Driver for very small brushless DC motors?
Phil Hobbs
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
--
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
John Larkin
<pcdhSpamM...@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
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
Baron
> 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
That sounds like the mirror motor I've seen in some laser printers.
--
Best Regards:
Baron.
lang...@fonz.dk
lang...@fonz.dk
> 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
>
how much current is needed?
-Lasse
o...@uakron.edu
From research I'm doing for a similar but slower project.
ATMEL APP NOTE AVR843
Microchip App Note 857
Any model airplane ESC
http://www.avrfreaks.net/index.php?name=PNphpBB2&file=printview&t=71674&start=20
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
Phil Hobbs
lang...@fonz.dk wrote:
--
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
Probably 100 mA--500 at most.
Cheers
Phil Hobbs
> 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.)
dagmarg...@yahoo.com
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.
--
Cheers,
James Arthur
Phil Hobbs
> (resonate). I never imagined how much until I tried a few.
>
> 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.
>
> --
> 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.
Spehro Pefhany
<pcdhSpamM...@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"
sp...@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
Bill Sloman
<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
John Larkin
wrote:
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
Phil Hobbs
Spehro Pefhany wrote:
> On Sun, 22 Nov 2009 15:48:00 -0500, the renowned Phil Hobbs
> <pcdhSpamM...@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
> <pcdhSpamM...@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
Spehro Pefhany
<pcdhSpamM...@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
--
Phil Hobbs
>> (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.
>
>>
>> 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.
Phil Hobbs
> On Sun, 22 Nov 2009 21:29:11 -0500, the renowned Phil Hobbs
> <pcdhSpamM...@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. ;) ]
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.
Jon Slaughter
> 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...
dagmarg...@yahoo.com
> dagmargoodb...@yahoo.com wrote:
> > On Nov 22, 5:43 pm, Phil Hobbs wrote:
> > 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
Phil Hobbs
>>
>>> 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.
> 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.
dagmarg...@yahoo.com
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Sun, 22 Nov 2009 17:23:32 -0800 (PST), dagmargoodb...@yahoo.com
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
Phil Hobbs
>>> (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.
>>
>>
>>
>>> 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
> 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.
dagmarg...@yahoo.com
> dagmargoodb...@yahoo.com wrote:
> > On Nov 22, 9:06 pm, Phil Hobbs wrote:
> >> 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
>
> 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.
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
Phil Hobbs
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...
dagmarg...@yahoo.com
> dagmargoodb...@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...
--
Cheers,
James Arthur
John Larkin
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
Phil Hobbs
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?
John Larkin
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
Phil Hobbs
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.
Phil Hobbs
(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.)
dagmarg...@yahoo.com
<pcdhSpamMeSensel...@electrooptical.net> wrote:
> dagmargoodb...@yahoo.com wrote:
> > On Nov 23, 12:42 am, Phil Hobbs wrote:
> >> dagmargoodb...@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
Fred Bartoli
> dagmarg...@yahoo.com wrote:
>
>> On Nov 23, 12:42 am, Phil Hobbs wrote:
>>> dagmargoodb...@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.
Bill Sloman
<pcdhSpamMeSensel...@electrooptical.net> wrote:
> Bill Slomanwrote:
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
lang...@fonz.dk
wrote:
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
John Larkin
<pcdhSpamM...@electrooptical.net> wrote:
Got any links to such motors?
Induction motors should have zero free-spin torque too, except for
windage.
>
>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!
Or it breaks up into little pieces, bad for the optics too.
> Must be bomb photographers.
>
Yup.
John
John Larkin
<pcdhSpamM...@electrooptical.net> wrote:
Pass the raw data to a PC and do it there?
We're starting to use an NXP uP, ARM architecture, 280 MHz or some
such, that has a floating-point vector co-processor. About $7.
John
Phil Hobbs
http://www.maxonmotor.com/product_overview_details_ec_motor.html
>
> Induction motors should have zero free-spin torque too, except for
> windage.
>
>>
>> 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!
>
> Or it breaks up into little pieces, bad for the optics too.
>
>> Must be bomb photographers.
>>
>
> Yup.
