Error in ode45 (line 115)

Question

Edward 2021-1-21

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/723358-error-in-ode45-line-115

评论： Edward 2021-1-21

I'm trying to solve a second-order ODE and am receiving the following errors:

Error in ode45 (line 115)
  odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0,
  options, varargin);
Error in code (line 17)
[out1,out2] = ode45(ode,[0 100],cond);

Here's my code:

syms phi(t) theta(t)
phid(t) = diff(phi,t);
thetad(t) = diff(theta,t);
q(t) = [phi(t);theta(t)];
qd(t) = diff(q,t);
qdd(t)= diff(qd,t);
a = 10;
b = 3;
y = 2;
d = 5;
M = [a+b*(sin(theta))^2 y*cos(theta);y*cos(theta) b];
C = [b*thetad*sin(theta)*cos(theta) -y*thetad*sin(theta)+b*phid*sin(theta);-b*phid*sin(theta) 0];
G = [0;-d*sin(theta)];
ode = 0 == M*qdd+C*qd+G;
cond = [q(0)==[1;-1], qd(0)==[0;0]];
[out1,out2] = ode45(ode,[0 100],cond);

I'm not very experienced with Matlab, so I apologize if this is an obvious mistake I'm making.

Thank you in advance!

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Answer 1

Walter Roberson 2021-1-21

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https://ww2.mathworks.cn/matlabcentral/answers/723358-error-in-ode45-line-115#answer_603213

ode45 can never be used for symbolic expressions. See dsolve() instead, or see the first example for odeFunction

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Walter Roberson 2021-1-21

That is not an error, that is a warning that dsolve does not know how to solve that system. Either dsolve does not have the algorithm for that system, or else there is no closed form solution for the system, or else the system is inconsistent for closed form.

You should follow the guidance in odeFunction first example.

Edward 2021-1-21