Negative Values in Step Response

Hi guys,

I just noticed something interesting on MATLAB.

I was working with a transfer function, and I inserted a delay using a Pade approximation, and plotted the step response. At first glance, it seems just like any other step response, however when i zoomed in on the origin, I noticed that it slightly dips into negative values before coming up.

I've never noticed this before. Anyone have any idea why it does this?

Answer 1

[Azzi Abdelmalek]

This is normal, when your transfer function contains positives zeros

example H(p)=(-p+2)/(p^2+2p+1)

step([-1 2],[1 2 1])

[Kenneth]

Hmm indeed it did have a positive zero. Why does it do this though? What does it mean?

[Rajiv Singh]

See: http://www.smp.uq.edu.au/people/YoniNazarathy/Control4406/resources/HoaggBernsteinNonMinimumPhaseZero.pdf

[Kenneth]

Interesting stuff, thanks for the info!