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Homework Help: Responses of a transfer function on MATLAB

  1. Apr 19, 2012 #1
    I'm given the following system:

    [tex]\frac { 3s+0.5 }{ { s }^{ 3 }+3{ s }^{ 2 }+8s } [/tex]

    I'm supposed to find the steady state error of the system by plotting the step and ramp responses, and then find the value when s tends to infinity.

    Here is the MATLAB code:

    Code (Text):
    sys1 = 3 + tf(0.5.*[1], [1 0]);
    sys2 = tf([1], [1 3 8]);
    sys = sys1*sys2

    subplot(1, 2, 1)

    sys3 = tf([1], [1 0]);
    subplot(1, 2, 2)
    title('Ramp Response')

    Here's the result: http://i.imgur.com/vXoXV.png

    As can be seen, the step and ramp responses are unstable. All the roots are on the left of the imaginary axis, so the system must be stable. I'm not sure where I went wrong.
  2. jcsd
  3. Apr 19, 2012 #2


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    Your system has a pole at the origin. So, it is unstable. With a unit feedback it can be made stable, depending of the gain (your root locus graph).
  4. Apr 19, 2012 #3
    So it's only stable if it's a closed loop? Thanks, that solved the unit step part.

    What about the ramp response? How do I make it stable?
    Last edited: Apr 19, 2012
  5. Apr 19, 2012 #4


    User Avatar

    The response of a stable system to a ramp input is a ramp, so it tends to infinity.
    If you have a pole at the origin, the response to a ramp will be a parabola, as your graph shows.
  6. Apr 20, 2012 #5
    Many thanks for the help.
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