Курчаткина Г.П., Стручкова С.Б. Обучение чтению литературы на англ.языке по спец. САУ для студентов старших курсов (2014) (1257062), страница 2
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washing machines which require the water to beswitched on and then, after a suitable time, switched off followed by the heaterbeing switched on and then, after a suitable time, switched off.In a closed-loop control system a signal indicating the state of the output of thesystem is fed back to the input where it is compared with what was required andthe difference is used to modify the output of the system so that it maintains theoutput at the required value. The term closed-loop refers to the loop created by thefeedback path.
Closed-loop systems have the advantage of being relativelyaccurate in matching the actual position to the required values. They are, however,more complex and so more costly with a greater chance of breakdown as aconsequence of the greater number of components.The difference between the open- and closed-loop systems can be illustrated bythe following example. Consider the heating of a room to some requiredtemperature using an electric fire with a selection switch which allows a 1 KW or a2 KW heating element to be selected.
The decision might be made, as a result ofexperience, that to obtain the required temperature it is only necessary to switch onthe 1 kW element. The room will heat up and reach a temperature which is9determined by the fact the 1 kW element is switched on. The temperature of theroom is thus controlled by an initial decision and no further adjustments are made.If there are changes in the conditions, perhaps someone opening a window, there isno way the heat output can be adjusted to compensate.
This is an example of openloop control in that there is no information fed back to the element to adjust it andto maintain a constant temperature. The heating system with the electric fire couldbe made a closed-loop system if the person has a thermometer and switches the 1KW and 2KW elements on or off to maintain the temperature of the room constant.In this situation there is feedback, the input to the system being adjusted accordingto whether its output is the required temperature.
Thus if a window is opened andthere is a sudden cold blast of air, the feedback signal changes because the roomtemperature changes and is fed back to modify the input to the system. The input tothe heating process depends on the deviation of the actual temperature fed backfrom the output of the system from the required temperature initially set. Thedifference between the two values is determined by a comparison - the person inthis case. A simple block diagram representation is given in Fig.
5.(2660)Figure 5. Room heating system10Task 16. Explain the difference between open-loop and closed-loop control.Task 17. Speak about the advantages and disadvantages of open-loop controlsystems.Task 18. Speak about the advantages and disadvantages of closed-loop controlsystems.Task 19. Render into English.Можно ли управлять, не используя обратную связь? В принципе, можно.В этом случае регулятор не получает никакой информации о реальномсостоянии объекта, поэтому должно быть точно известно, как этот объектсебя ведет. Только тогда можно заранее рассчитать, как им нужно управлять.Однако при этом нельзя гарантировать, что задание будет выполнено.
Такиесистемы называютразомкнутыми системами, поскольку информацияпередается не по замкнутому контуру, а только в одном направлении.Несмотря на недостатки, разомкнутые системы применяются на практике.Например, информационное табло на вокзале. Или простейшая системауправления двигателем, в которой не требуется очень точно поддерживатьчастоту вращения.
Однако с точки зрения теории управления разомкнутыесистемы малоинтересны.(786)Task 20. Search the Web for additional information on the topic of Unit 1. Usetexts A,B,C, and your findings to make a presentation on the basics of controlsystems.Task 21. Read the text, find definitions to the terms process variable, processupset, process instrumentation and remember them.Text D. Process Variables and Process InstrumentationThe parts of the process to be controlled are referred to as the process variables.Any condition that causes a process variable to deviate from the desired level or"set point" is referred to as a process upset.
Depending on the process, there aremany different kinds of process variables that we may want to control. Sometypical examples of process variables are liquid level, fluid flow rate, temperature,pressure, voltage rate or resistance (when working with electricity), density or PH11(when working with a chemical process), and position or speed (when workingwith mechanical systems).The devices that are used to monitor and control these variables are generallyreferred to as process instrumentation or simply instrumentation. Instruments havetwo basic jobs, and may combine both tasks into one device.
First, they measureand indicate the value of a process variable, which may then show up asinformation on a readout, gauge, recorder etc. Secondly, after comparing thevariable reading to some predetermined value (or set point), they send out acontrolling signal to another element in the system. Typically this might be a valveor heating element, which causes a change in the operation. Such a change mightbe a valve opening or closing, or a heating element increasing or decreasing thetemperature.