>
> John
>
Phil Hobbs
That would be quite possible, and the prototype will certainly be done
that way, via the HP 35665A and Prologix. I'd love the actual
instrument to be lighter weight than that, though--more like a Fluke 87,
which you can shove in a drawer for a year, then pull it out and it just
works.
>
> We're starting to use an NXP uP, ARM architecture, 280 MHz or some
> such, that has a floating-point vector co-processor. About $7.
>
> John
My hope would be that this could be done with a 16-bit PIC and some
SPI-attached SRAM, or something like that--battery-powered, anyway.
Do those NXPs have power control features that are reasonably easy to
use? Being able to crank it up to full speed (which I assume means a
watt or so) 1% of the time would be great. A nice development system, a
good library, and a decent display would really help. Does it run
COMMAND.COM? ;) (I'd happily settle for bash over telnet.)
Ideally I want to measure:
1. Wavelength,
2. Collimation,
3. Power,
4. Beam profile,
5. Wave aberrations through fourth order (i.e. spherical, astigmatism,
and coma),
6. Temporal coherence,
7. Spatial coherence,
8. AM and FM Noise, and
9. Pointing.
I have some idea how to do all these things with a simple optomechanical
gizmo and some not-too-ferocious software, though they won't all be in
the first version, that's for sure. (Some of them need 2D motion, for
one thing.)
It also won't do an equally good job on all of them, nor will it work
well on all beams. But the things it does do, have to be done really
really right--as the teacher might say, "The essence of Fluke nature,
Grasshopper, is no ambiguity: give trustworthy results or confess
failure." It sure won't happen by accident.
To get much further with it, I need some data on beams with accurately
known problems, which is my spare-time project for the next month or so.
I'll certainly try the VCR and hard disk ideas--I've used salvaged 2.5
inch hard disks to spin hologon scanners, which survive 3600 rpm with no
problems, but I'm not optimistic about how they'll do at spinning some
comparatively gigantic interferometer thing, much more slowly.
Maybe an unpowered HDD spun by hand, to start with.
whit3rd
<pcdhSpamMeSensel...@electrooptical.net> wrote:
> >> 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.
> 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...
This reminds me of a toy I used to love. It was a gyroscope, with
a wrap-string-on-shaft motor. Do you have to have electric
drive?
John Devereux
John's probably talking about the LPC3000 series, e.g.
<http://www.nxp.com/#/pip/pip=[pip=LPC3220_30_40_50_1]|pp=[t=pip,i=LPC3220_30_40_50_1]>
It's an ARM9, 266Mhz. At 208 MHz, the datasheet says 80mA @ 1.2V core
voltage. These sort of chips tend to have very good power control.
> A nice development system, a good library, and a decent display would
> really help. Does it run COMMAND.COM? ;) (I'd happily settle for bash
> over telnet.)
They have a good development ecosystem (free and non-free) and I believe
a linux distribution is available. It's BGA and you'll need external
SDRAM+flash though.
I am normally a strong advocate of the free GNU tools, but if you
especially want to do heavy duty floating point work you would likely
save time going with the official ARM (Keil) compiler.
[...]
--
John Devereux
Michael Wieser
Michael Wieser
<pcdhSpamM...@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
Toshiba TB6588FG
-
Michael Wieser
--
JosephKK
Hey, i still vote for the flywheel.
JosephKK
wrote:
Not the lowest friction bearing but certainly smooth during useful
life.
JosephKK
<pcdhSpamM...@electrooptical.net> wrote:
>Spehro Pefhany wrote:
>Spehro Pefhany wrote:
>> On Sun, 22 Nov 2009 15:48:00 -0500, the renowned 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
>> 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
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.
ChrisQ
>
> Hey, i still vote for the flywheel.
Reminds me of disk motors, as used in early electricity meters ?. All
you need then are two or three coils around the circumference. So long
as you can electronically isolate the windings on spin down, there
should be negligable effect on spin down. A metal disk is easy to
balance as well...
Computer dat drives use a what looks like a modded camcorder mechanism,
with flat rare earth magnetics on the shaft and ironless pancake coils
to drive them, should you need much torque. Usually a one chip drive
solution internally, though from the examples i've stripped down, often
hall sensor(s) as well...
Regards,
Chris