机械设计制造及其自动化专业英语第十七单元.doc

Unit 17 Feedback and Its Effect 1 The concept of feedback plays an important role in control system. Feedback is a major requirement of a closed-loop control system. Without feedback, a control system would not be able to achieve the accuracy and reliability that are practical required in most applications. In reality, the reasons for using feedback carry far more meaning than the simple one of comparing the input with the output in order to reduce the error. The reduction of system error is merely one of the many effect that feedback may bring upon a system. We shall now show that feedback also has effects on such system performance characteristics as stability, bandwidth, overall gain, impedance, and sensitivity. 反馈的概念在控制系统中扮演了一个重要的角色。在一个闭环控制系统中反馈是一个主要条件，没有反馈，一个控制系统将不能实现在实际应用中所需要的准确性和可靠性。现实中，使用反馈承载着更重大的意义与减小输入、输出误差相比。减小系统错误，仅仅是反馈带给系统的影响之一。我们现在举几个反馈也能影响系统的性能特征：稳定性、频宽、总增益、阻抗和灵敏度。 2 To understand the effects of feedback on a control system, it is essential that we examine this phenomenon with a broad mind. When feedback is deliberately introduced for the purpose of control, its existence is easily identified. However, there are numerous situations wherein a physical system that we normally recognize as an inherently non-feedback system may turn out to have feedback when it is observed in a certain manner. In general we can state that whenever a closed sequence of cause-and-effect relation exists among the variables of a system, feedback is said to exist. This viewpoint will inevitably admit feedback in a large number of systems that ordinarily would be identified as non-feedback systems. 为了理解反馈对控制系统的影响，从广义上说我们分析这个现象也是必要的。当反馈被有意的引入作为控制的目的，它的存在是不容易确定的。然而，在许多情况下，在一个物理系统中，我们通常认为一个本质上没有反馈的系统也许原来是有反馈的，当它被观察在一个特殊的方式下。通常，我们可以指出，只要系统的变量之间存在闭合序列的因果关系，就认为反馈存在。这个观点将不可避免地承认，许多反馈系统通常被认为是无反馈的系统。 3 We shall now investigate the effects of feedback on the various aspects of system performance.Without the necessary background and mathema