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From: sharpe@mecad.uta.edu (Todd Wittman Sharpe)
Subject: Re: Positioning w/DC motors
Message-ID: <1994Aug26.193508.19816@news.uta.edu>
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Date: Fri, 26 Aug 1994 19:35:08 GMT
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MikGreene (mikgreene@aol.com) wrote:
: On Fri, Aug 19, Lynwood wrote:
: >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.
: >Thanks very much,
: >Lynwood Wilson
: >lynwood@csn.org
: >P.S. It strikes me that I am unduly restricting the question.  What I
: >really want to know is this:  Given computer power and the positioning
: >problem, how can I save money on the hardware cost of the device by
: >spending on the software, spend up front to reduce the unit cost of
: >the device?
: >Thanks
: >Lynwood

I hate to say it but I don't think it's possible.  I"m working right
now with an anthropomorphic (typical serial arm) robot that uses
a 4 hp motor to support two links (weighing about 40 lbs apeice, a
40 lb motors and a 20 lb payload) with a 1:100 harmonic drive reduction.
I dont' think a 1/4 hp motor could handle the static load let alone
the dynamic.

Point of advice, don't go with stepper motors.  THey have very low winding
utilization efficiencies and torque to weight ratios compared with other 
motors.  DC servo brushed motors have low max torques and continuous stall
torques because the field windings are on the rotor in the interior of the
motor (heat dissipation kills it).  That leaves brushless and variable
AC motors.

If you're worried about cost, the variable AC motors are cheaper.  They
use induction of a soft iron core instead of the heavy rare earth metal
magnets of the brushless DC.  If you've heard of Staubli over in Europe,
they use variable AC and a zero backlash reduction on their R90's and 
R130's.  Even though the brushless DC has more starting torque, the
variable AC provides a very constant torque over the whole speed range
and weighs about 1/3 to 1/2 as much.

As for your theory about better controls to enable lower quality 
hardware, I don't think it's too viable.  The controls necessary
for most motors can be quite complex, espeically for varying the
frequency of variable AC (and feedback for stepper motors is worse,
although I don't know why you'd want to use feedback for a stepper
motor, kind of defeats the purpose of what it was designed for).
Most controllers enable you to control the motor with position, velocity,
and torque.  How you control that output depends on your own computing
possibilities.  I know it's possible to run control algorithms on a
PC so that's cheap

Good luck...

________________________________________________________________________
|Todd Sharpe				| Do not use a hatchet to       | 
|sharpe@mecad.uta.edu			|   remove a fly from your	|
|The University of Texas at Arlington	|     friend's forehead.	|
|MSME					|       - Chinese Proverb	|
|_______________________________________|_______________________________|

