Why am I getting an error when using the RLOCUS function to find the root locus of my system when using the Control System Toolbox 5.1 (R12.1)?

Why am I getting an error when using the RLOCUS function to find the root locus of my system when using the Control System Toolbox 5.1 (R12.1)?

Here is my system:

nume=[1 0 1];

dene=conv([1 0],[1 0 4]);

rlocus(nume,dene);

I get the following error upon running the above code

 ??? Error using ==> eig

 NaN or Inf prevents convergence.

RLOCUS works fine with other systems.

Answer 1

[MathWorks Support Team]

This is a bug in RLOCUS in the Control System Toolbox 5.1 (R12.1) that has been fixed in the Control System Toolbox 5.2 (R13).

As a workaround for R12.1, please explicitly specify the Gain value in the RLOCUS command.

nume=[1 0 1];

dene=conv([1 0],[1 0 4]);

sys = tf(nume,dene);

GainK=0;

rlocus(sys,GainK);