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例如，在冷卻系統(tǒng)中，控制器設置為在空間溫度達到78F時打開冷卻閥，在溫度降至76F時關閉閥。兩個溫度（2華氏度）之間的差值就是差分間隙。受控變量在兩個溫度之間波動?；镜碾p位置控制適用于許多應用。然而，為了進行嚴密的溫度控制，必須加速或定時循環(huán)?；緝晌豢刂圃诨緝晌豢刂浦?，控制器和終控制元件相互作用，無需來自機械或熱源的修改。其結(jié)果是受控設備的循環(huán)運行，以及受控變量在兩個值（開和關點）之間來回循環(huán)的情況，并受系統(tǒng)滯后的影響。在受控變量達到差速器兩個極限中的一個或另一個極限之前，控制器不能改變終控制元件的位置。因此，差速器是受控變量的小可能擺動。圖21顯示了典型的加熱系統(tǒng)循環(huán)模式。溫度（°F）OFF ON 75 74 73 72 71 70 69 68時間欠調(diào)條件差分刻度盤設置過調(diào)條件C2088
An example of differential gap would be in a cooling system in which the controller is set to open a cooling valve when the space temperature reaches 78F, and to close the valve when the temperature drops to 76F. The difference between the two temperatures (2 degrees F) is the differential gap. The controlled variable fluctuates between the two temperatures. Basic two-position control works well for many applications. For close temperature control, however, the cycling must be accelerated or timed. BASIC TWO-POSITION CONTROL In basic two-position control, the controller and the final control element interact without modification from a mechanical or thermal source. The result is cyclical operation of the controlled equipment and a condition in which the controlled variable cycles back and forth between two values (the on and off points) and is influenced by the lag in the system. The controller cannot change the position of the final control element until the controlled variable reaches one or the other of the two limits of the differential. For that reason, the differential is the minimum possible swing of the controlled variable. Figure 21 shows a typical heating system cycling pattern. TEMPERATURE (°F) OFF ON 75 74 73 72 71 70 69 68 TIME UNDERSHOOT CONDTION DIFFERENTIAL DIAL SETTING OVERSHOOT CONDTION C2088
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