On/off control is the simplest mode of automatic control. It has only two outputs—on (100%) or off (0%)—and only responds to the sign of the error—positive or negative; i.e., whether it is above or below the setpoint.
Because of an effect known as constant cycling, on/off control is not generally suitable for continuous automatic feedback control. If the control valve in Figure 300-4 were to remain completely open when the level is above setpoint, and completely closed when the level drops below setpoint, a constant cycling of valve position and level would result (see Figure 300-5). As with open loop control, the varying level resulting from constant cycling may be acceptable in some noncritical level applications.