基于單片機的閥門定位器設(shè)計與開發(fā),基于,單片機,閥門,定位器,設(shè)計,開發(fā) 西北工業(yè)大學(xué)明德學(xué)院本科畢業(yè)設(shè)計論文 英 文 翻 譯 系 別 自動化系 專 業(yè) 自動化 班 級 163003 學(xué)生姓名 張瑞 學(xué) 號 103654 指導(dǎo)教師 何振琦 PID與開關(guān)控制,為什么要使用閥門定位器? George Ballard Foxboro Company Portland, OR 關(guān)于過程控制回路中元素的討論,特別對于木材干燥窯，必須深入探究這些問題：PID和開關(guān)控制,閥特性和使用閥門定位器,以及各種用于木材行業(yè)類型的溫度測量元素。 在某個預(yù)定值，一個控制回路的作用是維持一個過程的輸出, 例如,某些類型木材的指定含水率。一個控制類型回路采用的是歷史木材干燥反饋回路,理想情況下,如果可以,我們將測量含水率的木材產(chǎn)品和飼料,反饋回一個控制裝置。該設(shè)備將比較測量水分和所需的水分,產(chǎn)生一個控制器輸出值到最后運營商——閥。閥門將反過來調(diào)節(jié)能量流的輸入，從而使得窯實測含水率到所需的濕度水平。 因此,這四個組件的循環(huán)因素為：測量,自動控制器,最后的算子,最后這個過程本身。 測量 我們最感興趣的是在木材干燥問題中控制水分含量,該變量幾乎從不直接測量和反饋到自動控制器，因此我們測量空氣溫度或微分溫度、或濕和干球溫度和這些變量成為控制變量。在某些情況下,含水率是通過推理計算來控制。 早期的自動干燥窯控制器是在干窯中采用填充熱系統(tǒng)測量和控制濕、干球溫度(空氣)的方法。這樣的溫度測量系統(tǒng)目前仍在廣泛使用。然而目前應(yīng)用最多的，是熱電偶和電阻溫度探測器(RTD),部分由于其固有的電子特性,而不是純粹的機械性質(zhì)熱系統(tǒng)了。所有三種類型的測量元素覆蓋廣泛的溫度，它當(dāng)然可以為任何領(lǐng)域提供可用于干燥窯控制的測量。比較表明，每一種方法都有其優(yōu)點和缺點,然而,隨著這種比較顯示的實際限制,精度和響應(yīng)數(shù)據(jù)可能會因供應(yīng)商的不同而有所不同。 控制模式 接下來將我們的注意力轉(zhuǎn)移到第二個元素的控制回路,即自動控制器。我們會記得在不損壞木材的條件下控制系統(tǒng)的終極目標(biāo)是在可能的最短的時間內(nèi)對干木材一些終點的測量。Shinskey的過程模型是基于干燥的原則，隨著空氣經(jīng)過物料被干從而相應(yīng)地減少對干燥空氣溫度蒸發(fā)的水分。當(dāng)然我們也接受wetbulb抑郁會創(chuàng)建一個驅(qū)動力從而對對水分傳輸?？諝飧稍锏臏囟群蜐袂虻臏囟?因為可以并且容易測量,因此作為一直的控制變量。而被操縱的變量是蒸汽流量,加載過程包括木材和空氣以及和它們相關(guān)的水分。其次,回顧早期窯控制、開關(guān)控制器On-Offor雙位閥被雇用的蒸汽,噴霧,和阻尼運營商。開關(guān)控制是最簡單的形式(或模式)的反饋控制,其他基本模式是比例、積分和導(dǎo)數(shù)(或PID)。 注意,在這兩種情況下,閥總是要么完全開啟要么完全關(guān)閉,測量周期一直保持不變。唯一的區(qū)別在于操作這兩個控制器是在不同頻率和振幅的測量周期下。它的主要缺點是開關(guān)控制正常狀態(tài)是恒定的循環(huán)。它的主要優(yōu)點是低成本, 通常這類控制器和閥門是便宜的。事實上，開關(guān)控制不需要控制器、繼電器或其他類似設(shè)備作為控制器功能的創(chuàng)建聯(lián)系人。是否可以接受開關(guān)控制取決于效果的循環(huán)在測量變量對產(chǎn)品和其他生產(chǎn)裝置的影響。不斷開放關(guān)閉循環(huán)的蒸汽閥門,無論他們是具有破壞性的鍋爐房還是蒸汽頭,都不會損害閥門本身。不斷循環(huán)的測量變量,即濕和干球溫度,如果足夠大,毫無疑問將有助于最終產(chǎn)品上的質(zhì)量差。早期開關(guān)窯控制器在驗收時可能發(fā)現(xiàn)由于我們只是做好一個工作的干燥，大多數(shù)人都會擁有一個小的數(shù)量的死區(qū)。 開關(guān)控制應(yīng)該只適用于以下三個條件存在,即: 1、精確的控制是不需要的,因為測量將不斷循環(huán)。 2、在這個過程中停歇時間必須足夠長，以防止閥門的過度磨損和損壞其他過程單元。 3、死區(qū)時間的比例時間常數(shù)過程必須小,以避免一個太大振幅的測量周期。 經(jīng)驗表明,木材干燥在這些條件下不表現(xiàn)出在所有時間和一定程度的比例作用是需要良好的溫度控制和閥門操作。比例控制是基于控制器響應(yīng)的大小原則在控制點和測量值的差異來防止閥和測量循環(huán)。它主要改進(jìn)開關(guān)控制,因為它有能力穩(wěn)定回路。它的主要缺點是會不可避免的偏移,但在窯相當(dāng)恒定載荷時和所需數(shù)量的比例控制很小時,抵消不是問題?？刂泣c是可以調(diào)整的，直到在期望價值和其后的設(shè)置點的測量是一個簡單的參考點的比例作用。除了數(shù)字控制，積分和導(dǎo)數(shù)模式的控制幾乎從不用于木材干燥操作。幾乎現(xiàn)代所有的數(shù)字控制器都是將PID算法應(yīng)用到各種控制計劃，在許多系統(tǒng)中的P、I和D都是自動確定使用自調(diào)整算法。 最終運營商 沒有任何控制模式的控制器將不會正確的控制,而且如果閥門大小的不當(dāng)、泄漏、粘黏,或者是有其他瑕疵也不會有正確的控制。因此,選擇閥適當(dāng)?shù)牟僮魈卣魇窃谠O(shè)計控制回路中一個最重要的階段。窯加熱和噴霧控制閥門必須符合過程及其管道以及蒸汽供應(yīng)源。以便在控制閥門中提供一致的控制,貢獻(xiàn)一定的增量使整個系統(tǒng)的壓力損失。這通常是隨意設(shè)置的增量，從25%-35%的總系統(tǒng)中損失。 大多數(shù)程序選擇和分級控制在閥門開始進(jìn)口和出口實施壓力,兩者幾乎從不保持不變。閥門一般選擇“以后”，選擇忽略這些因素的關(guān)鍵是最佳性能的過程。正如已經(jīng)提到的,早期的窯控制器通常的開關(guān)種類和開關(guān)控制門是適當(dāng)?shù)氖褂眠@些控制器。從古至今，成本對于許多人仍是非常重要的,正是由于如今許多窯具有更少的昂貴的開關(guān)閥甚至適當(dāng)類型的控制器，實際上這個過程是其本身規(guī)定的?；旧?控制閥包含兩個主要的組件、隔膜和閥組件運營商。致動器感應(yīng)位置內(nèi)的閥積極和快速的輸出任何控制器的改變。閥門動作氣開式和氣關(guān)式是決定的過程。有兩個基本類別的控制閥門 ,閥門開啟或關(guān)閉，內(nèi)部完全由一個反應(yīng)信號從控制器、比例閥提供一個應(yīng)對比例變化的信號。這是值得注意的,任何比例閥可以作為開關(guān)閥，但真實的是相反的。一些快開閥可以提供不同程度的比例控制,但在實際的安裝上一般令人不滿意的是計量設(shè)備。 線性閥特性中提供一個相當(dāng)寬的量程,雖然不是廣泛的平等,但在大多數(shù)情況下比例閥將允許使用常數(shù)控制設(shè)置。在實際實踐中,一個閥有兩個特點。一個是設(shè)計特性(剛剛討論)，另二個是安裝的特性——后者是最重要的。安裝的特點是流和中風(fēng)之間的關(guān)系當(dāng)閥門遭受到壓力降穿而保持不變。然而,在實際實踐中。 注意 當(dāng)比最低工作壓力下降到最大操作壓力降時,安裝特點的相同比例閥變得更線性,或者換句話說是閥門增益方法的一個常數(shù)。線性閥,另一方面,變得不那么線性，它的曲線是扭曲向的開關(guān)閥。因此,當(dāng)遇到這些條件,同等比例閥通常是選擇,解釋它。 