Newsgroups: comp.robotics
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From: kilian@cray.com (Alan Kilian)
Subject: Re: Stepper motors, Robot bases...
Message-ID: <1994Feb9.133224.4782@hemlock.cray.com>
Lines: 97
Nntp-Posting-Host: poplar12
Organization: Cray Research, Inc.
Date: 9 Feb 94 13:32:24 CST

> From: tallent@netcom.com (Mike Tallent)
> Subject: Stepper Motors, Robot Bases - 2 versus 3 wheels
> 
> I'm working on a design for a robot base that will support a robot about 
> 18 inches round and 30 inches tall. 

That's much larget than anything I have ever made.

> I would like to know if 2 wheels with a caster or 3 wheels without is better? 

I understand the 2 wheels and a castor, but what is the arrangement of 3 wheels?
I have used two wheels and two castors also for a very heavy robot where the
senter of gravity didn't work out properly.

> What method is usually used to link up stepper motors to wheels and what 
> about part sources? 

Ah, the largest problem for hobby robotics. How do I get some wheels on this
damn motor?
I farted around for years trying to get wheels (Or even a gear) on a motor
shaft. Drilling a hole in the wheel and using glue sometimes works.
Taking a piece of insulation from a large guage wire and pushing it onto the
motor shaft and then pushing a wheel onto that also works sometimes.
Soldering the wheel or gear onto the saft also sometimes works.
File the chrome off the shaft first. Also file the shaft square or some 
non round shape. Use one of those small Radio Shack torches to get enough heat.
Don't overheat the motor. (Yeah, easy as that sounds) Use acid flux from the
plumbing store and regular non core solder and you just might get a wheel to
stick if everything is right. Your motor might even work when you're done.

What I finally did was to buy a hobby lathe and vertical mill and I now
use the lathe to build small aluminum "mags" that fit the motor shaft
and a rubber tire from the model airplane store and have two setscrews on
opposite sides of the motor shaft. I file two flat spots on the shaft and
bingo only two hours metal work per wheel and I'm all set.
This works great. And for only $2000.00 (What the hell, some people buy
sailboats for god's sakes what a waste of money. And do you know what
people pay for a single day lift ticket to go downhill skiing? Yipes)
So if you are in town (Minneapolis MN) call me and we'll make some wheels.

> Any books on the subject (mechanical engineering of wheels, gears, and motors)
> would be helpful. 

_The complete handbook of robotics_ Edward L. Safford Jr. Tab books #1071 1978
_How to build you own robot pet_ Frank DaCosta Tab books #1141 1979
_Standard handbook of machine design_ Shigley & Mischke 1986 McGraw Hill
_Direct drive robots_ Asada & Youcef-Toumi 1987 MIT press
_The robot builder's bonanza_ Gordon McComb Tab books #2800 1987

> I would be interested in hearing from anyone out there that has 
> successfully built a robot base using stepper motors and gears that 
> resulted in enough torque. Please respond here or email me. Thanks!
>                                              tallent@netcom.com

I recently built a very small robot. It is an upright cylinder 3.5 inches
in diameter and 3.5 inched tall with a transparent hemisphere top.
The diameter was set by the hemispherical plastic dome I found at the 
local surplus shop. $0.85 for a green dome that just had to be the top
of a robot. I found some stepper motors with integrated gearboxes
from Clifton Precision for $10.00 each. These are 45 degree per step
4-phase unipolar 28 volt steppers with an axial 77.9:1 gearbox.
They are 0.75 inches in diameter and 1.5 inches long excluding the shaft.
I used the lathe to make a cylinder 1.0 Inches outside diameter and
0.75 inches inside diameter and 2.0 inches long. I drilled and tapped
so that I could use short bolts to attach this cylinder to a 3 inch diameter
plexiglas disk and I inserted the two motors into this cylinder and used two
setscrews to hold them in place. I also milled a channel near the center of 
the cylinder so that the motor wires exited the cylinder from the top.
I then made two "mag" wheels that coupled directly to the gearbox shaft
and had wheels! The wheels are 4.0 inches apart. Then I got four 2.0 inch
long 0.25 inch diameter threaded standoffs from the junk store and used
them as vertical supports to hold the dome. Two sets of 10 cell NiCd
batteries on each side of the robot nestled in perfectly for 12 Vdc power.
I then milled a Miniboard removing the serial connectors from one side
and the motor LEDs and power connector from the other side. (You need to
do a little repair work on the power side since you remove two traces
that are needed for the motor drivers) I then had a miniboard that was
less than 2.5 inches long and it fits on top of the batteries and inbetween
the vertical standoffs. And you can see it through the dome! It couldn't
have worked out better.

The miniboard drives all 8 phases of the two steppers. (You can figure
it out. It is possible trust me)

All it can do right now is draw a square version of a figure-8

I am now working on some "whiskers" so that it doesn't fall off the table.

It can run for about 30 minutes on a change of the NiCds.

See it at BEAM.

             -Alan Kilian
-- 
 -Alan Kilian           kilian@cray.com 612.683.5499 (Work) 612.721.3990 (Home)
 "Seeing is much more than the perception of images. 
  It is the application of knowledge" Some Visual Agnosia expert on NOVA
