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1、<p>  畢業設計(論文)外文資料翻譯</p><p>  系別: 電氣工程系 </p><p>  專業: 電氣化與自動化 </p><p>  班級: </p><p&

2、gt;  姓名: </p><p>  學號: </p><p>  外文出處: Specialized English For Architectural </p><p> 

3、 Electric Engineering and Automation </p><p>  附 件:1、外文原文;2、外文資料翻譯譯文。</p><p>  注:請將該封面與附件裝訂成冊。</p><p><b>  外文原文</b></p>&l

4、t;p>  Introductions to temperature control</p><p>  and PID controllers</p><p>  Process control system. </p><p>  Automatic process control is concerned with maintaining process

5、 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 </p><p&

6、gt;  taken, the important process variables-those related to safety, product quality, and production rates-will not achieve design conditions. </p><p>  In order to fix ideas, let us consider a heat excha

7、nger in which a process stream is heated by condensing steam. The process is sketched in Fig.1 </p><p>  Fig. 1 Heat exchanger</p><p>  The p

8、urpose of this unit is to heat the process fluid from some inlet temperature, Ti(t), up to a certain desired outlet temperature, T(t). As mentioned, the heating medium is condensing steam.</p><p>  The energ

9、y 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 and piping are well insulated.</p><p>  In this pr

10、ocess 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

11、 control the outlet process temperature to maintain its desired value.</p><p>  One way to accomplish this objective is by first measuring the temperature T(t) , then comparing it to its desired value, and,

12、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, if the temperature is above its desired value, then the steam valve can be

13、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</p><p>  To accomplish ~his objective a control syste

14、m must be designed and implemented. A possible control system and its basic components are shown in Fig.2.</p><p>  Fig. 2 Heat exchanger control loop</p><p>  The first thing to do is to measur

15、e the outlet temperaVare of the process stream. A sensor (thermocouple, thermistors, etc) does this. This sensor is connected physically to a transmitter, which takes the output from the sensor and converts it to a signa

16、l strong enough to be transmitter to a controller. The controller then receives the signal, which is related to the temperature, and compares it with desired value. Depending on this comparison, the controller decides wh

17、at to do to maintain the t</p><p>  The preceding paragraph presents the four basic components of all control systems. They are</p><p>  (1) sensor, also often called the primary element.</p&

18、gt;<p>  (2) transmitter, also called the secondary element.</p><p>  (3) controller, the "brain" of the control system.</p><p>  (4) final control system, often a control valve

19、 but not always. Other common final control elements are variable speed pumps, conveyors, and electric motors. </p><p>  The importance of these components is that they perform the three basic operations tha

20、t must be present in every control system. These operations are </p><p>  (1) Measurement (M) : Measuring the variable to be controlled is usually done by the combination of sensor and transmitter.</p>

21、<p>  (2) Decision (D): Based on the measurement, the controller must then decide what to do to maintain the variable at its desired value.</p><p>  (3) Action (A): As a result of the controller's

22、 decision, the system must then take an action. This is usually accomplished by the final control element. </p><p>  As mentioned, these three operations, M, D, and A, must be present in every control system

23、.</p><p>  PID controllers can be stand-alone controllers (also called single loop controllers), controllers in PLCs, embedded controllers, or software in Visual Basic or C# computer programs.</p><

24、;p>  PID controllers are process controllers with the following characteristics:</p><p>  Continuous process control</p><p>  Analog input (also known as "measuremem" or "Proce

25、ss Variable" or "PV")</p><p>  Analog output (referred to simply as "output")</p><p>  Setpoint (SP)</p><p>  Proportional (P), Integral (I), and/or Derivat

26、ive (D) constants</p><p>  Examples of "continuous process control" are temperature, pressure, flow, and level control. For example, controlling the heating of a tank. For simple control, you have

27、two temperature limit sensors (one low and one high) and then switch the heater on when the low temperature limit sensor tums on and then mm the heater off when the temperature rises to the high temperature limit sensor.

