Newsgroups: comp.robotics
Path: brunix!cat.cis.Brown.EDU!agate!howland.reston.ans.net!swrinde!elroy.jpl.nasa.gov!decwrl!netcomsv!netcom.com!nagle
From: nagle@netcom.com (John Nagle)
Subject: Re: Positioning w/DC motors
Message-ID: <nagleCusKFt.Bo8@netcom.com>
Organization: NETCOM On-line Communication Services (408 261-4700 guest)
References: <CurFon.1BC@csn.org>
Date: Fri, 19 Aug 1994 16:57:29 GMT
Lines: 32

lynwood@csn.org (Lynwood H. Wilson) writes:
>I'm looking for advice on using DC motors with encoders to position
>a heavy object in several axes.  The straightforward approach seems to
>be to use servos or steppers, but I am investigating the idea of using
>cheaper motors and computing power instead.  The device already has a
>computer that won't be doing anything during the positioning.  The 
>mechanical part of the job is shaping up nicely, and position sensing
>doesn't seem too hard.  The object weighs several hundred pounds, and 
>must be positioned within about 0.001 inch.  The problem as I (naively)
>see it seems to come down to how small an increment of rotation can I
>reliably get from a 1/4 hp (or so) motor.  Can't gear it down too far
>'cause I need to move fairly fast in another part of the cycle.
>I'd appreciate some advice from anyone with experience in this area,
>even if it's a recommendation to forget it.

      That's done routinely, but it isn't easy.  Motor drives for
larger motors are fairly expensive.  PMAC makes some good ones in
VMEbus form.  

      Basically, you need an H-bridge with a suitable power rating,
drive electronics to interface the H-bridge with the computer, protect
the H-bridge, and provide emergency stop circuitry, and control software
in the computer.  

      The trend is towards using "brushless DC" servomotors, which are
really AC motors, but these are interesting to drive.  DC motors have
annoying spots where the commutator switches.

      A good book on motor control theory is "Control System Design Guide",
by George Ellis (Academic Press, 1991).

					John Nagle
