Search All of the Math Forum:

Views expressed in these public forums are not endorsed by NCTM or The Math Forum.

Notice: We are no longer accepting new posts, but the forums will continue to be readable.

Topic: system identification question
Replies: 0

 Christopher Thomas Holcomb Posts: 2 Registered: 12/7/04
system identification question
Posted: Jun 28, 1996 9:10 AM

Hello,

I am new at using the system identification toolbox as well as system
identification theory in general. I am working with the equation:

dp/dt=-Wp+S(t)

where W is an nxn matrix, and p, dp/dt, and S(t) are nx1 column vectors.
This defines a system of differential equations. I have entered S(t) as
input and p(t) as output into the system, and I want to find the elements of
W.

I have tried to define a state-space model structure to import into ident
to define the structure of this system. It looks like:

F=[NaN NaN; NaN NaN]
G=[1 0; 0 1]
H=[1 0; 0 1]
D=zeros(2,2)
K=zeros(2,2)
x0=[0.5 0.5]'
ms2=modstruc(F, G, H, D, K, x0);
thguess=[1, 1, 1, 1]
th2=ms2th(ms2, 'c', thguess)

model=pem([p S], th2)

In this notation, F is equal to the W that I am looking for. I am
encountering three problems:

1.) If I use a thguess, it gives back exactly what I guessed as the
elements of F, even though I know they are wrong, because in a simple test
case, I know what W is already

2.) If I do not provide a thguess, it gives back zeros for all the NaN's.

3.) If I give it all of "the answers", namely I replace the NaN's in F with
the actual values in the system, the model output still does not match with
the actual output, in fact, the rootmean square deviation between the two is
given as NaN.

The third problem leads me to beleive that the state-space structure I am
defining is possibly wrong for my system.

Can anyone give me any advice or help?
Thanks for reading this far.

Chris Holcomb
Universtiy of Rochester Department of Physics and Astronomy