In open loop control, the controller adjusts the final control element without measuring the process. An example of open loop control is a cycle timer that operates a drain valve, as in the simple gas-liquid separation process shown in Figure 300-3. At predetermined intervals, the timer causes the drain valve to open even if there is nothing to drain.
A more common example of open loop control would be an automatic lawn sprinkler system. Here a clock timer opens a water valve for several minutes each day. It would not check to see if the lawn needed water and would even turn on the sprinklers in the rain. Open loop control like these examples is not widely used. Open loop control operating in a feed forward mode is frequently used along with closed loop control. Feed forward control is discussed in Section 342.