28、 This is similar to most home air conditioning & heating thermostats.</p><p>  In contrast, the PID controller would receive input as the actual temperature and control a valve that regulates the flow of

29、 gas to the heater. The PID controller automatically finds the correct (constant) flow of gas to the heater that keeps the temperature steady at the setpoint. Instead of the temperature bouncing back and forth between tw

30、o points, the temperature is held steady. If the setpoint is lowered, then the PID controller automatically reduces the amount of gas flowing to the heater.</p><p>  The analog input (measurement) is called

31、the "process variable" or "PV". You want the PV to be a highly accurate indication of the process parameter you are trying to control. For example, if you want to maintain a temperature of + or -- one

32、 degree then we typically strive for at least ten times that or one-tenth of a degree. If the analog input is a 12 bit analog input and the temperature range for the sensor is 0 to 400 degrees then our "theoretical&

33、quot; accuracy is calculated to be 400 degrees di</p><p>  The analog output is often simply referred to as "output". Often this is given as 0~100 percent. In this heating example, it would mean th

34、e valve is totally closed (0%) or totally open (100%).</p><p>  The setpoint (SP) is simply--what process value do you want. In this example--what temperature do you want the process at?</p><p>

35、  The PID controller's job is to maintain the output at a level so that there is no difference (error) between the process variable (PV) and the setpoint (SP).</p><p>  In Fig. 3, the valve could be cont

36、rolling the gas going to a heater, the chilling of a cooler, the pressure in a pipe, the flow through a pipe, the level in a tank, or any other process control system. What the PID controller is looking at is the differe

37、nce (or "error") between the PV and the SP. </p><p>  SETPOINT P,I,&D</p><p><b>  CONSTANTS</b></p><p>  Difference error PID control</p

38、><p><b>  algorithm</b></p><p>  process output</p><p><b>  variable</b></p><p>  Fig .3 PIDcontrol</p><p>  It looks a

39、t the absolute error and the rate of change of error. Absolute error means--is there a big difference in the PV and SP or a little difference? Rate of change of error means--is the difference between the PV or SP getting

40、 smaller or larger as time goes on. </p><p>  When there is a "process upset", meaning, when the process variable or the setpoint quickly changes--the PID controller has to quickly change the outp

41、ut to get the process variable back equal to the setpoint. If you have a walk-in cooler with a PID controller and someone opens the door and walks in, the temperature (process variable) could rise very quickly. Therefore

42、 the PID controller has to increase the cooling (output) to compensate for this rise in temperature.</p><p>  Once the PID controller has the process variable equal to the setpoint, a good PID controller wil

43、l not vary the output. You want the output to be very steady (not changing) . If the valve (motor, or other control element) is constantly changing, instead of maintaining a constant value, this could cause more wear on

44、the control element.</p><p>  So there are these two contradictory goals. Fast response (fast change in output) when there is a "process upset", but slow response (steady output) when the PV is clo

45、se to the setpoint.</p><p>  Note that the output often goes past (over shoots) the steady-state output to get the process back to the setpoint. For example, a cooler may normally have its cooling valve open

46、 34% to maintain zero degrees (after the cooler has been closed up and the temperature settled down). If someone opens the cooler, walks in, walks around to find something, then walks back out, and then closes the cooler

47、 door--the PID controller is freaking out because the temperature may have raised 20 degrees! So it ma</p><p>  Let's think about how to design a PID controller.</p><p>  We focus on the dif

48、ference (error) between the process variable (PV) and the setpoint (SP). There are three ways we can view the error.</p><p>  The absolute error</p><p>  This means how big is the difference bet

49、ween the PV and SP. If there is a small difference between the PV and the SP--then let's make a small change in the output. If there is a large difference in the PV and SP--then let's make a large change in the o

50、utput. Absolute error is the "proportional" (P) component of the PID controller.</p><p>  The sum of errors over time</p><p>  Give us a minute and we will show why simply looking at t

51、he absolute error (proportional) only is a problem. The sum of errors over time is important and is called the "integral" (I) component of the PID controller. Every time we run the PID algorithm we add the late

52、st error to the sum of errors. In other words Sum of Errors = Error 1 q- Error2 + Error3 + Error4 + .... </p><p>  The dead time</p><p>  Dead time refers to the delay between making a change i

53、n the output and seeing the change reflected in the PV. The classical example is getting your oven at the right temperature. When you first mm on the heat, it takes a while for the oven to "heat up". This is th

54、e dead time. If you set an initial temperature, wait for the oven to reach the initial temperature, and then you determine that you set the wrong temperature--then it will take a while for the oven to reach the new tempe

55、rature setpoint</p><p>  Absolute Error/Proportional</p><p>  One of the first ideas people usually have about designing an automatic process controller is what we call "proportional".

