[scilab-Users] System Identification with frfit()

[John Ryan]

Hi Mike,
Thanks for the suggestion; I'll give it a go. The system should be simple,
it's a DC motor driving a solid load via gears and belts. I think that the
complexity comes from stretching in the belts, lost motion in the gears and
nonlinear effects in the motor, which is why I was looking to identify it
empirically.
Regards, John

-----Original Message-----
From: Michael J. McCann [mailto:mjmccann at iee.org] 
Sent: 06 May 2008 12:20
To: users at lists.scilab.org
Subject: Re: [scilab-Users] System Identification with frfit()

John,
         If you know something about the underlying physics of the 
system, then that is, in my view (as a control and dynamic systems 
person), the place to start.
         I suspect that you can get the peak with a second order 
under damped-pair of poles, and fake the rest of the drop off or rise 
with first order (left half plane) poles and zeros.  It might be 
easier to build a model up from these basic components (as products 
in a transfer function) rather than try to get a mechanical solution 
method. However, it's possible you have a non-minimum phase system so 
it will be trickier.
Mike.
==============================================

At 10:53 2008-05-06, you wrote:
>Bonjour,
>I have measured the phase and magnitude response of a system and I 
>am trying to identify a model for it with frfit(). This works well 
>up to about 10Hz but then can be 20dB different in gain, 150degrees 
>in phase. Best fit is with order 3 or 4.
>My data is a log frequency sweep from 0.7Hz to 60Hz. It is a little 
>noisy (+/- 0.5dB, +/-3deg). The response has a 'peak' in the phase 
>between 7 and 20Hz and increasing gain over this region.
>I tried separating the model. First I divided the measured data by 
>the modeled response (to extract just the data that does not fit the 
>model) and then frfit() to the remaining data. This did not help as 
>the fit to the remaining data was just as bad.
>Can anybody suggest how to analyse this system?
>Thanks for any help,
>Regards, John


Dr Michael J. McCann,           Date: 2008.05.06        11:18gmt
BSc(Eng), DIC, PhD, CEng, FIEE.
McCann Science.
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