自动化专业英语课文翻译1.1

1、Lesson 1.1 Introduction to Process Control In recent years the performance requirements for process plant have become increasingly difficult to satisfy. 近年来，对过程系统的性能改善需求变得越来越困难. Stronger competition, tougher (更加严苛的) environmental and safety regulations (法规), and rapidly changing economic conditions

2、have been key factors in the tightening of plant product quality specifications (产品质量标准). 更为剧烈的竞争，更加严格的环境和平安标准，以及快速变化的经济条件都是制约工厂产品质量标准的关键因素。 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 wi

3、th smaller surge capacities (谐振能力) between the various processing units. 更为复杂的情况是，由于现代制造业朝着规模更大,集成度更高的方向开展,而使不同的加工环节之间的协调能力更低, 所以加工过程更难控制. Such plants give the operators little opportunity to prevent upsets (扰乱) from propagating from one unit to other interconnected units. 在这种工厂中，要想让一个生产环节出现的问题不对其相连

4、的另一个生产环节产生影响,几乎是不可能的. 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. 近年来，考虑到工业制造逐渐加强的平安、高效需求，过程控制这个课题变得越来越受重视. In fact, without process control it would no

5、t be possible to operate most modern processes safely and profitably (有利的), while satisfying plant quality standards. 实际上，对于大多数现代工业，要满足平安、高效，产品质量的要求，没有控制系统是不可能的 As an introduction to process control, consider the continuous (连续) stirred-tank heater (加热搅拌器) The inlet liquid stream has a mass flow rat

6、e w and a temperature Ti. The tank contents are well agitated (搅动) and heated by an electrical heater that provides Q watts (瓦特). 输入液态流体的质量流量率为w，温度为Ti. 槽内成分搅拌均匀，并且用电热器加热，功率为Q瓦特。 It is assumed that the inlet and outlet flow rates are identical (相同的) and that the liquid density (rho) remains constant,

7、 假设输入和输出流量率是相等的，并且液体密度保持恒定 that is, the temperature variations are small enough that the temperature dependence of can be neglected. 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 Under these conditions the volume V of liquid in the tank remains constant. 在这些条件下，槽内液体的体积保持恒定 The control objective for the stirred-tank he

8、ater is to keep the exit temperature T at a constant reference value TR. 加热搅拌器的控制目标是保持输出温度T在一个恒定参考值TR上 The reference value (参考值，给定值) is referred to as a set point (设定值，给定值) in control terminology (术语). Next we consider two questions. 在控制术语中，参考值指的是给定值. 下面我们考虑两个问题 Question 1: How much heat must be sup

9、plied to the stirred-tank-heater to heat the liquid from an inlet temperature Ti to an exit temperature TR? 把加热搅拌器内的液体从输入温度Ti加热到输出温度TR，需要多少热量？ To determine the required heat input for the design operating conditions, we need to write a steady-state (稳态) energy balance (能量平衡) for the liquid in the ta

10、nk. 要确定到达设计运行条件下的热量需求，我们需要写下槽内液体的稳态能量平衡式 In writing this balance, it is assumed that the tank is perfectly mixed and that heat losses are negligible. 在写平衡式之前，假设槽内是完美搅拌的，同时忽略热损耗 Under these conditions there are no temperature gradients (梯度) within the tank contents and consequently, the exit temperat

11、ure is equal to the temperature of the liquid in the tank. 在这些条件下，槽内成分的温度保持一致，因此，输出温度等于槽内液体温度 A steady-state energy balance for the tank indicates that the heat added is equal to the change in enthalpy (焓, 表示物质内能，可以理解为衡量物质温度的物理量) between the inlet and exit stream. 根据稳态能量守恒，参加的热量等于输入和输出流体之间的焓变化量 .Thu

12、s, if T is too high we would decrease w to reduce the energy input rate in the stirred tank relative to the mass flow rate and thereby reduce the exit temperature. 因此，如果温度太高，我们将降低流量率w，使得搅拌槽内的与质量流量率相关的能量输入速率减少，因此使输出温度得以降低 Place a heat exchanger on the inlet stream. The heat exchanger is intended to r

13、educe the disturbances in Ti and consequently reduce the variation in T. This approach is sometimes called “hog-tieing (把脚或腿束缚住，这里意思与 restraint 相近，解释为约束、束缚) an input. 方法7. 在输入流安置一个热交换器. 热交换器意图减少Ti的干扰，因此可以减少温度T的扰动. 这个方法有时又叫做输入束缚法. Method 8. Use a larger tank. If a larger tank is used, fluctuations (波

14、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. 方法8. 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

15、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. 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). 要指出的是这个方法类似于

16、化学实验室中水缸的使用，水缸大的热容量可以看作散热装置，因此可以为小型实验仪器提供一个恒温环境. Next, we will classify the eight control strategies of the previous section and discuss their relative advantages and disadvantages. 接下来，我们将给这8种控制方法进行分类，同时讨论他们各自的优缺点 Methods 1 and 3 are examples of feedback control (反应控制) strategies. In feedback contr

