wheels, increasing the normal force
and thus increasing the friction.
After searching for a suitable
tether, we settled on a terry cloth
towel (which we thought would have
a better coefficient of friction too).
Folding over the towel gave us a very
thick tether, so much that it was a bit
of a chore to begin feeding it through
the mechanism. The mechanism
ate up the tether with an initial test
on the ground floor.
For our next trick, we sat the bot
up in climbing position, had it grab a
hold of the tether, and we lifted it up.
Instead of sliding down the tether, the
bot held fast. When we activated the
climbing mechanism, the bot climbed
a little bit before the tether fell out of
It wasn’t perfect, but we had our
proof of concept. We were so
confident in the basic efficacy of our
mechanism, that we wanted to
demonstrate it in death-defying
fashion. We live on the third story of
our apartment building, and having
the bot ascend the tether over the
edge of the balcony would add the
same sense of urgency as the opening
scene of Cliffhanger. Fortunately for
us, our ribbon climber had a stronger
grip than Sly, and after a few heart-pounding seconds we brought the bot
back to safety.
We were pleased that our
mechanism was functional, but there
was still a lot of room for
improvement. The climbing
mechanism thus presented a golden
opportunity for a process that is of
critical importance to any successful
engineering project — optimization.
Optimization is the process of
improving the performance of your
project in a systematic way. You
pinpoint specific problems and solve
them. Optimization is what you do
after getting the basic project
working, and it’s what makes the
difference between something that is
functional and something that is truly
elegant and effective.
Our ribbon climber may have
been functional enough to survive a
cliffhanger, but relying on a thick
towel and having your ascent cut
short by losing your grip on the tether
meant that it wasn’t great. We didn’t
want something merely functional, we
wanted something as super effective
as a a Charizard’s flamethrower
unleashed against a grass-type.
One of the major problems we
noted with the first draft of our
mechanism was that the wheel
assemblies lacked rigidity, and so they
would flex and lose grip on the
ribbon. A solution to this problem was
to redesign the bracket mechanism for
the wheels. After rummaging through
some of our other VEX parts, we
found some long frame pieces that
would allow us to create a more rigid
and overall simpler structure. Our
redesigned mechanism certainly
worked better. Before, the climber
could only get enough of a grip on a
thick folded up towel to make any
sort of ascent. With the new rigid
bracket, the climber was able to
ascend the much thinner blue ribbon.
Another issue we noticed with
the mechanism was that the ribbon
tended to bunch up and fall from
above the wheels. One possible
solution we imagined was to add a
bracket behind the climber to feed the
ribbon through as the robot ate it up.
The bracket would help add some
tension to the ribbon and hopefully
avoid the bunching problem.
Upon initial testing, it did not
look like the bracket did much of
anything, but then we realized that
the reason was because the ribbon
lacked tension below the climber.
When we tensioned the ribbon by
holding down the bottom end, the
bracket worked like a charm, allowing
72 SERVO 12.2013
OPTIMIZING THE MECHANISM.