- Group 24:
- Samuel Nalbone
Mike Ornstein
Jill Wollenberg
How does this thing work anyway, what’s the big picture?
In order to compete in the challenge for maximum height that the weight is to be lifted, be knew we had to build something with a point of rotation further from the weight than necessary to complete the basic task of lifting the weight 2 inches. Starting from the clamped base, we built a support structure which would be just strong enough to support the loading at the end of the arm, without twisting or bending too far as to render the device inoperative. From there, we worked on the actual mechanism which would allow us to raise the weight an estimated 12 inches, using a four-bar linkage. This would allow us to maximize the range and power of the servo, while still allowing for the maximum lift. In addition to the torque from the servo, we also planned on using a counterweight to help our crane to achieve the lift that we desired. According to our calculations, taking into account the ounce per inch weight of the arm, we needed a counterweight equal to the remaining mass allotted – weight of the boom at a distance of ~20 inches to help generate the moment around the point of rotation to allow for the servo to lift the weight as we desired.
How was it supposed to work theoretically?
The 1lb weight is a constant 9 inches from the servo, meaning that there is a moment of 144oz-in that must be overcome to lift the weight. The servo will apply 42oz-in to the arm that will lift the weight, so there is another 102oz-in that must be overcome in order to cause a moment to lift the weight. To lift the weight a counterweight was placed on the other end of the arm, to create a moment that will cause the weight to lift. When the servo is turned on the left side of the arm, which will contact and lift the weight, it is at an angle of 50 degrees below the horizontal, so that the full angular range of the servomotor can be utilized. The entire torque of the servo would be used to lift the weight, especially when it is first turned on and the arm is at 50 degrees below the horizontal. All of the torque is necessary at this point, as the torque from the counterweight is lowest at this point, and at the point when it is 50 degrees above the horizontal. By using the entire angular rotation of the servo the weight can be lifted a total of 12.6 inches.
That’s nice. What’s cool about it?
We’ve got a four bar linkage. It’s neat because our moment arm is nice and continuous throughout the lift. This is beneficial when considering the fine tuning of our counterweight that can be accomplished with our easy to model system.
Also, our supporting structure was constructed using the bare minimum of support members, not only to decrease weight but to increase the effectiveness of the members that were included. Using a simple rectangular prism structure, solidified by additional members purely in tension, we were able to construct a light enough, yet still strong, supporting structure to support the load required at the end of the crane. From there, we decided to move forward with our idea that a four-bar linkage would minimize the changing moment that would be induced by the weight being touched by different points by whatever member was touching it. This also increased the rigidity of the actual moving members by doubling the number of load bearing members, so as to keep the plate which supported the weight parallel to the ground. The final custom part was how we went about mounting the servo to our support structure. Instead of taking a minimalist approach, we determined through the various design reviews that having something extremely secure and unmoving was more important than saving weight, so we used a custom made CNC aluminum plate to attach the servo to the main structure. This allowed for added rigidity, as well as a level of precision that was required by our design.
So what does the thing look like?
But did it work?
We flew through our first two design revisions lifting the weight great heights, but we were neglecting some of the rules. Our machine and its high lift potential were flattened with the necessity of a recyclable lift. We were able to lift the weight and bring it down all according to the rules, but we could not accomplish this twice without manually resetting our machine. This caveat proved deadly in the third design review. In order to abide by that particular rule, an entirely new system needed to be developed. That system fell short, and our innovations were Steifled. Given one more round, and some more creativity, the use of the four-bar linkage would have proved to be a winning design choice.