测控技术与仪器专业英语 第六章 化工出版社.ppt

1、An Overview for Control System,Control systems are everywhere around us and within us. *Many complex control systems are included among the functions of the human body. An elaborate control systems centered in the hypothalamus of the brain maintains body temperature at 37 degrees Celsius () despite

2、changes in physical activity and external ambience. In one control systemthe eyethe diameter of the pupil automatically adjusts to control the amount of light that reaches the retina. Another control system maintains the level of sodium ion concentration in the fluid that surrounds the individual ce

3、lls.,Threading a needle and driving an automobile are two ways in which the human body functions as a complex controller. The eyes are sensor that detects the position of the needle and thread, or of the automobile and the center of the road. A complex controller, the brain, compares the two positio

4、ns and determines which actions must be performed to accomplish the desired result. The body implements the control action by moving the thread or turning the steering wheel; an experienced driver will anticipate all type of disturbances to the system, such as a rough section of pavement or a slow-m

5、oving vehicle ahead. It would be very difficult to reproduce in an automatic controller the many judgments that an average person makes daily and unconsciously.,Control systems regulate temperature in home, school, and buildings of all types. They also affect the production of goods and services by

6、ensuring the purity and uniformity of the food we eat and by maintaining the quality of products from paper mills, steel mills, chemical plants, refineries, other types of manufacturing plants. Control systems help protect our environment by minimizing waste material that must be discarded, thus red

7、ucing manufacturing costs and minimizing the waste disposal problem. Sewage and waste treatment also requires the use of automatic control system.,A control system is any group of components that maintains a desired result or value. From the previous examples it is clear that a great variety of comp

8、onents may be a part of a single control system, whether they are electronic, mechanical, hydraulic, pneumatic, human, or any combination of these. The desired result is a value of some variable in the system, for example, the direction of an automobile, the temperature of a room, the level of liqui

9、d in a tank, or the pressure in a pipe. The variable whose value is controlled is called the controlled variable.,To achieve control, there must be another variable in the system that can influence the controlled variable. Most systems have several such variables. Control system maintains the desire

10、d result by manipulating the value of one of these influential variables. The variable that is manipulated is called the manipulated variable. The steering wheel of an automobile is an example of manipulated variable. An excellent idea of the scope of control systems is given in an Instrument Societ

11、y of America film, “Principles of Frequency Response,”1958,Control systems are becoming steadily more important in our society . We depend on them to such an extent that life would be unimaginable without them . Automatic control has increased the productivity of each worker by releasing skilled ope

12、rators from routine tasks and by increasing the amount of work done by each worker. Control systems improve the quality and uniformity of manufactured goods and services : many of the products we enjoy would be impossible to produce without automatic controls. Servo systems place tremendous power at

13、 our disposal, enabling us to control large equipment such as jet airplanes and ocean ships .,Control systems increase efficiency by reducing waste of materials and energy , an increasing advantage as we seek ways to preserve environment . Safety is yet another benefit of automatic control . Finally

14、 , control systems such as the household heating system and the automatic transmission provide us with increased comfort and convenience .,In summary , the benefits of automatic control fall into the following six broad categories . Increased productivity : Improve quality and uniformity : Increased

15、 efficiency : Power assistance : Safety : Comfort and convenience .,Control systems are classified in a number of different ways . They are classified as closed-loop , depending on whether or not feedback is used . They are classified as analog or digital , depending on the nature of the signals con

16、tinuous or discrete . They are divided into regulator systems and follow-up systems , depending on whether the setpoint is constant or changing. They are grouped into process control systems or machine control systems, depending on the industry they are used in processing or discrete-part manufactur

17、ing . Processing refers to industry they are used to produce products such as food, petroleum, chemicals, and electric power. Discrete-part manufacturing refers to industries that make parts and assemble products such as automobiles, airplanes, appliances, and computers. They are classified as conti

18、nuous or batch (or discrete) , depending on the flow of product from the process-continuous or intermittent and periodic. Finally, they are classified as centralized or distributed, depending on where the controller are locatedin a central control room or near the sensors and actuators. Additional c

19、ategories include servomechanisms, numerical control, robotics, batch control, sequential control, time-sequential control, time-sequenced control, event-sequenced control, and programmable controllers. These general categories are summarized below.,2.Classification of Control Systems,(1) feedback N

20、ot usedopen-loop Usedclosed-loop (2) Type of signal Continuousanalog Discretedigital (3) Setpoint Seldom changedregular system Frequently changedfollow-up system (4) Industry Processingprocess control a. Continuous system b. Batch systems Discrete-part manufacturingmachine control a. Numerical contr

21、ol b. Robotic control systems (5) Location of the controllers Central control roomcentralized control Near sensors and actuatorsdistributed control (6) Other categories Servomechanisms Sequential control a. Event-sequenced control b. Time-sequenced control Programmable controllers,3.Block Diagrams a

