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ODE- How to solve this equation??

Asked by inbal kashany on 1 Jun 2011
Hi Im trying to solve this differential equation:
dydt=((g0.*L-y)./tau)-((Pin./Esat)*(exp(y)-1));
by ODE45: [T Y]=ode45(@(t,y) fun(t,Y,It,I),[0 3],0);
but it doesnt work... I guess becouse it based on example3 from ODE45 help which looks like this: dydt = -f.*y + g,(y'(t) + f(t)y(t) = g(t))
but here I have the "exp(y)" which I dont know how to hadle with.
g0=R*a*n0*((I/I0)-1); I is a vector dependent on t. L,tau,Pin,Esat,R,a,n0,I0 are constant.
any suggestion may help. thanks

  1 Comment

Arnaud Miege
on 1 Jun 2011
Please reformat your code. See http://www.mathworks.co.uk/matlabcentral/answers/6200-tutorial-how-to-ask-a-question-on-answers-and-get-a-fast-answer

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3 Answers

Answer by Igor
on 1 Jun 2011

be exactly in y(t) definition and remember that y(t) is a COLUMN vector.
also in -- @(t,y) fun(t,Y,It,I) -- you use y and Y!

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Answer by Laura Proctor on 1 Jun 2011

The following code worked for me:
dydt=@(t,y)(((g0*L-y)/tau)-((Pin/Esat)*(exp(y)-1)));
[T Y]=ode45(dydt,[0 3],0);
But, I did have to make some guesses at the constant values, such as g0, L, tau, Pin, and Esat.

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g0 is not constant-
g0=R*a*n0*((I/I0)-1);
I=[zeros(10,1);ones(10,1);zeros(10,1);]; %step signal%%
It=linspace(0,30,30); %generate t for I
I=interp1(It,I,t); % Interpolate the data set (It,I) at time t
now I got the error: odearguments at 115 @(T,H)(((G0*L-H)/TAU)-((PIN/ESAT)*(EXP(H)-1))) returns a vector of length 30, but the length of initial conditions vector is 1. The vector returned by @(T,H)(((G0*L-H)/TAU)-((PIN/ESAT)*(EXP(H)-1))) and the initial conditions vector must have the same number of elements.
*h replace y
I'll be happy get an answer.

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Answer by inbal kashany on 2 Jun 2011

Thanks to both of you for your replay.
g0 is not constant-
g0=R*a*n0*((I/I0)-1);
I=[zeros(10,1);ones(10,1);zeros(10,1);]; %step signal%%
It=linspace(0,30,30); %generate t for I
I=interp1(It,I,t); % Interpolate the data set (It,I) at time t
now I got the error: odearguments at 115 @(T,H)(((G0*L-H)/TAU)-((PIN/ESAT)*(EXP(H)-1))) returns a vector of length 30, but the length of initial conditions vector is 1. The vector returned by @(T,H)(((G0*L-H)/TAU)-((PIN/ESAT)*(EXP(H)-1))) and the initial conditions vector must have the same number of elements.
*h replace y
I'll be happy get an answer.

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