Inner matrix dimensions must agree.

I am trying to solve the the equation below and keep getting the error that inner matrix dimensions don't agree. I've been looking at this for awhile and can't figure it out. K1 = 4x4 matrix, W = 4x4 matrix, TD = 4x3 matrix, tau = 3x1 matrix.

 u2 = inv(K1)*Winv*TD*inv(TD'*Winv*TD)*tau;

[David Goodmanson]

Hi m13, Assuming Winv is the obvious thing and is 4x4, this seems to work. Are your matrix dimensions perhaps not what you think they are?

[m13]

I believe the problem might actually be earlier in my simulation. I am trying to use simulink and am completely new to it. Is there any way to look at each variable output after running the model?

[David Goodmanson]

I don't have simulink but no doubt someone who does will weigh in.

Answer 1

[John D'Errico]

NEVER ASSUME ANYTHING. Never. When you have a problem like this, test those assumptions. Verify your facts. Because the fact is, MATLAB is not a liar. It tells the truth. If it tells you there is a matrix dimension mismatch, then that is because there is one.

whos K1 Winv TD tau

You have told us the shape of those variables. But not all of them. You told us that W is 4x4. Should we just assume that means the variable Winv is also 4x4?

If you are running this inside a function, verify the size of those matrices INSIDE the function. Why? Because people screw up. They change things without realizing they did. They overwrite variables with ones of a different size.

[m13]

This issue is in an embedded matlab function in my simulink simulation. Is there a way to see the results variables it is computing as it runs? not sure how to do that to be able to debug.

[John D'Errico]

I am in the same place as David, lacking simulink, I can't help you in that respect. The fact is your arrays do not conform in size for multiplication. So you need to track that down.