# Extract information about zeros of system from step response

Is there any way to know whether or not an unknown system has positive or negative zeros just from looking at the step response? This is the step response:

• If I remember correctly, then if the step response initially moves in the opposite direction, then your system is non-minimum phase (has an right half plane zero(s)). But I do not know the formal proof for this. Apr 8 '17 at 0:00

I found that I got a reasonable fit with the transfer function $G_p(s) = \frac{(s-1)^2}{(s+1.5)^2}$, though you could obviously do better by tweaking parameters.