The goal of the design project is to design, build, and test a structure that is required to traverse an obstacle course and lift a 1 pound weight a minimum of 2 inches using a provided servo motor. We were provided a set of materials to use, including several feet of 3003 aluminum strips of varying widths, an aluminum U-channel, aluminum rods, delrin, and 4-40 hardware. However, we were not limited to the provided hardware.
This first iteration of the crane was used to prototype many of our ideas. We decided to go through the smaller hole since it provided the shortest distance to the weight. We felt that going through the larger hole would require a more robust truss, and we decided that we could save weight by going through the smaller opening. The main issue with going through the smaller opening was the angle at which the “arm” must bend. We ended up having several issues with the bend in the arm due to torsion that we resolved in later design iterations, but we solved our problem here by the introduction of supports that attached from the main base to the arm to restrict some of the motion caused by torsion. We lifted the weight by using a counter weight system. Knowing that the servos could output roughly 72 ounce-inches, we attached 8 ounces to an 8-inch beam that then extended another four inches out to the lift the weight, which made lifting the beam significantly less taxing on the servo. The moment that the servo exerted was mostly used to hold the counterweight, requiring less once the weight was actually loaded on. It used 64 out of 72 available ounce-inches or 89%. Based on this theoretical calculation, we could lift the 5.7 inches assuming a 90 degree servo motion. We still had several issues with bending in the main arm, so we attached a flat piece of aluminum to the top of the base that extended out several inches to provide support for the arm. We also mitigated issues with torsion in the base by adding supports that ran between sections that experienced the most deflection, increasing the base rigidity.
In designing the base of the structure, the goal was to use the entire 6" x 6" so the moment that the base it applies to counteract the external forces on the truss structure uses the smallest forces. From there, the driving constraint of the base was the geometry of the playing field. The geometry of the platform was created so that the arm would go directly through the opening that we chose to reach the weight. The arm would go through the center of the hole in the horizontal direction and near the top of the hole in the vertical direction. This was done so that any deflections would not cause the arm to contact the barrier. The next concern of the construction was the structural rigidity of the base. We mainly used strips of aluminum bent down the center to increase the compression and bending load it was able to handle. The design started with 2 bent pieces to be put under tension and 2 under compression the support the upper arm that was attached to the top plane. These pieces allowed our long arm to act as a lever arm. The initial design also included 2 pieces attached in an X-shape to prevent relative motion of the bent pieces. This added to the torsional rigidity of the base. We also made minor modificaitons to the arm, adjusting how long the beams were to lift the weight.
For our third design review, we made a few small changes in our design from the second design review in order to make it lift higher while making it more consistent in its lifts. One issue that we noticed was the torsion in the arm while lifting the weight due to an imbalance of forces. To solve this issue, another piece of material was added to the base to add rigidity to arm where it attached. Another modification that was added was a support to the arm to attach to the base in order to decrease the deflection when the weight was being lifted. One final change we made was the addition of a few screws in areas that needed more secure attachments to that when the weight was being lifted, the structure as a whole would deform less. These changes allowed up to lift the weight a height of 3 and 3/8 inches. We were proud of the structure's design in how torsionally rigid the base was as well as how well the arm was able to lift the weight past the required 2 inches.
During each design review, we were able to lift the weight the appropriate amount. During our first design review, our structure was able to lift the weight a small amount above the required two inches. However, when lifting, our lifting arm made contact with the ground. It was also a few ounces above the maximum weight due to the additional supports that we needed to add to prevent torsional rotations that reduced the lifting height.
Our second design proved more successful with less deflection in the lifting arm. However, even with reduced deflections in the structure, our lifting arm still made contact with the ground. With the modifications we made in the third structure, we were able to lift the weight a total of 3 and 3/8 inches. Our minor modifications allowed it to fulfill all of the requirements including not making contact with the ground and being under the maximum 20 ounces with a weight of around 16 ounces.