Alt-BEAM Archive

Message #05641



To: beam@sgiblab.sgi.com
From: Sean Rigter rigter@cafe.net
Date: Wed, 11 Aug 1999 10:31:31 -0700
Subject: [alt-beam] Re: LEM videos now on-line.


Congratulations Bob,

Your whole LEM project (ie the physical design, shape and symmetry, sensors, the
documentation and video) shows great finesse. Since so far there have been only
kudos, I hope you are ready for some critical comments.

I found the concept of a net linear displacement (ie the useful motion to move the
LEM from point A to point B) of a triaxial wheeled platform difficult to grasp
until I realized it is similar to Steven Bolt's 2 wheel design of a pivoting motion
in which one wheel turns while the other is stalled. In Bolt's design, the 2
unidirectional motors are alternately turned on (in one direction) for a 180 degree
pivot around the stalled wheel resulting in a somewhat wobbly but on average linear
motion while the platform is continuously spinning around it's center (in the same
direction). This pivoting linear motion can be approximated with a conventional 2
wheel platform which reciprocates (not spin) around it's center. The main advantage
of Bolt's design is that it can reverse by turning on both motors and spinning in
place around it's vertical center and can therefore "back up" from walls and
corners. A conventional 2 wheeler with unidirectional motors can reverse only by
keeping one motor on for 360 degrees instead of 180 degrees and which does not allow
it to "back out" of a corner.

So in a 3 wheel platform one can approximate Bolt's linear motion with two wheels
spinning in the same direction while the third is reversed causing pivoting around
an off-center pivot point and resulting in a net linear displacement.

While it was possible to conceive of such linear motion, I had problems
understanding the frictional losses in such a 3 wheel design so I decided to build
one. I used 3 gear motors mounted horizonally with 1 inch diameter toy wheels
attached on the perimeter of a 5 inch circular platform. I used a simple tricore
with the 3 outputs connected to 3 bridge circuits. This results in a simple sequence
of 2 motors Forward (F=1,2)and one motor Reversed (3), followed by F=2,3 R=1 then
followed by F=3,1 and R=2 . The Tricore timing was adjusted to a rotation of
approximately 120 degrees around the pivot point

I found that net linear motion does results in my 3 motor design but is very
inefficient with a lot of wheel dragging and slipping resulting in high frictional
losses compared to, for example, Bolt's 2 motor design! These losses are caused by
the "toe in" of the 3 wheels which oppose all net linear motion. Even if one wheel
is held perfectly stationary causing the platform to rotate about that relatively
low friction pivot point , the sideways dragging and slipping between wheel and
surface (ie friction losses) of the other 2 wheels is very high.

So my question are:

Am I missing something?

Do you observe similar "wheel/surface" drag and slip by peeking on the "skirt" of
the LEM? (it is impossible for us to see this on the video)

In your opinion, would you say that the LEM design has an efficient linear motion (
getting from point A to point B)?

What do you think is the efficiency of useful linear motion of your 3 motor
triaxial LEM compared to a 2 motor axial + one caster platform controlled, for
example, by a Unicore circuit which has a similar Horse and Rider (embedded
Bicores) architecture.?

Do you think that by extrapolating the LEM design to a 2 motor or 4 motor design,
would this result in a more or less efficient linear motion?

I look forward to comparing my tentative results with your experience (but of course
you have the right to remain silent - grin)

regards

wilf

Bob Shannon wrote:

> Whew, that was too close...
>
> Ok, the LEM page has been updated to include the videos!
>
> (sorry no schematic yet...)
>
> The videos have been encoded with the new RealPlayer G2 codec, and a link for a
> free
> download has been provided.
>
> The 'Seeking' video shows the rather unusual form of phototropisim that is
> unique to
> these new multimotor symets. In this video you will see the LEM's reaction to a
> moving
> light pool. While having no prefered direction of travel, its strongly
> phototropic. LEM's
> behavior is also strongly effected by the overall light level, and some evidence
> of this is
> also shown when the light source is moved closer to the RJP.
>
> A second, shorter video shows LEM's logic powered and running with the motor
> drive Bicores disabled in a dark room so you can see the timing and sequencing
> of the microcore, and its mapping to the Bicore modules.
>
> The 'Seeking' video also has sound, one of LEM's best features is its cool
> 1950's sci-fi
> sound effects when bare gears are used as wheels. Check it out at:
>
> http://www.the-nest.com/bshannon/
>
> (Please let me know if you have any problems with the videos)

Home