On-Off Control v. Proportional Control

What Is The Difference Between On-Off Control and Proportional Control?

A proportional controller output isn’t just on or off. Its output can be adjusted to any value between 0% and 100% of power.

Just as with on-off control, the control direction is important. Reverse or inverse acting changes the output from 100% to 0% as the process rises if heating. Direct acting would go from 0% to 100% if cooling.

When correctly tuned, the controller can adjust its output to match the power that is required to keep the process stable at the setpoint. This is something an On-Off controller cannot achieve.

If Proportional Control Always Gives A More Stable Process, Why Would I Choose To Use On-Off Control?

The user must tune a proportional controller to match the characteristics of the process, which can be difficult in some cases.

On-off control is chosen for simplicity. The only parameters that need to be set are the switching differential and setpoint. On-off control is suitable if the response delay is short when the output is switched and the maximum rate of rise is small, or if a very stable process isn’t needed.

When Should I Use Proportional Control? 

Choose proportional control if stable control is required and other factors don’t prevent it. Also, there are some occasions where on-off cannot be used; for example, if the controller has a 0 to 10 volt or 0 to 20mA analogue output connected modulating valve, proportional control has to be used.