I don’t think this is something I would design my robot around. We’re compressing the ball about 3" and it doesn’t noticeably change the shape of the ball after it’s out of the shooter. If you compress it any more you’ll have to worry about things like stalling your shooting motors. Then you can try putting bigger wheels on the gain some linear speed while gearing down the motors. By then you’ll notice the shooter is clunky and doesn’t fit into tight places where you want it. Then you’ll realize wow, this change is causing more trouble than it’s worth. Smaller wheels, fine tuned compression, low gear ratios, and high rpm’s. Those are the constraints I’d be designing my shooter around.
I have managed to find to different places where you can get the balls from. Here are the links.
http://www.andymark.com/FRC16-p/am-3276.htm
Hope this helps.
I have a feeling that this is what the GDC wanted you to feel. It was no coincidence that from the material down to the name of the game piece it automatically scream catapult.
Well…
BALLISTA FOR THE WIN
Maybe.
Anyways, had to stop a rookie from picking at the skin of our boulder… It seems heavier than it looks.
Thanks for the suggestions! We were prototyping on the second day and grabbed whatever 6’’ wheels we had on hand to throw something together:D . Now that we’ve seen what plaction wheels can do to our ball, I think we’d be using Vexpro colsons or one of Andymark’s colored stealth wheels, depending on how they perform in a later prototype.
We were running the motors as hard as we could. I think we can safely assume that the motors were getting full voltage.
I wasn’t the one putting it in, from from my perspective, we were not vigorously pushing the ball in, nor were we especially gentle. We just put it forward and watched it shoot.
Is that a significant factor in damaging/shooting the Boulder? After all, the robot would be pushing the ball into the shooter with some amount of force anyhow.
Thanks for the advice! We were going to use smaller wheels, but we had a scrap piece of 2x1 handy to fit a 6’’ wheel shooter. Can you describe your “fine tuned compression”? I’ve always assumed that moving the 2 flywheels on the same shaft closer together would be enough to vary compression. Is that true?
What is the 1114 style articulating arm? A quick google search yields this thread.
Fine tuned compression: The point where the wheels don’t slip on the ball and it doesn’t leave black streaks, but it isn’t over compressed. It isn’t an exact science (for Ri3D at least…), but we found a compression of 3 inches gave us the results we were looking for. I can’t speak to moving the flywheels on the same shaft closer together and how that affects the compression. Making an easily adjustable prototype is the best way to determine this 
Simbot SS, 1114’s robot in 2008 inspired their 2014 Robot.
I’m more impressed that you actually have a shooter. Or maybe my team is just a bit slow off the mark.
A Ballista is a type of catapult, along with the Onager and Trebuchet are the 3 major types of catapult.
The More You Know!
/wavehandfromlefttoright
Just want to comment on this, we also found that 3 inches of compression is ideal. This ball can really fly.
Has FIRST been able to offer a good reason WHY basic foam balls are priced so highly?
- See more at: http://www.firstinspires.org/robotics/frc/blog/Kickoff-and-Arnold-Schwarzenegger#sthash.4XOPPEDm.dpuf
Not so basic. They had a lot of criteria to hold.
It may be that it takes longer for the wheels to grab the ball and shoot it when the ball is pushed forward gently allowing the wheels more time to abrade through the outer coating of the ball. This is just a hypothesis and you would have to specifically test for the difference.
Some new testing with our wheeled shooter for the GreenHorns showed some great results. We started playing basketball with one of our boulders around 3 AM (used the tower for our hoop… hopefully robots will be more accurate than me…)while waiting for the programmers and it got pretty beat up. When they were finally finished we set our robot up in an orientation that it was making 100% of its shots with that beat up ball. Next we introduced an untouched ball to the shooter and it was still successful.
The tests tell me that wheeled shooters aren’t terribly affected by wear on the boulder. For reference we were shooting from about 7-10 feet away from the tower.
We have already tried both a wheeled shooter and catapult prototype design with the option to change certain variables.
Compression on a wheeled shooter seemed optimal at 3" compression. We tested 2" also with poor results. We also used direct drive CIMs on an older AM wheel similar in texture to the stealth wheel.
We didnt even bother trying the plaction wheels because our goal isnt to create a shredder.
Shooters and catapults have already been built by many teams and there are a ton of old video clips from previous seasons.
IMO, building either is the trivial part.
Hm. It just seems like a foam ball, even built to specifications, shouldn’t be quite that pricey, although as always I appreciate the thought from FIRST that went into the game piece even when it seems basic!
I will testify that the catapult works, I will also testify that it creates one hell of an accuracy problem.
Please tell us more about the design of your catapult that has an accuracy problem.
I wouldn’t expect that to work either, but it’s worked for my team so far. I have a video here:
https://www.instagram.com/p/BAdNlxTjpJc/?taken-by=team293spike
I suggest you take a look at the end of the catapult. Do some slow motion video on it - even the slow motion that a Samsung phone can shoot. We did quite a bit of testing in 2014 and found the biggest contributor to inaccuracy from a catapult is when the ball ‘rolls’ off the ends of the catapult as it is shooting. As a result, we put wedges on the end of the catapult to keep the ball on until the end of motion.
In addition, keep close mind of how you are powering the catapult. If you are using motors, it will be inaccurate unless you are accounting for battery voltages, motor speeds, etc. If you are using some type of spring, get a fish scale so you can measure the spring tension and make sure it is consistent. Also insure you are shooting from a hard stop to another hard stop on the motion so that the catapult follows a proscribed motion each time.
People with wheels need to worry about slip, moment of inertia, and wheel velocity and how the ball is introduced to the shooter. Catapults just offer different challenges.