Problems with Arx-algorithm

[Flo89]

Hello,

I did some measurements and data analysis on a system. Now I want to let
octave calculate a transfer function with arx algorithm but I'm running into
problems.
Here's the code:

InputData =
dlmread('/home/flo/Dokumente/Octaveprojects/c_output_frequencies_high_speed.csv','\t');
input = InputData(:,1);
input_short = input(10 : 10 : end);
output = InputData(:,2);
output_short = output(10 : 10 : end);

sampletime = 1/(200000)*10;
t = 0 : sampletime :sampletime*(rows(input_short)-1);
dat = iddata(output_short, input_short, sampletime);

[sys, x0] = arx(dat,  'na',i, 'nb', l);
[y, t, x] = lsim (sys,input_short,t,x0);
figure
plot(t,y,'r');
hold on;
plot(t,output_short,'g');

I'm doing this for example in two for loops to find good values for na and
nb. But if I'm doing na = 6 and nb = 1 to nb = 6 theres always the same
simulated system. I doubt it will ever chance when changing the parameters. 
So my question is: What am I doing wrong in general?
And the second: I knot that the delay between input and output is around 100
sampletimes. Is nk then 100?

Here are images of the real system:
<http://octave.1599824.n4.nabble.com/file/n4664059/real_system.png> 
And here of the real output and the simulated one:
<http://octave.1599824.n4.nabble.com/file/n4664059/simulated.png> 

Thank you!



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