線性閥在工業(yè)應(yīng)用主要用途：應(yīng)該避免在過程運行范圍容許極限安裝一個閥門,獲得改變增益大于2或相對增益小于0.5。如果增益過高或過低,或者如果它改變太多的操作范圍,過程控制將非常困難。這個問題相對于使用閥門定位器是常見的。一個閥門定位器是一個裝置用于同一個閥操作符，在任何不平衡的力量閥體內(nèi)克服閥桿摩擦和準(zhǔn)確地定位閥盡管。一個定位器也可以用來改變閥特性,例如給一個線性閥特點的平等的比例閥或者改變閥的特點來響應(yīng)非線性的過程。它是用來關(guān)閉閥門執(zhí)行機構(gòu)或閥電機的回路循環(huán)，它將驅(qū)動電動機,直到一個機械閥桿位置的測量是對輸入信號的平衡控制器——換句話說閥門定位,它應(yīng)該是從控制器控制信號。因為一個定位器將采取行動克服干摩擦,從而消除或大大減少了死區(qū)。證明一個定位器組件閥門增益接近統(tǒng)一或成為至少一個恒定值,簡化了控制問題。一般來說它可能是一個閥門定位器將有利于各種控制回路可能例外的控制流或液體壓力。 結(jié)論 雖然簡單討論了溫度測量、控制模式和控制閥門這些幾乎沒有觸及過的科目,并提出結(jié)論和建議。關(guān)于溫度測量在窯、濕和drybulb都利用鉑電阻溫度探測器(有時匹配),建議在兩個熱電偶和熱系統(tǒng)。一般來說,控制推薦的類型是一種窄帶比例控制系統(tǒng)，完全模擬可能增加衍生到窄帶比例在數(shù)字系統(tǒng)控制中的比例。最后, 建議使用相同比例閥控制幾乎所有的溫度，作為額外的保險閥堅持和克服不可預(yù)見的壓力失衡,這種閥門可以配置定位器。 PID Versus On-off Control,Why Use A Valve Positioner? George Ballard Foxboro Company Portland, OR A discussion of the elements of a process control loop as specifically related to lumber dry kilns must delve into such subjects as PID versus On-Off control, valve characteristics and the use of valve positioners, and various types of temperature measuring elements employed in the lumber industry. The function of a control loop is to maintain a process output (or product) at some desired value, for example, a certain type of wood at a specified moisture content. The type a control loop historically employed in lumber drying is a feedback loop where, ideally, if we could, we would measure the moisture content of the wood product and feed that measurement back to a control device. That device would compare the measured moisture content to the desired moisture content and would produce a controller output to a final operator - a valve. The valve would in turn regulate the flow of energy entering the kiln which would drive the measured moisture content to the desired moisture level.The four components of this loop are, therefore, the measurement, the automatic controller, the final operator, and lastly the process itself. Measurement While we're most interested in the control of moisture content in lumber drying, that variable is almost never directly measured and fed back to the automatic controller. Instead we measure air temperature or differential temperature, or wet- and dry-bulb temperatures and those variables becomes the controlled variables. In some cases, moisture content is inferentially calculated and controlled. Early automatic dry kiln controllers employed filled thermal systems to measure and control both wet- and dry-bulb temperatures (of the air) within dry kilns. Such temperature measuring systems are still in widespread use. More recently, thermocouples and resistance temperature detectors (RTD's) are being employed, in part due to their inherent electronic characteristics as opposed to the purely mechanical nature of filled thermal systems.All three types of measuring elements cover a wide range of temperatures and it can rightly be assumed that any one of the three would provide a measurement that could be used for dry kiln control .Each has its advantages and disadvantages, however, as this comparison indicates .The practical limits, accuracies, and response figures may vary somewhat from supplier to supplier. Shifting our attention to the second element in the control loop, namely the automatic controller, it's well to keep in mind that the ultimate objective of the control system is to dry lumber to some measurable end point in the shortest possible time without causing damage to the wood. Shinskey's process model on drying was