Lesson 16 Automatic Control System,Section I Words and Phrases Section II Text and Translation Section III Exercises and Answers,词,英,汉,练,答,Next,End,Section I Words and Phases,variable 5vZEriEbln.数变数, 可变物, 变量adj.可变的, 不定的, 易变的, 数变量的 manipulate mE5nipjuleit vt.(熟练地)操作, 使用(机器等), 操纵(人或市价、市场) vt.(熟练地)操作,巧妙地处理 transmitter trAnz5mitEn.转送者,传导物, 发报机, 话筒, 发射机 tank tANkn.(盛液体, 气体的大容器)桶、箱、罐、槽, 池塘, 坦克, 战车, 柜, 储水池,Above,Topic,Next,End,Section I Words and Phases,deviation 7di:vi5eiFEnn.背离,偏离 feedback 5fi:dbAkn.无回授, 反馈, 反应 external signal自外部信号 disturbance dis5tE:bEnsn.骚动,扰动, 骚乱 stability stE5bilitin.稳定性 consequently 5kRnsIkwEntlIadv.从而, 因此 oscillation 7Csi5leiFEnn.摆动, 振动,Above,Topic,Next,End,Section II Text,An automatic control system1 is a preset closed-loop control2 system. An automatic control system has two process variables ass-ociated with it: a controlled variable and a manipulated variable. A controlled variable3 is the process variable that is maintained at a specified value or within a specified range. In the previous example （the water tank level control system）, the storage tank level is the controlled variable. A manipulated variable4 is the process variable that is acted on by the control system to maintain the controlled vari-able at the specified value or within the specified range. For the pre-vious example, the flow rate of the water supplied to the tank is the manipulated variable. Functions of Automatic Control In any automatic control system, the four basic functions that occur are: Measurement ，Comparison， Computation, Correction. In the water tank level control system in the example above, the lev-el transmitter measures the level within the tank. The level transmitt-er sends a signal representing the tank level to the level control dev-ice, where it is compared to a desired tank level. The level control device then computes how far to open the supply valve to correct,Above,Topic,Next,End,汉,Section II Text,any difference between actual and desired tank levels. Elements of Automatic Control The three functional elements needed to perform the functions of an automatic control system are: A measurement element, An err- or detection element and A final control element5. Relations between these elements and functions they perform in an automatic control system are shown in figure 16-1. The measuring element perform the measuring function by sending and evaluating the controlled var-iable. The error detection element first compares the value of the co-ntrolled variable to the desired value, and then signals an error if a deviation exists between the actual and desired values. The final control element responds to the error signal by correcting the manip-ulated variable of the System. Feedback Control An automatic controller is an error-sensitive6, selfcorrecting7 device. It takes a signal from the process and feeds it back into the process. Therefore, closed-loop control is referred to as feedback control.,Above,Topic,Next,End,汉,Section II Text Figure 16-1,Above,Topic,Next,End,汉,Section II Text,Figure 16-2 shows basic elements of a feedback control system as represented by a block diagram8. The functional relationships between these elements are easily seen. An important factor to remember is that the block diagram repre-sents flow paths of control signals, but does not represent flow of ene-rgy through the system or process.,Above,Topic,Next,End,汉,Section II Text,Below are several terms associated with the closed-loop block diagram. These elements are also called the “controller”. The feedb-ack elements are components needed to identify the functional relat-ionship between the feedback signal and the controlled output. The reference point is an external signal applied to the summing point of the control system to cause the plant to produce a specified action. This signal represents the desired value of a controlled variable and is also called the “setpoint9”. The controlled output is the quantity or condition of the plant which is controlled. This signal represents the controlled variable. The feedback signal is sent to the summing point and algebraically added to the reference input signal to obtain the actuating signal. The actuating signal represents the control action of the control loop and is equal to the algebraic sum of the reference input signal and feedback signal. This is also called the “error signal.“ The manipulated variable is the variable of the process acted upon to maintain the plant output (controlled variable) at the desired value. The disturbance is an undesirable input signal that upsets the value of the controlled output of the plant.,Above,Topic,Next,End,汉,Section II Text,Stability of Automatic Control Systems All control modes previously described can return a process va-riable to a steady value following a disturbance. This characteristic is called “stability.“ Control loops can be either stable or unstable. Instability is