测控技术与仪器专业英语 第六章 化工出版社

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 components 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 liquid 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 desired 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.
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 positions 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-moving 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 ensuring the purity and uniformity of thquality 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 reducing manufacturing costs and minimizing the waste disposal problem. Sewage and waste treatment also requires the use of automatic control system.
In summary , the benefits of automatic control fall into the following six broad categories . ① Increased productivity : ② Improve quality and uniformity : ③ Increased efficiency : ④ Power assistance : ⑤ Safety : ⑥ 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 —continuous 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 manufacturing . 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 continuous 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 located—in a central control room or near the sensors and actuators. Additional categories 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.
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An Overview for Control System
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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 changes in physical activity and external ambience. In one control system—the eye—the 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 cells.
An excellent idea of the scope of control systems is given in an Instrument Society 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 operators 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 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 , control systems such as the household heating system and the automatic transmission provide us with increased comfort and convenience .