nls convergence trouble

Hi,

Parameters assessment in R with nls doesn't work, though it works fine with
MS Excel with the internal solver :(


I use nls in R to determine two parameters (a,b) from experimental data.

       m         V        C0         Ce        Qe
1  0.0911 0.0021740  3987.581   27.11637  94.51206
2  0.0911 0.0021740  3987.581   27.41915  94.50484
3  0.0911 0.0021740  3987.581   27.89362  94.49352
4  0.0906 0.0021740  5981.370   82.98477 189.37739
5  0.0906 0.0021740  5981.370   84.46435 189.34188
6  0.0906 0.0021740  5981.370   85.33213 189.32106
7  0.0911 0.0021740  7975.161  192.54276 233.30310
8  0.0911 0.0021740  7975.161  196.52891 233.20797
9  0.0911 0.0021740  7975.161  203.07467 233.05176
10 0.0906 0.0021872  9968.951  357.49157 328.29824
11 0.0906 0.0021872  9968.951  368.47609 328.03306
12 0.0906 0.0021872  9968.951  379.18904 327.77444
13 0.0904 0.0021740 13956.532 1382.61955 350.33391
14 0.0904 0.0021740 13956.532 1389.64915 350.16485
15 0.0904 0.0021740 13956.532 1411.87726 349.63030
16 0.0902 0.0021740 15950.322 2592.90486 367.38460
17 0.0902 0.0021740 15950.322 2606.34599 367.06064
18 0.0902 0.0021740 15950.322 2639.54301 366.26053
19 0.0906 0.0021872 17835.817 3894.12224 336.57036
20 0.0906 0.0021872 17835.817 3950.35273 335.21289
21 0.0906 0.0021872 17835.817 3972.29367 334.68320

the model "LgmAltformula" is

Qe ~ (V + b * m * a + C0 * V * b - ((C0 * V * b)^2 + 2 * C0 *
   b * V^2 - 2 * C0 * V * m * a * b^2 + V^2 + 2 * V * m * a *
   b + (b * m * a)^2)^(1/2))/(2 * b * m)

the command in R is


nls(formula=LgmAltFormula,data=bois.DATA,start=list(a=300,b=0.01),trace=TRUE
,control=nls.control(minFactor=0.000000009))

R has difficulties to converge and stops after the maximum of iterations

64650.47 :  2.945876e+02 3.837609e+08
64650.45 :  2.945876e+02 4.022722e+09
64650.45 :  2.945876e+02 1.695669e+09
64650.45 :  2.945876e+02 5.103971e+08
64650.44 :  2.945876e+02 8.497431e+08
64650.41 :  2.945876e+02 1.515243e+09
64650.36 :  2.945877e+02 5.482744e+09
64650.36 :  2.945877e+02 2.152294e+09
64650.36 :  2.945877e+02 7.953167e+08
64650.35 :  2.945877e+02 7.625555e+07
Erreur dans nls(formula = LgmAltFormula, data = bois.DATA, start = list(a =
300,  :
 le nombre d'it?rations a d?pass? le maximum de 50


The parameters "a" and "b" are estimated to be 364 and 0.0126 with Excel
with the same data set.
I tried with the algorithm="port" with under and upper limits. One of the
parameter reaches the limit and the regression stops.

How can I succeed with R to make this regression?


Regards/Cordialement

-------------
Benoit Boulinguiez
Ph.D
Ecole de Chimie de Rennes (ENSCR) Bureau 1.20
Equipe CIP UMR CNRS 6226 "Sciences Chimiques de Rennes"
Campus de Beaulieu, 263 Avenue du G?n?ral Leclerc
35700 Rennes, France
Tel 33 (0)2 23 23 80 83
Fax 33 (0)2 23 23 81 20
http://www.ensc-rennes.fr/



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