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圖19：。閉環(huán)系統(tǒng)中的反饋。在此示例中，傳感元件測量排氣溫度并向控制器發(fā)送反饋信號。控制器將反饋信號與設定點進行比較。根據(jù)差異或偏差，控制器向閥門發(fā)出校正信號，閥門調(diào)節(jié)熱水流量以滿足工藝要求。因此，受控變量的變化反映了需求。傳感元件繼續(xù)測量排氣溫度的變化，并將新情況反饋給控制器，以便進行持續(xù)比較和校正。如上所述，自動控制系統(tǒng)使用反饋來減小偏差的大小并產(chǎn)生系統(tǒng)穩(wěn)定性。二次輸入，例如來自室外空氣補償傳感器的輸入，可以提供有關(guān)影響受控變量的干擾的信息。除了受控變量外，還使用一個輸入，使控制器能夠預測干擾的影響并對其進行補償，從而減少干擾對受控變量的影響。控制方法概述自動控制系統(tǒng)根據(jù)能量傳輸類型和用于執(zhí)行其功能的控制信號類型（模擬或數(shù)字）進行分類。自動控制系統(tǒng)常見的能源形式是電和壓縮空氣。系統(tǒng)可包括一種或兩種形式的能量。使用電能的系統(tǒng)由機電、電子或微處理器控制。氣動控制系統(tǒng)使用來自傳感器的不同氣壓作為控制器的輸入，控制器反過來向終控制元件發(fā)出氣動輸出信號。氣動、機電和電子系統(tǒng)執(zhí)行有限的預定控制功能和順序。基于微處理器的控制器對各種各樣的控制序列使用數(shù)字控制。

Fig. 19. Feedback in a Closed-Loop System. In this example, the sensing element measures the discharge air temperature and sends a feedback signal to the controller. The controller compares the feedback signal to the setpoint. Based on the difference, or deviation, the controller issues a corrective signal to a valve, which regulates the flow of hot water to meet the process demand. Changes in the controlled variable thus reflect the demand. The sensing element continues to measure changes in the discharge air temperature and feeds the new condition back into the controller for continuous comparison and correction. Automatic control systems use feedback to reduce the magnitude of the deviation and produce system stability as described above. A secondary input, such as the input from an outdoor air compensation sensor, can provide information about disturbances that affect the controlled variable. Using an input in addition to the controlled variable enables the controller to anticipate the effect of the disturbance and compensate for it, thus reducing the impact of disturbances on the controlled variable. CONTROL METHODS GENERAL An automatic control system is classified by the type of energy transmission and the type of control signal (analog or digital) it uses to perform its functions. The most common forms of energy for automatic control systems are electricity and compressed air. Systems may comprise one or both forms of energy. Systems that use electrical energy are electromechanical, electronic, or microprocessor controlled. Pneumatic control systems use varying air pressure from the sensor as input to a controller, which in turn produces a pneumatic output signal to a final control element. Pneumatic, electromechanical, and electronic systems perform limited, predetermined control functions and sequences. Microprocessor-based controllers use digital control for a wide variety of control sequences.