based on the principle that the dryer air temperature is reduced proportionally to the evaporation of moisture as the air passes over the material being dried. It's also accepted that the wetbulb depression creates a driving force for moisture transfer. The air dry- and wet-bulb temperatures, since they can readily be measured, have therefore beenthe customary controlled variables. The manipulated variable is steam flow, and the process loads include the wood and air with their associated moisture. Again, looking back on early kiln control, On-Off controllers with On-Offor two-position valves were employed on the steam, spray, and damper operators.On-Off control is the simplest form (or mode) of feedback control, the other basic modes being Proportional, Integral, and Derivative (or PID).Figure #5 shows the operation of an On-Off controller and an On-Off controller with a "Dead Band". Note that in both cases the valve is always either completely open or completely closed and that the measurement cycles constantly. The only difference in the operation of these two controllers is the difference in the frequency and amplitude of the measurement cycle. The principal disadvantage of On-Off control is that its normal condition is constant cycling. Its principal advantage is that it's low in cost, that is the controller and valve are usually inexpensive. On-Off control, in fact, does not even require a controller as the controller function can be created with contacts and relays or other such devices. Whether or not On-Off control is acceptable depends on the effect of cycling in the measured variable both on the product and on upsets to other process units. Constant open-to-shut cycling of steam valves, no matter whattheir characteristics, is disruptive to boiler houses and steam headers, not to mention damaging to the valves themselves. Constant cycling of the measured variable, namely wet- and dry-bulb temperatures, if large enough, no doubt contributes to poor quality in the final product. Early On-Off kiln controllers found acceptance probably due to the fact that most of them possessed a small amount of dead band and we simply were not doing as good a job of drying as we are today. On-Off control should only be applied to those situations where three conditions are present - namely: 1. Precise control must not be required because the measurement will constantly cycle. 2. Deadtime in the process must be long enough to prevent excessive valve wear and upset of other process units, and 3. The ratio of dead time to the time constant of the process must be small so as to prevent too large an amplitude in the measurement cycle. Experience has shown that lumber kilns do not exhibit these conditions at all times and a degree of proportional action is required for good temperature control and valve operation. Proportional control, based on the principle that the size of the controller response should be proportional to the difference between the control point and the measurement value, prevents valve and measurement cycling. It is a major improvement over On-Off control because of its ability to stabilize the loop. Its main disadvantage is its inevitable offset, but in kilns where loads are fairly constant and the required amount of proportional control is small, offset is not a problem. The control point can be adjusted until the measurement is at the desired value and thereafter the set point is simply a reference point for proportional