Re: Driver for very small brushless DC motors?

On Nov 22, 9:06 pm, Phil Hobbs wrote:
dagmargoodb...@xxxxxxxxx wrote:
On Nov 22, 5:43 pm, Phil Hobbs wrote:

I'm mostly interested in very smooth motion at small scales, which is
why I want an ironless BLDC.  The gizmo's operation will require a lot
of curve fitting to pull out the amplitude and phase of a
small-amplitude tone burst of about 10k cycles over about 5 degrees of
shaft rotation, once per rev.  Any cogging or other bad behaviour of the
motor will cause nasty spurious peaks in the spectrum, among other problems.

Steppers are never sufficiently well made to avoid periodic errors--I'm
at the level where I have to worry about whether the ball bearings are
smooth enough, or whether I need to use jewels, which would be fragile
and expensive enough to dim my enthusiasm quite a bit.  (A galvo is
another possibility, but those cost the Earth.)  My hope is that because
the balls' motion doesn't have the same period as the shaft rotation, I
can sort out the bearing junk from the desired signal.

In the real system, I'm expecting to have optical clues as to what the
actual motor phase is, but I'm not too worried about that at this point.

I'm currently gearing up to do a sanity test with a nice Maxon brush
motor from my junk box, a He-Ne, and an HP 35665A dynamic signal
analyzer to do the data acq and so on.  (I just got a Prologix
GPIB-Ethernet gizmo, so I don't have to use the floppy drive to get data
in and out.)

Cheers

Phil Hobbs

Even microstepped, steppers shake, rattle,&  roll.  And they sing
(resonate).  I never imagined how much until I tried a few.

As far as COTS, CD, DVD&  hard disk spindle motor drivers?  They use 3-
phase BLDC motors & integrated controllers.

Here's an old BLDC datasheet off ye old hard drive:
   http://www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MC34929

But won't you be wanting ultra-fine control over commutation, PWM,
position-interpolation and such?  You'll probably have to do that
yourself.

Atmel, Microchip, and Freescale all have good application notes on
BLDC-driving with uCs.

e.g. Atmel AVR444: Sensorless control of 3-phase brushless DC motors.



I'm actually just going to spin it up and do the measurement as it spins
down unpowered.  That way I should have zero cogging and no jitter due
to commutation.

The unpowered motor will still cog of course, just not nearly as much.

For just testing VCR spindles might be interesting. They're an
endangered species now, but they're 3-phase BLDC motors, with
integrated drivers, flywheels, and impressively low run-out bearings.
That level of precision & longevity has got to imply a certain
smoothness of rotation & lack of vibration too. Couldn't hurt,
anyhow.

Probably kid stuff by your standards.

--
Cheers,
James Arthur
.

Relevant Pages

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