Glossary · PLC and control programming
Controlled variable
Also known as: Control variable, Process variable (PV)
German: Regelgröße
In control engineering, the controlled variable is the physical quantity that a control loop measures and keeps at or near its setpoint, such as a temperature, pressure, level or speed.
- PLC programming
- Process industry
In one sentence
The controlled variable is the measured quantity a control loop keeps at its setpoint, such as temperature, pressure or speed.
Example
In a tank heating loop, the tank temperature is the controlled variable, the steam valve position is the manipulated variable and 80 °C is the setpoint.
How it applies
- Engineering: Every control loop starts with a clear choice of controlled variable, the sensor that measures it and its engineering unit and range. The controller compares it with the setpoint and adjusts the manipulated variable.
- Commissioning: Loop tuning is judged by how the controlled variable reacts to setpoint changes and disturbances: overshoot, settling time and steady-state error.
- Operation: Operators usually see the controlled variable, the setpoint and the controller output side by side on the HMI faceplate.
- Documentation: Terms get mixed up easily. Define in the glossary of the manual which quantity is the controlled variable, which is the manipulated variable and which is the setpoint, and use these names consistently in texts, HMI labels and loop diagrams.
Controlled variable vs. manipulated variable
The controlled variable is what the loop wants to keep stable (the temperature). The manipulated variable is what the controller changes to achieve this (the valve opening). In English, the phrase control variable is used for both in different sources, which is why the German list term Regelgröße is rendered here as controlled variable. A reliable Closed-loop control depends on a trustworthy measurement of the controlled variable, see Sensor data integrity.