[Brandon Holley]

Like most teams this season, we spent a majority of our build season developing and iterating our shooter. After weighing the pros/cons of catapults, shooter wheels, slingshots, etc.; we made a firm decision to go with a shooter wheel for scoring.

Concept/Initial Design:

We leveraged experience from 2006 to know we did not want a 2 wheeled shooter (meaning oppositely driven wheels on either side of a ball). We felt it would be mechanically simpler to go with a single wheeled system and could potentially provide a more repeatable system. One of the large benefits of a 2 wheeled system is the ability to get a lot of distance on a shot. For us, our strategy did not revolve around being able to score from a large distance away (ie: half field +). That helped nullify some of the benefits of a 2 wheel.

Right away we began prototyping. Our 2009 robot utilized a single wheeled, hooded shooter on a turret- how convenient. That was the first prototype we used. The lunacy balls were larger in diameter, and required very little compression because we did not fire them all that far. After a few tweaks, we had our first shots out of the 2009 robot and learned a lot instantly:
- Balls fed at different rates into the shooter went different distances.
- We needed a considerable amount more compression than initially thought (first guess was approximately 20% of the ball's diameter).
- Our 2009 shooter wheel (6" in diameter) was spinning far too slow to get the distance we required.

Our game strategy set our design goal for shooter distance to be approximately 3 feet past the outer diameter of the key. So if you drew an offset arc 3 feet away from the edge of the key, we wanted to make shots within that arc. With that metric, we were able to start solidifying design criteria for the design team.

We jumped right into our next shooter prototype. This time we wanted to start nailing down some specifics:
- Compression of the ball.
- Diameter versus rotational speed of the shooter wheel.
- A rough guess on angle for both fender shots and key shots

This next prototype allowed us to insert different shooter wheels, change the compression in the hood in 1/4" increments and of course adjust the speed the wheel spun at. We tried wheels varying in size from 4" up to 10". Using a variety of motorized devices ranging from a CIM motor, to a corded drill we varied the speed with each wheel size and compression number. We measured wheel speed using a strobe tachometer. The "money zone" seemed to be following combination:
- 2.0" ball compression
- 6.0" diameter shooter wheel - with a concave to center the ball as it came through the shooter
- 4000 RPM

We also came to the conclusion that backspin was key. It allowed shots to bank off the backboard and drive into the hoop. This was something our 2009 robot prototype proved quite well.

With these fundamental specs laid out, the design team took off and got down to work. The prototyping team continued to work and iron out some more of the specs, but the initial concept was secured.

Design Phase I - Hardware/Mechanical

Based on a team philosophy, we like to overdesign fundamental subsystems. We knew our CIM motors were going to be utilized in the drive system as usual. We also had sworn off the use of Banebot 775 motors in the off-season after incredibly devastating results using them in the 2011 season. This left us with only a few motor choices. The 0673 Fisher Price motors, or the RS 550s. Conveniently, these have very similar free speeds, and identical form factors, so we could begin designing the gearbox knowing it would be one of those two choices in the final design.

We geared our shooter to peak efficiency of the FP motor curve (our leading choice because it was a higher power motor). We ended up with a clean 4:1 gear ratio in the gearbox. Knowing we may want to dial that down or up as the season went on and we refined our shooter, the last set of reduction was done with a belt/pulley. This was able to be tweaked to finely dial in our shooter speed after the gearbox was assembled. Doing this also let us put our gearbox in front of the shooter wheel to help slim down the shooter.

As the detailed design progressed on the shooter, the prototype team continued. We tried to answer a few more questions:
- Weight of the shooter wheel vs. ball shot consistency
- Width of the shooter wheel vs. ball shot consistency
- Material of the shooter wheel
- Material of the shooter "hood"

We found that a heavier wheel did provide a noticeable increase in consistency. With this came the tradeoff of a longer spinup time. We felt the spin up time was a tradeoff that could be handled and decided to go for a shooter in the 3 lb. neighborhood.

We also noticed that a wider shooter wheel wasn't necessarily better than a skinnier one. However, making a wider shooter wheel with a concave in it, did improve consistency. The thought is that the ball tends to be more centered on the wheel for each shot, thus reducing left/right inconsistency.