action. Except in digital control, Integral and Derivative modes of control are rarely if ever used in lumber drying operations. Virtually all modern controllers,digital and otherwise, possess the ability to incorporate PID control into various control schemes and in many systems the values for P、I and D areautomatically determined through the use of self-tuning algorithms. FINAL OPERATORS No controller with whatever control modes will control properly, however, if the valve is sized improperly, leaks, sticks, or is otherwise defective. The selection of a valve, therefore, with the proper operating characteristics is one of the most important phases in designing a control loop. Kiln heating and spray control valves must work in concert with the process and its piping as well as the steam supply source. The control valve, in order to provide consistent control, must contribute a certain increment of the total system pressure loss. That increment is usually arbitrarily set at from 25 to 35% of the total system loss. Most procedures for selecting and sizing control valves begin with inlet and outlet pressures, both of which rarely if ever remain constant. Valves are usually selected "later" and are often selected ignoring those factors key to optimum performance of the process. As already noted, early kiln controllers were usually of the On-Off variety and On-Off control valves were appropriately employed with those controllers. Cost was and is still of paramount importance to many people and as a result many kilns today are equipped with the less expensive On-Off valves even when the type of controller and indeed the process itself dictates otherwise. Basically, the control valve consists of two major components, the diaphragm operator and the valve subassembly. The actuator should position the inner valve positively and quickly for any change in the controller output. The valve action, either air-to-open or air-to-close is determined by the process. There are two basic categories of control valves, namely two-position or On-Off in which the inner valve opens or closes completely to a response in signal from the controller, and proportioning valves which provide a proportional response to a change in signal from the controller. It is well to note that any proportional valve can be used as an On-Off valve but the opposite is not true. Some quick opening valves do provide a degree of proportional control but in actual installations are generally unsatisfactory as proportioning devices. provides wide rangeability and permits the use of constant control settings under varying pressure drop conditions. Linear valves are so designed that all openings of the valve will produce equal change in flow for equal incremental changes in lift. For example, if at 20% of its stroke a linear valve will pass 10 GPM, at 30% it will pass 15 GPM, at 40%, 20 GPM and so forth. The linear valve characteristic provides a fairly wide rangeability, though not as wide as the equal percentage valve, and under most circumstances will permit use of constant control settings when the pressure drop across it remains constant. In actual practice, however, a valve has two characteristics. One is the design characteristic (just discussed) and the second is the installed characteristic - the latter being the most important. The installed characteristic is the relationship between flow and stroke when the valve is subjected to the pressure drop across it remains