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1、Lesson 16 Automatic Control System,Section I Words and PhrasesSection II Text and TranslationSection 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 mE5nipjuleitvt.(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
2、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 5kRn
3、sIkwEntlIadv.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 vari
4、able3is 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 t
5、o 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: M
6、easurement ，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 t
7、ank 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 cont
8、rol 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 co
9、ntrolled 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 vari
10、able 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 TextFigure 16-1,Abo
11、ve,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 signa
12、ls, 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 functi
13、onal 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 calle
14、d the “setpoint9”. Thecontrolled outputis the quantity or condition of the plant which is controlled. This signal represents the controlled variable. Thefeedback signal is sent to the summing point and algebraically added to the reference input signal to obtain the actuating signal. Theactuating sig
15、nalrepresents 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
16、 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
17、a disturbance. This characteristic is called stability. Control loops can be either stable or unstable. Instability is caus-ed by a combination of process time lags discussed earlier and inhe-rent time lags within a control system. This results in slow response to changes in the controlled variable.
18、 Consequently, the controlled variable will continuously cycle around the setpoint value. Oscillation describes this characteristic. There are three types of oscillations th-at can occur in a control loop. They are decreasing amplitude10, con-stant amplitude11, and increasing amplitude12. Each is sh
19、own in Fig-ure 16-3. Decreasing amplitude (Figure 16-3A). These oscillations decrease in amplitude and eventually stop with a control system that opposes the change in the controlled variable. This is the condition desired in an automatic control system. Constant amplitude (Figure 16-3B). Action of the controller sustains oscillations of the controlled variable. The controlled variable will never reach a stable condition; therefore, this condition is not desired. Increasing amplitude (Figure 16-3C).,
