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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: <email@example.com> <firstname.lastname@example.org>
Date: Sun, 06 Oct 2002 12:26:25 GMT
NNTP-Posting-Date: Sun, 06 Oct 2002 05:26:25 PDT
Tony Williams wrote:
> In article <email@example.com>,
> Frank Bemelman wrote:
> > What we want is a moterized pot for a homemade amplifier,
> > so that it can be controlled both manually and by remotecontrol.
> > Since we lack mechanical skills (not me of course) we want it to
> > be mechanically simple. For cosmetic reasons we want the volume
> > knob to turn as well, when using the remote.
> > Okay. We use a size 17 steppermotor from an old 5.25 floppy
> > drive. Some circuit to convert to logic levels, a PIC
> > to decode etc. Now we have a signal to control a digital
> > volume control. Easy.
> Chuck Simmons is your man for this, but wouldn't
> it be easier to do with a brushless dc-motor?
Magnetically, step motors and brushless DC motors are identical. The
differece you usually see is that step motors have more poles so that
the steps are small. It is possible to drive either motor type with
currents that are essentially sinusoidal and continuously variable. This
is called microstepping. The driver to do this with a brushless DC motor
has 50% more power parts usually because common brushless motors are 3
phase. For a volume control, you need at least 20 steps over the
rotation of the pot and I think a step motor rescued from an old floppy
drive would do that possibly requiring half step mode. Integrated
circuit step motor drivers that support half step mode are inexpensive.
> One which provides a fixed amplitude rotor position
> signal, no matter how slowly the knob is turned.
Brushlees DC motors usually don't have a rotor position output. The
motors used as spindle motors in many CD and DVD players have linear
hall sensors which allow calculation of rotor position over one cycle of
the hall output. A program controlling motor position would have to have
a way of retaining gross rotor position across power cycles. I don't
know of any commercial parts that use the linear halls to get linear
position so you are on your own on that one. The Rohm drivers, for
example, use differential comparators to get a digital signal from the
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.
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.
... The times have been,
That, when the brains were out,
the man would die. ... Macbeth
Chuck Simmons firstname.lastname@example.org
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