Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!portal!cup.portal.com!mmm
From: mmm@cup.portal.com (Mark Robert Thorson)
Newsgroups: sci.electronics
Subject: Re: Stepper Motor
Message-ID: <19588@cup.portal.com>
Date: 18 Jun 89 00:16:01 GMT
References: <7137@cbnews.ATT.COM> <1137@cbnewsc.ATT.COM>
Distribution: usa
Organization: The Portal System (TM)
Lines: 32

You can get a really good book on designing stepper motor drive circuits
for free from Warner Electric.  I lost my copy, so I don't have their address
or phone.  (Perhaps someone on the net could provide it?)

BTW, today I got a newsletter from a maker of drive units (Anthem Automation,
I believe) which had an interesting tidbit of information.  It says that
most modern high-torque step motors are rated for case temperatures of
100 degrees C.  Mighty toasty, eh?  They also note that steppers generate
excess heat even when the shaft is not moving (i.e. when they are exerting
holding torque).

BTW.2, a friend of mine who is a machinist doesn't know how steppers and 
servos work, but he sure knows which ones he likes.  He says steppers are
crap.  Eventually they go bad, while servos last forever.  His exposure
to motion control systems comes from CNC (computerized numerical control)
systems used to run machine tools.  I suppose the mechanism of failure is
the gradual de-magnetization of the magnets in the step motors.

BTW.3, there are two main types of step motors:  permanent magnet steppers,
which have numerous small magnets embedded in the periphery of the rotor,
and variable reluctance step motors, which have an iron core with notches
cut in it.  Only PM step motors are subject to demagnetization, but they
are smaller and more efficient than the VR type.  Most vendors only offer PM
steppers, but Warner Electric specializes in the VR type.

BTW.4, an engineer at Allen-Bradley developed an interesting new use for
PM step motors.  He uses them as the transducers in control knobs on a console.
When someone twists a knob, the passage of the magnets over the coils creates
small magnetic pulses which can be amplified and converted into digital
signals.  Advantages of this technique include the ability to set knobs
to desired locations (by driving the motor as a motor) and a slight detent
action when the knob is turned by hand.