generating wrong Nyquist plot of tf

i have been trying to plot nyquist plot of following transfer funtion but i am getting wrong.

sys =(s+3)/(s^2*(s-3))

when i tries to plot it using control technique i am getting different plot

please help

[Paul]

What is your code and why do you think the result is wrong?

[keshav agrawal]

the code is

s = tf('s')
sys =(s+3)/(s^2*(s-3))
nyquist(sys)

the plot i got from this is

the nyquist plot i am expecting should be like this :-

Answer 1

[Paul]

Keep in mind that nyquist() doesn't draw the arc at infinity due to the double pole at the origin.

If you zoom in around the origin on the Nyquist plot produced by nyquist(), you'll see that the plot crosses the imaginary axis at a value of around 0.1*j and approaches the origin from the left, as in your sketch.

[keshav agrawal]

thank you for reply ... i am statisfied with your answer. but is there any function other than nyquist() that can show infinite semi circles.

[Paul]

None that I'm aware of.

Answer 2

[Girijashankar Sahoo]

sys=tf([0 0 1 3],[1 -3 0 0])

nyquist(sys)