Driver for very small brushless DC motors?

What a coincidence... I've been thinking about the same problem.

How about a small, cheap stepper. One could run it in microstep mode and tweak its drive waveform to get very smooth rotation; I know that works. Then couple it to the load platform through something torsionally compliant, like a spring or a rubber tube or a piece of piano wire or something. Maximize the mass of the load platform to make a mechanical lowpass filter.

Over the top, but I suppose one could make a multipole rotational lowpass filter by adding mass to the motor and/or insert an intermediate mass and use two compliant couplings. I've seen Collins-type mechanical filters like this, and it resembles a microstrip lowpass filter in concept.

The stepper gives exact, controllable rotational speed open-loop, which is nice. And small steppers are cheap and easy to drive.

We could program one of our multichannel arbs to test some motors and find a nice pre-distorted waveform that gives smooth rotation. I think adding some third harmonic is classic here, but whatever works. How would one instrument the resulting angular rotation? Optically, I guess, or maybe drive a variable capacitor?

John

That sounds like the mirror motor I've seen in some laser printers.

--
Best Regards:
                     Baron.

how much current is needed?

-Lasse

Phil,

From research I'm doing for a similar but slower project.

ATMEL APP NOTE AVR843 Microchip App Note 857

Any model airplane ESC

I've got a very similar problem right now, except I need to microstep a stalled 3 amp 3 phase brushless motor for controlled torque in a feedback loop.

Steve Roberts

-- Dr Philip C D Hobbs Principal ElectroOptical Innovations

55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net

Probably 100 mA--500 at most.

Cheers

Phil Hobbs

-- Dr Philip C D Hobbs Principal ElectroOptical Innovations

55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net

Mostly seems to be uC/DSP-based designs these days rather than ASICs.

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

-- Dr Philip C D Hobbs Principal ElectroOptical Innovations

55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net

So I'm sort of gathering. It's natural to want to save a chip when you're controlling a lot of motors, but it's a bit of a drag for proof-of-concept--I really just want to know whether the cogging can really be made low enough...with an ironless rotor, there have to be slip rings in there somewhere, to get the current to the rotor winding.

I suppose I could use a clutch, or a long floppy belt, or even an eddy current drive, but I'd really rather not--a little turntable attached to the shaft of a pancake motor is much more like it. If I do need a separate spindle, eddy current drive is probably next easiest--spin a small magnet near the edge of a brass turntable--but that would require a lot more mechanical fiddling than I'd like. On the other hand, it could use a cheap little brush motor with plain bearings...I'll have to think about it. I only need about 100-500 rpm, but it's got to be really really smooth.

Cheers

Phil Hobbs

-- Dr Philip C D Hobbs Principal ElectroOptical Innovations

55 Orchard Rd Briarcliff Manor NY 10510 845-480-2058 hobbs at electrooptical dot net

They're supposed to have "zero" cogging, but not sure offhand how close they really get. We're using them at approximately zero RPM.

Flywheel?

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Interesting, thanks. These little ones run best above 20k rpm, but I'd have worlds of fun making my little spinner balance well enough for that.

A bit of one, but the whole thing has to fit into a 1-inch diameter cylinder, *crossways*--it's for a laser beam diagnostic, so it has to go where the beam goes. I can make the turntable out of brass, which will help. I'm sort of liking the eddy current drive/brass turntable/jewel bearing approach, if it can be made shock resistant enough. Needle rollers, maybe--time for a Small Parts Inc. order.

I thought about using magnetic bearings, but that's really outside my comfort zone, and self-pressurized air bearings don't work at low speed.

[Note to self: I need to get some small machine tools, starting with a Sherline tabletop lathe/mill. Business is picking up, and I now have most of the test equipment I really need, so maybe I can do that soonish. My son is going off to get a BSME next year, so I can blame it on him. ;) ]

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

FS has many that have built in mosfets. They also have ones with external switches but unfortunately the 2-"phase" are being phased out...

Toshiba TB6588FG

- Michael Wieser

motors

spin-up,

motors

spin-up,

=20

Brass/bronze sleeve bearings have been traditionally used when that requirement is paramount. Millions of turntables, tape decks, and VCRs use them for that attribute. These devices also implement / take advantage of flywheel effects for the same reason.