How can I determine the gain that is needed to get a second order closed loop system to oscillate indefinitely?

So i created a bode plot using margin() function and then multiplied my closed loop transfer function with the gain margin that I got from margin(). I created new transfer function for the system using X= feedback(K*G,1). When I made the step response graph with step(X), it didn't seem to oscillate indefinitely. What's wrong in my method?

Thank you foreahead for your answers!

Sincerely Yours, Jani

Answer 1

[Arkadiy Turevskiy]

Can you paste your code. Seems like you are doing the right thing. Here is the code that does what you want.

s=tf('s')
ol=2/((s+1)*s*(s+10));
[gm,pm]=margin(ol);
step(feedback(ol,1),feedback(gm*ol,1),'r')
axis([0 30 0 2])
legend('original', 'multiplied by gm')