Re: [ATM] Stepper Stuff

[Nils Olof Carlin <nilsolof.carlin@telia.com>]

Matt,

I am sure you are right about the start-stop frequency and associated 
problems. But the inertia (and damping) I was thinking of would be in 
the mount and OTA rather than in the motor itself. But I suppose you 
need to cut and try to find what actually works and where the vibration 
frequencies are with a particular telescope.

Others have described success particularly at high speeds using 
"flywheels", that would certainly affect the start-stop frequency. 
Another approach is one I have used:

http://web.telia.com/~u41105032/Stepper/Stepper.htm

You might find it overkill (not to mention Rube Goldberg ;-) but it 
does eliminate vibrations even at low "step" frequencies.

thanks for the feedback,

Nils Olof


from my old days of designing stepper based disk drives , printers and
plotters for my day job as electronics design engineer , steppers of 
that
size and type have a start stop frequency in the hundreds of 
steps/second .
That meaning the rotor has time to come to a full stop between steps 
that
are lower frequency than the start stop.
The stepper gets into the slewing region only above the start/stop
frequency, and that is the only area of operation where there is any
smoothing out of steps due to inertia .
For step frequencies under the start stop frequency , there are 
actually
some frequency values where there is considerable rotor position 
overshoot
quite the opposite of smoothing out . Running under the start stop 
frequency
is the roughest area of operation on the torque vs. frequency stepper 
curve
. Use at least some microstepping to smooth out the vibrations or some 
other
means , like air dashpots, inertial disk dampers on the rotor , and 
plenty
of friction in the drive train. A common mistake is trying to 
eliminate
friction, which leads to an underdamped system with high overshoot .

best regards,
matt tudor

-----Original Message-----
From: Nils Olof Carlin <nilsolof.carlin@telia.com>
To: chaosopher23@yahoo.com <chaosopher23@yahoo.com>; atm@atmlist.net
<atm@atmlist.net>
Date: Monday, September 26, 2005 10:46 AM
Subject: Re: [ATM] Stepper Stuff


>Kevin,
>
>
>>>>
>The steppers I have are from old disk drives.  I have
>a couple from the old TEAC 1/2 height 5.25's that have
>some decent torque and work in 3.6 degree steps.
>That's 100 steps per revolution...  ok, that part's
>easy.
>
>I'm going to run the motor on a 8" f/6 Newt and
>another on a 13.25" f/20-something Cass-variation.
>Equatorial, maximum magnification and stability is
>desired, probably both forks with conical
>right-ascension axles.
>
>What's a good reduction for these motors so that they
>don't go too fast when doing one step at a time?>>>
>
>I would suggest that you aim for 25-30 fullsteps or halfsteps per
>second (the apparent motion of an object near the celestial equator 
is
>15 arcsec per second of time - 30 steps/sec is 1/2 arcsec per step) -
>there will be some inertia to smooth out the motion but there may be
>resonances in the telescope - perhaps in the secondary and spider, 
that
>can amplify vibrations.
>
>This means 1 turn per 3-4 sec or so - 15-20 turns per minute or 900-
>1200 per hour or 21600-28800 turns per 24 hr for fullstep (half that
>for halfstep), if I didn't miscalculate - this gives the total gear
>reduction to aim for. At least, don't go much below.
>
>Nils Olof
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