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Elastic line equation in Matlab - Error using Mupadengine/feval - Error in dsolve>mupadDsolve - Error in dsolve

Francesco Giuseppe Fornari
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Francesco Giuseppe Fornari Published in 2018-01-11 14:25:20Z

I can't understand the solution of multiple differential equations in Matlab. In this example, I'm trying to work with independent equations: the program should solve them separately (and it does, see below) and together (it doesn't!), shouldn't it?

N.B. The equations describe the elastic line according to Euler Bernoulli. I'd like to use them, just setting the appropriate intitial conditions and known data for each problem.

This is the part of code in common with both procedures:

Creating symbolic functions

syms v(z) w(z)
syms B Chiterm p mx by l A Eterm p bz

Differentiating

D1v = diff(v,z);
D2v = diff(v,z,2);
D3v = diff(v,z,3);
D4v = diff(v,z,4);
D1w = diff(w,z);
D2w = diff(w,z,2);

Setting known values (and parameters, like p)

by=-p;
Chiterm=0;
mx=0
bz=0;
Eterm=0

Defining differential equations

odetrasv = diff(B*(diff(v,z,2)+Chiterm),z,2)-mx-by == 0;
odelong = diff(A*(diff(w,z)-Eterm),z)+bz == 0;

Defining initial conditions

cond1 = B*D2v(l) == 0;
cond2 = D1v(0) == 0;
cond3 = v(0) == 0;
cond4 = v(l) == 0;
cond5 = D1w(l) == 0;
cond6 = w(0) == 0;

Here is the part of the code that solves the equations separately:

condstrasv = [cond1 cond2 cond3 cond4];
condslong = [cond5 cond6];
vSol(z) = dsolve(odetrasv, condstrasv);
wSol(z) = dsolve(odelong, condslong);
vSol = simplify(vSol)
wSol = simplify(wSol)

Here's the part that is not working. I'm trying to solve them togheter, since the next step is solving a system of multiple equations:

odes = [odetrasv; odelong];
conds = [cond1; cond2; cond3; cond4; cond5; cond6];
[vSol(z), wSol(z)] = dsolve(odes,conds)

Matlab returns:

Error using Mupadengine/feval - Invalid initial conditions
Error in dsolve>mupadDsolve - T=feval(symengine,'symobj::dsolve',sys....
Error in dsolve - sol = mupadDsolve(args, options);

Why are the initial conditions invalid in the second case? Solving without them works:

[vSol(z), wSol(z)] = dsolve(odes)
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