constant. In actual practice, however, a valve has two characteristics. One is the design characteristic (just discussed) and the second is the installed characteristic - the latter being the most important. The installed characteristic is the relationship between flow and stroke when the valve is subjected to the pressure conditions of the process. Figure #8 shows the relationships for both linear and equal percentage valves under varying pressure drop conditions. Note that as the ratio of minimum operating pressure drop to maximum operating pressure drop becomes more extreme, the installed characteristic of the equal percentage valves becomes more linear, or in other words the valve gain approaches a constant. ATTENTIONS The linear valve, on the other hand, becomes less linear and its curve is distorted toward that of an On-Off valve. Consequently, when these conditions are encountered, an equal percentage valve is usually chosen, explaining its predominant use over linear valves in industrial applications. A rule of thumb for permissible limits for the installed gain of a valve is that a change in gain larger than 2 or a relative gain smaller than 0.5 should be avoided in the process operating range. If the gain is too high or too low, or if it changes too much in the operating range, process control will become very difficult. The question relative to the use of valve positioners is frequently asked. A valve positioner is a device used on or in conjunction with a valve operator to overcome valve stem friction and accurately position the valve in spite of any unbalanced forces within the valve body. A positioner may also be used to alter valve characteristics, as for example to give a linear valve the characteristics of an equal percentage valve or perhaps to alter the valve characteristics in response to a non-linearity in the process. It is used to close the loop around the valve actuator or valve motor and it will drive the motor until a mechanical measurement of the valve stem position is balanced against the input signal from the controller - in other words the valve is positioned where it's supposed to be in accordance with the signal from the controller. Since a positioner will act to overcome stem friction, it thereby eliminates or greatly reduces dead band. It has been demonstrated that a positioner forces the valve gain closer to unity or at least a constant value which simplifies the control problem. In general it may be stated that a valve positioner will be helpful in every kind of control loop with the possible exception of the control of flow or liquid pressure. CONCLUSIONS While this short discussion of temperature measurement, control modes, and control valves has barely touched on these subjects, some general conclusions and recommendations are suggested.With regard to?temperature measurements in kilns, both wet- and drybulb, the use of platinum resistance temperature detectors (sometimes matched), is recommended over both thermocouples and filled thermal systems. In general, the type of control recommended is a form of narrow band .Proportional control for purely analog systems with the possible addition of Derivative to narrow band Proportional in digital system control. Finally, the use of equal percentage valves is recommended for virtually all temperature control. As extra insurance against valve sticking and to overcome unforeseen pressure imbalances, the valve may be equipped with a positioner. 8