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傳感器可以與控制器分離或作為控制器的一部分，并位于受控介質(zhì)中。傳感器測量受控變量的值，并將結(jié)果信號發(fā)送給控制器。控制器接收傳感器信號，將其與期望值或設(shè)定點進行比較，并生成校正信號以指導(dǎo)受控設(shè)備的操作。受控設(shè)備改變控制代理，以調(diào)節(jié)產(chǎn)生所需條件的控制設(shè)備的輸出。HVAC應(yīng)用程序使用兩種類型的控制回路：打開和關(guān)閉。開環(huán)系統(tǒng)假定受控條件和外部條件之間存在固定關(guān)系。開環(huán)控制的一個例子是根據(jù)室外空氣溫度傳感器的輸入控制周邊輻射加熱。當(dāng)室外空氣溫度降至規(guī)定設(shè)置時，循環(huán)泵和鍋爐通電，水溫或流量根據(jù)室外溫度按比例控制。開環(huán)系統(tǒng)不考慮內(nèi)部熱增量、滲透/滲出、太陽能增量或建筑中其他變化變量引起的空間條件變化。單獨的開環(huán)控制不能提供閉合控制，可能導(dǎo)致過熱或過熱。因此，開環(huán)系統(tǒng)在住宅或商業(yè)應(yīng)用中并不常見。閉環(huán)系統(tǒng)依靠測量受控變量來改變控制器輸出。圖19顯示了閉環(huán)系統(tǒng)的框圖。閉環(huán)控制的一個示例是管道中的排氣溫度，該溫度確定流向加熱盤管的熱水流量，以將排氣溫度保持在控制器設(shè)定值。

The sensor can be separate from or part of the controller and is located in the controlled medium. The sensor measures the value of the controlled variable and sends the resulting signal to the controller. The controller receives the sensor signal, compares it to the desired value, or setpoint, and generates a correction signal to direct the operation of the controlled device. The controlled device varies the control agent to regulate the output of the control equipment that produces the desired condition. HVAC applications use two types of control loops: open and closed. An open-loop system assumes a fixed relationship between a controlled condition and an external condition. An example of open-loop control would be the control of perimeter radiation heating based on an input from an outdoor air temperature sensor. A circulating pump and boiler are energized when an outdoor air temperature drops to a specified setting, and the water temperature or flow is proportionally controlled as a function of the outdoor temperature. An open-loop system does not take into account changing space conditions from internal heat gains, infiltration/exfiltration, solar gain, or other changing variables in the building. Open-loop control alone does not provide close control and may result in underheating or overheating. For this reason, open-loop systems are not common in residential or commercial applications. A closed-loop system relies on measurement of the controlled variable to vary the controller output. Figure 19 shows a block diagram of a closed-loop system. An example of closed-loop control would be the temperature of discharge air in a duct determining the flow of hot water to the heating coils to maintain the discharge temperature at a controller setpoint.