Glossary · Control theory and process control
Nyquist plot
Also known as: Nyquist diagram, Polar plot
German: Nyquist-Diagramm
In control engineering, a Nyquist plot shows the frequency response of an open control loop as a curve in the complex plane, with frequency as the parameter; its position relative to the critical point −1 indicates closed-loop stability according to the Nyquist criterion.
- Control theory
In one sentence
A Nyquist plot draws the open-loop frequency response in the complex plane; its path around the point −1 indicates closed-loop stability.
Example
In the controls course material, the Nyquist plot of the tuned speed loop passes well to the right of the point −1, showing adequate margins.
How it applies
- Engineering: The Nyquist criterion works even for loops with dead time and for open-loop unstable systems, where simpler rules fail. The shortest distance from the curve to −1 is a compact robustness measure.
- Commissioning: In practice, measurement tools more often display a Bode plot; Nyquist plots appear mainly in design and analysis software.
- Documentation: In training material, annotate the critical point and the margins directly in the diagram. Readers who aren't control engineers find the plot hard to read without this guidance.
Nyquist plot vs. Bode plot
Both show the same frequency response. The Nyquist plot combines magnitude and phase in one curve and makes the stability criterion visible; the Bode plot separates them over frequency and makes margins and resonances easier to read.