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Someone helps me please

  1. May 26, 2008 #1
    someone helps me please!!

    Hello all,

    This is the first time Ive stumbled across this site, but it appears to be extremely helpful. I am an elettronic engineering grad student, and in my research, I have run across the following 2nd order non linear differential equation. It is of the form:

    x'' + x = sinwt + s(x)

    where s(x) = abs(5*x+1)-abs(5*x-1) (saturation)
    and t=(0 2000)
    and w=[0:0.001:2.^0.5] (it's a variable )

    My questions are :
    1) How can I write and solve this problem using Matlab ?
    2) How can I plot this differential equation as a function of w ?
    after the plot, How can I find the max Local (Relative) Maxima and put these results in another graph always considering them as a function of w ?

    I hope there's someone who helps me...
  2. jcsd
  3. May 27, 2008 #2


    User Avatar
    Staff Emeritus
    Science Advisor

    If you wanted to know how to solve that equation, I could tell you. Unfortunately, I know nothing about using Matlab.
  4. May 28, 2008 #3
    Thank you....but i have to solve this problem only using matlab...
  5. May 28, 2008 #4

    First you need to transform your problem to a system of (two) first order differential equations by the substitution

    You will then get a problem of the type
    [tex]x'_{1}=x_{2}, x'_{2}=f(x) [/tex]

    After that create a m-file containing

    function xprim=xprim(t,x)

    When try to use the function ode45 or some other solver to solve problem.
    and use plot to plot the result
  6. Jun 18, 2008 #5
    I have another problem :
    I have to find, after solving that differential equation,a vector based only with the maximum values of its results...After that i have to plot this vector as a fuction of w...how can i do that ? :confused:
  7. Jun 18, 2008 #6
    Let me give you some invaluable advice: read the Matlab documentation. It's superb, and explains in detail how to approach such basic problems.
  8. Jun 18, 2008 #7
    Thank you for your "invaluable advice"...:uhh:
  9. Jun 18, 2008 #8
    You're enrolled in an electronic engineering graduate programme but are incapable of using the detailed examples given by typing 'doc ode45'? Sounds like the advice I gave you really would be invaluable if only you'd take it.

    Ninety-five percent of the time, a question about Matlab is answered in the Matlab documentation. For instance, precisely all of your questions are answered, with examples, in the documentation. Use it. It'll give you a better answer than anyone here can.
  10. Jun 18, 2008 #9

    it's hard for me twice : first of all i haven't time to study all the Matlab documentation and second i'm an italian girl that not understand english very well..i'm trying to do it by myself...trying to traslate all i read...so please don't kidding me ...
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