Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!dali.cs.montana.edu!uakari.primate.wisc.edu!sdd.hp.com!hp-pcd!hplsla!jamesv
From: jamesv@hplsla.HP.COM (James Vasil)
Newsgroups: comp.dsp
Subject: Re: Best z-trans. for CONTROL SYSTEMS?
Message-ID: <9360025@hplsla.HP.COM>
Date: 22 Apr 91 23:09:06 GMT
References: <1991Apr18.160222.21961@uservx.afwl.af.mil>
Organization: HP Lake Stevens, WA
Lines: 21

> What transformation is best for converting a transfer function in a CONTROL
> SYSTEM to the z-domain?  Right now I'm using the bilinear-z transform and
> I'm converting a system that MATLAB says is stable, but when I actually 
> implement the system on a DSP board, it is unstable.  Any help would be
> greatly appreciated!

The bilinear-z transform (or the related, pre-warped bilinear) is 
generally considered a good tranform to use when you are interested
in preserving the frequency domain charicteristics of your analog
filter.  Since the theory says that the bilinear transform of a stable
filter is a stable filter, I'd suggest you look into some of the
implementation details.  Some ideas to consider: is the transformation
being done correctly?  Is the analog filter really stable?  When the
filter coeficients are rounded, is this done in such a way so as to
keep poles inside the unit circle?  And so on.  

Regards & good luck,
James Vasil

Disclaimer: as always, these opinions are mine and not necessarily 
            those of my employer.