Analytical Evaluation of Design Alternatives

Evaluating design alternatives is an important part of communicating your process to anyone reading your design report, or Engineering Notebook (BEST). Your process should be explicit, and the justification (using analytical and mathematical approaches) needs to be well explained. As an example, we will go through our process for two different components of the design of our large scale prototype: the Motor Mounts, and the arm in the upper assembly.

Motor Mounts

When we began the design of the large scale prototype we knew each arm in the upper assembly required two motors to be mounted to each arm (there are four arms, and 8 motors). Knowing this, we needed to design a mount that would attach to the end of the arm enabling a motor to attach both above the arm and below the arm. 

In order to get a better understanding while you read this, here is a picture of the final Catia model of the motor mount.

Figure 1: The final design of the motor mount

The first thing we thought about was what material these mounts would need to be made out of. Initially, we considered manufacturing these mounts out of aluminum. However, due to manufacturing and repeatability issues (i.e. when you make one, can you make them all the same?) we explored alternative material possibilities. With this, the idea of creating the mounts out of ABS plastic came into consideration. 3D Printing the motor mounts could be a good idea, or it could be a bad idea. We had to consider what 3D printers we had access to, the printing processes, the material properties, along with the cost to purchase the material. Once we verified the ABS plastic would meet the required mechanical properties for the motor mount and the loads it would see, we were able to move forward with deciding to 3D print the mounts.

Part of the process in deciding if this new material would work was to do some basic calculations analyzing what kind of load the motor mount would see. Once we determined a maximum stress the part would undergo, we were able to verify the components of ABS plastic would fit. The next stage was to input the Catia file and material properties into ANSYS (an Engineering Software) to ensure the stresses and deformations calculated, were correct. Once verifying this in ANSYS we were able to move to the next stage. 

At this point, it was critical to print a rough mock-up of the motor mount and do some experimental load testing (See Figure 2 & 3 below).

Figure 2: The test pieces that were 3D printed for load testing

We set-up the motor mount test pieces and using a strip of metal and a fish scale applied approximately the maximum load it would see (20 lbs) to check for any cracking, or plastic deformation. This initial test helped to prove ABS plastic would satisfy all of the requirements.

 

Figure 3: The experimental set-up of the preliminary testing of the motor mounts

Now, the next stage was to come up with the actual design. Without giving you all the minor successions of change this design underwent many modifications. We had to look at adding structural pieces to create more support along the edges, we also had to account for how the 3D printer would actually print our piece ensuring the most strength was obtained. 

Arms

When we began the design of the upper assembly we had completed the design of the frame of the large scale prototype, and had the required dimensions. The first thing we had to decide on was the size of the components. The size of pipes, the reinforcement place (near the bend of the tubes) and the mounting plate (bottom where the pipes attach) all had to be decided on. Once huge design requirement we have for the large scale prototype is it must be under 55 pounds. With this being a huge constraint, not only do we have to ensure the design of the upper assembly will be able to withstand the loading and stresses during flight, we also have to optimize the strength-to-wait ratio because it must stay under 55 pounds. 

The first design of the arms was 2 inch aluminum tubing. We ran initial calculations for the maximum bending stress that would be seen at maximum load (20 pounds for each arm) and chose the aluminum based on those properties. We created the Catia model and ran the same stress calculations in ANSYS confirming the arm would not deform or fail during loading. Everything was finalized and ready for prototyping. When one of our team members who is manufacturing the entire large scale prototype put the tubing through the pipe bender in order to achieve the curve, because the die of the pipe bender is made for 2 inch pipe not 2 inch tubing, the tube didn't fit perfectly. When the tube went through the bender, the corner crinkled, causing the tube wall to buckle. So what does this mean? Basically, an entire redesign is needed. One thing in particular that matters in this case is to choose aluminum pipe that the company we are manufacturing this at has the corresponding size die for. We chose a new size aluminum piping (1-1/4" with 1.660" OD wall), and re-did all of our initial calculations. We put the new Catia model through ANSYS to verify everything would still satisfy the requirements and were ready to manufacture the arms again. The interesting thing about design is sometimes you can't predict what happens in testing. In a "vacuum" everything works, and in real-life unfortunately not everything goes as planned. The benefit of engineering is you can adapt to these unforeseen circumstances and redesign if failures occur. 

Below is the final design of the upper arm assembly (See Figure 4).

Figure 4: The final upper arm assembly

Closing

This excerpt is just a brief look into explaining a little of what we had to consider and evaluate during the process of this design. At each point in solidifying your design any designs you make should have proof behind why you are making them. Inserting initial basic calculations, initial testing experiments, and any research of what is required of your designs needs to be included in this section to fully demonstrate how you went from the brainstorming, to concept development, to final solution. There are some methods that could be helpful when evaluating different design alternatives, and this includes using Pugh charts or morphological matrices. As always, with design it is important to look into prior work and relevant patents that could apply to the design you are considering.

Don't forget to stay updated on our Twitter, and look for another blog post next week!