Servo - December 2013

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 the mechanism.

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.

Optimize Prime

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