Glossary · Control theory and process control
Phase margin
Also known as: Phase reserve
German: Phasenreserve
In control engineering, the phase margin is the additional phase lag an open control loop could tolerate at its gain crossover frequency (where the open-loop gain is 1) before the closed loop becomes unstable.
- Control theory
In one sentence
Phase margin is the extra phase lag a loop could tolerate at gain crossover before becoming unstable; it indicates damping.
Example
Adding a slower filter to a noisy speed measurement reduced the phase margin from 55 to 30 degrees, and the speed loop began to ring after load changes.
How it applies
- Engineering: Phase margin is a good indicator of damping and overshoot. Dead time, filters, sampling and communication Latency all add phase lag and reduce it, so moving a loop to a slower network or controller cycle can destabilize it.
- Commissioning: Read the phase margin from the open-loop Bode plot at the crossover frequency and record it with the commissioning data.
- Documentation: Changes to cycle times, filters or network architecture belong in impact analyses of control loops. System documentation should list which loops are sensitive to additional delay.
Phase margin vs. gain margin
Gain margin protects against gain changes, phase margin against additional delay. Both are needed; a specification that names only one is incomplete.