A two-position controller, also called an ON/OFF controller, is the most basic type of process controller.As the name implies, it provides an ON/OFF signal to the process's control element see Fig (2-1).Ideally, if the set point temperature is 70?F, the heater will turn ON when the temperature is less than 70?F and turn OFF when it is greater than 70?F, as the heater tries to keep the error (SP - PV) at zero.The heater turns ON when the temperature is below the set point and turns OFF when the temperature reaches an acceptable level.