We had experience selection shooter wheel treads in our 2009 robot. We had a lot of issues picking the right material for that shooter wheel. Critical issues involved:
- The "right" amount of grip with the ball
- Adhesion of the tread to the quickly rotating wheel
- Lifetime of the tread material

Like most engineering problems, there was a tradeoff in these materials as well. The softer materials tended to not last as long, and because they gripped the ball better, also tended to pull off the wheel more.

Our 2009 robot utilized banebots wheels because of their urethane tread. While we would not use it for a drive application, the grip those wheels have on game pieces seems to be very good. We decided to step outside the box a bit, and make a 100% custom shooter wheel.

After trying several materials in the shooter hood, we decided upon an aluminum hood, mainly for its rigidity over polycarbonate in similar thicknesses.

Design Phase II - Finalized Detailed Design/Software Development

The ground work for the shooter was laid out already. We had our gearbox. We had found more 0673 FP motors. We just needed to fill in a few of the blanks.
- Adjustment of the secondary hood
- How to make our shooter wheel

We knew we wanted adjustment in our hood because we knew our initial game strategy called for making shots from both the fender and the key. While we did find an angle that worked for both shots universally, we felt it was impractical and inefficient for key shots. We also saw that an adjustable hood could allow us to focus on controlling just a single speed on the shooter, while adjusting the angle. Initially our software team thought the hood angle would be easier to control than a precise shooter speed. So with that, we decided to make an infinitely adjustable hood driven by a leadscrew. This turned out to be a poor decision (see below).

We knew the diameter, width, weight and approximate profile we wanted our shooter wheel to have. Nothing like it existed off the shelf, so we went ahead and set out for a custom design. We wanted the weight to be on rim of the wheel for more rotational inertia. Our prototypes had proven this lead to more consistent shots. We used a thick walled aluminum cylinder for the bulk of the wheel. We machined plastic endcaps from solid PVC stock. The final piece was the tread. For this we leaned on our experience with urethane molding. I 3D printed a mold (complete with '125' logos in it) at work. The mold was in 3 pieces so we could pull the wheel out of the mold without an issue. The issue was the large concave in the wheel created a massive undercut around the entire diameter. If we id not make the mold separable, the wheel would probably be permanently stuck inside the mold.


As all of this happened- our software team began laying out the framework for their solution to controlling a spinning wheel. The used prototypes and old robots we had to come up with a plan. I do not want to misspeak, so I will bug our software lead to come on and post about the software development. As we all know, this was a huge part of the battle.

Design Refinement- Iteration/Software Development

We put the entire robot together in time for us to attend a scrimmage event in Suffield, CT. The robot performed well for the small amount of runtime and software development it had. As we do every year, we also learned a ton from the scrimmage.

The most critical from a mechanical point of view was that we had overestimated our need for a secondary hood adjustment. We were only shooting from 2 locations, and did not have a need for this large amount of adjustability. The leadscrew adjusting the hood was also a weak point in the design team's mind. Too many things could go wrong with it, so we decided to move to a 2 position, pneumatic hood.

With the shooter fully built and the time to really work on refining it, we wanted to ensure our shooter speed was tuned; both in a top speed mechanical sense, and from a software point of view.

The fully built shooter was highly efficient compared to it's less ideal prototype cousins. This meant, the shooter wheel speeds we initially geared for were actually too fast for our game strategy. Fender shots were only utilizing ~10% of the top speed, while key shots were in the ~50% range. We knew if we bumped that up to about 90%, we would gain a much wider range of adjustment and make our software team's lives easier.

Building in the functionality to change the final gear ratio with a belt was awesome. We made the swap with minimal impact to the timeline, or overall design, and we didn't look back all season. This is something we will most likely incorporate in fundamental subsystems moving forward.

An issue we experienced in the eliminations of the Boston Regional was slippage in our encoder we used for wheel speed feedback. The disc would consistently skip ticks which would result in the shooter pegging itself to max speed and us sending balls over the backboard.

For championships, we switched to an enclosed encoder which rode on an internal ball bearing. It was bullet proof, and we will use this style encoder in essential systems moving foward.

The software team developed a strong PID control system for our shooter wheel that provided consistent results throughout our competitions. We withheld our shooter system after ship to continue refining the control software. In the end, we had quite a bit of success with our shooter. It is extremely consistent, and resulted in very few mechanical issues over the season. The main reason being we ironed many of them out through prototyping, iteration and testing.

-Brando