This has the effect of changing the process enough to bring theprocess variable in line with the set point.(1199)12UNIT 2Task 1. Skim text A and speak about its main points.Task 2. Find the following word combinations in the text and give their Russianequivalents:to manually deal with all the process variables; an automatic process controlsystem; to do exactly what the name implies; depending on the location; regardlessof location; to vary in shape, size, and components; to control the flow of water inand out of a tank; is referred to as; to convert the motion into a signal; to bring theprocess variable back to set point.Task 3. Read and translate the text using a dictionary. Choose, underline andwrite out words and word combinations which are essential for your speciality.Remember them.Text A.
Basic Elements in Closed-loop ControlIn a simple situation, it may be possible for one operator to manually deal withall the process variables and control them, but processes become more complex,the number of process variables and upsets that must be dealt with increases.Additionally, as in the case of an offshore platform or gas processing facility, theseprocesses are physically spread out over a large area. In such cases, automaticprocess control systems are installed. An automatic process control system doesexactly what the name implies: it automatically controls a process and its variables.Depending on the location where they are installed, automatic process controlsvary in their shape, size, and components. However, regardless of location, allautomatic process control systems contain the following main elements:1.
A primary measuring element (sensor)2. A transmitting element (transducer)3. A controller, and4. A final control elementThe level control system in Fig.6 controls the flow of water into and out of aholding tank or reservoir. The desired level of the tank is referred to as the set13point. The demand on the tank causes the water level to drop and consequentlycauses a float in the tank to drop as well. The float is the primary measuringelement of this simple system. The primary element is the part of the system thatdetects a process upset or change in the value of a process variable. The primaryelement is linked to the transmitting element.The transmitting element convertsthe motion of the primary measuringelement into a signal (typically achange in pressure in a pneumaticline or an electric signal) whichrepresents the value of the processupset. This signal or value is thensent to (or input) to the controllerFigure 6.
Level control systemwhere it is compared to a valuerepresenting the desired set point. The difference between the two values (set pointand upset) is then computed, and another signal is sent out (output) to the finalcontrol element, in this case, a valve. This output signal is proportional to theamount of corrective action necessary by the valve to bring the process variable(water level) back to set point.(1809)Task 4. Answer the questions.1. Is it possible for one operator to manually deal with all the process variables?When?2.
In what situations are automatic process control systems used?3. What does an automatic process control system do?4. What do automatic process controls shape, size, and components depend on?5. What are the main elements of any automatic control system?6. What is the purpose of a primary measuring element?147.
What is the function of a transmitting element?8. Where is the difference between the set point and the process upset computed?9. What element is responsible for bringing the process variable back to set point?Task 5. Complete the text using the words below.Outputcontrollerprocessfinal control elementcorrectcylinderpowerelementcorrectscontrol actionsignalinputThe correction element or, as it is often called, the__________, produces a changein the __________ which aims to __________or change the controlled condition.The term actuator is used for the__________ of a correction unit that provides the__________ to carry out the__________. An example is a motor, withan__________ of a voltage to its armature coils and an __________ of a rotatingshaft which, via possibly a screw, rotates and __________ the position of aworkpiece.Another example is a hydraulic or pneumatic cylinder.
The__________ has a piston which can be moved along the cylinder depending on apressure __________ from the__________.Task 6. Identify the basic control loop elements in the case of 1) a personcontrolling the temperature of a room using electric fire, 2) a person controllingthe level of liquid inside a tank. Use Glossary if necessary.Controlled variable is _______________Set point (reference) is _______________Comparison element is _______________Error signal is _______________Controller is _______________Actuator is _______________Controlled process is _______________Sensor is _______________Task 7. Speak about the basic elements of a closed- loop control system.Task 8.
Label a primary measuring element and a final control element in Fig. 6.Task 9. Describe the level control system represented in Fig. 6.15Task 10. Skim text B and decide into how many sections or stages of thought itcan be divided. Make use of the author's paragraphing.Task 11.
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