河科大 机械制造专业英语 3.doc

Unit 16 Open-loop and Closed-loop Control Open-loop Control Systems (Nonfeedback Systems) 开环控制系统（无反馈系统） The word automatic implies that there is a certain amount of sophistication in the control system. By automatic, it generally means that the system is usually capable of adapting to a variety of operating conditions and is able to respond to a class of inputs satisfactorily. 自动化一词意味着在控制系统中有一定数量采用先进的技术，它通常意味着系统有普遍适应的能力，对于操作环境的变化能够响应一类令人满意的输入However, not any type of control system has the automatic feature. Usually, the automatic feature is achieved by feeding the output variable back and comparing it with the command signal. When a system does not have the feedback structure, it is called an open-loop system， which is the simplest and most economical type of control system. 然而，不是任何一个控制系统都具有自动化特征。通常，控制特征是通过反馈输出变量和比较控制信号获得的。当一个系统没有反馈结构，这称为开环系统，开环系统是最简单而且最经济的控制系统类型。Unfortunately ,open-loop control systems lack accuracy and versatility and can be used in none but the simplest types of applications. 不幸的是，除了最简单的应用类型，开环控制系统，由于缺乏精确度和多功能不被应用。 Consider, for example, control of the furnace for home heating. Let us assume that the furnace is equipped only with a timing device, which controls the on and off periods of the furnace. To regulate the temperature to the proper level, the human operator must estimate the amount of time required for the furnace to stay on and then set the timer accordingly. When the preset time is up, the furnace is turned off. 考虑一下，例如，用于家庭取暖的火炉的控制。假设火炉仅配有时间装置，控制火炉周期内的开启和关闭。操作者必须估计火炉达到理想温度所需要的时间，然而设置计时器，当达到预先设计的时间时火炉关闭， However, it is quite likely that the house temperature is either above or below the desired value， owing to inaccuracy in the estimate. Without further deliberation, it is quite apparent that this type of control is inaccurate and unreliable. 房间的温度也可能高于或者低于期望值，原因就是估计不精确不从长远考虑，很明显这样控制的类型是不精确不可靠的。 One reason for the inaccuracy lies in the fact that one may not know the exact characteristics of the furnace. The other factor is t