of a car, the controlled variable or system output is the angle of the front wheels. It must follow the system input ,the angle of the steering wheel, as closely as possible but at a much higher power level.
In the process industries, including refineries and chemical plants, there are many temperatures and level to be held to usually constant values in the presence of various disturbances. Of an electrical power generation plant, controlled values of voltage and frequency are outputs, but inside such a plant there are again many temperatures, level, pressures, and other variables to be controlled.
In aerospace, the control of aircraft, missiles, and satellites is an area of often very advanced systems.
One classification of control systems is the following:
⑴ Process control or regulator systems: The controlled variable, or output, must be held as close as possible to a usually constant desired value, or input, despite any disturbances.
⑵ Servomechanisms: The input varies and the output must be made to follow it as closely as possible.
Power steering is one example of the second class, equivalent to systems for positioning control surfaces on aircraft. Automated manufacturing machinery, such as numerically controlled machine tools, uses servos extensively for the control of positions or speeds.
This last example brings to mind the distinction between continuous and discrete systems. The latter are inherent in the use of digital computers for control.
The classification into linear and nonlinear control systems should also be mentioned at this point. Analysis and design are in general much simpler for the former, to which most of this book is devoted. Yet most systems become nonlinear if the variables move over wide enough ranges. The importance in practice of linear techniques relies on linearization based on the assumption that the variables stay close enough to a given operating point.
1.3 Open-Loop Control and Closed-Loop Control
To introduce the subject, it is useful to consider an example. In this example, let it be desired to maintain the actual water level c in the tank as close as possible to a desired level r. The desired level will be called the system input, and the actual level the controlled variable or system output. Water flows from the tank via a valve V0 and enters the tank from a supply via a control valve VC. The control valve is adjustable, either manually or by some type of actuator. This may be an electric motor or a hydraulic pneumatic cylinder. Very often it would be a pneumatic diaphragm actuator, in general, increasing the pneumatic pressure above the diaphragm pushes it down against a spring and increases value opening.
Open-Loop Control
In this from of control, the valve is adjusted to make output c equal to input r, but not readjusted continually to keep the two equal. Open-loop control, with certain safeguards added, is very common. For example, in the context of sequence control, that is, guiding a process through a sequence of predetermined steps. However, for systems such as the one at hand, this from of control will normally not yield high performance. A difference between input and output, a system error e= r – c would be expected to develop, due to two major effects:
Disturbances acting on the system
Parameter variations of the system
These are prime motivations for the use of feedback control. For the example, pressure variations upstream of VC and downstream of V0 can be important disturbances affecting inflow and outflow, and hence level. In a steel rolling mill, very large disturbance torques on the drive motors of the rolls when steel slabs enter or leave affect speeds.
For the water level example, a sudden or gradual change of flow resistance of the values due to foreign matter or value deposits represents a system parameter variation. In a broader context, not only are the values of the parameters of a process often not precisely known, but they may also change greatly with operating condition.
In an electrical power plant, parameter value are different at 20﹪ and 100﹪ of full power. In a valve, the relation between pressure drop and flow rate is often nonlinear, and as a result the resistance parameter of the valve changes with flow rate. Even if all parameter variations were known precisely, it would be complex, say in the case of the level example, to schedule the valve opening to follow time-varying desired levels.
Closed-Loop Control or Feedback Control
To improve performance, the operator could continuously readjust the valve based on observation of the system error e. A feedback control system in effect automates this action, as follows:
The output c is measured continuously and fed back to be compared with the input r. The error e = r – c is used to adjust the control valve by means of an actuator.
The feedback loop causes the system to take corrective action if output c ( actual level ) deviates from input r ( desired level ), whatever the reason.
A broad class of system can be represented by the block diagram shown in Fig. 1.1. The sensor in Fig. 1.1 measures the output c and, depending on type, represents it by an electrical, pneumatic, or mechanical signal. The input r is represented by a signal in the same form. The summing junction or error junction is a device that combines the input to it according to the signs associated with the arrows: e = r – c.

