Variable speed electric motor questions

I don't know if sci.engr.mech is the appropriate newsgroup for this inquiry; if anyone knows a more appropriate group, please let me
know.
Someone needs a way to run a small electric motor at variable speeds over a dynamic range of 100 and still produce a smooth motion. That is, once the motor is geared down properly, the shaft must turn anywhere from 5rpm to 0.5rpm without jerking. Torque is not likely to be an issue--I can turn the load to be driven with the fingers of one hand.
Can this be done with a DC motor, with the appropriate type of controller? What type?
Is it possible to run a stepper motor over that much of a range and somehow remove the jerkiness from the motion? (We could run the stepper motor at whatever absolute speed produced the best results, then gear it down appropriately; however, the speed of the motor would still vary by a factor of 100.)
This project must be done on the cheap, using surplus or home built equipment.
If the dynamic range is simply too large, then I suppose one alternative might be to use several interchangable gear reducers. What are the chances of finding gear reducers with dimensions similar enough that they can be swapped out?
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Paul Ciszek wrote:

A DC brushless servo motor will do it, although they are not cheap when new. maybe you want something like that:( in german, sorry):
http://www.faulhaber.de/zip_pdf/anleitung/2000/BED3564.pdf
If you want to go the hard way, build your own servo. Take an appropriate DC motor, attach a rotary encoder, maybe homebuilt optical. Connect everything to a PC and regulate it with homemade software.
Another way would be using a fast PID temperature controller and some appropriate interface electronics instead of the PC and software.
The easiest, cheapest motor that comes to my mind is a DC brushless fan for computers. They run without jerking and some of them have a integrated tach sensor (3 wire models). Available up to some watts.
good luck! Andreas Rutz
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There is a DC gearmotor available as surplus that will turn at 4.5 RPM with a 12V supply; my question is, what is the best method for slowing it down electrically? And can it be slowed down by a factor of 100 without chattering?

Even with a feedback loop, how do you control the speed? Do you cut the voltage? Use pulse width modulation? How do you get the motor to turn at less than full speed without it chattering?

I doubt they are really designed to run smoothly at 1% of full speed.
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snipped-for-privacy@nospam.com (Paul Ciszek) wrote in message wrote:

Yes and yes. (Someone stop me if I'm mistaken). You could probably do either (varying the voltage or PWM). You may need a capacitor with the PWM controller, depending on the motor specs.
Another option is getting an AC motor and inputing a variable frequency voltage. That might not be as cheap, but it would definately be easy to control.
Dave
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snipped-for-privacy@nospam.com (Paul Ciszek) wrote in message wrote:

Yes and yes. (Someone stop me if I'm mistaken). You could probably do either (varying the voltage or PWM). You may need a capacitor with the PWM controller, depending on the motor specs.
Another option is getting an AC motor and inputing a variable frequency voltage. That might not be as cheap, but it would definately be easy to control.
Dave
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snipped-for-privacy@nospam.com (Paul Ciszek) wrote in message wrote:

Yes and yes. (Someone stop me if I'm mistaken). You could probably do either (varying the voltage or PWM). You may need a capacitor with the PWM controller, depending on the motor specs.
Another option is getting an AC motor and inputing a variable frequency voltage. That might not be as cheap, but it would definately be easy to control.
Dave
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snipped-for-privacy@nospam.com (Paul Ciszek) wrote in message wrote:

Yes and yes. (Someone stop me if I'm mistaken). You could probably do either (varying the voltage or PWM). You may need a capacitor with the PWM controller, depending on the motor specs.
Another option is getting an AC motor and inputing a variable frequency voltage. That might not be as cheap, but it would definately be easy to control.
Dave
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A capacitive load on a PWM servo amplifier is in general a "very bad idea" (tm). It reduces the effective inductance of the load and can result in instability in the current loop. At the very least it will form an RLC tank circuit which will ring and may cause emissions problems. If filtering required, additional inductance can be added in series.
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I wasn't aware as far as PWM amps go. Good point. I've used caps with some very light loads (no amps) in some PWM applications, and it's been within spec of the IC. Guess this doesn't apply to amps.
Dave
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snipped-for-privacy@nospam.com (Paul Ciszek) wrote in message

A LINEAR microstepping controller for a stepper can do that given a bit of tuning for the mechanical parts of the system. See:
http://www.piclist.com/techref/io/stepper/linistep/smoother.htm
and
http://www.piclist.com/techref/io/stepper/linistep/index.htm
James.
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This is great; I have some friends who know all about PICS. Thanks.
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Small AC variable speed drives are now quite reasonably priced (couple of hundred US$ new). They can also now slow to a stop with near full torque. At the sort of revs you want a speed reducer is probably still needed simply to keep the motor size reasonable and the output as smooth as poss.
Mark.

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Dear Paul Ciszek:

There are such things as infinitely variable speed drives. I don't know how low they will go horsepower-wise...
URL:http://www.industrialnewsroom.com/fullstory/24019 as an example returned by google advanced with all the words infinite variable speed drive transmission fractional
David A. Smith
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100:1 is a relatively small speed range and should be easy with any PWM servo on a dc motor. Depending on your velocity stability requirements you may be able to use the motors back EMF as a velocity feedback, eliminating the need for a tach or encoder. See: http://www.elecdesign.com/Articles/Index.cfm?ArticleID !03 BTW, since you are talking cheap and dirty, have a look at surplus automobile electric window motors, and/or windshield wiper motors. Jeff
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