1.1In recent years the performance requirements for process plant have become increasingly difficult to satisfy. Stronger competition, tougher (更加严苛的) environmental and safety regulations (法规), and rapidly changing economic conditions have been key factors in the tightening of plant product quality specifications (产品质量规范).A further complication (复杂) is that modern processes have become more difficult to operate because of the trend toward larger, more highly integrated plants with smaller surge capacities (谐振能力) between the various processing units. Such plants give the operators little opportunity to prevent upsets (扰乱) from propagating from one unit to other interconnected units. In view of (考虑到，由于) the increased emphasis placed on safe, efficient plant operation, it is only natural that the subject of process control has become increasingly important in recent years. In fact, without process control it would not be possible to operate most modern processes safely and profitably (有利的), while satisfying plant quality standards.近年来，对过程系统的性能改善需求变得越来越困难.更为激烈的竞争，更加严格的环境和安全规范，以及快速变化的经济条件都是加强工厂产品质量规范的关键因素更为复杂的情况是，由于现代制造业朝着规模更大,集成度更高的方向发展,而使不同的加工环节之间的应变能力更低, 所以加工过程更难控制近年来，考虑到工业制造逐渐加强的安全、高效需求，过程控制这个课题变得越来越受重视. 实际上，对于大多数现代工业，要满足安全、高效，产品质量的要求，没有控制系统是不可能的.It is assumed that the inlet and outlet flow rates are identical (相同的) and that the liquid density ρ (rho) remains consant,that is, the temperature variations are small enough that the temperature dependence of ρ can be neglected. Under these conditions the volu me V of liquid in the tank remains constant.假设输入和输出流量是相等的，并且液体密度保持恒定，也就是说温度变化足够小，密度对温度的影响可以忽略不计. 在这些条件下，槽内液体的体积保持恒定Use a larger tank. If a larger tank is used, fluctuations (波动) in Ti will tend to be damped out (阻尼，衰减) due to the larger thermal capacitance of the tank contents. However, increased volume of tankage would be an expensive solution for an industrial plant due to the increased capital costs of the larger tank.Note that this approach is analogous to the use of water baths in chemistry laboratories where the large thermal capacitance of the bath serves as a heat sink (散热装置) and thus provides an isothermal (恒温的) environment for a small-scale research apparatus (仪器).使用一个更大的槽. 如果使用更大的槽，因为更大的热容，Ti的波动会趋向于衰减. 然而，体积增加使得开支增加，会使工厂系统的解决方案变得更加昂贵.要指出的是这个方法类似于化学实验室中水缸的使用，水缸的大热容量可以看作散热装置，因此可以为小型研究仪器提供一个恒温环境.Note that in feedforward control, the controlled variable T is not measured.在前馈控制中，被控变量T是没有被测量的.1.2The motivation of using feedback, illustrated (说明) by the examples in Section (1), is somewhat oversimplified.In these examples, the use of feedback is shown to be for the purpose of reducing the error between the reference input and the system output.However, the significance of theeffects of feedback in control systems is more complex than is demonstrated by these simple examples.The reduction of system error is merely (仅仅) one of the many important effects that feedback may have upon a system. We show in the following sections that feedback also has effects on such system performance characteristics as stability (稳定性), bandwidth (带宽), overall gain (总增益), disturbance (扰动), and sensitivity (灵敏度).第一节事例中，应用反馈的动机有些过于简单。

在这些例子中，应用反馈的目的是减小参考输入和系统输出间的误差。

然而，在控制系统中应用反馈的重要性要比这些简单例子所示的复杂得多. 减少系统误差只是反馈对系统产生的重要作用之一. 在下面的章节里，反馈还能对系统的下列运行特性产生影响：稳定性，带宽，总增益，扰动和灵敏度。

In a practical control system, G and H are functions of frequency, so the magnitude of 1+GH may be greater than 1 in one frequency range but less than 1 in another.Therefore, feedback could increase the system gain in one frequency range but decrease it in another.在实际的控制系统中，G和H都是频率的函数，因此1+GH的幅值在一种频段下可能增大系统的增益，而在另一频段下又可能减小系统的增益。

所以，反馈在一种频段下有可能会加大系统的增益，而在其它频段下减小系统的增益。

It can be demonstrated that one of the advantages of incorporating (包含) feedback is that it can stabilize an unstable system.可以证明，加入反馈的好处之一是能够使不稳定的系统稳定。

1.3An important consequence of feedback control is that it can cause oscillatory responses (振荡响应).If the oscillation has a small amplitude and damps out (衰减) quickly, then the control system performance is generally considered to be satisfactory.However, under certain circumstances the oscillations may be undamped (无衰减的) or even have an amplitude that increases with time until a physical limit is reached, such as a control valve being fully open or completely shut. In these situations, the closed-loop system is said to be unstable.反馈控制的一个重要的结果是会产生振荡响应。

如果振荡的幅值很小并且衰减很快，那么一般认为控制系统的运行状态是令人满意的。