Курчаткина Г.П., Стручкова С.Б. Обучение чтению литературы на англ.языке по спец. САУ для студентов старших курсов (2014) (1257062), страница 3
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Read the text carefully to know its content in more detail and completethe tasks that follow.Text B. Feedback ControlIn level control system, if the level increases above the set point, the automaticprocess control system will control the valve until it is partially closed or, ifnecessary, shut completely. If the level drops below the set point, the valve isopened, allowing more water to enter the reservoir. As the level of the waterchanges, this information is sensed by the level sensing primary element. This newlevel becomes information that is fed back into the control system. This process ofsensing the level of the water, comparing it against a desired value, making achange in the process to bring the process variable in line with the set point andthen feed back the new information to the level controller is commonly referred toas feedback control. Because of the cyclical or "looping" nature of feedbackcontrol, a process that uses feedback control is sometimes referred to as a"feedback loop".
Therefore, it can compensate unmeasured disturbances in theloop.Feedback control may be either manual or automatic. The difference betweenautomatic and manual control systems is that automatic control systems useinstruments and automatic control equipment to control the process, whereasmanual systems use an operator to make decisions and adjustments.
The systemdescribed is automatic, with little need for anyone to interact with the process onceit is placed into service. It may be desirable, however, to add additionalinstrumentation to give an operator updated information about the status of thesystem. For example, a chart or a strip recorder can be added to record processupsets over a period of time, or a warning signal can be added, activated by a veryhigh or low level in the tank.16The feedback is said to be negative feedback when the error signal is thedifference between the desired value and the measured output value from theprocess.
Positive feedback occurs when the reference value is added to themeasured value. A simple example of positive feedback is the characteristic of anelectric resistor. The higher its temperature is, the lower its resistance. Whencurrent passes through the resistor it becomes warm. A consequence of this is thatthe resistance decreases. This results in the current increasing. The increasedcurrent causes the resistor to become even warmer. A consequence of this is that itsresistance decreases further.
This results in an increase in current and so on. It is apositive feedback since the input, the current, is increased by the feedback from theoutput rather than maintained at a constant value (unstable system). Negativefeedback is able to stabilize the controlled system and maintains the output at aconstant value (stable system).(2281)Task 12. Explain the concepts feedback control and feedback loop in your ownwords the way you understand it.Task 13. Give each stage of thought a suitable title.Task 14. Condense the content of each stage of thought into one statement.Task 15.
Write a descriptive abstract of the text. Remember that an abstract doesnot give any details, nor does it make judgments about the work. Use PassiveConstructions.Task 16. Explain the difference between positive and negative feedback and givean example of each. Which of them provides a stable control system?Task 17. Search the Web for more feedback examples. Report your findings toothers in your group.Task 18. Render into English.А. В типичную систему управления входят объект, регулятор, привод идатчики.
Однако, набор этих элементов – еще не система. Для превращения всистему нужны каналы связи, через них идет обмен информацией между17элементами. Для передачи информации могут использоваться электрическийток, воздух (пневматические системы), жидкость (гидравлические системы),компьютерные сети. Взаимосвязанные элементы – это уже система, котораяобладает (за счет связей) особыми свойствами, которых нет у отдельныхэлементов и любой их комбинации.(455)Б. Информация в системе управления как бы «ходит по кругу»: регуляторвыдает сигнал управления на привод, который воздействует непосредственнона объект; затем информация об объекте через датчики возвращается обратнок регулятору и все начинается заново. Говорят, что в системе есть обратнаясвязь, то есть регулятор использует информацию о состоянии объекта длявыработки управления.
Системы с обратной связью называют замкнутыми,поскольку информация передается по замкнутому контуру.(420)Task 19. Look at the title of text C and the figures and decide what the text isabout.Task 20. Read the text carefully to know its content in more detail and explainthe difference between feedback and feed-forward control.Text C. Feed-forward ControlFeedback systems and feedback control loops are the most common type ofcontrol found. However, there are the disadvantages in using just feedback controlsystems in all applications. In a gas-fired water heater shown in Figure 7 thedesired temperature of the water in the tank is controlled by the use of a feedbackmechanism which controls the fuel gas supply to the gas-fired heater.
