train dynamics undefined variable and ode problem

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i need help about ode23 and subfunction topic
function xdot=train(t,x) alfa=0.3; beta=0.13; gama=0.16; Ua=1; Ub=2; xdot=[x(2);((2/pi)*(atan((x-4)/0.05)+atan((x-2)/0.05))-alfa-beta*abs(x(2))-gama*(x(2)^2)+Ua-Ub)];
and the plot function
[t,x]=ode23('train',[0,0.1],[0;0]); subplot(2,1,1);plot(t,x(:,1)) subplot(2,1,2); plot(t,x(:,2))
and the error is
??? Error using ==> odearguments at 116 TRAIN returns a vector of length 3, but the length of initial conditions vector is 2. The vector returned by TRAIN and the initial conditions vector must have the same number of elements.
Error in ==> ode23 at 172 [neq, tspan, ntspan, next, t0, tfinal, tdir, y0, f0, odeArgs, odeFcn, ...
Error in ==> main1 at 1 [t,x]=ode23('train',[0,0.1],[0;0]);

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Answers (3)

Walter Roberson
Walter Roberson on 25 Jan 2012
Your expression for xdot has two atan() calls with arguments that are expressions in x -- but x is a vector, so you get out a vector result the same length as x... but you also put x(2) at the front of that vector, resulting in a vector of length 3.
  1 Comment
sarngon on 26 Jan 2012
Hi can u help me pls i added a message to my ouestion

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sarngon on 26 Jan 2012
ok i change the matrix variable name as k(2)and kdot but now the prog gives such a error
codes function kdot=train(t,x) alfa=0.3; beta=0.13; gama=0.16; Ua=1; Ub=2; kdot=[k(2);(2/pi)*(atan((x-4)/0.05)+atan((x-2)/0.05))-(alfa+beta*abs(k(2))+gama*(k(2)^2))+Ua-Ub];
and the error line
??? Undefined function or method 'k' for input arguments of type 'double'.
Error in ==> train at 7 kdot=[k(2);(2/pi)*(atan((x-4)/0.05)+atan((x-2)/0.05))-(alfa+beta*abs(k(2))+gama*(k(2)^2))+Ua-Ub];

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devalaraju venkata naga amulya
Hello I want to know that how to know the ode's in a vehicle train model. we can calculate manually if the degrees of freedom is small like 5,10 . but what is it is lie 41DOF and more than that . is there any possible approach in matlab to directly code a script and get to know the equations for the system by coding anything in matlab. kindly help and suggest anything in this regard.
thank you in advance

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