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From: sasrer@unx.sas.com (Rodney Radford)
Subject: Re: Selecting DC motors for a differential drive system
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Date: Mon, 13 Jun 1994 18:54:31 GMT
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dkroy@bbn.com (Deb K. Roy) writes:
>I am planning to build a mobile robot with a differential drive
>system.  I have sent out for catalogs from a few surplus DC motor
>suppliers, but I have no idea what torque I need.  The robot will
>weigh 50lbs, have 5 inch diameter wheels and run on carpet indoors.
>How can I estimate the torque requirements of the motor?

>I recently found a cheap ($30) motorized car that children can ride, and took
>out the drive mechanism.  It seems to be powerful enough and the
>right speed.  The gears are plastic and I'm sure it's a pretty low grade
>DC motor, but it was built to carry heavy loads.  What are the disadvantages
>of using toy parts like this?  Do the gears wear down quickly?  Are
>cheaper motors more difficult to control?  Are they going to be less
>efficient?  One obvious problem is that it will be difficult to mount
>optical sensors for feedback.  How common is it to find complete DC
>gearhead motors with built in sensors?

I have a robotic platform built using motors/gearboxes from a children's
car as well. So far I have been *very* pleased with them. I have been able
to put up to 140lbs on the robotic platform and the motors were still able
to move the platform around without a problem (I was afraid to go above that
weight as the light weight aluminum frame was not designed for that kind of
stress). In my situation, I currently have about 45lbs on my robot (and that
counts 3 12v 8AH gel cells) and the motors draw about 1.5A when moving the
platform on relatively flat areas (like inside a house). I used 7" diameter
lawn-mower wheels, but I hope to replace them soon with something that are
a bit 'nicer' to hardwood floors....  ;-}

On my system, feedback was fairly easy - I have at least two choices. If your
system is similar to mine (probably as I have taken several cars apart and
all the motors/gearboxes have been similar in construction), you can either:

    - drill a hole in one of the plastic gears in your housing and put an
      IR emitter/detector pair on both sides of the gears. Alternatively,
      you can paint white/black stripes on the gear and use a reflective
      IR emitter/detector pair.

    - on my system, I had a small shaft (0.25") sticking out on the back
      side of the motor - enough to attach a disk for an opto-isolator.

Another solution is to monitor the high-frequency noise on the motor 
supply as this is generated by the brushes/commutator of the motor. I have
heard of systems using this (pass through a high-pass filter), but it 
would not work in my situation as I was PWM'ing the motor.

Regarding life of the motors - well so far they have worked pretty well
(about 4 months) and I can always replace them if they go out (about $30
each new for replacements).

Good luck - let me know what you finally decide...

--
---
Rodney Radford,     Senior Systems Developer,  Image Technology Group
sasrer@unx.sas.com  SAS Institute, Inc, Cary, NC 27513  (919) 677-8000 x7703

