>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

> 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

> 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 nethttp://electrooptical.net

> 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

>> 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

>  > 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

>>   >  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.

> 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

>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?

>> 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 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

>> 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.

>Phil Hobbs

>> On Nov 22, 5:43 pm, Phil Hobbs
>> <pcdhSpamMeSensel...@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

>> 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?

> <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 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

> >> 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.

>>>> 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

> >On Nov 22, 5:43 pm, Phil Hobbs
> ><pcdhSpamMeSensel...@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

>>> On Nov 22, 5:43 pm, Phil Hobbs
>>> <pcdhSpamMeSensel...@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

> > 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'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