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When implementing the control for my application I got this Nyquist plot which made me flabbergasted.

enter image description here

If I zoom on the point $-1 + 0j$ I get the following image

enter image description here

How do I interpret this contour going to infinity? Do I have 1 or 2 counter-clockwise turns around $-1 + 0j$?

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  • $\begingroup$ How many integrators does your open loop have? $\endgroup$ – fibonatic Jun 16 '19 at 5:47
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It seems you have 1 counter-clockwise turn around. One trajectory comes from the left, then turns around and then moves to the right as you change the parameters. I think this behavior is normal as you have 1 integrator (PID controller) and you have one pole in s=0. As you change the parameters you annulate some function chunks and you get at s=0. Then you continue changing your parameters and get backwards to the right, though this time the trajectory is different due to the different values your function is taking.

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