# Question about a PD controller.

## Main Question or Discussion Point

Hi, I'm taking a control engineering course and I've got a little doubt about PD controllers.

Here is my root locus plot: http://imageshack.us/photo/my-images/535/controlw.png/

I have the system defined by the poles in the root locus with some design specifications that are satisfied when the complex poles are in the same position as the green stars. In order to move the branches of the root locus a PD controller is used, so a zero appears in the plot. With a specific gain the specifications are satisfied as shown in the right plot.

My question is: the specifications are computed assuming a normalized second order system, so the transfer function of the final system would have only the two complex conjugate poles. I know that the leftmost pole can be ignored, but what about the remaining zero? Wouldn't it affect the response?

Any help will be appreciated!!

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