Driver for very small brushless DC motors?
in [Design]
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From: Phil Hobbs on 22 Nov 2009 22:33 John Larkin wrote: > On Sun, 22 Nov 2009 17:23:32 -0800 (PST), dagmargoodboat(a)yahoo.com > wrote: > >> On Nov 22, 5:43 pm, Phil Hobbs >> <pcdhSpamMeSensel...(a)electrooptical.net> wrote: >>> John Larkin wrote: >>> >>> > 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? >>> >>> I'm mostly interested in very smooth motion at small scales, which is >>> why I want an ironless BLDC. The gizmo's operation will require a lot >>> of curve fitting to pull out the amplitude and phase of a >>> small-amplitude tone burst of about 10k cycles over about 5 degrees of >>> shaft rotation, once per rev. Any cogging or other bad behaviour of the >>> motor will cause nasty spurious peaks in the spectrum, among other problems. >>> >>> Steppers are never sufficiently well made to avoid periodic errors--I'm >>> at the level where I have to worry about whether the ball bearings are >>> smooth enough, or whether I need to use jewels, which would be fragile >>> and expensive enough to dim my enthusiasm quite a bit. (A galvo is >>> another possibility, but those cost the Earth.) My hope is that because >>> the balls' motion doesn't have the same period as the shaft rotation, I >>> can sort out the bearing junk from the desired signal. >>> >>> In the real system, I'm expecting to have optical clues as to what the >>> actual motor phase is, but I'm not too worried about that at this point. >>> >>> I'm currently gearing up to do a sanity test with a nice Maxon brush >>> motor from my junk box, a He-Ne, and an HP 35665A dynamic signal >>> analyzer to do the data acq and so on. (I just got a Prologix >>> GPIB-Ethernet gizmo, so I don't have to use the floppy drive to get data >>> in and out.) >>> >>> Cheers >>> >>> Phil Hobbs >> >> Even microstepped, steppers shake, rattle,& roll. And they sing >> (resonate). I never imagined how much until I tried a few. > > But they can be silky-smooth if you drive them right, in the speed > range they like. > >> >> 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: >> http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MC34929 >> >> 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. > > 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 -- 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
From: Phil Hobbs on 22 Nov 2009 22:55 Spehro Pefhany wrote: > On Sun, 22 Nov 2009 21:29:11 -0500, the renowned Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> wrote: > >> 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. > > They're supposed to have "zero" cogging, but not sure offhand how > close they really get. We're using them at approximately zero RPM. 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. > >> 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. > > Flywheel? 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
From: Phil Hobbs on 22 Nov 2009 22:59 Bill Sloman wrote: > On Nov 22, 11:43 pm, Phil Hobbs > <pcdhSpamMeSensel...(a)electrooptical.net> wrote: >> John Larkin wrote: > > <snip> > >> Steppers are never sufficiently well made to avoid periodic errors--I'm >> at the level where I have to worry about whether the ball bearings are >> smooth enough, or whether I need to use jewels, which would be fragile >> and expensive enough to dim my enthusiasm quite a bit. (A galvo is >> another possibility, but those cost the Earth.) My hope is that because >> the balls' motion doesn't have the same period as the shaft rotation, I >> can sort out the bearing junk from the desired signal. > > 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 > > http://www.portescap.com/product-39-P010.html > > which does offer the 10mm diameter you ask for, but is much too long. > > -- > Bill Sloman, Nijmegen 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 -- 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
From: Jon Slaughter on 22 Nov 2009 23:34 Phil Hobbs wrote: > I have a partly-baked idea I'm exploring, for a simple laser beam > diagnostic tool. It needs a small brushless motor (less than 10 mm > diameter and 3 mm tall) with an ironless rotor. I have possible > motors in mind, but it seems that there are few integrated BLDC > controller/driver chips these days. I was going to use an Allegro > A8904, but it's now listed as "not recommended for new designs". :( > > I'd prefer to use a back-EMF controller rather than Hall sensors, > because I don't care too much about smoothness of motion during > spin-up, and sensorless motors are cheaper, particularly in such > small sizes. > Any recommendations for integrated BLDC controller/driver chips? > > Thanks > > Phil Hobbs FS has many that have built in mosfets. They also have ones with external switches but unfortunately the 2-"phase" are being phased out...
From: dagmargoodboat on 22 Nov 2009 23:43
On Nov 22, 9:06 pm, Phil Hobbs wrote: > dagmargoodb...(a)yahoo.com wrote: > > On Nov 22, 5:43 pm, Phil Hobbs wrote: > >> I'm mostly interested in very smooth motion at small scales, which is > >> why I want an ironless BLDC. The gizmo's operation will require a lot > >> of curve fitting to pull out the amplitude and phase of a > >> small-amplitude tone burst of about 10k cycles over about 5 degrees of > >> shaft rotation, once per rev. Any cogging or other bad behaviour of the > >> motor will cause nasty spurious peaks in the spectrum, among other problems. > > >> Steppers are never sufficiently well made to avoid periodic errors--I'm > >> at the level where I have to worry about whether the ball bearings are > >> smooth enough, or whether I need to use jewels, which would be fragile > >> and expensive enough to dim my enthusiasm quite a bit. (A galvo is > >> another possibility, but those cost the Earth.) My hope is that because > >> the balls' motion doesn't have the same period as the shaft rotation, I > >> can sort out the bearing junk from the desired signal. > > >> In the real system, I'm expecting to have optical clues as to what the > >> actual motor phase is, but I'm not too worried about that at this point. > > >> I'm currently gearing up to do a sanity test with a nice Maxon brush > >> motor from my junk box, a He-Ne, and an HP 35665A dynamic signal > >> analyzer to do the data acq and so on. (I just got a Prologix > >> GPIB-Ethernet gizmo, so I don't have to use the floppy drive to get data > >> in and out.) > > >> Cheers > > >> Phil Hobbs > > > 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: > > http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MC34929 > > > 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. > > 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. 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 |