Glossary · Control theory and process control
Bode plot
Also known as: Bode diagram
German: Bode-Diagramm
In control engineering, a Bode plot is a pair of graphs that shows the frequency response of a system: the magnitude in decibels and the phase in degrees, both plotted against frequency on a logarithmic scale.
- Control theory
In one sentence
A Bode plot shows a system's frequency response as magnitude and phase over logarithmic frequency; it is used to read gain and phase margin.
Example
The drive engineer records a Bode plot of the speed loop with the drive's measurement function and reads a phase margin of about 50 degrees at the crossover frequency.
How it applies
- Engineering: Bode plots let engineers read Gain margin and Phase margin of an open loop and see resonances, for example of a mechanical axis. Many servo drive commissioning tools measure and display them directly.
- Commissioning: Store the measured plots of critical loops with the commissioning record. A later measurement can then show whether mechanical wear or a changed load has shifted a resonance.
- Documentation: When a manual shows a Bode plot, label axes with units, state whether it shows the open or closed loop and name the operating point. Without that context, readers can't interpret the margins.
Bode plot vs. Nyquist plot
Both show the same frequency response. The Bode plot splits magnitude and phase into two graphs over frequency; the Nyquist plot shows the response as one curve in the complex plane, which makes the Nyquist stability criterion visible.