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Subject: JEB.00b
From: David P. Anderson dpa at io.isem.smu.edu
Date: Tue Aug 29 15:33:11 CDT 2000

Hi Eric & Barry

>>> cool beans!
> Thanks!  Barry and I have been having a blast "making chips" and
> working through the various little engineering challenges.  ;-)

green with envy I am.  I now have my little milling machine setup on 
a homemade bench next to the lathe, just looking for an excuse to 
make a lot of chips!  So far mostly just made jigs and things to
go on the milling machine itself.  Sort of like programming in Forth.

> Your SR04 is using a similar 2 big drive wheels and a tail castor
> scheme if I recall correctly, but I think it might be a bit wider
> than what we're doing with JEB.  Did you encounter many issues with
> COG getting outside your wheelbase?  Or with clever placement of
> batteries and other heavy things did that become a non-issue?

Yes and no... I tried to get the center of gravity about 1/2" behind
the main wheels, and as low as possible.  Lateral stability is ok and
robot has plenty of traction for climbing over cables and such, but the
platform tends to tump over on it's nose when going down a incline of
more than 20 degrees or so.

> JEB's wheelbase is dimensioned like this:
>      |----------- 7" ----------|
>                  |H|
>                  |H|              ---
>                  |H|               |
>                /  |  \             |
>               /   |   \            |
>             /     |     \          5"
>            /      |      \         |
>      ||  /        |        \  ||   |
>      || /         |         \ ||   |
>      ||___________|___________||  _|_
>      ||                       ||
>      ||                       ||
>      ||                       ||

SR04 is something like:

      |---------- 10.5" ---------|
                   |H|              ---
                   |H|               |
                 /  |  \             |
                /   |   \            |
              /     |     \          5"
             /      |      \         |
     |  |  /        |        \  |  | |
     |  | /         |         \ |  | |
     |  |___________|___________|  |_|_
     |  |                       |  |
     |  |                       |  |
     |  |                       |  |
The vertical pivot of the trailing caster is on the perimeter the
circle defined by the center of the platform and the two drive wheels.
All three wheels lay on the perimeter of a 10" circle.

> I was also curious about how SR04 in the real world does with its
> ground clearance.  What do you have, a little more than 1 inch?
> Does that seem to be enough to easily roll over crumpled socks?
> Right now we've got 4" wheels and mostly about 1.25" clearance.

The platform originally had about 1" clearance, and that seemed to work
fine for socks, thresholds, throw-rugs, cables, and so forth.  My main
goal was to keep all the gears and mechanics sealed up inside the main
body box where they couldn't snag anything, and to keep the bottom
of the robot as clean as possible, no bolts or edges hanging down.

When I added the gripper, it reduced the clearance to about 1/2", which
is not really quite enough.  The grippers are programmed to act as
bumper switches when they are retracted, so I get a lot more bumper
presses now from sock-like objects than previously.

> Just curious about some of your real world experiences.  I don't
> anticipate making any significant changes right now...

I was curious looking at your excellant photo-expose' how you are
going to mount the battery(ies?).  Since these represent most of the
weight on my robots, the placement seems to have a lot to do with the
way the robot manuevers.   My experience has been that the ideal CG for
a three wheeled robot is somewhere just behind the main axle in between
the two drive wheels, and the lower the better.

Borenstien in his "Where am I" paper in the section on wheel encoder
odometry suggest that one source of error in a three wheeled design
is to have too much weight on the tail wheel, which evidently causes
the main drive wheels to slide in a turn.  

What I've done on the last few robots is to use the batteries to balance
the rest of the robot/wheels/frame/motors.  In the case of the LegoBot,
we ended up splitting the motor battery into two packs.  You might think
about something similar.

Way cool,  can't wait to see it in action!


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