Re: Driver for very small brushless DC motors?

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Even microstepped, steppers shake, rattle, & roll. And they sing (resonate). I never imagined how much until I tried a few.

As far as COTS, CD, DVD & hard disk spindle motor drivers? They use 3- phase BLDC motors & integrated controllers.

Here's an old BLDC datasheet off ye old hard drive:

But won't you be wanting ultra-fine control over commutation, PWM, position-interpolation and such? You'll probably have to do that yourself.

Atmel, Microchip, and Freescale all have good application notes on BLDC-driving with uCs.

e.g. Atmel AVR444: Sensorless control of 3-phase brushless DC motors.

-- Cheers, James Arthur

I'm actually just going to spin it up and do the measurement as it spins down unpowered. That way I should have zero cogging and no jitter due to commutation.

Cheers

Phil Hobbs

Conceptually, steppers and brushless DC motors are identical, except that the brushless DC motor has got a rotational position sensor to control the current through the various windings. In both cases the windings are static and on the outside of the motor, which makes it easier to get rid of the heat.

Escap certainly used to sell a small stepper that was designed for microstepping and rotated tolerably smoothly when excited by sine/ cosine drive currents. It used a disc magnet rather like this part

which does offer the 10mm diameter you ask for, but is much too long.

-- Bill Sloman, Nijmegen

But they can be silky-smooth if you drive them right, in the speed range they like.

I think of a BLDC as a 3-pole stepper that hard commutates based on crappy Hall sensors. And I think of a stepper as a 100-pole BLDC that soft commutates using precisely the waveform that produces the smoothest rotation.

So there.

John

But iron-rotor steppers and BLDCs both cog like absolute mad on the scale I care about--the signal I'm looking for is the equivalent of ~10*6 cycles per rev, and I need to resolve 1/8 cycle or better. I can average out random stuff, or things like out-of-round ball bearings, but cogging is the same on every single revolution, so it survives averaging and looks just like signal.

Cheers

Phil Hobbs

I'd need several million steps per rev--accurate ones, not Marketing Microsteps--and there's no way to compensate the cogging caused by the iron in the rotor to that level, certainly not over time and temperature. Ironless BLDCs are not stepper-like in design--when the power goes off, they rotate completely freely, except for the bearings and slip rings.

Cheers

Phil Hobbs

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The unpowered motor will still cog of course, just not nearly as much.

For just testing VCR spindles might be interesting. They're an endangered species now, but they're 3-phase BLDC motors, with integrated drivers, flywheels, and impressively low run-out bearings. That level of precision & longevity has got to imply a certain smoothness of rotation & lack of vibration too. Couldn't hurt, anyhow.

Probably kid stuff by your standards.

-- Cheers, James Arthur

There are no iron teeth (or any other iron) in the rotor, so when it's unpowered, the only things left to cause angular acceleration are the bearings, the slip rings, air friction, and probably some slight eddy current loss due to remanent magnetization. They call it 'zero cogging'. Whether it's close enough to zero, I'm not sure.

Cheers

Phil Hobbs

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I was thinking both eddies and modulation of aerodynamic drag by / at the poles.

Also, even completely electrically open, the rotor poles form L-C tanks with their winding capacitances. I've no idea how much those will matter, but they'll suck a little energy at each pole crossing, and kick or drag, depending.

-- Cheers, James Arthur

True, but hopefully a small effect at a few hundred RPM--they'll be very far from resonance.

After all that build-up, I'd better go take some data, or people will start thinking I'm like that guy who used to brag all over Usenet about making diamonds by the pound...

Cheers

Phil Hobbs

Well shoot, if inertia's fair, the prototype's easy: VCR spindle with a single optical stripe...

-- Cheers, James Arthur

Then mimimize the amount of cogging and maximize its frequency, and mechanically lowpass filter the rotation. The imperfect microstep wiggles are essentially harmonics of the step rate, so are relatively easy to lowpass filter. A clever drive waveform will minimize the lower harmonics and make the filtering more effective.

If you really want to run in spindown mode (sounds mathematically messy to me!) use any junky motor and disconnect it during spindown. Some Bendix drive variant maybe, or a centrifugal clutch, or an air gap viscous coupling.

Air motors are very cool, except that they need compressed air. We're working with some guys who use a gas motor to spin the prism in a drum camera, at 20 KHz; 1.2 MRPM.

John

Interesting idea. I need something with a flattish top, that I can attach some various bits and pieces to (in order to do the measurements).

I've actually never taken a VCR apart in my life, I'm ashamed to say. (I've also hardly ever used one, except to show Veggie Tales to a Sunday school class once in a great while.)

What sort of bearings do they use?

Cheers

Phil Hobbs

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Disconnecting it is probably the ticket, just as you say, which is why I'm coming around to the eddy current drive. Linkages don't get much floppier than that, it's dead simple, and there's nothing to wear out or get broken.

The pattern I'm looking for is symmetrical with rotation, so it isn't too hard to get the deceleration rate to high accuracy. It's all about curve fitting, and I have 50k data points and only need 7 parameters. So if I can minimize the systematic errors (especially those harmonics of the rotation rate you mention), I should be in pretty good shape.

At 1.2 MRPM, the glass is probably becoming birefringent due to the stress! Must be bomb photographers.

Cheers

Phil Hobbs

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Ball bearings: super-fine, and low friction. Any wobble trashes the video, as you can imagine. I've got a few and couldn't detect any runout at all. Mr. Google says some VCRs use a bronze(?) sleeve on a steel post--I haven't seen any of those, but they can be very good too--and fluid dynamic pressure bearings, e.g.

For that matter an old hard drive spindle might work pretty well too; they've got super bearings.

Oh, and you could use more than one stripe--maybe one index stripe for repeatable positioning pickup, and a separate band of however many not- as-accurately-spaced stripes for speed control feedback.

-- Cheers, James Arthur

Or maybe you can use some old hard disk drive?

Speaking of microstepping, why not just go nano/picostepping and have a look at US motors:

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If you got desperate enough you might cannibalise an ESCAP stepper for the magnetic disk and its shaft and print your drive windings on a pair of small multilayer printed circuit boards.

The torque wouldn't be anything like as high, but you might get enough to do your job, and you wouldn't have any cogging.

-- Bill Sloman, Nijmegen

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turn it inside out, rotate magnets keep the (air) coils stationary and you don't need the slip rings

I wonder if it might be better to not even use magnets and try to make a simple induction motor instead

-Lasse

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Got any links to such motors?

Induction motors should have zero free-spin torque too, except for windage.

Or it breaks up into little pieces, bad for the optics too.

Yup.

John