Fig. 1.1 System block diagram

It is important to recognize that if the control system is any good, the error e will usually be small, ideally zero. Therefore, it is quite inadequate to operate an actuator. A task of the controller is to amplify the error signal. The controller output, however, will still be at a low power level. That is, voltage or pressure have been raised but current or airflow are still small. The power amplifier raises power to the levels needed for the actuator.
The plant or process has been taken to include the valve characteristics as well as the tank. In part this is related to the identification of a disturbance d in Fig. 1.1 as an additional input to the block diagram. For the level control , d could represent supply pressure variations upstream of the control valve.
译文
线性化动态模型
1.1介绍
在文章的第一部分，在一般的介绍之后，将讨论本文中水位控制系统的开环控制和闭环控制的概念。然后用一个例子来介绍控制系统中基本的分析与设计。
在文章的第二部分，将讨论拉普拉斯变换和系统传递函数的定义。这是一个系统动态特性的线性化模型，并且在文中的大部分它都是服务于基本的系统分析与设计。由相互联系的子系统组成的系统将用方块图化简的方法来获得其传递函数。这就完成了第二章所需要的框架，在第二章中导出了各种物理系统的传递函数。
1.2实例与控制系统的分类
实际上，控制系统无论是在应用的种类还是复杂程度上都存在许许多多的形式。家用的加热系统和热水器也是一种系统，它用来控制所希望的温度与实际温度之间的误差的。如果温度下降到低于一个设定值时，将接通一个恒定的加热源，直到温度上升到设定值的上限为止。由于它们的成本相对较低，继电器或通——断控制系统在实际应用中是相当普遍的。
控制系统的本质是控制变量在最大值与最小值之间不断的变化。许多应用这种控制的地方都是不够平滑或精确的。例如，在汽车动力驾驶装置中，控制变量或系统的输出是前轮的角度。它必须尽可能地跟踪系统输入——方向盘角度，但是功率水平更高。
在工业生产过程中，包括提炼厂和化学厂，有许多温度或液位在有干扰的情况下需要被控制在恒定值上。在一个发电厂中，控制变量电压和频率是输出，但是其它变量例如温度，液位，压力等也是需要控制的。
在航空，飞行器的控制，导弹，卫星等领域中都是先进的控制系统。
控制系统的分类如下：
（1）过程控制或恒值系统：尽管存在干扰，被控变量或叫输出必须尽可能保持在一个希望的常值也就是输入上。
（2）伺服系统：输出必须尽可能地跟随输入的变化。
动力驾驶装置是第二阶段的一个例子，相当于系统对飞机表面姿势的控制。自动化加工机器，例如数字式生产工具，在位置控制或速度控制上都广泛地应用伺服机构。
这最后的例子使人想起连续控制系统与离散控制系统的区别。最新的控制系统为数字式计算机控制系统。
在这里将会提及到线性控制系统和非线性控制系统。在文中，大部分的系统的分析与设计都是比较简单的。然而，变量变化时超过范围则系统将会变成非线性的。在实践中线性技术的重点在于依赖线性化立基于假设变量到给予的操作点足够的近。
1.3开环控制和闭环控制
为了介绍这个课题，我们将会用到一个实例。在这个例子中，将让桶中的实际水位c 尽可能地接近期望的水位r 。期望的水位叫做系统的输入，而实际水位叫做控制变量或系统的输出。水流经供水处的控制阀门VC，在经过阀门V0进入桶中。控制阀门是可调节的，既可以手动又可以作为执行器。这也许是电动机或是液压气缸。通常它是一个气压隔膜执行器，一般当增加的压力超过弹簧对隔膜的推力时，阀门就会打开。
开环控制
这种形式的控制，可以通过调节阀门使输出c 与输入r 相等，但不能连续重复调节使其保持相等。开环控制是非常普遍的。例如，下文中的顺序控制，那就是引导一个来使其通过预先决定的步骤。然而，对系统来说，这种形式的控制通常不能产生好的性能。输入/输出的误差为 e = r – c ，由此可见系统误差主要由两个方面影响：
1.系统的干扰作用
2.系统的参数变化
这些是使用反馈控制的主要原因。例如，压力改变上游的VC和下游的V0，这种干扰可以严重地影响进水量，出水量及水位。在一个钢滚磨机中，当钢坯进入或离开时作用在驱动马达上的非常大的干扰力矩影响速度。
对于水位的例子，由于外部物质或阀门沉渣所引起的阀门流阻的突然或逐渐的变化代表系统的参数变化。从更广的范围来看，它不仅过程参数不精确，而且还要改变工作环境。在发电厂中，参数值在满电的20%到100%之间。压力的下降和流通率的关系经常是非线性的，而且阀门阻力参数也能改变流通比率。即使全部的参数变化都是精确的，它也将是复杂的，如水位的例子，按照预定时间阀门开到跟随时间——改变期望的水位。
闭环控制或反馈控制
为了提高性能，操作者可以根据观察系统误差来连续重复调节阀门。实际上，一个反馈控制系统使其动作自动化，如下:
输出c是输入r经过连续测量与反馈而得到的。误差e = r – c 是由调节控制阀而产生的。
无论什么原因，如果输出c偏离输入r，那么反馈环就会对系统采取正确的动作。
系统大概的种类可由方框图来表示，如图1.1。图1.1中传感器测量的输出c由电力，液压或机械信号来表示。输入r也用相同形式的信号来表示。相加点或误差点是一种根据符号或箭头连接输入的装置:e = r – c。

以上为本篇毕业论文范文英文文献Linearized Dynamic Models的介绍部分。

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