From: "Christopher R. Carlen"
Subject: Re: How to increase PLL order?
Date: Mon, 09 Dec 2002 16:58:04 -0800
Organization: Sandia National Laboratories, Albuquerque, NM USA
NNTP-Posting-Date: Mon, 9 Dec 2002 23:56:15 +0000 (UTC)
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Jim Thompson wrote:
> On Mon, 09 Dec 2002 12:39:50 -0800,
> "Christopher R. Carlen" ,
> In Newsgroup: sci.electronics.design,
> Article: ,
> Entitled: "How to increase PLL order?",
> Wrote the following:
> |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:
> |Good day!
> Chris, Are you trying to get a lock on shaft position or it just
> speed control you are after?
> If the latter case I've used a normal PLL (with its own VCO) driving
> an EC (electronically commutated) motor.
Yeah, in application #1, I just need speed control. Thus, the PLL that
I'm using, which is about exactly what is explained in U113 which I can
understand reasonably well, works very well.
Application #2 will have two wheels each on their own motors. One will
turn at 350Hz and has a very narrow slit, about 1 degree of arc for an
optical shutter with about 8us open time. But 350Hz is too frequent for
a camera shutter to gate out only one pulse per 100ms (the laser period
that illuminates the subject), so we divide the frequency down by a
factor of 7 with an additional wheel, with a larger slit, running 50Hz.
This was all done mechanically before (in app. #1), but the gears and
bearings wear out too fast with the fast wheel running at 21000RPM. So
my plan is to electronically "gear" the motors (app. #2).
That requires absolute phase lock of the shaft positions, though not
very stringent phase jitter requirements since I can always detect the
fast wheel near the top position with my rotation sensor, and trigger
the lasers from that. So it should be well within my means with basic
PLL design, needing no exotics to acheive other-worldy phase jitter
performance. I have pretty much promised and am confident that with
some further study I can pull this one off.
Application #3, which may be beyond my means, is to run two wheels, both
close to 21000RPM or higher but counterrotating, with a difference
frequency of 10Hz. Absolute phase locking to achieve the slit overlap
at a repeatable and controlled absolute shaft angle. Considering that
we need less than 5% jitter timing the laser to when the slits overlap
in order to avoid excessive amplitude jitter in the signal, then this
represents a serious challenge! Also, the absolute shaft angle position
of the slit overlaps must meet that requirement as well. If it is
possible, this will take us to less than 4us of mechanical shutter
performance! This will be a test that I will do later after application
#2 is complete, and there is no loss if I can't make it work. Then
again, I just might hire one of you experts to help with the hard stuff
at that point.
Why do I need a mechanical shutter, considering the fact that there are
electro-optical shutters that can do sub 100ns or so? The reasons are
wavelengths, transmission efficiency, contrast ratio, and some others.
We need mechanical, trust me.
Thanks for the input.
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA