Re: Motor speed control via back-EMF detection

On Tue, 02 Sep 2008 07:51:26 -0700, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

Jim Thompson wrote:
On Mon, 01 Sep 2008 17:54:08 -0700, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

Jim Thompson wrote:
On Mon, 01 Sep 2008 17:36:33 -0700, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

Jim Thompson wrote:
On Mon, 01 Sep 2008 16:24:07 -0700, Joerg
<notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx> wrote:

Tim Wescott wrote:
John Larkin wrote:
On Sun, 31 Aug 2008 18:27:33 -0700, "Bob Eld" <nsmontassoc@xxxxxxxxx>
wrote:

"Phil Pemberton" <usenet08@xxxxxxxxxxxxx> wrote in message
news:kgmro5-jbh.ln1@xxxxxxxxxxxxxxxxxxxxx
Hi guys,
I've been tearing apart a Brother PT-1000 label printer with the
eventual
goal being to connect it to a desktop PC to print labels for my various
component storage boxes. I've figured out how the print head
communicates
with
the controller board, which just leaves the motor drive circuitry.

Naturally, being a low cost device (£15), the PT-1000 doesn't use
anything
remotely stepper-motor-like for the label feed. Instead, it uses a
cheap
Mabuchi DC pancake motor (an RF-300C-11440, for which I have yet to
find a
data***) and a speed reduction gearbox. The speed control is
performed
by a
ROHM BA6220 chip.

Ideally I'd like to eliminate the ROHM chip (seeing as it's
basically
unobtainium) and use a PIC of some description (probably a 12F675)
to do
PWM
speed control of the motor. But first I'd like to understand how the
ROHM
chip
manages to do what it does.

I found a data*** for the '6220 here:
<http://www.classiccmp.org/rtellason/chipdata/ba6220.pdf>
.. but like most ROHM datasheets, it doesn't say much about the
chip,
other
than that it uses back-EMF sensing and how to determine one of the two
external resistor values.

What I don't get is that there's no obvious way for the chip to
sense
back-EMF. Everything I've been able to find about bEMF sensing suggests
that
it's normally used with PWM control -- the motor is powered up for a
short
period of time, then in the off period the voltage across the motor is
sampled
and used to (roughly) determine the motor speed. Unless it's sensing
current,
but if it is, the "application circuit" (BA6220 data***, Fig. 2,
page 2)
doesn't look like any current sensing circuit I've ever seen. In
fact, it
looks like a voltage comparator, but the polarity of the voltage
reference
doesn't look quite right...

I've hooked the scope up to the driver IC's pins (and the motor
itself) and
didn't see anything that suggested the driver IC was switching the
power
on
and off. In fact, the voltage remained more or less constant, excepting
the
~50mV sine wave (plus one ~200mV spike per cycle) modulation that I
suspect is
being caused by the motion of the commutator relative to the brushes.

Can anyone shed some light on this?

Like I said, I'm getting rid of this thing anyway, but I'd
rather like
to
understand how the existing circuit works first, if at all possible...

Thanks,
--
Phil.
usenet08@xxxxxxxxxxxxx
http://www.philpem.me.uk/
If mail bounces, replace "08" with the last two digits of the current
year.

The motor current is sensed inside the 6620 chip with a 20:1 current
mirror.
Though not completely clear because actual components are left out of
the
block diagram, there are two transistors being driven from the internal
amplifier. They are the main parts of the current mirror. The motor
current
goes to the collector of one transistor and one twentieth of that
current
flows in the other transistor. The current ratio is established by the
mirror geometry from the ratio of collector areas in the chip.

This sensed current with the motor voltage is applied to the internal
op-amp
in a way that forms a back emf bridge. The bridge subtracts the motor
driving voltage from the current generated voltage leaving only the
back emf
which is proportional to speed. This can be shown with some simple
algebra.

This bridge concept allow DC motor back emf and thus speed to be
sensed in
the steady, DC state.



That's equivalent to driving the motor from a power supply that has a
negative output impedance. The negative-impedance supply cancels the
ohmic losses in the motor and in theory makes the motor speed
independent of load, and dependent only on the equivalent applied
voltage.

And ideal, lossless shunt or PM DC motor has perfect speed regulation
with a constant applied voltage.

John

The only trick is making the negative output impedance match the
armature impedance of the motor. I'm sure that there's a bit of
tweaking involved in getting the figure right, and in compensating the
amplifier so it's stable when you hang a motor off of it, but it's been
done for years.

AFAIK this is how the capstan speed of all but the cheapest or most
expensive cassette recorders was controlled.

They even did that on record players. Couldn't believe it until I saw it.
Not on my Rek-O-Cut (SP? after all these years :), it had a
synchronous motor and a tapered pulley so you could set the speed with
a strobe.

Ah, I remember the old strobe disk. Having to switch taper would somehow
hint that the line frequency wasn't all that stable out there. Mine only
had tapers for the various record speeds. Why a strobe disk came with it
I have no clue because if the speed was off you couldn't do anything
about it. Plus it was relying on line powered lights so how could it
ever show a deviation? It would be like trying to calibrate a frequency
counter with its own clock.
Mine tilted the motor mount ever so slightly to change the BELT
position on the pulley to set the speed accurately.

I still have it. I'll get it down from the upper storage shelving and
take a picture.

But against what would it compare? The TV station's V-sync? I can't
imagine they provided a crystal controlled strobe lamp with it.
Sunchronous motors have no slip. They are either in sync or stalled.

Variable pulley ratio... you dig?


No grok here :-)

Again:

Synchronous motor -> no slip -> always in sync with your utility's 60Hz.

Strobe disk -> lamp -> lamp also always in sync with utility.

So, where's the value in this measurement?

The AC line frequency is usually very stable, 10's of PPM around here,
plenty good enough to tweak a record player.

Really good turntables had sync motors and toothed belt drives, so
were dead on. Cheaper ones used shaded-pole motors and/or smooth
belts, neither of which is precise, so had tapered pulleys or other
means of speed trimming, which justifies using a strobe disk.

John



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