Glossary · Automation fundamentals, platforms and components
Steady state
Also known as: Equilibrium state
German: Stationärer Zustand
In control engineering, the steady state is the condition of a system in which, after transient effects have died out, its variables no longer change over time for constant inputs. The remaining difference between setpoint and process variable in this state is the steady-state error.
- Automation components
In one sentence
The steady state is the condition in which a system's variables stay constant after transients have died out; any remaining deviation is steady-state error.
Example
After the setpoint change, the temperature settles at 79.6 °C instead of 80 °C; the 0.4 K offset is the steady-state error of the P controller.
How it applies
- Engineering: Process gain is measured in steady state, and a controller with integral action removes steady-state error for constant setpoints and disturbances.
- Commissioning: Test results, calibration and energy measurements are only meaningful once the system has reached steady state. Define a settling criterion, for example staying within ±1 % for 60 s.
- Documentation: Performance data such as accuracy, energy use or throughput should state that they apply in steady state and under which conditions. Start-up and transition behavior are described separately.
Steady state vs. transient response
The Transient response describes how the system gets from one state to another (overshoot, settling time). Steady state is where it ends up. Both are needed to specify a control loop fully.