For our structure, codename MURDERATOR, we decided to make a box frame truss that would be as wide as possible. The thought was, much in the way that a beam twice as thick (in the bending direction) is eight times harder to bend, if we could separate the edges of our truss enough we could keep the bending to a minimum. A box itself can easily be collapsed, however, so as we constructed the frame we performed constant stress tests, applying small loads to the end of the structure. This allowed us to see how our structure deformed and where it needed the most reinforcement. It was a very loose, very dynamic engineering process. Over a dozen different reinforcing members were added – ones that needed to withstand compression were bent into angle, while ones that only needed to withstand tension were made of the narrowest strips of material. Creating a large, open frame, and only reinforcing where most necessary, allowed us to have a light, yet stable, structure.
We built the mount out of two pieces of heavy angle aluminum (actually the u-channel cut down the middle) and attached two vertical bars of angle to each. From the four vertical risers, we had four members of the same type as offsets, to give us access to the larger of the two holes in the obstacle. From here, four more beams extended through the rectangular hole in the obstacle, each about a half- inch from a corner. We planned the placement of the risers and offsets this way, so that we could make the most advantage of the opening. A quick way to summarize our essential structure would be “four up, four over, four out” Once the box frame was complete we set about reinforcing it, adding tensile and compressive members wherever deemed necessary.
At the end of the structure, the two top beams join with a triangular brace to the two longer lower beams, which have a cross-brace between them as well, which serves as the servo mount. Mounting the servo to just one piece of thin angle was found insufficient, so another piece of angle attached to the servo was spliced onto the inner edge of the outer lower beam (using a flat-angle-flat sandwich), which provided the servo mount with enough rigidity.
The lifting mechanism was a simple counterweighted lever, which we chose for its elegance and ease of fabrication. We made the lever arm of the counterweight as long as possible (more detail later) and then added as much weight as we could without causing the servo to backdrive in an idle position. The acting lever arm was made as short as possible to preserve
Because of the circular arc of the lifting arm, and the vertical track of the weight, we had to ensure that the end of the arm could touch the servo at its base position, at its highest position, and move freely along its range of motion. This was accomplished fairly elegantly (or sketchily, depending on how you view it) by having a simple L-shaped end to the arm, where a piece of angle was bolted at 90 degrees to the beam connected to the servo. After ensuring that it could touch the weight at both ends of its range of motion, we loosened the bolt connecting it so that, when it was passing through the middle of its range, it could deflect (elastically) inwards and permit the necessary motion.
Before the final design review, a bend was introduced into the tail of the counterweight because of clearance issues it was having with the ground. This allowed the servo to go through its entire range of motion without disqualifying us by hitting the ground.
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