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From: email@example.com (Bill Sloman)
Subject: Re: Closed loop stepper driver circuit
Date: 8 Jan 2003 08:33:48 -0800
NNTP-Posting-Date: 8 Jan 2003 16:33:48 GMT
"Kevin Doney" wrote in message news:<02pS9.20379$9N5.firstname.lastname@example.org>...
> 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.
The joke is that a DC motor is essentially a stepper motor that has
been turned inside out, with moving coils and a stationary magnetic
The commutator is both a position encoder and a multipole switch. At
relatively high current levels the commutator burns out too fast to be
much use, and we go over to the brushless DC motor which separates the
switching function and the encoding function so that it can use
relatively robust semiconductor switches.
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).
Inland Motor patented a scheme for using the back-EMF visible on the
drive coils of a stepper motor to detect the physical occurence of a
step. As a technique it is a bit hairy.
In theory, you ought to be able measure the changing inductance of the
stepper motor drive coils through a stepping cycle to exploit the
magnetic structure of the motor as an incremental encoder. I've never
of anybody actually doing it.
Bill Sloman, Nijmegen
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