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From: Tony Williams
Subject: Re: steppermotor as encoder -> presto, moterized pot.
Date: Mon, 07 Oct 2002 08:15:30 +0100
References: <email@example.com> <firstname.lastname@example.org> <3DA02BE5.AC8F6F93@webaccess.net>
NNTP-Posting-Date: Mon, 7 Oct 2002 07:20:44 +0000 (UTC)
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In article <3DA02BE5.AC8F6F93@webaccess.net>,
Chuck Simmons wrote:
> Tony Williams wrote:
> > One which provides a fixed amplitude rotor position
> > signal, no matter how slowly the knob is turned.
> Spindle motors from hard disk drives, BTW, do not have any rotor
> position sensor at all. This is a cost and space issue in hard drives.
> There are a number of patents on starting and running sensorless motors
> some of which are quite clever. One method of getting coarse rotor
> position needed to avoid the motor rocking backward during start is to
> measure the winding incremental inductance which tells you where on the
> hysteresis curve the stator magnetic state is. This is a little
> complicated for a volume control.
Yes I've been reading my stepper motor book (by
P.P Acarnley). I'm amazed that calculating rotor
position by inductance is cheaper than an actual
> A motor example assuming digital hall states. A 3 phase motor with 12
> poles will give 6 cycles per turn of one hall output. There are 6
> commutation states per cycle of the halls giving 36 states per rotation.
> This is a bit marginal for a volume control where you would like each
> comutation state to be 2db or less.
> Finally, a disadvantage of step motors and brushless motors is that at
> least one winding must be powered at all times. Although the current can
> be reduced when not stepping, some current is required. Otherwise, the
> motor will rotate on its own to the nearest magnetic detent.
I think that Frank is not driving an actual pot.
When the knob turns all he needs is a set of up/
/down pulses that will inc/dec a counter somewhere,
and the value in that counter sets the volume.
The counter can also be inc/dec'd by remote control
and in this situation the motor will be powered to
click the knob round, as a cosmetic exercise.
So pulses/rev, detents, and remembering the rotor
position would seem to be secondary things.
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