56、 Meaning, if the difference between the PV and SP is small--then let's make a small correction to the output. If the difference between the PV and SP is large-- then let's make a larger correction to the output.

57、This idea certainly makes sense.</p><p>  We simulated a proportional only controller in Microsoft Excel. Fig.4 is the chart showing the results of the first simulation (DEADTIME = 0, proportional only):<

58、/p><p>  Proportional and Integral Controllers</p><p>  The integral portion of the PID controller accounts for the offset problem in a proportional only controller. We have another Excel spreadshe

59、et that simulates a PID controller with proportional and integral control. Here (Fig. 5) is a chart of the first simulation with proportional and integral (DEADTIME :0, proportional = 0.4).</p><p>  As you c

60、an tell, the PI controller is much better than just the P controller. However, dead time of zero (as shown in the graph) is not common.</p><p>  Fig .4 The simulation chart</p><p>  Derivative C

61、ontrol</p><p>  Derivative control takes into consideration that if you change the output, then it takes tim for that change to be reflected in the input (PV).For example, let's take heating of the oven.

62、</p><p>  Fig.5The simulation chart </p><p>  If we start turning up the gas flow, it will take time for the heat to be produced, the heat to flow around the ove

63、n, and for the temperature sensor to detect the increased heat. Derivative control sort of "holds back" the PID controller because some increase in temperature will occur without needing to increase the output

64、further. Setting the derivative constant correctly allows you to become more aggressive with the P & I constants.</p><p>  2、外文資料翻譯譯文</p><p>  溫度控制簡介和PID控制器</p><p>  過程控制系統 自動過

65、程控制系統是指將被控量為溫度、壓力、流量、成份等類型的過程變量保持在理想的運行值的系統。過程實際上是動態的。變化總是會出現,此時如果不采取相應的措施,那些與安全、產品質量和生產率有關的重要變量就不能滿足設計要求。</p><p>  為了說明問題,讓我們來看一下熱交換器。流體在這個過程中被過熱蒸汽加熱,如圖1所示。</p><p>  這一裝置的主要目的是將流體由入口溫度乃(f)加熱到某一

66、期望的出口溫度T(f)。如前所述,加熱介質是過熱蒸汽。</p><p>  只要周圍沒有熱損耗,過程流體獲得的熱量就等于蒸汽釋放的熱量,即熱交換器和管道間的隔熱性很好。</p><p>  很多變量在這個過程中會發生變化,繼而導致出口溫度偏離期望值。如果出現這種情況,就該采取一些措施來校正偏差,其目的是保持出口溫度為期望值。</p><p>  實現該目的的一種方法

67、是首先測量r(0,然后與期望值相比較,由比較結果決定如何校正偏差。蒸汽的流量可用于偏差的校正。就是說,如果溫度高于期望值,就關小蒸汽閥來減小進入換熱器的蒸汽流量;若溫度低于期望值,就開大蒸汽閥,以增加進入換熱器的蒸汽流量。所有這些操作都可由操作員手工實現,操作很簡單,不會出現什么問題。但是,由于多數過程對象都有很多變量需要保持為某一期望值,就需要許多的操作員來進行校正。因此,我們想自動完成這種控制。就是說,我們想利用無需操作人員介入就可

68、以控制變量的設備。這就是所謂自動化的過程控制。</p><p>  為達到上述目標,就需要設計并實現一個系統。圖2所示為一個可行的控制系統及其基本構件。首先要做的是測量過程流體的出口溫度,這一任務由傳感器(熱電偶、熱電阻等)完成。將傳感器連接到變送器上,由變送器將傳感器的輸出信號轉換為足夠大的信號傳送給控制器??刂破鹘邮张c溫度相關的信號并與期望值比較。根據比較的結果,控制器確定保持溫度為期望值的控制作用?;谶@一

69、結果,控制器再發一信號給執行機構來控制蒸汽流量。</p><p>  下面介紹控制系統中的4種基本元件,分別是:</p><p>  (1)傳感器,也稱為一次元件。</p><p>  (2)變送器,也稱二次元件。</p><p>  (3)調節器,控制系統的“大腦”。</p><p>  (4)執行機構,通常是一個控

