1、 毕业设计（论文）外文资料翻译 系 别 ： 电气 工程 系 专业： 电气工程及其自动化 班级： 姓名： 学号： 外文出处： Specialized English For Architectural Electric Engineering and Automation 附 件： 1、外文原文； 2、外文资料翻译译文。 指导教师评语： 签字： 年 月 日 注：请将该封面与附件装订成册。 1、外文原文 Introductions to temperature control and PID controllers Process control system. Automatic process
2、 control is concerned with maintaining process variables temperatures pressures flows compositions, and the like at some desired operation value. Processes are dynamic in nature. Changes are always occurring, and if actions are not taken, the important process variables-those related to safety, prod
3、uct quality, and production rates-will not achieve design conditions. In order to fix ideas, let us consider a heat exchanger in which a process stream is heated by condensing steam. The process is sketched in Fig.1 Fig. 1 Heat exchanger The purpose of this unit is to heat the process fluid from som
4、e inlet temperature, Ti(t), up to a certain desired outlet temperature, T(t). As mentioned, the heating medium is condensing steam. The energy gained by the process fluid is equal to the heat released by the steam, provided there are no heat losses to surroundings, iii that is, the heat exchanger an
5、d piping are well insulated. In this process there are many variables that can change, causing the outlet temperature to deviate from its desired value. 21 If this happens, some action must be taken to correct for this deviation. That is, the objective is to control the outlet process temperature to
6、 maintain its desired value. One way to accomplish this objective is by first measuring the temperature T(t) , then comparing it to its desired value, and, based on this comparison, deciding what to do to correct for any deviation. The flow of steam can be used to correct for the deviation. This is,
7、 if the temperature is above its desired value, then the steam valve can be throttled back to cut the stearr flow (energy) to the heat exchanger. If the temperature is below its desired value, then the steam valve could be opened some more to increase the steam flow (energy) to the exchanger. All of
8、 these can be done manually by the operator, and since the procedure is fairly straightforward, it should present no problem. However, since in most process plants there are hundreds of variables that must be maintained at some desired value, this correction procedure would required a tremendous num
9、ber of operators. Consequently, we would like to accomplish this control automatically. That is, we want to have instnnnents that control the variables wJtbom requ)ring intervention from the operator. (si This is what we mean by automatic process control. To accomplish his objective a control system must be designed and implemented. A possible control system and its basic components are shown in Fig.2.
