From help-request at octave dot org Tue Jan 18 09:26:23 2005
Subject: [leasqr] help with parameters
From: "Pascal A. Dupuis" <Pascal dot Dupuis at esat dot kuleuven dot ac dot be>
To: help at octave dot org
Date: Tue, 18 Jan 2005 01:37:19 -0600

Hello,

I use leasqr to fit a complex frequency response to measurered points.
Could someone please help fit the parameters ?

Context: there are a number of frequencies contained in f, the vector
to be optimised is Xok = [Rse Rpe 1e9*Cce Rce], where the scaling on
the third parameter ensure it has more or less the same order of
magnitude as the others.
The target is an impedance, obtained as Rs+ (Cce // (Rpe + Rce)),
where Rce has a special frequency response.

Then I have:

# fitting func
F = "leasqr_warburg_func";
# Jacobian
dFdp = "leasqr_warburg_dfdp"; 
#scalar tolerance and number of iter
stol=0.001; niter=250;
#desired fractional precision in parameter estimates
minstep = [0.01; 0.01];
# weight vector
wt1 = ones(2*length(f), 1);
# for the numerical derivative. 
dp = .001*ones(size(Xok));
[f1, p1, kvg1, iter1, corp1, covp1, covr1, stdresid1, Z1, r21] = ...
      leasqr ([f;f], [real(Zr); imag(Zr)], 1.01*Xok, F, stol, niter,
      wt1, dp, dFdp, [minstep]);


The problem is that this always stop after 3 iterations. 

If a compute:
%# at solution
f0=leasqr_warburg_func([f;f], Xok);
norm(f0-[real(Zr); imag(Zr)])
ans =  3.7211e-10

%# move 1% away
f01=leasqr_warburg_func([f;f], 1.01*Xok);	
norm(f01-[real(Zr); imag(Zr)])
ans = 407.35

%# solution from leasqr
norm(f1-[real(Zr); imag(Zr)])
ans = 404.61

So, there was a progress, but I would expect further reduction of the
error norm! Which parameter should I change to force the convergence
to better values ? In particular, are stol absolute or relative ?

TIA

Pascal


-- 
Dr. ir. Pascal Dupuis
K. U. Leuven, ESAT/ELECTA (formerly ELEN):  http://www.esat.kuleuven.ac.be/
Kasteelpark Arenberg, 10; B-3001 Leuven-Heverlee, Belgium
Tel. +32-16-32 10 21 -- Fax +32-16-32 19 85



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