how to solve this error ?

Hi

I have this function then m and k are an input matrices having dimension (12,12) but an error message is displayed

like : Error using *

Inner matrix dimensions must agree.

Error in equabeam1 (line 9)

xdot = A*x+B*f;

Error in @(t,x)equabeam1(t,x,m,k)

Error in odearguments (line 87)

f0 = feval(ode,t0,y0,args{:}); % ODE15I sets args{1} to yp0.

function xdot = equabeam1(t,x,m,k)

% Function file for mass with spring.

% Position is first variable, velocity is second variable

freq=100; %frequency (Hz)

w=2*pi*freq;

A = [zeros(12,12),ones(12,12);-m^-1*k,zeros(12,12)];

B = [zeros(12,1);m^-1*ones(12,1)];

f =sin(w*t);

xdot = A*x+B*f;

end
[m,k,idof]=beam1(node,elt,pe,ie,opt)
[t,x] = ode45(@(t,x) equabeam1(t,x,m,k),[0,10],[0,0])

figure;

plot(t,x(:,1))

Please help me

Answer 1

[Walter Roberson]

You construct your A as 24 by 24, and you do a matrix multiplication by x which is going to be 2 x 1 because your x0 of [0,0] is length 2. There is no way to multiply a 24 x 24 by a 2 x 1.

Note: I see you use m^-1 where m is a 12 x 12 matrix. That is equivalent to doing inv(m) which is an error prone operation. Instead of

-m^-1*k

you should be using

-(m\k)

and instead of

m^-1*ones(12,1)

you should be using

m\ones(12,1)

[Mallouli Marwa]

What should i do about the dimension of x ?

[Mallouli Marwa]

If i change it by another x(12,12) ??

[Mallouli Marwa]

No (12,1)

[Walter Roberson]

It looks to me as if you need to redesign your function. Remember you need to have as many output values as you have x input values, but your code appears to me to be designed to output only half as many outputs as inputs. But then it is not clear to me what your m and k are or where you are using the velocity information your comments mention. Your m sort of looks like a mass matrix?? If so then there are other ways of handling those; see http://www.mathworks.com/help/matlab/ref/odeset.html#zmw57dd0e573778

[Mallouli Marwa]

m= mass matrix k= stiffness matrix