DC motor selection for REALLY low speeds

[sgc7of7]

Hi everyone.

I'm in the process of designing a telescope mount, and am now trying to figure out how to drive the thing. Ideally I would like to go with an off the shelf dc motor and gearhead setup, as this appears to be the cheapest option. My problem is, that I don't know how slowly you can run a dc motor before you start to cause issues - like cogging, or maybe burn-out, or who knows what. So I was hoping that you guys could give me some pointers...

Here's some specs that I think are required:

-The system must turn at around 1 revolution per day.
-Start-up torque, with safety factor included, is about 20Ncm.
-Normal running torque will be significantly less than above.
-Budget is around AU$200 per axis (this is the main problem!)

Stepper motors are not an option, unless someone can help me prove that the cogging of a micro-stepped stepper is comparable or less than that of a standard dc motor.

Becuase I've come in on this project three quarters of the way through, I'm a bit limited by the design so far - so a great big worm gear arrangement probably will be a little too tricky to implement (and would play havoc with the budget too I think).

I guess what I would primarily like to know (at this stage ) is how to figure out how slow a dc motor can safely go given the data provided by manufacturers such as mclennan.co.uk and pittmannet.com.

I'm sure there are a million other questions too, but I just don't know them yet! I've trawled the net and various textbooks too, so if anyone can suggest some good texts etc, that would be great.

 

[Danger]

Can't help you with most of your questions, unfortunately. I would, however, recommend looking into epicyclic gear trains. One alone can give you something on the order of 1,000:1 reduction, and you can stack them.
Russ might be your best source here; he has a bit of experience with telescopes.

 

[sgc7of7]

Thanks for the tip Danger - I'll have a look into epicyclic gear trains.

I've managed to find some good info on the Pittman website, and it seems that slowest you can run any of their servo-motors is about 500rpm, which means I still need a reduction of around 700 000:1. If I were to use a worm drive of say, 60:1, on there, I could get away with a gear train of about 11 000:1, which is in the realm of possibility (I've seen a 15000:1 gearbox made by Mclennan). It's just the cost that appears to be the killer! If Russ is hanging around, I'd love to hear your thoughts on driving telescopes...

 

[Danger]

Russ is always lurking somewhere; he's one of the Mentors.
Strangely enough, when I Google epicyclic trains all that turns up are various planetary set-ups such as in an automatic transmission. I have not been able to find anything at all about the one I'm thinking of. It consists of 2 sun gears back to back, connected by a single planetary. One of the suns has one more tooth than the other. That's for instance 100 teeth on one and 99 on the other. It's not an ideal situation because obviously there isn't a perfect pitch match between the 3 gears, but you can get some awesome ratios.

 

[AlephZero]

I don't know anything specifically about telescopes but I'm curious why you don't want to use stepper motors. I would have thought they were very suitable for this, since they are easy to control, high torque, and run efficiently at any speed from hundreds of steps/second right down to zero.

I would have thought the inertia of the scope and drive train would smooth out the "stepping" effect pretty well. You might want to consider the position errors caused by tolerances in the reduction gears (imperfect tooth profiles, gear wheels not perfectly centered, etc) and compare those with errors from a stepper motor drive.

Speculating, you might even be able to compensate for the errors in a cheap gearbox with clever software that drives the stepper at slightly variable speed to compensate. That sounds pretty simple, compared with trying to control the speed of an "analog" motor accurately.

One final thought (hope it's not so obvious as to be patronizing): cheap and simple mechanical devices that rotate accurately at 2 rev/day have been around for a long time. They are called clocks. Maybe high-rratio epicyclic gears etc are overkill for this application.

 

[Danger]

One final thought (hope it's not so obvious as to be patronizing): cheap and simple mechanical devices that rotate accurately at 2 rev/day have been around for a long time. They are called clocks. Maybe high-rratio epicyclic gears etc are overkill for this application.

It's not patronizing at all; sometimes we overlook the obvious. My only concern in this case would be getting enough torque out of a clock to move the telescope. Good post.

 

[sgc7of7]

I can't use stepper motors because the guy I'm building this for has said "NO STEPPERS". Which is unfortunate...

Clocks though, eh? Good suggestion, although as Danger said, I may need a little more torque than that required by the hour hand of a clock :) Will look into it though.

