Boulder inconsistency issue

[quillford]

Originally Posted by Max Boord

Thats what 4592 is running. we found 2 inches of compression was enough to shoot high goals from the low bar although I would not be surprised if 3-4 inches of compression would be more ideal. We currently run 3 4x7/8 colson wheels per side at 3370 RPM for most ranges. Also, the mechanism pushing the ball into the shooter wheels had a large effect on accuracy. Switching from pushing the ball by hand to a pneumatic indexer altered our RPM by about 200 to get the same shot.

Do you have a picture?

[Tom Line]

The short version:

If you look at a wheeled shooter as a simple ideal system, the only things that really come into play are:

1. Shooter moment of inertia.
2. Normal force on the ball.
3. Shooter wrap.

The first matters because the ball is generally in contact with the wheels for such a short period that the motor and control system really can't react to it. By the time your system sees a decrease in velocity and tries to account for it by increasing motor power, the ball is gone. That's why some teams use heavy shooter wheels or add flywheels.

The second item matters because the only thing that moves the ball is the wheel-to-ball friction. As the balls get more used, they become much softer. This decreases normal force on the ball, and decreases the friction force that moves the ball. You need to have enough force on the ball that the ball surface speed matches the wheel speed before it leaves the shooter. I.e. - wheel slip on the ball is bad because it will result in inconsistent distances. This was an important lesson that came out of 2012.

Shooter wrap matters because the longer the ball is in contact with the wheels, the more time it has to accelerate to the same speed as the wheels. 60 degrees versus 120 degrees may not seem like much visibly, but it is substantially more time that the wheels have to accelerate the ball so they arrive at the same speed. This was a lesson we learned from 2013. In-line small wheel shooters needed multiple wheels to match what a wrap-around single wheel shooter could do. This was actually a combination of all 3 of the issues listed above, but testing proved that wrap greatly helped on the slippery frisbee surface.

Use your old ball, and tune your shooter. Then put a new ball through it. The new ball will fly much further because it will be less squishy and have more normal force (more friction). Start increasing your compression. Eventually you will reach the cross-over point where the two are shooting the same.

Of course - there are a couple additional points. It takes some brute force to do this. Either a high initial RPM, a flywheel, shooter power, or all three. Consistently introducing the ball to the shooter in the same place each time is important.

[zinthorne]

Our shooter sees little to none or no variation between brand new and used balls. All shoot just as easily and are very accurate. The consistent shot has made our robot very competitive. The ball will alsways go in the same place 97% of the time and even when it does very it's only off by a few inches at most. We are using 2 775 pros for our shooter with 2 sets of 4 inch wheels. We are likely adding another 775 pro to aid in intake and make spin up time faster.

[Jaci]

We ran into a similar issue. The boulders on the field were significantly firmer than the ones we got in the KOP even before they were used.

Our shooter's holding mechanism relies on compression and stall torque. The main flywheels create such a strong vacuum when they spin up that they physically suck the boulder out the intake unless we run the intake in reverse, stalling the motor to hold the boulder in place. Because the boulders were much, much firmer, the MiniCIM direct couple that is our intake was not strong enough to hold the boulder in place, causing the shots to fire prematurely before the wheels got up to speed, resulting in poor shots.

In our case, it was a fairly easy fix. We adjusted the spacing on the intake's wheels to be further out, reaching further on the circumference of the boulder and therefore not causing as much compression, allowing us to stall the boulder into place. Image for reference

[waialua359]

Teams are lucky that they dont have to worry about limiting muzzle velocity like in 2006.
I have already seen some pretty fast (and dangerous) linear shooters, but at the same time can limit the effects that boulder inconsistencies can cause.
Potential inconsistency is the primary reason we went with a catapult design this season.

[Arhowk]

Originally Posted by quillford

We're running them with versaplanetaries 1:1 just for the ease of mounting. Are you shooting in the high goal with that much reduction?

Oh my bad I thought you were having intake issues. We use a 1:1 fly wheel shooter running from a CIM motor. We have a decent bit of aided compression with that as well (the shot goes through the intake motor which we drive forward when we shoot to get a cleaner shot). (That intake motor is the one running 30:1). Don't know how much ball difference there was at Lansing but we got first in qualifications so we did something right .

[Sperkowsky]

Boulder inconsistency really really hurt our team. Our issue wasn't accuracy related and we were testing with 8 different boulders throughout the season. Basically what was wrong was our pneumatic indexer. The indexer had a 3/4" stroke which worked fine for all 8 of our boulders. We even went to the Suffield Shakedown and played half the day without having one issue. Then what continued to happen was the ball would come in contact with the wheels rendering us useless. We tried switching to our next smallest wheels 2 7/8" BB wheels which worked for a few Matches until we met a boulder that was too small. Then the indexer couldn't hit the ball into the wheels. All in all it was not fun to try to figure out what kind of Boulders were on the field. We have already developed a pretty simple fix (longer cylinder farther back) but it's a little late. Luckily we have 3 off season competitions to play with it.

[Ninjastahr]

My team has the same problem (we were forced to low goal for all of the Iowa Regional). We compressed the ball more, and now it works much better with different balls. We use two 2" orange BaneBots wheels with a cylindrical piece of metal between them on each side (the piece of metal makes it so the wheels don't stall when the ball hits the wheels).

[Ether]

Originally Posted by Ninjastahr

We use two 2" orange BaneBots wheels with a cylindrical piece of metal between them on each side (the piece of metal makes it so the wheels don't stall when the ball hits the wheels).

This is a case where a picture would be worth a thousand words.

[Ninjastahr]

Here's a picture.