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From: Chuck Simmons
Organization: You jest.
X-Mailer: Mozilla 4.61 [en] (X11; U; Linux 2.0.33 i586)
Subject: Re: steppermotor as encoder -> presto, moterized pot.
References: <firstname.lastname@example.org> <email@example.com> <3DA05D28.CC5530C1@webaccess.net> <firstname.lastname@example.org>
Date: Sun, 06 Oct 2002 18:32:51 GMT
NNTP-Posting-Date: Sun, 06 Oct 2002 11:32:51 PDT
Frank Bemelman wrote:
> "Chuck Simmons" schreef in bericht
> > Frank sent me an email that I answered. At least part of my thinking
> > that went into the answer might be appropriate here. Frank's idea boils
> > down to using the BEMF from the stepper to sense motion whether by the
> > motor or by turning the knob. Since steppers usually have enormous
> > torque constants compared to spindle motors, this idea has considerable
> > merit. I could come up with a long list of problems but I think that
> > they all could be solved in the application without much cost. It takes
> > thinking it through and experimenting a bit.
> > I could take this on. I would use an AVR because I can usually wangle a
> > free AT90S8515 and the lowest grade seems to run at 10MHz (the specs say
> > it won't but I do it anyway). Thus the processor power is through the
> > roof for this application. It looks like a fun project however futile.
> I sent my reply as an email by accident. Here's my reply again, with
> your comment:
> > That last characteristic would perhaps be an advantage, volumeknobs often
> > have these detents on purpose. During the moments the stepper needs to
> > works as an encoder, I intended it to be disconnected from the drivers,
> > and use the coils as pickups. I measured a small stepper motor here,
> > and when giving it nice turns, it generates rather nice pulses. When
> > it slow, it all looks a bit shabby. Not to worry about, after all it
> > was just an idea, intended to be carried out by others ;)
> Not a bad idea. If people want to learn how to control anything, they
> need to learn to use the components. In your suggestion, you are
> proposing using the BEMF of the stepper. I haven't seen that done
> (though it is done with brushless DC motors) but since most steppers
> have high torque constants, it should work pretty well. It's a nice idea
> that just requires a little immagination and experimentation to fill in
> the details. BTW, I can get clean BEMF from a brushless DC motor PWM
> driven from about 100RPM up. Talk about ragged coil voltage! This says
> your stepper having at least 10 times the torque constant has a lot of
> potential for very low speed application like a volume control.
> To get a 100% tight interpretation of the stepper used as an encoder
> might be difficult, but to make it serve as a volume control, it may
> give enough satisfaction. Now we only need a volunteer that examines
> this. I don't have the authority to call in Tony, perhaps Will can
> pick this up.
> Hahaha, I love you guys picking up on silly ideas ;) Martin can
> do the analog stuff, the real volume controlling bit...
Some silly ideas are fun if nothing else. Many years ago a guy took the
guts out of an HP plotter to drive a pot that controlled the feedback to
a magnetically driven torsional pendulum. The guy didn't know about
compensation so the thing oscillated. He was trying to servo on the
average amplitude. I scratched my head for a while and then went back to
my cave and built a peak detector controlling two sample and holds the
second sampling the output of the first. The current sample was taken as
amplitude and the difference between the current and the last previous
sample was taken as rate of change. Those two terms were sufficient to
stabalize the servo. It seemed a bit silly but it worked. This was 10
years before I got into digital control.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons email@example.com
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