Steady state doesn't appear in step response plot

I Have the following specific transfer function:

bad_sys = tf([0 0 93 278], [1 20 12 0]);

And I want to plot the step response of its close loop (with negative feedback, no gain), and display the settling time. So I call:

step(bad_sys/(1+bad_sys));

But the plot I get don't dave the dashed line which represents the steady state value, and thus will not display the settling time when I choose this option (not defined):

This steady state line, and the option to display settling time works great for the following transfer function:

good_sys = tf([0 0 1 1], [1 2 2 2]);
step(good_sys/(1+good_sys));

I suggested that the problem is in bad_sys. So I called:

sisotool

And defined G in the architecture as bad_sys:

Now I let the magical sisotool to plot the step response (with the gain C=1). This should yield the same problem, but here I got the line I wanted and the option to display settling time:

So my question is, why the steady state line of bad_sys is not displayed (and I cant display settling time) via the step function?

Answer 1

[Aquatris]

The bad_sys works if you call the step function with;

step(minreal(bad_sys/(1+bad_sys)))

I am not sure about the exact reason but one possibility is, when you closed the loop without calling minreal() function, you basically do not allow pole-zero cancellation. Since bad_sys has a pole at zero, this causes problems (internally Matlab probably does not know what to do). good_sys on the other hand do not have a pole at zero so Matlab is happy to work with it.