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基于单片机的阀门定位器设计与开发,基于,单片机,阀门,定位器,设计,开发
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西北工业大学明德学院本科毕业设计论文英 文 翻 译 系 别 自动化系 专 业 自动化 班 级 163003 学生姓名 张瑞 学 号 103654 指导教师 何振琦 PID与开关控制,为什么要使用阀门定位器? George BallardFoxboro CompanyPortland, OR 关于过程控制回路中元素的讨论,特别对于木材干燥窑，必须深入探究这些问题：PID和开关控制,阀特性和使用阀门定位器,以及各种用于木材行业类型的温度测量元素。在某个预定值，一个控制回路的作用是维持一个过程的输出, 例如,某些类型木材的指定含水率。一个控制类型回路采用的是历史木材干燥反馈回路,理想情况下,如果可以,我们将测量含水率的木材产品和饲料,反馈回一个控制装置。该设备将比较测量水分和所需的水分,产生一个控制器输出值到最后运营商阀。阀门将反过来调节能量流的输入，从而使得窑实测含水率到所需的湿度水平。因此,这四个组件的循环因素为：测量,自动控制器,最后的算子,最后这个过程本身。测量 我们最感兴趣的是在木材干燥问题中控制水分含量,该变量几乎从不直接测量和反馈到自动控制器，因此我们测量空气温度或微分温度、或湿和干球温度和这些变量成为控制变量。在某些情况下,含水率是通过推理计算来控制。早期的自动干燥窑控制器是在干窑中采用填充热系统测量和控制湿、干球温度(空气)的方法。这样的温度测量系统目前仍在广泛使用。然而目前应用最多的，是热电偶和电阻温度探测器(RTD),部分由于其固有的电子特性,而不是纯粹的机械性质热系统了。所有三种类型的测量元素覆盖广泛的温度，它当然可以为任何领域提供可用于干燥窑控制的测量。比较表明，每一种方法都有其优点和缺点,然而,随着这种比较显示的实际限制,精度和响应数据可能会因供应商的不同而有所不同。控制模式接下来将我们的注意力转移到第二个元素的控制回路,即自动控制器。我们会记得在不损坏木材的条件下控制系统的终极目标是在可能的最短的时间内对干木材一些终点的测量。Shinskey的过程模型是基于干燥的原则，随着空气经过物料被干从而相应地减少对干燥空气温度蒸发的水分。当然我们也接受wetbulb抑郁会创建一个驱动力从而对对水分传输。空气干燥的温度和湿球的温度,因为可以并且容易测量,因此作为一直的控制变量。而被操纵的变量是蒸汽流量,加载过程包括木材和空气以及和它们相关的水分。其次,回顾早期窑控制、开关控制器On-Offor双位阀被雇用的蒸汽,喷雾,和阻尼运营商。开关控制是最简单的形式(或模式)的反馈控制,其他基本模式是比例、积分和导数(或PID)。注意,在这两种情况下,阀总是要么完全开启要么完全关闭,测量周期一直保持不变。唯一的区别在于操作这两个控制器是在不同频率和振幅的测量周期下。它的主要缺点是开关控制正常状态是恒定的循环。它的主要优点是低成本, 通常这类控制器和阀门是便宜的。事实上，开关控制不需要控制器、继电器或其他类似设备作为控制器功能的创建联系人。是否可以接受开关控制取决于效果的循环在测量变量对产品和其他生产装置的影响。不断开放关闭循环的蒸汽阀门,无论他们是具有破坏性的锅炉房还是蒸汽头,都不会损害阀门本身。不断循环的测量变量,即湿和干球温度,如果足够大,毫无疑问将有助于最终产品上的质量差。早期开关窑控制器在验收时可能发现由于我们只是做好一个工作的干燥，大多数人都会拥有一个小的数量的死区。开关控制应该只适用于以下三个条件存在,即:1、精确的控制是不需要的,因为测量将不断循环。2、在这个过程中停歇时间必须足够长，以防止阀门的过度磨损和损坏其他过程单元。3、死区时间的比例时间常数过程必须小,以避免一个太大振幅的测量周期。经验表明,木材干燥在这些条件下不表现出在所有时间和一定程度的比例作用是需要良好的温度控制和阀门操作。比例控制是基于控制器响应的大小原则在控制点和测量值的差异来防止阀和测量循环。它主要改进开关控制,因为它有能力稳定回路。它的主要缺点是会不可避免的偏移,但在窑相当恒定载荷时和所需数量的比例控制很小时,抵消不是问题。控制点是可以调整的，直到在期望价值和其后的设置点的测量是一个简单的参考点的比例作用。除了数字控制，积分和导数模式的控制几乎从不用于木材干燥操作。几乎现代所有的数字控制器都是将PID算法应用到各种控制计划，在许多系统中的P、I和D都是自动确定使用自调整算法。最终运营商没有任何控制模式的控制器将不会正确的控制,而且如果阀门大小的不当、泄漏、粘黏,或者是有其他瑕疵也不会有正确的控制。因此,选择阀适当的操作特征是在设计控制回路中一个最重要的阶段。窑加热和喷雾控制阀门必须符合过程及其管道以及蒸汽供应源。以便在控制阀门中提供一致的控制,贡献一定的增量使整个系统的压力损失。这通常是随意设置的增量，从25%-35%的总系统中损失。大多数程序选择和分级控制在阀门开始进口和出口实施压力,两者几乎从不保持不变。阀门一般选择“以后”，选择忽略这些因素的关键是最佳性能的过程。正如已经提到的,早期的窑控制器通常的开关种类和开关控制门是适当的使用这些控制器。