70、制閥,但并不全是。其他常用的執行機構有變速泵、傳送裝置和電動機。</p><p>  這些元件的重要性在于它們執行每個控制系統中都必不可少的3個基本操作,即:</p><p>  (1)測量:被控量的測量通常由傳感器和變送器共同完成。</p><p>  (2)決策:根據測量結果,為了維持輸出為期望值,控制器必須決定如何操作。 </p>

71、<p>  (3)操作: 根據控器的處理,系統必須執行某種操作,這通常由執行機構來完成.</p><p>  如上所述,每個控制系統都有M,D和A這3種操作.</p><p>  有些系統的決策任務簡單,而有些很復雜.設計控制系統的工程師必須確保所采取確保所采取的操作能影響被控變量,也就是說,該操作要影響測量值.否則,系統是不可控的,還會帶來許多危害.</p>

72、<p>  PID控制器可以是獨立控制器(也可以叫做單回路控制器),可編程控制器(PLCs)中的控制器,嵌入式控制器或者是用Vb或C#編寫的計算機程序軟件。</p><p>  PID控制器是過程控制器,它具有如下特征:</p><p><b>  連續過程控制;</b></p><p>  模擬輸入(也被稱為“測量量”或“過程變量”

73、或“PV”);</p><p>  模擬輸出(簡稱為“輸出”);</p><p><b>  基準點(SP);</b></p><p>  比例、積分以及/或者微分常數;</p><p>  “連續過程控制”的例子有溫度、壓力、流量及水位控制。例如:控制一個容器的熱量。對于簡單的控制,你使用兩個具有溫度限定功能的傳感器(

74、一個限定低溫,一個限定高溫)。當低溫限定傳感器接通時就會打開加熱器,當溫度升高到高溫限定傳感器時就會關 加熱器。這類似于大多數家庭使用的空調及供暖系統的溫度自動調節器。</p><p>  反過來,PID控制器能夠接受像實際溫度這樣的輸入,控制閥門,這個閥門能夠控制 進入加熱器的氣體流量。PID控制器自動地找到加熱器中氣體的合適流量,這樣就保持了溫度在基準點穩定。溫度穩定了,就不會在高低兩點間上下跳動了。如果基準

75、點降低,PID控制器就會自動降低加熱器中氣體的流量。如果基準點升高,PID控制器就會自動的增加加熱器中氣體的流量。同樣地,對于高溫,晴朗的天氣(當外界溫度高于加熱器時)及陰冷,多云的天氣,PID控制器都會自動調節。</p><p>  模擬輸入(測量量)也叫做“過程變量”或“PV'’。你希望PV能夠達到你所控制過程參數的高精確度。例如,如果我們想要保持溫度為+1度或—1度,我們至少要為此努力,使其精度保持

76、在0.1度。如果是一個12位的模擬輸入,傳感器的溫度范圍是從0度到400度,我們計算的理論精確度就是4096除400度=0.097656度。我們之所以說這是理論上因為我們假定溫度傳感器,電線及模擬轉換器上沒有噪音和誤差。還有其他的假定。例如,線性等等。即使是有大量的噪音和其他問題,按理論精確度的1/10計算,1度精確度的數值應該很容易得到的。</p><p>  模擬輸出經常被簡稱為“輸出”。經常在0%到100%

77、之間給出。在這個熱量的例子中閥門完全關閉(0%),完全打開(100%)。</p><p>  基準點(SP)很簡單,即你想要什么樣的過程量。在這個例子中一你想要過程處于怎樣的溫度。</p><p>  PID控制器的任務是維持輸出在一個程度上,這樣在過程變量(PV)和基準點(SP)上就沒有偏差(誤差)。</p><p>  在圖3中,閥門用來控制進入加熱器的氣體,冷

78、卻器的制冷,水管的壓力,水管的流量,容器的水位或其他的過程控制系統。</p><p>  PID控制器所觀察的是PV和SP之間的偏差(或誤差)。它觀察絕對偏差和偏差變換率。絕對偏差就是一'PV和SP之間偏差大還是小。偏差變換率就是——PV和SP之間的偏差隨著時間的變化是越來越小還是越來越大。</p><p>  如果存在過程擾動,即過程變量或基準點變化時一-PID控制器就要迅速改變