If thetemperature of the output water is too hot, the amount of fuel gas going to theheater is reduced. If the output water is too cool, the supply of fuel gas is increased.18Figure 7. Feedback control of gas-fired water heaterA simple feedback mechanism controlling the temperature of the water is notoperated until the temperature sensor detects a deviation from some set point. Inmany situations this is unacceptable because it is critical to maintain certainprocess variables within a very small range. Suppose that in the example of heatingwater the temperature range of the water in the vessel is critical. In instances suchas this, it may be necessary to use feed-forward control.
It is an open-loop strategyto predict and compensate as quickly as possible the known (measured)disturbances.An automatic feed-forward control system installed on the water heater isshown in Figure 8. In this example, a temperature sensing element is installed onthe water inlet (as opposed to the feedback system where it is installed on theoutlet).Figure 8. Feed-forward control of gas-fired water heaterIf the temperature of the incoming water changes, a signal is sent to the measuringelement, which is also a transmitter. The transmitter sends the signal to the19controlling element, where it is then compared to the set point.
If a difference isnoted, the controlling element sends a proportional control signal to the finalcontrolling element (the gas supply valve of the gas-fired heater), which thenincreases or decreases the temperature of the fire tube. The result of this feedforward control is that the temperature of the controlled variable (in this case, thetemperature of the heated water in the vessel) will be consistent. It is possible alsoto have a manual feed-forward control system. Here an operator would read atemperature gauge on the water inlet, mentally compare the figure to the set point,and adjust the gas-fired heater accordingly.The use of feed-forward control by itself is rare. In any process that might befound in industry, there are many possible disturbances that could arise.
If, forexample, the water output temperature were to change, a feed-forward controlsystem by itself would not have the capability to sense this situation.(2317)Task 21. Skim text D and speak about its main points.Task 22. Read the text carefully to know its content in more detail and completethe tasks that follow.Text D. Combination of Feedback/Feed-forward ControlIt can be concluded that feedback normally presents the most practicaltechnique for the development of a control system.
However, in a situation whenthe controlled plant is affected by a small number of dominant disturbances, whichcan be properly monitored (measured), the feedback could be supplemented by afeed-forward mechanism, responsible for dominant disturbances. Because of theshortcomings of both the feedback and feed-forward control systems, what weusually find is the two control systems combined together as shown in figure 9.The feed-forward control loop compensates for changes in the inlet watertemperature while the feedback control loop makes adjustments to compensate forother process disturbances. This combination of control loops allows for process20control which will compensate for both inlet water temperature changes as well asfor changes in the output water temperature.Figure 9. Feedback and feed-forward controlIn general, automatic control has two major objectives: firstly, closed-loopsystem stability has to be guaranteed by the means of control system; secondly, thebest possible performance during transient and steady state response has to beprovided by the controller.Control systems can be considered to fall in two main categories.
In a processcontrol system variables such as temperature, liquid level, fluid flow, and pressureare maintained constant. Thus in a chemical process there may be a need tomaintain the level of a liquid in a tank to a particular level or to a particulartemperature.
These variables are also commonly found in oil and gas industry. Inthis type of applications, the desired output for the controlled system is rare to bechanged and it is required to maintain the output at this value (regulationobjective). A servo control system involves consistent and accurate positioningsome moving parts or maintaining a constant speed. This might be, for example, amotor designed to run at a constant speed or a machining operation in which theposition, speed and operation of a tool is automatically controlled. In this type ofapplications, the desired output for the controlled system could be frequentlychanged.
Therefore, it is required to follow this change and to maintain the newdesired value (regulation plus tracking objectives).(1997)21Task 23. Ask questions to the text. Discuss the questions with the others in yourgroup.Task 24. Speak about 1)automatic control objectives; 2)two main categories ofcontrol systems.Task 25. Compare between process control and servo control systems. Give anexample for each.22UNIT 3Task 1. Skim text A and speak about its main points.Task 2.
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