This section explains ON/OFF control which is the simplest type of feedback control.
ON/OFF control means to turn ON or OFF the control section depending on whether the deviation (target value - measured value) is positive or negative.

Figure 2.1-1 shows how ON/OFF control changes the status of the control valve and liquid temperature.
As shown in Figure 2.1-1 (1), when the temperature of the liquid falls below the target value, the control valve is fully opened (ON) to feed steam.
When steam is fed, the liquid temperature does not rise immediately, but begins to rise after a while and then exceeds the target value as in (2). When the temperature exceeds the target value, the control valve is fully closed (OFF) to stop feeding steam.
However, the liquid temperature does not fall immediately but begins to do so after a while.

Under ON/OFF control, liquid temperature is not constant but varies repeatedly as shown in Figure 2.1-1.
P control (proportional control) was invented to solve this problem as follows.