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By Los Inginerios

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Darin Gray

on 20 July 2015

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Transcript of By Los Inginerios

Gandalf Memorial Bridge
Project Overview
The goal was to build a bridge capable of spanning four feet, stand at least one foot high, and have a two lane deck capable of supporting a Vex robot (also built by the group) loaded with three bricks (not built by us). After the initial design stages, we split into two teams to make the most of the limited time. Both teams communicated back and forth thoroughly, sometimes even assisting the other team with construction.
Gandalf Memorial Bridge
The first bridge up on its legs, Gandalf Memorial Bridge appears to have the thickest truss of the class. Preliminary testing showed that it could hold some weight, but the Tiananmin Square Special caused some unnerving vibrations while driving across. After consuming every last usable piece of wood and glue, the bridge is ready for its passengers...
Bridge Design
Our first introduction to serious bridge design was on the computer program "Bridge Designer 2015." We all used a specific type of truss system to span a set length, width, and height.
Scale Models
The drawing of the scale model gave us an accurate idea of how big the bridge should be. The constant questioning of the group building the model of our bridge also taught us to consider measurements in three dimensions; mostly concerning the width of the driving surface.
Building the other group's model made us realize that not everyone remembered to scale (or even specify) the thickness of beams. Luckily, we did a mostly accurate job on our drawing.
Bridge Building
When we started building our actual bridge, our bridge team made quick work of the initial planning. We started with the deck, laying out long planks to span about 5 feet, and a couple planks wide for stability. This was also checked several times to see if it was at lest twice the width of the Vex robot, and that it had ample room to turn around. We then started mass-producing parts for the truss: the primary frame made of 2x3 of the balsa rods, then other parts of the truss in 2x2 and 2x1 configurations. The robot was repeatedly taken over to the bridge to see if it was wide enough and if the truss had high enough of a ceiling.
Since the bridge was still
very
wobbly, we added additional beams to the bottom of the bridge. Then we added some simple towers, each just two planks at right angles with some reinforcement bars. At this point, the bridge was pretty sturdy. We tested its strength first with a static load of 1 brick, then a dynamic load using our robot with no bricks. It was discovered that it produced vibrations that rattled the entire bridge. With the few scraps remaining, we reinforced the bridge to the end.
Complications
Early in the design process, we came across several problems.
Robot Design
Before the Vex kits were available, we began planning how the bricks would be arranged, propulsion, and other design requirements. The end result is a compact tank and Battlebot champion "Tiananmin Square Special"(in honor of the man who stood in front of the tank column). We were able to build a very compact chassis and design; with the battery and both motors on the belly of the tank. The motors are connected directly to the drive wheels, so while space-saving, limited the potential top speeds of our tank.
Bridge Builder Challenge
The challenge was to build the cheapest bridge within a specific set of parameters. While the record holder for the class had a bridge totaling around $311k, the closest we could get was in the low $400k range. The extreme experimentation allowed us to see how different shapes and sizes of beams affected structural integrity, as well as where most of the strain was concentrated.
By Los Ingenierios
The first issue was that there was a severe shortage of chains and compatible gears to use for a tank tread. We competed with another group for the necessary parts. Luckily, a trade with another group early on yielded enough gears of the same size to solve our problems.
Another problem, more serious than the last, occurred after mounting the battery pack. It would work well separate from the vehicle, but once mounted onto the belly, would fail to provide power and rapidly overheat. It was later discovered that contacts on the bottom were touching four bolts holding the chassis together and shorting the 6 AA batteries inside. We moved the bolts and covered all the contacts with electrical tape to prevent the building from catching on fire. You're welcome.
Testing and Bot Performance
After rigorous testing (including vicious robot fights), we determined that our bot can turn on a dime, carry at least four bricks, traverse minor obstacles, flip larger vehicles on their sides, fit our bridge, and travel very slowly. It is very stable and, thanks to the wonders of electrical tape, not a fire hazard.
This Project has Gone __ Days Without an Accident...
Accidents during project:
Jared : cut on hand
Elvan : moderate cut on hand
Elvan : 2nd degree burn on arm
Chris: minor burn
a Group 1 company
Vex Robot Build Team:
Tony
Jared
Bridge Build Team:
Chris
Elvan
Juan
Bridge Builder Program
After the group used the computer program to design the best bridge, we decided that Juan's bridge was the most effective and the most efficient use of resources of the various options.
Thank you!
Bridge Analysis
Our bridge held the weight of the robot. Upon first placing the tank at the starting position, the deck started to bend inwards, but still held steady. The starting position was then moved off to one side, where it was closer to key structural elements. The vibrations caused by the track made the journey suspenseful.
The tank caused some post-last-minute problems. All of the battles and testing had drained the batteries to the point where the motors did not have sufficient power to turn. After changing batteries, it was ready. Some error occurred when driving the tank on the far side of the bridge, as his view was blocked. This almost caused him to drive the robot off the end of the bridge, take out a truss, and wear a hole through the deck.
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