finding the coordinate where imaginary value is zero in nyquist plot

Here I have plotted a nyquist plot and I want to find the coordinate where imaginary value is zero but I cant find it because it is very small. Is there any way I can find it specifically the coordinate where imaginary value is zero. I even zoomed it and pin-pointed but the imaginary value wont go to zero but very small values.

thank you for ur time

Answer 1

[Paul]

The doc page for nyquist shows how to use it to return the real (Re) and imaginary (Im) parts along with the frequency vector (w). That latter two can be used to determine value of w at the zero crossings of Im for w > 0 (not sure if you count w == 0 as a crossing), and that value of w can be used to determine Re. Functions like fsolve and interp1 might be helpful.

Or, you can think about how those crossing points relate to gain or phase margins. Matlab has functions that can compute those as well.