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From: "Jonathan Bromley"
Subject: Re: Closed loop stepper driver circuit
Date: Wed, 8 Jan 2003 16:50:01 -0000
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Reply-To: "Jonathan Bromley"
NNTP-Posting-Date: Wed, 8 Jan 2003 16:50:04 +0000 (UTC)
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> > Anyone know where I can get a stepper driver with encoder feedback or a
> > circuit drawing of one?
> Yes. They are called brushless DC motors, and the catch is that I
> don't know of any that offer you access to the encoder.
Hidden encoders (and switching electronics) are a feature of the
tiny motors integrated into disk-drive spindles, fans and suchlike.
If you go to a servo-motor supplier such as Maxon you will easily
get motors with (electrically) exposed commutation switches -
I prefer not to call them "encoders" because they have rather low
resolution and poor-ish accuracy.
> Once you've done this it makes much more sense to put the coils on the
> outside of the motor (where they can dissipate directly to ambient, as
> well as being easy to hook up) while the magnetic structure becomes
> the rotor, and you've got something that looks exactly like a stepper
> motor (apart from the encoder).
...and the fact that it has only six steps per revolution, and the
fact that the coils are very carefully shaped so it's possible to
make the torque almost independent of shaft angle if you tweak the
coil currents appropriately (noting carefully the difference
between trapezoidal and sinusoidal excitation)...
If you have a high-resolution shaft encoder and you can tolerate
a bit of messing-about on power-up to locate the zero index of
rotation, you can usually do without a separate encoder to do
the commutation switching - it can all be extracted from the
shaft encoder's position value.
Micrel offer (used to offer? it's a while since I looked)
controller chips aimed at 3-phase brushless motors for
disk spindle applications. Spindle speed is essentially
constant, you can live with a bit of fooling around at
start-up, and the chip can sense back EMF from the coils,
so it's possible to do without explicit commutation.
Every cent saved is a cent more profit, and two Hall
effect sensors represent quite a few cents :-)
Jonathan Bromley, Consultant
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