Driver for very small brushless DC motors?
in [Design]
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From: Spehro Pefhany on 22 Nov 2009 21:10 On Sun, 22 Nov 2009 15:48:00 -0500, the renowned Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> 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 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(a)interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
From: Bill Sloman on 22 Nov 2009 21:16 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
From: John Larkin on 22 Nov 2009 21:23 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
From: Phil Hobbs on 22 Nov 2009 21:29 Spehro Pefhany wrote: Spehro Pefhany wrote: > On Sun, 22 Nov 2009 15:48:00 -0500, the renowned Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> 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 > > Mostly seems to be uC/DSP-based designs these days rather than ASICs. > > > Best regards, > Spehro Pefhany -- 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 > On Sun, 22 Nov 2009 15:48:00 -0500, the renowned Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> 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 > > Mostly seems to be uC/DSP-based designs these days rather than ASICs. > 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 http://electrooptical.net
From: Spehro Pefhany on 22 Nov 2009 22:34
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. >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? >Cheers > >Phil Hobbs Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" speff(a)interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com |