Glossary · Control theory and process control
Stability margin
Also known as: Robustness margin
German: Stabilitätsreserve
In control engineering, the stability margin is the distance of a control loop from instability, expressed as gain margin, phase margin or the minimum distance of the open-loop Nyquist curve from the critical point.
- Control theory
In one sentence
Stability margin measures how far a control loop is from instability, usually as gain margin and phase margin.
Example
The design review requires every drive loop of the machine to keep a stability margin of at least 6 dB gain margin and 45 degrees phase margin at maximum load.
How it applies
- Engineering: Stability margins ensure that a loop stays stable when the process, the load or the timing changes within expected limits. Target values depend on the application and company practice.
- Verification: Margins can be computed from a model or measured on the machine. A measured margin at nominal conditions doesn't cover worst-case loads unless those are tested too.
- Documentation: When margins are requirements, write them as testable Verification criterion statements: which loop, which margin, at which operating point, measured how. Record the measured values in the commissioning report.
Stability margin vs. performance
Large margins make a loop robust but usually slower. Tuning balances margin against Rise time and disturbance rejection; a specification should state both.