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From: "John Jardine"
Subject: Re: How to increase PLL order?
Date: Mon, 9 Dec 2002 21:50:16 -0000
NNTP-Posting-Date: 9 Dec 2002 21:41:18 GMT
X-Newsreader: Microsoft Outlook Express 5.00.2615.200
Christopher R. Carlen wrote in message
> I am working with PLLs containing motors. I have been using a
> "zero-pole" loop filter with my motor/VCO, so that I have transfer
> functions of the elements of the loop which look something like this:
> Hmotor(s)=(1/(LC))/(s^2+(R/L)s+1/(LC)s)*N/Kv in (rad/V)
> Suggestions appreciated.
> Good day!
> Christopher R. Carlen
> Principal Laser/Optical Technologist
> Sandia National Laboratories CA USA
Offhand I would suggest that what you are wishing to do may be impossible.
The beauty of an idealised mathematical transfer function, is the infinite
connectedness of an input to it and the output from it.
The motors being mechanical devices will not care for this and will be
limited on a turn by turn basis by their non linear bearing chatter and
losses, windage loadings re speed, maybe (DC motor) brush loadings and
probably a host of other mechanical 'inadequacies'.
The accumulated result of this can only be an unresolvable 'error band' that
would translate into arbitary positional variations in a 'locked' system
sufficient that two motors cannot be run in a phased synchronisation
arrangement to the degree you are after. (sounds like you are after nearly
stepper motor precision).
Best looked at perhaps as trying to run two VCO's in defined phase
synchronism when each VCO suffers from different (unpredictable) amounts of
phase jitter. (eg trying to phase lock 2 poor Xtal oscillators) Or in the
motor's rotating masses case, they are best looked at perhaps as two fairly
hi-Q resonant circuits with non linear, mechanical losses sufficient to
cause uncorrelated variations in each working Q; hence a varying bandwidth
at controlled resonance resulting in an undefined control point no matter
how high the system gain.
I would guess that the best you will be able to get out of such a system
would be motors that are -nearly- locked in rotational speed with an
arbitary phase drift between them.
My two-pence worth!.
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