Team Members
- Ben Allen
- Santia Valerio
- Mana Heshmati
Design Overview
Our mechanism started from a “less is more” design approach. We choose to go through the smaller window to simplify our design and save weight. This forced us to be precise in our design. Our main tower has a purposely calculated taper and the angle of the boom is created by the “twist” on the top of the tower. All structural members have identical cross sections of “angle iron” form and all connections are riveted together. This is a much easier and secure fasting system than the machine screws used previously, but it allowed less room for trial and error as each connection was permanent. Our boom was two angle iron sections riveted together to make a U channel; however, we had to full box in some of the boom by using 4 angle iron pieces as reinforcement.
Design Improvement
This design set forth the correct mistakes that we made in the previous design. Our old design did not incorporate enough angled pieces and thus deformed and hit the wood obstacle. We were strongly focused on solving this issue and without a doubt we made a very stiff design that did not contact the obstacle. Something we did not think of a head of time was the added moment created by going through the small hole. Our old design went through the larger hole and interacted with the weight “straight” on, so there was no twisting of the boom. Our new design engaged the weight from the side so there for there was an added twisting moment due to lifting the weight. We added bracing and boxed in our boom, but we could not overcome enough of the twisting. We had to abandon our well designed lever arm and quickly try to come up with a counter weighted lifting arm. We hoped to offset some of the moment caused by the weight with another, opposite moment caused by a counter weight and thus minimize the twisting of the boom. Unfortunately this new design was hastily assembled and failed anyway.
The other main problem with our design is that we treated every connection as a bolted connection. The aluminum strip was too ductile to be modeled this way and instead we should have designed our connections as pin connections instead, with the extra degrees of freedom for each joint.
Calculations
In our design, the arm that is attached to the servo is approximately 2.5 inches long at the far side of the weight. The weight that will be lifted is one pound, equivalent to 16 ounces. The servo has a maximum torque of 40 oz-in and the mechanism must lift the weight a minimum of 2 inches. Therefore, we need approximately 2.5 inches of length on the arm to make a torque on the servo that is equivalent to 40 oz-in. Our calculations ignored friction between the lever arm and weight and the weight of the lever arm and servo horn itself.
For the rotation of the arm, a 2.5 inch arm rotated 100 degrees will move at least 2.5 inches. This is obtained from drawing an isosceles triangle where the equal sides are the 2.5 inch arm at its two extreme positions and the angle between them is 100 degrees. Then, from using the law of cosines the height it lifts can be calculated. This approach assumes the interaction between the lever arm and weight is simply coincident, however in the real world the interaction changes as the geometry of the device changes. We over built our design to compensate for this by extending the lever arm well past the weight.
These calculations did not work with our final model. Even while trying to model the “best case” scenario where the arm contacts the weight at the shortest distance and without friction, the servo could not lift the weight. Even worse is that when the servo rotated, the weight remained stationary and our boom arm twisted 90 degrees. We attempted to counter act this by using the supplied U channel as a counter weight. We could have made the counter weight part of the arm four times longer than the arm that engaged the weight, but this was deemed illegal. Instead we had to rely on the counter weight making a partial counter moment and hoped the arm wouldn’t twist too much. However, this design failed.
Interesting Aspects
Our mechanism has many original aspects which we are very proud of. However there are two key points which we focused on while constructing. The most interesting feature of our final product was the rivets which were used in place of screws. This held much more secure than the screws did previously and were also much easier to work with while building. Another interesting aspect of our mechanism is low weight of the final design. Without the servo motor and counterweight, the final assembly was approximately 5 ounces. The design of the boom was the key to the low weight which we were able to attain. We designed a skinny boom that was constructed from two strips of aluminum folded onto each other and riveted together. The two strips of aluminum together were very stiff while also lightweight.
Pictures
Video
Click here to watch a rendered video of our design
FEA
We ran a quick FEA analysis in SolidWorks. We used the weldment tool to make all the beams and created a custom structural member to replicated the bent aluminum strips. To simplify the process we did not model our fasteners and simply used global bonded connections.
