Final Design Update

Team Rubber Band Gunner had a final meeting on Wednesday, December 11, from 2:30 pm to 5 pm in Perry 321. In this meeting, the team completed the assembly, tested the mechanism, and made appropriate adjustments. All members of the team were present for this meeting, and all members participated equally in each aspect of the meeting.

Finishing the assembly was an easy process. The team purchased Epoxy adhesive to attach the servo horn to the gear. The adhesive was spread over the gear, the servo horn was placed on the gear, and pressure was put on the servo horn for about 30 minuets. To ensure that the servo horn and gear would have as little independent movement as possible, electrical tape was placed over the servo horn on the gear after the adhesive dried. Here is a close-up view of the attachment between the two parts:

The team also made a slight change in the mechanism's assembly from its initial assembly that was created in SolidWorks (shown in Week 3 Design Update). Instead of placing the Arduino Uno board on the side of the base, the bread board was placed on the side of the base. This adjustment allowed the Arduino board to connect to a computer without the USB cable possibly getting in the mechanism's way. The bread board is designed with adhesive on the bottom, so it was simply placed on the mechanism's base. Some electrical tape was also put on the launching arm to make it a bit sturdier. Here are some pictures of the completed mechanism:

Side 1

Side 2

Once testing began, the team noticed a clear problem. The mechanism could support one rubber band, but once two, three, and four rubber bands were placed onto the mechanism, the servo motor began to rotate on its own due the force from the rubber bands. Also, the force of the rubber bands caused the gear's position on the servo horn to shift slightly. After initial testing, it was clear that this specific mechanism should launch one rubber band at a time.

In order to allow the mechanism to launch one rubber band at a time, the Arduino code needed to be adjusted. The code was changed so that the servo motor alternated between two different positions: its initial "reload" position, and its position after launch. The difference between these two positions was made very large, so that servo motor can make a large rotation, and the rubber band is guaranteed to launch every time. If the servo motor does not rotate enough, then the rubber band will most likely remain on the gear. Here is the updated code:

And here is a video of the mechanism in action:

Overall, the mechanism performs what it is intended to do, which is launch rubber bands and indicate when it needs to be reloaded. However, it does not perform to its fullest extent because it is unable to hold multiple rubber bands.

After completing this project, there are a few changes that would be made if the team were to do it again. First, some simple changes would be made to the gear that was 3D printed. The center hole would be cut to fit directly on the servo motor, so that a servo horn or any adhesive would not be needed. Also, the edges of the gear would be rounded to make sure that rubber bands would not get caught on pointed edges while launching (a problem that doesn't occur often, but occasionally). Here is what the re-designed gear would look like:

A close-up of the center hole, cut to fit a servo motor

Also, if possible, a stronger, more powerful servo motor would need to be purchased, so that more rubber bands could be placed on the gear without the servo motor rotating on its own.