Glossary · Control theory and process control
Proportional controller
Also known as: P controller, Proportional action
German: P-Regler
In control engineering, a proportional controller produces an output proportional to the current control error, multiplied by the proportional gain; on its own it usually leaves a steady-state error when a load acts on the process.
- Control theory
In one sentence
A proportional controller's output is the control error times a gain; it reacts immediately but usually leaves a steady-state offset.
Example
A float valve that opens further the lower the tank level drops is a mechanical proportional controller, and the level settles slightly below the target at high demand.
How it applies
- Engineering: Proportional action is the backbone of most controllers. Higher gain means faster response and smaller offset, but too much gain leads to oscillation. Some systems express gain as a proportional band in percent (the inverse of gain).
- Commissioning: Pure P control is acceptable where a small offset doesn't matter or where the process integrates, such as some level and position loops.
- Documentation: State whether the parameter is gain or proportional band and in which units. Confusing the two inverts the effect of a change.
Proportional controller vs. PI controller
The proportional controller leaves an offset under load; the PI controller adds integral action to remove it. Choosing P control deliberately should be noted in the control narrative.