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From: email@example.com (Bill Sloman)
Subject: Re: Closed loop stepper driver circuit
Date: 8 Jan 2003 15:13:40 -0800
References: <02pS9.20379$9N5.firstname.lastname@example.org> <email@example.com>
NNTP-Posting-Date: 8 Jan 2003 23:13:40 GMT
"Jonathan Bromley" wrote in message news:...
> > > 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)...
I did say "like *a* stepper motor" not "like *every* stepper motor" -
there were some small ones with 45 degree steps, intended for clocks
and minature applications, and the cheap tin-can versions offer 7.5
degree steps, not the 1.8 degree steps you get on the good ones we
used at Cambridge Instruments, and we (different we) use now at
The point about shaping the coils echoes the Escap claims for one of
their ranges of stepper motors which are optimised for micro-stepping,
so that sinusoidal drive in quadrature really will give uniform
Bill Sloman, Nijmegen
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