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Hi Collin and List,<br>
I've owed you more info since the meeting, Collin. But I've been
mired in a sea of misinformation that would be alternately hilarious
and sad, so I'll just leave most of that out and report my
conclusions after poking into this some more. I wanted a little more
time to be a bit more definitive, but the executive summary starts
with some very basic details. I realize you know most or all of
these already by now and include it for the sake of other readers
who might be interested, but hopefully some of this is going to be
helpful to you.<br>
1) Without using back EMF, hall effect sensor signals, or a coil
and magnet (or maybe some clever communication with spirits of the
dead as mediators) you can forget about doing much of anything
reliably, let alone spinning up to and holding a high speed. If
there was a way to do that I'd have found it by now, having run many
nested for loops varying a god-awful collection of parameters. <br>
2) Controller chips for these motors are not designed to start up
reliably with high loads. They're designed to start up reliably with
fans or disks or the like attached to the spindles so the power of
the motor is to do with high speed. So the controller chip approach
is doomed for low speed and/or high load-medium speed work without
at least some means of external control allowing special case
handling of this scenario.<br>
3) You can't just run X volts DC through the coils without
significant cleverness: either PWM, current limiting, or kludgy
dependence on over-temperature shutoff of the motor power source.
These are 12v (or 5v for the 2.5 inch drives, I think) DC motors
that are completely dependent on their winding *impedance* to run at
reasonable current/power levels: stalled, or at very low speed there
is no "impedance", only the DC coil resistance and they emulate
tiny, enclosed toaster ovens. I think this aspect might be why some
folks on the Internet insist these are AC motors.<br>
My theory is that the vast majority of enthusiasts get one of
these motors, mess around for a while, then set it aside after
initial frustration or determining and being unwilling to implement
the necessary drive scheme. The small fraction (but no doubt large
number) of folks using these motors successfully only occasionally
provide quality information, and not a single source I've found so
far covers all the critically important bases. The vast majority of
folks out there are too smart to try to use these things at low
speed. (In my defense I haven't already been using stepper motors
'cause I couldn't arrange the mechanical coupling without having a
setup that is grossly inappropriate for calibrating a magnetometer!
But I'm rethinking that avenue now!!)<br>
From a standing start there is no back EMF or Hall effect
signals, of course, so the controller chip has to use "something" to
handle this scenario. The chip I'm using has an oscillator that
sequences through the winding energizing order until it starts
seeing the back EMF zero crossings. It appears the TI <a
href="http://datasheet.octopart.com/DRV11873PWPR-Texas-Instruments-datasheet-13024530.pdf">DRV11873</a>
fan controller chip you pointed to uses "sensorless, proprietary
control" for this. With this chip there is also "lock protection",
so if something prevents fast enough startup the chip will power off
the motor and optionally try again later.<br>
But also, with this TI chip <b>note well</b> that it doesn't
contain diodes to protect the semiconductor junctions from high back
EMF potentials. As per page 9 of the datasheet you MUST add external
schottkey diodes to protect the chip or the magic smoke will escape
with "high speed motors". But here's the possible "too good to be
true" aspect of this chip: It's in such a small package that proper
thermal connection to the metal pad on the bottom of the chip is
critical. Without that proper connection the chip is likely to
overheat for your (relatively high speed, presumably 12 volt)
application. So a custom PCB and good surface mount soldering might
be a must (also per page 9 of the datasheet). This is relatively
straight forward, but involves an hour or so of CAD work and about
two weeks to get the boards and solder paste stencil for a few
bucks. I or somebody at NCSU can solder a simple "breakout board"
for this part. An alternative to this might be to bond a heat sink
to the bottom of the chip and solder it "dead bug style". But those
are fine pitch pins and this would require professional grade
soldering skills and be delicate as all get out (but I know a pro
who might be able to do it for you). A hybrid alternative might be
dead bug soldering and fast moving air for cooling. It would be very
interesting to know from you or your buddies that have the physics
understanding to say whether blowing enough room temperature air at
the chip could work. There is pretty decent thermal info in the
datasheet. <br>
As for the CCW/CW question, it depends entirely on the control:
nothing in the motor itself limits it to rotation in one direction.
