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What I Have to do :

Perform the linearization in the vicinity of the operating point. Determine the linearized transfer

This is my non-linear operating point model ('op') with step :

enter image description here

This is my non-linear operating point model ('linmod'), where I have replaced step -> In and to workspace -> Out

enter image description here Parameter Kn = 948; For transfer function I did:

[num,den] = linmod('test');
G = tf(num,den);

     0.3333 s^2 + 83.33 s + 23.33
  --------------------------------------
  s^4 + 333.3 s^3 + 2.09e04 s^2 + 5833 s

If I change the value of Kn, and re-run simulation + linmod, I get the exact same transfer function.

Also, I do not quite understand in the mathlab help what is 

[A,B,C,D] = linmod['linmod',x,u]
I know that matrixes A,B,C,D are used in

x' = Ax + Bu and y = Cx + Du

But I do not know what to do from here.

Also a question :

Do I need to use linear or non-linear model for linmod ?

Smuk
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    Why doesn't it work? Explain – rayryeng Jan 19 '15 at 19:52
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    If you change the value of a parameter, you need to run `linmod` again. The transfer function doesn't get automatically updated. – am304 Jan 19 '15 at 22:25
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    As far as I see, your model is already a linear model. Maybe you can use a computer algebra system and compute the symbolic transfer function including all of your parameters. – Diphtong Jan 20 '15 at 07:21
  • Ok, after your edit it is not linear anymore. My comment is invalid now. Is this an mdof Duffing oscilator? – Diphtong Jan 20 '15 at 12:35
  • @Diphtong to be honest, I got so confused right now... If the linmod function is used to linearize the model, than I guess it's "not usefull" if I use the linear model, so I have replaced it. This is actually my school project, where I have to perform the linearization of my mechanical translational system – Smuk Jan 20 '15 at 12:44
  • Be careful when linearizing this model. Because of the cubic stiffness, there can be a "jump" in the frequency response. Make sure you are sufficiently far from this. Otherwise, your linear approximation will be bad. Do you have to use Simulink? Try to write down the equations on paper and perform the analysis and linearization by hand. Also take a look at [scholarpedia](http://www.scholarpedia.org/article/Duffing_oscillator) – Diphtong Jan 20 '15 at 13:08
  • Yes, as you can see on the top of the post, there are simulink models. And no, this is not Duffing oscilator – Smuk Jan 20 '15 at 14:43

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