Accuracy of single axis shooter

[TheRozb]

Hey All,

In our team's prototype, we have a single axis shooter (i.e. two wheels on an axle. Ball is fed through and there is a bent piece of sheet metal like a hood to direct it. Pretty much what every Ri3D shooter robot had). However, like most of the Ri3D robots, our precision and accuracy is pretty iffy. We've tuned it the best we could, but it still has pretty poor performance. Anyone had success with a single axis shooter? Ideas?

[Andrew_L]

For the most part, you can make any sound shooter reliable and accurate with the right execution and tuning. Rarely is one style objectively better than the others.

[Ginger Power]

I wouldn't say our (The GreenHorns) shooter was inaccurate. When the balls were fed at a slow enough rate (around 1 Fuel/second) the shooter put the Fuel in a pretty tight grouping. During the filming for the reveal video we were hitting with around 80% accuracy.

With that said, I'm assuming you're having consistency problems when trying to shoot at faster speeds then 1 Fuel/second. Two (relatively) easy solutions to this problem are to put more power into your shooter wheel by adding a motor, and/or adding a flywheel to increase the amount of inertia on your shooter.

General things you can do with a single axis shooter: make sure the compression of the ball is consistent throughout the contact with your shooter wheel. Make sure your wheel isn't slipping on the Fuel as you shoot. Run the wheel using closed loop control so the speed of your shooter wheel isn't dependent upon the voltage of your battery. Make sure your hood (whatever you're using opposite of the shooter wheel) is solidly mounted and can't flex.

It would help to know the exact situation your shooter was experiencing when you were having consistency issues. Did you have closed loop control, or were you powering the system with a drill? How finalized is your setup? Pictures would help even more, but I could understand if you didn't want to share those.

[Ekcrbe]

I made a post about shooters in a different thread that I think covers some of the general questions about what to consider when designing one.

Basically, there are no quick-fix answers, and it does involve a lot of trial and error, but from some FRC experience, I will attest that it can be done very well.

Try to diagnose the source of your inconsistency. Are your shots inconsistent left/right, long/short, and/or high/low? I know it may not be immediately apparent, but try to tell whether your shots are coming out at different speeds or different directions if you are missing the goal from front to back. I would hypothesize that some likely culprits are inconsistent positioning of the balls coming into the shooter (for left/right inaccuracy), poor speed control on the wheels (long/short), or too-sudden decompression of the balls upon release.

I've seen all three issues in shooters I had a hand in during my years, and I would say a few things.

• It's important to have your feeder send the balls into the shooter in the same position each time, not try to make the shooter align the balls into its preferred track. This is a recipe for shots to come out crooked. (edit: To elaborate on this, consider that if your feeder does put balls into the shooter in slightly different places, you want them to come out perfectly parallel, so that their total inaccuracy is only that very small difference, not something much larger. I like having a consistent profile across the width of the shooter to help with this, although others will say two wheels with a space in between can work just fine as well.)
  
• The importance of closed-loop speed control cannot be overstated. We had a lot of success in prior years with instituting a "speed check" which would disable the feeder into the shooter unless the wheel was within a small error range of the requested set point. You'll have to balance your required accuracy versus your desired throughput rate to determine what that acceptable range is.
  
• And I've found it nice, although other teams probably have made good shooters without caring for this point, to have the hood gradually decompress the ball in the last ~30 degrees or so and end with a short section facing tangent to its arc to help the balls return to their natural shape after being compressed by the wheels. If your hood cuts off suddenly and the balls leave contact with the wheel and hood at the same time, the balls can "pop" back into shape in unpredictable directions, causing vertical inaccuracy. This is likely the point that the most people will scoff at because they think it doesn't matter, and maybe it doesn't, but I've never regretted paying attention to this detail.

[chandrew]

It's important to have your feeder send the balls into the shooter in the same position each time, not try to make the shooter align the balls into its preferred track. This is a recipe for shots to come out crooked. (edit: To elaborate on this, consider that if your feeder does put balls into the shooter in slightly different places, you want them to come out perfectly parallel, so that their total inaccuracy is only that very small difference, not something much larger. I like having a consistent profile across the width of the shooter to help with this, although others will say two wheels with a space in between can work just fine as well.)

We had that issue and changing our feeder increased our accuracy significantly. Also you can try changing the compression/duration of contact of the ball by the wheel, the less compression by the ball, the less speed lost by the flywheel.