It would just take an alternative set of six coil energizing
sequences to turn the other way, but the sequence appears to be
fixed for a given chip for the cases I've seen.<br>
The 555 seems like a no-brainer for the PWM with this TI chip,
but IMO you could combine something like an ATTiny and a
potentiometer easier, with fewer components, and with the capability
to goose the other chip options and play with the info coming out of
the chip if that appears interesting later. <br>
I have a tube of <a
href="http://datasheet.octopart.com/TDF5140AP/C1%2C112-NXP-Semiconductors-datasheet-11806991.pdf">Philips
TDF5140A</a> chips and would be happy to drop a couple off so you
can bypass most of the horsing around exercises I've been doing with
raw FETs and H-bridge chips and at least get to the "seeing this
thing spin" state very quickly. These plain, 18 pin DIP chips have
internal schottkey diodes and smart current limiting so you can
connect a full 12 (or 5) volt supply for a given motor without
worrying about overheating. The chip itself can run hot, though,
without a heat sink. I use a 3/4x3/4" square copper heat sink and
the whole thing seems to stay in the 30-40C range. They cost about
$2.50 at quantity 10.<br>
The bottom line is that only the first drive motor I started with
would turn slowly but reliably (with heavy load) using a series of
+12 volt pulses on it's windings (i.e. just using FETs, not half
H-bridges). I just got super-unlucky at the start. The more
plentiful Western Digital drive motors I've been using since then
won't cooperate with blind control of their winding voltages. In
fact they won't cooperate with any combo of bipolar energizings of
one or two coils, except with the latter (using the typical sequence
mentioned below) they will turn reliably at a single speed for a
given load and pulse rate in some cases. But they won't reliably
start up to that speed: I have to spin the device close to the
target speed for it to reach equilibrium. And of course I'm talking
relatively low speeds: I can't spin an 8 ounce assembly of aluminum
to 1500 RPMs with one finger flick.<br>
In other words, I've found no way to just do it "blind", as so
many Internet sources about these motors suggest, except with one
motor that can't be removed from the disk drive frame, and I have no
clue about how that motor might differ (if anybody knows, please
enlighten us!).<br>
The Philips chip above is designed for three phase motors in a
"star" winding configuration. I can confirm that with a lightly
loaded motor it will spin the thing up very quickly and reliably,
depending only on the values of three caps that govern the start up
oscillator frequency and the expected min/max target speed and
electromechanical forces involved. Selecting these capacitor values
for uncommon situations is the tricky bit and I'm still
characterizing the limits of that for low speeds. Page 10 of the
datasheet shows the coil energizing sequence. If you use this
sequence and reasonable cap values (the datasheet suggests ones that
"just work" for me) and an unloaded motor doesn't immediately spin
up, then the winding connections are wrong. For the motors I'm using
the four pins are common, winding 1, 2, and 3 in sane order. I
suspect it would constitute inhumane treatment of engineers for a
motor to have other than low to high winding connection order (e.g.
wired "1 3 2"). As many other Internet sources point out, an ohm
meter will read 2X ohms between a pair of windings and 1X ohms
between common and one winding and that can be used to determine
which is common. If there are not four pins, you're on your own. :-)
No, seriously, apparently "delta" connections are used some times
and the key there would be three windings forming a triangle and no
common (i.e. Hall effect or spirits of the dead used in this case).
I've seen references to motors with two windings, but don't know how
they would work and wonder if they're just noise.<br>
And I knew almost zero about brushless DC motors just a few weeks
ago, so take all this with a lump of salt. And finally, here's the
most useful page I've seen (found by Erik Auschhaug). The three
links at the bottom should not be missed, but watch out for the fact
that these were written years ago, contain proprietary (and possibly
obsolete) tech references, etc. But the "Sensorless Startup"
paragraph in page 2 of the first Agile paper makes clear the
subtleties involved with that. And the second Agile paper goes into
even more detail about what's really going on at low speeds. (The
Freescale link no longer works, but suggests that TI and Philips/NXP
are not the only vendors making these chips)<br>
<br>
<a href="http://bldc.wikidot.com/p-esc-motor">http://bldc.wikidot.com/p-esc-motor</a><br>
<br>
Here's a picture of my setup using the "motor in plywood" mount Erik
made. This picture file is about 200kb, so I couldn't inline it or
it would exceed the 100kb per message limit for postings to this
list and block on moderation.<br>
<br>
<a
href="http://triembed.org/blog/wp-content/uploads/2014/07/turntable3.jpg">http://triembed.org/blog/wp-content/uploads/2014/07/turntable3.jpg</a><br>
<br>
If anybody is interested in access to the web site to upload stuff
like this just drop me a line.<br>
<br>
One more, possibly important detail about that Philips chip:<br>
<br>
Digikey is discontinuing this chip and when their stock is exhausted
or we reach September 30th they won't be for sale from them anymore.
I suspect that relatively speaking these chips are so outdated the
insides are gray. Also, if you just look up the part number Digikey
will claim zero available. You have to use their part number
568-5788-5-ND to find them.<br>
<br>
-Pete<br>
(PS your Google address is now an alias for the ones you subscribed
to the list last week and your postings using this address should go
through from now on without waiting for moderation: no need to
subscribe it too.)<br>
<br>
<br>
<div class="moz-cite-prefix">On 07/23/2014 09:45 PM, Collin Ladd
wrote:<br>
</div>
<blockquote
cite="mid:CAEyb0Jg6KSMkZq313GBtgy-FDs0g3neaiNCirRSm2=iZgjoN0w@mail.gmail.com"
type="cite">
<div dir="ltr">Hey guys,
<div><br>
</div>
<div>I was asking around at the last meeting about getting good
RPM performance out of a salvaged hard drive motor. I had to
do some shopping around for the lab, and I thought this might
be of interest to anyone with old hard drive motors sitting
around.</div>
<div><br>
</div>
<div> <a moz-do-not-send="true"
href="http://www.ti.com/lit/ds/symlink/drv11873.pdf"
target="_blank">This IC </a>sounds almost too good to be
true for me, but for about 2 bucks you get back EMF sensing
speed control, lock sensing and a handful of other features.
The best part is the low parts count required--just a few caps
and resistors. It takes a PWM input and is pretty
microcontroller friendly. I will probably just use a 555.</div>
<div><br>
</div>
<div>1.5 A cont isn't going to spin up anything huge, but TI has
a family of higher power ones... I think (haven't had time to
look around). <a moz-do-not-send="true"
href="%A0http://www.ti.com/product/DRV11873/compare"> http://www.ti.com/product/DRV11873/compare</a> <<
probably can find one here.</div>
<div><br>
</div>
<div>Depending on when I make our order, and how many I ruin
during prototyping, I might have a few extra to hand out at
the next meeting. </div>
<div><br>
</div>
<div>I read that HDD spindles are made to spin CCW only. Any
truth to that? </div>
<div><br>
</div>
<div>--Collin, the liquid metals guy</div>
</div>
<br>
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