ON/OFF control action - Principle

The ON/OFF control (two-position ) controller operates only when the measurement crosses the set point. In on/off control ,a valve would have only two positions fully opened or fully closed .As a result the output measurement always cycles above and below the set point.

Application of On/Off control

ON-OFF control is best applied to a large capacitance process that has essentially no dead time . Eg : large tank or bath

A system where dead time is short and rate of rise is slow can be handled adequately by ON/OFF control

There is some hysteresis or deadband between the make/break or set/reset or on/off values.

Alarm relays trip exactly at setpoint, plus or minus the hysteresis.

On-Off control might do that or it might split the hysteresis half above the setpoint and half below the setpoint, like this