17、ol, the process variable to be controlled is measured and the measurement is used to adjust another process variable which can be manipulated (操作，操纵). 方法1和3 是反应控制的例子. 在反应控制中，测量被控过程变量，该测量值用于调整另一个可以操做的过程变量.即测量变量，操作变量，测量变量用于调整操作变量. . It is important to make a distinction between negative feedback (负反应)

18、 and positive feedback (正反应). Negative feedback refers to the desirable situation where the corrective action taken by the controller tends to move the controlled variable toward the set point. 区分负反应和正反应很重要. 负反应是指期望到达的形势，控制器的校正作用使得被控变量趋于给定值. In contrast, when positive feedback exists, the controller

19、 tends to make things worse by forcing the controlled variable farther away from the set point. 相反地，当正反应存在时，控制器使局面变得更加糟糕，它使被控变量远离给定值 Thus, for the stirred-tank heater, if T is too high we would decrease Q (negative feedback) rather than increase Q (positive feedback). 因此，对于加热搅拌器来说，如果T太高，我们将减少输入Q负反应，

20、而不是增加输入热量Q正反应 . So far (到目前为止) we have considered only one source of process disturbance, fluctuations in Ti. We should also consider the possibility of disturbances in other process variables such as the ambient (周围的) temperature, which would affect heat losses from the tank. 到目前为止，我们仅仅考虑了Ti波动这一种干扰

21、源. 我们也应该考虑其他过程变量干扰的可能性,如会影响槽中散热量的环境温度 Recall that heat losses were assumed to be negligible earlier. 回忆一下前面我们假定热损失是可忽略的 Changes in process equipment are another possible source of disturbances. For example, the heater characteristics (特性) could change with time due to scaling (结垢) by the liquid. 过程设

22、备的变化是另一个可能的干扰源. 例如，加热器的特性会因为液体结垢而随时间变化 It is informative to examine the effects of these various types of disturbances on the feedforward and feedback control strategies discussed above. 考察这些不同类型的干扰对前馈和反应控制策略的影响是有益的 First, consider the feedforward control strategy of Method 2 where the disturbances

23、in Ti are measured and the measurements are used to adjust the manipulated variable Q. 首先，考虑方法2中的前馈控制方法,在这种方法中测量的是干扰Ti，并且测量用于调整可操作量Q From a theoretical point of view, this control scheme (方案) is capable of keeping the controlled variable T exactly at set point TR despite disturbances in Ti. 从理论上讲，尽管

24、存在干扰Ti, 这种控制方案有能力保持被控变量精确在给定值TR Ideally, if accurate measurements of Ti were available and if the adjustments in Q were made in an appropriate manner, then the corrective action taken by the heater would cancel out the effects of the disturbances before T is affected. 在理想情况下,如果对Ti的精确的测量是可得的,并且以一种适宜的

25、方法对Q进行调整，那么加热器的校正作用将在T被作用以前就抵消干扰的影响. Thus, in principle, feedforward control is capable of providing perfect control in the sense that the controlled variable would be maintained at the set point. 如此而言,从维持被控变量在给定值的意义上讲,前馈控制原那么上能够提供完美 (无差，没有误差)的控制. But how will this feedforward control strategy perfo

26、rm if disturbances occur in other process variables? 但是如果干扰源来自其他过程变量,这种前馈控制的策略如何发挥作用呢？ In particular, suppose that the flow rate w cannot be held constant but, instead, varies over time. In this situation, w would be considered a disturbance variable. 特别地,假设流量w不能维持恒定，而是随时间变化. 在这种情况下，w被看作是一个扰动变量 If w

27、 increases, then the exit temperature T will decrease unless the heater supplies more heat. 如果w增加,出口温度T将减少,除非加热器提供更多的热量 However, in the control strategy of Method 2 the heat input Q is maintained constants as long as Ti is constant. 然而,在方法2的控制策略中,只要Ti不变,热量输入值Q就维持恒定 Thus no corrective action would be

28、 taken for unmeasured flow disturbance. 因此,对没有测量的流量扰动就不会采取校正动作 In principle, we could deal with this situation by measuring both Ti and w and then adjusting Q to compensate for both of these disturbances. 原那么上说,处理这种情况,我们可以同时测量Ti和w,然后调整Q来同时补偿这两种扰动 However, as a practical matter it is generally unecon

29、omical to attempt to measure all potential disturbances. 然而,从实际出发,试图测量所有潜在的干扰一般来说是昂贵的 It would be more practical to use a combined feedforward-feedback control system, since feedback control provides corrective action for unmeasured disturbances, as discussed below. 既然反应控制可以对未知的干扰提供校正动作,采用前馈和反应组合的控制

30、策略将更加实际,正如我们下面将要讨论的那样 Consequently, in industrial applications feedforward control is normally used in combination with feedback control. 因此在工业应用中,前馈控制一般是和反应控制结合使用的 Next, we will consider how the feedback control strategy of Method 1 would perform in the presence of disturbances in Ti or w. 下面我们考虑扰动Ti或w出现的情况下，方法1的反应控