Pole Zero plot is giving wrong plot

Hi,

I've used 'pzmap' to create ploe zero plot of a transfer function. According to the transfer function used, all the poles of it should be on the imaginary axis. But this is not the case in the output.

code:- >> num=[3]; >> den=[1 0 10 0 9 0]; >> sys=tf(num,den)

Transfer function: 3 ------------------ s^5 + 10 s^3 + 9 s

>> pzmap(sys)

output:- http://postimage.org/image/1dx6mev9g/

Answer 1

[Wayne King]

Hi, The poles are all on the imaginary axis, look at the scale of your real axis, it is 10^{-16}

If you enter:

[p,z] = pzmap(sys);

You'll see the poles are:

      0          
   0.0000 + 3.0000i
   0.0000 - 3.0000i
  -0.0000 + 1.0000i
  -0.0000 - 1.0000i

Answer 2

[Honglei Chen]

If you look at the ticks of the real axes, you can see that the scale is 10e-16. So practically it is on the imaginary axis. If you want to see a better picture, you can do the following to rescale the axes

axis([-1 1 -5 5])

HTH