Glossary · Control theory and process control
Gain margin
Also known as: Amplitude margin
German: Amplitudenreserve
In control engineering, the gain margin is the factor by which the open-loop gain of a control loop could increase at the frequency where the phase reaches −180° before the closed loop becomes unstable.
- Control theory
In one sentence
Gain margin is how much the open-loop gain could rise at −180° phase before the closed loop becomes unstable.
Example
The Bode plot of a pressure loop shows a gain margin of 8 dB, so a doubling of process gain at high load would still leave the loop stable.
How it applies
- Engineering: Gain margin is usually expressed in decibels. It indicates how robust a loop is against changes in process gain, for example a valve whose characteristic is steeper in one range. Target values depend on the application and company practice.
- Commissioning: Read gain margin from a measured Bode plot of the open loop, together with the Phase margin; neither alone guarantees good behavior.
- Documentation: When a specification requires a minimum gain margin, state how it is measured and at which operating point, so verification can be repeated.
Gain margin vs. phase margin
Gain margin tolerates gain increases; Phase margin tolerates additional phase lag, such as extra dead time from a slower bus or filter. A loop can have a good gain margin and still be poorly damped because of a small phase margin.