Glossary · Automation fundamentals, platforms and components
Control deviation
Also known as: Control error, Error signal
German: Regelabweichung
In closed-loop control, the control deviation is the difference between the setpoint (reference variable) and the actual value of the controlled variable. The controller acts on this difference to reduce it.
- Automation components
- Standards
In one sentence
The control deviation is the difference between setpoint and actual value of the controlled variable that a controller works to reduce.
Example
The oven setpoint is 180 °C and the measured temperature is 172 °C, so the control deviation is 8 K and the controller increases heater power.
How it applies
- Engineering: Controller tuning decides how quickly and strongly the controller reacts to the control deviation. Integral action removes a remaining steady-state deviation.
- Operation: Large or persistent deviations are often alarmed, since they indicate disturbances, wrong tuning or equipment faults.
- Documentation: Specify sign conventions (setpoint minus actual value or the reverse), units and alarm limits. Trend displays and alarm texts should use the same wording as the control description.
- Commissioning: Recording setpoint, actual value and deviation in a trend during step tests shows whether the loop settles quickly and without oscillation.
Control deviation vs. offset
A persistent deviation that remains after the process has settled is called steady-state error or Offset in process control. It typically occurs with purely proportional controllers. The control deviation itself is the momentary difference at any time.