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控制空間內(nèi)溫度的理想方法是以所需的量來替換損失的熱量或置換獲得的熱量。使用基本的兩位控制，這樣的操作是不可能的，因?yàn)榧訜峄蚶鋮s系統(tǒng)要么完全打開，要么完全關(guān)閉，并且在任何特定時刻的輸送量要么太多，要么太少。然而，定時雙位置控制可以預(yù)測需求，并按百分比按時提供測量的加熱或冷卻量，以減少控制點(diǎn)波動。定時由電子控制中的熱預(yù)測器和電子和數(shù)字控制中的計時器完成。在定時雙位置控制中，控制器和終控制元件之間的基本交互作用與基本雙位置控制相同。然而，控制器對受控變量平均值的逐漸變化而不是周期性波動作出響應(yīng)。由于熱預(yù)測或時間比例特性導(dǎo)致機(jī)械設(shè)備的循環(huán)速度更快，因此可以減少或消除過沖和欠沖。結(jié)果是比基本雙位置控制更接近變量控制（圖23）。圖23：。比較基本兩位和定時兩位控制。熱預(yù)測在機(jī)電控制中，可通過在雙金屬傳感元件上添加熱預(yù)測器來實(shí)現(xiàn)定時雙位置控制。在加熱系統(tǒng)中，連接熱預(yù)測器，使其在雙金屬元件需要加熱時通電。溫度下降時，傳感元件會啟動加熱系統(tǒng)和熱量預(yù)測器。熱預(yù)測器提前將雙金屬元件加熱到其關(guān)閉點(diǎn)，并使加熱系統(tǒng)和熱預(yù)測器斷電。隨著環(huán)境溫度的下降，雙金屬元件加熱到關(guān)閉點(diǎn)所需的時間增加，冷卻時間減少。因此，熱預(yù)測器根據(jù)環(huán)境溫度自動改變開啟時間與關(guān)閉時間的比率。由于熱量僅供應(yīng)給傳感器，因此隨著熱量需求的增加，熱量預(yù)測功能會降低控制點(diǎn)。降低的控制點(diǎn)稱為“下垂”，在設(shè)計條件下保持較低的溫度，下面將對其進(jìn)行更深入的討論。在恒溫器關(guān)閉期間通電加熱器可實(shí)現(xiàn)對恒溫器冷卻的預(yù)期操作。在任何一種情況下，接通時間百分比都與系統(tǒng)負(fù)載成比例變化。

The ideal method of controlling the temperature in a space is to replace lost heat or displace gained heat in exactly the amount needed. With basic two-position control, such exact operation is impossible because the heating or cooling system is either full on or full off and the delivery at any specific instant is either too much or too little. Timed two-position control, however, anticipates requirements and delivers measured quantities of heating or cooling on a percentage on-time basis to reduce control point fluctuations. The timing is accomplished by a heat anticipator in electric controls and by a timer in electronic and digital controls. In timed two-position control, the basic interaction between the controller and the final control element is the same as for basic two-position control. However, the controller responds to gradual changes in the average value of the controlled variable rather than to cyclical fluctuations. Overshoot and undershoot are reduced or eliminated because the heat anticipation or time proportioning feature results in a faster cycling rate of the mechanical equipment. The result is closer control of the variable than is possible in basic twoposition control (Fig. 23). Fig. 23. Comparison of Basic Two-Position and Timed Two-Position Control. HEAT ANTICIPATION In electromechanical control, timed two-position control can be achieved by adding a heat anticipator to a bimetal sensing element. In a heating system, the heat anticipator is connected so that it energizes whenever the bimetal element calls for heat. On a drop in temperature, the sensing element acts to turn on both the heating system and the heat anticipator. The heat anticipator heats the bimetal element to its off point early and deenergizes the heating system and the heat anticipator. As the ambient temperature falls, the time required for the bimetal element to heat to the off point increases, and the cooling time decreases. Thus, the heat anticipator automatically changes the ratio of on time to off time as a function of ambient temperature. Because the heat is supplied to the sensor only, the heat anticipation feature lowers the control point as the heat requirement increases. The lowered control point, called “droop”, maintains a lower temperature at design conditions and is discussed more thoroughly in the following paragraphs. Energizing the heater during thermostat off periods accomplishes anticipating action in cooling thermostats. In either case, the percentage on-time varies in proportion to the system load.