MATLAB Nyquist plot error

Question

When i try to draw Nyquist plot for

1 / (s * (s^2+s+4) )

the curve does not move towards imaginary axis for w=infinity.
It moves away from imaginary axis in MATLAB. I get same errors when there is 's' term in denominator.

Answer 1

I don't think you understand what's happening:

the system you built starts at infinity because you have a pole at 0, then it moves to 0 as w goes to infinity. You can double check this with the bode plot. For your nyquist plot, use data cursor to track how things change.

Where your source of confusion likely lies is that the nyquist uses negative frequencies, thus, when it starts a -Inf, it is 0, as w approaches 0, it wraps around infinity, and then as w goes to +inf, it once again returns to 0. Confused? good! I hate nyquist plots, and I have little patience for them