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Under-damped second order system - CONTROL

  1. Mar 30, 2013 #1
    hey all,
    i'm stuck with the following designing problem (Control course) :

    1. The problem statement, all variables and given/known data

    given the location of the poles , find rise time , peak time, percentage maximum overshot and settling time for each pole. pole are:
    1 . pole at θ = 70 , ωn = 1

    2. pole at θ = 70 , ωn = 3

    3. pole at θ = 30 , ωn = 1

    4. pole at θ = 30 , ωn = 3

    2. Relevant equations

    1. how to calculate tp, tr, max o.s and ts for each pole ?

    2.how to find the damping ratio (zeta) ?

    3. which one of the poles is the best ?
  2. jcsd
  3. Mar 30, 2013 #2

    rude man

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    Gold Member

    I've never see poles described this way. If θ is the angle associated with the complex s plane, then not only are all four poles in the right-hand plane but they are not complex-conjugates.

    So - anybody understand this?
  4. Mar 31, 2013 #3
    A step in the right direction :)

    Is there no transfer function associated with the question?
    I think knowing the order of the system is useful.
    Maybe fourth pole means fourth order.

    I referred to this http://web.mit.edu/2.14/www/Handouts/PoleZero.pdf

    If when you do the nyquist plot the transfer function does not circle -1 then I think your system is stable.
    Sometimes this is called the nyquist criterion.

    Im going to have a go assuming the nominator of the transfer function is 1.

    I can't properly explain how to do this. But let me show a similar example:

    Putting this code in Matlab
    EDU>> Q2 = tf ([5],[1 204 800])
    figure(99); bode(Q2)
    figure(100); step(Q2)

    Generates this

    There are buttons for the parameters you asked for.

    So from what I know the solutions to the denominator of the quadratic are the roots.
    And the roots give you the location of the poles.

    So if you can figure out what the transfer function is, im happy to re-plot for you.

    Unfortunately I don't have a fuller answer for you: im studying Control Systems 1 at the moment myself. Hopefully by the time I get to Control Systems 2 I can answer this properly :)

    p.s. to get the fourier transform we substitute s=jw into the roots of the equation

    [tex]\frac{1}{s{}^{\wedge}2+204 s+800}=\frac{1}{(s+200)(s+4)}[/tex]
  5. Apr 1, 2013 #4

    rude man

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    Even after looking at your link I can't make head or tail of this way of describing pole/zero locations. Theta could be the angle from a pole on the real axis or from one of a complex-conjugate pole pair, for example. See figs. 7 and 8 in your reference.

    Do you have any previous material where you were given pole/zero locations in this arcane way?
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