Driver for very small brushless DC motors?
in [Design]
|
From: Phil Hobbs on 23 Nov 2009 01:24 John Larkin wrote: > On Mon, 23 Nov 2009 00:42:36 -0500, Phil Hobbs > <pcdhSpamMeSenseless(a)electrooptical.net> wrote: > >> dagmargoodboat(a)yahoo.com wrote: >>> On Nov 22, 9:23 pm, John Larkin >>> <jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote: >>>> On Sun, 22 Nov 2009 17:23:32 -0800 (PST), dagmargoodb...(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 >>> >>> Depends on how you drive 'em, of course, and how fast. >>> >>> I think of BLDCs and kin as linear motors--almost like a voice-coil >>> motor--wrapped around a spindle: drive them with sinusoids at low >>> speeds, and interpolate smoothly between positions. >>> >>> Or you can drive them all--steppers too--at high speeds with >>> rectangular or crapezoidal waveforms for higher torque,& the >>> mechanical low-pass of the rotor's inertia still yields smooth >>> rotation. >>> >>> Stepper resonances aren't a problem at all if you crawl, or if you >>> fly, but they sure are a pain at mid-band. >>> >>> But for super-fine angular resolution stepper poles just aren't >>> mechanically or magnetically accurate enough. >>> >>> I'd think ironless rotors would still have several once-per-rev >>> periodic errors, but at least they don't have a magnetized cog with 50 >>> hungry poles, lusting for iron fingertips across a small gap. >>> >>> So, that's my boneheaded appreciation of it. >>> >>> Phil's app sounds like it needs a 1,000,000 line optical encoder (or a >>> 100,000 line analog encoder and a 14-bit a/d)! >>> >>> -- >>> Cheers, >>> James Arthur >> >> Nah, just Newton's laws and good timing accuracy, hopefully. I used to >> pal around with a guy named Ed Yarmchuk, who invented self-servowriting >> for hard disks--he replaced insane laser interferometer spin-stands for >> writing the servo tracks, with a bit of drive firmware, good timing, and >> Mr. Newton. You couldn't make terabyte hard disks without it. He >> retired a year or so ago (very young). Smart guy. >> >> Cheers >> >> Phil Hobbs > > Right. If you had some sort of reference signal, equivalent to a > million-step encoder, rotational noise wouldn't matter... you could > timebase correct it out. And chance of mixing a reference signal with > the real thing, or having one on the side? > > That might get to be compute-intensive, but so is doing Fouriers > linked to a spindown system. > > Some truly constant-speed spinner sure would be nice. > > John > > > My processing algorithm will probably be something like this: (1) Count the fringes from the time they become visible till the time they go away; (2) Estimate the deceleration rate by comparing the transition rates in patches at the two ends of the pattern, and maybe a few pairs of patches in between, which gives the deceleration curve; (3) Knowing the wavelength and path length vs time (because we do by now), estimate the other things we care about. No big huge FFTs, I don't think. That could be grim on a PIC! (On the other hand, it doesn't matter if it takes 5 seconds or so to do the measurement.) 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: dagmargoodboat on 23 Nov 2009 02:11 On Nov 23, 1:08 am, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > dagmargoodb...(a)yahoo.com wrote: > > On Nov 23, 12:42 am, Phil Hobbs wrote: > >> dagmargoodb...(a)yahoo.com wrote: > > >>> Phil's app sounds like it needs a 1,000,000 line optical encoder (or a > >>> 100,000 line analog encoder and a 14-bit a/d)! > > >> Nah, just Newton's laws and good timing accuracy, hopefully. I used to > >> pal around with a guy named Ed Yarmchuk, who invented self-servowriting > >> for hard disks--he replaced insane laser interferometer spin-stands for > >> writing the servo tracks, with a bit of drive firmware, good timing, and > >> Mr. Newton. You couldn't make terabyte hard disks without it. He > >> retired a year or so ago (very young). Smart guy. > > > Well shoot, if inertia's fair, the prototype's easy: VCR spindle with > > a single optical stripe... > > 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? 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. http://www.freepatentsonline..com/4972283.html. 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
From: Fred Bartoli on 23 Nov 2009 06:13 Phil Hobbs a �crit : > dagmargoodboat(a)yahoo.com wrote: > >> On Nov 23, 12:42 am, Phil Hobbs wrote: >>> dagmargoodb...(a)yahoo.com wrote: >> >>>> Phil's app sounds like it needs a 1,000,000 line optical encoder (or a >>>> 100,000 line analog encoder and a 14-bit a/d)! >>> >>> >>> Nah, just Newton's laws and good timing accuracy, hopefully. I used to >>> pal around with a guy named Ed Yarmchuk, who invented self-servowriting >>> for hard disks--he replaced insane laser interferometer spin-stands for >>> writing the servo tracks, with a bit of drive firmware, good timing, and >>> Mr. Newton. You couldn't make terabyte hard disks without it. He >>> retired a year or so ago (very young). Smart guy. >> >> Well shoot, if inertia's fair, the prototype's easy: VCR spindle with >> a single optical stripe... >> > > 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 > 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: http://www.physikinstrumente.com/en/products/primages.php?sortnr=1000180&picview=2#gallery -- Thanks, Fred.
From: Bill Sloman on 23 Nov 2009 06:49 On Nov 23, 4:59 am, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > Bill Slomanwrote: > > 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. 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
From: langwadt on 23 Nov 2009 08:30
On 23 Nov., 04:59, Phil Hobbs <pcdhSpamMeSensel...(a)electrooptical.net> wrote: > 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 > 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 |