Warning: The closed-loop system is unstable.

I am getting this warning. It is saying that the function is unstable. The root locus plot indicates that the system is stable.

ctf =

10 s^3 + 1100 s^2 + 10000 s

-----------------------------------

s^4 + 210 s^3 + 11100 s^2 + 10000 s

Continuous-time transfer function.

>> rlocus(ctf)

>> [Gm]=margin(ctf)

Warning: The closed-loop system is unstable.

> In ctrlMsgUtils.warning (line 25)

In DynamicSystem/margin (line 65)

Gm =

Inf

Answer 1

[Walter Roberson]

There is no constant term in the denominator. There is a pole at 0, which is inside the unit circle, so it is unstable.

Answer 2

[Star Strider]

The system has a pole-zero cancellation at the origin:

s = tf('s');
ctf = (10*s^3 + 1100*s^2 + 10000*s) / (s^4 + 210*s^3 + 11100*s^2 + 10000*s);
[p,z] = pzmap(ctf)

Use minreal to resolve it and produce a stable system:

ctf = minreal(ctf)
Gm = margin(ctf)