Glossary · Automation fundamentals, platforms and components
Process variable
Also known as: PV, Controlled variable
German: Prozessgröße
In control engineering, a process variable (PV) is a measured physical quantity of a process, such as temperature, pressure, flow, level or speed, that is monitored or controlled. In a closed control loop, the controller compares the PV with the setpoint and adjusts the manipulated variable.
- Automation components
In one sentence
A process variable (PV) is a measured quantity such as temperature, pressure or flow that a control loop monitors and keeps at its setpoint.
Example
The PV of loop TIC-101 is the reactor temperature from a Pt100 sensor; the controller keeps it at a setpoint of 80 °C.
How it applies
- Engineering: Each PV has a sensor, a measuring range, an Engineering unit and usually alarm limits. The control error is the difference between Setpoint and PV.
- Operation: HMI faceplates show PV, setpoint and controller output together; trends of the PV are the first place to look when a loop misbehaves.
- Documentation: List each PV with tag, range, unit and alarm limits in the loop list, and use the same tag in manuals and HMI texts. A PV depends on sensor quality: include calibration intervals and what Sensor drift looks like in maintenance documentation.
Process variable vs. manipulated variable
The PV is what you measure (tank level). The manipulated variable is what the controller changes to influence it (inlet valve position). A Disturbance variable also influences the PV but is not set by the controller.