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The mechanism on our crane was designed in a way similar to that found on a bucketloader where the bucket attaches to the loaderframe. The way we did this is when the servo rotates, the point of attachment for the control rod moves farther away from the rotational axis. This creates a motion similar to a hydraulic piston’s contraction, so in order for our mechanism to lift, we had the attachment point for the control rod above the rotational axis with the forks themselves mounted below the rotational axis. This allowed all of the components to always be in tension which is the more stable configuration. After the mechanism had been created, there was an amount of iterative testing which allowed us the opportunity to optimize the amount of lift achieved by the forks, and to use the full range of motion in the servo in order to do so
Our mechanism had a .375 lb. counterweight which had no effective moment arm at the initial theta, and a 6.5" arm at the final theta. Our "forks" on the end of the arm had an effective arm of 5.625" in the initial state, and 6.5" in the final state. The control rod which acted on the fork assembly had moments due to the downward force of the rod as well as the longitudinal force. At the initial state, the rod was at 11 degrees from horizontal, at the final state, it was at 5 degrees. Using all of these numbers, I was able to derive that in the initial state, that the tension in the control rod was 1.375 lbs. which with our 1.125" lever arm on the servo itself works out to 54% of Tmax at θ0 at the final state, there was 1.72 lbs. which translates to 68% of Tmax at θfinal. Knowing that our servo was unable to utilize the whole 120 degrees, but only 100 due to contact with the substructure. We were able to ascertain from the geometry of the forks that after rotating through an angle of 23.92 degrees, that our mechanism had Lifttheoretical-max of 2.29". This however is in conflict with our figures from testing of greater than 3" and of our trial during the competition of 2.5" which leads me to believe that the deflection of our substructure allowed for greater travel in the mechanism, as even with 120 degrees of travel, Lift theoretical-greater-than-max of 2.81".
Interesting aspects of our mechanism include the use of a control rod in order to control the forks which were mounted at the other end of the boom. This allowed us to reinforce the motor mounting position as we had earlier had problems with deflection. All but those members which were in pure tension had 90 degree bends in them in order to improve the I value for that member. By shrinking and stretching, we were also able to achieve much stiffer members than if one simply attempted to use straight pieces. This allowed for much less deflection in key areas of our substructure.
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While making adjustments to the mechanism, we had a few ideas that were deemed unfeasible. However, through these mistakes we improved our mechanism and were able to make a better machine in the end. The first barrier we encountered was support; our structure seemed to lack enough to even be structurally sound, never mind support a weight. The original solution was to simply put the aluminum sheet supplied to us as the base. This idea was abandoned instead for a bent feet design that took less material and weight on our structure but provided the same integrity to the mechanism. Our second problem arose when our arm seemed to translate itself instead of the weight. One proposed solution was to build up and over the wood barrier and thus have a suspension bridge set up to support the arm. However, this was found to be against the rules as well as infeasible for staying within the weight restrictions. Yet even though the idea was scrapped, it did give way to the notion of bending the arms such that they would deflect less under a load. Our final main obstacle was lift: our servo seemed unable to provide enough force to lift the weight the specified distance. A counterweight attached to the forks at the end of the arm was considered yet was disregarded instead for a counterweight directly attached to the servo. This way, the arm would have less of a load to bear as would the overall structure.