79、輸出,這樣過程變量就返回到基準點。如果你有一個PID控制的可進入的冷凍裝置,某個人打開門進入,溫度(過程變量)將會迅速升高。因此,PID控制器不得不提高冷度(輸出)來補償這個溫度的升高。</p><p>  一旦過程變量等同于基準點,一個好的PID控制器就不會改變輸出。你所要的輸出就會穩定(不改變)。如果閥門(發動機或其他控制元件)不斷改變,而不是維持恒量,這將造成控制元件更多的磨損。</p>&l

80、t;p>  這樣就有了兩個矛盾的目標。當有“過程擾動”時能夠快速反應(快速改變輸出)。當PV接近基準點時就緩慢反應(平穩輸出)。</p><p>  我們注意到輸出量經常超過穩定狀態輸出使過程變量回到基準點。比如,一個制冷器通常打開它的制冷閥門的34%,就可以維持在零度(在制冷器關閉和溫度降低后)。如果某人打開制冷器,走進去,四處走,找東西,然后再走出來,再關上制冷器的門--------- PID控制器會非

81、?;钴S,因為溫度可能將上升20度。這樣制冷閥門就可能打開50%,75%甚至100%——目的是趕快降低制冷器的溫度——然后慢慢關閉制冷閥門到它的34%。</p><p>  讓我們思考一下如何設計一個PID控制器。</p><p>  我們主要集中在過程變量(PV)和基準點(SP)之間的偏差(誤差)上。有三種定義誤差的方式。</p><p><b>  絕對

82、偏差</b></p><p>  他說明的是PV和SP之間的偏差有多大。如果PV和SP之間偏差小——那我們就在輸出時作一個小的改變。如果PV和SP之間偏差大——那我們就在輸出時作一個大的改變。絕對偏差就是PID控制器的比例環節。</p><p><b>  累積誤差</b></p><p>  給我們點兒時間,我們將會明白為什么僅僅

83、簡單地觀察絕對偏差(比例環節)是一個問題。累積誤差是很重要的,我們把它稱為是PID控制器的積分環節。每次我們運行PID算法時,我們總會把最近的誤差添加到誤差總和中。換句話說,累積誤差二誤差1+誤差2+誤差3+誤差4+…。</p><p><b>  滯后時間</b></p><p>  滯后時間指的是PV引起的變化由發現到改變之間的延時。典型的例子就是調整你的烤爐在合

84、適的溫度。當你剛剛加熱的時候,烤爐熱起來需要一定時間。這就是滯后時間。如果你設置一個初始溫度,等待烤爐達到這個初始溫度,然后你認為你設定了錯誤的溫度,烤爐達到這個新的溫度基準點還需要一段時間。這也就被認為是PID控制器的微分環節。這就抑制了某些將來的變化因為輸出值已經發生了改變,但并不是受過程變量的影響。</p><p><b>  絕對偏差/比例環節</b></p><

85、p>  有關設計自動過程控制器,人們最初想法之一是設計比例環節。意思就是,如果PV和SP之間的偏差很小——那么我們就在輸出處作一個小的修改;如果PV和SP之間的偏差很大----那么我們就在輸出處作一個大的修改。當然這個想法是有意義的。</p><p>  我們在MicrosoftExcel僅對比例控制器進行仿真。圖4是顯示首次仿真結果的表格。(滯后時間二0,只含比例環節)</p><p&

86、gt;<b>  比例、積分控制器</b></p><p>  PID控制器中的積分環節是用來負責純比例控制器中的補償問題的。我們有另外一個Excel的擴展表格,表格上仿真的是一個具有比例積分功能的PID控制器。這里(Pig.5)是比例積分控制器最初的仿真表格(滯后時間=0,比例常數二0.4)。</p><p>  眾所周知,比例積分控制器要比僅有比例功能的比例控制器

87、好得多,但是等于0的滯后時間并不常見。</p><p><b>  微分控制</b></p><p>  微分控制器考慮的是:如果你改變輸出,那么要在輸入(PV)處反映這個改變就需要些時間。比如,讓我們拿烤爐的加熱為例。</p><p>  如果我們增大氣體的流量,那么從產生熱量,熱量分布烤爐的四周,到溫度傳感器檢測升高的溫度都將需要時間。PI

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