Servo - December 2013

70 SERVO 12.2013

Team 1079’s robot from the 2006

FIRST game Aim High. Part of the challenge of Aim High was to shoot balls through a high-mounted target.

Team 1079 created a delightfully effective shooter by powering two wheels by pulley arrangements and spacing them apart so that the foam ball would be squeezed between the wheels. We geared the wheels for speed and when ushered between the wheels by a feeder mechanism, the balls would shoot out at speeds that would make them hard to avoid in a game of dodgeball. In the same way the wheels could grip the ball to shoot it, a similar mechanism could grip a ribbon and climb it.

Unlike a softball shooter where attaining a high speed for the wheels is the goal, with a climber we were far more interested in torque. A high torque mechanism would allow the climber to hoist itself up the ribbon, and the resulting lower speed of the wheels would also make for a more controlled ascent.

With this design in mind, we determined that the VEX kit would be a suitable starting point. The kit had the gears needed for a good ratio, versatile and easy to use frame components, and intuitive controls. We also thought the VEX kit would present a particularly nice challenge with respect to ribbon climbing because the parts were not exactly ideal.

With the RoboGames event, weight was at a premium — the maximum weight was about 2. 2 pounds, and putting a score on the board was contingent upon not damaging the tape ribbon. The VEX kit — with its metal frame, bulky brain, and weighty wheels — is not exactly the portrait of a weight conscious bot. The extra weight, however, would make a good challenge because it meant we would have to do an even better job designing a strong climbing mechanism.

The first order of business was to sort out our gear ratio. For the best torque, we would want a small input gear (the one attached to the motor) and a large output gear (the one attached to the wheel). The VEX kit has a nice assortment of gears, but there were other limiting factors we had to consider.

We planned on using VEX wheels for the grippers on our climbing mechanism because the rubber tires would provide great friction. Since the large output gear would be on the same axle as the gripper wheel, the size of the gear had to be slightly smaller than that of the gripper wheel. We opted to use the mid-size green VEX wheels for our preferred grippers because even though there is a larger sized wheel, the mid-size wheels were more manageable to build a bracket around while still allowing for a serious gear ratio. With the wheels chosen, we went with the gears that were the biggest possible with that pairing.

With our climbing mechanism beginning to take shape, we needed to sort out what kind of base to put it on. Many ribbon climbers at RoboGames didn’t have any other mechanism aside from the climber — since the bot was only going up and down, a driving base wasn’t really a necessity. We thought that attaching our climber to a driving base would add an extra layer of challenge by making it more difficult to balance the bot, and push us to make a mechanism strong enough to lift the entire assembly. We realized that adding the extra weight wouldn’t align as perfectly with the goals of a RoboGames competitor, but we liked the idea of placing more emphasis on the mechanism itself.

Tsiolkovsky’s Eiffel Tower X110,000 may have been too heavy to be mechanically feasible, but we were confident that a driving VEX base could make the ascent.

Clif fhanger

Now that we had the design, all we had to do was everything. For the climber mechanism, we used one long frame piece as the base. Since the entire purpose of the mechanism was