Shooters: What have you learned?

[Jeffy]

Originally Posted by CalTran

Really? We did about the same thing with our original shooter, and we found DRASTIC variation between shots because of its explosive exit as it uncompressed...

Conveniently knowing both systems well allow me to answer.

Our shooter does not shoot the ball with a very high velocity (compared to most). The ball is compressed between a roller and a flat piece of poly carbonate. This allows the ball to be decompressed over a fairly large distance (~5 inches). So, it's not exactly an "explosive" exit like it is from a shooter with dual wheels spinning much faster than ours.

Morale of the story:
Allow the ball to compress and decompress over a distance rather than "instantly".

[CalTran]

Originally Posted by Jeffy

So, it's not exactly an "explosive" exit like it is from a shooter with dual wheels spinning much faster than ours.

Er, yeah? I don't recall comparing the exits from the two shooters...

[Kellen Hill]

In designing a shooter, you must design around the ball you will be shooting and what you will be shooting it into. In 2006, I think the Poof balls were much more consistent in their squishy-ness or it was just a lot easier to shoot at a 30inch diameter near vertical goal. Either way, shooters that year did not require as much work due to a more consistent game piece and/or a large goal to shoot at.

This year however, we are playing with a game piece that changes quite a bit throughout the regional and is not consistent in squishy-ness to start with, along with a much more difficult goal to shoot at (18 inches and horizontal). With these factors, creating a shooter that would make every shot without adjustment took a little more time and effort.

From what I have found, the top three options for having a consistent shot are:

1. Catapult (16 is the best that I've seen: http://www.chiefdelphi.com/forums/sh...d.php?t=103876)
This approach removes ball squishy-ness from the picture. While it's harder to get significant backspin, you can land the ball in the same spot every time if you do it right. The trick is the getting the thing lined up and the right distance away from the hoop.

2. Extended acceleration period (2949 using belts: http://www.chiefdelphi.com/forums/sh...highlight=2949)
With lower compression and extended acceleration, each ball will exit the shooter at a consistent speed regardless of squish. Backspin is tweakable based on your setup. 2949 had a good setup with their shooter on a turret and their laser targeting system.

3. Shoot it hard with a lot of backspin (341 and I believe many others: http://www.chiefdelphi.com/forums/sh...d.php?t=103579)
For this setup, you need a decent amount of compression for this years ball and I think it helps a significant amount to be a tall robot. I will definitely be inspecting 341's shooter at champs to see just how they did it. As long as you can hit the backboard in this case, the backspin should put the ball right into the hoop as long as you are lined up correctly.


As to what we (525) did this season, we had to find another way to do it. We didn't wind up having enough height (we are about 3 ft tall) or enough compression on our shooter to easily do what 341 did. We thought about speeding up the last stage of our feeding mechanism to do more of what 2949 did. And obviously a catapult would have been quite a drastic change from what we had to work with. Our shooter used a 6inch Colson with one FP 0801-0673 with just short of 2 inches of compression on the ball and an adjustable hood. We decided we had to adjust our shooter speed/hood angle on a per ball basis in order to have a consistent shot. To do this we added a compression load cell (http://search.digikey.com/us/en/prod...6948-ND/809394) to measure each balls squish factor. You can find more information about our setup for the load cell here: http://www.chiefdelphi.com/forums/sh...d.php?t=105394. This setup got us to the point where we hit 9/10 balls in tele-op for one of our finals matches at 10,000 Lakes, hopefully something we can carry on to champs.

I'm sure there are other options for having a consistent shot, bot these are the three designs that have stuck out to me.

I hope this is helpful and I hope I remember all of this for the next game involving shooting a ball.

Kellen Hill

[rbmj]

I would say shooting for the backboard increased our shot consistency significantly.

Also, if you use a flexible back plate, make sure its braced to handle the force from the ball. Ours would lose a lot of energy as it was only held in place at a single contact point by a winch. You could see the plate buckle somewhat as the ball forced it back.

1218 had a similar back plate if i remember correctly, but it was braced by a solid assembly with multiple contact bearings so they had greater range and accuracy.

[IndySam]

Originally Posted by hill

From what I have found, the top three options for having a consistent shot are:

1. Catapult

2. Extended acceleration period

3. Shoot it hard with a lot of backspin


Kellen Hill

You forgot #4 Fling-A-Pult originated by 3940 and used by 292, 829 and 217

[Kellen Hill]

Originally Posted by IndySam

You forgot #4 Fling-A-Pult originated by 3940 and used by 292, 829 and 217

What exactly differentiates a Fling-A-Pult from a catapult? It's just something I don't have experience with and haven't personally seen this season. After just watching a couple of 829's matches, it seemed like a pretty consistent shot. More details/pictures of your Fling-A-Pult would be helpful.

[Hugh Meyer]

PID in the Jaguar works well. It would consistently recover in about 0.5 seconds after a shot. See attached graph for details.

-Hugh

[IndySam]

Originally Posted by hill

What exactly differentiates a Fling-A-Pult from a catapult? It's just something I don't have experience with and haven't personally seen this season. After just watching a couple of 829's matches, it seemed like a pretty consistent shot. More details/pictures of your Fling-A-Pult would be helpful.

I posted a pick.

[Kellen Hill]

Originally Posted by IndySam

I posted a pick.

Looks like another great way to deal with ball inconsistency. Consider it added to my list!

[Hawiian Cadder]

Originally Posted by Jeffy

Conveniently knowing both systems well allow me to answer.

Our shooter does not shoot the ball with a very high velocity (compared to most). The ball is compressed between a roller and a flat piece of poly carbonate. This allows the ball to be decompressed over a fairly large distance (~5 inches). So, it's not exactly an "explosive" exit like it is from a shooter with dual wheels spinning much faster than ours.

Morale of the story:
Allow the ball to compress and decompress over a distance rather than "instantly".

That is probably a more complete and better analysis than just "moar compression" I think that using relatively smaller wheels (3.875inches) also made the compression behave differently than most other shooters.

[DominickC]

What have we learned? Cannons are awesome

[CalTran]

Originally Posted by DominickC

What have we learned? Cannons are awesome

I really want to feel your robot up at Champs...

[billbo911]

The attached Periodic Tasks.vi include compensation for different loop intervals. This allows you to set the priority/frequency that you want to update the shooter motor drive. It also includes Slew Rate limiting that compensates for the interval time. Lastly, I cleaned up some data types conflicts.

[EDIT 4/13/12]Ether has made some refinements to his document on the Bang-Bang control. This latest version for LabView reflects those changes. [/EDIT 4/13/12]