Why does this step response overshoot?

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Discussion Overview

The discussion revolves around the reasons for overshoot in the step response of a closed-loop transfer function. Participants explore the effects of poles and zeros on system behavior, particularly in the context of control system design and tuning.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions why their closed-loop transfer function exhibits overshoot despite having correctly placed poles.
  • Another participant asserts that there is no overshoot according to Wolfram Alpha, but later acknowledges a small overshoot due to a zero at s = -1.513.
  • There is a discussion about the role of zeros in contributing to overshoot, with one participant expressing confusion over how zeros can affect the system's response.
  • A participant emphasizes that the transient response is influenced not only by poles but also by the presence of a left-hand plane (LHP) zero.
  • Questions arise regarding the criteria for determining zeros and their impact on system performance, particularly in relation to LHP zeros.
  • Another participant inquires about adjusting zero locations while tuning a PID controller, indicating a desire to understand the interplay between pole and zero placement.
  • One participant requests more context about the plant and response criteria to better address the issue at hand.
  • A participant shares their specific response criteria, aiming for 0% overshoot and a settling time under 1 second, while expressing concerns about using only integral or derivative control actions.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the reasons for the observed overshoot, with multiple competing views regarding the influence of poles and zeros on the system's response. The discussion remains unresolved as participants explore different aspects of the problem.

Contextual Notes

There are limitations in the discussion regarding the clarity of the transfer function and the specific characteristics of the system being analyzed. The impact of the LHP zero on overshoot and the criteria for effective control design are also not fully resolved.

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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.
 
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With the given transfer function there is no overshoot.
Wolfram alpha agrees!
 
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).
 
But how come does zeroes apply overshoot? I thought it was only poles which affected the system
 
rude man said:
With the given transfer function there is no overshoot.
Wolfram alpha agrees!
You should check your entry. It does have overshoot.

215 said:
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.
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
 
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
 
How should i using a PID controlle be able to adjust the Zero location, while i am adjusting the location of the pole.
 
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?
 
  • #10
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 a lot of places that it is not recomended to use just I or D regulators.
 

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