inControl ep42 - inControl guide to ... the Nyquist criterion
Mar 16, 2026
A lively dive into Harry Nyquist's work, the birth of negative feedback, and why stability can suddenly turn to oscillation. The conversation traces historical Bell Labs breakthroughs and the geometric Nyquist plot idea. It highlights practical measurement, robustness measures like gain and phase margins, and modern extensions to nonlinear and multivariable analysis.
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How Bell Labs And Nyquist Sparked Breakthroughs
- Bell Labs funded patient, mission-driven research and arranged spaces for chance interactions that produced breakthroughs.
- A patent study found employees with most patents often shared meals with Harry Nyquist, who provoked ideas by asking good questions.
Use Nyquist Plot To Determine Closed Loop Stability
- To test loop stability, draw the Nyquist plot of the open-loop transfer over all frequencies and check encirclements of minus one.
- If the open-loop is stable, zero encirclements => closed-loop stable; each clockwise encirclement adds one unstable pole.
Always Declare Nyquist Conventions First
- Always state your Nyquist conventions and test them on a known example before trusting counts.
- Conventions (clockwise vs counterclockwise, critical point sign) change the encirclement arithmetic and are a common source of error.