从古至今，成本对于许多人仍是非常重要的,正是由于如今许多窑具有更少的昂贵的开关阀甚至适当类型的控制器，实际上这个过程是其本身规定的。基本上,控制阀包含两个主要的组件、隔膜和阀组件运营商。致动器感应位置内的阀积极和快速的输出任何控制器的改变。阀门动作气开式和气关式是决定的过程。有两个基本类别的控制阀门 ,阀门开启或关闭，内部完全由一个反应信号从控制器、比例阀提供一个应对比例变化的信号。这是值得注意的,任何比例阀可以作为开关阀，但真实的是相反的。一些快开阀可以提供不同程度的比例控制,但在实际的安装上一般令人不满意的是计量设备。线性阀特性中提供一个相当宽的量程,虽然不是广泛的平等,但在大多数情况下比例阀将允许使用常数控制设置。在实际实践中,一个阀有两个特点。一个是设计特性(刚刚讨论)，另二个是安装的特性后者是最重要的。安装的特点是流和中风之间的关系当阀门遭受到压力降穿而保持不变。然而,在实际实践中。 注意当比最低工作压力下降到最大操作压力降时,安装特点的相同比例阀变得更线性,或者换句话说是阀门增益方法的一个常数。线性阀,另一方面,变得不那么线性，它的曲线是扭曲向的开关阀。因此,当遇到这些条件,同等比例阀通常是选择,解释它。线性阀在工业应用主要用途：应该避免在过程运行范围容许极限安装一个阀门,获得改变增益大于2或相对增益小于0.5。如果增益过高或过低,或者如果它改变太多的操作范围,过程控制将非常困难。这个问题相对于使用阀门定位器是常见的。一个阀门定位器是一个装置用于同一个阀操作符，在任何不平衡的力量阀体内克服阀杆摩擦和准确地定位阀尽管。一个定位器也可以用来改变阀特性,例如给一个线性阀特点的平等的比例阀或者改变阀的特点来响应非线性的过程。它是用来关闭阀门执行机构或阀电机的回路循环，它将驱动电动机,直到一个机械阀杆位置的测量是对输入信号的平衡控制器换句话说阀门定位,它应该是从控制器控制信号。因为一个定位器将采取行动克服干摩擦,从而消除或大大减少了死区。证明一个定位器组件阀门增益接近统一或成为至少一个恒定值,简化了控制问题。一般来说它可能是一个阀门定位器将有利于各种控制回路可能例外的控制流或液体压力。结论虽然简单讨论了温度测量、控制模式和控制阀门这些几乎没有触及过的科目,并提出结论和建议。关于温度测量在窑、湿和drybulb都利用铂电阻温度探测器(有时匹配),建议在两个热电偶和热系统。一般来说,控制推荐的类型是一种窄带比例控制系统，完全模拟可能增加衍生到窄带比例在数字系统控制中的比例。最后, 建议使用相同比例阀控制几乎所有的温度，作为额外的保险阀坚持和克服不可预见的压力失衡,这种阀门可以配置定位器。PID Versus On-off Control,Why Use A Valve Positioner?George BallardFoxboro CompanyPortland, ORA 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. MeasurementWhile were 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 (RTDs) 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, its 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.Shinskeys 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. Its 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 its 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, and3. 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 beadjusted 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 OPERATORSNo 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 adegree of proportional control but in actual installations are generally unsatisfactory as proportioning vides 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 pressuredrop 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 andequal 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.ATTENTIONSThe 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
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