22、nd Transfer Functions,Although it is not unusual to find several kinds of components in a single control system ,or two systems with completely different kinds of components , anycontrol system can be described by a set of mathematical equationsthat define the characteristics of each component. A wi

23、de range ofcontrol problem-including process, machine tools, servomechanisms,space vehicles, traffic, and even economics-can be analyzed by thesame mathematical methods. The important feature of each component isthe effect it has on the system. The block diagram is a method ofrepresenting a control

24、system that retains only this importantfeature of each component. Signal lines indicate the input and outputsignals of the component. As show in Fig.6.1. Each component receives an input signal from some part of systemand produces an output signal for another part of the system. Thesignal can be ele

25、ctric current, voltage, air pressure ,liquid flow rate,liquid pressure, temperature, speed, acceleration, position,direction, or others. The signal paths can be electric wires,pneumatic tubes, hydraulic lines, mechanical linkages, or anythingthat transfers a signal from one component to another. The

26、 componentmay use some source of energy to increase the power of the outputsignal.,3.Block Diagrams and Transfer Functions,A block diagram consists of a block representing each component ina control system connected by lines that represent the signal paths.The drivers sense of provides the two input

27、 signals; the position ofthe automobile and the position of the center of the road. The drivercompares the two positions and determines the positions of thesteering wheel that will maintain the proper position of theautomobile . To implement the decision, the drivers hands and armsmove the steering

28、wheel to the new position. The automobile respondsto the change in steering wheel position with a corresponding changein direction. After a short time has elapsed, the new direction movesthe automobile to a new position. Thus, there is a time delay betweena change in position of the steering and the

29、 position of theautomobile. This time delay is include in the mathematical equation ofthe block representing the automobile.,The loop in the block diagram indicates a fundamental concept ofcontrol. The actual position of the automobile is used to determinethe correction necessary to maintain the des

30、ired position. This conceptis called feedback, and control systems with feedback are calledclose-loop control systems. Control system that do not have feedbackare called open-loop control system because their block diagram doesnot have a loop. The most important characteristic of a component is ther

31、elationship between the input signal and the output signal. Thisrelationship is expressed by the transfer function of the component,which is defined as the ratio of the output signal divided by theinput signal. (Mostly, it is the Laplace transform of the outputsignal divided by the Laplace transform

32、 of the input signal),4 Open-loop Control,An open-loop control system does not compare the actual result with the desired result to determine the control action. Instead, a calibrated setting-previously determined by some sort of calibration procedure or calculation-is used to obtain the desired res

33、ult. The needle valve with a calibrated dial shown in Fig.6.2 is an example of an open-loop control system. The calibration curve is usually obtained by measuring the flow rate for several dial settings. As the calibration curve indicates, different calibration lines are obtained for different press

34、ure drops. Assume that a flow rate of is desired and a setting of S is used. As long as the pressure drop across the valve remains equal to , the flow rate will remain . If the pressure drop changes to , the flow rate will change to . The open-loop control cannot correct for unexpected changes in th

35、e pressure drop. The firing of a rifle bullet is anther example of an open-loop control system .The desired result is to direct the bullet to the bulls-eye .The actual result is the direction of the bullet after the gun has been fired. The open-loop control occurs when the rifle is aimed at the bull

36、s-eye and the trigger is pulled . Once the bullet leaves the barrel ,it is on its own : If a sudden gust of wind comes up ,the direction will change and no correction will be possible .,The primary advantage of open-loop control is that it is less expensive than closed-loop control: it is not necess

37、ary to measure the actual result . In addition ,the controller is much simpler because corrective action based on the error is not required . The disadvantage of open-loop control is that errors caused by unexpected disturbances are not corrected . Often a human operator must correct slowly changing

38、 disturbances by manual adjustment . In this case , the operator is actually closing the loop by providing the feedback signal .,5 Close loop Control: Feedback,Feedback is the action of measuring the difference between the actual result and the desired result, and using that difference to drive the

39、actual result toward the desired result. The term feedback comes from the direction in which the measured value signal travels in the block diagram. The signal begins at the output of the controlled system and ends at the input to the controller. The output of the controller is the input to the cont

40、rolled system. Thus the measured value signal is fed back from the output of the controlled system to the input. The term closed loop refers to loop created by the feedback path.,6.Variable Name,The controlled variable (C) is the process output variable that is to controlled. In a process control sy

41、stem, the controlled variable is usually an output variable that s a good measure of the quality of the product. The most common controlled variables are position, velocity, temperature, pressure level and flow rate. The set point (SP) is the desired value of the controlled variable. The measured va

42、riable (Cm) is the measured value of the controlled variable. It is the output of the measuring means and usually differs from the actual of the controlled variable by a small amount. The error (E) is the difference between the setpoint and the measured value of the controlled variable . It is computed according to the equation E=SP-. The controller output (V) is the control action intended to drive the measured value of the controlled variable toward the setpoint value ,The control action depends on the error signal (E) and