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Process Automatic Control System

By Bruce Greene,2014-05-27 14:58
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Process Automatic Control System

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     Chapter 8 Process Automatic Control System 过程自动控制系统

     8.1 Illustrative System 图解系统

     Communication System 通信系统 AM broadcast system 调幅广播系统 audio音频的,

    声频的 microphone 扩音器, 麦克风 transmitter 发送器 receiver 接收器 antenna 天线

    processor 处理器 transmitting system 发送系统 receiving system 接收系统 Amplitude

    Modulation 调幅 amplitude 振幅 modulation 调制 radio frequency 无线电频率 射频

    lower frequency 低频 audible frequency 可听频率 radio frequency current 射频电流

    electromagnetic wave 电磁波 radiate 辐射 filter 过滤?滤波

     measurement system 测量系统 thermometer 温度计 linearizer 线性化电路

    register 寄存器 ice bath 冰浴器 proportional 成比例的?比例的 correction 修正?校

     error 误差 display register 显示寄存器 reproduction 再现 modify 修改?改变

    bimetallic strip 双金属条?,双金属片 mechanism 机构?机械装置

     bend 弯曲

     dial 刻度盘 mechanical switch 机械开关 radiator 散热器

     A Communication System The first system we shall illustrate is the communications system. The input can be speech, music, or data that is produced at one location and transmitted efficiently over long distances, permitting faithful recovery of the original input. The example chosen, the familiar AM broadcast system is shown schematically in Figure 8.1

     The letters AM stand for Amplitude Modulation. They mean that the amplitude or magnitude of a radio frequency (RF) signal is caused to vary according to the magnitude of a lower-frequency signal (audio, corresponding to audible frequencies).

     The function of transmitter in an AM broadcast system is to accept the input signal from an input device (microphone), use this signal to control the amplitude of a radio frequency signal( each broadcast station is assigned its own radio frequency), and drive the output device (the antenna) with a radio frequency current to produce electromagnetic waves radiating into space.

     The receiving system consists of an input device (the antenna), a processor (the receiver), and an output device (the loudspeaker).

     The functions of the receiver are to amplify or increase the strength of the relatively weak signal obtained from the antenna, to filter or select the desired radio frequency signal from the signals of all other broadcast stations, to recover the audio signal from the amplitude variations of the radio frequency signal, and to drive a loudspeaker with this audio signal.

     A Measurement system A second system is a measurement system. The purpose of this system is to acquire information from suitable transducers about the behavior of some physical system and to display this information to the observer. An example of such a system, a digital thermocouple thermometer, is shown in Figure8.2

     The pair of thermocouple junctions, one attached to the object whose temperature is to be measured, the other submerged in an ice bath (to establish a stable reference point), presents to the processor a voltage that depends on the temperature difference between the object to be measured and the ice bath.

     Because the thermocouple voltage is never exactly proportional to the temperature difference, a small correction must be applied to the thermocouple voltage to produce an analog voltage that is exactly proportional to the temperature.

     This correction is the role of the linearizer. The analog voltage from the thermocouple is first amplified (i.e., made larger) then linearized, and then converted to digital form. Finally, it is displayed in a digital display register as the output of the thermometer.

     Although a major goal of the communications system is to transmit a faithful reproduction of the source signal, a major goal of the measurement system is to produce numerically accurate data. In measurement system, therefore, one expects to be concerned with locating and removing any small errors that might be added to the signal at each step of the processing sequence.

     A Feedback Control system The third system is the feedback control system, in which information about the behavior of the output modifies the signals driving the system. In Figure 8.3 a thermostat is used to control the temperature of a room.

     In this case, the thermostat contains the input device for determining the room temperature (normally a bimetallic strip that bends as its temperature is varied), a mechanism for setting the desired temperature (the set point dial), and mechanical switches, activated by the bimetallic strip, which control the furnace.

     This familiar example, which in fact includes no electrical components other than a switch, was chosen to emphasize the feedback concept. Suppose the feedback line were disconnected in Figure 8.3 . That is ,suppose there were no mechanism for turning the furnace ON and OFF, the temperature of the room would either rise to some maximum (if the furnace were off all the time).

     Presuming that the maximum temperature is too hot for comfort while the minimum temperature is too cool, some controller is needed to turn the furnace ON and

    OFF. The controller might be a person who turns the heat ON when he feels cold and turns it OFF when he feels warm.

     Even at this level, the system (including the person) comprises a feedback control system, because information about the output (i.e.,the temperature of the room) is used to modify one of the driving signals (the on-off switch on the furnace).

     The thermostat is simply a piece of hardware that performs automatically what the temperature- sensitive person would do; namely, it turns the furnace ON when the temperature drops below the point where the person set the thermostat, and turns it OFF otherwise.

     There are many other feedback systems, including systems in which the processing of signal is done electronically. We shall encounter several electronic feedback systems later.

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