电气化与自动化专业毕业设计论文外文资料翻译.doc

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1、毕业设计（论文）外文资料翻译 系别： 电气工程系 专业： 电气化与自动化 班级： 姓名： 学号： 外文出处： Specialized English For Architectural Electric Engineering and Automation 附 件：1、外文原文；2、外文资料翻译译文。指导教师评语：签字： 年 月 日注：请将该封面与附件装订成册。1、 外文原文Introductions to temperature controland PID controllersProcess control system. Automatic process control is con

2、cerned with maintaining process 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 taken, the important process variables-those related to safety, product quality, an

3、d production rates-will not achieve design conditions. In order to fix ideas, let us consider a heat exchanger in which a process stream is heated by condensing steam. The process is sketched in Fig.1 Fig. 1 Heat exchanger The purpose of this unit is to heat the process fluid from some inlet tempera

4、ture, Ti(t), up to a certain desired outlet temperature, T(t). As mentioned, the heating medium is condensing steam. The energy 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 we

5、ll insulated. In this process 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 control the outlet process temperature to maintain its d

6、esired value. One way to accomplish this objective is by first measuring the temperature T(t) , then comparing it to its desired value, and, 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 tempera

7、ture is above its desired value, then the steam valve can be 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 some more to increase the steam flow (energy) to the exchanger. All of these can be d

8、one manually by the operator, and since the procedure is fairly straightforward, it should present no problem. However, since in most process plants there are hundreds of variables that must be maintained at some desired value, this correction procedure would required a tremendous number of operator

9、s. Consequently, we would like to accomplish this control automatically. That is, we want to have instnnnents that control the variables wJtbom requ)ring intervention from the operator. (si This is what we mean by automatic process control. To accomplish his objective a control system must be design

10、ed and implemented. A possible control system and its basic components are shown in Fig.2.Fig. 2 Heat exchanger control loopThe first thing to do is to measure the outlet temperaVare of the process stream. A sensor (thermocouple, thermistors, etc) does this. This sensor is connected physically to a

11、transmitter, which takes the output from the sensor and converts it to a signal 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 what

12、 to do to maintain the temperature at its desired value. Base on this decision, the controller then sends another signal to final control element, which in turn manipulates the steam flow.The preceding paragraph presents the four basic components of all control systems. They are (1) sensor, also oft

13、en called the primary element. (2) transmitter, also called the secondary element. (3) controller, the brain of the control system. (4) final control system, often a control valve but not always. Other common final control elements are variable speed pumps, conveyors, and electric motors. The import

14、ance of these components is that they perform the three basic operations that must be present in every control system. These operations are (1) Measurement (M) : Measuring the variable to be controlled is usually done by the combination of sensor and transmitter. (2) Decision (D): Based on the measu

15、rement, the controller must then decide what to do to maintain the variable at its desired value. (3) Action (A): As a result of the controllers decision, the system must then take an action. This is usually accomplished by the final control element. As mentioned, these three operations, M, D, and A

16、, must be present in every control system. 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. PID controllers are process controllers with the following characteristics:

17、 Continuous process control Analog input (also known as measuremem or Process Variable or PV) Analog output (referred to simply as output) Setpoint (SP) Proportional (P), Integral (I), and/or Derivative (D) constants Examples of continuous process control are temperature, pressure, flow, and level c

18、ontrol. For example, controlling the heating of a tank. For simple control, you have 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 se

19、nsor. This is similar to most home air conditioning & heating thermostats. In contrast, the PID controller would receive input as the actual temperature and control a valve that regulates the flow of gas to the heater. The PID controller automatically finds the correct (constant) flow of gas to the

20、heater that keeps the temperature steady at the setpoint. Instead of the temperature bouncing back and forth between two 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. If the setpoint is raise

21、d, then the PID controller automatically increases the amount of gas flowing to the heater. Likewise the PID controller would automatically for hot, sunny days (when it is hotter outside the heater) and for cold, cloudy days. The analog input (measurement) is called the process variable or PV. You w

22、ant 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 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 th

23、e temperature range for the sensor is 0 to 400 degrees then our theoretical accuracy is calculated to be 400 degrees divided by 4,096 (12 bits) =0.09765625 degrees. We say theoretical because it would assume there was no noise and error in our temperature sensor, wiring, and analog converter. There

24、are other assumptions such as linearity, etc. The point being-with 1/10 of a degree theoretical accuracy-even with the usual amount of noise and other problems- one degree of accuracy should easily be attainable. The analog output is often simply referred to as output. Often this is given as 0100 pe

