Hello,
actually the following code doesn't work:
s = tf('s')
phi = (2s +3)/(s+1).^2
nyquist(phi)
I have to ask what you want to calculate with this: phi = -1 + det[I+G(S)] = (2s +3)/(s+1)^2 ? I'm not quite sure what you want to do there. Now, to your questions:
1) As your system is MIMO (Multiple Input Multiple Output) because of the dimensions of B and C, your transfer function is acutally a transfer matrix (which input to which output) according to: G(s) = C*(s* I- A)^(-1)* B . Therefore MATLAB plots all four transfer functions (see the labels: Input .. to Output ..).
tf(ss(a,b,c,d)) =
From input 1 to output...
1
1: -----
s + 1
2: 0
From input 2 to output...
99
1: -----
s + 1
1
2: -----
s + 1
2) In your code is a little mistake I think. You forgot the w(k) in your numerator: G = (2 * w(k) *i+3)/((w(k)*i+1).^2) . Beside this fact, your numerical nyquist plot for this transfer function should be fine, but it won't start from w = 0, as logspace starts from 10^0 = 1 (if you're wondering why your plot is not complete).
I hope I could help you, best regards,
Stefan
