7.1.1 On/Off Control

 

On/Off control is the simplest means of control and simply turns heating power on when the PV is below setpoint and off when it is above setpoint. As a consequence, On/Off control leads to oscillation of the process variable.

 

This oscillation can affect the quality of the final product but may be used on non-critical processes. A degree of hysteresis must be set in On/Off control if the operation of the switching device is to be reduced and relay chatter is to be avoided. For heating, hysteresis is applied below the setpoint as shown in section 7.2.11.

 

If cooling is used, cooling power is turned on when the PV is above setpoint and off when it is below. For cooling, hysteresis is applied above the setpoint as shown in section 7.2.11.

 

It is suitable for controlling switching devices such as relays, contactors, or digital (logic) devices.