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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> <3DA02BE5.AC8F6F93@webaccess.net> <email@example.com>
Date: Mon, 07 Oct 2002 12:22:08 GMT
NNTP-Posting-Date: Mon, 07 Oct 2002 05:22:08 PDT
Tony Williams wrote:
> 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
> position sensor.
It's because to do the incremental inductance measurement simply costs
another sample and hold and mux channel on the A/D they have to have
anyway. As far as component cost, it is nearly free. At least one disk
drive company does not worry about the backward rock and starts the
motor as a stepper and switches to timed optimal commutation when the
motor is running fast enough to extract BEMF. The interconnect from the
electronics in a hard drive to the spindle motor is just three wires.
Contrast this with the 11 wires used in many DVD players to run the
> > 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.
In that case, there are optical incremental encoders available that are
dirt cheap. The code wheel is injection molded plastic. Some of them
have only a single output channel but there are quadrature types
available for very little more. The quadrature types are a bit nicer
because the order of the output states determines direction (up/down).
... 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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