I have found *a* solution - a motor with a 800:1 gearbox on the front, plugged into the 15000:1 gearbox I mentioned earlier. I'm not really happy with it, as you mentioned, Alephzero, all those gears are sure to introduce a heap of errors. Some sort of feedback loop may help to sort this out though (although I feel like I'm just handballing the problem onto the next person).

Slightly off topic, I've read on a few places the term "base speed" referring to DC motors - does anyone know what this means? Rated speed perhaps?

 

[AlephZero]

I can't use stepper motors because the guy I'm building this for has said "NO STEPPERS". Which is unfortunate...

Clocks though, eh? Good suggestion, although as Danger said, I may need a little more torque than that required by the hour hand of a clock :) Will look into it though.

Well, clockwork used to be the answer...

http://www.aas.org/~pboyce/mma/cooke-tel.htm

 

[sgc7of7]

Ahh, I thought you were talking about modifying an old wall clock!

 

[xez]

If you're really stuck, ask me and I'll send you plenty of more
specific links and references that I've accumulated over the
years.

My best suggestion for inspiring ideas / dialog is to take a
look at the various "ATM" sites (amateur telescope
makers / making) out there. There's a LOT of
good information on all kinds of homebrew motorized
drive mechanisms for various kinds of mounts / platforms
out there. There are all kinds of options from the simple
to the byzantine and the elegant to the rube goldberg
"wow I can't believe that works".

There's nothing inherently wrong with steppers; being
a fairly old time amateur astronomer myself, I realize
that the two objections are (a) complexity -- needing
a less common motor, and digital/analog/power drive
controller board electronics to drive it at the proper rates,
and (b) smoothness -- if you're tracking something at high
magnification, or doing long exposure astrophotography,
then the jitter due to the jerky steps / pulses can cause
irritating vibration / dancing / blurring of the object
even though the jitter motion may be quite small.
That being said, almost all the modern scope designs
for modestly big instruments including ones that
are excellent at astrophotography do use steppers and
feedback control system technology to control the
motors. In fact many scopes use ALTITUDE-AZIMUTH
mounts and it's ONLY the smart positioning/tracking
systems that keep them tracking anything at all. The
cost / size / mechanical inconvenience of an massive
equatorial mount is FAR worse a burden than something
effectively as cheap/simple as an alt/az 'dobsonian' like
mount added with the cost of the relatively more
complex stepper / computerized tracking & motion control
system and position sensing systems etc.
Also you can do things like micro-step steppers even
to the point of using them as somewhat linear thrust
motors in the space between one step and the next
so in effect it' acting more like a DC motor (which it
is, really, just one with several individual phases of
stators to control) than a digital 'step' motor.

You can get things called slow / synchronous motors that
are AC sine wave motors that are powered by household
power, and they run synchronously to the AC power
frequency which is only 60 or 50 Hz. Given that and very
modest up/down gearing, you can end up with very slow
speeds indeed, but you're limited to places where you have
sources of sine wave AC power to run them.

If you pick the appropriate DC motor you can run it
quite slowly, you just need to find one that's got plenty
of torque and can run with up to 12 hours * many days
per year of service duty in a near stalled condition with
very little energizing current if it's being used at much less
than it's usual rated speed. Generally that'll be a very
over-rated motor used at a fraction of its capacity.

There are also lots of ways to 'cheat' and get an excellently
smooth tracking with relatively cheap parts. One being
using an 'equatorial platform' which is basically just
a board / 'ramp' that the telescope sits on that happens
to point north and be tilted at the local angle of latitude.
Think a couple of doors screwed together by one set of
hinges, one laying flat, the other tilting open at whatever
angle is needed, and a brace set between them to keep
them open at the right angle. Just a crude description;
there are lots of designs for building them out of
anything from plywood to concrete etc. etc. check the
ATM articles.

Given such an adjustable equatorial platform you're
already north aligned and equatorially tilted, then you
set an alt-az mounted scope on it e.g. like a dobsonian.