25、rcent. In this heating example, it would mean the valve is totally closed (0%) or totally open (100%). The setpoint (SP) is simply-what process value do you want. In this example-what temperature do you want the process at? The PID controllers job is to maintain the output at a level so that there i

26、s no difference (error) between the process variable (PV) and the setpoint (SP).In Fig. 3, the valve could be controlling 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 contr

27、oller is looking at is the difference (or error) between the PV and the SP. SETPOINT P，I，&D CONSTANTS Difference error PID control algorithm process output variable Fig .3 PIDcontrolIt looks at the absolute error and the rate of change of error. Absolute error means-is there a big difference in the

28、PV and SP or a little difference? Rate of change of error means-is the difference between the PV or SP getting smaller or larger as time goes on. When there is a process upset, meaning, when the process variable or the setpoint quickly changes-the PID controller has to quickly change the output to g

29、et 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 the PID controller has to increase the cooling (output) to compensate for this rise in

30、 temperature. Once the PID controller has the process variable equal to the setpoint, a good PID controller will 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,

31、 this could cause more wear on the control element. 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 close to the setpoint. Note that the output often goes past (over shoots) the steady-s

32、tate output to get the process back to the setpoint. For example, a cooler may normally have its cooling valve open 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 b

33、ack out, and then closes the cooler door-the PID controller is freaking out because the temperature may have raised 20 degrees! So it may crank the cooling valve open to 50, 75, or even 100 percent-to hurry up and cool the cooler back down-before slowly closing the cooling valve back down to 34 perc

34、ent. Lets think about how to design a PID controller. We focus on the difference (error) between the process variable (PV) and the setpoint (SP). There are three ways we can view the error.The absolute error This means how big is the difference between the PV and SP. If there is a small difference b

35、etween the PV and the SP-then lets make a small change in the output. If there is a large difference in the PV and SP-then lets make a large change in the output. Absolute error is the proportional (P) component of the PID controller.The sum of errors over time Give us a minute and we will show why

36、simply looking at the 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 latest error to the sum of errors. In other words Sum of Errors = Error 1 q-

37、Error2 + Error3 + Error4 + . The dead time Dead time refers to the delay between making a change in 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

38、 is the 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 temperature setpoint. This is also referred to as the derivative (D) component o

39、f the PID controller. This holds some future changes back because the changes in the output have been made but are not reflected in the process variable yet.Absolute Error/Proportional One of the first ideas people usually have about designing an automatic process controller is what we call proporti

40、onal. Meaning, if the difference between the PV and SP is small-then lets make a small correction to the output. If the difference between the PV and SP is large- then lets make a larger correction to the output. This idea certainly makes sense. We simulated a proportional only controller in Microso

41、ft Excel. Fig.4 is the chart showing the results of the first simulation (DEADTIME = 0, proportional only):Proportional and Integral Controllers The integral portion of the PID controller accounts for the offset problem in a proportional only controller. We have another Excel spreadsheet that simula

42、tes 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). As you can tell, the PI controller is much better than just the P controller. However, dead time of zero (as shown in the gr

43、aph) is not common. Fig .4 The simulation chart Derivative ControlDerivative 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, lets take heating of the oven. Fig.5The simulation chart If we start turning u

44、p the gas flow, it will take time for the heat to be produced, the heat to flow around the oven, 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 outpu

45、t further. Setting the derivative constant correctly allows you to become more aggressive with the P & I constants.2、外文资料翻译译文温度控制简介和PID控制器过程控制系统 自动过程控制系统是指将被控量为温度、压力、流量、成份等类型的过程变量保持在理想的运行值的系统。过程实际上是动态的。变化总是会出现，此时如果不采取相应的措施，那些与安全、产品质量和生产率有关的重要变量就不能满足设计要求。为了说明问题，让我们来看一下热交换器。流体在这个过程中被过热蒸汽加热，如图1所示。这一装置的主要目的是将流体由入口温度乃(f)加热到某一期望的出口温度T(f)。如前所述，加热介质是过热蒸汽。只要周围没有热损耗，过程流体获得的热量就等于蒸汽释放的热量，即热交换器和管道间的隔热性很好。很多变量在这个过程中会发生变化，继而导致出口温度偏离期望值。如果出现这种情况，就该采取一些措施来校正偏差，其目的是保持出口温度为期望值。实现该目的的一种方法是首先测量r(