Someone helps me please

  • Thread starter dmg_80
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  • #1
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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...
thanks
 

Answers and Replies

  • #2
HallsofIvy
Science Advisor
Homework Helper
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If you wanted to know how to solve that equation, I could tell you. Unfortunately, I know nothing about using Matlab.
 
  • #3
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If you wanted to know how to solve that equation, I could tell you. Unfortunately, I know nothing about using Matlab.
Thank you....but i have to solve this problem only using matlab...
 
  • #4
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Hey

First you need to transform your problem to a system of (two) first order differential equations by the substitution
[tex]x_{1}=x[/tex]
and
[tex]x_{2}=x'[/tex]

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)
xprim=[x_2;f(x)]​

When try to use the function ode45 or some other solver to solve problem.
and use plot to plot the result
 
  • #5
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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:
 
  • #6
422
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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:
Let me give you some invaluable advice: read the Matlab documentation. It's superb, and explains in detail how to approach such basic problems.
 
  • #7
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Let me give you some invaluable advice: read the Matlab documentation. It's superb, and explains in detail how to approach such basic problems.
Thank you for your "invaluable advice"...:uhh:
 
  • #8
422
1
Thank you for your "invaluable advice"...:uhh:
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.
 
  • #9
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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.

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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