Then use a manual ALTITUDE adjustment on the scope
since ALTITUDE (now == declination given the
e.q. platform) never needs to change once you're pointed
at the object of interest. So then the only axis that
needs mechanical tracking is the AZIMUTH
(== right ascention now given the eq. mount). This
can be done ever so smoothly, cheaply with a
tangent arm drive. Get a long threaded rod with modestly
fine threads per inch of its length.
Fix one end of the threaded rod to your favorite motor,
DC, AC, synchronous, whatever. The motor and rod are
fixed to the platform and do not move with the scope.
Put a "nut" aka "follower" on the fixed position rotating
threaded rod, and hold the nut / follower from rotating
and have it 'push' tangentially against a rotation circle
attached to the azimuth (aka R.A.) axis of the scope.
As the motor turns, the nut/follower 'unscrews' along
the length of the threaded rod and pushes the scope
along. Disadvante being it can only track for a limited
period of time since the follower is moving in a line and
the scope is rotating, so the line is basically a chord of
the rotation circle of the scope and once the nut is at
maximum extension you have to mechanically 'reset'
the scope by rotating the tube mount relative to the base
so that the 'retracted' position now again points at the
object of interest and you're ready to track for another
length of time until you reset again. With even a modestly
slow motor and modestly resolute threaded rod you can
devise these to track for hours at a time which is usually
quite sufficient even for photography, though it's unsuited
for something that should function from dusk to dawn
entirely automatically. Of course you can add
altitude / declination motor drive if you want too, but
that's quite optional.

Another variant is to make the motor and threaded rod
so they move linearly on a slide as the rod screws against
a fixed nut/follower; in that case you use the end of
the screw rod itself to push tangentially against a
fin/driving point on the rotation axis of the scope so that
the tip of the screw pushes along the rotation of the
scope. Again, smooth, simple, and the drawback is
that it'll work over a certain angle of arc driving duration
before needing to be reset to bring the rod back to
the retracted position, bear the fin against the rod, and
rotate the scope relative to the fin for another period
(minutes, hours, depends on your mechanical design)
of driving.

Another simple / cheap option is again basically like
a dobsonian idea; instead of using a very very fine
toothed right ascention gear and a fine gear to rotate
against it, just abandon the gears and use a drive
circle attached to the rotation axis of the scope that's
not a gear at all but is just a very large diameter CIRCLE
that's FRICTION DRIVEN by the rotation of a small
motor mounted next to it. If you use something like a
small circular table top, circular piece of plastic
countertop, circle-cut plywood, etc. it's hardly difficult or
expensive to get a drive circle that's perhaps two, three,
four feet in diameter. Bicycle wheel, same idea.

It's not particularly in the way, heavy, or inconvenient
if your scope tube is even longer (e.g. 12" F/5 = 5' long
tube or 8" F/8 tube etc.), and the drive circle can even
be disassembled into two or four pieces if need be for
transport / storage compactness.

So given something like a 3' e.g. diameter drive circle,
that's 113" of circumference over 360 degrees of arc.
Drive against it with something like a 1" diameter
friction wheel and that's a 1:36 rotation reduction right
there from the gear ratios. So for the big wheel to
turn at 1/1440 RPM (1 turn/day) the small wheel
would turn at a rate of 1/40th RPM which is a lot more
easily achievable with common gear box reduced motors
due to the large drive circle to small drive rotor ratio,
and it'll help minimize the effect any irregularities in drive.
The friction drive against the drive circle is of course
easy to mechanically or electrically (solenoid/spring)
disengage for manual repositioning of the scope, and it's
advantageous since there is no precise alignment or
gearing of the scope to the drive motor, so no worries
about stripped gears or precise assembly.

 

[sgc7of7]

You can get things called slow / synchronous motors that
are AC sine wave motors that are powered by household
power, and they run synchronously to the AC power
frequency which is only 60 or 50 Hz. Given that and very
modest up/down gearing, you can end up with very slow
speeds indeed, but you're limited to places where you have
sources of sine wave AC power to run them.

If you pick the appropriate DC motor you can run it
quite slowly, you just need to find one that's got plenty
of torque and can run with up to 12 hours * many days
per year of service duty in a near stalled condition with
very little energizing current if it's being used at much less
than it's usual rated speed. Generally that'll be a very
over-rated motor used at a fraction of its capacity.

Wow - heaps of info there, thanks. I'm just going to pick on one or two points at this stage though...

First, I've looked at AC motors, but the mount needs to run from a portable power source. The addition of inverters etc makes things a little expensive.

So onto the DC motor comment. When you specify "little energizing current", is that to limit the rate at which the batteries powering the motor are drained, or is there another reason?

Thanks for all the other design info - I've read of some very weird designs...
I have it pretty much nutted out now though, I've found some reasonably priced, AGMA 10 quality wormsets, and will use a couple other spur gears to couple the motor to the worm. The only thing remaining is the motor selection. I'm considering a Como Drills 919D series geared motor, but I'll have to check out a few things with it...