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Why does this step response overshoot?

  1. May 17, 2014 #1

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    I have this transfer function which i don't know why it is overshooting due to an step input.

    http://snag.gy/Vh1SM.jpg

    Could someone explain why it does that.

    It's it a close loop transfer function, where i've designed a controller for it, but for some reason, eventhoug all my poles are placed correctly, it is not possible to get the response i want to.
     
  2. jcsd
  3. May 17, 2014 #2

    rude man

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    With the given transfer function there is no overshoot.
    Wolfram alpha agrees!
     
  4. May 17, 2014 #3

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  5. May 17, 2014 #4

    rude man

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    Oops, there is too a small overshoot. I couldn't see it on wolfram. It's due to the zero at s = -1.513. Sorry!
    You can cancel it by inserting a pole at the same point (s = - 1.513).
     
  6. May 17, 2014 #5

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    But how come does zeroes apply overshoot? I thought it was only poles which affected the system
     
  7. May 17, 2014 #6
    You should check your entry. It does have overshoot.

    The transient response of your system is not only determined by its poles. You have a LHP zero as well, which can have a significant impact on the overshoot of the step response of your system.

    Quick Google search for 'poles zeros transient' turned up:
    http://courses.engr.illinois.edu/ece486/documents/lecture_notes/effects_zero_pole.pdf
     
  8. May 17, 2014 #7

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    but what criteria should i apply for determining the zeroes, and how does it affect the system. I mean it is still a LHP zero??? so bad can't it be
     
  9. May 18, 2014 #8

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    How should i using a PID controlle be able to adjust the Zero location, while i am adjusting the location of the pole.
     
  10. May 18, 2014 #9

    rude man

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    You need to describe your problem more fully, not just talk about an isolated transfer function. What is your plant and what are the response criteria?
     
  11. May 18, 2014 #10

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    I tried to make a general problem, but seems to have forgotten where it began at.
    I posted my problem on another forum.

    http://snag.gy/OqhxH.jpg

    I hope this helps.
    My repsponse criterias is that I want an overshoot to be 0% and settling time be under 1 sec.
    Using Matlab tuning application i managed to do with only the I part of the controller. The problem is though i've read alot of places that it is not recomended to use just I or D regulators.
     
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